The Journal of
Physical Chemistry ~~~
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VOLUME 98, NUMBER 26, JUNE 30,1994
0 Copyright 1994 by the American Chemical Society
ARTICLES Direct Observation of CIDEP Generated through Enhanced Intersystem Crossing Yasuhiro Kobori, Akio Kawai, and Kinichi Obi' Department of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo 152, Japan Received: February 22, 1994'
The concentration dependence of free radicals (TEMPO) of enhanced absorptive chemically induced dynamic electron polarization (CIDEP) spectra was studied quantitatively to clarify the mechanism of CIDEP generation in excited singlet stateradical systems. It was experimentally demonstrated that the CIDEP intensity on the free radical was in proportion to the concentration of the spin-polarized free radical. The rate constant of the CIDEP generation was determined to be (6.3 f 0.2) X lo9 M-l s-l from the Stern-Volmer plots of CIDEP intensity in the coronene-TEMPO system in benzene. The absorptive CIDEP spectra were attributed to the free radicals which enhanced the SI-Tl intersystem crossing and were interpreted by introducing the stochasticLiouville equation into the radical-triplet pair mechanism with doublet precursor.
Introduction The quenching of excited states by molecules has been extensively investigated on many photochemical processes, such as energy transfer, triplet-triplet annihilation, quenching by radicals, and so on.' For example, the quenching of the lowest excited singlet states by free radicals has been discussed on the basis of the spin conservation rule and is interpreted in terms of the enhanced intersystem crossing (EISC).* The spin selectivity of this process leads us to observe the chemically induced dynamic electron polarization (CIDEP) through EISC. Recent experiments show that CIDEP is generated through interactions between excited molecules and free radicals. This phenomenon is explained by magnetic interaction acting on the potential surface of spin states of radical-triplet (RT) pairsH and is interpreted in terms of the radical-triplet pair mechanism (RTPM). According to RTPM, two types of CIDEP are generated on free radicals; one is the net emission (E) with an E / A (emission/absorption) pattern ( E E / A pattern) attributed to being thequartet precursor RTPM (QP-RTPM), and the other is an A + A / E pattern, the doublet precursor RTPM (DP-RTPM).5 In QP-RTPM, triplet molecules are quenched through the doublet spin states of RT pairs. During the triplet-
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* Abstract published in Aduance ACS Abstracts, June 1,
1994.
0022-3654/94/2098-6425%04.50/0
doublet interaction, the quartet and doublet spin states mix with each other by zero-field splitting (zfs) and hyperfine interactions. Thus, the quenching of the doublet pair induces an E E / A type of CIDEP. On the other hand, in DP-RTPM, EISC caused by radicals selectively yields the doublet spin states of RT pairs, and hence one observes the A A / E type of CIDEP which is opposite in sign to that of QP-RTPM. This conclusion was deduced from the qualitative comparison of CIDEP signals between those with and without triplet quenchers which removed the CIDEP signals due to QP-RTPM. Thus far, the mechanism of CIDEP generation has mainly dealt with radicals formed in photoreactions of diamagnetic molecules. CIDEP generation has been mostly interpreted in terms of the triplet mechanism" (TM) and radical pair mechanism1* (RPM), which give information on the spin multiplicity of the precursors of produced radicals. On the other hand, RTPM, recently proposed in the excited molecule-radical systems, deals with the dynamics of photoreactions of paramagnetic species, because quartet and doublet pairs generated in the excited molecule-radical systems are prototypesof the precursors in paramagnetic molecular systems. The interaction between excited molecules and radicals is often observed as EISC or triplet quenching with radicals in conventional photochemical systems. To obtain a deeper understanding of RTPM, it is desirable to
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0 1994 American Chemical Society
Kobori et al.
6426 The Journal of Physical Chemistry, Vol. 98, No. 26, 19#94 a' C.W. 3
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Figure 1. ESR spectra of TEMPO in the coronene -TEMPO system in benzene: (a) continuous wave ESR spectrum of TEMPO (2.1 mM); (b, d) time-resolved ESR spectra measured at 0.6 fl after 308-nm laser excitation. The concentration of coronene is 0.16 mM, and that of TEMPO is (b) 2.1, (c) 1.2, and (d) 0.7 mM. For each measurement, tram-stilbene (1.1 mM) was added as a triplet quencher. promote thequantitativestudy of CIDEP generation in the excited molecule-radical systems. In this paper, we report the dependence of the A + A / E type CIDEP intensity on free radical concentration in the system of coronene-TEMPO (2,2,6,64etramethyl- 1-piperidinyloxyl). We applied rate equations to analyze the generation of spin polarized free radicals which quenched the lowest excited singlet molecules. Using these equations, we explained the experimental results and obtained the rate constant (k,) for the CIDEP generation. As this rate constant was almost the same with that for EISC, we Concluded that this A A / E type of CIDEP was generated through EISC. The enhanced absorption of CIDEP in the excited singlet molecubradical systems was interpreted by introducing the stochastic-Liouville model into DP-RTPM.
