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

Abstract

The novel imidazole-based phenothiazine derivatives 3a-3e differing in the length of the alkyl chain at the imidazole substituents were effectively designed and synthesized. The effect of the alkyl chain on the photophysical properties of novel compounds was systematically investigated and supported by TD/DFT calculations. The photophysical characterization of 3a-3e compounds was performed in a series of solvents. Both absorption and fluorescence spectra exhibited positive solvatochromism. Phenothiazine derivatives 3a-3e have a noteworthy emission from blue to yellow region 450–600 nm, high quantum yields 41–93%, lifetimes in nanosecond scale (4.01–7.55 ns), and Stokes shifts in the range of 5688–7021 cm−1 in solvents of different polarity. The noticeable positive solvatochromism indicates the good charge separation in compounds. This work presents the variability of photophysical properties associated with a simple modification of the alkyl chain at the imidazole unit in 3a-3e compounds. Additionally, the ability of the novel compounds 3a-3e for cellular staining, and their cytotoxicity was investigated. Comparative lack of toxicity and appealing photophysical properties make them useful as molecular fluorescent dyes in biological applications.