Related Excited-State Energy-Transfer Processes

Related Excited-State Energy-Transfer Processes... 

The covalently linked or self-assembled systems constructed from free-base porphyrins or metalloposphyrins and various quenchers and related studies of excited-state energy-transfer and electron-transfer processes in such systems are a widely studied topic of contemporary supramolecular chemistry. There are two major reasons for the exceptional interest in the topic a challenge to synthesize artificial systems for harvesting solar energy of... 

Excited state electron transfer also needs electronic interaction between the two partners and obeys the same rules as electron transfer between ground state molecules (Marcus equation and related quantum mechanical elaborations [ 14]), taking into account that the excited state energy can be used, to a first approximation, as an extra free energy contribution for the occurrence of both oxidation and reduction processes [8]. 

When one metal ion is used as a donor for sensitizing the emission of a second accepting metal ion, the characteristic lifetimes r of their excited states, which are related to their deactivation rates by r = k l, are affected by the metal-to-metal communication process. This situation can be simply modeled for the special case of an isolated d-f pair, in which the d-block chromophore (M) sensitizes the neighboring lanthanide ion (Ln) thanks to an energy transfer process described by the rate constant k 1 ". In absence of energy transfer, excited states of the two isolated chromophores decay with their intrinsic deactivation rates kxl and kLn, respectively, which provides eqs. (32) and (33) yielding eqs. (34) and (35) after integration ... 

Compared with alcohols, which possess a high standard potential of oxidation, moderate nucleophilicity and weak basicity, amines very often serve as good electron donors and relatively strong bases and nucleophiles in chemical reactions. Photoinduced electron transfer (PET) processes, in which an amine donates an electron to the reaction partner in either its ground or excited electronic state, result in the formation of an amine substrate exciplex (Scheme 6.196).670 1224 The driving force for electron transfer is related to the standard potential of oxidation of the donor, the standard potential of reduction of the acceptor and the excited state energy of the absorbing partner (see Chapter 4). 

The promotion of one electron to an excited state upon light absorption generates an electron-hole pair in the frontier orbital system of the porphyrinic chro-mophore. This electron-hole pair is an exciton, and its generation introduces a nonpermanent (transitory) dipole in the chromophore. The energy transfer process is strongly related to the interactions between excitons in multiple chromophore assemblies. 

A powerful application of TRIR has been to use the z/(CO) and n(CN) vibrations of supramo-lecular coordination compounds to monitor electron- and energy-transfer processes in solution. For example, a combination of fast and ultrafast TRIR was used to show that excitation into the Re-MLCT transition of [(phen)(CO)3Re NC)Ru CN)(bpy)2] (phen= 1,10-phenanthroline) produces the Re-MLCT state, followed by energy transfer to the Ru-based MLCT state." For the related complex, (CO)5W(4,4 -bpy)W(CO)5 ns-TRIR was used to confirm that, on the IR... 

The process of vibrational excitation and deexcitation of a diatom in a collision with an atom represents a simplest example from the host of processes which are relevant to gas-phase chemical kinetics. Experimental techniques available now allow one to measure directly state-to-state energy transfer rate coefficients. Theoretically, it is possible to accomplish completely ab initio calculation of these coefficients. One can therefore, regard the existing models of the vibrational relaxation from a new standpoint as a means for helping to understand more clearly the dynamics of the energy transfer provided that all the models are related to a single fundamental principle. This is the Ehrenfest adiabatic principle as formulated by Landau and Teller in the application to the collisional vibrational transitions of diatomic molecules. 

The interaction of Z-state molecules with closed shell atoms and related energy transfer processes have been extensively studied and are well understood, on almost a quantitative level (see Ref. 49, and also the article by Beneventi et al. in this Volume). For open shell molecules not in Z-state the situation is more involved in addition to rotational excitation, transitions between spin-orbit states and/or A-doublet levels may occur. Ab initio calculations of the relevant PES s are obviously even more challenging than for closed shell species. 

The thermodynamic ability of an excited state to intervene in energy transfer processes is related to its zero-zero spectroscopic energy, From a kinetic viewpoint, bimolecular energy transfer processes involving encounters can formally be treated using a Marcus type approach, i.e. by equations like (2.19) and (2.20), with AG° = and A A [8]. 

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

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