The effect of locking π-conjugation in organoboron moieties in the structures of luminescent tetracoordinate boron complexes
A series of 8 luminescent borafluorene complexes were extensively studied both experimentally and theoretically in order to elucidate the effect of organoboron moiety rigidification on physicochemical properties of these compounds. Due to spiro geometry of boron atom, borafluorene and ligand units are perpendicularly aligned, what considerably affects the flexibility of the molecule as well as its solid-state structure. Through the comparative analysis with close diphenyl analogues, we show how these structural features influences thermal, photoluminescent and charge mobility behaviour of studied compounds. Crystal structural analysis revealed that molecules are mostly connected through C-H…O and C-H… π interactions formed between perpendicularly aligned borafluorene and ligand moieties from neighboured molecules, serving as complementary donor and acceptor of electron density, respectively. This also efficiently prevents molecules from engaging in unfavoured π-stacking contacts. Furthermore, structural analysis suggests that borafluorene complexes possess considerable degree of flexibility due to OBN heterocycle distortions and mutual borafluorene-ligand planes movements. Magnitude of these effects strictly depends on the ligand structure and may lead either in enhancing or lowering the quantum yield value with respect to BPh2 analogues, while the absorption and emission wavelength are slightly affected. The measured photophysical parameters for solid-state samples show that studied complexes are much better emitters in their crystalline states that in amorphous films. The TD-DFT and NTO calculations reveal significant change in frontier molecular distribution, with HOMO localized on borafluorene moiety. However, as the HOMO-LUMO transition is geometrically not favoured, excitation occurs from HOMO-1 localized on the ligand. Finally, aggregation effects are discussed based on supramolecular arrangement in crystal structures and charge transfer rates obtained from theoretical calculations in the framework of Marcus-Hush approximation. They suggest that borafluorene complexes are much better electron carriers with respect to non-annulated BPh2 complexes.