Femtosecond broadband time-resolved fluorescence and transient absorption study of the intramolecular charge transfer state of methyl 4-dimethylaminobenzoate

Abstract

A combined application of femtosecond broadband time-resolved fluorescence (fs-TRF), fluorescence anisotropy (fs-TRFA) and fs to microsecond (μs) transient absorption (TA) have been used to probe directly the dynamics, nature, formation and decay paths of the singlet intramolecular charge transfer (¹ICT) state of methyl 4-dimethylaminobenzoate (1a) in acetonitrile. The result reveals explicit evidence for a common electronic origin (the L_a nature) of the ¹ICT state and its precursor the locally excited (¹LE) state to account jointly for the dual florescence known to this system. It also shows that the ICT reaction from the ¹LE to ¹ICT state occurs with time constant of ∼0.8 ps and the ¹ICT state formed decays with a ∼1.9 ns time constant leading mainly to a ππ* natured triplet state (³T₁). The ³T₁ then relaxes with a ∼4 μs lifetime under deoxygenated condition resulting in full recovery of the ground state (S₀). As a case study, this work contributes novel experimental data for improved understanding of the mechanism of ICT reaction; it also reveals a distinct deactivation pattern for this prototype para-amino substituted aromatic carbonyl compound in acetonitrile.