Phosphorescence time scale

Molecular emission is referred to as luminescence or fluorescence and sometimes phosphorescence. While atomic emission is generally instantaneous on a time scale that is sub-picoseconds, molecular emission can involve excited states with finite, lifetimes on the order of nanoseconds to seconds. Similar molecules can have quite different excited state lifetimes and thus it should be possible to use both emission wavelength and emission apparent lifetime to characterize molecules. The instrumental requirements will be different from measurements of emission, only in detail but not in principles, shared by all emission techniques. 

On the contrary, at low temperatures and/or in a rigid medium, phosphorescence can be observed. The lifetime of the triplet state may, under these conditions, be long enough to observe phosphorescence on a time-scale up to seconds, even minutes or more. 

The major reasons for using intrinsic fluorescence and phosphorescence to study conformation are that these spectroscopies are extremely sensitive, they provide many specific parameters to correlate with physical structure, and they cover a wide time range, from picoseconds to seconds, which allows the study of a variety of different processes. The time scale of tyrosine fluorescence extends from picoseconds to a few nanoseconds, which is a good time window to obtain information about rotational diffusion, intermolecular association reactions, and conformational relaxation in the presence and absence of cofactors and substrates. Moreover, the time dependence of the fluorescence intensity and anisotropy decay can be used to test predictions from molecular dynamics.(167) In using tyrosine to study the dynamics of protein structure, it is particularly important that we begin to understand the basis for the anisotropy decay of tyrosine in terms of the potential motions of the phenol ring.(221) For example, the frequency of flips about the C -C bond of tyrosine appears to cover a time range from milliseconds to nanoseconds.(222)... 

Experiments involving anisotropy of phosphorescence or of the absorption of the triplet state rely upon the same principles as the measurement of fluorescence anisotropy. All are based upon the photoselection of molecules by polarized light and the randomization of polarization due to Brownian motion occurring on the time scale of the excited state. Anisotropy is defined as... 

The long lifetime of phosphorescence allows it to be used for processes which are slow—on the millisecond to microsecond time scale. Among these processes are the turnover time of enzymes and diffusion of large aggregates or smaller proteins in a restricted environment, such as, for example, proteins in membranes. Phosphorescence anisotropy is one method to study these processes, giving information on rotational diffusion. Quenching by external molecules is another potentially powerful method in this case it can lead to information on tryptophan location and the structural dynamics of the protein. 

It would be desirable to insert a probe into the polymer to ascertain the local environmental conditions. In addition to having microscopic dimensions, the probe must act as a timing device which specifies the time-scale of the observation. Such a probe is a fluorescent molecule. Its dimensions are about the size of a monomer residue, namely of the order of 10 A, and the lifetime of fluorescence, r, varies between about 10-9— 10"7 sec., depending on the fluorescent compound and the medium (9). Still longer time-scales, namely, 10"4—10 sec., are achieved with organic molecules in the phosphorescent state (21). 

Figure 24-6 Photoluminescence methods (fluorescence and phosphorescence). Fluorescence and phosphorescence result from absorption of electromagnetic radiation and then dissipation of the energy by emission of radiation (a). In (b), the absorption can cause excitation of the analyte to state 1 or state 2. Once excited, the excess energy can be lost by emission of a photon (luminescence, shown as solid line) or by nonradiative processes (dashed lines). The emission occurs over all angles, and the wavelengths emitted (c) correspond to energy differences between levels. The major distinction between fluorescence and phosphorescence is the time scale of emission, with fluorescence being prompt and phosphorescence being delayed.

Figure 1. Semilogarithmic decay curves of benzophenone phosphorescence in PMMA excited by 10-ns nitrogen laser pulse at 337 nm. Temperature and symbols for time scales are given beside the curves. (Reproduced from Reference 6. Copyright 1984 American Chemical Society.

P/VN and P-Np respectively. The onset temperature for P/ACE is in good agreement with that of the conventional value for Tg, consistent with the low frequency equivalence of the time-scale imposed by the phosphorescent label at this temperature in the PBA matrix. 

Even though the triplet appears localized, at least on a vibrational time scale, the apparent delayed fluorescence does indicate substantial mobility on the time scale of phosphorescence decay. We have examined these decays and found them to be quite nonlinear. With the exoeption pf the g-naphthyl polymer, the first half lives are about 1-2 x 10 seconds. 

A number of reports on phthalocyanines and porphyrins have been published. Spectral diffusion and thermal recovery of spectral holes burnt into phthalocyanine doped Shpol skii systems has been examined . An absorption, emission, and thermal lensing research on carboxylated zinc phthalocyanine shows the influence of dimerization on these properties. Fourier transformation of fluorescence and phosphorescence spectra of porphine in rare gas matrices has yielded much structural and electronic state data on this compound . Exciton splitting is an effect which is seen in the spectra of covalently linked porphyrins . A ps fluorescence study of the semirigid zinc porphyrin-viologen dyad has provided evidence for two dyad conformers . Spectral diffusion in organic glasses has been measured by observing the hole recovery kinetics over the time scale of 1 to 500 ms for zinc tetrabenzoporphyrin in PMMA . 

Microcrystalline benzophenone [38] and benzil [16] were two of the first systems studied by nanosecond diffuse reflectance flash photolysis. Both samples gave transient absorptions which were positively identified as triplet-triplet absorptions. In the case of benzophenone an absorption, centred at 540 nm, was obsejrved which has, within experimental error, identical kinetics to the phosphorescence decay, which is predominantly second order. In the case of benzil a transient absorption of 60% at 510 nm was observed after 354 nm excitation. The assignment as triplet-triplet absorption was made on the basis of the absorption and phosphorescence kinetics being virtually identical, namely a mixture of first and second order kinetics. Ikeda et al [39] have also studied microcrystalline benzophenone on the picosecond time scale. Another microcrystalline sample studied is 1,5-diphenyl-3-styryl-2-pyrazoline, in which the triplet-triplet transient absorption was identified within the microsecond time domain [15] (see figure 7(b)). However, as mentioned above (see section 4 and figure 5), the transient absorption due to the excited singlet state has been observed on a picosecond time domain [17]. 

Nitrobenzene and its derivatives have been used for many chemical reactions or as an efficient quencher of excited states [78,79] and the nature of the photophysical properties have been investigated both theoretically [80,81] and experimentally [82,83]. However, the photophysical properties of these molecules have not been revealed and remained to be mysterious. The reason for the mysterious nature is the very weak emissive character and weak transient absorption from the excited states. Under any conditions, neither fluorescence nor phosphorescence has been detected. The photophysical properties of nitrobenzene and its derivatives were investigated by the TG method in a fast time scale [84]. 

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