Barbara Cunha De Miranda

Gustavo A Garcia

François Gaie-Levrel

Ahmed Mahjoub

Thomas Gautier

Benjamin Fleury

Laurent Nahon

Pascal Pernot

Nathalie Carrasco

Molecular Isomer Identification of Titan's Tholins Organic Aerosols by Photoelectron/Photoion Coincidence Spectroscopy Coupled to VUV Synchrotron Radiation

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Titan, the largest moon of Saturn, has a dense atmosphere mainly composed of nitrogen (98%), methane, molecular hydrogen, traces of hydrocarbons and nitrogenous compounds. Atmospheric photochemistry leads to the production of microscopic particles forming the brownish haze that permanently surrounds Titan [START_REF] Waite | The Process of Tholin Formation in Titan's Upper Atmosphere[END_REF] . The ongoing Cassini-Huygens space mission provided first insights on the chemical composition of these atmospheric organic aerosols. Mid-and Far-IR spectroscopy confirmed some aliphatic signatures [START_REF] Anderson | Titan's Aerosol and Stratospheric Ice Opacities between 18 and 500 µm: Vertical and Spectral Characteristics from Cassini CIRS[END_REF][START_REF] Gautier | Mid-and Far-Infrared Absorption Spectroscopy of Titan's Aerosols Analogues[END_REF][START_REF] Rannou | Titan Haze Distribution and Optical Properties Retrieved from Recent Observations[END_REF][START_REF] Vinatier | Optical Constants of Titan's Stratospheric Aerosols in the 70-1500 cm-1 Spectral Range Constrained by Cassini/CIRS Observations[END_REF] , while the Aerosol Collector Pyrolyser experiment highlighted the presence of nitrogen in the refractory nucleus [START_REF] Israel | Complex Organic Matter in Titan's Atmospheric Aerosols from in Situ Pyrolysis and Analysis[END_REF] .

To complete this chemical overview, the knowledge about Titan's aerosols is improved by the study of analogous materials produced in the laboratory ("Titan's tholins"). The word "tholins" has been proposed in 1979 by Sagan and Khare [START_REF] Sagan | Tholins: Organic Chemistry of Interstellar Grains and Gas[END_REF] to name laboratory analogues of solid planetary aerosols. It comes from the Ancient Greek thólos and refers to the brownish color of the material.

These are most often synthesized by maintaining a continuous plasma discharge in gaseous N 2 -CH 4 mixtures [START_REF] Sagan | Tholins: Organic Chemistry of Interstellar Grains and Gas[END_REF][START_REF] Coll | Experimental Laboratory Simulation of Titan's Atmosphere: Aerosols and Gas Phase[END_REF][START_REF] Imanaka | Laboratory Experiments of Titan Tholin Formed in Cold Plasma at Various Pressures: Implications for Nitrogen-Containing Polycyclic Aromatic Compounds in Titan Haze[END_REF][START_REF] Szopa | Pampre: A Dusty Plasma Experiment for Titan's Tholins Production and Study[END_REF] , such as the PAMPRE reactor in LATMOS laboratory. This original setup is designed for atmospheric simulation, as the production and growth of the tholins occur directly in levitation inside the confined reactive plasma, avoiding any contact with a solid support during the growth process.

High-resolution mass spectrometry analysis has been performed in order to understand the C x H y N z chemical structure [START_REF] Gautier | Nitrogen Incorporation in Titan's Tholins Inferred by High Resolution Orbitrap Mass Spectrometry and Gas Chromatography-Mass Spectrometry[END_REF][START_REF] Pernot | Tholinomics: Chemical Analysis of Nitrogen-Rich Polymers[END_REF][START_REF] Hörst | Formation of Amino Acids and Nucleotide Bases in a Titan Atmosphere Simulation Experiment[END_REF][START_REF] Somogyi | Organic Environments on Saturn's Moon, Titan: Simulating Chemical Reactions and Analyzing Products by FT-ICR and Ion-Trap Mass Spectrometry[END_REF][START_REF] Somogyi | Chemical Ionization in the Atmosphere? A Model Study on Negatively Charged "Exotic" Ions Generated from Titan's Tholins by Ultrahigh Resolution Ms and Ms/Ms[END_REF][START_REF] Vuitton | Very High Resolution Mass Spectrometry of HCN Polymers and Tholins[END_REF] of Titan's tholins. It was found to contain a complex mixture of molecules: mass spectra were actually obtained with more than one species per nominal mass at every mass between m/z 50 and 800. Van Krevelen representations enabled to identify their molecular formula, polymeric patterns and to decompose the complex organic matter into only about ten polymeric families [START_REF] Gautier | Nitrogen Incorporation in Titan's Tholins Inferred by High Resolution Orbitrap Mass Spectrometry and Gas Chromatography-Mass Spectrometry[END_REF][START_REF] Pernot | Tholinomics: Chemical Analysis of Nitrogen-Rich Polymers[END_REF] . But high resolution mass spectrometry does not solve isomeric ambiguity and cannot provide the chemical structure of the molecules. Moreover, the extraction method relies on the solubility of the material, which has been found to be mostly insoluble, even in a polar solvent: only 20-30% can be dissolved in methanol, and less than 1% in toluene [START_REF] Carrasco | Chemical Characterization of Titan's Tholins: Solubility, Morphology and Molecular Structure Revisited[END_REF] . The material analysis has therefore to be completed by other extractions techniques, such as pyrolysis.

Previous studies involving Titan's tholins pyrolysis were performed [START_REF] Mcguigan | Analysis of Titan Tholin Pyrolysis Products by Comprehensive Two-Dimensional Gas Chromatography-Time-of-Flight Mass Spectrometry[END_REF][START_REF] Coll | Review and Latest Results of Laboratory Investigations of Titan's Aerosols[END_REF][START_REF] Khare | Organic Solids Produced by Electrical Discharge in Reducing Atmospheres: Tholin Molecular Analysis[END_REF] . In [START_REF] Mcguigan | Analysis of Titan Tholin Pyrolysis Products by Comprehensive Two-Dimensional Gas Chromatography-Time-of-Flight Mass Spectrometry[END_REF] tholins were pyrolysed at four temperatures, 250, 400, 600 and 900°C, and analyzed by GC-GC-MS. Surprisingly, at 250°C, very few species were detected, among them acetonitrile was the major one. Because of this limitation, the authors focused on identifying the volatiles compounds obtained at a 600°C pyrolysis, and found mainly pyrrole structures. Despite the strong interest of this primary study on tholins pyrolysis, coupled with a powerful GC-GC-MS analysis, the quasi non-detection of products at 250°C remains a major issue. Indeed, the pyrrole structures detected at 600°C are probably not representative of the original solid material, as it is known that the aromaticity of pyrolysates increases with the temperature [START_REF] Cable | Titan Tholins: Simulating Titan Organic Chemistry in the Cassini-Huygens Era[END_REF] . Actually, recent NMR studies on tholins analysis confirmed a low aromatic content of tholins [START_REF] Derenne | New Insights into the Structure and Chemistry of Titan's Tholins Via 13 C and 15 N Solid State Nuclear Magnetic Resonance Spectroscopy[END_REF][START_REF] He | NMR Identification of Hexamethylenetetramine and Its Precursor in Titan Tholins: Implications for Titan Prebiotic Chemistry[END_REF][START_REF] He | Structural Investigation of Titan Tholins by Solution-State 1H, 13C, and 15N NMR: One-Dimensional and Decoupling Experiments[END_REF] .

Previous thermal degradation studies of tholins confirmed the need for soft pyrolysis temperatures to address the issue of a tholins molecular identification representative of the chemical structure of the native non-heated material [START_REF] He | Thermal Degradation of Organics for Pyrolysis in Space: Titan's Atmospheric Aerosol Case Study[END_REF][START_REF] Nna-Mvondo | Thermal Characterization of Titan's Tholins by Simultaneous Tg-MS, DTA, DSC Analysis[END_REF] . The study by Nna Mvondo et al. [START_REF] Nna-Mvondo | Thermal Characterization of Titan's Tholins by Simultaneous Tg-MS, DTA, DSC Analysis[END_REF] pointed out that Titan's tholins contain more open-chain structures than ring-shaped structures and that cyclisation occurs during high temperature treatments. Similarly, He et al. [START_REF] He | Thermal Degradation of Organics for Pyrolysis in Space: Titan's Atmospheric Aerosol Case Study[END_REF] showed that the heated solid residue has an elemental content significantly modified for temperatures higher than 300°C. There is therefore a need for coupling a sensitive organic compound analytical technique to a soft temperature pyrolysis extraction. This is the purpose of the present work, in which we introduce an alternative analysis method based on PhotoElectron/PhotoIon Coincidence (PEPICO) techniques coupled with tuneable VUV synchrotron radiation, and using a soft pyrolysis extraction method, by heating the material at temperatures lower than 300°C. This experimental method has already been successfully employed for species identification, including isomer differentiation, in complex media [START_REF] Bodi | Mass-Resolved Isomer-Selective Chemical Analysis with Imaging Photoelectron Photoion Coincidence Spectroscopy[END_REF][START_REF] Felsmann | Electron Ionization, Photoionization and Photoelectron/Photoion Coincidence Spectroscopy in Mass-Spectrometric Investigations of a Low-Pressure Ethylene/Oxygen Flame[END_REF][START_REF] Kruger | Photoelectron-Photoion Coincidence Spectroscopy for Multiplexed Detection of Intermediate Species in a Flame[END_REF][START_REF] Oßwald | In Situ Flame Chemistry Tracing by Imaging Photoelectron Photoion Coincidence Spectroscopy[END_REF] and is applied here to improve the characterization of the chemical composition of Titan tholins.

Experimental Setup and Methodology

Sample synthesis

Tholins samples were produced in the PAMPRE radio-frequency capacitively coupled plasma reactor described in detail in Szopa et al. [START_REF] Szopa | Pampre: A Dusty Plasma Experiment for Titan's Tholins Production and Study[END_REF] . In this work, the experimental conditions at room temperature were a N 2 -CH 4 gas mixture containing 5.0 ± 0.1% of methane at a flow rate of 55.0 ± 0.1 sccm, a pressure of 0.9 mbar, and a plasma power of 30 W. During the production process, the particles were gently deposited in a glass vessel without any interaction with the substrate and were collected into microvials for further analysis.

Vaporization and chemical analysis on the VUV DESIRS-synchrotron beamline

The chemical analysis was performed at the DESIRS undulator beamline (Synchrotron SOLEIL, France) [START_REF] Nahon | Desirs: A State-of-the-Art VUV Beamline Featuring High Resolution and Variable Polarization for Spectroscopy and Dichroism at Soleil[END_REF] , on the SAPHIRS permanent molecular beam endstation, which is composed of two chambers, expansion and ionization, connected through a 0.7 mm skimmer [START_REF] Nahon | Desirs: A State-of-the-Art VUV Beamline Featuring High Resolution and Variable Polarization for Spectroscopy and Dichroism at Soleil[END_REF] .

