Extension Of Π-conjugation Research Articles

Novel electron-deficient quinoxalinedithienothiophene- and phenazinedithienothiophene-based photosensitizers: The effect of conjugation expansion on DSSC performance

Two novel electron-deficient π-conjugated moieties, quinoxalinedithienothiophene (QBTT) and phenazinedithieno-thiophene (PBTT) were designed and synthesized as the π-bridge for the construction of D-π-A and D-A1-π-A configured photosensitizers for DSSC applications. Five new metal free photosensitizers, namely QC5-m and PC5-n where m = 1–2 and n = 1–3 were designed and synthesized using carbazole as an electron-donor, benzothiadiazole (BTZ) as an auxiliary group and cyanoacylic acid as an electron-acceptor. Despite only lateral π-conjugation expansion of PBTT π-bridge, PC5-n based dyes show broader spectral absorption than those of QC5-m counterparts. Both electron-deficient QBTT and PBTT moieties are found to be a useful π-conjugated bridge to achieve broad spectral absorption and strong charge transfer of a photosensitizer. DSSCs based on these photosensitizers were fabricated and investigated which exhibited power conversion efficiency (PCE) in the range of 5.23–7.77% with a simple sandwich structure under AM 1.5 irradiation, 100 mW cm−1. Among DSSCs fabricated, PC5-1 based device afforded the best photovoltaic performance with PCE up to 7.77%, Voc = 692 mV, Jsc = 15.6 mA cm−2 and FF = 72%. Consistently, the EIS and IMVS studies showed that PC5-1 based device exhibited the most efficient electron transfer at the interface between the electrolyte and the sensitized TiO2 and the slowest charge recombination rate among the devices studied. Our results demonstrated that lateral π-conjugated expansion is found more useful than linear π-conjugated extension in enhancing absorption and device performance, and electron-deficient π-conjugated moieties are a promising π-bridge for efficient charge-transport for photosensitizing applications.

Synthesis of a near-infrared light-absorbing polymer based on thiophene-substituted Aza-BODIPY

An alternating conjugated polymer with thiophene-substituted aza-boron dipyrromethene (BODIPY) was synthesized in which the carbon atom at the meso-position in the ligand moiety was replaced by nitrogen. Initially, it was found that the synthesized polymer had a significant near-infrared (NIR) light-absorbing ability. In the absorption spectrum, a large absorption band (molar extinct coefficient emax = 48,000 M−1 cm−1) with a peak at 864 nm was detected in the deep NIR region, even above 1300 nm. From cyclic voltammetry (CV) data, the energy levels of the frontier orbitals were determined. Accordingly, the polymer had a deep lowest unoccupied molecular orbital (LUMO) level (−4.01 eV), and this value was similar to that of the monomer. This result indicates that extension of π-conjugation throughout the polymer main-chain influenced only the highest occupied molecular orbital (HOMO) level while preserving the LUMO. The alternating conjugated polymer was synthesized with thiophene-substituted aza-boron dipyrromethene. The synthesized polymer had the near-infrared light-absorbing ability (molar extinct coefficient: 48,000 M−1cm−1) in the region over 1300 nm with the peak at 864 nm. From the electrochemical data, it was found that the polymer had the deep lowest unoccupied molecular orbital (LUMO) level. This result means that only the highest occupied molecular orbital level was influenced and the LUMO can be preserved by extension of π-conjugation through the polymer main-chain.

Design of a New Small-Molecule Electron Acceptor Enables Efficient Polymer Solar Cells with High Fill Factor.

Improving the fill factor (FF) is known as a challenging issue in organic solar cells (OSCs). Herein, a strategy of extending the conjugated area of end-group is proposed for the molecular design of acceptor-donor-acceptor (A-D-A)-type small molecule acceptor (SMA), and an indaceno[1,2-b:5,6-b']dithiophene-based SMA, namely IDTN, by end-capping with the naphthyl fused 2-(3-oxocyclopentylidene)malononitrile is synthesized. Benefiting from the π-conjugation extension by fusing two phenyls, IDTN shows stronger molecular aggregation, more ordered packing structure, thus over one order of magnitude higher electron mobility relative to its counterpart. By utilizing the fluorinated polymer (PBDB-TF) as the electron donor, the corresponding device exhibits a high efficiency of 12.2% with a record-high FF of 0.78, which is approaching the theoretical limit of OSCs. Compared with the reference molecule, such a high FF in the IDTN system can be mainly attributed to the more ordered π-π packing of acceptor aggregates, higher domain purity and symmetric carrier transport in the blend. Hence, enlarging the conjugated area of the terminal-group in these A-D-A-type SMAs is a promising approach not only for enhancing the electron mobility, but also for improving the blend morphology, and both of them are conducive to the fill-factor breakthrough.

