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25th Anniversary Article: Progress in Chemistry and Applications of Functional Indigos for Organic Electronics

Eric Daniel Głowacki ,* Gundula Voss , and Niyazi Serdar Sariciftci

PO

RT Indigo and its derivatives are dyes and pigments with a long and distin-

guished history in organic chemistry. Recently, applications of this ‘old’ struc-ture as a functional organic building block for organic electronics applications have renewed interest in these molecules and their remarkable chemical and physical properties. Natural-origin indigos have been processed in fully bio-compatible fi eld effect transistors, operating with ambipolar mobilities up to 0.5 cm 2 /Vs and air-stability. The synthetic derivative isoindigo has emerged as one of the most successful building-blocks for semiconducting polymers for plastic solar cells with effi ciencies > 5%. Another isomer of indigo, epindo-lidione, has also been shown to be one of the best reported organic transistor materials in terms of mobility ( ∼ 2 cm 2 /Vs) and stability. This progress report aims to review very recent applications of indigoids in organic electronics, but especially to logically bridge together the hereto independent research directions on indigo, isoindigo, and other materials inspired by historical dye chemistry: a fi eld which was the root of the development of modern chem-istry in the fi rst place.

1 . Introduction

In modern science and technology of nanomaterials, interest in biological and bio-inspired materials is motivated by potential for biomedical integration and sustainable development. [ 1 ] Research in fi eld of organic semiconductor-based technologies has grown tremendously in the past decades. The success of organic photo-conductors in xerography in the 1960s–1990s [ 2,3 ] and the recent evolution of organic light emitting diodes [ 4,5 ] into a multi-billion dollar industry demonstrate relatively mature applications of organic semiconductors. Organic photovoltaics (OPVs), “plastic solar cells” have improved in power conversion effi ciency from ∼ 1% in 2000 [ 6–8 ] to values above 10% in the past years, [ 9–11 ] making OPV a serious player in the photovoltaic fi eld. Printable “plastic electronics” based on organic thin-fi lm transistors, syn-onymously-called organic fi eld-effect transistors (OFETs), have likewise been the topic of intensive research and commercial interest. [ 12–15 ] With the growing commercial success of organic semiconductor based technologies, and their potential for appli-cation at the interface of biomedical applications, evaluating bio-

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

DOI: 10.1002/adma.201302652

E. D. Głowacki, Dr. G. Voss, Prof. N. S. Sariciftci Linz Institute for Organic Solar Cells (LIOS)Physical Chemistry Johannes Kepler University Linz , Austria E-mail: eric_daniel.glowacki@jku.at

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inspired materials for organic electronics is crucial for producing sustainable and low-cost devices as well as enabling unique and biochemically-specifi c functionality. The search for such novel functional materials can lead us to reevaluate substances right under our noses, indeed things familiar since ancient times. The subject of this Progress Report is research in the indigo family of materials for organic electronics applications. We discuss progress in using indigos as organic semiconductors, under-standing their photophysics, and using them as functional synthetic building block for novel synthetic materials.

1.1 . Biocompatible Electronics and the Inspiration for Using Bio-Origin Dyes

In the past few years, a growing body of scientifi c work has focused on utilizing

the unique bio-integration and bio-functionality of organic con-ducting and semiconducting materials to fabricate devices for biomedical applications as well as various use-and-throw appli-cations. A few examples of biocompatible organic electronic devices are shown in Figure 1 . Some review articles have cov-ered this recent work. [ 16–19 ] In a series of papers in the years 2010–2013, we reported on our efforts to fabricate organic fi eld effect transistors using exclusively bio-compatible mate-rials. [ 18,20–22 ] Substrate materials used in these studies included cellulose-based polymers, gelatin, and caramelized glucose. Later we found that the natural resin shellac had superior prop-erties, and could be processed into robust substrates and could additionally be cast into thin fi lms with excellent insulating properties to function as the gate dielectric in the transistors. [ 23 ] The applicability of this resin in drug delivery applications made it particularly promising. A number of natural materials proved to function as gate insulators, including sugars like glucose and sucrose, as well as nucleobases guanine, adenine, cytosine, and thymine. A number of other groups have reported successful and creative demonstrations of other natural mate-rials for organic electronics. Paper is particularly attractive as a cheap and eco-friendly substrate material for OFETs [ 24–26 ] and OPVs. [ 27,28 ] Silk has been demonstrated to be a bio-resorbable material for implantable electronic devices, [ 29–31 ] and also can function as a gate dielectric for OFETs. [ 32 ] However, what was lacking in this work was natural-origin semiconductors with good performance. We initially explored β -carotene as a semi-conductor, however mobility remained low ( μ h ∼ 5 × 10 −4 cm 2 /Vs)

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Eric Daniel Głowacki com-pleted his MSc. degree (2010) in chemistry at the University of Rochester, in Rochester, New York, USA, in the group of Prof. Ching Tang. During the years 2007–2010, he worked at the Laboratory for Laser Energetics in Rochester. In 2013, he completed his PhD at the Linz Institute for Organic Solar Cells (LIOS),

under the supervision of Prof. Serdar Sariciftci. His research interests are functional dyes and pigments, as well as spectroscopy of organic materials.

Gundula Voss was born in 1947 near Rostock, Germany, where she also studied chemistry. She obtained a Ph.D. in 1978 from the University of Hamburg on heterocyclic chemistry. After graduation, she worked as research associate at the universities in Aachen and in Bayreuth, where she received a permanent position at the

Department of Organic Chemistry. In 2011 she became a research fellow at the Linz Institute for Organic Solar Cells (LIOS). Her present research focuses on the structure-function relationship of indigoids for organic electronics.

Niyazi Serdar Sariciftci is Ordinarius Professor for Physical Chemistry and the founding director of the Linz Institute for Organic Solar Cells (LIOS) at the Johannes Kepler University of Linz, Austria. He studied at the University of Vienna and graduated as PhD in physics in 1989. After two years postdoctoral study at the

University of Stuttgart, he joined the Institute for Polymers and Organic Solids at the University of California, Santa Barbara, USA, led by Prof. Alan J. Heeger. His major contri-butions are in the fi elds of photoinduced optical, magnetic resonance and transport phenomena in semiconducting and metallic polymers. He is the inventor of conjugated polymer/fullerene solar cells.

Figure 1. Some examples of biocompatible organic electronic devices. a) An example of OFETs fabricated using biodegradable and nontoxic materials, on caramelized glucose, and a gelatin drug carrier capsule – these are edible electronics. b) An array of OFETs fabricated on paper. Image courtesy of Hagen Klauk. [ 24 ] c) An electronic sensing element fabri-cated on natural bioresorbable silk, for food quality sensing. Reproduced with permission United States National Academy of Sciences. [ 30 ] d) An example of a microfabricated electrochemical transistor functioning as an ion pump to deliver neurotransmitters directly into neural tissues. Repro-duced with permission. [ 39 ] e) In vivo studies using organic conducting-polymer based transistors can be used for high-quality recordings of brain

[40]

and the material readily oxidizes and is thus unstable. [ 20,33 ] Upon surveying many conjugated molecules present in nature, we found that most natural chromophores are not dyes with large π -systems (with the carotenoids and porphyrins as the notable exceptions) but instead relatively small molecules that form pigments, i.e., they adopt their characteristic colors in the aggregated state. Examples include alizarin (madder root dye), napthoquinones, such as lawsone (Henna pigment), juglone (black walnut pigment), and indigos. Representatives of these classes of compounds are shown in Figure 2 . These examples are representative of the major categories of such dyes – in fact

activity. Reproduced with permission.