Tholins samples were heated in an in-vacuum oven placed inside the expansion chamber, and the resulting vapor was mixed with 1 bar of Argon and expanded through a 70 μm nozzle. The oven temperatures were chosen according to tholins thermal stability [START_REF] He | Thermal Degradation of Organics for Pyrolysis in Space: Titan's Atmospheric Aerosol Case Study[END_REF] . Temperature has to be above 150°C to go over the simple extraction of adsorbed water. But the native compounds in the aerosols are increasingly altered with temperature, showing thermic cracking effects. Beyond 300°C, both the elemental analysis and infrared signature of the residual heated sample significantly evolve, showing a drastic thermal evolution of the material [START_REF] He | Thermal Degradation of Organics for Pyrolysis in Space: Titan's Atmospheric Aerosol Case Study[END_REF] . Oven temperatures between 150 and 300°C were therefore chosen in the present study as the best compromise to extract as much representative molecules as possible without modifying significantly the material. In this range, three temperatures 180, 230 and 280°C were used in order to probe a possible change in the volatiles compounds collected according to the heating.

After traversing the skimmer, the vapor entered the ionization chamber of SAPHIRS where sits a double imaging PEPICO (i 2 PEPICO) spectrometer, named DELICIOUS III [START_REF] Garcia | Delicious III: A Mulipurpose Double Imaging Particle Coincidence Spectrometer for Gas Phase Vacuum Ultra-Violet Photodynamics Studies[END_REF] , which combines a velocity map imaging (VMI) apparatus [START_REF] Eppink | Velocity Map Imaging of Ions and Electrons Using Electrostatic Lenses: Application in Photoelectron and Photofragment Ion Imaging of Molecular Oxygen[END_REF] with a modified ion Wiley-McLaren time of flight analyser/imager. The coincidence scheme is used here to filter the electrons according only to the ion mass and thus to provide mass-selected photoelectron images. Abel inversion of the images [START_REF] Garcia | Two-Dimensional Charged Particle Image Inversion Using a Polar Basis Function Expansion[END_REF] yields the corresponding photoelectron spectra (PES) for all the ions in the mass spectra, in a multiplex manner, with an estimated resolution of 200 meV. The error bars on the experimental PES are estimated assuming a Poisson distribution on each independent pixel of the photoelectron images, and subsequent propagation of the error over the algebra operations performed by the Abel inversion algorithm. These error bars are then used by the least squares fit routine to estimate the statistical error associated to the relative isomeric abundance.

The molecular beam was ionized by the VUV synchrotron radiation at the center of the spectrometer. The monochromator was set to deliver 3 x 10 12 photons/sec with a resolution of 10 meV at 10 eV. A gas filter [START_REF] Mercier | Experimental and Theoretical Study of a Differentially Pumped Absorption Gas Cell Used as a Low Energy-Pass Filter in the Vacuum Ultraviolet Photon Energy Range[END_REF] located upstream the monochromator and filled with Ar ensured spectral purity by effectively absorbing the high harmonics radiation emission from the undulator. In practice, i 2 PEPICO data were recorded at the 4 fixed photon energies of 9.5 eV, 10.5 eV, 11.5 eV and 12.5 eV for a typical 7200 sec duration.

Molecular identification

Molecular identification of selected chemical compounds was achieved through the comparison of the cation electronic signatures in our experimental results with existing experimental data.

All C x H y N z O w species were considered: oxygenated compounds are part of the original tholins aerosols, as oxidation of the surface occurs in the time between production and characterization.

The oxygen content in our samples has been previously studied and quantified [START_REF] Cable | Titan Tholins: Simulating Titan Organic Chemistry in the Cassini-Huygens Era[END_REF][START_REF] Fleury | Influence of CO on Titan Atmospheric Reactivity[END_REF][START_REF] Sciamma-O'brien | Titan's Atmosphere: An Optimal Gas Mixture for Aerosol Production[END_REF] .

The mass resolving power of DELICIOUS III under the present experimental conditions (timefocusing mode operation) is estimated from the most intense peak at m/z 111 to be 𝑀 Δ𝑀 = 450 at the full width half maximum (FWHM). This value is not high enough to directly infer the chemical composition of the molecules.

Photoelectron spectra (PES)

The isomers identification is made by comparison between mass selected photoelectron spectra obtained in this work from the vaporized tholins and the He I, He II and Ne photoelectron spectra existing in the literature. This identification can be hindered due to the following obstacles: (1) the presence in the spectra of electronic structures corresponding to precursors of species produced by a dissociative photoionization process; (2) internal temperature effects (imperfect cooling) that may lead to hot bands, and in some cases to the presence of several conformers; (3) the lack of defined structure in the PES and the overlap between different isomers; (4) the absence of experimental PES data for the isolated molecules in the literature, and more commonly, the lack of fragment selected data to deduce the state-selected fragmentation pattern; and (5) the increasing number of possibilities with increasing mass.

Clearly the higher the mass, the more difficult it is to attribute the isomers, and therefore this work is dedicated to the isomer attribution of the lighter species of the tholins composition with m/z ≤ 69.

ii.

Orbitrap High Resolution Mass spectrometry analysis (OHR-MS)

Complementary analyses using OHR-MS were made with a hybrid linear trap/orbitrap mass spectrometer (LTQ orbitrap) of the polar species, following the methodology developed in [START_REF] Pernot | Tholinomics: Chemical Analysis of Nitrogen-Rich Polymers[END_REF] : a soft electrospray ionization, but limited to the soluble fraction of the sample, coupled to a high resolution mass analyzer. This analysis assumes that the soluble fraction is representative of the bulk tholins (as shown in [START_REF] Carrasco | Chemical Characterization of Titan's Tholins: Solubility, Morphology and Molecular Structure Revisited[END_REF] ). The lower mass limit of m/z 50 with this instrument precludes a full comparison with the PEPICO data for light species.

Results and discussion

Effect of the oven temperature on the volatiles compounds extracted from the tholins sample

Figure 1 (a-c) shows the mass spectra obtained at 10.5 eV of photon energy with DELICIOUS III for oven temperatures of (a) 180°C, (b) 230°C and (c) 280°C obtained with the same acquisition time. The detected ions are the same in the three mass spectra. The major quantitative difference between these results is the ratio between the heavier and lighter ions, which increases as a function of the temperature. Looking at the absolute scales in Figure 1 this increase correspond to both a gain of the heavier masses (>100 amu), and a loss of the lighter ones. While the temperature trend of the heavier masses is intuitive, the loss of signal on the lighter ones must be explained by the volatile fraction gradually disappearing with time, since the increase in temperature shown in Figs. 1a-1c followed a chronological order. In any case, we did not observe any significant temperature (or time) dependence on the measured PES on the most part of the species analyzed in this work, which leads us to conclude that the evolution of this ratio should be simply associated with the relative vapor pressure and quantity of these species. These effect are most probably attribute to the increase of the temperature, however we note that it could be partially attributed to a time effect, where the volatile compounds would gradually disappear, since the increase of the temperature follows the chronologic order of the data acquisition.

The invariability of the PES with the temperature indicates that the structure of the molecules extracted from the sample at temperatures between 180°C and 280°C is unchanged. Therefore, and in order to increase the data statistics, we added the data recorded at the two lowest oven temperatures, 180°C and 230°C. Even if we have no evidence of a possible degradation of the sample at 280°C, on a precautionary principle, we discarded, for the PES, the analysis of the corresponding data. First, the ion at m/z 17 is found unambiguously to be ammonia (NH 3 ) by comparing our photoelectron spectra with the one obtained by Locht et al. [START_REF] Locht | I) Photoelectron Spectroscopic Study of the X̃2a″2 State of NH3+ and ND3+: A Reanalysis and Evidence for the Coriolis Coupling between the Bending V2 and V4 Modes[END_REF] . Thanks to this attribution we could use 8 observed vibrational transitions to calibrate the kinetic energy scale of our spectra, leading to an absolute precision of 5 meV on the binding energy for all species. eV, along with those for the possible isomers found in the literature for the formaldehyde (CH 2 O) (resolution 80 meV)) [START_REF] Turner | Molecular Photoelectron Spectroscopy[END_REF] , diazene (N 2 H 2 ) [START_REF] Frost | The Photoelectron Spectra of Diazene, Diazene-D2, and Transmethyldiazene[END_REF] represented by red line and nitric oxide (NO) (resolution 15 meV) [START_REF] Natalis | Very High Vibrational Excitation, up to n′ = 32, in NO+ X 1σ+ Ions by UV Irradiation, Evidenced by Photoelectron Spectroscopy[END_REF] ionization energies represented in blue lines.

General comparison with OHR mass spectra

In the case of ion at m/z 30 several species with this m/z ratio could make a contribution to the observed PES. Comparison of the PES (Figure 4) found in the literature with our spectra suggests the presence of nitric oxide (NO) [START_REF] Natalis | Very High Vibrational Excitation, up to n′ = 32, in NO+ X 1σ+ Ions by UV Irradiation, Evidenced by Photoelectron Spectroscopy[END_REF] , and formaldehyde (CH 2 O) [START_REF] Takeshita | A Theoretical Study on the Ionic States with Analysis of Vibrational Levels of the Photoelectron Spectrum of Formaldehyde (CH2O)[END_REF] , the diazene (N 2 H 2 ) [START_REF] Frost | The Photoelectron Spectra of Diazene, Diazene-D2, and Transmethyldiazene[END_REF] photoelectron spectrum is also represented in Figure 4 (b), but the comparison suggests that if the diazene is present it is in small quantity, since our spectra are dominated by NO structures. A more complex example of the isomers identification in this work is illustrated in Figure 5 for the ion at m/z 31. Although we could readily identify the methylamine (CH 5 N) and probably nitrosyl hydride (HNO) through comparison with the experimental PES by Bieri et al. [START_REF] Bieri | 30.4nm He (II) Photoelectron Spectra of Organic Molecules[END_REF] and Baker et al. [START_REF] Baker | Vacuum Ultraviolet Photoelectron Spectroscopic Study of the NH2O and Hno Molecules[END_REF] , the region above 10.8 eV is unexplained. Other candidate such CH 3 O radical [START_REF] Dyke | Observation of the CH2OH Radical in the Gas Phase by Vacuum Ultraviolet Photoelectron Spectroscopy[END_REF][START_REF] Zhu | First Experimental Observation on Different Ionic States of Both Methylthio (CH3S) and Methoxy (CH3O) Radicals[END_REF] was also excluded by their cation's electronic footprint, and the fact that radicals are not expected in tholins. Alternatively, the structure above 10.8 eV could come from dissociative ionization of some unidentified heavier ion(s). For m/z 41 we could easily identify the contribution as acetonitrile (CH 3 CN), as illustrated on the comparison between our PES obtained at 12.5 eV and the He I PES of the acetonitrile obtained by Gochel-Dupuis et al. [START_REF] Gochel-Dupuis | High-Resolution He I Photoelectron Spectrum of Acetonitrile[END_REF] on Figure 5 (b). For the weak ion signal at m/z 42 (Figure 6) we identify the possible presence of 3H-diazirine (cyclic CH 2 N 2 ) [START_REF] Robin | Photoelectron Spectra of Small Rings. Iv. The Unsaturated Threemembered Rings[END_REF] , Cyanamide (NC-NH 2 ) [START_REF] Guimon | He1 Photoelectron and Theoretical Study of the Gas Phase Flash Pyrolysis of Tetrazole and Analysis of CN2H2 Energy Hypersurface[END_REF] and propene (C 3 H 6 ) [START_REF] Bieri | Valence Ionization Energies of Hydrocarbons[END_REF] and exclude the presence of cyclopropane (C 3 H 6 ) [START_REF] Redemacher | Photoelectron Spectra of Cyclopropane and Cyclopropene Compounds[END_REF] and diazomethane (linear CH 2 N 2 ) [START_REF] Bastide | Electronic States of the Radical Cations of the 1,3-Dipoles : HCNO, CH2N2, and N3h Studied by Photoelectron Spectroscopy[END_REF] . N 3 radical 53 is unlikely, but cannot be excluded, For binding energies greater than 11

eV we observed structures of unidentified origin. with those for the possible isomers found in the literature for 3H-diazirine (CH 2 N 2 ) (resolution 30 meV) (blue marker) (47), propene (C 3 H 6 ) (red line) (48), N 3 (pink line) [START_REF] Tarroni | Cyclic and Bent Electronic States of the N3+ Ion[END_REF] and cyanamide (CN 2 H 2 ) (green marker) [START_REF] Guimon | He1 Photoelectron and Theoretical Study of the Gas Phase Flash Pyrolysis of Tetrazole and Analysis of CN2H2 Energy Hypersurface[END_REF] .