Electronic π-Delocalization Boosts Catalytic Water Oxidation by Cu(II) Molecular Catalysts Heterogenized on Graphene Sheets.

A molecular water oxidation catalyst based on the copper complex of general formula [(Lpy)CuII]2-, 22-, (Lpy is 4-pyrenyl-1,2-phenylenebis(oxamidate) ligand) has been rationally designed and prepared to support a more extended π-conjugation through its structure in contrast with its homologue, the [(L)CuII]2- water oxidation catalyst, 12- (L is o-phenylenebis(oxamidate)). The catalytic performance of both catalysts has been comparatively studied in homogeneous phase and in heterogeneous phase by π-stacking anchorage to graphene-based electrodes. In the homogeneous system, the electronic perturbation provided by the pyrene functionality translates into a 150 mV lower overpotential for 22- with respect to 12- and an impressive increase in the kcat from 6 to 128 s-1. Upon anchorage, π-stacking interactions with the graphene sheets provide further π-delocalization that improves the catalytic performance of both catalysts. In this sense, 22- turned out to be the most active catalyst due to the double influence of both the pyrene and the graphene, displaying an overpotential of 538 mV, a kcat of 540 s-1 and producing more than 5300 TONs.

Wide Band Gap and Highly Conjugated Copolymers Incorporating 2-(Triisopropylsilylethynyl)thiophene-Substituted Benzodithiophene for Efficient Non-Fullerene Organic Solar Cells

Recent years have seen a rapid progress in the power conversion efficiencies (PCEs) of non-fullerene polymer solar cells (NF PSCs). However, the donor materials accordingly used are typical low or medium band gap polymers, some of which possess badly overlapped absorption spectra relative to the low band gap n-type acceptors, for example, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone)-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene) (ITIC). To obtain polymers simultaneously owning a wide band gap, a highly extended π-conjugation system, and a low-lying highest occupied molecular orbital (HOMO), a polymer (PBDTSi-TA) incorporating 2-(triisopropylsilylethynyl)thiophene substituted benzodithiophene (BDTSi) and fluorinated benzotriazole (FTAZ) units was designed and synthesized. PBDTSi-TA (Egopt = 1.92 eV) exhibits strong molecular aggregation properties and a lower-lying HOMO energy level compared to its structural analogues. When blended with ITIC and after device optimization with solvent vapor annealing in combination with a developed PDIN/BCP/Ag cathode structure, PSCs yielded a PCE of 7.51%, with Voc = 0.96 V. Moreover, a rather small energy loss (Eloss) of 0.6-0.63 eV was determined. For comparison, another polymer (PBDTSi-Qx) with a more-electron-deficient quinoxaline-based acceptor unit was also synthesized and applied to NF PSCs. Charge generation rate, exciton dissociation probabilities, dark leakage current, nanoscale morphology, and charge carrier mobilities have been evaluated to probe the reasons for the differentiated performances. The results suggest that PBDTSi-TA is a promising donor material for NF PSCs, and the molecular design strategy demonstrated here would be helpful for pursuing high-performance polymers for PSCs.

Luminescence Color Tuning from Blue to Near Infrared of Stable Luminescent Solid Materials Based on Bis-o-Carborane-Substituted Oligoacenes.

Aryl-substituted o-carboranes have shown highly efficient solid-state emission in previous studies. To demonstrate color tuning of the solid-state emission in an aryl-o-carborane-based system, bis-o-carborane-substituted oligoacenes were synthesized and their properties were systematically investigated. Optical and electrochemical measurements revealed efficient decreases in energy band gaps and lowest unoccupied molecular orbital (LUMO) levels by adding a number of fused benzene rings for the extension of π-conjugation. As a consequence, bright solid-state emission was observed in the region from blue to near infrared (NIR). Furthermore, various useful features were obtained from the modified o-carboranes as an optical material. The naphthalene derivatives exhibited aggregation-induced emission (AIE) and almost 100 % quantum efficiency in the crystalline state. Furthermore, it was shown that the tetracene derivative with NIR-emissive properties had high durability toward photo-bleaching under UV irradiation.

