Second Order Nonlinear Optical Performances of Polymers Containing Imidazole and Benzimidazole Chromophores

Summary: Condensation polymers based on two different NLO‐active chromophores, namely 2‐[4‐[(4‐N,N‐dihydroxyethylamino)phenylazo]phenyl]‐5(6)‐nitrobenzimidazole (BZI) and 2‐[4‐(4‐N,N‐dihydroxyethylamino)phenylazo]‐4,5‐dicyanoimidazole (IMI), have been synthesized and their basic properties measured. The second order NLO properties of poled polymers were investigated by SHG procedures at different temperatures (35, 65 and 85 °C) and exposure times. Resonance free d₃₃ coefficients (λ = 1 368 nm) took values in the range 1.8–4.0 pm · V^?1, but the time stability of their SHG signal was fairly acceptable, particularly for the high T_g (e.g. 188 °C) polymer for which a relaxation time (at 25 °C) of 2.8 years has been determined.

BZI and IMI chromophores.     

Tailoring Linear and Nonlinear Optical Properties of a Side‐Chain Liquid Crystalline Azo‐Polymethacrylate

The molecular orientation in a side‐chain liquid crystalline azopolymer with push–pull chromophores ( Pol‐PZ‐CN ) is studied by UV‐vis spectroscopy, refractive index and second harmonic generation measurements. Fresh films show homeotropic arrangement, but irradiation with polarized light induces “in‐plane” birefringence, which is enhanced by subsequent thermal annealing up to values as high as 0.34. The nonlinear response of Corona poled films with “in‐plane” anisotropy is governed by three independent NLO d_ij coefficients, evidencing symmetry different from the usual (C_∞v). For irradiated films, stable values d₃₁ = 1.4, d₃₂ = 0.9 and d₃₃ = 11 pm·V^?1 are measured at 1.9 µm. A noticeable higher d₃₁/d₃₂ ratio is obtained for preirradiated and annealed films. The proposed two‐step method consisting of light irradiation and heating is an effective strategy to tailor the polar molecular orientation of nonlinear liquid crystalline azopolymers.

     

NLO Behavior of Polymers Containing Y‐Shaped Chromophores

New polyurethanes containing Y‐shaped chromophores, symmetrical derivatives of 4‐(dicyanomethylene)‐2‐methyl‐6‐[p‐(dimethylamino)styryl]‐4H‐pyran, have been prepared. The polymers show high glass transition temperatures (T_g) and a good thermal stability. SHG measurements on poled polymer films of the synthesized polymers have been carried out and a maximum d₃₃ of 15 pm · V^?1 has been found at 1 368 nm fundamental wavelength. Time stability measurements on the most active polymer have shown that after the initial fast relaxation, the d₃₃ value remains constant at 80 °C for 60 d.

     

Nonlinear Optical Properties and Liquid‐Crystalline Behavior of New Polyesters with Dipole Moments Aligned Transverse to the Backbone

Side‐chain azobenzene‐based second‐order nonlinear optical (NLO) polyesters ( PNPAR , PNPAPAR , PPAR‐TC , PPAPAR‐TC ) containing nitro or tricyanovinyl groups as acceptors with their dipole moments aligned transverse to the polymer backbone, were synthesized and characterized, and their thermal properties, liquid‐crystalline (LC) phase behavior, and NLO properties studied. Polymers containing nitro groups as acceptors were found to exhibit LC properties, while those containing tricyanovinyl groups as acceptors did not. The LC polymers were of smectic nature. The polymer PNPAR with a nitro group, when poled at the LC phase‐transition temperature range, was found to exhibit higher NLO activity than that with a tricyanovinyl group ( PPAR‐TC ). When polymer PNPAPAR containing a nitro group, also exhibiting an LC phase, was poled at a temperature different from that of its transition temperature, the NLO activity observed was comparable to, and/or lower than, that of the similar polymer ( PPAPAR‐TC ) having a tricyanobvinyl group in the place of the nitro group. All the polymers were found to possess moderate temporal stability of second harmonic generation activity and were found to have stability over the whole process.     

Temperature‐Responsive Water‐Soluble Copolymers Based on 2‐Hydroxyethyl Acrylate and Butyl Acrylate

Amphiphilic copolymers have been synthesised by free radical copolymerisation of 2‐hydroxyethyl acrylate with butyl acrylate, the reactivity ratios of which indicate practically equal reactivity. The copolymers containing less than 30 mol‐% of BA were soluble in water and exhibited a LCST in aqueous solutions. It was found that the interaction between these copolymers and poly(acrylic acid) in aqueous solutions resulted in the formation of interpolymer complexes stabilised by hydrogen bonds and hydrophobic interactions. This interaction was significantly affected by solution pH and led to modification of the temperature‐responsive behaviour of the copolymers.

