Nondestructive Probing of a Photoswitchable Dithienylethene

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Nondestructive Probing of a Photoswitchable Dithienylethene Coupled to Plasmonic Nanostructures Christian Schörner, Daniela Wolf, Thorsten Schumacher, Peter Bauer, Mukundan Thelakkat, and Markus Lippitz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05558 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

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The Journal of Physical Chemistry

Nondestructive Probing of a Photoswitchable Dithienylethene Coupled to Plasmonic Nanostructures Christian Schörner,† Daniela Wolf,† Thorsten Schumacher,† Peter Bauer,‡ Mukundan Thelakkat,‡ and Markus Lippitz∗,† †Experimental Physics III, University of Bayreuth, Universit¨ atsstr. 30, 95440 Bayreuth, Germany ‡ Applied Functional Polymers, University of Bayreuth, Universit¨ atsstr. 30, 95440 Bayreuth, Germany E-mail: [email protected]

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Abstract We demonstrate read-out of the conformational state of photoswitchable molecules, without modifying that state by the read-out process itself. The ring-opening and ringclosing reaction do not only change the absorption spectrum of a molecular layer, but also its index of refraction. A thin layer of a dithienylethene derivative was combined with well-designed gold nanostructures. The particle plasmon resonance of these nanostructures is extremely sensitive to the photoinduced change in the environment and can therefore be used to probe the state of the photochromic switch. The probing wavelength now interrogates the plasmonic structure, not the molecular film, and can thus be conveniently placed in a transparent spectral range, e.g., the near infrared. We find good agreement between experiments and numerical simulations with regard to spectral signatures of the plasmon resonance.

Introduction Photochromic molecules can be reversibly transformed between an open and a closed form by light. 1–4 A significant change in the absorption spectrum is going along with this conformational rearrangement. Especially dithienylethenes are promising photochromic materials for optical and optoelectronic data storage and processing 5,6 because of their thermal stability, fatigue resistance, high sensitivity, rapid response on the picosecond timescale and their ability to undergo photochromic reactions in the solid state. 6 However, optical read-out of the photoswitches’ conformation always triggers the corresponding photochromic reaction with a certain quantum yield. Quenching of fluorescence by one state of the switch can be used to signal this state, 7–10 but the overlap of the absorption bands of fluorophore and switch still cause unwanted switching reactions. A non-destructive optical technique is therefore required for a broad range of applications. Here we demonstrate that the spectral position of a particle plasmon resonance is a convenient mediator of the conformational state. The plasmon resonance is the collective resonance of conduction electrons in noble metals, 2


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which occurs in the visible and near-infrared spectral range. 11,12 The resonance condition strongly depends on the dielectric properties of the surrounding as well as shape and size of the metal structure. The photochromic reactions of diarylethenes can take place even on the surface of noble metals. 13–16 For the case of plain metal surfaces the shift of the plasmon resonance upon photoisomerisation of covering diarylethenes was used as a nondestructive read-out method. 17,18 In contrast to plain surfaces, particles of a few tens of nanometers in size show a much more pronounced particle plasmon resonance, which is strongly influenced by photochromic molecules. 19–23 We set out to use the particle plasmon mediated signal to read out the isomerisation state without influencing it. We measure extinction by the plasmonic particle to determine the conformation of a molecular layer which does not absorb at the probe wavelength.

Experimental Section Samples The DCP molecule was synthesized according to a published procedure by a Suzuki coupling between 3-Bromo-5-(4-methylphenyl)-2-methylthiophene and the boronic acid monoester obtained from 3,5-Dibromo-2-methylthiophene. 24 The thin layers of photochromic DCP molecules on substrates were prepared as follows: The DCP molecules are dissolved in toluene with a concentration of 10−2 M. Subsequently 20 µl of the solution is pipetted onto a substrate (Menzel cover slip, ∅ 25 mm, # 1,5) and spin coated (12 s ramp up to 30 roations per second (rps), hold for 20 s, 12 s ramp down to 12 rps). This procedure results in thin films of DCP with thicknesses of approximately 60 nm, as determined with a profilometer (Veeco Dektak 150). The plasmonic nanostructures were fabricated by electron beam lithography. The nanorods were arranged in a grating structure, with a grating constant of 460 nm and a constant height of 30 nm. The width of the rods is varied from field to field from 130 nm to about 270 nm 3



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in 9 steps. Each field has a size of 80×80 µm. The structures were checked with a scanning electron microscope (Zeiss Leo 1530) after the fabrication. For the hybrid systems, combining photochromic DCP with the plasmonic nanostructures, a layer of DCP is spin coated on top of the fabricated gold gratings with the above recipe.

