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To discern multiple intertwined effects, a set of azobenzene-functionalized amide-imide block copolymers, azo(PA-co-PI)-x, where x is amide-block content, viz., [azoPA] = 25, 50, 75 mol %, was synthesized from 2,2-bis{4-[4-(4-aminophenyldiazenyl)phenoxy]phenyl}propane(azoBPA), 4,4'-oxydibenzoyl chloride (ODBC), and 4,4'-oxydiphthalic anhydride (OPDA). Including homopolymers (azoPA and azoPI), this series of amorphous azopolymers possesses a high glass-transition temperature (Tg > 210 °C) and a modulus (E' ∼ 1.23-2.50 GPa). Their photobending (ca. 23-90°) and photostress (ca. 250-380 kPa) were assessed in the form of cantilevers with a linearly polarized 445 nm light. Nonlinear composition/[azoPA] dependencies of the thermo- and photomechanical properties are correlated. As [azoPA] increases from 0 mol %; Tg, E', photostress, and photobending angle initially decrease to reach four separate minima for azo(PA-co-PI)-50; and then all increase with a higher [azoPA]. The trend considerations of film density, dynamic thermomechanical, Fourier transform infrared (FT-IR), and ultraviolet-visible (UV-vis) measurements implicate that (i) intermolecular association and intramolecular segmental mobility collectively influence the photomechanical outcomes and (ii) two types of hydrogen bonding (HB), namely, amide-amide [HB-AA] and amide-imide [HB-AI] coexist in azo(PA-co-PI)-x copolymers, with [HB-AI] being largely responsible for photomechanical outcomes of azo(PA-co-PI)-x with [azoPA] 40-50 mol %. We hypothesize that the "U-shaped" photomechanical effect apparently stems from the cooperative "unzipping" of H bonds in the [HB-AA]* excited state with H bonds in [HB-AI]* being stabilized by electrostatic interactions inherent in an excited intermolecular complex.

Photomechanical effects realized in azobenzene-functionalized polyimides have shown large deformation and an exceptional increase in photogenerated force output. Here, we synthesize and characterize the photomechanical output of a series of linear polyimide materials prepared with a bulky substituent, incorporated via the development of a new bis(azobenzene-diamine) monomer containing a 9,9-diphenylfluorene cardo structure (azoCBODA). All six azoCBODA-containing polyimides are amorphous and exhibit high glass transition temperatures (Tg) ranging from 298 to 358 °C, storage moduli ranging from 2.27 to 3.81 GPa (at 30 °C), and good thermal stability. The magnitude of the photoinduced mechanical response of the azobenzene-functionalized polyimide is correlated to the rotational freedom of the polyimide chains (resulting in extensive segmental mobility) and fractional free volume (FFV > 0.1).

A methodology that integrates a folding step into the conventional poly(amic acid)/polyimide film fabrication scheme is developed. It enables fabricating cross-linked polyimide (XCP2) films into a host of complex-shaped objects. Particularly unprecedented is that these origami (3D) objects can be unfolded into a 2D temporary shape under externally applied stress at

The classical "chair-twist boat-boat" conformational dynamics (CD) of cyclohexane is thermally activated. Here we report on the photoinduced/azobenzene-assisted CD of bilaterally fused cyclohexane moieties contributing to large photomechanical response of cross-linked azobenzene-functionalized polyimides (X-azoPI), based on 1,2,4,5-cyclohexane-tetracarboxylic-dianhydride (CHDA), exhibiting a photobending angle and photogenerated stress, up to ∼90° and 370 kPa, respectively. In contrast, X-azoPI containing planar pyromellitimide (PMDI) or cage-like bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic-diimide (BCDI) show smaller photomechanical responses. The superior photomechanical performance of X-azoPI with constrained cyclohexane-diimide (CHDI) units is attributed to an increased mobility of segments comprising "hinged" p-phenylene rings, azobenzene, and CHDI units in the cross-link sites. Blue light irradiation initiates the motions driven by photoisomerization/reorientation of azobenzenes connected to CHDI units, whose CD is then amplified, leading to longer-range segmental mobility, more local free volume, and culminating in large photoinduced bending. The trapping of redistributed CHDI's stereoisomers in X-azoPI backbone at

Utilization of [60]fullerosome monolayer grafting approach on both sides of graphene nanosheets giving the corresponding sandwiched nanostructures was demonstrated as a key intermediate for the con...

