This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 6063--6065 6063 Cite this: Chem. Commun., 2013, 49, 6063 A facile protection–deprotection route for obtaining indigo pigments as thin films and their applications in organic bulk heterojunctions† Eric Daniel Glowacki,* a Gundula Voss, a Kadir Demirak, b Marek Havlicek, a Nevsal Su ¨nger, b Aysu Ceren Okur, c Uwe Monkowius, d Jacek Ga ˛siorowski, a Lucia Leonat a and Niyazi Serdar Sariciftci a Indigo and its derivatives are industrially-important dyes known for centuries. The low solubility of these compounds limits their applications and hinders potential synthetic chemistry using indigo as a building-block. Herein we report attachment of the tert-butoxy carbonyl (tBOC) thermolabile protecting group to indigos, allowing their processing into neat thin films as well as mixed films with a semiconducting polymer. Photoinduced charge transfer is observed to and from these pigments and the polymer. Indigo and its derivatives constitute a family of natural-origin dyes and pigments of substantial industrial importance. 1 Indigo is usually processed via vat dyeing, exploiting the water-solubility of the reduced leuco indigo. Indigo is also used in its hydrogen-bonded pigment form for industrial coloring applications. Recent work exploring indigoids as organic semiconductors has shown that these materials afford ambipolar charge transport with mobilities in the range of 10 À2 –0.5 cm 2 V À1 s À1 with substantial operational stability under ambient conditions. 2–4 Both ‘old’ and new emerging applications motivate us to find a way to work with indigos in organic solvents. Herein we use the technique of attaching the thermolabile tert -butoxy carbonyl ( t BOC) protecting group to the indigo amine function, yielding a highly-soluble indigo derivative. These groups can be removed by heat treatment later, regenerating the original pigment (Fig. 1). We observed a variety of different H-bonded crystallites formed using this approach, with potential applications as functional organic nano- and microparticles for organic electronics. Mixing the soluble indigo precursors with a semiconducting polymer, poly- (3-hexylthiophene), P3HT, followed by thermal regeneration of the pigment, yields evidence of photoinduced charge transfer between the polymer and indigo. Starting from the 1990s, researchers at Ciba–Geigy have employed the t BOC group to form soluble latent pigments of diketopyrrolopyrroles, allowing dissolution of the latent pigment in organic matrices followed by heating to yield the stable pigment. 5 This method was disclosed in a 1993 patent for a wide range of pigment-forming molecules, including indigo. 6 Basic chemical data on 1b were reported for the first time in 2004, however no derivatives of indigo were reported. 7 In the field of organic electronics, the t BOC group has been employed to allow solution-processing of polyaniline 8 and solution-processing of solar cells using t BOC quinacridone 9 and transistor devices using quinacridone and diketo- pyrrolopyrroles. 10 We prepared pigments 1a–3a and 5a–6a from the precursor o-nitrobenzaldehyde. 11 Compound 4a was prepared accord- ing to Clark and Cooksey. 12 1b–6b were prepared by stirring the pigments 1a–6a in dichloromethane at room temperature with Z2 equivalents of t BOC 2 O and about one equivalent of dimethyl amino- pyridine (DMAP) for 1–3 days. Experimental details and analytical data for 1–6 can be found in the ESI.† This is the first time that indigo mixtures have been separated by column chromatography. To under- score the utility of this method, we obtained a historical sample from the early 1900s (Dyestuff museum, TU Dresden, Prof. Hartmann) consisting of a mixture of unsubstituted indigo, 5-bromoindigo, and Fig. 1 Protection and deprotection of indigo pigments (1a–6a) using tBOC to yield protected dyes 1b–6b. The table shows the compounds used in this study. *Unless otherwise specified, all R = H. a Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University, Linz, Austria. E-mail: eric_daniel.glowacki@jku.at b Solar Energy Institute, Ege University, Bornova-Izmir, Turkey c Dept. of Materials Science and Engineering, Sabanci University, Istanbul, Turkey d Institute of Inorganic Chemistry, Johannes Kepler University, Linz, Austria † Electronic supplementary information (ESI) available. CCDC 918485 and 918486. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc42889c Received 18th April 2013, Accepted 16th May 2013 DOI: 10.1039/c3cc42889c www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 17 May 2013. Downloaded by Johannes Kepler Universitat Linz on 11/06/2013 10:03:35. View Article Online View Journal | View Issue