Π-conjugated Bridge Research Articles

Theoretical study on the electronic structure and optical properties of carbazole-π-dimesitylborane as bipolar fluorophores for nondoped blue OLEDs

Molecules with D-π-A structures are drawing increased attention for applications in organic electronic devices due to their distinct optoelectronic properties. A study of a new series of bipolar fluorophores that have been chemically modified for use as highly efficient nondoped blue organic light-emitting diodes (OLEDs) has been carried out based on existing molecular structures and a literature survey. The aim of this study is to provide a profound interpretation of the optical and electronic properties and the structure–property relationships of a series of new bipolar fluorophores. The study also aims to predict the photophysical and optoelectronic properties of the new fluorophores. The density functional theory (DFT) has been confirmed as reliable, especially in predicting the properties of unknown products. The geometry and the electronic structure of these molecules in the ground state were studied with DFT and ab initio HF, whereas the lowest singlet excited-state geometries were optimized by ab initio singlet configuration interaction (CIS). The absorption and emission spectra, both in the gas phase and in THF, and the lowest singlet excited energies were calculated by employing the time-dependent density functional theory (TDDFT) and the polarizable continuum model (PCM). To precisely predict the charge-transporting and charge-confining properties of the new fluorophores, three-layered devices have been simulated. The results show that the molecular geometries, HOMOs, LUMOs, energy gaps, ionization potentials (IP), electron affinities (EA), radiative lifetimes (τ), absorption and emission spectra are all tuned by chemical modifications with different π-conjugated bridges. The results also show that these molecular materials could be used as bipolar light-emitting materials for blue and deep-blue OLEDs.

Evidence of electrochemical transduction of cation recognition by TEMPO derivatives

This work reports the first example of electrochemical cation binding transductionvianitroxyl groups. It shows the possibility to transduce a complexation without a π-conjugated bridge between the redox and the host moieties. As expected, we confirm that the host/redox probe distance is a key point for transduction.

Organic bulk heterojunction solar cells based on solution processable small molecules (A–π–A) featuring 2-(4-nitrophenyl) acrylonitrile acceptors and phthalimide-based π-linkers

A novel small molecule-based organic donor SM [(2Z,2′Z)-3,3′-(((1E,1′E)-(2-cyclohexyl-1,3-dioxoisoindoline-4,7-diyl)bis(ethene-2,1-diyl))bis(4,1-phenylene))bis(2-(4-nitrophenyl)-acrylonitrile)] featuring 2-(4-nitrophenyl)acrylonitrile as the acceptor and a π-conjugation bridge composed of phthalimide and styryl units, with an A–π–A type structure, has been synthesized. It showed a long wavelength absorption band having an absorption maximum around 635 nm and the optical bandgap was 1.63 eV, which is lower than most reported conjugated polymers, including poly(3-hexylthiophene) (P3HT). The photovoltaic properties were investigated by constructing bulk heterojunction organic solar cell devices using SM as the electron donor and fullerene derivatives, i.e. PC60BM and PC70BM as the electron acceptors with the device architecture ITO/PEDOT:PSS/SM:PC60BM or PC70BM/Al. The effect of the SM/fullerene derivative weight ratio and the processing solvent were carefully investigated to improve the performance of the organic solar cells. The optimized organic solar cell with SM:PC60BM and SM:PC70BM cast from THF solvent, at a weight ratio of 1 : 3 showed power conversion efficiencies (PCEs) of about 1.70% and 2.56%, respectively. The enhanced value of PCE for the BHJ photovoltaic device based on PC70BM is related to the better absorption of PC70BM in the visible region compared to that of PC60BM. The SM:PC70BM blends cast from a DIO–THF mixture and subsequent thermal annealing exhibited improved PCEs of 3.68% and 4.14%, respectively.

Bistriphenylamine-based organic sensitizers with high molar extinction coefficients for dye-sensitized solar cells

A series of novel bistriphenylamine-based organic sensitizers (coded as RC-dyes) with high molar extinction coefficients (>5.0 × 104 M−1 cm−1) have been synthesized for dye-sensitized solar cells (DSSCs). By changing the length of the π-conjugation bridge thiophene from zero to two between the donor and the acceptor part, the range of absorption and its corresponding photon-to-current conversion was modulated gradually to the low energy region in the visible spectrum. Based on the electrochemical and theoretical investigations on the electronic and geometrical structure, RC dyes were shown to be a promising candidate for efficient metal-free organic sensitizers with strong light absorption. Among RC dyes (11∼13) tested, the highest power conversion efficiency of 5.67% was obtained from RC-13 with a short circuit current of 10.78 mA cm−2, an open circuit voltage of 0.71 V, and a fill factor of 0.74 under standard AM 1.5 measurement conditions.

