J. Phys. Chem. 1995, 99, 102-104
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CIDNP from Photogenerated Geminate Radical Ion Pairs Hidden in Triplet-State Products Erik Schaffner and Hams Fischer" Physikalisch-Chemisches Znstitut der Universitat Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland Received: July 6, 1994@
Radical ion pairs generated by photoinduced electron transfer from Nfl-dimethyl- 1-naphthylamine to benzonitrile in acetonitrile solutions undergo reverse electron transfer to the singlet ground state and to the donor excited triplet state of the parent compounds. Both pathways lead to chemically induced nuclear polarization, but on a short time scale the triplet contribution is hidden in the triplet donor molecules. This leads to unusual CIDNP phases and time dependencies which are unravelled by FT NMR measurements and auxillary optical studies.
Introduction
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In highly polar media many cyclic photochemical electrontransfer reactions obey the mechanism shown in Figure 1.' Singlet excited donors D are quenched by ground-state acceptors A at or close to diffusion-controlled rates to generate solventseparated geminate singlet radical ion pairs. Driven by magnetic interactions of the radical ion electron and nuclear spins, these undergo intersystem crossing to the isoenergetic triplet state. On a nanosecond time scale diffusive reencounters allow reverse electron transfer. Singlet pairs regenerate the ground state, while from triplet pairs excited triplet donors result. Free ions are formed from pairs which escape the geminate processes and undergo the reverse electron transfer upon diffusive encounters in a much longer time scale of micro- to milliseconds. The spin-dependent intersystem crossing is the basis for magnetokinetic effects on triplet and free ion yields,',* CIDEP,3 and CIDNP in the electron-transfer systems. Here, we show that the lifetime of the donor triplet plays a crucial role for the separation of the geminate singlet and triplet pathways by time-resolved CIDNP. With pulse excitation and FT NMR one can observe the nuclear polarizations of the regenerated ground state species Do and A0 in a short-time regime where nongeminate processes do not yet contribute to a large e ~ t e n t . ~This . ~ geminate CIDNP usually results from both channels which lead to opposite signs and may cancel. In fact, due to the spin-sorting nature of the radical pair mechanism in high fields4 no CIDNP is observed for highly effective singlet and triplet reactions if 3D1has a submicrosecond lifetime and does not undergo reactions to products other than D0.6,7 Only grossly different reactivities of singlet and triplet ion pairs allow the observation of CIDNP from the dominant reverse electron transfer which is normally from the triplet because of energetic reasonse8 On the other hand, even for similar reactivities CIDNP from the singlet path can be expected if the lifetime of 3D1is larger than microseconds since then at short observation times the contribution from the triplet reaction is hidden in 3D1. The following presents an example for this behavior which to our knowledge has not yet been observed. Results and Discussion The experimental arrangement and procedures for timeresolved CIDNP studies were as described b e f ~ r e . ~For excitation we use here 30 mJ excimer laser pulses at 308 nm, for detection z12 rf pulses of 1.5 p s duration and a minimum @
Abstract published in Advance ACS Abstracts, November 15, 1994.
