Small particles of metals in solution often behave like electrodes although they are not connected to a battery which determines their potential. However, when a chemical reaction occurs in the solution of such particles intermediate free radicals may transfer electrons to them. The particles are thus charged chemically and are able to act as a metal electrode on cathodic potential. Electron transfer reactions become possible at these micro-electrodes which cannot be brought about by the radicals in the absence of the colloidal catalyst.

Mechanism of reactions on colloidal microelectrodes and size quantization effects

Investigations on the ultrafast electron injection mechanism from the dye alizarin to wide band gap semiconductor colloids in aqueous medium are presented, combined with detailed studies on population, depopulation, and relaxation phenomena in trap states and their influence on the injection process. Because of the very strong electronic coupling between dye and semiconductor in an alizarin/TiO2 system, a very fast electron injection from the excited dye to the conduction band of TiO2 is expected. Our measurements show an injection time τinj < 100 fs, suggesting that the electron transfer follows an adiabatic mechanism. Furthermore, we present experiments over a wide spectral range on the recombination reaction of the electron in the conduction band of the semiconductor colloid and the dye cation to the ground state. We find highly multiphasic recombination dynamics with time constants from 400 fs to the nanosecond time scale. The nonexponential character of the recombination reaction is attributed to fas...

The Role of Surface States in the Ultrafast Photoinduced Electron Transfer from Sensitizing Dye Molecules to Semiconductor Colloids

Electron transfer from organic dye molecules to semiconductor−colloidal systems is among the fastest reported charge-separation reactions. We present investigations on alizarin complexing the surface of TiO2 semiconductor colloids in solution. Because of the very strong electronic coupling between the sensitizer and the semiconductor in the alizarin/TiO2 system, very fast electron injection from the photoexcited dye to the conduction band of TiO2 occurs. The real-time observation of the injection process is achieved by transient absorption spectroscopy using a 19-fs excitation pulse provided by a pump pulse from a noncollinear optical parametric amplifier and a probe pulse from a quasi-chirp-free supercontinuum. An injection time τinj of 6 fs can be unambiguously derived in three different ways from the experimental data:  (i) analysis of individual transients at spectral positions without contributions from subsequent reactions (relaxation, recombination); (ii) global fitting procedure for 31 wavelengths...

http://nathan.instras.com/documentDB/paper-113.pdf

Real-time observation of photoinduced adiabatic electron transfer in strongly coupled dye/semiconductor colloidal systems with a 6 fs time constant

Electron injection and recombination dynamics of the dye fluorescein 27 adsorbed on a titanium dioxide (TiO2, anatase) surface were studied, following femtosecond pulse excitation of the 0−0 transition of the dye molecule. Nanosized colloidal particles (average diameter ∼ 2.4 nm) were employed as electron acceptors in optically transparent aqueous suspensions. Large changes of the dye absorption spectrum in the presence of TiO2 colloids confirmed strong adsorption of the dye to the TiO2 surface. The strong quenching of steady-state fluorescence under such conditions is correlated to the electron injection process. Transient absorption spectra and kinetics were recorded in the spectral region from 410 to 950 nm. While the transient absorption signal in the interval 410−600 nm originates primarily from the dye molecule, the observed signal in the range 600−950 nm is assigned to intra-band transitions or free carrier absorption of the electron injected into the semiconductor. The decay time of the stimulated...

Dynamics of Electron Injection and Recombination of Dye-Sensitized TiO2 Particles

The photophysics of three cyanine dyes (i) 1,1‘-diethyl-2,2‘-cyanine iodide (pseudoisocyanine, PIC), (ii) 3,3‘-didodecyldithia-2,2‘-carbocyanine bromide (dye 1), and (iii) 3,3‘-diethyldithia-2,2‘-carbocyanine iodide (dye 2) have been examined by picosecond-laser photolysis in aqueous and methanolic-aqueous media. At moderately high concentration, solutions of PIC in 5 M NaCl/water contain monomers, H-aggregates, and J-aggregates; dye 1 water/methanol solutions consist mostly of monomers and H-aggregates (dimers and higher n-mers); aqueous dye 2 solutions contain only monomers and dimers. Photolysis of H- and/or J-aggregates in PIC and dye 1 cyanine solutions leads to photobleaching of the respective aggregate absorption bands and subsequently decays by biphasic kinetics. Two mechanisms are discussed for the deactivation of excited aggregates. In the first, nonradiative decay of the excited singlet states results in considerable heating of the aggregates together with their surrounding solvent shells causi...

Photophysics of Cyanine Dyes: Subnanosecond Relaxation Dynamics in Monomers, Dimers, and H- and J-Aggregates in Solution

Reference LPI-ARTICLE-1985-008doi:10.1002/hlca.19850680623View record in Web of Science Record created on 2006-02-21, modified on 2016-08-08

Picosecond time-resolved studies of photosensitized electron injection in colloidal semiconductors

Previous studies on photooxidation of 9,10-diethoxyanthracene (DEA) with diphenyliodonium cation revealed dealkylation of DEA radical cation. Fluorescence, dealkylation quantum yield, and laser-flash transient absorption spectroscopic studies on DEA and its dimethoxy (DMA) and diisopropoxy (DIPA) analogs, probed solvent and substituent effects on this reaction. We reconsider our original inferences as to relative rates of dealkylation and back electron transfer; the results pose new questions, e.g., as to the role of orbital symmetry in controlling rate of back electron transfer.

Solution Chemistry of Dialkoxyanthracene Radical Cations

An unusual slow rise of fluorescence intensity from Coumarins 102, 314, and 334, observed under conditions of high intensity laser excitation prompted further investigation of their photophysics by photoquenching, time-resolved transient absorption spectroscopy and time- resolved amplified stimulated emission. The slow rise was found to be the consequence of multi-photon excitation. Principal fate of coumarin molecules so excited was shown to be intersystem crossing to the nonemissive triplet manifold. Rate of intersystem crossing was estimated as ca. 10 10 s -1 ; energy matching between a higher excited singlet state, S n , and a higher triplet, T n , is implicated.

Devendra K. Sharma

Papers

Picosecond time-resolved studies of photosensitized electron injection in colloidal semiconductors

Solution Chemistry of Dialkoxyanthracene Radical Cations

Photophysics of rigidized 7-aminocoumarin laser dyes