Harnessing substituent and aggregation-induced effects for color-tunable emission in borafluorenium ions

Once considered a “laboratory curiosity”, cyclic borenium ions have recently been shown to exhibit tunable emission and stimuli-responsive properties. Utilizing hexaphenylcarbodiphosphorane (CDP), a series of borafluorenium and 3,3′-dimethoxyborafluorenium ions were synthesized to determine the impacts of counteranion and substituent effects on the optical properties and stability of borafluorenium ions. Clear relationships were established between structure and properties, with emission wavelengths of the borafluorenium ions ranging from yellow (λ_em = 559 nm) to red (λ_em = 650 nm). By employing density functional theory, a possible mechanism for the observed luminescent behavior was proposed. These compounds were shown to exhibit aggregation-induced emission (AIE) properties and significant changes in solid-state emission color when compared to those observed in solution. The AIE properties of a CDP–borafluorenium ion with dimethoxy substitution were further explored by perturbing the temperature in solution, which resulted in a clear shift in emission wavelength from 563 nm (yellow, 20 °C) to 513 nm (green, −90 °C).