Chinese Chemical Letters  2016, Vol.27 Issue (04): 487-491   PDF    
Multiscale and multicomponent layer by layer assembly of optical thin films triggered by electrochemical coupling reactions of N-alkylcarbazoles
Jian Zhanga,b, Shu-Sen Kanga,b, Zhe Zhanga, Mao Lia     
a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China;
b University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: The integration of multiscale and multicomponent of molecules and nanoparticles into thin films for applications requires the abilities of controlled their processing and assembly, which has been an great challenge because of the difficulty in manipulating the various materials such as small molecules, complexes, polymers, and inorganic nanomaterials through synergetic combinations of chemical or physical fabrications. Eletropolymerization is of great significance to fabricate polymeric film materials straight on the conductive substrates with tunable morphologies and thicknesses. However, unlimited electrochemical reactions(polymerization) have been usually leading to disadvantageous in ill-defined structure and highly doped state. Thanks to finding of exceptional electrochemical reaction (oligomerization) of N-alkylcarbazole, electrochemical layer by layer assembly has emerged as a promising strategy for a wide library of applications. The capability of this strategy can manipulate various molecules and nanoparticles into the scale and component controllable thin films. Unlike other electropolymerizable precursors such as aniline and thiophene, the resulting di-N-alkylcarbazole is transparent in the visible light region and thus does not impair the intrinsic properties of the components in the film. This account highlights of the typical findings in investigating both single- and multi-components thin films as a forum for discussing new opportunities in exploiting novel designs and applications of optical thin films.
Key words: Layer-by-layer assembly     Carbazole     Electropolymerization     Optical films     Devices    
1. Introduction

The assembly and integration of electronic materials need a paradigm shift from single- to multi- functionalities for next generation electronics [1, 2, 3, 4]. It is well-known that it is heavily complicated to integrate the various electronic, electrical, magnetic, optical, biological and chemical properties through purely chemical synthesis. Therefore, how to manipulate the various welldefined materials such as small molecule, complex, polymer, and inorganic nanomaterials into thin films through synergetic combinations of chemical and physical fabrications has been remaining the great challenge. Although the evaporation and spincoating methods [5] among various techniques are most popular under currently active research and development, the compatibility of various materials has been always bothering for film forming processes. As a low-cost solution-processed method, eletropolymerization [6] can make the creation of molecular wire or nets possible for uniform conducting films on conductive substrates with tunable morphologies and thicknesses. However, over the past few decades, use of electropolymerization has been limited to applications, where ion-doping is required (i.e. electrochromism) [7, 8] because non-quantitative (unlimited) electrochemical reactions usually result in conducting conjugated polymers with an unavoidable highly doped state. In fact, the resulting conjugated polymers and highly doped states are often unnecessary (or disadvantageous) [9] for broadening applications of a wide library of building blocks. First of all, the efforts should be made to limit the doping state of resulting films through molecular design, which can prevent the unnecessary formation of a conducting conjugated polymer with sharp band gap and corresponding highly doping possibility. We developed a quantitatively (reaction-limited) electrochemical reaction of N-alkylcarbazole, which gives a formation of oligomer through electrochemical or chemical oxidation. As shown in Fig. 1, at a low potential (ca. 1.0 V vs. Ag/ Ag+ ), the radical of di-N-alkylcarbazolecation is more stable at 3, 30- position than those at 6, 60-position of di-N-alkylcarbazole, that extremely depresses the growing possibility of polymer chain of polycarbazole. Unlike the all of electroactive precursors such as thiophene [10], selenophene [11], aniline [12], pyrrole [13] et al., the resulting di-N-alkylcarbazole has the feature absorption below 350 nm with excellent transparence at the visible light region, which does not impair the optical properties of subsequent research of desirable building blocks in the film. This clean reagent less reaction is especially useful for constructing covalently linked polymers or their thin films through chemical or electrochemical processes. By utilizing this special reaction, therefore, various functional building blocks can be interconnected through N-alkylcarbazole as a bridge linker (Fig. 1). Here, we describe an account of researches focusing on the optical and electrical properties of thin films fabricated by various functional building blocks with pendentcarbazole. Special emphasis is that how to understand and solve the questions remaining of electronic materials and their applications from electrochemical approaches.

