Meta effect enables redder and larger Stokes shift chromophores by enhanced aromaticity reversal

Compact fluorescent molecules with meta-positioned donor (D) and acceptor (A) groups have recently demonstrated extraordinary photophysical properties such as red emission and large Stokes shifts for advanced bioimaging and optoelectronic applications. However, it remains elusive how such simple D–A configuration can give rise to these unique properties which para isomers lack. Herein, we unlock the fundamental photophysics and its origin of “meta effect” by systematically investigating series of para and meta chromophores with varying donor and acceptor structures. Compared to the para isomers, meta chromophores possess redder emission, larger Stokes shifts, stronger solvatochromism, solvent-dependent fluorescence quantum yields (FQYs), and lower extinction coefficients. We reveal that the redder emission and larger Stokes shift in meta chromophores stem from the enhanced aromaticity reversal and ensuing relief of excited-state antiaromaticity, amplified by solvation. Since large Stokes shifts hurt FQYs mainly via conical intersections, we propose feasible mitigation strategies to improve meta chromophores toward “all-in-one” fluorophores.