Phosphorescence, cyclodextrins

Solid-surface room-temperature phosphorescence (RTF) is a relatively new technique which has been used for organic trace analysis in several fields. However, the fundamental interactions needed for RTF are only partly understood. To clarify some of the interactions required for strong RTF, organic compounds adsorbed on several surfaces are being studied. Fluorescence quantum yield values, phosphorescence quantum yield values, and phosphorescence lifetime values were obtained for model compounds adsorbed on sodiiun acetate-sodium chloride mixtures and on a-cyclodextrin-sodium chloride mixtures. With the data obtained, the triplet formation efficiency and some of the rate constants related to the luminescence processes were calculated. This information clarified several of the interactions responsible for RTF from organic compounds adsorbed on sodium acetate-sodium chloride and a-cyclodextrin-sodium chloride mixtures. Work with silica gel chromatoplates has involved studying the effects of moisture, gases, and various solvents on the fluorescence and phosphorescence intensities. The net result of the study has been to improve the experimental conditions for enhanced sensitivity and selectivity in solid-surface luminescence analysis. 

Solid-surface luminescence analysis involves the measurement of fluorescence and phosphorescence of organic compounds adsorbed on solid materials. Several solid matrices such as filter paper, silica with a polyacrylate binder, sodium acetate, and cyclodextrins have been used in trace organic analysis. Recent monographs have considered the details of solid-surface luminescence analysis (1,2). Solid-surface room-temperature fluorescence (RTF) has been used for several years in organic trace analysis. However, solid-surface room-temperature phosphorescence (RTF) is a relatively new technique, and the experimental conditions for RTF are more critical than for RTF. 

Figure 2. (a) Room-temperature phosphorescence spectrum of benzo(e)pyrene on 80% a-cyclodextrin-NaCl in the presence of ben2o(a)pyrene. = 284 nm. (h) Room-temperature phosphorescence spectrum of benzo(e)pyrene on 80% a-cyclodextrin—NaCl. X - 284 nm. 

Cyclodextrins can solubilize hydrophobic molecules in aqueous media through complex formation (5-8). A nonpolar species prefers the protective environment of the CDx cavity to the hulk aqueous solvent. In addition, cyclodextrins create a degree of structural rigidity and molecular organization for the included species. As a result of these characteristics, these macrocycles are used in studies of fluorescence and phosphorescence enhancement (9-11), stereoselective catalysis (.12,13), and reverse-phase chromatographic separations of structurally similar molecules (14,15). These same complexing abilities make cyclodextrins useful in solvent extraction. 

Studies on the dynamics of complexation for guests with cyclodextrins have been carried out using ultrasonic relaxation,40 151 168 temperature jump experiments,57 169 183 stopped-flow,170,178,184 197 flash photolysis,57 198 202 NMR,203 205 fluorescence correlation spectroscopy,65 phosphorescence measurements,56,206 and fluorescence methods.45,207 In contrast to the studies with DNA described above, there are only a few examples in which different techniques were employed to study the binding dynamics of the same guest with CDs. This probably reflects that the choice of technique was based on the properties of the guests. The examples below are grouped either by a type of guest or under the description of a technique. 

The photochemical process built into 7 was encountered previously with regard to 2, Le, the capability of 02 in quenching excited states of sufficiently long lifetimes. In the case of 7, the process is so efficient that ambient levels of 02 completely kill off phosphorescence, even if the phosphor is enveloped by p-cyclodextrin. 

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

Surfactants were found to induce phosphorescence from 1-bromonaphthalene in aerated aqueous solutions of (3-CD [22], In the case of the 1 1 inclusion-complex formation between 7-cyclodextrin and acenaphthene in aqueous solution, bromoalcohols such as 2,3-dibromopropane-l-ol or 2-bromoethanol induced a decrease of the fluorescence and an enhancement of room-temperature phosphorescence of acenaphthene [23],... 

Cyclodextrins have been used for enhancing the phosphorescence of guests by encapsulation [12]. de Silva et al. employed cyclodextrins as transparent shields to protect the phosphor molecule sterically from contact with its environment (especially O2) while allowing access to photons. Sensing remains viable because the proton receptor module is not enveloped (Figure 9) [13]. The authors used this system as a phosphorescent PET (photoinduced electron transfer) chemosensor for... 

Selective enhancement of room temperatures phosphorescence is achieved by cyclodextrin treated cellulose s ubs t r a t e s, surface active agents3 . and for heterocyclic compounds by absorption on silica gel coated plates submerged in chloroform-containing solvent s3. ... 

Fig. 7. A three-input INHIBIT gate exemplified by the tetraanion 54 and /1-cyclodextrin (j8-CD). a With neither protons nor /1-CD present in the solution, phosphorescent output is low, because of both PET from the tertiary amine, and through intermolecular triplet-triplet collisions of the bromonaphthalene phosphor, b Addition of calcium ions leads to a reduction in the PET-based quenching of the phosphorescence - however, intermolecular collisions still lead to a low emission, c Shielding of the phosphor with /l-CD reduces intermolecular triplet annihilations, but quenching still occurs via PET. d Only with both Ca2+ and /1-CD present does the solution phosphoresce, e In any combination of Ca2+ and /1-CD, the solution will yield a low output in the presence of molecular oxygen (the INHIBIT stimulus), as a consequence of triplet-triplet collisions...

Generation and trapping of radical cations of a,co-diphenylpolyenes within the channels of pentasil zeolites provides an environment which allows these transient species to be spectroscopically characterized . Similarly, complexation of xanthone in cyclodextrin has made it possible for the triplet state of this molecule to be fully characterized . Association and dissociation processes are related to the dipoles developed in the complex and in solution. A unimodal Lorentzian lifetime distribution for 2-anilinonaphthalene-6-sulphonate B-cyclodextran inclusion complexes have been recovered by multifrequency phase-modulation fluorometry in the presence of the quenchers Cu, acrylamide, and I . Both the fluorescence and phosphorescence spectra of benzo[f]quinoline adsorbed on p-cyclodextrin/NaCl have been determined as a function of temperature . 

Turro to be a strong phosphor at room temperature in deaerated P-cyclodextrin solution (28). Indeed, halonaphthalenes were among the examples employed by early workers to establish the favourable heavy atom effect on low temperature phosphorescence (36,37). The other essential piece of inspiration came from... 

Phosphorescence is usually not observable in fluid solution even from compounds with high triplet yields, being overruled by nonradiative triplet deactivation and/or quenching by co-solutes such as O2. Cyclodextrin indusion is a way to overcome... 

Several other diaryl and alkylarylketones also exhibit room temperature phosphorescence in air equilibrated samples when included in silicalite [83c] or forming inclusion complexes with cyclodextrins [83c, 102], depending on the probe and cavity size. Both substrates provide some degree of protection from oxygen quenching, as well as imposed conformational restrictions that decrease the non-radiative mechanisms of deactivation. 

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