Formation and behavior of fluorescent Lewis acid-base exciplexes and triplexes between 3-aminostilbenes and aliphatic amines

The excited singlet states of trans-3-aminostilbene and its N-methyl derivatives are strongly fluorescent in cyclohexane solution and have large singlet state dipole moments. Addition of low concentrations of alkylamines results in a continuous red shift of the emission maximum and decreasing fluorescence intensity. Analysis of the fluorescence behavior using a combination of singular value decomposition with self-modeling and kinetic analysis provides evidence for the sequential formation of a 1:1 complex (exciplex) and 1:2 complex (triplex) between the excited stilbene and ground state alkylamine, both of which are strongly fluorescent. Both the formation and decay of the exciplex and triplex are dependent upon the extent of amine N-alkylation, primary amines forming the most stable exciplex and triplex. Similarly, N-aminoalkyl derivatives of the aminostilbenes form intramolecular exciplexes that in turn form 1:1 complexes with added amines. Addition of diaminoalkanes to the aminostilbenes results in sequential formation of 1:1 and 1:2 complexes rather than the formation of a triplex with a single molecule of the diaminoalkanes. Excited state complex formation is attributed to a Lewis acid-base interaction between the excited stilbene (lone pair acceptor) and ground state amine (lone pair donor). An alternative explanation for the red-shifted emission based on Suppan's theory of solvent dielectric enrichment is found to be incompatible with the experiment results.