Optical excitations of porphyrin J-aggregates

Synthetic Metals 155 (2005) 291–294

Optical excitations of porphyrin J-aggregates A. Tonizzo a,∗ , M. Cerminara a , G. Macchi a , F. Meinardi a , N. Periasamy c , P. Sozzani b , R. Tubino a a

INFM and Universit`a degli Studi di Milano-Bicocca, Dipartimento di Scienza dei Materiali, via Cozzi 53, I-20125 Milano, Italy b Universit` a degli Studi di Milano-Bicocca, Dipartimento di Scienza dei Materiali, via Cozzi 53, I-20125 Milano, Italy c Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, 400005 Mumbai, India Available online 4 November 2005

Abstract In this work, we will present a study of the optical properties of aggregated tetrakis(p-sulfonatophenyl)porphyrin (TSPP) in different environments which can have a dramatic influence on the spectroscopic features. We will show that when the porphyrins are free to self-organize (i.e. in saturated solutions) they show two distinct photoluminescence peaks, revealing the presence of two different species, with a relative population strongly dependent on the temperature. If aggregation occurs in an environment with restricted geometry (i.e. within the nanochannels of MCM-41) it is possible to modify the spontaneous self-assembly of the molecules and, as a consequence, the relative ratio of the two emissions intensity. We will report on the possibility to induce the preferential formation of the low-energy emitting species, aiming to tune in a controlled way the emission spectrum of the aggregate. © 2005 Elsevier B.V. All rights reserved. Keywords: Porphyrins; Photoluminescence; Collective phenomena; Inclusion compounds

1. Introduction Self-assembling processes of molecular components into large supramolecular structures are primarily investigated because of their involvement in many fundamental physicochemical as well as biological processes. The possibility of changing the mesoscopic structure of the resulting species through a proper choice of the molecular components opens the way to the design and synthesis of materials capable to exhibit specific properties and functions [1]. From this point of view, porphyrins are well suited building blocks because, depending on their electronic and steric properties, they can spontaneously self-assemble into dimers or higher aggregates through noncovalent interactions [2]. In particular, water-soluble porphyrins are very interesting because aggregation can be conveniently controlled by screening the charge repulsion by changes in the ionic strength and pH. Many relevant physicochemical properties of this class of compounds, including photophysical features, are strictly dependent on their aggregation state. On the basis of these properties, a challenging problem is the translation of the electron transfer high efficiency and speed,

∗

Corresponding author.

0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.09.005

often found for biological reactions, to the world of molecular photonic materials or photonic devices (i.e. biomimetics), aiming to optimize their performances. In particular, the tetrakis(psulfonatophenyl)porphyrin (TSPP) has recently received particular attention because it is able to form, under proper experimental conditions, highly ordered J- and/or H-aggregates [3–5]. The H- and J-aggregates correspond to the limiting cases of parallel monomeric units stacked face-to-face or edge-to-edge, respectively. According to the excitonic splitting theory [6], Haggregates exhibit a blue-shifted Soret absorption band, whereas J-aggregates are characterized by a red-shifted absorption band. In this paper, we present a detailed study of the spectroscopic properties of TSPP in monomeric and aggregated state, elucidating the role of aggregation on the excitonic properties. Furthermore, we examine the formation of porphyrin aggregates within a mesoporous cage compound (specifically an aluminosilicate mesostructure MCM-41), intended to create an encapsulated species where constraints in aggregate length and orientation, associated with the spatially constricted and directional character of the cavity, result in a new “ship-in-a-bottle-type” nanomaterial, possessing novel spectroscopic properties. Additionally, in this kind of compound the aggregates assume a more robust and manipulable physical form, which would be more suitable for optical and optoelectronic applications.

