Heteroaggregation and photoelectric conversion of porphyrins on a nanostructured TiO2 electrode

PII: S0968-5677(98)00099-6

Supramolecular Science 5 (1998) 669—674  1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/98/$19.00

Heteroaggregation and photoelectric conversion of porphyrins on a nanostructured TiO2 electrode Huihua Deng* and Zuhong Lu National Laboratory of Molecular and Biomolecular Electronics, Southeast University, Nanjing 210096, People’s Republic of China

The cosensitization of a nanostructured titanium dioxide (TiO ) electrode with tetrasulphonated porphyrins  (MTsPP; M"Zn, H ) is reported. Cosensitization greatly enhances the photocurrent response at the  Q-band of H TsPP and markedly decreases the photocurrent response in the Soret band of ZnTsPP. The  photoelectric behavior of the cosensitized TiO electrode is attributed to the formation of heteroaggregates  between H TsPP and ZnTsPP molecules on the positively charged TiO electrode by charge-transfer   interaction, resulting in the decrease of the surface concentration of the H TsPP dimer and the presence of  the low-lying charge-transfer state.  1998 Elsevier Science Limited. All rights reserved. (Keywords: charge-transfer interaction; cosensitization; heteroaggregation)

INTRODUCTION The nanocrystalline solar cell, recently developed by Gra¨tzel and coworkers, overcomes the low conversion efficiency of conventional photoelectrochemical cells or solid cells due to inefficient separation and transport of the photogenerated charge carriers\ or due to low absorbance. The nanocrystalline solar cell is based on a dye-sensitized colloidal TiO film with a very high  specific surface area. As a consequence, a large number of dye molecules can be adsorbed directly on to the electrode surface and simultaneously be in direct contact with the redox electrolyte, which results in efficient separation and transport of the photogenerated charge carriers and leads to efficient and rapid regeneration of the adsorbed dye molecules. If a sufficiently large surface area of the electrode can be provided, even a monolayer of dye molecules adsorbed on the electrode will absorb most of the incident light photons. In addition, the photogenerated excitons (or charge carriers), originating from the adsorbed monolayer of dye molecules on the microporous TiO electrode, directly transport into the  conduction band of the semiconductor electrode. This injection mechanism avoids the recombination loss of the photogenerated excitons while, diffusing toward the junction region, which is encountered in the multilayer cell in which the photogenerated charge carriers result from the exciton dissociation by a built-in field at the junction region. The liquid junction solar cells based on transparent and microporous TiO electrodes sensitized with  * Corresponding author. Centre for Advanced studies Science and Technology of Microstructure, Nanjing University, Nanjing 210093, People’s Republic of China

phthalocyanines and porphyrins had been fabricated in our laboratory\. Phthalocyanine and porphyrin compounds, whose chemical structures resemble chlorophyll derivatives, are chosen as photosensitizers because phthalocyanine shows strong absorption at the longer wavelength, and porphyrin shows very strong absorption and high quantum efficiency in photoelectric conversion in the region of 400—470 nm, which compensates for the lower absorbance and conversion efficiency of ruthenium bipyridine complex at the given region. In this paper, we first report the cosensitization of the nanostructured TiO electrode with tetrasulphonated  porphyrins (MTsPP; M"Zn, H ). The mechanism to  improve the photoelectric conversion by the cosensitization is also discussed.

EXPERIMENT DETAILS Preparation of the nanostructured ¹iO2 electrode TiO colloidal solutions were prepared by hydrolysis  of tetrabutyl titanate ((C H O) Ti) by a procedure that    described in ref. 7. The colloidal solution on addition of 2 wt% poly(vinyl alcohol) (PVA) was then concentrated to a desired density through vacuum rotation evaporation. After addition of 1.5 wt% Triton X-100, the concentrated solution (TiO content; 10 wt%) was  spin-coated on a freshly cleaned ITO (indium tin oxide; sheet resistance of 50 )/)) and transmission of 95% in the visible region) conducting glass substrate. The sheet resistance of the nanostructured TiO electrode, finally  obtained by a heating process that described in ref. 8, is 500 )/). Prior to dye sensitization, the nanostructured

SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998 669

Heteroaggregation and photoelectric conversion of porphyrins: H. Deng and Z. Lu TiO electrodes were soaked in HCl solution (pH"2)  and naturally dried. The acidified TiO electrode is posit ively charged on the surface. Sensitization and cosensitization MTsPPs were sensitized according to the methods described in ref. 14; and their molecular structures are shown in Figure 1a. When sensitized with H TsPP or  ZnTsPP dye molecules alone, the working electrode was obtained by plunging the bare TiO electrode into  a 5;10\ M solution of H TsPP or ZnTsPP in  dimethyl sulfoxide (DMSO) until the absorbance of the electrode showed no increase. When cosensitization with H TsPP and ZnTsPP, the working electrodes were ob tained by plunging the bare TiO electrode into a mixed  solution of H TsPP and ZnTsPP in DMSO until the  absorbance of the electrode showed no increase. The total concentration of the mixed solution was

5;10\ M. After the characteristic performances of the cosensitized TiO electrode had been measured, H TsPP   and ZnTsPP on the TiO electrode were washed away  using 0.001 M NaOH solution. The resulting solution was used to analyze the actual content of H TsPP and  ZnTsPP molecules adsorbed on the TiO electrode.  Fabrication of the liquid junction cell The liquid junction solar cell for measuring the photocurrent, as shown in Figure 1b, consists of the sensitized or cosensitized TiO electrode and a counter electrode  with an electrolyte solution in between containing 0.1 M KI and 0.05 M I in 0.001 M HClO . When light is   incident through the working electrode, the counter electrode is an ITO conducting glass, on which a thin layer of platinum had been coated by vapor vacuum deposition at 10\ Torr. When light is incident through the counter electrode (the back electrode), an ITO conducting glass is also used as the back electrode. Measurements The morphology of the TiO electrode was examined  by atomic force microscopy (AFM; DI Co., Nanoscope III). The film thickness of the TiO electrode was exam ined by scanning electron microscopy (SEM; JEOL, JSM-6300). The absorption spectra were recorded with a Shimadzu UV-2201 UV—visible spectrometer. The photocurrent was measured with a potentiostat (model CMBP-1). Monochromatic illumination was obtained using a 500 W xenon arc lamp in combination with a grating monochromator (model WPG3D). The light intensity was calibrated using a model LM-5 laser power meter made by the National Institute of Metrology, China.

RESULTS AND DISCUSSION Molecular heteroaggregation between H ¹sPP and  Zn¹sPP molecules on the cosensitized ¹iO electrode 

Figure 1 (a) The molecular structures of tetrasulphonated porphyrin (MTsPP; M"Zn, H ) and (b) the structure of the liquid junction solar  cell based on the cosensitized TiO electrode 

The nanostructured TiO electrode is composed of  interconnected particles (40—60 nm) and pores and has a thickness of 10 lm, as shown in Figure 2. The bare nanostructured TiO electrode shows the fundamental  absorption edge at 390 nm of anatase in the UV region, as shown in Figure 3. The specific surface area of the nanostructured TiO electrode is characterized by the  dimeric absorbance of GaTsPc molecules in the case of a single sensitization. The specific surface area of the TiO electrode of 370 was obtained by the mrthod de scribed in ref. 2. Cosensitization extends the absorbance of the electrode into the broader region (400—750 nm) in Figure 4 and the photocurrent response in the visible region stems from the absorbance of the dye molecules adsorbed on the TiO electrode.  In DMSO, the H TsPP absorption spectrum at a con centration of 5;10\ M consist of the Soret band with

670 SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998

Heteroaggregation and photoelectric conversion of porphyrins: H. Deng and Z. Lu

Figure 2 Surface morphology of a nanostructured TiO electrode by  AFM

Figure 3 Absorption spectra of the bare TiO electrode and H TsPP   and ZnTsPP dye molecules in DMSO

Figure 4 Absorption spectra of the TiO electrode sensitized with  with H TsPP (0.40) and H TsPP and ZnTsPP alone and cosensitized   ZnTsPP (0.60) dye molecules

a strong peak at 415 nm and the Q-band with four peaks at 512, 550, 593, 645 nm (see Figure 3). As shown in Figure 4, when adsorbed on the TiO electrode, the Soret  band of H TsPP has a red shift and is split as two peaks 

