Photo-Conducting Polymers That Are Not Based on Carbazole

When spiropyran is incorporated into plasticized poly (vinyl chloride) membranes and placed between two identical NaCl solutions and irradiated with ultraviolet light for long periods, the potential decreases [257]. This was shown by Ryba and Petranek to be a result of the spiran ring opening up [257]:

When the irradiation is interrupted and the membrane is irradiated with visible light, the potential returns to its original value, because the ring closes back to the spiran structure [125]. The addition of electron donors, like dialkyl aniline, to Kapton polyimide film

results in an enhancement of photocurrent by as much as five orders of magnitude, compared with the virgin material [258]. Freulich explains the mechanism of enhancement as a result of radiation absorption by the charge-transfer complex formed between the added electron donor and the imide portion of the polymer backbone. Excitations are followed by rapid and complete electron transfer from the donor to pyromelitimide to yield the radical anion of the polymer and the radical cation of the donor [258]. These species undergo rapid back-electron transfer. In other words, the dialkyl aniline donates one of the unpaired electrons in a typical photo-reduction reaction to the carbonyl group. The reaction is reversible and the photo-conduction is by a mechanism of the ion radical returning to the ground state. Studies of the thermochromic, solvatochromic, and photoconductive properties of 3-, 4-, 6-, and 9 poly [(butoxycarbonylinethyl urethane)-diacetylenes] result from changes from rod (red or blue) to coil (yellow) conformations of the polymer backbone [259]. Photo-excitations of the solutions of these polymers in the rod state result in a large transient photo-conductivity, while only very small conductivity signals are observed in the coil state. The thermochromic shift that occurs in going from the rod state at room temperature to the coil state at 65C is accompanied by a decrease in the photo conductivity. The large conductivity signal in the rod state is attributed to the formation of mobile charge carriers possibly via interchain charge transfer within aggregates. The decay of the photo conductivity is nonexponential and extends to microseconds. Wongetal. [260] reported that they prepared a soluble rigid-rod organometallic polymer containing electron-donating and electron-withdrawing trans-[-Pt-(tibutylphosphine)2-acetylene-R-acetylene-]n (where R = bithiazol-ediyl) groups. The polymer was formed by cuprous iodide-catalyzed dehydroha logenation reaction. The electron-donating and electron-withdrawing properties of the thiazole ring confer solubility to the polymer. This polyacetylene is luminescent with a singlet emission peak at 539 nm and photo-conducting. The glass transition temperature of the polymer is 215 and it shows relatively good thermal stability. The p-conjugation of the ligands extends into and through the metal core and the absorption peaks show a significant red-shift of 17–26 nm compared to the bithienyl counterparts due to the presence of the electron-withdrawing imine nitrogen atoms [260]. Kimura et al. [259] reported applying organic photochromic compounds to photochemical switching of metal-ion complexation and ionic conduction by combining photochromism with metal-ion binding property of crown ether derivatives. They synthesized vinyl polymers, incorporating a crowned spirobenzopyran moiety at the side chain:

The crowned spiropyran in the electrically neutral form can bind an alkali metal ion with the crown ether moiety. At the same time, the spirobenzopyran portion isomerizes to the corresponding merocyanine form photo-chemically. The zwitterionic merocyanine form of crowned spirobenzopyran moiety brings about a significant change in the metal-ion binding ability. This prompted the authors to apply the compound to photo-responsive ion-conductive materials. They observed that the ion conductivity was increased by ultraviolet light and decreased by visible light [259]. Chan and coworkers [269] prepared polystyrenes and poly (methyl methacrylate) s that contain metal complex cores:

When the polymers are doped with a hole-transporting triphenylamine, an enhancement in photo conductivity in the visible region is observed. This suggests that the metal complexes serve as photosensitizers instead of charge carriers [260]. Chan et al. [260] observed an electric field-dependent charge separation process in these polymers. It is described well by the Onsager’s theory of charge germinate recombination. This theory assumes that some fraction of absorbed photons produce bound thermalized electron-hole pairs that either recombine or dissociate under the combined effects of the Coulombic attraction and the electric field. The photo-generation efficiency is given as the product of the quantum yield of thermalized pair formation and the pair dissociation probability:

where Ig is a recursive formula given by

where I0) x= (1 exp(-x), F is the primary yield of thermalized bound pairs, r0 is the initial thermalization separation between the bound charges, er is the relative permeability, and E is the applied electric field strength. Jenekhe and de Paor [261] reported exciplex formation and photoelectron transfer between several n-type (electron accepting) p-conjugated rigid-rod polymers and donor triarylamine molecules. In particular, they reported an investigation of an n-type conjugated polymer poly (benzimido azobenzo phenanthroline ladder) [132]. No evidence was observed by them of a ground state charge transfer or any strong interactions between the conjugated polymer pairs. Transient absorption spectra of a blend of thin films in the 420–730 nm region were obtained at various time delays following photo excitation at 532 nm. Dramatically enhanced photo-induced bleaching in the 430–480 nm region was observed. Jenekhe and de Paor propose that they observed enhanced photo-bleaching in the blends and that it is a consequence of photo-induced electron transfer [261]. The electron transfer was illustrated as follows:

Molecular materials, such as 2-(2-hydroxyphenyl) benzoxazole and 2-(2-hydroxyphenyl) benzothiazole, which contain intramolecular hydrogen bonds are known to undergo excited state (charge transfer) intramolecular proton transfer upon photo-excitation.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم صناعة الشبابيك يواصل أعمال الإدامة لأحد أبواب حرم مرقد أبي الفضل العباس (عليه السلام)

ضمن فعاليات ملتقى النورين.. المجمع العلمي يقيم ندوة فكرية في بابل

المجمع العلمي يحيي ليلة القدر الأولى في مقام الإمام المهدي (عجّل الله فرجه)

مركز السيد جعفر الحلي الطبي يقدم الاستشارات والخدمات الصحية في ليالي القدر المباركة

المزيد

الآخبار الصحية

خمسة مؤشرات صحية مهملة قد تنقذ حياتك

متى تكون الحمى طبيعية؟

التدخين الإلكتروني يهدد قلبك.. دراسة جديدة تكشف مخاطر ارتفاع ضغط الدم

دراسة تكشف فائدة بعض أنواع الشاي في محاربة الاكتئاب

المزيد

الآخبار التكنلوجية

الجهاز المناعي يظهر دورا غير متوقع في صحة العظام

ابتكار من جامعة الشارقة منخفض التكلفة للحماية من الزلازل

اكتشاف تأثير غير متوقع للكاكاو على وظائف الدماغ

باحثون يرصدون أثر تغيّر المناخ في دم البشر

المزيد

مواضيع ذات صلة

Polymer-Based Solar Cells

Photoconductive Polymers Based on Carbazole

Photo-Conducting Polymers

Liquid Crystalline Alignment

Application to Optical Data Storage

Changes in Viscosity and Solubility of Polymeric Solutions

Photo-Isomerization of the Azo Group

Photo-Isomerization of the Olefinic Group

Copolymers of Methacrylates with Cholesteric Monomers

Polymers with Norbornadiene Moieties

Photo-Responsive Polymers

Polymers Designed to Cross-link Upon Irradiation with Laser Beams