J. Phys. Chem. 1995, 99, 17082-17084
17082
Electron Spin Polarization Transfer from Excited Triplet Porphyrins to a Nitroxide Radical via a Spin Exchange Mechanism Jun-ichi Fujisawa, Kazuyuki Ishii, Yasunori Ohba, Masamoto Iwaizumi, and Seigo Yamauchi" Institute for Chemical Reaction Science, Tohoku University, Katahira 2-1-I, Aobaku, Sendai 980, Japan Received: July 27, 1995; In Final Form: October 4, 1995@
CIDEP mechanisms in systems of excited triplet tetraphenylporphyrins (MTPP; M = H2, Mg, Zn, Cd) and TEMPO were studied by time-resolved EPR. It was found that dominant polarizations generated on TEMPO are net emissions for H2TPP and MgTPP and net absorptions for ZnTPP and CdTPP, which are the same as the polarizations of the corresponding triplet (TI) porphyrins. The polarizations were diminished with a spin-lattice relaxation time of TEMPO. A polarized TI signal of ZnTPP was observed in paraffin solution and was quenched by TEMPO. These facts clearly indicate that these polarizations are due to an electron spin polarization transfer mechanism. This is the first definitive demonstration of ESPT in triplet-doublet systems.
Introduction Transient radicals often exhibit anormalous EPR polarizations called chemically induced dynamic electron polarization (CIDEP) in an initial stage of photochemical reactions. Recently, CIDEP effects have been observed in several systems of excited triplet (TI) molecules and stable radicals by means of timeresolved EPR (TREPR) spectroscopy.'-* To explain the polarization peculiar to these triplet-doublet (T-D) systems two mechanisms have been proposed; one is a radical triplet pair mechanism (RTPM), and the other is an electron spin polarization transfer (ESPT) mechanism. In the former, the polarization arises from the mixing between quartet and doublet spin states during encounters of triplets and radicals. There are two kinds of RTPMs that give rise to a net polarization and a multiplet p~larization.~-~ These are just in correspondence with those of the ST- and STo mechanisms, respectively, in doubletdoublet systems. The RTPM polarization is determined by the multiplicity of a precursory excited state (SI or T I )and the sign of an exchange parameter J between the triplet and the radical. On the other hand, in the case of ESFT,'.? it is considered that the polarization of the T I state due to a spin-selective intersystem crossing (isc) is transfered to a stable radical via an energy transfer or a spin exchange interaction. Therefore, in ESPT the polarization generated on the stable radical should be the same as that of the excited triplet state, being in contrast with that due to RTPM. There have been many reports concerning RTPM, which now seems to be established from both experiment and theory. To our knowledge, however, very few investigations have been reported, and unambiguous experimental evidence has not been given yet for ESPT. In this paper, we report the first definitive observation of ESPT generated in the T-D system. Tetraphenylporphyrins (MTPP; M = H2, Mg, Zn, Cd) and TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl, free radical) were selected as polarization donors and an acceptor, respectively, by two reasons; the first is that both the Si and T I states of the examined porphyrins are lower in energy than the lowest excited doublet state (D1) of TEMPO, where no energy transfer is expected to occur. The second is that the triplet polarization does easily vary by changing central metals or the hydrogen atoms with less
* To whom correspondence should be addressed. Abstract published in Advance ACS Absfmcrs, November 1, 1995.
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modification of a T I (nn*)character of TPP. It is well-known that the whole T I polarizations are reflected on polarities at H,i, (AMs = &2) fields in the T I TREPR spectra, which are emissive for H2TPP9.l0and MgTPP10.11,13 and absorptive for ZnTPP9-I3 and CdTPP.I2 In these systems the porphyrins are excited and the polarized TEMPO signals were observed by means of the TREPR technique. We also examined an observation of the transient T I signal of ZnTPP together with that of TEMPO in solution.
