Low Band Gap Donor–Acceptor Conjugated Systems Based on 3‐Alkoxy or 3‐Pyrrolidino‐4‐cyanothiophene and Benzothiadiazole Units

3‐Hexyloxy‐4‐cyanothiophene, 3‐pyrrolidil‐4‐cyanothiophene, and 3,4‐ethylenedioxythiophene (EDOT) units are used with benzothiadiazole as building blocks for the development of three new conjugated donor–acceptor–donor (DAD) derivatives. The DAD molecules have the central acceptor part, which is formed by combining electron‐withdrawing cyano groups and the benzothiadiazole moiety, in common. Theoretical calculations and UV/Vis and electrochemical data reveal the key role of the end‐capped donor to tune the electronic properties of the derivatives. A study of the electropolymerization process of the three derivatives shows the strong influence of the donor parts on both the reactivity of the precursors and the electronic properties of the resulting polymers. Derivatives end‐capped with pyrrolidinocyano thiophene or EDOT units lead to films of polymers presenting low band gaps of around 0.9–1.4 eV. Upon oxidation, the two polymers present different behavior. In the presence of the pyrrolidinocyano thiophene moieties, oxidation leads to a blueshift of the absorption bands, whereas with EDOT units a classical redshift, giving high absorption in the near‐IR region, is observed for the oxidized states.     

Donor‐π‐Acceptors Containing the 10‐(1,3‐Dithiol‐2‐ylidene)anthracene Unit for Dye‐Sensitized Solar Cells

Two donor–acceptor molecular tweezers incorporating the 10‐(1,3‐dithiol‐2‐ylidene)anthracene unit as donor group and two cyanoacrylic units as accepting/anchoring groups are reported as metal‐free sensitizers for dye‐sensitized solar cells. By changing the phenyl spacer with 3,4‐ethylenedioxythiophene (EDOT) units, the absorption spectrum of the sensitizer is red‐shifted with a corresponding increase in the molar absorptivity. Density functional calculations confirmed the intramolecular charge‐transfer nature of the lowest‐energy absorption bands. The new dyes are highly distorted from planarity and are bound to the TiO₂ surface through the two anchoring groups in a unidentate binding form. A power‐conversion efficiency of 3.7 % was obtained with a volatile CH₃CN‐based electrolyte, under air mass 1.5 global sunlight. Photovoltage decay transients and ATR‐FTIR measurements allowed us to understand the photovoltaic performance, as well as the surface binding, of these new sensitizers.     

Shape‐Persistent Oligothienylene–Ethynylene‐Based Dendrimers: Synthesis,Spectroscopy and Electrochemical Characterization

A series of novel structurally well‐defined oligothienylene–ethynylene‐based dendritic macromolecules up to the 3rd generation (G3) were successfully synthesized by a combination of Pd‐catalyzed Sonogashira‐type cross‐coupling and oxidative homocoupling steps. Oxidative homocoupling of dendrons successfully afforded dendrimers up to the 2nd generation (G2). In contrast, the G3 dendrimer was effectively prepared by a four‐fold Sonogashira‐type cross‐coupling reaction. All compounds showed broad and structureless absorption and emission spectra arising from the presence of different π‐conjugated chromophores. With increasing generation, a bathochromic shift of the π–π* absorption band and an increase of the absorption coefficient were observed. The insertion of ethynylene groups into the conjugated dendrimer backbone resulted in a hypsochromic shift compared to all‐thiophene dendrimers reported earlier by our group. All dendritic compounds are fluorescent and showed moderate quantum efficiencies due to an effective intramolecular charge‐transfer (ICT) process. Cyclic voltammetry measurements also revealed the presence of multiple π‐conjugative pathways that show very broad oxidation waves for higher generations. HOMO–LUMO energy levels of these dendrons and dendrimers were estimated from optical and redox measurements and the calculated band gaps were within the range of 3.3 to 2.4 eV, typical for oligo‐ and polythiophenes. Electrochemical polymerizations of several desilylated compounds were performed and characterization of the films is reported. Preliminary bulk heterojunction solar cells that utilise these ethynylated dendrimers as the donor and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM[60]) as the acceptor showed moderate efficiencies ranging from 0.18–0.64 %.     

