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Picosecond dynamics of excited singlet states in organic microcrystals: Diffuse reflectance laser photolysis study
Volume 150, number 5
CHEMICAL PHYSICS LETTERS
23 September 1988
PICOSECOND DYNAMICS OF EXCITED SINGLET STATES IN ORGANIC MICROCRYSTALS: DIFFUSE REFLECTANCE LASER PHOTOLYSIS STUDY
Noriaki IKEDA ‘, Masanori KOSHIOKA, Hiroshi MASUHARA ’ Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan
and Keitaro YOSHIHARA Institutefor Molecular Science, Myodaiji, Okuzaki 444, Japan Received 23 May 1988; in final form 29 June 1988
Absorption spectra and picosecond dynamics of the singlet exciton states of benzil and pterphenyl in a microcrystal have been measured for the first time by analyzing the diffuse reflected spectra of the picosecond continuum.
1. Introduction
The laser photolysis method is now recognized as an important technique to study photoprimary processes of a variety of molecular systems. This conventional technique has been restricted to transparent samples; however, a diffuse reflectance laser photolysis method has been developed in recent years [I]. This method is powerful because it gives absorption spectra of transient species in opaque and scattering materials such as organic microcrystals, semiconductor as well as insoluble polymer powders, dyed fabrics, molecules adsorbed on silica gels, etc. Although the photophysics of organic crystals has been extensively studied, many of those studies have been done mainly by emission techniques. Energy transfer and exciton dynamics are processes studied in detail so far in organic solids, While most attention has been paid to the behavior of single crystals, studies of molecules in various crystalline environments are also interesting. Recently, we have succeeded in measuring the ’ Present address: Chemistry Department, College of General Education, Osaka University, Toyonaka, Osaka 560, Japan.
* To whom correspondence should be addressed.
452
transient absorption spectrum and its dynamics in the picosecond time range, for benzophenone microcrystals [ 2 1. Wilkinson et al. also studied the picosecond dynamics of triplet species by the pump and probe method at a single wavelength [ 1,3]. In the application of this methodology, however, there was no report on the excited singlet state until now. From this view point we have examined various systems and measured for the first time the absorption spectra of excited singlet states (so-called exciton states) of benzil and p-terphenyl microcrystals. Their formation dynamics in the picosecond time domain and related problems will be discussed.
2. Experimental A microcomputer-controlled picosecond diffuse reflectance laser photolysis system with a repetitive mode-locked Nd3+ :YAG laser was used, in which a double-beam optical arrangement was adopted. The details have been described in a previous paper [ 2 1. The sample, contained in a suprasil cell of 2 mm thickness, was excited with a single 355 nm pulse ( 17 ps, 1 mJ), and monitored by using the picosecond continuum as a wide-band analyzing light. The pi-
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
CHEMICAL PHYSICS LETTERS
Volume 150.number5
cosecond diffuse reflected light from the sample was detected by a multichannel photodiode array (MCPD 1) through a polychromator. A part of the continuum beam was detected by another polychromator with MCPD2, and was used as the reference for a spectral shape of the analyzing light and shotto-shot variations. The spectral data were averaged over several measurements. As the diffuse reflected light is rather weak, it was important to correct the contribution of sample emission. The transient absorption intensity for optically thick materials in diffuse reflectance laser photolysis was displayed as % absorption [4], defined as %abs.(l)=lOCl(l-
8))
(1)
where R and R,, represent the intensity of the diffuse reflected picosecond continuum with and without excitation, respectively. The origin of the time axis when exciting and monitoring pulses take the maximum temporal overlap was determined here by measuring the rise curve of the triplet absorption of benzophenone in the microcrystal. The zero time was set at the delay time when the %abs. is a half of the plateau value of the rise curve. In the previous paper, it was confirmed that the zero time of the benzophenone system was in agreement with that for the excited singlet pyrene in solution within experimental error [ 21. Transient absorption spectra in solution were measured by a conventional laser photolysis system with a repetitive mode-locked Nd3+ : YAG laser, the details of which have been described elsewhere [ 5 1. Emission spectra were measured by a laboratorymade spectrophotometer. A time-correlated single photon counting system with hydrogen discharge lamp was used to measure fluorescence decay time. Microcrystalline benzil (Tokyo Kasei GR grade) was recrystallized three times from ethanol, and pterphenyl (Dotite scintillation grade) was zone refined ( 100 pass). After purification, the samples were grounded in a mortar. Particle sizes ranged over several tens of microns. All experiments of the powder sample were performed under aerated conditions at room temperature, while the solution samples were bubbled with Nz gas.
