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-Cs and n-Cg matrices at 5 K as well as their geometry in the crystal and potential energy curves along the Ca(ehylefic)-aTyl torsional coordinate have been investigated giving us the opportunity to compare two diarylethylene systems: one in which the Ha(e*ylefic)-Haryl 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,-phenyl torsion mode (137) of a, symmetry in CZh. We can conclude that in the n-alkane lattice, the t-St molecule tends to be twisted around the C,-phenyl bonds and that its symmetry is either CU, or Ci, 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 Ca=C,y-Phenyl bending mode (~2 5 ) and Ca=Ca, stretching mode (~7 ) .These results lead us to the conclusion that only one rotamer of Di2PE is trapped in n-C6 and in n-C8. It is characterized by (i) a first electronic transition of marked mc* 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 C2h or Ci symmetry. Ah41 computation of the ground-state potential energy curves along the torsional coordinate (Ca-wl) suggest, in the case of t-St, the existence of a very shallow potential barrier to planarity (0.12 kcaYmol) with an equilibrium value for the torsional angle smaller than 20" (Ci symmetry).In the case of Di2PE, these calculations confirm the existence of two minima in the potential curve. The rotamer in which the H3-Ha. 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.

We have studied the quasiline electronic spectra (Shpolskii spectra) of the single-bond rotamers of trans-3-and trans-2-styrylphenanthrenes embedded in n-hexane and n-decane matrices at 5 K. With 3-styrylphenanthrene, in both n-paraffin host matrices, well-resolved absorption spectra due to the two almost isoenergetic s-trans and s-cis rotamers were obtained with comparable contribution in agreement with experiments conducted at room temperature by Bartocci et al. Vibronic analysis of the spectra indicates a localized phenanthrenic character for the S1 -SO transition of the s-trans rotamer and a delocalized stilbenic character in the transition of the s-cis rotamer. In the case of 2-styrylphenanthrene, in n-hexane and n-decane matrices, the two expected rotamers which were not detected in liquid-solution experiments could be clearly distinguished by their absorption and fluorescence spectra, thus giving conclusive proof for the separate existence of the s-cis and s-trans rotamers in this compound. Contrary to the case for 3-styrylphenanthrene, the vibronic analysis of 2-styrylphenanthrene spectra of both rotamers did not provide evidence for a clear-cut localized or delocalized character but the long fluorescence lifetime is in favor of a localized phenanthrenic character. Moreover, the n-decane matrix was seen to shift the rotameric equilibrium toward the postulated s-trans rotamer presumably as a result of specific Shpolskii matrix host-guest interactions. IntroductionWe have recently described1 the simultaneous coexistence of two single-bond rotamers of 2-styrylnaphthalene (2-StN) embedded in polycrystalline n-hydrocarbon matrices (Shpolskii matrices). In these systems, we detected two separate families of well-resolved electronic absorption spectra attributed respectively to the s-trans and s-cis rotamers (s-for single bond to differentiate from the ethylenic double-bond rotamerism) which differ in their naphthyl to ethylenic double-bond dihedral angle 8 (cf. Figure 1 for nomenclature). The value of the torsional angle 8 is close to 180' in the s-trans conformer and close to 0 ' in the s-cis form. In truns-diarylethylenes, due to the interplay between intramolecular steric hindrance and electronic interactions,z4 s-trans and s-cis conformers are expected to present various deviations from planarity depending on the angular difference between the planes of their three chromophoric units (the two aryl groups and the central double b~n d ) .~J As a corollary, s-trans and s-cis rotamers are found to differ also in the nature of their lowest excited electronic state SI. Thus in the s-trans conformers of the s-trans-1,24(2-naphthyl)ethylene (di2-N) and of s-trans-2-styrylnaphthalene (2-StN), the aryl groups and the double-bond bridge are almost coplanar, leading to an SI state of naphthalenic character (B-type state). These B-type states which characterize s-trans conformers were described in the initial studies of the linelike spectra of di2-N and 2-StN and of their am ana10gues.z~ Transitions involving these stat...

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.