Mechanophotonics: Flexible Single-Crystal Organic Waveguides and Circuits

Article abstract of DOI:10.1002/anie.202003820

We present the one-dimensional optical-waveguiding crystal dithieno[3,2-a:2′,3′-c]phenazine with a high aspect ratio, high mechanical flexibility, and selective self-absorbance of the blue part of its fluorescence (FL). While macrocrystals exhibit elasticity, microcrystals deposited at a glass surface behave more like plastic crystals due to significant surface adherence, making them suitable for constructing photonic circuits via micromechanical operation with an atomic-force-microscopy cantilever tip. The flexible crystalline waveguides display optical-path-dependent FL signals at the output termini in both straight and bent configurations, making them appropriate for wavelength-division multiplexing technologies. A reconfigurable 2×2-directional coupler fabricated via micromanipulation by combining two arc-shaped crystals splits the optical signal via evanescent coupling and delivers the signals at two output terminals with different splitting ratios. The presented mechanical micromanipulation technique could also be effectively extended to other flexible crystals.

Full text of DOI:10.1002/anie.202003820

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Accepted Article
Title: Mechano-Photonics: Flexible Single-Crystal Organic Waveguides
and Circuits
Authors: Rajadurai Chandrasekar, Mari Annadhasan, Abhijeet R.
Agrawal, Surojit Bhunia, Vuppu Vinay Pradeep, Sanjio S.
Zade, and Chilla Malla Reddy
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Mechanophotonics: Flexible Single-Crystal Organic Waveguides
and Circuits
Mari Annadhasan,[a],§ Abhijeet R. Agrawal,[b],§ Surojit Bhunia, Vuppu Vinay Pradeep, Sanjio S.
[b]
[a]
[
b],
[b],
[a],
Zade, * C. Malla Reddy, * and Rajadurai Chandrasekar *
[
a]
Dr. M. Annadhasan, V. V. Pradeep, Prof. R. Chandrasekar
Functional Molecular Nano/Micro Solids Laboratory
School of Chemistry, University of Hyderabad
Prof. C. R. Rao Road, Gachibowli, Hyderabad 500 046, Telangana, India
E-mail: r.chandrasekar@uohyd.ac.in
[
b]
A. R. Agrawal, S. Bhunia, Prof. S. S. Zade, Prof. C. M. Reddy
Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER), Kolkata, Mohanpur, West Bengal, 741246, India
E-mail: sanjiozade@gmail.com; cmallareddy@gmail.com
§
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Microfabrication of circuits by combining the mechanical
and photonic properties of flexible crystals is imperative for flexible
miniature photonic devices. The fundamental understanding of
molecular packing and energetics of intermolecular interactions is vital
to design crystals with intrinsic flexibility and optical attributes. We
present a unique one-dimensional optical waveguiding crystal of
dithieno[3,2-a:2′,3′-c]phenazine (1) with high aspect ratio displaying
high mechanical flexibility and selective self-absorbance of the blue
part of its fluorescence (FL). Though, macrocrystals exhibit elasticity,
microcrystals deposited at a glass surface behave incredibly like
plastic crystals due to significant surface adherence energy, making
them suitable for constructing photonic circuits via micromechanical
operation with atomic force microscopy cantilever tip. Phenomenally,
the flexible crystalline waveguides display optical path-dependent FL
signals at the output termini in both straight and bend configuration
suitable for wavelength division multiplexing technologies. A futuristic
reconfigurable
22
directional
coupler
fabricated
via
Scheme 1. Cartoon representation of a 22 directional coupler constructed from
two flexible organic crystals via mechanical micromanipulation with AFM
cantilever tip. The molecular movements at (011) plane during bending is shown
at the bottom right corner.
micromanipulation by combining two arc-shaped crystals split the
optical signal via evanescent coupling and deliver the signals at two
output terminals with different split ratios. The presented mechanical
micromanipulation technique could also be effectively extended to any
flexible crystals to design and carve complex photonic circuits.
performed as optical waveguides in straight and bent
geometries.[ The use of flexible crystal waveguides in photonic
micro-circuits is limited by lack of suitable manipulation technique
to morph microcrystal(s) into intricate shapes. Further, the
implementation of mechanical operation in microcrystals is rather
tricky due to their soft and fragile nature. Micromanipulation with
atomic force microscopy (AFM) cantilever tip has turned out to be
a promising technique[11] to mechanical shaping of microcrystals
without causing damage to them.
