Perspective pubs.acs.org/cm
Isoindigo, a Versatile Electron-Deficient Unit For High-Performance Organic Electronics Romain Stalder,‡ Jianguo Mei,‡ Kenneth R. Graham,‡ Leandro A. Estrada,† and John R. Reynolds*,† †
School of Chemistry and Biochemistry, School of Materials Science and Engineering, and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ The George and Josephine Butler Polymer Chemistry Laboratories, Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 32611, United States ABSTRACT: Isoindigo (iI) has proven successful as an electron-accepting building block for the preparation of electroactive materials for organic electronics. Its high yielding and scalable synthesis has enabled the rapid development of a large number of molecular and polymeric iI-based materials with remarkable physical properties. This perspective provides an overview of the fundamental properties of isoindigo and summarizes the progress in the development of new materials for varied electronic applications during the last 3 years, focusing in particular on organic photovoltaics (OPVs) and organic field effect transistors (OFETs). The fundamental electronic properties of isoindigo are discussed in the context of the substitution pattern effect (5,5′ vs 6,6′) on the frontier orbitals energies and optical properties. The development of molecular systems in the 6,6′-iI configuration for OPVs is examined with an emphasis on molecular design for improved electronic properties thanks to fine-tuning of the active layer morphology via crystallization control. Numerous copolymers of iI have been reported, with both electron-rich and electron-poor comonomers. The homopolymer of isoindigo displays electron-accepting and electrochromic properties and serves as a polymeric surrogate for fullerenes in all-polymer solar cells. The copolymers’ absorption profiles span the entire visible spectrum into the near-infrared, up to 900 nm. Bulk-heterojunction solar cells based on iI copolymers have reached up to 6.3% efficiency. While the effect of processing additives and cell architecture are important, the unique electronic properties of iI polymers also provide useful insight on energetic losses within blends with fullerenes. Selected copolymers also perform highly in air-stable field effect transistors, with p-type mobilities exceeding 3 cm2/(V s). New concepts concerning the effect of backbone curvature and side-chain branching or polarity have been investigated using iI copolymers. Additionally, some all-acceptor copolymers display n-type mobility. As the design of iI materials evolves, structural modifications of the iI core emerge, targeting ambipolar charge transport and enhanced backbone planarity. Overall, isoindigo provides the field of organic electronics with impressive performance as well as a valuable platform for structure−property relationship investigation. KEYWORDS: isoindigo, π-conjugated materials, electron-acceptor, organic photovoltaics, organic transistors
1. INTRODUCTION
reactivity and stability remain fewer than their electron-rich counterparts. Within the acceptor library,3 perylene4 and naphthalene5 diimides, bithiophene imides,6 thienopyrrole dione,7 or diketopyrrolopyrrole8 (DPP) are some of the most popular amide/imide-based acceptors employed to date. High OPV efficiencies approaching 8%9 and high charge carrier mobilities up to 8 cm2/(V s),10 along with excellent ambient stability and ambipolar behavior have been reported thanks to amide/imidebased materials.
Organic photovoltaics (OPVs), organic field-effect transistors (OFETs),1b organic light-emitting devices (OLEDs),1c and organic electrochromics (OECs)1d have all benefitted from increasing effort on the synthesis of conjugated molecules and polymers, with structural designs benefitting from increased understanding of the key parameters governing high performance. Some of these parameters such as ionization potentials (IPs) and electron affinities (EAs), optical gaps, and absorption profiles have been successfully tailored by conjugating electronrich (donor) and electron-deficient (acceptor) arenes within the electroactive core of conjugated materials.2 This strategy has proven effective given its ease of implementation through transition metal catalyzed cross-couplings. As a consequence, the number of donor and acceptor moieties is steadily increasing, although high performance acceptors with suitable 1a
© XXXX American Chemical Society
Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: July 5, 2013 Revised: October 4, 2013
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suspension of oxindole and isatin in acetic acid yield isoindigo quantitatively. Another route to obtain isoindigo derivatives from isatins uses tris(diethylamino)phosphines as deoxygenating agents to form the ketocarbenes of isatin which dimerize selectively into isoindigo.14,15 The acid-catalyzed isatin/oxindole condensation has been the preferred route for synthesizing materials since it was first introduced in the open11 and patent16 literatures for organic electronic applications. The attractive features of the first step are the availability of the brominated starting indoles and the near-quantitative yield of the condensation with little purification required, as depicted in Scheme 1. The
