J-Like Liquid-Crystalline and Crystalline States of Polyaniline

Article pubs.acs.org/JPCB

J‑Like Liquid-Crystalline and Crystalline States of Polyaniline Revealed by Thin, Highly Crystalline, and Strongly Oriented Films Natalia Gospodinova,*,† Elena Tomšík,† and Olga Omelchenko†,‡ †

Institute of Macromolecular Chemistry AS CR, 162 06 Prague 6, Czech Republic A. N. Frumkin Institute of Physical Chemistry and Electrochemistry RAS, 199071 Moscow, Russian Federation

‡

ABSTRACT: Liquid-crystalline state of dyes can be easy distinguished from the crystalline one by the appearance of characteristic long-wavelength optical absorption, the so-called J-band. Similarly, long-wavelength absorption of polyaniline is assumed to be the signature of its J-like liquid-crystalline state. When water evaporates from polyaniline adsorbed on a glass support during polymerization, the long-wavelength maximum at about 800 nm disappears, and one at 570 nm appears, characteristic for highly crystalline and strongly oriented thin film. Reversible red shift of long-wavelength absorption upon water adsorption is a significant feature of these films. By analogy with dyes, it is attributed to water-promoted superficial formation of J-like mesophase. Such surprising properties of wet films as propagation of chemical reduction and enhanced exciton transport, reported by us recently, can also be considered as a signature of the J-like liquid-crystalline state of polyaniline.

1. INTRODUCTION Self-assembly of dye molecules into the nematic liquidcrystalline phase (J-aggregates) was discovered in the 1930s1−4 but only recently has become one of the promising approaches to organize organic semiconductors.5 J-aggregates (fibril-like structures with a length up to a few hundred nanometers) are formed either upon concentrating solutions or adsorption at the air/liquid (solid/liquid) interface. Mesophase formation is accompanied by the appearance of a long-wavelength absorption (called the J-band), which is absent in the spectrum of molecularly dispersed dyes. The mechanism of formation of J-aggregates, their structure, as well as the origin of the J-band is far from being elucidated.5 Nevertheless, the crucial role of water-mediated hydrogen bonds for self-assembling of dyes has already been revealed by Jelley and Sheppard, who introduced the idea of “hydrated mesophases” for these structures.1,4 According to Sheppard, appearance of the J band, which results from the self-assembling of dyes, cannot be explained by π−π interactions in the direction perpendicular to the planes of benzene nuclei of dye molecules but by the intercalation of water molecules between hydrophilic substituents, allowing maintenance of the union of the conjugated molecules. The decisive influence of watermediated hydrogen bonding on the self-assembly and optical properties of J-aggregates of porphyrin-based dyes has been recently shown by McHale6,7 and Villari.8 It is very important to underline that the J-band is absent not only in the spectrum of molecularly dispersed dyes but also in that of dry crystalline dyes.4 However, the reversible appearance of this characteristic absorption is observed upon humidification of the dye crystals. This phenomenon has been explained by the superficial mesophase formation.4 © XXXX American Chemical Society

Thus, the advantage of J-aggregates in comparison to the other kinds of liquid crystals is the possibility of their easy detection by the appearance of the J-band. Polyaniline (PANI) possesses hydrophilic substituents (amino and imino groups) in π-conjugated moieties similarly to molecules able to self-assemble into J-aggregates. We have shown earlier that PANI can be obtained as thin films constituted by the highly crystalline fibrils oriented perpendicularly to the film surface.9,10 Moreover, the formation of films with unprecedented ordering (more than 70% of a crystalline phase and a strong chain orientation) takes place during water evaporation from PANI adsorbed on the solid support during synthesis. The water intercalation between the chains has been shown to be crucial for their fibril-like gathering. As it has been highlighted, the efficient chain hydration is promoted by the highly hydrated anions, whose presence in water is required both during synthesis and drying. It can be supposed that achieving such an organized system is possible only in the case of short polyaniline chains. Indeed, our recent study exploiting small-angle X-ray scattering shows that the diameter of PANI fibrils issued from the synthesis is about 4 nm, which corresponds to the maximal length of aniline octamer).11 Here, we show that the transformation of PANI adsorbed on a glass support into highly crystalline and strongly oriented film is accompanied by the disappearance of the absorption maximum at about 800 nm and the appearance of one at 570 nm. Moreover, reversible red shift of the long-wavelength Received: May 26, 2014 Revised: June 26, 2014

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dx.doi.org/10.1021/jp505150j | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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optical absorption is situated between the initial and final positions when equilibrium is not achieved.

absorption of PANI films to about 800 nm upon water adsorption is revealed. We explain this phenomenon by the superficial formation of J-like liquid-crystalline PANI. This idea is in agreement with the very intriguing behavior of the wet, thin, highly crystalline, and strongly oriented PANI films observed by us previously.10,12 Unprecedented propagation of the chemical reduction has been observed: the area of reduced (discolored) PANI exceeds more than 1 order of magnitude of the area of a direct contact with Fe as a reducer. Simple deposition of a water drop at the surface of a PANI film sandwiched between indium tin oxide (ITO) and Al electrodes has been found to result in 3-fold increase of the open circuit voltage (Voc).