Enabling multidirectional optical communications requires
crystal waveguides generating and transducing light signals of
different wavelengths, a technique called wavelength-division
multiplexing (WDM). Production of crystal waveguides with high
flexibility and WDM property is a challenging task. Use of flexible
microcrystals, possessing self-absorbance of part of fluorescence
9]
Crystals are usually brittle and inelastic.[1] Most of the crystal-
based devices which employ active micro-optical components
such as optical waveguides,[2] cavities, lasers, circuits, and
field-effect transistors[6] are all finished in rigid platforms. On the
contrary, the next-generation “smart” technologies demand
flexible devices with the forecasted market of USD 70 billion by
[3]
[4]
[5]
2
026.[7] Flexible devices require mechanically reconfigurable
crystals as active microcomponents. Flexible crystals are quite
rare. Therefore, the quest for such crystals with unusual
mechanical properties, for instance, elasticity (reversible
deformation) and plasticity (irreversible deformation) has recently
arisen to exploit them[8-10] for smart flexible device technologies.
The few known examples of millimetre-sized crystals
demonstrated their flexibility upon manipulation with sharp
mechanical objects.[ Crystals possessing interlocked host
structure, weak and dispersive interactions and mobile solvent
channels with the ability to store elastic energy generally show
reversible deformation in response to external mechanical
stress.[1,8] Some of the millimetre-long flexible crystals also
(
FL) is a promising approach to realise mechanically compliant
8]
WDM crystal waveguides and microphotonic circuits. However, to
the best of our knowledge, exploitation of flexible WDM
microcrystals to construct mechanically reconfigurable
waveguides and photonic circuits has not been realized yet.
1
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Figure 1. (a) Molecular structure of dithieno[3,2-a:2′,3′-c]phenazine 1. (b)
Schematic of the morphology of the crystal. (c) Images (video-grabs) showing
elastic bending of a single crystal: 1-3) first bending cycle under applied
mechanical stress using a forceps (on the left) and a metal needle (on the right).
4-6) the subsequent shape recovery of crystal upon stress withdrawal. (d)
Representative load-depth (P-h) curve corresponding to quantitative data
obtained by nanoindentation perpendicular to (01-1) face at 1 mN peak load.
Here, for the first time, we report the rare mechano-photonic
attributes, viz., mechanical flexibility coupled with WDM in single-
crystals of dithieno[3,2-a:2′,3′-c]phenazine (1) (Figure 1a). We
demonstrate, (i) structural basis for the extraordinary crystal
flexibility, (ii) mechanical compliance of crystals of 1 for bending
under micromanipulation conditions with AFM cantilever tip and
Figure 2. Crystal packing and elastic bending mechanism of 1. (a) ORTEP
diagram (with 50 % probability) and packing viewed along [100] and [011],
showing stacking and herringbone arrangement. (b) Plausible mechanism of
elastic bending involving elongation (left) and compression (right) of outer and
inner arcs, respectively, along the length of the bent crystal. Notice the
orientation of molecules on the outer (away from planarity) and inner arcs
(towards the planarity of herringbone structure). The molecules shown in grey
(
iii) mechanical assembly of two arc-shaped waveguides forming
shade in the packing diagrams (a, bottom-left) and (b) are from a row below and
overlap with weak interaction plane of top layer, making permanent slip difficult
along this plane during (elastic) mechanical bending. (c), (d) Energy frameworks
showing total intermolecular interaction energies. The thickness of tube
represents the magnitude of energy (scale factor is 35, tube size 100 and energy
cut-off is 5 kJ/mol). (e) π-stacking of molecules along crystal growth direction
[
12]
a 22 directional coupler
with two inputs and outputs, which
could split the transducing optical signals and deliver at two
different outputs with different split ratios (Scheme 1).
Synthesis procedure for compound
1 is provided in
supporting information (Scheme S1). Compound 1, which is an
important class of polycyclic conjugated molecule, was
characterized by various techniques, including the fluorescence
(Etotal =~  46 KJ/mol). (f) and (g) FESEM images of microcrystals of 1 in its
straight and bent geometries.
