Transannular Interactions in [ 2 . 2 ] Phanes as Studied by Magnetic Resonance and Optical Spectra

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.