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Pramita Mondal, [Jonathan P. Hill](https://orcid.org/0000-0002-4229-5842), Gouranga Manna, Sharmistha De Dalui, Edward A. Neal, Gary. J. Richards, [Katsuhiko Ariga](https://orcid.org/0000-0002-2445-2955), Yusuke Yamauchi, [Lok Kumar Shrestha](https://orcid.org/0000-0003-2680-6291), Somobrata Acharya

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[Self‐Assembled 2D Sheets of an Amphiphilic Sexiphenyl Exhibit Intense Polarized Blue Emission in the Solid State](https://mdr.nims.go.jp/datasets/218451aa-806b-4052-9b5d-84a2fbfb3101)

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Self‐Assembled 2D Sheets of an Amphiphilic Sexiphenyl Exhibit Intense Polarized Blue Emission in the Solid StateRESEARCH ARTICLEwww.advopticalmat.deSelf-Assembled 2D Sheets of an Amphiphilic SexiphenylExhibit Intense Polarized Blue Emission in the Solid StatePramita Mondal, Jonathan P. Hill,* Gouranga Manna, Sharmistha De Dalui,Edward A. Neal, Gary. J. Richards, Katsuhiko Ariga, Yusuke Yamauchi,Lok Kumar Shrestha, and Somobrata Acharya*Distinct from inorganic 2D nanomaterials (e.g., graphene, MoS2, etc,),self-assembled organic 2D materials are significant due to the uniqueadvantages of fine structure tunability and functionality. Electronically andphotofunctionally-active compounds (e.g., pentacene, p-sexiphenyl) are highlyattractive for applications but are limited by difficult handling and theexistence of only a few native structures of the compounds. Here, it is shownthat a p-sexiphenyl derivative incorporated into 2D multilamellar sheets withchromophores oriented in the layers exhibits a significant polarization ofphotoluminescence. Highly emissive, amphiphilic sexiphenyl withhydrophilic/hydrophobic groups self-assembles at air–water interface withchromophore orientation in the resulting films being controlled by monolayercompression. Large-area multilamellar structures with p-sexiphenylchromophores highly oriented against the 2D plane are prepared byconsecutive film transfers. The 2D sheets exhibit significant polarization ofphotoluminescence with polarization ratio 0.8 between orthogonal in-planeaxes. Notably, multilamellar structures cannot be established for the samep-sexiphenyl using thermotropic processing thus emphasizing the importanceof the 2D sheet multilayer synthesis process. Amphiphile design can beapplied for the tailored synthesis of large-area multilayered 2D sheets andused to construct highly efficient light-emitting devices.P. Mondal, S. De Dalui, S. AcharyaSchool of Applied and Interdisciplinary SciencesIndian Association for the Cultivation of ScienceJadavpur, Kolkata, West Bengal 700032, IndiaE-mail: camsa2@iacs.res.inJ. P. Hill, E. A. Neal, K. Ariga, L. K. ShresthaResearch Center for Materials NanoarchitectonicsNational Institute for Materials ScienceNamiki 1-1, Tsukuba, Ibaraki 305-0044, JapanE-mail: jonathan.hill@nims.go.jpThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adom.202400177© 2024 The Authors. Advanced Optical Materials published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution-NonCommercial-NoDerivs License,which permits use and distribution in any medium, provided the originalwork is properly cited, the use is non-commercial and no modificationsor adaptations are made.DOI: 10.1002/adom.2024001771. IntroductionAtomic scale ultrathin 2D nanosheets, suchas graphene,[1] hexagonal boron nitride,[2]or MXenes,[3] have drawn significant inter-est owing to their exceptional electronic,optical, thermal, and mechanical propertieswith strongly anisotropic features for appli-cations in flexible optoelectronics, catalysis,bioimaging, and energy storage.[4–8] De-spite the growing interest in this field,the synthesis of 2D assemblies, especiallybulk multilayer or freestanding materials,remains a challenging task. Notwith-standing the molecular phenomenon ofaggregation-induced emission (AIE),[9]solid-state organic 2D nanosheets do notusually exhibit intense fluorescence due toself-quenching[10] or aggregation-inducedquenching.[11] However, few-layered or-ganic 2D nanosheets exhibiting highphotoluminescence (PL) quantum yieldsoffer several advantages in certain organicoptoelectronic applications.