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many derivatives of each of these molecules are found in dif-ferent places throughout nature. Several expansive reviews of natural pigments exist. [ 34 ] These molecules contain carbonyl and amine groups in conjugated segments. Since under neutral pH

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Figure 2. Structures of representative natural-origin pigment-forming dyes used throughout history. In fact, very numerous derivatives of each of these compounds are found throughout nature in plants, animals, and fungi.

N

NO

OH

H

N

NO

OH

HBr

Br

O

O

OOH

O

O

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O

OOH

HO

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OO

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H

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O

OOH

H

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indigo

tyrian purple

polyporic acid

alizarin

lawsonejuglone

purpurin

conditions tautomeric forms featuring enol or imine character are unfavorable, carbonyl and amine groups are seen as inter-rupting π -conjugation. The prevailing notion among synthetic chemists is that when designing organic semiconducting small molecules or polymers, maximizing π -conjugation is crucial, and thus amine or carbonyl functional groups are avoided. [ 35–38 ] Thus the limited π -conjugation of such molecules led them to be overlooked by the organic electronic community, which sought molecules with extensive π -conjugation. Nevertheless, the large bathochromic shifts (50–80 nm) that occurs when these natural dyes aggregate into their pigment forms indicate

© 2013 WILEY-VCH Verlag GmAdv. Mater. 2013, 25, 6783–6800

that electronic coupling between molecules must be substan-tial. Indeed, a survey of the known crystal structures of indigo and alizarin, for example, reveals that these molecules pack with close and remarkably cofacial π – π stacking and therefore are arranged in a way that is considered favorable for charge transport. The subject of this progress report is how these mol-ecules can be applied to organic electronic applications.

2 . A Short History of Indigos

Along with the production of alcohol, bread, and cheese, the oldest ‘biotechnology’ known in human history is indigo dyeing. Archeological evidence shows that the practice of coloring fab-rics with indigo dyes existed since at least 2000 BC and was prac-ticed in different forms by civilizations in Asia, Europe, and the Americas. [ 41 ] The name originates from the Indus River, where the ancient Indus Valley civilization is thought to have fi rst mastered the dying process. The name “indigo” comes down to us from the ancient Greeks, who named indigo based on its origin. The ancient trade of Indigo and Tyrian Purple (primary constituent: 6,6’-dibromoindigo) production was perfected by Egyptian, Phoenician, Greek, and Roman craftsmen, whose techniques were carefully guarded secrets passed down from generation to generation. [ 41,42 ] Understanding the chemistry utilized by ancient indigo dyers is a major subject of interest for archeological chemists. [ 43,44 ] Indigo throughout history was the only available blue dye that could be used to color textiles, thus throughout ancient cultures garments colored with indigo and tyrian purple were extremely valuable and a symbol of wealth and power. The blue of indigo, mentioned in the Old Testament as “tekhelet” was the color of the Judaic priesthood. The underlying chemistry behind the Biblical indigo dying pro-cedure has been a subject of much study and debate and only recently satisfying explanations have been proposed. [ 34,45 ] In all cases of plant-origin indigo, indigo was produced from glyco-sidic precursors featuring an indoxyl ring attached to a sugar molecule ( Figure 3 ). The biochemical origin of such molecules is from the amino acid tryptophan. In Europe, the plant Isatis tinctoria , or Dyers’ Woad, was cultivated for its indigo-precur-sors. The ‘true’ indigo plant, Indigofera tinctoria , originating in India, contained 30 times more indigo precursor and was culti-vated in tropical climates ( Figure 4 a). The precursor compound is known is indican, and the synthetic pathway to indigo from indicant is shown in Figure 3 . Key steps involve the enzymatic cleavage of the glycosidic precursor, which was accomplished by fermenting the raw indigo plant matter. The resulting mol-ecule, Indoxyl, would then oxidatively dimerize in the pres-ence of oxygen, yielding a highly-insoluble blue indigo mass. Dying with the indigo was accomplished by reducing the indigo to produce the reduced form, known as leucoindigo, which is water-soluble. This process is called Vat dying, and is shown schematically in Figure 4 c. The reduced leucoindigo readily would penetrate textile fi bers dipped into the vat. Removing the textile would cause reoxidation in air of the leucoindigo to pro-duce insoluble blue indigo – now trapped within the fi bers. The chemical reduction of indigo in pre-modern times is thought to have been accomplished by several means, including sugar fermentation producing NADH as a reducing agent, or use of

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Figure 3. Synthetic pathways for indigo and tyrian purple from natural precursors. The upper-right fi gure shows the indigo numbering scheme accepted in most literature on indigos and used in this work. The bottom scheme shows the Baeyer-Drewsen indigo synthesis – a facile synthetic route to a wide variety of derivatized indigos.

NH

N

O

HIndoxyl

NBrH

Tyrindoxyl sulfate

NBrH

NBr

SMe

OSO3-M+

OH O

enzyme

enzyme

SMe SMe

oxidation

Tyriverdin

O2

Tyrindoxyl Tyrindoleninone

- CH3SSCH3N

NO

OBr

Br

H

HSCH3

SCH3

Acetone, -OH

o-Nitrobenzaldehyde

NO2

Baeyer and Drewsen "one-pot" Indigo synthesis (1882) :

H OH O

N

O

H

N

O

H

Indigo

D-GlucoseO

Indican

RNO2

H

O

N

O

H

N

O

H

-OH

aldol condensation product

RR R

lightN

O

H

N

O

H

Br

Br

Indigo

Tyrian Purple(6,6'-dibromoindigo)

Synthetic route of indigo fromglycosidic precursors in Indigofera tinctoria :

Tyrian purple from precursors in Muricidae snails :

6

54

7

3

21

2'

3' 4'5'

6'7'1'

metals like zinc as reducing agents. In modern times, the vat process used to make indigo blue jeans (which are the reason that indigo is the most-produced dyestuff worldwide today) is conceptually identical to the ancient vat dying procedure. Nowadays, reducing agents like sodium dithionite are used in alkaline water to produce vats of leucoindigo. The photo in Figure 4 c shows one of the authors along with colleagues vis-iting an indigo dyeing “ Blaufärberei ” in Bad Leonfelden, Aus-tria – one of the last surviving dying facilities doing traditional indigo dying as it had been done for centuries in central Europe. Tyrian Purple, the dibrominated derivative of indigo, originates not from plants but from animals. Several species of marine snails in the Mediterrenean Sea produce the purple 6,6′-dibro-moindigo in their hypobrachial gland (Figure 4 b). [ 41,46,47 ] The biosynthesis of tyrian purple is likewise shown in Figure 3 . Squeezing the glands of the snail releases the purple dye in tiny amounts. Thus thousands of snails need to be harvested to yield enough dye even to dye a small piece of textile, leading to

