Excimer trapping

Most simple aromatic polymers like polystyrene, polyvinylnaphthalene and poly(N-vinylcarbazole) exhibit both exciton migration and excimer trapping. 

---

The theoretical implications of (a) are fascinating since single step transfer as part of extensive e.e.m. would occur on a time-scale much faster than would be predicted by exchange of dipole transfer and would be of the magnitude typical of crystals which exhibit ground-state interaction. Furthermore, if (a) were the case then all excimer traps would be populated on this very fast time basis. However the observed behaviour shows a definite rise-time of the order of a nanosecond for the excimer state. 

On cooling films of pure polystyrene to 77K the ratio Ie/Im goes to almost zero (14). Assuming that excimer traps are preformed in the TTlm when cast, the population of such traps will remain constant, once the film is formed. Neither the exchange nor the dipole-dipole mechanism of energy transfer is particularly sensitive to temperature and thus it is difficult to see why the traps should not be populated at very low temperatures, if indeed the mechanism of population is by e.e.m. into the traps. 

In synthetic polymers, the interpretation is necessarily more difficult The form of Equation 4 and Equation 5 requires that the kinetics of formation and decay of complexes are modelled adequately by rate-constants and that they take place in a homogeneous medium. If, as in synthetic polymers, the population of excimer trap sites, may occur through energy migration or rotational diffusion, a rate-constant may not be an adequate representation of the process, some time-dependent parameter being required (see below.) Heterogeneity may also play an important role. Thus in earlier work the fluorescence decay of excimer-forming polymers was modelled adequately by a scheme based upon simple excimer kinetics to which had been added terms to account for the occurrence in co-polymers of monomer sites which, by their isolation, could not form excimers (4-10). For polymers which contain isotactic and syndiotactic sequences, or rather, are made up of meso and racemic triads (14), the kinetics may be similarly a superimposition of simple schemes appropriate for the different sequences. 

It has been suggested that in the solid state electronic energy migrates rapidly from the initially excited chromophore to excimer trap sites by either exciton diffusion or single-step mechanisms. 

Thus, both transient and photostationary trapping experiments may be interpreted Quantitatively if an expression for the donor excitation function G (t) is available. In Section 2.2 we outline the development of an expression for G (t) for an aryl vinyl polymer in dilute solution. Here energy migration from a single monomer donor state to a single excimer trap state is analyzed by a one-dimensional model. We will present the analysis in more detail than the subsequent discussion of various many body theories because the treatment is straightforward and concise. This will allow the fundamental approach to be understood more clearly without the extensive mathematics required for the many-body treatment. 

The dimensionless monomer donor and excimer trap concentrations are defined by... 

As well known, so-called excimer sites exist in poly-N-vinylcarbazole. It is well established that these excimer sites are the efficient traps for the singlet and triplet excitons, which migrate along the polymer chain. The structure of these sites are thought to be a special conformation having a pair of carbazolyl groups arranged parallel each other. 

It seems to be natural to suspect if these excimer forming sites were the effective trapping center also for hole carriers. 

When electron transfer sensitizers are bonded to polymers the sensitizer efficiency is in general reduced. This is caused by (a) loss of segment mobility, (b) enhanced excimer formation (energy trap), (c) enhanced side reactions, and... 

The only head-to-head polymer which has been examined for excimer fluorescence is polystyrene 25). Unfortunately, the synthetic route to this polymer leaves a number of stilbene-based structures in the sample, which have a lower-energy singlet state than either PS monomer (285 nm) or excimer (330 nm). Thus, fluorescence from these intrinsic stilbene traps was seen in the spectra of head-to-head PS in pure films and, to a lesser extent, in fluid solution. In the latter, the fluorescence of PS monomer was predominant, and the small amount of stilbene fluorescence was increased when a nonsolvent (methanol or cyclohexane) was added to the 2-methyl-tetrahydrofuran solution. In films of the polymer, stilbene fluorescence was the major spectral band, although some PS excimer fluorescence was also present in the spectrum. No monomer fluorescence at 285 nm was detected from films. Given the impure nature of the head-to-head PS sample, no conclusions on excimer formation in these systems could be drawn. 

These bacteria cannot in general oxidize water and must live on more readily oxidizable substrates such as hydrogen sulfide. The reaction centre for photosynthesis is a vesicle of some 600 A diameter, called the chromato-phore . This vesicle contains a protein of molecular weight around 70 kDa, four molecules of bacteriochlorophyll and two molecules of bacteriopheophy-tin (replacing the central Mg2+ atom by two H+ atoms), an atom Fe2+ in the form of ferrocytochrome, plus two quinones as electron acceptors, one of which may also be associated with an Fe2+. Two of the bacteriochlorophylls form a dimer which acts as the energy trap (this is similar to excimer formation). A molecule of bacteriopheophytin acts as the primary electron acceptor, then the electron is handed over in turn to the two quinones while the positive hole migrates to the ferrocytochrome, as shown in Figure 5.7. The detailed description of this simple photosynthetic system by means of X-ray diffraction has been a landmark in this field in recent years. 

The most important photochemical reaction of the bases of DNA is the dimerization of thymine, because this can take place not only between free molecules but also within the DNA chain when two such bases happen to be close together. Since the bases in a DNA strand form a continuous array of relatively close-packed molecules, energy transfer is efficient throughout the chain and any excitation energy will eventually find a trap such as a thymine dimer (or excimer) where the dimerization reaction can take place. 

For photodissociation experiments, a special probe was constructed to replace the 12.7 mm diameter direct insertion probe normally employed. It consists of a hollow stainless steel tube which has a 38 mm focal length quartz lens vacuum sealed at one end and an extended hollow probe tip at the other. The beam of a Lambda Physik excimer laser operating at 308 nm was passed through the probe and lens into the FTMS cell through a small hole in the center of the trap plate as shown in Figure 1. 

The ejection of atoms or molecules from the surface of solid in response to primary electronic excitation is referred to as electronically stimulated desorption (ESD) or desorption induced by electronic transitions (DIET). Localization of electronic excitations at the surface of RGS induces DIET of atoms both in excited and in ground states, excimers and ions. Most authors (see e.g. Refs. [8,11,23,30] and references therein) discuss their results on DIET from RGS in terms of three different desorption mechanisms namely (i) M-STE-induced desorption of ground-state atoms (ii) "cavity-ejection" (CE) mechanism of desorption of excited atoms and excimers induced by exciton self-trapping at surface and (iii) "dissociative recombination" (DR) mechanism of desorption of excimers induced by dissociative recombination of trapped holes with electrons. 

---