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Fig. 1.Schematic layer-by-layer assembly of small molecule, complex, polymer, carbon material and inorganic nanoparticle materials triggered by electrochemical coupling reactions of N-alkylcarbazoles for optical films and devices.
2. Electrochemical layer-by-layer assembly

Layer by layer (LbL) assembly could be the most versatile technique among many thin film preparation methods, which can assemble the materials into nanometric thin films at a variety of substances through mostly noncovalent interactions between pair of layers. For example, the polyelectrolyte multilayers can be fabricated through alternating deposition of polyanions and polycations [14]. Electrochemically fabricated thin films usually are conductive, stable and unsolved in normal solvent, making them possible for further fabrication of next layer of other materials. As a novel electrochemical driving force for LbL assembly, the different building blocks with pendent carbazoles (porphyrin 1, fullerene 2 and fluorine 3) [15] as shown in Fig. 2 were covalently immobilized into both homo- or hetero- thin films with efficient electronic interactions between pair of layers. In contrast to the conventional methods of thin filmpreparation, themolecularweights, solubilities and film-forming ability are not necessarily obstacles. Use of electrochemical coupling of N-alkylcarbazoles enabled the various materials includingsmallmolecule, polymer, complex andinorganic nanoparticle to be fixed into cross-linked thin films at a substrate surface from solutions of those components. The facile control process by courtesy of electrochemical advantages, the one-pot in situ switching LbL assembly can be achieved by combining oxidative and reductive electrochemical reactions [16]. Inparticular, the bipyridine-Ru complex bearing vinyl groups with reductive electrochemical activity and trifluoren 3 containing pendent carbazole units with oxidative electrochemical activity in one-pot can be electrochemically triggered by alternation of reductive and oxidative potentials. The electroactive sites and functional units of precursors are independent, allowing more flexibility in molecular design and convenient realization of other components assembly This fabrication process without moving or changing experimental gearswould provide powerful strategy for automated LbLmachines The gold nanoparticles 6 can be also integrated into organic/ inorganic hybrid thin film with controlled distribution ability through electrochemical assembly process in alternative solutions This gold nanoparticle layer can be applied as a counter electrode of photovoltaic device because the transparent gold nanoparticle film has a high conductivity value of 1.2 × 102 S cm-1. Significantly, the fully solution-processed photovoltaic device with simple structure of ITO/organic layer/gold nanoparticle electrode can be achieved [17]. This low-cost strategyprovides aneconomic route to fabricated electronic devices with low dependence on work function under vacuum-evaporation free condition. Similarly, multiple assembly of full inorganic nanoparticles was also investigated [18]. This study highlights the integration of multiple nanoparticles into bulky hybrid films or tandem multilayer structures, whichwas difficult to be obtained using traditional LbL technique. The resultant multinanoparticles filmswith desired sequences and thickness are highly stable and photoactive, exemplified by prototype single and doublelayered photodetectors.

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Fig. 2.Various building blocks with pendent carbazoles for electrochemical layer-by-layer assembly.
3. Typical applications of electrochemical LbL films

For low-cost solution-processed applications, the solubility and film-forming ability of materials have been always taking huge efforts of chemical synthesis from researches. Electrochemical synthesis enables in situ preparation of uniform polymer films directly on the surface of conductive substrates with controllable morphologies and thicknesses and without significant solubility dependence. For example, fullerene [19, 20, 21, 22] is one of the most investigated species due to its unique physicochemical properties It is well-known that the physical and chemical properties of the fullerene core strongly depend on their surface surgery (i.e. the number of substituents) [23, 24]. Therefore, the mono-substituted fullerene is superior to those of multi-substituted for maintaining the intrinsic properties of fullerene. In order to overcome the filmforming problem of mono-substituted fullerene with low molecular weight and low solubility, the electrochemical fabrication could be an ideal candidate. The quantitative (reaction-limited) electrochemical coupling reaction of carbazoles ensured well-defined fullerene polymer films [25]. Interestingly, this high content fullerene film (60 wt%) is amorphous and highly transparent with similarly physicochemical properties to pure fullerene. The work function of these fullerene films can be tuned through the electrochemical doping/dedoping processes [26], making them an ideal candidate as an interfacial layer forth inverted organic photovoltaic devices. The synthesis of fullerene polymers fills the void, which once existed for electrochemical preparation of nonconjugated polymers, and also represents a broad way to a high degree of control over the predictable molecular structure or nanostructure of a large library of building blocks. For electrochemically fabricated examples as an interfacial layer of light emitting devices or photovoltaic devices, the perylenebisimides (PBIs) film [27], gold nanoparticle film [28, 29], conjugated microporous polymer films [30, 31] were also developed.

