Chemistry Department Seminar

In a recent publication, Tung and Wu demonstrated that a group of Pt(II) terpyridyl acetylide complexes yield hydrogen upon photolysis in the presence of dihydropyridines. In addition, others have demonstrated that this class of Pt(II) complexes have excited state behavior that is strongly dependent on terpyridyl substitution and the nature of the fourth ligand. The mechanism of hydrogen production was postulated to involve H atom abstraction from the dihydropyridine by the Pt(II) complex excited state. Our efforts in this area have been to use time resolved absorption and emission spectroscopy to investigate the nature of the excited state of a group of Pt(II) complexes and to explore formation and decay of photoproducts formed in the presence of reductive quenchers.
Excited state behavior of four complexes was examined in detail : [(DMAtpy)Pt(II)Cl]+, [(mpt)PtCCPhCl]+, [(mpt)PtCCPhOMe]+ and [(mpt)PtCCPhMe]+ (DMAtpy = 4’-dimethy lamino-2,2’6’,2”-terpyridine; mpt = 4’-(p- methylphenyl)2,2’,6’,2”-terpyridine). Excited state lifetimes, emission maxima and emission quantum yields were strongly dependent on both the terpyridyl substituent and the phenylacetylide substituent.
The photoredox reactions of [(DMAtpy)PtCl]+ and [(mpt)PtCCPhCl]+ with a variety of reductive electron transfer quenchers was also examined. With quenchers such as N-methylphenothiazine and triethylamine, excited state quenching rate constants were in excess of 109 M-1s-1 and laser flash photolysis indicated that single electron transfer products were formed. When dihydropyridines, potential H atom donors, were used as quenchers, transient absorption spectra provided evidence that the species formed were the result of electron transfer quenching of the excited state of the Pt complex. The results suggest that formation of the Pt hydrido intermediate in H2 production with dihydropyridine quenchers involves protonation of the one electron reduced species formed in the excited state reaction.2