Glassy Liquid Crystalline Cyclic and Monodisperse p-conjugated Oligomers:

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1 Glassy Liquid Crystalline Cyclic and Monodisperse p-conjugated Oligomers: Self-Organized Solid Films for Optoelectronics Shaw H Chen University of Rochester 6 February

2 Liquid Crystals Identified with Optical Textures Nematic Cholesteric Smectic Columnar I. Dierking, Textures of Liquid Crystals, Wiley-VCH Verlag: Weinheim, Glassy Liquid Crystals, GLCs Crystals: positional order in a three-dimensional lattice Liquid Crystals: orientational order, or positional order in 1- or 2-D GLCs: liquid crystals frozen into solid state without crystallization All liquids expected to form glass at a sufficiently rapid cooling rate Most organics, e.g. liquid crystals, will crystallize on cooling from melt GLCs as an approach to self-organized solid films across a large area 2

3 C&E News, 17 January 1994: Cover Story on Advanced Energetic Materials 3

4 First-Generation Core-and-Pendant Glassy Liquid Crystals Cyclohexane core: Bicyclooctene core: COOR 1 COOR1 ROOC ROOC COOR COOR R 1 : R 2 : COOR 2 (CH 2 ) 2 O COO OCH 3 (CH 2 ) 2 O COO * CONHCH CH G 69 o C Ch 137 o C I 3 R: (CH 2 ) 3 O G 84 o C N 222 o C I CN Adamantane core: COOR Cubane core: ROOC COOR ROOC COOR ROOC R: (CH 2 ) 2 O COO R: ROOC CH 2 CH 3 (CH 2 ) 2 N N COOR G 86 o C S A 236 o C I G 95 o C S A 156 o C I N NO 2 R: (CH 2 ) 2 O C C G 77 o C N 190 o C I CN R: (CH 2 ) 4 O C G 50 o C N 126 o C I C CN Adv. Mater. 1996, 8,

5 High-Temperature Nematic GLCs OR ROOC COOR ( 12 ) G 82 o C N 347 o C I ROOC COOR COO COOR ROOC COOR ROOC O OOC COO NO 2 COOR ROOC ( 13 ) G 86 o C N 288 o C I ( 14 ) G 127 o C N 308 o C I COOR R : (CH 2 ) 3 O CN Chem. Mater. 2001, 15,

6 DSC Thermograms of Single-Component Liquid Crystals Conventional LC T m T i Unstable GLC T g T c T m T i Stable GLC T g T i Temperature 6

7 Morphological Stability of Liquid-Crystal and Amorphous Glasses Quantified by Maximum Crystallization Velocity, MCV T c,max /T m =0.93 ± 0.01 CV=0 at T m for all T c,max /T m =0.92 ± 0.03; T g /T m 2/3 (II) and (III) at 6 nm/s, comparable to slowly crystallizing isotactic polystyrene Top to bottom: 6.3 x 10 8 to 1.6 x 10 4 nm/s Shi and Chen, Liq. Cryst., 1995, 12, Naito and Miura, J. Phys. Chem. 1993, 97, ( II ) ( III ) Thermally processed GLC films have remained intact left at 25 o C for 22 years and counting 7

8 Linear and Circular Polarization of Light Wire-grid linear polarizer allows incident waves with electric fields perpendicular to wires to pass through without attenuation. The vertical components of waves with other orientations will be allowed as well. A linear polarizer and a quarter wave plate combine to form a circular polarizer, converting an unpolarized incident to circularly polarized light with handedness determined by the nature of quarter wave plate. 8

9 Selective Wavelength Reflection by a Left-Handed Cholesteric Film One Dimensional Photonic Band-Gap Unpolarized Incident Light Unpolarized Light, l l R Left-Handed Circularly Polarized Light, l=l R Right-Handed Circularly Polarized Light, l=l R Pitch Length Cholesteric film as a helical stack of quasinematic layers Three parameters governing optical properties: helical sense, pitch length, p, and average refractive index, n avg ; selective reflection wavelength, l R =p n avg Unpolarized light with l=l R on a left-handed film: left-handed circularly polarized component reflected, right-handed transmitted Complete reflection of unpolarized light with l=l R by a stack of left- and righthanded films with otherwise identical properties Incident light with l l R and any polarization state, transmitted through left- or right-handed film 9

