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Photonic Microrods Composed of Photoswitchable Molecules: Erasable Heterostructure Waveguides for Tunable Optical Modulation Dasari Venkatakrishnarao, Mahamad Ahmad Mohiddon, Naisa Chandrasekhar, and Rajadurai Chandrasekar* Nano- and microscale organic solids[1–4] are emerging as promising photonic materials to realize miniaturized device applications such as wave guides,[3–11] lasers,[12,13] resonators,[14,15] circuits,[16] and optical filters.[17] In the next level of complexity, responsive organic solids which can change their shape and dimensions with respect to external perturbation are known to exhibit switchable photonic properties.[5] Similarly a molecular solid which is acting as a reversible optical switch can be exploited to create erasable photonic structures reversibly to achieve complex optical functions. For example, one of the challenging research problems in organic photonics is fabricating tunable integrated device structures, which are erasable multiple times. Although the function of self-assembled organic optical wave guides are well known implanting tunable intensity modulators within the wave guide is challenging and unprecedented development in the area of organic photonics, because these modulators allow the amount of light passed to vary from maximum to minimum in a controlled manner. Technically a modulator integrated to a wave guide act as a light shutter and controls the light flow reversibly as a function of time. Scheme 1. Chemical structure of photoswitching molecules a) in its open (L-o) and b) closed A popularly known material for an external (L-c) states. c,d) Confocal micrographs of L-o microrod crystal grown from MeOH/CHCl3 solumodulator is lithium niobate (LiNbO3), which tion and its reversibly inter-conversion to L-c state microrod under UV/visible light, respectively. operates based on electrooptic effect.[18] In e) Graphical representation of various open and closed state heterostructures within a single microrod for delaying the propagating light. Scale bar is 10 µm. this material the refractive index (n) is suddenly increased by applying electric field thus slowing down the speed of light pulse, because νg = c/ng, found a simple yet promising reversible photochemical route where νg is the group velocity of light in a medium, c is the to fabricate “absorption mediated” erasable modulators which are implanted within organic crystal waveguides composed of velocity of light, and ng is the group refractive index. We have photoswitching molecules. These molecules undergo lightinduced reversible transformation between two distinct isomers D. Venkatakrishnarao, Dr. M. A. Mohiddon, having dissimilar n values, optical absorbance spectra, and Dr. N. Chandrasekhar, Prof. R. Chandrasekar colors.[19] Functional Molecular Micro/Nano Solids Laboratory In this work we have exploited the photoresponsive nature School of Chemistry of a well-known dithienylethene[18] molecule to fabricate University of Hyderabad Prof. C. R. Rao Road microrod waveguides and subsequently implanted a series GachiBowli, Hyderabad 500 046, India of erasable modulators multiple times within the waveguide E-mail: [email protected]; [email protected] (Scheme 1). In solid state the dithienylethene unit undergoes photochromic reversible cyclization (closed-state) and DOI: 10.1002/adom.201500106

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cyclo-reversion (open-state) reactions (response times of coloration and decoloration are less than ten picoseconds)[24] under exposure to UV and visible light, respectively (Scheme 1a–d). The open-state microrod, directly guides the visible light to the output end, which is typical for passive organic waveguides.[4,5,8] On the contrary, the closed-state wave guide absorbs the visible laser light, carrying out a time-dependent solid state cyclo-reversion reaction and thus delaying the output of the propagating light (τ). Based on this delay light principle, we have created a range of delay lines or modulators of different reaction volume within a single waveguide using laser writing and erasing technique (Scheme 1e). This technique was applied to implant or erase modulators multiple times within a microrod waveguide thereby fine-tuning the optical output intensity as a function of τ. The photoresponsive molecular switch, i.e., a back-to-back coupled 2,6-bis(pyrazolyl)pyridine (bpp) fragments bridged by a dithienylethene unit (closed-state; L-o) was prepared in five steps (Figure S1, Supporting Information). The diacetylenic derivative of dithienylethene was synthesized as per the reported procedure[19,20] and subsequently it was cross coupled with the 4-iodo-2,6-di(1H-pyrazol-1-yl)pyridine[21] under Sonogashira reaction condition to isolate target molecule L-o in good yields (Scheme 1a). The 1H, 19F-NMR spectroscopy and electron spray ionization time-of-fight mass spectrometry ESITOF-MS (m/z 835.16 [M + H]+) analyses clearly confirmed the formation of a photoswitching isomers. Further in the solution state L-o is colorless, while the closed-state (L-c) it displayed blue color (Figure S2, Supporting Information). Photoresponsive units in its L-o state aggregate in a mixture of methanol/ chloroform (2:1) via weak intermolecular forces to form needle like crystals. Examination of the single crystal X-ray structure (monoclinic; C2/c) (Figure 1a,b) revealed the presence of supramolecular interactions necessary for self-assembly. The solid state packing of L-o clearly exhibited the involvement of several hydrogen bond interactions such as F(2)···H(20) ≈ 2.259 Å, F(1)···H(21C) ≈ 2.323 Å, and F(5)···H(21A) ≈ 2.581 Å in the supramolecular self-assembly process. In the open-state the two bpp molecular planes nearly orthogonal to each other. This indicated that during the ring closure reaction of L-o to form L-c, the two bpp units undergo large movements leading to crystal defects. For the same reason we were unable to get the crystal structure of L-c. Microcrystals of L-o suitable for photonic studies were grown by drop casting a drop of crystallizing mother liquor on a glass slide followed by evaporating at room temperature (Scheme 1). Confocal and scanning electron microscopy (SEM) examinations of as prepared sample showed the formation of several colorless linear microrods composed of L-o molecules (Scheme 1c and Figure S3, Supporting Information). Upon UV light illumination, these colorless linear rods undergo electrocyclic reaction (without fluorescence) forming blue color bend rods composed of molecules of L-c (Scheme 1d and Figure 2e). The linear (colorless) to bend (blue) morphological change is easily reversible for several cycles upon exposure to UV and visible lights, respectively. Transmission electron microscopy (TEM) images of L-o and L-c microrods exhibited dark contrasts (Figure 2a,c) and selected area electron diffraction (SAED) indicated the crystalline and disordered states, respectively

