THEORETICAL INTERPRETATION OF THE ABSORPTION AND IONIZATION SPECTRA OF THE PARACYCLOPHANES

The ultraviolet absorption spectra of the [m-m]paracyclophanes are studied by utilizing an INDO/S-CI method. The [4.4] and higher paracyclophanes can be considered as two parallel p-xylenes and the spectrum is interpreted as a simple doubling of the xylene bands. For the [3.3]paracyclophane the first band observed in absorption is assigned to the second excited state, 1B3u, but with the first calculated state, 1B2g, borrowing intensity from higher states. This lowest 1B2g state may be responsible for an inflection on the first observed band and it is held responsible for the emission observed in concentrated benzene solution. By utilizing di-p-xylene as a model system for the [m-m]paracyclophanes, the dependence of the ultraviolet absorption spectra with the interring separation is systematically analyzed. [2.2]Paracyclophane, known to have a short interring separation (3.1 Å) and a boat-like distortion of each ring, presents a spectrum that is considerably more complex. All seven bands observed experimentally are considered in our theoretical assignment (i.e. <60000 cm-1). Our calculations utilize the crystallographic structure and the results indicate that the lowest band consists of two transitions, one allowed (1B3u) and one forbidden (1B2g), borrowing intensity from a higher 1B2u state via au the au vibrational mode (twisting of the rings in opposite directions around the major axis). This lowest state 1B2g is interpreted as responsible for the weak fluorescence and for the long axis polarized absorption. The observed phosphorescence of [2.2]paracyclophane is assigned to the emission 3B3g-1Ag. For the calculation of this emission energy the geometry of the excited 3B3g state is optimized by using the ab initio SCF gradient technique with the STO-3G basis set. The ionization spectrum of the [2.2]paracyclophane is calculated by using both the 1NDO/S-C1 method and the ab initio SCF procedure with a split-valence basis set. The interpretation of the spectrum of the paracyclophanes presented in this paper is perhaps the most consistent so far available and conforms very well with all known experimental characteristics. © 1990, American Chemical Society. All rights reserved.