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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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The interaction of IT-eleetrons in [2.2] phanes was studied both experimcntally and theoretically. The fluorescence and phosphorescence spectra were measured at liquid helium temperature; in addition, the zero field splitting parameters D and E were detcrmined by ODMR in zero field and by ordinary ESR at X·band. The results for the phanes with two identical aromatic units can be summarized as follows: The rather small reduction of the D and E values of the order of 10% with respect to the monomrrs indicates, in agreement with the theoretical treatment given in part II, that the two unpaired electrons of the excited triplet state have a high probability to be at a given time in the same half of the molecule. While the fluorescence spectra show the typical behaViour of emission spectra of dimers or excimers, the phosphorescence spectra CKhibit some remaining structure. This behaviour which indicates a somewhat weaker coupling among the triplet orbitals as compared to the singlet orbitals can also be understood on the basis of theoretical considerations. for a phane with two different aromatic units thl! behaviour is found t~ be more Similar to the corresponding aromatic monomer with the lower ex" cited states with some pcrtu~bation by the other part of the phane also.in agreement with theoretical expectation.

The emissio.n spectrum o.fpo.lycrystalline [22]paracyclo.phane shows a resolved vibronic structure ~ith a 241 cm-1 pro.gressio.n at He temperatures. The dependence orthe energy or this mode upon selective deuteration in co.mbinatio.n with results from FIR and Raman spectra could be used to identify the mode as a torsional dimer vibration_ The emission spectra could be simulated assuming a linear co.upling o.fthe to.rsio.nal mode to. the electronic transitions with co.upling strengths o.f S = 10 (fluorescence) and S = I3 (phosphorescence). This co.rrespo.nds to. an· equilibrium displacement o.fthe benzene rings under electronic excitatio.n by a torsional angie of 10.6 0 (Sl) and 12..1 0 (Tl)' in addition to the small torsion in the ground state So by about 3°.

The fluorescence and phosphorescence of kekulene in a host matrix of polycrystalline tetrachlorobenzene are investigated, together with the triplet zero field splitting parameters IDI and lEI obtained by ODMR in zero field. The D value is also calculated within a semi-empirical 1T-theory and compared with experiment. It could be shown that the triplet state energies of a number of different sites of kekulene in the host matrix and the zero field splitting parameters are related, in first order, by spin orbit interaction.