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Effect of Ring Strain on the Charge Transport of aRobust Norbornadiene – Quadricyclane-Based Molecular Photoswitch

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ABSTRACT

Integratingfunctional molecules into single-molecule devices isa key step toward the realization of future computing machines basedon the smallest possible components. In this context, photoswitchingmolecules that can make a transition between high and low conductivityin response to light are attractive candidates. Here we present thesynthesis and conductance properties of a new type of robust molecularphotothermal switch based on the norbornadiene (NB)–quadricyclane(QC) system. The transport through the molecule in the ON state isdominated by a pathway through the π-conjugated system, whichis no longer available when the system is switched to the OFF state.Interestingly, in the OFF state we find that the same pathway contributesonly 12% to the transport properties. We attribute this observationto the strained tetrahedral geometry of the QC. These results challengethe prevailing assumption that current will simply flow through theshortest through-bond path in a molecule.

No MeSH data available.


(a) Photoisomerizationof NB-1 (dark cyan) to QC-2 (orange) indicatedby the black arrow followed by UV/visspectroscopy. The photoisomerization was performed using a 405 nmlaser diode, and the sample was exposed to the irradiation every 10s until the complete conversion of NB-1 through the grayplots to QC-2 was reached. (b) Sketch of device geometryunder test in the STM break junction measurements. (c) No sign ofdegradation was observed in the accelerated stability test of the NB-1 (dark cyan maxima)/QC-2 (orange minima)system at elevated temperature (50 °C) sealed under nitrogenatmosphere; 124 photothermal switching cycles are shown.
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fig1: (a) Photoisomerizationof NB-1 (dark cyan) to QC-2 (orange) indicatedby the black arrow followed by UV/visspectroscopy. The photoisomerization was performed using a 405 nmlaser diode, and the sample was exposed to the irradiation every 10s until the complete conversion of NB-1 through the grayplots to QC-2 was reached. (b) Sketch of device geometryunder test in the STM break junction measurements. (c) No sign ofdegradation was observed in the accelerated stability test of the NB-1 (dark cyan maxima)/QC-2 (orange minima)system at elevated temperature (50 °C) sealed under nitrogenatmosphere; 124 photothermal switching cycles are shown.

Mentions: The photoswitchingmolecule (NB-1) was synthesizedthrough Sonogashira23 cross-coupling reactionstarting from 2,3-dibromonorbornadiene. Full synthetic details andcharacterization are presented in the SI. The photoisomerization process was examined in solution using NMRand UV–vis spectroscopy. The proton NMR for the photoisomerizationof NB-1 to QC-2 showed an alkenyl protonfor NB-1, whereas in QC-2 the alkenyl protonsdisappeared and new alkyl protons emerge (SI: Figure S3). UV–vis absorption spectroscopy of NB-1 was carried out in toluene, and the onset of absorption wavelengthwas found to be around 460 nm (SI: FigureS4(a)), which is significantly red-shifted from unsubstituted norbornadiene.This is attributed to the extended π-conjugation in NB-1, which enabled the photoisomerization with visible light. Irradiatinga toluene solution of NB-1 with visible light resultedin its photoisomerization to QC-2 with isosbestic pointsin the absorption spectra indicating a clean photoconversion (Figure 1(a)).


Effect of Ring Strain on the Charge Transport of aRobust Norbornadiene – Quadricyclane-Based Molecular Photoswitch
(a) Photoisomerizationof NB-1 (dark cyan) to QC-2 (orange) indicatedby the black arrow followed by UV/visspectroscopy. The photoisomerization was performed using a 405 nmlaser diode, and the sample was exposed to the irradiation every 10s until the complete conversion of NB-1 through the grayplots to QC-2 was reached. (b) Sketch of device geometryunder test in the STM break junction measurements. (c) No sign ofdegradation was observed in the accelerated stability test of the NB-1 (dark cyan maxima)/QC-2 (orange minima)system at elevated temperature (50 °C) sealed under nitrogenatmosphere; 124 photothermal switching cycles are shown.
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Related In: Results  -  Collection

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fig1: (a) Photoisomerizationof NB-1 (dark cyan) to QC-2 (orange) indicatedby the black arrow followed by UV/visspectroscopy. The photoisomerization was performed using a 405 nmlaser diode, and the sample was exposed to the irradiation every 10s until the complete conversion of NB-1 through the grayplots to QC-2 was reached. (b) Sketch of device geometryunder test in the STM break junction measurements. (c) No sign ofdegradation was observed in the accelerated stability test of the NB-1 (dark cyan maxima)/QC-2 (orange minima)system at elevated temperature (50 °C) sealed under nitrogenatmosphere; 124 photothermal switching cycles are shown.
Mentions: The photoswitchingmolecule (NB-1) was synthesizedthrough Sonogashira23 cross-coupling reactionstarting from 2,3-dibromonorbornadiene. Full synthetic details andcharacterization are presented in the SI. The photoisomerization process was examined in solution using NMRand UV–vis spectroscopy. The proton NMR for the photoisomerizationof NB-1 to QC-2 showed an alkenyl protonfor NB-1, whereas in QC-2 the alkenyl protonsdisappeared and new alkyl protons emerge (SI: Figure S3). UV–vis absorption spectroscopy of NB-1 was carried out in toluene, and the onset of absorption wavelengthwas found to be around 460 nm (SI: FigureS4(a)), which is significantly red-shifted from unsubstituted norbornadiene.This is attributed to the extended π-conjugation in NB-1, which enabled the photoisomerization with visible light. Irradiatinga toluene solution of NB-1 with visible light resultedin its photoisomerization to QC-2 with isosbestic pointsin the absorption spectra indicating a clean photoconversion (Figure 1(a)).

View Article: PubMed Central - PubMed

ABSTRACT

Integratingfunctional molecules into single-molecule devices isa key step toward the realization of future computing machines basedon the smallest possible components. In this context, photoswitchingmolecules that can make a transition between high and low conductivityin response to light are attractive candidates. Here we present thesynthesis and conductance properties of a new type of robust molecularphotothermal switch based on the norbornadiene (NB)–quadricyclane(QC) system. The transport through the molecule in the ON state isdominated by a pathway through the π-conjugated system, whichis no longer available when the system is switched to the OFF state.Interestingly, in the OFF state we find that the same pathway contributesonly 12% to the transport properties. We attribute this observationto the strained tetrahedral geometry of the QC. These results challengethe prevailing assumption that current will simply flow through theshortest through-bond path in a molecule.

No MeSH data available.