Enhanced Photoluminescence of Sulfur-Bridged Organic Chromophores
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Enhanced Photoluminescence of Sulfur-Bridged Organic Chromophores Peter R. Christensen,1 Élise Caron,1 Jeffery A. Nagle,2 Aini Bhatti,1 and Michael O. Wolf.1 1 2
Department of Chemistry, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada Department of Chemistry, Bowdoin College, Brunswick, Maine, 04011, United States
ABSTRACT A general approach to enhancing the photoluminescent quantum yield for a series of organic chromophores is presented. By bridging a chromophore symmetrically about a sulfur atom it was found that the photoluminescence could be systematically increased by oxidizing the bridge. Furthermore, the enhanced quantum yields were achieved without diminishing the solubility of these chromophores in common organic solvents. The photophysical characterization, as well as potential applications of these molecules will be discussed. INTRODUCTION Strongly photoluminescent organic molecules are of significant relevance to applications in light emitting devices (OLEDs) [1]. OLEDs have been realized as the next generation of display technology largely due to the ability to synthesize and manipulate organic molecules in solution, enabling large-area printing of these materials [2]. Furthermore, the mechanical flexibility of organic small molecules, and polymers has enabled the manufacture of correspondingly flexible fully functioning displays [3]. Some of the first OLED prototypes employed conjugated organic polymers as the light-emitting component [4]. For applications as printable materials, the viscosity of light emitting polymers is often preferred over less viscous small single molecule chromophores. However, the light emitting (quantum) efficiency, or quantum yield, of small molecule chromophores are typically far superior to that of polymers [5]. In general, conjugated polycyclic aromatic hydrocarbons (PAHs), such as thiophenes and benzenoids (i.e. naphthalene, pyrene, and anthracene) make up the core building blocks of light emitting small molecules. The photoluminescence (PL) quantum efficiency, and color (wavelength) of PAHs can be tuned by increasing or decreasing the effective length of conjugation [6]. While this may seem like a viable strategy for tuning the properties of light emitting small molecules, the problem with larger PAHs is that they become increasingly insoluble with size (i.e. sexithiophene, and pentacene) [7]. The ideal class of small molecule light emitters would be both optimally soluble, color tunable, and maintain the highest possible quantum efficiencies. As such, efficient, and generalized methods for controlling the luminescence of organic molecules are of great interest. We have recently discovered that by bridging a series of small organic molecules (oligothiophenes, naphthalene, and pyrene) symmetrically about a sulfur atom, the PL quantum yield can be systematically enhanced by oxidizing the bridging sulfur from S to SO, to SO2 (Figure 1) [8]. This enhancement in quantum yield was achieved without diminishing the solubility of these molecules, due largely to th
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