In an effort to probe the influence of structural isomerism on the excited-state properties of a naphthalene-based donor-acceptor (D-A) system, four 1-naphthalenecarbonitrile compounds with amine substituents in the 2-, 3-, and 4-positions were synthesized and their photophysical properties were examined. Specifically, the molecules 2-dimethylamino-1-naphthalenecarbonitrile (

Highly fluorescent trans- and cis-tris(fluorenylphenylamino)benzene stereo-isomeric derivatives, trans- and cis-tris(DPAF-C9), respectively, were designed and synthesized to minimize facile planar fluorene ring stacking in the solid state during thin-film device fabrications. Resulting tris-cup cis- and tris-chair trans-conformers were hypothesized to be capable of reducing direct packing contact of light-harvesting fluorene moieties from each other that should reduce the aggregation-related self-quenching effect derived from the π–π stacking packing arrangement. By the fact of closely matching photophysical characteristics of them to those of poly(N-vinylcarbazole) (PVK), we applied the former conformer as the secondary blue hole-transporting material in the study of its electroluminescence properties, using a tricomponent system of PVK‒cis-tris(DPAF-C9)‒tris(2-phenylpyridine)iridium (III) [Ir(ppy)3], as an example. Accordingly, several prototype single-layered or multilayered devices were fabricated with their electroluminescence and external quantum efficiency demonstrated. These analogous 3D-stereoconformers can also be used in other fluorescence-related thin-film applications.

A new class of high-temperature dipolar polymers based on sulfonylated poly(2,6-dimethyl-1,4-phenylene oxide) (SO2 -PPO) was synthesized by post-polymer functionalization. Owing to the efficient rotation of highly polar methylsulfonyl side groups below the glass transition temperature (Tg ≈220 °C), the dipolar polarization of these SO2 -PPOs was enhanced, and thus the dielectric constant was high. Consequently, the discharge energy density reached up to 22 J cm-3 . Owing to its high Tg , the SO2 -PPO25 sample also exhibited a low dielectric loss. For example, the dissipation factor (tan δ) was 0.003, and the discharge efficiency at 800 MV m-1 was 92 %. Therefore, these dipolar glass polymers are promising for high-temperature, high-energy-density, and low-loss electrical energy storage applications.

Two photon absorption (2PA) is of great interest across many disciplines and there has been a large effort to increase the two-photon cross section (σ2) via synthetic modification, especially by enhancing intramolecular charge-transfer (ICT). This work takes the previously studied (7-benzothiazol-2-yl-9,9-diethylfluoren-2-yl)diphenylamine (AF240), an asymmetric D-π-A chromophore, and intentionally appends a functional group (-OH, AF240-OH or -OCH3, AF240-OMe) to the 6-position of the fluorenyl π-bridge of the new chromophores. Electrochemical results in both dichloromethane and acetonitrile support stabilization of the highest occupied molecular orbital in the derivatives due to inductive electron donating effects of the hydroxy and methoxy groups. The lowest unoccupied molecular orbital is stabilized via intramolecular hydrogen bonding to the benzothiazole moiety in the case of AF240-OH. As previously observed for AF240, the steady-state emission spectra show significant solvatochromism as they broaden and red shift with increasing solvent polarity. The fluorescence lifetimes and quantum yields show that the non-radiative rate constant is increased for AF240-OH in all solvents, especially in nonpolar media. The results suggest there is forced intramolecular hydrogen bonding to the benzothiazole in nonpolar solvents because the solvent poorly solubilizes the hydroxy group. This increases the non-radiative decay rate constant (knr) via additional vibrational decay pathways. While not as dramatic, the increase in knr in polar solvents supports some deactivation via hydrogen bonding to the solvent. Steric effects are also observed in the methoxy derivative, which inhibits planarization of the benzothiazole with the fluorene, increasing the energy of the excited state. Ultrafast transient absorption spectroscopy in tetrahydrofuran solution supports stabilization of the excited state in a few ps as solvent and structural reorganizations occur. In the case of AF240-OH, no evidence of proton transfer is observed. The decrease in emission energies in the case of AF240-OH support increased ICT driven by higher degree of coplanarity and the quinoidal structure in the excited state. However, a moderate increase in the intrinsic 2PA cross-section is resulted. It is likely because of the two possible and competing solvent-stabilized ICT processes (PICT and TICT) in AF240-OH. Nevertheless, the strategic presence of a hydroxide group capable of intramolecular hydrogen bonding in AF240-OH provides a much broader 2PA sensitivity window than AF240.

Photomechanical effects in materials can directly convert light stimulus into mechanical work. The magnitude of the photogenerated work is directly associated with the material stiffness (modulus). Here, we report the synthesis of an azobenzene-functionalized polyamide (azoPA) and demonstrate its room-temperature photomechanical response. The comparatively large photomechanical response of this high stiffness material is enabled by light-induced, azobenzene-assisted dissociation of inter-chain hydrogen bonding within the polymer backbone. The photomechanical response of the azoPA is directly contrasted with an analogous azobenzene-functionalized polyimide (azoPI) to illustrate the influence of intermolecular interactions.

