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Organic Submicro Tubular Optical Waveguides: Self-Assembly, Diverse Geometries, Efficiency, and Remote Sensing Properties Naisa Chandrasekhar, Md Ahamad Mohiddon, and Rajadurai Chandrasekar* The art of the self-assembly of shape-defined 1D nano-/microscale solids from chemically synthesized π-conjugated organic molecules into shape-defined 1D nano-/microscale solids is one of the emerging areas of molecule-based nanoscience and technology.[1–6] π-Conjugated organic molecules self-assemble to form diverse 1D geometrical shapes such as wires, belts, fibers, rods, and tubes in a variety of solvents in various sizes.[2– 6] There has been rising interest over the years in guiding the optical waves in these 1D organic solid state materials for potential future application in the areas of nano-/microscale organic photonic devices, sensors, light-concentrating devices, waveguides, etc.[2,3] In particular, nano-/micro-scale tubular solids with hollow cross-sections have diverse features that make them excellent photonic building blocks, including inherent one-dimensionality, their capability to function above and below the diffraction limit, and the presence of low-refractive-index air media in and around the tube.[2,3c] The dielectric difference (norganics/nair ∼ 1.5/1) can also provide good optical confinement at the interface due to the occurrence of total internal reflection (TIR). Compared to polymer optical fibers, and lithographically fabricated ridge waveguides, chemically synthesized and self-assembled nano/submicroscale optical waveguiding tubes can be used to achieve laser spot sizes in the range of the wavelength of the incident light. Additionally, the dimension of the tubular objects can also be tuned by controlling the growth kinetics. Moreover, the long waveguiding organic tubes are useful to isolate the heat from the laser source to the target illumination area in the nano-/microdomain. Hitherto, in all the reported waveguide experiments,[2] primarily the molecular building block of the organic solid with an electronic absorption window in the range of input UV lasers was used. Hence the resultant organic solid was electronically excited to show higher wavelength photoluminescence (PL) as an output, where the light propagation direction was controlled by the confinement geometry of the self-assembled structure. N. Chandrasekhar, Prof. R. Chandrasekar Functional Molecular Nano-/Micro-Solids Laboratory School of Chemistry University of Hyderabad Prof. C. R. Rao Road Hyderabad, 500046, India E-mail: [email protected]; [email protected] Dr. M. A. Mohiddon Centre for Nanotechnology University of Hyderabad Prof. C. R. Rao Road, Hyderabad, 500046, India

DOI: 10.1002/adom.201200067

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Recently, Rubahn et al. reported on the incidence light angle-dependent fluorescence and light-scattering properties of p-hexaphenyl nanofibres.[7] Alternatively, to avoid PL, the use of low-energy photons smaller than the electronic absorption range of the organic nano-/microsolids is useful, since it might provide direct insight about the confinement/propagation details of guided laser photons at different dimensions and geometries with minimal optical loss. To our knowledge, there is no report available in the literature on the investigation of guided laser light propagation directions within the organic solids through Raman scattering effect.[8] Recently in our preliminary report,[3c] we showed the preparation of shape-shifting organic solids (1D↔2D sheets) and their dimensionality-dependent optical waveguiding behavior explored by monitoring the inelastically scattered photons. Generally, Raman scattering experiments involve two different wavelengths, the input laser photons and the frequency-shifted inelastically scattered Stokes photons (mostly Stokes-shifted according to Boltzmann distribution) corresponding to the vibration energy. In the absence of any electronic absorption of input photons by the organic solid, both elastically (Rayleigh) and inelastically scattered (Raman) photons will be created due to light–molecular matter interactions. Raman scattering is a weak process, so the scattering intensity is feeble. Hence, Raman scattering can be exploited to track the propagation path of the guided input light within the organic solids. Additionally, Raman photons can also be utilized to detect the nano/ microscale defect sites inherited by the less-ordered organic solids. In order to investigate the inter- and intra-tube optical waveguiding/transfer mechanism in organic tubes with linear, bent, crossed, and tip-to-tip geometries, we have envisioned coupling different laser lines (Ar+ 488 nm, Nd:YAG 532 nm, and He–Ne 633 nm) to the organic submicrotubes prepared by the solvent-assisted self-assembly of 4,4’-bis(2,6-di(1H-pyrazol1-yl)pyridin-4-yl)biphenyl 1 molecule.[3a] In this paper, we report a controlled self-assembly route to fabricate submicrotubes with diameters in the range of 800 nm–1.5 μm, with various lengths (7 μm to ∼400 μm) and geometries (linear, “C”-bent, “X”-crossed, and Tip-to-Tip) from molecule 1 (Scheme 1). The fabricated tubes were well characterized by using solid state UV–visible spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), fluorescence microscopy (FM), and confocal Raman microscopy (CRM). We also report, for the first time, a detailed farfield analyses on the confinement and guidance direction of input laser photons along these tubular solids with linear and bent geometries by mapping Rayleigh and Raman photon

