Highly Efficient Diels–Alder Crosslinkable Electro‐Optic Dendrimers for Electric‐Field Sensors

One of the most challenging tasks encountered in developing highly efficient electro‐optic (EO) devices is to find a material system that possesses all desirable properties such as large EO coefficients, good thermal and mechanical stability, and low optical loss. In order to meet this stringent requirement, we have developed a series of crosslinkable EO dendrimers using the standardized AJL8 ‐type chromophore as the center core and the furyl‐ and anthryl‐containing dendrons as the periphery. Upon adding a trismaleimide ( TMI ) dienophile, these dendrimers could be in‐situ crosslinked via the Diels–Alder cycloaddition and efficiently poled under a high electric field. Through this dynamic process, the spatially voided and π‐electron‐rich surrounding of the chromophore core changes into a dense and more aliphatic network, with the dipolar chromophore embedded and aligned inside. The resultant materials exhibit large EO coefficients (63–99 pm V^–1 at 1.31 μm), excellent temporal stability (the original r₃₃ values remain unchanged at 100 °C for more than 500 h), and blue‐shifted near‐IR absorption. With these combined desirable properties, a poled EOD2/TMI film could be processed through multiple lithographic and etching steps to fabricate a racetrack‐shaped micro‐ring resonator. By coupling this ring resonator with a side‐polished optical fiber, a novel broadband electric‐field sensor with high sensitivity of 100 mV m^–1 at 550 MHz was successfully demonstrated.     

Symmetric and Asymmetric Conjugated 3,3′‐Bipyridine Derivatives as a New Class of Third‐Order NLO Chromophores with an Enhanced Non‐resonant,Nonlinear Refractive Index in the Picosecond Range

We report on the synthesis and third‐order nonlinear optical (NLO) properties of new asymmetric (push–pull) and symmetric chromophores based on the 3,3′‐bipyridine core. The nonlinear refraction as well as the linear and nonlinear absorption of these compounds has been studied, in solution, by spectroscopy and picosecond single‐shot Z‐scan measurements. The results are very promising in terms of non‐resonant, nonlinear refractive index in the near infrared, particularly with enhancement of the (nonlinear efficiency/transparency) trade‐off afforded by the symmetrization of the chromophores. A new polymer with this structural design has also been investigated.     

Synthesis,Characterization, Two‐Photon Absorption,and Optical Limiting Properties of Ladder‐Type Oligo‐p‐phenylene‐Cored Chromophores

This paper reports on the two‐photon absorption (TPA) and related up‐converted emission properties of a novel series of chromophores containing ladder‐type oligo‐p‐phenylenes with various π‐conjugation lengths. The design and synthesis of these ladder‐type two‐photon chromophores are first discussed. An increase in the π‐conjugated length of the ladder‐type oligo‐p‐phenylene for these chromophores leads to an increase in TPA cross‐section together with an increased fluorescence quantum yield. These chromophores exhibit high fluorescence quantum yields because of the rigid planar structure of the ladder‐type oligomers. The chromophore with an enhanced TPA cross‐section together with an increased fluorescence quantum yield would provide significant benefits for two‐photon excited fluorescence based applications. An improved optical limiting behavior was also demonstrated using the ladder‐type pentaphenylene cored chromophore.     

Reversible Polarization Gratings on Thin Films of Polyoxetanes Bearing 4‐(N,N‐Diphenyl)amino‐4′‐nitroazobenzene Chromophores

Polarization gratings were fabricated by the two‐beam coupling method on the surface of a series of polyoxetanes containing 4‐(N,N‐diphenyl)amino‐4′‐nitroazobenzene pendants with the azobenzene chromophores either side‐on attached or end‐on attached through short or long spacers to the main chain. The dynamics of formation of the gratings or writings was studied in relation to the diffraction efficiency. The erasing behavior of the gratings was also examined, irradiating a linearly polarized single beam. The temporal stability of the diffraction efficiency was further studied after removing the coupled excitation beam. Both the mode of attachment of the chromophore to the main chain and the length of the side‐chain spacer greatly influenced the dynamic properties of the diffraction gratings. The presence of a long side‐chain spacer and side‐on attachment of the chromophore resulted in faster writing and erasing behavior. A preliminary study by solid‐state ¹³C NMR of the relaxational behavior of the main‐chain CH₂ groups and the pendant chromophores provided a reasonable picture to help explain the photo‐induced reorientation and relaxation of the azo chromophores at the molecular level.     