Scalable water-dispersible graphene (eGPNc) powders were prepared with consecutive chlorosulfonic acid (CSA)/H2O2 and methylmorpholine N-oxide monohydrate (NMMOm) treatments, followed by the common filtering, sonication, and drying processes. The yield of graphene from expanded graphite (EG) powders prepared by the combined CSA/H2O2 and NMMOm treatments (3.0 wt%) was more than five times that from only the NMMOm treatment (0.6 wt%). The produced eGPNc powders had an almost defect-free graphitic structure with good redispersibility in water and an electrical conductivity of 86.9 S/cm from the filtered eGPNc film. The eGPNc film was dispersed at 0.28 mg/mL in water after centrifugation of the 2 mg/mL aqueous solution at 5000 rpm. The aqueous eGPNc solution was utilized as the reaction medium for the in situ polymerization of pyrrole to produce the polypyrrole (PPy)/graphene nanocomposite. The capacitance of PPy measured from cyclic voltammetry (CV) was improved from 122.8 to 278.6 F/g by loading 1 wt% eGPNc onto the nanocomposite. The capacitance of PPy after 1000 CV cycles was improved from 54.0% to 91.0% by loading 3 wt% eGPNc onto the nanocomposite. This improvement in the capacitance and capacitance-stability is due to the in situ formation of PPy in the well-dispersed aqueous graphene solution. Thus, this simple in situ preparation of PPy with eGPNc in water demonstrated the potential for the diverse applications of water-dispersible eGPNc in various water-based systems such as conducting inks, silver wires, and watersoluble conducting polymers (e.g. poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PSS/PEDOT)) for improving their electron conductivity and stability.

3D-configurated donor-acceptor stereoisomers cis-cup-tris[C 60 >(DPAF-C 9 )] were found to exhibit photoswitchable dielectric amplification phenomena using white LED light excitation at GHz frequency. The observation was correlated to the photoactivation of the Au-layer resulting in plasmonic energy to enhance photoinduced intramolecular e−-transfer from DPAF to a C 60 > moiety. Accumulated plasmonic resonance energy at the near-field surface was effective to distribute negative charges along the outer (C 60 >)-derived fullerosome shell layer of the trilayered NPs.

We investigate the nonlinear optical properties of a series of two-photon absorbing chromophores in the femtosecond regime using both two-photon induced fluorescence spectroscopy and the Z-scan technique. We apply a simplified sum-over-states model to the experimental data to predict nonlinear refraction.

Stimulated Rayleigh–Bragg scattering (SRBS) in highly two-photon active chromophores’ solutions and in a doped polymer rod has been investigated. Under optimized experimental conditions, the energy conversion efficiency from the input 816-nm and ∼ 10 - n s pump pulses to the backward SRBS pulses was up to η = 58 % in the solution of AF450 chromophore in methyl methacrylate (MMA) liquid of 0.001 M concentration, while for the AF450 doped poly(methyl methacrylate) rod of the same concentration the measured value was only η = 0.16 % . Considering that the two-photon absorption properties of the chromophore molecules are basically the same in both cases, the above-mentioned results strongly suggested that the formation of the induced Bragg grating in a solution is mainly due to spatial redistribution of the dopant molecules driven by two-photon resonance enhanced gradient force. In contrast, the same dopant molecules are “frozen” in a solid matrix which prohibits the spatial redistribution of these molecules from forming an effective Bragg grating. Based on the high efficiency of backward SRBS generation as well as the coexisting two-photon absorption, the intense input laser beam will undergo a much stronger nonlinear attenuation. For the tested AF450/MMA solution sample of 0.004 M concentration, when the input laser pulse energy changed from 0.06 mJ to 3.7 mJ, the measured nonlinear transmissivity decreased from 0.82 to 0.11, showing a superior nonlinear attenuation performance with regard to the input pump laser beam.

Directly transducing light into work is attractive for remotely powering soft mechanisms designed from photoresponsive polymers, but presents challenges for achieving reliable actuation within a control framework. Here, we utilize azobenzene-functionalized polyimides to fabricate actuators characterized by mechanically discrete states. Irradiation initiates the photochemically induced, quasistatic deformation to advance the actuator to the edge of instability. Following this latency, ultrafast snap through actuation (∼10 ms-scale) ensues. Restricting the role of control to the attainment of the edge of instability, strategies for achieving repetitive actuation via multiplexed irradiation are demonstrated. Approaches are examined for modulating the latency of the actuator using an all-optical strategy as well as mechanical design of the actuator. Prototypical assemblies of these actuators in arrays are used to fabricate morphable surfaces and structures, which is aided by the realization that the ultrafast actuation is characterized by a high power-density on the order of ∼kW/m3.

