Emissions from both exciplexes and excited charge-transfer (CT) complexes have been observed for the fullerene-aromatic amine systems in nonaromatic solvents and solvent mixtures using steady-state fluorescence measurements. Support for the exciplex and CT emissions has also been obtained from the time-resolved fluorescence measurements. Both emissions are very sensitive to the polarity of the solvents. From the solvatochromic shifts of the emission maxima, dipole moments (µ) of the exciplexes and the excited CT complexes have been estimated. For C 60 -N,N-dimethylaniline (DMAN) and C 60 -N,N-diethylaniline (DEAN) systems, the µ values for the exciplexes and the excited CT complexes are in the ranges 10-12 and 25-27 D, respectively. Both species are thus very polar in nature. In aromatic solvents the fullerene-aromatic amine systems behave differently, without forming any exciplexes, though the fullerene emissions are quenched by the amines with almost diffusion-controlled rates. CT emissions are also not observed in the aromatic solvents. The π-electrons of the solvent molecules are supposed to be responsible for the unusual behavior of these solvents on the present systems.
The effect of surface modification on interfacial electron transfer (IET) dynamics into the surface states of ZrO(2) nanoparticles sensitized by quinizarin (Qz) and its derivatives has been carried out using time-resolved emission spectroscopy. The surface of ZrO(2) nanoparticles has been modified by sodium dodecyl benzyl sulfonate . We have observed that Qz's can form a strong charge-transfer (CT) complex with both unmodified and surface-modified (SM) ZrO(2) nanoparticles. We have confirmed electron injection into the surface states of ZrO(2) nanoparticles from the photoexcited Qz molecule in our earlier work (J. Phys. Chem. B 2004, 108, 4775; Langmuir 2004, 20, 7342). In the present investigation, we have observed electron injection from photoexcited Qz derivatives into the surface states of both unmodified and SM ZrO(2) nanoparticles and also detected CT emission. Monitoring CT emission, we have determined back electron transfer (BET) dynamics of the dye-nanoparticle systems. We have found that the BET rate for the QZs/ZrO(2) systems decreases as the relative driving force increases following Marcus inverted region kinetic behavior for an IET process. BET dynamics was found to be faster on SM ZrO(2) nanoparticles as compared to that of the unmodified (bare) one. Our time-resolved emission data indicates that upon surface modification the majority of the deeper trap states of ZrO(2) nanoparticles can be removed with the formation of some new shallower trap states in the band gap region.
The properties of one-electron reduced species of 1,4-naphthoquinone (NQ), 2-hydroxynaphthoquinone (2HNQ), 5hydroxynaphthoquinone (5HNQ) and 5,8-dihydroxynaphthoquionone (58DHNQ) in water-isopropanol (Pr'OH) (5 mol dm -,)acetone (1 mol dm-3) mixed solvent (MS), Pr'OH and water have been investigated by a pulse radiolysis technique. Semiquinones of 2HNQ produced at acidic pH resemble those produced in pure Pr'OH and have no absorption beyond 500 nm. These were assigned as neutral semiquinones. The semiquinones produced at higher pH ( > 11) were assigned as dianionic semiquinones. Both the dianionic semiquinones and the one-electron oxidised species of 2 HNQ have absorption beyond 600 nm, but to a different extent since the difference of two electrons between the species leads to a difference in the extent of conjugation of the aromatic ring with the oxygen atom of the OH group. The pKs of the parent molecules and the semiquinones were determined in MS. The one-electron reduction potentials (E') of the quinones were calculated and compared with those of other related quinones. The kinetics of formation and decay of both the one-electron reduced and the one-electron oxidised species have been calculated. The k, values for the reduced and the oxidised species were of the order of lo9 dm3 mol-' s-'. The corresponding k, values were of the order of lo8 dm3 mol-' s-' for the oxidised species, but the decay of the semiquinones was very slow. The effect of the position of the OH groups in NQ on the semiquinone characteristics has been discussed for 2HNQ, 5HNQ and 58DHNQ in MS and aqueous solutions.
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