Inside Back Cover: Infinite Polyiodide Chains in the Pyrroloperylene–Iodine Complex: Insights into the Starch–Iodine and Perylene–Iodine Complexes (Angew. Chem. Int. Ed. 28/2016)

COMMUNICATION Infinite Polyiodide I ∞δ– Chains in the Pyrroloperylene-Iodine Complex: Insights into the Starch-Iodine and Perylene-Iodine Complexes Sheri Madhu, Hayden A. Evans, Vicky V. T. Doan-Nguyen, John G. Labram, Guang Wu, Michael L. Chabinyc, Ram Seshadri,* and Fred Wudl* Abstract: We report the preparation and X-ray crystallographic characterization of the first crystalline homoatomic polymer chain, which is part of a semiconducting pyrroloperylene-iodine complex. The crystal structure contains infinite polyiodide I ∞ δ– . Interestingly, the structure of iodine within the insoluble, blue starch-iodine complex has long remained elusive, but has been speculated as having infinite chains of I. Close similarities in the low-wavenumber Raman spectra of the title compound and starch-iodine point to such infinite polyiodide chains in the latter as well. According to Saenger, 1 the starch-iodine complex was discovered by Colin and de Claubry, over two hundred years ago. 2 The complex is better described as amylose-iodine 1 and has continuously, over the years, been considered as a clathrate of polyiodide and carbohydrate. The nature of the polyiodide in this complex still remains to be fully characterized, and many of the references on iodine chain-containing compounds in the literature draw analogies with the elusive starch-iodine structure. For example, Kahr et al. 3 established the crystal structure of so-called herapathite: a quinine iodine complex with iodide trimers. Redel et al. 4 have reported a complex copper iodide with chains of up to 5 iodine atoms. Lin et al. 5 have reported bent anionic I 7 chains stabilized by complex multivalent organic ammonium cations. Schroder and coworkers 6 have reviewed the rich structural chemistry that emerges when macrocylic thioethers are employed to stabilize oligomeric iodine chains. Yin et al. 7 have reported a metal-organic framework based on Cu(II) cations and I 5− and I 7− chains. Kloo et al. 8 have also employed ab-initio techniques to study the structure and stability of short I n – moieties in the vacuum. These descriptions of iodide chains, as well as numerous others, are better described as oligomeric rather than polymeric chains of iodine, with lengths typically shorter than 10 iodine units and usually containing molecular iodine within the chain. 9 Indeed, a seminal resonance Raman spectroscopic study of starch iodine carried out in 1978 allowed the authors inferred the presence predominantly I 5– species. 10 A separate and highly relevant structural mystery involving Dr. Sheri Madhu, Hayden A. Evans, Dr. Vicky V.T. Doan-Nguyen, Prof. Dr. Ram Seshadri, Dr. Fred Wudl Materials Research Laboratory University of California, Santa Barbara, CA 93106 Hayden A. Evans, Dr. Guang Wu Department of Chemistry and Biochemistry University of California, Santa Barbara, CA 93106 Dr. Vicky V.T. Doan-Nguyen, Dr. John G. Labram California NanoSystems Institute University of California, Santa Barbara, CA 93106 Prof. Dr. Michael L. Chabinyc, Prof. Dr. Ram Seshadri, Prof. Dr. Fred Wudl Materials Department University of California, Santa Barbara, CA 93106 Supporting information for this article is given via a link at the end of the document. iodine is that of one of the earliest organic electronic conductors; the perylene-iodine charge transfer complex, 11 which to this day, has not been fully resolved. It is clear that the hydrocarbon is partially oxidized and the iodine shows hints of iodine and triiodide 12 but the latter are disordered in essentially, all examined cases. Here we relate these stories and shed light on the science of these functional organic materials. We have determined the structure of a pyrroloperylene-iodine complex that comprises helical, infinite iodine chains interspersed between π-stacks of the pyrroloperylene. The electrical conductivity of this compound is found to be close to 10 –2 S cm –1 at room temperature, and a change in the temperature coefficient of the conductivity at temperatures close to 150 K is suggested as corresponding to a structural transition. Based on similarities between the published low-frequency Raman spectroscopic signatures of starch-iodine and the spectrum measured on the title compound, it is suggested that starch-iodine must similarly comprise infinite iodine chains. Scheme 1. Synthetic route for Pyrroloperylene 3. Pyrroloperylene 3 was prepared in 80 % to 85 % yield by a simple Cadogan reaction shown in Scheme 1. 13 The preparation of the pyrroloperylene-iodide complex from solution by slow cooling was straightforward, and employed a benzene solution containing 15 mg (0.0566 mmol) of pyrroloperylene 3 and 51 mg (0.198 mmol) of iodine, kept at 65 °C in a vial with a screw cap. The solution was allowed to cool slowly from 65 °C to 30 °C over a period of five days in a doubly insulated container. Shiny needles with a dark yellow-green lustre were obtained in the bottom of the vial. The crystals were found to be quite stable at room temperature under STP conditions, but lost iodine at higher temperature (TGA in supporting information). The concentration most favorable for the formation of large (>1 mm) single crystals was 10 mg of pyrroloperylene and 35 mg of Iodine per 5 mL benzene solution. The dimensions of these crystals varied in length from 1 mm to 5 mm, the width from 100 μm to 200 μm, and thickness from 10 μm to 50 μm.