Abstract
The electronic spectrum of tetrathiafulvalene (TTF) oriented in a polyethylene film is characterized experimentally using synchrotron radiation linear dichroism from the near-UV to the vacuum-UV region to 56,500 cm–1 (177 nm). The non-planar TTF molecule is shown to have electronic transitions of significant intensity polarized along all three molecular symmetry axes. A procedure for processing linear dichroic data to obtain partial absorbance curves representing different polarization directions is exemplified. The overlapping contributions to eight individual spectral features are furthermore interpreted by the aid of time-dependent density functional theory using CAM-B3LYP/6-311++G(3df,3pd) with geometry optimized at the B3LYP / 6-311++G(3df,3pd) level within the C2v point group. Additional calculations using larger basis sets as well as other long-range corrected functionals (LC-ωPBE and ωB97XD) yielded similar results. The planar conformer with D2h symmetry is also considered and it is discussed that rapid equilibration via this transition state may contribute to the diffuse nature of some features of the electronic spectrum. The TTF molecule is a dynamic and 3-dimensional chromophore, and these characteristics may serve as a basis for understanding the optical properties of the numerous TTF-based materials finding applications in supramolecular chemistry, molecular electronics and beyond.
Highlights
The tetrathiafulvalene (TTF, Scheme 1) motif is renowned for its role in Bechgaard salts [1,2]
The TD–CAM-B3LYP calculations for boat-shape TTF were repeated with the larger basis sets AUG-cc-pVTZ and AUG-cc-pVQZ [46,47] and additional Time Dependent Density Functional Theory (TD-DFT) calculations were performed with the long-range corrected functionals LC-ωPBE [48,49] and ωB97XD [50], all yielding rather similar results (S7 – S11)
Interpretation of the experimental linear dichroic absorbance data show that TTF is a truly 3-dimensional chromophore with UV spectral contributions from electronic transitions polarized along all three molecular axes
Summary
The tetrathiafulvalene (TTF, Scheme 1) motif is renowned for its role in Bechgaard salts [1,2] These are organic superconductors that rely on charge-transfer complexes comprised of a TTF derived radical cation and an anion, which in the solid state can display low-temperature superconductivity. TTF is intrinsically connected with the term molecular electronics and with socalled Aviram−Ratner Rectifiers that were proposed in a seminal paper in 1974 [4]. Such compounds where a TTF donor is connected with an acceptor by a σ-bonded bridge have since been experimentally characterized; see for instance Ho et al [5]. Additional information is provided as Supplementary data, referred to in the ensuing text as S1 – S11
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