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Spectroscopy and relaxation dynamics of salicylideneaniline derivative aggregates encapsulated in MCM41 and SBA15 pores
Microporous and Mesoporous Materials 226 (2016) 34e43
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Spectroscopy and relaxation dynamics of salicylideneaniline derivative aggregates encapsulated in MCM41 and SBA15 pores lix Sa nchez b, Abderrazzak Douhal a, * Noemí Alarcos a, Fe a
Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S.N., 45071 Toledo, Spain b Instituto de Química Organica, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2015 Received in revised form 16 December 2015 Accepted 18 December 2015 Available online 29 December 2015
We report on steady-state and picosecond time-resolved emission studies of (E)-2-(2’-hydroxybenzyliden)amino-4-nitrophenol (HBA-4NP) interacting with MCM41, Al-MCM41 and SBA15. In these materials, the molecule shows spectral changes, with a broadening and shifts of absorption and emission bands, which depend on the concentration used in the preparation of the samples, indicating the aggregates formation. These caged monomers (M), H- and J-aggregates are in enol form at S0 and in keto one at S1, as a result of an excited-state intramolecular proton transfer (ESIPT) reaction. The K lifetime of the caged M is longer (3 ns) than the observed in solution (14 ps), due to the confinement effect on the radiationless pathways. The K lifetimes of J- and H-aggregates are 600 and 100 ps, respectively, and their € nsted acidity) of populations and stability depend on pore size and chemical composition (in terms of Bro the host. For MCM41 and Al-MCM41 ones, the formation of H- and J-aggregates is higher than using SBA15. This is due to a better encapsulation by the formers (85e95% MCM41/Al-MCM41 vs 60% SBA15). Doping MCM41 by Al allows more J-aggregates as result of strong interaction between the guest molecules and the Al-MCM41 framework. Thus, modulating the topological and structural properties of the mesoporous material leads to the formation of different encapsulated species, which could be of great interest to design nanophotonics devices based on this kind of materials. © 2015 Elsevier Inc. All rights reserved.
Keywords: Photobehavior H- and J-aggregates Confinement effect Intramolecular proton transfer Silica-based nanomaterials
́
1. Introduction Silica-based materials are being used in catalysis [1e5], photonics [6e9] and drugs delivery [9e12], to cite few of their applications. MCM41 and its metal doped derivatives are widely studied and have been used to encapsulate a large number of molecules [7,8,13e21]. The caged molecules (guests) show physical and chemical behaviors different from those observed in solutions [7,8,14,16e18,20,22e26]. Several trapped guests exhibit significant increases in the fluorescence signals due to the restriction produced by the hosts [16,22,25,27,28]. For example, Sudan I and hemicyanine (4-[4-(dimethylamino)styryl]1-n-alkylpyridinium bromide, interacting with zeolites and MCM41 showed large increases in the emission lifetime of the trapped and photoproduced keto species [16,22,28]. However, the encapsulation effect is not always so simple. Other molecules like cyanines [29e32], xanthenes
* Corresponding author. Tel.: þ34 925 265717. E-mail address: [email protected] (A. Douhal). http://dx.doi.org/10.1016/j.micromeso.2015.12.037 1387-1811/© 2015 Elsevier Inc. All rights reserved.
[33e35], rhodamines [36e38], pyrene [39e43], anthracene [44,45] or porphyrins [46e49], interacting with silica-based materials formed trapped aggregates in addition to the monomers. While, anthracene, pyrene and xanthene interacting with X and Y zeolites and MCM41 mesoporous materials, give place to the formation of H-aggregates [33e35,41e43,45]. In these complexes, the face-toface stacking produces a quenching in the emission signal of the system. On the other hand, molecules as cyanines, porphyrins and pyronines, produce J-aggregates within zeolites and mesoporous materials [31,32,47,49e51]. In this case, the obtained complexes exhibit a superradiant fluorescence due to a large excitonic coupling within the J-aggregates [50,51]. Clearly, the nature of the aggregates depends of the molecular structure of the guest, but also can be influenced by the type of silica-based material used. This has been observed for Nile Red (NR) interacting with different metaldoped MCM41 materials (X-MCM41) [21]. In this case, NR trapped molecules form both H- and J-aggregates within the mesoporous materials [21]. When MCM41 was doped with aluminum (Al-MCM41), the H- and J-aggregates formation (respect to the other X-MCM41 materials) was higher due to its large loading
N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
efficiency [21]. In presence of Ti or Zr (Ti-MCM41 or Zr-MCM41), the formation of J-aggregates was more favored than in the other XMCM41 materials [21]. Recently, we have reported on the spectroscopy and photodynamics of (E)-2-(2-hydroxybenzyliden)amino-4-nitrophenol (HBA4NP) interacting with NaX and NaY zeolites [52]. HBA-4NP, which does not form aggregates in solution, exhibits aggregation within these hosts. We observed that the formed composites contain caged enol structures in the forms of monomers, H- and J-aggregates which undergo an excited-state intramolecular protontransfer (ESIPT) reaction at S1 to produce keto (K) type phototautomers [52]. The fluorescence lifetime of caged K monomers is remarkably long (~6 ns inside of zeolites vs 14 ps in DCM solution) due to the confinement effect on the radiationless pathways. The relative population and emission lifetimes of H- or J-aggregates depend on the loading and initial concentration of the dye used to make the composites [52]. For a more understanding of these trapped populations, a spectroscopic study of HBA-4NP interacting with large pores like those of MCM41 (d z 25 Å) or SBA 15 (d z 60 Å), is necessary. Moreover, doping MCM41 with aluminum (Al) will also provide information on the role of this metal in the dye aggregates formation and related spectroscopy and dynamics. Herein, we report on the encapsulated effect of MCM41, AlMCM41 and SBA15 materials on the steady-state UVevisible diffuse transmittance and emission spectra as well as on the piconanosecond photobehavior of trapped HBA-4NP. The results indicate that the aggregates formation and their nature depend on the used materials. The lifetime of the aggregates (~100 ps and ~600 ps for H- and J-types, respectively) in their photoproduced keto forms are shorter than that of monomers (2.20e3.00 ns) due to excitonic coupling in the formers. Upon increasing the concentration of the encapsulated dye, the times become shorter. We compare these results with those obtained using NaX and NaY zeolites [52] and we conclude that MCM41, materials traps more aggregates as its pore diameter is larger than that of the zeolites, allowing a better packing between encapsulated molecules. For Al-MCM41 one, the J-aggregates formation is favored due to the strong interactions with the framework. For the SBA15 complexes, the unexpected results (low formation of aggregates despites its large pore diameter) resides on its weak loading ability. 2. Experimental section The synthesis of (E)-2-((2-hydroxybenzyliden)amino-4nitrophenol (HBA-4NP) is described in our previous report [52]. Dichloromethane (DCM, spectroscopic grade, 99.9%), and MCM41, Al-MCM41 and SBA15 materials were purchased from SigmaeAldrich, and used as received. HBA-4NP/mesoporous material composites were prepared by adding a 100 mg of a dried mesoporous material into 15 mL of HBA-4NP in DCM solution (2 103 M or 6 107 M). The suspension was stirred at room temperature during 24 h. The solid mixture was centrifuged and rinsed four times with pure DCM to remove the dye weakly bounded to the surfaces of the mesoporous material. The solid then was dried in vacuum at room temperature. The steady-state UVevisible absorption and fluorescence spectra have been recorded using JASCO V-670 and FluoroMax-4 (Jobin-Yvon) spectrophotometers, respectively. Picosecond emission decays were measured by a time-correlated single photon counting (TCSPC) system [53]. The sample was excited by a 40-ps pulsed diode laser centered at 371 or 433 nm (<5 mW, 40 MHz repetition rate) and instrument response function (IRF) of ~70 ps. For the excitation at 320 nm, we used the second harmonic of the output (640 nm) from a femtosecond optical parametric oscillator (Inspire Auto 100) pumped by 820 nm pulses (90 fs, 2.5 W, 80 MHz)
35
from a Ti:sapphire oscillator (Mai Tai HP, Spectra Physics). In these experiments (IRF z 30 ps) the fs-laser excitation was at very low power to avoid undesired photochemistry. The IRF of the system has been measured using a standard LUDOX (SigmaeAldrich) solution in 1 cm cell. The decays were deconvoluted and fitted to single or multi-exponential functions using the FLUOFIT package (PicoQuant) allowing single and global fits. The quality of the fit was estimated by c2, which was always below 1.2 and the distribution of the residues. The time-resolved emission spectra (TRES) were constructed form the single-wavelength measurement, and the zero time spectrum was taken as the one obtained at the intensity half-maximum corresponding to the rise of the excitation pulse. All experiments were done at 293 K. 3. Results and discussion 3.1. Steady-state observation 3.1.1. Diffuse transmittance spectra Fig. 1 shows the UVevisible absorption and diffuse transmittance (DT) spectra of HBA-4NP in a DCM solution, and interacting with MCM41 materials in a DCM suspension prepared from a diluted parent solution (6 107 M). In pure DCM, the molecule exhibits two absorption bands at 280 and 350 nm, which were assigned to S0(p) / S2(p*) and S0(p) / S1(p*) transitions of the enol (E) tautomer, respectively (Scheme 1A) [52]. Interacting with MCM41, the band shape and spectral position significantly differ from that observed in pure DCM, which suggests the presence of different electronic structures in the ground state as a result of interactions between the dye and the MCM41 framework. We anticipate that the observed bands at ~360, and ~310 and 410 nm are due to caged monomers (M), H-, and J-aggregates, respectively. This assignment is based on our previous report where HBA-4NP interacts with NaX and NaY zeolites, showing the presence of this kind of species [52]. In the following paragraphs, we present and discuss the steady-state and time-resolved data to support our attribution. Fig. 2 shows a comparison of the DT spectra of HBA-4NP interacting with MCM41, Al-MCM41 and SBA15 in DCM suspensions, using two different initial dye concentrations to make the composites: diluted (6 107 M) and concentrated (2 103 M) parent solutions. Upon addition of the mesoporous materials to the dyeDCM solutions, an immediate change in the powder color from
Fig. 1. Normalized (to the maximum intensity) UVevisible absorption, diffuse transmittance and fluorescence (lexc ¼ 370 nm) spectra of HBA-4NP (2 105 M) in a DCM solution (dashed line) and interacting with MCM41 in a DCM suspension (solid line) prepared from a diluted parent DCM solution (6 107 M).