Compared to above mentioned, the most successful application is fabrication of highly fluorescent thin films. The electrochemically fabricated conducting polymers including polycarbazole generally showed extremely weak or no fluorescence most probably due to structural defects and quenching by doping counter-ions during the electrochemical redox processes. Originally, the carbazole had been considering to be favorable for hole injection and transport in light emitting devices [32]. Advincula and coworkers [33] reported electrochemically fabricated luminescent films based on electrochemical coupling reaction of carbazoles. This polymer possesses polyfluorene backbone and pendentcarbazole, which plays role as electrochemically crosslinkable unit. As a result, the blue fluorescence properties defined by polymer main chain can be observed. Subsequently, Ma group [34] first reported a highly luminescent network films based on small molecule 2. Compared to polyfluorene, small molecule has own well-defined structure without ketonic defects. After the optimizing various electrochemical conditions (ca. solvent, ion species, potential, scan rate etc.) [35, 36], the resulting films exhibited strong blue luminescence with smooth morphology and excellent thermal stability. Significantly, the single-layer device without modification layer of pre-spin-coated PEDOT:PSS exhibited better performance (brightness of 4224 cd m-2 and efficiency of 0.47 cd A-1) than those of spin-coated films with help of PEDOT:PSS(brightness of 78 cd m-2 and efficiency of 0.11 cd A-1). The high performance of light emitting device (LED) was supposed most probably due to that the pendent cabazolesnot only serve as electro active linker, but also can balance the injection of hole and electron benefiting the electroluminescence efficiency [37].Furthermore, more highly blue, green 4 and red 5 fluorescent precursors with pendent carbazoles had been designed and synthesized and their full-color patterned films with sharp edge and high resolution were successfully realized on ITO strip [38]. By utilizing electrochemical LbL assembly, the red, green and blue fluorescent precursors were integrated into thin film with excited white light emitting [39, 40]. The highly cross-linked structure of luminescent chromophores facilitated energy transfer and resolved the notorious phase separation issue, achieving the stable color. The devices show excellent performance with maximum brightness of 13, 000 cdm-2 and high efficiency of 6.7 cd A-1. Taking account of electrochemical advantages with simplicity and low cost, this electrochemical fabrication for luminescent films could not only lead to a completely newtechnology for realization of RGB patterns for organic/polymer full-color display, but also can create the fabrication onarea-selective surface andcomplicated3Dgeometries [18, 41], which is superior to current technologies.

Besides LED applications, the breakthrough research of highly fluorescent EP films also thrived the thin film fluorescence sensors on both organic vapor [42, 43] and metal ion detections [44]. Based on the successful molecule design of molecule 2 with separated fluorescence unit and electroactive groups, numbers of derivatives with enhanced performances and multi-functionalities are explored. The metal-chelated moieties, namely 2, 20-bipyridine and 1, 10-phenanthrolinewereintroduced as a core, which combined the coordinate ability with EP and fluorescence properties, leading to an ideal fluorescence sensor of metal ion. The resulting films have an excellent fluorescence detection on Fe3+ ion ranges from 10-5 mol L-1 to 10-4 mol L-1. Conjugated microporous polymers (CMPs) are a class of π-conjugated porous frameworks connected by covalent bonds. The extended πconjugated system and inherent nanopores structure make them having numbers of exciting properties and potential applications. However, CMPs usually synthesized as insoluble and unprocessable powder, which limit their utility. Electrochemical coupling reaction of cabazoles was employed to address this issue by in situ synthesizing CMP and forming films on ITO substrates simultaneously [45]. The rigid precursor has a triphenylbenzene core and three peripheral cazaboles. The highly cross-linked stable CMP films can be obtained after electrochemical processes within controllable thickness, size and shape. These films can be utilized as versatile fluorescence sensor platforms of electron-rich and electron-poor arenes, metal ions, dopamine with featuring highly sensitive, rapid response, excellent selectivity and reusability. Electrochemically synthesized porous πnetworks were further investigated by means of introducing the AIE moiety tetraphenyl ethylene. The resulting porous EP films have an exceptional absolute fluorescence quantum yield as high as 40%, which is the highest record for AIE based porous materials [46]. By virtue of rapid photoinduced electron transfer, these thin films can detect explosives with enhanced sensitivity to low parts-per-million and good selectivity.

4. Conclusions and challenges

Electrochemically LbL assembly is a highly robust and versatile strategy as an emerging approach to manipulate the small molecule, complex, polymer and nanoparticle possibly including other large library of building blocks into homo-, periodic heteroand gradient hybrid- thin films on patterning 2D and 3D surface platforms with a high degree of controllable abilities in predictable structure, morphology and thickness. These films can be utilized as active layer, interfacial layer, and electrode of optical devices, indicating that the electrochemically fabrication can not only realize the high performance for practical applications, but also shows great potential strategy toward fully solution-processed devices. This electrochemical fabrication fills the void which once existed for electrochemical preparation of polymers films, and also represents a breakthrough in broadening the potential applications of electrochemical synthesis.

Since the electrochemical stimulus locally take place at interface of electrode, the substrate for layer by layer assembly needs to be conductive or semiconductive. Though the electrochemically fabricated thin film could fall off fromelectrode by pre-spin-coated PEDOT:PSS as an interlayer, the fabrication of free-standing films still remains great challenges. To date, the ion doping can be extremely restricted by special molecular design and electrochemical control, but the tiny doped state is hard to be completely purged, especially when the thickness is getting thick. Therefore, how to utilize this tiny doped species for practical applications could be another novel emerging research topic.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Nos. 21374115, 51573181, and 21504088), the Hundred Talents Program, CAS, China.

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