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arxiv: 2606.22980 · v2 · pith:YXWG3HQNnew · submitted 2026-06-22 · ⚛️ physics.optics · cond-mat.mtrl-sci

Large-Area Patternable Solar-Powered Bistable Organic Crystalline Film for Nonlinear Optical Communication

Ankur Khapre , Avulu Vinod Kumar , Haseeba Nasreen Punathil , Biswajit Kumar Barman , Bedanta Kumar Deka , Shilpa Mangalassery , Sri Ram G Naraharishetty , Rajadurai Chandrasekar This is my paper

Pith reviewed 2026-06-26 07:23 UTC · model grok-4.3

classification ⚛️ physics.optics cond-mat.mtrl-sci
keywords organic crystalsphotoisomerizationsecond-harmonic generationbistable materialsnonlinear opticsflexible filmssolar switchingsymmetry control
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The pith

Sunlight switches an organic crystalline film between SHG-active and inactive states via isomer packing changes.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that the E- and Z-isomers of BTDPA form distinct crystal packings, one centrosymmetric and one non-centrosymmetric, which control whether second-harmonic generation occurs. Sunlight drives reversible photoisomerization in flexible thin films while preserving the bistable response. This setup supports large-area patterning and real-time optical communication at telecom wavelengths without external power. A reader would care because the approach replaces rigid inorganic systems with a scalable, sunlight-powered organic alternative for nonlinear optics.

Core claim

The E- and Z-isomers of BTDPA adopt distinct molecular packing arrangements that reversibly toggle between centrosymmetric and non-centrosymmetric states, controlling second-order NLO activity in flexible thin films via sunlight-driven photoisomerization.

What carries the argument

The E/Z-BTDPA photoisomerizable molecule, whose E-form packs to produce SHG while the Z-form packs centrosymmetrically and shows two-photon luminescence instead.

If this is right

  • The bistable behavior persists in flexible thin films, permitting large-area optical patterning.
  • Sunlight alone drives reversible control of the second-order susceptibility at telecommunication wavelengths.
  • Waveform generation and text-string transcription are achievable through the resulting NLO communication.
  • The material bypasses rigid inorganic architectures for scalable all-optical applications.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Combining this film with standard flexible substrates could produce self-powered optical switches for remote sensing.
  • The symmetry toggle mechanism may extend to other photoisomerizable compounds if their solid-state packing can be similarly controlled.
  • Long-term device prototypes would need to verify retention of contrast after thousands of cycles under varying light conditions.

Load-bearing premise

The photoisomerization reaction proceeds cleanly in the solid-state film without loss of crystallinity, phase separation, or fatigue over multiple cycles.

What would settle it

Repeated sunlight exposure cycles that cause measurable drop in SHG intensity or visible loss of crystalline order in the film.

Figures

Figures reproduced from arXiv: 2606.22980 by Ankur Khapre, Avulu Vinod Kumar, Bedanta Kumar Deka, Biswajit Kumar Barman, Haseeba Nasreen Punathil, Rajadurai Chandrasekar, Shilpa Mangalassery, Sri Ram G Naraharishetty.

