Desymmetrization on electron-withdrawing groups in single benzene fluorophores for fine tuning of photophysical properties and applications

In this study, a series of unsymmetrical single-benzene fluorophore (SBF) derivatives were systematically synthesized by selectively disrupting the ester symmetry of standard diamino terephthalate-type analogs <i>via</i> alternative synthetic routes. Various electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) were introduced at one ester group position, yielding 12 A series compounds with emission wavelengths spanning from 440 to 578 nm. EDGs led to blue-shifts, while EWGs induced red-shifts relative to the symmetric parent compound. This trend can be attributed to electronic modulation of frontier molecular orbitals. Density functional theory (DFT) calculations confirmed that increasing EWG strength led to a gradual reduction in the HOMO-LUMO energy gap, in agreement with experimental emission trends. Although substituent effects were evident, the overall spectral shifts between symmetric and unsymmetric structures remained modest, indicating structural robustness of the core scaffold. In addition, a series of monoamino B series compounds (12 analogs) were prepared to examine the photophysical consequences of removing one amino group. These consistently showed blue-shifted emissions compared to the A series, attributed to reduced conjugation and secondary hydrogen bonding. Finally, ester hydrolysis of selected compounds generated C series carboxylic acids, enabling pH-dependent fluorescence switching. Notably, C-COMe exhibited multicolor emission-from blue to orange-across varying pH conditions. These results demonstrate a modular strategy for tuning emission behavior in SBFs through controlled symmetry and functionalization.