Photoexcitation of dye

This is essentially the same process responsible for deactivation of photoexcited cyanine dyes. 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

Fluorescence quenching may be dynamic, if the photochemical process is the result of a collision between the photoexcited indicator dye and the quencher species, or static, when the luminophore and the quencher are preassociated before photoexcitation of the former20. It may be easily demonstrated that dynamic quenching in isotropic 3-D medium obeys the so-called Stem-Volmer equation (2)21 ... 

Nanoparticles of Ti02 are deposited on to a glass support covered with a transparent conducting layer of tin-doped indium oxide (ITO). Each nanoparticle is coated with a monolayer of sensitising dye based on Ru(II). Photoexcitation of the dye results in the injection of an electron into the CB of the semiconductor. 

Figure 3.32. Energy level scheme of the device in Figure 3.31. Photoinduced electron transfer takes place from the photoexcited ruthenium dye into the Ti02 conduction band. The recombination directly back to the dye has to be suppressed. Instead, the current is directed through the circuit to the counterelectrode and the hole conductor that brings the electrons back via hopping transport. HTM hole transport material.

In the absence of the dye the photocurrent is observed only in relatively short-wavelength light in the region of intrinsic absorption of semiconductors and is caused by photoexcitation of the electron-hole system of the semiconductor (cf. Part III). For zinc oxide (Eg = 3.2 eV) this region corresponds to wavelengths shorter than 400 nm for gallium phosphide (Eg = 2.2 eV), shorter than 550 nm. 

Scheme 1 Relaxation and trapping dynamics of M0S2/DTDCI following photoexcitation of the dye.

The details of die operating principles of the dye-sensitized solar cell are given in Figure. 3. The photoexcitation of the metal-to-ligand charge transfer of the adsorbed sensitizer [Eq. (1)] leads to injection of electrons into the conduction band of the oxide [Eq. (2)]. The oxidized dye is subsequently reduced by electron donation from an electrolyte containing the iodide/triiodide redox system [Eq. 

Time-resolved fluorescence from sub-picosecond to the nanosecond time-scale of dye molecules like coumarins has been widely used to study solvation dynamics in liquids [1], As the dye is photoexcited, its dipole moment abruptly changes. Then by monitoring the time-dependent fluorescence energy one can have access to the solvent dynamical response to the charge reorganization in the dye. The microscopic interpretation of these experiments has greatly benefited from Molecular Dynamics (MD) studies [2], Recently, few experimental [3-5] and theoretical [6,7] works have been performed on solvation dynamics in liquid mixtures. A number of new phenomena can arise in mixtures which are not present in pure solvents, like association, mutual diffusion and preferential solvation [6]. We present here a... 

The semiconductor nanocrystallites work as electron acceptors from the photoexcited dye molecules, and the electron transfer as sensitization is influenced by electrostatic and chemical interactions between semiconductor surface and adsorbed dye molecules, e.g., correlation between oxidation potential of excited state of the adsorbed dye and potential of the conduction band level of the semiconductor, energetic and geometric overlapping integral between LUMO of dye molecule and the density of state distribution of the conduction band of semiconductor, and geometrical and molecular orbital change of the dye on the... 

More recently, Schuster [25] has demonstrated that cyanine dyes, i.e., cyanine borates or cyanine dye-borate mixtures, provide visible light activated initiation of free radical polymerization [26]. The photoexcited cyanine dye oxidizes alkyltriphenylborates by PET to produce the bleached reduced cyanine along with an alkyl radical. The alkyl radical can then initiate free radical polymerization [27], This visible light activated PET bond cleavage is of considerable importance in photoimaging and photocuring [28]. 

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