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Figure 4. a) Indigofera tinctoria , the tropical source of indigo. b) Snails fromc) The chemistry of vat dyeing. Indigos are reduced to produce an aqueouK + , Na + , or other metal counterions. The photo shows one of the authors vior brick indigo vats like this one were used for centuries in very much the s

the high value of the substance. Throughout the ancient world, purple became the symbol of power and wealth. The purple stripe on the toga was reserved in ancient Rome only for sena-tors and high-ranking offi cials. Indigo and its derivatives hold a special place in the history of modern organic chemistry. The pursuit of economically-competitive dyestuffs was one of the primary motivating factors for development of the fi eld in the 19 th century and the commercial raison d’être for industry giants such as BASF, AGFA, Ciba, and Hoechst. Many of the great founders and innovators of synthetic organic chemistry, such as William Perkin, Adolf von Baeyer, Emil Fischer, Heinrich Caro, and Carl Lieberman – were all associated with indigo chem-istry. [ 41,48 ] For his work on elucidating the structure of indigo, and its fi rst synthetic preparations, Adolf von Baeyer received the Nobel prize in chemistry in 1905. The Baeyer-Drewsen indigo synthesis, fi rst published in 1882, is one of the simplest ways for obtaining indigos synthetically. The reaction involves a base-catalyzed aldol condensation of o -nitrobenzaldehyde with

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the Muricidae and Thaisidae families are the source for tyrian purple dyes. s solution of indigo 1− and indigo 2− (known as leuco or “white” indigo) with siting a traditional indigo vat dying facility in Bad Leonfelden, Austria. Stone ame way in many parts of the world.

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Figure 5. a) A photo of an indigo transistor fabricated using TTC/Al 2 O 3 as the gate dielectric on top of a shellac resin substrate. b) UV-vis spectra of indigo in CHCl 3 solution compared with an evaporated thin-fi lm. c) X-ray diffraction taken normal to the substrate. The broad amor-phous peak originates from the glass substrate, only one indigo peak is visible at 2 θ = 11.06°, corresponding to the indigo [001] plane meaning that the molecules grow in the “Standing” orientation with stacking parallel to the substrate. d) Cyclic voltammetry of an indigo thin fi lm evaporated on ITO functioning as the working electrode. Reproduced with permission. [ 51 ]

acetone, followed by cyclization to 3-indolone and dimeriza-tion to indigo. [ 49 ] Substituted indigo derivatives can be obtained by using the corresponding substituted o -nitrobenzaldehydes. This synthesis is shown in Figure 3 . Other useful syntheses for substituted indigos include the Heumann-Pfl eger route, involving condensation of anilines with chloroacetic acid fol-lowed by cyclization in alkali conditions to yield indoxyl, which dimerizes to indigo, [ 42 ] and the method of Clark and Cooksey for preparing asymmetrically-substituted indigos from substi-tuted isatins and O-acetylindoxyl. [ 50 ]

3 . Indigo Semiconductors

We reported in 2012 in this journal that Indigo exhibits ambi-polar transport, with balanced electron and hole mobility of approx. 1 × 10 −2 cm 2 /Vs in OFETs. [ 51 ] The devices were fab-ricated using as many bio-origin materials as possible, with shellac resin serving as the substrate and Al 2 O 3 /tetratetracon-tane (TTC) serving as the gate dielectric onto which indigo was thermally evaporated. TTC is an oligoethylene present in many plants, famously present in the waxy part of the aloe vera plant. Biocompatible metals gold or silver were used for source/drain electrodes. These devices demonstrated that all-natural mate-rials could be used to fabricate transistors that were working on a reasonable level, on-par with many synthetic counterparts. A

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, WeinhAdv. Mater. 2013, 25, 6783–6800

photo of such a transistor sample is shown in Figure 5 a. Indigo, as other organic H-bonded pigments, is highly insoluble but readily sublimable and thus can be processed using vacuum evaporation. An important property of indigo and related materials is that they support reversible two-electron reduction and two-electron oxidation processes. Meas-uring cyclic voltammetry of indigo thin-fi lms evaporated onto an electrode, as shown in Figure 5 d, results in a quasi-reversible elec-trochemistry behavior. This is caused by the fact that the reduced or oxidized species are both highly soluble in the electrolyte solu-tion, thereby they fall away from the elec-trode and the current measured upon cycling back is smaller. From the electrochemistry it is apparent that indigo can be both reduced and oxidized at relatively low potentials, with a small electrochemical “bandgap” of 1.7 eV. This accounts for the balanced ambipolar charge injection from a single contact metal (in this case gold). One of the fi rst things that became apparent while fabricating tran-sistors with indigo was that performance was heavily dependent on the gate dielectric material. Devices would either show charac-teristic ambipolar transport or no transport whatsoever, depending on the dielectric. From XRD measurements on the fi lms it was apparent that indigo worked in OFETs only if the [001] plane was prominently developed in the growth direction, i.e. the molecules were

“Standing up” with the π -stacking parallel to the gate dielectric and thus oriented in the proper direction for transport in an OFET geometry (Figure 5 c). This favorable orientation could be achieved with aliphatic, hydrophobic dielectric materials such as TTC or vacuum-evaporated polyethylene. [ 52 ] Several other aliphatic polymers such a poly(butyl methacrylate) were also found to work. The crystal structure of indigo is shown in Figure 6 a. In the crystal structure, along the b-direction π – π is present, with an interplanar distance of ∼ 3.3 Å and distance between equivalent atom positions of 5.77 Å. Along the c-axis intermolecular H-bonding is present, with each indigo mol-ecule H-bonded to four of its neighbors with –NH–O= bond lengths of 2.11 Å. Along the a-axis there are only van der Waals contacts. The packing is relatively cofacial, especially when compared to the herringbone pattern present in many organic molecular semiconductors. Packing motifs enhancing cofacial stacking are considered favorable for charge transport. [ 53 ] Two modifi cations of indigo have been reported, based on single-crystal X-ray diffraction: The A form and B form. Both are very similar, consisting stacked molecules along the b-axis and the same intermolecular H-bonds, however with slightly dif-ferent alignment between neighboring stacks of molecules. [ 54 ] After obtaining interesting results with indigo, we turned to its dibromo derivative, tyrian purple. Though a natural-origin material, tyrian purple is not available commercially and thus we synthesized it according to Voss et al. [ 55 ] Tyrian purple