(
FL) spectroscopy. The FL quantum yield of compound 1 in
suggests that the pop-ins in molecular crystals occur due to
sudden movement of the tip due to abrupt yielding of the
compressed molecular layers underneath the tip when the force
exceeds their resistive force, leading to intermittent plastic
deformation.[13a,b]
The structure of 1 was determined from single-crystal X-ray
diffraction technique to establish a structural basis for the
exceptional mechanical flexibility. These needle-shaped crystals
which grow along [100] axis with two pairs of side faces, (0-
dichloromethane solution was found to be 14% which was
calculated with respect to quinine sulfate as reference (see
Supporting Information). Pale-yellow crystals of 1 were obtained
from dichloromethane solution with a hexane layer on top (see
Supporting Information). The macro-crystals could be bent into a
loop without breaking in a three-point bending test by applying
mechanical stress with a pair of forceps and a needle (Figure 1c,
labels 1-3 and video S1, in Supporting Information). Upon release
of the stress, the crystals restored to original shape on their own
and did not show any sign of permanent damage (or strain),
hence are elastic in nature with excellent mechanical flexibility
11)/(01-1) and (0-1-1)/(011), crystallize in the orthorhombic polar
space group P2 (Figure 2a,b and Figure S4). There is one
1 1 1
2 2
molecule in the asymmetric unit, with nearly flat molecular
conformation (Figure 2a). The molecule has a large aromatic
surface with two S and two N heteroatoms. In the crystal, the
molecules stack along [100] axis or crystal length to form
columns. Molecules in the adjacent stacked columns are tilted
with respect to each other to form a herringbone structure. The
columns with respect to each other are arranged such that the
weak interaction planes of one-row overlap with molecules from
below row, hence the structure is devoid of slip planes (see,
Figure 2a,b). As quantitative information of the intermolecular
interactions is crucial for understanding the mechanical flexibility
of crystals, we calculated the pairwise interaction energies from
the energy frameworks calculations using Crystal Explorer
software (see Figure 2c,d). The thickness of the tubes connecting
each pair of molecules is proportional to the total intermolecular
[
8]
(
Figure 1c, labels 4-6).
The crystals break when bent into
smaller loops. For instance, a crystal with a thickness of ~20 µm
broke when the diameter of the loop was decreased to around 0.8
mm (Supporting Information, Figure S1 and Video S2). The
maximum strain the bent crystals could withstand was in the
range of 2.5-2.8%. This is comparable to the strain in known
elastically bendable molecular crystals.[8] We further quantitatively
measured the mechanical properties of the crystals using
nanoindentation technique.[13c] Indentation tests were performed
on (01-1) face of the crystal with 0.2 mN/sec loading rate using a
peak load of 1 mN. The corresponding load (P) vs displacement
(
(
h) curves are shown in Figure 1d). The extracted elastic modulus
E) and hardness values (H) are 5.83  0.62 GPa and 0.26  0.01
GPa, (Figure 1d) in the loading region indicate sudden
displacement of the indenter tip at constant load. Literature
2
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Figure 3. (a)-(f) Confocal optical microscope images showing step-wise mechanical bending of a nearly straight crystal of length L ≈ 75.6 m (a and g) into an arc-
shape (f and m). (h)-(r) FL images of straight and bent crystals of 1 transducing light when excited at different positions. The yellow dotted arrows indicate the
direction of light propagation. (s)-(t) FL spectra of straight and bent crystals collected at the end of the crystals excited at different positions. (u) Table of input
distance-dependent FL colours at outputs 1 and 2.
interaction energy of the corresponding pair. The calculations
suggest that the most dominant interactions in the structure are
from aromatic stacking (Etotal =~ 46 KJ/mol). The other edge-to-
face aromatic interactions, involving CH···N (d/ Å, /: 2.74 Å,
insignificant plastic deformation before fracture. The large
dispersive π-stacking and weak intermolecular interactions
among adjacent molecules in other directions have been found in
most elastically bendable organic crystals. [ Such interactions
are proposed to act as structural buffers,[1a] thus accommodating
elastic strain in flexible elastic crystals, as in our case.