at 420 and 475 nm, and the strong Q-band absorption of H TsPP at 685 nm is observed and is red-shifted by  40 nm relative to that in DMSO. The red shifts of absorption spectra indicate the occurrence of a strong interaction between H TsPP molecules and the TiO electrode.   The band spilt of the Soret band indicates the formation of molecular aggregation of H TsPP on a nanostruc tured TiO electrode. Assuming that there is no higher  order molecular aggregates than dimers, H TsPP mainly  exists as a dimer on the TiO electrode. In contrast, for  H TsPP molecules on the cosensitized TiO electrode,   a peak at 480 nm with decreased intensity and a shoulder at 420 nm are observed, and the Q-band is red-shifted to 695 nm relative to that for sensitization with H TsPP  alone. Thus, cosensitization markedly decreases dimerization or aggregation of H TsPP molecules.  In DMSO, the absorption spectrum of ZnTsPP consists of the Soret band with a very strong peak at 423 nm and the Q-band with three peaks at 553, 595 and 642 nm (see Figure 3). Compared with the absorption of the porphyrin solution in DMSO, that of ZnTsPP adsorbed on the TiO electrode has red shifts at the Soret band  with a peak at 430 nm and Q-band with three peaks at 563, 610, 649 nm, as shown in Figure 4. This indicates that there is a strong interaction between ZnTsPP molecules and the TiO electrode. However, the band split is  not observed for the Soret band of ZnTsPP. Therefore, it is believed that ZnTsPP mostly exists as a monomer on the TiO electrode. In contrast, for ZnTsPP molecules  on the cosensitized TiO electrode, a peak at 430 nm for  the Soret band is observed and the Q-band is not observable. The spectral characteristics of the cosensitized TiO  electrode indicate that there is a strong interaction between H TsPP and ZnTsPP of the self-assembled film on  the cosensitized TiO electrode and an extensive orbital  mixing occurs. Therefore, the possible formation of the heteroaggregates between H TsPP and ZnTsPP molecu les is suggested. In the previous study, stable heteroaggregates can be obtained with porphyrins grafted with ionic substituents of opposite charges\ and also with anionic porphyrins with different central metals. In the former, the main driving force of heteroaggregation is the electrostatic attraction of peripheral substituent, whereas, in the latter, charge-transfer interaction of the porphyrin moieties is considered as the main driving force of hetero-aggregates composed of anionic zinc and gold porphyrins. The van der Waals attraction between the hydrophobic aromatic macrocycles also occurs in holding the individual components together. In addition, the planar geometry of porphyrins and phthalocyanines allows close contact (and therefore extensive orbital overlap) in the face-to-face configuration. In the case of the cosensitization, H TsPP and ZnTsPP molecules ad sorb on the nanostructured TiO electrode through  coulombic attraction via the negatively charged sulfonate (SO\) group. The positively charged surface of the  nanostructured TiO electrode partly shields the elec trostatic repulsion between H TsPP and ZnTsPP mol ecules. The reduction potentials of H TsPP and ZnTsPP 

SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998 671

Heteroaggregation and photoelectric conversion of porphyrins: H. Deng and Z. Lu are !1.06 and !1.16 V (vs. NHE), and the oxidation potentials of H TsPP and ZnTsPP are #1.10 and  #0.87 V (vs. NHE), respectively. This shows the presence of the charge-transfer interaction between H TsPP  and ZnTsPP molecules. Therefore, the charge-transfer interaction, together with the shielding interaction and the van der Waals attraction, leads to the formation of the PP/PP (PP refers to H TsPP; PP refers to ZnTsPP)  heteroaggregates on the positively charged TiO elec trode. The interaction between two porphyrin rings leads to a face-to-face molecular aggregation with their centers offset both in solutions and in crystals. On this basis, it is concluded that these red shifts of absorption are due to the adoption of a face-to-face stacking orientation and charge-transfer interaction in heteroaggregates of H TsPP and ZnTsPP molecules coadsorbed on the  nanostructured TiO electrode. As a result, an extended  and conjugated face-to-face system is formed with a norbital overlap. Such an extended and conjugated n system within the molecular heteroaggregates lowers the excitation energy of the n—n* and n—n* electron transition, which results in a red shift of the Soret band and Q-band absorption, as shown in Figure 4, respectively. The maximum of the short-circuit photocurrent occurs at x" 0.40 where the molar ratio of H TsPP to ZnTsPP  amounts to 1 : 1.5 and is close to 1 : 2. Thus, it is assumed that the PP—PP—PP heterotrimer is created at x"0.40. Photoelectric conversion of a nanostructured ¹iO  electrode cosensitized with H ¹sPP and Zn¹sPP dyes  Figure 5 shows the measured short-circuit photocurrent of the nanostructured TiO electrode sensitized with  a single H TsPP or ZnTsPP dye and cosensitized with  H TsPP (0.40) and ZnTsPP (0.60) dye molecules as a  function of wavelength for light incident through (a) the working electrode and (b) the back electrode. The photocurrent action spectra have been corrected for the absorption and scattering (ca. 10—15%) of incident light by the ITO conducting glass substrate. The photocurrent for light incident through the working electrode is generally larger than that for light incident through the back electrode, due to the light multi-reflection of the Ptcoated ITO counter electrode in the former and the longer distance across which the photogenerated charge carriers transport toward the TiO electrode in the latter.  Irrespective of whether light is incident through the working electrode or the counter electrode, the photocurrent action spectrum is different from the absorption spectrum of the working electrode. However, the photocurrent action spectrum in the former is always ‘in-phase’ with that in the latter irrespective of whether it was sensitized alone or cosensitized, indicating that the adsorbed dye molecules exist as a monolayer on the nanostructured TiO electrode or at least the thickness of the  adsorbed or coadsorbed layer is smaller than the diffusing length of the photogenerated charge carriers or excitons if dye molecules exist as a multilayer. Otherwise, the ‘in-phase’ relation will not hold according to the exciton dissociation theory, developed by Ghosh and

coworkers, which states that excitons must diffuse into the junction and are dissociated into free charge carriers by a built-in field in the junction region. Therefore, it is believed reasonably that the sensitized dye molecules exist as a monolayer self-assembled on the nanostructured TiO electrode. In addition, the ‘in phase’ relation between photocurrent action spectra in the above two cases excludes the possibility that the above action spectrum in the former results from the filter effect of the adsorbed multilayer of dye molecules. For sensitization with H TsPP dye alone, the absorb ance of the Soret band of H TsPP generates a photocur rent with two maxima of 300 nA cm\ at 420 and 450 nm, resulting in IPCEs of 24.3 and 10.8%, respectively; and that of the Q-band in the region of 640—750 nm generates a photocurrent with a maximum of 100 nA cm\ and an IPCE maximum of 1.20% at 680 nm, whereas the absorbance of the Q-bands in the region of 550—630 nm does not convert into a photocurrent. Here, IPCE"1243;I ;100%/(MR;j;P ), in 1!  which I and P are the short-circuit current density 1!  and the incident light power at the monochromatic wavelength (corrected for the absorption and scattering of incident light by the ITO glass), respectively. For sensitization with ZnTsPP dye molecules alone, the absorbance of the ZnTsPP Soret band in the region of 400—470 nm generates the strong photocurrent response with a photocurrent maximum of 1990 nA cm\ and an IPCE maximum of 99.4% at 430 nm, and that of the ZnTsPP Q-band in the region of 500—640 nm shows the weaker photocurrent response with two maximum of 320 nA cm\ at 560 nm and 210 nA cm\ at 610 nm (6.0 and 3.2% for IPCE). In contrast, cosensitization markedly decreases the monochromatic photocurrent response in the region of 400—610 nm, specially for the Soret band of the ZnTsPP dye, and at the moment greatly enhances the monochromatic photocurrent response of the Q-band of H TsPP in  the region of 640—760 nm. As shown in Figure 5a, at 430 nm, the photocurrent is reduced to 280 nA cm\ and an IPCE is reduced to 16.7%. At the moment, the photocurrents of 450 nA cm\ at 680 nm and 520 nA cm\ at 690 nm are obtained with five- and ten-fold improvement, respectively. Because the absorbance of ZnTsPP molecules at around 680 nm does not convert into a photocurrent, the photocurrent response at around 680 nm is attributed to the absorbance of the H TsPP  Q-band. In addition, cosensitization leads to the achievement of the photocurrent response in the broader region (400—760 nm). ¹he improving mechanism of photoelectric conversion for the ¹iO electrode cosensitized with H ¹sPP and   Zn¹sPP dye For the liquid junction cell based on the dye-sensitized TiO electrode, the cosensitization of the nanostructured  TiO electrode with H TsPP and ZnTsPP dye molecules   strikingly enhances the photocurrent response under monochromatic illumination in the Q-band of H TsPP 

