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

Daniela Pinheiro; Marta Pineiro; Adelino M Galvão; J Sérgio Seixas de Melo

doi:10.1039/d0sc04958a

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

Chem Sci. 2020 Oct 22;12(1):303-313. doi: 10.1039/d0sc04958a.

Authors

Daniela Pinheiro¹, Marta Pineiro¹, Adelino M Galvão², J Sérgio Seixas de Melo¹

Affiliations

¹ University of Coimbra, CQC, Department of Chemistry Rua Larga 3004-535 Coimbra Portugal sseixas@ci.uc.pt.
² Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico (IST), Universidade de Lisboa Av. Rovisco Pais 1049-001 Lisboa Portugal.

Abstract

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_1,2Ind 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'C₁Ind, two conformers are found to be present in the excited state.