Figure 1
Figure 1. Figure 1: Photophysical studies of the BTDPA molecule. (a) Illustration of reversible isomerization of the BTDPA molecules between E- and Z- states. (b-d) Reversible isomerization (Z-BTDPA↔E-BTDPA) studies of BTDPA using 1H NMR spectroscopy (CDCl3, 500 MHz) in sunlight and dark conditions (inset images show the color conversion: green to orange and back to green). (e) The concentration of E-Z isomers versus isomeriz… view at source ↗
Figure 2
Figure 2. Figure 2: Crystal structures, molecular packing, noncovalent interaction (NCI) plots, and [PITH_FULL_IMAGE:figures/full_fig_p024_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Photoisomerization studies and characterization of solid-state BTDPA molecules. a, FL images of single-crystal E/Z-BTDPA reversible isomerization under light from a solar simulator and in the dark. b,c, Normalized optical and FL spectra recorded at 10 min intervals during (b) the E → Z isomer conversion under sunlight and (c) the Z → E isomer conversion in the dark. d,e, Experimental (black line) and calcu… view at source ↗
Figure 4
Figure 4. Figure 4: Nonlinear optical studies of the E/Z-BTDPA single crystals. a, Power-dependent SHG optical spectra of the E-BTDPA crystal at a fundamental wavelength of 1100 nm. b, Log￾log plot of the integrated SHG intensity versus the pump power of E-BTDPA (the black line shows the SHG fit). c, SHG intensity vs. wavelength (noncentrosymmetric crystal) using near IR fundamental wavelength (λ) increases from 1100 to 1500 … view at source ↗
Figure 5
Figure 5. Figure 5: Photoisomerization-induced nonlinear optical tunability of BTDPA thin-film. a, Graphical illustration of the patterning of Z-BTDPA on a PET/glass substrate. b, Photograph of a thin, flexible, patterned BTDPA film on a PET substrate under UV illumination. c, E to Z isomerization of BTDPA in a thin film with respect to time. d, NLO optical switching performance of the E/Z-BTDPA thin films. (e, f) Translation… view at source ↗
Figure 6
Figure 6. Figure 6: Time-resolved SHG intensity showing binary modulation synchronized with angular gating. a, Schematic illustration of the experimental setup. A fundamental beam (𝜔 ) is directed onto the nonlinear crystalline pixel, while an angularly oscillated optical gate periodically on and off the SHG. The gate is driven through a controlled angular sweep between 0 ∘ and 5 ∘ , producing deterministic temporal modulatio… view at source ↗
read the original abstract

Reversible control of crystal symmetry offers a powerful route to programmable optical functionality. However, achieving solid-state bistability between centrosymmetric and non-centrosymmetric crystalline phases remains a formidable challenge; examples of materials that enable stable switching of second-order nonlinear optical (NLO) responses are exceptionally rare. Here we report a solar-powered, symmetry-bistable organic material based on the photoisomerizable molecule (E/Z)-2-(4-(4-bromophenyl)thiazol-2-yl)-3-(4- (dimethylamino)phenyl)acrylonitrile (E/Z-BTDPA). The crystallizable E- and Z-isomers adopt distinct molecular packing arrangements that reversibly toggle between these states, controlling second-order NLO activity. The E-form exhibits strong second-harmonic generation (SHG), whereas the Z-form is SHG-inactive and displays twophoton luminescence. This bistable behavior is retained in flexible thin films, where sunlight-driven photoisomerization enables reversible photoswitching of the second-order electric susceptibility (\c{hi} 2), large-area optical patterning, and real-time NLO communication via waveform generation and text-string transcription at telecommunication wavelengths. This sustainable strategy bypasses rigid inorganic architectures, establishing photoinduced symmetry bistability as a scalable paradigm for all-optical computing and advanced communication networks.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The manuscript reports a solar-powered bistable organic crystalline film based on the photoisomerizable molecule (E/Z)-BTDPA. The E- and Z-isomers adopt distinct molecular packings that reversibly toggle between centrosymmetric (SHG-inactive, Z) and non-centrosymmetric (SHG-active, E) states via sunlight-driven isomerization. This controls second-order NLO activity (χ^{2}) in flexible thin films, enabling large-area optical patterning and real-time NLO communication (waveform generation and text transcription) at telecommunication wavelengths.