2. EXPERIMENTAL SECTION Aniline (Acros Organics), ammonium peroxydisulfate (APS, Carlo ErbaReagenti), and formic and hydrochloric acids (Sigma-Aldrich), all of reagent quality, were used as purchased. Thin, highly ordered PANI films were obtained as described previously.9,10 A glass support was placed horizontally on the surface of the reaction medium, which hinders the deposition of macroscopic aggregates. The thickness of PANI films was determined by atomic force microscopy in tapping mode with a Digital Instruments MultiModeTM instrument coupled to a NanoScope IV controller. The UV−vis absorption spectra of PANI films were obtained with a Lambda 750 UV/vis Spectrometer (PerkinElmer). The aqueous solutions of formic and hydrochloric acids with pH of about 2 were used to study the evolution of UV−vis spectra of PANI films during immersion in acid medium.

Figure 2. Evolution of UV−vis absorption spectrum of PANI deposited on the glass support during polymerization upon drying resulting in the formation of the thin highly crystalline and strongly oriented film. Thickness of the dry film is 80 nm.

The evolution of the optical absorption spectrum of the thin polycrystalline PANI films upon immersion in acid aqueous solutions and distilled water, and upon humidification at ambient conditions has been monitored. Spectra of the films detected during the immersion in aqueous solution with pH of about 2 are presented in Figure 3.

3. RESULTS AND DISCUSSION As can be seen from the two-dimensional WAXS patterns (Figure 1, left) the location of the reflexes corresponding to the

Figure 1. Two-dimensional WAXS patterns detected in the crosssection of the film (80 nm thickness)10 (left) with a 3D molecular model of the PANI fibril (right). For the sake of simplicity, the fibrilar crystals are presented as the defect-free ones.

Figure 3. Evolution of UV−vis absorption spectrum of the thin (80 nm) highly ordered PANI film upon immersion in acid aqueous solution (from 1 to 4) and drying (from 4 to 1), where 1 corresponds to the dry film. Direction of the arrow corresponds to the increase of duration of drying or immersion.

interplane distance of 3.6 Å (π−π stacking) at the equator indicates that the aromatic nuclei are strongly perpendicular to the support surface.10 Such regularity in their position is ensured by the strong water-mediated hydrogen bonds between nitrogen atoms in the direction perpendicular to the support surface, as schematically shown in the 3D molecular model of the PANI fibril. As a consequence, the reflexes corresponding to the interplane distance ensured by the water-mediated hydrogen bonds (6.1 Å) are situated at the meridian. PANI adsorbed during synthesis on a glass support has a long-wavelength absorption at about 800 nm (green color). After drying up to equilibrium, the absorption maximum is shifted to 570 nm (violet color) (Figure 2). Long-wavelength

As it can be seen, with increasing duration of immersion, the position of the long-wavelength maximum progressively displaces to lower energy. After reaching the equilibrium, films become green. This change of the optical absorption spectra is reversible following drying of the films. It has to be noted that, at the same pH of the medium, red shift of the longwavelength optical absorption is stronger in aqueous solution of formic acid compared with that of a hydrochloride one, where λmax is below 800 nm (not shown here). Reversible displacement of long-wavelength maximum is observed when the dry film is immersed in distilled water B



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(Figure 4). However, the resulting color of the film is blue, not green (λmax is around 720 nm).

maximum upon water adsorption is considered to be the result of water-promoted superficial formation of the J-like mesophase. Observed by us previously, both propagation of chemical reduction in the wet films and substantial increase of Voc of the ITO/PANI/Al cell compared to that of the dry film can also be characteristic for J-like liquid-crystalline PANI.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +420 296 809 268. Fax: +420 296 809 410. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the Czech Grant Agency (No. 1300270S) is acknowledged. The authors are grateful to the Institute of Macromolecular Chemistry ASCR (Czech Republic) for the opportunity given to Olga Omelchenko to take part in UNESCO/IUPAC Postgraduate Course in Polymer Science.

Figure 4. UV−vis absorption spectra of the thin (80 nm) polycrystalline PANI film before and after immersion in distilled water.

