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Electronic Confinement Effect in Silica-Based Materials
C H A P T E R
16 Electronic Confinement Effect in Silica-Based Materials Boiko Cohen and Abderrazzak Douhal Department of Physical Chemistry, Faculty of Environmental Science and Biochemistry and INAMOL, University of Castilla la Mancha, Toledo, Spain
16.1 INTRODUCTION When a molecule is encapsulated within the nanochannels of silica-based material its spectral and photophysical properties may be altered significantly.1 This in turn may give rise to variations in optical band-gaps correlated with changes in the excited-state lifetimes, absorption, emission spectra, and quantum yields, for example, of the encapsulated guest molecules. When a tight host/guest confinement is possible (in other words when the size of the guest is comparable with the size of the host pore), these changes reflect variations in basicity/acidity and redox proprieties of the encapsulated guest, for example. The structural properties of channels and cavities with molecular sizes, like in zeolites have a strong influence on diffusion and solvation effects of reactant molecules, and they can become determinant for the catalytic activity of these materials.14 Diffusion, which involves the dynamics
Chemistry of Silica and Zeolite-Based Materials DOI: https://doi.org/10.1016/B978-0-12-817813-3.00016-X
of the molecule into the channels, is described by a statistic mechanical model which correlates well with experimental results.2,3,58 Solvation or cavity effects that involve physicochemical interactions between the zeolite pore and the guest molecule are the subject of the electronic confinement model.911 The apparent variations in the properties of the guest molecules cannot be explained only in terms of the influence of the host electrostatic field on the stabilization of polar guests. The classical type of interactions normally considered are the Coulomb interactions and coordination effects.9 The formers arise from a charge distribution along the silica framework owing to the partial ionic character of the aluminosilicate hosts. The generated Coulomb electric field into the cavities, might influence the chemical behavior of the guest molecules. The coordination effects, on the other hand, are produced by a Lewis acidbase type interaction among the guest and specific sites of the host, such as hydrogen-bond (HB)
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electron donoracceptor centers. The presence of these active sites in the host framework favors the adsorption of molecules.9 Weak electron interactions can account for forces of van der Waals type, which could produce and reinforce the “docking” of a molecule into a zeolite or mesoporous cavity. These interactions are further amplified by the surface curvature of the framework walls, which interact with the adsorbed guest molecules. An electronic confinement concept has been proposed based on the idea that the molecular orbitals (MOs) of the guests inside the host (zeolite) pores are not extended throughout the space but instead are limited to within the zeolite walls.9 It is expected that this confinement effect is stronger as the size of the confined guest approaches the host (zeolite) cage dimension, producing an energy increase in all MOs of the guest, particularly those that are more diffuse. Due to the partial covalent character of the aluminosilicate crystal, the electrons are not localized on the framework atoms, but they are partially delocalized through it. This causes the density of the orbitals of the guest molecule to suddenly drop to nearly zero when reaching the walls of the zeolite as a consequence of the short-range repulsion with the delocalized electronic clouds of the lattice. This situation implies a contraction of the orbitals of the adsorbed guest, with the consequent changes in its energy levels.9 The proposed electronic confinement model in mesoporous and zeolite materials draws similarity with the quantum confinement effect in semiconducting nanomaterials such as quantum dots and quantum nanowires, for example.
16.2 QUANTUM CONFINEMENT IN SEMICONDUCTOR CRYSTALS The quantum confinement effect arises essentially due to changes in the atomic
structure as a result of the influence of ultrasmall length scale on the energy-band structure of the semiconducting nanomaterials.12 For semiconductor crystals, the electronic excitation produces loosely bounded electron hole pairs (the MottWannier exciton) which are mostly delocalized over a length much longer than the lattice constant. As the size of the semiconductor material approaches the exciton Bohr diameter, its electronic properties start to change. In general, the Bohr radius of a particle is defined as12,13: aB 5 ε
m a0 m
(16.1)
where ε is the dielectric constant of the material, m* is the mass of the particle, m is the rest mass of the electron, and a0 is the Bohr radius of the hydrogen atom. When the particle size approaches the Bohr exciton radius, the quantum confinement effect causes an increase in the excitonic transition energy and a blue shift in the absorption and luminescence band gap energy.12,13 In addition to that, the quantum confinement leads to a collapse of the continuous energy bands of a bulk material into discrete, atomic-like energy levels. The discrete structure of the energy states leads to a discrete absorption spectrum, which is in contrast to the continuous absorption spectrum of a bulk semiconductor. A quantum confined structure is one in which the motion of charge carriers (electron and hole) is confined in one or more directions by potential barriers.14 According to the confinement direction, a quantum confined structure is classified into three categories as quantum well, quantum wire, and quantum dots or nanocrystals. When a higher number of dimensions is confined, more discrete energy levels can be found, in other words, carriers movement is strongly confined in a given dimension. In a bulk semiconductor, for example PbS or CdS quantum dots, the electron and hole are bound together by a screened Coulomb interaction to form a
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MottWannier exciton. By assuming the energy-band to be parabolic near the band gap (i.e., the effective mass approximation), the size-dependent shift (with respect to the bulk band-gap) in the exciton energy of a small cluster (where the cluster radius B the exciton radius) can be derived as15: EðRÞ 5 Eg 1
π2h2 1:786e2 2 0:248ERy 2 2m R2 εR (16.2)
where R is the cluster radius, 1=m 5 1=me 1 1=mh , me is the electron effective mass, mh is the hole effective mass, ε is the dielectric constant, and ERy is the effective Rydberg energy. The first term in the above equation is the band-gap of the bulk materials, the second represents the particle—in-a-box quantum localization energy and has a simple 1/R2 dependence, the third term is the Coulomb energy with a 1/R dependence, and the last one is the result of the spatial correlation effect. This last size-independent term is usually small but becomes significant for semiconductors with small dielectric constant.
16.3 ELECTRONIC CONFINEMENT MODEL IN SILICA-BASED MATERIALS The electronic confinement in silica-based materials is associated with the previous described quantum confinement, despite the electronic confinement being induced by the intrachannel surface and not by the exciton Bohr diameter. Moreover, quantum confinement effect cannot be expected to be quantitatively correct for very small clusters; in contrast, the electronic confinement effect focuses on both inorganic compounds and organic molecules within nanopores. Hence these clusters, loaded in inorganic hosts, must be extremely small, and may be even treated
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as single molecules in some cases.15 The new phenomena for such small clusters should be explained by the concepts derived from the electronic confinement theory. A confined model system can be built, for example, by locating the conjugated molecules parallel to two surfaces, which, for simplicity, may be simulated by two infinite planes located at a certain distance. In this case, the atomic orbitals can still have as the normal 2pz orbitals, a node at the molecular plane but are adapted to the confinement, which makes them absent beyond the limit of the plane.911,15 The model of electronic confinement is based on the Hu¨ckel molecular orbital (HMO) theory and accounts for the changes in the conjugated π systems as a result of the guest encapsulation in the nanocavities of zeolites (parallel to the walls of the channels for the strongest coupling between the host and the guest).9 For the development of the model the closed-shell single-determinant approximation is applied, which treats each electron as moving independently in a mean electrostatic field of the other electrons and the nuclei. In the applied approximation, only two empirical parameters are required—the Coloumb integral, α, and the resonance integral, β. For simplicity, the chosen guest is ethylene, which has only two electrons in the π system. In this case, the solutions for the bonding highest occupied molecular orbital (HOMO) and antibonding lowest unoccupied molecular orbital (LUMO) and their respective energies given by the HMO theory are9: jπi 5 jai 1 jbi; EHOMO 5 α 1 β
jπ i 5 jai 2 jbi; ELUMO 5 α 2 β
(16.3a) (16.3b)
where jai and jbi are the 2pz atomic orbitals on carbon atom a and b, respectively, jπi and jπ i are the bonding (HOMO) and antibonding (LUMO) π molecular orbitals, and EHOMO and ELUMO are the corresponding energies given as a function of α and β parameters. For the
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confined molecule, the two parallel planes are placed at a distance equal to the diameter of the cavity, while the guest is located equidistant (at d distance of each plane) and parallel to both planes. If one assumes that the symmetry of the system remains unchanged, then the solutions given by the HMO theory under the same approximation will have the same expression as for the unconfined system9: 0 0 0 π i 5 a0 i 1 b0 i; E0 (16.4a) HOMO 5 α 1 β 0 0 0 0 0 π i 5 a i 2 b i; E0 (16.4b) LUMO 5 α 2 β In these expressions, ja0 i and jb0 i are the 2pz atomic orbitals of the confined carbon atoms, jπ0 i and jπ0 i the respective bonding (HOMO) and antibonding (LUMO) π molecular orbitals, while α0 and β 0 are, respectively, the Coulombic and resonance integrals for the confined system. Following the boundary condition for the encapsulated system that the probability of finding an electron belonging to the guest molecule outside the cavity is zero (i.e., the difficulty of these electrons to penetrate through the surface electron clouds), the
jπ0 i and jπ0 i cannot be considered as true π orbitals. Although, like the normal π orbitals they have a node at the molecular plane and they also have to adapt to the boxing, which makes them disappear beyond the cavity limits. Next, to evaluate the energy changes as a result of the electronic confinement, one can use an expression that relates the energy variation of a given electronic state when confining the system, with the corresponding wave functions: Ð 2 ðxÞð@N Ψ 0 ÞðxÞ h S dSΨ Ð ΔE 5 2 (16.5) 2m V dτΨ ðxÞΨ 0 ðxÞ Here Ψ and Ψ 0 are the wave functions of a given energy state for the free and confined systems, respectively (Fig. 16.1A). It should be noted that while in the denominator the integration is over the cavity volume, V; in the nominator, it is performed over S, the cavity border surface. By taking into account the symmetry conditions imposed on the model, the energy
FIGURE 16.1 (A) Schematic representation of the Ψ function (see the text) for x and y fixed at any point of space. The thin line depicts the unconfined wave function and the thick line depicts the confined one. Dashed line represents the cavity limit. (B) Qualitative description of the π orbital energy spectrum for the unconfined (left-hand side) and confined (right-hand side) ethylene molecule. Source: Reprinted with permission from Zicovich-Wilson, C. M.; Corma, A.; Viruela, P. Electronic Confinement of Molecules in Microscopic Pores—A New Cconcept Which Contributes to Explain the Catalytic Activity of Zeolites. J. Phys. Chem. 1994, 98 (42), 1086310870. Copyright 1994 American Chemical Society.
