Synthesis, photophysical, electrochemical and computational investigation of dimethine and trimethine cyanine-based dyes

Abstract

Three novel cyanine-based dyes (D1-D3) were obtained by the carboxylic indolium salt the condensation reaction with three different donors, N,N-dimethylaniline, carbazole and indole derivatives. The chemical structures of these dyes were confirmed and fully characterized by IR, NMR, and HRMS. The UV–vis, fluorescence spectra, and cyclic voltammetry were conducted to give a preliminary insight into their optoelectronic properties. Generally, all synthesized dyes exhibited well-defined absorption in the visible region with high absorptivity and low band gap, giving λEx. max (516–564 nm), ε (45279–141011 M−1cm−1) and E0-0 (2.11–2.21 eV). Distinctive stokes shifts were observed for all dyes, with the highest value for the dimethine cyanine containing carbazole donor, D2, (2752 cm−1). The solvatochromic study showed hypsochromic effects and increased Stokes shifts with increasing solvent polarity. The electrochemical study on the other hand revealed having HOMO and LUMO energy levels, <-5.46, and > -3.58 eV, respectively, indicating their suitability as sensitizers in dye sensitized solar cells. The impact of different donors on the geometrical, photophysical, charge transfer and reorganization energy on the designed dyes has been investigated using Density Functional Theory (DFT) and time-dependent (TD-DFT). The theoretical and experimental results are in good agreement. The results indicate that the D1-D3 dyes have a small energy gap (2.50–2.77 eV) and their cationic counterpart absorption in the visible (480–576 nm). The modification of the donor suggests that these dyes would be suitable for dye-sensitized solar cells (DSCCs).