95676-89-4Relevant articles and documents
Fluorescent polyolefins by free radical post-reactor modification with functional nitroxides
Cicogna, Francesca,Coiai, Serena,Pinzino, Calogero,Ciardelli, Francesco,Passaglia, Elisa
, p. 695 - 702,8 (2012)
The 4-(1-naphthoate)-2,2,6,6-tetramethylpiperidine-1-oxyl (NfO-TEMPO) has been synthesized and successfully grafted in the melt onto a random poly(ethylene-co-1-ottene) copolymer. Functionalized polyolefins have been prepared by coupling reaction between NfO-TEMPO free radicals and macroradicals which have been formed by H-abstraction induced by the presence of a peroxide. In order to deepen insight into the functionalization mechanism, the reaction has been investigated by Electron Paramagnetic Resonance (EPR). EPR spectra collected during the reaction run, have evidenced the decrease of TEMPO signal as a consequence of temperature increasing. This decrease has been attributed to the formation of a covalent bond between macroradicals and nitroxide free radical. The resulting functionalized polyolefins, PO-g-(NfO-TEMPO), have been characterized by FT-IR and 1H NMR which has allowed to evaluate their functionalization degree, whereas UV-Vis and fluorescence spectroscopy have been used to investigate their optical properties. The comparison with low molecular weight model compounds, has allowed to state that our methodology can be conveniently adopted to prepare fluorescent polyolefins where the optical properties of the chromophore has been completely transferred to polymer backbone both in solution and in the condensed phase.
Intramolecular quenching of excited singlet states by stable nitroxyl radicals
Green,Simpson,Zhou,Ho,Blough
, p. 7337 - 7346 (2007/10/02)
Absorbance and steady-state and time-resolved fluorescence measurements were employed to examine the mechanism(s) of excited singlet state quenching by nitroxides in a series of nitroxide-fluorophore adducts. This work establishes the following: (1) the absorption and emission energies of the fluorophores are unaffected by the presence of the nitroxide substituent(s), and the residual emission that is observed from the adducts arises from the locally excited singlet of the fluorophore, not from charge recombination; (2) rate constants for intramolecular quenching by the nitroxides (k) are high (108-1010s-1) and decrease significantly with increasing nitroxide to fluorophore distance-however, relatively high rates of quenching (>108 s-1) are observed over distances as great as 12 ?; (3) F?rster energy transfer does not contribute significantly to the quenching due to the low values for the spectral overlap integrals; (4) the kq's do not increase proportionally to the solvent-dependent increases in the Dexter overlap integral, indicating that energy transfer by the Dexter mechanism is not responsible for the quenching; (5) the values of kq show no obvious correlation with the calculated free energies for photoinduced electron transfer, suggesting that this quenching pathway is also unimportant; (6) for hematoporphyrin-nitroxide adducts, which contain a fluorophore whose singlet energy is below that of the first excited state energy of the nitroxide (thus precluding energy transfer), significant rates of quenching are still observed; (7) for compounds with similar nitroxide-fluorophore distance, an approximately linear correlation is observed between the kq's of the paramagnetic compounds and the nonradiative rate constants of the diamagnetic reference compounds, suggesting that the nitroxide moiety catalyses a preexisting nonradiative pathway in the fluorophore. These results indicate that the quenching arises through electron exchange which causes relaxation of the (local) singlet state to the triplet and/or ground state of the fluorophore.
