Photoinduced electron transfer sensitization investigated by chemically induced dynamic nuclear polarizatioin (CIDNP)

The key step of photoinduced electron transfer sensitization (sensitizer, S; donor substrate, M; quencher, Q) is the transformation of a primarily formed auxiliary radical ion pair M(*+)S(*-) into the desired radical ion pair M(*+)Q(*-). The systems chosen exhibit striking CIDNP effects, which can only be explained by that pair substitution. An inversion of all polarization phases occurs when the quencher diethylfumarate is added to the two-component system 9-cyanophenanthrene/ trans-anethole (S/M) because the g-value differences have opposite signs for the first and second radical pair. With the system 9-cyanoanthracene/trans-anethole/fumarodinitrile (S/M/Q), the pair substitution is a necessary condition for observable CIDNP, and no polarizations can be detected when any of the three components is omitted. Closed-form expressions for the CIDNP intensities as functions of the first-order rate constant of pair substitution k are derived, which use the limiting polarizations from the first and the second radical pair to eliminate any explicit dependence on diffusion-related quantities and hyperfine coupling constants, leaving only k and the g-value differences of the two pairs as parameters. From fits of these expressions to the dependence of the polarizations on the concentration of Q, the unknown g values of M(*+) and S(*-) are estimated.