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Scheme 1. A) Molecular structures of (E)-2-((2-hydroxybenzyliden)amino-4-nitrophenol (HBA-4NP) tautomers and schematic representation of MCM41/Al-MCM41 and SBA15 mesoporous materials. The inserted pictures show the color changes encapsulation of HBA-4NP into the different mesoporous materials. B) Schematic representation (not in scale) of the allowed (full arrows) and not allowed (dotted arrows) absorption electronic transition of the monomer, H- and J-aggregates of HBA-4NP. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
white to yellow occurs, indicating an instantaneous interaction of the material with the dye leading to composites formation (powders picture in Scheme 1A). This effect is considerably more pronounced using Al-MCM41, indicating stronger interactions with this material. This result is in agreement with previous reports, € nsted acidity in mesoporous materials where the increase in the Bro doped with metal (Al, Ga, Ti, Zr) leads to stronger hosteguest interactions [21,54]. In order to know if the dye molecules are within the host pore or/and on its surface, we recorded the DT spectra of the MCM41 composites before and after washing several times with pure DCM (Fig. S1 in supplementary material). The result does not show any significant change in the spectra; suggesting that most of the observed signal is coming from the included dye molecules. A priori, those found adsorbed on the crystal surface must have a weaker adsorption energy, and thus easy to decomplex by successive washing of the composites. Moreover, the formed complexes have a great stability as their spectra do not show changes for a one year approximately (Fig. S2 in Supplementary Material). For diluted samples, the DT spectra of the samples using MCM41 and Al-MCM41 show three absorption maxima at 260, 310 and
400 nm, and a small shoulder at 360 nm. For a better understanding of these bands, the normalized DT spectra were deconvoluted into its constituting components, supposing Gaussian-shape bands (Fig. 3). The obtained spectra are composed of four bands centered at 257, 308, 366 and 410 nm for MCM41 and 260, 305, 354 and 406 nm for Al-MCM41 (Fig. 3A and B, respectively). When the dye is interacting with SBA15, only the absorption maxima at 260, 300 and 360 nm, with a tail at 400 nm, are observed (Fig. 2). In this case, the deconvoluted DT spectrum exhibits four bands centered at 256, 301, 355, and 400 nm, being the latter of low intensity (Fig. 3C). To understand the nature of these absorption bands, we recorded the DT spectra of samples made from a higher concentration of the dye (2 103 M) (Fig. 2). For these samples, the spectral position does not change, but the relative intensities of the bands show a variation in which the intensity at 360 nm decreases and that of 410 nm increases (Fig. 2 and S3 in supplementary material). This relative intensity change is due to the formation of more aggregates in the concentrated sample which is in agreement with previous reports where HBA-4NP and its parent molecule, salicylaldehyde azine (SAA), produce aggregates within faujasite zeolites [23,52]. The exciton coupling theory assigns the hypsochromic and
N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
37
Fig. 2. Normalized (to the maximum intensity) UVevisible diffuse transmittance and fluorescence spectra of HBA-4NP interacting with MCM41, Al-MCM41 and SBA15 in DCM suspension, using diluted (6 107 M) (dashed line) and concentrated (2 103 M) (solid line) parent HBA-4NP solutions. F(R) is the KubelkaeMunk function, F(R) ¼ ((1R)2)/2R, where R is the diffuse reflectance from a sample. For emission the excited wavelength was 370 nm.
bathochromic shifts of the absorption bands relatively to that of the monomers, to H- (face-to-face interactions) and J-aggregates (faceto-edge interactions), respectively (Scheme 1B) [55e57]. Therefore, we attribute the ~310 and ~410 nm bands to the absorption of encapsulated H- and J-aggregates, while the absorption at ~260 and ~360 nm is due to S0(p) / S2(p*) and S0(p) / S1(p*) transitions of caged E monomers, respectively (Scheme 1B). The relationships between the spectral shifts and type of aggregates that are formed, is explained in terms of different electronic coupling between the monomers involved in the aggregates. This coupling is related to the angle (a) formed between the axes connecting the dipole moment centers of two neighboring monomers which are disposed (stacked) in a parallel direction (Scheme 1B) [55]. For J-aggregates, the small angle (a) produces a strong orbital coupling between the molecules packing [51]. In these aggregates where only the S0/ S1 transition is allowed, the strong orbital coupling gives place to redshifted narrow absorption bands [51]. However, in our case, the heterogeneity of the system produces a widening of the absorption band as the aggregates population also depends on the specific and no specific interactions with the host. On the other hand, H-aggregates show an absorption due to S0/ S2 transition as that of S0/ S1 is forbidden, and whose width is affected by the heterogeneity of the composites [51,55]. This is in agreement with the observation at higher concentration, where the width of both bands increase (Table 1). On the other hand, the formation and stability of H- and J-aggregates depend of the morphology (pore size) and chemical composition (doping metal) of the used material [20,21,30,47,58]. In our case, the formation of H- and J-aggregates is higher for MCM41 complexes than for those of SBA15. So, comparing the relative absorption intensity of M, H- and J-aggregates in the three
Fig. 3. Spectral decomposition (supposing a Gaussian shape of the absorption band) of diffuse transmittance spectra of HBA-4NP interacting with (A) MCM41, (B) Al-MCM41 and (C) SBA15, using a diluted parent DCM solution (6 107 M). The inset gives information on the fitted and fitting bands of the experimental DT spectra.
composites, SBA15 ones have the lowest population of aggregates (Figs. 2 and 3). This is explained in term of a better encapsulation, as MCM41 and Al-MCM41 show a loading efficient of 85 and 95%, respectively, while that of SBA15 is of 60%. This is due to the large pore (diameter 60 Å vs 25 Å for MCM41) and lower BET surface area
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N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
Table 1 Values of the full width at half-maximum (FWHM) intensity of the emission band of monomers H-, and J-aggregates present in HBA-4NP interacting with MCM41, AlMCM41 and SBA15, using diluted and concentrated parent DCM solutions. Sample
Host
FWHM (cm1) H-aggregates
FWHM (cm1) Monomer
FWHM (cm1) J-aggregates
Diluted (6 107 M)
MCM41 Al-MCM41 SBA15 MCM41 Al-MCM41 SBA15
5490 5210 4500 7170 6733 6270
3430 3160 3990 3630 3560 3545
2470 3460 1190 3255 3660 2276
Concentrated (2 103 M)
(780 m2/g vs 1000 m2/g for MCM41), that presents SBA15 materials [54,59e61]. It is known that MCM41 and SBA15 materials have the same hexagonal structure with a surface that contains hydroxyl groups [61,62]. However, the SBA15 exhibits a considerable surface roughness, which is attributed to (SiO2)n islands on the surface and a lower presence of eOH groups [62,63]. It is believed that this is the reason why this material has a low energy surface, which leads to a lower loading [15,62,63]. Comparing MCM41 and Al-MCM41 complexes, the latter exhibits more presence of J-aggregates (absorption band at ~410 nm) (Figs. 2 and 3 and S3 in supplementary material). Al doping leads to charge defects formation and delocalization in the host framework, producing an increase in its acidity due to the generation of € nsted sites [1,64e66]. So, we believe that the interaction beBro tween the guest molecules and the acid sites of the framework produces more J-aggregates. This is in agreement with what we observed for HBA-4NP interacting with the faujasite zeolites (NaX and NaY) [52]. In that case, the J-aggregates formation was also favored being higher for NaX complexes where the amount of Al is greater (Si/Al ¼ 1.24 and 2.86 for NaX and NaY, respectively) [52].