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Experimental Section A conventional X-band ESR spectrometer (Varian E-1 12) was used to measure TR-ESR spectra. Transient ESR signals obtained without field modulation were transferred to a boxcar integrator (Stanford SR-250) for spectrum measurements or a transient memory (Iwatsu DM-901) for CIDEP decay profiles. The gate time of the boxcar integrator was opened for 0.5 ps after 0.6 p s from the laser pulse. The excitation light source was a XeCl excimer laser (Lambda Physiks LPX 100). Details of the equipment and method were described previou~ly.~Coronene (Tokyo Kasei) and trans-stilbene (Tokyo Kasei) were recrystallized from benzeneand n-hexane,respectively. TEMPO (Aldrich) was used as received. GR grade benzene (Tokyo Kasei) was used as a solvent without further purification. The solution was degassed by bubbling nitrogen gas and flowed through a quartz flat cell (0.3-mm interior space) in an ESR cavity. The lifetime of the lowest excited singlet state of coronene was obtained from the fluorescence decay measured in the same flow system as the ESR measurements. The fluorescence light passed through a monochromator (Nikon P250) was detected by a photomultiplier tube (Hamamatsu R928). Results 1. Radical Concentration Dependence of an A + A / E Type CIDEP. Figure 1 shows the radical (TEMPO) concentration effect on CIDEP spectra obtained in the system of coronene TEMPO in benzene by 308-nm laser excitation under the same
Figure 2. Plots of the peak heights of continuous wave ESR spectra against the concentrations of TEMPO in benzene.
experimental conditions. trans-Stilbene was added as a triplet quencher to eliminate the CIDEP signals of the E E / A type due to QP-RTPM in the coroneneTEMPO system. The top of the spectra shows a continuous wave (CW) ESR spectrum at a radical concentration of 2.1 mM. Microwave power was fixed at 50 mW in each measurement. Hyperfine peaks in TR-ESR spectra appeared at the same positions with those of TEMPO in the CW ESR spectrum. Hence the signals in CIDEP spectra are assigned to spin-polarized TEMPO radicals. CIDEP patterns of these spectra are almost the same, intense net absorptive spin polarization with a weak A / E hyperfine dependent one. 'This indicatesthat CIDEP is generated through the interaction between radicals and the lowest excited singlet molecules, as is discussed in the previous study.5 It is remarkable that the CIDEP intensity depends on the radical concentration; the CIDEP intensity becomes stronger with an increase in the radical Concentration. The increase in CIDEP intensity is thought to reflect the increase in the concentration of the spin-polarized free radical which has quenched the first excited singlet molecules. However, other factors to control the CIDEP intensities must be examined. The first is the state mixing in the RT pair. In the previous studies,4.5 it was explained that the spin polarization through RTPM was caused by the mixing of doublet (ID)) and quartet (IQ)) spin states of the RT pair due to the crossing of the potential curves. The high probability of state mixing makes both net and multiplet electron spin polarizations strong. Shushin6 proposed another mechanism of net CIDEP generation; the spin-lattice relaxation in RT pairs would induce the state mixing of ID) and IQ). In both mechanisms, the probability of state mixing is controlled by the magnitude of zfs interaction and diffusion coefficient. The CIDEP spectra in Figure 1 were measured by changing only the radical concentration, which did not affect zfs interaction and diffusion. The state mixing does not, therefore, contribute to the change of the CIDEP signal intensity. The second is the spin-spin interaction. The spin-spin relaxation is affected by the spin-spin interaction between radicalsa8 At a high concentration of radicals, this interaction shortens the spin-spin relaxation time ( Tz) and broadens the ESR spectrum. Figure 2 shows the plots of peak heights of CW ESR spectra of TEMPO against its concentration. The peak height rises in proportion to the concentration of TEMPO in this concentration range (0.7 mM) than [Sllf=0(