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2. Experimental Aqueous solutions of the porphyrin H2 TPPS4− (TSPP) were prepared by dissolving the tetrasodium salt (Strem Chemicals) in deionized water. Aggregation was started by addition of the proper amount of concentrated KCl solution up to the required concentration. MCM-41 was synthesized according to the procedure described in ref. [7]. The silylated MCM-41 and the inclusion compounds TSPP-mon/MCM-41 and TSPP-agg/MCM-41 (monomeric and aggregated TSPP within the modified MCM41, respectively) were prepared according to the procedure detailed in ref. [8]. Absorption spectra were recorded using a Cary 50 Scan UV–Visible spectrophotometer. Steady-state photoluminescence (PL) measurements were acquired using a monochromated Xe lamp as excitation source and a N2 -cooled CCD camera coupled with a Jobin–Yvon Triax 190 polichromator for the signal detection. PL spectra were recorded at various temperatures between 300 and 80 K using a bath cryostat, which ensures a reliable sample temperature control. 3. Results and discussion 3.1. Absorption spectra The absorption spectra of the TSPP solution as a function of the KCl concentration are shown in Fig. 1. At pH lower than 4, H2 TPPS4− is protonated at the nitrogen centers of the macrocycle to form the dianion H4 TPPS2− . The dianion at pH 2.5 has the Soret absorption band at 2.86 eV and the Q bands at 2.09 and 1.92 eV. As the concentration of KCl is increased, a relatively sharp absorption band emerges at 2.53 eV, accompanied by a reduction of the intensity of Soret band at 2.86 eV. In the Q-bands region, the dianion band at 1.92 eV decreases simultaneously with the emersion of a new

band at 1.75 eV, indicating the formation of a new species. The conversion of the dianion to a new species (absorbing at 2.53 and 1.75 eV) is quantitative as indicated by the isosbestic point at 1.87 eV. The narrow red-shifted absorbion band is a characteristic feature of the formation of J-aggregates. In the case of porphyrins, the J-aggregate has a structure in which the monomeric dianions are cofacially stacked but translated along the axis defined by opposite meso carbons such that the negatively charged sulfonato group of the porphyrin moiety is positioned over the positively charged macrocycle moiety. The role of cations (K+ ) in shielding the sulfonato group in the aggregate is also important to stabilize the J-aggregate [9]. In order to model the role of the aggregation, only homogeneous aggregates are considered, in which all the transition frequencies ωn are equal, and it is assumed that only transfer interactions between two neighboring molecules can occur (the interaction strength is denoted by J). Limiting the calculation only to nearest-neighbors dipole–dipole interaction is not too crude, especially if the aim is to study the fundamental physics of the system. Using this approximation, the oscillator strength between the ground state and the one-exciton k state is given by: fk,g = µ2k,g = k|M - |g

(1)

where µ is the dipole moment of the transition. It is useful to consider the special situation in which the molecular dipoles of neighboring molecules are arranged in an edge-to-edge configuration. This situation usually occurs in J-aggregates and it yields that almost the entire oscillator strength between the ground state and the one-exciton band resides in the transition to the k = 1 state: µ2k=1,g = 0.81(N + 1)µ2 for N  1. Thus, the absorption spectrum of the ordered chain is dominated by a peak at the position of the k = 1 state which, for N  1, occurs to a good approximation at w0 + 2J. Besides dominating the absorption spectrum, the k = 1 state, which has an oscillator strength in the order of N times the oscillator strength of a single molecule, leads to fast emission rates, roughly N times that of the monomer. For this reason, the k = 1 state is often also called a superradiant state and the process of fast emission is also called exciton superradiance, but it is better to refer to it as “cooperative spontaneous emission”, resulting from the fact that the emitting state has the dipoles of the individual molecules oscillating almost perfectly in phase. 3.2. Emission spectra

Fig. 1. Absorption spectra of the dianion H4 TPPS2− (10 ␮M) for different concentrations of KCl, pH 2.5. Arrows indicate the direction of changes upon increasing the concentration of KCl. In the inset, an enlargement of the isosbestic point at 1.87 eV is also reported.

