Radiationless process, intramolecular

The application of UV absorbers, i.e. compounds absorbing the harmful solar radiation, represents an effective solution of the problem (Rabek, 1990). The absorbed radiation is deactivated by intramolecular radiative and radiationless processes. The ideal UV absorber is expected to absorb all terrestrial UV-A and UV-B radiation but no radiation having wavelengths higher than 400 nm. Different classes of commercialized UV absorbers fulfil requirements on effective plastics protection. A group of UV absorbers acting by excited state intramolecular proton transfer (ESIPT) mechanism (Pospfsil and Nespurek, 1997) includes phenolic derivatives of benzophenone (37), various benzotriazoles, such as 38 or 39, and 1,3,5-triazine 40. Non-phenolic UV absorbers are represented by oxamide 41 and a-cyanoacrylate 42. 

In the more complex situation where intramolecular radiationless processes occur as well as radiative and collisional ones, the equation becomes... 

Although the radiationless processes appeared to be intramolecular in character, the presence of the solvent or rigid matrix presented an essential complication to a theoretical description of the radiationless phenomena. A complete theory of radiationless processes must begin with a description of electronic relaxation in isolated molecules. Once the isolated molecule case is properly understood, the effects of the external media can be considered and the bulk of the experimental data concerning radiationless transitions can be confronted. 

Let the basis set still be the BO states starting points. Sim we wish to focus upon all the diverse molecular phenomena which are classify as involving radiationless processes, it is necessary to center attention upon th molecule. This focus is best obtained by considering the effective Hamiltoniar Hett, for the molecule which accounts for all relaxation mechanisms other tha the intramolecular nonradiative decay. (The use of effective Hamiltonians is popular in considering the relaxation processes associated with studies of magnetic resonance 37L) For the present case, the effective Hamiltonian is 16>17)... 

UV absorbers (UVA) act by absorbing UV light to retard the photolysis of hydroperoxides. Typical examples are based on 2-hydroxybenzophenones (AO 28) and 2-hydroxybenzotriazoles (e.g.. Table 1, AOs 29-32) both are photo-stable with high molar absorptions over the region 300-360 nm. Their activity is based essentially on absorption of the harmful UV-radiation and its harmless dissipation as heat. For example, in 2-hydroxybenzophenones, UV-light induces intramolecular hydrogen transfer to yield an enol, which reverts back to the original ketone in a radiationless process, Reaction 4. 

Early studies of the photophysical radiationless processes of molecular systems were carried out on molecules in condensed media, liquids, rigid matrices, and high-pressure gases. This experimental situation introduces the complication associated with the presence of the possible occurrence of a number of different competing photophysical relaxation processes in the same molecular system in a fashion that mimics the complexity of a full photochemical reaction scheme. In order to study the primary photophysical radiationless transitions, it is optimal to consider experiments in which only the elementary individual processes of interest appear. Such investigations often involve the experimental determination of radiationless transition rates in isolated collision-free molecules. " For instance, collision-free experiments enable the consideration of the important phenomena of electronic relaxation and intramolecular vibrational redistribution. Studies on isolated molecules have greatly contributed to our... 

The lack of fluorescence of TAM dyes in solution is attributed to an extremely rapid, nomadiative deactivation process brought about by intramolecular rotation of the flexible aryl groups. Suppression of molecular rotation, by increasing the viscosity of the medium, by binding of the dye to a polymer or protein, or by selfassociation, diminishes this radiationless process. Coincident with this change in photophysical properties is often a dramatic increase in sensitivity toward photofading due to an increase in the quantum yield of the relatively long-lived and photochemically active triplet state [139-143]. The triplet state of the dye cation... 

As discussed in Section II, the Ij and CS2 molecules show intramolecular magnetic quenching while the radiationless process associated with the magnetic quenching of I2 involves the transition between the bound state and the dissociating state, the radiationless process associated with the magnetic quenching of CS2 involves the transition between the two bound states. 

Fig. 6.4 Modified Jablonski diagram for an organic molecule showing ground and excited states and intramolecular photophysical processes from excited states. Radiative ntK esses—fluorescence (hvf) and phosphorescence (hvp) are shown in straight lines, radiationless processes— internal conversion (1C), inter system crossing (ISC), and vibrational cascade (vc) are shown in wavy lines. Adapted with permission fiom (Smith MB, March J 2006 March s Advanced Organic Chemistry Reactions, Mechanisms and Stiucmres, 6th Ed., John Wiley, New York). Copyright (2007) John Wiley Sons...

Lefebvre, R. (1971) Preparation and decay of initial states in intramolecular radiationless processes. Chem. Phys. Lett., 8, 306. 

Chapter 3 provides a detailed description of radiationless processes in a statistical large molecule embedded in an inert medium. In this chapter, we are for the first time able to express the vibrational overlap between the electronic states under consideration in terms of intramolecular distributions in the full harmonic approximation taking into account the effects of vibrational frequency distortion, potential surface displacement, and the Duschinsky rotation. 

The intramolecular processes responsible for radiative and radiationless deactivation of excited states we have considered so far have been uni-molecular processes that is, the processes involve only one molecule and hence follow first-order kinetics. 

---