Synthesis and Characterization of Donor–π‐Acceptor‐Based Porphyrin Sensitizers: Potential Application of Dye‐Sensitized Solar Cells

New porphyrin sensitizers based on donor–π‐acceptor (D‐π‐A) approach have been designed, synthesized, characterized by various spectroscopic techniques and their photovoltaic properties explored. N,N′‐Diphenylamine acts as donor, the porphyrin is the π‐spacer, and either carboxylic acid or cyanoacryclic acid acts as acceptor. All compounds were characterized by using ¹H NMR spectroscopy, ESI‐MS, UV–visible emission spectroscopies as well as electrochemical methods. The presence of aromatic groups between porphyrin π‐plane and acceptor group push the absorption of both Soret and Q‐bands of porphyrin towards the red region. The electrochemical properties suggests that LUMO of these sensitizers above the TiO₂ conduction band. Finally, the device was fabricated using liquid redox electrolyte (I^?/I₃^?) and its efficiency was compared with that of a leading sensitizer.     

Controlling the Conformational Changes in Donor–Acceptor [4]‐Dendralenes through Intramolecular Charge‐Transfer Processes

The synthesis of two [4]‐dendralene compounds incorporating thiophene‐(p‐nitrophenyl) donor–acceptor units is presented. The dendralenes adopt two different conformers in solution and solid state and the transformation between the structures can be controlled by light and heat. The electron‐donating components of the dendralenes are represented by bromothienyl (in 13 ) and ethylenedioxythiophene(EDOT)‐thienyl (in 15 ) end‐groups. The most facile transformation involves the isomerisation of donor–acceptor conjugated systems ( a conformers) into structures in which only the thiophenes are conjugated ( b conformers), and this process is driven by ambient light. The structures of the two conformers of compound 13 are confirmed by single‐crystal X‐ray diffraction studies and the structural changes in both compounds have been monitored by ¹H NMR spectroscopy and absorption studies. The transformations were found to be first‐order processes with rate constants of k=0.0027 s^?1 and k=0.00022 s^?1 for 13 and 15 , respectively. Density functional theory calculations at the B3LYP/6‐31G* level give credence to the proposed mechanism for the a → b conversion, which involves photoinduced intramolecular charge transfer (ICT) as the key step. The EDOT derivative ( 15 ) can be polymerised by electrochemical oxidation and a combination of cyclic voltammetry and UV/Vis spectroelectrochemical experiments indicate that the a conformer can be trapped and stabilised in the solid state.     

NIR Absorbing Squaraines by Extension of the Conjugation with (Aminothiazolyl)ethenyl Groups

The bis{4‐{2‐[2‐(dialkylamino)thiazol‐5‐yl]ethenyl}‐2,6‐dihydroxyphenyl}squaraines 12a , b were synthesized from ethyl carbonochloridate ( 1 ) in six steps (Scheme). The donor–acceptor–donor systems 12a , b are dark blue dyes with absorption maxima in the NIR region, unless the measurements are performed in the presence of EtOH. In the latter case, the long‐wavelength band disappears, and the absorption in the UV region is strongly enhanced. The λ_max values in CHCl₃ and CHCl₃/EtOH differ by more than 450 nm. The completely reversible effect can be rationalized by the reversible degradation of intramolecular H‐bonds and a consequent torsion between the acceptor and the donor moieties.     

Asymmetric Donor–π‐Acceptor‐Type Benzo‐Fused Aza‐BODIPYs: Facile Synthesis and Colorimetric Properties

Novel aza‐diisoindolylmethene and their BF₂‐chelating complexes (benzo‐fused aza‐BODIPYs) were synthesized on a large scale and in a facile manner from phthalonitrile in tBuOK‐DMF solution. The unique asymmetric donor–π‐acceptor structure facilitates B? N bond detachment in the presence of trifluoroacetic acid (TFA) in dichloromethane, resulting in sharp color change from red to colorless, with over 250 nm hypsochromic shift in the absorption maximum. This colorimetric process can be reversed by adding a very small amount of proton‐accepting solvents or compounds. A ¹H and ¹¹B NMR spectroscopy study and also density functional theory (DFT) calculations suggest that TFA‐induced B? N bond cleavage may disrupt the whole π‐conjugation of the BODIPY molecule, resulting in significant colorimetric behavior.     

