Electronic configurations, time-resolved

Recently, the electron-transfer kinetics in the DSSC, shown as a schematic diagram in Fig. 10, have been under intensive investigation. Time-resolved laser spectroscopy measurements are used to study one of the most important primary processes—electron injection from dye photosensitizers into the conduction band of semiconductors [30-47]. The electron-transfer rate from the dye photosensitizer into the semiconductor depends on the configuration of the adsorbed dye photosensitizers on the semiconductor surface and the energy gap between the LUMO level of the dye photosensitizers and the conduction-band level of the semiconductor. For example, the rate constant for electron injection, kini, is given by Fermi s golden rule expression ... 

Time-resolved fluorescence spectroscopy of polar fluorescent probes that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section II of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies (Fg and Fe) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a time-dependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1). 

A time-resolved fluorescence measurement collects the emission spectra at regular time intervals after the excitation, defined at t=0, from which one constructs the normalized solvation dynamics response function, S(t) = [hv(t)—hv(oo)]/[hv(0)— hv(oo)] [55], In our simulations, hundreds of uncorrelated equilibrium molecular configurations with the electron in its ground state were selected as initial configurations (t=0). From each of these initial configurations, the electronic state is adiabatically promoted to the first excited state, the system is then propagated in... 

Fundamental questions related to the electronic configuration of the open or colored forms and the number and structures of the photomerocyanine isomers are considered on the basis of the results of continuous-wave (stationary) and time-resolved (picosecond, nanosecond, and millisecond) Raman experiments. For spironaphthoxazine photochromic compounds, the Raman spectra may be attributed to the TTC (trans-trans-cis) isomer having a dominant quinoidal electronic configuration. Surface-enhanced resonance Raman spectroscopy (SERRS) is demonstrated as a new analytical method for the study of the photodegradation process in solution for nitro-BIPS derivatives. The development of this method could lead to the identification of the photoproducts in thin polymer films or sol-gel matrices and ultimately to control of degradation. 

Redmond, R.W. and Braslavsky, S.E. (1988) Time-resolved thermal lensing and phosphorescence studies on photosensitized molecular oxygen formation. Influence of the electronic configuration of the sensitizer on sensitization efficiency, Chem. Phys. Lett., 148, 523-529. 

This chapter has focused almost exclusively on the electronic structures and dynamics associated with carotenoids in all-trans configurations. Although all-trans carotenoids often are selected by lightharvesting complexes, c/x-isomers tend to be employed by photosynthefic reaction centers, and it is important to understand both how the energetics and the kinetics are modified for cis conformers. One obvious problem in carrying out such studies, at least in vitro, is the thermal and photochemical instability of the cis forms. Nevertheless, the importance of these isomers in vision and in photoprotection provides strong motivation for extending spectroscopic and time-resolved studies to these systems. 

Jiang Y-S, Kurimoto Y, Shimamura T, Ko-chi N, Ohashi N, Mukai Y and Koyama Y (1996) Isolation by high-pressure liquid chromatography, configurational determination by H-NMR, and analyses of electronic absorption and Raman spectra of isomeric spheroidene. Biospectroscopy 2 47-58 Kandori H, Sasabe H and Mimuro M (1994) Direct determination of a lifetime of the Sj state of /J-carotene by femtosecond time-resolved fluorescence spectroscopy. J Am Chem Soc 116 2671-2672... 

Figure 5. Set of time-resolved UV-near-IR spectroscopic data (3.44-0.99 eV) following the femtosecond UV excitation of an aqueous sodium chloride solution ([H20]/[NaCl] = 55). An instrumental response of the pump-probe configuration at 1.77 eV (n-heptane) is also shown in the middle part of the figure. The ultra-short-lived components discriminated by UV and IR spectroscopy correspond to low or high excited CTTS states (CTTS, CTTS ), electron-atom pairs (Che pairs), and excited hydrated electrons (ehyd )- The spectral signature of relaxed electronic states (ground state of a hydrated electron, (ehyd) electron-cation pairs, a e hyd) observed in the red spectral region.

Figure 7. Spectral contributions of transient electronic configurations triggered by the femtosecond UV excitation of aqueous chloride ions. The relative spectral contributions are obtained from the computed analysis of time-resolved UV-IR femtosecond spectroscopic data. A first photophysical channel, including a non-adiabatic transition from a p-like excited hydrated electron state (e hydV to an s-like ground hydrated electron state. B spectral contributions of two well-defined transient fe Cl pairs. The presence of counterions (Na ) influences the dual behavior of these transient electronic configurations. C Direct identification of the spectral band assigned to near-infrared fe Cl pairs, made by using a cooled Optical Multichannel Analyzer (OMA 4) equipped with CCD detectors (1024 X...

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