CC BY
8
УДК 678.01
A. A. Olkhov, D.-J. Liaw, Y.-C. Huang, C.-H. Chang, B. M. Rumyantsev, T. A. Lozinova, V. P. Zubov, V. N. Bagratashvili, G. E. Zaikov, A. A. Ischenko
PHOTOELECTRON PROPERTIES AND PARAMAGNETISM OF POLYIMIDES BASED ON N,N,N',N'-SUBSTITUTED PARAPHENILENEDIAMINE AND DIANHYDRIDES
Key words: polyimides, electrophotographic method, ESR signal, polycondensation, photoelectric characteristics, charge transfer complex, quantum yield of photogeneration, photogeneration mechanism, cation - radical.
The photoelectric characteristics and paramagnetic properties of newly synthesized polyimides (PI) are investigated using the electrophotographic method and an ESR signal study. The polyimides based on N,N,N',N'-substituted paraphenylenediamine and dianhydrides, were prepared via the polycondensation. The polymers exhibit excellent solubility in common organic solvents, and have high thermal stability, Td10 = 726K in a nitrogen atmosphere and Tg = 413K. The PI films (3 /м thickness) exhibit photoelectric sensitivity in the UV, and visible spectral regions, due to charge transfer interactions between donor and acceptor fragments of the PI chains (formation of a charge transfer complex, CTC). Study of the photogeneration quantum yield field dependence provides the evidence that the photogeneration mechanism is the field assisted thermo-dissociation of radical ion pairs kinetically associated with the excited CTC. The second important mechanism of photogeneration is photostimulation of long-lived stable cation-radicals of the donor PI fragments, representing the hole (majority carriers) captured by deep centers (photostimulated currents). The accumulation of labile cation-radicals in the dark and photo processes leads to the dependence of photoelectric characteristics on the number of charge-discharge cycles of the sample. Paramagnetism of the air stored PI samples is determined by the air oxidized PI donor fragments (stable radicals). PI ESR spectra in solid state and solution are investigated as well as the effect of light excitation on those spectra. Correlations between ESR signal intensity and photoelectric sensitivity are observed.
Ключевые слова: полиамиды, электрофотографический метод, сигнал ЭПР, поликонденсация, фотоэлектрические характеристики, комплекс с переносом заряда, квантовый выход фотогенерации, механизм фотогенерации, катион-радикал.
Фотоэлектрические характеристики и парамагнитные свойства вновь синтезированных полиимидов (ПИ) исследованы с использованием электрофотографического метода и ЭПР. Полиимиды на основе N,N,N',N'-замещенных парафенилендиамина и диангидрида, были получены с помощью реакции поликонденсации. Полимеры обладают превосходной растворимостью в обычных органических растворителях и имеют высокую термическую стабильность, Td10 = 726K в атмосфере азота и Tg = 413K. Пленки ПИ (3 мкм толщиной) проявляют фотоэлектрическую чувствительность в УФ и видимой области спектра, за счет переноса заряда взаимодействия между донором и акцептором фрагментов ПИ цепей (формирование комплекса с переносом заряда, КПЗ). Исследование выхода зависимости фотогенерации квантового поля обеспечивает доказательство того, что механизм фотогенерации является следствием термо-диссоциации ион-радикальных пар кинетически связанных с возбужденным КПЗ. Вторым важным механизмом фотогенерации является фотостимуляция долгоживущих устойчивых катион-радикальных фрагментов доноров PI, представляющих «дырки» (основные носители), захваченные на глубинных центрах (центры фотостимуляции). Накопление лабильных катион-радикалов в темноте и в результате фото процессов зависит от фотоэлектрических характеристик и количества циклов зарядки-разрядки. Парамагнетизм долго хранящегося на воздухе образца PI определяется окислением ПИ-донорных фрагментов (стабильные радикалы). Спектры ЭПР были сняты на твердых пленках, а также было исследовано влияния светового возбуждения на ЭПР-спектры пленок. Наблюдается корреляция между интенсивностью сигнала ЭПР и фотоэлектрической чувствительностью.