For the ion at m/z 43 we could compare the PES obtained in this work with three isomers of C 2 H 5 N, the N-methylmethylenimine, C-methylmethylenimine and ethylenimine. Our experimental PES obtained at 10.5 eV could be explained by the presence of only Cmethylmethylenimine isomer as illustrated on the PES on Figure 7. However, we have to take into account two important points: (1) First one, despite the C-methylmethylenimine spectra was obtained by Bock et al [START_REF] Bock | Cycloalkanimine 2h-Azirin, 1-Azetin, 1-Pyrrolin, 1-Piperidein, Und 3,4,5,6-Tetrahydro-2H-Azepin: Darstellung Und Photoelektronen-Spektren[END_REF] with a higher resolution (18meV-25meV) than the one obtained in this work, we verify that we have more resolved electronic structures. In our point of view, we interpret this discrepancy as a higher temperature effect on the C-methylmethylenimine spectra due to their synthesis procedures that involves pyrolysis process without an adiabatic expansion afterwards [START_REF] Bock | Cycloalkanimine 2h-Azirin, 1-Azetin, 1-Pyrrolin, 1-Piperidein, Und 3,4,5,6-Tetrahydro-2H-Azepin: Darstellung Und Photoelektronen-Spektren[END_REF] .

(2) Second one, since this molecule is highly unstable, [START_REF] Frost | Pyrolysis of Trimethyl Hexahydro-S-Triazines. The Photoelectron Spectra of N-Methylmethylenimine, CH2 NCH3, and C-Methylmethylen[END_REF] showing a tendency to polymerize, we believe that if it is present it would be a product of thermal decomposition of some unspecified compound in the tholins. For medium-sized ions that can be studied with the OHR-MS technique (m/z > 50) but that are small enough to have a manageable number of possible isomers, it is interesting to compare the information obtained with the OHR-MS and VUV-MS analysis.

For instance, for the ion at m/z 56, the aminoacetonitrile (AAN) is the only candidate offered by the OHR-MS studies [START_REF] Pernot | Tholinomics: Chemical Analysis of Nitrogen-Rich Polymers[END_REF] . Several isomers with known electronic footprints have been considered

here to model the PES associated with this mass (Figure 10), including AAN. However, although the subsequent least squares fit predicts the presence of some amount of 2-propenal (C 3 H 4 O) and AAN, it is clear that most of the obtained PES is not well fitted or explained, including the region where AAN would contribute, and the binding energies above 11 eV.

Expanding further on the presence of AAN, Bellini et al. (55) recently recorded the fragmentation pattern of AAN + and found that, at the photon energy of 11.5 eV, the main fragment ion would be m/z 29 (around 25%), which it is not in agreement with our VUV-MS recorded at this energy, where we found a branching ratio of around 11%. Assuming that ion at m/z 29 comes exclusively from dissociative ionisation of AAN places an upper limit of 44% to the contribution of AAN to the m/z 56 channel. Of course, m/z 29 could come from dissociation of other parent ions (although not from HCO or C 2 H 5 since the presence of radicals is ruled out), and indeed the least squares fitting provides a AAN contribution well below 44 %. We note that AAN is highly thermolabile as reported by Bellini et al. (55), and decomposes at room temperature in a matter of hours so that not all of the AAN might survive the vaporization temperatures used in this work, while the softer electrospray method used in the OHR-MS is more favorable for fragile molecules, provided they are soluble. We cannot explain, however, why McGuigan et al. [START_REF] Mcguigan | Analysis of Titan Tholin Pyrolysis Products by Comprehensive Two-Dimensional Gas Chromatography-Time-of-Flight Mass Spectrometry[END_REF] using a high temperature pyrolysis found AAN to be a major species, In the same manner, ions at m/z 58 the ones tried here may contribute to the signal. In order to test for the presence of acetamidine, we have calculated its adiabatic ionization energy at the PBE0/aug-cc-pvdz DFT level, and obtained a value of 8.47 eV. The geometry change upon ionization is slight (less than 5 % for chemical bands and less than 6 % for angles), so one would expect a marked adiabatic transition.

As seen in Figure 11, this value is far from the main experimental band starting at 9.4 eV, but could be in agreement with the weak structure observed around 8.5 eV. Although more information than just one theoretical point is needed to corroborate its presence, we can nevertheless conclude that if acetamidine is present, it is only in very small concentrations. Since this molecule is predicted in the OHR-MS, the disagreement could be attributed to thermal decomposition. Note that the measured vapor pressures at 25°C by the ACD (176 torr) and EPISuite (22 torr) are relatively high so that, barring decomposition, we would expect a strong signal at our working temperatures. Indeed, acetamidine has been reported to decompose at 95°C to yield 70% of acetonitrile [START_REF] Crossland | A Convenient Preparation of Acetamidine[END_REF] , which is consistent with both the fact that acetamidine is not present (or in trace quantities) in our measurements, and that acetonitrile is detected in the measurements performed at 12.5 eV (see Figure For the ion at m/z 59 (PES shown in Figure 12), which dominates our VUV-MS (low mass side) at all temperatures (see Figure 1), a good fit of the PES could be achieved with the available isomers, and acetamide (C 2 H 5 NO) is found as the most abundant contribution. Although the presence of acetamide is in agreement with the OHR-MS data, it was found to be minor compared with the main species at m/z 59, guanidine (CH 5 N 3 ). However, guanidine is totally absent from the PES spectra. The experimental electron impact ionization energy of guanidine is 9.10 eV [START_REF] Loudon | Charge Location and Fragmentation in Mass Spectrometry. Ii. The Mass Spectra and Ionization Potentials of Some Methylguanidines[END_REF] , and its recently calculated adiabatic and vertical first ionization energies are 8.48 and

9.09 eV respectively [START_REF] Antol | Guanidine and Guanidinium Cation in the Excited State-Theoretical Investigation[END_REF] . As in the case of AAN, the reason for this discrepancy is found in the extraction methods. Guanidine is a fragile molecule which decomposes at temperatures above 160°C [START_REF] Mei | Thermal Decomposition Properties of Guanidine Nitrate and Basic Cupric Nitrate[END_REF] . The present thermal extraction method is therefore inappropriate for the detection of this specific molecule.

Discussion

The Aerosol Collector and Pyrolyser-Huygens instrument detected ammonia (NH 3 , m/z 17) and hydrogen cyanide (HCN, m/z 27) as the main released molecules by pyrolysis at 600°C of the aerosols in the atmosphere of Titan. In Coll et al [START_REF] Coll | Can Laboratory Tholins Mimic the Chemistry Producing Titan's Aerosols? A Review in Light of Acp Experimental Results[END_REF] it was shown that Titan's tholins produced in confirming the oxygen content of the samples [START_REF] Sciamma-O'brien | Titan's Atmosphere: An Optimal Gas Mixture for Aerosol Production[END_REF] . The O-bearing molecules are always found in these kinds of laboratory analogues [START_REF] Cable | Titan Tholins: Simulating Titan Organic Chemistry in the Cassini-Huygens Era[END_REF] . Pyrolysis process is discarded as the oven is under vacuum. A recent chemical analysis of the samples by XPS highlights that the O-bearing molecules are only present in the external layer of the solid sample, not in its core [START_REF] Carrasco | Laboratory Analogues Simulating Titan's Atmospheric Aerosols: Compared Chemical Compositions of Grains and Thin Films[END_REF] . They are produced when the sample is exposed to ambient air. High resolution mass spectrometry shows that the O-bearing molecules have a similar mass pattern to the rest of the sample [START_REF] Pernot | Tholinomics: Chemical Analysis of Nitrogen-Rich Polymers[END_REF] . Those are therefore no atmospheric volatile organic compounds adsorbed on the sample during the transfer, but result from an oxidation process. As they are considered as a contamination, we did not discuss further their formation in the manuscript.

The strong signature of ammonia in our analysis (Figure 3) is in agreement with the ACP data.

Ammonia is a volatile molecule, which possibly results from the pyrolysis of primary terminal amine functional group in the macromolecules composing tholins. Such an origin is consistent with the identification of light primary amines in our work. We detected the two simplest primary amines, methylamine and ethylamine at m/z 31 and 45, but also the amino-acetonitrile at m/z 56. It is well known that the amine functional group plays a key role in prebiotic chemistry.

The amino-acetonitrile molecule has moreover its amine-functional group in alpha-position regarding the nitrile group. The hydrolysis of this precursor leads to the formation of glycine, the smallest α-amino-acid. Amine functional groups can require specific targeted analytical techniques [START_REF] Cable | Identification of Primary Amines in Titan Tholins Using Microchip Nonaqueous Capillary Electrophoresis[END_REF] and the present analysis appears to be efficient for their detection and suggests an amine-origin of the NH 3 signature detected after pyrolysis of Titan's aerosols by the ACP instrument.

The second main molecule detected by ACP, hydrogen cyanide, could not be identified here because its ionization energy is higher than the photon energy used for this work. Tholins are known to bear nitrile functional groups through their mid-IR signature around ~2200 cm-1 [START_REF] Gautier | Mid-and Far-Infrared Absorption Spectroscopy of Titan's Aerosols Analogues[END_REF][START_REF] Quirico | New Experimental Constraints on the Composition and Structure of Tholins[END_REF] .

We have identified in this work at m/z 41 the presence of acetonitrile (CH 3 CN). A similar pyrolysis extraction from tholins was performed in [START_REF] He | Thermal Degradation of Organics for Pyrolysis in Space: Titan's Atmospheric Aerosol Case Study[END_REF] Comparison with the elemental formula of the main species detected by OHR-MS at the same mass units, when possible (m/z > 50), and extracted from tholins in methanol.

The molecules identified in this work are compared with the elemental formula of the main species detected by OHR-MS at the same mass unit for m/z > 50 in Table 1. First we notice that the species are mostly consistent in spite of the different extraction method, in agreement with the previous general comparison of the mass spectra (Figure 2).

The main difference is for m/z 59, corresponding to acetamide in the case of the soft-pyrolysis-MS-PES analysis and to CH 5 N 3 in the solvent-OHR-MS analysis, strongly suspected to be guanidine. Guanidine is a thermally fragile molecule that decomposes with pyrolysis-extraction methods even with the mild temperatures used here. This underlines the complementarity of OHR-MS and the resistive heating described here. Indeed species that are not seen in the former due to the lack of solubility can be observed by resistive heating combined with PES, and conversely, thermally fragile molecules that are not seen at high temperatures, are only seen in OHR-MS provided that they are soluble.

For the other species, the elemental compositions obtained with OHR-MS are consistent with the structure extracted from the MS-PES. We confirm the putative attribution made for m/z 56 in [START_REF] Pernot | Tholinomics: Chemical Analysis of Nitrogen-Rich Polymers[END_REF] as amino-acetonitrile. Moreover we highlight a N-aromatic structure for the species at m/z 69 with a triazole ring. This cycle involves two adjacent double-bonded nitrogen atoms, which are also seen in diazirine, a smaller structure identified at m/z 42. Tetrazolo [1,5-b]pyridazine, a gas product with similar chemical properties was moreover previously detected in the plasma discharge where tholins are produced [START_REF] Gautier | Nitrile Gas Chemistry in Titan Atmosphere[END_REF] . This specific N=N pattern is possibly a marker of a direct incorporation of N 2 (or N 2 + ) in the growing organic material composing tholins. This basic structure would deserve further investigation to interpret the N-aromatic signature globally detected in Titan's tholins [START_REF] Derenne | New Insights into the Structure and Chemistry of Titan's Tholins Via 13 C and 15 N Solid State Nuclear Magnetic Resonance Spectroscopy[END_REF][START_REF] Quirico | New Experimental Constraints on the Composition and Structure of Tholins[END_REF] and to understand the role of nitrogen in the organic growth in the atmosphere of Titan. The present knowledge on gas-phase nitrogen chemistry representative of Titan's atmosphere actually hardly explains the incorporation of two adjacent nitrogen atoms in the organic products [START_REF] Dutuit | Critical Review of N, N + , N 2 + , N ++ , and N 2 ++ Main Production Processes and Reactions of Relevance to Titan's Atmosphere[END_REF] .

Conclusion and perspectives

The chemical composition of the volatile fraction of Titan's tholins was studied by means of electron/ion coincidence techniques coupled to tunable VUV synchrotron radiation. A large number of ions were detected in the mass spectra and for each of them a photoelectron spectrum was obtained through the coincidence scheme. For some of the lighter ions (m/z ≤ 69), comparison with existing experimental PES yielded the structure, while for heavier ions no analysis was performed due to the lack of PES experimental data and the larger number of possibilities. This limitation highlights the importance of recording experimental data on the photoionization of isolated relevant molecules, such as electron spectroscopy, state-selected photodissociation and total ionization cross section for quantitative analysis. In addition, although thermolysis data could provide further speciation and also be invoked to explain the differences between HR-OMS and the resistive oven PEPICO experiments, the lack of literature on the subject and the expected overlap of the thermolysis products, especially at low masses, prevent us from attempting so.

Crossed analyses, addressing complementary fractions of the material have to be used to fully understand the complex composition of tholins. The analytical methodology developed in this work was focused on relatively mild conditions in order to ensure that the molecules extracted and softly ionized are representative of the bulk material: a temperature lower than 230°C and ionizing energies lower than 12.5 eV.

Beyond the present case of tholins analysis, the present methodology consisting in coupling a soft pyrolysis to an imaging PEPICO set-up associated to VUV synchrotron radiation could be extended to other solid materials, including organic powders, in a very complementary Way with other analytical techniques.

Figure 1 :

 1 Figure 1: Mass spectra obtained during 7200s at 10.5 eV photon energy at DESIRS beamline with an oven temperature of (a) 180°C, (b) 230°C and (c) 280°C.

Figure 2

 2 Figure 2 (a) shows a typical OHR mass spectrum of the soluble part of tholins aerosol. The OHR

Figure 2 : 69 Figure 3

 2693 Figure 2: (a) OHR mass spectrum (b) VUV Mass spectra (VUV-MS) obtained at 10.5 eV photon energy with an oven temperature of 180 o C on the DESIRS beamline with DELICIOUS III spectrometer.

Figure 3 :

 3 Figure 3: VUV-MS obtained at 10.5 eV at an oven temperature of 230 o C. The inset shows a close-up of the region

Figure 4 :

 4 Figure 4 : Photoelectron spectra for ion at m/z 30 obtained in this work (black line) at (a) hν = 10.5 eV and (b) hν = 11.5

Figure 5 :

 5 Figure 5: (a) Photoelectron spectra obtained in this work (black line) for m/z 31 at 11.5 eV photon energy and He I photoelectron spectra (red marker) obtained of methylamine (CH 5 N) by Bieri et al. 43 . (b) Photoelectron spectra obtained for m/z 41 at 11.5 eV photon energy and He I photoelectron spectra obtained of acetonitrile (CH 3 CN) by Gochel-Dupuis et al (resolution 25 meV) (red line) 44 .

Figure 6 :

 6 Figure 6 : Photoelectron spectrum for ion at m/z 42 obtained in this work at (a) hν = 10.5 eV and (b) hν = 11.5 eV, along

Figure 7 :

 7 Figure 7: Photoelectron spectrum for ion at m/z 43 obtained in this work at hν = 10.5 eV along with those for the possible isomers found in the literature for C 2 H 5 N, the N-methylmethylenimine (green markers), Cmethylmethylenimine (red markers)[START_REF] Bock | Cycloalkanimine 2h-Azirin, 1-Azetin, 1-Pyrrolin, 1-Piperidein, Und 3,4,5,6-Tetrahydro-2H-Azepin: Darstellung Und Photoelektronen-Spektren[END_REF] and ethylenimine[START_REF] Bieri | 30.4nm He (II) Photoelectron Spectra of Organic Molecules[END_REF] (blue markers).

Figure 8 :

 8 Figure 8 : Photoelectron spectrum for ion at m/z 44 obtained in this work at (a) hν = 10.5 eV and (b) hν = 11.5 eV (black line) and photoelectron spectrum for acetaldehyde (CH 3 CHO) obtained by Tam et al (red line) 59 , for propane (CH 3 CH 2 CH 3 ) by Bieri et al (resolution between 60 and 100 meV) (green line) 60 and for ethylene oxide (C 2 H 4 O) by Corderman et al (resolution 25 meV) (blue line) 61 .

Figure 9 (Figure 9 :

 99 Figure 9 (a) and (b) present the isomers identification conducted for the ion at m/z 45. Despite

  /z 45 at hν=11.5 eV (this work) He I PES of formamide (Asbrink et al) (b) PES from m/z 45 at hν=9.5 eV (this work) He I PES of ethylamine (Maruyama et al.) (a)

  while in our work it represents a minor fraction of the m/z 56 channel. Other structures like 2propenal (C 3 H 4 O) and cyclobutane (C 4 H 8 ) are likely, but a large portion of the PES remains unexplained so that other isomers than the ones named in Figure 10 must be present.

Figure 10 :

 10 Figure 10: (a) Photoelectron spectrum for ion at m/z 56 obtained in this work at hυ = 11.5 eV, along with those for the possible isomers found in the literature. 64-69 . The curves have been normalized to their integral over the displayed ionization energy range. (b) Experimental PES (black circles) and least squares fit of the linear combination of all isomers shown in panel (a) to the data. The contributions of each isomer to the data are shown in the inset.

Figure 11 :

 11 Figure 11: (a) Photoelectron spectrum for m/z 58 obtained in this work at hυ = 11.5 eV, along with those for the possible isomers found in the literature. 40, 60, 65, 71-75 The curves have been normalized to their integral over the displayed ionization energy range. The calculated adiabatic ionization of acetamidine (see text for details) has been marked with an arrow. (b) Experimental PES (black circles) and least squares fit of the linear combination of all isomers shown in panel (a) to the data. The contributions of each isomer to the data are shown in the inset.

Figure 12 :

 12 Figure 12: (a) Photoelectron spectrum for ion at m/z 59 obtained in this work at hν = 11.5 eV, along with those for the possible isomers found in the literature 63, 79-81 . The curves have been normalized to their integral over the displayed ionization energy range. (b) Experimental PES (black circles) and least squares fit of the linear combination of all isomers shown in panel (a) to the data. The contributions of each isomer to the data are shown in the inset. The dashed lines represent the range of energies where signal from guanidine should be seen according to literature 76-77 .

Figure 13

 13 Figure13shows the PES corresponding to ions at m/z 69. The lack of sharp structure in our

Figure 13 :

 13 Figure 13: (a) Photoelectron spectrum for ion at m/z 69 obtained in this work at hν = 11.5 eV, along with those for the possible isomers found in the literature 54, 63, 82-87 . The curves have been normalized to their integral over the displayed ionization energy range. (b) Experimental PES (black circles) and least squares fit of the linear combination of all isomers shown in panel (a) to the data. The contributions of each isomer to the data are shown in the inset.

  and 59 have been assigned by OHR-MS to acetamidine (C 2 H 6 N 2 ) and guanidine (CH 5 N 3 ), respectively. This cannot be fully verified in our PEPICO data since no information whatsoever was found on the photoionization of acetamidine, and only ionization energies exist for guanidine. From Figure11, we find, however, that several other isomers can contribute to the PES of m/z 58, without any species being predominant. Indeed, we

		m/z 56 This Work
		1-Butene (White 1974)
		1-Butene (Van der Meij 1988)
		cis-butene (White 1974)
		trans-butene (White 1974)
		isobutene (Kimura 1975)
		cyclobutane (Bischof 1970)
		2-propenal (von Niessen 1980)
		propargyl alcohol (von Niessen 1980)
		AAN (Lacombe 1989)
		8	9		10		11
			Ionization Energy (eV)
		Point Perc (%)	Isomer
		0 ± 5	1-Butene_1974
		6 ± 4	1-Butene_1988
		0 ± 5	cis_butene_1974
	Intensity (arb. units)	0 ± 4 2 ± 3 29 ± 1 0 ± 6 28 ± 6	trans_butene_1974 iso_butene_1975 2-propenal_1980 propargyl_OH_1980 AAN_1989
		8.5	9.0	9.5	10.0	10.5	11.0
			Ionization Energy (eV)

succeeded in obtaining a quite satisfactory fitting of the PES by a by a linear combination of all possible isomers. The departure from a perfect fitting, indicates that other molecules apart from