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Figure 1. a) Single crystal X-ray structure of L-o. b) Solid state packing of L-o molecules. The inset shows the intermolecular C–H···F interactions.

(Figure 2b,d). To study the optical wave guiding tendency through a laser confocal microscope, one of the L-o state linear microrods placed on glass slide was selected and converted into L-c state using an UV lamp. Interestingly, point illumination of visible laser beam at one of the tips of L-c state bent rod showed no immediate out coupled light (OFF state) at the other opposite tip end (Figure 2e) due to the absorption of the visible light at the point of illumination. On the contrary, the L-o microrod showed an immediate optical wave guiding behavior (ON state) (Figure 2f). Furthermore, when one of the ends of a bent L-c microrod (length ≈153 µm, width ≈4.5 µm and height not measured) was point focused (20× objective; power 3.7 mW) with a 488 nm Ar+ CW laser (Figure 3a,b), the molecules at the light–matter interaction volume switch back to open-state (L-o). As a result creating a microrod heterostructure consists of two coexisting open and closed molecular state domains. The length of the newly formed L-o domain after 20 s of laser illumination was ≈23 µm (Figure 3e). These two domains can be easily seen under confocal optical microscope due to a clear color difference between the open and closed molecular states. Interestingly, incessant laser illumination revealed the propagation of visible laser along the open state domain till the L-o and L-c interface, converting L-c molecules into open L-o state there by delaying the light propagation time (τ). For example, after ≈30, ≈40, ≈50 s time intervals, the open domain length increased concurrently to about ≈41, ≈59, ≈68 µm, respectively (Figure 3f–h). Finally, at ≈120 s the input light reached the output end, after virtually converting all of the L-c molecules into L-o and subsequently

