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Photophysical Properties of an Amphiphilic Cationic Hemicyanine Dye in Solution and Adsorbed on a TiO2 Mesoporous Film Elias Stathatos and Panagiotis Lianos* Engineering Science Department, University of Patras, GR-26500 Patras, Greece
Andre Laschewsky Departement de Chimie, Universite Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium Received May 6, 1996. In Final Form: October 11, 1996X The photophysical behavior of the amphiphilic cationic hemicyanine dye N-n-heptyl-4-(((dimethylamino)phenyl)ethenyl)pyridinium bromide (H7HC) has been examined in various environments with the intention to assess its state of adsorption on a TiO2 mesoporous film. In some solvents, such as short-chain alcohols or chloroform, the dye is dissolved as a monomer. In other solvents, like water and cyclohexane, it is found in concentration-dependent aggregate forms. H7HC monomer absorption and fluorescence spectra show a symmetric solvatochromic effect previously found for other amphiphilic hemicyanines (Fromherz, P. J. Phys. Chem., 1995, 99, 7188). Aggregates are of the H-type; i.e., they are formed by repulsive interaction and display a hypsochromic shift in their absorption spectra. When adsorbed on nanostructured mesoporous TiO2 films, H7HC is found in aggregate form, which evolves in the course of time by displaying a hypsochromic shift in the absorption maximum. Photoelectrochemical studies with an ITO-TiO2-H7HC electrode revealed that aggregates are responsible for the ability of the dye to photosensitize TiO2.
1. Introduction The photophysical properties of amphiphilic hemicyanine dyes are studied with much interest, since these substances are appropriate for several applications. Their strong solvatochromic effect1 has been exploited in fluorescence probing studies2 to monitor changes in membrane potential.3 Their tendency to aggregate4-6 has been analyzed in Langmuir-Blodgett films and monolayers at the air-water interface7,8 in the goal of producing materials with nonlinear optical properties.9-11 Dye aggregates are of particular importance in photography and xerographic reproduction as spectral sensitizers.12,13 In addition to such applications, surfactant hemicyanines, which possess both dyeing and amphiphilic properties,14 are convenient substances for studying fundamental photophysical phenomena such as J- and H-aggregate * To whom correspondence should be addressed: email, lianos@ upatras.gr. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Ephardt, H.; Fromherz, P. J. Phys. Chem. 1989, 93, 7717; 1993, 97, 4540. (2) (a) Loew, L. M.; Simpson, L.; Hassner, A.; Alexanian, V. J. Am. Chem. Soc. 1979, 101, 5439. (b) Grinvald, A.; Hildesheim, R.; Farber, I. C.; Auglister, L. Biophys. J. 1982, 39, 301. (c) Grinvald, A.; Salzberg, B. M.; Lev-Ram, V.; Hildesheim, R. Biophys. J. 1987, 51, 643. (3) Yuet, P. K.; Blankschtein, D. Langmuir 1995, 11, 1925. (4) Song, Q.; Evans, C. E.; Bohn, P. W. J. Phys. Chem. 1993, 97, 13736, and 12302. (5) Evans, C. E.; Bohn, P. W. J. Am. Chem. Soc. 1993, 115, 3306. (6) Wolthaus, L.; Gnade, M.; Mo¨bius, D. Adv. Mater. 1995, 7, 453. (7) Hall, R. A.; Thistlethwaite, P. J.; Grieser, F.; Kimizuka, N.; Kunitake, T. J. Phys. Chem. 1993, 97, 11974. (8) Vaidynathan, S.; Patterson, L. K.; Mo¨bius, D.; Gruniger, H. R. J. Phys. Chem. 1985, 89, 491. (9) Lupo, D.; Prass, W.; Scheunemann, U.; Laschewsky, A.; Ringsdorf, H.; Ledoux, I J. Opt. Soc. Am. 1988, 5, 300. (10) Ashwell, G. J.; Hargreaves, R. C.; Baldwin, C. E.; Bahra, G. S.; Brown, C. R. Nature 1992, 357, 393. (11) Laidlaw, W. M.; Denning, R. G.; Verbiest, T.; Chauchard, E.; Persoons, A. Nature 1993, 363, 59. (12) Law, K. -Y. Chem. Rev. 1993, 93, 449. (13) Kim, Y. -S.; Liang, K.; Law, K. -Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 984. (14) Lunkenheimer, K.; Laschewsky, A. Prog. Colloid Polym. Sci. 1992, 89, 239.
