Light-driven rotary molecular motors Augulis, Ramunas; Klok, Martin; Loosdrecht, Paul H.M. van; Feringa, B.L.

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1 University of Groningen Light-driven rotary moleular motors Augulis, Ramunas; Klok, Martin; Loosdreht, Paul H.M. van; Feringa, B.L. Published in: Physia Status Solidi (C) DI: /pss IMPRTANT NTE: You are advised to onsult the publisher's version (publisher's PDF) if you wish to ite from it. Please hek the doument version below. Doument Version Publisher's PDF, also known as Version of reord Publiation date: 2009 Link to publiation in University of Groningen/UMCG researh database Citation for published version (APA): Augulis, R., Klok, M., Loosdreht, P. H. M. V., & Feringa, B. (2009). Light-driven rotary moleular motors: an ultrafast optial study. Physia Status Solidi (C), 6(1), DI: /pss Copyright ther than for stritly personal use, it is not permitted to download or to forward/distribute the text or part of it without the onsent of the author(s) and/or opyright holder(s), unless the work is under an open ontent liense (like Creative Commons). Take-down poliy If you believe that this doument breahes opyright please ontat us providing details, and we will remove aess to the work immediately and investigate your laim. Downloaded from the University of Groningen/UMCG researh database (Pure): For tehnial reasons the number of authors shown on this over page is limited to 10 maximum. Download date:

2 phys. stat. sol. () 6, No. 1, (2009) / DI /pss Light-driven rotary moleular motors: an ultrafast optial study pss urrent topis in solid state physis Ramūnas Augulis *, 1, Martin Klok **, 2, Ben L. Feringa ***, 2, and Paul H. M. van Loosdreht ****, 1 1 Zernike Institute for Advaned Materials, University of Groningen, Nijenborgh 4, 9747AG Groningen, Netherlands 2 Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands Reeived 11 July 2008, revised 11 August 2008, aepted 12 August 2008 Published online 16 tober 2008 PACS B, Df, Hq, Hp, J, Qt **** Corresponding author: R.Augulis@rug.nl, Phone: , Fax: **** Martin.Klok@rug.nl, Phone: , Fax: **** B.L.Feringa@rug.nl, Phone: , Fax: **** P.H.M.van.Loosdreht@rug.nl, Phone: , Fax: Moleular rotary motors, though ommon in nature, were first synthesized rather reently. ne of the most promising ategories of light-driven rotary moleular motors whih allow for optial ontrol is based on helial overrowded alkenes. In this ategory of motors, the rotation of the motor s rotor involves four disrete steps: fast light-indued is-trans isomerisation is followed by a thermally ativated step, the same steps are repeated one more to omplete the full 360 yle. Though the rotation rate of the motors is mainly limited by the duration of thermally ativated steps, many important fators, suh as effiieny and diretionality are determined by the light-indued isomerisation. For the development of ultrafast moleular motors it is ruial to understand the mehanism and dynamis of the optially indued isomerisation steps. In this paper we will review reent time-resolved optial pump-probe experiments on hiral moleular motors and disuss the results in terms of potential energy landsapes for the ground and exited states of the motor moleules. 1 Introdution Control of motion at the moleular level using hemial and/or l methods is an emerging and exiting field of siene. There are many examples of ontrollable linear and partial rotary motion [1] in syntheti moleular systems, however, rotary moleular motors, though ommon in biologial systems, were first synthesized rather reently [2]. ne of the most promising ategories of moleular motors whih allow for optial ontrol is based on helial overrowded alkenes. The hemial struture of the motor whih is analysed in this paper is shown in Fig. 1. From here on in this paper, the lower symmetrial part of the moleule is referred to as stator, whereas the upper part is alled the rotor. Repetitive, unidiretional rotation around the entral arbon-arbon double bond involves four disrete steps, in turn ativated by light and temperature as shown in Fig. 1. Rotation is ahieved by fast light-indued is-trans isomerisation to a higher energy intermediate with inversed heliity and the methyl group in an equatorial orientation (in Fig. 1 transition from stable form a to unstable form b). This is followed by a thermally ativated helix inversion that relaxes the struture bak to the lowest energy onformation with the methyl in axial orientation (in Fig. 1. transition from unstable form b to stable form ). The diretion of rotation is governed by the absolute onfiguration at the stereogeni entre. In ase of symmetrial stator, form is equivalent to form a, though the rotor is turned by 180 with respet to its initial position. The seond 180 yle is equivalent to the first one photoisomerisation ( to d) is followed by a thermally ativated step (d to a). Two suh yles result in one full 360º rotation. Continuous irradiation at suffiiently high temperature results in ontinuous unidiretional rotation. The limiting fator for the maximum rotation rate is the duration of the thermally ativated steps. Sine the first publiation, a signifiant progress has been made in redution of the energy barriers: the barrier rossing time for thermally ativated steps was redued from days to a few hundreds nanoseonds [3-6]. Light-indued steps, on the other hand, remain less explored, understood, or optimized. Ultimately one would like to have moleular motors whih perform an optially indued full rotation on an utrashort

