Theoretical study of the effects of different substituents of tetrathiafulvalene derivatives on charge transport

Huixue Li,Zhifeng Li,Xiaofeng Wang

doi:10.1007/s11434-012-5222-z

Huixue Li, Zhifeng Li + Show 1 more

Open Access

https://doi.org/10.1007/s11434-012-5222-z

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Journal: Science Bulletin	Publication Date: May 14, 2012
Citations: 9	License type: CC BY 2.0

Affiliation: Tianshui Normal University

Abstract

Density functional theory calculations were carried out to investigate the charge transfer of four tetrathiafulvalene derivatives. Perfluorination of dibenzo-tetrathiafulvalene (DB-TTF) increased the reorganization energy and was considered disadvantageous for the charge-transport process. Fluorination lowered the frontier orbitals of the compound, favoring electron—rather than hole-transport due to the low injection barrier. While intra-ring substitution of carbons of benzene with N atoms did not increase the reorganization energy, it enforced thermodynamic stability and decreased the charge injection barrier due to lowering the frontier orbital. Calculation results also showed that introduction of NH2 to DB-TTF can change the crystal structure and charge mobility, thus providing a method with which to promote ɛ-stacked structures. Calculation of charge transfer integrals using site energy correction methods was found to be more suitable for perfluorinated DB-TTF because it exhibits remarkable polarization effects.

Highlights

Introduction of an amino group toDB-TTF to yield NH2DB-TTF changes the electronic configuration of the atoms
Han group [41,42] pointed out that there exit great difference for the charge transport along different directions in the identical organic semiconductor, they have performed some import calculations about the anisotropic mobility, and the results showed the charge mobility along a specific path can be improved one order of magnitude compared with the total mobility
Charge mobility was found to depend mainly on the monomer reorganization energy and the coupling matrix element between dimmers. Both the reorganization energy and transfer integral were calculated at the first-principles DFT level, and results showed that perfluorination increases the stability of radical anions, lowers the injection barrier of electrons, and increases the reorganization energy of the charge-transport process

Summary

Computational approaches

The incoherent hopping model is adopted to illustrate the charge transfer between a neutral molecule and a neighboring charged molecule in organic materials. Several studies have described how to obtain these parameters from first-principles calculations [9,10,11,24] Reorganization energy has both internal and external contributions, with the internal contribution arising from geometric changes in the geometry of the molecular dimer when electron transfer takes place, and the external contribution coming from changes in the surrounding media accompanying the charge transfer. A more direct and simpler way of calculating the transfer integral involves direct evaluation of the coupling element between frontier orbitals using the unperturbed density matrix of the dimer Fock operator. In this case, the transfer integral is calculated using the PW91PW91/6-31G* basis set, which has been proven to be simple, efficient, and reliable [31,32]. [34] and Edward et al [35] proposed the site energy correction (SEC) model, where the site energy Hii and charge transfer integral V are expressed as

Results and discussion

Frontier orbital energy level and reorganization energies

Charge transfer integral and mobility

Conclusion