J. Phys. Chem. C 2009, 113, 2961–2965
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Fluorescence Quantum Yield of Aromatic Hydrocarbon Crystals Ryuzi Katoh,*,† Kengo Suzuki,‡ Akihiro Furube,† Masahiro Kotani,§ and Katsumi Tokumaru| National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565; Systems DiVision, Hamamatsu Photonics K. K., 812 Joko-cho, Higashi-ku, Hamamatsu-city, 431-8196; Faculty of Science, Gakushuin UniVersity, Mejiro, Tokyo 171-8588; and UniVersity of Tsukuba, Tennodai, Tsukuba, Ibaraki 305-8577, Japan ReceiVed: August 28, 2008; ReVised Manuscript ReceiVed: December 2, 2008
We measured the fluorescence quantum yield (Φf) of several aromatic hydrocarbon crystals: p-terphenyl, trans-stilbene, anthracene, pyrene, and R-perylene. The Φf is reduced by chemical impurities, structural defects, and reabsorption of fluorescence. To minimize the effect of chemical impurities and structural defects, we evaluated the Φf of highly purified single crystals. We also measured the Φf of powder samples prepared by the mechanical milling of single crystals to assess the effect of reabsorption and structural defects induced by milling. We estimated the lower limit values of the Φf to be 0.80 for p-terphenyl, >0.65 for trans-stilbene, >0.64 for anthracene, 0.68 for pyrene, and 0.31 for R-perylene. Introduction The recent development of luminescent devices based on organic materials, including electroluminescence devices and lasers, has provided the potential for high quantum yield and variation of color.1-3 The high level of performance of these devices has been demonstrated with the synthesis and characterization of numerous molecules. Computational calculations are often used to characterize new molecules, and an evaluation of the fluorescence color in an isolated state can be made. On the contrary, the characterization of crystalline materials based on computational methods may not be accurate: since relatively weak intermolecular interactions affect electronic structure and hence luminescence properties, spectra and quantum yields that differ markedly between crystalline materials and isolated molecules can result. This problem can be overcome by characterizing molecules by means of direct evaluation of luminescence properties. The fluorescence properties of a material can be characterized by determining the fluorescence spectra, lifetime, and quantum yield. Fluorescence spectra can be readily obtained using a conventional spectrometer whereas the lifetime of fluorescence can be evaluated through time-resolved spectroscopy based on time-correlated single photon counting and streak camera techniques. By comparison, the fluorescence quantum yield (Φf) is more difficult to evaluate, especially for crystalline materials. For solutions, although absolute values for quantum yield are difficult to estimate, relative values can be obtained by comparison with the integrated spectra of the standard sample under the same experimental conditions.4 In such experiments, the shape of the sample should be the same as that of the standard. Thus, this technique is difficult to apply to crystalline samples. The recent proposal of an alternative technique, employing an integrating sphere equipped with a multichannel spectrometer to evaluate absolute values of fluorescence quantum yield, provides a solution for the characterization of * To whom correspondence should be addressed. E-mail:
[email protected]. † AIST. ‡ Hamamatsu Photonics K. K. § Gakushuin University. | University of Tsukuba.