[3,12] Similarlyto the case of inorganic 2D nanosheets,an important criterion to be fulfilled forG. MannaSurface Physics and Materials Science DivisionSaha Institute of Nuclear Physics1/AF Bidhan Nagar, Kolkata 700064, IndiaG. J. RichardsDepartment of Applied ChemistryGraduate School of Engineering and ScienceShibaura Institute of TechnologyFukasaku 307, Minuma-ku, Saitama-shi, Saitama 337-8570, JapanK. ArigaDepartment of Advanced Materials ScienceGraduate School of Frontier SciencesThe University of Tokyo5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, JapanY. YamauchiDepartment of Materials Process Engineering, Graduate School ofEngineeringNagoya UniversityNagoya 464-8603, JapanY. YamauchiDepartment of Chemical and Biomolecular EngineeringYonsei University50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South KoreaAdv. Optical Mater. 2024, 12, 2400177 2400177 (1 of 8) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbHhttp://www.advopticalmat.demailto:camsa2@iacs.res.inmailto:jonathan.hill@nims.go.jphttps://doi.org/10.1002/adom.202400177http://creativecommons.org/licenses/by-nc-nd/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadom.202400177&domain=pdf&date_stamp=2024-03-14www.advancedsciencenews.com www.advopticalmat.dethe applications of organic self-assembled 2D and 3D systemsis a high degree of polarization anisotropy in order to establishsuperior optical switching and display-device performance.[13,14]In particular, long-range orientational order of organic moleculescan promote control over the polarization anisotropy. For ex-ample, controlling long-range molecular order determines intra-assembly excitation delocalization and relaxation pathways, andis central to the design of artificial light-harvesting systems usingsynthetic molecular assemblies.[15,16] For 𝜋-electronic systems,the molecular orientation of the chromophores is usually per-pendicular to the long axis of, for instance, 1D fibers. This isessentially due to spontaneous 𝜋-stacking of the molecules usu-ally enforced by van der Waals interactions.[15,17] While molecularorientational control of excited state properties of spontaneouslyoriented supramolecular 𝜋-conjugated systems is a challengingtask, the precise tuning of molecular alignment can be achievedin organic 2D sheets using self-assembly at the air–water inter-face by controlling the surface pressure in Langmuir–Blodgett(L–B) technique, and by applying molecular design principles toaffect the orientation of molecules aggregated under interfacialconditions.The compound p-sexiphenyl (1,1′:4′,1′′:4′′,1′′′:4′′′,1′′′′:4′′′′,1′′′′′-sexiphenyl) was first prepared around a century agoand, despite subsequent difficulties with its synthesis andpurification,[18] it is regarded as a promising semiconductorand optoelectronic material due to its excellent thermal stabilityand high PL quantum yield in the solid state.[19,20] Sexiphenylexhibits a wide bandgap,[21] strong blue PL[22] and large holemobility,[21–23] and can be obtained in thin film form makingit suitable for applications in organic transistors[22] and otheroptical device architectures. Polarized electroluminescence hasbeen stimulated from an oriented p-sexiphenyl film,[24] whilefull-color applications are also achievable by converting the deepblue light generated by p-sexiphenyl into green and red usingthe proper dye layers and filters.[25,26] Based on the reportedproperties of its films, its extended linear molecular morphology,which suggests linear polarization activity, and a perceivedsynthetic flexibility, we have selected p-sexiphenyl as a chro-mophore for the design and assembly as 2D sheets using the LBmethod.In this work, we introduce an amphiphilic sexiphenyl deriva-tive ASP-1 (see Figure 1) designed specifically for self-assemblyat the air–water interface. Interestingly, the resulting multilayer2D sheets exhibit intense blue luminescence, an unusual featurefor planar chromophores.