Designing Non‐Centrosymmetric Molecular Crystals: Optimal Packing May Be Just One Carbon Away

Molecular nonlinear optical (NLO) crystals feature important advantages compared to inorganic counterparts, such as low dielectric constants, ultrafast response times, and large electro‐optic coefficients. Conjugated push–pull chromophores connecting electron‐donating with accepting groups are often employed in the design of these crystals. However, associated large molecular dipole moments induce antiparallel or centrosymmetric conformations in the solid‐state, which leads to NLO inactivity. The cation–anion hydrogen bond interactions of a hydroxy‐piperidino electron donor group are combined with increased van der Waals volume effects induced by an ethyl modification of the electron‐accepting moiety. This produces non‐centrosymmetric packing in the organic salt EHPSI‐4NBS ((E)‐1‐ethyl‐2‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)‐3,3‐dimethyl‐3H‐indol‐1‐ium 4‐nitrobenzenesulfonate). Converting a methyl group into ethyl changes the packing symmetry in the molecular crystal to switch on NLO activity. This behavior is attributed to the increased size of the ethyl group, which pushes apart the van der Waals contacts of the cation that lead to centrosymmetric packing in the methyl derivative. To test the NLO properties of EHPSI‐4NBS, THz generation experiments are performed at 1200 nm pump wavelength. Spectral amplitude similar to DAST ((E)‐4‐(4‐(dimethylamino)styryl)‐1‐methylpyridin‐1‐ium tosylate) crystal is observed with generation profile from 0 to 3.8 THz.     

Quantum chemical calculations,synthesis and properties of novel organic molecules with photoinduced intramolecular electron transfer and large change in electric dipole moment in the excited state

The topic of this paper is bipolar organic compounds containing both charged electron donor and electron acceptor groups interconnected by various kinds of bridges (–D–X–A⁺). Such betaines are subject to photoinduced intramolecular electron transfer (PIET), large change in dipole moment in the excited state and considerable hyperpolarisability, which causes large non‐linear optical effects in solutions and Longmuir–Blodgett (LB) films. Quantum chemical calculations (CNDO/S, ZINDO/S) of some types of betaines containing a 1,3‐indandione anion or diazole anion electron donor part and an N‐pyridinium cation electron acceptor part show that the HOMO and LUMO are strongly localized and an effective PIET takes place. The calculated change in dipole moment in the excited state, δμ=μ_g−μ_ex , is substantial (8–20 D) and in many cases the direction of μ is reversed. The character of the bridge X in betaines shows the largest effect on μ_g and δμ, especially in the case of X=p‐phenylene. The pyrazole anion is a better electron donor than the 1,3‐indandione anion. The synthesis of betaines containing a 1,3‐indandione anion part of an N‐pyridinium cation part interconnected directly or through a p‐phenylene bridge has been achieved and their electron absorption spectra have been investigated. Preliminary experiments confirm NLO effects in solutions and LB films. The synthesis of novel substituted betaines is unlimited and the synthesis of polymer‐bonded or surface‐bonded betaines is possible. Copyright © 1999 John Wiley & Sons, Ltd.     

Highly Branched Phosphorescent Dendrimers for Efficient Solution‐Processed Organic Light‐Emitting Diodes

Intermolecular interactions play a crucial role in the performance of organic light‐emitting diodes (OLEDs). Here we report the photophysical and electroluminescence properties of a fac‐tris(2‐phenylpyridyl)iridium(III ) cored dendrimer in which highly branched biphenyl dendrons are used to control the intermolecular interactions. The presence of fluorene surface groups improves the solubility and enhances the efficiency of photoluminescence, especially in the solid state. The emission peak of the dendrimer is around 530 nm with a PL quantum yield of 76 % in solution and 25 % in a film. The photophysical properties of this dendrimer are compared with a similar dendrimer with the same structure but without the fluorene surface groups. Dendrimer LEDs (DLEDs) are prepared using each dendrimer as a phosphorescent emitter blended in a 4,4′‐bis(N‐carbazolyl)biphenyl host. Device performance is improved significantly by the incorporation of an electron‐transporting layer of 1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene. A peak external quantum efficiency of 10 % (38 Cd A^–1) for the dendrimer without surface groups and 13 % (49.8 Cd A^–1) for the dendrimer with fluorene surface groups is achieved in the bilayer LEDs.     

