Donor-π Bridge-acceptor Research Articles

Insertion of a naphthalenediimide unit in a metal-free donor–acceptor organic sensitizer for efficiency enhancement of a dye-sensitized solar cell

Through the use of a planar conjugated naphthalenediimide (NDI) functionality, a novel, metal-free organic chromophore was designed, synthesized and characterized for application in dye-sensitized solar cells (DSSCs). The new sensitizer, 2-cyano-3-(5-(9-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)-2,7-dioctyl-1,3,6,8-tetraoxo-1,2,3,6,7,8-hexahydrobenzo[lmn] [3,8]phenanthrolin-4-yl)thiophen-2-yl)acrylic acid (coded as A1), was based on a donor–π-bridge–acceptor (D–π–A) module where a simple triphenylamine functionality served as an electron donor, a cyanoacrylic acid as an electron acceptor and anchoring group, and a NDI moiety as the π-bridge embedded between the two thiophene units. Because of the extensively conjugated NDI unit, the new dye A1 exhibited high extinction coefficient, tuned energy levels and improved DSSC performance when compared with DSSCs fabricated using the conventional sensitizer, 2-cyano-3-(5’-(4-(diphenylamino)phenyl)-[2,2’-bithiophen]-5-yl)acrylic acid (R1). Furthermore, DSSCs that were constructed using A1 showed an increase in electron lifetime when compared with DSSCs fabricated using R1. A1 demonstrated its suitability with a variety of electrolyte systems and DSSC power conversion efficiencies of 6.24%, 6.05% and 5.17% were recorded using cobalt-, solvent- and ionic liquid-based redox shuttles. To our knowledge, A1 is the first reported example in the literature where NDI unit has been embedded between thiophene functionalities to extend the π-conjugation in a given D–A system for DSSC applications.

A porphyrin derivative for the fabrication of highly stable and sensitive electrochemical sensor and its analytical applications

For improving electron transfer, the insertion of an electron-deficient group (2, 3-diphenylquinoxaline, DPQ) into different positions of the porphyrin (P) based donor-π bridge-acceptor (D-π-A) dye (FNE57) was designed to obtain two porphyrin derivatives (FNE58 and FNE59). The cyclic voltammetry and density functional theory (DFT) revealed that FNE59, with incorporating the DPQ unit adjacent to the acceptor, showed the best conjugated structure, lowest energy gap, and strongest electron transfer ability by comparing those of FNE57 and FNE58. Entrapped in tetraoctylammonium bromide (TOAB) on the glassy carbon electrode (GCE), a novel non-enzymatic sensor (TOAB/FNE59/GCE) based on FNE59 was fabricated. The sensor showed excellent electrocatalytic activities towards H2O2 and nitrite with rapid response, high sensitivity, and good stability. The electrocatalytic reduction of H2O2 provided a linear response in the range of 5.0−295.0μM with a low limit of detection (LOD) of 0.77μM and a high sensitivity of 3.35μA μM−1cm−2. The response for nitrite showed the sensitivity of 4.10μA μM−1cm−2 and a LOD of 0.63μM within nitrite concentration range from 2.0 to 250.0μM. Therefore, the strategy based on the design and synthesis of the materials paves a new avenue for the interface fabrication of non-enzymatic electrochemical sensor.

(Invited) Dye Sensitized Solar Cell Based on Dicarboxylic Acid Substituted Benzoporphyrin and the Effect of Pi-Linkers Towards Their Photovoltaic Performances