3.1.2. Emission spectra Fig. 1 shows the fluorescence spectra of HBA-4NP in a DCM solution and interacting with MCM41 in a DCM suspension using diluted sample. In solution, the molecule has an emission band maximum at 580 nm, assigned to the keto form, as a result of an ESIPT process in the initially excited enol form (Scheme 1) [52]. While, interacting with MCM41 materials, the emission band is shifted toward shorter wavelengths, indicating the formation of other species (caged aggregates and monomers). Fig. 2 shows a comparison of the fluorescence spectra of HBA4NP within MCM41, Al-MCM and SBA15 suspensions, using two different dye concentrations (diluted and concentrated sample). Using the diluted ones, the emission bands are shifted to shorter wavelengths when compared to the one in pure DCM (Figs. 1 and 2). This is explained in terms of an emission from keto forms not allowed twisting motion inside these materials. Comparable behavior was observed for a similar molecule, salicylaldehyde azine, encapsulated in NaX zeolite [23]. In that case, the maximum emission spectra changed from 580 in DCM solution to 505 nm in NaX complexes [23]. The fluorescence quantum yield of HBA-4NP in DCM solution is 3 103 M, which indicates the involvement of efficient nonradiative transitions in the photoproduced keto form [52]. When it is confined in the mesoporous materials and in spite of aggregates formation, the emission signal is about seven times larger than in DCM solution, consequence of a decrease in the nonradiative rate constants (Fig. S4). On the other hand, the absence of an emission band at 580 nm, reveals the lack of free keto at S1 which in turn reflects the absence of free enol structures at S0 in these suspensions. Thus, the observed emission bands from these composites are due to trapped HBA-4NP molecules by the used hosts. In order to explore the aggregates emission from these complexes, we recorded the spectra at higher concentrations (Fig. 2). The emission intensity maxima appear at 500, 505 and 475 nm for MCM41, Al-MCM41 and SBA15, respectively (Fig. 2). Moreover, the full width at half-maximum (FWHM) of these
Scheme 2. Schematic representation (not in scale) of the energetic diagram at ground and electronically excited state, of monomer (M), H- and J-aggregates of HBA-4NP interacting with the used mesoporous materials in DCM suspensions. For simplicity the ground- and excited-state levels of the three caged structures are put at the same level. Cartoon illustrating the caged structures (M, H- and J-aggregates) within MCM41/Al-MCM41 and SBA15 materials. For each species, we indicate the fluorescence lifetime.
N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
Fig. 4. Diffuse transmittance (solid line) and excitation spectra of HBA-4NP interacting with (A) MCM41, (B) Al-MCM41 and (C) SBA15 in a DCM suspension, using concentrated (2 103 M) parent solutions. The values of the observation wavelengths are shown in the inset.
emission bands (3760 cm1 for MCM41, 4000 cm1 for Al-MCM41, and 4140 cm1 for SBA15) are larger than those obtained for the diluted samples (3650 cm1 for MCM41, 3840 cm1 for Al-MCM41, and 4000 cm1 for SBA15). These results clearly reflect the presence
39
Fig. 5. Magic-angle emission decays of HBA-4NP interacting with (A) MCM41, (B) Al-MCM41 and (C) SBA15 in a DCM suspension, using (1) diluted (6 107 M) and (2) concentrated (2 103 M) parent HBA-4NP solutions. The samples were excited at 371 nm and observed at 500 nm. The solid lines are from the best-fit using exponential functions. IRF is the instrumental response function (70 ps).
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N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
of emission aggregates. As already anticipated, we suggest that the aggregates and monomers experience an ESIPT reaction at the S1 state, as the emission intensity maxima of the bands shows an abnormally Stokes-shift. Thus, the emission bands are due to the caged keto forms: K-monomers and K-aggregates (H- and J-type) (Scheme 2). 3.1.3. Excitation spectra The presence of these caged structures (monomer and aggregates) is also confirmed by recording fluorescence excitation spectra of the related samples at different wavelengths of observation (Fig. 4). The complexes were prepared using concentrated (2 103 M) parent DCM solutions of HBA-4NP. For all the complexes, observing at 450 nm (emission from caged K monomers), the obtained spectrum mainly corresponds to trapped E monomers which absorb at 360 nm (Fig. 4). When we observed at longer wavelengths (475e550 nm), a new absorption band at 400 nm corresponding to the J-aggregates appears (Fig. 4). The absorption band of H-aggregates should appear at higher energies (~310 nm), as the S0 / S1 transition is forbidden. The absorption in the Table 2 Values of time constants (ti), normalized (to 100) pre-exponential factors (ai) and fractional contributions (ci ¼ tiai) obtained from the fit of the emission decays of diluted sample of HBA-4NP (6 107 M) interacting with MCM41, Al-MCM41 and SBA15 in DCM suspensions upon excitation at 371 nm and observation as indicated (lobs). Host
lobs/nm
MCM41
450 475 500 530 570 450 475 500 530 570 450 475 500 530 570
Al-MCM41
SBA-15
t1/ps
100
90
122
a1%
c1%
45 44 44 45 46 50 49 51 52 51 42 40 43 46 47
10 11 10 11 12 11 10 11 12 12 11 11 12 16 17
t2/ps
570
540
700
a2%
c2%
39 40 37 38 37 38 41 39 37 38 26 24 22 20 19
37 38 38 37 38 39 39 37 35 36 25 23 21 19 20
t3/ns
2.30
2.20
3.00
a3%
c3%
16 16 19 17 17 12 10 10 11 11 32 36 35 34 34
53 51 50 52 52 50 51 52 53 52 64 66 67 65 63
Table 3 Values of time constants (ti), normalized (to 100) pre-exponential factors (ai) and fractional contributions (ci ¼ tiai) obtained from the fit of the emission decays of concentrated sample of HBA-4NP (2 103 M) interacting with MCM41, Al-MCM41 and SBA15 in DCM suspensions upon excitation at 371 nm and observation as indicated (lobs). Host
lobs/nm
MCM41
450 475 500 530 570 630 450 475 500 530 570 630 450 475 500 530 570 600
Al-MCM41
SBA-15
t1/ps
60
40
100
a1%
c1%
71 72 72 75 77 75 72 74 75 77 82 76 57 48 60 63 65 66
31 32 32 33 34 33 31 31 32 34 34 33 10 10 12 14 15 15
t2/ps
290
270
650
a2%
c2%
20 22 21 17 15 17 21 19 19 16 11 18 16 15 13 14 11 11
14 15 18 18 18 18 18 20 21 21 22 22 25 25 24 26 26 25
t3/ns
1.20
1.00
2.70
a3%
c3%
9 6 7 8 8 8 7 7 6 7 7 6 27 27 27 23 24 23
55 53 50 49 48 49 51 49 47 45 44 45 65 65 64 62 61 60
310e325 nm region, could correspond to the S0 / S2 absorption of these aggregates (Fig. 4). According to what we observed previously, the excitation spectra of the Al-MCM41 complexes indicate that the formation of the J-aggregates is larger than in other mesoporous material (MCM41 and SBA15), due to the strong interaction with the Al-doped material. Based on the above observation, we suggest that absorption and emission band maxima of the formed structures of HBA-4NP upon interaction with MCM-41, AlMCM41 and SBA15 materials in DCM suspensions are: monomers (360 and 465 nm), H-aggregates (310 and 500 nm) and J-aggregates (400 and ~500 nm). For a better understanding of their photobehavior, we investigated the time-resolved emission decays of these entities. 3.2. Picosecond studies Fig. 5 shows representative emission decays of the complexes collected at 500 nm and excited at 370 nm, for both diluted and concentrated samples. Tables 2 and 3 give the obtained time constants (ti), the pre-exponential factors (ai) and relative contributions (ci) normalized to 100 after a multiexponential global fits. In agreement with the observed results in the steady-state part, the obtained lifetime values depend on the loading and on the used mesoporous material (Fig. 5, and Tables 2 and 3). For diluted samples, we observed decay times of 100 ps, 570 ps and 2.30 ns for MCM41, 90 ps, 540 ps and 2.20 ns for Al-MCM41, and 122 ps, 700 ps and 3.00 ns for SBA15 (Table 2). The related amplitude and contribution of each component exhibit small variations with the observation wavelength (Table 2). The global contribution of the shortest component is ~10% for all complexes, that of the intermediate one, ~35e40% for MCM41 and Al-MCM41 and ~20e25% for SBA15, and that of the longest component, ~50% for MCM41 and AlMCM41 and ~65% for SBA15. Based on these results and taking into account that for diluted samples the caged monomer species is the main population, we assign the longest component (~3 ns) to the caged keto monomers emission. Previous studies showed that the lifetime of K monomer in pure DCM is 14 ps. [52]. This shortening in the emission lifetime is due to non-radiative processes produced by the presence of an electron withdrawing group (NO2), and the twisting motion around C]N bond which leads to the formation of further rotamers of the tautomers [52]. The encapsulation of the HBA-4NP in the mesoporous materials produces a molecular restriction to twisting motion, which decreases the radiationless transitions of the caged K species. This confinement effect was observed in other molecules where their inclusions into cavities or channels change the photodynamics, resulting in an increase in the lifetimes of the emitting forms, in comparison with the neat solution [7,16,20,23,25]. HBA-4NP interacting with faujasite zeolites, showed a lifetime of the caged K monomer even longer than in the present hosts (6 ns vs 3 ns). NaX and NaY cavities have small cavities (pore diameter of the zeolite ¼ 8 Å) not allowing twisting motion [52]. The intermediate and shorter components observed in the decays of the present composites are due to caged H- and Jaggregates of keto tautomers. In agreement with the exciton theory, we assign the shortest one to the H-aggregates [55,67]. These species, which are rather in an excited-state of higher energy (S2), are characterized by a very fast relaxation to the ground state. As a result, part of the excitation energy is lost in the non-radiative processes, leading to very low fluorescence quantum yield [67]. However, the J-aggregates which are populated at an excited-state of a lower energy (S1), are more stabilized showing a higher emission quantum yield [51,67]. Thus, the intermediate component is assigned to these species. Upon increasing the HBA-4NP concentration (2 103 M), we observed a shortening in the emission decays as a result of an increase in the monomeremonomer