Introduction
In the last two decades, tc-conjugated polymers have attracted considerable interests because of their potential applications in electrochromics [1-4], light emitting diodes [5-9], organic thin film transistors [1014], photovoltaic's [15-18], and polymer memory [1920]. Fluorene and its analogous derivatives have drawn much attention in optoelectronics because they generally have good solubility, high luminescent efficiency, and very good charge carrier mobility [21-24]. However, they are also known to have drawbacks such as unsatisfied thermal stability and the formation of excimers in the solid state [25, 26]. Essentially higher thermal, photochemical and radiation stability are inherent to polyimides (PIs) and their photoelectrical, optical and other properties are determined to a great extent by the electron charge transfer between donor and acceptor units of the polymer chain.
The exceptional photoelectric characteristics of organosoluble PIs based on triphenylamine and its derivatives [27], as well as their composites with the organic and inorganic semiconductors [28, 29] (the photoelectric sensitivity (PES) S, quantum yield of the charge carrier photogeneration p, drift length lD, and collection coefficient C(Z)) are the basis of their applications in a series of optoelectronic devices: photovoltaic and electroluminescent cells [30,31], photodetectors [32] and organic phototransistors [33]. In the present work, the photoelectric characteristics, paramagnetism and their correlation for the newly synthesized PI [34, 35] are investigated via the electrophotographic (EPG) method [36] and an electron spin resonance (ESR).
The effect of radical product (Poole-Frenkel pairs, PFP) on the photoelectric sensitivity (photostimulated currents) in PI films based on diphenylbenzidine is observed at the first time in [37, 38]. As shown in these works PFP is a triphenylamine
cation radical (TPA+) that is stabilized in the field of Cl-(Br- ) anion, generated by a free-radical photochemical reaction with participation of C2H2Cl4 or CBr4. Furthermore under photostimulation of long-lived PFP in TPA (red and near IR spectral range) or charge transfer complex absorption bands, a transformation into shortlived Onsager type ion-radical pair occurs, which undergo field-assisted thermal dissociation, resulting in charge carrier photogeneration. Similar processes with PFP formation and accumulation and their subsequent photostimulation are observed on the interface of organic polymer heterostructures [32].
In the present work photoelectric properties, paramagnetism and their correlation for newly synthesized organosoluble PI based on N,N,N',N' -substituted paraphenylenediamine with propeller-shaped triarylamine fragments are investigated. Their synthesis is described in [39]. All polymers with high molecular weight form flexible and strong films. The electrochromic properties of PI films are found characterized by their color alternation from pale-yellow in neutral form to green (for the single-oxidized state, cation radical) and finally blue for the second oxidized state under potential variation from 0 to 1.5 V in an electrochemical cells with a liquid electrolyte. Cyclovoltammetry of the polymer films shows two reversible redox couples at potentials of 0.91-0.99 V and 1.30-1.38 V. Investigation of the oxidation mechanism suggests that oxidation involves not only electron removal from the nitrogen atom but from alkyl-phenyl groups as well. The results of the work prove that electron rich triarylamine fragments are oxidized easily, forming stable cation-radicals. In the present work it is found that cation- radicals can be also generated in other processes: under dark hole injection from the corona charged polymer film surface (labile radicals) and in solid state samples stored in the air (ESR registrated stable radicals as a result of interaction with O2). The increase of ESR signal intensity is observed under short wavelength UV excitation of PI solutions indicating radical generation. On the basis of the revealed dependence of photoelectric sensitivity on labile radical accumulation the conclusion can be drawn regarding charge carrier photogeneration as a result of labile radical photostimulation in the absorption band (photostimulated currents).
Methods and materials
Synthesis of polyimides
The polyimides were prepared by a procedure similar to the one described in [39]. An example is described as follows (Scheme 1). To a stirred solution of 0.6g (0.883 mmol) of diamine in 5 mL DMAc, 0.382g (0.883 mmol) of 4,4'-
(Hexafluoroisopropylidene)diphthalic anhydride
(6FDA) was gradually added. The mixture was stirred at room temperature for 4 h in nitrogen atmosphere to form poly(amic acid). Chemical cyclodehydration was carried out by adding equal molar mixtures of acetic anhydride and pyridine into the poly(amic acid) solution under stirring at room temperature for 1 h, and the solution was then treated at 373K for 4 h. The polymer solution was poured into methanol. The precipitate was
collected by filtration, washed thoroughly with methanol, and then dried at 373K under vacuum.
ШЛ XjVi.
