Optical properties of quinolinium tetracyanoquinodimethanide, [Qn(TCNQ)2] and (N-methylphenazinium)x(phenazine)1-x (tetracyanoquinodimethane) [(NMP)x(Phen)1-x(TCNQ)]

We present here optical-absorption data of quinolinium ditetracyanoquinodimethanide [Qn(TCNQ)2] and (N-methylphenazinium)x(phenazine)1-x(tetracyanoquinodimethanide) [(NMP)x(Phen)1-x(TCNQ)] for 0.5 <_x<_ 0.6. These materials span the range of electronic structure from commensurate, to commensurate with confined soliton-antisoliton pairs (bipolarons), to incommensurate. The data show that these materials are semiconductors up to 300 K. The semiconducting gap, due mostly to a Peierls distortion, is weakly temperature dependent, decaying slowly as the temperature increases. This behavior, determined by the observation of the totally symmetric a, vibrational modes of the TCNQ molecule, is similar for all compositions studied and is suggested to be due to the effects of an external cation potential or to Coulomb effects, which vary little with composition. Substantial in-gap absorption is observed and is suggested to be due to the presence of quantum-nucleated and thermally generated bipolarons. The variation of higher-energy electronic transitions with composition corresponds to a crossover from commensurate to incommensurate electronic structure.