N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
41
Fig. 6. Normalized (to the maximum of intensity) magic-angle time-resolved emission spectra (TRES) of HBA-4NP interacting with MCM41 in a DCM suspension, using concentrated (2 103 M) parent HBA-4NP solutions gated at the indicated delay times after excitation at 371 nm. The inset gives the gating time of the spectra.
interactions (Fig. 5 and Table 3). Now, the lifetimes are 60 ps, 290 ps, 1.20 ns for MCM41, 40 ps, 270 ps, 1.00 ns for Al-MCM41, and 100 ps, 650 ps and 2.70 ns for SBA15 (Table 3). Fluorescence quenching processes take place between neighboring encapsulated molecules. For this reason, the Al-MCM41 complexes, where there is more amount of encapsulated dye (95% of loading), exhibit the shortest emission lifetimes (Fig. 5 and Fig. S5 in supplementary material). On the contrary, the SBA15 complexes present very small variations in their decays when changing the used dye concentration to make the composites. The reason of this is that in SBA15 the aggregates formation is lower due to the weaker loading efficiency (60%) and larger pore of this material (Fig. 5 and Fig. S5 in supplementary material). Similar behavior was observed for pyronine within the L zeolite where the change (quenching) of its emission lifetimes depended on dye loading [50]. This also was observed for HBA-4NP in NaX and NaY zeolites, as the loading efficient for these zeolites was different (20% for NaX and 90% for NaY) [52]. So, the HBA-4NP/NaY complexes showed the decay times shorter than those NaX ones [52]. To better understand the photodynamics of these complexes, we recorded time-resolved emission spectra (TRES). Fig. 6 exhibits the result exciting and gating the spectra of HBA-4NP/MCM41 complexes prepared from concentrated parent solution (2 103 M). We can distinguish three bands whose intensity maxima are 465, 485 and 530 nm. The band collected at longer gating times (3 ns), with intensity maximum at 465 nm and which, is due to the emission of the caged K monomer. The other bands with maxima at 485 and 530 nm are assigned to the caged K in Jand H-aggregates, respectively. The latters are characterized by a larger Stokes shift while the formers have a small one [55e57]. To further investigate the emission lifetime of the aggregates, we analyzed the decays upon femtosecond excitation at 320 (region of H-aggregates absorption) and 433 nm (region of J-aggregates absorption). Fig. 7 shows the decays recorded at 530 nm for HBA-4NP/MCM41 and HBA-4NP/Al-MCM41 composites using concentrated parent solutions. Comparing the 370 nm excitation results, upon excitation at 320 nm, the decay signal is shorter reflecting in the fastest decay of H-aggregates type, while upon excitation at 433 nm it is longer due to the J-aggregates contribution in the global signal. Table 4 shows the data fitting the emission decays of these complexes. The time values do not show significant
Fig. 7. Magic angle emission decays of HBA-4NP interacting with (A) MCM41 and (B) Al-MCM41 using concentrated (2 103 M) parent HBA-4NP solutions. The samples were excited at (1) 320 nm, (2) 370 nm and (3) 433 nm and observed at 530 nm. The solid lines are from the best-fit using exponential functions. IRF is the instrumental response function.
change relatively to the results exciting at 371 nm. However, the pre-exponential factors exhibit some variations. The shorter component (40e70 ps) shows their maximum amplitude when exciting at 320 nm (80e84%), while the intermediate one (250e290 ps) has their maximum upon exciting at 433 nm, being longer for the Al-MCM41 sample (~35%) than for that MCM41(~27%). Table S1 in supplementary material, shows the data of the emission decays using diluted samples and exciting at the same regions (320 and 433 nm). For diluted samples, a comparable behavior related to the amplitudes observed for each of the regions of different wavelength excitation, is observed. 4. Conclusions We showed here, that when HBA-4NP is interacting with MCM41, Al-MCM41 and SBA15, caged enol monomers, H- and Jaggregates are stabilized at S0. Using MCM41 and Al-MCM41 hosts,
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Table 4 Values of time constants (ti) and normalized (to 100) pre-exponential factors (ai) obtained from the fit of the emission decays of concentrated sample of HBA-4NP (2 103 M) interacting with MCM41 and Al-MCM41 in DCM suspensions upon excitation at 320 and 433 nm, and observation at different wavelengths (lobs). Concentrated samples Host
MCM41
Al-MCM41
lobs/nm 450 475 500 530 570 630 450 475 500 530 570 630
Excitation at 320 nm
t1 (ps)
50
40
a1 (%) 80 81 82 83 84 84 79 80 81 81 83 82
Excitation at 433 nm
t2 (ps)
260
250
a2 (%) 15 13 12 11 10 11 13 13 11 11 11 12
t3 (ns)
1.25
1.00
more H- and J-aggregates are trapped that when using SBA15, due to a higher loading efficient in the formers. Thus, for the SBA15 complexes the caged monomers are the predominant forms. The Al atoms presence in Al-MCM41 favors the J-aggregates formation as a € nsted result of interactions between the dye molecules and the Bro acid sites of the host framework. The emission spectra of the different complexes indicate that these species undergo an ESIPT reaction at S1 producing the corresponding K forms within the host. The emission decays are complexes due to the heterogeneity of the composites having different electronic structures. For the composites made from diluted DCM parent solution, the observed lifetime of the caged K monomer for the three complexes, is very long (2.2e3.0 ns) compared to that of solution (14 ps), due to the confinement effect. However, for H- and J-aggregates, the lifetimes show a shortening (100 and 600 ps, respectively). The results give new physical insight into the aggregation formation, which is affected by the pore size and the structural properties of the used host materials. Acknowledgments This work was supported by the MICINN and JCCM through projects: Consolider Ingenio 2010 (CSD2009-0050, MULTICAT), MAT2014-57646-P and PEII-2014-003-P. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.12.037. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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a3 (%) 5 6 6 6 6 5 8 7 8 8 6 6
t1 (ps)
60
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a1 (%) 65 65 65 67 68 70 63 63 61 64 70 72
t2 (ps)
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a2 (%) 26 26 27 26 27 24 31 32 35 33 27 28
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a3 (%) 9 9 8 7 5 6 6 5 4 3 3 2
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Spectroscopy and relaxation dynamics of salicylideneaniline derivative aggregates encapsulated in MCM41 and SBA15 pores lix Sa nchez b, Abderrazzak Douhal a, * Noemí Alarcos a, Fe a
Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S.N., 45071 Toledo, Spain b Instituto de Química Organica, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2015 Received in revised form 16 December 2015 Accepted 18 December 2015 Available online 29 December 2015
We report on steady-state and picosecond time-resolved emission studies of (E)-2-(2’-hydroxybenzyliden)amino-4-nitrophenol (HBA-4NP) interacting with MCM41, Al-MCM41 and SBA15. In these materials, the molecule shows spectral changes, with a broadening and shifts of absorption and emission bands, which depend on the concentration used in the preparation of the samples, indicating the aggregates formation. These caged monomers (M), H- and J-aggregates are in enol form at S0 and in keto one at S1, as a result of an excited-state intramolecular proton transfer (ESIPT) reaction. The K lifetime of the caged M is longer (3 ns) than the observed in solution (14 ps), due to the confinement effect on the radiationless pathways. The K lifetimes of J- and H-aggregates are 600 and 100 ps, respectively, and their € nsted acidity) of populations and stability depend on pore size and chemical composition (in terms of Bro the host. For MCM41 and Al-MCM41 ones, the formation of H- and J-aggregates is higher than using SBA15. This is due to a better encapsulation by the formers (85e95% MCM41/Al-MCM41 vs 60% SBA15). Doping MCM41 by Al allows more J-aggregates as result of strong interaction between the guest molecules and the Al-MCM41 framework. Thus, modulating the topological and structural properties of the mesoporous material leads to the formation of different encapsulated species, which could be of great interest to design nanophotonics devices based on this kind of materials. © 2015 Elsevier Inc. All rights reserved.
