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.