Three C60-DPAF conjugates were synthesized with one as a linear monoadduct and two as branched or starburst multiadducts in structure for the investigation of chromophore antenna number-dependent tunability of RF-responses at 0.5–18 GHz. The effect was photoinduced by the generation of surface plasmonic resonance (SPR) at the surface of trilayered core–shell nanoparticles (NPs) consisting of a highly magnetic γ-FeOx@AuNP core and an e–-polarizable C60-DPAF adduct-derived partially bilayered fullerosome outer shell. Observed large dielectric (permittivity) amplification phenomena, with the maximization on the starburst multiadduct-derived nanoparticles, were correlated to the photoactivation efficiency at the gold nanolayer. We hypothesized that the prolong accumulation (or trapping) of photogenerated SPR energy in the intermediate gold layer, sandwiched between the C60(>DPAF-C9)x-derived fullerosome membrane and the highly magnetic γ-FeOx NP core, was effectively released at the light-off stage to induce ...

With a view toward broadening the adaptive capability of polyimide-based systems that have been shown to be mechanically responsive to light, heat, and thermal-electrical stimuli, a simple diamine containing a highly polar ester–sulfonyl (ES) pendant was synthesized via a two-step route. It was polymerized with five common dianhydrides in N-methylpyrrolidinone to afford poly(amic acid), PAA, solutions, which were subsequently converted to a series of amorphous polyimides containing ester–sulfonyl (−CH2CH2SO2Me) pendants, generically designated as PI-ES, by either chemical imidization at room temperature in the same pot or heat treatment of PAA cast film at 175 °C. The chemically imidized polyimide films are tough and creasable, but the thermally imidized ones are brittle because of much lower molecular weights (GPC results). In addition, a series of thermally derived copolymers designated as PI-ES:A, which contains ES and carboxylic acid (A) pendants, were prepared from PI-ES via a retro-Michael reaction ...

We unexpectedly observed a large amplification of the dielectric properties associated with the photoswitching effect and the new unusual phenomenon of delayed photoinduced capacitor-like (i.e. electric polarization) behavior at the interface on samples of three-layered core-shell (γ-FeOx@AuNP)@[C60(DPAF-C9)](n)2 nanoparticles (NPs) in frequencies of 0.5-4.0 GHz. The detected relative dielectric constant amplification was initiated upon switching off the light followed by relaxation to give an excellent recyclability. These NPs having e(-)-polarizable fullerosomic structures located at the outer layer were fabricated from highly magnetic core-shell γ-FeOx@AuNPs. Surface-stabilized 2 in a core-shell structure was found to be capable of photoinducing the surface plasmonic resonance (SPR) effect by white LED light. The accumulated SPR energy was subsequently transferred to the partially bilayered C60(DPAF-C9) fullerosomic membrane layer in a near-field (∼1.5 nm) region without producing radiation heat. Since the monostatic SAR signal is dielectric property-dependent, we used these measurements to provide evidence of derived reflectivity changes on a surface coated with 2 at 0.5-4.0 GHz upon illumination of LED white light. We found that a high,99%, efficiency of response amplification in image amplitude can be achieved.

The two-photon absorbing dye AF240 [1, (7-benzothiazol-2-yl-9,9-diethylfluoren-2-yl)diphenylamine] is modified by adding bulky alkyl groups to the diphenylamino moiety. Three new compounds are synthesized which have ethyl groups in both ortho positions of each phenyl ring (2), t-butyl groups in one ortho position of each phenyl ring (3), and t-butyl groups in the para position of each phenyl ring (4). The dyes are examined in several aprotic solvents with varying polarity to observe the effects of the sterically hindering bulky groups on the ground and excited-state photophysical properties. While the ground state shows minimal solvent dependence, there is significant dependence on the fluorescence quantum yield and lifetime, as well as the excited-state energy levels. This effect is caused by the formation of an intramolecular charge-transfer (ICT) state, which is observed in the solvents more polar than n-hexane and supported by TD-DFT calculations. Electronic effects of ortho or para alkyl substitution should be similar, yet drastic differences are observed. A red shift in the fluorescence maximum is observed in 4 relative to 1, yet a blue shift occurs in 2 and 3 because the substituents at the sterically sensitive ortho-positions inhibit excited-state geometric relaxation and result in less ICT character than 1. Coupled with theoretical calculations, the data support a planar ICT (PICT) excited state where the diphenylamino nitrogen in an sp(2)-like geometry is integral with the plane containing the fluorene and benzothiazole moieties. Ultrafast transient absorption experiments show that ICT occurs rapidly (150 fs) followed by geometric and solvent relaxation in ∼ 1-4 ps to form the PICT or solvent-stabilized ICT (SSICT) state. This relaxation is not observed in non-polar n-hexane because the solvent dependent ICT state energy lies higher than the locally-excited (LE) state. Finally, formation of a triplet state (T1) is only efficiently observed in n-hexane for all four dyes.