[27,28] The interfacial self-assembly pro-Y. YamauchiAustralian Institute for Bioengineering and Nanotechnology (AIBN)The University of QueenslandBrisbane, QLD 4072, AustraliaL. K. ShresthaDepartment of Materials Science, Faculty of Pure and Applied SciencesUniversity of Tsukuba1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, JapanS. AcharyaTechnical Research Center (TRC)Indian Association for the Cultivation of ScienceJadavpur, Kolkata 700032, Indiacess of formation of the 2D sheets was monitored using atomicforce microscopy (AFM) performed at different stages of the as-sembly process. The multilayer of the 2D sheets of the sexiphenylderivatives form a lamellar structure based on the alternation oflayering of the amphiphile. Sexiphenyl molecules are stronglyaligned within the 2D sheets as a consequence of the deposi-tion process and the extended molecular morphology. This wasconfirmed by using polarized PL spectroscopy. The 2D sheetsexhibit large polarization anisotropy again indicating the strongalignment of ASP-1 molecules with orientation perpendicularto the long axes of the 2D sheets. Thus, the molecular designconcept in combination with the interfacial technique at the wa-ter surface can be considered a highly promising method forthe design and construction of polarized organic luminescent2D sheets.[29,30]2. Results and Discussion2.1. SynthesisSexiphenyl derivative ASP-1 was synthesized by a mixed Suzukicoupling reaction of two dendronized biphenyl derivatives (3,4,5-tris(n-dodecyloxybenzyl/ (3,4,5-tris(2-(2-(2-methoxyethoxy)ethox-y)ethoxy)benzyl dendrons; see Supporting Information for de-tails) with 4,4′-biphenylboronic acid followed by separation of thethree main products using column chromatography, gel perme-ation chromatography and a final “recrystallization” step fromtetrahydrofuran (THF) where ASP-1 was dissolved in hot THFand a gel monolith allowed to form by cooling at 5 °C. Subse-quent decantation of non-gelated THF or cold filtration led tohighly pure ASP-1, which is amphiphilic based on its hydropho-bic/hydrophilic substituents. Importantly, hydrophilic oligoethy-lene glycol units are essentially dissolved in the water subphasewhen spread at an air–water interface[31] while the hydropho-bic alkoxy-substituted group is repelled away from the watersubphase. This provides a strong basis for the assembly of themolecules especially where a lateral force can be applied to ma-nipulate the molecular spacing in situ. The p-sexiphenyl unit isalso highly hydrophobic and will reside away from the water sub-phase with inter-chromophore interactions (𝜋–𝜋, C─H…𝜋) pro-moting the formation of ordered layered structures. Figure 1ashows the chemical structure of ASP-1 and a graphical repre-sentation of the molecule, where the long rigid sexiphenyl chro-mophore is appended at its opposing extremities with dendron-like substituents (green cone = hydrophilic; red cone = hy-drophobic) attached through flexible benzyloxy linkages (indi-cated by balls). Although coplanarity of the phenyl rings is an-ticipated based on X-ray crystallographic measurements,[32] themolecules may be non-coplanar since they are known to undergolibrational motion in particular when not constrained by a crystallattice or at an interface. The compound is highly fluorescent inthe solution and solid states (Figures 1b,c, resp.), forms a gel inTHF (Figures 1d–g), forms self-assembled fibers under precipi-tation from solution (Figure 1h), and also forms a liquid crystalmesophase at elevated temperature (Figure 1i) the latter being as-signed as a lamellar tetragonal structure similar to that observedfor other dendron-rod-type molecules.[33]Adv. Optical Mater. 2024, 12, 2400177 2400177 (2 of 8) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 18, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202400177 by National Institute For, Wiley Online Library on [12/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 1. Structure and characteristics of ASP-1. a) Chemical structure and graphical representation of the sexiphenyl amphiphile ASP-1 used in thiswork. b) Solution of ASP-1 in CDCl3 (4 mg mL−1) under irradiation using 365 nm ultraviolet light eliciting intense white-blue fluorescence. c) Fluorescenceof ASP-1 in the solid state under irradiation with 365 nm UV lamp. d) Photograph of a solution of ASP-1 (2 mg mL−1 in tetrahydrofuran). e) Photographof the gel formed on cooling the solution in (d) for 2 h at 5 °C. f) Gel state of ASP-1 (10 mg mL−1 in tetrahydrofuran) formed during purification. g)Scanning electron microscopy (SEM) image of the xerogel formed from the 10 mg mL−1 gel state. h) Fibres formed by diffusing methanol into a toluenesolution of ASP-1. i) Fern-like optical texture (cross polarizers at 90°) of a solid sample of ASP-1 at 220 °C after cooling from an isotropic melt indicatesa tetragonal mesophase structure. (g) Schematic illustrating the limiting molecular area (A = 120 Å2) and its origin. (h) Method for preparation ofmulti-layered film by repetitive immersion and withdrawal