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the undissolved tubes act as nucleation sites and the dissolved molecules reassembled on the tube growth axes and increased the tube lengths. The obtained solution containing tubes was drop-cast on a glass substrate and dried at rt for SEM, AFM, and optical waveguiding studies. Interestingly, during the surface deposition, some of the longer tubes (ca. >100 μm) became curved (Figure S3, S9) and some short tubes formed various complex geometries such as X and V shapes. The SEM images of the self-assembled submicrotubes showed clear open-end features (Figure 1a). The AFM measurement revealed the hexagonal shape of the tubes (each facet width profile of ∼800 nm) and its diameter ∼1.5 μm (Figure 1c,d). In the solid state, the submicrotubes showed maximum absorbance at 330 nm (black line in Figure 1b) and emission at 451 nm (λex = 350 nm; blue line in Figure 1b). The shape of the solid state absorbance band of 1 showed that there is no possible molecular absorbance above λ∼ 420 nm (black line in Figure 1b). At first the waveguiding property of the blue fluorescent tubes under UV excitation was recorded using an FM set-up (Figure 1e,f). In this FM experiment, each isolated tube displayed a bright blue fluorescence spot at the tube open ends. Interestingly, two crossed tubes having “X” geometries showed bright blue fluorescence at the tubes’ crossing point and from the exit points of the tubes. These results demonstrate the waveguiding tendency of the tubes along their long axis (Figure 1e,f; Supporting Information Figure S1). Furthermore, the propensity of the submicrotubes to guide laser light was examined under CRM connected to a charge coupled device spectrophotometer (CCD) detector in a back-scattering geometry. A 100× objective [Numerical Aperture (NA) Scheme 1. Illustration of the experiment performed on a SNOM set-up to probe the optical wave = 0.95] was used to focus the laser beam propagation direction within the self-assembled organic tubes with different geometries. (488 or 633 nm) on the sample. The diameter of the focused laser spot on the sample signals. Optical coupling between two tubes arranged in crossed was ca. 680 nm. To study the waveguiding behavior of the and tip-to-tip geometries and the transmission efficiency is dissubmicrotubes deposited on a clean glass surface (Figure 2a), cussed as well. Finally, we also demonstrate the potential use a low energy 488 nm Ar+ source was used to circumvent any of these organic tubular waveguides for the remote (sensing) molecular electronic absorption and the resultant blue fluoelectronic excitation of a meso-tetratolylporphyrin 2 microsheet rescence. Orthogonal illumination of an input laser beam at located ca. 20 μm away from the laser source. one of the open ends of a tube (length ~ 100 μm) showed a The submicrotubes were fabricated as per our reported clear output light at the opposite open end due to a guided procedure,[3a] with a modification, in order to grow tubes of 1D optical wave transmission (Figure 2b,c). Furthermore, to varying lengths and geometries. In brief, in a test tube 0.2 mg study the scattering phenomena due to laser light–organic of compound 1 was dissolved in 1 mL of dichloromethane tube interactions, the optical images were recorded simul(DCM) (c ∼ 0.35 × 10−3 M) and the obtained clear solution was taneously using a color eyepiece video camera with 488 nm slowly evaporated at room temperature (rt) to get nearly monolong-pass edge filter (LPEF) to minimize Rayleigh-scattered dispersed tubes. To increase the tube lengths again 1 mL of photons. Interestingly, we observed strong inelastically scatDCM solvent was added to the solid. Here, possibly some of tered (low energy) green photons at the laser input point and

