Solution‐Processed Bulk‐Heterojunction Photovoltaic Cells Based on Dendritic and Star‐Shaped D‐π‐A Organic Dyes

A series of D ‐π‐A organic dendritic and star‐shaped molecules based on three various chromophores (i.e., the truxene nodes, triphenylamine moieties as the donor, and benzothiadiazole chromophore as the acceptor) and their corresponding model compounds are facilely developed. Their photophysical and electrochemical properties are investigated in detail by UV/Vis absorption and photoluminescent spectroscopy, and cyclic voltammetry. By changing the various conjugated spacers (i.e., single bond, double bond, and triple bond) among the three chromophores of dendritic series, their photophysical properties (that is, the one‐photon absorption range and two‐photon absorption cross‐section values) are effectively modulated. All D ‐π‐A conjugated oligomers show a broad and strong absorption band from 250 to 700 nm in thin films. Solution‐processed bulk‐heterojunction photovoltaic devices using our oligomer as donor and PCBM as acceptor are fabricated and measured. The power conversion efficiency of the devices based on our oligomers continuously increases from DBTTr to TRTD2A as a result of an increasing relative absorption intensity in longer wavelength region by changing the donor‐acceptor ratio and conjugated spacers between the donor and acceptor. The power conversion efficiency of the devices based on TRTD2A was 0.54 % under the illumination of AM 1.5 and 100 mW cm^?2, which is the highest value recorded based on D ‐π‐A conjugated oligomers containing triphenylamine moieties and benzothiadiazole chromophores with truxene to date.     

Second-order Nonlinear Optical Properties of a Series of Benzothiazole Derivatives

The second-order nonlinear optical (NLO) properties of a series of benzothiazole derivatives were studied by use of the ZINDO-SOS method.These chromophores are formed by a donor- π- bridge-acceptor system,based on a nitro group connected with benzothiazole as the acceptor and a hydroxyl-functional amino group as the donor.For the purpose of comparison,we also designed molecules in which nitrobenzene is an acceptor,The calculation results indicate that benzothiazole derivatives exhibit larger second-odrder polarizabilities than nitrobenzene derivatives.In order to clarify the origin of the NLO response of these chromophores,their electron properties were investigated as well.The benzothiazole derivatives are good candidates for application in electro-optical device due to their high optical nonlinearities,good thermal and photonic stability.     

Synthesis and Second‐Order Nonlinearities of Chiral Prolinol‐Substituted Chromophores

A new series of thiophene‐ and furan‐containing chromophores with a chiral prolinol donor and a sulfone acceptor has been synthesized. The UV‐vis absorptions, second‐order nonlinear optical properties, and X‐ray crystal structures are described.     

含1,8-二甲氧基-9,10-二氢蒽和巴比妥酸的发色团化合物的合成和NLO性质

在哌啶和醋酸催化下,1,8-二甲氧基-4,5-二甲酰基-9,10-二氢蒽和异佛尔酮巴比妥酸衍生物经Knoevenagel缩合合成了具有NLO性质的发色团化合物。在这些发色团中,环锁定的三烯和9,10-二氢蒽用作共轭桥,巴比妥酸和甲氧其分别作为吸电子和给电子基, 组成非共轭的两个D-π-A单位。溶剂变色法和紫外光谱研究证实它们比相应的参考物有较大的NLO活性并能保持与参考化合物相同的透光性。     

Charge‐Transfer Interactions in Tris‐Donor–Tris‐Acceptor Hexaarylbenzene Redox Chromophores

Symmetric‐ and asymmetric hexaarylbenzenes (HABs), each substituted with three electron‐donor triarylamine redox centers and three electron‐acceptor triarylborane redox centers, were synthesized by cobalt‐catalyzed cyclotrimerization, thereby forming compounds with six‐ and four donor–acceptor interactions, respectively. The electrochemical‐ and photophysical properties of these systems were investigated by cyclovoltammetry (CV), as well as by absorption‐ and fluorescence spectroscopy, and compared to a HAB that only contained one neighboring donor–acceptor pair. CV measurements of the asymmetric HAB show three oxidation peaks and three reduction peaks, whose peak‐separation is greatly influenced by the conducting salt, owing to ion‐pairing and shielding effects. Consequently, the peak‐separations cannot be interpreted in terms of the electronic couplings in the generated mixed‐valence species. Transient‐absorption spectra, fluorescence‐solvatochromism, and absorption spectra show that charge‐transfer states from the amine‐ to the boron centers are generated after optical excitation. The electronic donor–acceptor interactions are weak because the charge transfer has to occur predominantly through space. Moreover, the excitation energy of the localized excited charge‐transfer states can be redistributed between the aryl substituents of these multidimensional chromophores within the fluorescence lifetime (about 60 ns). This result was confirmed by steady‐state fluorescence‐anisotropy measurements, which further indicated symmetry‐breaking in the superficially symmetric HAB. Adding fluoride ions causes the boron centers to lose their accepting ability owing to complexation. Consequently, the charge‐transfer character in the donor–acceptor chromophores vanishes, as observed in both the absorption‐ and fluorescence spectra. However, the ability of the boron center as a fluoride sensor is strongly influenced by the moisture content of the solvent, possibly owing to the formation of hydrogen‐bonding interactions between water molecules and the fluoride anions.     

