White Light and Heat Sensitivity in a Pyridine-Based Polymer Blend

Article pubs.acs.org/JPCC

White Light and Heat Sensitivity in a Pyridine-Based Polymer Blend Evgenia Vaganova,*,† Ellen Wachtel,‡ Alex Goldberg,§,∥ and Shlomo Yitzchaik*,† †

Institute of Chemistry and the Farkas Center for Light-Induced Processes, The Hebrew University of Jerusalem, 91904, Jerusalem, Israel ‡ Chemical Research Support Unit, Weizmann Institute of Science, 76100, Rehovot, Israel § Accelrys Inc., San Diego, California 92121, United States S Supporting Information *

ABSTRACT: Sensing from the ultraviolet to the infrared is important for a number of scientific and industrial applications. Poly(4-vinyl pyridine) swollen in liquid pyridine functions as a photoconductive gel sensitive to irradiation in the ultraviolet. By blending poly(4-vinyl pyridine) with poly(4-vinyl pyridineco-butyl methacrylate), we have now succeeded in expanding the range of wavelength sensitivity of the gel to cover the whole visible spectrum. Furthermore, addition of a small amount of 4-hydroxypyridine to the polymer blend results in unusually high thermal sensitivity (TCR = (0.1−0.16)/1 °C). Spectroscopic measurements show that the combined processes of proton transfer and electron transfer, occurring in a DC electric field, contribute to the gel properties. The optimized system has potential application as a simple and inexpensive active layer in organic photovoltaic cells as well as a thermal sensor. find that, following extended application of a DC bias, the pyridine-based gel blend is both photoconductive and, with the addition of a small amount of 4-hydroxypyridine, highly thermosensitive. We used a number of spectroscopic techniques to probe the nature of the structural changes within the gel blend which occur in the presence of the electric field and which may account, in part, for its marked radiation sensitivity.

1. INTRODUCTION The development of optoelectronic devices using organic and polymeric molecules is one of the most significant challenges of 21st century electronics.1−4 Such devices would be inexpensive, easy to handle, and, at the same time, would permit miniaturization of components down to the nanometer scale. The design concepts of molecules and/or molecular aggregates, which are capable of modifying their electrical properties in response to irradiation, have been reviewed recently.5−12 The wavelength range of sensitivity of these devices is currently the subject of extensive study.13 On the other hand, thermosensitive materials comprised of organic compounds with metal fillers have also been introduced.14 Polymeric gels can function as electro- or thermoactuators, usually by contraction or expansion of the polymer chain due to charge redistribution.15,16 Recently, an effective photoconductive polymeric gel, based on the pyridine molecule, that is, poly(4-vinyl pyridine) (P(4VP)) in liquid pyridine (Py), molar ratio 1:1, was described.17 The P(4VP)/Py gel18 is basic with a pH of 9.1. Photoconductivity was observed in the gel under irradiation at 385 nm at the proton transfer center N+H, which formed on the polymer side chain.18 The conductivity was explained as being due to the proton mobility in the excited state and to the conjugated structure of the protonated species.18,19 To expand the spectral range of photosensitivity, we proposed introducing into the P(4VP)/Py gel different types of electron transfer centers in addition to the proton transfer centers. Here, we demonstrate this principle with the example of a polymer gel containing two polymers: poly(4-vinyl pyridine) and poly(4vinyl pyridine-co-butyl methacrylate) swollen in pyridine. We © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(4-vinyl pyridine) (P(4VP)) with a molecular weight of 50000 (Polyscience, Inc.) was carefully dried in a vacuum oven (10−3 Torr) at 25 °C during 3−4 weeks prior to use. The pyridine (Sigma) was anhydrous. Poly(4vinylpyridine-co-butyl methacrylate) (P(4VP-BMA), Aldrich), with a molecular weight of 337000 and 3% butyl methacrylate side chains, was used as received. For some samples, a small amount of 4-hydroxypyridine (Sigma) was added to the polymer blend. 2.2. Preparation of the Polymer Blend. The polymer blend was prepared by mixing P(4VP-BMA) with P(4VP) at weight ratio 20:80. The mixture of polymers (P(4VP)/P(4VPBMA)) was dissolved in an equal weight of liquid pyridine. For some samples, a small amount of 4-hydroxypyridine was added. The sample was stored in the dark for a period of weeks. All procedures involving the initial viscous solution and gel preparation were accomplished in a glovebox under a nitrogen Received: August 1, 2012 Revised: October 23, 2012 Published: October 23, 2012 25028

