Photochemical reactions of coenzyme PQQ (Pyrroloquinolinequinone) and analogues with benzyl alcohol derivatives via photoinduced electron transfer

Photochemical redox reactions of the trimethyl ester of coenzyme PQQ (PQQTM E) with benzyl alcohol derivatives (ArCH2OH), tetrahydrofuran, and 1,4-cycl ohexadiene occur efficiently under visible light irradiation in MeCN to yie ld PQQTMEH(2) (reduced PQQTME in the quinol form) and the corresponding deh ydrogenated products (ArCHO, furan, and benzene) quantitatively. A similar photochemical oxidation of benzyl alcohols also occurs with phenanthrolineq uinone derivatives (PTQ), benzoquinolinequinone derivatives (BQQ), and phen anthrenequinone (PQ). PQQTME and the analogues are essentially nonfluoresce nt in MeCN, and the photochemical reaction of the o-quinones is retarded si gnificantly in the presence of molecular oxygen. Transient absorption spect ra of the triplet excited states of the o-quinones were detected in laser f lash photolysis of the MeCN solutions. From the decay of T-T spectra were d etermined the lifetimes of the triplet excited states of the o-quinones. Th e quenching rate constants of the triplet excited states by benzyl alcohols agree with the observed rate constants of the photochemical reduction of t he o-quinones with the same substrates determined from the saturated depend ence of quantum yields on the benzyl alcohol concentrations. Such an agreem ent confirms that the photochemical reaction proceeds via the triplet excit ed state of the quinones. Dependence of the observed rate constants k(obs) of the photochemical redox reaction on the one-electron oxidation potential E-ox(o) of the substrates as well as the results of kinetic deuterium isot opic study indicates that the photochemical redox reactions between the o-q uinones and the substrates proceed via photoinduced electron transfer from the substrate to the triplet excited state of the o-quinone, followed by pr oton and hydrogen atom transfer to yield the quinol and the corresponding o xidation products. The transient absorption spectra of the radical ion pair formed in the photoinduced electron transfer have been detected successful ly in laser flash photolysis of the o-quinone-benzyl alcohol systems.