Polymer photophysical parameters

Photophysical Parameters of Some Polymer Hosts and Phosphorescent Dopants... 

The emission spectra of homobimetallic and heterobimetallic polymers 89 and 91 in PrCN at 77 K are shown in Figure 41 as examples. Details of the photophysical parameters are presented in Table 14. 

It was noted above that measurements of the lifetime of emission provide an alternative way of accessing the photophysical parameters of a polymer. The analogous approach in CL is to subject the oxidizing polymer to an external perturbation and then observe the change in CL as the system returns to the steady state (George, 1981). This will genemlly take the form of a short period of UV irradiation after which there is a burst of CL followed by decay, which may be analysed according to an assumed kinetic model, such as second-order... 

Photophysical and photochemical processes in polymer solids are extremely important in that they relate directly to the functions of photoresists and other molecular functional devices. These processes are influenced significantly by the molecular structure of the polymer matrix and its motion. As already discussed in Section 2.1.3, the reactivity of functional groups in polymer solids changes markedly at the glass transition temperature (Tg) of the matrix. Their reactivity is also affected by the / transition temperature, Tp, which corresponds to the relaxation of local motion modes of the main chain and by Ty, the temperature corresponding to the onset of side chain rotation. These transition temperatures can be detected also by other experimental techniques, such as dynamic viscoelasticity measurements, dielectric dispersion, and NMR spectroscopy. The values obtained depend on the frequency of the measurement. Since photochemical and photophysical parameters are measures of the motion of a polymer chain, they provide means to estimate experimentally the values of Tp and Tr. In homogeneous solids, reactions are related to the free volume distribution. This important theoretical parameter can be discussed on the basis of photophysical processes. 

Figure 4.14 Steady-state fluorescence emission speara of crystalline iPS obtained from gel and atactic PS film cast from chloroform at room temperature, excitation wavelength 257 nm Reprinted from Polymer, Volume 32, B. Wandelt, Correlation of photophysical parameters with conformational structure of crystalline iPS and comparison with data of atactic PS, 2708, copyright 1991, with permission from Elsevier Science)...

Polymer 18 was synthesized in order to verify whether the molecular weight is influenced by side chain groups (it is not), but a notable difference in A, (da-da ) and was observed (17 and 18) despite the fact that the FWHM (2,3(X)-2,5(X)cm- ) were relatively constant all along this series. And similarly, the FWHM for the emission band varied only from 5,300 to 5,600 cm" going from 17 to 18. The loose bolt effect does not apply here because the photophysical parameters increase for... 

Table III. Photophysical Parameters of the Polymers in Solution and in the Presence of Liposomes...

Other ongoing efforts in our group currently examine the effects that large aromatic cores impart to important photophysical parameters in relation to the design of highly fluorescent sensory polymers. From these studies, we have... 

A prime objective in postulation of a kinetic scheme to describe a physical system is the use of that scheme to derive kinetic parameters which in turn allow comparison between one system and another. In polymer photophysics, the use of copolymer systems allows in principle the derivation of individual rate constants by a series of extrapolations and thereby removal of the effects of the fixed but unknown local chromo-phore concentration imposed on a system by study of the homopol)nner alone. 

It should be possible, in principle, to separate the two contributions to by control of the temperature. Extensive experiments with solutions of aromatic chromophores unbound to polymer chains have shown that at sufficiently low temperatures, excimer formation will be diffiision controlled. At sufficiently high temperatures, the Birks dynamic equilibrium regime will be reached, and the binding energy of the system will be the important parameter. This treatment of the photophysics appears to work quite well for the free chromophores in solution and even for the end-labeled chains... 

A detailed evaluation of the structural parameters affecting the photophysical properties was performed for the didodecyloxy-substituted quinquephenyl rigid-flexible polyethers 60. More particularly, the odd-even effect was observed for dilute polymeric solutions by means of steady-state and time-resolved fluorescence anisotropy. The different orientations of the quinquephenyl chromophores, and subsequently of their luminescent dipoles, concerning the polymers with an odd number of methylene units x=7, 9, 11) and those with an even (x = 8, 10, 12) one, were found responsible for the observed strong deviations in frozen and dilute solutions, where the flexible aliphatic chains are forced to adopt a nearly sta ered, lower energy conformation. 

An excimer is an electronically excited molecular complex formed between two suitably oriented aromatic rings when one of them has been promoted to an excited state by absorption of energy. The normal characterization parameter is the ratio of the excimer fluorescence intensity to the monomer fluorescence intensity, R, where the monomer refers to the uncomplexed aromatic ring. Of importance for our work is that it is a common feature of the photophysical behavior of the aryl vinyl polymers, as described in a recent review [48]. The objective of this section is to demonstrate the sensitivity of excimer fluorescence to those variables expected to influence the free energy of mixing in polymer blends solubility parameters of the two components, concentration, temperature and molecular weight. 

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