Intramolecular energy migration

The fluorescence is measured in dilute solution and in glassy PMMA for polyesters in which 2,6-naphthalene dicarboxylate is the rigid unit, and (CF iy+t is the flexible spacer. The anisotropy in the rigid medium demonstrates the existence of intramolecular energy migration, which becomes more important as y decreases from 5 to 1. The Forster radius is about 12 A in the bichromophoric compounds and 14 A in the polyesters. 

Direct evidence of energy migration in the different films was found in polarized fluorescence emission studies. The fluorescence anisotropy indicated that intramolecular energy migration (with a Dexter hopping mechanism) existed in both the solution and spin-cast films. Further energy migration and transfer studies were... 

Studies of copolymer systems have shown that the extent of depolarization of fluorescence increases as the chromophore content of the macromolecule is extended in glassy media [29,44,45,47,104-107]. Furthermore estimates of intrinsic polarization obtained in certain copolymer systems are commensurate [29,44,45,47] with those evident in either steady state [108] or transient excitation [109] conditions. Such observations are consistent with the occurrence of intramolecular energy migration. In these studies it is significant that excitation flux conditions were such as to obviate significant contribution from triplet energy migration and resultant interference from delayed fluorescence. 

In order to determine whether energy migration makes a significant contribution to the photophysical behavior of P2VN and PS in dilute miscible blends, it is instructive to calculate the expected exdmer-to-monomer fluorescence quantum yield ratio in the absence of energy migration. To do so, it is first necessary to assume that intermolecular and non-adjacent intramolecular EFS are absent. In addition, the adjacent intramolecular EFS are assumed to be frozen into the aryl vinyl polymer and must be excited by direct absorption of a photon. Since the absorption spectrum of an EFS is no different from that of non-EFS chromophores, then the calculated fraction of rings within EFS is sufficient to determine the fluorescence ratio. 

Experimental data are also consistent with a dissociative oxygen insertion pathway initiated by the dissociation of the anion PhCOO-, followed by coordination of 02, intramolecular hydrogen migration, and recoordination of PhCOO-. This mechanism has been ruled out on the basis of density functional calculations that placed the free energy of the intermediate for O2 insertion at about 120 kJ/mol above the energy of the corresponding intermediate in the reductive elimination mechanism. 

---