Long Distance Enhancement of Nonlinear Optical Properties Using Low Concentration of Plasmonic Nanostructures in Dye Doped Monolithic Sol–Gel Materials

Monolithic sol–gel silica composites incorporating platinum‐based chromophores and various types of gold nanoparticles (AuNPs) are prepared and polished to high optical quality. Their photophysical properties are investigated. The glass materials show well‐defined localized surface plasmon resonance (SPR) absorbance from the visible to NIR. No redshifts of the AuNP plasmon absorption peaks due to the increase in nanoparticle doping concentration are observed in the glasses, proving that no or very small SPR coupling effects occur between the AuNPs. At 600 nm excitation, but not at 532 nm, the AuNPs improve the nonlinear absorption performance of glasses codoped with 50 × 10^?3 m of a Pt‐acetylide chromophore. The glasses doped with lower concentrations of AuNPs (2–5 μm average distance) and 50 × 10^?3 m in chromophore, show a marked improvement in nonlinear absorption, with no or only small improvement for the more highly AuNP doped glasses. This study shows the importance of excitation wavelength and nanoparticle concentration for composite systems employing AuNPs to improve two‐photon absorption of chromophores.     

Octupolar Films with Large Second Harmonic Generation and Electro‐Optical Effects

Thin films that benefit from efficient octupolar molecular packing are prepared for second harmonic generation (SHG) and electro‐optic (EO) applications. The films are composed of 1,3,5‐tricyano‐2, 4,6‐tris(p‐diethylaminostyryl)benzene (TTB) in a ploymethylmetacrylate (PMMA) matrix on aluminum/BK7 glass (Al/BK7) and polyimide/indium tin oxide (PI/ITO) substrates. Octupolar films prepared on both substrates display polycrystalline and cylindrical domains. The molecular orientation, SHG efficiencies, and EO coefficients of the crystalline domains are measured. In the cylinders, the molecular crystal planes are oriented perpendicularly to the major cylinder axis, whereas in the polycrystals, the planes are randomly oriented. While both structures exhibit high and stable SHG and EO efficiencies, the cylinders, in particular, exhibited a very large SHG, a large EO coefficient, and high thermal stability; these characteristics will be useful in second order nonlinear optical applications.     

Nanoscale Refractive Index Tuning of Siloxane‐Based Self‐Assembled Electro‐Optic Superlattices

The refractive indices of self‐assembled organic electro‐optic superlattices can be tuned by intercalating high‐Z optically transparent group 13 metal oxide sheets into the structures during the self‐assembly process. Microstructurally regular acentricity and sizable electro‐optic responses are retained in this straightforward synthetic procedure. This “one‐pot” all wet‐chemistry approach involves: i) layer‐by‐layer covalent self‐assembly of intrinsically acentric multilayers of high‐hyperpolarizability chromophores on inorganic oxide substrates, ii) protecting group cleavage to generate a large density of reactive surface hydroxyl sites, iii) self‐limiting capping of each chromophore layer with octachlorotrisiloxane, iv) deposition of metal oxide sheets derived from THF solutions of Ga(OⁱC₃H₇)₃ or In(OⁱC₃H₇)₃, and v) covalent capping of the resulting superlattices.     

Highly Efficient Organic THz Generator Pumped at Near‐Infrared: Quinolinium Single Crystals

A novel highly efficient ionic electro‐optic quinolinium single crystals for THz wave applications is reported. Acentric quinolinium derivatives, HMQ‐T (2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 4‐methylbenzenesulfonate) and HMQ‐MBS (2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 4‐methoxybenzenesulfonate) exhibit high order parameters cos³θ_p = 0.92 and cos³θ_p = 1.0, respectively, as well as a large macroscopic optical nonlinearity, which is in the range of the benchmark stilbazolium DAST (N,N‐dimethylamino‐N’‐methylstilbazolium 4‐methylbenzenesulfonate) and phenolic polyene OH1 (2‐(3‐(4‐hydroxystyryl)‐5,5‐dimethylcyclohex‐2‐enylidene)malononitrile) crystals. As‐grown unpolished bulk HMQ‐T crystals with a side length of about 6 mm and thickness of 0.56 mm exhibit 3.1 times higher THz generation efficiency than 0.37 mm thick OH1 crystals and about 8.4 times higher than 1 mm thick inorganic standard ZnTe crystals at the near‐infrared fundamental wavelength of 836 nm. Therefore, HMQ crystals with high order parameter obviously have a very high potential for high power THz‐wave generation and its applications.     

Highly Efficient Green‐Emitting Phosphorescent Iridium Dendrimers Based on Carbazole Dendrons

Green‐emitting iridium dendrimers with rigid hole‐transporting carbazole dendrons are designed, synthesized, and investigated. With second‐generation dendrons, the photoluminescence quantum yield of the dendrimers is up to 87 % in solution and 45 % in a film. High‐quality films of the dendrimers are fabricated by spin‐coating, producing highly efficient, non‐doped electrophosphorescent organic light‐emitting diodes (OLEDs). With a device structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid)/neat dendrimer/1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene/LiF/Al, a maximum external quantum efficiency (EQE) of 10.3 % and a maximum luminous efficiency of 34.7 cd A^–1 are realized. By doping the dendrimers into a carbazole‐based host, the maximum EQE can be further increased to 16.6 %. The integration of rigid hole‐transporting dendrons and phosphorescent complexes provides a new route to design highly efficient solution‐processable dendrimers for OLED applications.     