Experimental Section TREPR and steady-state EPR measurements were carried out at room temperature using a modified JEOL JES-FE2XG EPR spectrometer. TREPR spectra and time profiles of the signals were obtained by a NF BX-53 1 boxcar integrator and an Iwatsu DM-7200 digital memory, respectively. The details of the TREPR apparatus have been described previo~sly.'~ A Lambda Physik LPXlOOi excimer laser-pumped Lambda Physik LPD3OOO dye laser (A = 585 nm) was employed to excite the samples in the cavity. Only the porphyrin molecules are excited at this wavelength. H2TPP, MgTPP, ZnTPP, and CdTPP were synthesized according to the methods described in the literature^'^-^^ and were purified by recrystallization. TEMPO (Wako Pure Chemicals) was used as received. Spectrograde toluene (Wako Pure Chemicals) and liquid paraffin (Wako Pure Chemicals) were used as solvents without further purification. The concentrations of the porphyrins and TEMPO were 1 x and 5 x M in all the experiments, respectively. The solutions in quartz EPR tubes of 5 mm 0.d. were deaerated by repeated freeze-pump-thaw cycles on the vaccum line.
Results A steady-state EPR spectrum of TEMPO in toluene at room temperature was observed in the first derivative form with respect to the field (Figure la). The EPR spectrum is characterized by a triplet signal showing a hyperfine structure of one nitrogen (AN = 15.5 G, g = 2.0057). Figure lb,c showed the TREPR spectra observed at 0.1-0.3 and 2.0-2.2 ,us after the laser pulse, respectively, for the ZnTPP-TEMPO system at room temperature. The signal positions of these spectra were in agreement with those in the steady state EPR spectrum (Figure la), which indicates that the observed peaks are attributed to those of the polarized TEMPO radical. A time 0 1995 American Chemical Society
Letters
J. Phys. Chem., Vol. 99, No. 47, 1995 17083
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Figure 1. (a) Steady-state EPR spectrum of TEMPO and time-resolved EPR spectra observed in the ZnTPP-TEMPO system at (b) 0.1-0.3 ps and (c) 2.0-2.2 ps. These spectra were observed at room temperature in toluene.
A
Figure 3. Time profiles obtained at the central peak of the TEMPO signal for (a) the ZnTPP and (b) the HITPP-TEMPO systems in toluene.
TABLE 1: CIDEP Polarizations Observed for Triplet Porphyrins and TEMPO
t
polarizationsu H2TPP MgTPP ZnTPP CdTPP
first
second
Tlb
E E
E E E E
E E A A
A A
a E and A denote an emission and an absorption of the microwave. See refs 9-13.
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33806
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Figure 2. Time-resolved EPR spectra observed for the HITPP, MgTPP, and CdTPP-TEMPO systems at 0.1-0.3 ,us in toluene.
profile measured at the central peak was shown in Figure 2a. From the figure we find that the polarizations generated on TEMPO are composed of two parts; a polarity of the first polarization is absorption which is the same as that of TI ZnTPP. This polarization decays with t = 0.5 ps, being in agreement with the spin-lattice relaxation time of TEMP0.17318 The second polarization is emissive and decays slower with the decay time of ca. 18 ps at [TEMPO] = 5 mM. The time profile of the signal obtained in the HzTPPTEMPO system was shown in Figure 2b, where the first polarization is emissive this time and the second polarization is also emissive. Such behavior was realized in the TREPR spectra. In Figure 3 we summarized the TREPR spectra obtained for the HzTPP, MgTPP, and CdTPP-TEMPO systems at the earlier time (0.1-0.3 ps). These spectra reflect the first polarization in the systems; namely, absorptive for the CdTPP system and emissive for the HzTPP and MgTPP systems as summarized in Table 1. All these polarizations are fairly well in agreement with the TI polarizations of the corresponding porphyrins. From the analyses of the decay curves, it was found that these polarizations decay with the time constant o f t = 0.5 f 0.04 ps. For the second polarizations we observed emissive ones in all cases, where the decay times were much longer than those of the first polarizations.
