Sequential energy transfer

A type II CL reaction can be represented as sequential chemical and energy transfer reactions. 

Forde TS, Hanley QS (2005) Following FRET through five energy transfer steps spectroscopic photobleaching, recovery of spectra, and a sequential mechanism of FRET. Photo-chem Photobiol Sci 4 609-616... 

F ure 5.17 Sequential steps for a nonradiative energy transfer process (see the text). 

Similarly, the fluorescence intensity of the 1,4-disubstituted azine with ferrocene and pyrene units (17) can be reversibly modulated by sequential redox reactions of ferrocene moiety. In the neutral state, compound 17 displays weak fluorescence owing to the electron transfer from the ferrocenyl group to the excited pyrene unit or by energy transfer from the excited pyrene unit to the ferrocenyl unit. Oxidation of the ferrocenyl unit, however, leads to remarkable fluorescence enhancement. This is because the ferrocenium cation shows weak electron donating ability and also the corresponding spectral overlap becomes small.27... 

Upconversion lanthanide-containing nanophosphors, which emit higher-energy photons when excited by lower-energy photons have stirred increasing research interest in recent years. The predominant mechanisms of upconversion in nanophosphors are excited-state absorption (ESA), energy-transfer upconversion (ETU) and photon avalanche (PA) (Prasad, 2004 Auzel, 2005). In the ESA process, two photons are sequentially absorbed by the same ion,... 

Up-conversion is a process by which two photons of lower energy are subsequently converted into a luminescence photon of higher energy (typically, two IR photons giving rise to one visible photon, e.g. in Er111-containing compounds). This anti-Stokes process is usually observed for ions embedded in solids and is made possible by various mechanisms, such as the now classical excited state absorption mechanism (ESA), or sequential energy transfers (ETU for... 

Up-conversion relies on sequential absorption and luminescence with intermediate steps to generate shorter wavelengths. Hence, the presence of more than one metastable excited state is required the intermediate metastable states act as excitation reservoirs. One typical example is ground-state absorption followed by inter-mediate-state excitation, excited-state absorption, and final-state excitation to give the up-conversion (the intermediate states and final states are real states) [1, 35], There are many types of up-conversion mechanisms such as excited-state absorption, energy transfer up-conversion and cooperative up-conversion. All these up-conversion processes can be differentiated by studying the energy dependence, lifetime decay curve, power dependence, and concentration dependence by experimental measurements [36-39]. 

These processes can be explained by using a harmonic oscillator-like model (Fig. 13(b)). The energy transfer of an electron excites the molecule to a higher energy level and subsequent energy transfer by other electrons causes further excitation in a sequential process. The molecule dissociates when it exceeds the dissociation barrier. 

Figure 2.7 Schematic illustrations of the proposed mechanisms for electron transfer through a bridge (a) superexchange, in which electron transfer is a concerted process mediated through a bridge whose energy levels lie well outside of resonance with the donor orbitals (b) electron hopping, where the electron is transferred sequentially through a bridge whose energy levels lie in close resonance with the donor orbitals...

Figure 5.63 Two examples of multilayer systems where the energy transfer components are (a) coadsorbed, and (b) sequentially adsorbed. Reprinted with permission from D. M. Kaschak, J. T. Lean, C. C. Waraska, G. B. Saupe, H. Usame and T. E. Mallouk,. Am. Chem. Soc., 121, 3435 (1999). Copyright (1999) American Chemical Society...

A closely related tetrad featuring two porphyrin moieties and a single naphthoquinone acceptor has also been reported [13]. Excitation of either porphyrin moiety of C-P-P-Q in anisole solution is followed by rapid (>10 s ) singlet-singlet energy transfer between the two porphyrins, whose absorption and emission spectra are essentially identical. C-P- P-Q decays by photoinduced electron transfer to the quinone with a rate constant of 2.4 x 10 s. Sequential transfer of the radical cation hole to the second porphyrin, and then to the carotenoid yields a final C +-P-P-Q state with a quantum yield of 0.25 and a lifetime of 2.9 ps. 

Even though the principle of focused microwaves is efficient in terms of energy transfer, it has allowed for the use of only one flask at a time in most designs. Automation in some systems can enable the sequential use of flasks, but this characteristic can be viewed as a constraint. One recent development involves the use of four flasks at a time by symmetrically splitting the microwave energy among the flasks at the end of each waveguide. 

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