Radiationless deactivation of an intramolecular charge transfer excited state through hydrogen bonding:: Effect of molecular structure and hard-soft anionic character in the excited state

Energy-gap dependency for radiationless deactivation from excited states of various molecules having strong intramolecular charge transfer (ICT) character has been investigated by observing fluorescence quenching on addition of alcohols. Molecules having strong ICT excited states were classified into three groups: (a) molecules that underwent considerable fluorescence quenching by ethanol (quenching constant, K-SV > 20 M-1) and for which radiationless deactivation in protic solvents was much faster than anticipated from the ordinary energy-gap law observed in aprotic solvents, (b) molecules whose fluorescence exhibited substantial red shifts, and (c) molecules whose fluorescence were barely affected by the addition of ethanol (K-SV < 1 M-1) and for which the energy-gap dependences on radiationless deactivation in protic solvents were not so different from those in aprotic solvents. Typical fluorophores for each case, i.e., a, b, and c, were aminoanthraquinone, aminophthalimide, and aminocoumarin, respectively. Differences in the fluorescence quenching phenomena are discussed in terms of the molecular structure and the hard-soft anionic character of the excited states,,governed by changes in charge density on the carbonyl. oxygen. An excited molecule having a hard anionic character on a specific site within the molecule, classified as group a, was concluded to undergo considerable fluorescence quenching through an intermolecular hydrogen bonding interaction with an alcohol having a hard cationic character. On the other hand, fluorescence of an excited molecule having a soft anionic character, classified as group c, cannot be quenched well by an alcohol because of the weak interaction on the carbonyl oxygen. The anomalous behavior of the excited aminophthalimides (group b), which are classified as hard anions but do not undergo fluorescence quenching, suggested the possibility that molecular rigidity is another factor controlling the radiationless deactivation process induced by hydrogen bonding.