Upper Excited State Photophysics of Malachite Green in Solution and Films.

Relaxation pathways of upper excited electronic states of malachite green (MG) in ethanol and in films are studied by steady-state and time-resolved spectroscopic techniques. In contrast to ethanol, where MG emits weak short-lived spectrally well separated S2 and S1 fluorescence with the lifetimes ∼0.3 and ∼0.9 ps, MG films show a much stronger broadband fluorescence within 430-700 nm, revealing multiexponential kinetics with the characteristic decay times τ1 ≈ 1 ps, τ2 ≈ 10 ps, τ3 ≈ 0.05-0.8 ns, and τ4 ≈ 2-3 ns. By the analysis of spectroscopic responses of MG in ethanol and in films as well as by theoretical modeling, we demonstrate that significant increase of fluorescence lifetimes and substantial enhancement of fluorescence intensity in MG films are stipulated by the decrease of efficiency of the S2 → S1 and S1 → S0 internal conversion, which in turn is caused by hindrance of rotation of MG's phenyl rings controlling the S2/S1 and S1/S0 conical intersections. These findings indicate that MG films may become promising non-Kasha materials (with reasonable S2 emission) with numerous photophysical and photochemical applications.