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To fine-tune the delay time of the propagating visible light, we created a series of modulators (L-c domains) at periodic intervals along the length of a microrod (length ≈102 µm, width ≈5 µm and height not measured) using laser writing and erasing technique (Figure 4a and Video S1, Supporting Information). At first localized visible laser illumination at one of the ends of the L-c microrod showed out coupled light after a delay time (τ) of ≈130 s (Video S2, Supporting Information). To create modulator integrated organic waveguides exhibiting varying τ of the output light, the same rod in the closed state was laser (He:Ne 633 nm) focused and illuminated along the rod long axis to cover half of its length, thus selectively creating L-o molecular domains. The resultant photonic heterostructure consisted of 50% closed (blue) and 50% open (colorless) states domains coexFigure 2. a,c) TEM images of L-c and L-o state microrods, respectively. b,d) Selected area elec- isting in a single microrod (Figure 4b and tron diffraction (SAED) data of L-c and L-o state microrods, respectively. e) Optical micrographs Video S3, Supporting Information). For this of a bent L-c microrod displaying no immediate output signal upon irradiation with 633 nm wave guide, it is theoretically expected that laser, and f) a linear L-o microrod guiding the 633 nm laser signal. the delay time would be of τ/2, i.e., one-half of the completely closed structure (see Figure 4a), but within our experimental accuracy we found a ≈57 s delay forming a linear microrod (Figure 3i–p). This above experiment time. This difference in the value of τ is attributed to slight varifurther provided a hint to deliberately slowing down the propagating light using the different volumes of L-c domain and ations of the rod width along its longitudinal axis, thus slightly using them as intensity modulators. The field emission scandecreasing the concentration of L-c state molecules. Similarly, ning electron microscopy (FESEM) image of the same wave a series of modulators such as, 2 × (τ/3), 2 × (τ/5), 5 × (τ/7), guiding L-o rod, which was picked-up through atomic force 6 × (τ/10), were created on the same microrod multiple times cantilever manipulation and deposited on a carbon coated TEM (Figure 4c–f) and they showed out coupled light after the delay grid is shown in Figure 3q. This rod again reverts back to its time of ≈91, ≈66, ≈95, and ≈55 s, respectively (Videos S4–S6, bent form upon UV light illumination (Figure 3a). It is imporSupporting Information). Another challenging experiment is tant to mention here that increasing laser power also swiftly Raman imaging of these two coexisting domains in a single decreased the delay time by opening up all the L-c molecules. microrod. We have used a very low power visible laser (power: 1.78 µW) at very fast scanning rate (scan time: 100 s) to scan the sample area. The resultant spectrum barely contains Raman signals, but imaging of the broad inelastic signal clearly showed a high intensity signal from the L-o domain and no signal from the L-c domain (Figure S9, Supporting Information). Finally to further engineer the delay time of the output light down to seconds, a time series experiment was performed on a transmission mode laser confocal microscope (Figure 5a). In this experimental set up a thin glass slide (thickness: 140 µm) carrying a microrod (length ≈108 µm, width ≈3.1 µm, and height not measured) was kept on a stage. From the bottom, a 532 nm (Nd:YAG) laser was focused (5× objective) on the rod and a UV laser was point focused from the top. Here the UV laser objective (40×; NA: Figure 3. Delaying the light propagation time (τ) using an L-c microrod. a,q) FESEM images of L-c and L-o microrods deposited on TEM grids, respectively, shown in b) and p). b–p) Delayed 0.6) is movable in such a way that it can pervisible light propagation (τ = 120 s) through a L-o microrod and reversible interconversion of form two separate tasks: (i) focusing UV light at the centre of the rod to create L-c domain L-c to L-o. Ar+ ion 488 CW laser with 3.7 mW power.

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count was recorded until the signal intensity reaches a maximum saturation value. Initially the output signal counts were in thousands, which progressively increased to a maximum value of several thousand after a delay time (Figure S6). The above sequential experiments were repeated five times on the same rod at fixed laser powers, the average of the five experiments was taken and the normalized values were plotted (Figure 5b). Similarly, keeping the same UV laser power, the UV irradiation time was increased progressively to 2, 3, 4, 5 10, 15, 20, 25, and 30 s and the corresponding delay times were plotted (Figure 5b). The sigmoidal curves clearly showed that upon increasing the UV exposure time from 1 to 30 s the τ value progressively increased from ≈10 to ≈35 s. In order to further slightly raise the value of τ, keeping the UV exposure time constant, the UV laser power was increased to 20 µW and 30 µW and the wave guiding experiments were repeated at constant visible laser power. By doing so the τ values were fine tuned to the range from ≈10 s to a minute scale (Figure 5c,d). In summary we have demonstrated an unprecedented technique to implant erasable intensity modulators at different delay time with in a photoresponsive organic microrod wave guide by exploiting the open and closed state reversible molecular interconversion reactions. These modulators with different delay time can be used to fine-tune the output intensity of the waveguides. The speed of the propagating optical signal can also be modulated by creating various closed state molecular domains or delay lines along the wave guide. Furthermore fine tuning of the parameters such as UV laser light irradiation time, its power, visible laser power, and the dimension of the photoresponsive wave guide provides an opportunity to delay the optical signal down to molecular Figure 4. Tuning the delaying time using a series of modulators. a) An L-c microrod modulator switching time scales to achieve high-speed (length ≈102 µm) delays the output light by ≈130 s. b–f) The same rod integrated with moduresponse. Additionally, by utilizing the metal lators of different delay times ≈τ/2, 2 × ≈(τ/3), 2 × ≈(τ/5), 5 × ≈(τ/7), 6 × ≈(τ/10). The solid coordinating ability of bpp units available in straight lines are guides for the eye. L, luminescent metal ions can also be integrated on the surface of the microrods to create hybrid structures.[22,23] Experiments are underway in or modulator, and (ii) collecting 532 nm output signal from the other end of the microrod (Figure 5a). At first a UV laser of our lab to probe the modulation properties of this modulators 10 µW power was focused for 1 s at the centre of the microrod implanted organic wave guide using infrared radiation. to create a modulator, and then the UV laser was turned off. Afterwards, the same objective was immediately moved to the output terminal of the microrod and tightly focused. Later, Experimental Section coupling of a 532 laser (5× objective; power 11 µW) to the rod 1 H, 13C, and 19F NMR spectroscopic data were recorded on a Bruker DPX input and collection of the output signal with an UV objec500 spectrometer with solvent proton as an internal standard. Elemental tive was performed simultaneously. For every 0.3 s the output analysis was recorded on a Thermo Finnigan Flash EA 1112 analyzer signal was collected for the duration of 120 s and the CCD instrument. HR mass spectrometry was performed on a Shimadzu