     

Benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐terthiophene Copolymers Containing Styryl‐Triphenylamine Side Chains: Synthesis and Photovoltaic Performance Optimization with Fullerene Acceptors

A new two‐dimensional‐conjugated polymer (PBDTT3‐TPA) containing benzodithiophene (BDT) and a side chain isolation comonomer is designed and synthesized. Interestingly, PBDTT3‐TPA is compatible with higher lowest unoccupied molecular level (LUMO) acceptors of indene‐C₆₀ bisadduct (ICBA), and polymer solar cells based on PBDTT3‐TPA/ ICBA show an open‐circuit voltage (V_OC) of ca. 0.80 V and a power conversion efficiency of 2.48% under AM1.5G illumination of at 100 mW cm^?2. Furthermore, the energy loss in the corresponding fullerene acceptor devices is discussed, and the increase in the observed V_OC is explained quantitatively by the up‐shifted LUMO energy of ICBA (0.17 eV) and the reduced saturation current (J_SO) in the blends.

     

Atom Transfer Radical Polymerization and Third‐Order Nonlinear Optical Properties of New Azobenzene‐Containing Side‐Chain Polymers

The atom transfer radical polymerization (ATRP) technique has been successfully applied to synthesize a series of nonlinear optically (NLO) active homopolymers, 4‐(4‐nitrophenyl‐diazenyl) phenyl acrylate ( P ‐ NPAPA ) and 4‐(4‐methoxyphenyl‐diazenyl) phenyl acrylate ( P ‐ MPAPA ), containing azobenzene groups on the side chain. The third‐order NLO properties of the polymer films were measured by the degenerated four‐wave mixing (DFWM) technique. A dependence of the χ⁽³⁾ values and response times of polymers on their number‐average molecular weight and the electronic effect of the substituent (nitro‐ or methoxy‐) on the azobenzene group have been evidenced. The increasing χ⁽³⁾ value of the polymer films at the magnitude of about 10^?10 was displayed with increasing molecular weight and the presence of the push‐pull electronic system contributes much in enhancing the third‐order NLO susceptibility of polymers.

     

Self‐Assembly Behavior and Optical Properties of Poly(3‐thiopheneacetate)/Dialkyldimethylammonium Complexes

Poly(3‐thiopheneacetate)/dialkyldimethylammonium complexes (PTA‐Cn) were prepared by mixing regiorandom poly(3‐thiopheneacetic acid) with dialkyldimethylammonium bromide to study the effect of their self‐assembled structures on the optical properties. The lengths of the alkyl groups in the ammonium salts were varied from decyl (C10) to octadecyl (C18) groups. These complexes showed fully developed layer structures with ordered mesophases. PTA‐C16 and PTA‐C18 with longer alkyl groups showed side‐chain crystalline phases, and PTA‐C10, PTA‐C12, and PTA‐C14 with shorter alkyl groups showed side‐chain crystalline and liquid crystalline phases at room temperature. When PTA‐C18 was heated, a melting transition from side‐chain crystalline to smectic liquid crystalline phase was observed at 41.5 °C. The maximum absorption and emission wavelengths of these complexes increased with increasing alkyl chain length indicating that the π‐conjugated structure of the regiorandom thiophene backbone changes according to the alkyl groups.

     

Radical Copolymerization Reactivity of N‐Substituted Maleimides with α‐Substituted Styrenes with Various N‐ and α‐Substituents and the Thermal and Optical Properties of the Resulting Copolymers

The radical alternating copolymerization of N‐substituted maleimides (RMIs) with N‐methyl, n‐butyl, and 2‐ethylhexyl groups and styrene derivatives with various α‐substituents (RSs) is carried out. The yield and the molecular weight of the obtained copolymers significantly decrease with an increase in the size of the α‐substituent of RSs, but are independent of the N‐substituents. The glass‐transition temperature of the copolymers increases on the introduction of the large substituents into the RS repeating units. The high‐molecular‐weight poly(RMI‐alt‐RS)s are soluble in organic solvents and provide flexible and transparent films with excellent transparency by casting the solutions.

     

Amphiphilic Block Copolymers Containing Regioregular Poly(3‐hexylthiophene) and Poly(2‐ethyl‐2‐oxazoline)

Poly(3‐hexylthiophene)‐block‐poly(2‐ethyl‐2‐oxazoline) amphiphilic rod–coil diblock copolymers have been synthesized by a combination of Grignard metathesis (GRIM) and ring‐opening cationic polymerization. Diblock copolymers containing 5, 15, and 30 mol‐% poly(2‐ethyl‐2‐oxazoline) have been synthesized and characterized. The synthesized rod–coil block copolymers display nanofibrillar morphology where the density of the nanofibrills is dependent on the concentration of the poly(2‐ethyl‐2‐oxazoline) coil segment. The conductivity of the diblock copolymers was lowered from 200 to 35 S · cm^?1 with an increase in the content of the insulating poly(2‐ethyl‐2‐oxazoline) block. By contrast, the field‐effect mobility decreased by 2–3 orders of magnitude upon the incorporation of the poly(2‐ethyl‐2‐oxazoline) insulating segment.