As the macrocrystals are highly flexible, to demonstrate their
bendability in the microscale regime, we intended to mechanically
cut and alter the geometry of a straight microcrystal into an arc-
shape by applying force using AFM cantilever tip (Supporting
videos S3 and S4). Field emission electron microscope (FESEM)
images of a microcrystal of 1 obtained via slow evaporation
technique in acetonitrile/methanol exhibited its smooth surface in
straight and bent geometries (Figure 2f,g). A representative
nearly straight crystal of length L~ 75.6 m with a slight bent of
1]
137.92; 2.55 Å, 143.80), CH···π (2.84 Å) and S···S, in other
directions, are much weaker (significant Etotal values are ~-16, -17
KJ/mol and rest are below 10 KJ/mol; see supporting information,
Figure S2).
Recent reports by us and others suggest that the
exceptional elastic flexibility in organic crystals originates from
buffering regions in crystal structures, typically consisting of soft
interactions that accommodate elastic strain energy.[8] On the
other hand, crystals that with facile slip planes allow permanent
movement of molecules leading to plastic (permanent)
deformation. When a crystal is bent elastically, the outer arc is
under tensile stress while the inner arc is under compressive
stress. Recently, Worthy et al. examined the mechanism of an
elastically bendable crystal consisting herringbone structure
where molecules stack along the needle direction (via π-π, Cu-
π).[8d] Using an elaborate synchrotron diffraction study, they
established that the elastic bending occurs via rotation of
molecules with respect to each other. This mechanism could be
applied to our flexible crystal as it has similar herringbone
structure with dominant π-stacking interactions in the needle
direction. In a herringbone structure, the rotation of molecules
towards planarity facilitates compression at the inner arc while
rotation in the opposite direction facilitates elongation at the outer
arc of the crystal (Figure 2b). [8d] These changes store elastic
strain in the crystal. Upon release of external stress, the
molecules rotate in the reverse direction and regain original
structure (release the elastic strain).[8d] Our crystal does not have
any facile slip planes in the structure, which is consistent with its
5.5 m at the right terminal was selected (Figure 3a) and tip force
was applied to the right side of the crystal to displace the crystal
terminal to a distance of 8.6 m (Figure 3b). Similarly, the left side
of the crystal was displaced to about 12 m to reduce the gap
between the two crystal termini to 70.3 m (Figure 3c). Notably,
the crystal was not held by any external microholders during
mechanical operation with tip. Further, the crystal termini
remained displaced without regaining to its original straight-shape
even after retracting the cantilever from the crystal. The apparent
plasticity by the microcrystal suggests that crystal-substrate
adherence energy is more significant than shape-regaining
energy. By displacing the right and left side of the crystal further,
a nearly arc-shaped crystal geometry with the terminal-to-terminal
distance of 67.14 m was accomplished (Figure 3d-f). During the
micromechanical operation, the crystals achieved a strain of ~
2.8% (supporting information, Figure S3) without damage,
3
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demonstrating excellent flexibility. Crystal without external
damage at high strains is essential in optical waveguides, as the
presence of defects (tip-made or inherent) of sizes larger than the
half of the optical wavelength facilitates unwanted optical loss via
scattering.
with the bent crystal (Figure 3m-r). The FL images presented in
Figure 3t demonstrate the outcoupling of light of different colour
(as in a straight crystal) at both the termini of the crystal after
propagating in the bent direction. Remarkably, the FL signal
counts (15k) obtained at the output of flexible waveguide before
and after bending were nearly the same (Figures 3s and t). The
efficient transmission of optical signals of different colours by the
flexible crystal waveguide demonstrates its use in flexible WDM
technologies.
Earlier, the directional coupling behaviour was not observed
in two X-crossed straight organic waveguides;[
14]
however,
channel add filters have been made successfully using a
combination of curved and straight organic dye-based
nanofibres.[ Therefore, we envisioned to join two arc-shaped
optical waveguides by micromanipulation. It is expected that, in
the resultant organic directional coupler with four termini, the
curved junction will facilitate the transfer of light from one
waveguide to another, which is analogous to evanescent field
coupling [ and subsequent light transfer between two optical
fibres in the tapered region. We selected a representative crystal-
15]
16]
1
of L ≈ 206 m (Figure 4a). By pressing the crystal top and
bottom termini with AFM tip an arc-shaped crystal with a terminal-
to-terminal gap of C ≈ 156 m was produced (Figure 4b-d).