One of the latest reported amide-based acceptor for organic electronics is isoindigo (iI), an isomer of the well-known dye indigo. This dye benefits the field of organic electronics given its ease of synthesis in bulk quantities and many synthetic handles useful for materials development. These characteristics have encouraged multiple research groups to actively pursue the preparation of iI-based materials whose performances in OPVs and OFETs are among that of state-of-the-art materials. Numerous publications since its first report in the open literature have been released.11 It is the subject of this perspective to summarize the development of isoindigo within this 3-year span. First, in section 2, we describe some of the past and current considerations employed to synthesize and functionalize isoindigo. We offer insights on structure−property relationships and show how these drive the electronic structure of iI and some of its molecular derivatives. For example, we highlight recent density functional theory (DFT) calculations on iI small molecules and their correlation with experimentally measured photophysical and electrochemical properties. This is followed by current studies of molecular systems in section 3, directed toward solar cell applications wherein structurally tailored iIbased oligothiophenes were investigated as a means of controlling the active layer morphology. Next, the focus of this report shifts from the molecular to the macromolecular field. Conjugated polymers incorporating isoindigo are first addressed in section 4.1 by describing the simplest archetype: its homopolymer. We summarize its spectroelectrochemical properties and use in all-polymer solar cells. To date, an extended library of copolymers of isoindigo has been reported in the open literature by several research groups. Their repeat unit structures, optical properties, and frontier orbital energies (FOEs) are summarized in section 4.2 in light of their targeted applications, where most copolymers were reported as part of OPV or OFET studies. The OPV characteristics of an extensive set of iI copolymers in solar cells are listed in section 4.3 and used to illustrate key concepts such as spectral matching, effects of processing additives, and limits of FOE offsets. Some of these polymers were used in OFET studies and their transistor characteristics are summarized in section 4.4, along with those of the other isoindigo polymers reported exclusively for OFET applications. Structure− property relationships in OFETs, such as the effect of backbone curvature and the influence of side-chain amphiphilicity/ branching-position, were investigated. This report concludes with structural perspectives in section 5, focusing on chemical modifications of the isoindigo unit and potential applications, for which some reports have begun to appear in the open literature.
Scheme 1. Synthesis of 6,6′-Dibromoisoindigo Derivatives via Aldol Condensation of Isatins and Oxindolesa
a (a) CH3COOH, HCl cat., reflux, >95%. (b) RX, K2CO3, DMF, 90 °C >85%. (c) B2pin2, Pd(dppf)Cl2, KOAc, dioxane, 90 °C, >75%.
dibromoisoindigo derivative 1 is readily alkylated under basic conditions with an alkyl halide of choice. Potassium carbonate and sodium hydride have both been shown to afford the Nalkylated isoindigo derivatives 2 in high yields. The latter derivatives possess sufficient reactivity and solubility to be used in palladium-catalyzed cross couplings such as Stille or Suzuki couplings, as has been widely used to date. A few examples of direct arylation coupling17 of the dihalide of iI have also been reported.18,19 One further functionalization step to the diboron isoindigo derivative 3 takes place under Miyaura borylation conditions from the dibromo derivative 2. This allows isoindigo to be coupled to other halogenated aryl units under Suzuki coupling conditions. Figure 1a illustrates the molecular structure of isoindigo; it is composed of two oxindole rings centrosymmetrically conjugated to one another at their 3-carbons by a central double bond that binds two electron-withdrawing carbonyls and two electron-rich phenyl rings in trans-conformation. Extension of the conjugation can occur at the 6,6′ positions of the isoindigo ring, although other positions of the phenyl ring can be functionalized as well. In particular, the 5,5′ positions are readily accessible as the 5-bromoindoles are commercially available. The electronic effects of substitution pattern (5,5′ vs 6,6′) and substituent nature (phenyl, thienyl, dioxythienyl) are the subject of a recent study on molecular compounds carried out in our laboratories.20 These D−A−D compounds are designed to model the more extended π-systems present in D− A copolymers. The geometry optimization of N,N′-dimethylisoindigo (iI-Me) computed at the B3LYP/6-31G(d) DFT level revealed two stationary points: one is completely planar, while the other one displays a slight twist (∼15° angle) between the two oxindole rings along the central double bond. Both conformations are consistent with previously reported crystal structures and computed geometries. These point toward a
2. ISOINDIGO: SYNTHESIS AND FUNDAMENTAL PROPERTIES Indigo is one of the oldest known dyes, whose structure was first elucidated by Von Bayer at the end of the 19th century.12 Isoindigoa naturally occurring isomer of indigois present as a minor isomer of the indigoid dyes found in the leaves of Isatis tinctoria.13 The dyes are produced in the plant from oxidized derivatives of indole (indoxyl, dioxindole, isatin, or oxindole) in the presence of hydrolases and air. Synthetically, the indigos and indirubrins can be prepared via condensation of indoxyl, dioxindole, and/or isatin in alkaline solutions, but the reaction fails to produce isoindigo under such basic conditions. Instead, catalytic amounts of hydrochloric acid added to a B
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Figure 1. (a) Molecular structure of isoindigo, (b) crystal structure of 6,6′-T-iI-T, (c) electronic structures of the HOMO and LUMO of 6,6′-T-iI-T and its 5,5′-analog calculated via DFT.
