Method Article

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

DOI:

10.3791/57612

⸱

May 29th, 2018

 ,  ,  ,  ,  ,  , 
1Graduate School of Natural Science and Technology, Okayama University, 2Graduate School of Science, Kyoto University, 3Center for Computational Sciences, University of Tsukuba, 4Graduate School of Science, Kyushu University

In This Article

Summary

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Here, we present the protocols of differential-detection analyses of time-resolved infrared vibrational spectroscopy and electron diffraction which enable observations of the deformations of local structures around photoexcited molecules in a columnar liquid crystal, giving an atomic perspective on the relationship between the structure and the dynamics of this photoactive material.

Abstract

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We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational spectroscopy and time-resolved electron diffraction. Liquid crystal phase is an important state of matter that exists between the solid and liquid phases and it is common in natural systems as well as in organic electronics. Liquid crystals are orientationally ordered but loosely packed, and therefore, the internal conformations and alignments of the molecular components of LCs can be modified by external stimuli. Although advanced time-resolved diffraction techniques have revealed picosecond-scale molecular dynamics of single crystals and polycrystals, direct observations of packing structures and ultrafast dynamics of soft materials have been hampered by blurry diffraction patterns. Here, we report time-resolved IR vibrational spectroscopy and electron diffractometry to acquire ultrafast snapshots of a columnar LC material bearing a photoactive core moiety. Differential-detection analyses of the combination of time-resolved IR vibrational spectroscopy and electron diffraction are powerful tools for characterizing structures and photoinduced dynamics of soft materials.

Introduction

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Liquid crystals (LCs) have a variety of functions and are widely used in scientific and technological applications1,2,3,4,5,6. The behavior of LCs can be attributed to their orientational ordering as well as to the high mobility of their molecules. A molecular structure of LC materials is typically characterized by a mesogen core and long flexible carbon chains that ensure high mobility of the LC molecules. Under external stimuli7,8,9,10,11,12,13,14,15, such as light, electric fields, temperature changes, or mechanical pressure, small intra- and intermolecular motions of the LC molecules cause drastic structural reordering in the system, leading to its functional behavior. To understand the functions of LC materials, it is important to determine the molecular scale structure in the LC phase and identify the key motions of the molecular conformations and packing deformations.

X-ray diffraction (XRD) is commonly employed as a powerful tool for determining structures of LC materials16,17,18. However, the diffraction pattern originating from a functional stimuli-responsive core is often concealed by a broad halo pattern from the long carbon chains. An effective solution to this problem is provided by time-resolved diffraction analysis, which enables direct observations of molecular dynamics using photoexcitation. This technique extracts structural information about the photoresponsive aromatic moiety using the differences between the diffraction patterns obtained before and after photoexcitation. These differences provide the means both to remove the background noise and to directly observe the structural changes of interest. Analyses of the differential diffraction patterns reveal the modulated signals from the photoactive moiety alone, thereby excluding the deleterious diffraction from the non-photoresponsive carbon chains. A description of this method of differential diffraction analysis is provided in Hada, M. et al19.

Time-resolved diffraction measurements can provide structural information about the atomic rearrangements that occur during the phase transition in materials20,21,22,23,24,25,26,27,28,29 and chemical reactions among molecules30,31,32,33,34. With these applications in mind, remarkable progress has been made in the development of ultrabright and ultrashort-pulsed X-ray35,36 and electron37,38,39,40 sources. However, time-resolved diffraction has only been applied to simple, isolated molecules or to single- or poly-crystals, in which highly ordered inorganic lattice or organic molecules produce well-resolved diffraction patterns providing structural information. In contrast, ultrafast structural analyses of more complex soft materials have been hampered because of their less ordered phases. In this study, we demonstrate the use of time-resolved electron diffraction as well as transient absorption spectroscopy and time-resolved infrared (IR) vibrational spectroscopy to characterize the structural dynamics of photoactive LC materials using this diffraction-extracted methodology19.

