Insight into the Stokes shift, divergent solvatochromism and aggregation-induced emission of boron complexes with locked and unlocked benzophenanthridine ligands

Abstract

Two four-coordinate boron complexes, compounds 5 and 7, have been synthesized, using analogous benzophenanthridine-based ligands but showing different conformational constraints, and their optical properties have been investigated. Following a ligand design that takes into consideration the expansion of π-conjugation and the structural desymmetrization has proved to be a useful approach to obtain fluorophores with large Stokes shifts. The analysis of the Huang-Rhys parameters along with the vibrational modes have revealed that the incorporation of a conformationally free substructure entails an enhanced Stokes shift as a result of an increased geometry relaxation in the first excited state (S1) and the ground state (S0). Additionally, the sensitivity of the Stokes shift to the solvent polarity showed a divergent solvatochromism (bathochromism in emission spectra and hypsochromism in absorption spectra) whose origin has been ascribed by experimental and theoretical methods to the dissimilar extent of the stabilization of S0 and S1 states with respect to the corresponding Franck-Condon states. Besides, the formation of aggregates in dimethylformamide/water mixtures has been related to the solid state packing, determined by X-ray diffraction, to explain the aggregation-induced effects on the emissive properties of the reported organoboron complexes. The less conformationally restricted molecule, 7, displayed aggregation-induced emission (AIE) promoted upon blocking the motion of the peripheral rotor.