Monomer, dimer, and tetramer states in molybdenum complexes of tetracyanoquinodimethane

We report on the determination of monomers, dimers, and alternating states of tetracyanoquinodimethane (TCNQ) species in molybdenum complexes using optical absorption and Raman spectroscopy. The molecular crystals investigated are [(MoO)-O-VI(dtc)(3)](TCNQ) 1a, [Mo-V(dtc)(4)](TCNQ) 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 the mode of packing of TCNQ moieties in these crystals. The optical spectrum 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 dimerized state is evidenced by the presence of three absorptions: two excitonic transitions LE1 at 1.9 eV and LE2 at 3.4 eV and an intermolecular charge transfer band (CT) near 1.38 eV. The bands LE1 and LE2 are blue-shifted by Davydov effect with respect to their position in the monomer states. Furthermore, it turns out that dimers are totally eclipsed in this system. Despite the fact that complex 2b differs only from 2a 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 optical 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 charge transfer between TCNQ(0)and TCNQ(.-) suggests the presence of TCNQ(0)/TCNQ(.-)/TCNQ(.-)/TCNQ(0)-stacked tetramers.