Energy Transfer in a Light Harvesting Dendron

Lea NienhausISMS

06/17/14

Introduction

GoalTo 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

2Nienhaus et al. J. Phys. Chem. 2014

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 CycloadditionCy3 is linked via click chemistry between a terminal acetylene and the Cy3 azide resulting in a 1,2,3-triazole.

Cy3N

NN

NN N N N

NCy3 Cy3

O O

CuSO4, ascorbate

D Cy32-D

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Synthetic Strategy Dye coupling via azide and hydrazide reactions

Hydrazide couplingCy5 is linked via H+ catalyzed coupling of Cy5 hydrazide and an acetophenone resulting in a hydrazone.

N CH3Cy5 NH

HN

Cl- NH3

NN N N N

NCy3 Cy3

O

NNNCy3

N NN Cy3

Cy5

TEA, HCl

Cy32-D-Cy5Cy32-D

4

5

Spectroscopy

UV-Vis spectrum of Cy32-D-Cy5 and of the unreacted reagents

Comparison of the dendron/Cy3 emission with Cy3/Cy5 absorption

Possible energy transfer pathways in Cy32-D-Cy5.

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Emssion spectra of Cy32-D-Cy5 excitation at 330 nm, 532 nm, 642 nm

Spectroscopy

S.E.

Emssion spectra of Cy5-Dexcitation 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

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The aliphatic linkers allow for structural flexibility – resulting in many different conformationsallowing 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

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Thank You

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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

http://www.organic-chemistry.org/namedreactions/click-chemistry.shtm

5 nm

Rear Illumination

5 nm

5 nm

Lock-In Absorption

ImageCNT on silicon

Optically Assisted STM

Lock-InAmplifier

Cy3-ResonantLaser: 532 nm

ElectronicModulation

ControlElectronics

STM

Cy5-ResonantLaser: 660 nm

Figures courtesy of Gregory Scott

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532 nm - excites donorOff - ground state - established tunneling current

On - excited state - change in LDOS - change in Itunneling

Modulation between on and off resonant laser reduces heating effects

Frequency modulation: electronic modulation using a square wave to drive the laser diodeLock-in amplifier detects the current fluctuations at the modulation frequency

Beamsplitter

IR LaserMech. Mod.

Dichroic Mirror