Radiationless transfer of electronic excitation

Ermolaev, V. L., Bodunov, E. N., Sveshnikova, E. B., and Shakhverdov, T. A. (1977) Radiationless Transfer of Electronic Excitation Energy, Nauka. Leningrad. 

III. Transfer of Electronic Excitation Energy A. Radiationless Energy Transfer... 

The transfer of electronic excitation energy from one molecule to another is another phenomenon that is related to the concentration of potentially luminescent molecules. Energy transfer occurs quite frequently in nature, either by direct collision or even over distances as great as 50 A or more by a radiationless mechanism by which the excitation energy is transmitted from the molecules that are originally excited (donors) to the recipient molecules (acceptors). The efficiency with which an excited donor will transfer its excitation energy to an acceptor molecule (rather than fluoresce) is a function of the lifetime of the excited... 

Radiationless transitions such as internal conversion or intersystem crossing lead to the conversion of electronic energy to vibrational energy which is transferred randomly to the environment. Another important type of deexcitation involves the direct transfer of electronic excitation selectively to individual molecules present in relatively small concentrations in the environment (for summary of literature see Kasha, 1963). Of particular interest to photochemists are excitation transfer processes which involve a donor molecule in either its lowest singlet or lowest triplet excited state and an acceptor molecule in its ground state (usually singlet). Because certain multiplicity selection rules must be obeyed, only the processes shown in Eqs. (7)-(9) are allowed, where D and A stand for donor and acceptor respectively. 

Knowledge of photoiaduced electroa-transfer dyaamics is important to technological appUcations. The quantum efficiency, ( ), ie, the number of chemical events per number of photons absorbed of the desired electron-transfer photoreaction, reflects the competition between rate of the electron-transfer process, eg, from Z7, and the radiative and radiationless decay of the excited state, reflected ia the lifetime, T, of ZA ia abseace ofM. Thus,... 

The molecular ion will be of low symmetry and have an odd electron. It will have as many low-lying excited electronic states as necessary to form essentially a continuum. Radiationless transitions then will result in transfer of electronic energy into vibrational energy at times comparable to the periods of nuclear vibrations. 

All these results indicate that, with titanium-silicon binary oxides having low titanium contents the Ti ions are enriched near the surface regions, separated from each other, and present as tetrahedral species in the Si02 carrier matrices. In such species, the radiationless transfer of photon energy absorbed by Ti02 is suppressed because of the low coordination of the ions. As a result, the formation of the (electron-hole) ion pairs, i.e., the excited state of (Ti +- 0 ) complexes, is facihtated (200, 201). 

Type C comprises a large number of dye-photosensitized reactions and usually involves a radiationless transition from the excited singlet to a triplet state prior to free radical formation in subsequent reactions. They are not discussed in detail here, although the similarity of the dye-photosensitized reaction with the reactions photosensitized by uranyl ion is noteworthy. Attention must also be drawn to Simons excellent review 18) of reactions of electronically excited molecules in solution, in which photochemical reactions of type C, including those involving energy transfer, are dealt with thoroughly. 

Following photoexcitation using a laser pulse at 355 nm, emission is observed from the monolayers with an excited state lifetime (6.2 ps) that exceeds that of the complex in solution (1.4 ps). It appears that weak electronic coupling between the adsorbates and the electrode means that the excited states are not completely deactivated by radiationless energy transfer to the metal. As illustrated in Fig. 13, in the first report of its land, we used voltammetry at megavolt per second scan rates to directly probe the redox potentials and electron transfer characteristics of electronically excited species. 

Cyanide bridges can also be used to bridge Ru(bpy)2 groups to other ruthenium centers. Examples of such multimetallic compounds are the triruthenium complexes X(NH3)4Ru-NC-Ru(bpy)2 N-Ru(NH3)4Y" (X = NH3, py Y=NH3, py n = 4-6), which have a central Ru(II) bpy complex, and outer ammine or pyridine complexes that have Ru2(II), Ru(II, III), or Ru2(III) metal centers. These complexes are nonemissive with the Ru(bpy)2(CN)2 chromophore being completely quenched by the Ru(NH3)r and Ru(NH3)4py" moieties. This quenching is a result of radiationless deactivation of the excited state via low-lying remote d-n or intervalence states. For the semioxidized complex an additional possibility is that electron transfer steps convert the intermediate states to the end-to-end intervalence state. 

Although the number of EFS traps will be an important factor governing the observed fluorescence, a second factor of equal or sometimes greater importance is the phenomenon of electronic excitation transport (EET) [51, 58-61]. This involves the radiationless transfer of excitation energy, in the singlet state for compounds in this work, from one aromatic chromophore to another. This process may be viewed as a random walk with the rate of transfer between randomly oriented absorption and emission dipoles being given at each step by... 

Conical intersections (CIs) between electronic potential energy surfaces play a key mechanistic role in nonadiabatic molecular processes [1 ]. In this case the nuclear and electronic motions can couple and the energy exchange between the electrons and nuclei may become significant. In several important cases like dissociation, proton transfer, isomerization processes of polyatomic molecules or radiationless deactivation of the excited state systems [5,6] the CIs can provide very efficient channels for ultrafast interstate crossing on the femtosecond time scale. 

In Sections 7.3 and 7.4, the temperature dependence of radiationless transitions and the effect of deuteration on the lifetimes of excited electronic states are examined. In Section 7.5, a contribution to time-resolved spectroscopy is presented. In that section, we will discuss a problem dealing with transport phenomena of electronic excitations in doped molecular crystals. The theory of singlet excitation energy transfer uses an effective Hamiltonian to account for intramolecular excited-state depopulation and energy transfer by multistep migration among guest molecules. 

Another form of radiationless relaxation is internal conversion, in which a molecule in the ground vibrational level of an excited electronic state passes directly into a high vibrational energy level of a lower energy electronic state of the same spin state. By a combination of internal conversions and vibrational relaxations, a molecule in an excited electronic state may return to the ground electronic state without emitting a photon. A related form of radiationless relaxation is external conversion in which excess energy is transferred to the solvent or another component in the sample matrix. 

This case is shown in Fig. 10.6c and d where through absorption of light a photohole in the vb and a photoelectron in the cb are formed. The probability that interfacial electron transfer takes place, i.e. that a thermodynamically suitable electron donor is oxidized by the photohole of the vb depends (i) on the rate constant of the interfacial electron transfer, kET, (ii) on the concentration of the adsorbed electron donor, [Rads]. and (iii) on the rate constants of recombination of the electron-hole pair via radiative and radiationless transitions,Ykj. At steady-state of the electronically excited state, the quantum yield, Ox, ofinterfacial electron-transfer can be expressed in terms of rate constants ... 

When molecules absorb a photon and produce an electronic excited state, the energy can be dissipated in several ways luminescence, radiationless decay to the ground state, and photochemistry. Luminescence dominated the older literature because it was easy to observe. A good review of luminescence is in Volume 3 of David Dolphin s seven-volume series The Porphyrins. Picosecond laser spectroscopy allowed for exploration of the radiationless decay pathways, particularly the initial steps that compete with luminescence and lead to photochemistry. Two principal forms of radiationless decay lead to long-term metastables ligand ejection and electron transfer. 

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