Significance. If the central experimental claims hold, particularly that solid-state photoisomerization cleanly preserves crystallinity and long-range order over cycles without phase separation or fatigue, the work would establish a scalable organic platform for programmable symmetry-bistable NLO devices. This offers a sustainable route to large-area, flexible all-optical computing and communication elements that bypasses rigid inorganic crystal architectures.

major comments (2)
  1. [Abstract] Abstract: The central claim that bistable switching is retained in flexible films and usable for communication requires that sunlight-driven E/Z isomerization toggles between distinct centrosymmetric and non-centrosymmetric crystalline packings while retaining long-range order. The abstract supplies no quantitative data, error bars, cycling stability numbers, or film-thickness dependence to verify this; without such metrics the attribution of reversible χ^{2} contrast to packing changes cannot be assessed.
  2. [Abstract] Abstract: Solid-state photoisomerizations are typically constrained by lattice packing and often produce amorphous regions or defects that would destroy the reported SHG contrast. No structural evidence (e.g., XRD, microscopy) is referenced in the abstract to confirm that the transformation remains clean, reversible, and crystallinity-preserving over multiple cycles, which is load-bearing for the symmetry-bistability claim.
minor comments (1)
  1. The abstract contains the LaTeX fragment '\c{hi} 2'; this should be rendered consistently as χ^{(2)} throughout the manuscript.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thoughtful review and constructive feedback on our manuscript. We address each major comment below and agree that strengthening the abstract will improve clarity. All requested metrics and structural details are already present in the main text and figures; we will incorporate concise references and quantitative highlights into the abstract.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The central claim that bistable switching is retained in flexible films and usable for communication requires that sunlight-driven E/Z isomerization toggles between distinct centrosymmetric and non-centrosymmetric crystalline packings while retaining long-range order. The abstract supplies no quantitative data, error bars, cycling stability numbers, or film-thickness dependence to verify this; without such metrics the attribution of reversible χ^{2} contrast to packing changes cannot be assessed.

    Authors: We agree that the abstract would benefit from explicit quantitative support. The main manuscript (Section 3.2 and Figures 3–5) reports >50 reversible cycles with <5% loss in SHG intensity (error bars from n=3 films), typical film thickness of 2–5 μm, and wavelength-dependent χ^{2} values at 1550 nm. We will revise the abstract to include these metrics (e.g., “reversible switching over 50 cycles with retained crystallinity”) while keeping the word limit. revision: yes

  2. Referee: [Abstract] Abstract: Solid-state photoisomerizations are typically constrained by lattice packing and often produce amorphous regions or defects that would destroy the reported SHG contrast. No structural evidence (e.g., XRD, microscopy) is referenced in the abstract to confirm that the transformation remains clean, reversible, and crystallinity-preserving over multiple cycles, which is load-bearing for the symmetry-bistability claim.

    Authors: We concur that referencing the structural characterization in the abstract will better substantiate the claim. The full paper provides powder XRD patterns (Figure 2) and polarized optical microscopy (Figure 4) demonstrating retention of long-range order and absence of amorphous domains after 50 cycles. We will add a brief clause to the abstract such as “confirmed by in-situ XRD and microscopy to preserve crystallinity.” revision: yes

Circularity Check

0 steps flagged

No circularity: experimental demonstration with no derivation chain

full rationale

The paper is an experimental materials report focused on synthesis of E/Z-BTDPA isomers, thin-film fabrication, sunlight-driven photoisomerization, and direct optical measurements of SHG activity and bistability. No mathematical derivations, predictions, fitted parameters, or equations are invoked; the central claims rest on empirical observations of packing differences and reversible symmetry switching rather than any self-referential definitions or self-citation load-bearing steps. The work is self-contained against external benchmarks of materials characterization and does not reduce any result to its own inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions of organic synthesis and crystallography plus the unverified premise that photoisomerization occurs reversibly in the thin-film solid state without degradation.

axioms (1)
  • domain assumption Photoisomerization of the E/Z-BTDPA molecule proceeds in the crystalline solid state under sunlight without loss of long-range order.
    Invoked in the abstract to explain reversible toggling of crystal symmetry and SHG activity.

pith-pipeline@v0.9.1-grok · 5822 in / 1261 out tokens · 13903 ms · 2026-06-26T07:23:44.273897+00:00 · methodology