Solid-state powders of water-dispersible graphene (GPN) were prepared by treatment of methylmorpholine N-oxide monohydrate (NMMOm). Re-dispersion of GPN in water by simple sonication was successfully demonstrated using a highly concentrated aqueous GPN solution (0.284 mg mL−1) after centrifugation at 9000 rpm for 10 min. The produced GPN had the graphitic structure without defects, and its electrical conductivity was 94.7 S cm−1, as measured from a filtered GPN film. The spin-coated thin film from the aqueous GPN solution exhibited a single-layered structure, which was examined using scanning electron microscopy, atomic force microscopy, and scanning transmission electron microscopy (with selective area electron diffraction). The GPN was also dispersible in polar solvents such as dimethyl sulfoxide, dimethylformamide, ethanol, and tetrahydrofuran. The origin of good dispersity of GPN in polar solvents, including water, was discussed with reference to the high polar nature of NMMO. A nanocomposite system with a water-soluble poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was prepared on a glass substrate. A fourfold improvement in the electrical conductivity of PEDOT:PSS without deterioration of transmittance was achieved by adding 1 wt% GPN.

In recent years, photoacoustic tomography (PAT) is increasingly used in biomedical research, as it allows for direct visualization of optical absorption in deep tissue. In addition to vascular and hemodynamic imaging using endogenous contrasts, PAT is also capable of imaging neural and molecular dynamics with extrinsic contrasts. While near-infrared (NIR)-absorbing contrasts are preferred for deep tissue imaging, compared to visible-light-absorbing contrasts, they are much harder to design and synthesize with good environmental stability. We introduce here a new PAT mode which utilizes nonlinear multiphoton upconversion of NIR light in situ to visible light, thus exciting locally a dye that can generate strong photoacoustic signal. This approach allows to take advantage of a large library of visible-light-absorbing dyes that can enable functional imaging, such as imaging of voltage, oxygen, pH, and ion channel activities. Two types of upconversion materials are utilized in this work: 1) a two-photon absorbing and emitting dye that is efficiently excited by NIR nanosecond laser pulses to enable pulsed laser-based PAT (pulsed-PAT); and 2) rare-earth containing inorganic nanocrystals that absorb continuous-wave (CW) NIR light by sequential multiphoton absorption through real intermediate states to enable intensity-modulated CW laser-based PAT (CW-PAT). Since both cases produce highly localized nonlinear photoacoustic signal, which has very weak scattering in tissue, we can achieve high contrast 3-D volume imaging of deep tissues. In this study, we validated the principle of our approach in different PAT modes and successfully detected enhanced photoacoustic signals from a visible-light-absorbing dye embedded deep in tissue. With vast variety of functionalized organic dyes operating in the visible range, our mode of nonlinear photoacoustic imaging will find great applications in preclinical and clinical researches.

Wireless transduction of light into mechanical work manifested as shape-changing surfaces, adaptive structures, or actuators is a topic of considerable recent interest. In the work reported here, the photomechanical responses of a new series of crosslinked azobenzene-functionalized polyimides prepared with increasing backbone rigidity over a range of crosslinker concentrations are examined. The baseline properties of the materials were characterized with X-ray diffraction, thermal analysis, and photomechanical examination including cantilever bending experiments and tensile tests. Increasing the rigidity of the backbone repeat unit reduces the magnitude of the shape change (observed as cantilever deflection) but increases the magnitude of photogenerated stress (in tensile tests). The promise of these materials as wireless actuators was examined in photoinitiated snap-through experiments.

Three new isomeric diamines containing three, oxy-linked benzonitriles (3BCN), one of which is asymmetric (meta, para, or m, p), are synthesized in a 3-step sequence. Polycondensation of these diamines and four common dianhydrides (6FDA, OPDA, BTDA, and PMDA) in N,N-dimethylacetamide via poly(amic acid) precursors and thermal curing at temperatures up to 300 °C lead to three series of tough, creasable polyimide (PI) films (tensile moduli = 1.63 − 2.86 GPa). Among these PIs, two PMDA-based PIs possess relatively high crystallinity and two OPDA-based PIs, low crystallinity, whereas all 6FDA- and BTDA-based PIs, and m,m-3BCN-OPDA-PI are amorphous, readily soluble in common polar aprotic solvents. Thermally stable and having high Tg (216 − 341 °C), these PIs lose 5% weight around 493–503 °C in air and 463–492 °C in nitrogen. Dielectric properties have been evaluated by broadband dielectric spectroscopy (BDS) and electric displacement-electric-field (D-E) loop measurements. D-E loop results show an increase in high temperature permittivity (at 190 °C/1 kHz) from 2.9 (for parent PI CP2 with no nitrile group) to as high as 4.9 for these PIs, while keeping their dielectric loss relatively low. Thus, an increase in dipole moment density by the presence of three neighboring CN per repeat unit can increase the overall permittivity, which could be further enhanced by sub-Tg mobility of para-phenylene linkages (BDS results). Published 2014. J. Polym. Sci., Part A: Polym. Chem. 2014 J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 422–436