  "cold plasma" conditions, such as in the PAMPRE reactor, provided the best possible analogues to reproduce the volatiles released with the ACP experiment. Our work, focused on the light species extracted from PAMPRE tholins through soft pyrolysis conditions (T<240°C), provides further insights on this in-situ thermal extraction of light molecules and on the possible presence of larger structures in Titan's aerosols.The ions detected at m/z ≤ 69 in this work are summarized in Table1. Eleven ion signatures with one to five heavy atoms were detected. Most of them could be firmly identified with their corresponding PES. A few oxygen-bearing compounds, NO, CH 2 O (formaldehyde), C 2 H 4 O (acetaldehyde and ethylene oxide), HCONH 2 (formamide) and CH 3 CONH 2 , are reported,

			m/z 69 This Work
			1H-1,2,4-Triazole (Guimon 1980)
			Butyronitrile (Ohno1984)
			Isobutyronitrile (Ohno 1984)
			Isoxazole (Kobayashi 1982 )
			Propiolamide (Asbrink 1981)
			1H-Pyrrole,2,5-dihydro (Morishima 1975)
			1-pyrrolin (Bock 1987)
			1H-1,2,3-triazole (Cradock 1973)
			1H-1,2,4-triazole (Cradock 1973)
			Vinyl isocianate (Kirby 1978)
			Oxazole (Kobayashi 1982)
			(a)
		6	7	8	9	10	11
			Point Perc (%)	Isomer
		5	73 ± 8 0 ± 21	1H-1,2,4-Triazole_1980 butyronitrile_1984
			0 ± 6	isobutyronitrile_1984
	Intensity (arb. units)	4 3 2	0 ± 4 0 ± 8 0 ± 1 0 ± 5 0 ± 4 27 ± 8 0 ± 4	isoxazole_1982 propiolamide_1981 Pyrrole_1975 pyrrolin_1987 1H-1,2,3-triazole_1973 1H-1,2,4-triazole_1973 vinyl_isocianate_1978
		1
			(b)
		0
				8	9	10	11
				Binding Energy (eV)

Table 1 :

 1 Volatile

	m/z	Soft-Pyr-MS-PES	Solvent-
			OHR-MS
	17	NH 3	-
	30	NO
		CH 2 O (formaldehyde)
	31	CH 3 NH 2 (methylamine)	-

with a simple QMS analysis of the released molecules. No firm identification was possible in this case, but the m/z 27 ion signature could reasonably be attributed to HCN. The evolution of its ion intensity was monitored according to the increasing temperature, showing a progressive release of this molecule, actually negligible at soft temperatures (T<300°C). molecules with m/z ≤ 69 extracted from PAMPRE Titan's tholins after a soft heating at ~230°C and identified by PES spectroscopy in this work (Soft-Pyr-MS-PES).

Acknowledgments

We would like to thank the general technical staff of SOLEIL for running the facility under project n° 20110728 and 20120953. The authors wish to thank Isabelle Schmitz-Afonso and David Touboul from Institut de Chimie des Substances Naturelles (ICSN) for the OHR-MS measurements. B.F. PhD grant is supported by Région Ile-de-France (DIM-ACAV program). NC acknowledges the European Research Council for their financial support (ERC Starting Grant PRIMCHEM, grant agreement n°636829).

AUTHOR INFORMATION

Corresponding Author

Electronic mail: nathalie.carrasco@latmos.ipsl.fr

Present

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. NC supervised the project. BCM, GG, FGL, BF, AM, LN and NC contributed to the experimental campaigns on the synchrotron. TG and PP contributed to the HRMS measurements and comparisons.


Vineet Kumar

Zhiqiu Ye

Haodong Jiang

Yue Shi

Ke Li

Davy Gé

Dan Luo

Quanquan Mu

Yan Jun Liu
email: yjliu@sustech.edu.cn

Highly Stable, Pretilted Homeotropic Alignment of Liquid Crystals Enabled by In-Situ Self-Assembled, Dual-Wavelength Photoalignment

Keywords: liquid crystals, vertical alignment, self-assembly, azo dyes, photoalignment, polymer stabilization

Homeotropic alignment of liquid crystals (LCs) is one of the key modes used in commercial liquid crystal displays (LCDs) due to its advantages, such as high contrast ratios and fast response speed. Here, we report stable homeotropic LC alignment with a small pretilt angle in a nematic, negative LC host doped with additives including self-aligning agents, photoresponsive azo dyes and reactive mesogen via an in-situ self-assembled, dual-wavelength photoalignment approach. The underlying mechanism of stable, pretilted homeotropic LC alignment was further analyzed in detail. The achieved pretitled homeotropic LC cells are very stable against heat and UV light exposure and demonstrate excellent electro-optical characteristics. The experimental results show that such a noncontact, simple homeotropic alignment technique has big potential in many applications, such as high-quality LCDs, spatial light modulators, and other optoelectronic devices.

INTRODUCTION

Liquid crystal displays (LCDs) are ubiquitous in the present technological society ranging from day to day smart phones to different home appliances such as TVs, computers, etc. The wide use of LCDs is possible only because of the key advancement in LC alignment control technologies.

Homeotropic and homogeneous alignments of LCs have been commercially successful in device fabrications because of great advantages including high contrast ratio, fast response speed, simple and easy fabrication process. So far, mechanical rubbing of polyimide-coated conductive indium-tin-oxide (ITO) substrates is the major LC alignment technique. Rubbing of polyimide consists of multi-step processes such as coating, soft and hard baking along with the drawbacks like dust creation, surface damage or electrostatic charge generation, greatly limiting its application for high-quality displays. Several alternative noncontact alignment techniques have been proposed, such as photoalignment [START_REF] Ichimura | Photoalignment of Liquid-Crystal Systems[END_REF][START_REF] Fuh | Variable Liquid Crystal Pretilt Angles Generated by Photoalignment in Homeotropically Aligned Azo Dye-Doped Liquid Crystals[END_REF][START_REF] Fang | Photo-Reversible Liquid Crystal Alignment Using Azobenzene-Based Self-Assembled Monolayers: Comparison of the Bare Monolayer and Liquid Crystal Reorientation Dynamics[END_REF] , self-assembled monolayer (SAMs) [START_REF] Yi | High-Sensitivity Aminoazobenzene Chemisorbed Monolayers for Photoalignment of Liquid Crystals[END_REF][START_REF] Prompinit | Controlling Liquid Crystal Alignment Using Photocleavable Cyanobiphenyl Self-Assembled Monolayers[END_REF] , oxide evaporation [START_REF] Janning | Thin Film Surface Orientation for Liquid Crystals[END_REF] , nanopatterns [START_REF] Liu | Nanoimprinted Ultrafine Line and Space Nanogratings for Liquid Crystal Alignment[END_REF][START_REF] Atkuri | Developing Novel Liquid Crystal Technologies for Display and Photonic Applications[END_REF][START_REF] Si | Polarization-Insensitive Light Tuning Properties in Silver Nanorod Arrays Covered with Photoresponsive Liquid Crystals[END_REF] , etc. to get rid of the shortcomings involved in the rubbing technique. The noncontact LC alignment systems consisting of nanoparticles or surface active agents dispersion in the LC host have been reported to achieve macroscopic homeotropic or homogeneous alignments [START_REF] Hwang | Characteristics of Nanoparticle-Doped Homeotropic Liquid Crystal Devices[END_REF][START_REF] Porte | Tilted Alignment of MBBA Induced by Short-Chain Surfactants[END_REF][START_REF] Campanelli | Hexadecyltrimethylammonium Bromide[END_REF][START_REF] Kang | Surface-Anisotropy-Induced Linear Electro-optic Effect in a Nematic Liquid Crystal[END_REF][START_REF] Jeng | Tunable Pretilt Angles Based on Nanoparticles-Doped Planar Liquid-Crystal Cells[END_REF] . Surface active agents (amphiphilic molecules) like hexadecyl trimethyl ammonium bromide (HTAB) in negative LC hosts interact with ITO substrates and results in homeotropic alignment due to molecular self-assembly at the interfaces [START_REF] Kim | Vertical Alignment of Liquid Crystals Without Alignment Layers[END_REF] . The amphiphilic molecules usually contain a polar anchoring group and a flexible long alkyl chain, in which the polar anchoring group interacts with ITO surface, leaving the flexible long alkyl chain in the LC bulk to homeotropically align the LC molecules [START_REF] Russell | Alignment of Nematic Liquid Crystals Using Carbon Nanotube Films[END_REF][START_REF] Kuo | Application of Nanoparticle-Induced Vertical Alignment in Hybrid-Aligned Nematic Liquid Crystal Cell[END_REF][START_REF] Lu | Carbon Nanotube Doped Liquid Crystal OCB Cells: Physical and Electro-optical Properties[END_REF] . In addition, nanoparticles have been also reported as automatic LC alignment inducing agents, where plasmonic or nonplasmonic molecules like Au, Ni, ZnO, CuS, CdSe, POSS or Fullerene (C60), Carbohydrates based dopants [START_REF] Qi | Multiple Alignment Modes for Nematic Liquid Crystals Doped with Alkylthiol-Capped Gold Nanoparticles[END_REF][START_REF] Jeng | Nanoparticle-Induced Vertical Alignment in Liquid Crystal Cell[END_REF][START_REF] Yoon | Giant Surfactants for the Construction of Automatic Liquid Crystal Alignment Layers[END_REF] were mixed with LCs in small amounts. Upon cooling from isotropic to nematic phase of such LC-nanoparticles mixtures, phase separation occurs. A layer of selfaligning agents (SAAs) is formed, which further determines a specific orientation of LCs. In most cases, nanoparticles break the continuity of the rotational symmetry by producing defects, hence resulting in homeotropic alignment of LCs. The resulting LC cells possess remarkable electro-optical properties, such as memory effect, frequency modulation response, fast response and low driving voltage [START_REF] Shiraishi | Frequency Modulation Response of a Liquid-Crystal Electro-optic Device Doped with Nanoparticles[END_REF][START_REF] Lee | Effects of Carbon Nanosolids on the Electro-optical Properties of a Twisted Nematic Liquid-Crystal Host[END_REF][START_REF] Park | Homeotropic Alignment of Liquid Crystals on a Nano-Patterned Polyimide Surface Using Nanoimprint Lithography[END_REF][START_REF] Wang | Dependence of Pretilt Angle on Orientation and Conformation of Side Chain with Different Chemical Structure in Polyimide Film Surface[END_REF] . However, there also exist some unresolved issues, such as poor interactions with LCs, aggregation-induced light leakage, unstable LC alignment against heat, light and moisture, hence culminating in poor device quality. In order to resolve these drawbacks, we propose a new noncontact alignment technique that combines photoalignment with selfaligning agents (SAAs).

As known, photoalignment is an important and attractive noncontact LC alignment method due to photo-patternable characteristics [START_REF] Kawatsuki | Photo-alignment of Low-Molecular Liquid Crystals on Photo-Crosslinkable Polymer Liquid Crystalline Film by Linearly Polarised UV Light[END_REF][START_REF] Ikeda | Photomodulation of Liquid Crystal Orientations for Photonic Applications[END_REF][START_REF] Sato | A Photoresponsive Liquid Crystal Based on (1cyclohexenyl)phenyldiazene as a Close Analogue of Azobenzene[END_REF][START_REF] Yaroshchuk | Photoalignment of Liquid Crystals: Basics and Current Trends[END_REF][START_REF] Yu | Photocontrollable Liquid-Crystalline Actuators[END_REF] , clean and simple process. Recently, polyimide-free homeotropic or homogeneous photoalignment systems have been realized using photopolymerizable monomer or mixed monomers upon suitable light irradiation [START_REF] Mizusaki | Fabrication of Vertically Aligned Liquid Crystal Cell Without Using a Conventional Alignment Layer[END_REF][START_REF] He | Polyimide-Free Homogeneous Photoalignment Induced by Polymerisable Liquid Crystal Containing Cinnamate Moiety[END_REF][START_REF] Mizusaki | Fabrication of Vertical Alignment Liquid Crystal Cell Without Forming a Conventional Polyimide-Alignment Layer[END_REF] .