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Figure 6. a) Indigo crystal structure, measured by. [ 57 ] The interplanar spacing between two indigo molecules stacked along the b -axis is 3.35 Å, while the interatomic distances between equivalent positions is 5.77 Å. b) tyrian purple (6,6′-dibromoindigo) crystal structure, with interplanar spacing of 3.46 Å, and interatomic distances between equivalent posi-tions of 4.84 Å. c) Tyrian purple view along the b-axis (stacking direction). The [ 100 ] and other [h00] peaks are the only ones visible when tyrian purple is grown on hydrophobic gate dielectrics like TTC. This kind of stacking parallel to substrate is necessary for OFET transport.

behaved much like indigo in terms of electrochemistry and ambipolarity, however with mobility of 0.4 cm 2 /Vs. [ 52,56 ] Just like indigo, tyrian purple afforded transport only when depos-ited onto aliphatic gate dielectrics. The higher mobility of tyrian purple can be correlated with the enhanced, more cofacial

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packing in the solid state (Figure 6 b,c). However the higher observed mobility may be dictated by the fact that tyrian purple grew in larger grains than indigo on the same substrates, so the effect of less intergrain boundries in the transistor channel may play the crucial role. Due to substitution with bromine in the 6,6’ positions, both the HOMO and LUMO energies decrease. With a LUMO of 4.0 eV, tyrian purple demonstrated air stable electron transport in both OFETs and diode devices. Using the Baeyer-Drewsen indigo synthesis shown in Figure 3 , other substituted indigo derivatives can be obtained from the corre-sponding o -nitrobenzaldehydes. Recently 6,6’-dichloro indigo and 5-monobromoindigo were tested and preliminary results yielded ambipolar mobility in the 10 −2 cm 2 /Vs range. The former is known to crystallize with an interesting “brick wall” type packing, different from all other reported indigo structure which feature antiparallel stacks. Through appropriate sub-stitution to obtain different derivatives of indigo, a structure-mobility relationship can be found. A limitation to observed mobility in thin-fi lm OFET devices is the fact that fi lms are composed of crystalline grains of several hundred nanometers in size. Intergrain resistance is thought to be a primary hurdle for higher mobility. Fabrication of short-channel devices as well as measurements of single-crystal transistors must be carried to learn what the intrinsic mobility limits in this class of mate-rials are. Table 1 summarizes OFET device data for different indigo derivatives discussed in this progress report.

4 . Photophysics of Indigos

Indigo has always been intriguing due to its brilliant blue color, indeed it is the only naturally-available blue organic dye with suffi cient stability for practical applications. This explains the ubiquity of indigo as a dye throughout history. An important consideration about indigo optical properties is that indigo forms H-bonded pigment aggregates. Indigo in the pigment form demonstrates excellent light-fastness and stability, in solution however under the infl uence of ozone and hydroxyl radicals it will slowly oxidize, into the yellow-colored isatin and anthranilic acid. [ 61 ] Both compounds are considered nontoxic and are pre-sent in nature as well. These are important considerations when discussing indigo and its derivatives for disposable use-and-throw electronics applications or biointegratable devices.

Indigo undergoes a pronounced bathochromic shift of 50–60 nm in absorbance when going from isolated molecules in dilute solutions to aggregates. Indigo can be dissolved in some solvents such as chloroform and DMSO with a max-imum concentration of about 100 μ M. In such solutions the absorbance peak occurs at around 600 nm ( λ max tetrachloroethane = 605 nm, ε = 16 580). [ 62 ] Solvatochromic effects have also been reported. Increasing of solvent dielectric constant leads to bathochromic shift in absorbance, which has been interpreted as resulting from the improved stability of charge-separated resonance forms featuring C + -O − , for example. [ 62–64 ] Colloidal particles dispersed in water [ 65 ] or evaporated thin fi lms [ 66 ] show absorbance maxima around 650 nm. Highly-crystalline thin fi lms have even further shifted absorbance around 700 nm. [ 67 ] This is consistent with our fi ndings: the UV-Vis absorbance of Indigo in chloroform and in thin-fi lm form is compared in

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Table 1. Basic organic semiconductor benchmark data for indigo and some of its derivatives.

Name Structure HOMO [eV] a)

LUMO [eV] a)

E g [eV] b)

μ e [cm 2 /Vs]

μ h [cm 2 /Vs]

Ref.

Indigo

−5.5 −3.8 1.7 .01 .01 [ 51 ]

Tyrian Purple

−5.8 −4.0 1.8 0.4 0.4 [ 56,58 ]

Cibalackrot

−5.6 −3.5 2.1 .009 .005 [ 33 ]

Thioindigo

− − 1.9(opt.) 1 × 10 −4 6 × 10 −4 [ 51 ]

6,6’-dichlorodindigo

−5.4 −3.6 1.8 0.08 0.08 [ 59 ]

5-bromoindigo

−5.4 −3.8 1.6 .02 0.2 [ 59 ]

epindolidione

−5.6 −2.9 2.7 - 1.5 [ 60 ]

a) Estimated based on cyclic voltammetry measurements of evaporated thin fi lms. b) Electrochemical bandgap, with the exception of thioindigo, where the optical bandgap is given.

Figure 5 b. The extent of the bathochromic shift of isolated mol-ecules relative to solid state has been correlated with intermo-lecular H-bonding. Indigo with substituents in the 4,4′ and 7,7′ positions, causing steric blocking of intermolecular H-bonding, indeed show no bathochromic shift in absorbance when com-paring solid state with solution. [ 65 ] Theoretical calculations sup-port that intermolecular H-bonding lowers the energy of the LUMO and is responsible for the bathochromic shifts observed experimentally. [ 68,69 ] The key question about the optics of indigo is: Why is it blue? It is surprising that such a small molecule can have a chromophore absorbing at such low energies. Much experimental and early theoretical work was done by Lüttke and coworkers in the 1960s. [ 70 ] They adopted the approach of “stripping down” the indigo molecule, selectively removing the phenyl rings and other components and observing the changes in optical behavior. Their work ultimately concluded that the primary chromophore in indigoids is what they called the “H-chromophore”, a cross-conjugated system of two elec-tron-donors, such as NH groups, and acceptors: two carbonyl groups, Figure 7 a. Modern DFT calculations confi rm this pic-ture, where light absorption results in a transfer of electron density from the donor amines to acceptor carbonyl groups. This transfer of electron density increases the acidity of the NH protons and the basicity of the carbonyl groups. [ 68,69 ] One of the most well-known properties of indigos is that they are not