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changes of the one-electron states of the ethylene molecule then may be expressed as9: ΔEHOMO 5 E0HOMO 2 EHOMO 5 Δα 1 Δβ (16.6a) ΔELUMO 5 E0LUMO 2 ELUMO 5 Δα 2 Δβ (16.6b) where Δα and Δβ are (in atomic units): ÐN 1 2N dxdyaðx; y; dÞ@z a0 ðx; y; dÞ Δα 5 2 Ð d Ð N 2 dz dxdyaðx; y; zÞa0 ðx; y; zÞ 2N
0
(16.7a)
ÐN
dxdyaðx; y; dÞ@z b0 ðx; y; dÞ 1 Δβ 5 2 Ð d 2NÐ N 2 dz dxdyaðx; y; zÞa0 ðx; y; zÞ 0
2N
(16.7b) Here, aðx; y; zÞ and bðx; y; zÞ are the 2pz atomic orbitals on atom a and b, respectively; a0 ðx; y; zÞ and b0 ðx; y; zÞ define the equivalent atomic orbitals of the confined system; @z is the partial derivative with respect to the z direction and d is the distance between the guest molecule and the cavity border. By analyzing the two expressions, it is easily concluded that both Δα $ 0 and Δβ $ 0, which is explained by the simultaneous increasing/decreasing of a0 and b0 at the cavity limit. Finally, the Coulomb and resonance integrals of the confined system can be written as9: α0 5 α 1 Δα $ α
(16.8a)
β 0 5 β 1 Δβ $ β
(16.8b)
The developed model shows that when the energy gap between π and π* orbitals decreases (Fig. 16.1B), upon confinement of the guest, the π bond should become weaker than when the molecule is unconfined. In a further theoretical study, applying semiempirical calculations, which were interpreted by means of the HMO theory, the confining space of the cavities was modeled using a mica sheet with a moleculesurface distance in the ˚ .16 These studies have range of 1.54.0 A
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expanded the model beyond the ethylene molecule, by studying the confinement of larger aromatic organic molecules (benzene, naphthalene, and anthracene) in the mesoporous and nanoporous cavities of silica-based materials. In agreement with the previously developed model,911 the results from this study have shown that the encapsulation of the larger aromatic systems alters their electronic properties as a result of changes in the molecular orbital energies and band-gaps.16 Furthermore, this work has provided additional evidence that the HOMO is more sensitive to confinement than the LUMO, and the overall effect is a reduction of the band-gap of the frontier MOs when the moleculesurface distance is less than ca. ˚ .16 2.5 A
16.3.1 Ensemble Average and Single-Molecule Studies An experimental evidence of electronic confinement was reported for anthracene encapsulated within zeolites.17 The study showed a bathochromic shift of the 00 transition from 375 nm in 3-methylbutane to 396 nm in NaY and to 419 nm in ZSM-5 (Fig. 16.2A). This observation translates in a strong correlation between the size of the nanochannel and the value of the bathochromic shift—the smaller the pore size of the zeolite, the larger is the bathochromic shift of the 00 transition. The observed effect was also linked with the shortening of the fluorescence lifetime (from 4.5 ns for free anthracene to 3.2 ns within NaY zeolite and ,100 ps in mordenite or ZSM-5). Overall, these experiments have related the change in the spectroscopic properties of the encapsulated anthracene with the increase of the HOMO energy and with the reduction in the HOMOLUMO band gap (Fig. 16.2B).17 Other studies have provided additional experimental evidences for the electronic
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FIGURE 16.2 (A) Excitation and emission spectra, at room temperature, of anthraceneNaY (a), anthracenesodium mordenite (b), and anthraceneNaZSM-5 (c). (B) Plots of the antracene HOMO in the gas phase (a) and upon confine˚ (b). Source: Reprinted with permission from Marquez, F.; Garcia, H.; Palomares, ment inside two mica sheets separated by 5.4 A E.; Fernandez, L.; Corma, A. Spectroscopic Evidence in Support of the Molecular Orbital Confinement Concept: Case of Anthracene Incorporated in Zeolites. J. Am. Chem. Soc. 2000, 122 (27), 65206521. Copyright 2000 American Chemical Society.
nanoconfinement effect in MCM-41 and zeolite materials. Results, comparable to those for anthracene encapsulated within zeolitic frameworks, have been reported in steady-state and dynamic spectroscopic studies of Zn (phen)2(NO3)2, (phen 5 1, 10-phenanthroline), within nanoporous MCM-41.18 In similarity with the results for anthracene encapsulated within zeolite frameworks,17 the observed bathochromic shift of the 00 transitions, along with shortening of the excited-state lifetimes have been correlated with the reduction of the HOMOLUMO band-gap accompanied by the increased energies of the frontier orbitals. This trend indicates that the electronic confinement of organic molecules is also realized in larger cavities, such as the ones of MCM-41. The encapsulation of N,N0 -bis(2hydroxy-5-methylbenzylidene)-1,2-ethanediamine, a Schiff-base molecule, in the mesoporous MCM-41, has also given support that the electronic confinement effect is significant even in the larger channels of MCM-41. 19 In this study, the 00 transition of the guest in DMSO, which was reported at 23,041 cm 21
(2.86 eV), shifts to 21,413 cm 21 (2.66 eV) in MCM-41. The observed was again explained in terms of a decrease in the HOMOLUMO band-gap. Additionally, as the electrostatic effect could not explain the observed shift of the 00 transition from DMSO to that in MCM-41, the trend could only be rationalized by the molecular orbital confinement theory.19 Thus, as a result of the electronic nanoconfinement, all the energy levels of the guest Schiffbase molecules will increase in the nanoporous channels host. As the increase for the HOMO has been predicted to be more sensitive than the LUMO, the expected overall effect is a reduction on the band-gap of the frontier orbitals. These studies also argued that an asymmetric confinement is present with the guest molecules being much closer to one wall than to the other one.18,19 Thus, the motion of electrons will be more restricted in one direction (along the wall of the nanoporous channels) than in the other directions. Although not directly related to the electronic nanoconfinement effect, significant variations in the photophysical properties of pyronine molecules with
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different molecular sizes (pyronine Y and pyronine B) incorporated inside zeolite crystals that have similar channel sizes, such as zeolite L and AlPO 4 -5, were observed by means of fluorescence polarization.20 The experimental results indicate that the dye molecules’ spectroscopic properties are strongly affected by the framework structure of the host, including the specific topology of the zeolite channels, which induces an angle dependence of the guest fluorescence behavior. The observed change in the optical properties of both molecular systems was explained in terms of the confinement effect.20 When an amino derivative of 2-(20 -hydroxyphenyl)benzothiazole (HBTNH2) was encapsulated within MCM-41 and Al-doped MCM-41, a strong variation in its spectroscopic properties was detected, as revealed by steady-state and time-resolved techniques.21 The study did not report any effect of the confinement of the nanochannels on the excited state intramolecular proton transfer reaction in the enol structure of HBTNH2 to give the keto tautomer. On the other hand, the component assigned to the excited-state lifetime of the twisted keto form of HBTNH2 increases from 22 ps in acetonitrile solution to 4045 ps within the MCM-41 channel providing additional verification for the electronic confinement effect. Notably, this system provides an example for the more complex nature of the electronic confinement effect. In the examples presented above, the HOMOLUMO energy gap decreases upon encapsulation (Fig. 16.1B). Thus the lifetime of HBTNH2@MCM-41 is expected to decrease due to faster internal conversion following the energy gap law. However, the lifetime of HBTNH2@MCM-41 increased only mildly, showing that the overall effect is not strictly a result of the modification of HOMO and LUMO energies due to electronic confinement, but also arises from the restriction in the rotation of the molecule resulting in the slower deactivation of the excited keto state.
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Further experimental evidence for the electronic nanoconfinement on the properties of HBTNH2 but at the single-molecule level has also been reported (Fig. 16.3).22 Time- and spectrally-resolved studies at the singlemolecule level have demonstrated that depending on the method used for the guesthost complex preparation, different sites in the host framework (MCM-41) can be accessed by the guest.22 The averaged emission spectrum of the single-molecule complexed to Aldoped MCM-41 host (HBTNH2/Al-MCM-41) consists of a single band centered at 583 nm with a full width at half maximum (FWHM) of intensity of 64 nm (B1480 cm21). The emission spectrum was assigned to the trapped anion (deprotonated phenol of the dye. This spectrum is much narrower than the one of the keto form generated using the undoped MCM41 (B3250 cm21) samples (HBTNH2/MCM41). This difference, along with the narrower lifetime distribution, is a clear indication of a stronger confinement of the local nanoenvironment around the single anion of HBTNH2 in the Al-MCM-41 channels. The replacement of Si with Al atoms changes the electronic density distribution within the framework of MCM-41, which in turn affects the hostguest orbital interaction so that the resulting emission comes from a much narrower density of the states of the single confined chromophore. Similar dependence on the presence of Al in the MCM-41 framework has been reported for DY-630-MI, a hemicyanine dye.23 Studying its single-molecule behavior, showed that replacing a few Si41 ions with Al31 ones in the framework of regular MCM-41 changes the local electrostatic field within the nanocannels, which results in a more selective orientation of the DY-630-MI molecules. Comparing the distribution of the polarization (P) values for DY630-MI/Al-MCM-41 with those for the free molecule adsorbed on the glass coverslip and the DY-630-MI/MCM-41 complexes, a stronger shift (61%) toward the positive limit (P 5 0.71)
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FIGURE 16.3 Single-molecule microscopy results of (A) 2-(20 -hydroxyphenyl)benzothiazole (HBTNH2) covalently bonded to MCM-41, (B) silica nanoparticle, and (C) Al-MCM-41. Emission decays of HBTNH2 covalently bonded to (D) MCM-41, (E) silica, and (F) Al-MCM-41, along with the best nonlinear least-squares monoexponential fits (solid lines). (G), (H), and (I) are the distribution of the lifetimes from the monoexponential fit to the decays of single-molecule MCM41, silica and Al-MCM-41 composites, respectively. (J), (K), and (L) represent the average emission spectrum of a single HBTNH2 covalently bonded to a silica nanoparticle and Al-MCM-41 nanochannels, respectively. The dashed line in part (K) represents the Gaussian fit to the main contributing band in the average spectrum of single HBTNH2silica complexes in the presence of the strong base 1,8-diazabicyclo[5.4.0]undec-7-ene. Source: Adopted with permission from Cohen, B.; Sanchez, F.; Douhal, A. Mapping the Distribution of an Individual Chromophore Interacting with Silica-Based Nanomaterials. J. Am. Chem. Soc. 2010, 132 (15), 55075514. Copyright 2010 American Chemical Society.
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is observed (Fig. 16.4). This behavior is indicative of an increase in the number of more oriented single chromophores in the nanochannel due to the modified electrostatic environment around the encapsulated single molecule.23 The rigid frameworks of zeolites can act as a host in which silver clusters can be stabilized, enabling a high degree of control over their optoelectronic properties (Fig. 16.5).2427 The dependence of their electronic properties on spatial confinement was studied by characterizing the ionization potential of the clusters embedded in four different zeolite environments over a range of silver concentrations.25,26 The chosen faujasite frameworks (FAUX and FAUY) differ in the Si/Al ratio of the framework, while the two LTA zeolites differ in the
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alkali metal charge-balancing cation (Na1 and K1, respectively).25 The results revealed a strong influence of silver loading and of the host environment on the Ag cluster ionization potential (over a range of .0.5 eV), which is also correlated with the cluster’s optical and structural properties. Using zeolites as host, the photoluminescence quantum yields of the composites could be optimized to reach nearly 100% by careful tuning of the mobilities of nonframework metal cations. In a subsequent study, by measuring exclusively the local structure of the emissive Ag clusters in partially exchanged LTA zeolites, their functional structures have been identified.26 DFT modeling based on these detailed structures showed that the double positively charged Ag4(H2O)4
FIGURE 16.4
Molecular structure of DY-630-MI (A) and schematic representation of the structure of MCM-41 and AlMCM-41 (B). Emission polarization (P) distribution histograms for (C) free DY-630-MI and interacting with (D) MCM-41 and (E) Al-MCM-41 materials, respectively. Source: Reprinted with permission from Cohen, B.; Alvarez, C. M.; Carmona, N. A.; Organero, J. A.; Douhal, A. Single Molecule Photobehavior of a Chromophore Interacting With Silica-Based Nanomaterials. Phys. Chem. Chem. Phys. 2011, 13 (5), 18191826. Copyright 2011 Royal Chemical Society.
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FIGURE 16.5 Activation of luminescent Ag-clusters confined inside the sodalite cages of faujasite (FAU) and Linde-type A (LTA) zeolites. Source: Reprinted with permission from Coutino-Gonzalez, E.; Baekelant, W.; Steele, J. A.; Kim, C. W.; Roeffaers, M. B. J.; Hofkens, J. Silver Clusters in Zeolites: From Self-Assembly to Ground-Breaking Luminescent Properties. Acc. Chem. Res. 2017, 50 (9), 23532361. Copyright 2017 American Chemical Society.
and Ag4(H2O)2 clusters, in which water ligand molecules modulate the HOMOLUMO gap, behave as confined two-electron helium or alkaline earth-like superatom quantum systems that mainly emit via their long-lived lowest-lying triplet excited state. Thus, it was concluded that the mechanism of Ag clusters bright luminescence when encapsulated within different silica-based materials depends on the combined effects of the interaction with oxygen ligands, electron confinement, electrostatic interaction, and charge transfer to the surrounding silver atoms.26 Electronic nanoconfinement effect was demonstrated also for photoactivated luminescent defects in SiO2 nanoparticles.28,29 In studies of the photoluminescence of defect centers in single 11-nm SiO2 nanoparticles, it was suggested that the strongly inhomogeneous local chemical environment around the centers results in a broad and random variation of the single nanoparticle emission and excitation spectra, fluorescence lifetime, and quantum yield. Both the emission and excitation spectra of different luminescent centers have shown a shift within the range of 2.0 2 2.4 eV. Despite this broad variation, the shape of the emission spectrum
remains constant, which suggests that the fluorescence comes from the same type of defects. As a result, this work demonstrated evidence for the electronic nanoconfinement effect on the properties of the luminescent defects in the SiO2 nanoparticles.
16.3.2 Electronic Confinement Effect on Charge Transfer Reactions Although not as evident and straightforward, the electronic nanoconfinement might be also involved in the generation, confinement, and stabilization of reactive intermediates in the cavities and channels of zeolites and other silica-based materials 2,3,30 It should be noted that in this type of systems, due to the presence of charged species in the ground and excited states, clear distinction between the involved effects is difficult to be made.3 In the radical generation and stabilization processes, the rigidity of the zeolite environment restricts vibrational and rotational motion of the tightly fitted guest species, leading to a fast intramolecular relaxation process. Stabilization at room temperature was reported for
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electronically excited states, such as radicals, and radical ions. For example, a study has reported on a room temperature phosphorescence with lifetimes similar to those observed at 77K for aromatic species such as naphthalene and 9-ethylcarbazole confined in a channel-type zeolite L.3 In the initial model, the walls of the confining media, have been treated as entirely reflecting boundary without contributing directly to the photophysical properties of the guest. One of the first works that suggested a more active role of the host framework studied the encapsulation and interaction of toluene within several zeolite cavities and in presence of varying Al content.31 Ab initio calculations showing frontier molecular orbital energy modifications of a toluene when located inside a microporous cavity (zeolites Beta, ZSM-12, and ZSM-5) have been performed. The results have shown that the HOMO energy of toluene increases when going from the gas phase state to restricted microporous environments. It was demonstrated that the smaller the zeolite cavity the higher is the rise in the toluene HOMO energy. The effect of the chemical composition on the toluene HOMO energy was tested in the ZSM-5 zeolite by varying the Al content. The results obtained showed that the frontier orbital energy increases upon decreasing the Al content of the cluster. As a consequence of the confinement effect, it was suggested that the toluene reactivity in zeolite-catalyzed reactions is expected to change toward a more covalent behavior in which electronic transfer from the toluene molecule to an electronic acceptor will be more favored.31 In a subsequent work, upon entrapping photosensitizers within inorganic supports such as zeolites, the determination of the band-gap on the basis of UVvisible measurements showed the presence of electronic interactions between the host and the guest, which result in a lowering of the band-gap of the inorganic support.32 The optical band-gap
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energy was studied in a series of metallophthalocyanine (Pc) and 2,4,6-triphenylpyrylium (TP1) ions encapsulated within different hosts (Y zeolite, mesoporous MCM-41, TiO2SiO2 and SiO2) in order to gain better understanding of the interaction occurring between the entrapped complexes and the host support. The band-gaps calculated in this study for the used porous supports were: 4.28 eV for NaY, 2.65 eV for MCM-41, 4.8 eV for the mesoporous TiO2SiO2,and 3.65 eV for the pure SiO2.32 The entrapment of metallophthalocyanine complexes in the cages of NaY resulted in a notable decrease of the band-gap irrespective of the nature of the metal (on average 2 eV). This decrease was explained in terms of an electronic interaction of the complex with the support. Additionally, although smaller, differences were also induced by the nature of the metal. In this case, the band-gap increased in the order FePc , ZnPc , MnPc , CoPc , NiPc , CuPc ranging from 2.16 to 2.47 eV. Significant effect on the band-gap was also observed when TP1 was encapsulated within the hosts.32 The entrapping of TP1 in NaY zeolite decreased the host HOMO-LUMO band-gap to 2.83 eV, a value higher than that for metallophthalocyanines. Although smaller, the entrapping of these photosensitizers in TiO2SiO2 or SiO2 led also to a decrease of the band-gap. The FePc decreased the band-gap of TiO2SiO2 by about 1.8 eV, to 2.98 eV. Similarly to the NaY case, TP1 decreased the band-gap of TiO2SiO2 by a smaller value, to 3.65 eV. For the SiO2 host, the decrease of its band-gap was observed after deposition of the organic sensitizer. In comparison with the metallophtalocyanines, the observed band-gap decrease was smaller for FePc (to 2.81 eV) than for TP1 (to 2.65 eV). Finally, when MCM-41 was used as support, the decrease of the band-gap was smaller than for the other supports. It was insignificant for FePc (only 0.2 eV decrease to 2.63 eV). TP1 encapsulation resulted in a value
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of 2.51 eV for the band-gap of SiO2. Thus, the optical spectra and the band-gap calculations have shown the presence of an electronic interaction between the entrapped photosensitizers and the porous materials, with the interaction increasing with the increase in the confinement between the solid pore and the photosensitizer. Further studies of the photocatalytic activity of the metallophtalocyanine complexes toward the photodecomposition of dipropyl sulfide showed a good correlation with the band-gap values.32 Additional evidence for the active role of the support in the electronic confinement effect was demonstrated in a study of the spontaneous charge separation of trans-stilbene (t-St) upon incorporation within the 10 membered rings (10-MR) channel of H2.2-GaZSM-5 support.33 The framework structure of MFI zeolites, including GaZSM-5, contains two types of intersecting channels, both formed by 10MR apertures. The t-St molecule enters the channels through the pore openings of the available faces of the zeolite crystals. By comparing the behavior of t-St encapsulated in different ZSM-5 zeolites (Na6.6-AlZSM-5, H3.4-AlZSM-5 and H2.2-GaZSM-5) the study has demonstrated that t-St is present as a neutral molecule in Na6.6-AlZSM-5, while the accessible protons in the inner surface 10-MR channel of H3.4-AlZSM-5 and H2.2-GaZSM-5 are fundamental for the t-St ionization.33 The ionization efficiency depends both on the ionization potential values of the sorbate and on the ionizing power of the host. The ionization energy of molecules encapsulated in the condensed phase is expected to depend on the ionization potential in the gas phase, the energy of the antibonding orbital in a localized orbital state, and the polarization energy imposed by the confinement with surrounding ions.34 As a result of the electronic confinement effect, the tendency of a guest molecule encapsulated in the zeolite channels toward giving electrons to the acid sites will increase,
which in turn will enhance its reactivity.9,31 In the reported study, it was suggested that the energy of the antibonding orbital of the ZSM-5 supports should depend significantly on the metal content (Ga or Al) and only weakly on the nature of the extra framework cation.33 Since the electrostatic field is dependent on the cation size and on the Si/Al ratio along with the steric constraints in the 10-MR channel, it was argued that the combined effects of the high polarization energy in close proximity of accessible acid site SiO(H)Ga within 10-MR channel and favorable antibonding orbital energy of GaZSM-5 induce spontaneous ionization of t-St by lowering the ionization energy. As a result, no supplementary Lewis acid site is necessary to induce the t-St spontaneous ionization in both H2.2-GaZSM-5 and H3.4-AlZSM-5. The 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane[tet-a] nickel(II) complex has been studied when encapsulated in zeolite Y.35 The energy of HOMO and LUMO levels for the neat nickel(II) complex were reported to be 3.78 eV and 2.448 eV, respectively, while those of the encapsulated complex were 4.678 eV and 2.271 eV, respectively. The geometry of the neat complex was found to be octahedral as the two chlorine atoms are coordinated to the central metal ion in an axial position. However, the complex within the zeolite cage is in a distorted square planar geometry due to a counterion effect and steric hindrance of the zeolite Y on the complex. Thus, it was suggested that the changes in the energy levels of the frontier orbitals might be due to distortion in the geometry of the complex and the counterion effect of zeolite Y.35 Interactions between the complex and zeolite were considered as Coulombic and coordination effects. In the case of the neat complex, the HOMO lies at the axial chlorine atoms, and the LUMO lies at the nitrogen atoms of the tet-a ligand. However, both the HOMO and LUMO lie on the nickel and nitrogen