Label‐Free,Non‐Derivatization CRET Detection Platform for 6‐Mercaptopurine Based on the Distance‐Dependent Optical Properties of Gold Nanoparticles

A label‐free, non‐derivatization chemiluminescence resonance energy transfer (CRET) detection platform has been developed for the detection of the non‐fluorescent small molecule 6‐mercaptopurine. This CRET process arose from a chemiluminescent (CL) donor–acceptor system in which the reaction of bis(2,4,6‐trichlorophenyl)oxalate (TCPO)–H₂O₂–fluorescein (maximum emission at 521.6 nm) served as the donor and gold nanoparticles (AuNPs, maximum absorption at 520.0 nm) served as the acceptor. This process caused a significant decrease in the CL signal of the TCPO–H₂O₂–fluorescein reaction. The presence of 6‐mercaptopurine induced an aggregation of AuNPs with the assistance of Cu²⁺ ions through cooperative metal–ligand interactions that was accompanied by a distinct change in color and optical properties. The maximum absorption band of the AuNPs was red‐shifted to 721.0 nm and no longer overlapped with the CL spectrum of the reaction; as a result, the CL signal was restored. This CRET system exhibited a wide linear range, from 9.0 nmol L^?1 to 18.0 μmol L^?1, and a low detection limit (0.62 nmol L^?1) for 6‐mercaptopurine. The applicability of the proposed CRET system was evaluated by analysis of 6‐mercaptopurine in spiked human plasma samples.     

Organic dyes containing nonsubstituted aryl amino moieties and azobenzene units for dye‐sensitized solar cells

A series of novel sensitizers were successfully synthesized utilizing azobenzene as a π‐linkage unit for the D–π–A structure. A slight red shift on the absorption spectra and λ_onset of the sensitizers could be observed when the thienyl group was introduced to the acceptor moiety (A). In addition, replacing the donor moiety (D) from carbazole to diarylamino could lead to a negative shift (approximately 0.3 V) in the first oxidation potential. DFT calculation was also carried out and the trend of calculated HOMO–LUMO gaps was consistent to the experimental data obtained from the CV results ( DT1 < DT2 < DT3 < DT4 ). These sensitizers were then employed in dye‐sensitized solar cells to investigate their photovoltaic performances. Highest power conversion efficiency (PCE) of 0.84% was achieved for DT1 ‐based DSSC according to its most bathochromic absorption spectrum.     

Organic Photovoltaic Devices Based on a New Class of Oligothienylenevinylene Derivatives as Donor Materials

A new class of donor–acceptor‐containing oligothienylenevinylenes with a triphenylamine donor and a dicyanovinyl group as acceptor has been synthesized and characterized. By extending the oligothiophene backbone, both the optical bandgaps and the charge‐transport properties can be tuned. These oligothienylenevinylene derivatives show intense charge‐transfer absorption bands that cover the entire visible spectrum, with low optical bandgaps of approximately 1.64 eV. In addition, electrochemical studies reveal that these compounds possess relatively large ionization potentials of approximately 5.5 eV. On the basis of these newly developed dicyanovinyl‐substituted chromophores as donor materials and C₆₀ as acceptor material, bilayer organic photovoltaic devices have been fabricated, with the best device showing a high power conversion efficiency (PCE) of 2.0 %, with an open‐circuit voltage of 0.68 V and a fill factor of 0.60 after thermal annealing. The obvious morphology change with the formation of small domains in thin films and the reduction of series resistance are believed to be responsible for the dramatic performance improvement upon thermal annealing.     

