Phosphorescence decay time

Griseofulvin exhibits both fluorescence and luminescence. A report by Neely et al., (7) gives corrected fluorescence excitation (max. 295 nm) and emission (max. 420 nm) spectra, values for quantum efficiency of fluorescence (0.108) calculated fluorescence lifetime (0.663 nsec) and phosphorescence decay time (0.11 sec.). The fluorescence excitation and emission spectra are given in Figure 7. 

Figure 7. Phosphorescence decay times (t) for hair samples from eight blondhaired donors. The error bars represent 1 standard deviation from the mean value of t for 10 hair samples from each donor.

Two observations about the 0-0 phosphorescence band of PVCA emerge from these experiments. In the first place a clear resolution of this band is, in fact, achieved. In addition, the intensity of this band decreases relative to longer wavelength components by using long delay times on the order of several hundred milliseconds. An obvious corollary to this effect is that apparent phosphorescence decay times will depend upon the wavelength chosen for the measurement. Such effects are not large but they are readily measurable. 

An important observation is that when powder samples of N2 and NIO are exposed to isooctane or methanol at room teiiq[>erature and then cooled to 77 K, the phosphorescence decay times are substantially longer than the dry powder sample. This result points to solvent-induced swelling of the PM phases during sample preparation leading to increased N/N pair separations. These differences are maintained when the sample is cooled. ... 

The relaxation temperature is frequency dependent. In the nmr experiment the maximum corresponds to a methyl group rotational frequ cy of 10 Hz. In the phosphorescence quenching experiment, the triplet lifetimes are on the order of seconds. Impurity diffusion is coupled in an yet unknown way to the methyl group rotation. From a kinetic point of view, -35 is the temperature where the rate of impurity quenching exceeds that of other processes in limiting the phosphorescence decay time. 

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

Lifetime [3,9-11] based sensors rely on the determination of decay time of the fluorescence or phosphorescence. Typically, the fluorescence lifetime is 2-20 ps and phosphorescence lifetime is 1 ps to 10 s. Lifetime-based sensors utilize the fact that analytes influence the lifetime of the fluorophore. Thus all dynamic quenchers of luminescence or suitable quenchers can be assayed this way. The relationship between lifetimes in the absence (t0) and presence (t) of a quencher is given by Stern and Volmer ... 

The transient response of luminescent substances to pulsed excitation can be captured in the time domain by sampling enough data points within the time span of the decay. For example, fast digitizers are commonly employed to store phosphorescence decays. If fast digitizers are unavailable, time-correlated single-photon counting can be used to monitor fluorescence decays. 

Essentially nothing is known about tyrosine phosphorescence at ambient temperatures. In frozen solution, tyrosine residues have a phosphorescence decay of seconds. We would expect, however, a decay of milliseconds or shorter at ambient temperature. Observation of tyrosine phosphorescence from proteins in liquid solution will undoubtedly require efficient removal of oxygen. Nevertheless, it could be fruitful to explore ambient temperature measurements, since the phosphorescence decay could extend the range of observation of excited-state dynamics into the microsecond, or even millisecond, time range. 

Berger and Vanderkooi(88) studied the depolarization of tryptophan from tobacco mosaic virus. The major subunit of the coat protein contains three tryptophans. The phosphorescence decay is non-single-exponential. At 22°C the lifetime of the long component decays with a time constant of 22 ms, and at 3°C the lifetime is 61 ms. The anisotropy decay is clearly not singleexponential and was consistent with the known geometry of the virus. 

For chromium containing lilac chlorite two types of luminescence were observed at 15 K phosphorescence at 14,518 and 1,547 cm with a decay time of 60 ps and a fluorescence band at about 13,850-13,500 cm with a decay time of several microseconds. For green chlorite weak fluorescence at 13,900 cm and phosphorescence at 14,320 and 14,665 cm were observed at room tern-... 

Thymidine phosphate and cytidine phosphate do not phosphoresce in a rigid ethylene glycol-water glass at 77°K109 when directly excited, but thymidine which has lost its proton at Nx does have a triplet which phosphoresces with a decay time of 0.50 sec at high pH uridine also fluoresces with a similar decay time. The quenching of purine fluorescence and appearance of T" fluorescence in UV-irradiated DNA and poly dAT (as well as U" fluorescence in poly rAU) was attributed by Rahn et al.109 to proton transfer from thymine to adenine. This quenches adenine fluorescence and enhances thymine fluorescence. 

The lifetime of triplet acetone at 25° in the vapor phase, as measured from the rate of decay of phosphorescence, is 0.0002 sec,318 so that the rate of decay is 5 x 103 sec-1. This figure represents the sum of the rates of all decay processes. Since the data at 40° 308 indicate that decomposition and internal conversion of triplet acetone occur approximately 40 times as fast as emission, the radiative lifetime must be on the order of 0.01 sec. Measurements of the rate of phosphorescence decay from solid acetone at 77°K, where all activated fragmentation and most radiationless decay normally disappear, have actually yielded values approximately one-tenth as large as that obtained in the gas phase at room temperature.319 The most recent measurements of the lifetime of triplet acetone at 77°K in frozen glasses does indeed yield an estimate of 0.01 sec for the radiative lifetime of triplet acetone.318... 

Sj - S0, the interconversion into the triplet state Tlf is also possible. The characteristic time of the phosphorescence decay of MP, on the other hand, is rather large and amounts to 10-2 s. (For a review of the physical and chemical properties of MP, see, for example, ref. 52.)... 

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