Energy Transfer in a Light Harvesting Dendron Lea Nienhaus ISMS06/17/14

Introduction Goal To investigate energy transfer by single molecule absorption detected by scanning tunneling microscopy Advantages detection of absorption directly by change of local density of states during absorption technique is not fluorescence based no diffraction limit – sub-nanometer resolution of absorption is possible 2 Nienhaus et al. J. Phys. Chem Single molecule absorption detected by scanning tunneling microscopy image of a PbS quantum dot excited with a 532 nm laser

Synthetic Strategy  Dye coupling via azide and hydrazide reactions  Azide-Alkyne Huisgen Cycloaddition Cy3 is linked via click chemistry between a terminal acetylene and the Cy3 azide resulting in a 1,2,3-triazole. 3

Synthetic Strategy  Dye coupling via azide and hydrazide reactions  Hydrazide coupling Cy5 is linked via H + catalyzed coupling of Cy5 hydrazide and an acetophenone resulting in a hydrazone. 4

5 Spectroscopy UV-Vis spectrum of Cy3 2 -D-Cy5 and of the unreacted reagents Comparison of the dendron/Cy3 emission with Cy3/Cy5 absorption Possible energy transfer pathways in Cy3 2 -D-Cy5.

6 Emssion spectra of Cy3 2 -D-Cy5 excitation at 330 nm, 532 nm, 642 nm Spectroscopy S.E. Emssion spectra of Cy5-D excitation at 330 nm, 632 nm Energy transfer efficiencies based on emission sprectroscopy peak integrals:  Cy3 – Cy5: 41 %  D – Cy3- Cy5: 41%  D – Cy3: 52%  D – Cy5: 20%

Possible Conformations Caused by Structural Flexibility 7 The aliphatic linkers allow for structural flexibility – resulting in many different conformations allowing for single molecule studies in different conformations and environments. SVD analysis of the absorption spectroscopy shows that there are no large shifts when the molecule is deposited onto a transparent metal substrate.

Acknowledgements Dr. Dustin Gross Dr. Zheng Xue Prof. Martin Gruebele Prof. Jeff Moore 8

Thank You 9

Click Reaction Acidic acetylene H is substituted by copper Metal coordinates with electrons on nitrogen atom while displacing one of the copper ligands resulting in copper-azide-acetylide Cyclization takes place Product is formed and catalyst regenerated

5 nm Rear Illumination 5 nm Lock-In Absorption Image CNT on silicon Optically Assisted STM Lock-In Amplifier Cy3-Resonant Laser: 532 nm Electronic Modulation Control Electronics STM Cy5-Resonant Laser: 660 nm Figures courtesy of Gregory Scott nm - excites donor Off - ground state - established tunneling current On - excited state - change in LDOS - change in I tunneling Modulation between on and off resonant laser reduces heating effects Frequency modulation: electronic modulation using a square wave to drive the laser diode Lock-in amplifier detects the current fluctuations at the modulation frequency Beamsplitter IR Laser Mech. Mod. Dichroic Mirror