A series of green fluorescent protein chromophore analogues incorporating nitrogen‐donor units—carbazole (Cz), phenothiazine (PTZ), phenoxazine (PXZ), and acridine (ACR)—was synthesized to elucidate the impact of donor strength and substitution topology on excited‐state dynamics. The para ‐substituted derivatives display strong intramolecular charge–transfer (ICT) absorption and efficient emission, whereas the meta ‐substituted analogues exhibit weaker ground‐state conjugation but significantly larger Stokes shifts and enhanced charge separation. Among the donors, PXZ and PTZ are particularly effective due to their distorted heterocyclic geometries, which facilitate HOMO–LUMO decoupling. The exceptionally large Stokes shifts observed in PTZ derivatives are attributed to extensive excited‐state relaxation involving quasi‐equatorial to quasi‐axial (eq → ax) conformer interconversion, a mechanism confirmed by the emergence of blue‐shifted, structured emission at 77 K. Additionally, p ‐Cz exhibits enhanced fluorescence in polar solvents driven by the amino conjugation effect, while p ‐ACR demonstrates reversible mechanofluorochromism. These findings provide a comprehensive framework for designing organic luminophores with tunable ICT emission and intrinsic large Stokes shifts for advanced photonic and sensing applications.
Han et al. (Sun,) studied this question.