10 Examples of Reflective Coloration in Nature 10

11 Cholesteric Glassy Liquid Crystals C O O ( C H 2 ) 2 O C O O * C O N H C H C H 3 N C O O C O ( C H 2 ) 3 O O C C O O ( C H 2 ) 3 O C O O C N G 85 o C Ch 216 o C I NC OOC O * OOC COO COO * * O O COO COO CN CN G 73 o C Ch 295 o C I Enantiomeric chiral-nematic GLCs yield opposite handedness and selective reflection in UV-region Chem. Mater. 2003, 15, Chem. Mater. 2008, 20,

12 Statistical Synthesis of Cholesteric GLCs HOOC HOOC Nm 1 -OH : Ch 1 -OH = 2:1 COOH Nm 1 OOC DEADC / PPh 3 Nm 1 OOC COONm 1 + Nm 1 OOC ( 19, 20% ) Nm 1 OOC ( 20, 31% ) COOCh 1 Ch 1 : (CH 2 ) 2 O COO * CONHCH CH 3 Nm 1 : (CH 2 ) 3 O COO OCH 3 Chem. Mater. 1999, 11,

13 Deterministic Synthesis of Cholesteric GLCs - I HOOC HOOC COOH O (1) (CH 3 CO) 2 O (2) SOCl 2 53 % O O COCl Nm 2 OOC Nm 2 -OH DMAP/Et 3 N 52 % HOOC COONm 2 Nm 2 : Ch 1 : (CH 2 ) 3 O (CH 2 ) 2 O COO CN S or R * CONHCH CH 3 Nm 2 -OH / DEADC / PPh 3 Ch 1 -OH / DEADC / PPh 3 52 % Nm 2 OOC Nm 2 OOC COONm 2 COONm 2 Nm 2 OOC Ch 1 OOC ( 21 ) G 63 o C N 183 o C I ( 22-S or R ) G 75 o C I Adv. Mater. 2000, 12,

14 Deterministic Synthesis of Cholesteric GLCs II and III Chem. Mater. 2003, 15,

15 Deterministic Synthesis of Cholesteric GLCs - IV COOH Ch 2 -OH COOCh 2 DEADC, PPh 3 HOOC COOH Ch 2 OOC COOCh 2 ( 26 ) G 73 o C Ch 295 o C I Ch 2 : * O COO CN Chem. Mater. 2008, 20,

16 Highly Circularly Polarized Fluorescence from a Chiral-Nematic GLC Film Doped with a Laser Dye g e = 2(I L I R )/(I L +I R ), g e 2 Nature 1999, 397,

17 Transmittance, % Reflectivity, % Tunable Reflection of Unpolarized Incident Light Using Mixtures of Enantiometric Chiral-Nematic GLCs at Varying Compositions mm 4 mm 8 mm / 0 76 / / Wavelength, nm 17

18 Transmittance, % Transmittance, % Circular Polarizers and Notch Filters with Chiral-Nematic GLCs a) RCP b) LCP LCP RCP Wavelength, nm Nearly 100% circular polarization Notch filter with a contrast ratio > 5000:1 Spectral tunability by mixing enantiomeric chiral-nematic GLCs at varying ratios Adv. Mater. 2000, 12,

19 Broadband Reflectors via Racemization of a Chiral Dopant in a Chiral-Nematic GLC Host with 140 mw/cm 2 at 334 nm at 100 o C Molar ratio at 1.00:4.36:0.55 T g = 65, T c =156 o C; 22 mm film Adv. Mater. 1999, 11,

20 Motivation for Monodisperse p-conjugated Oligomers Conjugated polymers widely explored for photonics & electronics Distributed chain length and composition, kinks, bends Purification, processing, alignment can be quite challenging Monodisperse conjugated oligomers Structural uniformity, solubility, purity Ease of processing and characterization: understanding of structureproperty relationships, conducive to practical applications Glass transition? Crystallization? Liquid crystallinity? Synthesis? Glass transition is currently regarded as the deepest unsolved problem in solid state theory. K. F. Freed, Acct. Chem. Research, 2011, 44, Referring to glass formation in isotropic polymer fluids, let alone ordered fluids such as liquid crystals, be they small molecules or polymers 20

21 Hairy-Rod Approach to GLC Conjugated Oligomers Rigid rods: high melting point to obscure inherent liquid crystallinity Aliphatic pendants: meltability and solubility, film preparation by spincasting from solution F(Pr)5F(MB)2 Endothermic F(MB)5 G N I G N I G N G N N G G Chem. Mater , 542. F(Pr)5F(MB)2 I I F(MB)10F(EH)2 N F(MB)10F(EH)2 0 F(MB) o Temperature, C 400 Adv. Mater. 2003, 15,

