A time-dependent density functional theory investigation on the nature of the electronic transitions involved in the nonlinear optical response of [Ru(CF3CO2)3T] (T = 4′-(C6H4-p-NBu2)-2,2′:6′,2″-terpyridine)

Abstract

We report a theoretical study based on density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on the nature and role of the absorption bands involved in the nonlinear optical response of the complexes [Ru(CF₃CO₂)₃T] (T = T1, T2; T1 = 4′-(C₆H₄-p-NBu₂)-2,2′:6′,2″-terpyridine, T2 = 4′-(C₆H₄-p-NMe₂)-2,2′:6′,2″-terpyridine). Geometry optimizations, performed without any symmetry constraints, confirm a twisting of the –C₆H₄-p-NBu₂ moiety with respect to the plane of the chelated terpyridine. Despite this lack of strong π interaction, TDDFT excited states calculations of the electronic spectrum in solution provide evidence of a relevant role of the NBu₂ donor group in the low-energy LMCT band at 911 nm. Calculations also show that the two bands at higher energy (508 and 455 nm) are not attributable only to LMCT and ILCT transitions but to a mixing of ILCT/MLCT and ILCT/π−π* transitions, respectively. The 911 nm LMCT band, appearing at lower wavelength of the second harmonic (670 nm) of the EFISH experiment, controls the negative value of the second-order NLO response. This is confirmed by our calculations of the static component β⁰_zzz of the quadratic hyperpolarizability tensor, showing a large positive value. In addition we have found that the increase of the dipole moment upon excitation occurs, in all the characterized transitions, along the dipole moment axis, thus explaining why the EFISH and solvatochromic experimental values of the quadratic hyperpolarizability agree as sign and value.