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of inelastically scattered (green) photons by the tube molecules. Additionally, to record the inelastic Raman photons spectroscopically, the scattered photons at the laser–tube interaction point were collected through a 488 LPEF. The obtained intense Raman active vibration modes (1225, 1293, 1367, and 1609 cm−1) within a broad background spectrum (λmax ∼ 590 nm) from a single tube clearly confirmed the guided 1D propagation of 488 nm input light (Rayleigh) along the tube axis (Figure 3). Furthermore, irradiation of the laser beam at the right and left ends of the tube resulted in the propagation of Rayleigh light to the opposite directions. While irradiation of laser at the centre of the tube showed light propagation to both open ends of the tube (Supporting Information Figure S2). Similar experiments performed on tubes of different lengths from 7 to 400 μm confirmed guided light propagation at different length scales (Supporting Information Figure S2 and S9). Furthermore to quantify the output intensity of guided input photons (Rayleigh), the experiments were performed on a transmission-mode CRM setup using a Nd:YAG 532 nm laser (Scheme 1). The glass coverslip containing the sample was kept over a movFigure 1. a) SEM micrograph of tubes with open ends. b) UV–visible diffuse reflection absorpable piezo scanner and one of the selected tube tion (black line) and emission spectrum (blue line) of tubes. c) AFM topography image of a selected hexagonal tube. d) Cross-section profile measurement of the tube shown in open ends was bottom illuminated with the laser source using a 5× objective (Scheme 1). (c). e,f) Fluorescence microscopy images of tubes under UV light. To probe the coupled input light propagation paths along the tube and its interaction with tube molecules, the Raman scattered also at the opposite end of the tube (Figure 2d,e). The collected photons were collected through a 570 LPEF by scanning the images with and without filters confirmed the propagation of entire area of the tube with a fixed top 100× objective and the elastically scattered photons (blue) and subsequent generation signal was sent to a CCD counter through an optical fiber (Scheme 1). To compare the scattered light propagation pattern and its transfer efficiency, organic tubes arranged in various geometries such as linear, bent, X-crossed tubes, and “V” shaped tubes were used. Illumination by a 532 laser beam (spot size ∼ 5 μm) at one of the open ends of a linear tube (Figure 4-Ia) showed propagation of guided laser photons to its opposite end (see bright- and dark-field images: Figure 4-Ib,c and Supporting Information Figure S6). The distribution of Raman photons along the tube was mapped in 3D from the recorded spectrum (Figure 3 and Supporting Information Figure S7). Here, the Figure 2. a,b) Bright-field images of a submicrotube without and with 488 nm Ar+ laser beam obtained strong Raman peak at 1609 cm−1 irradiation, respectively. c,d) Dark-field images of a tube irradiated with a laser, without and (C=C stretch) from the tube was used to with 488 nm LPEF, respectively. e) Waveguiding organic tube images obtained by focusing the laser beam at six different positions. The red arrows show the laser input point. The red and map the propagation path of the input light along the waveguiding tube. The intensity of blue circles denote the laser input and scattered wave output points.

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Figure 3. Raman spectra of tubes at different input wavelengths. The left inset shows the Stokes-shifted Raman signals in the 500–2000 cm−1 region. The right inset shows the dark-field images of tubes (without filter) displaying guided optical waves at different input wavelengths (488, 532, and 633 nm).