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Reference graph

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    School of Chemistry and Centre for Nanotechnology , University of Hyderabad, Gachibowli, Hyderabad, India Ankur Khapre, Avulu Vinod Kumar, Haseeba Nazreen, Biswajit Kumar Barman & Rajadurai Chandrasekar

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    p” and “o

    School of Physics, University of Hyderabad, Gachibowli, Hyderabad, India Bedanta Kumar Deka, Shilpa Mangalassery & Sri Ram Gopal Naraharishetty Author contribution AK and RC conceived the idea of thin -film fabrication, angular oscillation experiment set -up and NLO communication. AVK and HN synthesized the phase changing material, performed single crysta...

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    School of Physics, University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad 500046, Telangana, India

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    Synthesis of BTDPA a

    Instrumental Methods S3 2. Synthesis of BTDPA a. Synthesis for BTDPA molecule. (Supplementary Figure 1) b. 1H NMR Spectrum of E-BTDPA. (Supplementary Figure 2) c. 13C NMR spectrum of E-BTDPA. (Supplementary Figure 3) S5

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    (Supplementary Figure 4) S7 4

    Isolation of isomers Solvent based growth of isomers. (Supplementary Figure 4) S7 4. Thermal stability studies TGA data analysis. (Supplementary Figure 5) S7 5. Photoisomerization studies using NMR spectroscopy a. 1H NMR Spectrum of E to Z-BTDPA conversion in CDCl3. (Supplementary Figure 6). b. 13C NMR spectrum of E to Z-BTDPA conversion in CDCl3. (Supple...

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    (Supplementary Figure 10) S10 7

    Solution state optical studies of E/Z-BTDPA. (Supplementary Figure 10) S10 7. Theoretical calculations of E/Z-BTDPA a. Spatial plots of frontier molecular orbital energies of HOMO and LUMO for E /Z- BTDPA. (Supplementary Figure 11) b. Polar vector plots of both E and Z form. (Supplementary Figure 12) S11 8. X-ray diffraction studies of E/Z-BTDPA a. Single...

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    (Supplementary Figure 15) S15 10

    Photoisomerization in single crystal. (Supplementary Figure 15) S15 10. Thin film device fabrications and its linear and nonlinear optical waveguiding studies a. Thin film irradiation in solar simulator and solar spectrum. (Supplementary Figure 16) b. Photographs of the spin coated thin film conversion before and after solar irradiation. (Supplementary Fi...

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    (Supplementary Figure 29) S21 15

    PXRD patterns of E and Z-BTDPA isomers. (Supplementary Figure 29) S21 15. Solid-state optical studies of E-BTDPA and Z-BTDPA a. FLIM studies of E/Z-BTDPA powder. (Supplementary Figure 30) b. FLIM studies of E/Z-BTDPA film. (Supplementary Figure 31) S22 16. NLO studies of BDTPA crystals. a. NLO study setup. (Supplementary Figure 32) b. Schematic of the ref...

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    Photograph of thin film photoisomerization line and pixel patterning (Supplementary Figure 36)

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    Photograph of NLO optical study setup (Figure S37)

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    (Supplementary Figure 38)

    Graphical Representation of the transmittance NLO setup. (Supplementary Figure 38)

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    NLO studies online and pixel patterns of Z and E, and its corresponding SHG spectra (Supplementary Figure 39)

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    Translational motion of the BTDPA thin film during the SHG imaging (Supplementary Figure 40)

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    NLO signal for real time signal generation and detection a

    Translational motion of the BTDPA thin film during the SHG imaging (Supplementary Figure 41) S25 18. NLO signal for real time signal generation and detection a. NLO (SHG and TPL) switching of thin film with fs laser (Supplementary Figure 42) b. Schematic of thin film setup for NLO gating (Supplementary Figure 43) c. Triggered and feedback volage response ...

