Fluorescent Rotary Switches: Four- vs Three-Substituted Phthalimide Boron Difluoride Schiff Base Complexes.

The influence of the substitution pattern in phthalimide boron difluoride Schiff base complexes as fluorescent molecular rotors has been investigated. Due to their ground-state zwitterionic structures, they have exhibited negative solvatochromism in absorption and blue-green emission with moderate to satisfactory photoluminescence quantum yields in solution. Ground-state and excited-state theoretical calculations and time-resolved emission spectroscopy revealed that the excited-state rotation is triggered by planar-induced charge transfer, resulting in switched emission toward the green region. Fluorescence lifetime measurements and species-associated emission spectra exhibited two emitting excited species in equilibrium via a planar transition-state barrier. The substitution pattern models showed different behavior in solid-state mechanochromic switching and were analyzed by subcell unit packing obtained from X-ray structure data. We have attempted to gain in-depth insight into the fluorescence mechanism and photoluminescence properties associated with the substitution pattern of the phthalimide motif in order to understand the structure-property-function relationship.

Colour-tuneable solid-state fluorescence of crystalline powders formed from push-pull substituted 2,5-diphenyl-stilbenes.

The solid-state fluorescence (SSF) of eight DPA-DPS-EWG derivatives (DPA = diphenylamino, DPS = 2,5-diphenyl-stilbene building block, EWG = electron withdrawing group) was studied. Varying the strength of the EWG enabled the tuning of the LUMO energy within a range broader than 1 eV, while the simultaneous changes of HOMO energy were less than 0.1 eV, according to cyclic voltammetry. The fluorescence maxima in dichloromethane laid between 483 and 752 nm and exhibited monoexponential decay and a photoluminescence quantum yield (PLQY) always higher than 35%. Six derivatives with a SSF PLQY higher than 10% in polycrystalline powder form continuously covered the range from 475 to 733 nm. Three components of SSF multiexponential decay, obtained by time-resolved fluorescence spectroscopy, were ascribed to exciton migration to nonfluorescent traps, and monomer-like and aggregate fluorescence. The character of the emitting aggregates was evaluated by quantum chemical modelling based on time-dependent density functional theory computations, carried out on the dimer arrangements obtained by X-ray diffractometry of the single crystals.