Phosphorescence studies

To minimize quenching of triplets due to bimolecular collisions, phosphorescence studies are generally carried out in rigid glasses at 77°K. Some materials which may be used to produce these glasses are listed in Table... 

Excited-state lifetimes can be measured directly by monitoring the decay of luminescence, but impurities present affect both the lifetime and the luminescence spectrum. Also, because low temperatures are necessary for phosphorescence studies, the excited-state properties determined may differ from those at room temperature. 

In the case of carboxypeptidase B, Shaklai et al.(2lT> compared the relative contributions to the protein phosphorescence from tyrosine and tryptophan for the apoenzyme, the zinc-containing metalloenzyme in the absence of substrate, the metalloenzyme in the presence of the substrate iV-acetyl-L-arginine, and the metalloenzyme in the presence of the specific inhibitor L-arginine. The tyrosine tryptophan emission ratio of the metalloenzyme was about a factor of four smaller than that of the apoenzyme. Binding of either the substrate or the inhibitor led to an increase in the emission ratio to a value similar to that of the apoenzyme. The change in the tyrosine tryptophan phosphorescence ratio was attributed to an interaction between a tyrosine and the catalytically essential zinc. The emission ratio was also studied as a function of pH. The titration data are difficult to interpret, however, because a Tris buffer was used and the ionization of Tris is strongly temperature dependent. In general, the use of Tris buffers for phosphorescence studies should be avoided. 

N. Zisapel, N. Shaklai, and M. Sokolovsky, Metal-tyrosyl interaction in carboxypeptidase Phosphorescence studies, FEBS Lett. 51, 262-265 (1975). 

R. M. Purkey and W. C. Galley, Phosphorescence studies of environmental heterogeneity for tryptophyl residues in proteins, Biochemistry 9, 3569-3574 (1970). 

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-... 

The only materials that have been examined for this purpose to date have been dienes and other polyenes.12 The major liability of these substances is their chemical reactivity. The finding that 1,3,5-trans-hexatriene quenches the phosphorescent emission from phenanthrene but does not have a significant effect on the fluorescent emission from 1,3,5,7-tetraphenylisobenzofuran indicates that chemical reactivity may not be too severe a problem, and that triplet quenchers may be effective tools for mechanistic work in ECL. Here too, further study is indicated. Unfortunately, or perhaps significantly, the compounds that produce the brightest emission on redox stimulation—1,3,4,7-tetra-phenylisobenzofuran and rubrene—have no detectable phosphorescence. Studies with sensitizers and quenchers have failed to produce any evidence regarding their triplet levels.12 Thus the mechanism of anni-hilative emission remains uncertain. 

Triplet state acidities, pKj, have been obtained by Porter and his school from phosphorescence studies, using flash photolysis techniques. The singlet, triplet and ground state acidity constants of some organic molecules are given in Table 4.2. 

For the Eu3+ phosphorescence studies special precautions had to be taken since Eu3+ salts precipitate as Eu(OH)3 when added to basic solutions. In addition, the Eu3+ phosphorescence in H20 is weak because of a high probability for radiationless transitions (11). This can be overcome by using D20 as solvent instead of H20. An undoped zeolite gel was pre-... 

In Linde A and sodalite syntheses the signal grew to about 20 times its initial intensity. In other systems, such as faujasite, the increase was somewhat smaller. The increase seemed to depend upon the Si/Al ratio of the resultant zeolite crystals—i.e., the smallest increase occurred for mordenite crystallizations having an Si/Al ratio of 5 (for Linde A and sodalite Si/Al = 1). No Fe3+ phosphorescence was observed in the liquid phase of the gel. In three experiments carried out under identical conditions Fe3+ phosphorescence studies of the growth kinetics gave identical results (induction periods equal within 5%, Fe3+ intensity increase on crystallization equal within 10%). 

Because of the forbiddeness of the transition, Tx —> S0 + ho, the natural phosphorescent lifetime, t°, of a triplet state is long—from approximately 10"3 sec for an n,it triplet to 30 sec for a rr,n aromatic triplet. At room temperature in solution, phosphorescence is often not observed because ISC of Tx to S0 and quenching of Tx by impurities and molecular 02 (see below) competes effectively with phosphorescence. Therefore most phosphorescence studies must be carried out at low temperatures in carefully purified, outgassed, rigid media. Under these conditions the quantum yield of phosphorescence, 9P, defined by Equation 13.10, is often high and approaches 1.0 for some aromatic carbonyls. 

The role of the triplet state in biological and biochemical systems continues to receive wide attention. Photoexcited triplet states of prophins and derivatives (389), prophyrins (390), aromatic amino acids (390), aromatic amines (391), and monoanionic thymine (392) have been observed at 77°K. Shiga and Piette (393) confirmed the interpretation of ESR data on triplet-state excitation in proteins by a simultaneous phosphorescence study. 

As shown for the transformations of the cyclopentenyl methyl ketones (3a-d) to mixtures of (4a-d) and (5a-d) (Scheme 1), the ODPM rearrangement occurs from the lowest electronically excited p,y-enone triplet state. - The ir. ir configuration has been assigned to this state on the basis of CNDO-MO calculations, phosphorescence studies at 77 and mechanistic examinations. The phosphorescence data together with sensitization experiments have revealed a Ti energy range of 289-310 kJ mol (4.18 kJ = 1.0 kcal) for (3a-c) and a much lower Ti of 253 kJ mol for (3d). ... 

Phosphorescence studies in isobutylene-methyl methacrylate-1-naphthyl-methyl methacrylate co-polymer provided evidence to show that in very dilute solution the chain collapses into more compact structures, and intramolecular excimer formation in poly-(2-vinylnaphthalene) has been shown to exhibit non-Stokes-Einstein behaviour. Laser photolysis of polymers containing phenanthryl groups, such as poly-(9-vinylphenanthrene), indicates the presence of plural dimer sites having different geometries owing to the stacking effect of phenanthrene chromophores. In poly-(2-naphthyl methacrylate). 

Because of the significant contribution of fluorescence and phosphorescence studies to gas-phase and solution photochemistry, photoluminescence investigations of oxide catalysts have also been carried out. Valuable information characterizing the surface structures of oxides and their excited states has been obtained by w orkers including Vol kenshtein et al. (S), Tench and Pott (9). Pott and Stock (70) Kazansky, et al. 11, 12), Coluccia 13), Anpo et al 14-17), Texter et al 18), Iwasawa 19, 20), Iwamoto et al 21), Moser and Gratzel (22), Becker and Bard 23), Weller et al 24), Tanguay and Suib (25), and others 26-32). 

The properties of reverse micelles are of considerable interest at the present time. Amongst photochemical studies reported in this area are the behaviour of indole alkanoic acids and tryptamine in sodium dioctyl succinate, fluorescence and phosphorescence studies of A0T/H20/alkane systems using a variety of probes, photoionization of alkylphenothiazine sulphonates in reversed micelles, and interfacial interaction of probes with AOT inverted micelles. ... 

Parallel phosphorescence studies indicated that the O—O band for cinnamic acid occurred at 20,000 cm-1. Tsuda 43> reported a value of 17,600 cm-1. In the same study Kikuchi and Nakamura 42> measured the quenching of the phosphorescence of cinnamic acid with a group of typical sensitizers, among which were 2-nitro-fluorene and -nitroaniline. The critical separation distance between cinnamic acid and sensitizers for effective quenching was found to be 10A. 

Richtol, H. H., and A. Belorit Simultaneous Donor Quenching and Acceptor Sensitization in Phosphorescence Studies of Triplet Energy Transfer The Biacetyl-Benzil System. J. Chem. Phys. 45, 35 (1966). 

Redmond, R.W. and Braslavsky, S.E. (1988) Time-resolved thermal lensing and phosphorescence studies on photosensitized molecular oxygen formation. Influence of the electronic configuration of the sensitizer on sensitization efficiency, Chem. Phys. Lett., 148, 523-529. 

A19. Arduini, A., Strambini, G., Dillio, C, Aceto, A., Storto, S., and Federici, G., Tryptophan environments in glutathione transferase of human placenta from temperature-dependent phosphorescence studies. Biochim. Biophys. Acta 999, 203-207 (1989). 

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