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atoms of the complex. After encapsulation, the HOMO level is stabilized at a lower energy while the LUMO is positioned at a higher energy level and as a result the energy gap increases. The increase in the energy gap was experimentally supported by the absorption spectrum of the encapsulated complex. This one shows a shift toward the blue region of the spectrum, i.e. towards the high energy region, which indicates that structural distortion contributed to the increase in the energy gap, which further confirms that the complex in zeolite would certainly favor oxidation.35 Several theoretical works have also evaluated the energetics of electron transfer molecular systems encapsulated within different silica-based materials.36,37 A multiscale simulation was used to study the mechanism for the suppression of charge recombination of 9-mesityl-10-methylacridinium ion (MesAcr1) in the mesoporous aluminosilicate Al-MCM-41.36 Experimental works have provided evidence that the charge-transfer excited-state lifetime of MesAcr1 can be dramatically extended via encapsulation in Al-MCM-41.38,39 The simulations have shown that solvation in acetonitrile (MeCN) and encapsulation in Al-MCM-41 have opposing effects on the reorganization energy (λ) of MesAcr1. While the solvation in ACN substantially raises the reorganization energy, the encapsulation lowers it (Fig. 16.6). Thus it was concluded that the longer lifetime of the solvent-free MesAcr1@Al-MCM-41 results from the smaller solvent reorganization energy inside the mesoporous silicaalumina as compared in a MeCN solution. This is because the smaller the value of λ of electron transfer, the slower the rate becomes of the back electron transfer in the Marcus inverted region, where the driving force of back electron transfer is much larger than the λ value of electron transfer (Fig. 16.6). Finally, in a recent theoretical study, the confinement effects of a zeolite framework on the adsorption of a bidentate 4,40-bipyridine (44BPY) ligand on the straight channel of
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H-ZSM-5 have been investigated by density functional theory calculations using different functionals, such as B3LYP, B3LYP-D3, M062X, M06-2X-D3, along with the MP2 method with two basis sets 6-31 1 G* and 6-3111G (2d,2p).37 The straight channel was simulated by a realistic cluster model of 32 tetrahedra (T), having two aluminum atoms located in the straight (closed region, cl) and intersection regions (open region, op), respectively. In that way, the two Al atoms are located at positions sufficiently distant from each other allowing the bidentate 44BPY to interact via its N atoms with the two Brønsted acid sites of the zeolite cavity. The results have demonstrated that the B3LYP functional cannot be used to compute adsorption energies in the case of 44BPY adsorption in the H-ZSM-5 zeolite, while the M06-2X adsorption energies obtained using the 631 1 G* basis set were comparable to those calculated at the MP2 level with the extended 6-3111(2d,2p) basis set. Additionally, it was shown that the stability of the 44BPY adsorption complexes implicated in a proton transfer reaction between the Brønsted acid sites of HZSM-5 and the 44BPY ligand is completely governed by dispersive van der Waals interactions. With dimensions that are comparable to the channel diameter of H-ZSM-5, the 44BPY bidentate ligand can be used as a probe molecule to evaluate the influence of the steric constraint and dispersive van der Waals interactions exerted by the zeolite framework on the energetics of adsorption complexes. The results from the study have shown that steric constraints destabilize the 44BPY adsorption complexes, especially in the confined closed region, while in contrast, the dispersive van der Waals interactions contribute to the stabilization of these complexes and fully overweigh the steric constraints. The energetics of the potential energy surface of the proton transfer reactions allow to exclude completely the presence of the monodentate ion pair complex (44BPYH1/32T2)cl associated with a very
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FIGURE 16.6 Marcus curves for CR in photoexcited MesAcr1 in (A) the gas phase, (B) MeCN solution at room temperature, and (C) encapsulated in solvent-free Al-MCM-41 at room temperature. The different environments have negligible influence on the driving force for back electron transfer, but the reorganization energy is raised in MeCN solution and depressed within Al-MCM-41. Source: Reprinted with permission from Garcia, N.A.; Kowalczyk, T. Extension of Intramolecular Charge-Transfer State Lifetime by Encapsulation in Porous Frameworks. J. Phys. Chem. C 2017, 121 (38), 2067320679. Copyright 2017 American Chemical Society.
small, shallow local minimum separated from the bidentate ion pair complex by a very small barrier (less than 0.5 kcal/mol). On the other hand, the minimum of pathway related to (44BPYH1/32T2)op is separated from the minimum corresponding to 44BPYH221/32T22 by a small barrier of 2.2 and 4.2 kcal/mol with and without zero-point energy correction, respectively, suggesting the presence of a monodentate ion pair complex. Consequently, equilibrium between almost isoenergetic mono (44BPYH1/32T2)op and bidentate 44BPYH221/32T22 ion pair complexes could be
established. The results clearly showed that the adsorption energy for all complexes is substantially governed by the confinement effects executed through steric constraints and dispersive van der Waals interactions.
16.4 IMPLICATIONS The implications of the electronic confinement effect on chemical reactivity are obvious if one considers that changes in the energy levels of the guest molecule could imply a
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preactivation of the molecule when residing in the pore of mesoporous material or in the zeolite cavity. Encapsulation in a narrow zeolite channel offers advantages in terms of separating the molecules, improving the chemical stability of radicals, and allowing the individual moiety to be addressed experimentally. However, electronic nanoconfinement has additional implications in terms affecting the energetics of the host, guest, and the resulting hostguest interactions that go beyond catalysis. One such example is the encapsulation of different drugs, since mesoporous materials and zeolites have been proposed as drug nanocarriers. It is very feasible that the formation of such hybrid complexes would affect the properties of the guestdrug molecule and hence might modify its activity. This effect can be associated with either favorable increase in efficiency or on the contrary complete loss of overall efficiency. Similar arguments can be made for the possible photonic application. As commented in this chapter, for the charge-transfer systems there is a high possibility for preactivation and stabilization of charge-separated states of the molecular systems upon their encapsulation within the silica-based materials channels. This process can result in improvement of the desired optoelectronic properties of the encapsulated hybrid hostguest system. It should be noted that although the electronic confinement has been treated as a separate effect, it rarely acts independently but mostly in conjunction with other interactions or processes, such as Hbonding, diffusion, and electrostatic interactions. As such, improved theoretical models are required along with better designed experiments to fully comprehend the extent of the electronic nanoconfinement effect in silicabased materials.
Acknowledgment This work was supported by MINECO through projects MAT2017-82288-C2-1-P, and MAT2014-57646-P.
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References 1. Alarcos, N.; Cohen, B.; Ziolek, M.; Douhal, A. Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation, MA, USA. Chem. Rev. 2017, 117 (22), 1363913720. 2. Hashimoto, S. Optical Spectroscopy and Microscopy Studies on the Spatial Distribution and Reaction Dynamics in Zeolites. J. Phys. Chem. Lett. 2011, 2 (5), 509519. 3. Hashimoto, S. Zeolite Photochemistry: Impact of Zeolites on Photochemistry and Feedback From Photochemistry to Zeolite Science. J. Photochem. Photobiol. C: Photochem. Rev. 2003, 4 (1), 1949. 4. Tabacchi, G. Supramolecular Organization in Confined Nanospaces. Chemphyschem 2018, 19 (11), 12491297. 5. Michaelis, J.; Brauchle, C. Reporters in the Nanoworld: Diffusion of Single Molecules in Mesoporous Materials. Chem. Soc. Rev. 2010, 39 (12), 47314740. 6. Higgins, D. A.; Park, S. C.; Tran-Ba, K.-H.; Ito, T. Single-Molecule Investigations of Morphology and Mass Transport Dynamics in Nanostructured Materials. Ann. Rev. Anal. Chem. 2015, 8, 193216. 7. Karger, J. Transport Phenomena in Nanoporous Materials. Chemphyschem 2015, 16 (1), 2451. 8. Karger, J.; Ruthven, D. M. Diffusion in Nanoporous Materials: Fundamental Principles, Insights and Challenges. New J. Chem. 2016, 40 (5), 40274048. 9. Zicovich-Wilson, C. M.; Corma, A.; Viruela, P. Electronic Confinement of Molecules in Microscopic Pores—A New Cconcept Which Contributes to Explain the Catalytic Activity of Zeolites. J. Phys. Chem. 1994, 98 (42), 1086310870. 10. Zicovich-Wilson, C. M.; Planelles, J. H.; Jaskolski, W. Spatially Confined Simple QuantumMechanical Systems. Int. J. Quant. Chem. 1994, 50 (6), 429444. 11. Planelles, J.; Zicovich-Wilson, C.; Jaskolski, W.; Corma, A. Semiempirical Hamiltonians for Spatially Confined pi-Electron Systems. Int. J. Quant. Chem. 1996, 60 (5), 971981. 12. Yoffe, A. D. Semiconductor Quantum Dots and Related Systems: Electronic, Optical, Luminescence and Related Properties of Low Dimensional Systems. Adv. Phys. 2001, 50 (1), 1208. 13. Yoffe, A. D. Low-Dimensional Systems: Quantum Size Effects and Electronic Properties of Semiconductor Microcrystallites (Zero-Dimensional Systems) and Some Quasi-Two-Dimensional Systems. Adv. Phys. 1993, 42 (2), 173262. 14. Miller, D. A. B.; Chemla, D. S.; Damen, T. C.; Gossard, A. C.; Wiegmann, W.; Wood, T. H.; Burrus, C. A. BandEdge Electroabsorption in Quantum Well Structures: The Quantum-Confined Stark Effect. Phys. Rev. Lett. 1984, 53 (22), 21732176.
