Excitation-energy transfer

Energy transfer from an excited molecule (donor) to another that is chemically different (acceptor) according to 

When the process can repeat itself so that the excitation migrates over several molecules, it is called excitation transport or energy migration. 

Radiative and non-radiative transfers have different effects on the characteristics of fluorescence emission from the donor, which allows us to make a distinction between these two types of transfer. Tables 4.5 and 4.6 summarize the effects of heterotransfer and homotransfer, respectively. 

The factors governing the efficiency of radiative and non-radiative transfers are not the same (apart from the spectral overlap, of course, which is required for both processes) (see Table 4.7). 

In a crystal of identical chromophores, for example, a naphthalene crystal, local excitations may be transferred to other sites, just like electrons or holes. Excitation energy transfer (EET) is also called electron-hole pair transfer because the excited electron and remaining hole are transferred simultaneously to the other atom or molecule. Degeneracy appears in a finite system with two identical chromophores. In a repetitive system, the excitations may be delocalized, but this is not always the case. Devices for solar light harvesting, and natural antenna systems are examples of such repetitive systems. It is important to understand their properties. 

We will discuss in this section two radiationless decay mechanisms available to a molecule in the excited state (see Section 1.4.3). These processes involve the transfer of excitation energy or of an electron from an excited molecule, the donor, D, to a molecule, A, in its ground state. The energy transfer may occur, either to a ground-state donor molecule D, energy migration, or to a different molecule, the acceptor, A, sensitization  

The changes in the electronic states of the donor and acceptor are visualized in Table 3.1 . In an electron transfer process (top) an electron is transferred from the excited donor D to the acceptor. There are also situations where an electron is transferred from the donor D to the excited acceptor, A, Equation (3.4). 

In the case of excitation energy transfer two distinct types of mechanisms can take place. When a weak interaction can occur between the transition moments of the radiative and A A transitions, energy transfer can take place via dipole-dipole interactions. This mechanism occurs frequently in energy transfer processes between a donor in the excited singlet state and an acceptor in the ground state. The rate of energy transfer, itu A, is then described by Eq. (3.5) first derived by Fcirster 1  

From Eq. (3.6) we observe that the rate of energy transfer by electron exchange mechanism decreases exponentially with 2R/L. Thus, it will be negligibly small as R increases more than on the order of one or two molecular diameters. Hence the effective distances for Dexter energy transfer range between 10 and 15 A. 

Both the Forster and the Dexter energy transfer mechanisms require spectral overlap of the donor emission spectrum and the acceptor absorption spectrum. However, energy transfer is known to occur even in the absence of spectral overlap, resulting in effective quenching of excited states. As an example, we can cite the quenching of the fluorescence of aromatic hydrocarbons by dienes, a process which involves thermal deactivation of an excited state encounter complex, or exciplex, between D and A (Eq. (3.7))  

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Chemical properties of deposited monolayers have been studied in various ways. The degree of ionization of a substituted coumarin film deposited on quartz was determined as a function of the pH of a solution in contact with the film, from which comparison with Gouy-Chapman theory (see Section V-2) could be made [151]. Several studies have been made of the UV-induced polymerization of monolayers (as well as of multilayers) of diacetylene amphiphiles (see Refs. 168, 169). Excitation energy transfer has been observed in a mixed monolayer of donor and acceptor molecules in stearic acid [170]. Electrical properties have been of interest, particularly the possibility that a suitably asymmetric film might be a unidirectional conductor, that is, a rectifier (see Refs. 171, 172). Optical properties of interest include the ability to make planar optical waveguides of thick LB films [173, 174]. 

Agranovioh V M and Galanin M D 1982 Electronic Excitation Energy Transfer in Condensed Maffer (Amsterdam Elsevier/North-Flolland)... 

We have payed much attention to the investigations of excitation energy transfer in PCSs since the analysis of disclosed effects affords an entrance to the study of certain photochemical processes in PCSs. As to the photochemistry of PCSs, it is considered one of the promising fields of study not only from the theoretical but also from the practical point of view. 

The sensor detection of EEPs is methodically more complicated than the detection of atoms and radicals. With atoms and radicals being adsorbed on the surface of semiconductor oxide films, their electrical conductivity varies merely due to the adsorption in the charged form. If the case is that EEPs interact with an oxide surface, at least two mechanisms of sensor electrical conductivity changes can take place. One mechanism is associated with the effects of charged adsorption and the other is connected with the excitation energy transfer to the electron... 

