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

Photolysis of N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) in liquid a lcohols was studied by means of a time-resolved transient conductivity tech nique and the influence of an external magnetic field. We have shown that n o 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 form ation via quenching of excited (TMPD)-T-3* molecules in their triplet state by electron-accepting solutes. Quenching of the singlet excited state (TMP D)-T-1*, on the other hand, leads to fast recombining singlet ion pairs wit h no resulting free ion current. Competition between the two processes is a ffected 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 quan tum 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.