favorable electronic communication of the phenyl and the ketopyrrole carbonyl π-electrons. Also, their theoretical energetic difference at the B3LYP/6-311+G(2d,p)//B3LYP/ 6-31G(d) level is smaller than kT, hinting that the packing motif presents sufficient energetic compensation for the small planarization barrier. Figure 1b displays the molecular structure of N,N′-dihexyl6,6′-bisthienylisoindigo (6,6′-T-iI-T) from the X-ray diffraction of crystals which were grown by vapor diffusion of acetonitrile (poor solvent) into a chloroform solution (good solvent, ca. 10 mg/mL). In the crystal structure, the iI unit is planar with a small dihedral angle (∼6°) between the thiophene and iI; however, the theoretical ground state (GS) minimum presents isoindigo in twisted conformation. The small energetic disparity between twisted and planar structures (105 >106 106−107 106 105 106 106 105−106 105−106 105 105−106 104 103 106−107 106−107 104 105
Vt (V) 6
−20 −2 3 −18 −18 −10 −2 −5 −5 −10 −4 −6 −4 −16 −28 −30 −10 −25
ref 39 40 39 41 41 40 42 42 42 42 40 40 40 40 40 40 40 40 49 49
that both polymers showed excellent ambient operation characteristics and P(iI-T2) devices were even stable at high humidity (RH = 60%). The authors attributed the superior charge transport properties of P(iI-T2) to better molecular packing, higher molecular weight and centro-symmetric configuration. Pei and co-workers further studied the influence of polymer symmetry and backbone curvature on field effect transistors based on a set of ten iI-based polymers, depicted in Figure 9b.40 They found that polymers with different symmetry and backbone curvature exhibited different lamellar packing and crystallinity as revealed by AFM and grazing incidence X-ray diffraction (GXID) analyses, and further proposed the “molecular docking” concept as illustrated in Figure 9c. The latter concept emphasizes the reduced steric hindrance from small nonalkylated units, allowing them to “dock into” the cavities formed by large aromatic cores, similar to the “brick layer” packing of TIPS pentacene.53 Four out-of-plane diffractions were observed for the polymers with centrosymmetric donors (entries 14, 19, 22, 26, 33, 40), and the intensity of the peaks increased upon annealing. In sharp contrast, all polymers containing axisymmetric donor blocks (entries 8, 23, 30, 32) showed little or no out-ofplane diffractions, indicating the absence of lamellar packing in these polymer thin films. According to the out-of-plane GIXD data, d-spacings of polymers with centrosymmetric donors are significantly smaller than those of the polymers with axisymmetric donors. The FET performance correlates with the proposed molecular docking strategy. The performance of centrosymmetric polymers in general is orders of magnitude higher than that of axisymmetric isoindigo polymers. In a separate study with more extended donor moieties, Wang and co-workers also suggested that a higher backbone curvature led to decreased FET mobilities.49 Taking a currently unconventional strategy, Bao and coworkers replaced the OD group (entry 12) with siloxaneterminated hexyl chain (RSiO, entry 13) resulting in the new iIJ
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Figure 10. Structures of iI copolymers with customized branched alkyl and siloxane solubilizing groups.
Figure 9. (a) Structures of isoindigo-based copolymers; (b) centrosymmetric and axisymmetric donors of the copolymers; (c) proposed interpolymer docking model; (d) cartoon presentation of polymers with centrosymmetric donors; (e) cartoon presentation of polymers with axisymmetric donors. Adapted with permission from ref 40. Copyright 2012 American Chemical Society.
co-bithiophene alternating polymer depicted in Figure 10.41 It was proposed that closer packing of the conjugated polymer backbones would be achievable by moving the branching sites away from backbone. Additionally, highly flexible siloxane groups would still provide good solubility for solution processing. Measured by GIXD, the π−π stacking distance was shortened from 3.75 Å with conventional chains to 3.58 Å with the siloxane chains, confirming the hypothesis. The 2D-GIXD images shown in Figure 11 display two different textures. With conventional OD chains, a lamellar structure typical of conjugated polymers is observed, and the πstacking planes are perpendicular to the substrate. In contrast, the siloxane chains induce two kinds of textures corresponding to π-stacking planes both normal and parallel to the substrate surface. Further analysis found a longer crystalline coherence length when the siloxane chains were used. Since charge transport in polycrystalline polymer FETs occurs parallel to the substrate surface, it was argued that close packed crystallites of either parallel or perpendicular orientations could favor the overall in-plane migration by reducing the risk of carriers being trapped at grain boundaries. Correspondingly, transistors gave a high average hole mobility of 2 cm2/(V s) for the siloxaneterminated polymer, which is about six times higher than the reference polymer with OD branched alkyl chains.
Figure 11. GIXD images of (A) the OD-chain P(iI-T2) and (B) the siloxane-terminated P(iI-T2) annealed at 130 °C. Adapted with permission from ref 41. Copyright 2011 American Chemical Society.
