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

Abstract

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.

Solvent-tuned charge-transfer properties of chiral Pt(ii) complex and TCNQ˙− anion adducts

“…418 nm in the visible range, which is in line with the reported electronic excitation spectrum of the TCNQc À anion, and agrees well with previous interpretations of the calculated excitation energies. 47,48,65,66 During a titration experiment, LiTCNQ at various ratios is added to a MeOH solution of (À)-1 (Fig. 5).…”

“…TCNQc À anion, so the CT band observed at 860 nm is tentatively assigned to a charge transfer transition from the Pt(II) complex cation to the TCNQc À anion. 65,66 Complex (À)-1 is emissive in both MeOH and H 2 O solution. During the addition of LiTCNQ, different attenuation rates of emission intensity have been observed.…”

Importance of Vibronic Effects in the UV–Vis Spectrum of the 7,7,8,8-Tetracyanoquinodimethane Anion

“…Metal–organic complexes with 7,7,8,8-tetracyanoquinodimethane (TCNQ – ) as counterion form solid-state materials consisting of supramolecular stacks that strongly absorb light in the whole visible range. − The metal–organic moiety absorbs in the near-UV and short-wavelength visible range of the absorption spectrum, and the strong absorption is extended to longer wavelengths by the TCNQ – anion leading to black absorber materials suitable for solar cell applications. − However, despite absorbing over the entire visible spectrum, the TCNQ – anion does not exhibit any electronic transitions in the visible region. , Here we investigate whether vibronic effects arising from quantum nuclear motion can explain this conundrum.…”

“…The vibrational and structural broadenings of the peaks are then considered by using either a Gaussian or Lorentzian line shape of a given width for each transition. This empirical line broadening does not contain any physical information but is widely used, faute de mieux , for visualizing the spectrum …”

“…We apply the method to simulate the UV–vis spectrum of TCNQ – , which absorbs light in almost the whole visible range even though it has no spin-allowed electronic transitions in that region. The TCNQ – anion has one electronic transition in the near-infrared (near-IR) region with a wavelength of 850 nm and one transition at 395 nm. ,,− Due to Franck–Condon effects, the vibronic coupling leads to a broad absorption band with several peaks in the visible range. ,− Fluorescence measurements on the TCNQ – anion in solution show a weak fluorescence suggesting that internal conversion (IC) or intersystem crossing (ISC) processes are very fast. ,, …”