dx.doi.org/10.1021/jp3076063 | J. Phys. Chem. C 2012, 116, 25028−25033

The Journal of Physical Chemistry C

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described in the Experimental Section. The internal DC electric field, applied at least 0.5 h prior to the beginning of the measurements, exceeded 1 kV/cm. The resistance of the gel under bias increases with time: with glass/ITO electrodes the initial resistance of a 300 μm thick film is approximately 5−6 Mohm, whereas in the saturated state it reaches 8−10 Mohm. With plastic/ITO electrodes the resistance is consistently lower. The photoconductive response of the gel to the light of an incandescent lamp is shown in Figure 1. During the

atmosphere. The gelation of the viscous solution occurs spontaneously and its progress was evaluated visually. 2.3. Electrical Conductivity Measurement. For the electrical conductivity measurements, a 300 μm thick film of polymer gel, prepared as described above, was sandwiched between glass/ITO (SPI Inc., 30−60 ohm) or ITO-covered plastic sheets (produced in the HUJI Nanocenter). The area of the gel was 1 cm2. Photoinduced or thermally induced conductivity changes were measured with a multimeter (197 autoranging microvolt DVM, Keithley), which was also used as a source of DC bias. Bias was applied for at least 0.5 h prior to measurement. For the photoconductivity measurements, white light of different intensities was produced with an incandescent hand flash (KT-2.2 V, 0.47A; intensity distribution at the sample: 400 nm, 3 mW; 500 nm, 9 mW; 600 nm, 7 mW; 700 nm, 10 mW) or a ceiling fluorescent lamp (Lithonia Lighting, 4 ft tube guard T8; intensity distribution at the sample: 400 nm, 74 μW; 500 nm, 45 μW; 600 nm, 46 μW; 700 nm, 36 μW). The intensity distributions were measured using an optical power and energy meter, PM 100D (THORLAB). Chopping was achieved with a manual shutter. The temperaturecontrolled cells that were used included S.C.T.-cell (Shimadzu), TCC-controller (Shimadzu), and tempcontrol −37 (Zeiss) and depended on the type of measurement being made. For all measurements, the room temperature was stabilized at 22.5 ± 0.1 °C. 2.4. Spectroscopy. Absorption spectra were recorded with a UV-5301PC spectrophotometer (Shimadzu). Excitation and photoluminescence spectra were measured on a Shimadzu RF5301PC spectrofluorimeter; collection was at 90° to the direction of the light source. The resolution of the emission and excitation spectra was 1 nm; the resolution of the absorption spectra was 2 nm. Samples of the gel were placed between indium−tin oxide (ITO) covered glass slides (SPI Inc.; Kintec Company) or between quartz slides (Chemglass, U.S.A.). The applied bias was 3.7 V. The temperature of observation was stabilized at 23.5 ± 0.1 °C. FTIR spectra of free-standing films were measured using the Tensor ∑7 unit (Pike technologies). Each measurement was repeated three times in different areas of the film. 2.5. Modeling. DFT modeling of the pyridine and butyl methacrylate side chain moieties were performed with the density functional theory (DFT) code DMol3 that uses the numerical basis sets, as implemented in Materials Studio 5.5, provided by Accelrys.19,20 A double numerical polarized (DNP) basis set was employed that includes all occupied atomic orbitals, plus a second set of valence atomic orbitals, polarized D-valence orbitals, and a p-orbital on the hydrogen atoms. The Perdew−Burke−Ernzerhof (PBE) gradient corrected functional, that depends on the electron density and its derivative, was applied.20 This functional, based on the Perdew model to correct the local-density approximation,21 provides a correction that leaves only 1% error in the exchange energy. It has a strong physical background, reliable numerical performance and is frequently used in DFT calculations.22,23 The spin unrestricted approach was applied with all electrons being considered explicitly.

