Charge–transfer organic crystals: Molecular vibrations and spectroscopic effects of electron–molecular vibration coupling of the strong electron acceptor TCNQF4

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Abstract
A fundamental aspect in the study of the charge–transfer (CT) organic crystals with ionic or partially ionic ground states is the investigation of the spectroscopic effect of the electron–molecular vibration coupling. 7,7,8,8‐tetracyano‐2,3,5,6‐tetrafluoroquinodimethane (TCNQF4), an electron acceptor much stronger than TCNQ, is an outstanding component of many interesting CT systems. A thorough vibrational analysis of the title compound and of its monomeric radical anion is reported. The analysis is based on the Raman depolarization ratio measurements and on infrared data of solutions of both neutral and ionic species as well as on polarized infrared spectra of oriented crystals of the neutral molecule. The vibrational assignment, completed by a normal coordinate analysis (NCA), brings to the identification of the ionization frequency shifts and to the choice of the fundamentals (b1uν19 and b2uν33) whose frequencies are diagnostic of the degree of charge transfer for a TCNQF4 moiety partner of a CT system. Through a combined use of a RHF‐CNDO/S(CT) electronic calculation and of the eigenvectors of the radical anion given by the NCA, the numerical evaluation of the linear e–mv coupling constants is carried out. The relative values of these constants are extracted, by applying the dimer model, from the vibronic features of the infrared spectrum of the low temperature phase of Rb–TCNQF4. This compound is recognized, by comparison with the alkali salts of TCNQ, to display a phase transition at T≂130 K from a regular to a dimerized stack structure. From the analysis of all these data it is possible to characterize the spectroscopic vibronic effects related to e–mv coupling in CT systems of TCNQF4. This is successfully verified by the segregated dimerized stack system of DBTTF–TCNQF4 and applied to the not yet characterized system TSF–TCNQF4 which is recognized here as a fully ionic complex with segregated dimerized stack motif. As a consequence, through the vibrational spectra, TCNQF4 can be fruitfully used as a sensitive probe for structural and electronic characterization of its CT complexes

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