In recent years, dye sensitized solar cells have attracted considerable attention due to their low cost and potential for getting higher light harvesting efficiency.1 Porphyrin derivatives have immerged as a promising candidate for DSSC due their photo stability, broad absorption within solar spectrum with high molar extinction coefficients and tunability of electrochemical properties toward synthetic modifications. Extensive studies have been done on meso-substituted porphyrins as sensitizers in DSSC.2 In 2014, highest efficiency of 13% has been archived by installing donor–Π-bridge–acceptor at the porphyrin meso-position.3Due to limited synthetic methods, there are very few β functionalized porphyrin used in DSSC. In this work, we report the synthesis, properties and photovoltaic performance of novel series of benzoporphyrins bearing two carboxyl anchoring groups through different П-linkers at their 23 and 24 positions. Palladium catalyzed double Heck reaction followed by electrocyclization/dehydrogenation strategy was used to obtain various 23, 24-functionalized benzoporphyrin from 2,3-dibromoporphyrin.4 The dyes bearing vinylene linkage and phenyl acetylene linkage at 23 and 24 position of benzoporphyrin displayed much broader and red shifted B and Q bands in UV-Vis absorption compared to its phenyl linker substituted derivative. Cyclic voltammetry results of all studied compounds confirms the thermodynamic feasiblity of electron injection from their excited states to the CB of TiO2 and regeneration of the resulting porphyrin radical cation by redox mediators. However, the photovoltaic performance of the cells sensitized with vinylene linker substituted benzoporphyrin showed highest power conversion efficiency with highest J sc and V oc values. On the other hand the photovoltaic performance of the cells sensitized with phenyl linker substituted benzoporphyrin showed lowest power conversion efficiency in this series. Our results reflect light harvesting efficiency (LHE), length and flexibility of the linker and dye loading mainly determined the difference in power conversion efficiency in this series. DFT calculation provided support that the electron density distribution in frontier molecular orbitals of vinylene linker substituted benzoporphyrin and phenyl acetylene substituted benzoporphyrin could facilitate the electron injection to semiconductor compared to phenyl linker substituted benzoporphyrin. (1) Clifford, J. N.; Martinez-Ferrero, E.; Viterisi, A.; Palomares, E. Chem. Soc. Rev. 2011, 40, 1635. (2) Higashino, T.; Imahori, H. Dalton Trans. 2015, 44, 448. (3) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Nat. Chem. 2014, 6, 242. (4) Jinadasa, R. G. W.; Fang, Y.; Kumar, S.; Osinski, A. J.; Jiang, X.; Ziegler, C. J.; Kadish, K. M.; Wang, H. J. Org. Chem.[just accepted], DOI: 10.1021/acs.joc.5b01906 Nov., 18, 2015. Figure 1

Thieno[3,4-b]pyrazine as an Electron Deficient π-Bridge in D-A-π-A DSCs.

Thieno[3,4-b]pyrazine (TPz) is examined as an electron deficient π-bridge enabling near-infrared (NIR) spectral access in dye-sensitized solar cells (DSCs). Seven dissymmetric dyes for DSCs were synthesized (NL2-NL8) with TPz as the π-bridge utilizing palladium catalyzed C-H activation methodology. C-H bond cross-coupling was uniquely effective among the cross-couplings and electrophilic aromatic substitution reactions analyzed in monofunctionalizing the TPz building block. The TPz-based NL2-NL8 dyes examine the effects of various donors, π-spacers, and acceptors within the donor-π bridge-acceptor (D-π-A) dye design. Proaromatic TPz stabilizes the excited-state oxidation potential (E(s+/s*)) of the dyes by maintaining aromaticity upon excitation of the dye molecule. This leads to concise conjugated systems capable of accessing the NIR region. Through judicious structural modifications, dye band gaps were reduced to 1.48 eV, and power conversion efficiencies (PCEs) reached 7.1% in this first generation TPz-dye series.