22 Film Morphology and Polarized OLED Device Structure a 50 m m b Glassy-Amorphous 50 m m Polydomain Glassy-Nematic c Mg:Ag at 20:1 (200 nm) LiF (0.5 nm) TPBI (50 nm) Oligo(fluorene) (50 nm) Rubbed PEDOT:PSS (20 nm) ITO (50 nm) 50 m m Monodomain Glassy-Nematic 22

23 Absorbance Electroluminescence, a.u. Polarized Absorption and Emission of Monodomain Films on Rubbed PEDOT/PSS nm film F(MB)10F(EH)2 A S = 0.82 PL Photoluminescence, a.u nm film F(MB)10F(EH)2 EL Luminance, cd/m Applied Voltage, V A ^ PL ^ Wavelength, nm EL ^ Wavelength, nm Adv. Mater. 2003, 15,

24 Molecular Structures of Donors and Acceptors for Polarized OLEDs F(Pr)5F(MB)2 I II S S N N S III N N S IV Chem. Mater. 2003, 15,

25 Electroluminescence, normalized Linearly Polarized Fluorescent OLEDs via Förster Energy Transfer from Heptafluorene 1.0 a) EL I at 2.0 wt% II at 2.0 wt% III at 3.8 wt% 0.5 EL ^ Wavelength, nm 25

26 Electroluminescence, normalized Linearly Polarized White-Light Fluorescent OLEDs 1.0 b) IV at 0.5 wt% IV at 0.8 wt% IV at 1.0 wt% EL 0.5 EL^ Wavelength, nm Adv. Mater. 2004, 16,

27 Linearly Polarized Phosphorescent OLEDs Organic Electronics 2011, 12, 15-21; Ibid. 2014, 15,

28 28

29 Chem. Mater. 2005, 17,

30 Monodisperse Oligofluorenes with a Varying Degree of Pendant Chirality C-702 C-612 C-522 C-432 JACS, 2002, 124, 8338; Ibid. 2003, 125,

31 Right-Handed Helical Stacking of Chiral Oligofluorenes R 1 R 1 R 1 R 1 R 3 R 3 R 2 R 3 R 2 R 3 31

32 EL Intensity, normalized Circularly Polarized Fluorescent OLED Comprising a 70-nm-thick C-522 Film [turn-on voltage less than 5V; luminance yield, 0.94 cd/a at 20 ma/cm 2 ] 1.0 I L g e I R Wavelength, nm Wavelength, nm 32

33 Light Amplification by Stimulated Emission of Radiation Lasing Identified by Five Criteria Clear evidence of threshold in output energy as a function of pump energy with a greater slope above threshold than below Spatial and temporal coherence, highly directional, and sharply focused Significant spectral line narrowing, less than 1 to several nm Existence of laser cavity resonance, including mirrorless cavity presented by a cholesteric liquid crystal film Strong output beam regardless of polarization state 33

34 Apparatus for Characterization of GLC Lasers Reference detector Output detector l/2 l/4 wave-plate wave-plate Linear polarizer Focusing lens GLC laser Lens condenser Nd:YAG l = 532 nm, t =35 ps, Rep. Rate =10 Hz 34

35 Host and Guest Molecules for Fabrication of Lasers OF-r G 104 o C N 304 o C I Chem. Mater. 2003, 15, 4352; Adv. Mater. 2004, 16, 783. F(MB)5-N G 92 o C N 171 o C I F(MB)5-Ch G 91 o C Ch 173 o C I Chem. Mater. 2003, 15, 542. J. Am. Chem. Soc. 2002, 124,

36 Reflectance, % Fluorescence, Normalized Output, nj Robust Circularly Polarized Lasers S N N S S OF-r: G 104 o C N 304 o C I F(MB)5-N: G 92 o C N 171 o C I F(MB)5-Ch: G 91 o C Ch 173 o C I (a) Reflectance Fluorescence Lasing (b) =6.8 mj/cm 2 =1.3 % Wavelength, nm 100 μm Input, mj Appl. Phys. Lett. 2009, 94,

37 Reflectance, % Output, nj Cholesteric GLC Laser Chemical composition: 1.5 wt% OF-r in F(MB)5-Ch:F(MB)5-N at a 24.0:76.0 mass ratio (a) Reflectance Fluorescence Lasing Wavelength, nm Fluorescence, Normalized (b) =6.8 mj/cm 2 =1.3 % Input, mj 100 μm Green curve: Reflection spectrum Black curve: OF-r fluorescence spectrum from nematic GLC F(MB)5-N film Red curve: lasing peak at 635 nm with a pump fluence of 121 mj/cm 2 at 10 Hz Lasing output energy as a function of pump energy Monodomain character of the cholesteric GLC film verified with a polarizing optical micrograph included as the inset 37