Moreno M, Aramburu JA, Barriuso MT (2003) Electronic Properties and Bonding in Transition Metal Complexes Influence of Pressure 106 127-152 Morita M, Buddhudu S, Rau D, Murakami S (2004) Photoluminescence and Excitation Energy Transfer of Rare Earth Ions in Nanoporous Xerogel and Sol-Gel SiC>2 Glasses 107 115-143... 

Mullineaux, C. W. (1992). Excitation energy transfer from phycobilisomes to photosystem-I in a cyanobacterium. Biochim Biophys Acta 1100(3) 285-292. 

The next group of bimolecular interactions (3) shown in Table 1, includes noncontact interactions, in which fluorescence quenching occurs due to radiative and nonradiative excitation energy transfer [1, 2, 13, 25, 26]. Energy transfer from an excited molecule (donor) to another molecule (acceptor), which is chemically different and is not in contact with the donor, may be presented according to the scheme ... 

Gulis IM, Komiak AI, Tomin VI (1978) The electronic excitation energy transfer at conditions of dye spectra inhomogeneous broadening. Izv Akad Nauk SSSR, ser fiz 42 307-312... 

Hsu CP (2009) The electronic couplings in electron transfer and excitation energy transfer. Acc Chem Res 42 509-518... 

Agranovich, V. M. and Galanin, M. D. (1982). Electronic Excitation Energy Transfer in Condensed Matter. North-Holland Publishing Company, Amsterdam. 

Haas, E. and Steinberg, I. (1984). Intramolecular dynamics of chain molecules monitored by fluctuations in efficiency of excitation energy transfer. Biophys. J. 46, 429-37. 

In the following scheme the difference between intra- and inter-molecular electronic excitation energy transfer is summarized (as formulated in 2>) ... 

Sukhan has used PTAB cationic micelles to enhance the CL reaction of 4-diethylaminophthalohydrazide with oxygen and Co(II) in the presence of fluorescein as sensitizer [48], This enhancement is mainly due to electron-excited energy transfer from the donor (4-diethylaminophthalohydrazide) to the acceptor (fluorescein). The addition of fluorescein combined with the presence of PTAB reduces the detection limit of Co(II) by a factor of 6. The method was successfully applied in the determination of Co in tap water samples. 

Concept The rates of long-range electron transfer (ET) and excitation energy transfer (EET) processes between a pair of chromo-phores (redox couple) may be strongly facilitated by the presence of an intervening non-conjugated medium, such as saturated hydrocarbon bridges, solvent molecules and n-stacks, e.g.,... 

Through-Bond-Mediated Electronic Excitation Energy Transfer... 

Figure 23. Two principal mechanisms of excitation energy transfer (EET). (a) The Forster dipole-dipole mechanism, in which the active electrons, one and two, remain, respectively, on D and A throughout the process, (b) In the (Dexter) exchange mechanism, electrons one and two exchange locations.

A. Charas, J. Morgado, J.M.G. Martinho, A. Fedorov, L. Alcacer, and F. Cacialli, Excitation energy transfer and spatial exciton confinement in polyfluorene blends for application in light-emitting diodes, J. Mater. Chem., 12 3523-3527, 2002. 

Saini S, Srinivas G, Bagchi B (2009) Distance and orientation dependence of excitation energy transfer from molecular systems to metal nanoparticles. J Phys Chem B 113 1817-32... 

May V (2009) Beyond the Forster theory of excitation energy transfer importance of higher-order processes in supramolecular antenna systems. Dalton Trans 45 10086-105... 

Excitation energy transfer in dendritic host-guest donor-acceptor systems. Chem Phys Chem 3 1005-1013... 

Fig. 4.14. Schematic representation of the (A) Coulombic and (B) exchange mechanisms of excitation energy transfer. Cl Coulombic interaction EE electron exchange.

Baumann J. and Fayes M. D. (1986) Excitation Energy Transfer in Disordered Two-Dimensional and Anisotropic Three-Dimensional Systems Effects of Spatial Geometry on Time-Resolved Observables, J. Chem. Phys. 85, 4087-4107. 

Valeur B. (1989) Intramolecular Excitation Energy Transfer in Bichromophoric Molecules, in Jameson D. and Reinhart G. D. (Eds), Fluorescent Biomolecules,... 

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