2. CALCULATION METHODS
All calculations in this work were performed using Gaussian 09 software. The ground state geometry optimizations of all 12 dye molecules shown in Figure 1 were performed at the B3LYP41, 42/6-31G (d, p) level. There were no restrictions on the symmetry of the molecules during the optimization. In order to consider the solvent effect, dichloromethane was chosen that was used in the experiment, and the solvent model was the Polarizable Continuum Model (PCM). Standard convergence criteria were used, i.e. the SCF, gradient, and energy convergence was set to be 10-8, 10-4, and 10-5 a.u., respectively. Frequency calculations were performed at the same theoretical level as geometry optimization and the results showed that all the optimized structures are at the minimum on the potential energy surface.
Using the optimized structures we performed TD-DFT calculations to obtain the absorption spectra of the lowest 50 single-excited states for the twelve dye molecules in dichloromethane. In the TD-DFT calculations, the range separation functional ωB97XD43 was used and 6-311G(d,p) basis set44-46 was used for all atoms. Note that our previous work showed that this basis set has a good balance between accuracy and computing time.47