A1 A2
XrcTT
Scheme 1 - An example of polyamides synthesis
Electrophotographic study
The electrophotographic method involves the study of the kinetics of dark and photoinduced decay of the surface potential in polymer films that are charged in the field of positive or negative corona discharge [36]. The maximum potential of charging V and the rate of the potential dark decay (dV/dt)D are determined by the dark conductivity of the films: higher dark conductivity results in a lower value of V and higher (dV/dt)D. The photoinduced decay potential (1/I)(dV/dt) (where I is the intensity of excitation) is determined by the rate of the capacitor photodischarging due to ionic contact (aero ions on the film surface) of one electrode with a glass conducting support (indium tin oxide, ITO) as the second electrode on which the induced charge has the opposite sign. The potential photodischarging rate depends on the effective charge carrier photogeneration quantum yield in the film volume (xerographic output, peff), carrier collection efficiency of the electrodes C(Z) and the portion of absorbed exciting light, P:
(1/I) [dV/dt - (dV/dt)D)] = (ed/ee0) Peff P (1)
where: peff = PC(Z); C(Z) - charge carrier collection efficiency, Z = ^Vx/d2 - ratio of the carrier drift length (lD = ^Vx/d) to the film thickness d, ^ - carrier drift mobility, t - carrier lifetime, e - electron charge. The function C(Z) for strong and weak absorption is given in [36], from which it follows that for Z < 1, C(Z) = Z, and for Z > 1, C(Z) = 1 (strong absorption) and C(Z) = 1/2 for weak absorption. It is observed [39], that the P and C(Z) values of the polymers are usually strongly dependent on the field strength E = V/d. The accuracy of the P measurement is determined by the accuracy of the I, P and film thickness d measurements and is estimated as ~20%. The photoelectric sensitivity (PES) S ([m2/J]) is defined as the reciprocal value of the half decay exposure time ti/2 of the initial charging potential V:
S = (Iti/2) = (Peff Pde)/[E(hv)Veeo],
(2)
where E(hv) - photon excitation energy. The accuracy of the S value measurement is estimated to be ~10% and this value was determined by the accuracy of the excitation intensity (I) measurements. Thus, the electrophotographic method makes it possible to obtain the following photoelectric characteristics of polymer samples: photoelectric sensitivity, the carrier photogeneration quantum yield (1), and from the field dependence of peff (E) it is possible to estimate carrier drift length: at a field strength of
E0, for which the change of the C(Z) dependence from C(Z) = Z to C(Z) = const is observed, the drift length is equal to the film thickness, lD = ^E0 т = d.
The experimental setup makes it possible to determine both the optical density of the sample DX under monochromatic or integral excitation, and to measure the influence of the ionic contact field on the sample. Knowing the optical density, it is possible to estimate the portion of the absorbed excitation light energy:
P = 1 - exp[- (D-Do)], (3)
where D is the film optical density and D0 -is the equivalent optical density as a result of light reflection from the front and rear sample surfaces as well as light scattering. The sign of the majority carriers can be determined via the comparison of PES values for the positive (S+) and negative (S-) corona charging of the free surface under the heterogeneous excitation by absorbed UV light: for S+ > S- , the majority carriers are holes, while for S- > S+ the majority carriers are electrons. PI films of 3 /m thickness are prepared by casting of polymer solutions in chlorinated solvents onto conducting ITO glass supports followed by drying under ambient conditions at 50-100 °C . The PIs under study possess good solubility and excellent film-forming properties.
Experimental results and discussion
Study of the photoelectric sensitivity of the films prepared with PI samples and its connection with the charge transfer complex formation
Photoelectric sensitivity for films prepared with PI samples is observed and the charge carrier photogeneration quantum yield is determined for the films of the new class of PI based on N,N,N',N'- substituted paraphenylene-diamine (electron-donor fragment D) and dianhydrides (electron-acceptor fragment A). The PI series are denoted as PI Al to PI A5 (see Scheme 1). A study of the PES spectral dependence S(X) shows that the highest sensitivity (up to 30 m2/J) is observed within the UV region (200 - 400 nm). In the visible region (400 -700 nm) there is a PES band that collapses to a long-wave edge (Fig.l).