Figure 4. Far-field CCD images of waveguiding (I) linear, (II) bent, and (III) X crossed tubes upon point irradiation with a 532 nm laser: I. a) Bright-field image of a linear tube. b,c) Brightand dark-field images of a tube irradiated at the right open end with laser. d,e) 3D projection of the distribution of filtered scattered photons along the tube. II. a) Bright-field image of a bent tube with an angle 30°. b,c) Bright- and dark-field images of a bent tube irradiated at the one of the open end with laser. d,e) 3D projection of the distribution of filtered scattered photons along the tube. III. a) Bright-field image of two crossed tubes, labeled 1 and 2. b,c) Bright- and dark-field images of two crossed tubes displaying guided output light at points B,C, and D upon laser irradiation at one of the open ends (A) of tube 1. d,e) 3D projection of the distribution of filtered (570 nm LPEF) scattered photons in the X-crossed tubes. The red and blue circles denote the scattered wave input and output points. k is the wave propagation vector.

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the photons was estimated by integrating the output signal intensity as a function of time and dividing the output light (Io) by the input light (II). Mapping analyses in the presence of 570 LPEF showed low Rayleigh photons only at the input end (Figure 4-Id), whereas the Raman mapping showed ca. 33% signal intensity at the output tube exit compared to the input end intensity (Figure 4-Ie). This revealed a preferential interaction of the input photons with molecules located at the tube open ends, generating more Ramanscattered photons. In order to test the possibility to bend the light propagation direction, the waveguiding experiments were performed in a “C” shaped bent tube with a bending angle of ca. 30° from the tube axis (Figure 4-IIa). Interestingly, pointing the laser spot at one of the open ends of a tube showed a clear output light at the other open end (Figure 4-IIb,c). This observation evidently demonstrates that the output light direction can be bent through an organic tubular optical waveguide with curved geometry. With 570 LPEF, almost no Rayleigh line was found at the output end (Figure 4-IId) as a result of strong inelastic scattering generating ca. 45% Raman photon intensity at the output end (Figure 4e). The variation in the Raman photon intensity in the linear and bent tubes clearly points out the difference in the molecular environment/ population at the tube ends. Interestingly, in contrary to a linear tube, the whole body of the bent tube showed intense Raman scattering as a result of the bending inducing defect sites and the resultant favorable molecular orientation. Additionally, to study the optical coupling behavior of two tubes in contact, a waveguiding experiment was carried out in a geometry in which two tubes were “X” crossed (Figure 4-IIIa). The launching point of the input light is labeled A and the three output points are labeled B, C and D (Figure 4-IIIb,c). Illumination of one of the open end features (at A) of a crossed tube showed strong output light from the same tube at B as well as from the other tube open ends at C and D, which is not directly illuminated (Figure 4-IIIb,c). This finding establishes that the input signal can be split and send in three different directions in the submicroscale, when two tubes are in contact. Mapping studies confirmed the inelastic interaction of the propagating photons along the tube and also at the exit points B–D of tube-2. (Figure 4-IIIe). Almost no Rayleigh light was detected at the tube exits B, C and

