Phosphorescence viscosity

The long lifetime has important consequences on the decay rates. First, we consider what affects the nonradiative rates (knr) which change the yields of fluorescence and phosphorescence. The nonradiative decay rate is often enhanced in molecules which have flexible constituents (the so-called loose-bolt effect). Therefore, both fluorescence and phosphorescence yields are generally larger for rigid molecules than flexible molecules. For the same reason, a rigid environment will increase the emission yields hence both fluorescence and phosphorescence yields often increase with increasing viscosity. 

The rotational mobility of human low-density (LDL) and very-low-density (VLDL) lipoproteins was studied as a function of viscosity and temperature in the range of —90 to — 50°C.(86)The rotational behavior for LDL is represented by a single correlation time, consistent with the overall rotation of a spherical rigid particle as the source of the phosphorescence depolarization. For VLDL, internal peptide motions dominate the depolarization profile. 

Reticulum ATPase [105,106], Owing to the long-lived nature of the triplet state, Eosin derivatives are suitable to study protein dynamics in the microsecond-millisecond range. Rotational correlation times are obtained by monitoring the time-dependent anisotropy of the probe s phosphorescence [107-112] and/or the recovery of the ground state absorption [113— 118] or fluorescence [119-122], The decay of the anisotropy allows determination of the mobility of the protein chain that cover the binding site and the rotational diffusion of the protein, the latter being a function of the size and shape of the protein, the viscosity of the medium, and the temperature. 

Exercise 28-3 The fluorescence of many substances can be quenched (diminished or even prevented) by a variety of means. Explain how concentration, temperature, viscosity, and presence of dissolved oxygen and impurities may affect the degree of fluorescence observed for solutions of a fluorescent material. Would you expect similar effects on phosphorescence Explain. 

Spin, Mossbauer, fluorescent and phosphorescent labels were introduced into the various portions of the system being studied. They were covalently bound to the RC surface groups, adsorbed by the hydrophobic segments of the protein and membrane, and 57Fe atoms were incorporated by way of biosynthesis into iron-containing proteins. Then, in the same samples, the dependence on temperature, moisture content and viscosity was measured for the label mobility and the rate constant of electron transfer... 

If we assume that the HT form is excited mainly with the 530-nm pulse, there are three possible candidates for the mechanism that governs the decay rate of the fast component (1) intersystem crossing from the excited singlet state of HT to HT triplet (2) trans-cis isomerization and (3) internal conversion to the ground state. The first possibility is excluded, based on the absence of a reasonable yield of phosphorescence, even at low temperatures. Possibility 3 is not very plausible because of the existence of the additional long component and the strong dependence on the viscosity of the solvent. Trans-cis isomerization is the most reasonable possibility. 

The effect of the heavy-atom substituents, bromine and iodine, on the electron-donor aniline in the electron-transfer reaction with thiopyronine triplet has been investigated by flash spectroscopy in solvents of different viscosity and polarity. Triplet quenching and radical yields are presented in Table 30. The results are analysed in terms of decay constants of an intermediate triplet exciplex. The influence of an external heavy-atom effect on the phosphorescence spectra of quinoxaline and 2,3-dichloroquinoxaline has also been reported. The influence of chloride ion on the decay rate of Methylene Blue triplet in 0.01 M acid in the... 

---