J. Phys. Chem. B 1997, 101, 10661-10665
10661
Effect of Polymethylene-Chain Dynamics on the Lifetime of a Charge-Separated Biradical Studied by RYDMR Spectroscopy Kentaro Enjo, Kiminori Maeda, Hisao Murai,* and Tohru Azumi Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-77, Japan
Yoshifumi Tanimoto Department of Chemistry, Faculty of Science, Hiroshima UniVersity, Hiroshima 730, Japan ReceiVed: June 2, 1997; In Final Form: October 13, 1997X
The novel solvent-viscosity dependence of the time-resolved reaction-yield-detected magnetic resonance (RYDMR) spectra is observed for the polymethylene-linked biradical generated in the photoinduced intramolecular electron-transfer reaction between N,N-dimethylaniline (DMA) and phenanthrene (Ph) in the compound of the type Ph-(CH2)10-O-(CH2)2-DMA. With decreasing the solvent viscosity, the line width and peak intensity of the RYDMR B0 (external magnetic field) spectrum increases and decreases, respectively. This result is interpreted in terms of the decrease of the lifetime of intermediate ionic biradicals at lower viscosity. This explicitly shows that the lifetime of the ionic biradical is governed by the reencounter frequency of the two terminal radicals determined by the polymethylene-chain dynamics. The lifetime of the spin states in the ionic biradical is estimated to be less than 2 ns.
Introduction The nature of polymethylene-linked biradicals generated in the photochemical reactions has recently attracted considerable attention. The dynamics (or lifetime) of biradicals is governed by factors such as spin dynamics, polymethylene-chain dynamics, and the reactivities of two terminal radicals.1 The lifetime of the polymethylene-linked neutral biradical (e.g. biradicals generated in the photocleavage of cycloalkanones) is governed by the rate of the recombination reaction from the singlet state because the triplet states (initial spin state of neutral biradicals) do not decay by the recombination process. In this viewpoint it is generally believed that the restriction of the escaping process by the polymethylene chain forces the lifetime of neutral biradicals long enough to develop the modulation of intersystem crossing (ISC) so that a significant magnetic field effect is observed.2 In a high magnetic field, the electronic Zeeman splitting of triplet sublevels makes both T+ and T- states energetically isolated from the singlet state and only the T0 state mixes with the singlet state. Therefore, the rate-determining step for the decay of neutral biradicals depends on the polymethylene-chain dynamics that determines the reencounter frequency of the two terminal radicals, S-T0 mixing process, and the relaxation from T( states to the T0 state.1-3 In contrast, in some ionic biradicals such as A--(CH2)n+ D generated in the photoinduced intramolecular electrontransfer reactions in polymethylene-linked donor-acceptor systems,1,4-9 a back electron transfer process takes place in both its singlet (initial spin state) and triplet manifolds. Intramolecular back electron transfer from the singlet state leads to the ground state by way of a partially radiative process, and that from the triplet state generates the triplet excited state of the chain molecule. Therefore, polymethylene-chain dynamics affects the lifetime of the ionic biradicals stated above differently. It is thus important to investigate how the polymethylenechain dynamics affects the lifetime of the ionic biradical. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, December 1, 1997.
S1089-5647(97)01785-9 CCC: $14.00
In the past two decades, experimental and theoretical investigations of the external magnetic field effects on the spin dynamics in the polymethylene-linked ionic biradicals of this type have been given by Weller, Staerk et al.,4-6 and Tanimoto et al.7-9 Although these studies provide a qualitative interpretation for the effect of polymethylene-chain dynamics on the value of the spin exchange interaction (Jeff), these data were not informative enough to clarify the effect of the polymethylenechain dynamics on the lifetime of the ionic biradical. Dynamics of biradicals might seem to be best analyzed by ESR spectroscopy. However, the ionic biradicals stated above are so shortlived that they are hard to detect by conventional ESR spectroscopy. Recently developed reaction-yield-detected magnetic resonance (RYDMR) spectroscopy10,11 is capable of observing ESR spectra of radical pairs that exist on a time scale of a few nanoseconds (e.g. reaction center of the photosynthetic bacteria or nonlinked donor-acceptor systems involving exciplexes). Further, the RYDMR spectrum gives detailed information on the spin multiplicity and the kinetic process of the radical pair. We, in the present work, apply the time-resolved RYDMR spectroscopy to the ionic biradical generated in the intramolecular electron-transfer reaction in polymethylene-linked phenanthrene (Ph)/N,N-dimethylaniline (DMA) system, Ph-(CH2)10O-(CH2)2-DMA. Here, the fluorescence from the exciplex created via the singlet state of the ionic biradical is monitored as a reaction yield. To understand the effect of polymethylenechain dynamics on the line shape of the RYDMR B0 (external magnetic field) spectrum and RYDMR B1 (microwave field) spectrum, we control the chain dynamics by changing the solvent viscosity. In this way, the relation between the polymethylene-chain dynamics (i.e. the reencounter frequency of two terminal radicals) and the lifetime of ionic biradical is studied. Experimental Section 2-[4-(Dimethylamino)phenyl]ethyl 10-(9-phenanthryl)decyl ether (abbreviated hereafter Ph-(CH2)10-O-(CH2)2-DMA) © 1997 American Chemical Society
10662 J. Phys. Chem. B, Vol. 101, No. 50, 1997
Enjo et al.