Preparation of Functional Hybrid Glass Material from Platinum (II) Complexes for Broadband Nonlinear Absorption of Light

The synthesis of trans‐di(arylalkynyl)diphosphine platinum(II) complexes bearing trialkoxysilane groups is described, as well as the preparation of siloxane‐based hybrid materials from organometallic chromophores through a modified sol–gel process. Glass materials prepared from trans‐[P(n–Bu)₃]₂Pt[(C≡C–p–C₆H₄–C≡C–p–C₆H₄–CH₂O(CO)NH(CH₂)₃Si(OC₂H₅)₃]₂ generally show spectral transmittance, absorption and luminescence similar to that of solutions reported in the literature. Measurements of optical power limiting for the hybrid glass are carried out, and show broadband nonlinear absorption throughout the whole visible wavelength range with clamping values in the range 0.2–7 µJ at 120 mM chromophore concentration. The sol–gel process using urethane‐propyltriethoxysilane‐functionalized chromophores as precursors appears to be a valid method for formation of robust silicate materials with grafted diarylethynyl Pt(II) complexes for OPL devices.     

DNA Lipoplex‐Based Light‐Harvesting Antennae

Natural light‐harvesting complexes are operated through the well‐designed self‐assembly of pigments with large protein complexes in a thylakoid lipid bilayer. However, a long‐range, directed transfer of excitation energy has not been achieved in artificial systems because the nanoscale arrangement of chromophores into stable micrometer‐scale structures is highly challenging. Here the multiscale assembly of chromophores for excited energy transfer through the arrangement of chromophores on nanoscale DNA templates followed by their incorporation into larger multilamellar lipid structures is reported. Single‐strand 10 nucleotide DNA molecules containing a terminal residue linked with three different chromophores are hybridized with their complementary 30 nucleotide matrix DNA strand. Due to the short DNA sequences, the energy transfer of the DNA‐templated chromophore arrays is limited at 4 °C. However, the incorporation of DNA‐templated chromophores into lipid‐DNA complexes dramatically increases both of the efficiencies and antenna effects of the single and two‐step energy transfers at room temperature through the structural stabilization and the secondary assembly of DNA between the interstitial spaces of multilamellar lipid structures. The findings suggest that the supramolecular alignment of DNA‐templated chromophores, which has never been explored previously, can be a very promising route toward directed, long‐range light harvesting.     

Control of Charge Transport in Iridium(III) Complex‐Cored Carbazole Dendrimers by Generation and Structural Modification

Here, the charge transporting properties of a family of highly phosphorescent iridium(III) complex‐cored carbazole dendrimers designed to have improved charge transport by incorporating carbazole units into the dendrons are studied. Firstly, the effect of the dendrimer generation and the role of dendron for materials with one dendron per ligand of the core are considered. It is shown, in contrast to previously reported light‐emitting dendrimers, that in this case the carbazolyl‐based dendrons have an active role in charge transport. Next, the effect on the charge transport of attaching two dendrons per ligand to the dendrimer core is explored. In this latter case, for the so called “double dendron” material a highly non‐dispersive charge transport behavior is observed, together with a time‐of‐flight mobility of the order of 10^?3 cm² V^?1 s^?1. Furthermore the lowest energetic disorder parameter (σ) ever reported for a solution‐processed conjugated organic material is found, σ < 20 meV.     

Crosslinkable organic glasses with quadratic nonlinear optical activity

In this paper, the use of a pure benzimidazole based dimethacrylic push–pull chromophore has been investigated for the preparation of thin films exhibiting quadratic nonlinear optical (NLO) properties. To stabilize these NLO properties, the chromophore orientation has been frozen in a noncentrosymmetric arrangement by cross-linking the material using thermal polymerization induced by suitable initiators. The films present a very stable quadratic optical activity up to T = 152 °C with SHG coefficient d₃₃ of 14 pm/V measured at 1.9 μm.     