Spectroscopy The DCP molecules were switched with white light (Ocean Optics DH-2000) filtered by appropriate filters. For ring closing UV bandpass filters (Schott UG11 or AHF Brightline HC 292/27) were used resulting in wavelengths 270-380 nm or 278-306 nm for the intended UV illumination. Ring opening of the DCP molecule was achieved by using a longpass filter (Schott OG550) resulting in a visible illumination with wavelengths 550-800 nm overlapping with the DCP absorption. The absorbance spectra of the DCP layer used for the Kramers-Kronig analysis were recorded with an Agilent Cary 5000 UV-VIS-NIR spectrometer, where an empty Menzel cover slip was set in the reference arm. To measure the plasmonic extinction spectra, the beam of the white light source was polarised perpendicular to the nanorods and focused onto the sample with a spot of approximately 50 µm in diameter. An integrated widefield microscope allows us to address individual fields on the substrate. The transmitted light is collected from a fiber and detected by a spectrometer (Ocean Optics USB2000+). The extinction spectrum is calculated as the negative decadic logarithm of the transmission. For the reference measurement required for calculating the transmission, the probing light passes an empty part of the substrate. For probing the hybrid sample with a narrow spectrum, the white light was filtered by bandpass filters (Chroma ET845/55M or Semrock TBP01700/13). The fast switching of the DCP layer to the open form during the measurement can be induced by the light of an external light-emitting diode (Thorlabs MCWHF1 Cold White High-Power LED), which is filtered by a Schott OG550 and focused on the sample such that 4


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it covers the entire probe beam focus.

Simulations For the numerical simulations, a finite element method 25 was used. The substrate was modeled by a refractive index of n = 1.5. For the gold nanostructures, the dielectric data from Johnson and Christy 26 was used. An individual rod was inserted in the simulation box and periodic boundary conditions were applied to solve the problem for an infinitely extended grating structure and plane-wave excitation. The photochromic layer was implemented using the calculated complex refractive index (see Fig. 1), obtained from a singly subtractive Kramers-Kronig analysis. For the open form of the dithienylethene, the reference point (800 nm / 1,54) was used, which is estimated from similar values in the publication of M. Irie. 5 For the closed form, the reference point (100 nm / 1,48) was used, assuming a similar refractive index in the UV for both forms of DCP. The agreement of our simulations with the experimental results (e.g., spectral positions of the plasmon resonances) indicates that the model is valid.

Results and discussion Absorption and dispersion of DCP Let us start by discussing the linear optical properties of the dithienylethene derivative 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene considered in our experiments, referred to hereafter as DCP. All experiments are carried out with DCP spin-coated on a glass substrate. A comparison between the photochromism of different dithienylethenes (including ours) in solution and the crystalline phase can be found in the literature. 2 From the absorption spectrum the imaginary part κ of the refractive index is calculated (Fig. 1a) using the known film thickness (65 nm). To compute the real part n of the refractive index by the Kramers-Kronig-relation, 27 we assume a flat absorption spectrum above and below the mea5



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sured interval, as indicated by the dotted lines in Fig. 1a. This approach neglects possible absorption bands of the DCP molecule in the middle and far ultraviolet range which we did not record. However, we are only interested in the refractive index in the near infrared range, because this is our desired spectral position of the plasmon resonance (see below). According to the Kramers-Kronig-relation, this huge difference in spectral range implicates that the neglected absorption bands in the ultraviolet range do not affect the shape of the refractive index in the near infrared range significantly. As expected, the real part of the refractive index shows a dispersive shape which extends far beyond the absorption peak of DCP around 600 nm (Fig. 1b). Upon photo-switching of the molecules, the refractive index changes in the order of ∆n ≈ 0.06 at a wavelength of 800 nm. This is the same order of magnitude as reported for similar derivatives in thin films. 5 The fatigue resistance of the DCP molecule, i.e. the ability to be switched reversibly several times, was recently investigated using fluorescent perylene bisimide units covalently linked to the DCP unit, yielding typically several thousands of swichting cycles. 8,9 Generally dithienylethenes like DCP are classified as thermally stable, 4 so that thermally induced re-opening is not expected once the molecule is in the closed form.

