Quantum-chemical calculations of electronic spectra absorption: ab initio or semiempirical methods?

In order to develop approaches to predict the spectral properties of organic dyes for solar cells the test calculations of typical π-conjugated systems with various structural fragments, have been performed. Among the structural elements there are benzene, oxazole, oxsadiazole, thiophene and coumarin fragments. Nitro, methoxy, dimethylamino and diethylamino groups are substituents. According to the obtained experimental data, the diethylamino group at position 7 of the coumarin moiety gave the highest bato- and hyperchromic shifts of spectra. Experimental absorption spectra were measured in various solvents with different polarity. Among them are cyclohexane, dimethoxyethane, tetrahydrofuran, methanol, acetonitrile, and dimethylsulfoxide. It has been shown that there are no significant changes in the λ(max) absorption of coumarin-based compounds when the solvent is changed to a more polar one, but there is a noticeable tendency to increase the intensity of the absorption spectra. For the theoretical interpretations of electronic spectra absorption ab initio density functional theory (DFT) as well as semi-empirical methods (PPP/CIS, ZINDO/S, AM1/CIS) were used. In the DFT calculations the functionals B3LYP, CAM-B3LYP, M06-2x, PBE1PBE, wB97XD were used. A significant discrepancy in the estimations of electronic excitations is demonstrated by the DFT for the different functionals. It is concluded that DFT calculations should be performed by using both B3LYP and CAM-B3LYP (or M06-2x) functionals. Linear response theory has been used to evaluate the solvatochromic properties of π-conjugated compounds in the DFT frameworks. In general it was found that semi-empirical approaches especially π-electron method PPP/CIS and all valence method ZINDO/S can provide adequate estimations of excitation energies of π-conjugated dyes for moderate computer resources.