0022-365419512099-0102$09.00/0
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Figure 1. Energy level diagram for the Nfl-dimethyl-1-naphthylamine
(D)/benzonitrile (A) photosystem. 560 ns delay. The donor is NJV-dimethyl-1-naphthylamine, which gives little side products,6J0 the acceptor is benzonitrile, and the solvents are acetonitrile and acetonitrile-d3. The lifetime of 3D1 was shortened by adding 1,3-~yclohexadieneas efficient triplet quencher. From the redox properties E(D+/D) = 0.75 V," E(A-/A) = -2.35 V,12 ro = 0.7 nm, and the Rehm-Weller equation13the initial foreward electron transfer is estimated to be about -57 kJ/mol exergonic and thus diffusion ~0ntrolled.l~ The initial radical ion pairs are about 294 kJ/mol above the ground state and can populate the donor (237 kJ/m0lI4)but not the acceptor (322 kJlm01~~) triplet. Hence, the scheme of Figure 1 applies. Moreover, one expects the formation of 3D1to be more efficient than the reverse electron transfer to the ground state since for the latter reaction the energy gap is well within the Marcus inverted region.I6 Figure 2a shows geminate nuclear polarizations obtained for acetonitrile-& solutions containing 5 x M N,N-dimethyl1-naphthylamineand 9 x M benzonitrile directly after the laser pulse. By comparison with the dark spectra of Figure 2c,d the large emission is due to the donor CH3 groups (2.86 ppm), and the smaller enhanced absorptions belong to the donor ring protons (7.12, 7.47, 7.85, and 8.22 ppm) presumably those at positions 2 and 4." The ring protons of the acceptor benzonitrile (7.54 and 7.72 ppm) do not exhibit CIDNP effects. With the aid of Kaptein's rule^,^^,^^ g(D+) = 2.0029; g(A-) CZ 2.00275,18a(CH3) = +8 G,6 a(Hz,4) -= 0 and pair formation by singlet quenching the signs of the donor polarizations are easily explained by a dominating singlet reverse electron transfer of the geminate radical ion pairs. On the other hand, for the acceptor no polarization is expected since at the high concentra0 1995 American Chemical Society
Photogenerated Geminate Radical Ion Pairs a
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J. Phys. Chem., Vol. 99, No. 1, 1995 103
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Figure 2. (a) Geminate CIDNP for [D] = 5 x M, [A] = 9 x in acetonitrile-& (complete presaturation, 28 FID averaged). (b) Geminate CIDNP for [D]= 5 x M, [A] = 9 x M and 9.4 x M 1,3-cyclohexadiene. (c) Dark spectrum for [D] = 5 x M, [A] = [Q]= 0. (d) Dark spectrum for (b).
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Figure 3. Time dependencies of the donor polarization in the system Nfl-dimethyl- 1-naphthylaminelbenzonitrile for different concentrations of the triplet quencher 1,3-~yclohexadiene.
rationalized in terms of processes which are well established: For zero quencher concentration the initial singlet polarization is partially cancelled in time by the incoming triplet contribution, but there is no phase inversion because of fast nuclear relaxation in the triplet state.& For the highest quencher concentration there is a corresponding but weaker cancellation of the initial triplet polarization which is due to degenerate exchange of the donor cation radicals5p6 analogous to reaction 1. For the intermediate Concentration a sign inversion occurs because the overall reverse electron-transfer process is dominated by the triplet pathway. Several control experiments support our interpretation: Solutions of only the donor in acetonitrile did show CIDNP (emission of the donor CH3 protons) but much weaker than shown in Figures 2 and 3. Small enhanced absorptions of the same protons were observed in the absence of the acceptor benzonitrile but with 1,3-~yclohexadienepresent. This shows that side reactions occur but do not strongly perturb the major pathway. This is confirmed by the results of additional optical observations obtained by laser-flash photolysis.z1 Acetonitrile solutions with [D] = 7 x M and [A] = 10-1 M gave rise to transient absorptions with maxima at 560 and about 290 nm. Both decayed in second-order processes with first half-lives of 6 and 16 ,us, respectively. The 560 nm absorption was also observed in acceptor free solutions both in acetonitrile and in hexane, while the 290 nm absorption was weaker for acetonitrile and absent for hexane solvent. Hence, we attribute the 560 nm signal to 3D1and the absorption at shorter wavelengths to the radical ions. Adding 1,3-~yclohexadieneshortened the lifetime of the 560 nm transient to about 800 ns for [Q] = 2 x M, while the 290 nm radical absorption was not affected. Hence, as inferred from the CIDNP results electron-transfer reactions of lD1 with solvent and triplet quencher are minor for the conditions used for Figures 2 and 3. Furthermore, quencher concentrations above M are sufficient to shorten the lifetime of 3D1below microseconds so that the dominant triplet reverse electron transfer of the primary radical ion pair becomes observable. The complete analysis of the time and triplet quencher dependent nuclear polarizations will reveal the reactivities of singlet and triplet ion pairs and rate geminate constants for the radical ion reactions and is in progress.
