Electronic excitation energy fluorescence

Numerous autoxidation reactions of aliphatic and araliphatic hydrocarbons, ketones, and esters have been found to be accompanied by chemiluminescence (for reviews see D, p. 19 14>) generally of low intensity and quantum yield. This weak chemiluminescence can be measured by means of modern equipment, especially when fluorescers are used to transform the electronic excitation energy of the triplet carbonyl compounds formed as primary reaction products. It is therefore possible to use it for analytical purposes 35>, e.g. to measure the efficiency of inhibitors as well as initiators in autoxidation of polymer hydrocarbons 14), and in mechanistic studies of radical chain reactions. 

The theory of resonance transfer of electronic excitation energy between donor and acceptor molecules of suitable spectroscopic properties was first presented by Forster.(7) According to this theory, the rate constant for singlet energy transfer from an excited donor to a chromophore acceptor which may or may not be fluorescent is proportional to r 6, where r is the distance... 

In photoassociating solvents (e.g., benzene and its alkyl derivatives) the transfer of electronic excitation energy from solvent to fluorescent solute is characterized37,63 by rate constants kt which are approximately twice those computed from the less approximate form of Eq. (7) ... 

When deciding to study the dynamics of electronic excitation energy transfer in molecular systems by conventional spectroscopic techniques (in contrast to those based on non-linear properties such as photon echo spectroscopy) one has the choice between time-resolved fluorescence and transient absorption. This choice is not inconsequential because the two techniques do not necessarily monitor the same populations. Fluorescence is a very sensitive technique, in the sense that single photons can be detected. In contrast to transient absorption, it monitors solely excited state populations this is the reason for our choice. But, when dealing with DNA components whose quantum yield is as low as 10-4, [3,30] such experiments are far from trivial. 

Several interesting theoretical papers have appeared dealing with molecular dynamics and excimer formation in polymer systems. Frank and coworkers have developed a model to describe the transport of electronic excitation energy in polymer chains. The theory applies to an isolated chain with a small concentration of randomly placed chromophores, and a three-dimensional transport model was used to solve the problem which is based on a diagrammatic expansion of the transport Green function. (The Green function is related to time-dependent and photostationary depolarization and to transient and steady-state trap fluorescence.) The analysis is shown to be... 

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... 

Radiative transfer is an unfortunate complication in many electronic energy transfer experiments and it is difficult either to eliminate or make satisfactory allowance for this effect. Martinho and d Olveira have studied in detail the influence of radiative transport on observations of electronic excitation energy transfer. In particular they have analyzed the effects of radiative transport on measured fluorescence decay curves for concentrated solutions. An experimental study of the influence of radiative transport on energy transfer from excited fluorene to pyrene it occurs n-hexane relates closely with this work . Kawski et have... 

One difficulty frequently encountered in fluorescence is that of fluorescence quenching by many substances. These are substances that, in effect, compete for the electronic excitation energy and decrease the quantum yield (the efficiency of conversion of absorbed radiation to fluorescent radiation— see below). Iodide ion is an extremely effective quencher. Iodide and bromide substituent groups decrease... 

Fig. 1.1. Electronic and vibrational energy levels (schematic, rotational are omitted for simplicity), So singlet ground state, Sj excited singlet state (higher electronically excited states are omitted in this figure), T triplet electronically excited energy state. Straight lines symbolise radiative processes (absorption (Ab) as well as emission F, fluorescence P, phosphorescence). Wavy lines give the radiationless transitions, ic, internal conversion isc, intersystem crossing sd, radiationless deactivation te, thermal equilibration.

---