3 p s s 182 R. Augulis et al.: Light-driven rotary moleular motors timesale. This means that one has to overome or speedup the temperature assisted steps, and to understand the dynamis of the optially indued steps. Reently, a quantum hemial study of potential energy profiles along the torsional angle for the ground and the lowest exited states for a number of moleular motors has been arried out [7]. The simulations, indeed, reveal asymmetrial potential landsapes, whih lead to rotation of the rotor during optial exitation-relaxation yle. a d Δ Δ Figure 1 Moleular struture and rotational yle of light driven unidiretional moleular motor. Time-resolved transient absorption measurements is a valuable tool for the exploration of light-indued isomerisation step dynamis of the motors, but additional information is needed to determine relationships between optial spetra and ertain geometrial onfigurations. ne of the ways to gain additional data is to slow-down the thermally ativated isomerisation steps by reduing the temperature and inreasing the visosity of the solvent [6, 8]. This way, it is possible to measure optial properties of the unstable forms (Fig. 1, b and d) in CW regime and use them as referene points for ertain geometrial onfigurations in time-resolved full-speed measurements at room temperature. This paper will review reent time-resolved optial pump-probe experiments and use the data to qualitatively desribe the route of the rotation proess in the potential landsape of the moleules. b 2 Experiments and results 2.1 Experimental setup and samples Femtoseond time-resolved pump-probe experiments were performed by using a 1 khz Ti:sapphire amplified laser system (Hurriane, Spetra Physis) and two non-ollinearly pumped optial parametri amplifiers (NPA s) (TPAS- White, Light Conversion Ltd.). The laser system produes 120 fs, 700 µj pulses, entered at 800 nm. Two portions of about 250 µj of the laser output are employed to generate sub-50fs pulses in NPA s, tuneable in a region of nm. The output of eah NPA is frequeny doubled in a 0.2 mm BB rystal, and reompressed in a double-pass CaF 2 prism ompressor, whih, in addition, also separates the seond harmonis signal from the fundamental frequeny. The energy of the pulses in the UV range ( nm) an be attenuated in a range of 0 50 nj. ne of the NPA s was used for exitation, the seond one for probing. Sine the spetral width of the pulses beomes narrower (1 2 nm) after doubling, the pulse duration goes up to approximately 100 fs, limiting the time resolution of the whole system to about 150 fs. For measurements in the visible spetral range, one of the NPA s is used without frequeny doubling. For a broad band probe and measurements in a nm range, a white light ontinuum ( nm) is generated by fousing 800 nm 200 µj pulses in a pressurized (5 bar) xenon ell. The full spetrum is reorded using an optial multihannel analyzer (MA) system or a monohromator and photodiodes are used for a narrowband probing. All measurements are performed in magi angle onfiguration. The exitation pulse energy was attenuated to 10 nj, whih orresponds to approximately 1 photon absorbed per 50 motor moleules. At suh exitation energies, the probability of multi-photon absorption in the solvent or motors was negligible. All the measurements were performed at room temperature with the motor moleules dissolved in hexane. A magneti pin was used to stir the solution in a quartz ell and redue the effets of heating and photodegradation. No suh effets were observed under the desribed experimental onditions. 2.2 Experimental results The absorption spetrum of the stable form (Fig. 1, states a and ) of the motors features a broad absorption band entered at 360 nm. The spetrum of the unstable form (Fig. 1, states b and d) measured ryogenially in a visous medium is rather different the absorption band is entered at around 460 nm [8]. Using these data, we employed the optial pump-probe tehnique to determine the timesale of stable-to-unstable form transition. The sample was exited with 360 nm light pulses, mathing the near-uv absorption band peak of the stable form while the transient absorption was measured in the range form 320 nm to 520 nm. The transient absorption spetrum after 50 ps delay is shown in Fig. 2a. The spetrum losely mathes the differene of the CW spetra of

