Excimer fluorescence transitions

Figure B8.2.1 shows the fluorescence spectra of DIPHANT in a polybutadiene matrix. The h/lu ratios turned out to be significantly lower than in solution, which means that the internal rotation of the probe is restricted in such a relatively rigid polymer matrix. The fluorescence intensity of the monomer is approximately constant at temperatures ranging from —100 to —20 °C, which indicates that the probe motions are hindered, and then decreases with a concomitant increase in the excimer fluorescence. The onset of probe mobility, detected by the start of the decrease in the monomer intensity and lifetime occurs at about —20 °C, i.e. well above the low-frequency static reference temperature Tg (glass transition temperature) of the polybutadiene sample, which is —91 °C (measured at 1 Hz). This temperature shift shows the strong dependence of the apparent polymer flexibility on the characteristic frequency of the experimental technique. This frequency is the reciprocal of the monomer excited-state...

Georgescauld D., Desmasez J. P., Lapouyade R., Babeau A., Richard H. and Winnik M. (1980) Intramolecular Excimer Fluorescence A New Probe of Phase Transitions in Synthetic Phospholipid Membranes, Photochem. Photobiol. 31, 539-545. 

The excimer fluorescence (with respect to the excited vdW dimer emission) is red shifted and structureless because the emission is terminated in a repulsive ground-state potential energy surface (Figure 15). For parallel transition moments, emission from the out-of-phase exciton state to the ground state is forbidden and for the in-phase exciton state emission is allowed [28a]. It should be noted, however, that the forbidden emission from the out-of-phase exciton state is expected to have a similar transition dipole moment as the Lb So emission. The actual dynamics of the initially excited vdW dimer depend on the energy gap and the coupling strength between the primary excited (LE) state and the excimer state. 

A. K. Soutar, H. J. Pownall, A. S. Hu, and L. C. Smith, Phase Transitions in Bilamellar Vesicles. Measurements by Pyrene Excimer Fluorescence and Effect on Transacylation by Lecithin Cholesterol Acyltransferase, Biochemistry 13, 2828-2836 (1974). 

Reprinted from Polymer, Volume 33, B. Wandelt, D.J.S. Birch, R.E. Imhof, R.A. Pethrick, Time-resolved excimer fluorescence studies as a probe of the coil collapse transition and phase separation in isotactic PS/BA gel, 3561, copyright 1992, with... 

Frank CW (1975) Observation of relaxation processes near glass-transition by means of excimer fluorescence. Macromolecules 8(3) 305-310. doi 10.1021/ma60045a012... 

STUDIES OF RATES OF CONFORMATIONAL TRANSITIONS BY EXCIMER FLUORESCENCE... 

Intramolecular excimer fluorescence involves a conformational transition furing the lifetime of an excited chromophore and it can, therefore, used to study conformational transitions on the timescale of 10 ° sec. We have used this technique to study the behavior of copolymers and to define the dependence of the rate of conformational transitions on the viscosity of the medium. 

As noted in the introductory section, our purpose is to use the excimer fluorescence technique to study the flexibility of various pol3nner chains in polymer matrices. One question which arises is to know if the probe undergoes a conformational change via a free-volume dependent molecular relaxation process related to the glass transition of the polymer matrix. The free volume of a polymer at a given temperature should qualitatively relate to its Tg value. 

Through this study, we have shown that the excimer fluorescence technique of a probe simply dispersed in a macromolecular medium provides detailed information about the molecular motions of the polymer. Although the intramolecular rotational process reflects the glass transition of the host medium, within the same matrix, the absolute values of the experimental correlation times, differ for each probe. In Table 3... 

Electronic transitions of the carbazole chromophores afford a highly sensitive probe for studies of carbazole-containing polymers in respect of their absorption and emission characteristics as well as of either inherent or induced circular dichroism. They have been conveniently employed to describe microstructural characteristics of carbazole-containing polymers, including homopolymers of carbazolated a-aminoacids and of vinylated carbazoles with chiral substituents. Data obtained are entirely consistent with interpretation of polymer structures derived from related UV absorption, excimer fluorescence emission and NMR studies and are especially valuable as a completely new indication of copolymer sequence distributions. 

Ikkai, T Kondo, H. Excimer fluorescence to monitor transitions of the association-dissociation equilibria induced by dilution of proteins composed of subunits. J. Biochem. Biophys. Methods 1996,33, 55-58. 

There are many possibilities to use these complex formations in fluorescence sensing. If the excimer is not formed, we observe emission of the monomer only, and upon its formation there appears characteristic emission of the excimer. We just need to make a sensor, in which its free and target-bound forms differ in the ability of reporter dye to form excimers and the fluorescence spectra will report on the sensing event. Since we will observe transition between two spectroscopic forms, the analyte binding will result in increase in intensity of one of the forms and decrease of the other form with the observation of isoemissive point [22]. 

A short excursion into the physics and spectroscopy of intermolecular interactions is intended to illustrate the effects of fluorescence spectra change on the transition of dye molecules from liquid solvents to solid environments, on the change of polarity and hydration in these environments, and on the formation of excited-state complexes (excimers and exciplexes). 

In the absence of the reverse absorption the radiative transition probability fquantum yield of fluorescence qmC) and the decay constant l/r (C)= 2 [Pg.200]

The dynamic RIS model, which was proposed before to investigate the dynamics of local conformational transitions in polymers, is elaborated to formulate the increase in the number of excimer-forming sites through rotational sampling. Application of the model to the meso and racemic diads in PS confirms the fact that conformational mobility of the chain plays a major role in intramolecular exclmer formation. Comparison with experiments demonstrates that the decay of the monomer fluorescence in styrene dimers is predominantly governed by the process of conformational transitions. 

The lack of vibrational structure is proof that the ground state excimer is dissociative, Le. it does not exist5 at all remember that the molecular fluorescence of naphthalene, for example (Figure 3.28, p. 55) shows a structure which is related to the transitions from the v = 0 level of the excited state to the various discrete vibrational levels of the S0 ground state. 

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