Charge Transfer and Electron‐Molecular Vibration Coupling in Tetramerized Quasi‐lD Semiconductors

Abstract

Electron‐molecular vibration coupling in tetramerized organic ion‐radical semiconductors is considered. Eigenvalues and eigenfunctions of the molecular tetramer with two electrons are calculated in terms of extended Hubbard model parameters. The energy inequivalence of the monomer sites is introduced explicitly. The complex frequency dependent conductivity of the tetramerized compounds is found to produce a number of sharp maxima due to indirect excitation of the totally symmetric (ag) internal molecular modes of vibrations. These peaks are shown to have a fine structure if the charge density is not shared equally by the constituent molecules. The calculated reflectance is compared with the measured one for TEA (TCNQ)2 single crystals.

Application of the Dimer and Tetramer Models for Interpretation of the IR Spectra of TEA(TCNQ)2

“…These peaks are shown to have a fine structure if the charge density is not shared equally by the constituent molecules [5]. The fine structure of the activated ag modes was observed experimentally in TEA(TCNQ)2 [6][7][8]1].…”

“…This is why other models, in particular a model of quasi-isolated tetramers should be acceptable for interpretation of the polarized reflection spectra of the salt. The tetramer with two electrons has been calculated by Yartsev [5] in terms of extended Hubbard model parameters. The frequency dependent conductivity may be found as H e r e N t i s t h e n u m b e r o f t e t r a m e r s p e r u n i t v o l u m e , γa d e n o t e s t h e n a t u r a l w i d t h of the originally uncoupled vibrational modes, A and B are parameters describing the tetramer geometry and "j are the reduced CT electronic polarizabilities.…”

“…Contrary to that approach, as the other authors' simplification relative to γα does not seem to be substantial, we attempted to fit natural widths γα as well. The tetramer model was tested by Yartsev [5] for the experimental data of Belousov et al [8] at 300 K only.…”

“…It was assumed that the natural widths of both components of the vibronic bands are the same. We chose U/V equal to 0.4 as assumed by Yartsev [5] for TEA(TCNQ) 2 and U = 0.6 eV. Then using the matrix elements from the same paper we obtain the optimal values of ΓCT, ωα', ωα n , γαa ' = γ α " , g α 'a n d g « " , w h e r e t h e f o r m e r p a r a m e t e r s r e l a t e t o the main (wide) components of the doublets and the latter -to the narrow components.…”

Electron-molecular vibration coupling in (DMtTTF)Br and (o-DMTTF)2[W6O19] salts studied by vibrational spectroscopy

“…Subsequently, the isolated cluster model was also successfully applied to systems having greater than two TCNQ molecules [10][11][12]. Fitting the IR spectra of various TCNQ salts via multiple models afforded the unique opportunity to experimentally determine fundamental molecular parameters such as the EMV coupling constants and compare them to those obtained through theoretical calculations [13].…”