A highly sensitive fluorescent viscosity sensor.

Variation at the 3' position of fluorescein via Suzuki-Miyaura cross-coupling with aryl and heteroaryl moieties gave a family of anthofluoresceins whose spectroscopic properties were studied. The 1-methylindole derivative gave the highest quantum yield and was observed to behave as a molecular rotor, displaying marked variations in fluorescent intensities with viscosity and offering possible application in cellular sensing and fluorescent polarisation assays.