38 Reflectance, % Output, nj Cholesteric Fluid LC Laser Chemical composition: 2.0 wt% OF-r in CB-15:ZLI at a 35.6:64.4 mass ratio (a) Reflectance Fluorescence Lasing Wavelength, nm Green curve: Reflection spectrum Black curve: OF-r fluorescence spectrum from nematic LC ZLI film Red curve: lasing peak at 658 nm with a pump energy of 30 mj/cm 2 at 10 Hz Fluorescence, Normalized (b) =7.0 mj/cm 2 =5.2 % Input, mj 100 μm Lasing output energy as a function of pump energy Monodomain character of the fluid CLC film verified with a polarizing optical micrograph included as the inset. 38

39 Output energy, nj Output energy, nj Temporal Stability of Lasing Output Cholesteric GLC Laser: OF-r at 2.0 wt% in F(MB)5-Ch:F(MB)5-N at 24.1:75.9 mass ratio Cholesteric fluid LC Laser: OF-r at 2.0 wt% in CB-15:ZLI at 35.6:64.4 mass ratio mj/cm mj/cm 2 72 mj/cm mj/cm 2 Decay rate increases with pump energy 186 mj/cm mj/cm Time, sec Heating via optical pumping Light-induced pitch dilation Laser-induced fluid flow Time, sec Disruption of fluid LC structure and orientational order of OF-r molecules; GLC films remained robust Appl. Phys. Lett. 2009, 94,

40 Cholesteric GLC Film with a Lateral Pitch Gradient Appl. Phys. Lett. 2011, 98, Thermally activated molecular diffusion across interface of 14-mm-thick film at 220 o C for 62 h before cooling to 25 o C F(MB)5-Ch:F(MB)5-N mixtures at 29.0:71.0 mass ratio doped with 2.5 wt% OF-r F(MB)5-Ch:F(MB)5-N mixtures at 20.0:80.0 mass ratio doped with 2.5 wt% OF-r Bands capable of lasing 400 μm Grandjean-Cano band Grandjean-Cano line 40

41 Lasing intensity, a.u. Reflectance, % Lasing intensity, a.u. Stop-Bands and Lasing Spectra in an Arbitrary Grandjean-Cano Band (a) (b) 75 (b) (a) (b x x x mm Wavelength, nm Each Grandjean-Cano band is characterized by a constant value of an apparent helical pitch length, as evidenced by o Three reflection spectra in (b) correspond to the three positions identified as X s in (a) o Three overlapping lasing peaks 41

42 Emission intensity, a.u. A Cholesteric GLC Film with Lateral Pitch Gradient Capable of Multiple Lasing Wavelengths on Demand within a Single Film Bands capable of lasing 400 μm Wavelength, nm 42

43 Slope efficiency, % Slope Efficiency and Lasing Threshold of Spatially Resolved Lasers 2.0 (a) Lasing wavelength, nm Lasing threshold, mj/cm (b) Lasing wavelength, nm Maximum slope efficiency at 1.5% superior to the best value of 0.5% reported to date for gradient-pitch cholesteric fluid LC lasers Observed thresholds, G, are the lowest of all gradient-pitch cholesteric fluid and glassy liquid crystal lasers reported to date The slope efficiency profile largely tracks fluorescence spectrum of OF-r 43

44 Conclusions Two approaches to morphologically stable GLCs with elevated phase transition temperatures, core-pendant and hairy-rod platforms Conformational multiplicity underpinning versatile molecular design concepts, resulting in self-organized solid films for robust optoelectronic devises Cholesteric and Nematic GLC films demonstrated for Circular polarizers, optical notch filters, polarized photoluminescence Polarized fluorescent and phosphorescent OLEDs Anisotropic Organic Field-Effect Transistors Robust solid-state lasers with spatial resolution, rendering multiple wavelengths on demand of a single film 44

45 Acknowledgments Drs. H. P. Chen, Y. Geng, D. Katsis, J. J. Ou, J. U. Wallace, A. Trajkovska, S. W. Culligan, A. C. Chen, C. Kim, L. Zeng, S. K. Wei, H. Shi, J. C. Mastrangelo, and B. M. Conger. We are grateful for the generous support spanning over a quarter century at the University of Rochester by the U.S. National Science Foundation, Department of Defense, Department of Energy, New York State Energy Research and Development Authority, the Japanese Ministry of International Trade and Industry, and Eastman Kodak Company. Additional funding was provided by the U.S. Department of Energy Office of Inertial Confinement Fusion under cooperative agreements with Laboratory for Laser Energetics 45