Light is a readily available and sustainable energy source. Transduction of light into mechanical work or electricity in functional materials, composites, or systems has other potential advantages derived from the ability to remotely, spatially, and temporally control triggering by light. Toward this end, this work examines photoinduced piezoelectric (photopiezoelectric) effects in laminate composites prepared from photoresponsive polymeric materials and the piezoelectric polymer polyvinylidene fluoride (PVDF). In the geometry studied here, photopiezoelectric conversion is shown to strongly depend on the photomechanical properties inherent to the azobenzene-functionalized polyimides. Based on prior examinations of photomechanical effects in azobenzene-functionalized polyimides, this investigation focuses on amorphous materials and systematically varies the concentration of azobenzene in the copolymers. The baseline photomechanical response of the set of polyimides is characterized in cantilever deflection experiments. To improve the photomechanical response of the materials and enhance the electrical conversion, the polyimides are drawn to increase the magnitude of the deflection as well as photogenerated stress. In laminate composites, the photomechanical response of the materials in sequenced light exposure is shown to transduce light energy into electrical energy. The frequency of the photopiezoelectric response of the composite can match the frequency of the sequenced light exposing the films.

Photomechanical effects in polymeric materials directly convert input photonic energy into a macroscopic mechanical output. The photoinitiated mechanical output of these materials is typically dominated by classical mechanics, primarily derived from the material stiffness and sample geometry. Accordingly, large magnitude shape change (e.g., motion) is typically traded for large magnitude force generation. Here, we report on the systematic preparation and comparison of photomechanical effects in a set of isomerically varied linear and cross-linked azobenzene-functionalized materials that demonstrate the critical role of segmental mobility (evident in the magnitude of the β-transition) to assimilate the typically exclusive properties of large force generation and large shape change in a single material.

We report the observation of backward stimulated Mie scattering (SMS) due to light-field induced spatial redistribution of self-assembled nanospheres of a two-photon resonant organic chromophore in water, pumped by ∼10-ns laser pulses of ∼816-nm wavelength. The pump-energy threshold for generating backward stimulated scattering in such a system is remarkably lower than that in pure water. The gain of backscattering originates from an induced Bragg grating that reflects partial energy from the pump beam into the backward Mie scattering beam. Based on the experimental fact that the time-delay of the SMS pulse onset depends on both the pump level and the viscosity of the solvent, a physical model of SMS generation is proposed. Our experimental results have shown that the major contribution to the formation of an induced Bragg grating is spatial redistribution of nanoparticles suspended in the liquid. These nanoparticles are driven by a force that is proportional to the intensity gradient of the standing-wave field resulting from interference between the forward pump beam and the backward Mie scattering beam. When the nanoparticle motion is frozen in a gel-like medium, no SMS is observed, which experimentally supports the validity of the proposed physical model.

Azobenzene-functionalized polyimide materials can directly transduce light into mechanical force. Here, we examine the impact of polymer backbone rigidity on the photomechanical response in a series of linear, azobenzene-functionalized polymers. The rigidity of the backbone was varied by the polymerization of five dianhydride monomers with a newly synthesized diamine (azoBPA-diamine). The azobenzene-functionalized linear polymers exhibit glass transition temperatures (Tg) ranging from 276 to 307 °C and maintain excellent thermal stability. The photomechanical response of these materials was characterized by photoinduced cantilever bending as well as direct measurement of photogenerated stress upon exposure to linearly polarized, 445 nm light. Increasing the rigidity of the polymer backbone increases the magnitude of stress that is generated but decreases the angle of cantilever deflection.