© 2013 WILEY-VCH Verlag GmAdv. Mater. 2013, 25, 6783–6800

luminescent, with fl uorescence yield values φ f ∼ 1 × 10 −3 . [ 71–74 ] Instead, rapid internal conversion occurs with yields of >0.99, which is thought to be the reason for the exceptional photo-chemical stability of indigo. There is some controversy con-cerning the mechanism responsible for this rapid and effi cient internal conversion. The theory that is supported by the largest body of experimental evidence is that ultrafast proton transfer occurs in the indigo excited singlet state, leading to a tautom-erism of the keto form to the enol form. The enol form then rap-idly interconverts back to the more stable keto form, thereby dis-sipating the energy of the photoexcitation. This process is illus-trated in Figure 7 b. The theory of excited state deactivation via ultrafast proton transfer is supported by several experimental studies. [ 71,73 ] Some studies have concluded that single proton transfer occurs, instead of the concerted double-proton transfer leading to keto-enol tauromerism. [ 75,76 ] In all cases, however, deactiviation of the excited state is attributed to proton transfer. However at least one experimental study concluded that the evidence for proton transfer is lacking. [ 77 ] When the –NH protons are eliminated, by attaching substituents like methyl groups, what is found is that fl uorescence increases and com-petes with photochemical trans-cis isomerism, with the latter as the dominant mechanism, Figure 7 c. [ 78–81 ] Photoisomerism is accompanied by a signifi cant change in molecular geometry. This photomechanical effect can be exploited in light crystal

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Figure 7. Photophysics of indigos. a) The “H-chromophore”, responsible for the low-energy absorption of indigo, consisting of a cross-conjugated system of electron donor units, X, and carbonyl group acceptors. b) Indigo is well-known for its low fl uorescence yield and rapid internal conversion. The internal conversion pathway is attributed to ultrafast proton transfer. Two models involving a keto-enol tautomerism with one- or two-proton transfer exist. c) exchanging the –NH protons for other substituents, such as methyl groups, yields a molecule which readily undergoes reversible trans-cis isomerism as the dominant photophyscal process. d) Removing protons and blocking possible photoisomerization, as in the case of the Cibalackrot molecule, yields in turn a molecule with effi cient fl uorescence.

N

NO

OH

H

N

NO

OH

H

N

NOH

HO

di-enol indigoN

NO

OH

H

light absorption

*

S0

S1

Double proton transfer mechanism

N

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H

single proton transfer mechanism

N

NO

HO

H mono-enol indigo

*a

N

N

O

O

N

NO

OH

H

Cl

O

indigo

Cibalackrot

N

NO

ON N

O O

φf= 0.93

green light

Δ

N,N'-dimethylindigo

b

c- proton pathway blocked- trans-cis isomerism blocked

trans-cis isomerism

optoelectronics as a switch [ 82 ] and in light-responsive mem-branes. [ 83 ] Photoisomerism is normally prevented in indigo due to strong intramolecular H-bonding between –NH protons and the carbonyl groups. An interesting consideration, then, is to eliminate the protons responsible for excited state deactivation, but also to “lock” the indigo in the trans confi guration using a covalently-bound bridge. One such compound is Cibalackrot, a derivative of indigo fi rst reported in 1914. [ 84 ] Cibalackrot was reported to have a φ f of 0.93 [ 81 ] in methylcyclohexane and 0.76 in dioxane. [ 85 ] We synthesized Cibalackrot via con-densation of indigo with phenylacetyl chloride (Figure 7 d) according to [ 84 ] and fabricated transistors and diodes with the material. The electrochemistry of cibalackrot is very similar to that of indigo, with two-electron reduction and two-electron oxidations present. Transistors are similarly ambipolar, though

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mobility remained low, in the 1 × 10 −3 cm 2 /Vs range. Diodes could be fabricated as well, and upon charge injection in for-ward bias red electroluminescence was observed, in contrast to indigo diodes which emitted no light. [ 33 ] The high fl uores-cence yield, combined with ambipolar transport, recommend cibalackrot as an interesting building block for electrolumines-cent or light harvesting device applications.

4.1 . Photoinduced Charge Transfer

Photoinduced charge transfer is the fundamental operating principle behind donor-acceptor OPV cells. Despite the ubiquity of indigo, only limited studies concerning some indigo deriva-tive dyes (never NH containing pigment-forming compounds)

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Figure 8. a) Schematic showing photoinduced electron transfer from P3HT to indigo, and photoinduced hole transfer from photoexcited indigo to P3HT. b) Photoluminescence quenching of P3HT with indigo in solution. c) Light-induced electron-spin resonance of a P3HT/5,5′-dibromoindigo blend, excited at wavelengths to selectively excite P3HT or the indigo.

have been reported in the past on photoinduced charge transfer reactions. In two independent studies, both published in 1986, several N,N ′-disubstituted indigos were reported to undergo photoreduction in the presence of suitable electron donor molecules, as well as photooxidation in the presence of accep-tors. [ 86,87 ] This behavior of readily participating in photoinduced electron transfer reactions as a donor and acceptor is rational-ized by the low oxidation potential and high electron affi nity of N,N ′-disubstituted indigos, whose electrochemical behavior is very similar to indigo itself. In our recent research, we have endeavored to establish if similar photoinduced charge transfer reactions occur with the H-bond forming indigo derivatives, with the ultimate application of OPV in mind. Recently we found that H-bonded indigo as well as derivatives such as 5,5’-dibro-moindigo undergo photoinduced charge transfer ( Figure 8 a) to and from the semiconducting polymer P3HT, poly(3-hexylth-iophene). Small amounts of indigo in CHCl 3 solution quench the photoluminescence of P3HT (Figure 8 b). Blends of P3HT and 5,5′-dibromoindigo were prepared using the latent pig-ment technique described later in section 7. These blends were measured using light-induced electron spin resonance, L-ESR, a useful technique [ 88–90 ] for establishing the existence of pho-toinduced charge transfer. Upon photoexcitation using a green laser, which excites the P3HT, a double-signal originating from the radical cation on the P3HT and the radical anion on the 5,5′-dibromoindigo is observed. This is the classic signature of photoinduced charge transfer in donor-acceptor blends. Inter-estingly, selectively exciting the 5,5′-dibromoindigo by using red excitation (690 nm) also yielded a double-signal, indicating that hole transfer from the photoexcited indigo pigment to P3HT had occurred (Figure 8 c). This suggests that in a donor-acceptor blend, absorption from indigo could potentially contribute to photocurrent. This fi nding is also interesting from the perspec-tive of photophysics of donor/acceptor blends, as the existence of photoinduced charge transfer from the indigo indicates that this process effectively competes with the very rapid internal conversion mechanism. Light-induced ESR measurements are carried out at low temperatures (60K in this case), thus it is not clear at this point how effectively photoinduced charge transfer might compete with internal conversion at room tem-perature. Initial attempts to fabricate bulk heterojunction P3HT/indigo solar cells were successful in producing working devices however with low effi ciency: this is described in more detail in section 7. A wide range of indigo derivatives electrochemically furnish reductions at relatively low potentials (corresponding to a LUMO level in the –3.6–4.0eV range) suggesting that indigos are a promising materials family to explore as acceptor, n-type materials for bulk heterojunction OPV. Other H-bonded pig-ment molecules have emerged as candidates for single-layer organic solar cells, in particular the indigo-related material quin-acridone [ 91 ] – however the rapid internal conversion in indigos limit the amount of excitons that can be separated into electron-hole pairs in the absence of a donor/acceptor interface.