Figure 1. Experimental setup for the time-resolved RYDMR measurement.
was synthesized as reported previously.8,9 Acetonitrile (MeCN: η ) 0.38 cP, ) 37.5) and ethylene glycol (EtG: η ) 26.1 cP, ) 37.7) were used as received. The macroscopic viscosity of the solvent was controlled by changing the mixing ratio of EtG and MeCN with almost the same bulk dielectric properties. The sample solutions (1 × 10-4 mol dm-3) were deoxygenated by the nitrogen-bubbling method and flowed through a gap between two optical pipes in a quartz tube installed in an X-band ESR cavity (TE001 cylindrical mode). All the experiments were performed at room temperature (∼20 degree °C). The schematic diagram of the experimental setup for the timeresolved RYDMR measurement is shown in Figure 1. An Nd: YAG laser (Spectra Physics GCR-150, fwhm of ca. 5 ns and wavelength of 266 nm) with a repetition rate of 10 Hz and an output of ca. 20 mJ was used as an excitation source. Exciplex fluorescence (λ > 450 nm) was detected by a photomultiplier tube (Hamamatsu R928). A UV cutoff filter (Toshiba Y-450) was installed to remove the short-wavelength fluorescence (λ < 450 nm) which is attributed to locally excited states of Ph and DMA. The conceptual diagram for the timing between laser flashes and microwave pulses is expressed in Figure 2. The microwave generated by a Gunn diode was pulsed by a PIN-switch and then amplified by a TWTA (Applied System 127X, maximum output of 2 kW) and finally introduced to the cavity on alternate laser flashes. The PIN-switch was turned on 500 ns before the laser flash, and the duration of the microwave pulse was 1.5 µs. Therefore, the laser excitation and the observation were performed during the microwave duration. The time traces of the exciplex fluorescence in the presence and absence of microwave irradiation were accumulated separately using a digital oscilloscope (LeCroy 9350) to improve the signal-tonoise ratio of the measurement. The intensity of the RYDMR signal (field effect, FE) is defined as
FE ) (Ion - Ioff)/Ioff
(1)
where Ion and Ioff denote the exciplex fluorescence intensity in the presence and absence of microwave irradiation, respectively (see Figure 2). The microwave field amplitude (B1) was estimated in a manner described in a previous paper.12 Two kinds of RYDMR spectra were investigated in this report. One is the B0 spectrum, which is obtained by plotting field effects as a function of the external magnetic field (B0) while keeping B1 constant. The other is the B1 spectrum. This
Figure 2. Diagrammatic representation for the timing between laser flashes and microwave pulses. The time traces shown in the lower part are the observed exciplex fluorescence in the presence (Ion) and absence (Ioff) of microwave irradiation and the RYDMR signal (Ion - Ioff), respectively.
SCHEME 1. Reaction Pathways for Photoinduced Intramolecular Electron Transfer Reaction in Ph-(CH2)10-O-(CH2)2-DMA System
is obtained by monitoring the intensity of the peak position in the B0 spectrum (B0 ≈ 343 mT) with varying the power of microwave. Actually the B1 spectrum means the B1 dependence of the field effect at exact resonance of the B0 spectrum.11 Results and Discussion Scheme 1 shows primary processes in the photoinduced reaction of the Ph-(CH2)10-O-(CH2)2-DMA system in polar solvents.9 After excitation of the acceptor molecule Ph in the system, an end-to-end electron-transfer reaction from DMA to
Polymethylene-Chain Dynamics
J. Phys. Chem. B, Vol. 101, No. 50, 1997 10663
Figure 4. RYDMR B0 spectra obtained for the Ph-(CH2)10-O(CH2)2-DMA system in a solution of 5:1 mixture of EtG and MeCN with different B1 field, 9 and 6 mT. Stick diagram in the figure shows the hyperfine structure of the DMA cation.