Inspired by Bao’s discovery, Pei and co-workers set to systematically move the branching site away from the conjugated backbone (entries 15−18).42 A series of new isoindigo-bithiophene polymers was prepared with three newly customized branched alkyl chains: C3-DD, C4-DD, and C5DD as depicted in Figure 10. It was noted that the C3-DD and C4-DD monomers are significantly less polar than the OD and C2-DD ones, arguably because the polar amide groups in the isoindigo core are less shielded in the former case by the alkyl side chains as the branching sites are further away. The π−πstacking distances were found to be 3.75, 3.61, 3.57, and 3.57 Å for OD, C2-DD, C3-DD, and C4-DD, respectively. This observation strongly supports the benefits of moving the alkyl branching site away from the conjugated backbone. The K
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average hole mobilities measured for these polymers from bottom-gate/top-contact FETs are 0.66, 0.28, 2.98, and 1.44 cm2/(V s), respectively. The authors proposed that the lack of correlation between stacking distance and hole mobilities could be a result of different stacking conformations related to the supramolecular organization of the polymers. So far, only a handful of examples of iI polymers designed as n-type materials have been reported. Poly(iI) was designed as an all-acceptor polymer although electron mobility under the space charge limited model was low. Leclerc and co-workers have reported all-acceptor copolymers of iI using TTz, TPD, and DPP as electron-deficient comonomers (Table 2, entries 46−49).19,33 FET electron mobilities up to 3.5 × 10−3 cm2/ (V s) were achieved with P(iI-BTPD).
Scheme 2. Synthesis of the Fluorinated Isoindigo Unit and the Corresponding Copolymera
5. STRUCTURAL PERSPECTIVES This report shows that the availability and versatility of 6,6′dibromo isoindigo from commercially available precursors has enabled the rapid development of a multitude of compounds tailored around the 6,6′-isoindigo motif. Substitutions on carbons 5 and 5′ have yet to be fully explored. The few 5,5′ molecules studied help the community to better understand the electronic effects of isoindigo and may stimulate further research on the utility of these systemsdespite the πconjugation break. Indeed, previous reports have indicated a faster PET and slower charge recombination in metasubstituted arylenes compared with those in para-configuration in polar enough media,54 due to their near barrier-less forward intramolecular CT (ICT) and high barrier charge recombination (CR) lying in the inverted region of the Marcus parabola.55 This feature opens the door for the further exploration of 5,5′isoindigo derivatives as a less expensive alternative to the already established 6,6′-analogues for the generation of potentially useful active materials for organic electronics. As chemists pursue structural improvements of isoindigobased materials, they begin to fine-tune the chemical structure of the isoindigo unit itself. A different isoindigo derivative was reported where the two benzene rings were modified via the introduction of fluorine atoms at the 7,7′-positions.56 Scheme 2 shows the synthesis of the new fluorinated isoindigo compound, which was designed as monomer for copolymers applied to charge transport in OFETs. The synthesis of the fluorinated isoindigo starts from the commercially available 3bromo-2-fluoroaniline (4) and follows the early Sandmeyer synthesis of isatins from anilines,57 with some modification.58 The isatin 5 was then reduced to the corresponding oxindole 6 by the Wolff−Kishner−Huang method. Condensation of the oxindole and isatin ensues as for isoindigo, and alkylation under more basic conditions affords the alkylated 7,7′-difluoro-6,6′dibromoisoindigo compounds (7) ready for copolymerization. Diketopyrrolopyrole (DPP) based conjugated polymers have usually shown good ambipolar charge transport properties. In contrast, isoindigo-based polymers have seldom exhibited good electron mobility. By attaching electron-withdrawing fluorine atoms onto isoindigo-core, Pei and co-workers affected the LUMO level of PFII2T to about 0.18 eV lower than that of the nonfluorinated reference polymer, as measured by cyclic voltammetry. For the first time, the fluorinated isoindigobithiophene polymer field-effect transistors fabricated under ambient conditions exhibited electron mobility up to 0.43 cm2/ (V s), while the hole mobility remained high up to 1.85 cm2/(V s). Conceivably, this new isoindigo building block will be used
(a) NH2OH·HCl, CCl3CH(OH)2, H2SO4 (aq.), 130 °C; (b) conc. H2SO4, 70 °C, 61% for two steps; (c) (i) N2H4·H2O (85%), EtOH, reflux; (ii) t-BuONa, EtOH, reflux, 56%; (d) 5, AcOH/HCl, reflux, 24 h, 72%; (e) 15-(3-iodopropyl)nonacosane, powder KOH, DMSO/ THF (1:1), 20 °C, 88%; (f) 5,5′-bis(trimethylstannyl)-2,2′-bithiophene, Pd2dba3, P(o-tol)3, toluene, 110 °C, 48 h, 99%. Adapted with permission from ref 56. Copyright 2012 American Chemical Society a
in the future in other copolymers to achieve higher electron mobility. Among the imide acceptors used in conjugated systems, the earlier version of diketopyrrolopyrrole was first flanked with phenyl rings (Figure 12a), which were quickly replaced by
Figure 12. (a) Structures of the phenyl and thienyl derivatives of DPP and (b) structures of isoindigo compared to thienoisoindigo.
