Theoretical Studies of Transannular Interactions. I. Benzene Excimer Fluorescence and the Singlet States of the Paracyclophanes

The changes in the electronic transition energies of benzene which occur on dimerization have been calculated. It is shown that the energy-level splittings due to the interactions between neutral-excitation states are too small to explain the observed anomalous emission from concentrated benzene solutions and the absorption spectra of the paracyclophanes. To extend the theory, the eight-electron problem is treated for configurationally interacting neutral-excitation and charge-resonance states. Intermolecular overlap is included in a way consistent with the use of a core potential in the Goeppert-Mayer—Sklar representation. Use is also made of Hückel LCAO molecular orbital wavefunctions and a linear combination of four Slater carbon-atom 2pπ wavefunctions fit to an SCF function. The anomalous emission from benzene solutions can now be understood to arise from transitions from the lowest excimer state of α-level parentage for benzene molecules ∼3 or ∼3.3 Å apart when ground-state repulsion is taken into account. The solution absorption spectra of the paracyclophanes are interpreted within this framework of neutral-excitation—charge-resonance configuration interaction. Finally, dimer symmetries different from D6h, have been invesigated in an attempt to understand the absorption spectra of single crystals of [2.2] paracyclophane.