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I u lOOG Figure 4. Time-resolved EPR spectra observed in liquid paraffin for M). the ZnTPP system (a) without and (c) with TEMPO (5 x The simulated spectrum (b) was obtained with the zfs parameters of D = 0.930 GHz and E = 0 GHz, and the isc ratio of Px:Py:Pz = 0:O:l. The insert shows the time profiles of T I polarizations of ZnTPP (a’) without and (c’) with TEMPO at the field positions indicated by the arrows in the spectra, (a) and (c), respectively.
We next carried out TREPR measurements in the ZnTPP system using liquid paraffin as a solvent of high viscosity. The TREPR spectra and the time profiles of the polarized signals of TI ZnTPP were observed as shown in Figure 4, where the spectrum of T I ZnTPP is observed together with the CIDEP of TEMPO. From the figure it was shown that the polarizations of T I ZnTPP and TEMPO are both absorptive. We also observed the decrease and the increase in the TREPR signals of T I ZnTPP and TEMPO, respectively, with increasing a concentration of TEMPO. Further, from the analyses of the
17084 J. Phys. Chem., Vol. 99, No. 47, 1995 decay curves (insert of Figure 4), the polarized signal of ZnTPP was found to be quenched by TEMPO, being the decay times as 0.28 i 0.02 ps (without TEMPO) and 0.21 i 0.02 ps (with TEMPO; 5 x M). These results clearly indicate that the first polarization of TEMPO is related to the T I polarization of ZnTPP.
Discussion The results for the first polarizations observed for TEMPO are summarized as the following: (1) Polarities of the polarizations were the same as those of the corresponding T I porphyrins. (2) The polarizations had the same decay rates as the spinlattice relaxation (0.5 ps) of TEMPO. (3) The polarization appeared together with the quenching of the polarized T I signal of prophyrin. It is known from pulsed EPR studiesZoon the electron-transfer reaction between ZnTPP and quinones that the spin-lattice relaxation time of T I ZnTPP is ca. 20 ns in ethanol, which is much shorter than that (0.5 ps) of TEMPO. Therefore, the polarization due to ESPT should decay with the spin-lattice relaxation time of TEMPO, in contrast to the case of RTPM whose polarization decays with the T I decay time. The of SI porphyrins are 0.065, 2.7, 9.2, and 13.6 ns for CdTPP, ZnTPP, MgTPP, and H2TPP, respectively, which denies a possibility that the first absorptive polarizations observed for the ZnTPP and CdTPP-TEMPO systems are due to RTPM with the SI Consequently all these results for the first polarizations directly lead us to the conclusion that these polarizations are due to electron spin polarization transfers (ESPT) from the polarized T I porphyrins. Two mechanisms, an energy-transfer mechanism (ETM) and a spin-exchange mechanism (SEM), have been proposed for ESPT-generated polarizations and are discussed minutely in ref 2. In our case as the acceptor, TEMPO has the higher excited energy (k 18 000 cm-’)23than those (11 000- 13 000 cm-1)22,23 of the T I porphyrins; the energy transfer does not occur. Therefore, the SEM is important for the ESPT between porphyrins and TEMPO. For the second polarization, we observed that the polarizations are emissive in all cases. The signals had decay times (&I) that depend on the triplet lifetimes (ST) of the porphyrins as 2.0 ps (kz-I) and 1.7 ,us (TT) for the ZnTPP-TEMPO system in paraffin solution (Figure 4). These facts apparently indicate that the polarizations are due to RTPM with the T I precursor. Our preliminary result that these polarizations increased with lowering temperature supports this conclusion. The reason ESPT was dominantly observed for the cases of porphyrins is not clearcut. However, we suggest that raveraging of the zero-field splitting of T I porphyrins by rotational motions is less sufficient in porphyrins than the other smaller molecules, which makes the polarizations larger and the spin-lattice relaxations slower in the T I states. These conditions are needed
Letters for the observations of ESPT as well as the triplet mechanism in doublet-doublet systems and were realized by the result that the TREPR spectrum of the T I porphyrin was observed in the paraffin solution.