Facile and Reversible Electrogeneration of Porphyrin Trianions and Tetraanions in Nonaqueous Media.

The first examples for the facile, reversible, and stepwise electrogeneration of triply ring-reduced porphyrin macrocycles are presented. The investigated compounds are represented as MTPP(NO2)(PE)6, MTTP(PE)8, NiTPP(NO2)(Ph)4, and MTPP(CN)4, where TTP and TPP are the dianions of tetratolylporphyrin and tetraphenylporphyrin, respectively, NO2, phenylethynyl (PE), and CN are substituents at the β-pyrrole positions of the macrocycle, and M = CuII, NiII, ZnII, CoII, or 2H. Each porphyrin undergoes three or four reductions within the negative potential limit of the electrochemical solvent. The UV-visible spectra of the first three reduction products were characterized by means of thin-layer UV-vis spectroelectrochemistry, and the generation of multianionic porphyrins is interpreted in terms of extensive stabilization of the LUMOs due to the electron-withdrawing and/or extended π-conjugation of the β-substituents.

Ladder-type Heteroarenes: Up to 15 Rings with Five Imide Groups.

A series of novel imide-functionalized ladder-type heteroarenes with well-defined structure and controllable conjugation lengths were synthesized and characterized. The synthetic route shows remarkable efficacy for constructing the electron-deficient ladder backbones. π-Conjugation extension leads to narrowed band gaps with enhanced electron affinities. The ladder arenes are incorporated into organic thin-film transistors, and show encouraging electron mobilities of 0.013-0.045 cm2 V-1 s-1 . The heteroarenes reported here provide a remarkable platform for fundamental physicochemical studies and materials innovation in organic electronics.

Synthesis and photophysical properties of cyclometalated 4′-phenyl-2,2′:6′,2″-terpyridyl Pt(II) chloride complexes with different aryl substituents

A series of 4′-phenyl-2,2′:6′,2″-terpyridyl Pt(II) chloride complexes bearing different aryl substituents (1a–1f) was designed and synthesized. The photophysical properties of these complexes have been systematically investigated. All complexes exhibit strong 1π,π* absorption bands in the UV region; and broad, structureless metal-to-ligand charge transfer (1MLCT) absorption bands in the visible region. When excited at the charge-transfer absorption band, the complexes exhibit room temperature luminescence (λem=555–614nm) in CH3CN. The emissions of complexes 1c–1f with electron-donating or extended π-conjugation substituents are assigned to the 3MLCT states. Meanwhile, the emitting state of 1a and 1b exhibits a significant intraligand 3π,π* character. Complexes 1a–1d exhibit moderate triplet transient absorptions from visible to NIR region, where reverse saturable absorption (RSA) occurs. Their photophysical properties have been investigated with the aim to provide a basis for elucidating the structure-property correlations and developing new nonlinear optical materials.

Polyyne bridged AIE luminogens with red emission: design, synthesis, properties and applications.

Construction of a donor-acceptor (D-A) structure and extension of π-conjugation are the commonly used strategies to shift the emission of luminophores to the red region. However, molecules with high conjugation and a strong D-A effect tend to show weak emission due to the severe π-π interactions and twisted intramolecular charge transfer (TICT) effects. The turn-on characteristic of AIE luminogens (AIEgens) will also be lost due to the conjugation-enhanced emission in the solution state. Herein, a polyyne-bridged AIE luminogen (2TPE-4E) with long wavelength absorption and red emission has been afforded. Despite its large π-conjugation, 2TPE-4E suffers from no emission quenching resulted from strong π-π interactions and twisted intramolecular charge transfer effects. The strong red emission and the high photostability of 2TPE-4E inspired us to use it for targeted-imaging of cancer cells and monitoring the receptor mediated endocytosis process.

Photochromic Torsional Switch (PTS): a light-driven actuator for the dynamic tuning of π-conjugation extension.

Here we present a molecular architecture that can reversibly change the geometric conformation of its π-system backbone via irradiation with two different wavelengths. The proposed 'molecular actuator' consists of a photoswitchable azobenzene orthogonally connected to a π-conjugated bithiophene by both direct and aliphatic linker-assisted bonding. Upon exposure to 350 nm light, the trans azobenzene moiety isomerizes to its cis form, causing the bithiophene to assume a semiplanar anti conformation (extended π-conjugation). Exposure to 254 nm light promotes the isomerization of the azobenzene unit back to its initial extended trans conformation, thus forcing the bithiophene fragment to twist out of coplanarity (restricted π-conjugation). The molecular conformation of the bithiophene was characterized using steady-state UV-vis and nuclear magnetic resonance spectroscopy, as well as ab initio computations. The proposed molecular design could be envisaged as a π-conjugation modulator, which has potential to be incorporated into extended linear π-systems, i.e. via the terminal α-thiophene positions, and used to tune their optical and electronic properties.

