Molecular Rotors as Fluorescent Viscosity Sensors: Molecular Design, Polarity Sensitivity, Dipole Moments Changes, Screening Solvents, and Deactivation Channel of the Excited States
A library of new fluorescent molecular rotors (FMRs) for viscosity sensing was synthesized. The sensitivity of the fluorescence emission toward solvent viscosity and polarity was investigated by using UV/Vis absorption, fluorescence emission spectra, and theoretical calculations. For the new FMRs, red-shifted emissions at 620 nm, Stokes shifts of 170 nm, and up to 40-fold fluorescence enhancement upon increasing the viscosity of solvents were observed (cf. known FMRs with emissions at 491 nm, Stokes shift of 33 nm and fivefold emission enhancement). By using solvents with high viscosity but low polarity, for example, polyethylene glycol (PEG-400) or dimethyl silicone oil, rotors previously identified as non-FMRs with ethylene glycol/glycerol show FMR properties. We found triphenylamine (TPA)-based rotors showed solvent-polarity-dependent emission, but also FMR properties. This is contrary to the known theory of FMRs. One of the TPA-based rotors shows the highest x value of 0.88 (versus 0.6 for known FMRs). The emissive excited states of the FMRs proved to be locally excited (LE) states and twisted intramolecular charge-transfer (TICT) states were identified as dark states through trifluoroacetic acid titration and DFT/time-dependent DFT calculations, which show that the nonradiative decay channel of the excited FMRs is the rotation about the dicyanovinyl C=C double bonds in the S1 state, not rotation around C–C single bonds. Extension of the π conjugation of the rotors increases the rotation barrier around the C=C double bond in the S1 state (to ca. 50 kJ mol–1 or higher), thus the nonradiative channel is blocked and higher fluorescence quantum yields are observed for the new rotors. Our results will be useful for future design of FMRs as fluorescent viscosity sensors.