PHOTOPHYSICS OF POLY(PARACYCLOPHAN-1-ENE) AND DERIVATIVES - EVIDENCE FOR INTRACHAIN ENERGY-TRANSFER AND CHROMOPHORE AGGREGATION

The monomer [2.2]paracyclophan-1-ene (2) reacts with the Schrock initiator Mo(NAr)(OC(CF3)(2)Me)(2)-(CHCMe(2)Ph) (1, Ar = 2,6-diisopropylphenyl), via a living and cis-specific ring-opening metathesis polymerization (ROMP) mechanism, to give poly2 in essentially quantitative yield. This polymerization is significantly slower than those observed with monomers built on the norbornene skeleton, The monomers 9-[(tert-butyldimethylsilyl)oxy][2 .2]paracyclophan-1-ene (3) and 9-(n-butyl)-9-[(tert-butyldimethylsilyl)oxy][2.2]paracyclophan-1-ene (4) react similarly, but the resulting polymers are significantly more soluble as a result of the increased saturated content of the polymer chain. Depending on the manner of monomer addition, it is possible to prepare either random or block copolymers (of narrow polydispersity). Poly2, poly3, and poly4 obtained with 1 are stereoregular main-chain polystilbenes with close structural links to poly(p-phenylenevinylene). Poly2 and poly3 in particular have a strong, vibronically defined, red-shifted component in their photoluminescence spectra which depends strongly on the polymer stereochemistry, proximity of chromophores, backbone substitution, ability of the medium to solvate the polymer coils, and length of the stilbene-containing segments. These data suggest that chromophore cooperativity, or aggregation, is responsible for the lower energy fluorescence band. Fluorescence depolarization and lifetime measurements are consistent with significant energy transfer from excited monomeric stilbenes to ''aggregated'' sites. Poly4 is sufficiently substituted that chromophore cooperativity is observed only in the film state and in poor solvents which contract coil dimensions. The oxidative photocyclization of the stilbene components to phenanthrene (poly5) proceeds quantitatively using iodine and a large excess of propylene oxide. This structural change causes a significant decrease in the poly:ner's hydrodynamic volume, and importantly, no evidence of chromophore cooperativity has been observed in poly5.