The Charge Transfer Processes in Polymers

Charge transfer in polymers is either electronic (transfer of electrons or of positive charges alone) or it is ionic (transfer of protons or larger charged species). Electronic conduction can be also of two types. One type is conduction due to diffusion of electrons that are not localized on any particular molecule (this is usually found in liquids or in gases). The other type can be by conduction due to positive or negative charges that are localized on any particular part of the molecules. Such charges can be exchanges between like polymeric molecules (or between segments of single polymeric molecules).

This can occur without any net energy loss (resonant charge transfer). It was shown experimentally that the electrical conductivity in many polymeric materials, subjected to short irradiation pulses, consists initially of a “prompt” component. That means that very rapid transfer of a considerable amount of charge takes place over a comparatively short distance ( 100 A ˚). The movement of the charge is then terminated as a result of trapping in “shallow” traps [90, 93, 97]. This is followed by a “delayed” component that is very temperature-dependent and probably indicates a thermally activated charge-hopping process between the shallow traps. This continues until terminated (after 1 mm) by trapping in deep traps or by recombination [90, 93, 97]. There is a major difference between eximers of polymers and those of small molecules. The difference is that at least in some polymers a large part of the excitation of the excimer site appears to be a result of singlet energy migration [93]. Also, in polymeric materials with a number of identical chromophores, either in the backbone or as pendant groups, when photons are absorbed, the excited states cannot be considered as localized. In simple cases of rigid lattices, the excitations are distributed over the entire volume of the material as a wave-like linear combination of local excitations [87, 90, 91, 93]. They are referred to as tight-binding excitations [90, 91, 93]. As one might expect, excimer formations in polymers depend upon the properties of the chromophores and upon their location on the polymeric chain [90]. In addition, polymer tacticity, conformation, and distance between chromophores can greatly affect the formation of eximers. Also, it is possible to distinguish between two different types of energy transfers in polymeric materials. The transfer of excitation can take place either from or to large molecules from small ones. Thus, for instance, a polymer transfer of the excitation energy can be localized from a chromophore on one polymeric chain to another. An example of a transfer to a small molecule is an energy transfer from a polymer, like polystyrene to a scintillator molecule, like 1,4-bis[2-(5-phenyloxazolyl)] benzene shown below [95]:

More than that, transfer can also take place from one group of atoms, or from a chromophore, located on a polymeric chain in one section of the molecule, intramolecularly, to another one located at another section of the same polymer. Thus, in copolymers from monomers with two different chromophores groups, the energy absorbed by one group of chromophores can be transferred to the chromophores from the other group. This can take place by either Foster or exchange mechanism. The possibility of energy transfer from one chromophore to an adjacent different chromophore in polymeric chains depends to a large extent upon the lifetime of the excitation and its alternative modes of deactivation. For this reason, the most readily observed form of energy migration is one that occurs through the mechanism of the triplet [88, 90, 93, 97]. Intermolecular energy transfer from one polymeric material to another while the molecules are in solution or in the melt can also take place [17]. This was demonstrated on an intramolecular excimer and exiplex formation in solutions of polyesters containing naphthalene or carbazole moieties in their chemical structures [98]. In general, the migrations of energy in polymers are somewhat more complex, because chain folding and conformations are additional factors that enter into the picture. The separation between interacting units can be affected by the composition of the polymer, the geometry of the polymeric chains, and the flexibility of the backbones [99]. There are two limiting cases for the effects of polymer folding on energy transfer efficiency. Folding of a polymer before excitation into a conformation in which the sensitizers are held within a hydrophobic pocket improves the efficiency of energy migration. This takes place with a large number of intramolecular hops or when bond interactions intervene between the sensitizer and the ultimate trap [93]. If the polymers are flexible, however, they can also bend after photo-excitation to bring otherwise distant chromophores close enough so that energy can hop from one to the other, skipping intervening units and thereby considerably shortening the effective migration distance along an individual polymer chain [93]. For flexible polymers in solvents that promote folding, this motion can take place even faster than excited state decay [99]. Intramolecular singlet energy migration can also proceed via electronic coupling through the bonds that form the polymer backbone. In a random walk, the excitation energy migrates without directional control, moving back and forth along a chain or across space. Through-space interactions between pendant chromophores are also common in polymers with large numbers of absorbing units [18]. One should also include movement of excitation across folds or loops that can form in polymeric chains. Such folds can be the result of packing into crystalline domains or simply from temporary collisions. In principle, the excitation can be localized for some finite time (however small) on a particular chromophore before it is transferred to another one in the chain. Guellet [94] defines intramolecular energy migration as any process that involves more than one exchange of excitation energy between spectroscopically identical chromophores attached by covalent bonds to a polymeric chain [12]. He further terms “energy transfer” as a single step migration between two chromophores, while one that involves several or more chromophores as “energy migration” [93]. The polymers with multiple sensitizers offer several routes for energy migration. This can be illustrated as follows [99]:

