Addison, Kiri

Browne, Wesley R.

Cnossen, Arjen

Conyard, Jamie

Feringa, Bernard

Heisler, Ismael A.

Meech, Stephen R.

English

Light-driven molecular motors convert light into mechanical energy via excited state reactions. In this work we follow sub-picosecond primary events in the cycle of a two-stroke unidirectional motor by fluorescence up-conversion and transient absorption

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   University of GroningenUltrafast ignition of a uni-directional molecular motorConyard, Jamie; Addison, Kiri; Heisler, Ismael A.; Cnossen, Arjen; Browne, Wesley R.;Meech, Stephen R.; Feringa, BernardPublished in:Proceedings of the XVIIIth International Conference on Ultrafast PhenomenaIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2013Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Conyard, J., Addison, K., Heisler, I. A., Cnossen, A., Browne, W. R., Meech, S. R., & Feringa, B. (2013).Ultrafast ignition of a uni-directional molecular motor. In M. Chergui, S. Cundiff, A. Taylor, R. D. Vivie-Riedle, & K. Yamanouchi (Eds.), Proceedings of the XVIIIth International Conference on UltrafastPhenomena (41 ed., Vol. 41, pp. 05016-p.1-05016-p.3). University of Groningen, Stratingh Institute forChemistry.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 13-02-2023   Ultrafast ignition of a uni-directional molecular motor Jamie Conyard,1 Kiri Addison,1 Ismael A. Heisler,1 Arjen Cnossen,2 Wesley R. Browne,2 Ben L. Feringa2 and Stephen R. Meech1  aSchool of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ  bStratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands  Abstract. Light-driven molecular motors convert light into mechanical energy via excited state reactions. In this work we follow sub-picosecond primary events in the cycle of a two-stroke unidirectional motor by fluorescence up-conversion and transient absorption.  1 Introduction  Light driven molecular machines are designed to convert photon energy into mechanical energy, which can often be achieved through an excited state isomerisation reaction. For the operation of molecular motors it is also clearly essential to exercise control over the direction of motion. Unidirectional operation of light-driven molecular motors was first reported by Feringa and co-workers in their studies of chiral overcrowded alkenes (Fig. 1)[1]  Step 1hThermal HelixInversionStep 2C1'C9RotorStator1 (thermally stable) 1' (thermally unstable) 1 (thermally stable)   Figure 1 Structure of the molecular motor studied, indicating the rotor and stator of the motor linked by the double bond axle. The initial photochemical step is followed by the thermal stroke  In these molecules absorption gives rise to an isomerisation step which is followed by a thermal inversion in which the configuration at the stereogenic centre leads to rotation with an overwhelming preference for one direction. Subsequent sequential photo- and thermal steps result in one complete cycle. Key questions to be addressed include the rate of the isomerisation, its sensitivity to solvent friction, the extent to which the reaction occurs on the excited state surface and the possibility of controlling the rate of isomerisation through the use of shaped pulses.     EPJ Web of ConferencesDOI: 10.1051/C© Owned by the authors, published by EDP Sciences, 2013,epjconf 201/05016 (2013)4134105016         This  is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20134105016EPJ Web of Conferences 2 Results and Discussion  Time resolved fluorescence was measured at wavelengths spanning the emission spectrum. The decay on blue edge is ultrafast, revealing components on the order of 100 fs. As the observation wavelength increases the decay time lengthens, but no risetime is registered on the red edge of the spectrum (Fig. 2). Superimposed on the fast fluorescence decay is an oscillatory component which can be analysed in terms of one highly damped (90 fs) low frequency mode (113 cm-1)  and one less damped (220 fs) higher frequency mode(180 cm-1);  these are illustrated in the Fourier transform (inset). Such oscillations in the fluorescence are characteristic of vibrational modes in the excited state which are displaced with respect to the ground state. Ultrafast fluorescence is particularly well suited to the detection of such excited state modes  -0.3 0.0 0.3 0.6 0.9 1.20.00.20.40.60.81.0Normalized EmissionTime / ps(b)0 50 100 150 200 250 300 350 400 450 50005101520AmplitudeWavenumber / cm-1 Figure. 