INSIGHTS INTO THE SOLVATO-/THERMO-PROMOTED INTRAMOLECULAR ELECTRON TRANSFER IN A TTF-σ-TCNQ DYAD WITH AN EXTREMELY LOW HOMO–LUMO GAP

The low-lying states of an organic donor-σ-acceptor dyad, i.e. tetrathiafulvalene-σ-tetracyano-p-quinodimethane (TTF-σ-TCNQ), in gas phase and in various solvents have been investigated by means of hybrid DFT calculations in combination with the conductor-like polarizable continuum model to describe solvent effects. It has been shown that the dyad, though preferring a closed-shell singlet ground state with an eclipsed conformation in gas phase, adopts the charge-separated zwitterionic states with an extended conformation (TTF+-σ-TCNQ-), i.e. open-shell singlet biradical ground state immediately followed by triplet biradical state, in polar solvent ( CH3CN and CH2Cl2 ) as a result of the intramolecular electron transfer (ET) stimulated by solvent polarization. The degree of such intramolecular ET is so strongly dependent on the polarity (dielectric constant) of solvent that the zwitterionic biradical states become more stable with respect to the closed-shell singlet state with increasing polarity of the solvent. As such, the dyad should show a higher ratio of biradicals in more polar solvent and/or at higher temperature and, hence, is chameleonic in nature.

CHAMELEON GROUND STATE AND EXCITED STATES OF EDT-TTF-IM-F4TCNQ RADICAL DYAD IN DIFFERENT ENVIRONMENTS

We have performed a systematic density functional study on the ground-state electronic structure and excited states of a representative D-σ-A dyad, i.e. EDT-TTF-Im-F4TCNQ π-radical, in vacuo and in different conventional solvents (toluene, THF, DMF and DMSO) by using some popular hybrid density functionals (B3LYP, M05, M05-2X, PBE0 and BMK). It has been shown that the M05 and B3LYP functionals perform the best in predicting the intramolecular charge-transfer (ICT) pertaining to both the ground state and excited states of the dyad. The amphoteric dyad is liable to solvent-promoted ICT from its EDT-TTF-Im donor (D) to F4TCNQ acceptor (A), adopting a charge-unseparated ground state D-A• in vacuo, a partially zwitterionic ground state [D-A]• in nonpolar toluene solvent, and a fully zwitterionic ground state D•+-A- in such polar solvents as THF, DMF and DMSO. Owing to its solvent-dependent chameleon ground state, excited states of the dyad in solvents also exhibit remarkable dependence on solvent polarity, as revealed by TDDFT calculations. Furthermore, cluster model calculations revealed that intermolecular charge-transfer readily occurs between the dyads, accounting for the observed zwitterionic charge state in solid state and solid-state semiconductivity.

THEORETICAL STUDY OF NITROGEN MONOXIDE ADSORPTION ON RHODIUM CLUSTERS AT DIFFERENT SITES

The adsorption of nitrogen monoxide NO with charged and neutral [Formula: see text] clusters at atop, bridge, and threefold hollow sites had been investigated by density functional theory calculations. The results showed that rhodium clusters had strong orbital interactions with NO and formed the complex [ Rh n NO ]-/0/+. The stretching vibrational frequencies of the N–O bonds changed with the different adsorption sites and clusters sizes. The interactions between rhodium clusters and NO molecular could be described through the donation and back-donation of their frontier orbitals. The more back donation from Rh to NO , the weaker the N–O bonds, exhibiting that the lengthening of the N–O bond length and the lowering of its vibrational frequency. In general, the donation and back-donation interactions followed the tendencies: anionic > neutral > cationic, big size > small size, threefold hollow site > bridge site > atop site.