Platinum-Functionalized Chiral Molecular Squares as Light-Emitting Materials
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Platinum-Functionalized Chiral Molecular Squares as Light-Emitting Materials Lin Zhang,┼ Yu-Hua Niu,╪ Alex K.-Y. Jen, ╪,* and Wenbin Lin┼,* ┼Department of Chemistry, CB#3290, University of North Carolina, Chapel Hill, NC 27599, U.S.A. ╪Department of Materials Science and Engineering, Box 352120, University of Washington, Seattle, WA 98195, U.S.A.
ABSTRACT A family of new chiral metallocycles based on Pt(II) diimine metallocornors and bis(acetylene) bridging ligands have been synthesized, and characterized by a variety of techniques including 1H and 13C NMR, UV-visible, luminescence, infrared, and circular dichroism (CD) spectroscopies, and mass spectrometry. All metallocycles exhibit very strong phosphorescence with quantum yields of 8.3 to 15.7%. Chiral Pt(II)-based molecular squares were used as the light-emitting layer in multiplayer devices, and a maximum brightness of 5470 cd/m2 with a maximum luminous efficiency of 0.93 cd/A was achieved.
INTRODUCTION Since Fujita and Stang’s seminal work on highly efficient self-assembly of small molecular polygons in the middle of 1990’s [1], the synthesis of nanoscopic metallocycles has attracted tremendous interests over the past decade. These metallosupramolecular systems can exhibit interesting functions that are not manifested in the constituent building blocks, including cage-directed stabilization of labile molecules and stereoselective synthesis [2], molecular microporosity [3], and tuning of enantioselectivity of asymmetric catalysts [4]. Metallocycles and metallocages also hold great promise as interesting electrooptical materials owing to their nanoscopic and even mesoscopic nature [5]. The incorporation of metal centers however often introduces non-radiative decay pathways and thus can have deleterious effects in photophysical properties of metallosupramolecular systems. Electroluminescent (EL) devices based on cyclometalated complexes of Ir and Pt have received much attention due to their high quantum efficiencies [6]. Many studies have been reported on the photoluminescence (PL) of (diimine)Pt(acetylide)2 [7] and the 3MLCT[Pt π*(diimine)] nature of emissions is now well established [8,9]. We hypothesize that Pt(diimine) moieties can be used as a metallocorner to construct emissive metallocycles by exploiting the 3MLCT[Pt π*(diimine)] nature of their emissions. Herein we wish to report the first highly electroluminescent molecular squares.
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EXPERIMENTAL DETAILS Chiral molecular triangle [Pt(4,4’-dtbPy)(L1)]3 (1a) and square [Pt(4,4’-dtbPy)(L1)]4 (2a) were synthesized in 29% and 22% yield via self-assembly between equimolar BINOL-derived 4,4’-bis(alkynyl) linear bridging ligand L1-H2 and Pt(4,4’-dtbPy)Cl2 in the presence of CuI catalyst and diethylamine at room temperature (Scheme 1). In contrast, a similar reaction between equimolar L2-H2 and Pt(4,4’-dtbPy)Cl2 only afforded molecular square [Pt(4,4’-dtbPy)(L2)]4 (2b) in 17% yield. Molecular squares 2a and 2b can also be
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Scheme 1. Synthesis of metallocycles 1a, 2a, and 2b t
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