thiophene rings as a means of reducing backbone twisting and strengthening the D−A interaction between the electronwithdrawing ketopyrroles and their electron-rich counterpart.8 This structural modification led to the version of the acceptor most commonly referred to as DPP nowadays. Following this trend, a likely modification of the isoindigo unit would consist of replacing the benzene rings with thiophenes. This was outlined early on in the patent literature16 and has now been reported in the open literature.59−61 Briefly, the thieno version of isoindigo, thienoisoindigo, is synthesized by amination of 3-bromothiophene followed by ring closing L
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were fabricated under ambient conditions with stable operation over time. It was also observed that annealing over 150 °C is usually required to access these high mobilities. Since this may limit practical applications on flexible substrates, future designs of high mobility iI-based polymers with processing conditions compatible with industrial processes on low-cost substrates would be of high impact. New structures resulting from the synthetic modification of the iI core are emerging. Given the high performance of polymers incorporating such promising building blocks, they will likely receive considerable attention as iI-based materials evolve.
using oxalyl chloride to afford the diketopyrrole precursor. Its condensation can be effected using similar steps as for isoindigo, or by using Lawesson’s reagent. After bromination of the thiophene rings at their 2-positions, the compound is ready for polymerization. To date, a few copolymers with various electron-rich compounds have been reported by Janssen and co-workers 59 and Seki and co-workers,60 demonstrating significantly reduced bandgaps down to 0.9 eV. Hole mobilities in OFETs made using the latter D−A polymers are in the 10−2 to 10−4 cm2/(V s) range. An interesting copolymer reported by McCulloch and co-workers alternated thienoisoindigo and benzothiadiazole in an effort to foster electron mobility.61 This successful approach afforded balanced ambipolar charge transport in OFETs with mobilities of both types of carriers exceeding 0.1 cm2/(V s). In contrast to other iI-based materials,20 DFT calculations found that the electron density of not only the HOMO but also the LUMO were well delocalized along the backbone of the repeat unit, which is consistent with the observed ambipolar behavior.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors.
6. CONCLUSIONS AND OUTLOOK Within three years since the first report of isoindigo as valuable electron acceptor for organic electronics, close to 10 conjugated small molecules and 50 fully conjugated polymers incorporating iI have been reported in the open literature. The rapid development of such systems has likely been facilitated by the high yields and scalability of the 6,6′-dibromo-isoindigo synthesis from commercially available compounds, which is a tremendous advantage for industrial applications. In molecular systems, DFT calculations were correlated to experimental data gathered by X-ray crystallography, spectroscopy, and electrochemistry. This helps understand the fundamental electronic properties of isoindigo, such as the low-lying FOE levels, the extent of backbone planarity and the effect of substitution patterns. To date, some photophysical aspects pertaining to its weak fluorescence have been considered but have yet to be fully elucidated. Isoindigo-based small molecules proved to be a useful template to study structure−property relationships in organic molecular PVs. Through series of molecular designs and PV processing, modest initial PCEs of 1.3% were increased to relatively high PCEs up to 3.7%. Isoindigo-based small molecules helped highlight the importance of both morphology and interface optimization in molecular OPVs. With the advent of the “longer small molecules” with DADAD-type structures as reported by Bazan and co-workers,62 there is potential for even higher PV efficiency for iI-based small molecules. The OPV development of iI-based copolymers takes advantage of their extended absorption up to 900 nm and their favorable behavior in blends with fullerenes when processing additives