Rhodamines, excitation transfer

ZnO (suspension) sensitizes the photoreduction of Ag" by xanthene dyes such as uranin and rhodamine B. In this reaction, ZnO plays the role of a medium to facilitate the efficient electron transfer from excited dye molecules to Ag" adsortei on the surface. The electron is transferred into the conduction band of ZnO and from there it reacts with Ag. In homogeneous solution, the transfer of an electron from the excited dye has little driving force as the potential of the Ag /Ag system is —1.8 V (Sect. 2.3). It seems that sufficient binding energy of the silver atom formed is available in the reduction of adsorbed Ag" ions, i.e. the redox potential of the silver couple is more positive under these circumstances. 

The rhodamine B-bound complex of Ir1 (387) shows only minor alterations in the absorption spectrum of bound rhodamine B as opposed to free dye however, its fluorescence is strongly quenched.626 Fluorescence is intense when the rhodamine dye is attached to an Ir111 center. The authors conclude that the excited-state quenching mechanism is via electron transfer. 

The optical properties of organic dyes (Fig. ld-f, Table 1) are controlled by the nature of the electronic transition(s) involved [4], The emission occurs either from an electronic state delocalized over the whole chromophore (the corresponding fluorophores are termed here as resonant or mesomeric dyes) or from a charge transfer (CT) state formed via intramolecular charge transfer (ICT) from the initially excited electronic state (the corresponding fluorophores are referred to as CT dyes) [4], Bioanalytically relevant fluorophores like fluoresceins, rhodamines, most 4,4 -difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPY dyes), and cyanines (symmetric... 

Two donor-acceptor systems were examined, with Coumarin 1 (Cl) and 9-aminoacridine (9AA) as donors and Rhodamine 6G (R6G) as the acceptor. Initial experiments were performed to compare the amount of transfer observed in bulk solution and in particles made of the same material. Glycerol was chosen as the solvent, mainly because of its low vapor pressure and high viscosity. The low vapor pressure was necessary so that particles would be relatively stable in size, and the high viscosity ensures that the excited donor is essentially stationary for the lifetime of the excited state. The concentrations used were chosen to minimize donor reabsorption and to make the extinction of the donor considerably larger than the extinction of the acceptor at the excitation wavelength. Excitation wavelengths of either 365 or 387 nm were used in the experiments. Concentration ratios, donor to acceptor, of 10 1 and... 

Fig. 16 Addition of 0.017-nmol aliquots of a rhodamine B-labeled streptavidin and b Texas Red-X-labeled streptavidin to 1.51 nmol of 43. Energy transfer observed in both cases with amplified emission of the dyes to the light-harvesting conjugated polymers. Direct excitation of the dyes at 575 and 585 nm correspond to 0.100 nmol of streptavidin.

For the excited singlet state of rhodamine as product state the free energy plot of the reverse electron transfer from the reduced dye to the hole (dashed curve in Fig. 31) is a mirror image to the free energy plot of the forward reaction relative to AG° =0. We immediately see from Figs. 31 and 32 that this reverse reaction is very fast at phenanthrene and slower at chrysene. It is still slower at anthracene and extremely slow at perylene. At phenanthrene this reverse reaction can compete with the dissociation of the hole from the reduced dye as is borne out by the recombination controlled current in this system (Fig. 27). 

Kaplan and Jortner [164] have observed dipole—dipole energy transfer between the second excited state of rhodamine 6G and 2,5-bis(5 -f-butyl-2-benzoxazolyl)thiophene in ethanol. The donor excited state lifetime was estimated to be 0.19 ps based on energy transfer by Forster kinetics. 

Fluorescence depolarisation by energy transfer (rather than rotational relaxation) between donor molecules of the same type can occur. Eisenthal [174] excited solutions of rhodamine 6G (9 mmol dm-3) in glycerol with 530 nm light from a frequency-doubled neodymium laser. The polarisation... 

Macrae and Wright (96) demonstrated that visible light irradiation of xanthene dyes (eosin, erythrosin, rhodamine B, or RB) in ethanolic solutions of 4-(N,N-diethylamino)benzene-diazonium chloride (as the zinc chloride double salt) resulted in decomposition of the diazonium salt. Electron transfer from the dye excited state(s) to the diazonium salt was postulated and dye-diazonium salt ion pair formation in the ground state was shown to be important. Similar dyes and diazonium salts were claimed by Cerwonka (97) in a photopolymerization process in which vinyl monomers (vinylpyrrolidone, bis(acrylamide)) were crosslinked by visible light. Initiation occurs by the sequence of reactions in eqs. 40-42 ... 

Membranes fusion can be studied using the energy-transfer mechanism. In fact, membrane vesicles labeled with both NBD and rhodamine probes are fused with unlabeled vesicles. In the labeled vesicles, upon excitation of NBD at 470 nm, emission from rhodamine is observed at 585 nm as a result of energy transfer from NBD to rhodamine. The average distance separating the donor from the acceptor molecules increases with fusion of the vesicules, thereby decreasing the energy-transfer efficiency (Struck et al. 1981). 

Rigidity and Florescence The more rigid the molecule, the less likely that low energy vibrations are initiated by transfer of energy from the first excited state before fluorescence can take place. The effect of a more rigid structure of the rho-damine series has been clearly demonstrated in Rhodamine 101 (10) which has its... 

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