Computational Studies of the Electronic Absorption Spectrum of [(2,2′;6′,2″-Terpyridine)–Pt(II)–OH] [7,7,8,8-Tetracyanoquinodimethane] Complex

The electronic excitation spectrum of the [(2,2′;6′,2″-terpyridine)–platinum­(II)–OH] [7,7,8,8-tetracyanoquinodimethane] ([Pt­(trpy)­OH]­TCNQ) complex has been studied at the linear-response approximate coupled-cluster singles and doubles (CC2) level using triple-ζ basis sets augmented with polarization functions (TZVP). The calculated ultraviolet–visible (UV–vis) spectrum of the [Pt­(trpy)­OH]­TCNQ complex is compared with the UV–vis spectrum measured for [Pt­(tbtrpy)­OH]­TCNQ (tbtrpy = 4,4′,4″-^<i>t</i>Bu₃-2,2′;6′,2″-terpyridine) in dichloromethane (CH₂Cl₂) solution. The UV–vis spectrum is also compared with the calculated UV–vis spectra of [Pt­(trpy)­OH]⁺ and of the neutral and negatively charged TCNQ species. In contrast to previous interpretations, the CC2 calculations suggest that the [Pt­(trpy)­OH]­TCNQ complex is dissociated into [Pt­(trpy)­OH]⁺ and TCNQ^– when dissolved in CH₂Cl₂. The computed electronic excitation energies of [Pt­(trpy)­OH]⁺ provide information about the charge-transfer excitations between the Pt­(II) metal center and the ligands. The UV–vis spectra were also calculated at the linear-response time-dependent density functional theory (TDDFT) level using the B3LYP, BHLYP, and CAM-B3LYP functionals in combination with TZVP quality basis sets. For the TCNQ species, the TDDFT calculations yield slightly smaller excitation energies than obtained at the CC2 level, whereas for [Pt­(trpy)­OH]⁺ the CC2 excitation energies are slightly smaller than the TDDFT ones. For the [Pt­(trpy)­OH]­TCNQ complex, the B3LYP calculations yield spurious low-lying excited states rendering the spectral assignment using B3LYP data difficult. The low-energy part of the electronic excitation spectrum for the [Pt­(trpy)­OH]­TCNQ complex calculated at the BHLYP and CAM-B3LYP levels is reminiscent of the CC2 one because the larger amount of Hartree–Fock exchange and the long-range correction of the potential blue shifts the excitation energies