The PL spectra excited at Soret frequencies for the TSPP solution (10 ␮M), pristine and in presence of KCl, are shown in Fig. 2. For the monomer solution, exciting the TSPP molecules on the main absorption band (i.e. at 2.86 eV), the PL spectrum is constituted by a single peak at 1.85 eV. Upon addition of KCl, aggregation is induced: exciting the sample at 2.53 eV, it is possible to selectively excite the aggregate. The resulting emission is characterized by a peak with maximum located at 1.74 eV. Again, the position of the band is red-shifted with respect to the single molecule: this observation, coupled with the fact that the

A. Tonizzo et al. / Synthetic Metals 155 (2005) 291–294

Fig. 2. PL spectra of the dianion H4 TPPS2− (dotted line, λex = 2.86 eV, [H4 TPPS2− ] = 10 ␮M and pH 2.5) and of the J-aggregate sample at 300, 160 and 80 K (dashed lines, λex = 2.53 eV, [H4 TPPS2− ] = 10 ␮M, pH 2.5 and [KCl] = 0.2 M). In the inset linewidth (σ 2 ) of the 1.74 eV peaks as a function of the temperature.

line is narrower with respect to the single molecule, suggests the attribution of this line to the formation of J-aggregates. The formation of a delocalized exciton leads to a PL line narrowing because of a leveling of the energy fluctuations on the sites over which the excitation is shared [10]. From statistical considerations it is possible to demonstrate that the linewidth of the emission peak (σ) is inversely proportional to the square root of the number of molecules over which the exciton is delocalized (N). N is not a directly accessible quantity because it does not coincide with the physical dimension of the aggregates but it can be limited both by phonons and by structural defects. At high temperatures, the main role in limiting the maximum number of molecules emitting in phase is played by phonons. In the most simplified approach, N is given by a compromise between the molecule interactions, which allow for exciton delocalization, and the thermal fluctuations which tend to break cooperation (N = Ebond /kT, where Ebond is the stabilization energy of the aggregate) [11]. As a consequence, a linear relationship should hold also between σ 2 and T in the temperature range where coherence is limited by phonons. In this way, the dependence over N is discharged in the dependence of the quantity of interest over the temperature, which can be easily measured. In Fig. 2 are also reported the PL spectra for a TSPP solution (10 ␮M), recorded at various temperatures between 300 and 80 K. By deconvolving the emission bands with Gaussian curves, it is possible to precisely determine the values of position, intensity and width of the peaks. Linewidth data fit well the predicted linear relationship between σ 2 and T from 240 down to about 120 K (see inset of Fig. 2), quantitatively confirming exciton delocalization. At lower temperatures, the static disorder due to structural defects and impurities is the main factor disrupting exciton coherence and σ saturates toward a constant value related only to the sample quality. In fact, at low temperature, a small but significant deviation from the linear trend can be seen.

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Fig. 3. PL spectra of the J-aggregate sample in the temperature range 160–80 K. In the inset PL spectra of the J-aggregate sample at 80 K. Slow cooled sample (solid line), sample cooled with the cryostat (dotted line) and fast cooled sample (dashed line). [H4 TPPS2− ] = 10 ␮M, pH 2.5 and [KCl] = 0.2 M.

The number of molecules emitting in phase can be tentatively evaluated by using the relationship between N and Ebond reported above. The decrease in the emission energy due to aggregation from a monomeric molecule in solution is for the porphyrin ca. 800 meV, and gives an approximate estimation of the Ebond energy. Using this value, N increases from 4 at 240 K to 8 at 120 K, where σ, and as a consequence also the maximum number of coupled molecules, saturates. Moreover, lowering the sample below 160 K results in the emersion of a new peak in the photoluminescence spectra, whose intensity grows by further lowering the temperature, as depicted in Fig. 3. This second emission band is probably due a different species; in fact, it is possible to increase/decrease the relative intensity ratio of the two peaks by varying the speed of cooling. The inset of Fig. 3 shows that the low-energy shoulder is more prominent in slow cooling samples, thus suggesting that slow cooling allows the second species to be formed with higher probability. Fast cooling, on the contrary, blocks the system in a metastable situation, due to the fact that the molecules do not have the time to organize themselves before the solvent freezes. The low-energy peak remains quite broad by lowering the temperature while the width of the high-energy one decreases, as reported above. That seems to be a fingerprint of emission from localized states lying below the bottom of the excitonic band, which act as radiative traps for the delocalized excitons. The temperature dependence of this emission can be explained considering the occurrence of thermally activated detrapping processes [12]. In a two level system in which the lower lying state can be thermally depopulated, the temperature dependence PL(T) of the intensity of the emission is described by the law: I 1 = I0 1 + C e−∆E/kT