of a mica substrate through the compressed monolayer of ASP-1 at the air–water interface.2.2. Monolayer Formation and StructureThe surface pressure (𝜋) versus area/molecule (A) isotherm(Figure S1a, Supporting Information, red curve) shows a phasetransition from a 2D liquid-expanded phase at low surface pres-sure to liquid condensed phase, which collapses at ≈65 mN m−1.A limiting molecular area of ≈120 Å2 per molecule was extractedfrom the 𝜋−A isotherm curve, which is substantially lower thanthe area expected for a single ASP-1 molecule calculated using aminimum energy conformation (≈1210 Å2; see also Figure S2,Supporting Information). The calculated area per molecule issimilar to those reported for molecules also substituted with hy-drophilic dendron-like substituents and is a characteristic of theinteraction of such dendrons with the subphase where moleculesstand orthogonal to the subphase surface leading to low valuesof limiting molecular area.[34] We have carried out surface poten-tial measurements of ASP-1 derivatives at the air–water interfaceduring the uniaxial compression process (Figure S1a, SupportingInformation, blue curve). An optimal dipole moment is expectedfor the parallel arrangement of ASP-1 derivatives molecules. Thesurface potential (ΔV) versus area (A) isotherm does not show achange at the initial stage of compression, which is larger thanthe limiting area per molecule observed in the 𝜋–A isothermcurve. The 𝜋–A isotherm reflects van der Waals interactions be-tween the hydrocarbon chains and 𝜋–𝜋 interactions of the ASP-1derivatives, while theΔV–A isotherm is dominated by long-rangedipole-dipole interactions.[35] A rapid compression-induced in-crease of the effective molecular dipole moment is observed athigher surface pressure suggesting a preferred orientation of theASP-1 dipole moment perpendicular to the lifting direction ofthe LB films. We have investigated the film stability by perform-ing successive compression-expansion isotherm cycles below thecollapse pressure as shown in Figure S1a (Supporting Informa-tion) blue curve.The dependence of film dimensions and morphology of theself-assembled monolayers of ASP-1 was observed by liftingthe monolayer at different surface pressures selected accord-ing to the 𝜋-A isotherm. Atomic force microscopy (AFM) im-ages (Figure 2) reveal the formation of narrow interconnectedfiber-like structures at low surface pressures (5 mN m−1; seeFigures 2a,g). With increasing surface pressure, the fibrous struc-tures gradually merge eventually yielding a homogeneous 2Dsheet structure at a surface pressure of 35 mN m−1 (Figures 2e,k).Figure 2 also shows AFM images of the monolayer lifted at 10mN m−1 (Figure 2b,h), 15 mN m−1 (Figure 2c,i), and 25 mN m−1(Figure 2d,j) revealing a progressive shrinking and elimination ofvoid spaces in the monolayer by lateral merging of fibers. Smallvoids remain in the monolayer film at surface pressure 𝜋 = 25mN m−1 but these are absent in the uniform film obtained at 35mN m−1 (Figure 2e,k). AFM height profile measurements revealAdv. Optical Mater. 2024, 12, 2400177 2400177 (3 of 8) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 18, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202400177 by National Institute For, Wiley Online Library on [12/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 2. Atomic force microscopy (tapping mode, AFM tip (Si) radius: <10 nm) images of ASP-1 at different surface pressures. a) 5 mN m−1 b) 10 mNm−1, c) 15 mN m−1, d) 25 mN m−1, e) 35 mN m−1, f) dropcast film (10−6 m). Corresponding higher magnification AFM images are shown in g) 5 mNm−1, h) 10 mN m−1, i) 15 mN m−1, j) 25 mN m−1, k) 35 mN m−1 and l) dropcast films (10−6 m). Height profile analyses for AFM images (indicated byblue lines/arrows): m) 5 mN m−1, n) 10 mN m−1, o) 15 mN m−1, p) 25 mN m−1 and q) dropcast film (10−6 m). Data indicates monolayer height ≈3.5 ±0.5 nm at all surface pressures (also in the drop-cast film) although measured heights of soft materials are generally underestimated by this method[36]and are also affected by water layer on mica substrate.