A set of 12 new polyimides (PIs) with one or three polar CN dipoles directly attached to the aromatic diamine part were synthesized and their electric energy storage properties were studied using broadband dielectric spectroscopy (BDS) and electric displacement–electric field (D–E) loop measurements to determine their potential for high temperature film capacitors for aerospace applications. It was found that adding highly polar nitrile groups to the PI structure increased permittivity and thus electrical energy storage, especially at high temperatures, and 3 CN dipoles were better than 1 CN dipole. Below the glass transition temperature (Tg), a weak γ transition was observed around −100 °C and a broad β transition was observed between 100 and 150 °C. It was the β (i.e., precursor dipolar motion before long-range segmental motion, or glass transition), rather than the γ sub-Tg transition that substantially increased the permittivity of PIs. From the BDS results on PIs having 3 nitrile groups, the enhancement in permittivity from permanent dipoles decreased with dianhydride in the order of pyromellitic dianhydride (PMDA) > 4,4′-oxydiphthalic dianhydride (OPDA) > 1,1,1,3,3,3-hexafluoropropane dianhydride (6FDA) > 4,4′-benzophenonetetracarboxylic dianhydride (BTDA). Meanwhile, the increase in permittivity also decreased in the order of para–para, meta–para, and meta–meta linkage in the diamine, suggesting that the para–para linkage favored easier dipole rotation than the meta–meta linkage. From the D–E loop study, the PIs with a combination of PMDA dianhydride and a para–para linkage exhibited the highest discharged energy density and a reasonably low loss.

A new unsymmetrical diamine, 2-(3-aminophenoxy)-6-(4-aminophenoxy)benzonitrile (3,4-APBN), is synthesized via two consecutive SNAr reactions and the temperature-dependent reactivity of the fluorides in 2,6-difluorobenzonitrile, whose first SNAr reaction occurs at 70 °C and second, at 100 °C, allowing timing control of reaction sequence and circumventing the transetherification side reaction. Thus, a series of polyimides (PIs) is prepared from the polymerization of 3,4-APBN with five common dianhydrides (6FDA, DSDA, OPDA, BTDA, and PMDA). For comparison, a second series is also prepared from two symmetrical diamines ([2,6-bis(3-aminophenoxy)benzonitrile (3,3-APBN) and 2,6-bis(4-aminophenoxy)benzonitrile (4,4-APBN)] and 6FDA or PMDA. The processability of the poly(amic acids) (PAAs), for the first series is greatly improved since their solution viscosities are much lower than PAAs based on symmetrical diamines. Besides having high glass-transition temperatures (249–332 °C), and thermal stability [5% weight loss in the range of 505–542 °C (air) and 512–546 °C (nitrogen)], these PIs form tough, transparent and flexible films that have a tensile-strength range of 82.1–121.3 MPa, elongations-at-break of 5.33–9.81%, and tensile moduli of 2.11–2.97 GPa. Their film dielectric constants are 3.08–3.62 at 10 kHz, moderately higher than that (2.92) of analogous PI (CP2) without nitrile groups. Overall, we found that the reduction of structural symmetry in repeat units can improve the polymer processibility as well as increasing their dielectric constants. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4998–5011

CP2 polyimide (prepd. from 6FDA and 1,3-bis (3-aminophenoxy) benzene) was blended with (1-50 wt.%) detonation nanodiamonds (DND, pristine, acetone-washed, and 4-(2,4,6-trimethylphenoxy) benzoic acid-functionalized), and the blends were evaluated as thin films for its potential utility in high-energy-density capacitors that would have stable dielectric properties over a wide temperature range (-55 to 300°C) and at frequencies up to or greater than 100 kHz. Both the dielectric storage and loss increased substantially with DND content. Surface functionalization (with the above benzoic acid derivative) significantly reduced the dielectric loss, while the use of acetone-washed DNDs had no effect on the dielectric loss. DND was also blended with CP2 via in-situ polymerization and found to have little effect on the dielectric properties.

Spin-cast films of dyes (donor-π-donor, donor-π-acceptor, and acceptor-π-acceptor type, where the donor is Ph2N-, the acceptor is 2-benzothiazoyl, and the π-linker is 9,9-diethylfluorene) blended with nonconjugated bisphthalimides were prepared. Upon visible-light excitation of the dyes, quenching of the excited state occurs by exciplex formation between dye and bisphthalimide molecules or, in some cases, by excimer formation or aggregation-induced emission between two dye molecules. The extent of exciplex formation is dependent on the driving force, which can be calculated using the energy difference between the lowest unoccupied molecular orbitals (LUMOs) of the dyes and bisphthalimides. The results show that complete exciplex formation occurs when this driving force is greater than 0.57 eV whereas partial exciplex formation occurs when the driving force is between 0.28 and 0.57 eV. The exciplex emission energies can also be predicted by calculating the difference between the LUMO level of the bisphthalimide and the highest occupied molecular orbital (HOMO) of the dye. These calculated values, which were obtained from the electrochemically determined energy levels, showed good agreement with the observed emission energies. The exciplex lifetimes were found to be significantly longer than the lifetimes of the lone dyes. These exciplexes formed from nonlinked donors and acceptors in the solid state might have potential uses in nonlinear photonics.

