Solvatochromic shift of donor-acceptor substituted bithiophene in solvents of different polarity: quantum chemical and molecular dynamics simulations.

The dependence of excitation energies of the solvatochromic dye, 5-dimethylamino-5'-nitro-2,2'-bithiophene (Me(2)N-2T-NO(2)) on the solvent polarity is demonstrated by time-dependent density functional theory (TD-DFT) calculations in combination with molecular dynamics (MD) simulations. Three kinds of solvation models, namely, the continuum dielectric model, the discrete approach, and the combined discrete/continuum strategy, are employed to calculate the lowest dipole-allowed excitation energies of Me(2)N-2T-NO(2) in seven solvents with the dielectric constant, epsilon, ranging from 2.23 to 111.00. Our calculations demonstrate the limitations of the continuum dielectric model in predicting the solvatochromic shift of Me(2)N-2T-NO(2) in very polar solvents with epsilon > 35. The accuracy of the explicit solvent model is largely limited by the size of supermolecular cluster. The combined discrete/continuum solvent model gives a satisfactory description of the bathochromic shift of Me(2)N-2T-NO(2) with increasing solvent polarity, in agreement with the experimental observations.