Dynamics and equilibrium for the formation of fluorescent Lewis acid–base exciplexes and triplexes between 9-aminophenanthrene and aliphatic amines

Abstract

The excited singlet states of 9-aminophenanthrene and its N-aminoalkyl derivatives are strongly fluorescent in cyclohexane. Addition of low concentrations of Et₃N, Pr₂NH, or PrNH₂ results in a red shift of the emission maximum and moderately decreased 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 1 ∶ 1 (exciplex) and 1 ∶ 2 (triplex) complexes between the excited 9-aminophenanthrene 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. Rate constants and equilibrium constants for complex formation follow the order 1° ∼ 2° > 3°, analogous to that for the formation of ground-state complexes between amines and the soft Lewis base HgBr₂. Similarly, N-aminoalkyl derivatives of 9-aminophenanthrene form intramolecular exciplexes. Excited-state complex formation is attributed to a Lewis acid–base interaction between the excited aminophenanthrene (lone-pair acceptor) and ground-state amine (lone-pair donor). The factors which determine the stability of excited-state Lewis acid–base complexes are characteristic of the specific excited-state acceptor. No universal scale of lone-pair donor strength can be expected to describe the formation of such complexes.