BODIPY associates in organic matrices: Spectral properties, photostability and evaluation as OLED emitters

In the present study four BODIPY (boron dipyrromethene: 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes with π-extended substituents in C-8 position were investigated in solvents and polymer mediums. High aggregation degree was observed for the dyes in the solid state. Association and twisted intramolecular charge transfer processes were found to affect the spectral properties of the compounds causing bathochromic shifts in absorption and fluorescence spectra. The extension of substituent π-conjugation gains molecular association evoked presumably by π-π interaction between the substituents of the adjacent molecules. Photostability of the complexes in different forms was analyzed and the distorted form stabilized by polymer matrix was found to be the most stable. The substituent nature did not affect strongly the photostability of dyes. Displacement of monomer-associate equilibrium in hybrid materials with polymethylmethacrylate and poly(9-vinylcarbazole) was exploited for tuning spectral characteristics of the materials. Two dyes readily forming aggregates at the lowest concentrations were applied for the fabrication of organic light-emitting diodes. The fabricated devices exhibited electroluminescence in the appropriate spectral ranges with moderate efficiency.

Conjugated Oligothiophene Derivatives Based on Bithiophene with Unsaturated Bonds as Building Blocks for Solution-Processed Bulk Heterojunction Organic Solar Cells.

A new building block ATVTA that uses stiff carbon-carbon triple bonds (A) on 1,2-di(2-thienyl)-ethene (TVT) has been developed. Oligothiophene derivatives S-01 with a TVT unit, S-02 with a 5,5'-diethynyl-2,2'-dithienyl (AT2) unit and S-03 with ATVTA were synthesized to compare their effects in a systematic study. Due to the better π-conjugation extension of the TVT unit, S-01 exhibits the most red-shifted absorption profile among them, whereas S-02 possesses the deepest HOMO level. While the HOMO level of S-03 is down-shifted by 0.02 eV relative to that of S-01, the alkyne linkages can effectively down-shift the HOMO level. By replacing the terminal units of S-03 with stronger electron acceptors, S-04 and S-05 exhibited broader absorption profiles and lower HOMO levels than those of S-03. Organic solar cells based on these molecules were fabricated and an S-03:PC60 BM (1:1, w/w) based device afforded the highest Voc value of 0.96 V and a power conversion efficiency (PCE) of 2.19 %.

Nafion-Modified PEDOT:PSS as a Transparent Hole-Transporting Layer for High-Performance Crystalline-Si/Organic Heterojunction Solar Cells with Improved Light Soaking Stability.

We demonstrate the chemistry of amphiphilic perfluorosulfonic copolymer Nafion-coated conductive poly(3,4-ethyelenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and its effect on the photovoltaic performance of PEDOT:PSS/crystalline Si (c-Si) heterojunction solar cells. The highly hydrophilic sulfonate group of insulating, chemically stable Nafion interacts with PSS in PEDOT:PSS, which reduce the Coulombic interaction between PEDOT and PSS. The highly hydrophobic fluorocarbon backbone of Nafion favorably interacts with hydrophobic PEDOT of PEDOT:PSS. These factors give rise to the extension of π-conjugation of PEDOT chains. Silver paste used as a top grid electrode diffused into the Nafion layer and contacted with underneath Nafion-modified PEDOT:PSS layer. As a consequent, solution-processed Nafion-coated PEDOT:PSS/c-Si heterojunction solar cells exhibited a higher power conversion efficiency of 14.0% with better stability for light soaking rather than that of the pristine PEDOT:PSS/c-Si device by adjusting the layer thickness of Nafion. These findings originate from the chemical stability of hydrophobic fluorocarbon backbone of Nafion, diffusivity of silver paste into Nafion and contact with PEDOT:PSS, and Nafion as an antireflection layer.

Boron complexes of aromatic ring fused iminopyrrolyl ligands: synthesis, structure, and luminescence properties.

