Effects of Solvent Dielectric Constant and Viscosity on Two Rotational Relaxation Paths of Excited 9-(Dicyanovinyl) Julolidine

The understanding of the interplay between micro environment and molecular rotors is helpful for designing and developing of molecular sensors of local physical properties. We present a study on the two rotational relaxation paths of excited 9-(dicyanovinyl) julolidine in several solvents. One rotational path (C-C single-bond rotation, tau(b)) quickly leads to the formation of a twisted state. The other path (C=C double-bond rotation, tau(c)) shows that the populations go back to the ground state directly via a conical intersection between the S-1 and ground state. The increase in the solvent dielectric constant shows little effect on the tau(b) lifetime for its small energy barrier (<0.01 eV), but lifetime is increased in larger dielectric constant solvents due to the larger energy gap at conical intersection. Both tau(b) and tau(c) are increased greatly with the increased solvent viscosity. tau(b) is more sensitive to viscosity than tau(c) may be due to its larger rotational moiety.