J. Phys. Chem. 1981, 85, 3957-3960
CuNa-Y proceeds also on Na-Y. The Cu(I1) ions introduced into zeolite framework would make it easy to generate the defects, and hence CuNa-Y has much more reactive oxygen than Na-Y. The mobility of protons in intrazeolitic water has so far been fairly well investigated, but that of oxygen has never
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been reported. The results in the present work would also be useful in a study on the properties of zeolitic water.
Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research, No. 56750569, from the Ministry of Education, Science and Culture of Japan.
Ultrafast Intersystem Crossing in Some Intramolecular Heteroexcimers Tadashl Okada,’ Ichiro Karakl, Eli1 Matsuzawa, Noboru Mataga,” Yoshiteru Sakata,t and Solchl Misumit Department of Chemisity, Faculty of Engineering Science, Osaka University, Toyonaka 560, Osaka Japan and The Instltute of Scleniific and Industrlal Research, Osaka University, Suiia, Osaka 565, Japan (Receive& September 30, 198 1)
Picosecond laser photolysis studies upon intramolecular heteroexcimer systems, (l-pyrenyl)-(CH.Jn-(amine), have been carried out uncovering the very fast generation of the triplet state localized in the pyrene moiety via the intramolecular heteroexcimer state. The intersystem crossing rate depends rather strongly upon the mutual configuration of donor and acceptor groups as well as the solvent polarity.
Introduction The generation of triplet states from the charge transfer (CT) excited state of EDA complexes is a well-known phen0mena.l It has been observed in some EDA complexes that intersystem crossing (isc) from the S1 (CT) state to the T1state of the donor or acceptor is enhanced by CT interaction even in the absence of heavy atom or paramagnetic species in the comp1ex.l The mechanism for the enhancement of isc in EDA complexes, however, is not yet completely elucidated. On the other hand, the charge-transfer quenching of fluorescence also frequently leads to efficient production of the T1state of the donor or acceptor. The problem of triplet formation in heteroexcimer systems has been a subject of recent lively investigations by means of the nanosecond laser photolysis methode2 There have been some controversies concerning the “fast” isc of the excited CT system.2 It has been suggested that, in some typical heteroexcimer systems, the crossing to the triplet precedes the generation of the relaxed heteroexcimer state in nonpolar solvent^.^ However, such a “fast” isc mechanism has been rejected on the basis of the results of picosecond and nanosecond laser photolysis studies in the case of anthracene- and pyrene-diethylaniline heteroexcimers in nonpolar solvent^.^ In polar solvents, however, where photodissociation into solvated ion radicals is predominant, relatively fast triplet generation within ca. 10 ns is possible owing to geminate re~ombination.~The formation of triplet geminate pairs from the singlet pair within ca. 10 ns seems to be caused by the hyperfine coupling mechanism between unpaired electron spins and nuclear spins of the radical pain6 This mechanism has been confirmed by the magnetic field effect upon triplet generation from the geminate ion pair.7 We have found by means of picosecond laser spectroscopy extremely fast triplet generations from the CT state of some intramolecular heteroexcimer systems as indicated in I-IV. Isc within 30 ps has been observed in the case of the most rapid generation. Results of these investigations will be described in the following together with those +The Institute of Scientific and Industrial Research.
of picosecond laser photolysis studies upon the relatively fast isc of some intermolecular heteroexcimer systems such as pyrene-secondary amine and pyrene-Dabco systems. We have obtained quite similar results also for the intramolecular heteroexcimer systems of anthracene and amine with structures analogous to I-IV, although these results are not included in the present report. Experimental Section Time-resolved absorbance spectra in the picosecond and nanosecond time regions were obtained with a mode-locked ruby laser system.8 A pulsed nitrogen gas laser photolysis systemgset up in this laboratory was used for measurement (1) McGlynn, S. P.; Azumi, T.;Kinoshita, M. “Molecular Spectroscopy of the Triplet State”;Prentice Hall: New York, 1969; pp 284-325. (2) Mataga, N.; Ottolenghi, M. ”Molecular Association Including Molecular Complex”;Foster, R., Ed.; Academic Press: New York, 1979; Vol. 11, pp 65-71. (3) Orbach, N.; Nomos, J.; Ottolenghi, M. J. Phys. Chern. 1973, 77, 2831-6. (4) Nishimura, T.;Nakashima, N.; Mataga, N. Chem. Phys. Lett. 1977, 46, 334-8. (5) Orbach, N.; Ottolenghi, M. “The Exciplex”;Gordon, M.; Ware, W. R., Ed.; Academic Press: New York, 1975; pp 75-111. (6) Brocklehurst, B. Chem. Phys. Lett. 1974, 28, 357-60. (7) Schulter, K.; Staerk, H.; Weller, A.; Werner, H.-J.;Nickel, B. 2. Phys. Chern. (Frankfurt am Main) 1976, 101, 371-90. (8) Okada, T.; Migita, M.; Mataga, N.; Sakata, Y.; Misumi, S.J . Am. Chem. SOC. In press. (9) Yasoshima, S.;Masuhara, H.; Mataga, N.; Suzaki, H.; Uchida, T.; Minami, S. J. Spectrosc. SOC.Jpn. 1981, 30, 98-100.