672 SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998

Heteroaggregation and photoelectric conversion of porphyrins: H. Deng and Z. Lu

Figure 6 Energy level scheme of H TsPP/ZnTsPP heteroaggregates on the TiO electrode. S , the lowest singlet state; S , ground state; T ,  triplet state; CT, charge-transfer state. E "E , ox (ZnTsPP) E , !2   red (H TsPP) 

Figure 5 Photocurrent action spectra of the liquid junction cell based on the TiO electrode sensitized with a single dye or cosensitized with  and ZnTsPP (0.60) dye for light incident through (a) the H TsPP (0.40)  working electrode and (b) the back (counter) electrode

(640—760 nm) and markedly decreases the monochromatic photocurrent response in the region of 400—610 nm, especially for the Soret band of the ZnTsPP dye. This is attributed to the formation of heteroaggregates (e.g. heterodimer or heterotrimer) between H TsPP  and ZnTsPP molecules adsorbed on the cosensitized TiO electrode with the positively charged surface.  First, the formation of the PP/PP heteroaggregates prevents H TsPP molecules from forming homodimers  (e.g. self-aggregation), in which most of the excited excitons or the photogenerated charge carriers are quenched by the rapid internal conversion, resulting in the strong enhancement of the photocurrent response in the H TsPP Q-band. Thus, the decrease of the dimer  leads to the decrease of the deactivated possibility of the excited dye molecules through the rapid internal conversion. In addition, the formation of the PP/PP heteroaggregates results in an extended and conjugated face-to-face system formed with a n orbital overlap between H TsPP and ZnTsPP and aids to generate  low-lying charge-transfer states (CT states) in the cosensitization. The low-lying charge-transfer state was reported to exist in the heterodimer composed of the tetra(4- carboxyphenyl)porphyrin (ZnPPTC) and tetra(N-methylpyridyl) porphyrin (H TMyPP) and ap propriately explain the quenching of ZnPPTC at the Soret band and the increase of H TMyPP in the Q-band. Here,  the oxidation and reduction potentials of H TsPP and 

ZnTsPP show that there is the low-lying CT state in PP/PP heteroaggreagtes. As shown in Figure 6, the lowlying CT state, the charge-transfer energy of which is equal to 1.93 eV (corresponding to 644 nm) from the relation E "E , ox (ZnTsPP)—E , red (H TsPP), !2    locates under the lowest singlet state (2.05 eV) of ZnTsPP and above the lowest singlet state (1.92 eV) of H TsPP  (energy levels of the lowest singlet state and triplet state of H TsPP and ZnTsPP are shown in Figure 6). The  low-lying CT state leads to the strong quenching of the photocurrent in the 400—600 nm range, particularly for the Soret band of ZnTsPP molecules. The photoelectric behavior of the cosensitized TiO electrode with H TsPP   and ZnTsPP molecules can be understood by the presence of a low-lying CT state between H TsPP and  ZnTsPP molecules. The presence of the low-lying CT state is also derived from the fluorescence emission of the TiO colloidal  solution cosensitized with H TsPP and ZnTsPP. Strik ingly, emission from the heteroaggregates in the region of 400—470 nm is quenched at room temperature while that in the region of 640—740 nm is increased with a small red-shift (3 nm) of the H TsPP on the cosensitized TiO   electrode with respect to that of the H TsPP monomer.  The observed red-shifts of the absorbance and fluorescence of PP/PP aggregates with respect to the absorbance and fluorescence of H TsPP monomers are  attributed to charge-transfer interaction and singlet exciton coupling between the excited states of H TsPP  and ZnTsPP planar ring moieties, which stems from the energy difference between the lowest excited singlet states of H TsPP and ZnTsPP molecules. In some covalently  bound porphyrin dimers, this exciton coupling is responsible for a significant blue-shift of the Soret band in the absorption spectrum. In our experiment, this blue-shift can be off-set by the red-shift caused by charge-transfer interactions. A similar effect has been found in the electronic spectra of a stratibisporphyrin. CONCLUSION Cosensitization with H TsPP and ZnTsPP extends the  absorbance of the TiO electrode. In particular, the 

SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998 673

Heteroaggregation and photoelectric conversion of porphyrins: H. Deng and Z. Lu photocurrent response at the Q-band of H TsPP is strik ingly enhanced with a five- and a ten-fold improvement at 680 and 690 nm and, at the moment, the photocurrent response at the Soret band of ZnTsPP is markedly decreased with a six-fold decrease at 430 nm. The photoelectric behavior of the cosensitized TiO electrode is  attributed to the formation of PP/PP heteroaggregates during the cosensitization of the TiO electrode, resulting  in the decrease of the surface concentration of H TsPP  dimers and the presence of the low-lying charge-transfer state. The low-lying charge-transfer state of 1.93 eV, which locates under the lowest singlet state (2.05 eV) of ZnTsPP and above the lowest singlet state (1.92 eV) of H TsPP, quenches the photogenerated charge car riers of the Soret band of ZnTsPP. The formation of the PP/PP heteroaggregate prevents H TsPP from self aggregating, resulting in a striking improvement of the photocurrent response in the Q-band of H TsPP  (640—760 nm).

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China.

REFERENCES 1 O’Regan, B., Moser, J., Anderson, M. and Gra¨tzel, M. J. Phys. Chem. 1990, 94, 8720 2 O’Regan, B. and Gra¨tzel, M. Nature 1991, 253, 737

3 Shatrma, G.D., Mathur, S.C. and Dube, D.C. J. Mater. Sci. 1991, 26, 6547 4 Nevin, W.A. and Chamberlain, G.A. J. Appl. Phys. 1991, 69, 4324 5 Gerischer, H. in ‘Photoelectrochemistry, Photocatalysis and Photoreaction’, (Ed. M. Schiavello), Reidel, Dordrecht, 1985 6 Nevin, W.A. and Chamberlain, G.A. J. Appl. Phys. 1990, 68, 5247 7 Shen, Y.C., Wang, L., Lu, Z.H., Wei, Y., Zhou, Q.F., Mao, H.F. and Xu, H.J. ¹hin Solid Films 1995, 257, 144 8 Deng, H., Mao, H.F., Liang, B.J., Shen, Y.C., Lu, Z.H. and Xu, H.J. J. Photochem. Photobiol. A: Chem. 1996, 99, 71 9 Deng, H., Lu, Z.H., Mao, H.F. and Xu, H.J. Chem. Phys. 1997, 221, 323 10 Deng, H., Mao, H.F., Lu, Z.H., Li, J.M. and Xu, H.J. J. Photochem. Photobiol. A: Chem. 1997 (to be published) 11 Kay, A. and Gratzel, M. J. Phys. Chem. 1993, 97, 6272 12 Kay, A., Humphry-Baker, R. and Gra¨tzel, M. J. Phys. Chem. 1994, 98, 952 13 Nazeeruddin, M.K., Kay, A., Rodicio, I., Humphry-Baker, R., Muller, E., Liska, P., Vlachopoulos, N. and Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382 14 Fleischer, E.D., Palmer, J.M. and Svivastara, T.S. J. Am. Chem. Soc. 1971, 93, 3162 15 Shimidzu, T. and Iyoda, T. Chem. ¸ett. 1981, 853 16 Ojadi, E., Scizer, R. and Linschitz, H. J. Am. Chem. Soc. 1985, 107, 7783 17 van Willigen, H., Das, U., Ojadi, E. and Linschitz, H. J. Am. Chem. Soc. 1985, 107, 7784 18 Geiger, D.K. and Kelly, C.A. Inorg. Chim. Acta 1988, 154, 137 19 Segawa, H., Nishino, H., Kamikana, T. and Honda, K. Chem. ¸ett. 1989, 1917 20 Kalyanasundaram, K. and Neumann-Spallart, M. J. Phys. Chem. 1982, 86, 5163 21 Ltuate, C.A. and Sanders, K.M. J. Am. Chem. Soc. 1990, 112, 5525 22 Ghosh, A.K., Morel, D.L., Feng, T., Shaw, R. S. and Rowe, C.A. J. Appl. Phys. 1974, 45, 230 23 Ghosh, A.K. and Feng, T., J. Appl. Phys. 1978, 49, 5982 24 Hofstra, U., Koehorst, R.B.M. and Schaafsma, T.J. Chem. Phys. ¸ett. 1986, 130, 555 25 Chang, C.K. Adv. Chem. Ser. 1979, 173, 162 26 Kagan, N.E., Mauzerall, D. and Merrifield, R.B. J. Am. Chem. Soc. 1977, 99, 5484

674 SUPRAMOLECULAR SCIENCE Volume 5 Numbers 5—6 1998