     

n‐Type Copolymers with Fluorene and 1,3,4‐Heterodiazole Moieties

Summary: The successful realization of n‐channel field‐effect transistors requires the application of semiconducting polymers with high electron mobility (n‐type). However, reports on n‐type polymers are rather scarce in the literature. Therefore, the development of polymers with suitable electron transport properties is particularly challenging for the synthetic chemistry. Main chain polymers with strong acceptor units, such as 1,3,4‐heterodiazoles, are potential candidates for electron transport materials in electronic devices. The fluorene unit is another ring system with interesting physical and chemical properties, which is often used in rigid‐rod, main chain polymers. The present work introduces the synthesis of organo‐soluble copolymers consisting of alternating fluorene‐, 1,3,4‐heterodiazole, and, in some cases, additional 2,5‐dialkoxyphenylene units in the main chain. The reported synthesis involves modified classical polycondensation as well as the tetrazole route. We demonstrate the possibility of exchanging oxygen in the heteroaromatic ring with sulfur using Lawesson's reagent during the ring closure reaction. An alternative structure of the heterocyclic ring with N‐phenyl in the oxygen position is feasible using the tetrazole synthetic route. The chemical and electrochemical properties of the copolyfluorenes are investigated in detail. Some of the synthesized copolyfluorenes have also been used for the preparation of “electron‐only” devices enabling the calculation of the electron mobilities. Further, an organic field‐effect transistor (OFET) characteristic was shown.

OFET characteristics of the polymer 12b .     

New Second‐Order Nonlinear Optical Polymers Derived from AB2 and AB Monomers via Sonogashira Coupling Reaction

In this paper, a facile route was designed to prepare a new AB₂‐type polymer P1 via simple Sonogashira coupling reaction, also its corresponding linear analog ( P2 ) was obtained from AB monomer for comparison. Despite the relatively lower loading density of the effective chromophore moieties, P1 demonstrated higher second‐harmonic coefficient (153.9 pm · V^?1) than that of P2 (98.2 pm · V^?1), due to the three‐dimensional spatial isolation effect of the hyperbranched structure. The good results of P2 also indicated that the ladder shape may help to solve the problem existing in main‐chain polymers, the low poling efficiency.

     

Syntheses of New 3,6‐Carbazole‐Based Donor/Acceptor Conjugated Copolymers for Optoelectronic Device Applications

The syntheses, properties, and optoelectronic device characteristics of four new 3,6‐carbazole‐based donor/acceptor conjugated copolymers are reported. Such copolymers are used to explore the effects of acceptor strength and backbone coplanarity on the electronic and optoelectronic properties. The optical bandgaps of the studied copolymers are PCzQ (2.29 eV) > PCzDTQ (1.91 eV) > PCzTP (1.75 eV) > PCzDTTP (1.49 eV), which are much smaller than the parent poly(3,6‐carbazole). The power conversion efficiency of the photovoltaic cells fabricated from blends of copolymer/PC₆₁BM or PC₇₁BM reached 1.01 and 1.73% by varying the film thickness or blend ratio. The experimental results suggest the potential application of 3,6‐carbazole acceptor conjugated copolymers in optoelectronic devices.

     

Synthesis and Characterization of Conducting Copolymers of (S)‐2‐Methylbutyl‐2‐(3‐thienyl)acetate with Pyrrole and Thiophene

Polymerization of methylbutyl‐2‐(3‐thienyl)acetate (MBTA) was achieved by constant current electrolysis at low temperature. Subsequently, the syntheses of block copolymers of polyMBTA were accomplished in the presence of either pyrrole or thiophene by constant potential electrolysis. Moreover, the copolymer of MBTA with thiophene was obtained with constant potential electrolysis.

The synthesis of monomer MBTA.     

RAFT Synthesis and Solution Properties of pH‐Responsive Styrenic‐Based AB Diblock Copolymers of 4‐Vinylbenzyltrimethylphosphonium Chloride with N,N‐Dimethylbenzylvinylamine

The RAFT synthesis and solution properties of AB block copolymers of 4‐vinylbenzyltrimethylphosphonium chloride (TMP) and N,N‐dimethylbenzylvinylamine (DMBVA) is described. The pH‐dependent self‐assembly properties of the AB diblock copolymers were examined using of ¹H NMR, DLS, and fluorescence spectroscopy. The size of the polymeric aggregates depends on the block copolymer composition/molecular mass. The self assembly is completely reversible, as predicted from the tunable hydrophilicity/hydrophobicity of the DMBVA residues. The AB diblock copolymers can be effectively locked in the self‐assembled state using a straightforward core crosslinking reaction between the tertiary amine residues of DMBVA and difunctional 1,4‐bis(bromomethyl)benzene.