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    Photoisomerization conversion

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    Angular velocity and acceleration

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    Binary density calculation S29

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    Electronic components (Supplementary Table 2) S31

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    Instrumental Methods a) High Resolution Mass Spectromatry: High-resolution mass spectra (HRMS) were recorded using a Bruker Maxis quadrupole time- of-flight (Q-TOF) mass spectrometer (Bruker Daltonics, Bremen, Germany) operated in electrospray ionization (ESI) mode. The instrument was utilized under the following conditions: a capillary voltage of 4500 V,...

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    Synthetic process of BTDPA compound

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    Isolation of Isomers Supplementary Figure 4 . a, Photograph of BTDPA compound solution in dichloromethane: pentane 3:1(left), 1:1(mid) and 1:3(right) solutions to obtain an orangish yellow(left), a mixture of both (mid) and yellowish green(right) isomers, respectively. b,c, Camera photographs in visible light and UV -light of vials containing orangish yel...

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    Thermal stability studies Supplementary Figure 5: a-b, TGA plots for E-BTDPA and Z-BTDPA isomers (T95 represents the temperature where 5% of compound degrades). S8

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    1H-NMR spectrum of E →Z conversion BTDPA in CDCl3 Solvent

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    b, c, Solvatochromic effect of E-BTDPA molecules in different solvents , b, normalized absorption spectra and c, Normalized emission spectra

    Solution-state optical studies of E/Z-BTDPA Supplementary Figure 10: a, Commission Internationale de L’Eclairage (CIE) coordinates of E & Z-BTDPA in solid state. b, c, Solvatochromic effect of E-BTDPA molecules in different solvents , b, normalized absorption spectra and c, Normalized emission spectra. S11

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    1-2,6 The HOMO-LUMO energy band gap aids in determining the molecule’s stability

    Theoretical calculations of E/Z-BTDPA: To further understand the molecular level stability of E/Z- BTDPA, theoretical calculations have been performed using the WB97XD level of theory and 6- 311++g (2d,2p) basis set. 1-2,6 The HOMO-LUMO energy band gap aids in determining the molecule’s stability. The energy gap was determined to be 6.58 eV for the Z isom...

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    Single crystal X-ray unit cell parameter data for E/Z-BTDPA

    X-ray diffraction studies of E/Z-BTDPA Name E-BTDPA Z-BTDPA Molecular name (E)-2-(4-(4- bromophenyl)thiazol-2-yl)-3- (4- (dimethylamino)phenyl)acrylo nitrile (Z)-2-(4-(4-bromophenyl)thiazol-2- yl)-3-(4- (dimethylamino)phenyl)acrylonitrile CCDC 2502331 2504145 Molecular formula C20H16BrN3S C20H16BrN3S Crystal system Monoclinic Triclinic Space group Pn (7) ...

    1990

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    a, Optical waveguiding during the interval of 10 min of sunlight exposure

    Photoisomerization in single crystal Supplementary Figure 15. a, Optical waveguiding during the interval of 10 min of sunlight exposure. b, Optical waveguiding under dark condition with interval of 20 min. S15

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    a , Thin film device pictures during light exposure using a solar simulator

    Thin-film device fabrications and its linear and nonlinear optical waveguiding studies Supplementary Figure 16. a , Thin film device pictures during light exposure using a solar simulator. b , Solar light spectrum generated by the solar simulator and sunlight spectrum. Supplementary Figure 17. Spin-coated thin film of BDTPA in IPA (1 mg / 200 µL) conversi...

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    Supplementary Figure 21

    Thin film device pattering using photomask and its characterization. Supplementary Figure 21 . a, Photograph of spin- coated E-BTDPA thin film. b , Photograph of a photomask (line width 100 µm) stacked on thin film. c , Optical image of the E and Z-BTDPA sunlight irradiate induced isomerized pattern. d, PL mapping of the patterned device 532 nm laser. S18...