23 September 1988
3. Results and discussion On the photophysics of benzil, it is known that dual fluorescence and phosphorescence is observed in glassy solution between 77 and 300 K, assigned to matrix-imposed near-skewed and trans-planar geometries [ 61. The terms “skewed” and “planar” used here refer to the dihedral angle between the planes containing the benzoyl groups. The geometry of the benzil ground state is reported to be skewed in fluid media (98” in benzene at 25°C [7]) and in the crystal ( 111’36’ [ 81). In this study, the emission spectrum of microcrystalline benzil at room temperature was observed at 525 nm, which is assigned to be phosphorescence of the skewed form, but no fluorescence (440 nm) is appreciable within the detection limit of our fluorometer. On the other hand, the emission spectrum of benzil in cyclohexane solution was observed at 500 and 565 nm, due to fluorescence and phosphorescence of the trans-planar form, respectively. These were in good agreement with the above reference data [ 61. The fluorescence lifetime of benzil in solution was determined here to be 2.2 ns by the single photon counting method. Figs. 1 and 2 show a series of corrected time-resolved absorption spectra of benzil in the microcrystal and in solution, respectively. Although the spectra in the microcrystal are broader and shifted compared to those in solution, a similar spectral change occurred but with a different time constant. The transient absorption at 500-5 10 nm of benzil in the microcrystal (fig. 1) agrees with the band obtained by nanosecond diffuse reflectance laser photolysis of the same sample [ 9,101 and nanosecond transmittance laser photolysis of its single crystal [ 111. The transient absorption at 490 nm of benzil in cyclohexane solution (fig. 2) also agrees with that obtained by the conventional flash photolysis [ 10,121. Those bands are already assigned to the T,tT1 transitions of benzil. A peak shift of 20 nm between triplet transitions in cyclohexane solution and in the microcrystal seems to originate from a difference in geometry of benzil in each phase. The other transient (525 nm) observed at an early delay time in solution can be assigned to the excited singlet state since its decay time agrees with the fluorescence lifetime. This is the first measurement of the S,+S, absorption spectrum of benzil in solution, as far as we 453
420
500
580 30
Wavelengt~/nm
Fig. 1. Transient abortion spectra of benzil micr~s~ls tained by the diffuse reflectance laser phot~lysis method.
ob-
know, Similarly, the transient at 560 nm observed in microc~sta~ine benzil can probably be assigned to the excited singlet state. The fact that the fluorescence in the microcrystal was very weak and not detectable, is consistent with a more rapid disappearance of this transient in the time-resolved absorption compared with the case of solution. The observed peak shift in the S,tS, absorption is also explained by geometrical difference. From these results, it is concluded that those fast and slow transients are due to the S1 and T1 states in each phase, respectively. It is well known that the intersystem crossing (EC) rate of some organic carbonyl compounds such as benzophenone and fluorenone is dependent on the solvent, which means that this process is sensitive to the surrounding environment [ 13- 15 1. However, in the case of benzophenone there was no appreciable difference of the triplet rise curve between solid and solution phases within experimental error [ 2 1. As can be seen from figs. 1 and 2, the ISC rate of benzil 454
23 September 1988
CHEMICAL PHYSICS LETTERS
Volume 150, number 5
500 600 Wavelength /w-n
700
Fig. 2. Transient ab~~tian spectra of benzil in ~y~l~hexane solution obtained by the transmittan~ laser photolysis meth~.