formation,15,16 energy transfer in organized molecular assemblies,17,18 etc. In the present work we have studied some photophysical properties of N-n-heptyl-4-(((dimethylamino)phenyl)ethenyl)pyridinium bromide14, abbreviated as H7HC (see top of Figure 1), in several different solvents with the intention to understand its properties when adsorbed on nanostructured oxide semiconductor electrodes. For this reason, some photoconductivity measurements have also been performed. We have thus structured this article in the following parts: we first demonstrate the solvatochromic effect and the tendency of the dye to form clusters in pure solvents by steady-state absorption and fluorescence spectroscopy; then we study systems obtained by adsorption of the dye on colloidal TiO2 particles or on a nanostructured titania film which is also employed as a working electrode in photoconductivity studies. 2. Materials and Methods Organic solvents, Triton X-100, and titanium isopropoxide (Aldrich) were of the best quality available and used as received. N-n-Heptyl-4-(((dimethylamino)phenyl)ethenyl)pyridinium bromide (H7HC) was synthesized by condensation of the corresponding N-alkylated 4-methylpyridines with 4-(dimethylamino)benzaldehyde and it was purified by repeated recrystallization from ethanol.14 Millipore water was used in all experiments. ITO-glasses were purchased from FLABEG GmbH, Siemensstrasse 1-3, D-90766 Fuerth, Germany. Aqueous colloidal TiO2 particles were made by mixing titanium isopropoxide with water followed by addition of 37% HCl at the volume ratio water/alkoxide/acid 6.2:1:0.04.19 TiO2 mesoporous films were made by dipping a plain glass or an ITO (glass with deposited indium-tin-oxide film) slide into a cyclohexane solution containing Triton X-100 reverse micelles (15) Hada, H.; Yonezawa, Y.; Inaba, H. Ber. Bunsenges. Phys. Chem. 1981, 85, 425. (16) Ma, S.; Lu, X.; Song, J.; Liu, L.; Peng, W.; Zhou, J.; Yu, Z.; Wang, W.; Zhang, Z. Langmuir 1995, 11, 2751. (17) Sato, T.; Kurahashi, M.; Yonezawa, Y. Langmuir 1993, 9, 3395. (18) Papoutsi, D.; Bekiari, V.; Stathatos, E.; Lianos, P.; Laschewsky, A. Langmuir, 1995, 11, 4355. (19) O’Regan, B.; Moser, J.; Anderson, M.; Gratzel, M. J. Phys. Chem. 1990. 94, 8720.
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Table 1. Absorbance and Fluorescence Maxima of H7HC in Solvents of Different Polarity
solvent water methanol ethanol butanol pentanol decanol cyclohexane formamide acetonitrile acetone ethyl acetate chloroform b
absorbance fluorescence relative dielectric maximum maximum fluorescence (nm) (nm) intensity constantb 78.4 32.6 24.3 15.8 13.9 8.1 2 109.5 37.5 20.7 6 4.8
∼450 478 483 491 491 480 455 479 472 475 460 495
610 610 600 600 600 590 560 615 610 610 580 583
22 70 306 584 613 1000 140 216 40 97 853 515
a All data were recorded at ambient temperature (∼20 °C). Values tabulated as provided by solvent manufacturer.
and titanium isopropoxide.18,20 The films were slowly heated up to 450 °C so that all organic material was burnt out. The remaining TiO2 film consists of uniform spherical particles of a size of a few tens of nanometers.21 This mesoporous structure possesses an important capacity to adsorb several organic and inorganic species.21,22 When the titania film was dipped into an aqueous solution of H7HC, it was readily covered by a thin orange film of the dye. Absorption spectra were recorded with a Perkin-Elmer Lambda-15 spectrophotometer. Fluorescence measurements were made with a home-assembled spectrofluorometer using ORIEL parts. Photoconductivity measurements were made with an AMEL 2049 potentiostat. The working electrode served as cell window. A calomel electrode was used as reference and a platinum wire as counter electrode.