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15. Zhang, Z. L.; Sun, W.; Cheng, P. Spectroscopic and Theoretical Studies of Quantum and Electronic Confinement Effects in Nanostructured Materials. Molecules 2003, 8 (1). 16. Zhang, L. Z.; Cheng, P. Theoretical Studies of the Electronic Properties of Confined Aromatic Molecules in Support of Electronic Confinement Effect. Physchemcomm 2003, 6, 6266. 17. Marquez, F.; Garcia, H.; Palomares, E.; Fernandez, L.; Corma, A. Spectroscopic Evidence in Support of the Molecular Orbital Confinement Concept: Case of Anthracene Incorporated in Zeolites. J. Am. Chem. Soc. 2000, 122 (27), 65206521. 18. Zhang, L. Z.; Cheng, P.; Liao, D. Z. Electronic Confinement of Organic Molecules in Confined Spaces: A Spectroscopic Study of Zn(phen)2(NO3)2 Loaded MCM-41. J. Chem. Phys. 2002, 117 (13), 59595962. 19. Zhang, L. Z.; Xiong, Y.; Cheng, P.; Tang, G. Q.; Liao, D. Z. Molecular Orbital Confinement of a Schiff Base Molecule in the Nanoporous Channels of MCM-41 Host. Chem. Phys. Lett. 2002, 358 (3-4), 278283. 20. Lim, H.; Cheong, H.; Choi, S.; Choi, Y. N.; Lee, J. S. Optical Properties of Organic Dye Molecules Incorporated within Different Zeolitic Structures. J. Korean Phys. Soc. 2011, 58 (4), 10351038. 21. Cohen, B.; Wang, S.; Organero, J. A.; Campo, L. F.; Sanchez, F.; Douhal, A. Femtosecond Fluorescence Dynamics of a Proton-Transfer Dye Interacting with Silica-Based Nanomaterials. J. Phys. Chem. C 2010, 114 (14), 62816289. 22. Cohen, B.; Sanchez, F.; Douhal, A. Mapping the Distribution of an Individual Chromophore Interacting with Silica-Based Nanomaterials. J. Am. Chem. Soc. 2010, 132 (15), 55075514. 23. Cohen, B.; Alvarez, C. M.; Carmona, N. A.; Organero, J. A.; Douhal, A. Single Molecule Photobehavior of a Chromophore Interacting With Silica-Based Nanomaterials. Phys. Chem. Chem. Phys. 2011, 13 (5), 18191826. 24. Coutino-Gonzalez, E.; Baekelant, W.; Steele, J. A.; Kim, C. W.; Roeffaers, M. B. J.; Hofkens, J. Silver Clusters in Zeolites: From Self-Assembly to Ground-Breaking Luminescent Properties. Acc. Chem. Res. 2017, 50 (9), 23532361. 25. Fenwick, O.; Coutino-Gonzalez, E.; Grandjean, D.; Baekelant, W.; Richard, F.; Bonacchi, S.; De Vos, D.; Lievens, P.; Roeffaers, M.; Hofkens, J.; Samori, P. Tuning the Energetics and Tailoring the Optical Properties of Silver Clusters Confined in Zeolites. Nat. Mater. 2016, 15 (9), 10171022. 26. Aghakhani, S.; Grandjean, D.; Baekelant, W.; CoutinoGonzalez, E.; Fron, E.; Kvashnina, K.; Roeffaers, M. B. J.; Hofkens, J.; Sets, B. F.; Lievens, P. Atomic Scale Reversible Opto-Structural Switching of Few
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Atom Luminescent Silver Clusters Confined in LTA Zeolites. Nanoscale 2018, 10 (24), 1146711476. Baekelant, W.; Aghakhani, S.; Coutino-Gonzalez, E.; Kennes, K.; D’Acapito, F.; Grandjean, D.; Van der Auweraer, M.; Lievens, P.; Roeffaers, M. B. J.; Hofkens, J.; Steele, J. A. Shaping the Optical Properties of Silver Clusters Inside Zeolite A via GuestHostGuest Interactions. J. Phys. Chem. Lett. 2018, 9 (18), 53445350. Tarpani, L.; Ruhlandt, D.; Latterini, L.; Haehnel, D.; Gregor, I.; Enderlein, J.; Chizhik, A. I. Photoactivation of Luminescent Centers in Single SiO2 Nanoparticles. Nano Lett. 2016, 16 (7), 43124316. Chizhik, A. M.; Tarpani, L.; Latterini, L.; Gregor, I.; Enderlein, J.; Chizhik, A. I. Photoluminescence of a Single Quantum Emitter in a Strongly Inhomogeneous Chemical Environment. Phys. Chem. Chem. Phys. 2015, 17 (22), 1499415000. Hashimoto, S.; Saitoh, M.; Taira, N.; Schmidt, W.; Hirai, K.; Tomioka, H. Emission Spectroscopic Investigation of Triplet Diarylcarbene Generated in Molecular Sieve VPI-5. J. Phys. Chem. B 2005, 109 (43), 2040720414. Corma, A.; Garcia, H.; Sastre, G.; Viruela, P. M. Activation of Molecules in Confined Spaces: An Approach to ZeoliteGuest Supramolecular Systems. J. Phys. Chem. B 1997, 101 (23), 45754582. Cojocaru, B.; Neatu, S.; Parvulescu, V. I.; Dumbuya, K.; Steinruck, H. P.; Gottfried, J. M.; Aprile, C.; Garcia, H.; Scaiano, J. C. Band Gap Effect on the Photocatalytic Activity of Supramolecular Structures Obtained by Entrapping Photosensitizers in Different Inorganic Supports. Phys. Chem. Chem. Phys. 2009, 11 (27), 55695577. Moissette, A.; Lobo, R. F.; Vezin, H.; Al-Majnouni, K. A.; Bre´mard, C. Long Lived Charge Separated States Induced by trans-Stilbene Incorporation in the Pores of Brønsted Acidic HZSM-5 Zeolites: Effect of Gallium on the Spontaneous Ionization Process. J. Phys. Chem. C 2010, 114 (22), 1028010290. Hirata, Y.; Mataga, N. Electron Photoejection and Related Phenomena in Solutions—Ultrafast Laser Photolysis Studies. Prog. React. Kinet. 1993, 18 (3), 273308. Mohan, S. C.; Bhattacharjee, D.; Deka, R. C.; Jothivenkatachalam, K. Combined Experimental and Theoretical Investigations on the Encapsulation of Nickel (II)tet-a Complex in Zeolite Y and Its Photocatalytic Activity. Rsc Adv. 2016, 6 (75), 7121471222. Garcia, N. A.; Kowalczyk, T. Extension of Intramolecular Charge-Transfer State Lifetime by Encapsulation in Porous Frameworks. J. Phys. Chem. C 2017, 121 (38), 2067320679. Castella`-Ventura, M.; Moissette, A.; Kassab, E. A Theoretical Study of the Confinement Effects on the
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Energetics and Vibrational Properties of 4,40 -Bipyridine Adsorption on H-ZSM-5 Zeolite. Phys. Chem. Chem. Phys. 2018, 20 (9), 63546364. 38. Fukuzumi, S.; Doi, K.; Itoh, A.; Suenobu, T.; Ohkubo, K.; Yamada, Y.; Karlin, K. D. Formation of a LongLived Electron-Transfer State in Mesoporous SilicaAlumina Composites Enhances Photocatalytic
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Oxygenation Reactivity. Proc. Nat. Acad. Sci. U.S.A. 2012, 109 (39), 1557215577. 39. Fukuzumi, S.; Itoh, A.; Suenobu, T.; Ohkubo, K. Formation of the Long-Lived Charge-Separated State of the 9-Mesityl-10-methylacridinium Cation Incorporated into Mesoporous Aluminosilicate at High Temperatures. J. Phys. Chem. C 2014, 118 (41), 2418824196.
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16 Electronic Confinement Effect in Silica-Based Materials Boiko Cohen and Abderrazzak Douhal Department of Physical Chemistry, Faculty of Environmental Science and Biochemistry and INAMOL, University of Castilla la Mancha, Toledo, Spain
16.1 INTRODUCTION When a molecule is encapsulated within the nanochannels of silica-based material its spectral and photophysical properties may be altered significantly.1 This in turn may give rise to variations in optical band-gaps correlated with changes in the excited-state lifetimes, absorption, emission spectra, and quantum yields, for example, of the encapsulated guest molecules. When a tight host/guest confinement is possible (in other words when the size of the guest is comparable with the size of the host pore), these changes reflect variations in basicity/acidity and redox proprieties of the encapsulated guest, for example. The structural properties of channels and cavities with molecular sizes, like in zeolites have a strong influence on diffusion and solvation effects of reactant molecules, and they can become determinant for the catalytic activity of these materials.14 Diffusion, which involves the dynamics
Chemistry of Silica and Zeolite-Based Materials DOI: https://doi.org/10.1016/B978-0-12-817813-3.00016-X
of the molecule into the channels, is described by a statistic mechanical model which correlates well with experimental results.2,3,58 Solvation or cavity effects that involve physicochemical interactions between the zeolite pore and the guest molecule are the subject of the electronic confinement model.911 The apparent variations in the properties of the guest molecules cannot be explained only in terms of the influence of the host electrostatic field on the stabilization of polar guests. The classical type of interactions normally considered are the Coulomb interactions and coordination effects.9 The formers arise from a charge distribution along the silica framework owing to the partial ionic character of the aluminosilicate hosts. The generated Coulomb electric field into the cavities, might influence the chemical behavior of the guest molecules. The coordination effects, on the other hand, are produced by a Lewis acidbase type interaction among the guest and specific sites of the host, such as hydrogen-bond (HB)
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electron donoracceptor centers. The presence of these active sites in the host framework favors the adsorption of molecules.9 Weak electron interactions can account for forces of van der Waals type, which could produce and reinforce the “docking” of a molecule into a zeolite or mesoporous cavity. These interactions are further amplified by the surface curvature of the framework walls, which interact with the adsorbed guest molecules. An electronic confinement concept has been proposed based on the idea that the molecular orbitals (MOs) of the guests inside the host (zeolite) pores are not extended throughout the space but instead are limited to within the zeolite walls.9 It is expected that this confinement effect is stronger as the size of the confined guest approaches the host (zeolite) cage dimension, producing an energy increase in all MOs of the guest, particularly those that are more diffuse. Due to the partial covalent character of the aluminosilicate crystal, the electrons are not localized on the framework atoms, but they are partially delocalized through it. This causes the density of the orbitals of the guest molecule to suddenly drop to nearly zero when reaching the walls of the zeolite as a consequence of the short-range repulsion with the delocalized electronic clouds of the lattice. This situation implies a contraction of the orbitals of the adsorbed guest, with the consequent changes in its energy levels.9 The proposed electronic confinement model in mesoporous and zeolite materials draws similarity with the quantum confinement effect in semiconducting nanomaterials such as quantum dots and quantum nanowires, for example.
16.2 QUANTUM CONFINEMENT IN SEMICONDUCTOR CRYSTALS The quantum confinement effect arises essentially due to changes in the atomic
structure as a result of the influence of ultrasmall length scale on the energy-band structure of the semiconducting nanomaterials.12 For semiconductor crystals, the electronic excitation produces loosely bounded electron hole pairs (the MottWannier exciton) which are mostly delocalized over a length much longer than the lattice constant. As the size of the semiconductor material approaches the exciton Bohr diameter, its electronic properties start to change. In general, the Bohr radius of a particle is defined as12,13: aB 5 ε
m a0 m
(16.1)
where ε is the dielectric constant of the material, m* is the mass of the particle, m is the rest mass of the electron, and a0 is the Bohr radius of the hydrogen atom. When the particle size approaches the Bohr exciton radius, the quantum confinement effect causes an increase in the excitonic transition energy and a blue shift in the absorption and luminescence band gap energy.12,13 In addition to that, the quantum confinement leads to a collapse of the continuous energy bands of a bulk material into discrete, atomic-like energy levels. The discrete structure of the energy states leads to a discrete absorption spectrum, which is in contrast to the continuous absorption spectrum of a bulk semiconductor. A quantum confined structure is one in which the motion of charge carriers (electron and hole) is confined in one or more directions by potential barriers.14 According to the confinement direction, a quantum confined structure is classified into three categories as quantum well, quantum wire, and quantum dots or nanocrystals. When a higher number of dimensions is confined, more discrete energy levels can be found, in other words, carriers movement is strongly confined in a given dimension. In a bulk semiconductor, for example PbS or CdS quantum dots, the electron and hole are bound together by a screened Coulomb interaction to form a
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MottWannier exciton. By assuming the energy-band to be parabolic near the band gap (i.e., the effective mass approximation), the size-dependent shift (with respect to the bulk band-gap) in the exciton energy of a small cluster (where the cluster radius B the exciton radius) can be derived as15: EðRÞ 5 Eg 1
π2h2 1:786e2 2 0:248ERy 2 2m R2 εR (16.2)
where R is the cluster radius, 1=m 5 1=me 1 1=mh , me is the electron effective mass, mh is the hole effective mass, ε is the dielectric constant, and ERy is the effective Rydberg energy. The first term in the above equation is the band-gap of the bulk materials, the second represents the particle—in-a-box quantum localization energy and has a simple 1/R2 dependence, the third term is the Coulomb energy with a 1/R dependence, and the last one is the result of the spatial correlation effect. This last size-independent term is usually small but becomes significant for semiconductors with small dielectric constant.
