Ab inito study on triplet excitation energy transfer in photosynthetic light-harvesting complexes

We have studied the triplet energy transfer (TET) for photosynthetic light-harvesting complexes, the bacterial light-harvesting complex II (LH2) of Rhodospirillum molischianum and Rhodopseudomonas acidophila, and the peridinin-chlorophyll a protein (PCP) from Amphidinium carterae. The electronic coupling factor was calculated with the recently developed fragment spin difference scheme (You and Hsu, J. Chem. Phys. 2010, 133, 074105), which is a general computational scheme that yields the overall coupling under the Hamiltonian employed. The TET rates were estimated based on the couplings obtained. For all light-harvesting complexes studied, there exist nanosecond triplet energy transfer from the chlorophylls to the carotenoids. This result supports a direct triplet quenching mechanism for the photoprotection function of carotenoids. The TET rates are similar for a broad range of carotenoid triplet state energy, which implies a general and robust TET quenching role for carotenoids in photosynthesis. This result is also consistent with the weak dependence of TET kinetics on the type or the number of π conjugation lengths in the carotenoids and their analogues reported in the literature. We have also explored the possibility of forming triplet excitons in these complexes. In B850 of LH2 or the peridinin cluster in PCP, it is unlikely to have triplet exciton since the energy differences of any two neighboring molecules are likely to be much larger than their TET couplings. Our results provide theoretical limits to the possible photophysics in the light-harvesting complexes.     

A strategy to design highly efficient porphyrin sensitizers for dye-sensitized solar cells

We designed highly efficient porphyrin sensitizers with two phenyl groups at meso-positions of the macrocycle bearing two ortho-substituted long alkoxyl chains for dye-sensitized solar cells; the ortho-substituted devices exhibit significantly enhanced photovoltaic performances with the best porphyrin, LD14, showing J(SC) = 19.167 mA cm(-2), V(OC) = 0.736 V, FF = 0.711, and overall power conversion efficiency η = 10.17%.     

Heteroleptic ruthenium sensitizers that contain an ancillary bipyridine ligand tethered with hydrocarbon chains for efficient dye-sensitized solar cells

New heteroleptic ruthenium complexes have been synthesized and used as the sensitizers for dye‐sensitized solar cells (DSSCs). The ancillary bipyridine ligand contains rigid aromatic segments (fluorene‐, carbazole‐, or dithieno[3,2‐b:2′,3′‐d]pyrrole‐substituted bipyridine) tethered with a hydrophobic hexyl substituent. The conjugated aromatic segment results in significant bathochromic shift and hyperchromic effects in these complexes compared with Z907 (cis‐[Ru LL′ (NCS)₂]; L =4,4′‐dicarboxylic acid‐2,2′‐bipyridine, L′ =4,4′‐dinonyl‐2,2′‐ bipyridine). The long hydrocarbon chains help to suppress the dark current if appropriately disposed. DSSCs that use these complexes exhibit very impressive conversion efficiencies (5.94 to 6.91 %) that surpass that of Z907 ‐based (6.36 %) DSSCs and are comparable with that of N719 ‐based standard cells (7.13 %; N719 =cis‐di(thiocyanato)bis(2,2′‐bipyridyl‐4,4′‐dicarboxylato)ruthenium(II) bis(tetrabutylammonium)) fabricated and measured under similar conditions (active area: 0.5×0.5 cm²; AM 1.5 sunlight).     

Enhancing the stiffness of electrospun nanofiber scaffolds with a controlled surface coating and mineralization

A new method was developed to coat hydroxyapatite (HAp) onto electrospun poly(lactic-co-glycolic acid) (PLGA) nanofibers for tendon-to-bone insertion site repair applications. Prior to mineralization, chitosan and heparin were covalently immobilized onto the surface of the fibers to accelerate the nucleation of bone-like HAp crystals. Uniform coatings of HAp were obtained by immersing the nanofiber scaffolds into a modified, 10-fold-concentrated simulated body fluid (m10SBF) for different periods of time. The new method resulted in thicker and denser coatings of mineral on the fibers compared to those produced by previously reported methods. Scanning electron microscopy measurements confirmed the formation of nanoscale HAp particles on the fibers. A mechanical property assessment demonstrated a higher stiffness with respect to previous coating methods. A combination of the nanoscale fibrous structure and bonelike mineral coating could mimic the structure, composition, and function of mineralized tissues.     

Solving EDL configuration near a dissimilarly charged protrusions surface model using perturbation method

In this work, an elementary, novel dissimilarly charged protrusions (DCP) surface model in an electrolyte solution considering simultaneously the complexity of both surface morphology and surface charged condition, which are concerned frequently on a biological cell membrane, on a modified micro-particle surface, or in a lab-on-a-chip biosensor device, is proposed. Based on Fourier series and the perturbation technique, the configuration of electrical double layer (EDL) near this complicated charged surface model is successfully solved semi-analytically. The numerical calculation reveals that, the methodology suggested in present study could deal with charged surface systems of arbitrary geography and of arbitrary charge distribution. In the analysis, three special subjects are discussed, including an isolated dissimilarly charged protrusion, the effect of protrusions, and the effect of dissimilarly charged condition on protrusions.     

Electrophoresis of a particle at an arbitrary surface potential and double layer thickness: importance of nonuniformly charged conditions

Recent advances in material science and technology yield not only various kinds of nano- and sub-micro-scaled particles but also particles of various charged conditions such as Janus particles. The characterization of these particles can be challenging because conventional electrophoresis theory is usually based on drastic assumptions that are unable to realistically describe the actual situation. In this study, the influence of the nonuniform charged conditions on the surface of a particle at an arbitrary level of surface potential and double layer thickness on its electrophoretic behavior is investigated for the first time in the literature taking account of the effect of double-layer polarization. Several important results are observed. For instance, for the same averaged surface potential, the mobility of a nonuniformly charged particle is generally smaller than that of a uniformly charged particle, and the difference between the two depends upon the thickness of double layer. This implies that using the conventional electrophoresis theory may result in appreciable deviation, which can be on the order of ca. 20%. In addition, the nonuniform surface charge can yield double vortex in the vicinity of a particle by breaking the symmetric of the flow field, which has potential applications in mixing and/or regulating the medium confined in a submicrometer-sized space, where conventional mixing devices are inapplicable.     

Curvature sorting of peripheral proteins on solid-supported wavy membranes

Cellular membrane deformation and the associated redistribution of membrane-bound proteins are important aspects of membrane function. Current model membrane approaches for studying curvature sensing are limited to positive curvatures and often require complex and delicate experimental setups. To overcome these challenges, we fabricated a wavy substrate by imposing a range of curvatures onto an adhering lipid bilayer membrane. We examined the curvature sorting of several peripheral proteins binding to the wavy membrane and observed them to partition into distinct regions of curvature. Furthermore, single-molecule imaging experiments suggested that the curvature sensing of proteins on low-curvature substrates requires cooperative interactions.     

Induction of the Columnar Phase of Unconventional Dendrimers by Breaking the C2 Symmetry of Molecules

Two triazine‐based unconventional dendrimers were prepared and characterized by ¹H and ¹³C NMR spectroscopy, mass spectrometry, and elemental analysis. Differential scanning calorimetry, polarizing microscopy, and powder XRD studies showed that these dendrimers display columnar liquid‐crystalline phases during thermal treatment. This is ascribable to breaking of their C₂ symmetry. The molecular conformations of prepared dendrimers were obtained by computer simulation with the MM3 model of the CaChe program in the gas phase. The simulation showed that the conformations of the prepared dendrimers are rather flat and disfavor formation of the LC phase. However, due to C₂‐symmetry breaking, the prepared dendrimers have structural isomers in the solid state and thus show the desired columnar phases. This new strategy should be applicable to other types of unconventional dendrimers with rigid frameworks.     

Enantiotropic Nematics From Cross‐Like 1,2,4,5‐Tetrakis(4′‐alkyl‐4‐ethynylbiphenyl)benzenes and Their Biaxiality Studies

The theoretically predicted optimum length/breadth/width ratio for maximizing shape biaxiality was investigated experimentally by the facile and successful synthesis of cross‐shaped compound 3 , which showed enantiomeric nematic phase behavior. This cross‐like core structure could alternatively be viewed as two fused V‐shaped mesogens, which have recently immerged as a new direction in biaxial nematic research, at the bending tips that can act as a new structure for biaxial investigations. Whilst the thermal analysis data of compound 3 did not meet the expected theoretical values for biaxial nematics, surface‐induced biaxiality was evidenced by optical studies. Cluster‐size analysis within the nematic phase of compound 3 revealed the formation of meta‐cybotactic nematics, which approached the cluster sizes of cybotactic nematics. The split small‐angle 2D X‐ray diffraction patterns of magnetic‐field‐aligned samples indicated that the nematic phase was composed of small smectic C‐like clusters with the tilting of molecules within the clusters. The wide‐temperature‐range enantiomeric nematic phase of cross‐like compound 3 enabled the molecular skeleton to serve as an alternative skeleton to bent‐rod mesogens, which exhibited nematic phases with the potential competition of transitions to higher‐order liquid‐crystalline phases and crystallization, for future biaxial investigations.