0022-3654/81/2085-3957$01.25/00 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 26, 1981
CH
H @cH2Nt9
@cH"03 1,
-33ps
500ps
1
300ps
I
400
500
00
500
Air
3
500
0
I
I
500
Flgure 1. Time-resolved absorbance spectra of some intramolecular heteroexcimer systems. Delay tlmes after excitation are denoted in the figures. The absorbance is not corrected for the shot-to-shot variations of the exciting laser power. Correction was made in the evaluation of the lifetime of the heteroexcimer or the rise time of the triplet.
of the absorbance of the triplet state in the microsecond region. The sample solutions were deaerated by freezepump-thaw cycles or deoxygenated by irrigating with a purified nitrogen gas stream. Measurements were carried out a t room temperature (26 f 1 "C).
Results and Discussion Time-resolved absorbance spectra of intramolecular heteroexcimer systems in ethyl ether solutions are indicated in Figure 1. The absorption band with maximum at 415 nm is due to the triplet state localized in the pyrene moiety and the spectra around 500 nm are due to the CT (heteroexcimer) state except for the bands a t 33 ps for I, 30 ps for 11, and -33 ps for I11 where the spectra are the superposition of S, SI band of the pyrene part and the absorption band of the CT state. One can see from Figure 1 that, with an increase of the delay time, the absorbance of the triplet state of the pyrene moiety increases in accordance with the decrease of the absorbance of the CT state and the rate of this time-dependent spectral change is considerably different from system to system. The reaction scheme relevant to the above spectral change may be written as
-
'(A*-D)
= XCT
1
-
( A -D+)
kT.
3 ) t
( A -D)
'(A-D)
where kCT, kT, k N ,and k F represent the rate constants of heteroexcimer formation, isc of heteroexcimer, and radiationless and radiative processes to the ground state, respectively. The value of kT was estimated by means of the following equations: T = ( k , + kN + kF)-' (2) &- = kTT where 4T is the quantum yield for triplet-state formation and T is the lifetime of the heteroexcimer, obtained from
the time dependence of the transient absorbance. The lifetimes of the heteroexcimers in ether were TI = 1 ns, ~~1 = 3.3 ns, = 170 ps, and ~ ' " 1 = 10 ns. The kT values obtained are given in Table I. One can see from the table that the compounds with one CH2 group where the two moieties are close but not parallel have much larger kT values compared to compounds with n = 2 or 3 where the formation of a sandwich-type heteroexcimer is possib1e.'*l2 For the intermolecular systems, Delouis et al. reported that the triplet state of pyrene was formed with a quantum yield near unity when the pyrene fluorescence was quenched by 1,4-diazabicyclo[2.2.2]octane(Dabco) in cyclohexane s01ution.l~ In relation to this result, however, we confirmed that, when pyrene fluorescence was quenched by N,N'-dimethylpiperazine (DMP), a strong heteroexcimer fluorescence was observed. We measured also the time-resolved absorbance spectra of these systems in benzene solutions. The triplet state of pyrene was produced from CT state with rise time of 2.3 ns for the pyrene-Dabco system, while no triplet state was detected at the delay time of a few nanoseconds in the pyrene-DMP system. It is well-known that the pyrene-N,N-dimethylaniline system in nonpolar solvents forms stable heteroexcimer while the heteroexcimers of pyrene-primary or secondary amine systems undergo hydrogen-atom transfer from amine to pyrene as well as relatively fast formation of the pyrene triplet state.I4 From the results of the deuteration (10) Masaki, S.; Okada, T.; Mataga, N.; Sakata, Y.; Misumi, S. Bull. Chem. SOC.Jpn. 1976,49, 1277-83. (11) Okada, T.; Saito, T.; Mataga, N.; Sakata, Y.; Misumi, S. Bull. Chem. SOC.Jpn. 1977,50, 331-6. (12) Migita, M.; Okada, T.;Mataga, N.; Sakata, Y.; Misumi, S.; Nakashima, N.; Yoshihara, K. Chem. Phys. Lett. 1980, 72, 229-32. Bull. Chem. SOC.Jpn. In press. (13) Delouis, J. F.; Delaire, J. A.; Ivanoff, N. Chem. Phys. Lett. 1979, 61, 343-6.
The Journal of Physical Chemistry, Vol. 85, No. 26, 1981 3959
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r-T lI
TABLE I: Intersystem Crossing Rate in Some Heteroexcimer Systems compd solvent kT1S-I I ethyl ether 8 x 108 111 ethyl ether 2.7 X 10' 112 ethyl ether < 4 x 10' 113 ethyl ether
