Linear, two- and four-armed pyridine-decorated thiazolo[5,4-d]thiazole fluorophores: Synthesis, photophysical study and computational investigation

A novel family of thiazolo[5,4-d]thiazole (TzTz) fluorophores having a general formula (py)n-π-TzTz-π-(py)n (n = 1 or 2, py = pyridine) is herein reported. The investigated compounds are decorated with alkoxy-substituted pyridine scaffolds bearing chains with different lengths (a = methoxy, b = 2-ethylhexan-1-oxy, c = 2-decyltetradecan-1-oxy) that are connected at either the para- or meta-position(s) of the phenyl bridging ring. The compounds feature either a linear (n = 1, para-substituted, series 8), two-armed (n = 1, meta-substituted series 9) or four-armed (n = 2, meta-substituted series 10) molecular architecture. Steady-state and time-resolved photophysical investigation is employed to characterize the optical properties of the compounds in both diluted CH2Cl2 and CH2Cl2:trifluoroacetic acid (TFA) 95:5 solutions, as well as in thin-film in poly(methyl methacrylate) (PMMA) matrix at 20 wt% doping. All compounds display electronic transitions with mainly π-π* nature at both solution and solid state. However, while both peripheral pyridyl-phenyl moieties and the π-TzTz-π core participate in the absorption transitions, the emission involves mostly the π-TzTz-π core. A clear effect of the positional isomerism on the photophysical properties is observed, with the protonation of the peripheral pyridines offering a further modulation. Thus, these results provide some useful hints on the molecular design of TzTz-based compounds as promising photoactive materials.