Unveiling the Effect of π-Chain on Competitive Radiative and Non-Radiative Decay in Pyrene-Based Schiff Bases: ESIPT-AIE Fluorophores

Organic fluorophores combining excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) offer enhanced photoluminescence properties. Herein, we have adopted a significant distance strategy for asymmetric pyrene Schiff bases (HPBC and 1HPBC) to disclose their concurrent fluorescence mechanisms. In DMSO, both molecules showed two competing ESIPT and C═N photoisomerization processes at the E*-state, while K*-rotation is kinetically and thermodynamically unfavorable. In HPBC, the rigid dimeric Schiff base units endowed strong intramolecular H-bonding at the S₁ state, which blocked the N–N assisted rotation of the adjacent C═N bond and suppressed the nonradiative E*-relaxation. Thus, ESIPT, as the dominant relaxation pathway, led to a comparatively emissive solution. The enhanced solid-state K*-emission is attributed to the thermodynamically more favorable ESIPT, restricted free rotations, and antiparallel stacking. Conversely, flexible 1HPBC, as a monomeric Schiff base, considerably lowered the energy barrier for nonradiative relaxation, thereby leading to more competitive ESIPT and C═N rotation decay channels. On the basis of this computationally predicted fluorescence quenching mechanism of 1HPBC, it is proposed that the solid-state emission enhancement may occur through combined restricted intramolecular rotation and ESIPT processes, which requires experimental validation. Hence, it is found that simple structural modifications can efficiently tune the fluorescence mechanisms, providing a theoretical reference for the regulation of ESIPT-AIE systems.