Similarly, a bent crystal-2 with a terminal-to-terminal distance of
(
Chord) C ≈ 173 m was created and positioned adjacent to arc-
shaped crystal-1 (Figure 4e) such that the mid-point of two arcs is
touching each other to facilitate the light transfer. Further, fine-
tuning of crystal-2 with the tip provided a terminal-to-terminal gap
of C ≈ 166 m (Figure 4f). The radius of curvature (R) of both
crystal-1 and -2 in the two-coupled waveguide geometry is R ≈
Figure 4. (a)-(d). Confocal optical microscope images showing the
micromanipulation of a crystal with AFM cantilever tip. A crystal of length L ≈
206 m (a) was first approached and pressed with a tip. (b) The tip was retracted,
leaving the crystal bent at the contact point. (c) The same procedure was
applied again to bend the crystal further. (d) Similarly, the other end of the
crystal also pressed, resulting in an arc-shaped crystal. (e) A second arc-shaped
crystal-2 was made and placed adjacent to the crystal-1 shown in (d). (f)
Retraction of the tip after pressing of crystal-2 with a shortest end-to-end
distance of 166 m. (g) Radius (R) of curvature of two-coupled crystal
waveguides (Directional coupler) showing optical path length between two
coupled waveguides. The yellow rectangle shows coupling junction. (h) FESEM
image of a directional coupler. The inset shows the widths of the waveguides at
the junction. (i) A close-up view FESEM image of the junction shown in (h).
172.5 and 202.5 m, respectively (Figure 4g). The FESEM image
of the directional coupler exhibited width (W) of ≈ 2.9 and 2.4 m
for crystal-1 and -2, respectively at the coupling junction without
any noticeable gap (Figure 4h and i). The geometry of this single-
crystalline 22 directional coupler (2-inputs and 2-outputs) is such
that coupling of light at any one of four termini (crystal-1 or -2) will
transduce FL light to two of its opposite termini of crystal-1 and -
2, one via direct out-coupling and the other via the junction
between both waveguides. The method presents one of the
efficient ways to split optical signals in different intensities.
For optical waveguiding studies, a confocal microscopy set-
up in a transmission mode geometry was used (see Supporting
information). We used the same single flexible microcrystal
To investigate the directional coupling property of coupled
crystalline waveguides, we excited one of the terminals (left) of
crystal-1 (Figure 5a, see input-1), which transduced the blue FL
signal towards coupling junction, wherein the FL signal (after
reabsorbance) was efficiently split into two, and out coupled as
green FL at termini-1a and -1b. Remarkably, no FL signal was
detected at terminal1c, which confirms the directional coupling in
coupled crystal waveguides. The corresponding FL spectra
recorded at input-1, primary output-1a and secondary output-1b
showed a variation of the signal strengths and weak optical
(
L~75.6 m) before (Fig. 3g-l) and after (Figure 3m-r) bending it.
Excitation (bottom objective 60) of the left terminal of the straight
crystal with a 405 nm laser exhibited a bright blue FL at the site
of excitation (Figure 3h). The corresponding FL spectrum
(
4
collected with a top 150 objective) showed spectral maxima at
96, 532 and a shoulder around 600 nm (Figure 3s). Unusually,
at the crystal termini (output-1), a green FL was out coupled with
the disappearance of 496 nm peak in the FL spectrum (Figure
3h,s). The outcoupled green light confirms that the crystal is
modes. Within
a waveguide, the direct light transmission
acting as an active optical waveguide. The green light originates
as a result of complete self-absorption of the light corresponding
to max ≈ 496 nm peak (from 460 to 506 nm) during transduction
of blue FL along the waveguide (Figure 3s and Figures S11 and
efficiency from input-1 to output-1a was about 11%. The splitting
ratio that is the distribution of FL signal intensity among primary
(
1a) and secondary (1b) outputs was about 90/10. Similar optical
S12).[2e] The crystal was excited at a distance of 58.7, 40.4, 19.6,
excitation at the right terminal of waveguiding crystal-1 produced
(Figure 5b, see input 2) two split FL signals which were out
coupled at termini 2a and 2b with a splitting ratio of 72 and 28,
respectively. At terminal 2c no signal was recorded, reaffirming
the efficient directional coupling of the fabricated photonic circuit.
