Asymmetric boron-complexes containing keto-isoindolinyl and pyridyl groups: solvatochromic fluorescence, efficient solid-state emission and DFT calculations

Abstract

A small series of keto-isoindolinyl and pyridyl-containing boron-complexes are synthesized and characterized. Their optical properties are described based on UV-Vis absorption and both steady-state and time-resolved fluorescence spectroscopy. DFT and TD-DFT calculations are also presented to support experimental data. The difluoroboron complexes 1a and 2a display moderate fluorescence quantum yields both in aprotic solvents with different polarity and in the solid-state. Replacement of the fluoride group with the phenyl group at the boron atom to form 1b and 2b results in efficient nonradiative decay of the S₁ state and hence a drastic decrease in fluorescence intensity. The maximum emission wavelength (λ_em) and fluorescence lifetime (τ_F) of 2a incorporating a phenyl-substituent at the isoindonyl moiety are found to be very sensitive to the solvent (a bathochromic shift of 82 nm and an 8-fold increase in τ_F in aprotic polar solvents). The longer emission wavelength in polar solvents compared with that in low-polar solvents is due to the intramolecular charge-transfer (ICT) character of the excited state from the phenyl-isoindolyl unit to the pyridyl unit. The Stokes shift of 2a exhibits a linear increase as a function of the solvent orientation parameter, reaching ∼7890 cm⁻¹ in methanol.