FROM RACEMIC MIXTURES OF CHIRAL PI-DONOR MOLECULES TO MIXED STACKS OF H-BONDED CENTROSYMMETRICAL DIMERS OF CATION AND ANION-RADICALS WITH SINGLET-TRIPLET EXCITATIONS - THE EXAMPLE OF [(+/-)ME3TTF-C-ASTERISK-H(MEOH).+]2[TCNQ.-]2 (TTF = TETRATHIAFULVALENE - TCNQ = TETRACYANOQUINODIMETHANE)

Lithiation of Me3TTF, and subsequent reaction with ClCOMe followed by reduction with NaBH4 afford the racemic alcohol (+/-)Me3TTF-C*H(Me)OH. Single crystals of the title compound [(+/-)Me3TTF-C*H(Me)OH.+]2[TCNQ.-]2 were obtained, and their structure was determined by X-ray diffraction, revealing an unusual type of mixed stacking where the chiral pi-donor and the acceptor molecule are associated within a ... DDAA... sequence. The room temperature ESR spectrum is characteristic of two different triplet Frenkel exciton systems. From the analysis of the single-crystal ESR measurements the singlet-triplet activation energy and the fine structure tensor orientation were determined for both triplet excitations. The relationship between these parameters and the structure allowed us to propose an interpretation of the nature of both excitations. The role of the hydrogen bonding between the dimers of donors and acceptors is discussed.