Figure 1. Resistance changes of a 300 μm thick film of the P(4VP)/ P(4VP-BMA)/Py gel (area 1 cm2) placed between two glass/ITO electrodes. The sample was placed under applied bias 3.7 V for 2 h prior to measurement. The light source was a hand flash lamp (2.2 V, 0.47A) producing two different intensities: 1.2 and 2.0 mW/cm2 in the plane of the sample. The down pointing arrows indicate light on and the up pointing arrows indicate light off. The temperature in the room of observation was stabilized at 22.5 ± 0.1 °C. ΔR/R for 1.2 and 2.0 mW/cm2 was 1.8 and 2.8%, respectively.

measurement, the temperature recorded by the thermocouple was stable at 22.5 ± 0.5 °C. The radiation-induced resistance changes were fully reversible. Changing the weight ratio of the blended polymers P(4VP-BMA) and P(4VP) from 20:80 results in both reduced response and longer decay time for the resistance changes. For this optimized polymer composition, the response reached a sensitivity (ΔR/R) of 1.8 and 2.8% for 1.2 and 2.0 mW/cm2, respectively. In contrast to the P(4VP)/ Py gel, which is primarily sensitive to UV light at 385 ± 10 nm, and the P(4VP-BMA)/Py gel, which is preferentially sensitive to far red light at 700 ± 10 nm, Figure 1 demonstrates that the gel formed by blending the two polymers is sensitive to the whole range of visible light. Similar results were obtained when the light source was fluorescent, showing independence of the particular white light spectrum (see SI, movie). To try to understand this change in wavelength sensitivity, we have measured the absorption and excitation−emission spectra of the gel. 3.2. UV−Visible Absorption and Fluorescence Spectroscopy of the P(4VP)/P(4VP-BMA)/Py Gel. A comparison of the UV−visible absorption spectra of the P(4VP)/Py gel17 and the gel formed from the polymer blend, prepared as described in the Experimental Section, is presented in Figure 2. The tail of the absorption spectrum of the P(4VP)/Py gel reaches ∼550 nm, while that of the polymer blend gel extends throughout the whole visible range until approximately 870 nm. Absorption at ∼700 nm is well-known for systems containing oxides of nitrogen,24 while absorption between 600 and 700 nm

3. RESULTS 3.1. Photoelectrical Properties of the (P4VP)/P(4VPBMA)/Py Gel Blend. A 300 μm thick film of the P(4VP)/ P(4VP-BMA)/Py gel (area 1 cm2) was placed between two glass/ITO electrodes and irradiated with white light as 25029



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Significant changes were observed in both the emission and the excitation spectra of the gel blend when measurements were conducted under applied DC bias. A single high intensity, narrow excitation peak at 405 nm appeared, with emission maximum at 460 nm. In order to determine if these electric field induced changes are structural in origin, FTIR spectroscopy was used. 3.3. Effect of a DC Bias on the FTIR Spectra of the (P4VP)/P(4VP-BMA)/Py Gel. We measured the FTIR spectrum of the gel before and after applying a DC bias of ∼1 kV/cm for 2 h. The most pronounced changes occur in the wavelength region sensitive to hydrogen-bonded hydroxyl and NH groups (3600−3000 cm−1; Figure 4a) and in the region of the fundamental vibrations of the pyridine ring and the CO carbonyl stretching vibration (Figure 4b). The difference spectra show an increase in intensity of the broad peaks at 3367 cm−1 (N−H stretching vibration) and 3262 cm−1 (O−H stretch).31 Marked changes also occurred in the region of the CO stretching vibration due to the DC bias: the intensity of absorption at 1720 cm−1 is significantly reduced and a high intensity, broad absorption at 1680 cm−1 appeared instead (Figure 4b, inset). Absorption at 1680 cm−1 is known to be due to vibration of the ionized form of CO.32 The sharp peaks due to the distortional vibrations of the pyridine ring at ∼1440 cm−1 (semicircle stretch) and 1580 cm−1 (quadrant ring stretch) are reduced in intensity after application of the field. 3.4. DFT Modeling. Both the UV−visible absorption spectra (Figure 2) and the difference FTIR spectra (Figure 4) suggest the increased presence, due to DC bias, of a hydrogen-bonded complex: protonated pyridine side chain/ ionized carbonyl oxygen, RxNH+−RyO−. DFT modeling was performed to investigate the stereochemistry and energetics of such a complex. The energy-optimized geometry of a hydrogen-bonded heterodimer between a pyridine side chain and a butyl methacrylate side chain is shown in Figure 5. Significant distances are shown in green. The C−C double bond between the polymer main chain and the pyridine ring is relatively short, 1.386 Å, and is evidence for conjugation. The nitrogen atom is protonated, with a N−H distance of 1.538 Å. Continuing to the butyl methacrylate side chain, the distance from the proton to the oxygen atom is 1.066 Å, which suggests that this interaction is more similar to covalent bonding than to hydrogen bonding. There are indications from DFT modeling that, in principle, very strong DC fields are able to shift the position of the proton in the N−H interacting group. 3.5. Thermoelectrical Properties of the P(4VP)/P(4VPBMA)/Py Gel Blend. For some polymer blend samples, a small amount of 4-hydroxypyridine was added. A 300 μm thick layer of the gel blend was placed between two plastic/ITO electrodes and kept under bias of 3.7 V DC (internal field > 1 kV/cm) during at least 0.5 h. The area of the gel was 1 cm2. Resistance measurements show that the gel behaves as a thermoresponsive material. The response of the gel to a varying heat source is presented in Figure 6. The response is completely reversible and the sensitivity (or temperature coefficient of resistance, TCR) is ΔR/R/°C ∼ 6−16%/1 °C. The decay time of the response is 2−5 s. In the online Supporting Information, a movie showing the experimental measurement of the light and thermal sensitivity is included.