[37] Films shown in (e) and (k) were smooth with no measurable nanometric features.a uniform average height of ≈3.5 ± 0.5 nm at all surface pres-sures (Figures 2m–p). The constant height of the films indicatesthe consolidation of the fibers into a final 2D sheet of similarthickness. Interestingly, drop-casting of a chloroform solution ofASP-1 onto mica yields a nanofiber network (Figure 2f,l) simi-lar to that observed at low surface pressures and the gel fibers ofASP-1 formed in THF (see Figure 1). However, 2D sheet couldnot be formed by simple drop-casting indicating that processingof ASP-1 at the air−water interface is important for their synthe-sis.2.3. Optical Properties of ASP-1 SheetElectronic absorption (UV–vis) spectra of monolayer LB films de-posited at various surface pressures and a UV–vis spectrum ofASP-1 in chloroform solution (10−6 m) are shown in Figure 3a.In solution, ASP-1 exhibits a broad peak at 320 nm assigned tothe 𝜋→𝜋∗ electronic transition of the sexiphenyl chromophore.In monolayer films, the corresponding absorption peak is red-shifted by ≈20 nm and a weak additional absorbance appears at390 nm. These minor changes indicate J-type aggregation causedby specific molecular processes at the air–water interface. Theabsorbance of monolayer films increases as the surface pressureincreases without variation of the peak position indicating an in-creasing density of chromophore molecules per area of the liftedfilms, and that there is no variation in intermolecular interactionswithin the monolayers. PL spectra of the same ASP-1 monolayersand solution are shown in Figure 3b. In solution, the spectrumcontains two overlapping peaks at ≈388, 405 nm with a shoulderat ≈432 nm (Figure 3b). The spectra of the monolayer LB filmsFigure 3. Photophysical properties and structure of ASP-1 in solution andfilm states. a) Electronic absorption (UV–vis) spectra of ASP-1 in chloro-form solution (10−6 m) and in monolayer ASP-1 films lifted at the surfacepressures shown. The ≈20 nm red shift in 𝜆max LB films suggests aggre-gation of the chromophores at air–water interface. b) Photoluminescencespectra of ASP-1 in chloroform solution (10−6 m) and monolayer filmslifted at the surface pressures shown. c) UV–vis absorption spectra of LBmultilayer ASP-1 films deposited at 25 mN m−1. Layer number is given. d)Photoluminescence spectra of LB multilayer ASP-1 films with increasinglayer number. Inset: photographs of LB films under 365 nm UV illumina-tion. Layer number shown, deposited at 25 mN m−1.Adv. Optical Mater. 2024, 12, 2400177 2400177 (4 of 8) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 18, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202400177 by National Institute For, Wiley Online Library on [12/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deof ASP-1 contain three better-resolved peaks at 407, 431, and 454with a shoulder at 490 nm (Figure 3b). PL peaks of the mono-layers can be assigned to contributions from vibronic transitionsat 3.11 eV (0–0), 2. 93 eV (0–1) (dominant), 2.76 eV (0–2), and aweaker feature around 2.58 eV (0–3).[19,38] UV–vis absorption andPL spectra of the multilayered LB films lifted at 25 mN m−1 areshown in Figure 3c,d, respectively. UV−vis absorption spectra ofmultilayer films contain a broad peak at ≈340 nm (same as for themonolayers), which hardly shifts with increasing layer number.Absorbance of the LB films increases uniformly with increasinglayer number due to the increasing thickness of the multilayerfilm (Figure 3c). PL spectra of multilayer LB films of ASP-1 con-tain peaks at 407, 431, 454, and 490 nm with a layer-number-coupled increase in intensity (Figure 3d). A lack of any substan-tial variation in absorbance and PL peaks indicate that the struc-ture of the films persists in the Langmuir monolayers and upontransfer as multilayer LB films.[39] Multilayer films of 2D sheetsdeposited on a quartz substrate exhibit strong blue emission un-der 365 nm UV illumination (see photograph inset of Figure 3d,also Figure 1b,c).ASP-1 exhibits a PL quantum yield (PLQY) of ≈55% (Table S1,Supporting Information) in solution. PL quantum yields of amonolayer, LB-formed 2D sheets, and drop-cast film are lowerprobably due to non-radiative dissipation caused by the close 𝜋–𝜋approach of the sexiphenyl moieties. LB-formed 2D sheets liftedat different surface pressures of 10 and 25 mN m−1 have PL quan-tum yields of 12.79% and 40.51%, respectively. This enhance-ment of PL quantum yield with increasing surface pressure in-dicates that PL is quenched in the small aggregates of ASP-1formed at low pressure at the air−water interface but is enhancedat higher surface pressure during 2D sheet formation. Drop-castfilm has PL quantum yield (16.1%) similar to that of a monolayerfilm deposited at 10 mN m−1, which is not surprising based ontheir similar coverages and morphologies (see Figure 2). Fluo-rescence microscopy of a 3-layer film lifted at a surface pressureof 25 mN m−1 (Figure S3, Supporting Information) indicates in-creasing optical response with increasing layer number as is alsoobvious from the PL emission from multilayer films shown inthe inset of Figure 3d.2.4. Structure of Multilayer ASP-1 SheetBulk orientation of chromophores contained in self-assembledstructures is an important parameter in any potential applica-tions of organic self-assembled materials.