are used. The highest OPV efficiency reported to date is 6.3%. High Voc is also achievable up to 1.03 eV so far, correlating well with their low-lying HOMO energy levels. Isoindigo-based copolymers are also interesting candidates for the development of analytical methods designed to understand the energetic processes occurring within the BHJ, since energy offsets between the p- and n-type components are small. In the field of OFETs, isoindigo-based polymers have achieved astonishing progress over the course of 2 years through side-chain engineering and backbone selection. The hole mobility is now up to 3.6 cm2/(V s), and the electron mobility is up to 0.43 cm2/(V s). Importantly, these devices
Notes
The authors declare no competing financial interest. Biographies John R. Reynolds is a Professor of Chemistry and Biochemistry and Materials Science and Engineering at the Georgia Institute of Technology and serves as a member of the Center for Organic Photonics and Electronics (COPE). His research interests have involved electrically conducting and electroactive conjugated polymers for over 30 years with work focused to the development of new polymers in order to control their optoelectronic and redox properties for applications spanning visible and infrared light electrochromism, photovoltaics, charge storage, and light emission. Romain Stalder is a Postdoctoral Fellow in Prof. Gregory P. Roth’s group at the Sanford−Burnham Medical Research Institute (SBMRI). He completed his undergraduate studies in France followed by a Master’s Degree in Chemical Engineering in Bordeaux. He joined Prof. John Reynolds’ group at the University of Florida in 2006, where his doctorate work encompassed the synthesis and characterization of organic molecules and polymers for electrochromic and photovoltaic applications. In 2012, Romain began his postdoctoral appointment at the SBMRI, where he develops microfluidic electrosynthetic methodologies. Jianguo Mei is currently a Postdoctoral Fellow in Professor Zhenan Bao’s group at Stanford University. A native of China, he received his college education from the Hefei University of Technology and obtained his Bachelor’s and Master’s Degrees in Chemical Engineering. He performed his doctoral training (2005−2010) with Professor John R. Reynolds at the University of Florida, where he gained expertise in synthesis and characterization of organic electroactive materials for electronic devices. During 2011−2012, he was a Principal Investigator at DuPont Research and Development Center in Shanghai, China. Prior to that, he was a Camille and Henry Dreyfus Environmental Chemistry Postdoctoral Fellow at Stanford University. Kenneth R. Graham is currently working as a joint Postdoctoral Fellow with Prof. Michael McGehee at Stanford and Prof. Aram Amassian at KAUST. Kenneth received his BS in chemistry from the University of North Carolina at Chapel Hill in 2006. Kenneth received his Ph.D. in chemistry in 2011 from the University of Florida, working with Prof. John Reynolds’ group, where his research focused on controlling morphology in polymer-based light emitting diodes and organic photovoltaics. In his postdoctoral appointment, he continues work in M
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(20) Estrada, L. A.; Stalder, R.; Abboud, K. A.; Risko, C.; Brédas, J.L.; Reynolds, J. R. Macromolecules 2013, DOI: 10.1021/ma4013829. (21) Bialas, D.; Suraru, S.-L.; Schmidt, R.; Würthner, F. Org. Biomol. Chem. 2011, 9, 6127. (22) Graham, K. R.; Mei, J.; Stalder, R.; Shim, J. W.; Cheun, H.; Steffy, F.; So, F.; Kippelen, B.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2011, 3, 1210. (23) Graham, K. R.; Wieruszewski, P.; Stalder, R.; Hartell, M.; Mei, J.; So, F.; Reynolds, J. R. Adv. Funct. Mater. 2012, 22, 4801. (24) Graham, K. R.; Stalder, R.; Wieruszewski, P.; Patel, D. G.; Salazar, D. H.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2013, 5, 63. (25) Addadi, L.; Berkovitch-Yellin, Z.; Weissbuch, I.; van Mil, J.; Shirnon, L. J. W.; Lahav, M.; Leiserowitz, L. Angew. Chem., Int. Ed. 1985, 24, 466. (26) Yassin, A.; Leriche, P.; Allain, M.; Roncali, J. New J. Chem. 2013, 37, 502. (27) Walker, B.; Tomayo, A. B.; Dang, X. D.; Zalar, P.; Seo, J. H.; Garcia, A.; Tantiwiwat, M.; Nguyen, T. Q. Adv. Funct. Mater. 