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Protocol

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1.Time-Resolved Infrared Vibrational Spectroscopy

  1. Sample preparation
    1. Solution: Dissolve the π-extended cyclooctatetraene (π-COT) molecules into dichloromethane with proper concentration (1 mmol/L).
    2. LC phase: Melt the π-COT powder on a calcium fluoride (CaF2) substrate using hot plate at the temperature of 100 °C. Cool the sample at a room temperature.
      Note: We need to choose a material (CaF2 or barium fluoride (BaF2)) that is transparent in mid-IR range.
  2. Apparatus setting-up
    1. Switch on the titanium sapphire (Ti:sapphire) laser and the chirped pulse amplifier. Thermally stabilize them for several hours.
    2. Make sure the alignments are correct. Check the power and stability of the ultraviolet (UV) pump and the mid-IR probe and re-align the optical path if necessary. The optical setup of the time-resolved infrared spectroscopy is provided in Figure 5.
    3. Cool the HgCdTe IR detector array using liquid nitrogen. Make sure that the spectrometer is properly located so that reasonable amount of light is detected in the range of interest. Calibrate the spectrometer using absorption spectra of well-known materials such as polystyrene or polyethylene terephthalate.
    4. Mount a sample which shows large photo-induced transient response (Si wafer (1 mm) or Re(bpy)(CO)3Cl/CH3CN solution) on the sample holder. Locate the pump-probe delay to a positive value and optimize the amount of the transient signal by stirring the pump beam to ensure the pump-probe overlap.
    5. Find the time origin setting by taking long-range scan on the pump-probe delay using the home-built program (Figure 6). Check the position where the transient signal start to emerge.
    6. Check the dynamics of symmetric and anti-symmetric vibration of CO stretching in Re(bpy)(CO)3Cl whose dipole moments are orthogonal. Note that both should show exactly same dynamics when the magic angle condition is properly met.
  3. Measurement and data acquisition
    1. Solution: Mount the home-built flow cell. Setup the bubbling device with inert gas (nitrogen (N2) or Argon (Ar)) if necessary. LC phase: Mount the spin-coated π-COT sample with the substrate on the motorized stage to continuously move the laser spots on the sample to minimize the laser-induced damage.
    2. Doublecheck the time zero position with the sample.
    3. Set the scan range of the pump-probe delay properly (start, end, and step).
    4. Choose a directory to save data.
    5. Start the data collection with the home-built program.
      NOTE: The data are recorded automatically in the directory.

2. Time-Resolved Electron Diffraction

  1. Fabrication of sample substrate
    1. Purchase a silicon (001) wafer (200 µm thick), both sides of which are pre-covered with 30-nm-thick silicon-rich silicon nitride (Si3N4, or simply SiN) film (Figure 11A). Cut the SiN/Si/SiN wafer in square (15 × 15 mm2).
    2. Irradiate with Ar cluster ion beams41 at the fluence of 2.5 × 1016 ions/cm2 onto one of the sides of the SiN/Si/SiN wafer though a metal mask (Figure 12), which is sufficient to remove the 30-nm-thick SiN film (Figure 11B,C).
      NOTE: An alternative method to remove SiN film is plasma etching or ion beams etching.
    3. Prepare potassium hydroxide (KOH)aqueous solution at a concentration of 28%.
    4. Put the wafer into KOH solution at a temperature of 60-70 °C for 1-2 days (Figure 11D), which perform further etching of the Si wafer via isotropic chemical etching42.
      NOTE: The etching rate for Si by KOH solution is much faster than that for SiN, so the SiN thin film remains as self-supporting membranes (Figure 11E).
    5. Clean the wafer with SiN membranes in deionized water and dry it with nitrogen gas.
  2. Sample preparation
    1. Dissolve the π-COT molecules in chloroform at a concentration of 10 mg/mL.
    2. Program the spin-coater: accelerate to 2000 rpm in 5 s, keep the rotation for 30 s, and stop the rotation. Spin-coat the π-COT solution onto the SiN membrane substrate as shown in Figure 11F.
      NOTE: A proper wafer size for spin-coating must be more than 10 × 10 mm2, since the surface tension sometimes interferes with spin-coating of materials on smaller wafers, for example, a SiN membrane grid for transmission electron microscopy.
    3. Put the sample coated on the SiN membrane substrate on a hotplate at a temperature of 100 °C, melt it, and cool it gradually to room temperature (Figure 11G).
  3. Measurements
    1. Mount the sample on the sample holder with a screw and put the sample holder in the vacuum chamber (sample chamber).
    2. Seal the vacuum chamber with a lid and switch on a rotary pump to evacuate the chamber until a vacuum level of less than 1000 Pa. Then, switch on the molecular turbo pumps until the electron-gun chamber is at the vacuum level of ~10-6 Pa (typically for more than 12 h).
    3. Switch on the Ti:sapphire laser and the chirp pulse amplifier, and thermally stabilize them for more than 1 h. The experimental setup of the time-resolved electron diffraction is provided in Figure 9. Set the repetition rate to 500 Hz.
    4. Switch on the thriller of the charge-coupled device (CCD) camera and cool it to 10 °C.
    5. Switch on the electrical power supply and adjust the voltage to 75 kV.
      NOTE: The leak current of the power supply should not fluctuate out of the 0.1 µA range.
    6. Special overlap. Open the laboratory-coded automatic program (Figure 10A) and set the exposure time (50 ms). Find the electron beam position with a pinhole equipped in the sample holder using the program by setting Start type to Z_overlap for the overlap of Z-axis and Y_overlap and pressing Start button.
    7. Set the electron beam at the pinhole position and align the pump laser with the reflected pump light by the pinhole.
    8. Measure the time-zero position with an inorganic material (Bi2Te3) on the sample holder using a laboratory-coded automatic program (Figure 10B) by setting Start type to Time-resolved and pressing Start button. For this process, adjust the pump fluence to 2 mJ/cm2.
    9. Insert the Faraday cup to the pass of the electron beam and measure the fluence of the electron beam with a laboratory-built picoammeter and adjust it by rotating the adjustable ND filter on the probe line. Adjust thefluence of the pump pulse by rotating the waveplate on the pump line.
    10. Move to the sample position and set the exposure time of the CCD camera. Obtain the electron diffraction image using the laboratory-coded automatic program (Figure 10B) by setting Start type to Single and pressing Start button.
    11. Switch on the Peltier element of the CCD camera and cool it down to the temperature of -20 °C.
    12. Set the time-step and number of steps for the time-resolved measurements. Obtain the time-resolved electron diffraction images using the laboratory-coded automatic program (Figure 10B) by setting Start type to Time-resolved and pressing Start button.
    13. Obtain the time-resolved background image with the electron acceleration power supply switching off using the laboratory-coded automatic program (Figure 10B) by setting Start type to Time-resolved and pressing Start button.