4 Contributed Artile phys. stat. sol. () 6, No. 1 (2009) 183 the unstable and stable forms [6, 8] strongly indiating that the light-indued isomerisation step is ompleted within 50 ps from the exitation. To explore the photoisomerisation dynamis in more detail, the transient absorption dynamis at 375 nm and 450 nm probe wavelengths, mathing the absorption bands of the stable and unstable forms, respetively, were measured. The time traes, shown in Figs. 2b and 2, reveal a two-step proess. Firstly, in both ases, the signal grows with a time onstant of approximately 2 ps. Seondly, the signal deays with a time onstant of 16 ps to a ertain level and remains ompletely unhanged for hundreds of pioseonds, whih is not surprising, sine the duration of the thermally ativated step of this motor, as reported in [6], is about 3-4 miroseonds. a) b) ΔD (normalized) ΔD (normalized) ΔD (normalized) e) f) probe at 800 nm - probe at 375 nm - λ (nm) ) d) probe at 450 nm probe at 590 nm ΔD (normalized) ΔD (normalized) ΔD (normalized) - probe at 520 nm - Figure 2 Transient absorption dynamis of moleular motor at various probe wavelengths. The exitation wavelength is 360 nm in all ases. Sine the two-step proess desribed above is diffiult to interpret in terms of an isomerisation pathway, additional measurements were performed in the green-red side of the visible spetrum ( nm). Neither stable nor unstable form has absorption above 550 nm when in the ground state, so the non-zero transient absorption in this spetral region an be observed only when the motor moleule is in the exited state. Thus, transient absorption in this region is a seletive tool for probing the exited state. Pump-probe dynamis measured at 590 nm still reveals a two-step proess (see Fig. 2d). First, indued absorption appears instantaneously (within the resolution of the measurements), but ontinues to grow with a time onstant of about 2 ps. Then the signal deays to 0 with a time onstant of 12 ps. At even longer probe wavelength (800 nm) the dynamis is different again (Fig. 2e). In this ase, indued absorption appears instantaneously and deays mono-exponentially with a time onstant of 1.7 ps. The CW fluoresene spetrum of the motor features a single broad peak in the viinity of 500 nm [8]. Stimulated emission at the fluoresene wavelength an be used as a tool for observation of fluoresene dynamis. A negative peak in the initial part of the transient absorption trae at 520 nm (Fig. 2f) is indeed aused by stimulated emission, but unfortunately, the signal is overshadowed by strong indued absorption, whih grows with a time onstant of about 2 ps and afterwards deays to a ertain level with a time onstant of 16 ps. To summarize, all the pump-probe kinetis an be aurately fitted with a sum of two exponential deays of 1.7 and 16 or 12 ps with different relative weights for different probe wavelengths. 2.3 Interpretation of the results n the basis of the results desribed above a simple qualitative model of the motor rotation an be built. A qualitative diagram of potential landsapes of the ground and exited states of the motor is shown in Fig. 3. Initially, the motor persists in state 1 (equivalent to the state a in Fig. 1), the global potential minimum of the ground state, whih is equivalent to the state a in Fig. 1. The lowest energy absorption band (S 0 -S 1 transition), in this ase, is at approximately 360 nm, but absorption to higher lying states (<300 nm) is also possible. By absorbing a photon, the moleule undergoes transition to the exited state S 1 and relaxes to the loal minimum 2. Absorption, aused by the S 0 -S 1 transition is bleahed, but indued absorption to the S 2 sate an be observed in the near-infrared (800 nm). Further, the motor rosses a small barrier and undergoes a transition to a lower potential minimum 3. In this state the absorption in near-infrared disappears, sine state 3 is energetially lower than state 2, but at somewhat higher energies (590 nm in Fig. 2d), indued absorption is still observable. From both, state 2 and state 3, emission an be stimulated. However, in state 3, the indued absorption wavelength almost oinides with the emission wavelength, thus stimulated emission an be only observed while the motor is in state 2 (Fig. 2f). From state 3 the motor relaxes to the ground state and rotates until the loal potential minimum 4 is reahed (equivalent to form b in Fig. 1.). Relaxation of the signal at 590 nm (Fig 2d) indiates the deay of the exited state, while the dynamis at 375 nm and 450 nm shows the formation of state 4, thus the differene of 4 ps in the slower omponent of the dynamis (12 and 16 ps) is aused by the rotation down the potential slope of the ground state before state 4 is reahed.

