Influence of Structural Isomerism on the Photophysical Properties of a Series of Donor-Acceptor 1-Naphthalenecarbonitrile Derivatives Possessing Amine Substituents.

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 (2DA), 2-(1-piperidinyl)-1-naphthalenecarbonitrile (2P), 3-dimethylamino-1-naphthalenecarbonitrile (3DA), and 4-(1-piperidinyl)-1-naphthalenecarbonitrile (4P) were studied. The substitution position of the amine donor has a significant impact on both the ground-state absorption and excited-state properties of the complexes in toluene solution. The energy, band shape, and extinction coefficient of the ground-state absorption spectra are highly dependent on the substitution position of the amine donor. All of the derivatives exhibit fluorescence at room temperature. The fluorescence observed from 2DA, 2P, and 3DA demonstrates a vibronic structure with all three molecules possessing Stokes shifts on the order of 40 nm, whereas the fluorescence observed from 4P is broad and has a Stokes shift 2 times greater than the other derivatives. The fluorescence lifetimes, fluorescence quantum yields, and intersystem crossing quantum yields vary greatly with the substitution position of the amine donor. 2DA and 2P display intermediate fluorescence lifetimes (2.7 ns) and fluorescence quantum yields (0.20) while possessing the greatest intersystem quantum yield (0.80). 3DA has a much greater fluorescence lifetime (16.9 ns) and fluorescence quantum yield (0.82) at the expense of the intersystem crossing quantum yield (0.12). 4P has the shortest lifetime (0.53 ns), with the lowest fluorescence and intersystem crossing quantum yields (<0.05). The singlet-triplet energy gaps are nearly identical for 2DA, 2P, and 3DA with values on the order of 0.70 eV. This singlet-triplet gap is larger in 4P, with a calculated value of 0.94 eV. The triplet-triplet absorption spectra of 2DA, 2P, and 3DA are similar. Broad peaks in the UV and visible regions with maxima around 330 and 500 nm characterize all three spectra. The triplet excited-state extinction coefficient values for 3DA were found to be 1.5 times larger than those in 2DA and 2P. The triplet-triplet absorption spectrum of 4P is markedly different from the triplet-triplet absorption spectra of the other derivatives. The spectrum is broad, with the four local maxima observed at 374, 445, 624, and 774 nm. All four molecules display delayed fluorescence and laser-power-dependent triplet excited-state decay kinetics, indicating the involvement of triplet-triplet annihilation in the deactivation of the triplet excited states. Both the intrinsic triplet lifetimes and triplet-triplet annihilation rate constants were determined. These values are similar for all of the derivatives with triplet lifetimes on the order of 100 µs and diffusion-controlled rates of triplet-triplet annihilation.

Mechanism of stimulated Mie scattering: Light-induced redistribution of self-assembled nanospheres of two-photon absorbing chromophore.

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.

Synthesis and Intramolecular Energy- and Electron-Transfer of 3D-Conformeric Tris(fluorenyl-[60]fullerenylfluorene) Derivatives.

New 3D conformers were synthesized to show a nanomolecular configuration with geometrically branched 2-diphenylaminofluorene (DPAF-C2M) chromophores using a symmetrical 1,3,5-triaminobenzene ring as the center core for the connection of three fused DPAF-C2M moieties. The design led to a class of cis-cup-tris[(DPAF-C2M)-C60(>DPAF-C9)] 3D conformers with three bisadduct-analogous cages per nanomolecule facing at the same side of the geometrical molecular cis-cup-shape structure. A sequential synthetic route was described to afford this 3D configurated conformer in a high yield with various spectroscopic characterizations. In principle, a nanostructure with a non-coplanar 3D configuration in design should minimize the direct contact or π-stacking of fluorene rings with each other during molecular packing to the formation of fullerosome array. It may also prevent the self-quenching effect of its photoexcited states in solids. Photophysical properties of this cis-cup-conformer were also investigated.

Effects of intramolecular hydrogen bonding and sterically forced non-coplanarity on organic donor/acceptor two-photon-absorbing molecules.

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.

3D-Conformer of Tris[60]fullerenylated cis-Tris(diphenylamino-fluorene) as Photoswitchable Charge-Polarizer on GHz-Responsive Trilayered Core-Shell Dielectric Nanoparticles.

Novel 3D-configurated stereoisomers cis-cup-tris[C60>(DPAF-C9)] and trans-chair-tris[C60>(DPAF-C9)] were designed and synthesized in good yields. The former, with three C60> cages per molecule facing at the same side of the geometrical molecular cup-shape, was proposed to provide excellent binding interaction forces at the gold surface of core-shell γ-FeOx@AuNP nanoparticles and to direct the subsequent formation of a fullerene cage array (defined as fullerosome). Upon photoactivation of the Au-layer and cis-cup-tris[C60>(DPAF-C9)] itself, the degree of photoinduced intramolecular e--transfer from DPAF to a C60> moiety was found to be largely enhanced by the accumulated plasmonic resonance energy at the near-field surface. Distribution of resulting negative charges along the outer (C60>)-derived fullerosome shell layer of the trilayered NPs was correlated with the detected photoswitchable dielectric amplification phenomena using white LED light at 1.0 GHz.

High-Temperature and High-Energy-Density Dipolar Glass Polymers Based on Sulfonylated Poly(2,6-dimethyl-1,4-phenylene oxide).

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.

Steric hindrance inhibits excited-state relaxation and lowers the extent of intramolecular charge transfer in two-photon absorbing dyes.

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.

Contactless, photoinitiated snap-through in azobenzene-functionalized polymers.

Photomechanical effects in polymeric materials and composites transduce light into mechanical work. The ability to control the intensity, polarization, placement, and duration of light irradiation is a distinctive and potentially useful tool to tailor the location, magnitude, and directionality of photogenerated mechanical work. Unfortunately, the work generated from photoresponsive materials is often slow and yields very small power densities, which diminish their potential use in applications. Here, we investigate photoinitiated snap-through in bistable arches formed from samples composed of azobenzene-functionalized polymers (both amorphous polyimides and liquid crystal polymer networks) and report orders-of-magnitude enhancement in actuation rates (approaching 10(2) mm/s) and powers (as much as 1 kW/m(3)). The contactless, ultra-fast actuation is observed at irradiation intensities <<100 mW/cm(2). Due to the bistability and symmetry of the snap-through, reversible and bidirectional actuation is demonstrated. A model is developed to elucidate the underlying mechanics of the snap-through, specifically focusing on isolating the role of sample geometry, mechanical properties of the materials, and photomechanical strain. Using light to trigger contactless, ultrafast actuation in an otherwise passive structure is a potentially versatile tool to use in mechanical design at the micro-, meso-, and millimeter scales as actuators, as well as switches that can be triggered from large standoff distances, impulse generators for microvehicles, microfluidic valves and mixers in laboratory-on-chip devices, and adaptive optical elements.

Broadband Two-Photon Absorption Characteristics of Highly Photostable Fluorenyl-Dicyanoethylenylated [60]Fullerene Dyads.

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.

Frequency-Driven Self-Organized Helical Superstructures Loaded with Mesogen-Grafted Silica Nanoparticles.

Adding colloidal nanoparticles into liquid-crystal media has become a promising pathway either to enhance or to introduce novel properties for improved device performance. Here we designed and synthesized new colloidal hybrid silica nanoparticles passivated with a mesogenic monolayer on the surface to facilitate their organo-solubility and compatibility in a liquid-crystal host. The resulting nanoparticles were identified by 1 H NMR spectroscopy, TEM, TGA, and UV/Vis techniques, and the hybrid nanoparticles were doped into a dual-frequency cholesteric liquid-crystal host to appraise both their compatibility with the host and the effect of the doping concentration on their electro-optical properties. Interestingly, the silica-nanoparticle-doped liquid-crystalline nanocomposites were found to be able to dynamically self-organize into a helical configuration and exhibit multi-stability, that is, homeotropic (transparent), focal conic (opaque), and planar states (partially transparent), depending on the frequency applied at sustained low voltage. Significantly, a higher contrast ratio between the transparent state and scattering state was accomplished in the nanoparticle-embedded liquid-crystal systems.

Synthesis of Photoswitchable Magnetic Au-Fullerosome Hybrid Nanomaterials for Permittivity Enhancement Applications.

We designed and synthesized several nanomaterials 3 of three-layered core-shell (γ-FeOx@AuNP)@[C60(>DPAF-C9) 1 or 2]n nanoparticles (NPs). These NPs having e(-)-polarizable fullerosome structures located at the outer layer were fabricated from highly magnetic core-shell γ-FeOx@AuNPs. Fullerosomic polarization of 3 was found to be capable of causing a large amplification of material permittivity that is also associated with the photoswitching effect in the frequency range of 0.5-4.0 GHz. Multilayered synthetic construction allows Förster resonance energy transfer (FRET) of photoinduced accumulative surface plasmon resonance (SPR) energy in the gold layer to the partially bilayered C60(>DPAF-C9) 1 or 2-derived fullerosome membrane shell layer in a near-field of direct contact without producing radiation heat, which is commonly associated with SPR.

Synthesis and photoluminescent properties of geometrically hindered cis-tris(diphenylaminofluorene) as precursors to light-emitting devices.

A novel highly luminescent tris-fluorenyl ring-interconnected chromophore tris(DPAF-C9) was synthesized using a C3 symmetrical triaminobenzene core as the synthon. This structure bears three light-harvesting 2-diphenylamino-9,9-dialkylfluorenyl (DPAF) ring moieties with each attached by two branched 3',5',5'-trimethylhexyl (C9) arms. A major stereoisomer was chromatographically isolated and characterized to possess a 3D structural configuration of cis-conformer in a cup-form. Molecular calculation at B3LYP/6-31G* level revealed the unexpected stability of this cis-cup-conformer of tris(DPAF-C9) better than that of the stereoisomer in a propeller-form and the trans-conformer. The structural geometry is proposed to be capable of minimizing the aggregation related self-quenching effect in the condensed phase. Fluorescence emission wavelength of tris(DPAF-C9) was found to be in a close range to that of PVK that led to its potential uses as the secondary blue hole-transporting material for enhancing the device property toward the modulation of PLED performance.

Photopiezoelectric composites of azobenzene-functionalized polyimides and polyvinylidene fluoride.

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.

Symmetry- and solvent-dependent photophysics of fluorenes containing donor and acceptor groups.

Three two-photon absorption (2PA) dyes (donor-π-donor (DPA2F), donor-π-acceptor (AF240), and acceptor-π-acceptor (BT2F); specifically, D is Ph2N-, A is 2-benzothiazoyl, and the π-linker is 9,9-diethylfluorene) are examined in a variety of aprotic solvents. Because the 2PA cross section is sensitive to the polarity of the local environment, this report examines the solvent-dependent linear photophysics of the dyes, which are important to understand before probing more complex solid-state systems. The symmetrical dyes show little solvent dependence; however, AF240 has significant solvatochromism observed in the fluorescence spectra and lifetimes and also the transient absorption spectra. A 114 nm bathochromic shift is observed in the fluorescence maximum when going from n-hexane to acetonitrile, whereas the lifetimes increase from 1.25 to 3.12 ns. The excited-state dipole moment for AF240 is found to be 20.1 D using the Lippert equation, with smaller values observed for the symmetrical dyes. Additionally, the femtosecond transient absorption (TA) spectra at time zero show little solvent dependence for DPA2F or BT2F, but AF240 shows a 52 nm hypsochromic shift from n-hexane to acetonitrile. Coupled with the solvatochromism in the fluorescence and large excited-state dipole moment, this is attributed to formation of an intramolecular charge-transfer (ICT) state in polar solvents. By 10 ps in AF240, the maximum TA in acetonitrile has shifted 30 nm, providing direct evidence of a solvent-stabilized ICT state, whose formation occurs in 0.85-2.71 ps, depending on solvent. However, AF240 in nonpolar solvents and the symmetrical dyes in all solvents show essentially no shifts due to a predominantly locally excited (LE) state. Preliminary temperature-dependent fluorescence using frozen glass media supports significant solvent reorganization around the AF240 excited state in polar solvents, and may also support a twisted intramolecular charge-transfer (TICT)-state contribution to the stabilization. Finally, time-dependent density functional theory calculations support ICT in AF240 in polar media and also allow prediction of the 2PA cross sections in the 0-0 band, which are much larger for AF240 than the symmetrical dyes.

Exciplex formation in blended spin-cast films of fluorene-linked dyes and bisphthalimide quenchers.

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.

Synthesis of photoresponsive dual NIR two-photon absorptive [60]fullerene triads and tetrads.

Broadband nonlinear optical (NLO) organic nanostructures exhibiting both ultrafast photoresponse and a large cross-section of two-photon absorption throughout a wide NIR spectrum may make them suitable for use as nonlinear biophotonic materials. We report here the synthesis and characterization of two C60-(antenna)(x) analogous compounds as branched triad C60(>DPAF-C18)(>CPAF-C(2M)) and tetrad C60(>DPAF-C18)(>CPAF-C(2M))2 nanostructures. These compounds showed approximately equal extinction coefficients of optical absorption over 400-550 nm that corresponds to near-IR two-photon based excitation wavelengths at 780-1,100 nm. Accordingly, they may be utilized as potential precursor candidates to the active-core structures of photosensitizing nanodrugs for 2γ-PDT in the biological optical window of 800-1,050 nm.

Magnetocurrent of charge-polarizable C60-diphenylaminofluorene monoadduct-derived magnetic nanocomposites.

We demonstrated the strategy of a nanocomposite design by the incorporation of both a delocalized π-electrons system in a closely bound acceptor-donor analogue chromophore, based on charge-polarizable C(60)(>DPAF-C(9)) nanostructure 1, and spin-polarized d-electrons in the form of γ-FeO(x) nanoparticles. Facile intramolecular electron transfer from the DPAF-C(9) donor moiety to the C(60) acceptor cage of 1 upon activation to the excited state with a long lifetime of the charge-separated state forms a possible mechanism to integrate semiconducting and magnetic properties in a single system. We observed an appreciable magnetocurrent (MC) of C(60)(>DPAF-C(9))-encapsulated magnetic γ-FeO(x) nanoparticles in PMMA matrix upon applying a magnetic field from 0 to 300 mT at either 77 K (12% MC) or 300 K (4.5% MC). Interestingly, the detailed analysis of magnetocurrent curve profiles taken at 77 K allowed us to conclude that the measured magnetocurrent may be attributed to the contributions from magnetic field-dependent excited-state populations in semiconducting structure (density-based MC), magnetism from magnetic structure (mobility-based MC), and product of density and mobility-based MC components (π-d electronic coupling). At the higher temperature region up to 300 K, the semiconducting mechanism dominated the determining factor of measured magnetocurrent. This experimental observation indicated the feasibility of combining delocalized π electrons and spin-polarized d electrons through charge transfer to induce internally coupled dual mobility- and density-based MC through the modulation of spin polarization and excited states in semiconducting/magnetic hybrid materials.

Plasmonic enhancement of the two photon absorption cross section of an organic chromophore using polyelectrolyte-coated gold nanorods.

The effect of plasmonic enhancement on the two-photon absorption cross section of organic chromophores attached to polyelectrolyte-coated gold nanorods was investigated. The magnitudes of such enhancements were confirmed using single and two photon excitations of the chromophore molecules bound to polyelectrolyte-coated gold nanorods. By synthesizing two-, four-, six-, and eight-polyelectrolyte layer coated nanorods of a particular aspect ratio, the distance dependence of the evanescent electromagnetic field on molecular two-photon absorption was observed. Enhancements of 40-fold were observed for the chromophores nearest to the surface.

Role of Alicyclic Conformation-Isomerization in the Photomechanical Performance of Azobenzene-Functionalized Cross-Linked Polyimides Containing Tetra-Substituted Cyclohexane Moieties.

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 Troom is implicated for the observed photothermal memory.

Intermolecular Interactions and Intramolecular Motions in Photomechanical Effect: Nonlinear Thermo- and Photomechanical Behaviors of Azobenzene-Functionalized Amide-Imide Block Copolymers.

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 %, and [HB-AA] for [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.