The studies of the absorption and fluorescence spectra of single crystals of (2, 2)paracyclophane in the region 3300ndash;3100 Aring; reported in a previous paper have been extended. Wellhyphen;characterized samples with the (hk0) developed crystal plane were used. In absorption two distinct spectra were obtained in the two polarization components. The fluorescence was strongly polarized in one direction (parallel to the fourfoldcaxis) and represents the mirror image of the equally polarized absorption component. It was concluded that the observations prove conclusively that the differently polarized spectral components represent transitions to two different electronic states, with an energy difference of 369 cmminus;1. The lowesthyphen;energy transition certainly contains the 0ndash;0 line and all available evidence supports the conclusion that also the second transition has allowed character. The only interpretation consistent with all observed features of the spectra is as follows. The two observed states are the gerade dimer states (B2gandB3ginD2h) being the outhyphen;ofhyphen;phase combinations of theB2uandB1ubenzene moiety states. The transitions have allowed character due to a torsional distortion of the molecule in the excited state. The interpretation is fully consistent with theoretical energy calculations.
展开▼
