Controlling optoelectronic properties of carbazole-pyridinium luminophores via kinetic polymorphism

Abstract

This study demonstrates the use of kinetic polymorphism to modulate the optoelectronic properties of the donor–acceptor luminophore KL1421 without chemical modification. By employing a swift precipitation method, we selectively isolated three distinct polymorphs arising from specific rotational conformers of the pyridinium subunit, as confirmed by structural and vibrational analysis. These structural variations result in tunable band gaps (2.80–2.95 eV) and high solid-state quantum yields (61–80%), where geometric constraints on the donor–acceptor twist angle directly influence HOMO–LUMO overlap. Furthermore, temperature-dependent photoluminescence reveals that out-of-plane pyridinium rotamers, which restrict π–π⁺ interactions, promote thermally activated non-radiative transitions via triplet traps, distinguishing the mechanism from delayed fluorescence and highlighting the potential of kinetic control for tailoring functional organic materials.