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Here, the tube-1 output is almost pointed towards tube-2 input, facilitating increase in the optical coupling and guiding efficiency from points B to C. To calculate the efficiency of the guided laser light, the output light (Rayleigh) was collected without a 570 nm LPEF and 2D mapped. Additionally, to avoid CCD saturation a low energy laser power was used. For the quantification, the input and output photon counts of tube-2 at points B and C was used. One of the advantages of this geometry is that at the parallel coupling point B, the output laser light diameter from tube-1 exit is close to the diameter of tube-2 and is nearly pointed towards tube-2, which improves the coupling efficiency of the tubes. However, during the calculation one needs to take into account of the slight geometrical (slight deviation of the tube-2 output laser spot Figure 5. Far-field CCD images of two tubes in tip-to-tip contact used for the calculation of size compared to tube-2 diameter), angular optical waveguiding efficiency: a) Bright-field image of two tubes in contact. b,c) Bright and (tube acceptance angle, and source angular dark-field images of two tubes irradiated at one of the open end (A) with laser. d) 3D projecdistribution), and Fresnel (partial reflection tion of the distribution of Raman scattered photons in the tubes after 570 nm LPE filtration. e) 2D projection of the propagating Rayleigh photons from point A to B and from B to C imaged and transmission of light) losses, at point [10] after without filter using a low intensity laser power to avoid detector saturation. f) A plot of Rayeligh B. Ignoring all these optical losses, intensity as a fucntion of tube length at points A, B, and C corresponding to (e). The blue circle the background corrections, the estimated denotes the output point. The red circle denotes the input point. optical coupling efficiency (C/B) of tube-2 (even at 135° coupling geometry) was ca. 58% (Figure 5-e,f). A plot of Rayleigh photon CCD counts (obtained D with the filter (Figure 4-IIId). The directional optical coupling without 570 LPEF) versus tube length showed the effect of varibehavior of tubes shows that during the propagation of elastic ation in the optical coupling efficiency when the light is couphotons from point A to B of tube-1, due to the contact made pled perpendicular (source to tube-1) and parallel (tube-1 to by tube-2 with a similar refractive index, most of the coupled tube-2) to the tube. Additionally, the Raman mapping clearly photons were transferred from tube-1 and tube-2 efficiently confirmed the light output at both points B and C. Further, this at the tube junction. Similar experiments performed on two result revealed the possibility to transfer optical waves from one or three crossed tubes using a 488 nm laser line also showed organic tube to another tube via the air gap. strong coupling between the tubes (Supporting Information Finally, to exploit these tubular waveguides assembled from Figure S4 and S5). 1 for remote excitation of fluorescent microsolids, a meso-tetraThe optical transmission efficiency of an organic submicrotolylporphyrin molecule 2[11] was selected (Figure 6a). Injecting tube depends crucially on the relative position of the tube with 1 mL of water into 2 mL tetrahydrofuran (THF) solution of 2 respect to the laser beam. Due to the submicroscale diameter (0.2 mg/mL) readily formed nanoaggregates. The SEM and of the tube, direct in-plane coupling of the laser light with the TEM images of the aggregates displayed the formation of monotube is experimentally challenging and not straightforward. dispersed hexagonal microsheets composed of 2 (Figure 6c–e). Therefore in all the above experiments (Figure 4; Cases I-III) In the solid state, the sheets showed a maximum absorption the input laser light was coupled to the tube in an orthogonal at 431 nm and five additional weak absorptions from 475 to manner. As a result strong unwanted scattering at the inci675 nm (see purple line, Figure 7b). Electronic excitation of dence point was observed, leading to a decrease in the source a sheet with a 488 nm Ar+ laser readily displayed red fluoreslight coupling efficiency with the tube. Moreover, geometrical cence bands at 653 and 722 nm (see red line, Figure 6b). In losses before coupling cannot be avoided since the diamorder to make a tube–sheet hybrid geometry, the hexagonal eter of the laser spot is much larger than the tube diameter. sheets were deposited on a substrate containing submicroHence, to estimate to the optical transmission efficiency, a diftubes obtained from 1. For a remote excitation experiment, a ferent approach was adopted. For this, two tubes (numbered sheet (C) in close contact with a one of the open tube ends (B) 1 and 2) arranged in a “V”-shaped geometry with an angle was identified (Figure 7a). Orthogonal-point illumination of a of ca. 135° was identified (Figure 5-a). The labels A, B and C 488 nm laser beam at input point A of the tube displayed a denote the input point of tube-1, the meeting point of tube-1’s localized red luminescence from the microsheet, which is in output end and tube-2’s input end, and the final output end contact with the output far end of the tube B (at a distance of ca. of tube-2, respectively. The calculated gap between tube-1’s 20 μm). This model experiment clearly demonstrates that these output and tube-2’s input points (at B) was ca. 1 μm (Figure 5-a). tubular waveguides can be used for remote electronic excitaInterestingly, orthogonal illumination of a laser beam at point tion of fluorescent nano/microassemblies which are sensitive A showed two output lights, at points B and C (Figure 5-b,c).

