Chain Dynamics Cause the Disappearance of Spin-Correlated Radical Pair Polarization in Flexible Biradicals

Abstract

X-band time-resolved electron paramagnetic resonance (TREPR) spectra of 1,16-and 1,21-acylalkyl biradicals, obtained in toluene solution at low temperatures (187-253 K), are reported. The spectra show a strong temperature dependence in their patterns of chemically induced electron spin polarization. The spin-correlated radical pair (SCRP) mechanism dominates at high temperatures, while the radical pair mechanism (RPM) is the main pattern at lower temperatures. It is postulated that the SCRP spectrum is suppressed at the lower temperature due to molecular motion on a time scale which modulates the exchange interaction (J) between the unpaired electrons. Simultaneously the RPM is enhanced by this motion. Average values of J calculated from end-to-end distance distributions for the C 21 biradical show that in the absence of this dynamic effect, an intense SCRP spectrum would be present at the lower temperature. The results demonstrate that the interpretation of average J couplings may be difficult in temperature regions where these motional effects dominate the appearance of the spectrum.In photochemical reactions that produce geminate radical pairs (RPs) in environments with restricted diffusion, the spin angular momenta of the unpaired electrons can play a large role in the observed lifetimes of the intermediates and in the product distributions. 1 Examples of such systems include surface-bound RPs, 2 radical ion pairs bound by a Coulombic potential, 3 ionpair complexes, 4 micelle-confined RPs, 5 RPs produced in cyclodextrin host-guest complexes, 6 and covalently linked biradicals. 7 The rate of diffusion of the radical centers with respect to each other is also an important parameter affecting these observations. Phenomena such as magnetic field effects on lifetimes 8 and chemically induced electron spin polarization 9 (CIDEP) are also observed because of the interplay of spin and diffusive dynamics in the confined RP. The application of timeresolved optical 10 and electron paramagnetic resonance 11 (EPR) techniques to these systems has provided new structural and kinetic information to be obtained and allowed the mechanisms of these reactions to be better understood.The time scale of EPR spectroscopy and the magnitude of the magnetic interactions in organic RPs make the technique sensitive to motional effects. Chemical exchange processes in RPs 12 or modulation of the exchange interaction (J) in stable biradicals 13 have been studied by steady-state EPR spectroscopy for more than 30 years. However, dynamic effects in timeresolved EPR (TREPR) spectra of confined RPs, which exhibit non-Boltzmann spin-state populations due to CIDEP mechanisms, have only recently been studied. New experimental results and theoretical developments from our laboratory 14 and others 15 have shown that the effect of motion on different time scales presents challenges in the interpretation of magnetic field effects 16 and TREPR spectra of biradicals and micellar RPs and that more sophisticated theories and computational methods are requi...

Intramolecular photoinduced electron transfer in a hydrogen bonded zinc(II) porphyrin-dinitrobenzene complex by time-resolved electron paramagnetic resonance spectroscopy

“…In the isotropic phase, however, an absorptive signal is observed. It is noteworthy, that a recent study [27] suggests that the disappearance of SCRP polarization can be caused by chain dynamics. In the present case, in the isotropic phase, molecular dynamics may account for the thermal spectrum.…”

Time‐Resolved (CW) Electron Paramagnetic Resonance Spectroscopy: An Overview of the Technique and Its Use in Organic Photochemistry

“…The three triplet levels are labeled T', To and T-, while the (normally) empty singlet level is labeled S. The exchange interaction J is defined as half the energy gap between To and S. determination of both the sign and the magnitude of J. Recently, we have focused our attention on the consequences of motional or dynamic effects in SCRP spectra (30)(31)(32).…”

Diffusion-model analysis of effective CIDEP distance in solvent-separated radical-ion pair

“…Recent theoretical studies have shown, however, that dynamic effects of the molecular diffusion motions influence the observed SCRP spectra line shape and make it impossible to define the effective polarization distances and the magnitudes of J for the radical pairs such as in micellar solution systems [25,[28][29][30][31][32][33][34][35][36]. On the other hand, in homogeneous liquid solution systems, no analyses and predictions have been done to discuss reasonable or effective CIDEP distances for the conventional RPM polarization generations, in which molecular translation diffusive motions may play a predominant role in a stochastic manner.…”