A Light‐Blue Phosphorescent Dendrimer for Efficient Solution‐Processed Light‐Emitting Diodes

We describe the preparation of a dendrimer that is solution‐processible and contains 2‐ethylhexyloxy surface groups, biphenyl‐based dendrons, and a fac‐tris[2‐(2,4‐difluorophenyl)pyridyl]iridium(III ) core. The homoleptic complex is highly luminescent and the color of emission is similar to the heteroleptic iridium(III ) complex, bis[2‐(2,4‐difluorophenyl)pyridyl]picolinate iridium(III ) (FIrpic). To avoid the change in emission color that would arise from attaching a conjugated dendron to the ligand, the conjugation between the dendron and the ligand is decoupled by separating them with an ethane linkage. Bilayer devices containing a light‐emitting layer comprised of a 30 wt.‐% blend of the dendrimer in 1,3‐bis(N‐carbazolyl)benzene (mCP) and a 1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene electron‐transport layer have external quantum and power efficiencies, respectively, of 10.4 % and 11 lm W^–1 at 100 cd m^–2 and 6.4 V. These efficiencies are higher than those reported for more complex device structures prepared via evaporation that contain FIrpic blended with mCP as the emitting layer, showing the advantage of using a dendritic structure to control processing and intermolecular interactions. The external quantum efficiency of 10.4 % corresponds to the maximum achievable efficiency based on the photoluminescence quantum yield of the emissive film and the standard out‐coupling of light from the device.     

Room Temperature Multifunctional Organophosphorus Gels and Liquid Crystals

A new series of extended dithieno[3,2‐b:2',3'‐d]phospholes with dendritic and non‐dendritic architectures involving phenylenevinylene as well as Fréchet‐type dendrons is presented. Modification of the phosphorus center with Pd allows for the generation of a dimeric dendrimer with Pd‐center. The synthetic strategy employed balances the rigid main scaffold with the flexibility of the dendrons in order to keep control of supramolecular self‐organization features. All the structures show a high photoluminescence in both solution and solid state, which is further intensified via energy transfer from the dendrons to the core. In terms of self‐organization in solution, three of the derivatives which bear an extended phosphole unit as common moiety are able to gel a variety of organic solvents at room temperature independently from the nature of their substituents. Notwithstanding, the dimeric dendrimer with Fréchet‐type dendrons is only able to display gel properties at low temperature. All gels exhibit pronounced photoluminescence properties that can be tuned by variation of the solvent and the temperature. In absence of solvent, the phosphole derivatives exhibit, moreover, liquid‐crystalline mesomorphism features. While three of the compounds present stable and highly luminescent columnar hexagonal phases at room temperature, the fourth species was found to be crystalline in the thermal range up to its isotropic state. Finally as proof of concept, the multifunctionality of these materials is demonstrated in an electrochromic device.     

Comparative Studies on Optical,Redox, and Photovoltaic Properties of a Series of D–A–D and Analogous D–A Chromophores

A series of new symmetrical donor‐acceptor‐donor (D?A?D) dyes based on s‐indacene‐1,3,5,7(2H,6H)‐tetraone as an acceptor unit containing varying electron donating moieties and analogous donor‐acceptor (D?A) chromophores with indane‐1,3‐dione as an acceptor are synthesized. By employing these two sets of dyes, the influence of a scaffold change from unsymmetric push‐pull (D?A) to symmetrical (D?A?D) systems on optical, electrochemical, and photovoltaic properties are explored. Detailed comparative studies reveal favorable optical characteristics and considerably decreased bandgaps for the D?A?D dyes compared to those of the reference D?A chromophores. Accordingly, the evaluation of the present dyes as donor materials in bulk heterojunction (BHJ) solar cells in combination with fullerene derivatives PC₆₁BM or PC₇₁BM as acceptors afforded significantly improved performance for devices based on D?A?D blends (up to a factor of 4 compared to the respective D‐A reference) with power conversion efficiencies of up to 2.8%. In less polar solvents such as toluene, some of the novel D?A?D chromophores exhibit unexpectedly high fluorescence quantum yields Φ_em of up to unity, in striking contrast to their weakly fluorescent D‐A counterparts.     

Poled Sol–Gel Materials With Heterocycle Push–Pull Chromophores that Confer Enhanced Second‐Order Optical Nonlinearity

Hybrid organic–inorganic materials doped with zwitterionic push–pull chromophores with high hyperpolarizability have been synthesized by a sol–gel procedure. A large chromophore concentration was reached by using N‐(hydroxyethyl)carbazole as a physical spacer (preventing the dye aggregation). Spin‐coated doped films were electrically poled and second harmonic generation measurements performed in situ. During the thermally assisted poling under a N₂ atmosphere, only the carbazole molecules degraded. Second‐harmonic generation measurements gave an estimation of the nonlinear coefficient, r₃₃, of 38 pm V^–1 at 1064 nm.