Charge recombination dynamics of geminate ion pairs formed by electron transfer quenching of molecules in an upper excited state

An investigation of the charge recombination (CR) dynamics of geminate ion pairs formed upon electron transfer quenching of azulene, benz[a]azulene, a nd xanthione in the second singlet excited state by several electron donors , using ultrafast time resolved spectroscopy and photoconductivity is repor ted. The ion pairs have two possible CR pathways: (i) a highly exergonic CR to the neutral ground state or (ii) a moderately exergonic CR leading to t he formation of the neutral acceptor in the first singlet excited state. Th is investigation shows strong evidence of the predominance of the second pa thway. CR in ion pairs formed with the azulenes is faster by a factor of mo re than 50 than in ion pairs having a similar energy but with the first CR pathway only. The electron transfer quenching of xanthione in the second si nglet excited state by several weak donors does not lead to a significant r eduction of the tripler yield of this molecule. The relevance of these resu lts to explain the absence of the inverted region in highly exergonic bimol ecular charge separation reactions is discussed.