No abstract

Photoinduced charge separation and electron transfer are the fundamental phenomena involved [1±4] in the processes of light energy storage for photosynthesis in plants, xerography, photovoltaic (PV) energy conversion, etc. For the efficient photovoltaic harvesting of light energy, it is necessary, however, to have well-stabilized charge-transfer products. One impending problem in achieving a long-lived photoinduced charge transfer in an artificial system is its competing backtransfer reaction. The stability of the charge-separated state, on many occasions, is facilitated by carrier delocalization and by the spatial separation of electrons and holes, [1,3] and researchers often employ heterogeneous media [2,4] for this purpose. We report here the preparation of a group of (zinc, lead) phosphate glass±C 60 heterostructures that show a long-lived photoinduced electron transfer from the divalent lead center (Pb 2+ ) to the fullerene, upon 230±400 nm ultraviolet irradiation, which creates Pb 3+ -type hole centers and C 60 fullerene related anions in the system. Both the steady-state photoinduced absorption (PIA) and photoinduced electron spin resonance (PIESR) spectra of the irradiated system support the above view. An analysis of the carrier dynamics suggests that the electrons migrate from the hole sites to the fullerene sites in a tunneling mode. The charge-separated state thus formed in these systems has a lifetime ranging from a few hours to days, which is longer than those hitherto reported for various donor±acceptor systems. Figure 1 shows a photograph of one of the samples of the (Zn, Pb) phosphate glass±C 60 composites. Steady-state PIA spectra recorded at 300 K, in the region k = 250±1250 nm both before and after 90 min of UV-vis irradiation of the (Zn, Pb) phosphate glass±C 60 composite having a zinc oxide (ZnO) content of 40 mol-% and a fullerene (C 60 ) concentratioñ 5.0 10 ±5 M, are shown in Figure 2 (along with the absorption spectrum of a similarly irradiated base glass). Prior to irradiation, the composite exhibited a weak and broad absorption band at around 520 nm and another, less distinct, broad band around 830 nm.Considering the energy and weakness of the transition at 520 nm, one can easily recognize [5,6] it to be the highest-occupied molecular orbital to lowest-unoccupied molecular orbital (HOMO±LUMO) forbidden transition of the incorpo-COMMUNICATIONS Adv. Mater. 2003, 15, No. 3, February 5 Fig. 1. Photograph of a C 60 -doped (Zn, Pb)±phosphate glass sample.Fig. 2. Steady-state PIA spectra of a C 60 -doped (zinc, lead) phosphate glass (with ZnO = 40 mol-%; C 60 fullerene concentration = 5 10 ±5 M), along with the absorption spectrum of a similarly irradiated base glass (thickness, d = 0.2 cm, for both samples), recorded at 300 K, as a function of time. Inset: a plot of PIA intensity versus time.

Luminescence properties of a series of samples of Y2O3:Eu+3 red phosphor of particle sizes ranging from 50 to 300 nm were investigated as a function of time to decipher the long-standing mystery of the effect of reduction of particle size on the luminescence efficiency of the phosphor. The samples were found to lose luminescence efficiency and suffer a change in the excitation profile with time. Infrared studies showed that although the samples at their freshly prepared stage were almost free from contaminated water, on aging in air at room temperature, they absorbed the latter. The phenomenon of hydration-induced coupling of the excitonic state of Y2O3 with one of its Fu modes was detected in the case of the aged sample, which was shown to be instrumental in introducing newer nonradiative channels in the system. Because of larger surface-to-volume ratio, the effect was most pronounced in the case of nanocrystalline samples.

The present work emphasizes the effect of alkali (NaOH) solution on macromolecular parameters of sisal fiber. The macromolecular parameters of alkali treated sisal fiber were computed along with air-dried sisal fiber by treating fiber as a non-ideal two-phase system characterized by continuous variation of electron density at the phase boundary. Small angle Xray scattering (SAXS) technique was employed to evaluate the macromolecular parameters, using correlation functions, applying the theories of Vonk, Ruland and Misra et al. The finite value of width of transition layer suggests to treat the fiber to be a non-ideal two-phased system. The evaluated macromolecular parameters are: transversal periodicity, specific inner surface, volume fractions of matter and void phases, transversal lengths in both phases, range of inhomogeneity, volume fraction of transition layer and length of coherence.

A dark red colored composite of C 60 and (zinc, bismuth) borate glass was synthesized by solid-state reaction between C 60 and frits of the glass at 650-700 o C in an argon atmosphere completely free from oxygen and moisture. An unusual change in color and the absorption spectrum of the glass upon composite formation with C 60 indicated that the incorporated C 60 underwent some sort of interaction with the glass. Such fullerene molecules bonded to relatively smaller units of bismuth-boron network, were extracted out by eluting a powdered sample with toluene. The composite itself and the toluene extract there of, were then characterized by ultraviolet-visible-near-infrared, infrared, and mass spectral studies. The results showed that the bismuth ions of the bismuth-boron network were bonded with the C 60 cages through direct donation of their lone pair of electrons to the latter. The phenomenon of addition of boron to C 60 via an "oxygen bridge," which was observed in our earlier work, was also detected in this case. Studies on the nonlinear optical properties of the composite exhibited a moderate value of third order nonlinear susceptibility (3) (1.5 × 10 −11 esu) and optical limiting properties of the composite. The result showed that the material had the prospect of being used in nonlinear optical devices.