High‐Performance Dipolar Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety in the π‐Conjugation Framework for Dye‐Sensitized Solar Cells

We report here the synthesis and electrochemical and photophysical properties of a series of easily prepared dipolar organic dyes and their application in dye‐sensitized solar cells (DSSCs). For the six organic dyes, the molecular structures comprised 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 incorporation of the electron‐deficient diphenylquinoxaline moiety effectively reduces the energy gap of the dyes and broadly extends the spectral coverage. DSSCs based on dye 6 produced the best overall cell performance of 7.35 %, which translates to approximately 79 % of the intrinsic efficiency of the DSSCs based on the standard N719 dye under identical experimental conditions. The high performance of DSSCs based on dye 6 among the six dyes explored is attributed to the combined effects of high dye loading on a TiO₂ surface, rapid dye regeneration, and effective retardation of charge recombination.     

Ferrocene‐Donor and 4,5‐Dicyanoimidazole‐Acceptor Moieties in Charge‐Transfer Chromophores with π Linkers Tailored for Second‐Order Nonlinear Optics

A series of new nonlinear optical chromophores ( 1 – 15 ) that were comprised of ferrocene‐donor and 4,5‐dicyanoimidazole‐acceptor moieties and various π linkers of different length were synthesized. Support for the presence of significant D ? A interactions in these NLO‐phores was obtained from the evaluation of the quinoid character of the 1,4‐phenylene moieties and their electronic absorption spectra, which featured intense high‐energy (HE) bands that were accompanied by less‐intense low‐energy (LE) bands. The redox behavior of these compounds was investigated by cyclic voltammetry (CV) and by rotating‐disc voltammetry (RDV); their electrochemical gaps decreased steadily from 2.64 to 2.09 V. In addition to the experimentally obtained data, DFT calculations of their absorption spectra, HOMO/LUMO levels, and second‐order polarizabilities (β) (?2ω,ω,ω) were performed. A structure–property relationship study that was performed by systematically altering the π linker revealed that the intramolecular charge‐transfer and nonlinear optical properties of these inorganic–organic hybrid D? π? A systems ( 1 – 15 ) were primarily affected by: 1) The presence of olefinic/acetylenic subunits; 2) the length of the π linker; and 3) the spatial arrangement (planarity) of the π linker.     

Modulation of electronic properties of π‐conjugated copolymers derived from naphtho[1,2‐b:5,6‐b′]dithiophene donor unit: A structure‐property relationship study

A set of three donor‐acceptor conjugated (D‐A) copolymers were designed and synthesized via Stille cross‐coupling reactions with the aim of modulating the optical and electronic properties of a newly emerged naphtho[1,2‐b:5,6‐b′]dithiophene donor unit for polymer solar cell (PSCs) applications. The PTNDTT‐BT , PTNDTT‐BTz , and PTNDTT‐DPP polymers incorporated naphtho[1,2‐b:5,6‐b′]dithiophene ( NDT ) as the donor and 2,2′‐bithiazole ( BTz ), benzo[1,2,5]thiadiazole ( BT ), and pyrrolo[3,4‐c]pyrrole‐1,4(2H,5H)‐dione ( DPP ), as the acceptor units. A number of experimental techniques such as differential scanning calorimetry, thermogravimetry, UV–vis absorption spectroscopy, cyclic voltammetry, X‐ray diffraction, and atomic force microscopy were used to determine the thermal, optical, electrochemical, and morphological properties of the copolymers. By introducing acceptors of varying electron withdrawing strengths, the optical band gaps of these copolymers were effectively tuned between 1.58 and 1.9 eV and their HOMO and LUMO energy levels were varied between ?5.14 to ?5.26 eV and ?3.13 to ?3.5 eV, respectively. The spin‐coated polymer thin film exhibited p‐channel field‐effect transistor properties with hole mobilities of 2.73 × 10^?3 to 7.9 × 10^?5 cm² V^?1 s^?1. Initial bulk‐heterojunction PSCs fabricated using the copolymers as electron donor materials and [6,6]‐phenyl C71 butyric acid methyl ester (PC₇₁BM) as the acceptor resulted in power conversion efficiencies in the range of 0.67–1.67%. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2948–2958     