S , m2/J
2520 1510 5-
220
1,0 0,8 -0,6 -0,4 -0,2
400
x, nm
600
800
Fig. 1 - Spectra of the photoelectric sensitivity Sx(1): (+ charging, V = 30-36 V; the number of cycles N > 10) and optical density Dx(2) for film PI A2 (thickness d = 3 ^m)
Comparison of the PES spectral dependence with the absorption spectra of the films indicates that the PES in the visible region is due to the formation of weak donor-acceptor (D-A) charge-transfer complexes (CTC) with absorption maxima in the region of 400 -600 nm [37]. The maxima and band absorption edge of the CTC are determined. The most clearly expressed CTC bands are observed for the PI A2 and PI A3 films with flat absorption maxima in the 500- 560 nm region. For the PI A1, PI A4 and PI A5 films the CTC bands are essentially weaker with flat maxima shifted to short wavelengths of 420 - 480 nm. The energy position of the long-wavelength absorption band edge of the CTC and PES is determined, which is an analogue of the band gap for semiconductors, Eg, that allows the relative affinity energy values for the acceptor fragments EA to be estimated (at the same values of ionization potential of the donor fragments, ID = 7.0 - Ph = 5.5 eV, Ph -polarization energy of holes) from the following expression:
Eg = Id - Ea - (Ph + Pe), (4)
where Pe is the polarization energy for electrons.
Table 1 - Eg and (Ea + Pe) values (in eV) for the PI A1 - PI A5 film samples; Pe = 1.5 eV
PI A1 Eg = 2.2* (Ед+Pe) = 3.3*
PI А2 1.9 3.6
PI A3 2.0 3.5
PI A4 2.6 2.9
PI A5 2.4 3.1
* Estimated average uncertainty for the Еg and (Ea+P,) values is 0.1 eV.
As shown in Table 1, the highest (EA + Ре) values are observed for the PI A2 and PI A3 acceptor fragments (3.6 and 3.5 eV, respectively) which are characterized by the lowest Eg (1.9 and 2.0 eV) and the most pronounced CTC band as well as high PES in the visible region (up to 5 - 20 m2/J). The PI A1, PI A4 and PI A5 films possess lower EA values, weaker CTC bands and lower PES (of approximately 1- 4 m2/J).
The distinct spectral peaks (in the region of 440-480 nm, 540-560 nm and 640-660 nm) are registered for PI A1, PI A3, PI A5 (absorption spectrum) and PI A2 (PES spectrum, Fig.1). In this work they are ascribed to the formation and accumulation of the cation - radicals (D+) (and perhaps anion-radicals (A-)) of polymer fragments that arise in the PI as a result of the dark- and photo-processes [38]. Some evidence of this assumption is the PES found in the red spectral region (X > 600 nm, outside the CTC band) with a weak maximum in the triphenylamine cation - radical absorption band (640-660 nm) due to its photostimulation [38].
The photogeneration quantum yield for the UV (PI A1-PI A5) and visible spectral region (PI A1) is determined. By varying the charging potential V, a nonlinear field dependence of the photogeneration quantum yield P(E) ~ En (Fig. 2) is revealed. The exponent n increases with increasing excitation wavelength A from n ~ 1.2 to n ~ 1.8 as X varies from 257 to 547 nm, indicating that photogeneration occurs via the field
D
— carriers
(5)
assisted thermo-dissociation (FATD) of ion pairs (IP), kinetically coupled with the excited states of the CTC: FATD
CTC + huCT ^ CTC* ^ [D+... A-] (holes)
E, T
^ .electron/quantum
10 '
region only V is increase; the growth of PES is very small or negligible.
10"
10 4
Fig. 2 - Field dependence of the charge carrier photogeneration quantum yield P(E) for the PI A2 (1), PI A1 (2, 4-6) and PI A3 (3) films, under excitation by monochromatic light: 257 nm (2) and 365 nm (1, 3, and 4); 436 nm (5) and 547 nm (6). Positive charging (N > 10), the field changed by the time variation of corona charging. p in electrons/quanta, E in V/cm
The highest p values in the UV region (up to 0.1 in a field E = 5.7105 V/cm) are obtained for films PI A3 and PI A2 (P = 0.02, E = 105 V/cm). Using the geminate recombination Onsager model to interpret the field dependence of P(E), it is possible to determine the ion-pair parameters: the initial yield Ф0 and initial separation r0. For PI A1 with increasing excitation wavelength (from 257 to 547 nm) the value of r0 decreases from 3.6 - 4.5 nm to 2.0 nm, and the value of Ф0 increases from 0.2 to 0.7. However, under excitation in the red spectral region (640-680 nm, outside the CTC band), the value of r0 increases to 3.0 nm, which suggests that under photostimulation of the labile cation-radical an IP1 is formed that differs from the IP in scheme (3). Comparison of the field dependences of S and p leads to the conclusion that for the PI films drift length of the generated carriers (holes) lD > d (3 |im) for E > 105 V/cm and hence it require to an estimate of дт > 310-9 cm2/V. The majority carriers in the studied PI are - holes that are under free surface excitation by strongly absorbed UV light, S + > S-.
