ABSTRACT A comparative absorption study between the flexible molecule and its blocked homologue confirms that the twist around the C-N bond, in the ground state, of “anilines” in solution is the common origin of the anomalous temperature effects: - the collapse of the second electronic band for N,N-dialkylanilines - the anomalous red shift (ARS) of the first band for p-substituted N,N-dialkylanilines. This twist is due to the complexation by hydrogen bonding between “anilines” and water traces present as impurity in the aprotic solvents. The twisted conformation in the ground state yields a charge transfer character CT of the singlet excited state :S2* (case of N,N dialkylanilines) or S1* (case of substituted anilines). The CT character causes spectroscopic anomalies of emission: S2*(CT) => (Iph /F1, )S2* > (Iph /F1 )S1* S1*(CT) => dual fluorescence. Similar twisted complexes - and so similar anomalies - are obtained, at room temperature, in rigid hydrogen bonded matrices, such as polyvinyl-alcohol (PVA), in both cases (N,N-dialkylanilines and p-substituted N,N-dialkyIanilines). The simulation by intermolecular interactions calculations confirms the twisting influence of water molecules on the amine in the ground state. The importance of the twisting is seen to increase as more water molecules -up to three cluster- around the amine. |