The out coupled FL intensity at 2a was about 6%. The splitting
ratio at the secondary outputs, 1b and 2b is related to the optical
and 0 m from the right terminal (Figure 3i-l). The corresponding
spectra exhibited green, green, blue-green and blue FL at the
right terminal (output-1) and blue-green, green, green, and green
FL signals at the left terminal (output -2) (Figure 3u). The intensity
of the 496 nm FL peak also increased while moving the excitation
positions from 75.6 to 0 m due to decrease of the optical path
length (Figure 3s). We have repeated the waveguiding studies
4
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Figure 5. (a)-(d) From top: Cartoon representation of two coupled crystalline waveguides acting as a 22 directional coupler with two inputs and two outputs when
excited at any one of the four termini, confocal optical microscope and FL images and FL spectra. The red crosses represent no signal at the corresponding output.
The yellow colour arrows show the light transducing directions. The numbers in red colours show the optical path-dependent light splitting ratio of a directional
coupler.
path length (Lop) of propagating light (Figure 4g), which is 66 m
for the former and 90 m for the latter. An increase of optical path
length decreases the split ratio due to increased optical loss in
addition to coupling loss at the coupling junction. Excitation of
one of the termini of crystal-2 either at input-3 or -4 also produced
corresponding out coupled split signals at 3a/3b or 4a/4b, with
split ratios of 85/15 and 90/10, respectively (Figure 5c,d). As
observed earlier in Figure 5a and 5b, the variation of the
secondary output intensities indicates different optical path
lengths taken by the transmitted signals. Further, depending upon
the light input terminal, the same output exhibited entirely different
optical modes (though poorly resolved) in the FL spectra, which
further confirms the optical coupling between the two crystal
waveguides (Figure S13).
crystals for efficiently splitting the optical signals with different split
ratios. The presented proof-of-principle experiments which
combine state-of-the-art mechanical micromanipulation and
photonic studies of flexible microcrystals will hitch and expand the
research frontiers of two diverse disciplines, namely crystal
engineering[ and organic photonics
17]
[2-5]
.
Acknowledgements
RC thanks SERB-New Delhi (CRG-2018/001551) and UoH-IoE.
SSZ acknowledges SERB-New Delhi for financial support
(
(
CRG/2018/002784).
DST/SJF/CSA02/201415) for Swarnajayanti fellowship. ARA
CMR
thanks
DST,
India
In conclusion, the presented one-dimensional macrocrystal
thanks IISER Kolkata and SB thanks DST-INSPIRE for
fellowships.
(
1) with a high aspect ratio, formed by dithieno[3,2-a:2′,3′-c]
phenazine has demonstrated elasticity in a three-point bending
test. However, micro-crystals of 1 which are deposited at a glass
surface behaved like plastic crystals while applying AFM
cantilever tip pressure owing to strong adhesion force between
the surface and crystal which overpowered the crystal shape
regaining energy. The discriminatory self-absorbance of part of
the FL from 460 to 506 nm by the flexible crystal waveguides can
be used to switch output signal colours (from green to blue) for
WDM technologies by varying the optical path lengths of the
transducing light. The crystal waveguides deposited at the
surface can be mechanically bent with different chord lengths
using AFM cantilever tip for creating unique flexible WDMs.
Similarly, intricate photonic device structures such as 22
directional couplers can be produced by utilising two flexible
Keywords: Flexible Crystals • Mechano-photonics • Optical
waveguides • Directional Coupler • Organic Photonics
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Angewandte Chemie International Edition
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Flexible microcrystals of dithieno[3,2-a:2′,3′-c]phenazine exhibit excellent optical waveguiding and wavelength division multiplexing
properties in both straight and bent geometries during mechanical micromanipulation with AFM cantilever tip. Phenomenally, a futuristic
22 directional coupler made from two bent crystals via micromanipulation split the optical signal in two different split ratios depending
upon the optical propagation path length.
7
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