Four alkyl-substituted thiophene-3-carboxylate containing donor–acceptor (D–A) copolymers were designed, synthesized, and characterized. Thiophene-3-carboxylate was used as a weak electron acceptor unit in the copolymers to provide a deeper highest occupied molecular orbital (HOMO) level for obtaining a higher open-circuit voltage in polymer solar cells (PSCs). The resulting bulk heterojunction PSCs, made of the copolymers and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), exhibited different short circuit currents (JSCs) and open-circuit voltages (VOCs), depending on the length of alkyl side-chain in the thiophene-3-carboxylate unit. Among all fabricated photovoltaic (PV) devices, PC2:PC71BM (1:1 wt. ratio) showed the highest efficiency with the highest JSC of 10.5 mA/cm2. Although PC5:PC71BM (1:1) displayed the highest VOC of 0.93 V, the device efficiency was observed to be poor, which is due to poor nanophase segregation. This comparison shows that the side-chain of thiophene carboxylate in these copolymers plays a very important role in the device efficiency.

　Tricyanofuran-based small molecules were synthesized through Knoevenagel reaction with formyl heteroaromatic donating moieties and 2-cyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) as a strong acceptor. UV–vis absorption demonstrated that the combination of TCF with donating units resulted in an enhanced intra-molecular charge transfer (ICT) transition, which led to long wavelength absorption in the chromophores. The absorption coefficients and the molecular energy levels of the chromophores can be tuned effectively by employing different donating groups. The chromophores were used in conjunction with [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) to fabricate organic heterojunction photovoltaic cells. This solution-processed bulk heterojunction (BHJ) solar cells exhibited a power conversion efficiency (PCE) of 2.44%, a short-circuit current density (JSC) of 8.74 mA/cm2, and an open-circuit voltage (VOC) of 0.93 V under simulated air mass 1.5 global irradiation (100 mW/cm2).

　The synthesis and photophysical characterization of a PPV-type copolymer containing a fluorene derivative alternated with thiophene units is presented: poly(9,9’-dioctylfluorene-thiophene) (LAPPS29). Photophysical studies demonstrated that in the solid state only preformed ground state aggregates are responsible for exciton formation. These aggregates are formed with a wide range of size distribution. The emission from isolated segments is quenched either by resonant energy transfer, or by migration processes. Also, the main photovoltaic parameters are discussed in connection with the photophysical behavior.

　By adding silver ion to the non-fluorescent solution of a pyridine functionalized α-cyanostilbene derivative, an immediate gelation accompanying a high-contrast fluorescence turn-on was effectively generated. The silver ion-coordinated highly fluorescent gel underwent reverse gel-to-sol transition restoring the non-fluorescent sol state upon reaction with tetrabutylammonium fluoride. Silver nanoparticles embedded in the fluorescent gel film were fabricated by in situ UV-light irradiation, which induced the surface plasmon effect on the fluorescence emission.

　A new donor-acceptor copolymer, containing benzodithiophene (BDT) and methyl thiophene-3-carboxylate (3MT) units, is designed and synthesized for polymer solar cells (PSCs). The 3MT unit is used as an electron acceptor unit in this copolymer to provide a lower highest occupied molecular orbital (HOMO) level for obtaining polymer solar cells with a higher open-circuit voltage (VOC). The resulting bulk heterojunction PSC made of the copolymer and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibits a power conversion efficiency (PCE) up to 4.52%, a short circuit current (JSC) of 10.5 mA·cm-2, and a VOC of 0.86 V.

　A dual-emissive biopolymeric amphiphile (GC–HBO) has been prepared by densely conjugating hydrophilic glycol chitosan (GC) with a phototautomerizable hydrophobic dye (a derivative of 2-(2′-hydroxyphenyl)benzoxazole, HBO) showing excited-state intramolecular proton transfer (ESIPT). Ratiometric modulation of phototautomeric (enol–keto) dual emission, governed by the chromophoric aggregation state, was studied with a low-molecular-weight model compound (GA–HBO) and was well correlated with the nanoparticle-forming behavior of GC–HBO via amphiphilic self-assembly in water. It has been found that the promoted chromophoric aggregation at higher concentration or at basic-to-neutral pH assists the nanostructure formation to generate predominant keto emission while the enol emission is intensified at lower concentration or with decreasing pH by disintegration of self-assembled structure. The assembly-related advantageous potential of GC–HBO for probing self-concentration and surrounding pH has been demonstrated with the ratiometric determination of critical particulation concentration (0.04 kg m−3) and near-physiological pH sensitivity (pKa ∼ 5.1).

　A facile approach to make an efficient hybrid bulk heterojunction photovoltaic device with lead sulfide nanocrystals and a low-bandgap polymer is demonstarted, resulting in a power conversion efficiency of about 2–3%.

　This study aimed at elucidating the self-assembly structure of newly synthesized three-armed discotic liquid crystal molecules (DLCs) which give rise to small-angle X-ray scattering (SAXS) profiles that have difficulties in the direct determination of the symmetry of long-range intercolumnar lattice. The self-assembly structure of newly synthesized two analogues of DLCs, viz. 1,3,5-tris[2-(4-dodecyloxyphenyl)-oxadiazol-5-yl]benzene (TDOB) and 1,3,5-tris[2-(3,4,5-tris-dodecyloxyphenyl)-oxadiazol-5-yl]benzene (TTDOB), being discerned only in the number of dodecyloxyphenyl tails on each oxadiazol arm, were elucidated by comparative analyzes between experimentally observed SAXS profiles and those generated from a computational method. TTDOB molecules exhibited a typical SAXS profile of a hexagonal columnar mesophase, but TDOB molecules showed an ambiguous one hard to analyze. We found that, in TDOB molecules, the low degree of branching caused the localized conjugated electrons, which leads to weakening of interdisc interactions of core part and loosing the packing of disc molecules. And the free space between arms, afforded by a single branch on each arm, and electrostatic interactions between opposite charges on neighboring molecules which come from the localized electrons, allow TDOB columns to pack more closely than the disc diameter, then to form an interdigitated columnar structure. Such self-assembly structure is thought to be the result of the balance of various intermolecular interactions, so the self-assembly structures were tried to explain through a relative contribution of each intermolecular interaction component, that is, individual interaction energy values that we can calculate. With our proposed approaches, it is expected to widen our understanding of the self-assembly structures of various materials including DLCs.

　Enhanced photorefractive (PR) response in nanocrystal-polymer nanocomposite PR devices at the telecommunication wavelength of 1.3 𝜇m1.3 μm is achieved using facile surface treatment of PbS nanocrystals functionalized with thermally cleavable ligands. The cleavage of the ligands, by heat treatment, results in significant improvement in both photocharge generation and PR response, as demonstrated by photoconductivity and two-beam coupling (TBC) experiments, respectively. The TBC experiment shows that the optical gain coefficient is improved drastically (about 63%) at an applied electric field of 84 V/𝜇m84 V/μm.

　This work was supported by the U.S. Air Force Office of Scientific Research and also by NSF through Grant No. DMS-0702372. The authors thank K. R. Choudhury for his active help in different stages of the study.

　Excited state intramolecular proton transfer (ESIPT) and subsequent intramolecular charge transfer (ICT) dynamics of a 2-(2′-hydroxyphenyl)benzoxazole derivative conjugated with an electron withdrawing group (HBOCE) in solutions and a polymer film has been investigated by femtosecond time-resolved fluorescence (TRF) and TRF spectra measurements without the conventional spectral reconstruction method. TRF with high enough resolution (<100 fs) reveals that the ESIPT dynamics of HBOCE in liquids proceeds by at least two time constants of ∼250 fs and ∼1.2 ps. The relative amplitude of the slower picosecond component is smaller in the polymer film than that in solution. Conformational heterogeneity in the ground state originating from the dispersion of the dihedral angle between the phenolic and benzoxazole groups is invoked to account for the dispersive ESIPT dynamics in liquids. From the TRF spectra of both the enol and keto isomers, we have identified the ICT reaction of the keto isomer occurring subsequent to the ESIPT. The ICT proceeds also by two time constants of near instantaneous and 2.7 ps. Since the ICT dynamics of HBOCE is rather close to the polar solvation dynamics, we argue that the ICT is barrierless and determined mostly by the solvent fluctuation.

　Gelation-enhanced fluorescence: The fluorescence emission intensity of the organogel formed by a salicylanilide-containing organogelator is 10–50 times higher than that of the corresponding solution phase (see picture). The fluorescence enhancement is due to restricted intramolecular rotation in the gel state, which prevents access to the twisted intramolecular charge-transfer state after excited-state intramolecular proton transfer.