     

Synthesis and Photovoltaic Properties of Conjugated Copolymers with Benzo[1,2‐b:4,5‐b′]dithiophene and Bis(thiophene)phthalimide Units

Two new donor/acceptor copolymers PBDTPhBT1 and PBDTPhBT2 with alternating benzodithiophene ( BDT ) and bisthiophene‐phthalimide ( PhBT ) units were synthesized by Stille coupling reaction. The copolymers showed identical optical bandgaps of 1.98 eV. The HOMO energy levels are ?5.35 and ?5.32 eV for PBDTPhBT1 and PBDTPhBT2 , respectively. The bulk heterojunction solar cell devices based on these copolymers as donors and PC₇₁BM as acceptor displayed high open‐circuit voltages of 0.90–0.93 V and achieved power conversion efficiency of 1.54% under the illumination of AM 1.5, 100 mW · cm^?2.

     

Synthesis and Properties of Novel Sulfide‐Containing Aniline Copolymers as a Cathode Material for Li‐Ion Batteries

A novel sulfide‐containing aniline, 4‐amino‐dihydrobenzothiophene (ADBT), is synthesized by the alkylbromination of 1,2‐dimethyl‐3‐nitrobenzene. Copolymers of aniline and ADBT, P(An‐co‐ADBT), are prepared by chemical oxidative polymerization and tested as a cathode material for Li‐ion batteries. The results show that the copolymerization activity of ADBT is lower than that of aniline. The charge/discharge tests show that the P(An‐co‐ADBT)‐2 gives well‐defined and sloping plateaus in the potential rang of 3.0–4.0 V and quite a high capacity of 104.7 mAh g^?1. The P(An‐co‐ADBT)‐2 also shows high cycling stability and coulombic efficiency. After 50 cycles, the discharge capacity of the P(An‐co‐ADBT)‐2 still maintains 87.8% of the capacity obtained at the initial cycle.     

Synthesis and Optical Properties of π‐Conjugated Polymers Composed of Benzo[1,2‐b:4,5‐b′]dithiophene and Thiophenes Bearing Electron‐Deficient Ethenyl Groups in the Side Chains

π‐Conjugated polymers composed of dialkoxybenzo[1,2‐b:4,5‐b′]dithiophene and thiophenes bearing cyano‐alkoxycarbonylethenyl [? CH?C(CO₂R)CN] and bis(alkoxycarbonyl)ethenyl groups [? CH?C(CO₂R)₂] were prepared. The optical properties of the obtained polymers were investigated by UV‐vis and photoluminescence spectroscopy to demonstrate that the absorption maxima of the polymers were tunable by varying the substituent of thiophene unit. The photoluminescence wavelengths and intensities of the polymer in solution were significantly dependent on the polymer side chain. The HOMO energy level of the polymer was lowered by up to ?5.51 eV by introducing electron‐deficient cyano groups. Polymer solar cells based on the new polymers were fabricated to achieve a PCE of 1.90%.

     

A Polymeric Drug Depot Based on 7‐(2′‐Methacryloyloxyethoxy)‐4‐methylcoumarin Copolymers for Photoinduced Release of 5‐Fluorouracil Designed for the Treatment of Secondary Cataracts

The synthesis of a polymeric, photoresponsive drug‐delivery system on the basis of 4‐methylcoumarin‐side‐chain‐functionalized methacrylic copolymers with different degrees of functionalization is reported. Drug‐release experiments in solution indicated a maximum release of 22 µg of 5‐fluorouracil per 1 mg of polymer. Polymer analogous photochemical drug immobilization does not lead to observable intermolecular cross‐linking. Miscibility experiments with PMMA suggest that the bulk polymerization of homogeneous, transparent polymer sheets for intraocular lens fabrication, using MMA as the matrix monomer and incorporating the polymer‐drug conjugates as a functional component, is possible.

     

Thieno[3,4‐c]pyrrole‐4,6‐dione‐Based Polymers for Optoelectronic Applications

Over the last decade, great progress has been made in the field of organic electronics. Advancements in organic syntheses as well as in device engineering enabled preparation of polymer solar cells with power conversion efficiency (PCE) exceeding 8%–9%. In search for new polymers suitable for photovoltaic applications, push–pull polymers containing thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) motif as an electron deficient (pull) unit emerged as very promising candidates. This Trend Article summarizes research on TPD‐based polymers with a special emphasis on the structure–property relationships.