Quantum yield for fluorescence

The intensity of fluorescence. If, is proportional to the amount of the radiation from the excitation source that is absorbed and the quantum yield for fluorescence... 

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 have been recorded for four aryl-substituted isoindoles rmder conditions of electrochemical stimulation. Electrochemiluminescence, which was easily visible in daylight, was measured at a concentration of 2-10 mM of emitter in V jV-dimethylformamide with platinum electrodes. Emission spectra due to electrochemi-luminescence and to fluorescence were found to be identical, and quantum yields for fluorescence were obtained by irradiation with a calibrated Hght source. Values are given in Table X. As with peak potentials determined by cyclic voltammetry, the results of luminescence studies are interpreted in terms of radical ion intermediates. ... 

In contrast to borazine, the three corresponding excited singlet states of benzene have a much wider spread of absorbing wavelengths and exhibit easily distinguished vibrational fine structure. Many photolysis experiments have been performed using laser lines tuned to selective excite a particular vibrational level of a particular excited state of benzene. Such experiments are more difficult with borazine. The triplet states of benzene have been located experimentally and quantum yields for fluorescence and phosphorescence at various wavelengths and pressure conditions have been determined. 

Most of the energy associated with an incident x-ray or y-ray is absorbed by ejected electrons. These secondary electrons are ejected with sufficient energy to cause further ionizations or excitations. The consequences of excitations may not represent permanent change, as the molecule may just return to the ground state by emission or may dissipate the excess energy by radiationless decay. In the gas phase, excitations often lead to molecular dissociations. In condensed matter, new relaxation pathways combined with the cage effect greatly curtail permanent dissociation. Specifically in DNA, it is known that the quantum yields for fluorescence are very small and relaxation is very fast [6]. For these reasons, the present emphasis will be on the effects of ionizations. 

In at least one case, pyrazine, it is known that the ratio of the quantum yields for fluorescence and phosphorescence is comparable in the gas60 (at 1 mm) and a mixed crystal.61 However, a detailed comparison should take into account the medium-dependent vibronic interaction with states nearby to those that emit (see Sect. V). 

We can then evaluate the quantum yield for fluorescence by integrating eq. (12-48) over all time, 0 < t < oo ... 

Figure 13.3 shows the absorption and fluorescence spectra of anthracene in ethanol for the SQ Sx transitions. Since the transitions are allowed, the intensity of light emitted is almost as strong as the intensity of light absorbed. The quantum yield for fluorescence, 8f, is defined by Equation 13.9. 

A simple two level model was proposed for BDN (20) to explain the origin of the nonlinearity obtained by tuning onto the resonance in that material. A similar model may explain the results observed here, although in the model proposed in reference 20 a high quantum yield for fluorescence is important, whereas these materials do not fluoresce (9). 

The extent to which the IC process is blocked by vibrational deficiency can be ascertained by the quantum yield for fluorescence, <1>F. Such information can be extracted from the measured fluorescence lifetimes, X[, of the vibronic levels of the Sj state, providing the oscillator strength of the S i < S0 transition is known. The radiative lifetime, based on the transition strength, rr, can calculated from the oscillator strength through the expression, xr 1.5//V2. With a measured oscillator strength of / 1.2 x 10 4 and an average transition frequency of... 

Quantum yields for fluorescence from the naphthalene unit (< m) and excimer (< e) for a,cd-dinaphthylalkanes in degassed cyclohexane solution at 20°C... 

The quantum yield for fluorescence is low and decreases on increasing the solvent polarity. This may be due to photoionisation playing a more important part in solvents having a high polarity. The fluorescence lifetime remains short in all solvents. The fluorescence characteristics of [36 n = 1], low quantum yield of fluorescence, short fluorescence lifetimes and the wavelength... 

Diffusion controlled processes for H+/Lewis acid generation upon both OP and TP excitation can be of minor importance if the onium compound comprises a part of the TP chromophore. Compound 179 is such a chromophore. This compound combines a distyrylbenzene with end capped amino groups. The latter bears a sulfonium group in m-position. OPA of 179 occurs at 392 nm (e = 5.5 x 104 M em ). TPA was investigated between 705 and 850 nm. The TPA cross section of 179 is 690 GM (Fig. 3.68a). Furthermore, the quantum yield of OP initiated generation of H+ is about 0.5, while the quantum yield for fluorescence is significantly less (f = 0.013) [137, 225]. 

Monomers and dimers exhibit maximal emission at 314-324 nm upon excitation at 272-280 nm. Quantum yields for fluorescence, denoted by Q, are collected in Table I. Both monomers exhibit increasing fluorescence as the solvent changes from water to trifluoroethanol to dioxane. There is no significant difference in the emission of the two monomers in a common solvent. Dimers show less intense emission that do monomers. There is a further reduction in emission upon acetylation of all phenolic and aliphatic hydroxyl groups. COMPLEX FORMATION by POLY(VINYLPYRROLIDONE)... 

Ihe sample of poly(vinylpyrrolidone) used in these experiments has an intrinsic viscosity of 1.72 dl/g in water at 30° C. In the presence of 0.0014 M (+)-catechin, the intrinsic viscosity falls to 0.13 dl/g. Since it is the (+)-catechin that induces the collapse of the poly(vinylpyrrolidone) chain, it is assumed that this molecule finds itself in an environment that is not completely aqueous, but instead has a high local concentration of poly(vinyl-pyrrolidone) segments. The solvent dependence of the quantum yield for fluorescence, reported in Table I, shows that Q increases as (+)-catechin is transferred from water to dioxane. Collapse of the poly(vinylpyrrolidone) chain about (+)-catechin therefore produces an increase in Q, which is responsible for I/l(0) > 1 in Table II. 

However, changes in the absorption spectrum are not the only effects expected and seen in the behavior of dyes on metallic (silver) surfaces. It has been known for some time now that proximity to a silver surface brings about major changes in the lifetimes and quantum yields for fluorescence of molecules. For an early work in that field, see Kuhn" and for a excellent theoretical treatment. Chance et Within the framework of classical electrodynamics, it was shown that the metal opens dissipative channels into which the excited molecule can discharge its energy. The theoretically calculated lifetimes fully agreed with those measured, up to distances of about 10 nm from the surface. 

---