Isomerization dynamics of the 2-phenylazo-1,3-dimethylimidazolium cation photoexcited to the S2 (π, π*) state as studied by transient absorption spectroscopy in the time domain of 10−13 to 103 seconds

Abstract

Photoinduced isomerization of a novel photochromic cation, [2PA-Mmim]⁺ (2-phenylazo-1,3-dimethylimidazolium cation), was studied by optical spectroscopic methods. The UV-Vis absorption spectra of the [2PA-Mmim]⁺ cation show two prominent bands starting around 410 and 520 nm, corresponding to the S₀–S₂ (π, π*) and S₀–S₁ (n, π*) transitions, respectively. The photoisomerization mechanism is studied by femtosecond time-resolved transient absorption experiments performed after S₀–S₂ (π, π*) excitation in several solvents with different viscosity, including ionic liquids. The transient absorption signals at two representative wavelengths were fitted by bi-exponential functions, which yield four decay components. The photoisomerization mechanism is discussed in light of the relaxation schemes available for azobenzene. Only one of the components depends on the solvent viscosity and it changes from 1.2 ps (dichloromethane, 0.4 cP) to 5.6 ps ([Bmim][BF₄], 93 cP). This component is assigned to a molecule at the S₁ state, which is responsible for the “rotational” isomerization. The weak dependence on the solvent viscosity of this component is explained in terms of local change in the viscosity as a result of local heating due to excess energy released at S₂–S₁ internal conversion. The other three components of ∼0.4, 1.0 and 10 ps are attributed to relaxation processes of the molecule at S₂, S₁ and S₀ states, respectively. The quantum yields for the forward E–Zphotoisomerization are ∼0.15 after S₂ excitation. The backward Z–Eisomerization is slow with a lifetime of 1 hour and an activation energy of 91 kJ mol⁻¹ through an “inversion” mechanism.