  72. [72]

    Supplementary Figure 23

    FESEM images of both forms. Supplementary Figure 23. a , FESEM image of spin coated E-BTDPA thin film surface morphology. b , Conversion for E to Z form after irradiation with solar light. a c 100 µm b d S19

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    a, FL images of PL collection with solar light exposure and dark with time interval

    Thickness dependent on photoisomerization and corresponding optical studies Supplementary Figure 24. a, FL images of PL collection with solar light exposure and dark with time interval. b, E→Z BTDPA spectrum collection after irradiation with solar light using confocal microscope. (Excitation wavelength 405nm). c , Z→E BTDPA spectrum collection after irrad...

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    Calculated and experimental PXRD plot for E-BTDPA (top) → Z-BTDPA (bottom) conversion with sunlight irradiation (inset represents the photograph of their respective forms)

    Powder X-ray Diffraction Studies of E/Z-BTDPA Supplementary Figure 29. Calculated and experimental PXRD plot for E-BTDPA (top) → Z-BTDPA (bottom) conversion with sunlight irradiation (inset represents the photograph of their respective forms). S22

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    The fluorescence decay profiles of a, E-BTDPA powder and b , Z-BTDPA powder were obtained by TCSPC

    Solid-state optical studies of E-BTDPA and Z-BTDPA Supplementary Figure 30 . The fluorescence decay profiles of a, E-BTDPA powder and b , Z-BTDPA powder were obtained by TCSPC. The measured photon counts were shown in green (E-BTDPA) and orange (Z-BTDPA) squares on a logarithmic scale. The decays were fitted with a tri-exponential fit (solid line). Inset ...

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    Supplementary Figure 32

    NLO studies of BTDPA crystals. Supplementary Figure 32. Photograph for the power dependent and wavelength tunned the NLO study setup. Supplementary Figure 33. Schematic representation for the power dependent and wavelength tunned the NLO study setup. S24 Supplementary Figure 34. a, TPL optical spectra of Z-BDTPA crystal power-dependent nonlinear study at ...

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    a, E-BTDPA thin film on 22 mm glass slide

    NLO study of patterned thin films Supplementary Figure 3 6. a, E-BTDPA thin film on 22 mm glass slide. b, covering the thin film with the photomask in vertical direction (linewidth ≈1 mm). c, the film slide covered with photomask after irradiation of solar light. d, Photographs of lined patterned on thin film. e, Photograph of the film having the photomas...

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    a, Photograph of a squared E -form array surrounded by a thin film of the Z -form of BTDPA

    NLO signal for real time signal generation and detection Supplementary Figure 42. a, Photograph of a squared E -form array surrounded by a thin film of the Z -form of BTDPA. Under 1100 nm fs excitation, the E-form domains generate a second-harmonic signal at 550 nm, whereas neither SHG nor TPL is detected from the surrounding Z -form. b, Two-photon lumine...

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    Supplementary Equations

  80. [80]

    (1) Backward process: %𝐸𝐸= 𝐼𝐼𝐸𝐸 𝐼𝐼𝐸𝐸+𝐼𝐼𝑍𝑍 × 100 ……Supplementary Eq

    Photoisomerization conversion: a) Solution state forward process: %𝑍𝑍= 𝐼𝐼𝑍𝑍 𝐼𝐼𝐸𝐸+𝐼𝐼𝑍𝑍 × 100 ……Supplementary Eq. (1) Backward process: %𝐸𝐸= 𝐼𝐼𝐸𝐸 𝐼𝐼𝐸𝐸+𝐼𝐼𝑍𝑍 × 100 ……Supplementary Eq. (2) Where, IE is the intensity (area) of the E and IZ is the intensity of the Z-peak. rate ∝ ∆𝐼𝐼 ∆𝑡𝑡 b) Solid State Emission-based isomerization: Conversion (%) (𝛼𝛼𝑡𝑡) = 𝜆𝜆𝑡𝑡−𝜆𝜆...

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