A BF2 Chelate Exhibiting Excimer‐like Fluorescence with an Unusually Large Stokes Shift in the Crystalline Phase

• Published in: Chemistry : a European journal Vol. 29; no. 31; pp. e202300216 - n/a
• Main Authors: Bozdemir, Özgür Altan, Woodford, Owen J., Waddell, Paul G., Harriman, Anthony
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 02.06.2023
• Subjects: Absorption; Absorption spectra; boron(III) chelates; Charge density; Chemistry; Coupling (molecular); Crystal structure; Crystallography; Crystals; Dipoles; Emission; excimer emission; Excimers; excitonic coupling; Excitons; Fluorescence; Hydrogen bonds; photophysics; Single crystals; solid-state fluorescence
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202300216

The target mono‐BF2 complex is weakly emissive in fluid solution because radiationless decay of the excited‐singlet state is promoted through an intramolecular N⋅⋅⋅H−N hydrogen bond. The lack of mirror symmetry for this compound is attributed to vibronic effects, as reported previously for the bis‐BF2 complex (BOPHY). Red‐shifted fluorescence is observed from single crystals, the emission quantum yield approaching 30 % with a fluorescence lifetime of 2 ns. The large Stokes shift of 5,700 cm−1 helps minimize self‐absorption. Crystallography indicates that the internal fold and twist angles are increased substantially in the crystal, but the hydrogen bond is weakened relative to solution. The crystal structure is compiled from pairs of head‐to‐tail molecules having a shift of ca. 4.1 Å and closest approach of ca. 3.5 Å. These molecular pairs are arranged in columns, which, in turn, assemble into sheets. The proximity favors excitonic coupling between individual molecules, with the coupling strength obtained by analysis of the absorption spectrum reaching ca. 1,000 cm−1. Both the ideal dipole approximation and the extended dipole methodology seriously overestimate the coupling strength, but the atomic transition charge density procedure leads to good agreement with experiment. Emission is attributed to the closely coupled molecular pair functioning in an excimer‐like manner with the exciton trapped in a local minimum. Increasing temperature causes a slight blue shift and loss of fluorescence. Excimer emission: Crystals of the target compound exhibit bright emission in the red region for which the spectral profile is broad and well separated from absorption transitions. The solid‐state structure comprises tightly grouped molecular pairs that assemble into highly ordered columns while facilitating excimer formation at ambient temperature.