A highly efficient sensor molecule emitting in the near infrared (NIR):: 3,5-distyryl-8-(p-dimethylaminophenyl)difluoroboradiaza-s-indacene

The spectroscopic properties and the photophysical behaviour of difluoroboradiaza-s-indacene 1, especially designed for the near infrared (NIR) spectral region and equipped with a p-dimethylaminophenyl group at the meso-position, were studied by steady-state and time-resolved optical spectroscopy. Solvent-dependent measurements revealed that for 1, excited state deactivation is governed by population of a non-emissive charge transfer excited state ((CT)-C-1) as the solvent polarity increases, whereas reference compound 2 shows strong fluorescence from a locally excited state ((LE)-L-1) in all the solvents employed. Accordingly, protonation of 1 completely suppresses the quenching excited state charge transfer process and leads to strong enhancement of fluorescence in the NIR, distinguishing 1 as a very sensitive fluorescent sensor molecule for pH or solvent acidity in this favourable wavelength region.