Time-dependent phosphorescence spectra

Time-Dependent Phosphorescence Spectra of Polymers. Although a strong similarity exists between phosphorescence spectra of vinyl aromatic polymers and the corresponding monomeric analogues, it is interesting to focus attention upon the differences which exist between these spectra. A sample comparison is provided in Figure 1 between NEC and PVCA both as dilute solutions in MTHF at 77 K (23). Both spectra are recorded using comparable conditions and instrument parameters. 

Because phosphorescence is normally observed only in rigid media, proton transfer in the lowest triplet state is rarely observed, in real time, in the pH dependence of the phosphorescence spectrum. 

Dependencies of luminescence bands (both fluorescence and phosphorescence), anisotropy of emission, and its lifetime on a frequency of excitation, when fluorescence is excited at the red edge of absorption spectrum. Panel a of Fig. 5 shows the fluorescence spectra at different excitations for the solutes with the 0-0 transitions close to vI vn, and vra frequencies. Spectral location of all shown fluorescence bands is different and stable in time of experiment and during lifetime of fluorescence (panel b)... 

Luminescence decay curves are also often used to verify that samples do not contain impurities. The absence of impurities can be established if the luminescence decay curve is exponential and if the spectrum does not change with time after pulsed excitation. However, in some cases, the luminescence decay curve can be nonexponential even if all of the luminescing solutes are chemically identical. This occurs for molecules with luminescence lifetimes that depend upon the local environment. In an amorphous matrix, there is a variation in solute luminescence lifetimes. Therefore, the luminescence decay curve can be used as a measure of the interaction of the solute with the solvent and as a probe of the micro-environment. Nag-Chaudhuri and Augenstein (10) used this technique in their studies of the phosphorescence of amino acids and proteins, and we have used it to study the effects of polymer matrices on the phosphorescence of aromatic hydrocarbons (ll). 

Here v is the wave number, e (v) the molar decadic extinction coefficient of the acceptor and (v) the spectrum of the donor emission (fluorescence or phosphorescence) measured in quanta per wave-number interval and normalized to unity on the same scale. N is the number of molecules per millimole, n the refractive index of the surrounding medium and the intrinsic life time of the excited donor state, k is a numerical factor representing the orientation dependence of dipole-dipole interaction. Its average is /2/3 = 0.816 for fast Brownian rotation of both molecules and 0.690 for random but rigid orientations . 

The phosphorescence is usually measmed at 77 K to limit ffie temperature-dependent nonradiative deactivations, and a time delay is added between a pulsed excitation and the measurement to discard any residual fluorescence. The phosphorescence is typically located at the lower energy end of the fluorescence spectrum. 

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