By the use of the dual beam picosecond absorption spectroscopy technique, the initial events in the energy relaxation process of Cu(II) and Ag(II) protoporphyrin IX dimethyl esters were studied. The energy decay channels were found to be as follows. Cu(II) (Z=29) complex: After picosecond excitation, a Franck–Condon state of 2S1 is populated which decays into 2T1 within 8 psec. This state relaxes with a time constant of 450–460 psec to 2T1 and 4T1 equilibrium state from which phosphorescence is emitted. Ag(II) (Z=47) porphyrin: Picosecond excitation to the 2S1 state results in energy relaxation to 2T1 state within 8 psec which is followed by equilibration between 2T1 and 4T1 with a time constant of 11–12 psec. Subsequently the relaxation proceeds via nonradiative channels to 2Td and 4Td or 2(d,π*) states.

Recombination processes in InP have been studied using picosecond-time-resolved photoluminescence (PL). The technique makes it possible to measure the intrinsic surface recombination velocity (SRV) and the bulk lifetime~directly and independently.The results show that both pand n-type InP(110) etched surfaces have similarly low SRV, contrary to commonly accepted values. Moreover, it is found that ntype InP is distinguished by a very long nonradiative lifetime~" , (320 ns) and the bulk recombination process is mainly radiative. On the other hand, the~" , of p-type InP is very small ( 33 ns), apparently due to a high concentration of deep traps, and nonradiative bulk recombination is dominant. These results are discussed in view of other measurements and models. The SRV of metal/InP interfaces shows a strong dependence on the reactivity of the metal-semiconductor anion pair, which resembles the dependence found for the Schottky-barrier height at these interfaces. These measurements are compared to results also obtained in this work for UHV-cleaved surfaces.

The chemical compositions and structures of organic-inorganic interfaces in mesostructurally ordered conjugated polymer-titania nanocomposites are shown to have a predominant influence on their photovoltaic properties. Such interfaces can be controlled by using surfactant structure-directing agents (SDAs) with different architectures and molecular weights to promote contact between the highly hydrophobic electron-donating conjugated polymer species and hydrophilic electron-accepting titania frameworks. A combination of small-angle X-ray scattering (SAXS), scanning and transmission electron microscopy (SEM, TEM), and solid-state NMR spectroscopy yields insights on the compositions, structures, and distributions of inorganic and organic species within the materials over multiple length scales. Two-dimensional NMR analyses establish the molecular-level interactions between the different SDA blocks, the conjugated polymer, and the titania framework, which are correlated with steady-state and time-resolved photoluminescence measurements of the photoexcitation dynamics of the conjugated polymer and macroscopic photocurrent generation in photovoltaic devices. Molecular understanding of the compositions and chemical interactions at organic-inorganic interfaces are shown to enable the design, synthesis, and control of the photovoltaic properties of hybrid functional materials.

No abstract

The nanosecond and picosecond resolved dual fluorescences of p-dimethylaminobenzonitrile (DAB), in various solvents and glasses excited by 266 nm 20 ps FWHM laser pulses, have been investigated. Pulse-limited rise times are exhibited by the b*-state emission whose decay in turn feeds directly the risetime of a*-state emission at 440–600 nm in most solvents studied. The a*-state emission was monitored at 520–600 nm in order to eliminate contribution from the b*-state. Within the experimental resolution, the b*-state fluorescence decay times vary approximately linearly with solvent viscosity. The a*-state fluorescence decay times vary with both solvent and temperature, and may reflect either thermally assisted intersystem crossing from the solvated singlet a*-state (presumably of twisted internal charge transfer character) to a corresponding solvated triplet of slightly higher energy, or a thermally activated internal conversion of the 1TICT to the ground state.