Dependence of the two-photon absorption cross section on the conjugation of the phenylacetylene linker in dipolar donor-bridge-acceptor chromophores

The nonlinear optical properties of four isomeric dipolar two-photon chromophores are compared. The chromophores consist of a carbazole electron donor coupled to a naphthalimide electron acceptor by a phenylacetylene bridge. By variation of the connectivity of the bridge at the phenyl groups, four compounds with 0, 1, and 2 meta linkages are synthesized. The linear and nonlinear optical properties of these compounds are measured. Despite similar linear absorption cross sections, the two-photon absorption cross section delta of the all-meta compound is almost a factor of 10 lower than the all-para compound. By taking the detailed molecular conformations into account in order to calculate accurate dipole moment changes, we find that the decrease in delta results largely from the decreased charge transfer ability with increasing number of meta linkages. We find that a two-state model can be used to predict semiquantitatively the observed trend in delta on the basis of the linear optical properties of the molecules. This work illustrates the dramatic effect the ground-state polarizability can have on the nonlinear optical response of organic compounds and also provides a way to quantify the ability of meta linkages to inhibit charge transfer in their ground-state configuration.     

The [2+2] Cycloaddition‐Retroelectrocyclization (CA‐RE) Click Reaction: Facile Access to Molecular and Polymeric Push‐Pull Chromophores

The [2+2] cycloaddition‐retroelectrocyclization (CA‐RE) reaction between electron‐rich alkynes and electron‐deficient alkenes is an efficient procedure to create nonplanar donor–acceptor (D‐A) chromophores in both molecular and polymeric platforms. They feature attractive properties including intramolecular charge‐transfer (ICT) bands, nonlinear optical properties, and redox activities for use in next‐generation electronic and optoelectronic devices. This Review summarizes the development of the CA‐RE reaction, starting from the initial reports with organometallic compounds to the extension to purely organic systems. The structural requirements for rapid, high‐yielding transformations with true click chemistry character are illustrated by examples that include the broad alkyne and alkene substitution modes. The CA‐RE click reaction has been successfully applied to polymer synthesis, with the resulting polymeric push‐pull chromophores finding many interesting applications.     

Synthetic,Optical and Theoretical Study of Alternating Ethylenedioxythiophene–Pyridine Oligomers: Evolution from Planar Conjugated to Helicoidal Structure towards a Chiral Configuration

A series of alternating 3,4‐ethylenedioxythiophene–alkynylpyridine oligomers (DA)_n with increased solubility are synthesized and their photophysical properties and nonlinear optical properties are investigated. Their quadratic polarizabilities are determined from hyper‐Rayleigh scattering experiments to obtain information on their conformations in solution. These chromophores, based on the alternation of electron‐rich (D) and electron‐deficient (A) moieties, exhibit optical properties that arise from the combination of dipolar and helicoidal features in the (DA)_n homologue series where n=1–4. The transition from dipolar conjugated planar structures (n=1, 2) to helicoidal structures (n=3, 4) is clearly evidenced by results from symmetry‐sensitive second‐order nonlinear optical experiments. This suggests an approach towards highly efficient chiral chromophores for second‐order nonlinear optics. Interestingly, this structural evolution also has significant impact on the photophysical properties: both absorption and fluorescence emission show bathochromic and hyperchromic shifts with increasing number of repeating units in the dipolar planar derivatives (n=1–2) but show saturation effects in the helicoidal structures (n=2–4). In addition, the helicoidal structures show sizeable two‐photon absorption at 700–750 nm (40–100 GM) for compounds lacking either electron‐donating or electron‐withdrawing substituents.     

Theoretical Study on Second‐order Nonlinear Optical Properties of Substituted Thiazole Derivatives

On the basis of ZINDO program, we have designed a program to calculate the nonlinear second‐order polarizability β_yk and β_μ according to the SOS expression. The second‐order nonlinear optical properties of 4‐nitro‐4′‐dimethylamino‐stilbene and a series of its thiazole derivatives were studied. The calculated results were that: When replacing a benzene ring in 4‐nitro‐4′‐dimethylamino‐stilbene by a thiazole ring, the influence on β values depends on the position of thiazole ring. When the thiazole ring connects with nitro group (acceptor), the β values increase significantly compared with corresponding stilbene derivatives. The β values of 2‐(p‐donor‐β‐styryl)‐5‐nitro‐thiazole derivatives (2–7) are larger than those of 2‐(p‐nitro‐β‐styryl)‐5‐donor‐thiazole derivatives (8–13) and 2‐(p‐donor‐phenyl)‐azo‐5‐nitro‐thiazole derivatives (14–19). The 2‐(p‐donor‐β‐styryl)‐5‐nitro‐thiazole derivatives (2–7) are good candidates as chromophores duo to their high nonlinearities and potential good thermal stability.     

