Dipolar Ferrocene and Ruthenocene Second-Order Nonlinear Optical Chromophores: A Time-Dependent Density Functional Theory Investigation of Their Absorption Spectra

The origin of the two prominent solvatochromic near-UV/visible/near-IR absorptions observed for donor-(pi-bridge)-acceptor chromophores with ferrocene donors has been investigated using TD-DFT methods. Both chromophores with relatively weak (4-nitrophenyl) and strong acceptors (1,3-diethyl-2-thiobarbituric acid and 3-dicyanomethylidene-2,3-dihydrobenzothiophene-1,1-dioxide) were considered, as were ferrocene and octamethylferrocene donors. Computational predictions of optical properties made using the B3PW91 functional were found to be in good agreement with experimental data. The calculations reveal a complex orbital picture that varies from compound to compound, contribution of multiple configurations to some of the important states, and significant contributions from more than one transition to the experimentally observed bands. Natural transition orbitals have been used to gain an understanding of the charge redistribution associated with the transitions. The relatively weak low-energy bands of the ferrocene derivatives were generally found to have both d-d and metal-to-pi-bridge/acceptor charge-transfer character. The stronger higher energy bands were found to be associated with charge transfer from cyclopentadienyl rings and the pi bridge toward the acceptor group. The experimental spectra of ruthenocene chromophores differ significantly from those of the analogous ferrocene chromophores; however, the calculations reproduce the key differences and indicate a similar origin for the contributing transitions.