Hydrodynamic fluorescence emission behavior of molecular rotor-based vinyl polymers used as viscosity sensors

A molecular rotor compound containing an allyl group (aMR) was synthesized and copolymerized with the vinyl monomers methyl methacrylate (MMA), styrene (St), and 2-hydroxyethyl methacrylate (HEMA). The absorption and fluorescence (FL) emission maxima of the solution of aMR in THF were observed at 445 and 485 nm, respectively. The FL quantum yield (FLQY) of aMR in THF was quite low (0.21%). However, it was found that the FL intensity significantly increased as the viscosity of the fluid increased. This change was expressed using the Förster-Hoffmann equation, log I = C + x log η, where I and η are the FL intensity of the probe and the viscosity of the fluid, respectively. The value of x was determined to be 0.66 and 0.53 for the methanol/ethylene glycol mixtures and alcohols, respectively. The absorption and FL emission spectra of the aMR-incorporated vinyl polymers were almost similar to those of aMR. The FLQYs of P(MMA-co-aMR), P(St-co-aMR), P(HEMA-co-MMA-co-aMR) films were 4.9, 4.3, and 5.1%, respectively. These FLQYs are ∼10 times of the values obtained with their solutions. As soon as the polymer film came into contact with the fluids, its FL emission started to decrease and reached an equilibrium state after a few minutes. It was found that the equilibrium FL emission became weaker as the viscosity of fluid decreased. The FL intensity ratio was also expressed using the modified Förster-Hoffmann equation. The x values of P(MMA-co-aMR) for alcohols and methanol/ethylene glycol mixtures, P(St-co-aMR) for hydrocarbons, and P(HEMA-co-MMA-co-aMR) for the methanol/ethylene glycol mixtures were 0.26, 0.24, 0.29, and 0.53 respectively.