Excited-State Design DOI: 10.1002/anie.201404867 A Strap Strategy for Construction of an Excited-State Intramolecular Proton Transfer (ESIPT) System with Dual Fluorescence** Naoya Suzuki, Aiko Fukazawa, Kazuhiko Nagura, Shohei Saito, Hirotaka Kitoh-Nishioka, Daisuke Yokogawa, Stephan Irle,* and Shigehiro Yamaguchi* Abstract: An amine-embedded flexible alkyl strap has been incorporated into an emissive boryl-substituted dithienylpyr- role skeleton as a new entity of excited-state intramolecular proton transfer (ESIPT) chromophores. The p-electron system shows a dual emission, which covers a wide range of the visible region depending on the solvent polarity. The incorporation of the aminoalkyl strap as well as the terminal boryl groups efficiently stabilize the zwitterionic excited-state species result- ing from the ESIPT even in an aqueous medium. Luminescence is one of the fundamental properties that lead to attractive molecular functions of organic compounds. Construction of novel p-conjugated skeletons that can yield characteristic luminescence properties would have a signifi- cant impact on a broad spectrum of chemistry including not only organic electronics, but also bioimaging and sensors. Among the various fascinating luminescence properties, we now focus the attention on a fluorescence based on the excited-state intramolecular proton transfer (ESIPT). [1] The ESIPT is a photochemical process that produces a tautomer with a totally different electronic structure from the initial excited form. Consequently, the ESIPT state gives rise to a fluorescence with an anomalously large Stokes shift. In addition, the ESIPT chromophores often exhibit a dual emission originating from both the initial excited form and the proton-transferred tautomer, which covers a broad wave- length range. These features of the ESIPT can be the basis of various applications including white light-emitting materi- als, [2–4] polymorph-dependent light-emitting materials, [5] and fluorescent probes. [6–9] However, as the formation of an intramolecular hydrogen bond is a prerequisite of the ESIPT, the ESIPT chromophores have been limited to only a few variations, such as planar molecules that consist of a phenol or aniline scaffold as a hydrogen-bond donor and an imino/azo- nitrogen- or carbonyl-oxygen-containing ring skeleton as a hydrogen-bond acceptor. The ESIPT in most of these molecules is based on a keto–enol type tautomerization (Figure 1 a). An exception is 2-(hydroxyphenyl)imidazo[1,2- a]pyridine, whose ESIPT state adopts a zwitterionic form instead of a keto form. [5, 10] We now disclose a new design of the ESIPT chromophore, which does not rely on the keto–enol tautomerism, but is based on a strap strategy making use of a functional flexible chain. To equip the chromophore with an alkyl chain strap has been used as a simple strategy to constrain a conformation of the p-conjugated skeleton [11–14] or to insulate it. [15–18] In contrast, our idea is to employ an alkyl strap embedding an amine moiety, thereby endowing it with the function of a base. We install this strap in an emissive p-conjugated scaffold bearing a relatively acidic proton (Figure 1 b). As the p skeleton, we chose a pyrrole-containing oligoarene. The appropriate chain length of the aminoalkyl strap would ensure the formation of an intramolecular hydrogen bond between a nitrogen atom in the strap and the pyrrole NÀH proton. In the excited state, the acidity of the pyrrole NÀH would increase [19] similar to that of a phenol OÀH. [20] This behavior would be enhanced by introduction of an electron- accepting group in the pyrrole-containing p-conjugated skeleton to impart a donor–acceptor type character. Based on this consideration, we designed compound 1 that has an electron-donating dithienylpyrrole skeleton with electron- accepting boryl groups at the termini. We found that the ESIPT indeed occurred in this molecule to produce a zwitter- Figure 1. Designs of the excited-state proton transfer (ESIPT) systems based on a) a keto–enol type tautomerism and b) a strap strategy with a basic flexible chain. c) Chemical structures of compounds 1–3. [*] N. Suzuki, Prof.Dr. A. Fukazawa, Dr. K. Nagura, Dr. S. Saito, Dr. H. Kitoh-Nishioka, Prof. Dr. D. Yokogawa, Prof. Dr. S. Irle, Prof. Dr. S. Yamaguchi Institute of Transformative Bio-molecules (WPI-ITbM) and Department of Chemistry, Graduate School of Science Nagoya University Furo, Chikusa, Nagoya 464-8602 (Japan) E-mail: sirle@chem.nagoya-u.ac.jp yamaguchi@chem.nagoya-u.ac.jp [**] This work was partly supported by JST, CREST (S.Y. and S.I.). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201404867. A ngewandte Chemi e 1 Angew. Chem. Int. Ed. 2014, 53,1–6  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü