Spectroscopic Study of the Effect of High Pressures on Organic Charge-Transfer Complexes in Solution

Abstract

The charge-transfer solution spectra of tetracyanoethylene with benzene, hexamethylbenzene, diphenyl, and naphthalene have been observed as a function of pressure up to a maximum of 10 000 atm. The association constants and charge-transfer transition oscillator strengths have been determined for pressures up to 4000 atm in the case of the benzene—tetracyanoethylene complex. A large increase in transition oscillator strength with pressure is observed for the benzene complex; this is in accord with the theoretical predictions of Mulliken. A small decrease in association constant with pressure for this complex is tentatively interpreted in terms of complexing between the tetracyanoethylene and the solvent, methylene chloride. The observed frequency shifts with pressure, and the band splitting of the charge-transfer spectra of the diphenyl and naphthalene complexes are discussed. It has previously been suggested that solutions of charge-transfer complexes contain orientational isomers of those complexes; this theory receives a measure of support from the pressure measurements.