Influence of various electron-donating triarylamine groups in BODIPY sensitizers on the performance of dye-sensitized solar cells

Four donor-π bridge-acceptor structured boron dipyrromethene type sensitizers bearing triarylamine donors with different rigidities were synthesized and applied in dye-sensitized solar cells. The influence of different triarylamine donors on the optical, electrochemical properties and photovoltaic performances of sensitizers was systematically investigated. It was shown that the photovoltaic performance of boron dipyrromethene type sensitizer-based cell increased with increasing the fluorescence quantum yield and fluorescence lifetime of the corresponding sensitizers, which are believed to be closely related to the rigidities of the donor groups in the molecule. The best performance was realized for the cell based on rigid 9-phenyl-carbazole-substituted boron dipyrromethene sensitizer (ZH-b) with a fluorescence quantum yield of 0.516 and a fluorescence lifetime of 4.02 ns, resulting in a short circuit photocurrent density of 14.10 mA/cm2 and an overall conversion efficiency of 4.42%, which are fairly good results achieved for boron dipyrromethene type sensitizer-based solar cell.

Atypical organic dyes used as sensitizers for efficient dye-sensitized solar cells

Since their invention, dye-sensitized solar cells (DSSCs) have received enormous research attention from scientists with different background due to the possibility of low-cost production and fabrication on flexible substrates. One of the most important components in DSSCs is the sensitizing dye, including metal-complexes and metal-free organic dyes. The donor-π bridge-acceptor (D-π-A) structure is the mainstream in the design of organic sensitizers because it facilitates efficient charge-transfer toward the TiO2 upon excitation. However, some sensitizers that do not follow this structure, or have some modification upon this structure, also present good efficiency when applied in DSSC devices. This review summarizes these atypical dyes in order to inspire more diverse designs toward highly efficient DSSCs.

Synthesis of push–pull porphyrin dyes with dimethylaminonaphthalene electron-donating groups and their application to dye-sensitized solar cells

Two new donor–π-bridge–acceptor zinc porphyrins with dimethylaminonaphthalene electron donating moieties, coded T1 and T2, were synthesized and used as sensitizers in dye sensitized solar cells (DSSCs).

Efficiency and stability of spectral sensitization of boron-doped-diamond electrodes through covalent anchoring of a donor-acceptor organic chromophore (P1).

A novel procedure is developed for chemical modification of H-terminated B-doped diamond surfaces with a donor-π-bridge-acceptor molecule (P1). A cathodic photocurrent near 1 μA cm(-2) flows under 1 Sun (AM 1.5) illumination at the interface between the diamond electrode and aqueous electrolyte solution containing dimethylviologen (electron mediator). The efficiency of this new electrode outperforms that of the non-covalently modified diamond with the same dye. The found external quantum efficiency of the P1-sensitized diamond is not far from that of the flat titania electrode sensitized by a standard organometallic dye used in solar cells. However, the P1 dye, both pure and diamond-anchored, shows significant instability during illumination by solar light. The degradation is a two-stage process in which the initially photo-generated products further decompose in complicated dark reactions. These findings need to be taken into account for optimization of organic chromophores for solar cells in general.

Donor–π-bridge–acceptor fluorescent polymers based on thiophene and triphenylamine derivatives as solution processable electrochromic materials

A series of novel donor–π-bridge–acceptor type conjugated polymers bearing 3,3-bis((2-ethylhexyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine and triphenylamine (TPA) derivatives are synthesized via Stille coupling. The optical, fluorescent, electrochemical, spectroelectrochemical and electrochromic switching properties are characterized. The polymers display excellent solubility in low-boiling point organic solvents. Different electron-withdrawing pendent groups attached on TPA exert great influence on polymer’s fluorescent and electrochromic properties. All polymers show large transmittance changes in near-infrared region and reveal fast switching from one second to several seconds.

The effect of intramolecular donor–acceptor moieties with donor–π-bridge–acceptor structure on the solar photovoltaic performance

A series of intramolecular donor-acceptor polymers containing different contents of (E)-1-(2-ethylhexyl)-6,9- dioctyl-2-(2-(thiophen-3-yl)vinyl)-1H-phenanthro(9,10-d)imidazole (thiophene-DOPI) moiety and 4,4-diethylhexyl- cyclopenta(2,1-b:3,4-b!)dithiophene (CPDT) unit was synthesized via Grignard metathesis (GRIM) polymerization. The synthesized random copolymers and homopolymer of thiophene-DOPI contain the donor--bridge-acceptor conjugated structure to tune the absorption spectra and energy levels of the resultant polymers. UV-vis spectra of the three polymer films exhibit panchromatic absorptions ranging from 300 to 1100 nm and low band gaps from 1.38 to 1.51 eV. It is found that more thiophene-DOPI moieties result in the decrease of band gap and lower the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) values of polymers. Photovoltaic performance results indicate that if the content of the intramolecular donor-acceptor moiety is high enough, the copolymer structure may be better than homopolymer due to more light-harvesting afforded by both monomer units.