Figure 1. Normalized absorption spectra of 10-5 M H7HC in various solvents: (1) water; (2) methanol; (3) chloroform.
3. Results and Discussion 3.1. Absorption and Steady-State Fluorescence Spectra in Various Solvents. H7HC was dissolved in various solvents at concentration 10-5 M (unless otherwise indicated), and its absorption and fluorescence spectra were recorded. All spectra were structureless. The positions of the maxima are listed in Table 1 and Figures 1 and 2. The fluorescence maxima are red-shifted in polar solvents. This tendency is in perfect agreement with the solvent dielectric constant. Thus the lowest-wavelength maximum is at 560 nm found in cyclohexane. The longestwavelength maximum is at 615 nm found in formamide. A red shift in polar solvents is a common phenomenon observed with dyes showing a solvatochromic effect, and it is due to the lowering of the energy of the emitting state due to interaction with polar molecules. The absorption behavior, however, was rather complicated. Hemicyanines are expected to give a blue-shifted absorption maximum in polar solvents.1,23 Indeed, alcoholic solvents, from pentanol to methanol, do agree with this model. Water also fits in qualitatively. However, the absorbance maximum at 450 nm is too low to come from a solvatochromic effect only. As seen in Figure 1, the dispersion of the absorprtion peak in water is very large, much larger than in any other solvent, and the position of the maximum is therefore hard to define with accuracy. Solubility in water is limited; therefore, strong blue shift in absorption could be due to aggregation. Such a possibility is supported by the dispersion of the absorption spectrum. The unexpected position of absorbance maximum in (20) Bekiari, V.; Lianos, P. J. Colloid Interface Sci., in press. (21) Stathatos, E.; Lianos, P.; Del Monte, F.; Levy, D.; Tsiouvras, D. J Sol-Gel Sci. Technol., in press. (22) Work in preparation. (23) Fromherz, P. J. Phys. Chem. 1995, 99, 7188.
Figure 2. Normalized fluorescence spectra of 10-5 M H7HC in various solvents: (1) cyclohexane; and (2) formamide.
cyclohexane (cf. Table 1) can be also understood in terms of aggregation as H7HC is even less soluble in cyclohexane than it is in water. In this respect, the anomalous position of the absorption maximum in decanol could be also due to aggregation (cf. Table 1). A similar conclusion has been previously derived by Fromherz23 in order to explain deviations observed with decanol and octanol. Hemicyanines are notorious for their tendency to aggregate. We have therefore proceeded by studying H7HC absorption spectra in water and cyclohexane as a function of the dye concentration. In water, the solubility range at room temperature extends up to 5 × 10-3 M. From zero to 2 × 10-3 M, the absorbance spectrum is not affected by dye concentration. Above 2 × 10-3 M aggregation is indicated by a new absorption peak at shorter wavelength (see Figure 3), i.e., at about 425 nm. When analogues of H7HC with shorter aliphatic chains are used,22 i.e. when the dye becomes more hydrophilic, solubility in water increases and the new peak appears at higher concentrations. Apparently, there exists a critical aggregation concentra-
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Figure 3. Normalized absorption spectra of H7HC in water at different concentrations: (1) 5 × 10-3M; (2) 10-3 M.
Figure 4. Normalized absorption spectra of H7HC in cyclohexane at different concentrations: (1) 5 × 10-5 M; (2) 10-5 M.