5 p s s 184 R. Augulis et al.: Light-driven rotary moleular motors E S n S 2 S 1 S 0 1 0deg ps 3 12 ps 4 ps Rotation angle ~µs deg Figure 3 Shemati representation of the potential landsape and rotation mehanism of the moleular motor. Rotation angle and energy sales are arbitrary. Thik arrows indiate the rotation pathway, while thin dashed arrows indiate the optial transitions probed. The final step whih ompletes the 180 yle is the thermally ativated barrier rossing (transition from state 4 to state 1, or form in Fig. 1.). In our ase, state 1 is equivalent to state 1, but the rotor is rotated by 180. The duration of this step is on the order of miroseonds [6], thus, in our experiments the transition dynamis is not explored. The model desribed above doesn t take into aount additional relaxation pathways or possibilities of triplet formation. Triplet formation an be negleted, sine the time, spent in the exited state is very short (<20 ps). A ouple of examples of alternative pathways, whih don t result in motor rotation, ould be relaxation from state 2 diretly to the ground state or thermally ativated barrier rossing from state 4 to state 1. If relaxation from state 2 is the only alternative pathway, then, assuming that relaxation time from state 2 is the same as from state 3, it is possible to estimate the quantum yield of the isomerisation. With suh assumptions, the estimation for the urrent motor is about 85%. Although the measurements provide the time and energy sales of the proesses, the ruial, but rather unertain part of rotation analysis is to make orret assignments between geometry and optial properties. Additional data, suh as, for example, numerial simulations [7], would be very useful for verifiation and quantifiation of the model. Currently, the researh is ontinued by using time-resolved linear dihroism data, whih provides more insight about the intramoleular movement, and modified motors with different rotation time and energy sales. Referenes [1] W. R. Browne and B. L. Feringa, Nature Nanoteh. 1, 25 (2006). [2] N. Koumura, R. W. J. Zijlstra, R. A. van Delden, N. Harada, and B. L. Feringa, Nature 401, 152 (1999). [3] N. Koumura, E. M. Geertsema, A. Meetsma, and B. L. Feringa, J. Am. Chem. So. 122, (2000). [4] N. Koumura, E. M. Geertsema, M. B. van Gelder, A. Meetsma, and B. L. Feringa, J. Am. Chem. So. 124, 5037 (2002). [5] M. M. Pollard, M. Klok, D. Pijper, and B. L. Feringa, Adv. Funt. Mater. 17, 718 (2007). [6] M. Klok, N. Boyle, M. Prye, A. Meetsma, W. R. Browne, and B. L. Feringa, J. Am. Chem. So., DI: /ja (2008). [7] A. Kazaryan, J. Heuver, and M. Filatov, Chem. Eur. J. (aepted for publiation). [8] M. Klok, R. Augulis, W. R. Browne, P. H. M. van Loosdreht, and B. L. Feringa (in preparation). 3 Summary and onlusions n the basis of the results of ultrafast time-resolved pump-probe experiments a rotation model for light-driven moleular motor is suggested (Fig. 3). The data shows that the motor ompletes the light-indued isomerisation step within 20 ps, thus the thermally ativated isomerisation step is the main limiting fator for the rotation rate.