Figure 3. (a) Time-resolved RYDMR B0 spectra obtained for the Ph(CH2)10-O-(CH2)2-DMA system in a solution of 5:1 mixture of EtG and MeCN at B1 ) 9 mT. Delay time of sampling for each of the observed spectrum is shown. (b) Normalized RYDMR B0 spectra for each delay time of sampling. 1Ph*
(the excited singlet state of Ph) occurs, which leads to the formation of the singlet state of the ionic biradical. The intramolecular electron-transfer reaction from 1DMA* to Ph also takes place, but the excitation of Ph is dominant in the applied excitation wavelength (λ ) 266 nm). Further, the contribution of 1DMA* is insignificant because the lifetime of 1DMA* in the absence of Ph (∼3 ns) is much shorter than that of 1Ph* (∼50 ns).8,9 The intermolecular electron-transfer reaction is not likely to occur in the time region of the observation in view of the low concentration of the compound in the solution. The exciplex is generated directly from the locally excited singlet state of the chain molecule or formed indirectly via the singlet biradical. The formation of the triplet biradical from the singlet one is brought about by the singlet-triplet spin conversion (ISC). This process competes with the formation of the exciplex from the singlet biradical. The back electron transfer from the triplet biradical populates the lowest excited triplet state of Ph (3Ph*).8,9 The rate of ISC can be modulated by the microwave field (B1) under the resonance conditions of the biradical. The intensity of the exciplex fluorescence is directly influenced by the modulation of ISC because the emitting exciplex is predominantly generated from the singlet biradical in highly polar solvents (e.g. MeCN).4,9 Time-resolved B0 spectra obtained for the Ph-(CH2)10-O(CH2)2-DMA system in a solution of a 5:1 mixture of EtG and MeCN are shown in Figure 3a. As shown in Figure 2, a positive field effect (Ion - Ioff > 0) gradually appears after the
Figure 5. Dependence of the RYDMR B0 spectrum on the solvent viscosity observed in the Ph-(CH2)10-O-(CH2)2-DMA system. Mixtures of EtG and MeCN with different volume per volume ratio were used as solvents.
appearance of the exciplex fluorescence. Positive field effects, which correspond to the increase of the population of the singlet biradical, indicate the deceleration of singlet-triplet spin conversion by microwave irradiation under resonance conditions.10-12 This phenomenon is called “spin locking”.13 Figure 3b demonstrates that the line width of the observed spectrum is almost independent of the delay time of sampling (fwhm is about 18 mT). Figure 4 gives B0 spectra obtained for the Ph-(CH2)10-O(CH2)2-DMA system with a different B1 field, 9 and 6 mT. It is seen that both the intensity of the B0 spectrum and the line width drastically decrease as the applied B1 decreases. In addition, their line widths are larger than the total width of the hyperfine structure of the present system, the DMA cation and the Ph anion. The total widths of the hyperfine structure of the DMA cation and Ph anion are 9.20 and 2.22 mT, respectively (only the stick diagram of the DMA cation is shown in the figure). These results indicate that the observed spectra exhibit power broadening due to high B1 field strength. We could not obtain the B0 spectrum in good signal-to-noise ratio under the B1 field strength less than 3 mT. The B0 spectra observed for the Ph-(CH2)10-O-(CH2)2DMA system in solvents of different viscosity are shown in
10664 J. Phys. Chem. B, Vol. 101, No. 50, 1997
Enjo et al.