5 . Isoindigo

Isoindigo is an isomer of indigo, with a blue-shifted absorption relative to indigo – Isoindigo appears yellow. [ 64,92 ] Though not

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utilized extensively for dyestuff applications, isoindigo deriva-tives have been extensively studied as pharmaceuticals, being a component of some anti-cancer drugs, for example. [ 93–95 ] The molecule can be synthesized as shown in Figure 9 , via a con-densation of isatin and oxindole. [ 96 ] The ready availability of

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Figure 9. The facile synthesis of isoindigo from fusion of isatin and oxin-dole in acetic acid. Brominated isatin and oxindole can be coupled to yield dibromoisoindigos that are readily amenable to further substitution.

N

O

H

ON

O

H

NO

H

NO

H

CH3COOH

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isatin oxindole

isoindigo

many substituted isatins enhances the molecular palette that can be achieved using isoindigos. Isoindigo is a completely planar molecule, known to form infi nite H-bonded chains and a “bricks-in-a-wall” solid-state structure. [ 97 ] Isoindigo was fi rst applied in the fi eld of organic semiconductors in 2010, where Reynolds et al. recognized that isoindigo, as a planar aromatic electron-defi cient unit, can be incorporated with electron-rich thiophenes moieties to produce a conjugated molecule with an optical bandgap of 1.7 eV. They demonstrated that their donor-acceptor-donor oligomer could be fabricated into bulk hetero-junction solar cells with PCBM providing a power conversion effi ciency (PCE) of 1.76%. [ 98 ] The group of M. Andersson pub-lished in 2011 copolymers of benzodithiophenes with isoindigo units, which could be used in polymer solar cells to achieve PCE up to 4.22% [ 99 ] and copolymers with terthiophene giving a PCE of 6.3%. [ 95 ] The same year, the fi rst OFETs using isoindigo were published. Lei et al. reported a copolymer (IIDT) of isoin-digo and bithiophene with a hole mobility of 0.79 cm 2 /Vs. [ 100 ] A follow-up publication in 2012 showed that the same polymer was optimized to yield a mobility of 1 cm 2 /Vs. [ 101 ] The same group later demonstrated ambipolar transport using fl uori-nated isoindigo units, which could support hole mobility of 1.85 cm 2 /Vs and electron mobility of 0.43 cm 2 /Vs. [ 102 ] The elec-tron-withdrawing fl uorine atoms lower the reduction potential of the polymer and thus lower the transport level for electrons. Throughout the literature on isoindigo-containing polymers it appears that the polymers show quasi-reversible oxidation and reduction. Other copolymers containing donor-units of dith-ienosilole and anthracene have been demonstrated in OFETs and OPVs. [ 103 ] Two reports utilizing n-type isoindigo-con-taining polymers as acceptors in polymer-polymer solar cells show some promise, though PCEs remain below 1%. [ 104,105 ] Ashraf et al. recently synthesized derivatives of isoindigo con-taining thiophene rings instead of phenyls, and produced copolymers of this novel unit with benzothiadiazole that sup-ported ambipolar transport with a mobility of 0.1 cm 2 /Vs. [ 106 ] Isoindigo-containing copolymers with anthracene and naph-thalene units have also been demonstrated in OFETs and OPVs. [ 107 ] To our knowledge, the highest reported PCE (6.51%) featuring isoindigo is for an unusual polymer consisting of a benzodithiophene backbone with pendent isoindigo/thiophene side groups. [ 108 ] The highest fi eld effect mobility in isoindigo-based polymers was recently reported by the group of Z. Bao, who showed that by attaching siloxane-termined side-chains to the IIDDT polymer, a hole mobility of 2.48 cm 2 /Vs could be achieved. [ 109 ] A summary of isoindigo materials reported in

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the literature for organic electronics applications is shown in Figure 10 .

5.1 . Indigo vs. Isoindigo – Some Refl ections

In a review article entitled “A renaissance of color: New struc-tures and building blocks for organic electronics”, M. Robb et al. discuss the recent resurgence of interest in many dyes and pigments as functional building blocks for organic semicon-ductors. [ 110 ] In this work, the authors state that indigo does not possess the characteristics of a good building block owing to its discontinuous π -conjugation and remark that “its widespread application in materials for organic electronics is limited.” [ 110 ] The authors conclude that isoindigo should be considered as a superior building block due to the ease with which is can par-ticipate in forming fully-conjugated molecules. Indeed this is true in terms of maximizing potential intramolecular conjuga-tion. Intramolecular π -conjugation has been considered requi-site in the designing of organic semiconducting materials since the pioneering work of Nobel laureates Heeger, McDiarmid, and Shirakawa. The manipulation of optical and transport properties has primarily been accomplished through the syn-thetic modifi cation of individual molecules or macromolecules. This line of research has led to a relatively sophisticated under-standing of tailoring molecular structure to achieve a specifi c set of properties. It is known that mobility along continuous π -conjugated molecules, such as along a polymer chain, can be very high. [ 35,37,38,111,112 ] Indeed the 2D extrapolation of this concept is the realization of graphene layers, which can sup-port mobility as high as 15,000 cm 2 /Vs. [ 113 ] However, in typical organic semiconductor devices, it is inter- and not intramolec-ular conduction that ultimately limits mobility. Recent studies of H-bonded indigo and its derivatives underline that, though they are not well-conjugated intramolecularily according to the prevailing design rules for organic semiconductors, they form crystalline solids with a considerable overlap integral between neighboring molecules. Ultimately it is the charge transfer integral between molecules that facilitates conduction. [ 114 ] In the case of natural-origin pigments, as well as many synthetic pigments, intermolecular conjugation is driven by interplay of hydrogen-bonding and π – π interactions, leading to organic solids with high lattice energies. This research suggests that ideas from organic crystal engineering, regularly applied in the pigment industry, [ 115–118 ] might be used to design organic solids with desired properties such as optimal π – π overlap as well as tailor desired optical properties. [ 91 ] The idea of “crystal-engineered” organic semiconductors has appeared in the litera-ture in a limited way, [ 119,120 ] nevertheless, indigo-type molecules should invigorate this concept.

6 . Epindolidione

Epindolidione is another structural isomer of indigo, featuring a four six-membered ring fused system analogous to tetracene ( Figure 11 ). Epindolidione was fi rst reported in 1934, and was synthesized with the intention of comparing it to indigo. [ 121 ] After arriving at the target compound via a complex multi-step

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Figure 10. Isoindigo-based materials reported for OFETs (left column) and OPVs (right column). References are shown in brackets.

