MONOMER, DIMER, AND TETRAMER STATES IN MOLYBDENUM COMPLEXES OF TETRACYANOQUINODIMETHANE

We report on the determination of monomers, dimers, and alternating st ates of tetracyanoquinodimethane (TCNQ) species in molybdenum complexe s using optical absorption and Raman spectroscopy. The molecular cryst als investigated are [(MoO)-O-VI(dtc)(3)](TCNQ) 1a, [Mo-V(dtc)(4)](TCN Q) 2a and 2b, and [Mo-V(dtc)(4)](TCNQ)(2) 3a (a, dtc = Et(2)NCS(2); b, dtc = Me(2)NCS(2)). Optical and micro-Raman experiments performed at 50, 78, and 293 K yield information about the electronic states and th e mode of packing of TCNQ moieties in these crystals. The optical spec trum of 2a presents the characteristic bands of the isolated monomeric species TCNQ(.-) at 1.46, 1.63, and 3.02 eV. In complex la, the dimer ized state is evidenced by the presence of three absorptions: two exci tonic transitions LE1 at 1.9 eV and LE2 at 3.4 eV and an intermolecula r charge transfer band (CT) near 1.38 eV. The bands LE1 and LE2 are bl ue-shifted by Davydov effect with respect to their position in the mon omer states. Furthermore, it turns out that dimers are totally eclipse d in this system. Despite the fact that complex 2b differs only from 2 a by a methyl substitution in the organometallic cation, we find that this salt presents dimer species which are well characterized by Raman spectroscopy. A coupling between the CT and LE1 bands establishes the presence of slipped dimers. At low temperature, however, Raman and op tical spectra indicate a probable transition towards an eclipsed dimer state. Our optical spectra show that a perfect dimerization state is not possible in complex 3a since there is no excitonic band at 1.9 eV. However, the occurrence of a CT band at 1.23 eV corresponding to a ch arge transfer between TCNQ(0)and TCNQ(.-) suggests the presence of TCN Q(0)/TCNQ(.-)/TCNQ(.-)/TCNQ(0)-stacked tetramers.