Synthesis and optical and electrochemical properties of novel copolymers containing alternating 2,3‐divinylquinoxaline and hole‐transporting units

For the enhancement of charge affinity, electron‐affinitive 2,3‐divinylquinoxaline and a series of hole‐transporting chromophores (iminodibenzyl, phenothiazine, dihexyloxybenzene, and didodecyloxydistyrylbenzene) were incorporated alternately into the polymeric main chain. The resulting copolymers ( P1 – P4 ) were basically amorphous materials and were thermally stable below 300 °C. The electronic structures, photoluminescence, and electrochemical properties of these copolymers were mainly determined by the electron‐donating chromophores in the backbone. They showed significant positive solvatochromism in formic acid. An electrochemical study revealed that they exhibited lower band gaps (<2.3 eV) due to alternating donor and acceptor conjugated units (push–pull structure). Single‐layer light‐emitting diodes of aluminum, P1 – P4 , and indium tin oxide glass were fabricated, and preliminary electroluminescence spectra showed that P1 , P3 , and P4 were orange‐emitting materials. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4570–4580, 2002     

Dipyrido[3,2‐a:2′,3′‐c]phenazine‐Based Donor–Acceptor Aromatic Heterocyclic Compounds with Thienyl and Triphenylamino Chromophores at the 2,7‐ and/or 10,13‐Positions

A series of T‐ and H‐shaped donor–acceptor (D–A) types of dipyrido[3,2‐a:2′,3′‐c]phenazine (DPPZ)‐based molecules, extended by thienyl and triphenylamino chromophores at the 2,7‐(bottom) and/or 10,13‐positions (top), have been designed and prepared successfully. Synthetic, structural, thermal, spectral, and computational comparisons have been carried out for related compounds because of their adjustable intramolecular charge‐transfer properties. It is noted that a pair of structural isomers ( 5 and 6 ) has been obtained, respectively, where distinguishable UV/Vis and fluorescence spectra, electrochemical activity, thermal stability, and bandgaps are observed. Furthermore, compounds 6 , 8 , 10 , 11 , 13 , and 15 exhibit excellent thermal stability, and the Td₁₀ values for them are found to range from 524 to 646 °C, which can be regarded as one of the best groups of thermally stable compounds among organic small molecules. In addition, theoretical calculations were performed, and the structure–property relationships were examined to reveal the effects of the position and number of donor arms on the DPPZ acceptor core.     

Bandgap engineering of indenofluorene‐based conjugated copolymers with pendant donor‐π‐acceptor chromophores for photovoltaic applications

Three narrow‐band‐gap conjugated copolymers based on indenofluorene and triphenylamine with pendant donor‐π‐acceptor chromophores were successfully synthesized by post‐functionalization approach. All the polymers have good solubility in common solvents and excellent thermal stability. The photophysical properties, energy levels and band gaps of the polymers were well manipulated by introducing different acceptor groups onto the end of their conjugated side chains. By using different acceptor groups, the band gaps of the polymers were narrowed from 1.86 to 1.53 eV by lowering their lowest unoccupied molecular orbital levels, whereas their relatively deep highest occupied molecular orbital levels of approximately ?5.35 eV were maintained. Bulk‐heterojunction solar cells with these polymers as electron donors and (6,6)‐phenyl‐C₇₁‐butyric acid methyl ester as acceptor showed power conversion efficiencies as high as 3.1% and high open circuit voltages more than 0.88 eV. The relationships between the performance and film morphology, energy levels, charge mobilities were discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

The Effect of Branching on the One- and Two-Photon Absorption,Cell Viability,and Localization of Cationic Triarylborane Chromophores with Dipolar versus Octupolar Charge Distributions for Cellular Imaging

Two different chromophores, namely a dipolar and an octupolar system, were prepared and their linear and nonlinear optical properties as well as their bioimaging capabilities were compared. Both contain triphenylamine as the donor and a triarylborane as the acceptor, the latter modified with cationic trimethylammonio groups to provide solubility in aqueous media. The octupolar system exhibits a much higher two-photon brightness, and also better cell viability and enhanced selectivity for lysosomes compared with the dipolar chromophore. Furthermore, both dyes were applied in two-photon excited fluorescence (TPEF) live-cell imaging.     