tion (2 mM in the case of H7HC) above which the dye forms structured aggregates, possibly micellar structures. However, as already said, aggregates may also exist below critical concentration, possibly, involving fewer molecular species. This is again in agreement with a model previously proposed by Fromherz23 where amphiphilic hemicyanine dyes might form quasi-micellar structures providing an environment similar to a lipid bilayer.23,24 The present data are not conclusive about the exact state of aggregation of H7HC in water, and more studies are needed. Hall et al.7 have studied a longer-chain analogue of the same hemicyanine dye that forms monolayers at the air-water interface. By increasing surface pressure, they detected an absorbance band at around 425 nm and assigned it to H-aggregates. Song et al.4 studied other analogues in Langmuir-Blodgett (LB) films and detected aggregates absorbing at much smaller wavelength, i.e., 340 nm, and they also assigned them to H-aggregates. Cross et al.25 managed to shift aggregate absorption by introducing a molecular spacer in hemicyanine monolayers. It is obvious that the absorbance maximum of the aggregates depends on the conditions of the aggregate formation. H-aggregates correspond to repulsive interaction between aggregating dipoles.5 Thus the degree of orientation and the extent of aggregation will define the final absorbance maximum. The shortest wavelength absorbance goes with large and highly oriented structures as LB films (cf. ref 4). Micelles or aggregates of a few molecules should give absorbance at longer wavelengths since the degree of orientation is then lower. In conclusion, we note that the particular behavior of H7HC in aqueous solutions is dictated by its tendency to form clusters. H7HC dissolves in cyclohexane up to 5 × 10-5 M. For concentrations below 10-5 M, the absorbance spectrum shows a single peak with a maximum at 455 nm (see Figure 4). In the concentration range between 10-5 and 5 × 10-5 M, we observe a second absorption peak with a maximum at 390 nm. This spectral behavior clearly demonstrates formation of aggregates with varying degree of structure and orientation as in the case of water. However, contrary to aqueous solutions, the aggregated species in cyclohexane
possibly contain a well-defined number of monomers. For this reason, absorption peaks are finely dispersed (Figure 4). The spectral behavior of H7HC in the other solvents reported in Table 1 can be rationalized by the existence or absence of aggregates. As judged by the absence of aggregates, short and medium chain alcohols as well as chloroform are apparently the best solvents for this hemicyanine dye. The finding that no irregularities are observed in fluorescence maxima, whatever the solvent, is due to the fact that hemicyanine aggregates do not fluoresce.5 It is only the monomer that fluoresces even when only the aggregate is excited, apparently by energy transfer to the monomer. However, energy transfer depends on the quality and degree of aggregation and on the proximity of the excited aggregate to a monomer. For this reason, fluorescence intensity has been found relatively low even in cyclohexane and extremely low in water. The relative fluorescence intensities shown in Table 1 demonstrate these trends. Selective measurements of fluorescence quantum yields are also pointing to the same direction. In conclusion, H7HC shows a symmetric solvatochromic behavior as known for other hemicyanine dyes.23 However, there are many solvents where deviations are observed from symmetric behavior demonstrating themselves as extensive blue shifts in the absorption maxima. Characteristic examples are the absorption spectra in aqueous and in cyclohexane solutions. The concentration dependence of the absorption maximum clearly indicates the tendency of this dye to form clusters in these, as well as in many other, solvents. 3.2. H7HC on TiO2 Colloidal Particles and Nanostructured Mesoporous Films. H7HC has been dissolved in ethanol at concentration 2 × 10-5 M. To this solution we have added up to 5% by volume of the TiO2 colloidal solution described in Section 2 and observed the variation in the absorption and fluorescence spectrum. Figure 5 shows that the presence of TiO2 in the solution decreases H7HC absorbance, indicating that the dye is adsorbed on the colloidal titanium dioxide particles. Even though a large percentage of the dye has been adsorbed, as seen from the extensive decrease of the absorbance, there is no indication of aggregate formation, since no new peaks show up in Figure 5. The fluorescence intensity
(24) Fromherz, P.; Schenk, O. Biochim. Biophys. Acta 1994, 1191, 299. (25) Cross, G. H.; Cade, N. A.; Girling, I. R.; Peterson, I. R.; Andrews, D. C. J. Chem. Phys. 1986, 86, 1061.
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Figure 5. Absorption spectra of 2 × 10-5 M H7HC in ethanol in the presence of TiO2 colloidal particles. The absorption at short wavelengths is characteristic of TiO2.
