Rigidifying Fluorescent Linkers by Metal−Organic Framework Formation for Fluorescence Blue Shift and Quantum Yield Enhancement Zhangwen Wei, †,∥ Zhi-Yuan Gu, †,∥ Ravi K. Arvapally, §,∥ Ying-Pin Chen, †,‡ Roy N. McDougald, Jr., § Joshua F. Ivy, § Andrey A. Yakovenko, † Dawei Feng, † Mohammad A. Omary,* ,§ and Hong-Cai Zhou* ,†,‡ † Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States ‡ Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77842, United States § Department of Chemistry, University of North Texas, Box 305070, Denton, Texas 76203-5070, United States * S Supporting Information ABSTRACT: We demonstrate that rigidifying the structure of fluorescent linkers by structurally constraining them in metal− organic frameworks (MOFs) to control their conformation effectively tunes the fluorescence energy and enhances the quantum yield. Thus, a new tetraphenylethylene-based zirconium MOF exhibits a deep-blue fluorescent emission at 470 nm with a unity quantum yield (99.9 ± 0.5%) under Ar, representing ca. 3600 cm −1 blue shift and doubled radiative decay efficiency vs the linker precursor. An anomalous increase in the fluorescence lifetime and relative intensity takes place upon heating the solid MOF from cryogenic to ambient temperatures. The origin of these unusual photoluminescence properties is attributed to twisted linker conformation, intramolecular hindrance, and framework rigidity. ■ INTRODUCTION Fluorescent solid materials have attracted significant attention because of their wide applications especially as inorganic and organic light-emitting diodes (LEDs and OLEDs, respectively) and solid state sensors. 1 Discovering new fluorescent materials with intriguing properties, such as stimuli responsiveness and high porosity, will facilitate the development of functional materials enriching the current inorganic and organic solid semiconductors. Despite the large diversity of small organic fluorescent molecules, which provide almost infinite potential candidates for fluorescent and phosphorescent solids, these materials usually suffer from self-quenching and the consequent low quantum yield of their photo- or electroluminescence. 2 Recently, it has been shown that it is possible to turn on the fluorescence by building the fluorophore within metal−organic frameworks (MOFs). 3 Herein we propose a MOF with rigidified fluorescent linkers to effectively tune the frontier orbital energy gap (or semiconductor band gap) and improve the photoluminescence quantum yield, dramatically to attain unity. MOFs, constructed from inorganic metal-containing nodes and organic linkers bearing large internal surface areas, diverse structures, and versatile functionalities, 2,4 can be promising candidates as tunable OLED emitters and luminescent sensors. Their luminescence originates from metal cations, most commonly lanthanides, the organic linkers, or charge transfers between the two. 2,5 Rigidifying linkers in MOFs has two distinctive advantages. First, the linkers can adopt some special conformations that would otherwise be impossible, hence producing different fluorescence and/or absorption energies. Second, the linkers fixed in the porous frameworks have longer intermolecular separations and, as a result, can increase photoluminescence quantum yield due to decreased self-quenching. It will be of great scientific and technological significance to show a proof- of-concept demonstration that rigidifying fluorescent linkers by MOF formation would efficiently and substantially tune the electronic transition energies and raise the quantum yields. As a conventional fluorophore, tetraphenylethylene (TPE), Figure 1a, is well-known for its aggregation-induced emission (AIE) character. 6 Nevertheless, only several pioneering papers have discussed the utilization of TPE in MOFs to turn-on its fluorescence. 3 On the other hand, further applications of reported TPE-based MOFs are limited due to their moisture- sensitivity originating from the labile coordination bonds between divalent metal cations and carboxylate linkers. Here we undertake a combined structural/spectroscopic study of a new extended TPE-based linker and a robust tetravalent zirconium MOF thereof (PCN-94, where PCN stands for “porous coordination network”) that exhibits remarkably high fluorescence quantum yield in the solid state, among other unusual photophysical properties. We designed Received: January 22, 2014 Article pubs.acs.org/JACS © XXXX American Chemical Society A dx.doi.org/10.1021/ja5006866 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX