Paracyclophanes as model compounds for strongly interacting pi-systems. Part 1....

In this work we describe a study of the ground and first excited state structures and energetics of a dihydroxy-derivative of [2.2]paracyclophane (PC), the pseudo-ortho-dihydroxy[2.2]paracyclophane (o-DHPC), also termed 4,12-dihydroxy[2.2]paracyclophane. In order to understand the electronic interactions between the two pi-systems, the molecule is investigated by REMPI spectroscopy in a free jet and by quantum chemical calculations. REMPI-spectra of the cluster with one water molecule were also obtained and aid in the interpretation. The origin of the S(1) <-- S(0) transition lies at 31,483 cm(-1) (3.903 eV) for o-DHPC and 31,263 cm(-1) (3.876 eV) for the o-DHPC x H(2)O cluster. An adiabatic excitation energy of 3.87 eV was computed for the S(1) <-- S(0) transition in o-DHPC. The SCS-CC2 calculations deviate by less than 0.1 eV for the adiabatic excitation energies of PC, o-DHPC and the related aromatic molecules benzene and phenol. Considerable activity in a breathing vibration of 190 cm(-1) is found in the S(1) state of o-DHPC and o-DHPC x H(2)O, in agreement with the computed SCS-CC2 value of 185 cm(-1). Further vibrations appear at +11 cm(-1) and +54 cm(-1) in o-DHPC. The computations and the available experimental data of the parent PC show that both PC and o-DHPC are rather flexible with respect to motions of the benzene moieties.While PC has a double minimum potential energy with respect to the torsional motion, a single-minimum structure is found for the ground state of o-DHPC. The geometry change upon excitation is less pronounced in o-DHPC as compared to PC. Two of the three possible rotational conformers of the OH groups were found to have similar energies, but spectral hole burning shows that the spectra are dominated by a single rotamer.