16.3 ELECTRONIC CONFINEMENT MODEL IN SILICA-BASED MATERIALS The electronic confinement in silica-based materials is associated with the previous described quantum confinement, despite the electronic confinement being induced by the intrachannel surface and not by the exciton Bohr diameter. Moreover, quantum confinement effect cannot be expected to be quantitatively correct for very small clusters; in contrast, the electronic confinement effect focuses on both inorganic compounds and organic molecules within nanopores. Hence these clusters, loaded in inorganic hosts, must be extremely small, and may be even treated
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as single molecules in some cases.15 The new phenomena for such small clusters should be explained by the concepts derived from the electronic confinement theory. A confined model system can be built, for example, by locating the conjugated molecules parallel to two surfaces, which, for simplicity, may be simulated by two infinite planes located at a certain distance. In this case, the atomic orbitals can still have as the normal 2pz orbitals, a node at the molecular plane but are adapted to the confinement, which makes them absent beyond the limit of the plane.911,15 The model of electronic confinement is based on the Hu¨ckel molecular orbital (HMO) theory and accounts for the changes in the conjugated π systems as a result of the guest encapsulation in the nanocavities of zeolites (parallel to the walls of the channels for the strongest coupling between the host and the guest).9 For the development of the model the closed-shell single-determinant approximation is applied, which treats each electron as moving independently in a mean electrostatic field of the other electrons and the nuclei. In the applied approximation, only two empirical parameters are required—the Coloumb integral, α, and the resonance integral, β. For simplicity, the chosen guest is ethylene, which has only two electrons in the π system. In this case, the solutions for the bonding highest occupied molecular orbital (HOMO) and antibonding lowest unoccupied molecular orbital (LUMO) and their respective energies given by the HMO theory are9: jπi 5 jai 1 jbi; EHOMO 5 α 1 β
jπ i 5 jai 2 jbi; ELUMO 5 α 2 β
(16.3a) (16.3b)
where jai and jbi are the 2pz atomic orbitals on carbon atom a and b, respectively, jπi and jπ i are the bonding (HOMO) and antibonding (LUMO) π molecular orbitals, and EHOMO and ELUMO are the corresponding energies given as a function of α and β parameters. For the
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confined molecule, the two parallel planes are placed at a distance equal to the diameter of the cavity, while the guest is located equidistant (at d distance of each plane) and parallel to both planes. If one assumes that the symmetry of the system remains unchanged, then the solutions given by the HMO theory under the same approximation will have the same expression as for the unconfined system9: 0 0 0 π i 5 a0 i 1 b0 i; E0 (16.4a) HOMO 5 α 1 β 0 0 0 0 0 π i 5 a i 2 b i; E0 (16.4b) LUMO 5 α 2 β In these expressions, ja0 i and jb0 i are the 2pz atomic orbitals of the confined carbon atoms, jπ0 i and jπ0 i the respective bonding (HOMO) and antibonding (LUMO) π molecular orbitals, while α0 and β 0 are, respectively, the Coulombic and resonance integrals for the confined system. Following the boundary condition for the encapsulated system that the probability of finding an electron belonging to the guest molecule outside the cavity is zero (i.e., the difficulty of these electrons to penetrate through the surface electron clouds), the
jπ0 i and jπ0 i cannot be considered as true π orbitals. Although, like the normal π orbitals they have a node at the molecular plane and they also have to adapt to the boxing, which makes them disappear beyond the cavity limits. Next, to evaluate the energy changes as a result of the electronic confinement, one can use an expression that relates the energy variation of a given electronic state when confining the system, with the corresponding wave functions: Ð 2 ðxÞð@N Ψ 0 ÞðxÞ h S dSΨ Ð ΔE 5 2 (16.5) 2m V dτΨ ðxÞΨ 0 ðxÞ Here Ψ and Ψ 0 are the wave functions of a given energy state for the free and confined systems, respectively (Fig. 16.1A). It should be noted that while in the denominator the integration is over the cavity volume, V; in the nominator, it is performed over S, the cavity border surface. By taking into account the symmetry conditions imposed on the model, the energy
FIGURE 16.1 (A) Schematic representation of the Ψ function (see the text) for x and y fixed at any point of space. The thin line depicts the unconfined wave function and the thick line depicts the confined one. Dashed line represents the cavity limit. (B) Qualitative description of the π orbital energy spectrum for the unconfined (left-hand side) and confined (right-hand side) ethylene molecule. Source: Reprinted with permission from Zicovich-Wilson, C. M.; Corma, A.; Viruela, P. Electronic Confinement of Molecules in Microscopic Pores—A New Cconcept Which Contributes to Explain the Catalytic Activity of Zeolites. J. Phys. Chem. 1994, 98 (42), 1086310870. Copyright 1994 American Chemical Society.
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changes of the one-electron states of the ethylene molecule then may be expressed as9: ΔEHOMO 5 E0HOMO 2 EHOMO 5 Δα 1 Δβ (16.6a) ΔELUMO 5 E0LUMO 2 ELUMO 5 Δα 2 Δβ (16.6b) where Δα and Δβ are (in atomic units): ÐN 1 2N dxdyaðx; y; dÞ@z a0 ðx; y; dÞ Δα 5 2 Ð d Ð N 2 dz dxdyaðx; y; zÞa0 ðx; y; zÞ 2N
0
(16.7a)
ÐN
dxdyaðx; y; dÞ@z b0 ðx; y; dÞ 1 Δβ 5 2 Ð d 2NÐ N 2 dz dxdyaðx; y; zÞa0 ðx; y; zÞ 0
2N
(16.7b) Here, aðx; y; zÞ and bðx; y; zÞ are the 2pz atomic orbitals on atom a and b, respectively; a0 ðx; y; zÞ and b0 ðx; y; zÞ define the equivalent atomic orbitals of the confined system; @z is the partial derivative with respect to the z direction and d is the distance between the guest molecule and the cavity border. By analyzing the two expressions, it is easily concluded that both Δα $ 0 and Δβ $ 0, which is explained by the simultaneous increasing/decreasing of a0 and b0 at the cavity limit. Finally, the Coulomb and resonance integrals of the confined system can be written as9: α0 5 α 1 Δα $ α
(16.8a)
β 0 5 β 1 Δβ $ β
(16.8b)
The developed model shows that when the energy gap between π and π* orbitals decreases (Fig. 16.1B), upon confinement of the guest, the π bond should become weaker than when the molecule is unconfined. In a further theoretical study, applying semiempirical calculations, which were interpreted by means of the HMO theory, the confining space of the cavities was modeled using a mica sheet with a moleculesurface distance in the ˚ .16 These studies have range of 1.54.0 A
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expanded the model beyond the ethylene molecule, by studying the confinement of larger aromatic organic molecules (benzene, naphthalene, and anthracene) in the mesoporous and nanoporous cavities of silica-based materials. In agreement with the previously developed model,911 the results from this study have shown that the encapsulation of the larger aromatic systems alters their electronic properties as a result of changes in the molecular orbital energies and band-gaps.16 Furthermore, this work has provided additional evidence that the HOMO is more sensitive to confinement than the LUMO, and the overall effect is a reduction of the band-gap of the frontier MOs when the moleculesurface distance is less than ca. ˚ .16 2.5 A
16.3.1 Ensemble Average and Single-Molecule Studies An experimental evidence of electronic confinement was reported for anthracene encapsulated within zeolites.17 The study showed a bathochromic shift of the 00 transition from 375 nm in 3-methylbutane to 396 nm in NaY and to 419 nm in ZSM-5 (Fig. 16.2A). This observation translates in a strong correlation between the size of the nanochannel and the value of the bathochromic shift—the smaller the pore size of the zeolite, the larger is the bathochromic shift of the 00 transition. The observed effect was also linked with the shortening of the fluorescence lifetime (from 4.5 ns for free anthracene to 3.2 ns within NaY zeolite and ,100 ps in mordenite or ZSM-5). Overall, these experiments have related the change in the spectroscopic properties of the encapsulated anthracene with the increase of the HOMO energy and with the reduction in the HOMOLUMO band gap (Fig. 16.2B).17 Other studies have provided additional experimental evidences for the electronic
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FIGURE 16.2 (A) Excitation and emission spectra, at room temperature, of anthraceneNaY (a), anthracenesodium mordenite (b), and anthraceneNaZSM-5 (c). (B) Plots of the antracene HOMO in the gas phase (a) and upon confine˚ (b). Source: Reprinted with permission from Marquez, F.; Garcia, H.; Palomares, ment inside two mica sheets separated by 5.4 A E.; Fernandez, L.; Corma, A. Spectroscopic Evidence in Support of the Molecular Orbital Confinement Concept: Case of Anthracene Incorporated in Zeolites. J. Am. Chem. Soc. 2000, 122 (27), 65206521. Copyright 2000 American Chemical Society.
nanoconfinement effect in MCM-41 and zeolite materials. Results, comparable to those for anthracene encapsulated within zeolitic frameworks, have been reported in steady-state and dynamic spectroscopic studies of Zn (phen)2(NO3)2, (phen 5 1, 10-phenanthroline), within nanoporous MCM-41.18 In similarity with the results for anthracene encapsulated within zeolite frameworks,17 the observed bathochromic shift of the 00 transitions, along with shortening of the excited-state lifetimes have been correlated with the reduction of the HOMOLUMO band-gap accompanied by the increased energies of the frontier orbitals. This trend indicates that the electronic confinement of organic molecules is also realized in larger cavities, such as the ones of MCM-41. The encapsulation of N,N0 -bis(2hydroxy-5-methylbenzylidene)-1,2-ethanediamine, a Schiff-base molecule, in the mesoporous MCM-41, has also given support that the electronic confinement effect is significant even in the larger channels of MCM-41. 19 In this study, the 00 transition of the guest in DMSO, which was reported at 23,041 cm 21
(2.86 eV), shifts to 21,413 cm 21 (2.66 eV) in MCM-41. The observed was again explained in terms of a decrease in the HOMOLUMO band-gap. Additionally, as the electrostatic effect could not explain the observed shift of the 00 transition from DMSO to that in MCM-41, the trend could only be rationalized by the molecular orbital confinement theory.19 Thus, as a result of the electronic nanoconfinement, all the energy levels of the guest Schiffbase molecules will increase in the nanoporous channels host. As the increase for the HOMO has been predicted to be more sensitive than the LUMO, the expected overall effect is a reduction on the band-gap of the frontier orbitals. These studies also argued that an asymmetric confinement is present with the guest molecules being much closer to one wall than to the other one.18,19 Thus, the motion of electrons will be more restricted in one direction (along the wall of the nanoporous channels) than in the other directions. Although not directly related to the electronic nanoconfinement effect, significant variations in the photophysical properties of pyronine molecules with
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different molecular sizes (pyronine Y and pyronine B) incorporated inside zeolite crystals that have similar channel sizes, such as zeolite L and AlPO 4 -5, were observed by means of fluorescence polarization.20 The experimental results indicate that the dye molecules’ spectroscopic properties are strongly affected by the framework structure of the host, including the specific topology of the zeolite channels, which induces an angle dependence of the guest fluorescence behavior. The observed change in the optical properties of both molecular systems was explained in terms of the confinement effect.20 When an amino derivative of 2-(20 -hydroxyphenyl)benzothiazole (HBTNH2) was encapsulated within MCM-41 and Al-doped MCM-41, a strong variation in its spectroscopic properties was detected, as revealed by steady-state and time-resolved techniques.21 The study did not report any effect of the confinement of the nanochannels on the excited state intramolecular proton transfer reaction in the enol structure of HBTNH2 to give the keto tautomer. On the other hand, the component assigned to the excited-state lifetime of the twisted keto form of HBTNH2 increases from 22 ps in acetonitrile solution to 4045 ps within the MCM-41 channel providing additional verification for the electronic confinement effect. Notably, this system provides an example for the more complex nature of the electronic confinement effect. In the examples presented above, the HOMOLUMO energy gap decreases upon encapsulation (Fig. 16.1B). Thus the lifetime of HBTNH2@MCM-41 is expected to decrease due to faster internal conversion following the energy gap law. However, the lifetime of HBTNH2@MCM-41 increased only mildly, showing that the overall effect is not strictly a result of the modification of HOMO and LUMO energies due to electronic confinement, but also arises from the restriction in the rotation of the molecule resulting in the slower deactivation of the excited keto state.