Figure 6. Absorption spectra of freshly prepared titaniaH7HC sample (1) and after 3 days (2).
also decreased in the presence of TiO2. The percentage of decrease of fluorescence was equal to that of the decrease of absorbance at the absorption maximum. It is then obvious that the dye is adsorbed on TiO2 colloidal particles in monomeric form and that in this form its fluorescence is not quenched by titanium dioxide. H7HC has also been adsorbed on films made of TiO2 nanoparticles, as described in section 2, and formed colored, transparent films. Thus its absorption spectra were recorded in the transmission mode. The first important finding was that these films did not fluoresce. The absorption maximum appeared at about the same position as in aqueous and cyclohexane solutions (cf. Figures 1, 3, 4, and 6). The spectrum of Figure 6 was recorded with a fresh sample. If the same sample is examined 3 days later, absorption at shorter wavelength is found increased at the expense of the original peak seen in Figure 6. In view of the above findings, we conclude that H7HC is in the form of aggregates when adsorbed on the TiO2 film. However, the aggregation number as well
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Figure 7. I-V curve for the ITO-TiO2-H7HC electrode (1) in dark and (2) with illumination at 450 nm.
Figure 8. Action spectrum at zero bias for the freshly-prepared ITO-TiO2-H7HC electrode. Recording is, of course, done only above 350 nm since the glass window is not transparent at shorter wavelengths.
as the degree of orientation of the monomers slowly increases and the absorption is progressively shifted to shorter wavelengths. The aggregation of H7HC justifies low levels of fluorescence emission, but not the complete loss. This finding suggests that a quenching mechanism by electron transfer to titania particles takes place in these films. Electron transfer from the dye to the semiconductor would then be more efficient than energy transfer from the aggregate to the monomer. Electron transfer should occur only from aggregates. For this reason, pure monomers in alcoholic solutions are not quenched by TiO2 colloidal particles. Absorption spectra of H7HC adsorbed on nanostructured TiO2 films were also studied in thin cells were the film was in contact with a 10-3 M water solution of the dye. This system was examined since the photoactive film would be used in a similar configuration in photoconductivity measurements. The cell was constructed by
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gluing the slide holding the film with another glass slide such as to put the film on the inside of the cell. Even though, the cell thickness is a fraction of a millimeter, the absorption spectrum was dominated by the absorption of the solution (cf. Figure 3), since it was highly concentrated. A progressive evolution towards shorter-wavelength absorption was again observed, and it was in this case faster. In conclusion, when H7HC is adsorbed on a nanostructured TiO2 film, it forms aggregates which progressively evolve to bigger and better-oriented entities. Electron transfer occurs from the aggregates to the semiconductor that results in complete quenching of the fluorescence of the dye. 3.3. Photoconductivity measurements. A TiO2 mesoporous film was deposited on a transparent-conductive ITO glass slide by the method presented in section 2. When dipped into a 10-3 M aqueous H7HC solution for a few seconds, it was covered by a colored transparent film. It was then used as working electrode and as a window in the photoconductivity cell. The electrolyte solution consisted of 5 × 10-4 M H7HC plus 0.1 M KCl.15,26 The illuminated area was approximately 1 cm2. Figure 7 shows the I-V curves recorded in the dark and by illumination at 450 nm. All V values are measured with respect to the calomel reference electrode. We note an important displacement of the photoresponce toward positive values indicating the electron-donating capacity (26) Haraguchi, A.; Yonezawa, Y.; Hanawa, R. Photochem. Photobiol. 1990, 52, 307.
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of H7HC. Action spectra were recorded at both anodic and cathodic bias. No important differences in the shape of the curve have been detected. The peak value, of course, depended on applied voltage according to the data of Figure 7. Figure 8 shows an action spectrum recorded at zero bias. We note that the maximum is around 450 nm, which is a clear indication that electron transfer exclusively occurs from the aggregates. Photoconductivity measurements are further pursued in our laboratory. 4. Conclusions The aggregation behavior of the amphiphilic cationic hemicyanine H7HC in different solvents and TiO2 nanostructured films has been studied. H7HC was found to form concentration-dependent H-aggregates in poor solvents. Fresh samples consist of aggregates of low aggregation number which evolve to larger and better oriented ones demonstrating a hypsochromic effect. TiO2 nanostructured mesoporous films with adsorbed H7HC are photoconductive. Photoconductivity is caused by electron transfer from the aggregate to the semiconductor. Acknowledgment. We are grateful to Dr. M. Hof for very interesting discussions. We are also grateful to the University of Patras technicians D. Kosmatos and G. Hadjipanayiotou for their technical help. We gratefully acknowledge financial aid from HCM Project CHRXCT-0273. LA960446M
