Investigation of non-Rehm-Weller kinetics in the electron transfer from trivalent phosphorus compounds to singlet excited sensitizers

Singlet excited states (S-1(*) and S-1(+*)) of neutral and monocationic sensitizers, S and S+, respectively, were quenched by electron transfer (ET) from a variety of trivalent phosphorus compounds (Z(3)P). The quenching rate constants k(q), which are equal to the rate constants k(ET) of the ET from Z(3)P to S-1(*) or S-1(+*), were determined by the Stern-Volmer method. The logarithm of k(ET) was plotted against free-energy change G(0) of the ET. The plot deviated upward from the line predicted by the Rehm-Weller (RW) theory in the endothermic region, the deviation being larger in the ET to a neutral acceptor S-1(*) than in the ET to a cationic acceptor S-1(+*). Such a kinetic behavior is in sharp contrast to that observed in the ET from amines (R3N), where the ET to either neutral or cationic acceptor takes place according to the RW prediction. The ET from a donor, Z(3)P or R3N, to a neutral acceptor S-1(*) is a charge-separation type, during which electrostatic attraction between the donor and the acceptor is generated, whereas the ET to a cationic acceptor S-1(+*) is a charge-shift type, which results in neither electrostatic attraction nor repulsion. Difference in kinetics-energetics relationship by the type of ET, which is not recognized in the ET from R3N donor, becomes visible when Z(3)P is used as a donor. Copyright (c) 2013 John Wiley & Sons, Ltd.