in the microcrystal is one order of ma~itude faster than in solution. This is probably due to some differences, not in en~ronm~nt~ effects, but in electronic structure. For example, a contribution of o-n interaction inducing the ISC process depends on the structure, skewed on planar. The conformational change in the excited singlet state of benzil in solution is also an interesting subject. Since the observed transient is assigned to be the excited singlet in the tram-planar form as described above and no appreciable spectral change was observed, the conformational relaxation of benzoyl group seems to be completed within the excitation pulse width. Fig. 3 shows a transient absorption spectrum at 200 ps after excitation of microcrystalline pterphenyl and its temporal dependence. This is assigned to be the S,,cS, absorption, since it is in agreement with the spectra obtained by transmittance laser photolysis of its single crystal [ 161 and in solution [ 17 ] except its broadening. The decay time of the excited singlet ab-
Volume 150, number 5
CHEMICAL PHYSICS LETTERS
23 September 1988
(a) 80 33ps ._
i
0 640
600
520 Wavelength
/ nm
01 450
550
500 Wavelength
600
/nm
Fig. 4. Transient absorption spectrum at 100 ps of sodium 1-pyrenylsulfonate included in P_cyclodextrin powder.
0
4
2
6
Time1 ns
Fig. 3. Transient‘absorption spectrum at 33 ps ofpterphenyl mi crocrystals (a) and its rise and decay curves observed at 560 nrr (b).
sorption in the microcrystal practically agreed with the fluorescence lifetime (2.4 ns) determined by single photon counting method. The triplet absorption was not observed even at several nanoseconds, although it can be observed in solution. In the crystal phase, a high density laser excitation will usually bring about an efficient singlet-singlet annihilation. Our success in measuring the excited singlet absorption seems to be due to a low quantum yield of excited singlet benzil and a small molar extinction coefficient of pterphenyl at 355 nm. Both conditions lead to a rather low exciton density. Actually, it is difficult to measure the Q-S, absorption spectra of aromatic molecular crystals with a large molar extinction coefficient at this excitation wavelength. Concerning this problem we examined the cyclodextrin inclusion complex. Fig. 4 shows a transient absorption spectrum at 100 ps of sodium I-pyrenylsulfonate included in p-cyclodextrin in powder form (the mole ratio of host to
guest is x 200). Since the fluorescence spectrum of this inclusion compound shows both monomer and excimer bands of pyrenyl group, the observed transient absorption should be due to both species (monomer and excimer). Actually, both bands were observed at around 490 nm [ 181. In the present case, the pyrene chromophores included are isolated from each other by the cyclodextrin cavity except for a small amount of the inclusion complex with two pyrenyl groups. Therefore, the annihilation of excited states is considered to be inefficient, so that the S,t S, spectrum was easily measured. For an application of the picosecond diffuse reflectance laser spectroscopy to opaque and optically thick samples, the following points should be considered. The time resolution of this method might be lower than that of the transmittance laser photolysis. This is affected by the scattering coefficient and absorption coefficient of the ground state, and of transients. We are examining this problem by computer simulation of model systems, by which the time resolution of the present method will be discussed shortly. In conclusion, we have performed, for the first time, a picosecond transient absorption spectral and kinetic measurement of singlet exciton states of organic microcrystals by the diffuse reflectance laser photolysis method.
Acknowledgement We thank Professor N. Mataga and Dr. H. Miyasaka of Osaka University for permitting the use 455
Volume 150, number 5
CHEMICAL PHYSICS LETTERS
of the picosecond transmittance laser photolysis system. We also acknowledge Dr. Y. Takagi and Dr. M. Sumitani for their technical help of YAG laser maintenance. This work was supported by the Joint Studies Program (1986-1987) of the Institute for Molecular Science and also partly by a Grant-in-Aid from the Japanese Ministry of Education, Science, and Culture to HM and NI (61470006,62612507).
References [ 1 ] F. Wilkinson, J. Chem. Sot. Faraday Trans. II 82 (1986) 2073; R.W. Kessler and F. Wilkinson, J. Chem. Sot. Faraday Trans I 77 ( 1981) 309, and references therein. [2] N. Ikeda, K. Imagi, H. Masuhara, N. Nakashima and K. Yoshihara, Chem. Phys. Letters 140 ( 1987) 28 1. [3] F. Wilkinson, C.J. Willsher, P.A. Lcicester, J.R.M. Barrand M.J.C. Smith, J. Chem.Soc.Chem.Commun. (1986) 1216. [4] R.W. Kessler, G. Krabichler, S. Uhl, D. Oelkrug, W.P. Hagan, J. Hyslop and F. Wilkinson, Opt. Acta 30 (1983) 1099. [5] H. Masuhara, N. Ikeda, H. Miyasaka and N. Mataga, J. Spectrosc. Sot. Japan 31 ( 1982) 19; H. Miyasaka, H. Masuhara and N. Mataga, Laser Chem. 1 (1983) 357.