Figure 2. Normalized UV−visible absorption spectra of a 1:1 molar ratio P(4VP)/Py gel (dashed line); the P(4VP)/P(4VP-BMA)/Py gel, prepared as described above (dotted line), and the same film after being kept under 3.7 V DC bias during 2 h (bold line). The 300 μm thick gel layers were sandwiched between two ITO-coated glass slides.

is known for charge transfer25 via hydrogen bonds. When DC bias is applied to the gel blend for 1 h, a new broad absorption band centered at ∼1000 nm appears. We suggest that this band may be assigned to the ionized hydrogen-bonded complex: protonated pyridine side chain/carbonyl oxygen, RxNH+−RyO−.26 However, we cannot exclude the possibility that the absorption is due to oxidation of pyridine side chains with the concomitant formation of an NO group on the pyridine nitrogen.27−30 Emission and excitation spectra of the polymer blend gel are presented in Figure 3. They have weak similarity to the excitation and emission spectra of a P(4VP)/Py gel, which displays emission at 440 nm by excitation at 385 nm; emission at 504 nm by excitation at 440 nm; and emission at 525 nm by excitation at 460 nm.17 In contrast to the P(4VP)/Py gel, the polymer blend gel displays two maxima in the excitation spectra at 405 and 440 nm, with a single emission peak at 460 nm.

Figure 3. Excitation (left, solid plots) and emission (right, dashed plots) spectra of the P(4VP)/P(4VP-BMA)/Py gel (300 μm) on a glass/ITO substrate prior to application of the DC bias (plots without circles) and in the presence of the 3.7 V DC bias (plots with circles). The fixed emission wavelength employed to measure the excitation spectrum was 454 nm before DC bias, and 465 nm in the presence of DC bias. The fixed excitation wavelength employed to measure the emission spectrum was 442 nm before DC bias and 403 nm in the presence of DC bias.

4. DISCUSSION The electrical conductivity of the P(4VP)/P(4VP-BMA)/Py gel blend described here is sensitive to both irradiation by white 25030



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Figure 4. FTIR spectra of a free-standing thin film (10 μm) of the P(4VP)/P(4VP-BMA)/Py gel before (dashed curve) and after application of DC bias (2 h, internal field about 1 kV/cm; thin solid curve) in the frequency range characteristic of hydrogen-bonded hydroxyl and NH groups (LHS, 3700−2600 cm−1), in the frequency range of the fundamental vibrations of the pyridine ring (RHS, 1620−1400 cm −1), and in the range of the C O stretching vibration (1800−1600 cm−1 RHS inset, enlarged). The thick solid line is the difference spectrum before and after application of the DC field.

Figure 6. Change of resistance upon heating of a 200 μm thick layer of the P4VP/P(4VP-BMA) gel, with a small amount of hydroxypyridine added, placed between two ITO-coated plastic substrates. Bias voltage of 3.7 V was applied, beginning 0.5hr prior to resistance measurements. A T-type thermocouple with accuracy 0.1 °C was placed on the electrode surface and used for temperature control. The first two curves were produced by the approach of a student’s hand at two different distances from the gel sample, 20 and 5 cm. The third curve was produced by the student breathing on the sample. In each case, the heating lasted for ∼5 s. The temperature in the room of observation was stabilized at 22.5 ± 0.1 °C.