Conclusion We have observed two kinds of CIDEPs in the triplet porphyrins-TEMPO systems. For the first polarizations we offer definitive evidence that these are due to the electron spin polarization transfer (ESPT) via a spin-exchange mechanism for the first time. The second polarizations observed as emissions were assigned as those due to the radical-triplet pair mechanism (RTPM) with the triplet precursors. Slow rotational motions of the porphyrins, which play an important role in ESPT, are indicated from the facts that the T I spectrum was observed in a paraffin solution and that ESPT dominated over RTPM . Acknowledgment. This work was supported by Grant-inAid for Scientific Research No. 4242102 and No. 07230204 from the Ministry of Education, Science and Culture, Japan. References and Notes (1) Imamura. T.; Onitsuka, 0.;Obi, K. J . Phys. Chem. 1986, 90.6741. (2) Jenks, W. S.; Turro, N. J. Res. Chem. Intermed. 1990. 13, 237. (3) Blattler. C.; Jent, F.; Paul, H. Chem. Phys. Letr. 1990, 166, 375. (4) Kawai. A.; Okutsu, T.; Obi, K. J . Phys. Chem. 1991. 95. 9130. ( 5 ) Kawai, A.: Obi, K. J . Phys. Chem. 1992. 96, 52. (6) Kawai, A.; Obi. K. Res. Chem. Inrermed. 1993, 19, 865. (7) Kobori, Y.; Kawai, A.: Obi, K. J . Phys. Chem. 1994, 98. 6425. (8) Turro, N. J.: Khudyakov, I. V.: Bossmann, S. H.; Dwyer, D. W. J. Phjs. Chem. 1993, 97, 1138. (9) Levanon. H. Rev. Chem. Intermed. 1987, 8. 287. (10) Levanon. H.; Norris. J. R. Chem. Reu. 1978. 78. 185. (11) Angerhofer, A.; Toporowicz, M.; Bowman, M. K.: Norris. J. R.; Levanon, H. J . Phys. Chem. 1988, 92, 7164. (12) Yamauchi. S.: Fujisawa. J., private communication. (13) Yamauchi. S.: Ueda, T.; Satoh, M.; Akiyama, K.; Tero-kubota. S.: Ikegami, Y.; Iwaizumi, M. J . Photochem. Phorobiol. A: Chem. 1992, 65, 177. (14) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher. J.: Assour. J.: Korsakoff. L. J . Or,?. Chem. 1967. 32. 476. (15) Douough. G. D.; Miller, J. R.;Huennenkens. F. M. J . Am. Chem. Soc. 1951, 73, 4315. (16) Baum. S. J.: Plane, R. A. J . Am. Chem. Soc. 1966, 88, 910. (17) Schwartz. R. N.; Jones, L. L.: Bowman, M. K. J . Phys. Chem. 1979, 83, 3429. (18) Turro. N. J.; Khudyakov, I. V.: Dwyer. D. W. J . Phys. Chrm. 1993. 97, 10530. (19) The observed slow component in Figure 4 is due to a thermalized triplet of ZnTPP. (20) Willigen, H. V.; Levstein, P. R.: Ebersole, M. H. Chem. Rev. 1993, 93. 173. (21) Harriman, A.; Porter, G.; Searle, N. J . Chem. Soc., Faraday Trans. 2 1975, 75, 1515. (22) Harriman, A. J . Chem. Soc., Faraday Trans. 2 1981, 77. 1281. (23) Kuzmin, V. A.; Tatikolov. A. S. Chem. Phys. Lett. 1978, 53, 606. (24) Harriman, A. J . Chem. Soc.. Faraday Trans. 2 1980, 76, 1978.
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