456
23 September 1988
[6] T.-S. Fang and L.A. Singer, Chem. Phys. Letters 60 (1978) 117. [ 71 C.W.N. Cumper and A.P. Thurston, J. Chem. Sot. Perkin Trans. II (1972) 106. [ 81 C.J. Brown and R. Sadanaga, Acta Cryst. 18 ( 1958) 158. [ 91 F. Wilkinson and C.J. Willsher, Appl. Spectry. 38 (1984) 897. [ 101 N. Ikeda, K. Imagi, M. Hayabuchi and H. Masuhara, The 52th Annual Meeting of the Chemical Society ofJapan, Abstract I, (1986) 290; unpublished data. [ 111J.M. Morris and K. Yoshihara, Mol. Phys. 36 ( 1978) 993. [ 121 G. Porter and M.W. Windsor, Proc. Roy. Sot. A 245 (1958) 238: G. Krishna, J. Bhattacharya, J. Bandopadhyay and SC. Bcra, J. Photochem. Photobiol. 40 (1987) 47. [ 131 R.M. Hochstrasser, H. Lutz and G.W. Scott, Chem. Phys. Letters 24 (1974) 162; B.I. Greene, R.M. Hochstrasserand R.B. Weisman, J. Chem. Phys. 70 (1979) 1247. [ 141 T. Kobayashi and S. Nagakura, Chem. Phys. Letters 43 (1979) 429. [ 151 H. Miyasaka and N. Mataga, unpublished results (1987). [ 161 E. Morikawa, K. Shikichi, R. Katoh and M. Kotani, Chem. Phys. Letters 131 (1986) 209. [ 171 H. Miyasaka, Ph.D. Thesis, Osaka University ( 1985). [ 181 H. Masuhara, H. Shioyama, T. Saito, K. Hamada, S. YasoshimaandN. Mataga, J. Phys. Chem. 88 (1984) 5868.
CHEMICAL PHYSICS LETTERS
23 September 1988
PICOSECOND DYNAMICS OF EXCITED SINGLET STATES IN ORGANIC MICROCRYSTALS: DIFFUSE REFLECTANCE LASER PHOTOLYSIS STUDY
Noriaki IKEDA ‘, Masanori KOSHIOKA, Hiroshi MASUHARA ’ Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan
and Keitaro YOSHIHARA Institutefor Molecular Science, Myodaiji, Okuzaki 444, Japan Received 23 May 1988; in final form 29 June 1988
Absorption spectra and picosecond dynamics of the singlet exciton states of benzil and pterphenyl in a microcrystal have been measured for the first time by analyzing the diffuse reflected spectra of the picosecond continuum.
1. Introduction
The laser photolysis method is now recognized as an important technique to study photoprimary processes of a variety of molecular systems. This conventional technique has been restricted to transparent samples; however, a diffuse reflectance laser photolysis method has been developed in recent years [I]. This method is powerful because it gives absorption spectra of transient species in opaque and scattering materials such as organic microcrystals, semiconductor as well as insoluble polymer powders, dyed fabrics, molecules adsorbed on silica gels, etc. Although the photophysics of organic crystals has been extensively studied, many of those studies have been done mainly by emission techniques. Energy transfer and exciton dynamics are processes studied in detail so far in organic solids, While most attention has been paid to the behavior of single crystals, studies of molecules in various crystalline environments are also interesting. Recently, we have succeeded in measuring the ’ Present address: Chemistry Department, College of General Education, Osaka University, Toyonaka, Osaka 560, Japan.
* To whom correspondence should be addressed.
452
transient absorption spectrum and its dynamics in the picosecond time range, for benzophenone microcrystals [ 2 1. Wilkinson et al. also studied the picosecond dynamics of triplet species by the pump and probe method at a single wavelength [ 1,3]. In the application of this methodology, however, there was no report on the excited singlet state until now. From this view point we have examined various systems and measured for the first time the absorption spectra of excited singlet states (so-called exciton states) of benzil and p-terphenyl microcrystals. Their formation dynamics in the picosecond time domain and related problems will be discussed.
