Photophysics and rotational diffusion of hydrophilic molecule in polymer and polyols.

In this work we report the photophysics and rotational diffusion of a hydrophilic solute 7-(N,N'-diethylamino)coumarin-3-carboxylic acid (7-DCCA) in four protic solvents: poly(ethylene glycol), ethylene glycol, tetraethylene glycol, and glycerol, with variation of temperature. The cumulative effect of polarity, viscosity, and structural features of these solvents, as well as specific solute-solvent interaction on the photophysical properties of 7-DCCA was discussed. We observed significant differences in both steady-state and time-resolved emission properties. Estimation of activation energy of viscous flow and activation energy of nonradiative decay reinforce our assumption of a cumulative effect. It was observed that, in all solvents, H-bonding interactions are mainly responsible for changing the spectral properties. Study of rotational relaxation behavior demonstrates superstick boundary condition to be operative in ethylene glycol. It is due to the H-bonding interaction between 7-DCCA and ethylene glycol. Similarly, stick boundary condition is followed in case of tetraethylene glycol at 278 K and further from 293 K. Convergence to the stick boundary is observed in case of poly(ethylene glycol). These changes can be attributed to the change in structural organization in both poly(ethylene glycol) and tetraethylene glycol.