individual polymers are very different: P(4VP)/Py is basic with photoinduced response in the ultraviolet, whereas P(4VPBMA)/Py, which is acidic (pH 5.7), responds to irradiation with wavelength between 700 and 800 nm. The photosensitivity of the gel blend is only observed following extended application of a DC bias (internal electric field ∼ 1 kV/cm). As described above, the resistance of the gel increases with time under bias, the absolute value at saturation depending on whether glass or plastic is used for the electrode material. We used spectroscopic techniques to probe the nature of the

Figure 5. DFT-optimized, hydrogen-bonded dimer of the side chains of 4-vinyl pyridine and butyl methacrylate; significant distances are marked with green color, oxygens are marked with red color, and the nitrogen is marked with blue color.

light and, with the addition of 4-hydroxypyridine, to thermal perturbation . The light sensitivities of gels prepared from the 25031



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groups; (3) increased hydrogen bonding and a more rigid molecular organization. The optimized system demonstrates, for the first time, that a polymer gel may have potential application as the active layer in light or thermal sensing devices.

structural changes within the gel blend that occur in the presence of the electric field and which may account, in part, for its marked radiation sensitivity. The measured wavelength cutoff of absorption in the UV− visible wavelength region shows that the bandgap of the polymer blend gel is smaller than that of P(4VP)/Py alone. However, after maintaining the gel for 2 h under bias, the additional strong absorption that is observed at ∼1000 nm is evidence that a structural rearrangement has taken place. The excitation spectral narrowing and intensity increase demonstrate the appearance of a strong electric dipole with a more rigid molecular organization.33 The larger value of the Stokes shift, −0.432 eV, under applied DC bias instead of 0.072 eV in the absence of the field, argues for an excited state reaction,34 which requires increased energy. At the level of interacting molecular groups, we observe in the infrared that the CO of the butyl methacrylate side chain shows a population shift from neutral to ionic species following application of DC bias. The ionized state of the oxygen atom in the CO group is visible through changes in the characteristic stretching vibration of “free” carbonyl,32 originally at 1722 cm−1 . In addition, the red-shift of the carbonyl stretching vibration is evidence of hydrogen bonding with adjacent groups.34 The broad and intense vibration centered at ∼1663 cm−1 can be assigned to ionized carbonyl groups acting with different strength proton donors.34,35 With respect to the pyridine side chains, the increase in the broad, asymmetric absorption band at 3400 cm−1 (width 259 cm−1) indicates increased formation of pyridinium ions31,34 on the polymer side chains. A decrease in the vibrational intensity of the distortion modes of the pyridine ring is also observed. Based on the spectroscopic data and DFT modeling presented here, a picture of the polymer gel blend emerges. The effect of the electric field on the preformed, hydrogen bonded P(4VP)/P(4VP-BMA)/Py gel results in the formation of additional pyridinium groups; additional hydrogen-bonded and ionic interactions between pyridine side chains and between pyridine and butyl methacrylate side chains. The electric field contributes to structural reorganization and macroscopic ordering, while at the same time increasing the gel electrical resistance to a saturation value. Further work will be necessary to determine how these structural changes contribute to the observed light and heat sensitive electrical conductivity. Nevertheless, it appears that the magnitude of the observed thermal response (TCR ∼ (0.1−0.16)/1 °C) is significantly larger than those reported in the literature for either polymer film sensors or Cermet sensors available commercially.36

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ASSOCIATED CONTENT

S Supporting Information *

A movie demonstrating heat and light sensitivity. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Present Address ∥

Schrödinger Inc., 8910 University Center Lane, Suite 270, San Diego, CA 92122. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the internal fund “Baby Seeds” 2012, HUJI. E.V. gratefully acknowledges financial support from the Israel Ministry for Immigrant Absorption. E.V. thanks students Alina Gankin, Evgenyi Mervinetsky, and Avi Keninsberg for their help with sample preparation.

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REFERENCES

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5. CONCLUSIONS Soft polymer gels constitute a particularly flexible state of matter in which the tuning of molecular electronic and geometrical properties is markedly enhanced by collective effects. The P(4VP)/P(4VP-BMA)/Py polymer blend gel described here is a dielectric material which, following exposure to prolonged DC bias, displays changes in electrical conductivity in response to white light and, with the addition of a small amount of 4-hydroxypyridine, also to low level thermal perturbation. Spectroscopic data show that this behavior derives in part from a combination of the following field-induced processes: (1) appearance of ionic species with associated electron transfer; (2) increased proton transfer to the nitrogen atom of the pyridine ring, producing pyridinium 25032



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White Light and Heat Sensitivity in a Pyridine-Based Polymer Blend

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