Keywords: Photobehavior H- and J-aggregates Confinement effect Intramolecular proton transfer Silica-based nanomaterials
́
1. Introduction Silica-based materials are being used in catalysis [1e5], photonics [6e9] and drugs delivery [9e12], to cite few of their applications. MCM41 and its metal doped derivatives are widely studied and have been used to encapsulate a large number of molecules [7,8,13e21]. The caged molecules (guests) show physical and chemical behaviors different from those observed in solutions [7,8,14,16e18,20,22e26]. Several trapped guests exhibit significant increases in the fluorescence signals due to the restriction produced by the hosts [16,22,25,27,28]. For example, Sudan I and hemicyanine (4-[4-(dimethylamino)styryl]1-n-alkylpyridinium bromide, interacting with zeolites and MCM41 showed large increases in the emission lifetime of the trapped and photoproduced keto species [16,22,28]. However, the encapsulation effect is not always so simple. Other molecules like cyanines [29e32], xanthenes
* Corresponding author. Tel.: þ34 925 265717. E-mail address: [email protected] (A. Douhal). http://dx.doi.org/10.1016/j.micromeso.2015.12.037 1387-1811/© 2015 Elsevier Inc. All rights reserved.
[33e35], rhodamines [36e38], pyrene [39e43], anthracene [44,45] or porphyrins [46e49], interacting with silica-based materials formed trapped aggregates in addition to the monomers. While, anthracene, pyrene and xanthene interacting with X and Y zeolites and MCM41 mesoporous materials, give place to the formation of H-aggregates [33e35,41e43,45]. In these complexes, the face-toface stacking produces a quenching in the emission signal of the system. On the other hand, molecules as cyanines, porphyrins and pyronines, produce J-aggregates within zeolites and mesoporous materials [31,32,47,49e51]. In this case, the obtained complexes exhibit a superradiant fluorescence due to a large excitonic coupling within the J-aggregates [50,51]. Clearly, the nature of the aggregates depends of the molecular structure of the guest, but also can be influenced by the type of silica-based material used. This has been observed for Nile Red (NR) interacting with different metaldoped MCM41 materials (X-MCM41) [21]. In this case, NR trapped molecules form both H- and J-aggregates within the mesoporous materials [21]. When MCM41 was doped with aluminum (Al-MCM41), the H- and J-aggregates formation (respect to the other X-MCM41 materials) was higher due to its large loading
N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
efficiency [21]. In presence of Ti or Zr (Ti-MCM41 or Zr-MCM41), the formation of J-aggregates was more favored than in the other XMCM41 materials [21]. Recently, we have reported on the spectroscopy and photodynamics of (E)-2-(2-hydroxybenzyliden)amino-4-nitrophenol (HBA4NP) interacting with NaX and NaY zeolites [52]. HBA-4NP, which does not form aggregates in solution, exhibits aggregation within these hosts. We observed that the formed composites contain caged enol structures in the forms of monomers, H- and J-aggregates which undergo an excited-state intramolecular protontransfer (ESIPT) reaction at S1 to produce keto (K) type phototautomers [52]. The fluorescence lifetime of caged K monomers is remarkably long (~6 ns inside of zeolites vs 14 ps in DCM solution) due to the confinement effect on the radiationless pathways. The relative population and emission lifetimes of H- or J-aggregates depend on the loading and initial concentration of the dye used to make the composites [52]. For a more understanding of these trapped populations, a spectroscopic study of HBA-4NP interacting with large pores like those of MCM41 (d z 25 Å) or SBA 15 (d z 60 Å), is necessary. Moreover, doping MCM41 with aluminum (Al) will also provide information on the role of this metal in the dye aggregates formation and related spectroscopy and dynamics. Herein, we report on the encapsulated effect of MCM41, AlMCM41 and SBA15 materials on the steady-state UVevisible diffuse transmittance and emission spectra as well as on the piconanosecond photobehavior of trapped HBA-4NP. The results indicate that the aggregates formation and their nature depend on the used materials. The lifetime of the aggregates (~100 ps and ~600 ps for H- and J-types, respectively) in their photoproduced keto forms are shorter than that of monomers (2.20e3.00 ns) due to excitonic coupling in the formers. Upon increasing the concentration of the encapsulated dye, the times become shorter. We compare these results with those obtained using NaX and NaY zeolites [52] and we conclude that MCM41, materials traps more aggregates as its pore diameter is larger than that of the zeolites, allowing a better packing between encapsulated molecules. For Al-MCM41 one, the J-aggregates formation is favored due to the strong interactions with the framework. For the SBA15 complexes, the unexpected results (low formation of aggregates despites its large pore diameter) resides on its weak loading ability. 2. Experimental section The synthesis of (E)-2-((2-hydroxybenzyliden)amino-4nitrophenol (HBA-4NP) is described in our previous report [52]. Dichloromethane (DCM, spectroscopic grade, 99.9%), and MCM41, Al-MCM41 and SBA15 materials were purchased from SigmaeAldrich, and used as received. HBA-4NP/mesoporous material composites were prepared by adding a 100 mg of a dried mesoporous material into 15 mL of HBA-4NP in DCM solution (2 103 M or 6 107 M). The suspension was stirred at room temperature during 24 h. The solid mixture was centrifuged and rinsed four times with pure DCM to remove the dye weakly bounded to the surfaces of the mesoporous material. The solid then was dried in vacuum at room temperature. The steady-state UVevisible absorption and fluorescence spectra have been recorded using JASCO V-670 and FluoroMax-4 (Jobin-Yvon) spectrophotometers, respectively. Picosecond emission decays were measured by a time-correlated single photon counting (TCSPC) system [53]. The sample was excited by a 40-ps pulsed diode laser centered at 371 or 433 nm (<5 mW, 40 MHz repetition rate) and instrument response function (IRF) of ~70 ps. For the excitation at 320 nm, we used the second harmonic of the output (640 nm) from a femtosecond optical parametric oscillator (Inspire Auto 100) pumped by 820 nm pulses (90 fs, 2.5 W, 80 MHz)
35
from a Ti:sapphire oscillator (Mai Tai HP, Spectra Physics). In these experiments (IRF z 30 ps) the fs-laser excitation was at very low power to avoid undesired photochemistry. The IRF of the system has been measured using a standard LUDOX (SigmaeAldrich) solution in 1 cm cell. The decays were deconvoluted and fitted to single or multi-exponential functions using the FLUOFIT package (PicoQuant) allowing single and global fits. The quality of the fit was estimated by c2, which was always below 1.2 and the distribution of the residues. The time-resolved emission spectra (TRES) were constructed form the single-wavelength measurement, and the zero time spectrum was taken as the one obtained at the intensity half-maximum corresponding to the rise of the excitation pulse. All experiments were done at 293 K. 3. Results and discussion 3.1. Steady-state observation 3.1.1. Diffuse transmittance spectra Fig. 1 shows the UVevisible absorption and diffuse transmittance (DT) spectra of HBA-4NP in a DCM solution, and interacting with MCM41 materials in a DCM suspension prepared from a diluted parent solution (6 107 M). In pure DCM, the molecule exhibits two absorption bands at 280 and 350 nm, which were assigned to S0(p) / S2(p*) and S0(p) / S1(p*) transitions of the enol (E) tautomer, respectively (Scheme 1A) [52]. Interacting with MCM41, the band shape and spectral position significantly differ from that observed in pure DCM, which suggests the presence of different electronic structures in the ground state as a result of interactions between the dye and the MCM41 framework. We anticipate that the observed bands at ~360, and ~310 and 410 nm are due to caged monomers (M), H-, and J-aggregates, respectively. This assignment is based on our previous report where HBA-4NP interacts with NaX and NaY zeolites, showing the presence of this kind of species [52]. In the following paragraphs, we present and discuss the steady-state and time-resolved data to support our attribution. Fig. 2 shows a comparison of the DT spectra of HBA-4NP interacting with MCM41, Al-MCM41 and SBA15 in DCM suspensions, using two different initial dye concentrations to make the composites: diluted (6 107 M) and concentrated (2 103 M) parent solutions. Upon addition of the mesoporous materials to the dyeDCM solutions, an immediate change in the powder color from
Fig. 1. Normalized (to the maximum intensity) UVevisible absorption, diffuse transmittance and fluorescence (lexc ¼ 370 nm) spectra of HBA-4NP (2 105 M) in a DCM solution (dashed line) and interacting with MCM41 in a DCM suspension (solid line) prepared from a diluted parent DCM solution (6 107 M).
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N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
Scheme 1. A) Molecular structures of (E)-2-((2-hydroxybenzyliden)amino-4-nitrophenol (HBA-4NP) tautomers and schematic representation of MCM41/Al-MCM41 and SBA15 mesoporous materials. The inserted pictures show the color changes encapsulation of HBA-4NP into the different mesoporous materials. B) Schematic representation (not in scale) of the allowed (full arrows) and not allowed (dotted arrows) absorption electronic transition of the monomer, H- and J-aggregates of HBA-4NP. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
white to yellow occurs, indicating an instantaneous interaction of the material with the dye leading to composites formation (powders picture in Scheme 1A). This effect is considerably more pronounced using Al-MCM41, indicating stronger interactions with this material. This result is in agreement with previous reports, € nsted acidity in mesoporous materials where the increase in the Bro doped with metal (Al, Ga, Ti, Zr) leads to stronger hosteguest interactions [21,54]. In order to know if the dye molecules are within the host pore or/and on its surface, we recorded the DT spectra of the MCM41 composites before and after washing several times with pure DCM (Fig. S1 in supplementary material). The result does not show any significant change in the spectra; suggesting that most of the observed signal is coming from the included dye molecules. A priori, those found adsorbed on the crystal surface must have a weaker adsorption energy, and thus easy to decomplex by successive washing of the composites. Moreover, the formed complexes have a great stability as their spectra do not show changes for a one year approximately (Fig. S2 in Supplementary Material). For diluted samples, the DT spectra of the samples using MCM41 and Al-MCM41 show three absorption maxima at 260, 310 and
400 nm, and a small shoulder at 360 nm. For a better understanding of these bands, the normalized DT spectra were deconvoluted into its constituting components, supposing Gaussian-shape bands (Fig. 3). The obtained spectra are composed of four bands centered at 257, 308, 366 and 410 nm for MCM41 and 260, 305, 354 and 406 nm for Al-MCM41 (Fig. 3A and B, respectively). When the dye is interacting with SBA15, only the absorption maxima at 260, 300 and 360 nm, with a tail at 400 nm, are observed (Fig. 2). In this case, the deconvoluted DT spectrum exhibits four bands centered at 256, 301, 355, and 400 nm, being the latter of low intensity (Fig. 3C). To understand the nature of these absorption bands, we recorded the DT spectra of samples made from a higher concentration of the dye (2 103 M) (Fig. 2). For these samples, the spectral position does not change, but the relative intensities of the bands show a variation in which the intensity at 360 nm decreases and that of 410 nm increases (Fig. 2 and S3 in supplementary material). This relative intensity change is due to the formation of more aggregates in the concentrated sample which is in agreement with previous reports where HBA-4NP and its parent molecule, salicylaldehyde azine (SAA), produce aggregates within faujasite zeolites [23,52]. The exciton coupling theory assigns the hypsochromic and
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37
Fig. 2. Normalized (to the maximum intensity) UVevisible diffuse transmittance and fluorescence spectra of HBA-4NP interacting with MCM41, Al-MCM41 and SBA15 in DCM suspension, using diluted (6 107 M) (dashed line) and concentrated (2 103 M) (solid line) parent HBA-4NP solutions. F(R) is the KubelkaeMunk function, F(R) ¼ ((1R)2)/2R, where R is the diffuse reflectance from a sample. For emission the excited wavelength was 370 nm.
bathochromic shifts of the absorption bands relatively to that of the monomers, to H- (face-to-face interactions) and J-aggregates (faceto-edge interactions), respectively (Scheme 1B) [55e57]. Therefore, we attribute the ~310 and ~410 nm bands to the absorption of encapsulated H- and J-aggregates, while the absorption at ~260 and ~360 nm is due to S0(p) / S2(p*) and S0(p) / S1(p*) transitions of caged E monomers, respectively (Scheme 1B). The relationships between the spectral shifts and type of aggregates that are formed, is explained in terms of different electronic coupling between the monomers involved in the aggregates. This coupling is related to the angle (a) formed between the axes connecting the dipole moment centers of two neighboring monomers which are disposed (stacked) in a parallel direction (Scheme 1B) [55]. For J-aggregates, the small angle (a) produces a strong orbital coupling between the molecules packing [51]. In these aggregates where only the S0/ S1 transition is allowed, the strong orbital coupling gives place to redshifted narrow absorption bands [51]. However, in our case, the heterogeneity of the system produces a widening of the absorption band as the aggregates population also depends on the specific and no specific interactions with the host. On the other hand, H-aggregates show an absorption due to S0/ S2 transition as that of S0/ S1 is forbidden, and whose width is affected by the heterogeneity of the composites [51,55]. This is in agreement with the observation at higher concentration, where the width of both bands increase (Table 1). On the other hand, the formation and stability of H- and J-aggregates depend of the morphology (pore size) and chemical composition (doping metal) of the used material [20,21,30,47,58]. In our case, the formation of H- and J-aggregates is higher for MCM41 complexes than for those of SBA15. So, comparing the relative absorption intensity of M, H- and J-aggregates in the three
Fig. 3. Spectral decomposition (supposing a Gaussian shape of the absorption band) of diffuse transmittance spectra of HBA-4NP interacting with (A) MCM41, (B) Al-MCM41 and (C) SBA15, using a diluted parent DCM solution (6 107 M). The inset gives information on the fitted and fitting bands of the experimental DT spectra.
composites, SBA15 ones have the lowest population of aggregates (Figs. 2 and 3). This is explained in term of a better encapsulation, as MCM41 and Al-MCM41 show a loading efficient of 85 and 95%, respectively, while that of SBA15 is of 60%. This is due to the large pore (diameter 60 Å vs 25 Å for MCM41) and lower BET surface area
38
N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
Table 1 Values of the full width at half-maximum (FWHM) intensity of the emission band of monomers H-, and J-aggregates present in HBA-4NP interacting with MCM41, AlMCM41 and SBA15, using diluted and concentrated parent DCM solutions. Sample
Host
FWHM (cm1) H-aggregates
FWHM (cm1) Monomer
FWHM (cm1) J-aggregates
Diluted (6 107 M)
MCM41 Al-MCM41 SBA15 MCM41 Al-MCM41 SBA15
5490 5210 4500 7170 6733 6270
3430 3160 3990 3630 3560 3545
2470 3460 1190 3255 3660 2276
Concentrated (2 103 M)
(780 m2/g vs 1000 m2/g for MCM41), that presents SBA15 materials [54,59e61]. It is known that MCM41 and SBA15 materials have the same hexagonal structure with a surface that contains hydroxyl groups [61,62]. However, the SBA15 exhibits a considerable surface roughness, which is attributed to (SiO2)n islands on the surface and a lower presence of eOH groups [62,63]. It is believed that this is the reason why this material has a low energy surface, which leads to a lower loading [15,62,63]. Comparing MCM41 and Al-MCM41 complexes, the latter exhibits more presence of J-aggregates (absorption band at ~410 nm) (Figs. 2 and 3 and S3 in supplementary material). Al doping leads to charge defects formation and delocalization in the host framework, producing an increase in its acidity due to the generation of € nsted sites [1,64e66]. So, we believe that the interaction beBro tween the guest molecules and the acid sites of the framework produces more J-aggregates. This is in agreement with what we observed for HBA-4NP interacting with the faujasite zeolites (NaX and NaY) [52]. In that case, the J-aggregates formation was also favored being higher for NaX complexes where the amount of Al is greater (Si/Al ¼ 1.24 and 2.86 for NaX and NaY, respectively) [52].