2009, 19, 3063. (28) Yang, M.; Chen, X.; Zou, Y.; Pan, C.; Liu, B.; Zhong, H. J. Mater. Sci. 2013, 48, 1014. (29) Ying, W.; Guo, F.; Li, J.; Zhang, Q.; Wu, W.; Tian, H.; Hua, J. ACS Appl. Mater. Interfaces 2012, 4, 4215. (30) Stalder, R.; Mei, J.; Subbiah, J.; Grand, C.; Estrada, L. A.; So, F.; Reynolds, J. R. Macromolecules 2011, 44, 6303. (31) (a) DuBois, C. J.; Reynolds, J. R. Adv. Mater. 2002, 14, 1844. (b) DuBois, C. J.; Abboud, K. A.; Reynolds, J. R. J. Phys. Chem. B 2004, 108, 8550. (c) Galand, E. M.; Kim, Y.-G.; Mwaura, J. K.; Jones, A. G.; McCarley, T. D.; Shrotriya, V.; Yang, Y.; Reynolds, J. R. Macromolecules 2006, 39, 9132. (d) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem. Soc. 2006, 128, 12714. (e) Beaujuge, P. M.; Reynolds, J. R. Chem. Rev. 2010, 110, 268. (32) Thompson, B. C.; Fréchet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (33) Allard, N.; Najari, A.; Pouliot, J.-R.; Pron, A.; Grenier, F.; Leclerc, M. Polym. Chem. 2012, 3, 2875. (34) Stalder, R.; Mei, J.; Reynolds, J. R. Macromolecules 2010, 43, 8348. (35) Liu, B.; Zou, Y.; Peng, B.; Zhao, B.; Huang, K.; Pan, C. Polym. Chem. 2011, 2, 1156. (36) Wang, E.; Ma, Z.; Zhang, Z.; Henriksson, P.; Inganäs, O.; Zhang, F.; Andersson, M. R. Chem. Commun. 2011, 47, 4908. (37) Ma, Z.; Wang, E.; Vandewal, K.; Andersson, M. R.; Zhang, F. Appl. Phys. Lett. 2011, 99, 143302. (38) Zhang, G.; Fu, Y.; Xie, Z.; Zhang, Q. Macromolecules 2011, 44, 1414. (39) Lei, T.; Cao, Y.; Fan, Y.; Liu, C.-J.; Yuan, S.-C.; Pei, J. J. Am. Chem. Soc. 2011, 133, 6099. (40) Lei, T.; Cao, Y.; Zhou, X.; Peng, Y.; Bian, J.; Pei, J. Chem. Mater. 2012, 24, 1762. (41) Mei, J.; Kim, D. H.; Ayzner, A. L.; Toney, M. F.; Bao, Z. J. Am. Chem. Soc. 2011, 133, 20130. (42) Lei, T.; Dou, J.-H.; Pei, J. Adv. Mater. 2012, 24, 6457−6461. (43) Estrada, L. A.; Liu, D. Y.; Salazar, D. H.; Dyer, A. L.; Reynolds, J. R. Macromolecules 2012, 45, 8211. (44) Wang, E.; Ma, Z.; Zhang, Z.; Vandewal, K.; Henriksson, P.; Inganäs, O.; Zhang, F.; Andersson, M. R. J. Am. Chem. Soc. 2011, 133, 14244. (45) Stalder, R.; Grand, C.; Subbiah, J.; So, F.; Reynolds, J. R. Polym. Chem. 2012, 3, 89. (46) Xu, X.; Cai, P.; Lu, Y.; Choon, N. S.; Chen, J.; Hu, X.; Ong, B. S. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 424. (47) Ma, Z.; Wang, E.; Jarvid, M. E.; Henriksson, P.; Inganäs, O. J. Mater. Chem. 2012, 22, 2306. (48) Hu, C.; Fu, Y.; Li, S.; Xie, Z.; Zhang, Q. Polym. Chem. 2012, 3, 2949. (49) Deng, Y.; Chen, Y.; Zhang, X.; Tian, H.; Bao, C.; Yan, D.; Geng, Y.; Wang, F. Macromolecules 2012, 45, 8621.
organic photovoltaics with an emphasis on understanding the molecular and energetic aspects of the donor−acceptor interface. Leandro A. Estrada recently joined the workforce at Solvay Specialty Polymers. He grew up in Venezuela, where he completed his ́ undergraduate studies in Chemistry at the Universidad Simón Bolivar, Caracas. He then joined the Ph.D. Program at Bowling Green State University, where he studied the photophysical properties of chromophoric molecular systems under the supervision of Prof. Pavel Anzenbacher and later Professors Douglas C. Neckers and Massimo Olivucci. In 2010, Leandro began his postdoctoral studies with Prof. John Reynolds at the University of Florida and later at the Georgia Institute of Technology. His time was mostly devoted to the synthetic development of π-conjugated polymers for charge storage and the fundamental understanding of the electronic structure of isoindigo-based materials.
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ACKNOWLEDGMENTS J.R.R. gratefully acknowledges the Air Force Office of Scientific Research (FA9550-09-1-0320) and the Office of Naval Research (N00014-11-1-0245) for financial support. R.S. and K.R.G. acknowledge the University Alumni Awards Program for a fellowship.
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REFERENCES