Acknowledgment. Support by the Swiss National Science Foundation and experimental help of Dr.Yu. P. Tsentalovich are gratefully acknowledged. References and Notes
tions necessary for effective quenching of 'D1 of 9 x IOd2 M the degenerate electron transfer of the escaping polarized ions
*A'- f A - *A iA'-
(1)
will cancel the geminate polarizations already in the submicrosecond time ~ c a l e . ~ . ~ At first sight the dominance of the singlet reaction is surprising. However, it is caused by the longlevity of 3D1which is not detected by NMR19 and hides the triplet contribution. Figure 2b shows a CIDNP spectrum taken with a sample containing 9.4 x M of the triplet quencher 1,3-cyclohexadiene (ET = 219.4 kJ/mol < &(D) = 237 kJ/mol) besides D and A in the same concentrations as used for Figure 2a. Now the CIDNP phases are inverted as expected for a dominant triplet reaction. Figure 3 displays the time dependencies of the donor methyl group polarization in absolute concentration unitsz0for three quencher concentrations. Qualitatively they are easily
(1) (a) Steiner, U. E.; WOW, H.-J. Photochem. Photophys. CRC Press: Boca Raton, FL, 1991; Vol. IV. (b) Schulten, K.; Stae&, H.; Weller, A. Z. Phys. Chem. 1976,101,371. (c) Michel-Beyerle,M. E.; Haberkom, R.; Bube, W.; Steffens, E.; Schrader, H.; Neusser, H. J.; Schlag, E. W.; Seidlitz, H. Chem. Phys. 1976, 17, 139. (d) Orbach, N.; Ottolenghi, M. Chem. Phys. Lett. 1975, 35, 175. (2) Weller, A. Z. Phys. Chem. N. F. 1982, 130, 129. (3) (a) Kaptein, R.; Oosterhoff, J. L. Chem. Phys. Lett. 1969, 4, 195. (b) Adrian, F. J. J . Chem. Phys. 1971, 54, 3918. (c) Atkins, P. W.; McLauchlan, K. A. In Lepley, A. R., Closs, G. L., Eds.; Chemically Induced Magnetic Polaiizution; Wiley-Interscience: New York, 1973. (d) Wong, S. K.; Hutchinson, D. A.; Wan, J. K. S. J . Chem. Phys. 1973, 58, 985. (e) Pedersen, J. B.; Freed, J. H. J. Chem. Phys. 1973, 58, 2746. (4) (a) Closs, G. L. Adv. Magn. Reson. 1974, 7, 157. (b) Kaptein, R. J . Am. Chem. SOC. 1972, 94,6251,6262. (c) Adrian, F. J. J . Chem. Phys. 1970, 53, 3374; 1971, 54, 3912. (d) Freed, J. H.; Pedersen, J. B. Adv. Magn. Reson. 1976,8, 1. (e) Salikhov, K. M.; Molin, Yu. N.; Sagdeev, R. Z.; Buchachenko, A. L. Spin Polarization and Magnetic Field Effects in Radical Reactions; Elsevier: Amsterdam, 1984. ( 5 ) (a) Closs, G. L.; Sitzmann, E. V. J . Am. Chem. SOC. 1981, 103, 3217. (b) Closs, G. L.; Miller, R. J.; Redwine, 0. D. Acc. Chem. Res. 1985, 18, 196.
Schaffner and Fischer
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(16) Vauthey, E.; Suppan, P.; Haselbach, E. Helv. Chim. Acta 1988, 71, 99. (17) LucchiN, V.; Wells, P. R. Org. Magn. Reson. 1976, 8, 137. (18) For the unknown g value of the benzonitrile radical anion we adopted the one of the benzene anion extrapolated to T = 294 K [a]. Since 1,3-dicyanosubstitution in the radical anion of cyclo[3.3.3]azine does not affect the g value [b] we expect only a very small shift from the benzene to the benzonitrile anion. (a) Jones, M. T.; Metz, S.; Kuechler, T. C. Mol. Phys. 1977, 33, 717. (b) Gerson, F.; Jachimowisz, J.; Leaver, D. J . Am. Chem. SOC.1973, 95, 6702. (19) The resonance frequencies are shifted out of range by hf interaction which are close to those of the radical ions: McLachlan, A. B. Mol. Phys. 1962, 5, 51. (20) Comparison of integrals of CIDNP and dark spectra knowing the ratio of irradiated to detected volume. With the donor ground-state concentration of 5 x M and the relative difference of spin state populations hv/2kT = 1.62 x the absolute dark signal for the six methyl x 6 = 4.86 x protons corresponds to 5 x M x 1.62 x mol of transitionsk. (21) (a) Tsentalovich, Yu. P.; Fischer, H. J . Chem. SOC., Perkin Trans. 2 1994,729. (b) Salzmann, M.; Tsentalovich, Yu. P.; Fischer, H. J. Chem. SOC., Perkin Trans. 2 1994, 2119.
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