Molecular radiationless deactivation

Molecular fluorescence and, to a lesser extent, phosphorescence have been used for the direct or indirect quantitative analysis of analytes in a variety of matrices. A direct quantitative analysis is feasible when the analyte s quantum yield for fluorescence or phosphorescence is favorable. When the analyte is not fluorescent or phosphorescent or when the quantum yield for fluorescence or phosphorescence is unfavorable, an indirect analysis may be feasible. One approach to an indirect analysis is to react the analyte with a reagent, forming a product with fluorescent properties. Another approach is to measure a decrease in fluorescence when the analyte is added to a solution containing a fluorescent molecule. A decrease in fluorescence is observed when the reaction between the analyte and the fluorescent species enhances radiationless deactivation, or produces a nonfluorescent product. The application of fluorescence and phosphorescence to inorganic and organic analytes is considered in this section. 

Emission spectra of radical cations are obtained by vacuum UV ionization and subsequent laser excitation in noble-gas matrices (see below), or by electron-impact ionization of a beam of neutral parent molecules at energies above the first ionic excited state. After internal conversion to the first excited state, emission may compete more or less successfully with radiationless deactivation. If the experiment is carried out on a supersonic molecular beam one obtains highly resolved emission spectra which, in the case of small molecules, may contain sufficient information to allow a determination of the molecular structure. 

The intramolecular processes responsible for radiative and radiationless deactivation of excited states we have considered so far have been uni-molecular processes that is, the processes involve only one molecule and hence follow first-order kinetics. 

Notwithstanding the excellent analytical features inherent in molecular phosphorimetric measurements, their use has been impeded by the need for cumbersome cryogenic temperature techniques. The ability to stabilize the "triplet state" at room temperature by immobilization of the phosphor on a solid support [69,70] or in a liquid solution using an "ordered medium" [71] has opened new avenues for phosphorescence studies and analytical phosphorimetry. Room-temperature phosphorescence (RTF) has so far been used for the determination of trace amounts of many organic compounds of biochemical interest [69,72]. Retention of the phosphorescent species on a solid support housed in a flow-cell is an excellent way of "anchoring" it in order to avoid radiationless deactivation. A configuration such as that shown in Fig. 2.13.4 was used to implement a sensor based on this principle in order to determine aluminium in clinical samples (dialysis fluids and concen-... 

P. Celani, M. Garavelli, S. Ottani, F. Bernardi, M. A. Robb, and M. Olivucci,/. Am. Cbem. Soc., 117,11584 (1995). Molecular Trigger for the Radiationless Deactivation of Photoex-cited Conjugated Hydrocarbons. 

A quencher should have an excitation energy lower than that of the donor species and the appropriate electronic configuration. Transfer of excitation energy proceeds by radiative or radiationless deactivation of the donor molecular entity. Radiative energy transfer (also called trivial energy transfer) consists of light emission by the donor molecule and reabsorption of the emitted light by the acceptor molecular entity. 

The CT excited states usually are not emissive because the low energy gap and the strong distortion with respect to the ground state favor the occurrence of radiationless deactivation. Furthermore, the presence of the low-energy CT excited states causes rapid radiationless decay of the upper lying, potentially luminescent excited states localized on the molecular components. Therefore, rotaxanes and catenanes based on CT interactions usually do not exhibit any luminescence. For example, the intense emission band with maximum at 320 nm (t = 2.5 ns) exhibited by macrocycle 7 [1 la] is no longer present in catenane [23]. 

A theoretical paper discusses the nature of solvent effects which affect the deactivation of A 02 . The effect of hydrostatic pressure on the radiationless deactivation of 03 in solution has also been reported . The perturbing effects of the solvents H2O, DjO, and QH5CH3 on the luminescence rate constant of O2 up to 100 atm provide evidence for pardcipation of complexes involving both the ground and excited states of molecular oxygen . The application of a collision complex model has been applied to the interpretation of photophysical quenching of 03 in liquids by 4-amino-TEMPO . 

In competition with radiationless deactivation, energy can also be lost in the form of radiation (F, P) [3], [28], Fluorescence occurs with molecules that either (1) have extended n-systems (such as polycondensed aromatics) (2) do not permit deactivation by torsional or rotational motion of parts of the molecule or (3) have no heavy atoms as substituents [32], 33]. In addition to these molecular properties, the environment also plays a part. Thus, the fluorescence intensity increases at low temperature and in solid matrices. This is even more important in phosphorescence, where the T] -> So transition is in fact spin-forbidden. If a higher vibrational level (v >0) is occupied in Ti, in accordance with the Boltzmann equation (Eq. 5),. so-called delayed fluorescence [28] can occur by backward intersystem crossing [T, S,(b = 0) S ]. 

Celani P, GaraveUi M, Ottani S, Bemardi F, Robb MA, OHvucci M. Molecular tri er for radiationless deactivation of photoexcited conjugated hydrocarbons. J Am Chem Soc. 1995 117 11584-11585. 

Modern experimental measurements and the new computational techniques just discussed are now providing results that can rationalize issues such as the efficiency of 1C at a surface crossing, the competition with fluorescence when an excited state barrier is present, and the relationship between the molecular structure at the intersection and the structure of the photoproducts. Experiments on isolated molecules in cold-matrices or expanding-jets have revealed the presence of thermally activated fast radiationless decay channels. For example, Christensen et al. have proposed that (under isolated conditions in a cool-jet) trans — cis motion in all-tra 5-octa-1.3,5,7-tetraene (all-trow -OT) induces the opening of an efficient nonadiabatic radiationless deactivation channel on Si (2Ag). We now discuss this experiment and complementary theoretical results that illustrate the way in which theory and experiment can be used in concert. 

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. 

A term in photochemistry and photophysics describing an isoenergetic radiationless transition between two electronic states having different multiphcities. Such a process often results in the formation of a vibrationally excited molecular entity, at the lower electronic state, which then usually deactivates to its lowest vibrational energy level. See also Internal Conversion Fluorescence... 

Excited-state relaxation can proceed spontaneously in monomolecular processes or can be stimulated by a molecular entity (quencher) that deactivates (quenches) an excited state of another molecular entity, by energy transfer, electron transfer, or a chemical mechanism [lj.The quenching is mostly a bimolecular radiationless process (the exception is a quencher built into the reactant molecule), which either regenerates the reactant molecule dissipating an energy excess or generates a photochemical reaction product (Figure 4.1). 

An electronically excited atom must lose its energy either by emission of radiation or by collisional deactivation chemical decomposition is not possible, and radiationless degradation (involving an increase in translational energy) is extremely improbable. At low enough pressures, therefore, fluorescent emission is expected from all atoms. Many molecular species, however, either do not exhibit fluorescence or fluoresce weakly even when bimolecu-lar reaction or physical deactivation does not occur. Some general principles... 

Internal conversion A photophysical process. Isoenergetic radiationless transition between two electronic states of the same multiplicity. When the transition results in a vibrationally excited molecular entity in the lower electronic state, this usually undergoes deactivation to its lowest vibrational level, provided the final state is not unstable to dissociation. 

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