Reinvestigation of electronic structure and electric properties of large betaine molecules: A combined long-range-corrected DFT and coupled-cluster study

In this study we discuss the electric properties of imidazole pyridinic betaine family of molecules. We present the results of calculations of dipole moments corresponding to the ground and to the lowest-lying electronic excited state as well as first hyperpolarizability. The two quantities have been determined using high-level ab initio quantum-chemical approaches including the second-order Møller–Plesset perturbation theory and coupled-cluster methods. It is shown that the Hartree–Fock approximation, employed quite recently to describe photophysical properties for this class of compounds [J.M.F. Pinheiro, P.H.R. Peixoto, C.P. de Melo, Chem. Phys. Lett. 463 (2008) 172], yields qualitatively and quantitatively incorrect results of molecular first hyperpolarizability as well as excited state dipole moments. We also show that as far as static first hyperpolarizability is concerned the studied compounds are challenging for long-range-corrected density functional theory.