Triplet molecular probes

Motion of the dispersed naththalene triplet probe in P-Np is evident at 180K. By analogy with previous observations of the PMMA system(]Q) such mobility might be concomitant with the onset of the g-relaxation. It is apparent from dynamic mechanical data at IHz that resolution of the a and g relaxations is difficult(M). it may be that the use of molecular probes and labels such as 1-VN (as described above) offers a more sensitive sensory exploration of the matrix than is afforded by macroscopic techniques. It is also possible that the degrees of freedom for motion imbued in the matrix do not result from the motion of ester groups as generally inferred in PMA and PMMA for the g-relaxation but that conformational changes within the it-butyl substituent itself, constitute sufficient release of free volume within the matrix to allow rotational mobility of both the 1-VN label and the dispersed naphthalene probe. 

In this chapter, we concentrate on luminescence approaches that are most suitable for stilbenes to be molecular probes and are based on specific and nonspecific labeling, competition, solvatochromism, experimental molecular dynamics, and singlet-singlet and triplet-triplet energy transfer. A general survey is made of the physical principles and application of the fluorescence probe methods stressing on latest developments in this area. 

In the intermediate domain of values for the parameters, an exact solution requires the specific inspection of each configuration of the system. It is obvious that such an exact theoretical analysis is impossible, and that it is necessary to dispose of credible procedures for numerical simulation as probes to test the validity of the various inevitable approximations. We summarize, in Section IV.B.l below, the mean-field theories currently used for random binary alloys, and we establish the formalism for them in order to discuss better approximations to the experimental observations. In Section IV.B.2, we apply these theories to the physical systems of our interest 2D excitons in layered crystals, with examples of triplet excitons in the well-known binary system of an isotopically mixed crystal of naphthalene, currently denoted as Nds-Nha. After discussing the drawbacks of treating short-range coulombic excitons in the mean-field scheme at all concentrations (in contrast with the retarded interactions discussed in Section IV.A, which are perfectly adapted to the mean-field treatment), we propose a theory for treating all concentrations, in the scheme of the molecular CPA (MCPA) method using a cell... 

The traditional fluorescence and electron-spin resonance methods for recording molecular collisions do not allow the study of translational diffusion and rare encounters of molecules in a viscous media because of the short characteristic times of these methods. To measure the rate constants of rare encounters between macromolecules and to investigate the translation diffusion of labelled proteins and probes in a medium of high viscosity (like biomembranes), a new triplet-photochrome labeling technique has been developed (Mekler and Likhtenshtein, 1986 Mekler and Umarova, 1988 Likhtenshtein, 1993 Papper and Likhtenshtein, 2001). 

Figure 131 The EL device used to probe triplet bi-molecular excited states and molecular structures of FPtl and mCP (a). Comparison of the room temperature EL (solid line) and PL (dotted line) spectra of the device with a 30 K spectrum of Fptl (dashed line) (b). The EL spectrum of the device at different temperatures (c). Reprinted from Ref. 99. Copyright 2003 with permission from Elsevier.

Since the QRLPP has a fairly high electron temperature, the normal equilibrium between the Cs2 molecules and the Cs atoms is affected in the QRLPP. This means that the bound Cs2 (singlet) density is decreased while the unbound Cs2 (triplets) density is increased due to the QRLPP. We have indeed observed that the transient destruction of molecular state by the pulsed QRLPP can be measured by positioning the cw probe beam close to or in coincidence with the pulsed laser beam. The transient absorption signals observed in the probe beam, with long transient time constants (for example, on the order of 1 msec) provide a new method to study diffusion dynamics in a vapor system (33). 

A study of the comparative rates of triplet exciton migration has been carried out on molecularly doped polymers and on vinyl aromatic polymers. In both cases specific rate constants for triplet exciton migration were estimated from rate constants for triplet-triplet annihilation. The rate data were obtained by using a laser pulse-optical probe method to determine triplet concentrations directly by triplet-triplet absorption. It is found that triplet exciton migration rates for polymers are ten-fold to one-hundred-fold larger than those for doped polymer matrices probably due to the more dense local chromophore concentrations in the former. 

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