[40,41] This aspect hasbeen probed here for ASP-1 using linear dichroism (LD) spec-troscopy on films obtained during the fibers to 2D sheet trans-formation to monitor the intrinsic dipole moment alignmentin the molecular assembly. The LD spectrum of ASP-1 film(Figure 4a) contains an intense negative peak at 337 nm with aweaker negative shoulder (associated with J-type aggregation) at389 nm. The negative LD signal indicates a greater perpendicu-lar (A⊥) than parallel (A∥) absorption of the incident polarizedlight. LD intensity increases upon transformation from fibers to2D sheets (Figure 4b) due both to increased chromophore den-sity and higher alignment of the chromophores. In this case, theelongated nature of the ASP-1 amphiphile results in alignmentof the chromophores within the film upon compression at theFigure 4. Linear dichroism (LD) of 59-layered ASP-1 LB films lifted at dif-ferent surface pressures. a) LD and UV−vis spectra of ASP-1 2D sheetsof multilayered films. Significant negative LD signal indicating greaterperpendicular (A⊥) than parallel (A∥) absorption of polarized light. b)Surface pressure dependency of LD. c) Compressed monolayer of ASP-1molecules with SP long axes parallel to compressing barriers (responsiblefor the negative LD response).water surface. Since the substrate for LB deposition is orientedparallel with the barrier, films of ASP-1 lifted from the air–waterinterface have their component molecules aligned perpendicu-lar to the direction of substrate dipping. Moreover, the molec-ular dipole moment (estimated to be 2.45 D using DFT calcu-lations at the B3LYP/6-31G** level; see Figure S2b, SupportingInformation) coincides approximately with the long axis of theASP-1 molecules so that the transition dipole moment (TDM)should be similarly oriented.[42,43] Increasing LD intensity withincreasing surface pressure while peak position is maintained atthe same wavelength suggests a compression-induced orienta-tion of the dipole moments in the 2D sheets. These factors ac-count for the large increasingly negative response of ASP-1 inthe 2D sheets where chromophores are aligned perpendicular tothe sheet long axis (established by the substrate dipping directionduring LB multilayer fabrication).Having established the alignment of the molecules within the2D sheet based on limiting molecular area (Figure 5a), dippingdirection (Figure 5b) and LD spectroscopy (Figure 4), grazing in-cidence X-ray diffraction (GIXD) of a multilayer LB film was car-ried out using synchrotron radiation in order to further define thestructure of ASP-1 2D sheets (Figure 5c). An alternating lamellartype structure of the film is implied based on the LB approachfor multilayer films (here n = 50 layers) since hydrophilic andhydrophobic moieties will be segregated in the resulting multi-layered nanostructure (Y-type film structure[44,45]). In this case,Adv. Optical Mater. 2024, 12, 2400177 2400177 (5 of 8) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 18, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202400177 by National Institute For, Wiley Online Library on [12/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 5. Structure of multilayered ASP-1 LB films. a) Schematic illustrat-ing the limiting molecular area (A = 120 Å2) and its origin. b) Method forpreparation of multi-layered film by repetitive immersion and withdrawalof a mica substrate through the compressed monolayer of ASP-1 at theair–water interface. c) Synchrotron GIXD scattering pattern of a 50-layer LBfilm lifted at 𝜋 = 20 mN m−1 Major reflections indicate a lamellar structure.The [001] reflection is obscured beyond the beamstop. d) Bilayer structureof the ASP-1 LB film assembly established based on amphiphilic ASP-1in the LB method. Important