Infrared, optical, and luminescent properties of quasi-one-dimensional organic charge transfer salts and polymers.

摘要

Polarized infrared and optical reflectance spectroscopies were used to study the structural phase transitions in two quarter-filled semiconducting organic charge transfer salts, N-propylquinolinium(TCNQ){dollar}sb2{dollar} and N-dimethyl thiomorpholinium(TCNQ){dollar}sb2{dollar}; and two half-filled salts, Potassium-TCNQ and Rubidium-TCNQ.; The spectra of NPrQn(TCNQ){dollar}sb2{dollar} display characteristics common to quarter-filled charge transfer salts. Various cluster models were applied to describe the electron-phonon coupling at 300 K. The weakly metallic transport properties above {dollar}Tsb{lcub}c{rcub}{dollar} in NPrQn(TCNQ){dollar}sb2{dollar} are attributed to the uniform charge distribution within the tetramer and the high degree of overlap between the intra- and inter-tetramer charge transfer bands. Despite evidence for a larger intra-tetramer transfer integral below {dollar}Tsb{lcub}c{rcub},{dollar} the transport properties are dominated by charge localization, as evidenced by the doublet pattern in the vibrational features.; In the DMTM(TCNQ){dollar}sb2{dollar} salt, measurements were made on several faces of large single crystal samples. The electron-phonon coupling behavior was analyzed within the framework of both an isolated dimer model and a twofold commensurate charge density wave model. Our data, polarized close to the b crystallographic axis, do not support the idea that the unusual transport properties of semiconducting DMTM(TCNQ){dollar}sb2{dollar} are caused by low-energy inter-chain charge transfer.; The spectra of two closely related half-filled salts, K-TCNQ and Rb-TCNQ, were also investigated. The {dollar}Asb{lcub}g{rcub}{dollar} features change gradually with temperature in the potassium sample, whereas in the rubidium sample little contrast was observed. The far infrared {dollar}Asb{lcub}g{rcub}{dollar} vibrational features in K-TCNQ continue to change gradually above {dollar}Tsb{lcub}c{rcub}{dollar}. In contrast, the {dollar}Asb{lcub}g{rcub}{dollar} vibrational modes in Rb-TCNQ vanish almost completely at {dollar}Tsb{lcub}c{rcub}.{dollar} These observations suggest that while the phase transition is strongly first-order for Rb-TCNQ, it is closer to second-order in K-TCNQ. The temperature dependence of the lattice modes is also discussed.; Finally, optical absorption, photo- and electro-luminescence measurements were performed on a series of semiconducting polymers based upon phenylene and thiophene. Visible light emission was observed from two materials with discrete emitter units, suggesting that intra-chain mobility of the charge defect may not be essential to radiative recombination of the exciton.