crystalline samples.5-10 By using this technique, spectra are measured using excitation light with and without sample. The Φf can be evaluated by comparing the decrease in excitation light intensity with the increase in fluorescence intensity. This technique has been used to evaluate the Φf of various film samples, and careful comparison of values with those generated in previous studies reinforces the reliability of the integrating sphere method. 5-10 For crystalline samples, several factors influence the reliable evaluation of the Φf, despite application of the integrating sphere method. Although the apparent quantum yield can be obtained using the integrated sphere method, this value is not necessarily the intrinsic value of the sample material. This discrepancy occurs because the fluorescence properties of crystalline samples are sensitive to chemical impurities, structural defects, and reabsorption of fluorescence. Excitons migrating in the crystal are readily captured by impurities and structural defects thereby causing fluorescence quenching of the host material. For example, excitons in anthracene crystals can be quenched by tetracene at 10 µs), indicating ultrahigh purity.14,15 For R-perylene crystals, the carrier lifetime was small (10 µs), signals due to electrons were not observed.16 This finding indicates the presence of impurities at a low level in the bulk of the crystals. Photoconductivity measurements were not performed for pyrene crystals. Fluorescence quantum yields were measured by using a commercial spectrometer equipped with an integrating sphere of 3.3 in. in radius (Hamamatsu, C9920-02). Excitation was provided with a xenon lamp after passing through a monochromator. Excitation wavelengths (fwhm < 10 nm) were set around the peak of the lowest excited state. Samples, single crystals and powders, were placed in a quartz cell (Hamamatsu, A1009501) and were attached to the bottom of sphere’s wall. Mechanically milled samples were prepared by hand using a spatula on the quartz cell. Excitation light was irradiated from the top of the sphere and was incident normal to the cleavage plane of the single crystal samples. Emitted fluorescence was detected with a multichannel spectrometer through the side port of the sphere. In the sphere, a baffle between the sample holder and the detection port was placed to avoid direct observation of the scattered excitation light and the fluorescence from the sample. Time-resolved fluorescence spectra were measured with a commercial spectrophotometer (IBH, FluoroCube). Excitation was provided with a pulsed laser diode (λex ) 408 nm). Results and Discussion Fluorescence Quantum Yield of Crystalline Samples. Figure 1 shows fluorescence spectra of single crystals of p-terphenyl, trans-stilbene, anthracene, pyrene, and R-perylene together with the apparent Φf obtained. Absorption spectra of p-terphenyl,17 anthracene,18 pyrene,19 and R-perylene20 crystals are also shown. For the trans-stilbene crystal, no absorption spectra were available. The typical size of crystals was approximately 5 × 5 × 0.5 mm3. As mentioned above, the Φf is suppressed by chemical impurities, structural defects, and reabsorption. Thus, the Φf obtained here give lower limit values. When the Φf is higher than the values obtained for other samples, this value is regarded as close to the intrinsic value. Quantum yield measurements were performed for unpurified commercial products of anthracene to investigate the effect of quality of crystals on the Φf. These studies used unpurified powderlike samples. Figure 2 shows fluorescence spectra together with the Φf for several samples. All samples gave a lower quantum yield compared with the highly purified sample shown in Figure 1, indicating that quality of sample is important
Figure 1. Fluorescence (solid line) and absorption (dotted line) spectra for single crystals of p-terphenyl, trans-stilbene, anthracene, pyrene, and R-perylene. The apparent Φf values are also shown.
Figure 2. Fluorescence spectra and the apparent Φf for several unpurified commercial products of anthracene: (a) Nacalai 03029; (b) Nacalai 03028; (c) Tokyo Kasei A0092; (d) Aldrich 141062; (e) Nacalai 03030; (f) Nacalai 03103; (g) Aldrich A89200; (h) Tokyo Kasei A0495; (i) Merck, scintillation grade.
for the reliable estimation of the Φf. Additional fluorescence peaks at longer wavelengths are found in spectra (a)-(e) (Figure 2). These peaks are caused by the fluorescence from impurities after efficient energy transfer from anthracene excitons. The spectra (f)-(i) on the contrary did not show any additional peaks but showed a low Φf caused by energy transfer to unemitted impurities or structural defects. The Φf can be used as the reference for the quality of organic materials, the concentration of impurities and structural defects, because these are not easy to identify from fluorescence spectra. Reabsorption, which is absorption of fluorescence by the sample, affects the apparent values of fluorescence properties by suppression of the shorter wavelength region of the fluorescence spectrum, increasing the fluorescence lifetime and decreasing the quantum yield. The effects of reabsorption have an adverse effect on the reliability of fluorescence spectroscopy for the characterization of organic materials, and many studies
Φf of Aromatic Hydrocarbon Crystals
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Figure 4. Fluorescence spectra, change in the apparent Φf during the mechanical milling process, and microscopy images for p-terphenyl.