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Results

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We chose a saddle-shaped π-COT skeleton43,44 as a photoactive core unit of the LC molecule, because it forms a well-defined columnar stacking structure and because the central eight-membered COT ring is expected to show a photoinduced conformational change into a flat form owing to the excited-state aromaticity19,45. Synthetic process of this material is provided in previo...

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Discussion

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The crucial step of the process during the time-resolved electron diffraction measurements is maintaining the high voltage (75 keV) without current fluctuation since the distance between the photocathode and anode plate is only ~10 mm. If the current fluctuates above the range of 0.1 µA before or during the experiments, increase the acceleration voltage up to 90 keV to discharge and set it again to 75 keV. This conditioning process has to be done until the current fluctuates in the range of 0.1 µA. The proper d...

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Disclosures

$$\rightleftharpoonup{xx}$$ $$\longleftharp{xx}$$, $$\longrightharp{xx}$$,

The authors have nothing to disclose.

Acknowledgements

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We thank Dr. S. Tanaka at Tokyo Institute of Technology for time-resolved IR vibrational spectroscopy measurements and Prof. M. Hara and Dr. K. Matsuo at Nagoya University for XRD measurements. We also thanks Prof. S. Yamaguchi at Nagoya University, Prof. R. Herges at Kiel University and Prof. R. J. D. Miller at the Max Planck Institute for the Structure and Dynamics of Matter for valuable discussion.