A very common arrangement is for the photosensitive groups to be aligned outside of a spiral arrangement of the polymeric chain in close enough proximity to each other for energy transfer. Also, as mentioned earlier, folding of a polymer before excitation into such a conformation that the sensitizers are held within a hydrophobic pocket improves the efficiency of energy migration when a large number of intramolecular hops. Efficiency of energy migration is also helped through-bond interactions that intervene between the sensitizer and the ultimate trap [99]. Also, as mentioned before, flexible polymer frameworks can bend the polymeric chains in such a manner as to bring otherwise distant chromophores close enough together so that after excitation the energy can hop from one to another. In such a case, the energy migration can skip intervening units and thereby considerably shorten the effective migration distance along a single polymer chain. As stated above, for flexible polymers in solvents that promote folding, this motion can be even faster than excited state decay [99]. Intermolecular energy migration can also occur between two different polymeric molecules. Thus, for instance, Turro et al. [95] investigated inter- and intramolecular energy transfer in poly(styrene sulfonate). They found that excimer formation between adjacent phenyl groups is a dominant reaction both along a single chain and between two different chains [95]. At low densities of excited states, singlet energy transfer between a sensitizer and its nearest quencher (perhaps on another chain) dominates, whereas at high excited state densities, energy migration takes place through the series of donors [99]. Guellet quotes Webber, who reported that he used the following equation (that he called crude but useful) to obtain rough estimates of the energy migration diffusion rate along the polymer backbone [94]:

where DQ is the normal diffusion constant of the quencher and kg is the energy migration diffusion rate along the polymer. In some aromatic vinyl polymers, excimer emission can occur after an initial excitation of an aromatic chromophore. This is followed by intramolecular singlet energy migration, either along the polymer chain, or intermolecularly along the chromophores. Here too, it can be through different chains in a polymer that is in bulk form and the chains are in close proximity to each other. The process generally continues until the excitation is trapped at some chain conformation that is suitable for excimer formation. Such a chain conformation is referred to as eximer-forming site. If the polymer is in solution and viscosity is low, interconversion of chain conformations proceeds fairly rapidly. In such cases, the lifetimes of any particular conformation are limited by the collision processes as well as by the magnitude of the rotational barriers with respect to thermal energy [93]. In the solid state, however, the rotational freedom of the polymeric chain is considerably reduced. Large-scale conformational changes are unlikely. There still is the possibility, however, that adjacent chromophores will be in a marginal eximer-forming site [94].

اخر الاخبار

اخبار العتبة العباسية المقدسة

للشروع بصيانتها وترميمها..متحف الكفيل يستلم الدفعة الرابعة من مقتنيات العتبة العسكرية المقدسة

على مدار اليوم.. قسم الشؤون الدينية يقدّم خدماته التبليغية والإرشادية خلال شهر رمضان المبارك

شعبة التوجيه الديني تنظّم مسابقة ميدانية بذكرى ولادة الإمام الحسن المجتبى (عليه السلام)

قسم التطوير يقيم سلسلة من الورش التدريبية عن الأمن السيبراني لملاكاته

المزيد

الآخبار الصحية

مركب طبيعي في القهوة يحمينا من التهابات اللثة

داء السكري لا يرفع السكر فقط.. بل يزيد خطر الإصابة بأمراض القلب!

اكتشاف سبب غير متوقع لضعف الذاكرة والتركيز

نصائح علمية لتقوية دماغك وتقليل التدهور العقلي

المزيد

الآخبار التكنلوجية

باحثون يرصدون أثر تغيّر المناخ في دم البشر

الجميع يحلم.. لماذا يتذكر البعض أحلامهم بينما ينسى آخرون؟

حرق الأمونيا.. كيف يساعد محطات الكهرباء في اليابان على إزالة الكربون؟

الشمس تقذف نتوءا عملاقا باتجاه قطبها الشمالي

المزيد

مواضيع ذات صلة

Polymers with Functional Chalcone Groups

Polymers with Pendant Cinnamoyl Functional Groups

Polymers That Photocross-link by Formation of Cyclobutane Rings

Photocross-linkable Polymers

Photosensitizers

The Electron Transfer Process

Energy Transfer Process

Interaction of Light with Organic Molecules

The Nature of Light

Electricity-Conducting Polymers

Immobilized Reagents

Immobilized Nonenzymatic Catalysts