Photopolymerization of monomers upon linearly polarized light irradiation results in uniform homogeneous alignment of LCs. The resulting alignment is quite stable with improved electrooptical properties. In addition, it only requires low processing temperature below 200 ℃, making it compatible for flexible LCDs. To realize the use of photoalignment process in combination with SAAs systems, photochromic materials (for instance, the azo-dyes) will be quite suitable due to reversible cooperative conformational changes of azobenzene upon light irradiation of different wavelengths [START_REF] Park | Functional Supramolecular Assemblies Derived from Dendritic Building Blocks[END_REF][START_REF] Kosa | Light-Induced Liquid Crystallinity[END_REF][START_REF] Tanaka | Thermal and Photoinduced Liquid Crystalline Phase Transitions with a Rod-Disc Alternative Change in the Molecular Shape[END_REF][START_REF] Wang | Photoresponsive Behaviors of Smectic Liquid Crystals Tuned by an Azobenzene Chromophore[END_REF][START_REF] Wu | Reversible Phase Transition and Rapid Switching of Azobenzene-Doped Cholesteric Liquid Crystals with a Single Laser[END_REF][START_REF] Wie | Torsional Mechanical Responses in Azobenzene Functionalized Liquid Crystalline Polymer Networks[END_REF] . Azo-dye molecules in combination with the amphiphilic or hydrophilic molecules can self-assemble on the ITO interfaces, which can further tune the anchoring properties or boundary conditions of the interfaces for specific orientation of LCs with light irradiation [START_REF] Kundu | In situ Homeotropic Alignment of Nematic Liquid Crystals Based on Photoisomerization of Azo-Dye, Physical Adsorption of Aggregates, and Consequent Topographical Modification[END_REF][START_REF] Yoon | Photopolymerization of Reactive Amphiphiles: Automatic and Robust Vertical Alignment Layers of Liquid Crystals with a Strong Surface Anchoring Energy[END_REF] . Furthermore, the doping of reactive mesogen (RM) molecules in the system along with azo-dyes has been reported to enhance alignment stabilization upon UV irradiation, resulting in irreversible orientation of LCs [START_REF] Pozhidaev | Photoaligned Ferroelectric Liquid Crystal Passive Matrix Display with Memorized Gray Scale[END_REF][START_REF] Yaroshchuk | Stabilization of Liquid Crystal Photoaligning Layers by Reactive Mesogens[END_REF][START_REF] Guo | Polymer and Azo-Dye Composite: a Photo-Alignment Layer for Liquid Crystals[END_REF] .

In this work, we report highly stable, pretilted homeotropic LC alignment in a nematic, negative LC host doped with SAA molecules, azo dyes and RM additives via an in-situ self-assembled, dual-wavelength photoalignment approach. Our approach combines two different noncontact LC alignment methods (self-assembly and photoalignment) with two major processes. In the first process, the amphiphilic molecules interact with ITO surface, spontaneously resulting in homeotropic LC alignment. In the second process, the photoresponsive azo-dyes sensitive to visible light and UV-polymerizable RM molecules are utilized to perform dual-wavelength photoalignment with the purpose to create a small pretilt angle and stabilize alignment, respectively. A uniform pretilt angle of 0.7 has been achieved at the end of dual-wavelength irradiation process and the obtained alignment is quite stable against heat and light treatment. Such a noncontact, simple homeotropic alignment technique is very promising for applications including high-quality LCDs, spatial light modulators, and other optoelectronic devices.

DESIGN PRINCIPLE

Figure 1 schematically describes the design principle of our proposed in-situ self-assembled, dual-wavelength photoalignment that combines self-assembly and dual-wavelength photoalignment. First, in-situ homeotropic LC alignment occurs due to random distribution of SAAs and azo-dye molecules adsorbed at the ITO surface upon the infiltration of all the wellmixed materials, as shown in Figure 1a. In our design, we purposely choose a photochromic azo dye that is sensitive to the visible light. Upon linearly polarized visible light (LPVL) irradiation, the azo dye can reorient the LC molecules due to its dichroic absorption and subsequent trans-cis isomerization. Continuous irradiation of LPVL increases the population of azo-dye molecules falling under perpendicular direction of light polarization, which makes it less likely to go under transition for isomerization and results in the average orientation of azo-dye molecules perpendicular to the polarization direction of the light. As a result, upon LPVL irradiation, the isotropic layer of adsorbed azo dyes on the ITO surface becomes anisotropic in nature and induces LC alignment perpendicular to the light polarization direction. Figure 1b shows the oblique LPVL (λ1) treatment to induce LCs' photoalignment. For simplicity, a composite layer (labelled by the light green color) of SAA and azo dye molecules is formed due to interfacial adsorption on the ITO surface, which further results in self-assembly-induced homeotropic LC alignment, as shown in Figure 1b. Separately, random distributions of azo-dye molecules at the surfaces are represented by yellow distorted circular dots in a box shown below Figure 1b. LPVL treatment is carried out using a tilted light source at 65° with respect to the substrate normal.

Upon LPVL treatment, a small pretilt angle will be created from the normal orientation of LC director, as shown in Figure 1c. The observed LC director alignment is perpendicular to the polarization direction of light as confirmed by using quarter wave plates. The corresponding azodye surface becomes anisotropic in nature as represented by azo-dye in yellow color elongated eclipses aligned along one direction due to LPVL treatment (see the box below Figure 1c), which is responsible for the pretilt angle generation. RM molecules uniformly distributed in the LC bulk remains unperturbed at the end of LPVL photoalignment process due to the screening of UV-light by using a bandpass filter. At this stage, the alignment is reversible in nature, meaning a nonstable alignment state. The stabilization of the alignment can be achieved by RM polymerization via UV-light irradiation. The LPVL and UV-light treatment processes are therefore referred as dual-wavelength photoalignment. The RM polymerization is initiated by a photoinitiator used in trace amounts, upon unpolarized UV-light (λ2) irradiation at room temperature [START_REF] Yaroshchuk | Stabilization of Liquid Crystal Photoaligning Layers by Reactive Mesogens[END_REF][START_REF] Guo | Polymer and Azo-Dye Composite: a Photo-Alignment Layer for Liquid Crystals[END_REF][START_REF] Glushchenko | Memory Effect in Filled Nematic Liquid Crystals[END_REF][START_REF] Kumar | Dual Wavelength in situ Photoalignment for Stable Planar Alignment of Nematic Liquid Crystals[END_REF] . Stabilization of LC alignment by RM polymerization results in the formation of a thin polymer layer on top of the composite layer of SAA and azo-dye molecules, as shown in Figure 1d. The RM stabilization layer seizes the freedom of azo-dyes reorientation capability influenced by reversible photoisomerization process [START_REF] Kumar | Dual Wavelength in situ Photoalignment for Stable Planar Alignment of Nematic Liquid Crystals[END_REF][START_REF] Nasrollahi | Polyimide-Free Planar Alignment of Nematic Liquid Crystals: Sequential Interfacial Modifications through Dual-Wavelength in Situ Photoalignment[END_REF] . Here onwards, the new RM layer commands the LC alignment and endures irreversible LC orientation along specific direction.

The formed RM layer over the composite layer of additives is represented in light brown color, as shown below Figure 1d. Therefore, our proposed in-situ self-assembled photoalignment approach paves a new way to control the processes of LC alignment and stabilization, separately, and prevents the inevitable intermeddling when only a single wavelength of light is used.

EXPERIMENTAL SECTION

Materials.

A nematic LC, HNG715611-00 (Jiangsu Hecheng Advanced Materials Co.) with negative dielectric anisotropy was used as the host LC in our experiments. Amphiphilic molecules, 4'-octylbiphenyl-4-carboxylic acid (OBA) (MedChemExpress) and lauryl gallate (LG) (Sigma Aldrich), were used as the SAAs for in situ self-assembled LC photoalignment.

Visible-light-sensitive azo-dye methyl red (MR) (Sigma Aldrich) and UV-polymerizable reactive mesogen RM257 (Jiangsu Hecheng Advanced Materials Co.) were used as photoalignment and stabilizing agents of LC alignment. Trace amount of photoinitiator irgacure 651 (Jiangsu Hecheng Advanced Materials Co.) was doped in the system in order to initiate the photopolymerization of RM257. All the chemicals involved in this system were used as received without any further modifications or purifications. One-bottle approach of this experimental system consists of 0.1 wt% of LG or OBA, 0.2 wt% of MR and 0.1 wt% of RM257 with trace amount of irgacure 651 in the LC host and were homogeneously mixed by stirring at the isotropic temperature for 15 mins. The details of LC host and entities involved in the experiment were shown in Figure 2 with their chemical structures. The UV-Vis absorption spectrum of MR was presented in Figure S1 in the supporting information. MR constituents extended -conjugation along with electron donating and withdrawing functional groups, which shift absorption band in the visible light spectrum range [START_REF] Liu | Optically Switchable Gratings Based on Azo-Dye-Doped, Polymer-Dispersed Liquid Crystals[END_REF][START_REF] Liu | All-Optical Modulation of Localized Surface Plasmon Coupling in a Hybrid System Composed of Photo-Switchable Gratings and Au Nanodisk Arrays[END_REF][START_REF] Liu | Azo-Dye-Doped Absorbing Photonic Crystals with Purely Imaginary Refractive Index Contrast and All-Optically Switchable Diffraction Properties[END_REF] . The MR absorption ranges from 390 to 550 nm, showing significant visible light absorption that is purposely suitable for our designed dual-wavelength in-situ photoalignment approach. The photochromic properties of the azo-dye under the influence of LPVL irradiation were also examined using the absorption spectra of azo-dye coated film on the ITO substrate before and after photoalignment (see Figure S2). ). The LC cell was switched under cross-polarizer and applied voltage of square wave mode with a frequency of 1 KHz using a power amplifier (PZD350A, Trek). The response time was measured using an oscilloscope (DPO2024, Tektronix).