This work is supported by the Japanese Science Technology (JST), PRESTO, for funding the projects "Molecular technology and creation of new functions" (Grant Number of JPMJPR13KD, JPMJPR12K5, and JPMJPR16P6) and "Chemical conversion of light energy". This work is also partially supported by JSPS Grant Numbers JP15H02103, JP17K17893, JP15H05482, JP17H05258, JP26107004, and JP17H06375.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Chirped pulse amplifierSpectra Physics Inc.Spitfire ACEFor time-resolved IR vibration spectroscopy
Chirped pulse amplifier Spectra Physics Inc.Spitfire XPFor time-resolved electron diffractometry
Femtosecond laserSpectra Physics Inc.TsunamiFor time-resolved IR vibration spectroscopy
Femtosecond laserSpectra Physics Inc.TsunamiFor time-resolved electron diffractometry
Optical parametric amplifierLight Conversion Ltd.TOPAS prime
64-channel mercury cadmium tellurium IR detector arrayInfrared Systems Development CorporationFPAS-6416-D
FT-IR spectrometerShimadzu CorporationIR Prestige-21
High voltage supplyMatsusada precisionHER-100N0.1
Rotary pumpEdwardsRV12
Molecular turbo pumpsAgilent Technologies Japan, Ltd.Twis Torr 304FS
Vacuum gaugesPfeiffer vacuum systems gmbhPKR251For ICF70 flange
Vacuum monitorsPfeiffer vacuum systems gmbhTPG261
Fiber coupled CCD cameraAndor Technology Ltd.iKon-L HF
BaF2 and CaF2 substratesPier opticsThickness 3 mm
AgGaS2 crystalPhototechnica CorporationCustom-order
BBO crystalsTokyo Instruments, Inc.SHG θ=29.2 deg
THG θ=44.3 deg
calcite crystalsTokyo Instruments, Inc.Thickness 1mm
Optical mirrorsThorlabsPF10-03-F01
PF10-03-M01
UM10-45A		Al coat mirrors
Au coat mirrors
Ultrafast mirrors
Optical mirrorsHIKARI,Inc.Broadband mirrors
Dichroic mirrorsHIKARI,Inc.Custom-order
Reflection: 266 nm
Transmission: 400, 800 nm
Optical chopperNewport Corporation3501 optical chopper
Optical shuttersThorlabs Inc.SH05/M
SC10
Optical shuttersSURUGA SEIKI CO.,LTD.F116-1
Beam splittersThorlabs Inc.BSS11R
Fused-silica lensesThorlabs Inc.LA4663
LA4184
BaF2 lensThorlabs Inc.LA0606-E
Polarized mirrorsSigmakoki Co.,LtdCustom-order
Designed for 800 nm
Reflection: s-polarized light
Transmission : p-polarized light
Half waveplateThorlabs Inc.WPH05M-808
Mirror mountsThorlabs Inc.POLARIS-K1
KM100		Kinematic mirror mounts
Mirror mountsSigmakoki Co.,LtdMHAN-30M
MHAN-30S		Gimbal mirror mounts
Mirror mountsNewport CorporationACG-3K-NLGimbal mirror mounts
Variable ND filtersThorlabs Inc.NDC-25C-2M
Beam splitter mountsThorlabs Inc.KM100S
Lens mountsThorlabs Inc.LMR1/M
Rotational mountsThorlabs Inc.RSP1/M
RetroreflectorEdmund Optics63.5MM X 30" EN-AL 
spectrometersocean photonicsUSB-4000
Power meterOphir30A-SHUsed for intensity monitor of CPA
Power meterThorlabs Inc.S120VC
PM100USB		Used for intensity measurements of pump pulse
PhotodiodesThorlabs Inc.DET36A/M
DET25K/M
DC power supplyTEXIOPW18-1.8AQUsed for magnetic lens
Magnetic lensNissei ETC Co.,LtdCustom-order
StagesNewport CorporationM-MVN80V6
LTAHLPPV6		Used for magnetic lens
Stage controllerNewport CorporationSMC100
Stages Sigmakoki Co.,LtdSGSP20-35(X)
SGSP20-85(X)		Used for sample position
Stages Sigmakoki Co.,LtdSGSP26-200(X)
OSMS26-300(X)		Used for delay time generator
Stage controllerSigmakoki Co.,LtdSHOT-304GS
PicoammeterLaboratory built
spin coaterMIKASA Co.,Ltd1H-D7
hot plateIKA® C-MAG HP7
SiN waferSilson LtdCustom-order
KOH aqueous solution (50%)Hiroshima Wako Co.,Ltd.168-20455
ChloroformHiroshima Wako Co.,Ltd.038-18495
DichloromethaneHiroshima Wako Co.,Ltd.132-02456
Personal computers for the controlling programsEpson CorporateEndeavor MR7300E-L32-bit operation system
Program for the control the equipmentNational Instruments CorporationLabview2016
Program for the data analysisThe MathWorks, Inc.Matlab2015b

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Tags

Time Resolved IR SpectroscopyTime Resolved Electron DiffractionDifferential Detection AnalysisUltrafast Molecular DynamicsLiquid Crystal PhasePhotoactive Core MoietyStructural Dynamics ObservationElectron Diffraction PatternsVibrational Spectroscopy

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