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Further experimental evidence for the electronic nanoconfinement on the properties of HBTNH2 but at the single-molecule level has also been reported (Fig. 16.3).22 Time- and spectrally-resolved studies at the singlemolecule level have demonstrated that depending on the method used for the guesthost complex preparation, different sites in the host framework (MCM-41) can be accessed by the guest.22 The averaged emission spectrum of the single-molecule complexed to Aldoped MCM-41 host (HBTNH2/Al-MCM-41) consists of a single band centered at 583 nm with a full width at half maximum (FWHM) of intensity of 64 nm (B1480 cm21). The emission spectrum was assigned to the trapped anion (deprotonated phenol of the dye. This spectrum is much narrower than the one of the keto form generated using the undoped MCM41 (B3250 cm21) samples (HBTNH2/MCM41). This difference, along with the narrower lifetime distribution, is a clear indication of a stronger confinement of the local nanoenvironment around the single anion of HBTNH2 in the Al-MCM-41 channels. The replacement of Si with Al atoms changes the electronic density distribution within the framework of MCM-41, which in turn affects the hostguest orbital interaction so that the resulting emission comes from a much narrower density of the states of the single confined chromophore. Similar dependence on the presence of Al in the MCM-41 framework has been reported for DY-630-MI, a hemicyanine dye.23 Studying its single-molecule behavior, showed that replacing a few Si41 ions with Al31 ones in the framework of regular MCM-41 changes the local electrostatic field within the nanocannels, which results in a more selective orientation of the DY-630-MI molecules. Comparing the distribution of the polarization (P) values for DY630-MI/Al-MCM-41 with those for the free molecule adsorbed on the glass coverslip and the DY-630-MI/MCM-41 complexes, a stronger shift (61%) toward the positive limit (P 5 0.71)
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FIGURE 16.3 Single-molecule microscopy results of (A) 2-(20 -hydroxyphenyl)benzothiazole (HBTNH2) covalently bonded to MCM-41, (B) silica nanoparticle, and (C) Al-MCM-41. Emission decays of HBTNH2 covalently bonded to (D) MCM-41, (E) silica, and (F) Al-MCM-41, along with the best nonlinear least-squares monoexponential fits (solid lines). (G), (H), and (I) are the distribution of the lifetimes from the monoexponential fit to the decays of single-molecule MCM41, silica and Al-MCM-41 composites, respectively. (J), (K), and (L) represent the average emission spectrum of a single HBTNH2 covalently bonded to a silica nanoparticle and Al-MCM-41 nanochannels, respectively. The dashed line in part (K) represents the Gaussian fit to the main contributing band in the average spectrum of single HBTNH2silica complexes in the presence of the strong base 1,8-diazabicyclo[5.4.0]undec-7-ene. Source: Adopted with permission from Cohen, B.; Sanchez, F.; Douhal, A. Mapping the Distribution of an Individual Chromophore Interacting with Silica-Based Nanomaterials. J. Am. Chem. Soc. 2010, 132 (15), 55075514. Copyright 2010 American Chemical Society.
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is observed (Fig. 16.4). This behavior is indicative of an increase in the number of more oriented single chromophores in the nanochannel due to the modified electrostatic environment around the encapsulated single molecule.23 The rigid frameworks of zeolites can act as a host in which silver clusters can be stabilized, enabling a high degree of control over their optoelectronic properties (Fig. 16.5).2427 The dependence of their electronic properties on spatial confinement was studied by characterizing the ionization potential of the clusters embedded in four different zeolite environments over a range of silver concentrations.25,26 The chosen faujasite frameworks (FAUX and FAUY) differ in the Si/Al ratio of the framework, while the two LTA zeolites differ in the
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alkali metal charge-balancing cation (Na1 and K1, respectively).25 The results revealed a strong influence of silver loading and of the host environment on the Ag cluster ionization potential (over a range of .0.5 eV), which is also correlated with the cluster’s optical and structural properties. Using zeolites as host, the photoluminescence quantum yields of the composites could be optimized to reach nearly 100% by careful tuning of the mobilities of nonframework metal cations. In a subsequent study, by measuring exclusively the local structure of the emissive Ag clusters in partially exchanged LTA zeolites, their functional structures have been identified.26 DFT modeling based on these detailed structures showed that the double positively charged Ag4(H2O)4
FIGURE 16.4
Molecular structure of DY-630-MI (A) and schematic representation of the structure of MCM-41 and AlMCM-41 (B). Emission polarization (P) distribution histograms for (C) free DY-630-MI and interacting with (D) MCM-41 and (E) Al-MCM-41 materials, respectively. Source: Reprinted with permission from Cohen, B.; Alvarez, C. M.; Carmona, N. A.; Organero, J. A.; Douhal, A. Single Molecule Photobehavior of a Chromophore Interacting With Silica-Based Nanomaterials. Phys. Chem. Chem. Phys. 2011, 13 (5), 18191826. Copyright 2011 Royal Chemical Society.
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FIGURE 16.5 Activation of luminescent Ag-clusters confined inside the sodalite cages of faujasite (FAU) and Linde-type A (LTA) zeolites. Source: Reprinted with permission from Coutino-Gonzalez, E.; Baekelant, W.; Steele, J. A.; Kim, C. W.; Roeffaers, M. B. J.; Hofkens, J. Silver Clusters in Zeolites: From Self-Assembly to Ground-Breaking Luminescent Properties. Acc. Chem. Res. 2017, 50 (9), 23532361. Copyright 2017 American Chemical Society.
and Ag4(H2O)2 clusters, in which water ligand molecules modulate the HOMOLUMO gap, behave as confined two-electron helium or alkaline earth-like superatom quantum systems that mainly emit via their long-lived lowest-lying triplet excited state. Thus, it was concluded that the mechanism of Ag clusters bright luminescence when encapsulated within different silica-based materials depends on the combined effects of the interaction with oxygen ligands, electron confinement, electrostatic interaction, and charge transfer to the surrounding silver atoms.26 Electronic nanoconfinement effect was demonstrated also for photoactivated luminescent defects in SiO2 nanoparticles.28,29 In studies of the photoluminescence of defect centers in single 11-nm SiO2 nanoparticles, it was suggested that the strongly inhomogeneous local chemical environment around the centers results in a broad and random variation of the single nanoparticle emission and excitation spectra, fluorescence lifetime, and quantum yield. Both the emission and excitation spectra of different luminescent centers have shown a shift within the range of 2.0 2 2.4 eV. Despite this broad variation, the shape of the emission spectrum
remains constant, which suggests that the fluorescence comes from the same type of defects. As a result, this work demonstrated evidence for the electronic nanoconfinement effect on the properties of the luminescent defects in the SiO2 nanoparticles.
