Temperature Dependence of Photophysical Processes

Photochemical reactions competing with photophysical processes often require thermal activation in order to cross small barriers in the S, or T state. (See Section 6.1.) In general, such reactions may be suppressed at low temperatures and photophysical processes would then be favored. This is by far the most important temperature effect on photophysical processes although the following effects should also be considered. 

Another type of temperature dependence that may be thought of as competition between temperature-dependent and temperature-independent processes has been observed for substituted anthracenes. It is due to the fact that the Tj state of anthracene is energetically very close to the lowest excited singlet state S. When Tj is just above S the intersystem crossing Si-v Tj will be associated with a barrier . The rate constant of this transition can be expressed in the form of a normal Arrhenius equation k = g- a/ Tand the intersystem crossing becomes temperature dependent. This situation is found in 9- and 9,10-substituted anthracenes. (Cf. Example 5.3.) The fluorescence quantum yield 4 p increases strongly with decreasing temperature. In other substituted anthracenes as well as in anthracene itself T2 

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The recovery time following upon bleaching induced by Si-Sji absorption of p-terphenyl in cyclohexane at room temperature is found to be about 380 ps. The temperature dependence of photophysical processes in perylene, tetracene, and some derivatives is shown to arise from thermal activation of the S1-T2 intersystem crossing process and also for the Si-Sq internal conversion, which is particularly important in rubrene. ... 

Phillips, D., Roberts, A. J., Soutar, I. Transient decay studies of photophysical processes in aromatic polymers. V Temperature dependence of excimer formation and decay in copolymers of 1-vinyl naphthalene. As yet unpublished 1980... 

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. 

Emeline A. V., Polikhova S., Andreev N., Ryabchnk V. K. and Serpone N. (2002b), Photoinduced processes in heterogeneons gas-solid systems. Temperature dependence (100-600 K) and modelling of a snrface chemical reaction on zirconia that triggers photophysical events in the solid , J. Phys. Chem. B 106, 5956-5966. 

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