Subtle Structural Variations within Solvent-Free D–A-Type Organic Crystals for Tuning Emission Wavelength up to 47 nm Shift

Organic crystals have emerged as a promising platform for designing luminescent materials; yet, achieving tuneable fluorescence through subtle structural changes remains exceptionally rare. In this work, we report the fluorescence modulation caused by subtle structural changes in a solvent-free framework, HOF-FJU-101, assembled by a D–A molecule 3,4-bis­(bis­(4-bromophenyl)­amino)­cyclobut-3-ene-1,2-dione (o-SQDPA-Br). HOF-FJU-101 is a dense hydrogen-bonded organic framework interconnected through hydrogen bonding, π···π interactions, and conjugation between bromine atoms and benzene rings. Switching the crystallization mother liquor from dimethyl sulfoxide to dichloromethane, acetone, and N,N-dimethylformamide, different isomorphism HOF-FJU-101 crystals were obtained with subtle structure changes in both intermolecular Br4···Ph C and intramolecular C27–Br4 distances due to the contraction deformation of heavy atom Br, which affects the electron transfer from diphenylamine donor to the squaric acid ring acceptor, resulting in significantly differentiated luminescence behavior. The tuneable emission wavelength range extended from 651 to 604 nm, with a maximum of up to 47 nm shifts.