Figure 6. RYDMR B0 spectra calculated from eqs 2 (B1 ) 9 mT) with different values of κ
Figure 5. This change of the solvent viscosity should modulate the reencounter frequency of two terminal radicals. The peak intensity of the observed B0 spectrum decreases with decreasing the solvent viscosity, whereas the line width of the B0 spectrum increases. To interpret the origins of the observed solvent-viscosity dependence, we simulate the B0 spectrum and B1 spectrum with the exponential model reported by M. E. Michel-Beyerle et al.14,15 In this model the ionic biradical is created at t ) 0 in the singlet state, and the spin-selective recombination processes follow. The calculated FE is given by
FE(B0,B1) )
( ){
2 1 B1
2B 2 eff
∑
(2J)2 + κ2
λ)(1(2J
+ λgµBB1)2 + κ2
}
-2
(2)
where
Beff ) xB12 + (B0 - pω/gµB)2 1 κ ) (kS + kT) 2 In this solution the isotropic hyperfine interaction is treated as a small perturbation of the system. Beff is the net magnetic field in the rotating frame. µB is the Bohr magneton. ks () ksg + kse) and kT are the recombination rate constants from the singlet and triplet states in the biradical, respectively. ks is composed of the rate constants of the charge recombination to the ground state (ksg) and the formation of the singlet exciplex (kse). κ shows the damping rate of total spin states in the biradical under the following conditions.
(kS + kT)(FSS +
∑
i)+1,0,-1
(kS - kT)(FSS -
∑
i)+1,0,-1
FTiTi) .1
(3)
FTiTi)
where FSS and FTT show the populations in the singlet and triplet manifolds of the spin state, respectively. Here, we call the value of κ-1 the lifetime of the spin states in the ionic biradical. The line shapes of the B0 spectrum and B1 spectrum are both severely affected by the relative magnitudes of κ and |2J|.15 Calculations that demonstrate the effect of κ -1 on the line shape of the B0 spectrum are presented in Figure 6a. As κ -1 decreases, the peak intensity and line width of the calculated B0 spectrum decreases and increases, respectively. These results provide an interpretation of the origin of the observed solventviscosity dependence as follows: Polymethylene-chain motion
-1
(|2J| ) 2 mT) (a) and |2J| (κ
-1
) 2 ns) (b).
is quickened due to the decrease of solvent viscosity, which causes the increase of the reencounter frequency of the two terminal radicals. Consequently, the lifetime of the spin states in the ionic biradical, which recombines regardless of its spin multiplicity, decreases. We thus conclude that the lifetime of the ionic biradical is governed by the reencounter frequency of the two terminal radicals determined by the polymethylene-chain dynamics. The analysis of the line shape of B0 spectra becomes quite complicated when several parameters take values of about the same order of magnitude.15 Under the present experimental conditions, however, the absolute value of the spin exchange interaction |2J| is much smaller than κ and B1. This implies that the line shape of the B0 spectrum is insensitive to |2J|. Therefore, the correction of |2J| does not influence the above discussion except for the slight change of the peak intensity of the B0 spectrum (see Figure 6b). To obtain further experimental support for confirming that the observed solvent-viscosity dependence is explained due to the change of κ -1, we measure the solvent-viscosity dependence of the B1 spectrum. Experimental and calculated solventviscosity dependences of the B1 spectrum are shown in Figure 7. The calculated B1 spectrum also provides good agreement with the experimental one. In the case of a highly viscous solvent (EtG: MeCN ) 5:1), the sign of the field effects in the low B1 field (B1 < 2 mT) is negative, and then it changes to positive as the B1 field becomes larger than 2 mT, as shown in Figure 7a. This provides an “S-shaped” curve on the B1 field. The negative field effect (Ion - Ioff < 0) is due to the acceleration of ISC by the transition between the triplet sublevels.10-12 On the other hand, the curves of B1 spectra observed in solvents that are less viscous than EtG:MeCN ) 5:1 show no negative field effect. These line shapes indicate that there is almost no acceleration of ISC (no negative field effect) for the region of low B1 field, and only the deceleration (positive field effect) at high B1 field is observed (only the “spin locking” could be observed). The calculated B1 spectrum in Figure 7b shows that the decrease of κ -1 causes the decrease of the intensity of field effects at the overall B1 field. The intensity of negative field effects is especially sensitive to the decrease of κ -1, and the line at low B1 field becomes positive as κ -1 changes from 2 to 1 ns. This shows that the line shape of the B1 spectrum loses the S-shape if κ -1 decreases. These results and the above analysis are also consistent with the conclusion that the lifetime of the ionic biradical is governed by the reencounter frequency of the two terminal radicals determined by the polymethylenechain dynamics. Moreover, we recognize that the lifetime of the spin states in the ionic biradical is less than 2 ns under the
Polymethylene-Chain Dynamics
J. Phys. Chem. B, Vol. 101, No. 50, 1997 10665 polymethylene-linked ionic biradical and is successfully simulated with the exponential model reported by M. E. MichelBeyerle et al. The observed biradical is generated by the photoinduced intramolecular electron-transfer reaction of the system Ph-(CH2)10-O-(CH2)2-DMA. The theoretical treatment of the results explicitly shows that the lifetime of the spin states in the ionic biradical is governed by the reencounter frequency of the two terminal radicals determined by the polymethylene-chain dynamics. This is because the back electron transfer of the ionic biradical takes place in the present system regardless of its spin multiplicity, unlike neutral biradicals. Furthermore, the polymethylene-chain dynamics induces the back-and-forth reactions of the singlet biradical to the exciplex. That allows the long-time observation of the ionic biradical. Acknowledgment. The authors would like to express their grateful acknowledgment to Professor V. F. Tarasov for stimulating discussions. This research was supported by a Grantin-Aid for Development Scientific Research (No. 07554064 and 08740437) and a Grant-in-Aid on Priority-Area-Research on “Photoreaction Dynamics” (No. 07228206 and 07228241) from the Ministry of Education, Science, and Culture of Japan. References and Notes
Figure 7. Experimental (a) and calculated (b) solvent-viscosity dependence of the RYDMR B1 spectrum. The parameters are the same as used in the calculation of the RYDMR B0 spectrum (see Figure 6a).