We designed and synthesized amphiphilic oligo(ethylene glycolated) C60-(light-harvesting fluorene antenna) nanostructures C60(>DPAF-EG12C1) by incorporating a donor–acceptor antenna component of C60–DPAF with highly hydrophilic oligo(ethylene glycol) (EGn) side-chains to increase the amphiphilicity of the molecule. The chain length of EGn arms was selected to match with the overall molecular radius and to achieve a balance between the hydrophobicity and hydrophilicity of resulting molecules Molecular self-assembly of C60(>DPAF-EG12C1) forming bilayered fullerosome nanovesicles led to the observation of ultrafast femtosecond nonlinear optical (NLO) absorption characteristics. Two-photon absorption phenomena were found to be the dominating photophysical events showing a large molar concentration-insensitive 2PA cross-section value equivalent to 8500 GM in a form of vesicles, on average, at the estimated effective nanovesicle concentration as low as 5.5 × 10−8 MV (molecular molar concentration of 5.0 × 10−4 M) in H2O.

We synthesized four C60-(light-harvesting antenna) dyads C60 (>CPAF-Cn) (n = 4, 9, 12, or 18) 1-Cn for the investigation of their broadband nonlinear absorption effect. Since we have previously demonstrated their high function as two-photon absorption (2PA) materials at 1000 nm, a different 2PA wavelength of 780 nm was applied in the study. The combined data taken at two different wavelength ranges substantiated the broadband characteristics of 1-Cn. We proposed that the observed broadband absorptions may be attributed by a partial π-conjugation between the C60 > cage and CPAF-Cn moieties, via endinitrile tautomeric resonance, giving a resonance state with enhanced molecular conjugation. This transient state could increase its 2PA and excited-state absorption at 800 nm. In addition, a trend of concentration-dependent 2PA cross-section (σ2 ) and excited-state absorption magnitude was detected showing a higher σ value at a lower concentration that was correlated to increasing molecular separation with less aggregation for dyads C60(>CPAF-C18) and C60(>CPAF-C9), as better 2PA and excited-state absorbers.

Recently, we have reported that azobenzene-functionalized polyimide materials exhibit a substantial increase in photogenerated stress in comparison to azobenzene-functionalized acrylate-based liquid crystal polymer networks. Here, we show that pre-straining the azo-CP2-20 material further increases the magnitude of the photomechanical response visualized as bending in the cantilever geometry as well as in direct measurements of photogenerated stress. Additionally, we also report on the ability to optically fix both optically and mechanically generated shapes in azo-CP2-20. The optically fixable shape memory of azo-CP2-20 is enabled by the introduction of excess free volume during thermal processing as the material was pre-strained.

Photoresponsive polymers convert a light stimulus input into a mechanical output (work). Photoinduced conformational changes, such as within azobenzene, dictate molecular-level distortions that sum...

Polymer-grafted inorganic nanoparticles are being developed for a diverse array of applications, ranging from drug delivery to multifunctional composites. In many instances, performance of these core-shell hybrids is limited by relatively broad distributions of size and composition, as well as the presence of impurities, such as unattached polymer chains. Herein, further synthetic improvements, and associated characterization techniques, to enhance the fraction of the grafted polystyrene shell on silica hybrid nanoparticles are discussed. We found that during surface-initiated atom transfer radical polymerization (SI-ATRP) from the silica nanoparticles, thermal self-initiation of styrene produces unattached polymer chains. Size exclusion chromatography afforded a facile approach to quantify the mass of the unattached polymer, and provide a substantial refinement to estimates of chain graft density beyond traditionally-used approaches, such as thermogravimetry. This fraction of unattached polymer is still present even after post-polymerization work-up via precipitation and re-dissolution. Removal necessitates additional procedures, such as high speed centrifugation. Selection of a lower polymerization temperature, in concert with a more reactive Cu complex, significantly reduces the amount of unattached polystyrene impurity. The improved polymerization conditions and post-polymerization purification provide more refined polystyrene-grafted silica nanoparticles to clarify structure-property relationships of these core-shell hybrids.

A new bis(4-aminophenoxy) monomer containing a two-photon absorbing (2PA) and fluorescent diphenylaminodiethylfluorene–benzothiazole chromophore (AF240) was synthesized and used as a comonomer in preparing a series of heat-resistant, 2PA-active polyimides. Highly organo-soluble, these polymers easily formed optically clear, but nonfluorescent, films that contained covalently bound, AF240-like dye in concentrations up to ∼1.0 M. For comparison purposes, a model compound (AF349) with phthalimido end-caps was also prepared. From the fluorescence data, the presence of phthalimido moieties in both the model compound and the polymers drastically quenched the fluorescence emission from the 2PA moieties of these materials in solution as well as in solid state. Their intrinsic (at 780 nm and with 160 fs pulses) and effective (at 800 nm and 8 ns pulses) two-photon properties in THF solutions (0.02 M), and film samples (40–65 μm thick) were determined by a direct nonlinear transmission technique. Thus, their intrins...