Knowledge Bank

Photoconductivity in a dilute rigid solution of tetramethylparaphenylenediamine (TMPD) in 3-methylpentane at $77\circ K$ was the subject of a recent preliminary $report1$. Since then, photocurrents have been found in other cases as well. However our main effort has been directed towards an understanding of the phenomenon through a detailed investigation of the TMPD-3 methylpentane system. Included is a study of the wavelength dependence of photocurrent quantum yields as a function of wavelength and the field dependence of the photocurrents produced at different wavelengths. Fields up to $8\times 105$ volts cm are employed. The results agree in a gross fashion with a recently obtained measure of the wavelength dependence of absolute quantum yields for photooxidation in this $system2$ which shows an almost threshold type increase in efficiency toward the middle of the near ultraviolet band of TMPD. The contrasting lack of wavelength dependence of quantum yields for fluorescence and phosphorescence indicates that neither the thermally equilibrated excited singlet state nor triplet state mediate in the one electron $photoionization2$. Instead it is believed that the path towards ionization proceeds from vibronic levels of the upper state via intermediate solute-solvent charge transfer states into a final, very long lived charge transfer state---namely Wursters Blue plus trapped electron. In the presence of a field this is accompanied by a detectable flow of current. After some initial photooxidation, new strong photoelectric signals are observed in the previously inactive spectral regions of the very near ultraviolet and visible. Both transient and nearly constant photocurrents are observed---the latter only in the near ultraviolet. To date these induced photocurrents can be understood on the basis of a model which proposes both deep and shallow electron traps in the rigid glass which when occupied possess very high ionizing efficiencies. Such trapped electrons in glasses are reminiscent of trapped electron spectra observed by Linschitz and $coworkers3$ in other systems.