2. Experimental A microcomputer-controlled picosecond diffuse reflectance laser photolysis system with a repetitive mode-locked Nd3+ :YAG laser was used, in which a double-beam optical arrangement was adopted. The details have been described in a previous paper [ 2 1. The sample, contained in a suprasil cell of 2 mm thickness, was excited with a single 355 nm pulse ( 17 ps, 1 mJ), and monitored by using the picosecond continuum as a wide-band analyzing light. The pi-
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
CHEMICAL PHYSICS LETTERS
Volume 150.number5
cosecond diffuse reflected light from the sample was detected by a multichannel photodiode array (MCPD 1) through a polychromator. A part of the continuum beam was detected by another polychromator with MCPD2, and was used as the reference for a spectral shape of the analyzing light and shotto-shot variations. The spectral data were averaged over several measurements. As the diffuse reflected light is rather weak, it was important to correct the contribution of sample emission. The transient absorption intensity for optically thick materials in diffuse reflectance laser photolysis was displayed as % absorption [4], defined as %abs.(l)=lOCl(l-
8))
(1)
where R and R,, represent the intensity of the diffuse reflected picosecond continuum with and without excitation, respectively. The origin of the time axis when exciting and monitoring pulses take the maximum temporal overlap was determined here by measuring the rise curve of the triplet absorption of benzophenone in the microcrystal. The zero time was set at the delay time when the %abs. is a half of the plateau value of the rise curve. In the previous paper, it was confirmed that the zero time of the benzophenone system was in agreement with that for the excited singlet pyrene in solution within experimental error [ 21. Transient absorption spectra in solution were measured by a conventional laser photolysis system with a repetitive mode-locked Nd3+ : YAG laser, the details of which have been described elsewhere [ 5 1. Emission spectra were measured by a laboratorymade spectrophotometer. A time-correlated single photon counting system with hydrogen discharge lamp was used to measure fluorescence decay time. Microcrystalline benzil (Tokyo Kasei GR grade) was recrystallized three times from ethanol, and pterphenyl (Dotite scintillation grade) was zone refined ( 100 pass). After purification, the samples were grounded in a mortar. Particle sizes ranged over several tens of microns. All experiments of the powder sample were performed under aerated conditions at room temperature, while the solution samples were bubbled with Nz gas.
23 September 1988
3. Results and discussion On the photophysics of benzil, it is known that dual fluorescence and phosphorescence is observed in glassy solution between 77 and 300 K, assigned to matrix-imposed near-skewed and trans-planar geometries [ 61. The terms “skewed” and “planar” used here refer to the dihedral angle between the planes containing the benzoyl groups. The geometry of the benzil ground state is reported to be skewed in fluid media (98” in benzene at 25°C [7]) and in the crystal ( 111’36’ [ 81). In this study, the emission spectrum of microcrystalline benzil at room temperature was observed at 525 nm, which is assigned to be phosphorescence of the skewed form, but no fluorescence (440 nm) is appreciable within the detection limit of our fluorometer. On the other hand, the emission spectrum of benzil in cyclohexane solution was observed at 500 and 565 nm, due to fluorescence and phosphorescence of the trans-planar form, respectively. These were in good agreement with the above reference data [ 61. The fluorescence lifetime of benzil in solution was determined here to be 2.2 ns by the single photon counting method. Figs. 1 and 2 show a series of corrected time-resolved absorption spectra of benzil in the microcrystal and in solution, respectively. Although the spectra in the microcrystal are broader and shifted compared to those in solution, a similar spectral change occurred but with a different time constant. The transient absorption at 500-5 10 nm of benzil in the microcrystal (fig. 1) agrees with the band obtained by nanosecond diffuse reflectance laser photolysis of the same sample [ 9,101 and nanosecond transmittance laser photolysis of its single crystal [ 111. The transient absorption at 490 nm of benzil in cyclohexane solution (fig. 2) also agrees with that obtained by the conventional flash photolysis [ 10,121. Those bands are already assigned to the T,tT1 transitions of benzil. A peak shift of 20 nm between triplet transitions in cyclohexane solution and in the microcrystal seems to originate from a difference in geometry of benzil in each phase. The other transient (525 nm) observed at an early delay time in solution can be assigned to the excited singlet state since its decay time agrees with the fluorescence lifetime. This is the first measurement of the S,+S, absorption spectrum of benzil in solution, as far as we 453
420
500
580 30
Wavelengt~/nm
Fig. 1. Transient abortion spectra of benzil micr~s~ls tained by the diffuse reflectance laser phot~lysis method.