Rational Design,Synthesis, and Optical Properties of Film‐Forming,Near‐Infrared Absorbing,and Fluorescent Chromophores with Multidonors and Large Heterocyclic Acceptors

A new series of film‐forming, low‐bandgap chromophores ( 1 a,b and 2 a,b ) were rationally designed with aid of a computational study, and then synthesized and characterized. To realize absorption and emission above the 1000 nm wavelength, the molecular design focuses on lowering the LUMO level by fusing common heterocyclic units into a large conjugated core that acts an electron acceptor and increasing the charge transfer by attaching the multiple electron‐donating groups at the appropriate positions of the acceptor core. The chromophores have bandgap levels of 1.27–0.71 eV, and accordingly absorb at 746–1003 nm and emit at 1035–1290 nm in solution. By design, the relatively high molecular weight (up to 2400 g mol^?1) and non‐coplanar structure allow these near‐infrared (NIR) chromophores to be readily spin‐coated as uniform thin films and doped with other organic semiconductors for potential device applications. Doping with [6,6]‐phenyl‐C₆₁ butyric acid methyl ester leads to a red shift in the absorption only for 1 a and 2 a . An interesting NIR electrochromism was found for 2 a , with absorption being turned on at 1034 nm when electrochemically switched (at 1000 mV) from its neutral state to a radical cation state. Furthermore, a large Stokes shift (256–318 nm) is also unique for this multidonor–acceptor type of chromophore, indicating a significant structural difference between the ground state and the excited state. Photoluminescence of the film of 2 a was further probed at variable temperatures and the results strongly suggest that the restriction of bond rotations certainly helps to diminish non‐radiative decay and thus enhance the luminescence of these large chromophores.     

D–π–A Triarylboron Compounds with Tunable Push–Pull Character Achieved by Modification of Both the Donor and Acceptor Moieties

The push–pull character of a series of donor–bithienyl–acceptor compounds has been tuned by adopting triphenylamine or 1,1,7,7‐tetramethyljulolidine as a donor and B(2,6‐Me₂‐4‐RC₆H₂)₂ (R=Me, C₆F₅ or 3,5‐(CF₃)₂C₆H₃) or B[2,4,6‐(CF₃)₃C₆H₂]₂ as an acceptor. Ir‐catalyzed C?H borylation was utilized in the derivatization of the boryl acceptors and the tetramethyljulolidine donor. The donor and acceptor strengths were evaluated by electrochemical and photophysical measurements. In solution, the compound with the strongest acceptor, B[2,4,6‐(CF₃)₃C₆H₂]₂ ((FMes)₂B), has strongly quenched emission, while all other compounds show efficient green to red (Φ_F=0.80–1.00) or near‐IR (NIR; Φ_F=0.27–0.48) emission, depending on solvent. Notably, this study presents the first examples of efficient NIR emission from three‐coordinate boron compounds. Efficient solid‐state red emission was observed for some derivatives, and interesting aggregation‐induced emission of the (FMes)₂B‐containing compound was studied. Moreover, each compound showed a strong and clearly visible response to fluoride addition, with either a large emission‐color change or turn‐on fluorescence.     

Theoretical characterization and design of small molecule donor material containing naphthodithiophene central unit for efficient organic solar cells

To seek for high‐performance small molecule donor materials used in heterojunction solar cell, six acceptor–donor–acceptor small molecules based on naphtho[2,3‐b:6,7‐b′]dithiophene ( NDT ) units with different acceptor units were designed and characterized using density functional theory and time‐dependent density functional theory. Their geometries, electronic structures, photophysical, and charge transport properties have been scrutinized comparing with the reported donor material NDT(TDPP)₂ ( TDPP = thiophene‐capped diketopyrrolopyrrole). The open circuit voltage (V_oc), energetic driving force(ΔE_L‐L), and exciton binding energy (E_b) were also provided to give an elementary understanding on their cell performance. The results reveal that the frontier molecular orbitals of 3–7 match well with the acceptor material PC₆₁BM , and compounds 3–5 were found to exhibit the comparable performances to 1 and show promising potential in organic solar cells. In particular, comparing with 1 , system 7 with naphthobisthiadiazole acceptor unit displays broader absorption spectrum, higher V_oc, lower E_b, and similar carrier mobility. An in‐depth insight into the nature of the involved excited states based on transition density matrix and charge density difference indicates that all S₁ states are mainly intramolecular charge transfer states with the charge transfer from central NDT unit to bilateral acceptor units, and also imply that the exciton of 7 can be dissociated easily due to its large extent of the charge transfer. In a word, 7 maybe superior to 1 and may act as a promising donor candidate for organic solar cell. © 2013 Wiley Periodicals, Inc.     