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COMMUNICATION Figure 5. A tunable modulator implanted optical µ-wave guide. a) Schematics of the experimental set-up used for tuning the delay time of the output light by creating L-c domains (modulators) of different volumes. b) A plot of normalized CCD counts of 532 nm light output as function of delay time displays the effect of modulators created by 10 µW UV laser exposure for 1–5, 10, 15, 20, 25, and 30 s. c,d) Similarly normalized plots for irradiation with 20 and 30 UV µW laser power at various duration. LCMS-2010A mass spectrometer. UV–visible absorption spectra were recorded on a SHIMADZU-UV-3600 UV–vis-NIR Spectrophotometer. For thin-layer chromatography (TLC), silica gel plates Merck 60 F254 were used and compounds were visualized by irradiation with UV light. Raman Microscopy: Raman spectra of the samples were recorded on a Wi-Tec alpha 300 AR confocal Raman spectrometer equipped with a Peltier-cooled CCD detector. Using a 300 grooves mm−1 grating BLZ = 750 nm, the accumulation time was typically 10 s and integration time was typically 0.0223 s. Ten accumulations was performed for acquiring a single spectrum. Argon ion (Ar+) laser operating at 488 nm or He–Ne 633 line were used as an excitation source for the Raman scattering. All measurements were performed at amibent condition. Laser Confocal Optical Microscopy: The time series experiments were carried out using both the bottom illumination and top collection set-ups of the Wi-Tec alpha 200 SNOM laser confocal optical microscope (T-LCOM) facility equipped with a Peltier-cooled CCD detector. Both UV 355 and Nd:YAG 532 nm lasers were employed for the experiments. The collection of Rayleigh signals from the output of the microrod was performed by an upright microscope UV objective (40×; NA: 0.6) and a bottom inverted microscope 5× objective was used as an input source (maximum output power is 40 mW). The same UV objective was used for focusing UV beam at three different powers (10, 20, and 30 µW) for creating modulators with different delay times. At each of this laser power, the UV laser expose time was also varied at 1, 2, 3, 4, 5, 10, 15, 20, 25, and 30 s to further fine-tune the delay time. The 532 nm laser beam was coupled (spot size ≈ 680 nm) to the microrod for the passive wave guiding studies through the integrated organic modulator. Throughout the experiment all the X, Y, and Z values were kept constant for accurate measurements. Before starting the experiment the X, Y, and

Adv. Optical Mater. 2015, DOI: 10.1002/adom.201500106

Z values at the output microrod end were all set to zero and this values were optimized (X = −9.8, Y = 41.0, and Z = 0) to focus the UV laser on the middle of the microrod and again to reach the same output point with zero coordinate values. The output Rayleigh signal collection was performed using a UV objective for every 0.3 ms and the signal was sent to a CCD detector through a multimode optical fibre of diameter 100 µm (core). The time taken to complete one time series was from 90–120 s and the experiment was done for five times and the spectra were summed up to get a new spectrum showing the accurate time to open the microrod. The data were processed by Wi-Tec project 2.10 software as follows. The spectra collected at the output terminal has strong 532 nm peak along with faint broad hump extended across 100 nm. For studying the passive wave guiding nature of the rods, the area under the 532 nm peak was extracted and its variation as function of time is plotted with respect to UV laser power. As the 532 nm laser exposed time increases, the strength of the signal collected at output terminal also increases and it reaches to saturation, when the tube is completely opened. Laser power was estimated using THOR Labs power meter.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported UGC UPE 2 project. We thank the Centre for Nanotechnology (CFN), University of Hyderabad (UoH) for providing

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www.MaterialsViews.com the TEM and SNOM facilities. D.V. and N.C. thank CSIR-New Delhi for SRFs. Received: February 16, 2015 Revised: March 3, 2015 Published online:

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