Effect of labile cation - radical accumulation during repeated charge - discharge cycles on the photoelectric characteristics of the PI films; the observation of the photostimulated currents (PSC)
A strong dependence of the photoelectric characteristics of the samples (the potential of charging, PES in the red region, Sred) on the number of chargedischarge cycles N is found: as N varies from 1 to 10, V, the Sred values significantly increase (by approximately several multiples), Fig. 3. In the UV
V, V
30
20
Sk
0,40,2-
Fig. 3 - Photoelectric sensitivity in the UV (X = 365 nm) Sx (1, 2) and red region (X > 600 nm) Sred (5), as well as the maximal charge potential V (3, 4, 6) versus the number of charge-discharge cycles N for films PI A5 (2, 3) and PI A2 (1, 4-6); + charging
Usually the absence of surface charging is associated with the dark injection of carriers from the electrodes into the bulk of the film (holes in the case of a positively charged free surface), leading to a sharp increase in dark conductivity. The increase in the V value at the positive charge indicates blocking of the dark hole injection from the free surface when N > 3 - 4. The most probable reason for this effect is the appearance of a positively charged layer of labile cation - radicals in the bulk film near the electrode (electrode polarization). The latter are holes (h+) trapped by deep centers:
dark injection capture
+ Electrode +D ^ h+ (mobile hole) ^ cation - radical (6)
The accumulation and stabilization of the cation - radicals near the electrode leads not only to stoppage of the dark hole injection, but also to a reduction of the dark conductivity, an increase of V and the observation of photostimulated currents (PSC), which manifest themselves as a growth of PES in the red spectral range Sred, outside the absorption band of the CTC (absorption of cation - radicals), Fig. 3:
FATD
Cation - radical + hu ers (PSC)
IPi
— curri-
(7)
The growth of Sred with increasing N (Fig. 3) is partly due to its field dependence caused by the growth of the field strength according to E = V/d. Therefore, it was specifically confirmed that, in the red region the pure field dependence due to the ion pair IP1 FATD (7) has the following form: p ~ En (n = 1.35-1.60) and, by (2), S ~ En-1, i.e. it is weakly dependent on E, so that the growth of Sred(N) is partly due to the accumulation of
S, m'/J
V
->
the cation - radicals. Under conditions when V does not depend on N (for N > 10), the V value changes with variation of the corona discharge time. In some cases (Fig. 3) the growth of Sred was observed at constant V which indicates the effect of the cation - radical accumulation.
It should be noted that photostimulated currents are observed not only in long wavelength spectral region but also in shorter wavelength region where radicals absorb. This is evidenced by the appearance of corresponding maxima in both PES and absorption spectra (Fig. 1) in blue and yellow-green spectral ranges which are ascribed above to cation - radicals. For these maxima the positive electric field effect on optical density (electrochromic effect [39]) is observed due to cation -radical formation in photoprocess (5) but not to Stark effect on CTC absorption.
The observation of PSC indicates a long cation
- radical lifetime, t > (ctI)-1 (ct - absorption cross section). If ct = 10-17 - 10-16 cm2 [40], I = 1015 cm-2 s-1, an estimate of t > 10 s is obtained. It should be noted that the results of this study indicate that there is a range of cation - radical states from labile (t = 1-10 s) to a fully stable ones (t > 103 s) which participation in the process of photostimulated generation differ significantly. Similar cation - radicals and related photo-stimulated currents for the PI based on substituted triphenylamines resulting from irreversible photochemical transformation of free-radical type with the halogen hydrocarbons are observed in [38].
Paramagnetism of polyimides in solid state and solution
For PI samples with varying the PI acceptor unit while retaining the same donor, stored at the air for long time interval (several months) the paramagnetism at room temperature is observed. A signal of electron paramagnetic resonance (ESR) with the apparent g-factor value g = 2.0022 is observed both in solid state (powder sample) and in solution (solvent: tetrachloroethane (TCE) and chloroform mixture), Fig. 4. The identity of the ESR signal for the solid state PIs with various acceptor units suggests unpaired electron localization on the donor polymer chain unit, i.e. cation
- radical formation. The relative integral ESR signal intensities (Int), registered for various solid state PI samples are given in Table 2.