Synthesis and nonlinear optical properties of donor-acceptor dyes based on triphenylpyrazolines as a donor block and dicyanoisophorone as acceptor

A series of donor-acceptor dyes based on polyfluoro-substituted triarylpyrazolines (as a donor block) and a dicyanoisophorone group (as an acceptor) were synthesized using the Knoevenagel condensation. The dyes have an absorption in the region of 509–514 nm and intense luminescence at 648–663 nm in chloroform with a large Stokes shift (up to 4410 cm^–1). Based on the synthesized dyes, chromophore–polymer (guest–host) films were obtained in a polycarbonate matrix with a chromophore content up to 27 wt.%. Poling of chromophore–polymer films was carried out in an electric field of a corona discharge and the coefficient of nonlinear optical response d₃₃ was measured by the second-harmonic generation method of the fundamental frequency of a Nd-YAG laser (1064 nm). The obtained films have a high initial thermal stability and a nonlinear optical response up to 80 pm V^–1, which persists up to 115 °С.

     

Near‐Infrared Quadrupolar Chromophores Combining Three‐Coordinate Boron‐Based Superdonor and Superacceptor Units

Herein, two new quadrupolar acceptor‐π‐donor‐π‐acceptor (A‐π‐D‐π‐A) chromophores have been prepared featuring a strongly electron‐donating diborene core and strongly electron‐accepting dimesitylboryl (BMes₂) and bis(2,4,6‐tris(trifluoromethyl)phenyl)boryl (B^FMes₂) end groups. Analysis of the compounds by NMR spectroscopy, X‐ray crystallography, cyclic voltammetry, and UV/Vis‐NIR absorption and emission spectroscopy indicated that the compounds have extended conjugated π‐systems spanning their B₄C₈ cores. The combination of exceptionally potent π‐donor (diborene) and π‐acceptor (diarylboryl) groups, both based on trigonal boron, leads to very small HOMO–LUMO gaps, resulting in strong absorption in the near‐IR region with maxima in THF at 840 and 1092 nm and very high extinction coefficients of ca. 120 000 m ^?1 cm^?1. Both molecules also display weak near‐IR fluorescence with small Stokes shifts.     

Conjugated polymers with broad absorption: Synthesis and application in polymer solar cells

A series of main chain donor‐acceptor low‐bandgap conjugated polymers were designed, synthesized, and used for the fabrication of polymer solar cells. The absorption spectra of low‐bandgap conjugated polymers were tuned by the ratio of three copolymerization monomers. The polymers in films exhibited broad absorption ranging from 300 to 1000 nm with optical bandgaps of around 1.40 eV. All of the polymers have been investigated as an electron donor in photovoltaic cells blending with PCBM ([6, 6]‐phenyl C61‐butyric acid methyl ester) as an electron acceptor and power conversion efficiencies (PCEs) of 1.32–1.8% have been obtained. As for P1 , PCE increases from 1.67 to 2.44% after adding 1,8‐diiodooctance as an additive. The higher PCEs are probably because of better phase separation of blend films. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2571–2578, 2010     

Axially Substituted Silicon Phthalocyanine as Electron Donor in a Dyad and Triad with Azafullerene as Electron Acceptor for Photoinduced Charge Separation

