Spectroscopic and Theoretical Analysis of <i>N</i> -Ethylphenothiazine Derivatives: Alkyl Chain Length Effects on Solvatochromism, Fluorescence Efficiency, and Excited-State Polarity

The newly symmetrically substituted by carbazole units <i>N</i>-ethylphenothiazine derivatives <b>4a</b>-<b>4g</b>, which vary in the length of the alkyl chain at the carbazole substituents, were designed and synthesized. By methodically investigating the impact of the alkyl chain at the carbazole unit on the photophysical properties of these novel compounds, with support from TD/DFT calculations, their photophysical characteristics were analyzed across a range of solvents with varying polarity. In hexane through to acetonitrile, the fluorescent properties of compounds <b>4a</b>-<b>4g</b> result in light emission from blue to pale green. The moderate charge-transfer in compounds is indicated by the positive solvatochromism observed in their emission spectra. The derivatives with medium-length alkyl chains (C4-C6) demonstrated optimal photophysical performance, including high quantum yields (up to 81%), long excited-state lifetimes, and strong absorption coefficients. Low-temperature measurements confirmed the persistence of fluorescence and the emergence of phosphorescence bands. Dipole moment analysis using Lippert-Mataga plots revealed a gradual increase in excited-state polarity with increasing alkyl chain length. Cytotoxicity tests confirmed that all derivatives were nontoxic up to 25 μM across various cancer and normal cell lines. Despite limited passive membrane permeability, fluorescence microscopy under mild permeabilizing conditions (Tween 20) revealed that derivatives <b>4b</b> and <b>4c</b> could serve as selective fluorescent probes for membrane integrity. The results underscore the importance of alkyl chain engineering in tailoring molecular properties, making these compounds promising candidates for optoelectronic and photonic applications. They hold promise for bioimaging applications, particularly in membrane-targeted diagnostics or responsive drug-delivery systems.