Research Progress on the Application of D-A-π-A Featured Sensitizer in the Dye Sensitized Solar Cells

The donor-π bridge-acceptor (D-π-A) configuration becomes mainstream in the design of organic sensitizers due to its enhancement of the intramolecular photo-generated charge-transfer. On the basis of traditional D-π-A structure, researchers incorporate additional electron-withdrawing unites into the π bridge as internal acceptor, termed as D-A-π-A configuration, which displays better performance. This review summarizes the molecular design (since 2012) of sensitizers based on D-A-π-A configuration and their application in the dye sensitized solar cells. Keywords D-A-π-A; solar cell; sensitizer

The position effect of electron-deficient quinoxaline moiety in porphyrin based sensitizers

An electron-deficient group, 2,3-diphenylquinoxaline (DPQ), is incorporated as an auxiliary acceptor into the different positions of the porphyrin (Por) based donor-π bridge-acceptor (D-π-A) dye (FNE57) to construct D-A′-Por-π-A (FNE58) and D-Por-A′-π-A (FNE59) configurations. The incorporation of DPQ unit between the donor and porphyrin unit has negligible influence on the absorption property, whereas the DPQ unit located between the porphyrin unit and acceptor significantly increases the absorbance for the Soret band and the valley between the Soret and Q bands. Theoretical calculation reveals that incorporating the DPQ unit adjacent to the acceptor is more advantageous to delocalize the lowest unoccupied molecular orbital and enhance the electronic asymmetry, which facilitates the intramolecular charge transfer. The effect of DPQ unit and its linkage position on the performance of related quasi-solid-state dye-sensitized solar cells (DSSCs) is systematically investigated. The quasi-solid-state DSSC with sensitizer FNE59 displays a power conversion efficiency of 6.02%, which is 23% and 51% higher than those for FNE57 and FNE58 based DSSCs. Our studies facilitate the understanding of the crucial importance of molecular engineering and pave a new path to design novel porphyrin based sensitizers for highly efficient DSSCs.

Precise identification and manipulation of adsorption geometry of donor-π-acceptor dye on nanocrystalline TiO₂ films for improved photovoltaics.

Adsorption geometry of dye molecules on nanocrystalline TiO2 plays a central role in dye-sensitized solar cells, enabling effective sunlight absorption, fast electron injection, optimized interface band offsets, and stable photovoltaic performance. However, precise determination of dye binding geometry and proportion has been challenging due to complexity and sensitivity at interfaces. Here employing combined vibrational spectrometry and density functional calculations, we identify typical adsorption configurations of widely adopted cyanoacrylic donor-π bridge-acceptor dyes on nanocrystalline TiO2. Binding mode switching from bidentate bridging to hydrogen-bonded monodentate configuration with Ti-N bonding has been observed when dye-sensitizing solution becomes more basic. Raman and infrared spectroscopy measurements confirm this configuration switch and determine quantitatively the proportion of competing binding geometries, with vibration peaks assigned using density functional theory calculations. We further found that the proportion of dye-binding configurations can be manipulated by adjusting pH value of dye-sensitizing solutions. Controlling molecular adsorption density and configurations led to enhanced energy conversion efficiency from 2.4% to 6.1% for the fabricated dye-sensitized solar cells, providing a simple method to improve photovoltaic performance by suppressing unfavorable binding configurations in solar cell applications.

