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infrared (in both the high-temperature photostimulation of the high-conductivity state and the roomtemperature photo stimulation of the low-conductivity state) is in accord with the hypothesis of surface sorption; the blue light will be heavily absorbed in the surface layers, whereas the infrared will be absorbed approximately homogeneously throughout the volume.(5) The return to the low-conductivity state at room temperature occurs only in the presence of air, and not in the presence of nitrogen or helium.(6) The exponential portions of Curves 1, 2, and 4 of Fig. 6 all indicate a common activation energy of 0.44 ev, whereas Curve 3 has an activation energy of 0.22 ev, just one-half that of the others. If 0.44 ev is the activation energy of the donor levels, the measured exponential slope should correspond to 0.44 ev if acceptor levels are present, but to 0.22 ev if acceptor levels are absent. Assuming the oxygen-sorption process, these results are explainable. Curves 1 and 2 were measured under conditions in which adsorbed oxygen THE JOURNAL OF CHEMICAL PHYSICS was present. By subjecting the crystal to high temperature, flowing helium, and light, before measuring Curve 3, all of the oxygen acceptors were desorbed and the subsequent curve corresponds to the case where donors only are present. Curve 3 shows that at high temperature the conductivity decreases as if acceptors were again being introduced. The following Curve 4 possesses a slope corresponding to the activation energy with acceptors present. The identity of these acceptors is uncertain; oxygen should have been absent since the measurements were made in flowing helium, but the fact that' the photoinduced dark conductivity decays slowly in'helium indicates that enough oxygen may still be present to produce the observed effects. ACKNOWLEDGMENT

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The emission spectra and the zero field splitting parameters D and E of the first excited triplet states of [2.21 (4,4') diphenylophane 3 and of the two stereoisomeric syn-and anti [2.2] (2,7) fluorenophanes 4a and 4b are investigated and compared with those of the corresponding monomers. The results are interpreted in terms of the total intersystem bond density df between the two subunits which is the sum over the bond densities dl! between the 2pz-atomic orbitals of the various aromatic C atoms i,j of the two subunits (df = '£i,jdl!). I. Experimental resultsI_n several preceding papers we have studied the transanular 7r·electron interaction in a number of [2.2} phanes including several isomeric naphthalenophanes, phenanthrenophane and pyrenophane [1][2][3].In this paper we report on the emission spectra and the zero field splitting parameters D and E of the first excited triplet states of the monomers 4,4'-dimethyldiphenyl 1 and 2,7-dimethylfluorene 2 as well as on those of the corresponding [2.21 phanes, i.e.

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