The condensation reactions of 2-formylindole (1) or 2-formylphenanthro[9,10-c]pyrrole (2) with various aromatic amines afforded the corresponding phenyl or phenanthrene ring fused mono-/bis-iminopyrrole ligand precursors 3-8, which, upon reaction with BPh3 in an appropriate molar ratio, led to the new mono- and diboron chelate compounds Ph2B[NC8H5C(H)[double bond, length as m-dash]N-2,6-Ar] (Ar = 2,6-iPr2C6H39; C6H510), Ph2B[(NC8H5C(H)[double bond, length as m-dash]N)2-1,4-C6H4]BPh211, Ph2B(NC16H9C(H)[double bond, length as m-dash]N-Ar) (Ar = 2,6-iPr2C6H312; C6H513), and Ph2B[(NC16H9C(H)[double bond, length as m-dash]N)2-1,4-C6H4]BPh214, respectively. Boron complexes 12-14, containing a phenanthrene fragment fused to the pyrrolyl C3-C4 bond, are highly fluorescent in solution, with quantum efficiencies of 37%, 61% and 58% (in THF), respectively, their emission colours ranging from blue to orange depending on the extension of π-conjugation. Complexes 9-11, containing a benzene fragment fused to the pyrrolyl C4-C5 bond, are much weaker emitters, exhibiting quantum efficiencies of 10%, 7% and 6%, respectively. DFT and TDDFT calculations showed that 2,6-iPr2C6H3N-substituents or, to a smaller extent, the indolyl group prevent a planar geometry of the ligand in the excited state and reveal the existence of a low energy weak band in all the indolyl complexes, which is responsible for the different optical properties. Non-doped single-layer light-emitting diodes (OLEDs) were fabricated with complexes 9-14, deposited by spin coating, that of complex 13 revealing a maximum luminance of 198 cd m-2.

Porphyrins Containing a Triphenylamine Donor and up to Eight Alkoxy Chains for Dye-Sensitized Solar Cells: A High Efficiency of 10.9%.

Porphyrins are promising DSSC sensitizers due to their structural similarity to chlorophylls as well as their tunable strong absorption. Herein, a novel D-π-A porphyrin dye XW14 containing a strongly electron-donating triphenylamine moiety as the electron donor was designed and synthesized. To avoid undesirably decreased Voc caused by dye aggregation effect, two methoxy or hexyloxy chains were introduced to the para positions of the triphenylamine moiety to afford XW15 and XW16, respectively. To further extend the absorption to a longer wavelength, a benzothiadiazole unit was introduced as an auxiliary acceptor to furnish XW17. Compared with XW14, the introduction of additional methoxy or hexyloxy groups in XW15 and XW16 red-shift the onset wavelengths from 760 to 780 and 790 nm, respectively. More impressively, XW17 has a more extended π-conjugation framework, and thus, it exhibits a much broader IPCE spectrum with an extremely red-shifted onset wavelength of 830 nm, resulting in the highest Jsc (18.79 mA cm(-2)). On the other hand, the hexyloxy chains are favorable for suppressing the dye aggregation effect, and thus XW16 shows the highest Voc of 734 mV. As a result, XW16 and XW17 demonstrate photovoltaic efficiencies of 9.1 and 9.5%, respectively, higher than those of XW14 (8.6%) and XW15 (8.7%), and obviously higher than that of 7.94% for our previously reported dye, XW4. On the basis of optimized porphyrin dye XW17, we used a nonporphyrin dye with a high Voc and strong absorption around 500 nm (WS-5) as the cosensitizer to improve the Voc from 700 to 748 mV, with synergistical Jsc enhancement from 18.79 to 20.30 mA cm(-2). Thus, the efficiency was dramatically enhanced to 10.9%, which is among the highest efficiencies obtained for the DSSCs based on traditional iodine electrolyte. In addition, the DSSCs based on XW17 + WS-5 exhibit good photostability, which is beneficial for practical applications.

A comparative spectroelectrochemistry of homo- and copolymerization of pyrrole and N-methylpyrrole with indole on a gold electrode

