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