regions of the ‘Y‘-type layered structure areindicated.the compound has the form dendron(A)-rod-dendron(B) (by anal-ogy with dendron-rod-coil structures[46]) where sexiphenyl is the“rod”. The alternating lamellar structure of the multilayer filmand its dimensions give rise to the observed GIXD reflections asshown in Figure 5c. It is a lamellar structure based on a 9.2 nmthick bilayer. A lamellar structure is also supported by transmis-sion electron microscopy (TEM) observations (Figure S4, Sup-porting Information). Based on the characteristics of ASP-1 2Dsheets, a model of the nanostructured film is shown in Figure 5d.It is interesting to compare this lamellar phase with the struc-ture of ASP-1 in its bulk solid state obtained under thermotropicconditions (i.e., in the absence of an air–water interface). Inthat case, although a peak appears at a similar position to themonolayer films at 4.51 nm in the scattering pattern of as-isolated ASP-1 (possibly suggesting that the material has a sim-ilar alternating lamellar structure), that pattern also presents apeak having a base dimension of 6.1 nm following cooling of asample of ASP-1 from an isotropic state (≈270 °C) (Figure S5,Supporting Information). That is, while some compounds areknown to form thermotropic bilayer structures based on molecu-lar amphiphilicity,[47] ASP-1 does not. The obvious difference instructure of ASP-1 under thermotropic conditions highlights theimportance of using the air–water interface to obtain uniquelynanostructured materials. In this case, it is also important to notethat the LB method is performed under ambient conditions whilethermotropic processing of ASP-1 to an isotropic state involveselevated temperatures and inevitably risks decomposition of thesubject molecules.2.5. PL Polarization in ASP-1 FilmsBased on the molecular orientation of ASP-1 within multilay-ered 2D sheets (Figure 5d), we performed polarization-angle-dependent photoluminescence spectroscopy. The sheet exhibitspolarized PL with greatest emission intensity at ≈0° and 180°,and weakest emission at angles ≈90° and 270°. This indicatesthat polarization of the ASP-1 emission at 407, 431, and 454 nmis strongly directed parallel to the 2D sheets, with a variation in in-tensity dependent on the polarizer angle (see Figure 6a).[15] Start-ing at 0°, emission intensity is gradually attenuated as the po-larizers are rotated to a vertical position (90°) where it reachesa minimum, as shown in Figure 6b. PL intensity then graduallyincreases returning to the starting value by rotation of the po-larizers to 180°. According to polarized PL spectra,[48] the mul-tilayer 2D sheet of ASP-1 molecules exhibits extremely stronganisotropy. The intensity ratio, Ir = (I∥ − I⊥)/ (I∥+ I⊥), versuspolarization angle, was used to determine the polarization ra-tio (Ir), which was found to be ≈0.8. (Figure 6c). This high po-larization ratio indicates a well-oriented and uniformly orderedstructure of the 1D ASP-1 molecules contained in the LB films sothis 2D sheet has excellent potential as a polarized luminescentmaterial.[14,48,49] A schematic of the PL polarization measurementis shown in Figure 6d, and polarization of the 2D sheets was ver-ified using an optical microscope equipped with cross-polarizers(Figure 6e,f). The 2D sheets appear bright under parallel polar-ization conditions but are dark under cross-polarization.Large values of Ir have been reported for other materials usu-ally associated with a highly oriented internal structure. In ourprevious work,[13,15,16] we have obtained similar values of polar-ization from materials with structures based on liquid crystalalignment of nanomaterials or self-assembled fibers (0.8 – 0.9).Other workers have also reported useful levels of PL polarizationratio as high as 0.95 in polymers,[50,51] self-assembled organicnanomaterials[52,53], and in solid state nanomaterials.