Figure 3. Fluorescence spectra and the apparent Φf for powder samples prepared by the mechanical milling of p-terphenyl, trans-stilbene, anthracene, pyrene, and R-perylene. Dotted lines show the spectra of single crystals.
attempt to minimize these effects.21 However, no reliable methods have been established. For single crystals, emitted photons in the bulk of the crystal can propagate in the crystal because of reflection caused by a large difference in the refractive index between crystal and air. As a result, the effects of reabsorption become more pronounced. On this basis, the adverse effects of reabsorption effect are expected to be less pronounced in small crystals. Thus, we measured the Φf of powder samples prepared by the mechanical milling of the single crystals. Figure 3 shows the fluorescence spectra of the powder samples prepared by the milling process of p-terphenyl, transstilbene, anthracene, pyrene, and R-perylene together with the apparent Φf. For p-terphenyl and trans-stilbene, the fluorescence spectra at the shorter wavelength region are more pronounced after milling, indicating suppression of the reabsorption effect. For anthracene, although the fluorescence spectrum at the shorter wavelength region was more pronounced after milling, intensity at the longer wavelength region was also more pronounced. Together, these results suggest that fluorescent defects are introduced by the milling process. For pyrene and R-perylene, the difference in fluorescence spectra before and after milling was not pronounced. For these materials, fluorescence comes from the relaxed excimer state. Thus, the effect of reabsorption is not pronounced because the fluorescence spectra shift toward longer wavelengths and this reduces the overlap between the absorption and fluorescence spectra (see Figure 1). The effect of the milling process on the Φf differed among the compounds tested. For p-terphenyl, the Φf was markedly enhanced. This finding was clearly due to suppression of the reabsorption effect by the milling process without introduction of structural defects. For anthracene and R-perylene, the milling process efficiently suppresses the Φf. In this case, suppression of the Φf is due to the introduction of structural defects by the milling process. By comparison, for trans-stilbene and pyrene, the Φf is similar before and after the milling process. For transstilbene, the spectral shape change is indicative of suppression of the reabsorption effect and suggests that the introduction of structural defects by the milling process overrides enhancement of the Φf due to suppression of the reabsorption effect. For
Figure 5. Fluorescence spectra, change in the Φf during the mechanical milling process, and fluorescence microscopy images for anthracene.
pyrene, as mentioned above, the reabsorption effect is not important, and thus, the Φf is not affected by the milling process. The effect of the milling process was investigated by measuring the Φf for p-terphenyl and anthracene during milling. Figure 4 shows the fluorescence spectra and the Φf together with the fluorescence microscopy images for p-terphenyl. An initial study used single crystals with a size of 1.5 × 5 × 0.5 mm3. The milled sample of approximately 1 mm in size shows enhancement of the Φf. This finding suggests that fluorescence emitted in the bulk of the crystal can migrate at least several millimeters by the waveguide effect. During the milling process, the Φf increases gradually with decreasing size of the powder. At the end of the milling process, the spectra of powder with a size of around 0.1 mm is similar to the spectra of solutions,22 suggesting that the reabsorption effect is effectively suppressed. Figure 5 shows the fluorescence spectra and the Φf together with the fluorescence microscopy images of anthracene. The anthracene crystals were prepared by the sublimation method and had a size of 2 × 5 × 0.2 mm3. Spectra were normalized at the peak of 445 nm. The fluorescence image of the flake sample before milling revealed fluorescence emitted strongly at the edge of the crystal. This indicates that fluorescence generated in the bulk of the crystal propagates in the specimen and emerges at the edge of the crystal. Fluorescence spectra at the shorter wavelength region became more pronounced by the milling process, suggesting suppression of the reabsorption effect. However, the Φf decreased dramatically from 0.64 to 0.27 by the milling process, and the fluorescence spectra at the
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Katoh et al. TABLE 1: Fluorescence Quantum Yields Obtained in Single Crystals and in Powder Prepared by the Milling of Single Crystalsa crystal compound
single crystal
powder
solution
p-terphenyl trans-stilbene anthracene pyrene perylene
0.67 0.63 0.64 0.68 0.31
0.8 0.65 0.18 0.67 0.18
0.93b 0.08b 0.28c 0.75d 0.02d
a
Data for solutions were obtained from references. b Reference 28. Reference 10. d Reference 29. The Φf of pyrene and perylene in solution are for excimer fluorescence. c
Figure 6. Time-resolved fluorescence spectra of single crystals and powder of R-perylene. Spectra were normalized at longer wavelength region.