Experimental and Theoretical Studies on Organic D‐π‐A Systems Containing Three‐Coordinate Boron Moieties as both π‐Donor and π‐Acceptor

Four linear π-conjugated systems with 1,3-diethyl-1,3,2-benzodiazaborolyl [C(6)H(4)(NEt)(2)B] as a π-donor at one end and dimesitylboryl (BMes(2)) as a π-acceptor at the other end were synthesized. These unusual push-pull systems contain phenylene (-1,4-C(6)H(4)-; 1), biphenylene (-4,4'-(1,1'-C(6)H(4))(2)-; 2), thiophene (-2,5-C(4)H(2)S-; 3), and dithiophene (-5,5'-(2,2'-C(4)H(2)S)(2)-; 4) as π-conjugated bridges and different types of three-coordinate boron moieties serving as both π-donor and π-acceptor. Molecular structures of 2, 3, and 4 were determined by single-crystal X-ray diffraction. Photophysical studies on these systems reveal blue-green fluorescence in all compounds. The Stokes shifts for 1, 2, and 3 are notably large at 7820-9760 cm(-1) in THF and 5430-6210 cm(-1) in cyclohexane, whereas the Stokes shift for 4 is significantly smaller at 5510 cm(-1) in THF and 2450 cm(-1) in cyclohexane. Calculations on model systems 1'-4' show the HOMO to be mainly diazaborolyl in character and the LUMO to be dominated by the empty p orbital at the boron atom of the BMes(2) group. However, there are considerable dithiophene bridge contributions to both orbitals in 4'. From the experimental data and MO calculations, the π-electron-donating strength of the 1,3-diethyl-1,3,2-benzodiazaborolyl group was found to lie between that of methoxy and dimethylamino groups. TD-DFT calculations on 1'-4', using B3LYP and CAM-B3LYP functionals, provide insight into the absorption and emission processes. B3LYP predicts that both the absorption and emission processes have strong charge-transfer character. CAM-B3LYP which, unlike B3LYP, contains the physics necessary to describe charge-transfer excitations, predicts only a limited amount of charge transfer upon absorption, but somewhat more upon emission. The excited-state (S(1)) geometries show the borolyl group to be significantly altered compared to the ground-state (S(0)) geometries. This borolyl group reorganization in the excited state is believed to be responsible for the large Stokes shifts in organic systems containing benzodiazaborolyl groups in these and related compounds.

Syntheses of organic dyes based on phenothiazine as photosensitizers and effects of their π-conjugated bridges on the photovoltaic performances of dye-sensitized solar cells

Three organic dyes, 2-cyano-3-(N-hexylphenothiazine-3-yl)acrylic acid (H-PTZ), 2-cyano-3-(5′-(2-(Nhexylphenothiazine-3-yl)vinyl)-2,2′-bithiophen-5-yl)acrylic acid (H-PTZ-BT), and 2-cyano-3-(5-(4-((4-(2-(N-hexylphenothiazine-3-yl)vinyl)phenyl)(phenyl)amino)phenyl)thiophen-2-yl)acrylic acid (H-PTZ-TPA-T), based on the phenothiazine (PTZ) unit as an electron donor moiety and cyanoacrylic acid unit as an electron acceptor moiety were synthesized to study the effect of a π-conjugated bridge of the dye on the performance of the resulting dye-sensitized solar cells devices. The thiophene unit and triphenylamine were introduced as a π-conjugated bridge. The H-PTZTPA-T-sensitized cell showed the highest overall conversion efficiency of 4.01% (J sc : 9.64 mA·cm−2; V oc : 0.69 V; FF: 0.60), while the H-PTZ-sensitized cell showed the lowest value of 3.57% (J sc : 7.76 mA·cm−2; V oc : 0.70 V; FF: 0.65) among the cells based on the three organic dyes compared to 5.09% for the cell based on the N719 dye (J sc : 12.77 mA·cm−2; V oc : 0.72 V; FF: 0.55) under AM 1.5 illumination (100 mW·cm−2).