RESULTS AND DISCUSSION

From our designed idea, once the LC cell was loaded with the mixture at a temperature above the clearing point (TNI) and then slowly cooled down to the room temperature, homeotropic alignment was spontaneously formed inside the LC cell. The hydrophilic ends of the SAA molecules, acting as an anchoring unit, interact with the ITO surface, while the SAA molecules' flexible alkyl chains possess affinity to the LC bulk and orientate perpendicular to the surface, further influencing the overall LC alignment. The POM and macroscopic images in Figure 3a illustrate the pristine homeotropic alignment of LCs before dual-wavelength treatment. Figure 3a1 shows the observed POM images under two crossed polarizers at 0 sample stage rotation with no applied voltage, illustrating a complete dark state. The top right inset in Figure 3a1 shows the conoscopic image, confirming the homeotropic LC alignment. Figure 3a2 and 3a3 shows the POM images at 0 and 45 sample stage rotations with applied voltage of 5 V. A nonuniform bright field can be clearly observed from both POM images, indicating random distribution of LC directors upon electrical switching, respectively. At the initial homeotropic state, all the LC molecules are aligned perpendicularly to the substrate without pretilt angle. Upon applying a driving voltage, the LC molecules will re-orientate randomly without any preferred and uniform direction, hence resulting in an observed nonuniform bright field. Figure 3b shows the case after the LPVL treatment. Figure 3b1 demonstrates similar observed phenomena as Figure 3a1, confirming that the homeotropic alignment was still kept after the LPVL treatment. However, in contrast to Figure 3a2 and 3a3, Figure 3b2 and 3b3 demonstrates completely dark and uniform bright POM images for the LPVL-irradiated circular area under the same driving voltage of 5 V, indicating the creation of a pretilt angle. Only with a pretilt angle does uniform, unidirectional re-orientation of all LC molecules occur upon switching. The generated pretilt angle was measured to be 0.5 upon LPVL treatment with the exposure dose of 10.8 J/cm 2 (6 mW/cm 2 for 30 mins). A black tape mask with a circular opening hole was used to obtain the LPVL photoalignment effect, illustrating clearly distinguishable aligned and nonaligned areas, as shown in the bottom right insets in Figure 3b2 and 3b3. The achieved monodomain LC alignment in the circular region confirms uniform pretilt orientation of LCs in the self-assembled homeotropic cell. The obtained LPVL photoalignment effect is reversible in nature and can be easily erased even at room temperature in several hours or overnight due to the thermal relaxation of the azodye molecules. A well aligned cell after LPVL photoalignment got randomized after keeping it for overnight at ambient environment, as shown in Figure S2 in the supporting information. For practical applications, stabilization of LC alignment is the essential prerequisite criteria.

Therefore, RM polymerization was further carried out under UV-light irradiation in order to stabilize the LC alignment. Figure 3c shows the case after the UV-light treatment. Again, Figure 3c1 demonstrates similar observed phenomena as Figure 3a1 and 3b1, confirming that there is no alignment change after both LPVL and UV-light treatment. Correspondingly, Figure 3c2 and3c3 shows similar behaviors to Figure 3b2 and 3b3. The RM monomers were polymerized using unpolarized UV light with the exposure dose of 40.5 J/cm 2 (15 mW/cm 2 for 45 mins) at room temperature, essentially higher than that used for alignment process to ensure the complete polymerization quickly and minimize any alignment deformation. During the stabilization process, a thin polymer layer was formed over the composite layer of the SAA and azo-dye additives, which further modified the interfaces and rendered irreversible, permanent alignment of LCs. A large-area, uniform monodomain pretilted homeotropic alignment can be finally achieved, as shown in the bottom right insets in Figure 3c. Stability of LC alignment against extreme heat and light exposure were further tested, respectively. Heat exposure at 110 ℃ for several hours slowly degrades the LC alignment starting from the boundaries near the spacers as shown in Figure S3. Gradually, LC alignment over the large area was lost; it might be caused by the freed azo-dye molecules, which diffuse through the thin polymerized RM layer from the ITO/LC interface, gradually goes into the bulk LC, and damages the uniform alignment during the heating process. However, this testing temperature is far above the clearing point of the widely used LCs. In contrast, unpolarized UV-light treatment with uniform intensity at 30 mW/cm 2 for 2 hours at room temperature did not significantly affect the LC alignment, and stability was intact over the large area as shown in Figure S4. As a result, the efficient stable homeotropic LC alignment was successfully demonstrated using our proposed in-situ self-assembled photoalignment approach. The room temperature stability of obtained LC alignment was permanent with the increased pretilt angle of 0.7 from 0.5 after RM stabilization. The polymer stabilization layer is also known to reduce transient disclinations or defect formation and hence exhibits enhanced switching behavior for LC devices. A pretilt angle will define a well determined direction for the LC reorientation once the electric field is applied and hence it can improve the threshold voltage and response time for the device [START_REF] Kim | Stabilization of the Liquid Crystal Director in the Patterned Vertical Alignment Mode Though Formation of Pretilt Angle by Reactive Mesogen[END_REF] . However, a large pretilt angle can lead to a light leakage at the dark state. The light leakage will then lower the contrast ratio of the device. Therefore, homeotropic LC alignment with a small pretilt angle is highly desired, which can not only improve electro-optic characteristics but also keep a high contrast ratio of a device. It is worth mentioning that the pretilt angle could be further optimized by varying LPVL irradiation dose and RM concentrations. To further confirm the formation of the stabilization layer and understand the minute morphological details, we carried out the FESEM observation of the ITO substrate surface. The

(c 1 ) (c 2 ) (c 3 ) (a 1 ) (a 2 ) (a 3 ) 0 V 0 V 0 V 5 V 5 V 5 V 5 V 5 V 5 V
observed surface morphologies for the top and bottom ITO substrates were almost same. Figure 4 shows the surface morphologies of the top substrate observed under FESEM for the samples with the RM257 concentrations of 0.05 wt% and 0.1 wt%, respectively. From Figure 4, a thin RM stabilization layer can be clearly observed over the ITO substrates. For the case of low RM257 concentration in Figure 4a1 and 4a2, there are some isolated protrusions like RM aggregates decorated over the stabilization layer. The RM aggregates have an averaged particle size of 200-400 nm. The random distribution of RM aggregates corresponds to the morphological evidence of a factor responsible for homeotropic alignment as it breaks the boundary symmetry conditions of LCs. Morphological observations imply that the nature of alignment formation could be similar to SAMs or nanoparticle doping systems that exert homeotropic alignment of LCs [START_REF] Kim | Asymmetric Fullerene Nanosurfactant: Interface Engineering for Automatic Molecular Alignments[END_REF][START_REF] Yoon | Construction of Polymer-Stabilized Automatic Multidomain Vertical Molecular Alignment Layers with Pretilt Angles by Photopolymerizing Dendritic Monomers under Electric Fields[END_REF] . The low RM257 concentration case could not provide much information regarding the anisotropic nature of RM stabilization layer or LC director orientation.

In order to further examine the morphological details and LC director orientation, a sample with high RM257 concentration of 0.1 wt% was used for the alignment stabilization. Figure 4b1 and 4b2 shows the top and cross-sectional views of the top ITO substrate for the sample with the high RM257 concentration. The top view image of RM polymer layer in Figure 4b1 shows the orientation of polymer fibers or bundles running along one direction, which is perpendicular to the polarization direction of LPVL and concurrent to the LC director orientation. The crosssectional image in Figure 4b2 clearly displays the presence of vertically oriented RM polymer fibers closely packed together. The vertically tilted orientation of columnar RM polymer fibers was formed due to tilted homeotropic orientation of LC directors, which guided the polymer templating during polymerization along the vertical direction. The tilted vertically oriented polymer fibers support the experimental observations of alignment stabilization for in situ selfassembled homeotropic cell using dual-wavelength approach. Polymer layers formed at the ITO/LC interfaces significantly improves the anchoring strength and leads to stabilization of LC alignment. As reported [START_REF] Moon | Effects of Surface Modification with Reactive Mesogen on the Anchoring Strength of Liquid Crystals[END_REF][START_REF] Yoon | Photopolymerization of Reactive Amphiphiles: Automatic and Robust Vertical Alignment Layers of Liquid Crystals with a Strong Surface Anchoring Energy[END_REF] , during the RM polymerization process, polymerization-induced phase separation occurs, and the polymerized RM aggregates will migrate towards the interface and immobilized over the SAA layer, forming the stabilization layer for the irreversible homeotropic alignment. The formed RM stabilization layer increases the hardness and roughness of the alignment surface, hence increasing the surface anchoring energy of the alignment layer.

The passive nature of polymer layers makes it resistant to the damaging factors which can perturb the LC alignment conditions. It has been reported that such systems with RM-stabilized LC alignment can maintain the anchoring strength under the applied AC-field stress test for 24 hr, as it is a crucial factor to determine industrial applications of LC devices [START_REF] Nasrollahi | Polyimide-Free Planar Alignment of Nematic Liquid Crystals: Sequential Interfacial Modifications through Dual-Wavelength in Situ Photoalignment[END_REF] . If the anchoring strength is not strong enough, the relaxation process will become very slow. As reported [START_REF] Nasrollahi | Polyimide-Free Planar Alignment of Nematic Liquid Crystals: Sequential Interfacial Modifications through Dual-Wavelength in Situ Photoalignment[END_REF] the RM stabilization will be higher than that of the pristine cell without polymer stabilization [START_REF] Lim | Enhanced Contrast Ratio of Homogeneously Aligned Liquid Crystal Displays by Controlling the Surface-Anchoring Strength[END_REF] .

The increase in the anchoring energy of RM stabilization layer compared to the bare ITO layer can be attributed to the increased surface roughness [START_REF] Tie | Reducing Driving Voltage and Securing Electro-optic Reliability of In-plane Switching Liquid Crystal Display by Applying Polysulfone Photoalignment Layer with Photo-reactive Mesogens[END_REF] . From Figure 5, the significant change in the contact angle confirms the hydrophilic to hydrophobic transition of surface nature, which supports the evidence of pretilted homeotropic alignment stabilization by RM layer. The electro-optical properties of the pretilted homeotropic LC cell were finally tested. The uniformity of LC alignment was characterized by switching the cell with a driving voltage of 5 V and placing it under two crossed polarizers. By keeping the LC cell at fixed position while rotating either polarizer or analyzer, sinusoidal variation of darkness and brightness at the interval of 45° can be observed, as shown in Figure 6a, confirming the uniaxial tilted orientation of LC director. Figure 6b shows the voltage-dependent transmittance (V-T) curve of the cell, which shows the similar characteristics as conventional PI-based systems. The threshold voltage was ~2.4 V, which was lower than that of the PI based systems as reported previously [START_REF] Son | In situ Self-Assembled Homeotropic Alignment Layer for Fast-Switching Liquid Crystal Devices[END_REF][START_REF] Son | Self-assembled Vertical Alignment Layer of Bifunctional Amphiphiles for Fast Electro-optical Switching of Nematic Liquid Crystals[END_REF] . The transmittance increases as the applied voltage was increased from 0 to 3.5 V, and then gradually decreased to almost saturated state at 5 V. The gradual decrease in the transmittance after 3.5 V may be due to the large cell gap, which needs to be optimized in accordance with the d∆n parameter. Moreover, the voltage-dependent response time of the pretilted homeotropic cell was measured under 1 KHz AC-voltage with 3.5 Vpp amplitude. The measured rising and falling time from Figures 6c and6d was ~ 68 and ~ 38 ms, respectively. The response time for this device is expected to be faster than the PI-based devices since the LC molecules are in direct contact with the ITO surface and also contains predetermined director tilt with biased nature of alignment. A high contrast ratio of 593:1 for the device was achieved. These obtained results are in good agreement with the expectations and could be further optimized for specifically designed systems based on in-situ self-assembled homeotropic alignment.

The surface anchoring energy is an important parameter for a LC cell as it affects not only the LC alignment but also the electro-optic properties (for instance, threshold voltage and response time). Various experimental techniques have been developed for measuring the anchoring energy.