S

S

O

O

C2H5

C4H9

C2H5

C4H9

S

N O

NO

C6H13

C6H13

C6H13

C6H13

Sn

PBDT-TIT

[99]

N O

NO

C10H21

C8H17

C8H17

C10H21

SS

n

IIDDT

[101]

N O

NO

C6H13

C8H17

C8H17

C6H13

SS

P3TI

S

C8H17C8H17

[95]

S

S

O

O

C2H5

C4H9

C2H5

C4H9

S

S

N

O

2-ethylhexyl

N

O

2-ethylhexyl

PBDT-TID

[108]

NO

N O

C4H9

C2H5

C2H5

C4H9

S S

Si2-ethylhexyl 2-ethylhexyl

n

NO

N O

C4H9

C2H5

C2H5

C4H9

S

SS

SC6H13

C6H13

[98]

[103]

N OR

NOR

SS

n

PFII2TF

R=C14H29

C14H29

F

[102]

μh = 1 cm2/Vs

μh = 1.85 cm2/Vsμe = 0.43 cm2/Vs

PCE = 1.76%

PCE = 4 %

PCE = 6.51%

PCE = 6.3%

PCE = 4.22%

N O

NO

C10H21

C8H17

C8H17

C10H21

S

S

NS

N

n

μe =μh = 0.1 cm2/Vs

[106]

N O

NO

C10H21

C8H17

C8H17

C10H21 n

PCE = 1.13%

μh = 0.013 cm2/Vs

[107]

PISD-ANT

IGT-BT

synthesis, the researchers were surprised that epindolidione was a yellow and highly-fl uorescent compound and thus did not resemble indigo at all. Interest in epindolidione reemerged in the 1960s following the commercial success of the structur-ally-related quinacridone pigments. In 1967, Jaffe and Matrick reported essentially the same synthetic methodology of their successful quinacridone synthesis to produce epindolidione (Figure 11 b). The compound was the subject of several pat-ents: for electrophotographic processes, [ 122 ] a 1984 patent from Kodak disclosing the use of epindolidione among a long list of suitable organic light-emitting diode materials, [ 123 ] and inkjet and other pigment formulations. [ 124 ] A 1989 paper from the Kodak labs likewise reported that evaporated epindolidione thin-fi lms demonstrated photoconductivity response and an

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estimated hole mobility of approx. 0.01 cm 2 /Vs. [ 125 ] Interest-ingly, it was found in 1995 that epindolidione can be produced from the direct thermal isomerization of indigo in vacuum at 460 °C (Figure 11 a). [ 126 ]

This thermal rearrangement to form epindolidione was found to proceed with 80% yield. The authors also noted the remarkable stability of the epindolidione pigment at tempera-tures up to 1000 °C. We reported earlier this year in this journal a study comparing the performance of hydrogen-bonded pig-ments epindolidione and quinacridone with their well-known acene analogs, i.e., tetracene and pentacene. [ 60 ] We found that epindolidione shows similar electrochemical behavior to indigo, featuring quasi-reversible two-electron reduction and oxida-tion. The high crystal lattice energy, caused by interplay of π – π

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Figure 12. a) Transfer characteristic of an epindolidione OFET using 32 nm Al 2 O 3 passivated with TTC as the gate dielectric. Mobility in this device is 1.5 cm 2 /Vs for holes. b) The same OFET device after 100 days in air. The degradation of mobility is about 20%, however on/off ratio and threshold voltage do not change signifi cantly. c) A comparison of mobility and on/off ratio over time in air for epindolidione OFETs compared with tetracene ones. Reproduced with permission. [ 60 ]

Figure 11. a) Direct synthesis of epindolidione via the thermal rearrange-ment of indigo. b) Synthesis from dihydroxy fumaric acid condensation with aniline, followed by cyclization.

N

O

H

N

O

H

460 oC

N

NO

OH

H

HO

OHH3CO

O

OCH3

O

NH2

a)

b)

stacking and H-bonding, is refl ected in the high temperature of sublimation, which was found via thermogravimetric analysis to be 404 °C, versus 280 °C for tetracene. The performance of epindolidione in OFETs demonstrated a fi eld-effect mobility for holes of 1.5 cm 2 /Vs. A transfer characteristic of an epin-dolidione OFET fabricated using Al 2 O 3 passivated with tetra-tetracontane is shown in Figure 12 a. The OFET devices were stored and measured in air over the course of 140 days and were found to have outstanding stability, especially compared to tetracene which degrades rapidly in air. Mobility and on/off ratio of epindolidione were found to decay to 80% of the initial values within ∼ 85 days of testing in air (Figure 12 b,c), placing epindolidione along the most air-stable OFET semiconductors. This degradation stabilizes at this level, and the devices initially published in [ 60 ] were measured in May 2013 and found to still retain a mobility of 0.7 cm 2 /Vs after 325 days in air. As in the case of other indigo derivatives, the polycrystalline nature of the fi lm and intergrain resistance is believed to be the primary limitation in measuring higher mobilities. Nevertheless, epin-dolidione is currently one of the best OFET materials reported in terms of performance and stability. In terms of molecular design of organic semiconducting materials, epindolidione rep-resents a potentially useful bridge between indigos and linear fused-ring semiconducting materials.

7 . Solubilization and Functionalization of Indigo

Indigo, as all H-bonded pigments, has very poor solubility in organic solvents. This limits their thin-fi lm processability to vacuum-evaporation. Additionally, it seriously hampers any synthetic chemical reactions that might be used to derivatize indigo. Recently, we employed the technique of creating a latent pigment of indigo using thermolabile protecting groups to allow solution processing as well as to generate a form of indigo amenable to further chemical modifi cation. [ 127 ] The thermola-bile protection group chosen is the tert -butoxy carbonyl (tBOC) group, well-established in the fi eld of peptide chemistry for protecting the reactive amine function. [ 128 ] In 1992, researchers at the Ciba-Geigy pigment division in Basel disclosed in a patent on producing soluble precursors, or “latent pigments”, using the tBOC group that allowed the solution processing of

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dyes that could be regenerated into H-bonded pigments. They applied this technique for their diketopyrrolopyrrole class of pigments, as well as quinacridones for industrial coloring appli-cations. [ 129,130 ] This method relies on the conversion of the NH group into a carbamate, thereby breaking the intermolecular

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Figure 13. Synthesis of the latent pigment of indigo – N,N′-ditBOC indigo. Films of the latter can be solution processed and then heated to regenerate H-bonded fi lms of the parent indigo pigment.

Figure 14. Molecular structures of N,N′-ditBOC derivatives obtained from single crystal diffraction.

