Deep in blue with green chemistry: influence of solvent and chain length on the behaviour of N- and N,N′- alkyl indigo derivatives

Using green chemistry procedures the synthesis of N-alkyl (NC(n)Ind) and N,N′-dialkyl (N,N′C(n)Ind) indigo derivatives, with n = 1–3, 6, 8, 12 and 18, was undertaken, leading to compounds with blueish to greenish colors in solution. The effect of the alkyl chain length on the spectral (including color) and photophysical properties of the compounds was explored. This was done with solvents of different viscosities and polarities (dielectric constants). From time-resolved fluorescence and femtosecond-transient absorption (fs-TA) for the NC(n)Ind derivatives with n = 1 and 2, the decays are, in methylcyclohexane (MCH) and n-dodecane, single-exponential, while in 2-methyltetrahydrofuran (2MeTHF) they are bi-exponential. The excited state proton transfer (ESPT) is ultrafast (<1 ps) for NC(1,2)Ind in MCH and n-dodecane, supported by time-dependent density functional theory (TDDFT) calculations, thus showing that both the chain length and solvent influence the ESPT process. For N,N′C(n)Ind, from time-resolved experiments, and with the exception of the shortest member of the series, N,N′C(1)Ind, two conformers are found to be present in the excited state.