Here we used the recently suggested a reliable RV (retardation vs voltage) techniques [START_REF] Yokoyama | A Novel Method for Determining the Anchoring Energy Function at a Nematic Liquid Crystal-Wall Interface from Director Distributions at High Fields[END_REF]61 . Figure 7 plots the measured (R/R0-1)(V-V') as a function of voltage. The experiment was carried out with a He-Ne laser (λ = 633 nm) at room temperature. By fitting the slope of the linear curve, the anchoring strength is deduced between Vmin and Vmax. Here, Vmin = 9 V (~ 4Vth) and Vmax = 20 V have been used to calculate the anchoring energy of the 10 m cells with good linear fitting. The obtained anchoring strength for a homeotropic cell before light treatment and after RM stabilization were 4.41010 -5 J/m 2 and 1.41810 -4 J/m 2 , respectively. The change of nearly one order of magnitude in the anchoring energy shows that surface anchoring energy of RM stabilized cell is much stronger than the pristine one, indicating big potential for many applications, such as high-quality LCDs, spatial light modulators, and other optoelectronic devices.

CONCLUSION

In summary, we have successfully demonstrated highly stable homeotropic LC alignment with a small pretilt angle in a nematic, negative LC host doped with additives including self-aligning agents, photoresponsive azo dyes and reactive mesogen via an in-situ self-assembled, dualwavelength photoalignment approach. The underlying mechanism of stable, pretilted homeotropic LC alignment has been discussed in detail. Amphiphilic SAA molecules resulted in self-assembled homeotropic LC alignment that pave the way for polyimide-free LC alignment.

More importantly, the addition of photoresponsive azo-dye and RM molecules in the materials system took critical roles in subsequent interfacial modification using dual-wavelength approach to achieve in-situ pretitled photoalignment and stabilization, respectively.

Figure 1 .

 1 Figure 1. Schematic illustration of our proposed in-situ self-assembled, dual-wavelength photoalignment. (a) In-situ self-assembled pristine homeotropic cell prior to any light treatment that contains randomly distributed additives in the LC and ITO/LC interfaces after adsorption. Additives like SAA molecules, azo-dyes and RMs are represented by a blue color rectangular block containing red circular polar unit and black flexible alkyl chain, zigzag green molecules and brown circular dots, respectively. Elongated eclipses in purple color represent the LC molecules. (b) Oblique LPVL treatment of the azo dye with λ1 wavelength to create a pretilt angle in homeotropic cell. The below box illustrates the corresponding isotropic azo-dye surface. (c) Uniformly pretilted homeotropic LC alignment with anisotropic azo-dye composite layer at surface oriented perpendicular to the polarization direction of visible light. Corresponding azo-dye anisotropic surface due to LPVL treatment and photoreorientation boxed below which possess reversible nature and can be randomized to initial stage upon thermal relaxation or light exposure. (d) Alignment stabilization by RM polymerization using UV light with λ2 wavelength in nematic phase of LCs, RMlayer forms thin polymer coating (orange color) that controls the alignment. The anisotropic azo-dye surface becomes irreversible in nature since a polymer layer is formed on top of the composite layer due to UV-light induced RM polymerization and further seizes their reorientation property of the azo dye molecules, as shown in the

Figure 2 .

 2 Figure 2. Details of liquid crystal and chemical additives used in the system as one-bottle experimental approach.

  HNG715600-100 (∆ε = -12.2, ∆n = 0.153) Cr -30 C N 88 C I

  It is clearly visible that the absorption spectrum exhibits a considerable decrease near 470 nm and increase near 380 nm, reflecting the occurrence of a trans-cis isomerization process. The isomerization process upon LPVL irradiation significantly decreases the population density of the cis-form molecules and simultaneously increases the population density of the trans-form molecules. The change of the absorption spectra indicates the formation of an anisotropic azo-dye layer, which could further induce the LC alignment.LC Cell Fabrications.Commercially available ITO-coated conductive glass substrates were used in our experiments. They were cleaned by sonication in a detergent solution, deionized water, acetone and 2-isopropanol (IPA) in sequence at least twice and then dried in the oven at 80 ℃ overnight. Two pieces of the dried ITO glass, without any coatings, were assembled to a LC cell with the optical adhesive NOA65. The cell gap was controlled to be 10 µm using the ball spacer. The homogeneous host-guest mixture was injected in the cells by the capillary action at the isotropic temperature of 110 ℃ and then slowly cooled down to the room temperature. Homeotropic alignment of LCs was spontaneously achieved upon completion of the coolingdown process. Various material recipes of different ratios and concentrations have been tested in order to achieve the optimal alignment condition.In-Situ Alignment Control using Photoalignment. The in-situ self-assembled homeotropic LC cells were further subjected to LPVL treatment at isotropic temperature. Visible light illumination of the cell was carried out using high pressure 200 W Hg lamp (OmniCure Series 2000) as a light source and Instec mK2000 temperature controller with hot stage to maintain elevated temperature of 110 ℃. The collimated LPVL irradiation was carried out with an oblique incident angle of 65° with respect to the cell normal. An optical bandpass filter (400-700 nm, Thorlabs) was used to ensure the only visible light irradiation of the cell for photoalignment. A mask with a circular hole was used to demonstrate uniform monodomain pretilt alignment for VA cell at the end of photoalignment step was shown in the later section. The obtained pretilt angle of homeotropic cell was measured using modified LC rotation method with an optical setup assembled in the laboratory and using the instrument PAMS 200 (Sesim Photonics Technology, S. Korea).Stabilization of Tilted Homeotropic Alignment.As the idea of dual-wavelength photoalignment suggests, after the completion of in-situ photoalignment by visible light, the second-step irradiation with UV light at room temperature was performed to stabilize the LC alignment.Upon UV light irradiation, photopolymerization of RM molecules took place and a stabilization layer was formed on top of the composite layer of azo-dye and SAA molecules. The UV light illumination was done by UV-LED point light source (LOTS, UVEC-4II) in order to initiate the RM polymerization and polymer stabilization layer formation to attain irreversible alignment of LCs. The stabilization of tilted homeotropic alignment was achieved upon completion of RM polymerization using UV light exposure significantly cures the RM molecules. The UV light source with the wavelength range of 350-380 nm was used for RM polymerization.Characterization of the Polymer Stabilization Layer.The achieved homeotropic cells were kept in a co-solvent system of hexane and chloroform for 24 hours to remove the LCs, unreacted monomers and other additives. Cells were dismantled using sharp edge blades, top and bottom ITO substrates were separated for FESEM and other characterizations. The surface morphologies were observed under field emission scanning electron microscope (FESEM; GeminiSEM 300-71-10, Zeiss) in order to analyze the RM stabilization layers. The contact angle was measured with a contact angle detector (Powereach JC2000-99B, Shanghai Zhongchen Digital Technology Apparatus Co.) that helps in obtaining the information regarding the wettability nature of the RM polymer layer.Optical characterizations.The absorption spectrum for methyl red azo-dye ~ 10 -5 M solution in ethanol was acquired using a UV-Vis spectrophotometer (Jasco, ARSN-733). Polarized optical microscope (POM) was used for LC alignment and its texture monitoring. POM instrument (Nikon Eclipse, Ci POL) was equipped with a Nikon DS-Fi2 CCD camera for optical image acquisition. The electro-optical properties were measured using a 633-nm He-Ne laser (Research Electro-Optics Inc.), a photodetector (PM100A, Thorlabs), a function generator (DG4102, Rigol). The voltage-response time optical properties were measured using a LC electro-optical complex tester equipped with a white light source (ZKY-LCDE0-2, Chengdu Shijizhongke Equipment Co.

Figure 3 .

 3 Figure 3. Polarized optical images of a pristine (a1-a3), LPVL-treated (b1-b3), UV-treated (c1-c3) homeotropic cell at 0 sample stage rotation with no driving voltage (a1, b1, and c1), at 0 sample stage rotation with the driving voltage of 5 V (a2, b2, and c2), and at 45 sample stage rotation with the driving voltage of 5 V (a3, b3, and c3). The top right insets in 3a1 and 3b1 show the conoscopic image. All the bottom right insets illustrate the complete sample photos observed under two crossed polarizers. The scale bar at the bottom left corner in each image is 100 m.

  , the AC-field stress test for an IPS cell without RM stabilization showed defect creation and weak anchoring strength of LCs at the surface; in contrast, the same test with RM stabilization accomplished fast and complete relaxation of LCs, confirming the sufficiently strong anchoring induced by the RM layer. The RM stabilization layer therefore acts as a new alignment layer with both strong anchoring strength and irreversible nature as required for most of the LCD devices.

Figure 4 .

 4 Figure 4. Low-(a1) and high-magnification (a2) cross-sectional SEM images illustrating the surface morphologies of the sample with the RM257 concentration of 0.05 wt%, which were observed with the sample stage tilted at 10. Top (b1) and cross-sectional (b2) SEM images for the sample with the RM257 concentration of 0.1 wt%.

  -stabilization surfaces with low and high RM257 concentrations, respectively. The contact angle increases with the polymerization of RM at the ITO/air interface because it decreases the hydrophilicity of the ITO surface due to polymer coating. Increasing RM concentration further reduces the hydrophilicity and tends to be hydrophobic in nature. The modified nature of the coated surface renders ideal support for the homeotropic alignment. In other words, the anchoring energy of bare ITO surface was significantly changed from enabling homogeneous orientation of LCs to homeotropic. It is expected that the anchoring energy after

Figure 5 .Figure 6 .

 56 Figure 5. Measured contact angles for the bare ITO surface (a), RM-stabilization surfaces with low (b) and high (c) RM257 concentrations, respectively. Water with the volume of 1 mL was used as the solvent for the contact angle measurement.

Figure 7 .

 7 Figure 7. Measured anchoring energy for the homeotropic cell before (pristine) and after RM stabilization with the pretilt angle from the voltage-dependent RV curves.

TOC In-situ self-assembled homeotropic alignment RM-stabilized pretilted homeotropic alignment Dual-wavelength photoalignment Dark state Bright state Dark state Bright state Schematic illustration Experimental results

  The achieved pretitled homeotropic LC cells are very stable against heat and UV light exposure and demonstrate excellent electro-optical characteristics. Our proposed in-situ self-assembled, dual-wavelength photoalignment technique is simple and efficient, which could be the game changer in terms of production cost, processing time and complexity compared to the mechanical rubbing. Such a noncontact, simple homeotropic alignment technique has big potential in many applications, such as high-quality LCDs, spatial light modulators, and other optoelectronic devices. The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/xxxxxxx. UV-Vis absorption spectrum of the azo dye solution, and polarized photographs of the LC cells for stability test.NotesThe authors declare no competing financial interest. 61 Nie, X.; Lin, Y.-H.; Wu, T. X.; Wang, H.; Ge, Z.; Wu, S. T. Polar Anchoring Energy Measurement of Vertically Aligned Liquid-Crystal Cells. J. Appl. Phys. 2005, 98, 013516.

	ASSOCIATED CONTENT
	Supporting Information.

ACKNOWLEDGMENTS

This work was supported in part by Natural Science Foundation of Guangdong Province (Grant No. 2017A030313034 and 2018A030310224), Shenzhen Science and Technology Innovation Commission (Grant No. JCYJ20170817111349280, JCYJ20180305180635082, and GJHZ20180928155207206), Open Fund of State Key Laboratory of Applied Optics (Grant No. SKLAO-201904), and Guangdong Innovative and Entrepreneurial Research Team Program