Robust Circular Polarized Emission from Nanoscopic Single-Molecule Sources: Application to Solid State Devices
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Robust Circular Polarized Emission from Nanoscopic Single-Molecule Sources: Application to Solid State Devices Ruthanne Hassey, Ellen J. Swain, Nathan I. Hammer, Emily L. Richards, Dhandapani Venkataraman, and Michael D. Barnes Chemistry, University of Massachusetts, 710 North Pleasant, Amherst, MA, 01003
ABSTRACT Controlling the polarization state of fluorescence emission in solid state devices is an important goal in optical display technologies. High-purity (right or left) circular polarized emission is particularly desirable because an arbitrary linearly polarized state can be generated with much higher efficiency (lower loss) as compared with a non-polarized fluorescence input. Here we discuss observation of resonant chiroptical effects (fluorescence detected circular dichroism (FDCD), and circular polarized luminescence (CPL)) from single (bridgedtriarylamine) helicene molecules in a solid-film format. In the FDCD experiment using 457-nm excitation, single-molecule fluorescence from enantiomerically pure helicene samples show surprising distributions of dissymmetry (g)-parameters centered near zero but with a significant contribution from molecules showing an almost perfectly pure response to either right or left circularly polarized light. Experiments combining a well-defined circularly polarized excitation (either right or left) with decomposition of the fluorescence into left- and right-circular polarization component show only a weak correlation between the dissymmetry (sign and magnitude) of the CPL with the polarization state of the input. Current efforts are directed at wavelength resolved CPL, FDCD at wavelengths that are more closely match to bulk circular dichroism features, and orientational dependence of FDCD and CPL. These results provide new insight into chiroptical properties of chiral fluorophores at the single molecule level and suggest new optical device possibilities. INTRODUCTION In recent years, there has been a growing interest in developing organic molecules that are capable of emitting circularly polarized light because of their potential applications in display devices [1-3], optical storage devices [4], and in asymmetric photochemical synthesis [5,6]. Recently, Venkataraman and Riehl demonstrated the synthesis and bulk chiroptical characterization of a new kind of helicene molecule based on a bridged triarylamine structure where the Right (P) - or Left (M) - handed helical structure is enforced by the presence of a camphanate group [7]. These new molecules exhibited both a relatively strong bulk circular dichroism (CD) response and an impressive circularly polarized luminescence (CPL) emission. Here, we explore the chiroptical properties of these bridged triarylamine helicene molecules at the single molecule level by studying fluorescence detected circular dichroism (FDCD) and CPL.
EXPERIMENT The pure M- and P-type helicene diastereomers were synthesized as published previously and their purity was verified by 1H NMR [8]. Figure 1 shows the chemical structure of these
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