longer wavelength region became more pronounced. These findings imply that exciton traps are formed by the milling process. There are several candidates for exciton traps induced by the milling process, including surface adsorbed oxygen molecules, surface defects, and defects in the bulk of the crystal. As shown in Figure 5, the suppression of the Φf occurs for millimeter-sized particles. Thus, the formation of surface traps is unlikely because the exciton diffusion length in anthracene crystals is approximately 100 nm,23 which is markedly shorter than the size of the milled crystals (>1 mm). We observed no fluorescence intensity change of powder samples under an atmosphere of nitrogen. This finding suggests that structural defects formed in the bulk of the crystals are likely an exciton trap. After the milling process, fluorescence at the longer wavelength region becomes more pronounced. This enhanced spectrum is consistent with the spectrum of the anthracene excimer.24 This finding implies that excitons in the anthracene crystals are trapped by the dimer state of the bulk of the crystals, introduced by the mechanical milling process, and are emitted as excimer fluorescence. The effect of this phenomenon is a reduction in the Φf. For R-perylene, the Φf decreases dramatically as a result of the milling process. We examined time-resolved fluorescence to determine the cause of the reduction in the Φf. Figure 6 shows the temporal change in fluorescence spectra of a single crystal and powder sample of R-perylene. The spectra of the single crystal show a strong broad peak at 570 nm, which is assigned to the excimer state, and a small peak at 475 nm. The smaller peak can be assigned to the fluorescence from free excitons derived from the unrelaxed state before changing to the excimer.25 The spectral shapes did not change with time, suggesting that the free and excimer exciton states are in thermal equilibrium.25 By comparison, the spectra of the powder sample showed the same strong peak at 570 nm but absence of the small fluorescence peak at around 475 nm and a more
pronounced shoulder at 520 nm, which is assigned to fluorescence from the monomer defect sites.26 The fluorescence at around 520 nm was more pronounced during the early time region. This indicates that spectral changes induced by milling are due to the introduction of monomer defects. Thus, effective quenching of fluorescence from free excitons occurs by trapping of the excitons by the monomer defect sites and subsequently emission of fluorescence at around 520 nm. Evaluation of Lower Limit Values of Fluorescence Quantum Yields. The Φf obtained for single crystals and powder samples together with the Φf previously reported for solutions are listed in Table 1. For all compounds, the Φf values of solutions are different to the Φf values determined for the crystalline phase, with the exception of pyrene. This finding suggests that a small difference in the molecular properties of solutions and crystals leads to a dramatic difference in the Φf. It should be noted that the values of the Φf obtained for solutions cannot used as the Φf value of crystals. p-Terphenyl. The Φf for p-terphenyl in solution is very high (0.95) whereas slightly lower values were observed for single crystals (0.67) and powder samples (0.8). As mentioned above, the spectral shape of the fluorescence emission is similar for solutions and powder samples, suggesting that the effect of reabsorption is not significant. Thus, the Φf of the powder sample is close to the intrinsic value. The similarity of the spectra for solutions and powder samples suggests that intermolecular interaction is not strong. By comparison, the Φf values are slightly different. This implies the presence of specific internal conversion channels in the crystalline phase. trans-Stilbene. The Φf for trans-stilbene in solution is markedly lower (0.08) than the Φf for single crystals (0.63) because photoisomerization to the cis-form via an excited state occurs efficiently in solution.29 The Φf of molecules in a rigid matrix at lower temperature is high,30 suggesting that transstilbene has a high Φf in the absence of photoisomerization. In the crystalline phase, this photoisomerization reaction is suppressed because of the dense packing of molecules leading to realization of a higher Φf value. After mechanical milling, suppression of the reabsorption effect is confirmed by a spectral shape change at the shorter wavelength region. However, milling had no effect on the Φf, suggesting that exciton quenching centers, most likely in the form of structural defects, were introduced by the milling process. This suggests that suppression of the reabsorption effect can compensate for the effect of the defect formation. Thus, the Φf obtained for single crystals is slightly smaller than the intrinsic value. Anthracene. The Φf for anthracene in solution is lower (0.28) than the Φf for single crystals (0.64). The low Φf of anthracene in solution is based on efficient intersystem crossing (ISC) from the singlet excited state to the second triplet state.31 Efficient
Φf of Aromatic Hydrocarbon Crystals ISC for the anthracene molecule in solution is possible as the energy level of the singlet excited state (ES1 ) 3.29 eV) is slightly higher than the energy level of the second triplet state (ET2 ) 3.24 eV). Similarly, some derivatives of anthracene also show a relatively high Φf that is related to the difference between ES1 and ET2. A relatively high Φf was observed for crystalline anthracene, suggesting that ES1 is higher than ET2. For anthracene crystals, the energy levels of the lowest excited state were reported to be ES1 ) 3.11 eV and ET1 ) 1.83 eV.32 The absorption spectrum due to triplet excitons in single crystals of anthracene has recently been reported, and a broad peak at around 620 nm was observed.33 By comparing this absorption spectrum with the spectrum of the triplet excited state in solution, the ET2 is estimated to be ET2 ) 3.51 eV. This energy level is higher than the energy level of the singlet excited state (ES1 ) 3.11 eV). Thus, the higher Φf of single crystals of anthracene can be explained by inefficient ISC. As mentioned above, the Φf decreases dramatically as a result of the milling process, which is caused by formation of structural defects in the bulk of the crystal. Thus, the Φf obtained for single crystals is slightly smaller than the intrinsic value. Pyrene. The fluorescence spectrum of pyrene crystals is very broad, and there is no evidence of vibrational structures because fluorescence emission originates from the excimer state of pyrene in the crystal. For diluted solutions of pyrene, fluorescence from isolated molecules shows distinct vibrational structures. By comparison, concentrated solutions of pyrene show fluorescence originating from the excimer state, which is similar to the origin of fluorescence in the crystal. The Φf of the pyrene excimer in solution is estimated to be 0.75, which is similar to the Φf of the excimer in pure single pyrene crystals (0.68). As mentioned above, fluorescence emission originates from the excimer state of pyrene in pure single crystals, which occurs after relaxation from the monomer state. Because the fluorescence emission peak shifts markedly from the absorption tail, the reabsorption effect is unlikely to occur. As expected, the fluorescence spectral shape did not change by the milling process (Figure 3). The milling process did not alter the Φf for pyrene crystals thereby suggesting the absence of structural defects introduced by milling and the absence of a strong reabsorption effect. Thus, Φf obtained for single crystals (0.68) is close to the intrinsic value. r-Perylene. Similar to the finding for pyrene crystals, fluorescence from R-perylene crystals comes from the excimer state. In solution, fluorescence from the excimer state was observed in concentrated solutions of perylene. The fluorescence lifetime τfluo and the Φf were estimated to be τfluo ) 20 ns and Φf ) 0.02, respectively.28 By using these values, the radiative lifetime τrad can be estimated to be τrad ) 1000 ns. For the single crystal, τrad can be estimated to be τrad ) 200 ns using the values of τfluo ) 60 ns and Φf ) 0.31. This difference of τrad implies that the electronic structure of the excimer in solution is different from that in crystals. This finding is consistent with a recent computational study of perylene dimers and clusters.34 The Φf of perylene crystals decreases dramatically from 0.31 to 0.18 by the milling process. As discussed above, this is due to formation of structural defects acting as exciton traps. As discussed for pyrene, excimer fluorescence emission occurs at a substantially longer wavelength than absorption because of stabilization resulting from excimer formation. Accordingly, no strong reabsorption is expected. Thus, the Φf obtained for single crystals (0.31) is close to the intrinsic value.