Table 2 - ESR signal integral intensities at T=293K for PI solid state samples. Int values are standardized with sample weights; correlation of PES in red spectral region Sred for fresh prepared PI films with ESR signal of air stored PI samples (concentration of stable radicals), V=10V, N=1; correlation of PES in near UV region S for fresh prepared PI films with ESR signal of air stored PI samples (concentration of stable radicals), V=10 V, N=Nmax
It should be noted that in spite of signal intensity depending on acceptor unit structure its correlation with electron affinity value EA (Table 1) as well as PI charge transfer (CT) interaction is not observed. Indeed, the values for PIs A2 and A5 are the highest. CT interaction differs significantly: it is much stronger for A2 than for A5 with a very weak CT band. Therefore dark stable radical formation is not connected directly with CT interaction between PI units likely because of its insignificant intensity even for A2 so that the additional interaction (for example, with O2) is required.
The Int of PI ESR signals registered in solution are comparable with that of PI solid state (powder) samples if one takes into account sample weight (Fig. 4). Computer simulation of the A2 and A5 signals observed in solution using the SimFonia (Bruker) program shows that these signals are described well within the framework of the radical model with 4 equivalent protons (Fig. 4). In the case of A2 the simulation of experimental ESR spectra is also possible considering the interaction with two equivalent protons and a nitrogen atom 14N but the difference between experimental and simulated spectra for A5 is appreciably higher in this case than in the first model.
The effect of light excitation on the ESR signal intensity in diluted solution is found to depend on the excitation spectral composition. Under solution excitation by near UV light the ESR signal intensity increases (without deformation) after initial minutes of excitation (Fig. 5). Thus, it can be concluded that the generation of PI radicals occurs under near UV excitation. Under solvent excitation by long wavelength light (X > 580-640 nm, OS-14, KS-14 glass filters) the intensity of the dark signal does not change significantly.
The revealed photoelectric sensitivity (PES) of freshly prepared PI films in the red spectral region (X > 600 nm, out of CT band) Sred and near UV region S correlates well with the ESR signal intensity, Int (dark stable radical concentration in the sample volume), when the initial surface charge potential V under single corona charging is considered (Table 2).
However it is found that such correlation with stable radical concentration is absent both in red and near UV region for the films prepared from the air stored PI samples (oxidized samples) for which ESR signal is observed. The absence of such correlation indicates that stable radicals do not display photostimulated currents which are due mainly to labile radicals. The correlation of PES with stable radical concentration for freshly prepared PI films can be explained by the fact that the formation of both labile and stable radicals is determined mainly by the oxidation ability of PI donor fragments depending on PI acceptor fragment structure.
Such difference in functionalities of labile and stable radicals can be explained by the variable oxidation degree of donor fragments. Labile radicals, originating under corona charging or UV excitation which are assumed above to be trapped holes and so can be considered as incompletely oxidized donors (partially oxidized donors) [D+e-], in which electrons and cations are incompletely separated, their energetic level being localized in forbidden gap. These states are not assigned to the ion-radical pairs because their geminate recombi-
Polyimides PI A1 PI A2 PI A3 PI A4 PI A5
Int 19 338 49 59 341
Sred, a.u. 0.03 0.40 0.01 <0.015 0.30
S, a.u. 0.15 10.0 1.4 1.5 4.0
nation is hindered if electron being localized on some acceptor. So these states possess relatively high lifetime and radicals become labile. It is for these states the photostimulation and charge carrier photogeneration is possible that occurs under complete photooxidation of donor and geminate ion-radical pair formation. On the other hand stable radicals which appear in air stored PI samples are likely to be completely oxidized donor (with completely separated components) and stabilized by stable anion [D+ A- (O2)]. So they are not able to further oxidation and photostimulation.