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Due to the high specific surface of the thin polycrystalline PANI films, contribution of the optical properties into overall ones should be significant. As it has been shown previously,10 hydration of PANI is more efficient in the presence of the highly hydrated ions. For this reason, water diffusion into the film is more efficient in the presence of more hydrated (position in Hofmeister series) formate anions. Despite the lack of systematic studies about the influence of pH of aqueous medium on the formation of J-aggregates, available data show that this process is much more efficient in acidic medium than in neutral or alkaline ones.4,13,14 For this reason, probably, we observe stronger red shift of the longwavelength absorption for the thin polycrystalline PANI film after immersion in the acid medium than that monitored after treatment with distilled water. The data presented herein can be furthermore discussed in terms of the structure of J-aggregated PANI and their relation to the long-wavelength optical absorption. The fashion of oligomer arrangement in J-aggregated PANI should be quite similar to that in highly crystalline films. Namely, stacking of the oligomers through hydrogen bonds in the direction parallel to ring planes should largely dominate their assembly via interactions of π-systems of the aromatic rings in the direction perpendicular to their planes. It has to be underlined, however, that neither crystalline polyaniline nor crystalline dyes produces the J-band despite the strong exciton coupling. Probably, the liquid-crystalline nature of J-aggregates should be taken into consideration to elucidate the origin of the J-band.

ABBREVIATIONS PANI, polyaniline; WAXS, wide-angle X-ray scattering; ITO, indium tin oxide; Voc, open circuit voltage

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REFERENCES

(1) Jelley, E. E. Molecular, Nematic and Crystal States of I:I′-Diethylψ-Cyanine Chloride. Nature 1937, 139, 631−632. (2) Scheibe, G.; Kandler, L. Anisotropie Organischer Farbstoffrnolekiile. Nebenvalenz-Bindung als Energiefibertriger. Naturwissenschaften 1938, 26, 412−413. (3) Sheppard, S. E. The Structure of the Mesomorphic Phase of Certain Cyanine Dyes. Science 1941, 93, 42−43. (4) Sheppard, S. E. The Effects of Environment and Aggregation on the Absorption Spectra of Dyes. Rev. Mod. Phys. 1942, 14, 303−340. (5) Würthner, F.; Kaiser, T. E.; Saha-Möller, C. R. J-Aggregates: From Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials. Angew. Chem. Int.Ed. 2011, 50, 3376−3410. (6) Rich, C. C.; McHale, J. L. Influence of Hydrogen Bonding on Excitonic Coupling and Hierarchal Structure of a Light-Harvesting Porphyrin Aggregate. Phys. Chem. Chem. Phys. 2012, 14, 2362−2374. (7) Rich, C. C.; McHale, J. L. Resonance Raman Spectra of Individual Excitonically Coupled Chromophore Aggregates. J. Phys. Chem. C 2013, 117, 10856−10865. (8) Villari, V.; Mineo, P.; Scamporrino, E.; Micalia, N. Role of the Hydrogen-Bond in Porphyrin J-Aggregates. RSC Adv. 2012, 2, 12989− 12998. (9) Gospodinova, N.; Dorey, S.; Anokhin, D.; Ivanov, D.; Romanova, J.; Kolev, H. (Centre National de la Recherche Scientifique and Universite de Haute-Alsace). Method of preparing polyaniline films and highly self-oriented films obtained. French patent application, 2,928,646, March, 13, 2008. (10) Gospodinova, N.; Muşat, V.; Kolev, H.; Romanova, J. New Insight into the Redox Behavior of Polyaniline. Synth. Met. 2011, 161, 2510−2513. (11) Omelchenko, O.; Tomšík, E.; Zhigunov, A.; Guskova, O.; Gribkova, O.; Gospodinova, N. J-Like Supramolecular Assemblies of Polyaniline in Water. Macromol. Chem. Phys. 2013, 214, 2739−2743. (12) Gospodinova, N.; Tomšík, E.; Romanova, J. Thin Mesoporous Polyaniline Films Manifesting a Water-Promoted Photovoltaic Effect. Chemical Papers 2013, 67, 972−978. (13) Akins, D. L.; Zhu, H.-R.; Guo, C. Aggregation of TetraarylSubstituted Porphyrins in Homogeneous Solution. J. Phys. Chem. 1996, 100, 5420−5425.

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CONCLUSION In summary, formation of the thin, highly ordered PANI films constituted by fibrilar crystals is accompanied by consecutive displacement of the optical absorption maximum from about 800 to 570 nm. This process takes place upon water evaporation from PANI adsorbed on the glass support during polymerization. Reversible red shift of the long-wavelength optical maximum to 800 nm is observed after water penetration in the films. The red-shifted optical absorption is assumed to be the signature of the J-like liquid-crystalline state of PANI, which is prerequisite for the formation of the fibril-like crystals that build highly ordered PANI films. By analogy with the crystalline dyes, reversible red shift of the long-wavelength optical C



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(14) Akins, D. L.; Ö zçelik, S.; Zhu, H.-R.; Guo, C. Fluorescence Decay Kinetics and Structure of Aggregated Tetrakis(psulfonatophenyl)porphyrin. J. Phys. Chem. 1996, 100, 14390−14396.

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J-Like Liquid-Crystalline and Crystalline States of Polyaniline

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