TD-DFT investigation of the electronic spectra of (DOEO)₄HgBr₄· TCE salt

Abstract

In this work the, intrinsic properties of the ground and excited states of the DOEO 0 neutral molecule and the DOEO þ cation are studied;The ground-state optimised structures and energies were obtained using the density functional theory (DFT) as approximated by the various functionals (B3LYP, B3PW91, and LSDA) in restricted and unrestricted models for neutral DOEO 0 and DOEO þ cation, respectively. The character and energy of the first 30 singlet-singlet electronic transitions were investigated by applying the time-dependent DFT approximations, TD-B3LYP, TD-B3PW91, and TD-LSDA, to the corresponding optimised ground-state geometries. Moreover, excites states were calculated for DOEO dimer with one hole at the TD-B3LYP/3-21 þ G(d) level of theory; the initial geometry of the dimer was taken from X-ray data. A comparison is made between the different theoretical methodologies used to study the electronic ground and excited states. We also report the polarised optical conductivity spectra (500-18,000 cm À1 ) and the absorption spectra (400-47,000 cm À1 ) of (DOEO) 4 HgBr 4 Á TCE salt.

Spectroscopic fingerprints for charge localization in the organic semiconductor (DOEO)4[HgBr4]·TCE

“…A sharp increase of the sample resistivity below 70 K was observed in [6][7][8][9] and confirmed in special measurements for the crystal subjected to ESR studies. The room temperature dc conductivity of the crystal measured along the adirection within the ab plane was R 300 = 5 S/cm, while the perpendicular one was smaller by four orders of magnitude [6].…”

“…This continuous "transition" of the electrical dimensionality is accompanied with the localization of holes that, in turn, leads to dielectric polarization of the sample. The additional evidence of localization (besides the resistance behaviour itself [6][7][8][9]) is a significant shift of the XPS signals towards lower kinetic energies, progressively down to 140 K. The temperature dependence of the electrical anisotropy parameter R  /R  (triangles in Fig. 6) thus correlates with the temperature dependence of the level shifts in the XPS spectra (circles).…”

“…The crystal structure and low concentration of holes (one hole per molecular dimer [6][7][8][9]) explain the small value for the observed long range intermolecular exchange interaction because intermolecular distances exceed ~ 10 Å. The occurrence of antiferromagnetic long and short range order may explain the existence of a second ESR line 2 as an antiferromagnetic resonance.…”

“…The work function of a semiconductor, however, is not solely determined by the ionization energy but also by the position of the Fermi level at the surface, which may be shifted by the occupation of surface states in the gap. From the estimated value of Φ and assuming an ionization energy E I = 4.8 eV for the neutral DOEO molecules [7], it is possible to give an estimate of the expected position for the HOMO level, at a binding energy of 0.7±0.2 eV. Figure 7 shows a series of UPS spectra of the (DOEO) 4 [HgBr 4 ]·TCE crystal taken between 250 and 40 K in the valence-band region.…”

“…Synthesis, crystal structure, electrical and optical properties of the (DOEO) 4 [HgBr 4 ]·TCE semiconductors were described in detail [6][7][8][9]. Raman scattering investigations showed that the crystalsare quasi-one dimensional (1D)conductors down to 140 K,whereas below this temperature the conductivity behavior becomes similar to the one typical forquasi-2D compounds [9].…”