16.3.2 Electronic Confinement Effect on Charge Transfer Reactions Although not as evident and straightforward, the electronic nanoconfinement might be also involved in the generation, confinement, and stabilization of reactive intermediates in the cavities and channels of zeolites and other silica-based materials 2,3,30 It should be noted that in this type of systems, due to the presence of charged species in the ground and excited states, clear distinction between the involved effects is difficult to be made.3 In the radical generation and stabilization processes, the rigidity of the zeolite environment restricts vibrational and rotational motion of the tightly fitted guest species, leading to a fast intramolecular relaxation process. Stabilization at room temperature was reported for
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electronically excited states, such as radicals, and radical ions. For example, a study has reported on a room temperature phosphorescence with lifetimes similar to those observed at 77K for aromatic species such as naphthalene and 9-ethylcarbazole confined in a channel-type zeolite L.3 In the initial model, the walls of the confining media, have been treated as entirely reflecting boundary without contributing directly to the photophysical properties of the guest. One of the first works that suggested a more active role of the host framework studied the encapsulation and interaction of toluene within several zeolite cavities and in presence of varying Al content.31 Ab initio calculations showing frontier molecular orbital energy modifications of a toluene when located inside a microporous cavity (zeolites Beta, ZSM-12, and ZSM-5) have been performed. The results have shown that the HOMO energy of toluene increases when going from the gas phase state to restricted microporous environments. It was demonstrated that the smaller the zeolite cavity the higher is the rise in the toluene HOMO energy. The effect of the chemical composition on the toluene HOMO energy was tested in the ZSM-5 zeolite by varying the Al content. The results obtained showed that the frontier orbital energy increases upon decreasing the Al content of the cluster. As a consequence of the confinement effect, it was suggested that the toluene reactivity in zeolite-catalyzed reactions is expected to change toward a more covalent behavior in which electronic transfer from the toluene molecule to an electronic acceptor will be more favored.31 In a subsequent work, upon entrapping photosensitizers within inorganic supports such as zeolites, the determination of the band-gap on the basis of UVvisible measurements showed the presence of electronic interactions between the host and the guest, which result in a lowering of the band-gap of the inorganic support.32 The optical band-gap
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energy was studied in a series of metallophthalocyanine (Pc) and 2,4,6-triphenylpyrylium (TP1) ions encapsulated within different hosts (Y zeolite, mesoporous MCM-41, TiO2SiO2 and SiO2) in order to gain better understanding of the interaction occurring between the entrapped complexes and the host support. The band-gaps calculated in this study for the used porous supports were: 4.28 eV for NaY, 2.65 eV for MCM-41, 4.8 eV for the mesoporous TiO2SiO2,and 3.65 eV for the pure SiO2.32 The entrapment of metallophthalocyanine complexes in the cages of NaY resulted in a notable decrease of the band-gap irrespective of the nature of the metal (on average 2 eV). This decrease was explained in terms of an electronic interaction of the complex with the support. Additionally, although smaller, differences were also induced by the nature of the metal. In this case, the band-gap increased in the order FePc , ZnPc , MnPc , CoPc , NiPc , CuPc ranging from 2.16 to 2.47 eV. Significant effect on the band-gap was also observed when TP1 was encapsulated within the hosts.32 The entrapping of TP1 in NaY zeolite decreased the host HOMO-LUMO band-gap to 2.83 eV, a value higher than that for metallophthalocyanines. Although smaller, the entrapping of these photosensitizers in TiO2SiO2 or SiO2 led also to a decrease of the band-gap. The FePc decreased the band-gap of TiO2SiO2 by about 1.8 eV, to 2.98 eV. Similarly to the NaY case, TP1 decreased the band-gap of TiO2SiO2 by a smaller value, to 3.65 eV. For the SiO2 host, the decrease of its band-gap was observed after deposition of the organic sensitizer. In comparison with the metallophtalocyanines, the observed band-gap decrease was smaller for FePc (to 2.81 eV) than for TP1 (to 2.65 eV). Finally, when MCM-41 was used as support, the decrease of the band-gap was smaller than for the other supports. It was insignificant for FePc (only 0.2 eV decrease to 2.63 eV). TP1 encapsulation resulted in a value
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of 2.51 eV for the band-gap of SiO2. Thus, the optical spectra and the band-gap calculations have shown the presence of an electronic interaction between the entrapped photosensitizers and the porous materials, with the interaction increasing with the increase in the confinement between the solid pore and the photosensitizer. Further studies of the photocatalytic activity of the metallophtalocyanine complexes toward the photodecomposition of dipropyl sulfide showed a good correlation with the band-gap values.32 Additional evidence for the active role of the support in the electronic confinement effect was demonstrated in a study of the spontaneous charge separation of trans-stilbene (t-St) upon incorporation within the 10 membered rings (10-MR) channel of H2.2-GaZSM-5 support.33 The framework structure of MFI zeolites, including GaZSM-5, contains two types of intersecting channels, both formed by 10MR apertures. The t-St molecule enters the channels through the pore openings of the available faces of the zeolite crystals. By comparing the behavior of t-St encapsulated in different ZSM-5 zeolites (Na6.6-AlZSM-5, H3.4-AlZSM-5 and H2.2-GaZSM-5) the study has demonstrated that t-St is present as a neutral molecule in Na6.6-AlZSM-5, while the accessible protons in the inner surface 10-MR channel of H3.4-AlZSM-5 and H2.2-GaZSM-5 are fundamental for the t-St ionization.33 The ionization efficiency depends both on the ionization potential values of the sorbate and on the ionizing power of the host. The ionization energy of molecules encapsulated in the condensed phase is expected to depend on the ionization potential in the gas phase, the energy of the antibonding orbital in a localized orbital state, and the polarization energy imposed by the confinement with surrounding ions.34 As a result of the electronic confinement effect, the tendency of a guest molecule encapsulated in the zeolite channels toward giving electrons to the acid sites will increase,
which in turn will enhance its reactivity.9,31 In the reported study, it was suggested that the energy of the antibonding orbital of the ZSM-5 supports should depend significantly on the metal content (Ga or Al) and only weakly on the nature of the extra framework cation.33 Since the electrostatic field is dependent on the cation size and on the Si/Al ratio along with the steric constraints in the 10-MR channel, it was argued that the combined effects of the high polarization energy in close proximity of accessible acid site SiO(H)Ga within 10-MR channel and favorable antibonding orbital energy of GaZSM-5 induce spontaneous ionization of t-St by lowering the ionization energy. As a result, no supplementary Lewis acid site is necessary to induce the t-St spontaneous ionization in both H2.2-GaZSM-5 and H3.4-AlZSM-5. The 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane[tet-a] nickel(II) complex has been studied when encapsulated in zeolite Y.35 The energy of HOMO and LUMO levels for the neat nickel(II) complex were reported to be 3.78 eV and 2.448 eV, respectively, while those of the encapsulated complex were 4.678 eV and 2.271 eV, respectively. The geometry of the neat complex was found to be octahedral as the two chlorine atoms are coordinated to the central metal ion in an axial position. However, the complex within the zeolite cage is in a distorted square planar geometry due to a counterion effect and steric hindrance of the zeolite Y on the complex. Thus, it was suggested that the changes in the energy levels of the frontier orbitals might be due to distortion in the geometry of the complex and the counterion effect of zeolite Y.35 Interactions between the complex and zeolite were considered as Coulombic and coordination effects. In the case of the neat complex, the HOMO lies at the axial chlorine atoms, and the LUMO lies at the nitrogen atoms of the tet-a ligand. However, both the HOMO and LUMO lie on the nickel and nitrogen
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atoms of the complex. After encapsulation, the HOMO level is stabilized at a lower energy while the LUMO is positioned at a higher energy level and as a result the energy gap increases. The increase in the energy gap was experimentally supported by the absorption spectrum of the encapsulated complex. This one shows a shift toward the blue region of the spectrum, i.e. towards the high energy region, which indicates that structural distortion contributed to the increase in the energy gap, which further confirms that the complex in zeolite would certainly favor oxidation.35 Several theoretical works have also evaluated the energetics of electron transfer molecular systems encapsulated within different silica-based materials.36,37 A multiscale simulation was used to study the mechanism for the suppression of charge recombination of 9-mesityl-10-methylacridinium ion (MesAcr1) in the mesoporous aluminosilicate Al-MCM-41.36 Experimental works have provided evidence that the charge-transfer excited-state lifetime of MesAcr1 can be dramatically extended via encapsulation in Al-MCM-41.38,39 The simulations have shown that solvation in acetonitrile (MeCN) and encapsulation in Al-MCM-41 have opposing effects on the reorganization energy (λ) of MesAcr1. While the solvation in ACN substantially raises the reorganization energy, the encapsulation lowers it (Fig. 16.6). Thus it was concluded that the longer lifetime of the solvent-free MesAcr1@Al-MCM-41 results from the smaller solvent reorganization energy inside the mesoporous silicaalumina as compared in a MeCN solution. This is because the smaller the value of λ of electron transfer, the slower the rate becomes of the back electron transfer in the Marcus inverted region, where the driving force of back electron transfer is much larger than the λ value of electron transfer (Fig. 16.6). Finally, in a recent theoretical study, the confinement effects of a zeolite framework on the adsorption of a bidentate 4,40-bipyridine (44BPY) ligand on the straight channel of
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H-ZSM-5 have been investigated by density functional theory calculations using different functionals, such as B3LYP, B3LYP-D3, M062X, M06-2X-D3, along with the MP2 method with two basis sets 6-31 1 G* and 6-3111G (2d,2p).37 The straight channel was simulated by a realistic cluster model of 32 tetrahedra (T), having two aluminum atoms located in the straight (closed region, cl) and intersection regions (open region, op), respectively. In that way, the two Al atoms are located at positions sufficiently distant from each other allowing the bidentate 44BPY to interact via its N atoms with the two Brønsted acid sites of the zeolite cavity. The results have demonstrated that the B3LYP functional cannot be used to compute adsorption energies in the case of 44BPY adsorption in the H-ZSM-5 zeolite, while the M06-2X adsorption energies obtained using the 631 1 G* basis set were comparable to those calculated at the MP2 level with the extended 6-3111(2d,2p) basis set. Additionally, it was shown that the stability of the 44BPY adsorption complexes implicated in a proton transfer reaction between the Brønsted acid sites of HZSM-5 and the 44BPY ligand is completely governed by dispersive van der Waals interactions. With dimensions that are comparable to the channel diameter of H-ZSM-5, the 44BPY bidentate ligand can be used as a probe molecule to evaluate the influence of the steric constraint and dispersive van der Waals interactions exerted by the zeolite framework on the energetics of adsorption complexes. The results from the study have shown that steric constraints destabilize the 44BPY adsorption complexes, especially in the confined closed region, while in contrast, the dispersive van der Waals interactions contribute to the stabilization of these complexes and fully overweigh the steric constraints. The energetics of the potential energy surface of the proton transfer reactions allow to exclude completely the presence of the monodentate ion pair complex (44BPYH1/32T2)cl associated with a very
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FIGURE 16.6 Marcus curves for CR in photoexcited MesAcr1 in (A) the gas phase, (B) MeCN solution at room temperature, and (C) encapsulated in solvent-free Al-MCM-41 at room temperature. The different environments have negligible influence on the driving force for back electron transfer, but the reorganization energy is raised in MeCN solution and depressed within Al-MCM-41. Source: Reprinted with permission from Garcia, N.A.; Kowalczyk, T. Extension of Intramolecular Charge-Transfer State Lifetime by Encapsulation in Porous Frameworks. J. Phys. Chem. C 2017, 121 (38), 2067320679. Copyright 2017 American Chemical Society.
small, shallow local minimum separated from the bidentate ion pair complex by a very small barrier (less than 0.5 kcal/mol). On the other hand, the minimum of pathway related to (44BPYH1/32T2)op is separated from the minimum corresponding to 44BPYH221/32T22 by a small barrier of 2.2 and 4.2 kcal/mol with and without zero-point energy correction, respectively, suggesting the presence of a monodentate ion pair complex. Consequently, equilibrium between almost isoenergetic mono (44BPYH1/32T2)op and bidentate 44BPYH221/32T22 ion pair complexes could be
established. The results clearly showed that the adsorption energy for all complexes is substantially governed by the confinement effects executed through steric constraints and dispersive van der Waals interactions.
16.4 IMPLICATIONS The implications of the electronic confinement effect on chemical reactivity are obvious if one considers that changes in the energy levels of the guest molecule could imply a
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REFERENCES
preactivation of the molecule when residing in the pore of mesoporous material or in the zeolite cavity. Encapsulation in a narrow zeolite channel offers advantages in terms of separating the molecules, improving the chemical stability of radicals, and allowing the individual moiety to be addressed experimentally. However, electronic nanoconfinement has additional implications in terms affecting the energetics of the host, guest, and the resulting hostguest interactions that go beyond catalysis. One such example is the encapsulation of different drugs, since mesoporous materials and zeolites have been proposed as drug nanocarriers. It is very feasible that the formation of such hybrid complexes would affect the properties of the guestdrug molecule and hence might modify its activity. This effect can be associated with either favorable increase in efficiency or on the contrary complete loss of overall efficiency. Similar arguments can be made for the possible photonic application. As commented in this chapter, for the charge-transfer systems there is a high possibility for preactivation and stabilization of charge-separated states of the molecular systems upon their encapsulation within the silica-based materials channels. This process can result in improvement of the desired optoelectronic properties of the encapsulated hybrid hostguest system. It should be noted that although the electronic confinement has been treated as a separate effect, it rarely acts independently but mostly in conjunction with other interactions or processes, such as Hbonding, diffusion, and electrostatic interactions. As such, improved theoretical models are required along with better designed experiments to fully comprehend the extent of the electronic nanoconfinement effect in silicabased materials.
Acknowledgment This work was supported by MINECO through projects MAT2017-82288-C2-1-P, and MAT2014-57646-P.
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