Electron Transfer Reactions Involving Light Vincenzo Balzani, Fabrizio Bollella, Mauro Ciano, and Mauro Maestri lstituto Chimico "G. Ciamician", University of Bologna and istituto FRAE-CNR, Bologna, Italy
A substantial fraction of the chemical literature is currently dealing with reactions involving light. Electron transfer reactions, which are fundamental to all chemistry, are particularly important for the connection between chemistry and light. In this paper we will discuss the involvement of light in electron transfer reactions, with particular emphasis on the important role that can be played in these processes by suitable chemical species called sensitizers. Fundamental Principles Light can he involved in many electron transfer processes. The best known cases are those in which light is used as a reactant (photochemical reactions, eqn. (1)) ( I ) , while less known are perhaps the cases in which light is generated as a product (chemiluminescence reactions, eqn. (2)) ( 2 ) . A+B+hu-A-+B+
(1)
A-+B+-A+B+hv
(2)
1""
1
con wr.1m
0 '
light into
A+B
chemicai energy
Of course, the involvement of light in both processes occurs via the formation of electronically excited states. Assuming A,
the the two pr&eises may be reformulated as follows.' A+B+hu-*A+B-A-+Bt
(3)
A+Bt-*A+B-A+B+hu
(4)
An electronicallv excited state usuallv differs from the Figure 1. Schematic diagram showing three possible ways in which light can t~mv-puodinzg r t ~ u i d.t.,te I B I ~ , I Wt'.,r I I Ih;~vin:; ~. .nl 1 I,., 1rw1 be involved in electron transferprocesses. energy . I ~ I ~ ~ I ~ . I ~ ~t.rlni~:~l I \ ; : , I M I ,I 111 Ie i n i a I U W ~ I I in :i 111gl1 t ~ htjnrlin
