Quantum yield radiationless deactivation

Intersystem crossing from Si is a rather rapid process (kisc — 109s l), so that it may compete with fluorescence and reduce its quantum yield. Radiationless deactivation of... 

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. 

From these equations one can approximate kf for benzophenone to be 5 x 10s sec-1. This, however, is the expected rate constant for fluorescence, which should be in competition with radiationless deactivation of the excited state kd. In actuality no fluorescence is observed for benzophenone although the fluorescence techniques are sensitive enough to detect fluorescence occurring with a quantum yield as low as 0/ = 0.001. Therefore kd must be at least 1000 times greater than kf We have... 

Pyrimidine (1,3-diazine) and pyrazine (1,4-diazine) exhibit weak fluorescences73,74 in solutions or as vapors at room temperature, and strong phosphorescences 76-79 in dilute solid solutions at low temperatures (77 or 90°K). The phosphorescent quantum yields have never been accurately measured in these solid solutions. In the vapor phase or in ordinary solutions, at room temperature, these two compounds do not phosphoresce. Radiationless deactivation processes must be considered again and a deactivation through an isomer cannot be excluded. 

Marginal fluorescence quantum yields (1%) are generally observed though 25 and 33 fluorescence with 8% and 14% yields, respectively. Such low quantum yields are indicative of the effective competition of radiationless processes such as the Si —> Tj ISC and fast internal conversion (Si —> S0). The rate constants for radiative decay of Si (kF) range from 8 x 106 to 1.3 x 108 s-1, and the nonradiative decay rate constants (fcNR) range from 1.9 x 108 to 3.5 x 109 s / The nonradiative deactivation pathway is thus six times faster than the radiative one for 33 (anti) and about 110 times faster for 32 (syn). 

Dyes whose fluorescence intensity increases on binding to DNA (e.g. 3 and 4) have especially high potential as DNA marker or detector molecules. In the absence of DNA the relatively low fluorescence quantum yield of these dyes results from a radiationless deactivation of the excited state by conformational changes or acid-base reactions with the solvent. On association with DNA, however, significant suppression of the conformational flexibility and a shielding of the dye from solvent molecules within the complex occurs, leading to an increase of the emission intensity. 

Bimolecular deactivation (pathway vii, Fig. 1) of electronically excited species can compete with the other pathways available for decay of the energy, including emission of luminescent radiation. Quenching of this kind thus reduces the intensity of fluorescence or phosphorescence. Considerable information about the efficiencies of radiative and radiationless processes can be obtained from a study of the kinetic dependence of emission intensity on concentrations of emitting and quenching species. The intensity of emission corresponds closely to the quantum yield, a concept explored in Sect. 7. In the present section we shall concentrate on the kinetic aspects, and first consider the application of stationary-state methods to fluorescence (or phosphorescence) quenching, and then discuss the lifetimes of luminescent emission under nonstationary conditions. 

If there are leakages from the excited state due to radiationless deactivation processes, Eq. (6) must be used with k = — f, where is the fluorescence quantum yield of A in the absence of reaction. These leakages will increase the last term of the summation in Eq. (14) and hence lower the global conversion efficiency. In the above example, if 0f = 0.5 (k = 0.5), the flux ratio rp is increased from 0.98 to 0.99 and the global conversion efficiency at maximum power is slightly reduced from 0.71 to 0.70. 

The choice of donor and acceptor dyes used in this study was determined by several criteria Surface concentrations had to be varied over as large a range as possible. For this reason only cationic dyes could be employed. The spectral overlap between donor emission and acceptor absorption also had to be as large as possible. The donor dyes should have a sufficiently high fluorescence quantum yield while the deactivation of the acceptor dyes should be predominantly radiationless. 

The absorption behavior of the photoactive component does not say anything about the behavior of the excited state. Does it deactivate rapidly in the singlet channel with fluorescence or radiationless Does it undergo fast intersystem crossing into the triplet manifold What is the chemistry of all these excited states If we have the opportunity to measure them, fluorescence and phosphorescence spectra will supply us with lifetime and quantum yield data. A comprehensive up-to-date collection of photophysical data can be found in Murov s Handbook of Photochemistry (see Table 2). 

The participation of higher excited singlet states (Sn, n > 1) of molecules in photophysical (Sn SQ fluorescence (FL)) or photochemical (photoinduced electron transfer (PET), isomerization, etc.) processes, which compete with radiationless deactivation, manifests itself in the dependence of the quantum yield (q>) and FL spectra on the wavelength of the exciting light (the violation of the Vavilov law). Such processes were first shown for the FL of azulene solutions due to the transition from the second excited level to the ground state S2 -> S0. ... 

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