Shift of the plasmon resonance The spectral position of a particle plasmon resonance is especially sensitive to the index of refraction of the surrounding medium, but also depends on the size of the nanoparticle. 12,28 We fabricated grating-like patches of gold nanorods of different width between 130 nm and 270 nm (Fig. 2a), with a plasmon resonance wavelength between 690 nm and 790 nm in air. That way, we cover the spectral region of large variation in index of refraction when photoswitching DCP. The nanorods (height 30 nm) have been completely covered by a DCP film of 65 nm height. For more information see experimental section. We measured the extinction spectra for the uncovered field, as well as covered with both the open and the closed form of the DCP layer, as shown in Fig. 2b. The light was 7




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polarised perpendicular to the nanorods’ long axis. Additionally, the pure absorption of the photochromic film measured aside from the nanostructured fields is also plotted as a reference. The plasmon resonance leads to an additional peak in the spectrum, clearly separated from the molecules’ absorption band in the visible range. Due to this separation of both bands, we do not expect effects like enhanced cycloreversion reactions 21 or quenching of the excited state 29,30 to play a major role in our hybrid system. The exact spectral position of the plasmon resonance peak depends on the index of refraction of the environment. Varying the width of the metal rods (Fig. 2c), the peak shifts from 700 nm to 820 nm. We observed systematically for all fields a shift in the plasmon resonance upon switching the conformation of the molecular layer, as expected from the change in refractive index of the DCP layer shown in Fig. 1b. Our experimental spectra agree with finite element simulations 25 shown in Fig. 2d. In particular the spectral position of the plasmon resonances, including the relative shifts with respect to each other, are well reproduced. This confirms that the calculated refractive indexes for both isomers of the dithienylethene used in the simulation are consistent with the experiment in the near infrared region. The differences in spectral width of the plasmon resonance are due to slight inhomogeneities of the nanostructures fabricated by electron beam lithography.

Nondestructive probing The measurements and simulations demonstrate a remarkable change of the plasmon resonance upon photo-switching at wavelengths in the near infrared spectral region, far away from the DCP absorption band. This can be seen more clearly when calculating the difference of the extinction ∆E between open and closed form of the DCP layer. It is plotted for the case of field 7 (200 nm width of the nanorods), together with the absorption change of the pure DCP-film, in Fig. 3a. While the change in extinction at the DCP absorption band around 600 nm is almost unaffected, the hybrid system shows an additional peak around 9




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850 nm, which is only about a factor of three weaker than the direct absorption peak. The pure dithienylethene has quasi no change of extinction in this spectral region. For the other fields similar maxima were measured, slightly shifting due to the changing geometry (circles in Fig. 3a). This spectrally shifted second extinction feature allows us to detect the photochromic state at a wavelength where the molecules themselves do not absorb. We now probe the hybrid system around 850 nm and the pure DCP layer around 615 nm wavelength at similar excitation intensities (see markers in Fig. 3a) and monitor the resulting extinction change over several hours. Both systems were prepared with DCP in its closed form, having the characteristic absorption band in the visible region. Subsequently the samples were illuminated with the intended probe spectrum. The measured extinction was averaged in the ranges of maximum modulation (615 ± 10) nm and (850 ± 10) nm, respectively, and the difference in extinction ∆E was calculated as a function of time (Fig. 3c), until the sample was switched to the open form by filtered white light. When probing the pure DCP layer in the visible range, a temporal decrease of the difference in extinction ∆E can clearly be observed. Illuminating the photochromic molecules around 600 nm to read out their state induces ring-opening reactions which bleach the measured signal. A different behavior was observed for the hybrid system probed in the near infrared. In this case quasi no change of extinction was detected for several hours. The near infrared photons used to probe the plasmon resonance position are not able to switch the molecules. The plasmonic structure allows thus non-destructive probing of the photochromic state.

Conclusions We demonstrated by experiments and simulations that the plasmon resonance of suitably designed gold nanostructures is sensitive to the photochromic state of a surrounding dithienylethene derivative even in the near infrared spectral range. Here we exploit that the 11



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dispersive refractive index change arising from photo-switching DCP extends far beyond its absorption band in the visible range. An important application is nondestructive probing of the DCP state in the hybrid system. This approach is not restricted to a fixed wavelength, thus being very flexible. Engineering of nanostructures will enable us to select the desired wavelength of maximum modulation out of a large range of possible values. A further reduction of the layer thickness down to the monolayer level will highlight the advantage of plasmonic near-field light-matter interaction, as even the binding of a single protein could be tracked by its influence on the particle plasmon resonance. 31 Extinction spectroscopy of hybrid plasmonic structures promises to be a viable alternative to surface-enhanced Raman spectroscopy of similar structures. 32,33

Acknowledgement The authors gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (GRK1640).

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Nondestructive Probing of a Photoswitchable Dithienylethene

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