present experimental conditions. This conclusion does not contradict the observation of the long-lived RYDMR signal (Ion - Ioff) shown in Figure 2 because the generation of the singlet biradical from the exciplex (kes) takes place9 (the fluorescence lifetime of the exciplex is ca. 70 ns at 343 mT). Back-andforth reactions of the singlet biradical to the exciplex induced by the polymethylene-chain dynamics make the apparent lifetime of the ionic biradical (lifetime of the observed RYDMR signal) longer, although the lifetime of the spin states in the biradical is estimated to be less than 2 ns. Conclusion Solvent-viscosity dependence of both the RYDMR B0 spectrum and the RYDMR B1 spectrum is obtained from the
(1) Steiner, U. E.; Ulrich, T. Chem. ReV. (Washington, D.C.) 1989, 89, 51. (2) Tanimoto, Y.; Takashima, M.; Hasegawa, K.; Itoh, M. Chem. Phys. Lett. 1987, 137, 330. (3) Zimmt, M. B.; Doubleday, C., Jr.; Turro N. J. J. Am. Chem. Soc. 1986, 108, 3618. (4) Staerk, H.; Ku¨hnle, W.; Treichel, R.; Weller, A. Chem. Phys. Lett. 1985, 118, 19. (5) Busmann, H.-G.; Staerk, H.; Weller, A. J. Chem. Phys. 1989, 91, 4098. (6) Staerk, H.; Busmann, H.-G.; Ku¨hnle, W.; Treichel, R. J. Phys. Chem. 1991, 95, 1906. (7) Tanimoto, Y.; Okada, N.; Itoh, M.; Iwai, K.; Sugiura, K.; Takemura, F.; Nakagaki, R.; Nagakura, S. Chem. Phys. Lett. 1987, 136, 42. (8) Tanimoto, Y.; Hasegawa, K.; Okada, N.; Itoh, M.; Iwai, K.; Sugiura, K.; Takemura, F.; Nakagaki, R.; Nagakura, S. J. Phys. Chem. 1989, 93, 3586. (9) Cao, H.; Fujiwara, T.; Haino, T.; Fukazawa, Y.; Tung, C.-H.; Tanimoto, Y. Bull. Chem. Soc. Jpn. 1996, 69, 1. (10) Wasielewski, M. R.; Norris, J. R.; Bowman, M. K. J. Chem. Soc., Faraday Discuss. 1984, 78, 279. (11) (a) Batchelor, S. N.; McLauchlan, K. A.; Shkrob, I. A. Mol. Phys. 1992, 75, 501. (b) Batchelor, S. N.; McLauchlan, K. A.; Shkrob, I. A. Mol. Phys. 1992, 75, 531. (12) Maeda, K.; Araki, Y.; Kamata, Y.; Enjo, K.; Murai, H.; Azumi, T. Chem. Phys. Lett. 1996, 262, 110. (13) Koptyug, A. V.; Saik, V. O.; Anisimov, O. A.; Molin, Yu. N. Chem. Phys. 1989, 138, 173. (14) Lersch, W.; Michel-Beyerle, M. E. Chem. Phys. 1983, 78, 115. (15) Lersch, W.; Michel-Beyerle, M. E. Chem. Phys. Lett. 1987, 136, 346.