3.1.2. Emission spectra Fig. 1 shows the fluorescence spectra of HBA-4NP in a DCM solution and interacting with MCM41 in a DCM suspension using diluted sample. In solution, the molecule has an emission band maximum at 580 nm, assigned to the keto form, as a result of an ESIPT process in the initially excited enol form (Scheme 1) [52]. While, interacting with MCM41 materials, the emission band is shifted toward shorter wavelengths, indicating the formation of other species (caged aggregates and monomers). Fig. 2 shows a comparison of the fluorescence spectra of HBA4NP within MCM41, Al-MCM and SBA15 suspensions, using two different dye concentrations (diluted and concentrated sample). Using the diluted ones, the emission bands are shifted to shorter wavelengths when compared to the one in pure DCM (Figs. 1 and 2). This is explained in terms of an emission from keto forms not allowed twisting motion inside these materials. Comparable behavior was observed for a similar molecule, salicylaldehyde azine, encapsulated in NaX zeolite [23]. In that case, the maximum emission spectra changed from 580 in DCM solution to 505 nm in NaX complexes [23]. The fluorescence quantum yield of HBA-4NP in DCM solution is 3 103 M, which indicates the involvement of efficient nonradiative transitions in the photoproduced keto form [52]. When it is confined in the mesoporous materials and in spite of aggregates formation, the emission signal is about seven times larger than in DCM solution, consequence of a decrease in the nonradiative rate constants (Fig. S4). On the other hand, the absence of an emission band at 580 nm, reveals the lack of free keto at S1 which in turn reflects the absence of free enol structures at S0 in these suspensions. Thus, the observed emission bands from these composites are due to trapped HBA-4NP molecules by the used hosts. In order to explore the aggregates emission from these complexes, we recorded the spectra at higher concentrations (Fig. 2). The emission intensity maxima appear at 500, 505 and 475 nm for MCM41, Al-MCM41 and SBA15, respectively (Fig. 2). Moreover, the full width at half-maximum (FWHM) of these
Scheme 2. Schematic representation (not in scale) of the energetic diagram at ground and electronically excited state, of monomer (M), H- and J-aggregates of HBA-4NP interacting with the used mesoporous materials in DCM suspensions. For simplicity the ground- and excited-state levels of the three caged structures are put at the same level. Cartoon illustrating the caged structures (M, H- and J-aggregates) within MCM41/Al-MCM41 and SBA15 materials. For each species, we indicate the fluorescence lifetime.
N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
Fig. 4. Diffuse transmittance (solid line) and excitation spectra of HBA-4NP interacting with (A) MCM41, (B) Al-MCM41 and (C) SBA15 in a DCM suspension, using concentrated (2 103 M) parent solutions. The values of the observation wavelengths are shown in the inset.
emission bands (3760 cm1 for MCM41, 4000 cm1 for Al-MCM41, and 4140 cm1 for SBA15) are larger than those obtained for the diluted samples (3650 cm1 for MCM41, 3840 cm1 for Al-MCM41, and 4000 cm1 for SBA15). These results clearly reflect the presence
39
Fig. 5. Magic-angle emission decays of HBA-4NP interacting with (A) MCM41, (B) Al-MCM41 and (C) SBA15 in a DCM suspension, using (1) diluted (6 107 M) and (2) concentrated (2 103 M) parent HBA-4NP solutions. The samples were excited at 371 nm and observed at 500 nm. The solid lines are from the best-fit using exponential functions. IRF is the instrumental response function (70 ps).
40
N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
of emission aggregates. As already anticipated, we suggest that the aggregates and monomers experience an ESIPT reaction at the S1 state, as the emission intensity maxima of the bands shows an abnormally Stokes-shift. Thus, the emission bands are due to the caged keto forms: K-monomers and K-aggregates (H- and J-type) (Scheme 2). 3.1.3. Excitation spectra The presence of these caged structures (monomer and aggregates) is also confirmed by recording fluorescence excitation spectra of the related samples at different wavelengths of observation (Fig. 4). The complexes were prepared using concentrated (2 103 M) parent DCM solutions of HBA-4NP. For all the complexes, observing at 450 nm (emission from caged K monomers), the obtained spectrum mainly corresponds to trapped E monomers which absorb at 360 nm (Fig. 4). When we observed at longer wavelengths (475e550 nm), a new absorption band at 400 nm corresponding to the J-aggregates appears (Fig. 4). The absorption band of H-aggregates should appear at higher energies (~310 nm), as the S0 / S1 transition is forbidden. The absorption in the Table 2 Values of time constants (ti), normalized (to 100) pre-exponential factors (ai) and fractional contributions (ci ¼ tiai) obtained from the fit of the emission decays of diluted sample of HBA-4NP (6 107 M) interacting with MCM41, Al-MCM41 and SBA15 in DCM suspensions upon excitation at 371 nm and observation as indicated (lobs). Host
lobs/nm
MCM41
450 475 500 530 570 450 475 500 530 570 450 475 500 530 570
Al-MCM41
SBA-15
t1/ps
100
90
122
a1%
c1%
45 44 44 45 46 50 49 51 52 51 42 40 43 46 47
10 11 10 11 12 11 10 11 12 12 11 11 12 16 17
t2/ps
570
540
700
a2%
c2%
39 40 37 38 37 38 41 39 37 38 26 24 22 20 19
37 38 38 37 38 39 39 37 35 36 25 23 21 19 20
t3/ns
2.30
2.20
3.00
a3%
c3%
16 16 19 17 17 12 10 10 11 11 32 36 35 34 34
53 51 50 52 52 50 51 52 53 52 64 66 67 65 63
Table 3 Values of time constants (ti), normalized (to 100) pre-exponential factors (ai) and fractional contributions (ci ¼ tiai) obtained from the fit of the emission decays of concentrated sample of HBA-4NP (2 103 M) interacting with MCM41, Al-MCM41 and SBA15 in DCM suspensions upon excitation at 371 nm and observation as indicated (lobs). Host
lobs/nm
MCM41
450 475 500 530 570 630 450 475 500 530 570 630 450 475 500 530 570 600
Al-MCM41
SBA-15
t1/ps
60
40
100
a1%
c1%
71 72 72 75 77 75 72 74 75 77 82 76 57 48 60 63 65 66
31 32 32 33 34 33 31 31 32 34 34 33 10 10 12 14 15 15
t2/ps
290
270
650
a2%
c2%
20 22 21 17 15 17 21 19 19 16 11 18 16 15 13 14 11 11
14 15 18 18 18 18 18 20 21 21 22 22 25 25 24 26 26 25
t3/ns
1.20
1.00
2.70
a3%
c3%
9 6 7 8 8 8 7 7 6 7 7 6 27 27 27 23 24 23
55 53 50 49 48 49 51 49 47 45 44 45 65 65 64 62 61 60
310e325 nm region, could correspond to the S0 / S2 absorption of these aggregates (Fig. 4). According to what we observed previously, the excitation spectra of the Al-MCM41 complexes indicate that the formation of the J-aggregates is larger than in other mesoporous material (MCM41 and SBA15), due to the strong interaction with the Al-doped material. Based on the above observation, we suggest that absorption and emission band maxima of the formed structures of HBA-4NP upon interaction with MCM-41, AlMCM41 and SBA15 materials in DCM suspensions are: monomers (360 and 465 nm), H-aggregates (310 and 500 nm) and J-aggregates (400 and ~500 nm). For a better understanding of their photobehavior, we investigated the time-resolved emission decays of these entities. 3.2. Picosecond studies Fig. 5 shows representative emission decays of the complexes collected at 500 nm and excited at 370 nm, for both diluted and concentrated samples. Tables 2 and 3 give the obtained time constants (ti), the pre-exponential factors (ai) and relative contributions (ci) normalized to 100 after a multiexponential global fits. In agreement with the observed results in the steady-state part, the obtained lifetime values depend on the loading and on the used mesoporous material (Fig. 5, and Tables 2 and 3). For diluted samples, we observed decay times of 100 ps, 570 ps and 2.30 ns for MCM41, 90 ps, 540 ps and 2.20 ns for Al-MCM41, and 122 ps, 700 ps and 3.00 ns for SBA15 (Table 2). The related amplitude and contribution of each component exhibit small variations with the observation wavelength (Table 2). The global contribution of the shortest component is ~10% for all complexes, that of the intermediate one, ~35e40% for MCM41 and Al-MCM41 and ~20e25% for SBA15, and that of the longest component, ~50% for MCM41 and AlMCM41 and ~65% for SBA15. Based on these results and taking into account that for diluted samples the caged monomer species is the main population, we assign the longest component (~3 ns) to the caged keto monomers