[54,55] Thisdemonstrates that ASP-1 is competitive with state-of-the-art ma-terials in terms of its PL polarization ratio characteristics, but ithas additional benefits such as ease of preparation and flexibil-ity of the synthesis concept. That is, it is a simple matter to varythe active element (here sexiphenyl) to obtain polarized emis-sion in a wide variety of organic chromophores in turn allowingcoupling of this property with other salient characteristics. Forexample, sexiphenyl ASP-1 also exhibits semiconductivity withdiode-like behavior in its 2D sheet form (see Figure S6, Support-ing Information) as well as an easy first reduction at −1.3 V (seeFigure S7, Supporting Information). These additional propertiesimply that the presented treatment of chromophores for forma-tion of 2D sheet might be applied for incorporation of these ma-terials in electronic devices aimed at diverse uses including sens-ing, switching, or data storage. Our current research is aimed atdeveloping such multifunctional materials systems for advancedoptoelectronic applications.3. ConclusionThe amphiphilic linear sexiphenyl derivative ASP-1 assemblesduring compression at air–water interface forming uniform large2D sheets whose film packing properties improve with increas-ing applied surface pressure. ASP-1 exhibits large PL quantumAdv. Optical Mater. 2024, 12, 2400177 2400177 (6 of 8) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 18, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202400177 by National Institute For, Wiley Online Library on [12/11/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 6. Film structure and polarization of photoluminescence (PL) in ASP-1 LB multilayer films. a–c) ASP-1 molecules are forced with long axesparallel to film but perpendicular to substrate (mica) dipping direction. Blue arrows in c indicate approximate molecular dipole axis. d) Film viewedfrom top showing relative orientation of molecular long axes (and bulk dipole) against dipping direction. e) Geometry of measurement of fluorescencepolarization and representative fluorescence images. f) Dependence of the PL spectra of an ASP-1 LB film as a function of the polarizer rotation anglein a multilayer film lifted at a surface pressure of 25 mN m−1. g) Variation in PL intensity at 454 nm of ASP-1 LB film as a function of polarizer rotationangle from 0° to 270°. h) Angle-resolved polarization (Ir) of the PL spectra.yields in solution and when contained in multilayer film. Inter-estingly, multilayered films of ASP-1 obtained using the layer-by-layer (LbL) method form a lamellar Y-type structure as demon-strated by GIXD analysis using synchrotron radiation. Orienta-tion of the molecules in the films probed using LD spectroscopyreveals that sexiphenyl chromophores are aligned perpendicu-lar to the film lifting direction. Based on the molecular align-ment, multilayer LB film lifted at higher surface pressure exhibitsstrong fluorescence anisotropy with PL polarized parallel to the2D sheet lifting direction, as indicated by polarized PL spectra.The large value of the polarization ratio clearly establishes thehigh degree of molecular alignment of ASP-1 molecules withinthe 2D sheets obtained by using the LbL method at the air–waterinterface, which is a much easier-to-implement and better con-trollable method than substrate rubbing combined with vacuumdeposition techniques used previously.[24] The ease of film prepa-ration and its optical properties described here make ASP-1 LB-LbL film an excellent candidate as a polarized luminescent mate-rial for applications including OLEDs, display devices, and opticalinformation storage.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work was partly supported by World Premier International Re-search Center Initiative (WPI Initiative), MEXT, Japan. The authors aregrateful to JST-ERATO Yamauchi Materials Space-Tectonics Project (JPM-JER2003) and the Queensland Node of the Australian National Fabrica-tion Facility (ANFF-Q). S.A. acknowledges SERB grant SERB-STAR grant#STR/2020/000053. The authors acknowledge Technical Research Centre(TRC) of IACS for experimental support. Photon Factory, KEK, Tsukuba,Japan is gratefully acknowledged for the GIXD measurements using syn-chrotron radiation.[Correction added on 25th April, 2024 after online publication: Afillia-tions of Dr. Yusuke Yamauchi, Dr. Lok Kumar Shrestha and Dr. SomobrataAcharya was updated in this version].Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available in the sup-plementary material of this article.Keywords2D nanosheets, amphiphile, polarized emission, self-assembly, sexiphenylAdv. Optical Mater. 2024, 12, 2400177 2400177 (7 of 8) © 2024 The Authors. 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Optical Properties of ASP-1 Sheet 2.4. Structure of Multilayer ASP-1 Sheet 2.5. PL Polarization in ASP-1 Films 3. Conclusion Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords