Photosynthetic Antenna–Reaction Center Mimicry by Using Boron Dipyrromethene Sensitizers

Various molecular and supramolecular systems have been synthesized and characterized recently to mimic the functions of photosynthesis, in which solar energy conversion is achieved. Artificial photosynthesis consists of light‐harvesting and charge‐separation processes together with catalytic units of water oxidation and reduction. Among the organic molecules, derivatives of BF₂‐chelated dipyrromethene (BODIPY), “porphyrin’s little sister”, have been widely used in constructing these artificial photosynthetic models due to their unique properties. In these photosynthetic models, BODIPYs act as not only excellent antenna molecules, but also as electron‐donor and ‐acceptor molecules in both the covalently linked molecular and supramolecular systems formed by axial coordination, hydrogen bonding, or crown ether complexation. The relationships between the structures and photochemical reactivities of these novel molecular and supramolecular systems are discussed in relation to the efficiency of charge separation and charge recombination. Femto‐ and nanosecond transient absorption and photoelectrochemical techniques have been employed in these studies to give clear evidence for the occurrence of energy‐ and electron‐transfer reactions and to determine their rates and efficiencies.     

Carbon Dots: Bottom‐Up Syntheses,Properties, and Light‐Harvesting Applications

The development of cost‐effective and environmentally friendly photocatalysts and photosensitizers has received tremendous attention because of their potential utilization in solar‐light‐harvesting applications. In this respect, carbon dots (CDs) prepared by bottom‐up methods have been considered to be promising light‐harvesting materials. Through their preparation from various molecular precursors and synthetic methods, CDs exhibit excellent optical and charge‐transfer properties. Furthermore, their photophysical properties can be readily optimized and enhanced by means of doping, functionalization, and post‐synthetic treatment. In this review, we summarize the recent progress in CDs synthesized using bottom‐up approaches. These CDs exhibit strong light absorption and unique electron donor/acceptor capabilities for light‐harvesting applications. We anticipate that this review will provide new insights into novel types of photosensitizers and photocatalysts for a wide range of applications.     

Effect of the linkage position in ruthenium dyes on the photoelectrochemical perfomance

A series of newly designed Ru-dyes constructed with [Ru(bpy)(bpp)Cl]⁺ (bpp?=?2,6-bis(N-pyrazolyl)pyridine, bpy?=?bipyridine) as template were evaluated by studying the frontier molecular orbital energy, charge distribution, absorption spectrum, light harvesting efficiency, as well as photogenerated current density by using (time-dependent) density functional theory. The calculation results reveal that the light harvesting efficiency and current density are enhanced with only one linkage group attached onto the 4 site of pyridine ligand. Mounting the linkage groups to 3 or 3′ position on pyridine ligand introduces super strong UV absorption.     

Ultrafast Relaxation Processes of Conjugated Polymer Nanoparticles in the Presence of Au Nanoparticles

Considering the importance of conjugated polymer nanoparticles, major emphasis has been given for designing and understanding the energy transfer and charge transfer processes of organic‐inorganic hybrids for light harvesting applications. In the present study, we have designed an aqueous solution‐based light harvesting system using conjugated polymer nanoparticles (poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene], MEH‐PPV) and Au nanoparticles. The change in photo‐induced processes in the presence of metal nanoparticles are studied by steady‐state absorption, time‐resolved emission, time‐resolved fluorescence up‐conversion, ultrafast anisotropy and femtosecond transient absorption spectroscopy. Global and target analysis of transient absorption data validate the creation of a collective delocalized state in polymer nanoparticles, and the time scale for excitation energy funnelling from S₁ state to low lying collective delocalized state (CL_s) is 18 ps. Then, the electron transfer from the CL_s state to Au NP occurs with a time constant of 150 ps. The 815 ps long lived charge transfer (CT) state signifies the charge transfer from the CL_s state of polymer nanoparticles to Au NP. Such basic understanding of relaxation processes in hybrid systems is very important for designing inorganic‐organic hybrid light‐harvesting systems.     

Multichromophoric Organic Molecules Encapsulated in Polymer Nanoparticles for Artificial Light Harvesting

We designed a self‐assembled multichromophoric organic molecular arrangement inside polymer nanoparticles for light‐harvesting antenna materials. The self‐assembled molecular arrangement of quaterthiophene molecules was found to be an efficient light‐absorbing antenna material, followed by energy transfer to Nile red (NR) dye molecules, which was confined in polymer nanoparticles. The efficiency of the antenna effect was found to be 3.2 and the effective molar extinction coefficient of acceptor dye molecules was found to be enhanced, which indicates an efficient light‐harvesting system. Based on this energy‐transfer process, tunable photo emission and white light emission has been generated with 14 % quantum yield. Such self‐assembled oligothiophene–NR systems encapsulated in polymer nanoparticles may open up new possibilities for fabrication of artificial light harvesting system.     

DFT and TD-DFT study of benzene and borazines containing chromophores for DSSC materials

In this investigation, we have designed a series of benzene and borazines containing chromophores for employing in dye-sensitized solar cells (DSSCs). The optimized structures and photo-physical properties of these molecules have been explored by using the density functional theory method (B3LYP/6-311++G(d,p)). These dyes consist of electron-donor (benzene, borazine, fluorinated borazine) and -acceptor/anchoring (tricyanovinyl), connected by the π-conjugated linker as an electron spacer. The Natural Bond Orbital (NBO) analysis has also been employed for studying the origin of charge transfer. The time-dependent density functional theory (TD-DFT) method has also been used to calculate the electronic absorption spectra of these molecules. The maximum absorption wavelengths assign to HOMO → LUMO transition. The electronic coupling constant, electron injection and light harvesting efficiency have been computed by first principle researches. This revealed that the studied molecules would be efficient photosensitizers.     

Switching the photoinduced processes in host-guest complexes of β-cyclodextrin-substituted silicon(IV) phthalocyanines and a tetrasulfonated porphyrin

Porphyrins and phthalocyanines are two attractive classes of functional dyes for the construction of artificial light harvesting and charge separation molecular systems. The assembly of these components by supramolecular approach is of particular interest as this provides a facile route to build multi-chromophoric arrays with various architectures and tuneable photophysical properties. We report herein a series of host-guest complexes formed between a tetrasulfonated porphyrin and several silicon(IV) phthalocyanines substituted axially with two permethylated β-cyclodextrin units via different spacers. As shown by electronic absorption and fluorescence spectroscopic methods, the two components bind spontaneously in a 1:1 manner in water with large binding constants in the range of 1.1 × 10(7) to 3.5 × 10(8) M(-1). The photophysical properties of the resulting supramolecular complexes have also been studied in detail using steady-state and time-resolved optical spectroscopic methods. It has been found that two major photoinduced processes, namely fluorescence resonance energy transfer and charge transfer are involved which are controlled by the spacer between the β-cyclodextrin units and the silicon centre of phthalocyanine. Despite the fact that charge transfer is a thermodynamically favourable process for all the complexes, only the ones with a tetraethylene glycol or oxo linker exhibit an efficient charge transfer from the excited phthalocyanine to the porphyrin entity. The lifetimes of the corresponding charge-separated states have been determined to be 200 and 70 ps by picosecond pump-probe experiments.     

Highly efficient organic indolocarbazole dye in different acceptor units for optoelectronic applications—a first principle study

A series of novel organic dyes (ICZA1, ICZA2, ICZA3, ICZA4) with D-π-A structural configuration incorporating indolo[3,2,1-jk]carbazole moiety as donor (D) unit, thiophene as π-linker and 2-cyanoacrylic acid as acceptor unit were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Indolo[3,2,1-jk]carbazole-based D-π-A dyes composed of different acceptor groups were designed. By modulating acceptor unit, the efficiency of D-π-A dye-based dye-sensitized solar cells (DSSCs) can be further improved. In the present work, four novel push-pull organic dyes only differing in electron acceptor, have been designed based on the experimental literature value of IC-2. In order to further improve the light harvesting capability of indolo[3,2,1-jk]carbazole dyes, the acceptor influence on the dye performance were examined. The NLO property of the designed dye molecules can be derived as polarizability and hyperpolarizability. The calculated value of ICZA2 dye is the best candidate for NLO properties. Furthermore, the designed organic dyes exhibit good photovoltaic performance of charge transfer characteristics, driving force of electron injection, dye regeneration, global reactivity, and light harvesting efficiency (LHE). From the calculated value of ICZA4 dye, it has been identified as a good candidate for DSSCs applications. Finally, it is concluded that the both ICZA2 and ICZA4 dyes theoretically agrees well with the experimental value of IC-2 dye. Hence, the dyes ICZA2 and ICZA4 can serve as an excellent electron withdrawing groups for NLO and DSSCs applications.     

Synthesis and structural study of two new heparin-like hexasaccharides

Two new heparin-like hexasaccharides, 5 and 6, have been synthesised using a convergent block strategy and their solution conformations have been determined by NMR spectroscopy and molecular modelling. Both hexasaccharides contain the basic structural motif of the regular region of heparin but with negative charge distributions which have been designed to get insight into the mechanism of fibroblast growth factors (FGFs) activation.     

给电子基团对吲哚染料电子结构和吸收光谱的影响

利用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT), 分别基于B3LYP和PBE1PBE方法研究了一系列含有不同给电子基团的吲哚染料分子(ID1-ID3)的电子结构和吸收光谱性质. 重点比较了不同电子给体对染料的分子结构、吸收光谱以及其在电池中的光伏性能的影响. 结果表明从ID1、ID2到ID3, 随着电子给体中苯环数目的增加, 吲哚分子上的共轭效应逐渐增大, 导致吲哚分子最高占据分子轨道-最低未占据分子轨道(HOMO-LUMO)之间的能隙变窄, 分子的吸收光谱发生红移. 染料分子的吸收光谱和LUMO能级分别影响染料的吸光效率和光电转化过程中电子的注入过程, 从而使其二者成为决定电池光伏性能的重要参数. 综合考虑上述两个参数对电池性能的贡献, 通过理论研究证实, 在ID1-ID3系列染料中, ID3具有较长的吸收谱带、较大的分子消光系数和合适的LUMO能级, 从而表现出最为优越的光伏性能, 这与实验得出的结论很好地吻合.     

Probing the efficiency of electron transfer through porphyrin-based molecular wires

Electron transfer over long distances is important for many future applications in molecular electronics and solar energy harvesting. In these contexts, it is of great interest to find molecular systems that are able to efficiently mediate electrons in a controlled manner over nanometer distances, that is, structures that function as molecular wires. Here we investigate a series of butadiyne-linked porphyrin oligomers with ferrocene and fullerene (C60) terminals separated by one, two, or four porphyrin units (Pn, n = 1, 2, or 4). When the porphyrin oligomer bridges are photoexcited, long-range charge separated states are formed through a series of electron-transfer steps and the rates of photoinduced charge separation and charge recombination in these systems were elucidated using time-resolved absorption and emission measurements. The rates of long-range charge recombination, through these conjugated porphyrin oligomers, are remarkably fast (kCR2 = 15 - 1.3 x 108 s-1) and exhibit very weak distance dependence, particularly comparing the systems with n = 2 and n = 4. The observation that the porphyrin tetramer mediates fast long-range charge transfer, over 65 A, is significant for the application of these structures as molecular wires.     

A Series of Iron(II)-NHC Sensitizers with Remarkable Power Conversion Efficiency in Photoelectrochemical Cells**

A series of six new Fe(II)NHC-carboxylic sensitizers with their ancillary ligand decorated with functions of varied electronic properties have been designed with the aim to increase the metal-to- surface charge separation and light harvesting in iron-based dye-sensitized solar cells (DSSCs). ARM130 scored the highest efficiency ever reported for an iron-sensitized solar cell (1.83 %) using Mg²⁺ and NBu₄I-based electrolyte and a thick 20 μm TiO₂ anode. Computational modelling, transient absorption spectroscopy and electrochemical impedance spectroscopy (EIS) revealed that the electronic properties induced by the dimethoxyphenyl-substituted NHC ligand of ARM130 led to the best combination of electron injection yield and spectral sensitivity breadth.     

Single-chain and monolayered conjugated polymers for molecular electronics

Exploring the charge transport properties and electronic functions of molecules is of primary interest in the area of molecular electronics. Conjugated polymers (CPs) represent an attractive class of molecular candidates, benefiting from their outstanding optoelectronic properties. However, they have been less studied compared with the small-molecule family, mainly due to the difficulties in incorporating CPs into molecular junctions. In this review, we present a summary on how to fabricate CP-based singlechain and monolayered junctions, then discuss the transport behaviors of CPs in different junction architectures and finally introduce the potential applications of CPs in molecular-scale electronic devices. Although the research on CP-based molecular electronics is still at the initial stage, it is widely accepted that (1) CP chains are able to mediate long-range charge transport if their molecular electronic structures are properly designed, which makes them potential molecular wires, and (2) the intrinsic optoelectronic properties of CPs and the possibility of incorporating desirable functionalities by synthetic strategies imply the potential of employing tailor-made polymeric components as alternatives to small molecules for future molecular-scale electronics.     

Overcoming excitonic bottleneck in organic solar cells: electronic structure and spectra of novel semiconducting donor-acceptor block copolymers

A novel class of organic semiconductors aimed at improving the efficiency of organic photovoltaic devices is investigated by an ab initio electronic structure theory. Two conjugated block copolymers composed of chemically bound donor and acceptor blocks show substantial charge transfer upon photoexcitation, suggesting that the optically excited states are separated charge pairs rather than strongly interacting charges forming excitons. In contrast, little charge transfer is seen in the ground electronic state. The optical cross-sections of the charge separated states are quite high due to a good overlap of the tails of the ground and excited state wavefunctions. The absorption spectra of the systems cover visible spectrum and extend to the infrared, suggesting good prospects for light harvesting. The calculation results indicate that the proposed class of semiconducting molecular heterojunctions may overcome the exciton bottleneck problem in organic photovoltaic materials.     

Exploring the potential of tetraazaacene derivatives as photovoltaic materials with enhanced photovoltaic parameters

A series of D-π-A type molecules have been designed for their potential use in organic photovoltaic devices. Photovoltaic and optoelectronic properties of newly designed molecules have been explored by comparing with a reference molecule R comprising of the central core (2,3,8,9-tetrakis(thiophen-2-ylethynyl)-5,7,10,12-tetrakis((trimethylsilyl)ethynyl)pyrazino[2,3-b]phenazine) and π-bridge (thiophene). The end groups are (2-(2-ethylidene-3-oxo-2,3-dihydro-1H-inden-1 ylidene)malononitrile), (2-ethylidenemalonitrile), (methyl 2-cyanoacrylate) and (3-methyl-5-methylene-2-thioxothiazolidin-4-one) in the newly designed molecules. Among the investigated molecules M1 and M2 exhibit a broad absorption range of 627 and 626 nm with respect to the reference. All the designed molecules exhibited a lower bandgap as compared to R which indicates a better transfer of electron density from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO). The reorganization energy values show that all designed molecules have efficient charge transport capability. This study proves that end-capped acceptor modification is an effective strategy for designing optimistic molecule for high performance future organic solar cells fabrication.     

Band gap tuning of narrow‐polydispersity two‐dimensional conductive polymers with electroactive side‐chains

A series of two‐dimensional donor–acceptor–donor (D1–A(D2)) type of conducting polymers (CPs) all with electroactive bulky side chain structure has been designed, synthesized, and investigated by introducing the donor–acceptor (D1–A) thiophene–quinoxaline moiety in the main chain alongside and additional donor and hole transporting units in the side chain. All the UV‐vis spectra of the 2D polymers, PTPQT, PFPQT, and PCPQT, each with triphenylamine, fluorene, and carbazole units as the D2 side chain, possess strong intramolecular charge transfer absorption, thus resulting in better light harvesting. Their optical and electronic properties were thoroughly explored experimentally and computationally. The effect of molecular weight of the narrow polydispersity polymers on their optoelectronic property was studied in detail. In summary, the 2‐D CPs show potential for use as an active material in optoelectronic devices. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1217–1227     

Interfacial charge transfer absorption: Application to metal–molecule assemblies

Optically induced charge transfer between adsorbed molecules and a metal electrode was predicted by Hush to lead to new electronic absorption features, but has been only rarely observed experimentally. Interfacial charge transfer absorption (IFCTA) provides information concerning the barriers to charge transfer between molecules and the metal/semiconductor and the magnitude of the electronic coupling and could thus provide a powerful tool for understanding interfacial charge-transfer kinetics. Here, we utilize a previously published model [C. Creutz, B.S. Brunschwig, N. Sutin, J. Phys. Chem. B 109 (2005) 10251] to predict IFCTA spectra of metal–molecule assemblies and compare the literature observations to these predictions. We conclude that, in general, the electronic coupling between molecular adsorbates and the metal levels is so small that IFCTA is not detectable. However, few experiments designed to detect IFCTA have been done. We suggest approaches to optimizing the conditions for observing the process.     

Structure–Performance Correlations of Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety for Dye‐Sensitized Solar Cells

The high performances of dye‐sensitized solar cells (DSSCs) based on seven new dyes are disclosed. Herein, the synthesis and electrochemical and photophysical properties of a series of intentionally designed dipolar organic dyes and their application in DSSCs are reported. The molecular structures of the seven organic dyes are composed of a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron‐deficient diphenylquinoxaline moiety integrated in the π‐conjugated spacer between the electron donor and acceptor moieties. The DSSCs based on the dye DJ104 gave the best overall cell performance of 8.06 %; the efficiency of the DSSC based on the standard N719 dye under the same experimental conditions was 8.82 %. The spectral coverage of incident photon‐to‐electron conversion efficiencies extends to the onset at the near‐infrared region due to strong internal charge‐transfer transition as well as the effect of electron‐deficient diphenylquinoxaline to lower the energy gap in these organic dyes. A combined tetraphenyl segment as a hydrophobic barrier in these organic dyes effectively slows down the charge recombination from TiO₂ to the electrolyte and boosts the photovoltage, comparable to their Ru^II counterparts. Detailed spectroscopic studies have revealed the dye structure–cell performance correlations, to allow future design of efficient light‐harvesting organic dyes.     

Stability,spectroscopic constants,and dissociation of CO2+: A theoretical study

Stability, spectroscopic constants, and dissociation of CO²⁺ have been studied in detail using ab initio MP2, CCSD and CCSD(T) methods, and density functional B3LYP method. The stability and the ambiguity between the ground and metastable state of the molecular dication have been discussed. The spectroscopic constants of the molecular dication have been compared with the experimental and theoretical values wherever available. Various charge symmetric and charge asymmetric dissociation pathways of CO²⁺ have been investigated. After dissociation, the fragmented atoms and ions are considered to be either in their ground or in their metastable state. Interesting results have been obtained for the charge symmetric and charge asymmetric dissociation of the diatomic dication. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Simulations of DNA coiling around a synthetic supramolecular cylinder that binds in the DNA major groove

In this work we present the results of a molecular simulation study of the interaction between a tetracationic bis iron(II) supramolecular cylinder, [Fe2(C25H20N4)3]4+, and DNA. This supramolecular cylinder has been shown to bind in the major groove of DNA and to induce dramatic coiling of the DNA. The simulations have been designed to elucidate the interactions that lead the cylinder to target the major groove and that drive the subsequent DNA conformational changes. Three sets of multi-nanosecond simulations have been performed: one of the uncomplexed d(CCCCCTTTTTCC) d(GGAAAAAGGGGG) dodecamer; one of this DNA complexed with the cylinder molecule; and one of this DNA complexed with a neutralised version of the cylinder. Coiling of the DNA was observed in the DNA-cylinder simulations, giving insight into the molecular level nature of the supramolecular coiling observed experimentally. The cylinder charge was found not to be essential for the DNA coiling, which implies that the DNA response is moderated by the short range interactions that define the molecular shape. Cylinder charge did, however, affect the integrity of the DNA duplex, to the extent that, under some circumstances, the tetracationic cylinder induced defects in the DNA base pairing at locations adjacent to the cylinder binding site.