ob-
know, Similarly, the transient at 560 nm observed in microc~sta~ine benzil can probably be assigned to the excited singlet state. The fact that the fluorescence in the microcrystal was very weak and not detectable, is consistent with a more rapid disappearance of this transient in the time-resolved absorption compared with the case of solution. The observed peak shift in the S,tS, absorption is also explained by geometrical difference. From these results, it is concluded that those fast and slow transients are due to the S1 and T1 states in each phase, respectively. It is well known that the intersystem crossing (EC) rate of some organic carbonyl compounds such as benzophenone and fluorenone is dependent on the solvent, which means that this process is sensitive to the surrounding environment [ 13- 15 1. However, in the case of benzophenone there was no appreciable difference of the triplet rise curve between solid and solution phases within experimental error [ 2 1. As can be seen from figs. 1 and 2, the ISC rate of benzil 454
23 September 1988
CHEMICAL PHYSICS LETTERS
Volume 150, number 5
500 600 Wavelength /w-n
700
Fig. 2. Transient ab~~tian spectra of benzil in ~y~l~hexane solution obtained by the transmittan~ laser photolysis meth~.
in the microcrystal is one order of ma~itude faster than in solution. This is probably due to some differences, not in en~ronm~nt~ effects, but in electronic structure. For example, a contribution of o-n interaction inducing the ISC process depends on the structure, skewed on planar. The conformational change in the excited singlet state of benzil in solution is also an interesting subject. Since the observed transient is assigned to be the excited singlet in the tram-planar form as described above and no appreciable spectral change was observed, the conformational relaxation of benzoyl group seems to be completed within the excitation pulse width. Fig. 3 shows a transient absorption spectrum at 200 ps after excitation of microcrystalline pterphenyl and its temporal dependence. This is assigned to be the S,,cS, absorption, since it is in agreement with the spectra obtained by transmittance laser photolysis of its single crystal [ 161 and in solution [ 17 ] except its broadening. The decay time of the excited singlet ab-
Volume 150, number 5
CHEMICAL PHYSICS LETTERS
23 September 1988
(a) 80 33ps ._
i
0 640
600
520 Wavelength
/ nm
01 450
550
500 Wavelength
600
/nm
Fig. 4. Transient absorption spectrum at 100 ps of sodium 1-pyrenylsulfonate included in P_cyclodextrin powder.
0
4
2
6
Time1 ns
Fig. 3. Transient‘absorption spectrum at 33 ps ofpterphenyl mi crocrystals (a) and its rise and decay curves observed at 560 nrr (b).
sorption in the microcrystal practically agreed with the fluorescence lifetime (2.4 ns) determined by single photon counting method. The triplet absorption was not observed even at several nanoseconds, although it can be observed in solution. In the crystal phase, a high density laser excitation will usually bring about an efficient singlet-singlet annihilation. Our success in measuring the excited singlet absorption seems to be due to a low quantum yield of excited singlet benzil and a small molar extinction coefficient of pterphenyl at 355 nm. Both conditions lead to a rather low exciton density. Actually, it is difficult to measure the Q-S, absorption spectra of aromatic molecular crystals with a large molar extinction coefficient at this excitation wavelength. Concerning this problem we examined the cyclodextrin inclusion complex. Fig. 4 shows a transient absorption spectrum at 100 ps of sodium I-pyrenylsulfonate included in p-cyclodextrin in powder form (the mole ratio of host to
guest is x 200). Since the fluorescence spectrum of this inclusion compound shows both monomer and excimer bands of pyrenyl group, the observed transient absorption should be due to both species (monomer and excimer). Actually, both bands were observed at around 490 nm [ 181. In the present case, the pyrene chromophores included are isolated from each other by the cyclodextrin cavity except for a small amount of the inclusion complex with two pyrenyl groups. Therefore, the annihilation of excited states is considered to be inefficient, so that the S,t S, spectrum was easily measured. For an application of the picosecond diffuse reflectance laser spectroscopy to opaque and optically thick samples, the following points should be considered. The time resolution of this method might be lower than that of the transmittance laser photolysis. This is affected by the scattering coefficient and absorption coefficient of the ground state, and of transients. We are examining this problem by computer simulation of model systems, by which the time resolution of the present method will be discussed shortly. In conclusion, we have performed, for the first time, a picosecond transient absorption spectral and kinetic measurement of singlet exciton states of organic microcrystals by the diffuse reflectance laser photolysis method.