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laser lines in organic molecular tubes showed efficient propagation of laser light and subsequent generation of weak Raman scattered photons. The Raman scattering was used to probe the input light propagation path and also to understand the interaction of light with the molecular building blocks of the organic tube. Furthermore, upon increasing the incidence wavelength from 488 nm, 532 nm and 633 nm, the wavelength of the broad emission band also shifted (ca. 100 nm) to low energies with a λmax at ∼ 590 nm, ∼630, and ∼705 nm, respectively (Figure 3 and Supporting Information Figure S8). Additionally an efficient optical coupling between two tubes arranged in a “V”shaped geometry was demonstrated. This experiment showed that a proper geometry may increase the optical transmission efficiency >58%. It is also important to mention here that, apart from experimental factors, the percentage of light output is also Figure 6. a) A meso-tetratolylporphyrin molecule 2. b) The absorption and emission spectra of dependent upon the diameter, length, and 2. c) SEM image of a bunch of monodispersed hexagonal sheets composed of 2 self-assembled geometry of the tubes. A recent study showed in DCM/water. d) A close-view of a single hexagonal sheet. e) TEM image of hexagonal sheets that a complex diameter-dependent angular of 2. distribution for Raman intensity variation in semiconductor nanowire.[9] The observed strong Raman-scattered peaks (due to Raman-active molecular to laser source heat and its intense beam. The composition of vibrations) at the tube ends indirectly points out the propagatube and sheet was further spectroscopically mapped from the tion of guided laser photons to the tube ends. Finally, these corresponding Raman/PL spectra, respectively (Figure 7e–h). experiments guarantee that these organic tubes can be used in In summary, our original experiments demonstrate for the the nano/micro regime to transmit light waves from a source first time the possibility to guide and transfer optical waves in to a remote location for communications and sensing as well self-assembled organic tubular solids of various geometries. as for spot illumination. As the light source is remote, the long The optical waveguiding experiments performed with different waveguiding tube transmits the light but segregates the heat from the laser source to the illumination point, an important consideration for lighting heat-sensitive nano/submicro targets/objects (see Supporting Information Figure S3 and S9). Furthermore, the experiments performed for remote electronic excitation of a red PL hexagonal sheet using a ca. 20 μm long tubular waveguide (Figure 7) clearly exemplify its remote sensing potential down to the nano/microscale domain.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Figure 7. Tube–sheet hybrid geometry used for remote sensing experiments. a) Bright-field image of a waveguiding tube of 1 in contact with a hexagonal sheet of 2. The red circle indicates the 488 nm laser beam input point. The red dotted rectangle shows the sheet area. b) The dark-field image. c) The bright-field image obtained with a 488 LPEF without point laser irradiation. d) The dark-field image displaying Raman scattering from the tube and red PL from the sheet obtained with a 488 LPEF. e) Raman image of the tube shown in (a). f) PL image of the sheet shown in (a). g) The combined color-coded Raman/PL image of tube and sheet. h) The marker peaks used for the Raman/PL imaging are shown in green (Raman) and red (PL) arrows in the total spectrum.

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Acknowledgements This work was supported by UGC-UPE2 and CSIR-New Delhi (01/2409/1-/EMR-II) projects. We thank the Centre for Nanotechnology (CFN), University of Hyderabad for providing the TEM and

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Received: December 20, 2012 Revised: March 3, 2013 Published online: April 12, 2013

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SNOM facility. NC thanks CSIR-New Delhi for a SRF. Corrections were made to all pages of this manuscript on April 25, 2013.

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