Synthesis of EDOT-containing organic dyes via one-pot, four-component Suzuki–Miyaura coupling and the evaluation of their photovoltaic properties

We have developed a one-pot, four-component Suzuki–Miyaura coupling for the synthesis of thiophene-based donor–π-bridge–acceptor (D–π–A) dyes for dye-sensitized solar cells. Two D–π–A dyes, 19 and 20, retaining 3,4-ethylenedioxythiophene (EDOT) beside a cyanoacrylic acid moiety, and one D–π–A dye 21 without EDOT (reference compound) were rapidly synthesized in accordance with the developed procedure. The measurement of the absorption spectra and the electrochemical properties of synthesized dyes, and the photovoltaic properties of solar cells that were prepared using the synthesized dyes 19–21, revealed that the dyes retaining only one EDOT beside a cyanoacrylic acid moiety would exert a high Jsc.

New conjugated organic dyes with various electron donors: One- and two-photon excited fluorescence, and bioimaging

Six new fluorescent donor-π bridge-acceptor (D-π-A) structural molecules with various electron donors were synthesized and fully characterized. Influence of electron-donating strength on optical properties was investigated by the systematic alteration of the electron donors. The optical properties of the dyes were successfully tuned by choosing different electron donors. All the dyes in solution show strong one-photon excited fluorescence (OPEF) and high quantum yield. With increasing electron-donating strength of donors, dye 5 with diphenylamine and dye 6 with (4,4′-diethoxyphenyl)amino substituents, exhibit strong two-photon excited fluorescence (TPEF), and the large two-photon absorption (TPA) cross-section values are 1378 GM for 5 and 1654 GM for 6. The structure–property relationships were detailed through quantum chemical calculations and X-ray crystallography. The results of living cell imaging experiments show the values of them in one- or two-photon fluorescence microscopy bioimaging applications.

Synthesis and catalytic properties of novel POM@TiO2 composite materials

Donor–π-bridge–acceptor (D–π–A) type polyoxometalates (POMs) were self-assembled for the first time on the surface of titanium dioxide (TiO2) nanoparticles through the layer-by-layer (LBL) method. The obtained composite materials POM@TiO2 were characterized by Transmission electron microscopy (TEM), Fourier transform IR spectroscopy (FTIR), Raman spectrum and energy dispersive X-ray (EDX) spectroscopy. Catalytic properties of POM@TiO2 were also investigated by treating organic pollutants (typically, removal of 40mL 20mgL−1 methylene blue (MB) by 10mg POM@TiO2 was up to 99.5% within 3min under ambient conditions and the photodegradation efficiency was obviously higher than bare TiO2 nanoparticles under irradiation).

Novel Main Chain Polymeric Metal Complexes Based on Zn(II) or Cd(II) with Fluorene and 8-Hydroxyquinoline Ligand: Synthesis, Characterization and Photovoltaic Application in DSSCs

Four novel donor–π-bridge–acceptor(D–π–A) main chain polymeric metal complexes (P1, P2, P3 and P4) as dye sensitizers for dye-sensitized solar cells (DSSCs) based on Zn(II) or Cd(II) complexes with fluorene and 8-hydroxyquinoline ligand have been synthesized by the Heck coupling and characterized by FT-IR, Elemental analysis and GPC. The optical properties, thermal properties, electrochemical properties and photovoltaic properties of these polymeric metal complexes have been investigated and studied by TGA, DSC, C–V curves and I–V curves. DSSCs based on P1 and P3 show good photovoltaic performances and the corresponding power conversion efficiency (PCE) are up to 1.99 % (Jsc = 4.59 mA cm−2, Voc = 0.68 V, FF = 0.636) and 2.05 % (Jsc = 5.25 mA cm−2, Voc = 0.60 eV, FF = 0.65) under simulated AM 1.5 G solar irradiation (100 mW cm−2). Besides, they showed good stabilities with thermal decomposition Temperatures are 421, 441, 413, 385 °C, respectively, which are suitable for DSSCs.