Fig. 4 - Overhead - ESR signals observed for dry PI solid state samples (on account of the 1 mg sample). In the center - ESR signals of PI solutions in tetrachloroethane-chloroform mixture with a PI concentration of 0.1% wt. Bottom - spectra simulated with the use of the SimFonia (Bruker) program. Simulation parameters: aH = 5.5 G for the A2 signal and aH = 5.0 G for the A5 one, with line widths of 4.6 G and 5.3 G, respectively
Fig. 5 - Effect of near-UV excitation on the A2 ESR signal (PI solution in tetrachloroethane-chloroform mixture with concentration 0.6% wt)
Conclusion
Novel polyimides based on N,N,N',N'-substituted paraphenylenediamine and dianhydrides were prepared via polycondensation reaction. The polymers exhibited excellent solubility in common organic
solvents, and had high thermal stability, for example, Tdl0 = 726K in nitrogen atmosphere and Tg = 413K.
Photoelectric sensitivity of the freshly prepared PI films (3 |j.m thickness) in the UV, and visible spectral regions was observed, due to charge transfer interactions between the donor and the acceptor fragments of the PI chains (formation of the CTC). Study of the field dependence of the photogeneration quantum yield provided evidence that the photogeneration mechanism is the field assisted thermodissociation of radical ion pairs kinetically associated with the excited CTC.
The second important mechanism of photogeneration is photostimulation of long-lived labile cation - radicals of the donor PI fragments, representing hole (majority carriers) captured by deep centers (photostimulated currents). The accumulation of cation - radicals in the dark and photoprocesses leads to the dependence of photoelectric characteristics on the number of charge-discharge cycles of the sample.
The paramagnetism of the PIs investigated in dark conditions in the air for the solid state samples and their solutions was found by the ESR method. The effect of UV excitation on the ESR signal intensity of PI solutions was observed. For the solid state samples the correlation of this result with the photoelectric sensitivity of the fresh prepared PI films was established.
Acknowledgements
This work was finally supported by Ministry of Science and Technology, Grant No. 13-02-12409 OFI-M2 and Grant No. 15-59-32401 RT-omi and Grant of the Government of the Russian Federation for the Support of Scientific Investigations under the Supervision of Leading Scientists, Contract No. 14.B25.31.0019).
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© A. A. Olkhov - Ph.D., Senior Researcher, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia, aolkhov72@yandex.ru; D.-J. Liaw - Prof., Department of Chemical Engineering, National Taiwan University of Science and Technology; Y.-C. Huang - Prof., Department of Chemical Engineering, National Taiwan University of Science and Technology; C.-H. Chang - Prof., Department of Chemical Engineering, National Taiwan University of Science and Technology; B. M. Rumyantsev - Ph. D, Senior Researcher Emanuel Institute of Biochemical Physics, RAS; T. A. Lozinova - Ph. D, Senior Researcher Emanuel Institute of Biochemical Physics, RAS; V. P. Zubov - Dr.sc.(chemistry), Prof., Moscow Lomonosov State University of Fine Chemical Technologies; V. N. Bagratashvili - Dr.sc. (physics - mathematic), Prof. Institute on Laser and Information Technologies, RAS; G. E. Zaikov - Doctor of Chemistry, Full Professor of Plastics Technology Department, KNRTU, Kazan, Russia,
ov_stoyanov@mail.ru; A. A. Ischenko - Dr.sc.(chemistry), Prof., Moscow Lomonosov State University of Fine Chemical Technology.
© А. А. Ольхов - канд. техн. наук, ст. науч. сотр., Институт химической физики им. Н.Н. Семенова РАН, Москва, Россия, aolkhov72@yandex.ru; D.-J. Liaw - Prof., Department of Chemical Engineering, National Taiwan University of Science and Technology; Y.-C. Huang - Prof., Department of Chemical Engineering, National Taiwan University of Science and Technology; C.-H. Chang - Prof., Department of Chemical Engineering, National Taiwan University of Science and Technology; Б. М. Румянцев - к.х.н., старший научный сотрудник Института биохимической физики им. Н.М. Эммануэля РАН; Т. А. Лозинова - к.х.н., старший научный сотрудник Института биохимической физики им. Н.М. Эммануэля РАН; В. П. Зубов - д.х.н., проф. Московского государственного университета тонких химических технологий им. М.В. Ломоносова; В. Н. Баграташвили - д-р физ.-мат. наук, проф. зав. лаб. Института проблем лазерных технологий РАН; Г. Е. Заиков - д-р хим. наук, проф. каф. технологии пластических масс, КНИТУ, Казань, Россия, ov_stoyanov@mail.ru; А. А. Ищенко - д.х.н., проф., зав. кафедрой аналитической химии Московского государственного университета тонких химических технологий им. М.В. Ломоносова.