2. Time resolved fluorescence observed for the motor molecule of Fig. 1. Wavelengths span the spectrum from 455 nm (dark blue) to 556 nm (red). The inset shows a Fourier transform of the oscillatory part of the fluorescence. 20 1.0 1 ps 2 ps 4 ps 0.8 10 ps15 udetiplmOD m 0.610 ed A0.4A / zialΔ m5 orN 0.20 0.0400 450 500 550 600 -2 0 2 4 6 8 10 12 14 16 18 20Wavelength / nm Time / ps  Figure 3. Transient absorption data for the motor in the visible region The wavelength resolved time resolved fluorescence can be converted into time dependent emission spectra, which reveal that the spectral mean frequency shifts to the red as the integrated emission intensity collapses on a time scale of a few hundred femtoseconds. Subsequent to these fast events the fluorescence decays with a time constant of 1.4 ps. The oscillatory features evident in the wavelength resolved data of Fig. 2 translate to an oscillation in both the intensity and the mean frequency of the time resolved spectrum[2]. This shows that the low frequency modes excited are coupled to the electronic transition moment.  Transient absorption measurements reveal two spectral features (Fig. 3) with distinct kinetics. The lowest energy one is formed rapidly (within the 300 fs time resolution of the apparatus) and 05016-p.2(b)  Figure 4. Schematic representation of the primary steps in a molecular motor References [1] N. Koumura, R. W. J. Zijlstra, R. A. van Delden, N. Harada, and B.L. Feringa Light-driven monodirectional molecular rotor. Nature 401 (6749), 152  (1999) [2] J. Conyard, K. Addison, I. A. Heisler, A. Cnossen, W. R. Browne, B L. Feringa and S. R. Meech Ultrafast Dynamics in the Ignition Phase of Molecular Rotary Motors, Nature Chem. 4, 457 (2012). [3] Kazaryan, A. et al., Understanding the Dynamics Behind the Photoisomerization of a Light-Driven Fluorene Molecular Rotary Motor. J. Phys. Chem. A 114, 5058 (2010)  decays with a 1.5±0.3 ps decay constant. The higher energy feature is evidently a mixture of the 1.5 ps transient, stimulated emission a very long lived (> 100 ps) transient and a ground state bleach. We assign the 1.5 ps state to the relaxed excited state observed in the fluorescence data.  Based on these data we assign the primary events in the photoactivated stroke of a molecular motor following electronic excitation as follows. The bright Franck-Condon excited state relaxes in a few hundred femtoseconds to a less bright relaxed state. The high rate suggests a small scale energetically downhill structural transformation, possible involving pyramidalization at the C9 carbon, as suggested by quantum chemical calculation[3]. The relaxed state populated in ca 200 fs and relaxes back to the ground state in 1.5 ps. Coherently excited vibrational modes modulate the transition moment, connecting the dark and bright (F-C) states. These assignments are illustrated schematically in Fig. 4. It is interesting that the coherently excited low frequency modes relax on the sub 100 fs time scale, setting an upper limit to the time available for exerting coherent control over the primary photochemical step. One route to the optical control of motor efficiency is through shaped light pulses, which may direct the excited state population towards or away from a conical intersection. However, the very fast damping of the lowest frequency modes suggests that the timescale in which this can be achieved is small. As an alternative to optical control of the rate and efficiency of motor operation we have investigated the effect of chemical modulation. Electron withdrawing (cyano, chloro) and donating (methoxy) groups were substituted on the 4′ position on the naphthalenic ring. The effect of the substitution on the ground state structure was assessed. The data are complex but electron withdrawing groups accelerate the decay of the FC state, but extend the lifetime of the dark state. These data are being analysed in terms of modifications to the shape of the PES such as that described in Figure 4. XVIIIth International Conference on Ultrafast Phenomena05016-p.3

Ultrafast ignition of a uni-directional molecular motor

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Ultrafast ignition of a uni-directional molecular motor

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