The synthesis of a donor–acceptor silicon phthalocyanine (SiPc)‐azafullerene (C₅₉N) dyad 1 and of the first acceptor–donor–acceptor C₅₉N‐SiPc‐C₅₉N dumbbell triad 2 was accomplished. The two C₅₉N‐based materials were comprehensively characterized with the aid of NMR spectroscopy, MALDI‐MS as well as DFT calculations and their redox and photophysical properties were evaluated with CV and steady‐state and time‐resolved absorption and photoluminescence spectroscopy measurements. Notably, femtosecond transient absorption spectroscopy assays revealed that both dyad 1 and triad 2 undergo, after selective photoexcitation of the SiPc moiety, photoinduced electron transfer from the singlet excited state of the SiPc moiety to the azafullerene counterpart to produce the charge‐separated state, with lifetimes of 660 ps, in the case of dyad 1 , and 810 ps, in the case of triad 2 . The current results are expected to have significant implications en route to the design of advanced C₅₉N‐based donor–acceptor systems targeting energy conversion applications.     

Self‐Assembled Aggregates and Molecular Bilayer Films of a Double‐Chain Fullerene Lipid: Structure and Electrochemistry

The self‐aggregation behavior of C₆₀ fullerenes that bear two octadecyl chains (lipid 1 ) as well as the structures and electrochemical properties of cast films of 1 are described. We also examined the self‐aggregation behavior in organic solvents of three previously reported compounds: C₆₀ with three each of hexadecyl (lipid 2 ), tetradecyl (lipid 3 ), or dodecyl (lipid 4 ) chains. The fullerene lipids in alcohols spontaneously formed spherical aggregates, whose diameters are related to the alkyl‐chain lengths, concentrations of the fullerene lipids, and the solvent polarity. The morphologies of the aggregates showed temperature dependence. Cast films of 1 formed multimolecular bilayer structures that undergo a phase transition typical of lipid bilayer membranes. The electrochemistry of cast films of 1 on an electrode in aqueous medium exhibits temperature dependence.     

Photoconductivity in Metal–Organic Framework (MOF) Thin Films

Photoconductivity is a characteristic property of semi‐conductors. Herein, we present a photo‐conducting crystalline metal–organic framework (MOF) thin film with an on–off photocurrent ratio of two orders of magnitude. These oriented, surface‐mounted MOF thin films (SURMOFs), contain porphyrin in the framework backbone and C₆₀ guests, loaded in the pores using a layer‐by‐layer process. By comparison with results obtained for reference MOF structures and based on DFT calculations, we conclude that donor–acceptor interactions between the porphyrin of the host MOF and the C₆₀ guests give rise to a rapid charge separation. Subsequently, holes and electrons are transported through separate channels formed by porphyrin and by C₆₀, respectively. The ability to tune the properties and energy levels of the porphyrin and fullerene, along with the controlled organization of donor–acceptor pairs in this regular framework offers potential to increase the photoconduction on–off ratio.     

Theoretical investigation of charge transfer excitation and charge recombination in acenaphthylene–tetracyanoethylene complex

Ab initio calculations were performed to investigate the charge separation and charge recombination processes in the photoinduced electron transfer reaction between tetracyanoethylene and acenaphthylene. The excited states of the charge‐balanced electron donor–acceptor complex and the singlet state of ion pair complex were studied by employing configuration interaction singles method. The equilibrium geometry of electron donor–acceptor complex was obtained by the second‐order Møller–Plesset method, with the interaction energy corrected by the counterpoise method. The theoretical study of ground state and excited states of electron donor–acceptor complex in this work reveals that the S₁ and S₂ states of the electron donor–acceptor complexes are excited charge transfer states, and charge transfer absorptions that corresponds to the S₀ → S₁ and S₀ → S₂ transitions arise from π–π* excitations. The charge recombination in the ion pair complex will produce the charge‐balanced ground state or excited triplet state. According to the generalized Mulliken–Hush model, the electron coupling matrix elements of the charge separation process and the charge recombination process were obtained. Based on the continuum model, charge transfer absorption and charge transfer emission in the polar solvent of 1,2‐dichloroethane were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 23–35, 2003