CIDEP of 2-Chloroxanthone in 2-Propanol: Excitation Wavelength Dependence and Effect of Cross Relaxation

Abstract

The pattern of a chemically induced dynamic electron spin polarization (CIDEP) spectrum of 2-chloroxanthone (2-ClXn) in 2-propanol depends on the excitation wavelength. To clarify the cause of this intriguing phenomenon, we have investigated the details of the CIDEP and the transient visible absorption of 2-ClXn in 2-propanol produced by excitation at 355 or 308 nm. The CIDEP spectrum is strongly temperature dependent, which is mainly due to the temperature dependence of the net polarization. The net polarization consists of a fast rising emissive polarization and a slow rising absorptive polarization, both at 308 and 355 nm. However, the relative contributions of the two polarizations are different depending on the excitation wavelength, with more emissive polarization at 308 nm. From the analyses of the time profiles of the CIDEP signals and the results of quenching experiments with 2,6-di-tert-butylphenol, it is concluded that the emissive polarization arises from the triplet mechanism (TM) due to the reaction of highly excited 2-ClXn and the absorptive polarization arises from a slow reaction of the triplet molecules in thermal equilibrium. When excited at 355 nm the phase of the CIDEP spectrum of the 2-hydroxy-2-propyl radical due to the singlet−triplet mixing radical pair mechanism (ST0M−RPM) is inverted at later times. It is shown by simulation that large inverted signals can be produced by the electron−nuclear cross relaxation mechanism under proper conditions.