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″-(t)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.