emission. Previous studies showed that the lifetime of K monomer in pure DCM is 14 ps. [52]. This shortening in the emission lifetime is due to non-radiative processes produced by the presence of an electron withdrawing group (NO2), and the twisting motion around C]N bond which leads to the formation of further rotamers of the tautomers [52]. The encapsulation of the HBA-4NP in the mesoporous materials produces a molecular restriction to twisting motion, which decreases the radiationless transitions of the caged K species. This confinement effect was observed in other molecules where their inclusions into cavities or channels change the photodynamics, resulting in an increase in the lifetimes of the emitting forms, in comparison with the neat solution [7,16,20,23,25]. HBA-4NP interacting with faujasite zeolites, showed a lifetime of the caged K monomer even longer than in the present hosts (6 ns vs 3 ns). NaX and NaY cavities have small cavities (pore diameter of the zeolite ¼ 8 Å) not allowing twisting motion [52]. The intermediate and shorter components observed in the decays of the present composites are due to caged H- and Jaggregates of keto tautomers. In agreement with the exciton theory, we assign the shortest one to the H-aggregates [55,67]. These species, which are rather in an excited-state of higher energy (S2), are characterized by a very fast relaxation to the ground state. As a result, part of the excitation energy is lost in the non-radiative processes, leading to very low fluorescence quantum yield [67]. However, the J-aggregates which are populated at an excited-state of a lower energy (S1), are more stabilized showing a higher emission quantum yield [51,67]. Thus, the intermediate component is assigned to these species. Upon increasing the HBA-4NP concentration (2 103 M), we observed a shortening in the emission decays as a result of an increase in the monomeremonomer
N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
41
Fig. 6. Normalized (to the maximum of intensity) magic-angle time-resolved emission spectra (TRES) of HBA-4NP interacting with MCM41 in a DCM suspension, using concentrated (2 103 M) parent HBA-4NP solutions gated at the indicated delay times after excitation at 371 nm. The inset gives the gating time of the spectra.
interactions (Fig. 5 and Table 3). Now, the lifetimes are 60 ps, 290 ps, 1.20 ns for MCM41, 40 ps, 270 ps, 1.00 ns for Al-MCM41, and 100 ps, 650 ps and 2.70 ns for SBA15 (Table 3). Fluorescence quenching processes take place between neighboring encapsulated molecules. For this reason, the Al-MCM41 complexes, where there is more amount of encapsulated dye (95% of loading), exhibit the shortest emission lifetimes (Fig. 5 and Fig. S5 in supplementary material). On the contrary, the SBA15 complexes present very small variations in their decays when changing the used dye concentration to make the composites. The reason of this is that in SBA15 the aggregates formation is lower due to the weaker loading efficiency (60%) and larger pore of this material (Fig. 5 and Fig. S5 in supplementary material). Similar behavior was observed for pyronine within the L zeolite where the change (quenching) of its emission lifetimes depended on dye loading [50]. This also was observed for HBA-4NP in NaX and NaY zeolites, as the loading efficient for these zeolites was different (20% for NaX and 90% for NaY) [52]. So, the HBA-4NP/NaY complexes showed the decay times shorter than those NaX ones [52]. To better understand the photodynamics of these complexes, we recorded time-resolved emission spectra (TRES). Fig. 6 exhibits the result exciting and gating the spectra of HBA-4NP/MCM41 complexes prepared from concentrated parent solution (2 103 M). We can distinguish three bands whose intensity maxima are 465, 485 and 530 nm. The band collected at longer gating times (3 ns), with intensity maximum at 465 nm and which, is due to the emission of the caged K monomer. The other bands with maxima at 485 and 530 nm are assigned to the caged K in Jand H-aggregates, respectively. The latters are characterized by a larger Stokes shift while the formers have a small one [55e57]. To further investigate the emission lifetime of the aggregates, we analyzed the decays upon femtosecond excitation at 320 (region of H-aggregates absorption) and 433 nm (region of J-aggregates absorption). Fig. 7 shows the decays recorded at 530 nm for HBA-4NP/MCM41 and HBA-4NP/Al-MCM41 composites using concentrated parent solutions. Comparing the 370 nm excitation results, upon excitation at 320 nm, the decay signal is shorter reflecting in the fastest decay of H-aggregates type, while upon excitation at 433 nm it is longer due to the J-aggregates contribution in the global signal. Table 4 shows the data fitting the emission decays of these complexes. The time values do not show significant
Fig. 7. Magic angle emission decays of HBA-4NP interacting with (A) MCM41 and (B) Al-MCM41 using concentrated (2 103 M) parent HBA-4NP solutions. The samples were excited at (1) 320 nm, (2) 370 nm and (3) 433 nm and observed at 530 nm. The solid lines are from the best-fit using exponential functions. IRF is the instrumental response function.
change relatively to the results exciting at 371 nm. However, the pre-exponential factors exhibit some variations. The shorter component (40e70 ps) shows their maximum amplitude when exciting at 320 nm (80e84%), while the intermediate one (250e290 ps) has their maximum upon exciting at 433 nm, being longer for the Al-MCM41 sample (~35%) than for that MCM41(~27%). Table S1 in supplementary material, shows the data of the emission decays using diluted samples and exciting at the same regions (320 and 433 nm). For diluted samples, a comparable behavior related to the amplitudes observed for each of the regions of different wavelength excitation, is observed. 4. Conclusions We showed here, that when HBA-4NP is interacting with MCM41, Al-MCM41 and SBA15, caged enol monomers, H- and Jaggregates are stabilized at S0. Using MCM41 and Al-MCM41 hosts,
42
N. Alarcos et al. / Microporous and Mesoporous Materials 226 (2016) 34e43
Table 4 Values of time constants (ti) and normalized (to 100) pre-exponential factors (ai) obtained from the fit of the emission decays of concentrated sample of HBA-4NP (2 103 M) interacting with MCM41 and Al-MCM41 in DCM suspensions upon excitation at 320 and 433 nm, and observation at different wavelengths (lobs). Concentrated samples Host
MCM41
Al-MCM41
lobs/nm 450 475 500 530 570 630 450 475 500 530 570 630
Excitation at 320 nm
t1 (ps)
50
40
a1 (%) 80 81 82 83 84 84 79 80 81 81 83 82
Excitation at 433 nm
t2 (ps)
260
250
a2 (%) 15 13 12 11 10 11 13 13 11 11 11 12
t3 (ns)
1.25
1.00
more H- and J-aggregates are trapped that when using SBA15, due to a higher loading efficient in the formers. Thus, for the SBA15 complexes the caged monomers are the predominant forms. The Al atoms presence in Al-MCM41 favors the J-aggregates formation as a € nsted result of interactions between the dye molecules and the Bro acid sites of the host framework. The emission spectra of the different complexes indicate that these species undergo an ESIPT reaction at S1 producing the corresponding K forms within the host. The emission decays are complexes due to the heterogeneity of the composites having different electronic structures. For the composites made from diluted DCM parent solution, the observed lifetime of the caged K monomer for the three complexes, is very long (2.2e3.0 ns) compared to that of solution (14 ps), due to the confinement effect. However, for H- and J-aggregates, the lifetimes show a shortening (100 and 600 ps, respectively). The results give new physical insight into the aggregation formation, which is affected by the pore size and the structural properties of the used host materials. Acknowledgments This work was supported by the MICINN and JCCM through projects: Consolider Ingenio 2010 (CSD2009-0050, MULTICAT), MAT2014-57646-P and PEII-2014-003-P. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.12.037. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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a3 (%) 5 6 6 6 6 5 8 7 8 8 6 6
t1 (ps)
60
70
a1 (%) 65 65 65 67 68 70 63 63 61 64 70 72
t2 (ps)
290
290
a2 (%) 26 26 27 26 27 24 31 32 35 33 27 28
t3 (ns)
1.30
1.20
a3 (%) 9 9 8 7 5 6 6 5 4 3 3 2
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