Mechanism of TMPD photolysis in alcohols. Spin-dependent ion recombination and photoconductivity

Photolysis of N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) in liquid alcohols was studied by means of a time-resolved transient conductivity technique and the influence of an external magnetic field. We have shown that no single-photon ionization of TMPD occurs at energies up to 5.5 eV. Instead, the observed photocurrent is a result of diffusion assisted ion pair formation via quenching of excited (TMPD)-T-3* molecules in their triplet state by electron-accepting solutes. Quenching of the singlet excited state (TMPD)-T-1*, on the other hand, leads to fast recombining singlet ion pairs with no resulting free ion current. Competition between the two processes is affected by the external magnetic field and is observed as a magnetic field effect on the photocurrent. The rate constants of quenching of the singlet and triplet excited states of TMPD* by acetone in isopropanol were found to be 3.2 x 10(9) M-1 s(-1) and 2.5 x 10(8) M-1 s(-1), respectively. The quantum yield of free ions originating from the (TMPD)-T-3* precursor is close to 80% in the absence of magnetic field, while the yield from the (TMPD)-T-1* precursor is negligibly small.