On the applicability of the molecular dynamics SCC-DFTB treatment on optical spectra simulations for thiophene and phenyl containing oligomers

Abstract

The applicability of the semiempirical Self-Consistent Charge Density-Functional Based Tight Binding approach (SCC-DFTB) is tested and compared with the reference ab initio Møller–Plesset perturbation theory up to the second-order (MP2) and density functional theory (DFT). In this context, the calculations of 2-phenylthiophene and 2,2′-bithiophene molecules are presented with respect to the optimal geometries and torsional potentials. The vibrational broadenings of absorption spectra based on the combined on-the-fly molecular dynamics (MD) simulations were analyzed for the studied model molecules as well as for selected representative oligomeres or star-shaped molecules built from the thiophene and phenyl units. Our calculations showed that the SCC-DFTB method has the tendency to planarize the mutual distortion between the thiophene–thiophene or thiophene–phenyl units and to overestimate the energy barriers for perpendicular structures with respect to the benchmark MP2 and DFT results. The semiempirical TD-B3LYP//MD(SCC-DFTB) or ZINDO//MD(SCC-DFTB) simulations at a temperature of 300K produce narrower absorption spectroscopic bands comparing to the reference TD-B3LYP//MD(B3LYP) results. The temperature increase to 600K ensures the higher population of distorted MD(SCC-DFTB) geometries and consequently a more realistic shape of the absorption bands with respect to the internal vibrational and rotational modes.