Does the change in the length of the alkyl chain bring us closer to the compounds with the expected photophysical and biological properties? – Studies based on D-π-D-A imidazole-phenothiazine system

•Five imidazole-based phenothiazine derivatives were synthesised with different alkyl chain lengths at the imidazole unit.•Photophysical properties of the compounds were systematically studied and compared with DTF calculations.•The solvatochromic effect of compounds was extensively discussed and collated with DFT.•The ability of novel compounds for cellular staining was tested.