H-bonding and attaching highly-solubilizing tert -butyl groups – resulting in soluble dye molecules. ( Figure 13 ) The dye can be processed using organic solutions and heat and or acid treat-ment can remove the tBOC groups when desired. A signifi cant advantage of tBOC groups is that they decompose into gaseous products: CO 2 and isobutene. Upon deprotection, the dyes aggregate via H-bonding, thus regenerating the pigment. Some interesting, but limited, applications of this technique have also been used in the fi eld of organic electronics, such as making solution-processible and photopatternable polyaniline, [ 131 ] solution-processed quinacridone/fullerene solar cells, [ 132 ] and diketopyrrolopyrrole polymers for OFET applications. [ 133 ] We applied this tBOC protect-deprotect technique to a range of sub-stituted indigo derivatives, and were able to prepare solution-processed fi lms as well as mixed blends with polymers like P3HT. The tBOC indigo derivatives are red or purple in color, having hypsochromically-shifted absorption by about 50 nm relative to the parent compounds. The N,N′-ditBOC indigo has a λ max = 546 nm, ε = 6786. This shift is ostensibly related to the breakdown of the H-chromophore – as turning the NH groups into carbamates signifi cantly decreases their electron-donating character. X-ray crystal structures we obtained [ 134 ] for N,N’-ditBOC indigo and 5,5′-dibromo N,N’-ditBOC indigo revealed that the molecules are highly distorted around the central double bond between indole rings, with the two bulky tBOC groups on one face of the molecule pushing both indole rings away, yielding a “propeller-shaped” molecule ( Figure 14 ) similar in shape to N,N′-dimethylindigo. [ 135 ] As in the case of other indigos without intramolecular H-bonding (see section 4), N,N′-ditBOC indigos demonstrate reversible trans-cis photoi-somerism in solution.

Organic transistors and organic solar cells using soluble tBOC indigo precursors showed that following deprotection and annealing, the indigo pigments formed very large crystal-lites, on the size order of tens of microns or even larger. Thus fi lms were very rough and discontinuous, making fabrication of transistor devices diffi cult and observed mobilities were very low. With blends of tBOC indigos and P3HT spin-cast together to form homogenous thin fi lms, which when heated produced also indigo pigment crystallites within the polymer matrix. Photoinduced charge transfer in such blends was verifi ed by photoluminescence quenching measurements and light-induced electron spin resonance (see Section 4). Working OPV devices were obtained in this way, however photocurrents

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were in the range of several hundred microamps/cm 2 under simulated solar illumination. The issue limiting photocurrent was found to be the large size of indigo domains, which were on the size order of 1–3 μ m. Indeed polymer/fullerene blends with similar rough intermixing are known to yield low pho-tocurrents in the same range or lower. [ 136,137 ] With the proper substitution on the indigo molecules and optimization of pro-cessing conditions we believe that obtaining fi ner nanomor-phology with phase separation on the order of tens of nanom-eters, which is considered optimal for bulk heterojunctions, [ 8,9 ] can be obtained.

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Figure 15. "Extending the indigos" – derivitization reactions possible with soluble N,N′-ditBOC indigos.

N

N

O

O

O O

O O

S

S

N

N

O

O

O O

O O

N

N

O

O

O O

O O

O

O

H

H

N

N

O

O

O O

O O

Br

BrS B

OH

OH

tert-BuLi, -78 oCO

H N

Suzuki

Sonogashira

formylation

Pd0

Pd0, CuI

7.1 . Chemistry with Solubilized Indigo

Indigo protected with the tBOC group can easily be purifi ed by column chromatography. [ 127 ] Mixtures of 5-bromoindigo, 5,5′-dibromoindigo, and indigo – which would be very diffi cult to separate by other means – can be functionalized with tBOC protecting groups and readily separated chromatographically. More importantly, solubilized N,N′-ditBOC indigos lend them-selves well to further synthetic derivation that would be other-wise impossible with the highly-insoluble H-bonded indigos. 5,5′-dibromoindigo, which can be produced by direct bromi-nation of indigo, or 6,6′-dibromoindigo, prepared using [ 55 ] can be functionalized by tBOC and then coupled to other synthetic building blocks via Suzuki or Sonogashira chemistry. We were successful in obtaining several “extended” indigos using this chemistry, with some examples shown in Figure 15 .

8 . Conclusions and Future Perspectives

Indigo and its derivatives are molecules with a long history in chemistry and society. Today they are returning to the attention of science as air-stable, high-mobility organic semiconductor materials. The recent interest in ‘old’ dyestuff chemistry, par-ticularly natural-origin dyes, may usher in a new generation of organic semiconducting materials and devices with inherent biocompatibility and sustainability; as well as unique bio-functionality. The concept that a large intermolecular transfer integral within H-bonded organic crystals may be as good a

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criteria for charge transport as intramolecular conjugation is in conjugated polymers may be an important point arising from research on indigos. In this Progress report, we endeavored to give a snapshot of recent work on indigo, epindolidione, and derivatized isoindigo and tBOC indigo, and place this in the context of organic electronics research today. Recent work on indigo materials raises many questions and opens up several avenues of future perspectives. Crystal engineering organic semiconductors, where supramolecular effects are relied upon to enhance intermolecular conjugation, is a new concept for organic electronics which has substantial potential to pro-duce materials with high stability. The importance of organic crystal engineering has been recognized in the fi eld of indus-trial pigment technologies – this body of knowledge is poised to be directly applied to organic semiconductor design. There-fore the fi rst future perspective for this family of H-bonded organic semiconductors is an in-depth understanding of the molecular structure vs. crystalline packing vs. mobility relation-ship. Once certain principles about solid state structure vs. per-formance are understood, materials chemists must come into play to create novel H-bonded materials, “extended” indigos that utilize crystal engineering principles and molecular struc-ture to achieve target properties. The fi nal goal of these efforts is organic electronics with high performance, stability, and simple, cheap, and sustainable chemical syntheses routes for the active materials. Both indigo and isoindigo materials have proven successful in OFETs, however, solar cell materials based on H-bonded indigos are still lacking. Isoindigo as a building-block in polymers for organic photovoltaics has proven to be

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very successful. The ultrafast internal conversion of the excited state of H-bonded indigo molecules, however, may make it dif-fi cult for them to produce photocurrent with reasonable effi -ciency. Whether the fast radiationless processes responsible for the stability of this materials class is an obstacle for optoelec-tronics applications has yet to be determined. Results with high photocurrent yields in the related H-bonded quinacridone sug-gest that such materials could, in fact, be useful in solar cells. [ 91 ] Finally, a major future perspective for this class of materials is biointegration. The fi eld of OFET-based bioanalyte detectors and organic semiconductor and conductor in vivo biomedical devices is growing at present. Devices utilizing the inherent biocompatibility of many indigo and indigo-like materials com-bined with the useful chemical handles, i.e . amine, carbonyl groups, have yet to be realized.

Acknowledgements This article is part of an ongoing series celebrating the 25 th anniversary of Advanced Materials . Research at the Johannes Kepler University is supported by the Austrian Science Foundation, FWF, within the Wittgenstein Prize of N. S. Sariciftci and the Translation Research Project TRP 294-N19 “Indigo: From ancient dye to modern high-performance organic electronics circuits”. We are grateful to Mihai Irimia-Vladu and Siegfried Bauer for stimulating discussions. We thank the Wagner family, traditional indigo dyers, in Bad Leonfelden, Austria, for their hospitality and collaboration in bridging science, art, history, and culture.

Received: June 10, 2013 Revised: July 27, 2013

Published online: October 22, 2013

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