Photoconductivity in Rigid Organic Solutions. III. The Triplet Intermediate and a Kinetic Model Involving Secondary Matrix Trap Photoionization

Abstract

The original studies of photoconductivity in rigid organic solutions [G. E. Johnson and A. C. Albrecht, J. Chem. Phys. 44, 3162, 3179 (1966)] have been extended to a detailed experimental and theoretical analysis of the rise kinetics of the photocurrents. The principal system studied is an ∼ 10−3 M solution of N , N , N′ , N′ ‐tetramethylparaphenylenediamine (TMPD) in 3‐methylpentane (3‐MP) at 77°K. In the earlier work it was found that the charge‐carrier production is a biphotonic step involving a one‐photon‐produced intermediate state which did not appear to be the triplet state. In the present work it is found that under monochromatic excitation the triplet‐state intermediate is clearly indicated. However, when broad‐band ionizing light is used, entirely different rise behavior is seen which, as before, can exhibit, at low light levels, rise times which considerably exceed the triplet lifetime. However, under conditions in which the matrix‐trapped electrons are either stabilized (by O2 or CO2) or destabilized (lower viscosities) the triplet‐state intermediate is evident under all conditions of ionizing excitation. A kinetic model is developed which embraces these various observations while still maintaining the triplet intermediate under all conditions. Central to the model is the role of matrix‐trapped electrons which find themselves variously distributed across the Coulomb potentials of their partner cationic molecular centers. It is argued how the position of the trapped electron in the Coulomb well is a major factor in determining its dark lifetime, its photoionization cross section, but not its photon‐capture cross section for detrapping. An important distinction is made between detrapping which simply mobilizes the electron leading to charge recombination and detrapping which leads to full charge separation and conductivity. The model accounts for the new kinetics seen under broad‐band excitation through the relatively increased contribution to the photocurrent of the photoionization of matrix‐trapped electrons in the Coulomb well. The rate constant for the exponential photocurrent rise is now strongly light‐intensity dependent and reflects a composite of individual processes which include ionization of the triplet state as the first‐produced intermediate state. Some parameters are determined including estimates of the trapping efficiencies of matrix cavities and the ionization efficiency of matrix‐trapped electrons near the edge of the Coulomb well. The kinetic model is fairly complete and provides the analytic context calling for numerous additional kinetic studies. While the present model has been able to incorporate the rise kinetics into a triplet‐state intermediate model, it is still not able to explain the unusual excitation spectrum seen in the previous work for the first photon step. For this reason an alternate possibility that parallel biphotonic paths towards ionization are occurring is examined—one involving a nontriplet intermediate (a charge‐transfer state). This calls for a shift from one path to another depending on the characteristics of the ionizing radiation. Until certain definitive experiments are carried out this second mechanism must remain in contention, whenever matrix trapping is important.