Organic sensitizers from D–π–A to D–A–π–A: effect of the internal electron-withdrawing units on molecular absorption, energy levels and photovoltaic performances

The high performance and low cost of dye-sensitized solar cells (DSSCs) have drawn great interest from both academic and industrial circles. The research on exploring novel efficient sensitizers, especially on inexpensive metal-free pure organic dyes, has never been suspended. The donor-π bridge-acceptor (D-π-A) configuration is mainstream in the design of organic sensitizers due to its convenient modulation of the intramolecular charge-transfer nature. Recently, it has been found that incorporation of additional electron-withdrawing units (such as benzothiadiazole, benzotriazole, quinoxaline, phthalimide, diketopyrrolopyrrole, thienopyrazine, thiazole, triazine, cyanovinyl, cyano- and fluoro-substituted phenyl) into the π bridge as internal acceptors, termed the D-A-π-A configuration, displays several advantages such as tuning of the molecular energy levels, red-shift of the charge-transfer absorption band, and distinct improvement of photovoltaic performance and stability. We apply the D-A-π-A concept broadly to the organic sensitizers containing additional electron-withdrawing units between electron donors and acceptors. This review is projected to summarize the category of pure organic sensitizers on the basis of the D-A-π-A feature. By comparing the structure-property relationship of typical photovoltaic D-A-π-A dyes, the important guidelines in the design of such materials are highlighted.

Al3+-induced far-red fluorescence enhancement of conjugated polymer nanoparticles and its application in live cell imaging

Fluorescent nanoparticles (NPs) for Al(3+) sensing with high selectivity were developed from a type of carbazole-based conjugated polymer with a two-dimensional donor-π bridge-acceptor (D-π-A) structure. These NPs are characterized by their small particle diameter (∼18 nm), far-red fluorescence emission (centered ∼710 nm), and Al(3+)-induced fluorescence enhancement with high selectivity owing to an Al(3+)-triggered inhibition on the intramolecular charge transfer (ICT) processes between the conjugated backbone and the pendant acceptors. This type of nanoparticle is easily suspended in aqueous solutions, indicating their practical applicability in physiological media, and their ability for intracellular Al(3+) sensing was confirmed. As compared to other types of conjugated polymer based probes showing metal ion mediated fluorescence quenching, these as-prepared NPs possess analyte-enhanced fluorescence emission, which is analytically favored in terms of sensitivity and selectivity. Fluorescence emission with wavelengths in the biological window of maximum optical transparency (∼700 to 1000 nm) is expected to impart a salient advantage for biological detection applications to these as-prepared probes. The superior features of merit of this new type of fluorescent probe, together with the validation of practicability for intracellular Al(3+) ion sensing, are indicative of their potential for application in fluorescence-based imaging and sensing, such as investigations on Al(3+)-related physiological and pathological processes.

Molecular engineering for panchromatic absorbing oligothiophene donor–π–acceptor organic semiconductors

A series of three novel organic semiconductor molecules based on the donor–π-bridge–acceptor (D–π–A) modular design have been designed, synthesised and characterised. These small organic molecules have a common donor (triphenylamine), π-bridge (an alkylated tetrathiophene) and various acceptors to provide tuning of optical and electronic properties. The examined acceptors were dicyanovinylidene, cyanopyridone and oxoindenemalononitrile groups. These compounds were highly soluble in common organic solvents, such as chloroform, toluene and chlorobenzene. As acceptor strength increased from dicyanovinylidene to cyanopyridone to oxoindenemalononitrile groups, the wavelength of the longest wavelength absorption maximum increased (518, 587, 619nm (solution)), the respective extinction coefficients increased (4.9×104, 6.3×104, 7.4×104M−1cm−1) and with increasing acceptor strength the band gap narrowed (1.63, 1.42 1.38eV). Panchromatic absorbance was observed for the compounds comprising the cyanopyridone and oxoindenemalononitrile acceptors.