(1) (a) Molecular OPVs: Hains, A. W.; Liang, Z.; Woodhouse, M. A.; Gregg, B. A. Chem. Rev. 2010, 110, 6689. (b) Polymeric OPVs: Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Chem. Rev. 2009, 109, 5868. (c) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev. 2012, 112, 2208. (d) Zhong, C.; Duan, C.; Huang, F.; Wu, H.; Cao, Y. Chem. Mater. 2011, 23, 326. (e) Beaujuge, P. M.; Reynolds, J. R. Chem. Rev. 2010, 110, 268. (2) Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. Acc. Chem. Res. 2010, 43, 1396. (3) Anthony, J. E.; Facchetti, A.; Heeney, M.; Marder, S. R.; Zhan, X. Adv. Mater. 2010, 22, 3876. (4) Huang, C.; Barlow, S.; Marder, S. R. J. Org. Chem. 2011, 76, 2386. (5) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.; Facchetti, A. Nature 2009, 457, 679. (6) Guo, X.; Ortiz, R. P.; Zheng, Y.; Hu, Y.; Noh, Y.-Y.; Baeg, K.-J.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2011, 133, 1405. (7) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C. E.; So, F.; Reynolds, J. R. J. Am. Chem. Soc. 2011, 133, 10062. (8) Nielsen, C. B.; Turbiez, M.; McCulloch, I. Adv. Mater. 2013, 25, 1859. (9) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S.; Lai, T.; Reynolds, J. R.; So, F. Nat. Photonics 2011, 6, 115. (10) Chen, H.; Guo, Y.; Yu, G.; Zhao, Y.; Zhang, J.; Gao, D.; Liu, H.; Liu, Y. Adv. Mater. 2012, 24, 4618. (11) Mei, J.; Graham, K. R.; Stalder, R.; Reynolds, J. R. Org. Lett. 2010, 12, 660. (12) Baeyer, A.; Drewsen, V. Ber. Deut. Chem. Ges. 1882, 15, 2856. (13) Maugard, T.; Enaud, E.; Choisy, P.; Dominique, M. Phytochemistry 2001, 58, 897. (14) Bogdanov, A. V.; Mironov, V. F.; Musin, L. I.; Buzykin, B. I.; Konovalov, A. I. Russ. J. Gen. Chem. 2008, 78, 1977. (15) Bogdanov, A. V.; Mironov, V. F.; Musin, L. I.; Musin, R. Z. Synthesis 2010, 3268. (16) Flores, J.-C.; Berens, U.; Bienewald, F.; Kirnerand, H. J.; Turbiez, M. G. R. (CIBA) Ketopyrroles as organic semiconductors. PCT Int. Appl. WO 2009053291 A1, 2009; 150: 495343 (17) Mercier, L. G.; Leclerc, M. Acc. Chem. Res. 2013, 46, 1597. (18) Kuwabara, J.; Nohara, Y.; Choi, S. J.; Fujinami, Y.; Lu, W.; Yoshimura, K.; Oguma, J.; Suenobu, K.; Kanbara, T. Polym. Chem. 2013, 4, 947. (19) Grenier, F.; Berrouard, P.; Pouliot, J.-R.; Tseng, H.-R.; Heeger, A. J.; Leclerc, M. Polym. Chem. 2013, 4, 1836. N
dx.doi.org/10.1021/cm402219v | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Perspective
(50) Cao, K.; Wu, Z.; Li, S.; Sun, B.; Zhang, G.; Zhang, Q. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 94. (51) (a) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater. 2011, 23, 2367. (b) Hansen, W. N.; Hansen, G. J. Phys. Rev. A 1987, 1396. (c) Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2000, 298, 97. (52) Vandewal, K.; Ma, Z.; Bergqvist, J.; Tang, J.; Wang, E.; Henriksson, P.; Tvingstedt, K.; Andersson, M. R.; Zhang, F.; Inganäs, O. Adv. Funct. Mater. 2012, 22, 3480. (53) (a) Mannsfeld, S. C. B.; Tang, M. L.; Bao, Z. Adv. Mater. 2011, 23, 127. (b) Anthony, J. E. Chem. Rev. 2006, 106, 5028. (54) (a) Gaab, K. M.; Thompson, A. L.; Xu, J.; Martínez, T. J.; Bardeen, C. J. J. Am. Chem. Soc. 2003, 125, 9288. (b) Thompson, A. L.; Gaab, K. M.; Xu, J.; Bardeen, C. J.; Martínez, T. J. J. Phys. Chem. A 2004, 108, 671. (c) Zimmerman, H. E. J. Am. Chem. Soc. 1995, 117, 8988. (d) Havinga, E.; De Jong, R. O.; Dorst, W. Rec. Trav. Chim. PaysBas 1956, 75, 378. (e) Molina-Ontoria, A.; Fernández, G.; Wielopolski, M.; Atienza, C.; Sánchez, L.; Gouloumis, A.; Clark, T.; Martín, N.; Guldi, D. M. J. Am. Chem. Soc. 2009, 131, 12218. (f) González-Rodríguez, D.; Torres, T.; Herranz, M. A.; Echegoyen, L.; Carbonell, E.; Guldi, D. M. Chem.Eur. J. 2008, 14, 7670. (55) For insights in the utilization of this concept for the design of chromophoric relays with efficient charge photogeneration, see the following: (a) Wasielewski, M. R. Chem. Rev. 1992, 92, 935. (b) Piotrowiak, P. Chem Soc. Rev. 1999, 28, 143. (56) Lei, T.; Dou, J.-H.; Ma, Z.-J.; Yao, C.; Liu, C.; Wang, J.-Y.; Pei, J. J. Am. Chem. Soc. 2012, 134, 20025. (57) Sandmeyer. Helv. Chim. Acta 1919, 2, 237. (58) Marvel, C. S.; Hiers, G. S. Org. Synth. 1925, 5, 71. (59) Van Pruissen, G. W. P.; Gholamrezaie, F.; Wienk, M. M.; Janssen, R. A. J. J. Mater. Chem. 2012, 22, 20387. (60) Koizumi, Y.; Ide, M.; Saeki, A.; Vijayakumar, C.; Balan, B.; Kawamoto, M.; Seki, S. Polym. Chem. 2013, 4, 484. (61) Ashraf, R. S.; Kronemeijer, A. J.; James, D. I.; Sirringhaus, H.; McCulloch, I. Chem. Commun. 2012, 48, 3939. (62) Henson, Z. B.; Welch, G. C.; van der Poll, T.; Bazan, G. C. J. Am. Chem. Soc. 2012, 134, 3766.
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