Electrochemical formation and properties of intrinsically conducting homo- and copolymers of polypyrrole (PPy), poly-N-methylpyrrole (PNMPy), polyindole (PIn), poly(pyrrole-indole) [P(Py-In)], poly(N-methylpyrrole-indole) [P(NMPy-In)] on gold electrode were studied comparatively. Characterization of the samples was performed by cyclic voltammetry, in situ UV–Vis and FTIR spectroscopy methods, in situ resistance measurements and scanning electron microscopy (SEM) techniques. The voltammograms exhibited different behavior for various concentration ratios of the monomer in the feed with redox peaks observed at different positions. Positive shifts of the oxidation peaks were observed for copolymers synthesized using higher concentrations of monomer during the electropolymerization. In situ UV–Vis and FTIR spectroscopy results showed a spectroscopic behavior of the copolymers intermediate between those of the homopolymers. The resistance of copolymers generally grew with increasing (In)/(Py) and (In)/(NMPy) concentration ratio. This was most likely due to a smaller amount of pyrrole units in the copolymer backbone yielding a less extended π-conjugation system along the copolymer backbone. In comparison, the resistance of P(NMPy-In) was higher than that of P(Py-In) copolymers. The morphology of polymers and copolymers was studied and compared using SEM. The SEM micrographs showed similarities between homo- and copolymers. A mechanism was suggested for the formation of copolymers proceeding via radical cations of indole, pyrrole and that of the possibly formed copolymer.

Coplanar Oligo(p-phenylenedisilenylene)s as Si═Si Analogues of Oligo(p-phenylenevinylene)s: Evidence for Extended π-Conjugation through the Carbon and Silicon π-Frameworks.

A series of oligo(p-phenylenedisilenylene)s (Si-OPVs 1-4), silicon analogues of oligo(p-phenylenevinylene)s, up to the tetramer have been synthesized and isolated by the introduction of a newly developed protecting group [(HexO)MEind] for improving their solubility. The experimental and theoretical studies of the Si-OPVs 1-4 demonstrate the fully extended π-conjugation of the Si-OPV main chains. Single crystal X-ray analyses of the monomer 1 and the dimer 2 revealed the highly coplanar Si-OPV backbones facilitating the effective extension of the π-conjugation, which has further been validated by the significant increases in the absorption maxima from 465 nm for the monomer 1 to 610 nm for the tetramer 4. The absorption maxima exhibit an excellent fit to Meier's equation, leading to the estimation of an effective conjugation length (ECL) of 9 repeat units (nECL = 9) and the absorption maximum of 635 nm for the infinite chain (λ∞ = 635 nm). In sharp contrast to other nonemissive disilenes, the Si-OPVs 2-4 show an intense fluorescence from 613 to 668 nm at room temperature with the quantum yields up to 0.48. All the data presented here provide the first evidence for the efficient extended π-conjugation between the Si═Si double bonds and the carbon π-electron systems over the entire Si-OPV skeleton. This study reveals the possibility for developing the conjugated disilene π-systems, in which the Si═Si double bonds would be promising building blocks, significantly optimizing the intrinsic photophysical and electrochemical properties of the carbon-based π-conjugated materials.

Long-Range π-Conjugation in Phenothiazine-containing Donor-Acceptor Dyes for Application in Dye-Sensitized Solar Cells.

Four organic donor-π-bridge-acceptor dyes containing phenothiazine as a spacer and cyanoacrylic acid as an acceptor were synthesized and tested as sensitizers in dye-sensitized solar cells (DSCs). The influence of iodide- and cobalt-based redox electrolytes on the photovoltaic device performance was investigated. In these new dyes, systematic π-conjugation was extended by inserting one or two phenothiazine moieties and investigated within the context of the resulting photoinduced charge-transfer properties. A detailed investigation, including transient absorption spectroscopy and quantum chemical methods, provided important information on the role of extended π-conjugation on the photophysical properties and photovoltaic device performance. Overall, the results showed that the extension of π-conjugation by one phenothiazine unit resulted in the best device performance owing to reduced recombination rates, whereas extension by two phenothiazine units reduced dye adsorption on TiO2 probably owing to the increase in molecular size. The performance of the dyes in DSCs was found to be a complex interaction between dye structure and size.

Synthesis of π-Conjugated Polymers Containing Dibenzosilepin Moieties with Pentacoordinate Silicon

Abstract Benzo[h]quinolyldibenzo[b,f]silepin containing pentacoordinate silicon was incorporated into π-conjugated polymers. Substituent effects were investigated from UV–vis absorption and photoluminescence spectra. The extension of π-conjugation involving pentacoordinate silicon was suggested from the characteristic absorption and emission bands through the polymer chain at meta-positions toward silicon in the silepin rings. The polymers containing the fluorinated silicon showed bathochromic shift compared to that from the methylated silicon-containing polymers. Polymerization at para-positions toward silicon resulted in the formation of short conjugation length. Finally, the emission band from the charge-transfer transition was observed in photoluminescence spectra of the fluorinated polymer.