LC-BLYP Calculations of the Structures and Photophysical Properties of [1,3]Thiazolo[4,5-b]pyrazine Derivatives in Cyclohexane and Methanol.

Applications of the B3LYP and LC-BLYP functionals with the 6-311+G(d,p) basis set for predicting absorption and fluorescence spectra of benzothiazole and 11 [1,3]thiazolo[4,5-b]pyrazine (TPy) derivatives in implicit solvents (cyclohexane and methanol) were tested and compared with experimental results. The damping parameter of LC-BLYP was tuned for simulating absorption and fluorescence spectra of each compound. For the 11 TPy derivatives, four and seven compounds are small- and medium-sized TPy derivatives, respectively. From this calculation, TD-B3LYP can be used to simulate the absorption spectra of the small-sized compounds in cyclohexane and methanol, but it gives poor results for the fluorescence spectra in cyclohexane and methanol. Contrarily, the LC-BLYP functional with the optimal damping parameter can reproduce the absorption and fluorescence spectra of all compounds in cyclohexane and methanol. The values of the damping parameter of LC-BLYP depend on the types of substituents and solvents. Moreover, the conformations of the compounds do not affect the spectra, and the molecular size and molecular dipole moment do not affect the damping parameter. In addition, types and the number of substitutions attached to the phenyl rings of TPy as well as solvent polarity play the key roles in the long-wavelength fluorescence emission and fluorescence brightness of the compounds. The intramolecular charge transfer of the compounds containing the -NMe2 and -CN groups is predominant in methanol, resulting in a large Stokes shift and oscillator strength.