SHPOLSKII SPECTRA OF TRANS-1,2-DI(2'-PYRIDYL)ETHYLENE - IDENTIFICATION AND EVIDENCE FOR ALMOST PLANAR STRUCTURE AND PI-TYPE HMO FOR THE TRAPPED ROTAMER

The resolved absorption, fluorescence excitation, and fluorescence spectra of trans-stilbene (t-St) and trans-1,2-di(2'-pyridyl)ethylene (Di2PE) in polycrystalline n-C-6 and n-C-8 matrices at 5 K as well as their geometry in the crystal and potential energy curves along the C-alpha(ethylenic)-aryl torsional coordinate have been investigated giving us the opportunity to compare two diarylethylene systems: one in which the H-alpha(ethylenic)-H-aryl repulsion is on and one in which this interaction is off. In the case of t-St, some lines previously not observed in matrices were assigned to transitions involving two quanta of the C-alpha-phenyl torsion mode (nu(37)) of a(u) symmetry in C-2h. We can conclude that in the n-alkane lattice, the t-St molecule tends to be twisted around the C(a)lpha-phenyl bonds and that its symmetry is either C-2h or C-i as in the pure crystal. The vibronic structures of the sharp line excitation and fluorescence spectra of Di2PE are almost identical to those of t-St except for a slightly lower activity of the in-plane C(a)lpha=C-alpha'-Phenyl bending mode (nu(25)) and C(a)lpha=C-alpha' stretching mode (nu(7)). These results lead us to the conclusion that only one rotamer of Di2PE is trapped in n-C-6 and in n-C-8. It is characterized by (i) a first electronic transition of marked pi pi* character with no clear evidence for a contribution from the nonbonding electrons localized on the intracyclic nitrogen atoms and (ii) a conformation similar to that of t-St, with C-2h or C-i symmetry. AM1 computation of the ground-state potential energy curves along the torsional coordinate (C(a)lpha-aryl) suggest, in the case of t-St, the existence of a very shallow potential barrier to planarity (0.12 kcal/mol) with an equilibrium value for the torsional angle smaller than 20 degrees (C-i symmetry). In the case of Di2PE, these calculations confirm the existence of two minima in the potential curve. The rotamer in which the H-3-H-alpha' repulsion is relieved is found to be the most stable, in agreement with the X-ray diffraction results. Compared to the other rotamer, it is characterized by a more shallow potential around the equilibrium and a smaller barrier to planarity. It has been identified with the rotamer trapped in the n-alkane matrices.