Linear‐dendritic block copolymer hosts for the encapsulation of electro‐optic chromophores via H‐Bonding

Linear‐dendritic block copolymer hosts were synthesized by end‐functionalizing poly(methylmethacrylate) with dendrons that acted as hydrogen‐bonding acceptors for nonlinear optical chromophores. Second harmonic generation experiments indicate that the d₃₃ coefficients and maximum chromophore loading are increased in linear‐dendritic block copolymer hosts over comparable homopolymer hosts. Transmission electron microscopy shows 5–10 nm chromophore domains, confirming the effective spatial dispersion of the chromophores. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5017–5026, 2009     

Synthesis,Two‐Photon Absorption and Optical Limiting Properties of Multi‐branched Styryl Derivatives Based on 1,3,5‐Triazine

Five new multi‐branched two‐photon absorption triazine chromophores ( T1 – T5 ) with different donor strength, conjugation length, and direction of charge transfer have been designed and synthesized. The one‐photon fluorescence, fluorescence quantum yields, and two‐photon properties have been investigated. The two‐photon absorption (2PA) cross sections measured by the open aperture Z‐scan technique were determined to be 447, 854, 1023, 603, and 766 GM for T1 , T2 , T3 , T4 , and T5 , respectively. This result indicates that their 2PA cross section values (σ) increase with increasing electron‐donating strength of the end group, extending the conjugation length of the system, and introducing electron‐withdrawing perfluoroalkyl as side groups to the end donor. In addition, the σ value of T5 is also larger than that of T1 , which provides evidence that the σ value is relative to the direction of charge transfer (from the ends to the center of the molecule or from the center to the ends). Moreover, significant enhancement of the two‐photon absorption cross section was achieved by introducing a thiophene moiety to a conjugated CC bond. At the same time, the optical limiting behavior for these chromophores was studied by using a focused 800 nm laser beam with pulses of 140 fs duration. It was found that these molecules also exhibit good optical limiting properties. These initial results clearly demonstrate that multi‐branched triazine chromophores are a highly suitable class of two‐photon absorbing materials.     

Nonlinear optical side‐chain polymers post‐functionalized with high‐β chromophores exhibiting large electro‐optic property

Electro‐optic side‐chain polymers have been synthesized by the post‐functionalization of methacrylate isocyanate polymers with novel phenyl vinylene thiophene vinylene bridge (FTC) nonlinear optical chromophores. For this application, FTC‐based chromophores were modified in their electronic donor structure, exhibiting much larger molecular hyperpolarizabilities compared with the benchmark FTC. Of these new chromophores, absorption spectra, hyper‐Rayleigh scattering experiment, and thermal analysis were carried out to confirm availability as effective nonlinear optical units for electro‐optic side‐chain polymers. The electro‐optic coefficients (r₃₃) of obtained polymers were investigated in the process of in situ poling by monitoring the temperature, current flow, poling field, and electro‐optic signal. Compared with the nonsubstituted analogue, benxyloxy modified FTC chromophore significantly achieved higher nonlinear optical property, exhibiting molecular hyperpolarizability at 1.9 μm of 4600 × 10^?30 esu and an r₃₃ value of 150 pm/V at the wavelength of 1.31 μm. Synthesized electro‐optic polymers showed high glass transition temperature (T_g), so that the temporal stability examination exhibited >78% of the electro‐optic intensity remaining at 85 °C over 500 h. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Discovery of Novel D‐A and D‐π‐A Structured Carbazole‐Based Formazan Dyes: Synthesis,Characterization, and Spectroscopic Studies

In the current study, new carbazole‐based formazan dyes, D‐A and D‐π‐A, were synthesized, and their spectroscopic properties were studied for the first time. For this aim, carbazole aldehyde compounds were modified by the derivatization of carbazole, a natural electron‐donor compound, from 3‐ and 9‐position. Then, hydrazone derivatives were synthesized from these aldehyde derivatives. Finally, D‐A ( 5A–C ) and D‐π‐A ( 6A–C ) carbazole–formazan dyes were obtained by the interaction of the hydrazone compounds with p‐substituted aniline. After characterization of the structures of these compounds, photophysical properties of the carbazole–formazans were studied in the different polarity media (i.e., acetonitrile, toluene, and chloroform) in order to detect the solvent effects. Because of the π‐conjugated bridge and the electron acceptor nitro group at the para position, D‐π‐A structured carbazole–formazan dye 6C showed the highest Stokes shift of 155 nm.