Acknowledgement We thank Professor N. Mataga and Dr. H. Miyasaka of Osaka University for permitting the use 455
Volume 150, number 5
CHEMICAL PHYSICS LETTERS
of the picosecond transmittance laser photolysis system. We also acknowledge Dr. Y. Takagi and Dr. M. Sumitani for their technical help of YAG laser maintenance. This work was supported by the Joint Studies Program (1986-1987) of the Institute for Molecular Science and also partly by a Grant-in-Aid from the Japanese Ministry of Education, Science, and Culture to HM and NI (61470006,62612507).
References [ 1 ] F. Wilkinson, J. Chem. Sot. Faraday Trans. II 82 (1986) 2073; R.W. Kessler and F. Wilkinson, J. Chem. Sot. Faraday Trans I 77 ( 1981) 309, and references therein. [2] N. Ikeda, K. Imagi, H. Masuhara, N. Nakashima and K. Yoshihara, Chem. Phys. Letters 140 ( 1987) 28 1. [3] F. Wilkinson, C.J. Willsher, P.A. Lcicester, J.R.M. Barrand M.J.C. Smith, J. Chem.Soc.Chem.Commun. (1986) 1216. [4] R.W. Kessler, G. Krabichler, S. Uhl, D. Oelkrug, W.P. Hagan, J. Hyslop and F. Wilkinson, Opt. Acta 30 (1983) 1099. [5] H. Masuhara, N. Ikeda, H. Miyasaka and N. Mataga, J. Spectrosc. Sot. Japan 31 ( 1982) 19; H. Miyasaka, H. Masuhara and N. Mataga, Laser Chem. 1 (1983) 357.
456
23 September 1988
[6] T.-S. Fang and L.A. Singer, Chem. Phys. Letters 60 (1978) 117. [ 71 C.W.N. Cumper and A.P. Thurston, J. Chem. Sot. Perkin Trans. II (1972) 106. [ 81 C.J. Brown and R. Sadanaga, Acta Cryst. 18 ( 1958) 158. [ 91 F. Wilkinson and C.J. Willsher, Appl. Spectry. 38 (1984) 897. [ 101 N. Ikeda, K. Imagi, M. Hayabuchi and H. Masuhara, The 52th Annual Meeting of the Chemical Society ofJapan, Abstract I, (1986) 290; unpublished data. [ 111J.M. Morris and K. Yoshihara, Mol. Phys. 36 ( 1978) 993. [ 121 G. Porter and M.W. Windsor, Proc. Roy. Sot. A 245 (1958) 238: G. Krishna, J. Bhattacharya, J. Bandopadhyay and SC. Bcra, J. Photochem. Photobiol. 40 (1987) 47. [ 131 R.M. Hochstrasser, H. Lutz and G.W. Scott, Chem. Phys. Letters 24 (1974) 162; B.I. Greene, R.M. Hochstrasserand R.B. Weisman, J. Chem. Phys. 70 (1979) 1247. [ 141 T. Kobayashi and S. Nagakura, Chem. Phys. Letters 43 (1979) 429. [ 151 H. Miyasaka and N. Mataga, unpublished results (1987). [ 161 E. Morikawa, K. Shikichi, R. Katoh and M. Kotani, Chem. Phys. Letters 131 (1986) 209. [ 171 H. Miyasaka, Ph.D. Thesis, Osaka University ( 1985). [ 181 H. Masuhara, H. Shioyama, T. Saito, K. Hamada, S. YasoshimaandN. Mataga, J. Phys. Chem. 88 (1984) 5868.