ELECTRON-NUCLEAR CROSS-RELAXATION EFFECT ON THE PHOTOCHEMICAL-REACTION OF BENZALDEHYDE AS STUDIED BY CIDNP AND DNP

The magnetic field dependence of CIDNP was measured for the photolysis of benzaldehyde. At high and low fields, respectively, the CIDNP behavior is satisfactorily interpreted by the S-T0 and S-T- mixing of radical pair mechanism (RPM). At around 325mT the CIDNP of the aldehyde proton is emissive, which cannot be interpreted by RPM. In order to understand this anomalous behavior, we observe the dynamic nuclear polarization. The DNP sign shows that the cross-relaxation mechanism of the intermediate free radical is DELTAm = 2. With this result, the emissive CIDNP at 325 mT is interpreted by the triplet mechanism (TM) with cross-relaxation-induced polarization transfer from electron spin to nuclear spin. Further, the time-resolved DNP is observed with laser excitation and switching of microwaves. With pulse excitation, the features of CIDNP and DNP are different (rom that with continuous light excitation. This is interpreted as due to the associated increase of the initial concentration of the ketyl radical. From the time dependence of the DNP intensity, the lifetime of the ketyl radical is estimated to be of the order of hundreds of nanoseconds.