Quantum fluorescent properties

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. 

In most cases, the linear absorption is measured with standard spectrometers, and the fluorescence properties are obtained with commercially available spectrofluo-rometers using reference samples with well-known <1>F for calibration of the fluorescence quantum yield. In the ultraviolet and visible range, there are many well-known fluorescence quantum yield standards. Anthracene in ethanol (Cresyl Violet in methanol (commonly used reference samples for wavelengths of 350-650 nm. For wavelengths longer than 650 nm, there is a lack of fluorescence references. Recently, a photochemically stable, D-ji-D polymethine molecule has been proposed as a fluorescence standard near 800 nm [57]. This molecule, PD 2631 (chemical structure shown in Fig. 5) in ethanol, has linear absorption and fluorescence spectra of the reference PD 2631 in ethanol to... 

The aim of this Chapter is to review a method by which fluorescence properties of organic dyes can, in general, be predicted and understood at a microscopic (nm scale) by interfacing quantum methods with classical molecular dynamics (MD) methods. Some review of our extensive applications [1] of this method to the widely exploited intrinsic fluorescence probe in proteins, the amino acid tryptophan (Trp) will be followed by a discussion of electrochromic membrane voltagesensing dyes. 

A fluorophore in the proximity of the NP senses the altered EM-field and its fluorescence properties are consequently modified. There are (at least) two enhancement effects an increase in the excitation of the fluorophore and an increase in its quantum efficiency (QE). The increased excitation of the fluorophore is directly proportional to the to the square of the strength of the E-field and is a function of the wavelength and relative position of the NP. The maximum enhancement of this type is achieved if /.res equals the peak absorption wavelength of the dye. 

Dendrimers can be used to effectively coat and passivate fluorescent quantum dots to make biocompatible surfaces for coupling proteins or other biomolecules. In addition, the ability of dendrimers to contain guest molecules within their three-dimensional structure also has led to the creation of dendrimer-metal nanoclusters having fluorescent properties. In both applications, dendrimers are used to envelop metal or semiconductor nanoparticles that possess fluorescent properties useful for biological detection. 

Table 4.1 Fluorescence properties of some representative compounds. The fluorescence quantum yields are measured in solution at room temperature...

In summary, the encapsulation of cyanine dyes in CB7 causes either an increase or a decrease in quantum yields and brightness but in general increases photostability. Enhancements in fluorescence intensity by about one order of magnitude or more were observed [46]. These fluorescence property changes are utilized for the development of sensors, where the fluorescent dye may serve as a probe to signal the binding of an analyte. No reports were found on the encapsulation of squaraines in CBs. 

Among several fluorescence properties, fluorescence quantum yield and lifetime are the two most important characteristics of a fluorophore. Studies of the effects of the silica nanomatrixes on these two characteristics reveal the mechanism of enhanced fluorescence intensity of DDSNs. 

The effects of substitution and solvent polarity on the fluorescence properties of trans-9-styrylanthracenes 69a-k in terms of Stokes shift and fluorescence quantum yields have been summarized in Table 15. The fluorescence quantum yields in cyclohexane solution generally are about 0.5, exceptions with lower quantum yields (0.27) being the N,N-dimethylamino and nitro derivatives. For nonpolar substituted trons-9-styrylanthracenes in acetonitrile solution, the quantum yields are of the same order of magnitude as in cyclohexane. By contrast, the fluorescence quantum yields for trans-9-styrylanthracenes substituted by polar groups are drastically reduced in acetonitrile, as would be expected for bichromophoric excited state species of polar character (cf. Section III.B). 

The fluorescence properties of 2,2-diaryl-substituted l-(9-anthryl)-ethylenes 87c-e differ markedly from those of 87a, b by a decrease in the quantum yields of emission, and by the loss of vibrational fine structure of the emission spectra, which is associated with a dramatic increase of the Stokes shifts. For the 2,2-diphenyl derivative 87c in cyclohexane solution, the quantum yield is 0.29, and the Stokes shift is 5600cm-1. For 9-anthryl-ethylene 87e, in which the formal conjugation has been extended by a terminal methylene group, the quantum yield in cyclohexane is as low as... 

Exceptional fluorescence properties also characterize the ri.s-isomer 38e. Unsubstituted cis-l,2-di-9-anthrylethylene 38a and its monosubstituted derivatives such as 38b are nonfluorescent at room temperature. By contrast, cis-dianthrylethylene 38e does fluoresce with quantum yields of 0.0018, 0.0042, and 0.0064 in cyclohexane, dichloromethane, and acetonitrile, respectively. The emission is structureless (see Figure 18), and is associated with a solvent-independent Stokes shift of about 6000cm-1. As the molecular geometry of 38e is characterized by overlapping anthracene systems [80], the structureless emission may be attributable to an intramolecular excimer state. 

Detection of the B6 vitamers is complicated by the low levels at which they occur in foods (102,103). The sensitivity and specificity of the detection methods is therefore critical. All of the principal B6 vitamers are UV absorbers (70). Although their spectra are similar in 0.1 M hydrochloric acid, this is not the case at higher pH. These vitamers fluoresce naturally in slightly acidic to neutral solution and under strongly alkaline conditions (42,70). However, the individual vitamers exhibit some qualitative dissimilarities in their fluorescence spectra and significant differences in the intensities of their quantum fluorescence response PLP is significantly less fluorescent than the other five vitamers. In general, fluorescence is the preferred method of detection, due to its increased sensitivity and specificity relative to UV absorbance. Derivatization has been used to enhance and standardize the fluorescence properties of the B6 vitamers. Detailed reviews of the spectral properties of the B6 vitamers have been published (102,103). 

The fluorescent properties of TRITC (mixed isomers) include an absorbance maximum at about 544 nm and an emission wavelength of 570 nm. Fluorescent quenching of the molecule is possible. Under concentrated conditions, rhodamine-to-rhodamine interactions result in self-quenching, which reduces its luminescence yield. This phenomenon can occur with TRITC-tagged molecules, as well. If derivatization of a protein is done at too high a level, the resultant quantum yield of the conjugate will be depressed from expected values. Typically, modifications of proteins involve adding no more than 8-10 rhodamine molecules per molecule of protein, with a 4-5 substitution level considered optimal. 

The increasing pertubation of the fullerene ir-type electron system leads to a reduction of the symmetry, e.g., from /h to C2v or Cs, and converts some vibronic-forbidden states to allowed states. Besides the absorptivity in the UV-Vis, the fluorescence properties of the adducts are also influenced, as well as the fluorescence lifetime, the emission band positions, and especially the quantum yield, which increases compared with that of C6o [61,65-67,75,88-90,92]. 

The fluorescence properties of free 2AP are simple. AJablonski diagram of 2AP (Fig. 13.IB) computed with time-dependent density functional theory (TDDFT) finds a dominant singlet excited state transition from S() to at 292 nm (Jean and Hall, 2001). In solution, the free nucleobase has a fluorescence excitation maximum of 305 nm and an emission maximum of 360 nm at pH 7. Its quantum yield is not high 0.68 at pH 7.0 in 100 mM NaCl, 25 °C. Its fluorescence lifetime in aqueous solution is 10 ns at 22 °C and is described by a single exponential decay. 

Sandin, P., Wilhelmsson, L. M., Lincoln, P., Powers, V. E. C., Brown, T., and Albinsson, B. (2005). Fluorescent properties of DNA base analogue tC upon incorporation into DNA — negligible influence of neighbouring bases on fluorescence quantum yield. Nucleic Acids Res. 33, 5019-5025. 

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