J. Phys. Chem. C, Vol. 113, No. 7, 2009 2965 Conclusion The Φf of highly purified aromatic hydrocarbon crystals p-terphenyl, trans-stilbene, anthracene, pyrene, and R-perylene has been evaluated. The Φf is reduced by chemical impurities, structural defects, and reabsorption. Thus, we measured the Φf of single crystals and powder samples prepared by the milling of single crystals. Our findings reveal the estimated lower limit values of the Φf to be 0.80 for p-terphenyl, >0.65 for transstilbene, >0.64 for anthracene, 0.68 for pyrene, and 0.31 for R-perylene. References and Notes (1) Bru¨tting, W. Physics of Organic Semiconductors; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2005. (2) Hung, L. S.; Chen, C. H. Mater. Sci. Eng. R. 2002, 39, 143–222. (3) Mu¨llen, K.; Scherf, U. Organic Light Emitting DeVices; WileyVCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2006. (4) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107. (5) Greenham, N. C.; Samuel, I. D. W.; Hayes, G. R.; Phillips, R. T.; Kessener, Y. A. R. R.; Moratti, S. C.; Holmes, A. B.; Friend, R. H. Chem. Phys. Lett. 1995, 241, 89. (6) de Mello, J. C.; Wittman, H. F.; Friend, R. H. AdV. Mater. 1997, 9, 230. (7) Pa˚lsson, L.-O.; Monkman, A. P. AdV. Mater. 2002, 14, 757. (8) Ahn, T.-S.; Al-Kaysi, R. O.; Mu¨ller, A. M.; Wentz, K. M.; Bardeen, C. J. ReV. Sci. Instrum. 2007, 78, 086105. (9) Kawamura, Y.; Sasabe, H.; Adachi, C. Jpn. J. Appl. Phys. 2004, 43, 7729. (10) Endo, A.; Suzuki, K.; Yoshihara, T.; Tobita, S.; Yahiro, M.; Adachi, C. Chem. Phys. Lett. 2008, 460, 155. (11) Benz, K.; Wolf, H. C. Z. Naturforsch. 1964, 19, 177. (12) Kotani, M. Proc. Intl. Symp. Org. DeVices IPAP Conf. Ser. 2005, 9, 23. (13) Kotani, M.; Kakinuma, K.; Yoshimura, M.; Ishii, K.; Yamazaki, S.; Kobori, T.; Okuyama, H.; Kobayashi, H.; Tada, H. Chem. Phys. 2006, 325, 160. (14) Katoh, R.; Kotani, M. Chem. Phys. Lett. 1990, 166, 258. (15) Morikawa, E.; Isono, Y.; Kotani, M. J. Phys. Chem. 1978, 78, 2691. (16) Katoh, R.; Kotani, M. Chem. Phys. Lett. 1990, 174, 541. (17) Wakayama, N. I.; Matsuzaki, S.; Mizuno, M. Chem. Phys. Lett. 1980, 74, 37. (18) Nishimura, H.; Yamaoka, T.; Hattori, K.; Matsui, A.; Mizuno, K. J. Phys. Soc. Jpn. 1985, 54, 4370. (19) Tanaka, J. Bull. Chem. Soc. Jpn. 1965, 38, 86. (20) Hochstrasser, R. M. J. Chem. Phys. 1964, 40, 2559. (21) Nozue, Y.; Goto, T. J. Phys. Soc. Jpn. 1989, 58, 1831. (22) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic Press: New York, 1971. (23) Yago, T.; Tamaki, Y.; Furube, A.; Katoh, R. Chem. Phys. Phys. Chem. 2008, 10, 4435. (24) Rettig, W.; Paeplow, B.; Herbst, P.; Mu¨llen, K.; Desvergne, J.-P.; Bouas-Laurent, H. New J. Chem. 1999, 23, 453. (25) Nishimura, H.; Yamaoka, T.; Mizuno, K.; Iemura, M.; Matsui, A. J. Phys. Soc. Jpn. 1984, 53, 3999. (26) Furube, A.; Murai, M.; Tamaki, Y.; Watanabe, S.; Katoh, R. J. Phys. Chem. A 2006, 110, 6465. (27) Saltiel, J.; D’Agostino, J.; Megarity, E. D.; Metts, L.; Neuberger, K. R.; Wrighton, M.; Zafiriou, O. C. Org. Photochem. 1973, 3, 1. (28) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970. (29) Katoh, R.; Sinha, S.; Murata, S.; Tachiya, M. J. Photochem. Photobiol. A: Chem. 2001, 145, 23. (30) Sumitani, M.; Nakashima, N.; Yoshihara, K. Chem. Phys. Lett. 1977, 51, 183. (31) Fukumura, H.; Kikuchi, K.; Koike, K.; Kokubun, H. J. Photochem. Photobiol. A: Chem. 1988, 42, 283. (32) Karl, N. Landort-Bernstein Numerical Data and Fundamental Relationships in Science and Technology, New Series, Vol. 17; Springer: Berlin, Germany, 1985. (33) Katoh, R.; Tamaki, Y.; Furube, A. J. Photochem. Photobiol. A: Chem. 2006, 183, 267. (34) Velardez, G. F.; Lemke, H. T.; Breiby, D. W.; Nielsen, M. M.; Møller, K. B.; Henriksen, N. E. J. Phys. Chem. A, in press.
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