(2)

where I0 is the emission intensity at T = 0 K, E the activation energy of the thermally activated process, k the Boltzmann constant and C is a constant. The experimental data are found to fit well with Eq. (2), when the last intensity value is used

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Fig. 4. PL spectra of TSPP-agg/MCM-41 in the temperature range 300–80 K. In the inset PL spectra of monomeric TSPP, 10 ␮M, pH 2.5 (dash-dot-dot line); aggregated TSPP, 10 ␮M, pH 2.5 and [KCl] = 0.2 M (dash-dot line); TSPPmon/MCM-41 (dashed line); TSPP-agg/MCM-41 (solid line).

for I0 . The value obtained for the activation energy is in good agreement with the separation energy between the two emitting states (about 63 meV). This supports a two level system scheme formed by the localized state and the bottom of the exciton band. PL spectra for solution phase and composite systems were also acquired. In the inset of Fig. 4, the PL spectra recorded at RT of the inclusion compound are reported with those of the reference solution samples (monomer and J-aggregate). It appears clearly that the encapsulated free-base monomer exhibits a red shift for its Q-region band when compared to the Q band of the solution monomeric species. A red shift for the emission is also found for the encapsulated aggregate when compared to the solution aggregate. Such shifts can be explained in terms of intra-molecular charge transfer caused by host–guest interaction, which is connected to steric effects associated with the pore structure within modified MCM-41. Additionally, the broad structure of the Q band for TSPPmon/MCM-41 can be interpreted as site-specific emissions for encapsulated TSPP, suggesting that the monomeric species is distributed at various positions within the cage and experiences, as a result, a range of perturbations. By the same argument, the similarity between the spectrum of TSPP-agg/MCM-41 and solution phase aggregate can be interpreted as site-specific perturbations. Fig. 4 shows the behavior of the emission band of aggregated TSPP included within modified MCM-41 at different temper-

atures. The low-energy shoulder considerably gains intensity by decreasing the temperature up to completely hide the highenergy peak. Thus, the silica matrix does not make the formation of linear aggregate easier but it favors the species responsible of the intra-gap localized emission. It is in fact anticipated that alternative structures, with respect to the linear ones, would exist within the modified MCM-41 cavity, with electrostatic interaction between the sulfonato groups and the ammonium ions of the silylation reagent contributing to the stability of the structure. Moreover, such structures would be impacted by the electrostatic interaction between negative sulfonatophenyl groups and positive macrocycle groups within the interacting molecules. These considerations let us conclude that the functionalization with APTES, and therefore the affinity with the interior structure of the mesoporous silica, seems to play a key role in the spontaneous aggregation of the included molecules. 4. Conclusions In summary, evidence of a collective behavior is obtained for porphyrinic molecular aggregates suggesting the possibility to observe a superradiant behavior. Furthermore, it has been possible to modify the spectroscopic and structural properties of the aggregated species by changing the preparation conditions, in particular a new nanocomposite material was prepared, whose properties derive both from the energy levels of the included molecules and from the confinement imposed by the host material. More detailed investigation, using such techniques as picosecond photoluminescence, into the properties of the species responsible of the intra-gap emission should provide insight into the intermolecular energy transfer processes among the various species. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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