Novel unsymmetrical push–pull squaraine chromophores for solution processed small molecule bulk heterojunction solar cells

New unsymmetrical low band-gap push–pull squaraine chromophores, [TPA-bisHT-HP]-X-Ind [X: SQ, diCNSQ], which are composed of triphenylamine electron donating, π-conjugated dithiophene-pyrrole bridging, and squaraine-indol electron withdrawing moieties, for their application in solution processed BHJ solar cell. The [TPA-bisHT-HP]-diCNSQ-Ind having the stronger electron withdrawing moiety exhibits significantly the red-shifted absorption spectrum due to its strong intramolecular charge transfer in solution as well as in solid-state. The bulk-heterojunction photovoltaic devices fabricated with [TPA-bisHT-HP]-diCNSQ-Ind/C71-PCBM film exhibited a PCE of 2.05% (±0.07) with short-circuit current (Jsc)=9.05mA/cm2, fill factor (FF)=0.33, and open-circuit voltage (Voc)=0.69V, which is ∼3 times higher efficiency compared to [TPA-bisHT-HP]-SQ-Ind/C71-PCBM BHJ films.

Electrochemical and spectroelectrochemical behavior of planar binuclear naphthalocyanines

The electrochemical and spectroelectrochemical behavior of two planar binuclear naphthalocyanines containing benzene and naphthalene π-conjugated bridges was investigated for the first time. Their intense absorption in the near infrared region was observed even in a low polar solvent such as o-dichlorobenzene after electrochemical reduction at a potential of -0.4 V vs. SCE, which was not enough to reduce the complexes into their first reduction state. These data and EPR studies allow us to conclude that the electrochemically generated form is the true neutral form of the synthesized compounds whereas the suppression of the near-infrared bands is a result of the presence of their cation radicals as one-electron oxidized forms.

Very Long-Distance Magnetic Coupling in a Dicopper(II) Metallacyclophane with Extended π-Conjugated Diphenylethyne Bridges

Self-assembly of the rigid rodlike ligand N,N'-4,4'-diphenylethynebis(oxamate) (dpeba) and Cu(2+) ions affords a novel dinuclear copper(II) metallacyclophane (nBu(4)N)(4)[Cu(2)(dpeba)(2)]·4MeOH·2Et(2)O (1) featuring a very long intermetallic distance (r = 15.0 Å). Magnetic susceptibility measurements for 1 reveal a moderately weak but nonnegligible intramolecular antiferromagnetic coupling between the two metal centers across the double para-substituted diphenylethynediamidate bridge (J = -3.9 cm(-1); H = -JS(1)S(2), where S(1) = S(2) = S(Cu) = (1)/(2)). Density functional electronic structure calculations on 1 support the occurrence of a spin polarization mechanism.

Novel Trumpet-Shaped Conjugation Bridge (Carbon Nanocone) for Nonlinear Optical Materials

Trumpet-shaped carbon nanocone (CNC) is used as a π-conjugated bridge to design high-performance nonlinear optical (NLO) material. Owing to the attractive trumpet-shaped structure, O2N–CNC–NH2 exhibits nonlinear optical behavior that is both surprisingly and qualitatively distinct from conventional π-conjugated organic species. It has been shown that the electron-acceptor nitryl (−NO2) at the apex of the trumpet and the electron-donor amino (−NH2) at the bottom edge of the trumpet to form O2N–CNC–NH2 is beneficial to improve the NLO response of CNC. Significantly, the first hyperpolarizabilities (βtot) of O2N–CNC–NH2 show an arc-shaped change as −NH2 substituting different H atoms along the arc-shaped bottom edge of CNC. Interestingly, the βtot values of H–CNC–NH2 also show an arc-shaped change. By comparison of the βtot values of O2N–CNC–NH2 and H–CNC–NH2, it has been found that no matter which bottom H of H–CNC–H is substituted by NH2, the βtot value of H–CNC–NH2 will be increased about 1.6 times after ...

Effects of aromatic π-conjugated bridges on optical and photovoltaic properties of N, N-diphenylhydrazone-based metal-free organic dyes

A series of novel D–π–A hydrazone dyes ( HB, HP, HF and HT) containing an N, N-diphenylhydrazone donor and 2-cyanoacetic acid acceptor linked by a different aromatic bridge (benzene, pyrrole, furan, and thiophene) have been designed and synthesized to evaluate the aromatic bridge effects on the photophysical, electrochemical and the photovoltaic properties of the hydrazone-sensitized TiO 2 solar cells. Each of the hydrazone-based dyes exhibited different adsorption behavior, frontier molecular orbitals, and photovoltaic performance depending on the identity of aromatic π-conjugated bridges. Specifically, HP-sensitized TiO 2 solar cell showed an obviously higher photocurrent, photovoltage, and power conversion efficiency than other three dyes. We interpret that these results were stemmed from light-harvesting abilities, the charge recombination rate and electron lifetime of different aromatic bridges. Optical spectroscopy, cyclic voltammetry, density functional theory calculations, electrochemical impedance spectra, and photovoltaic measurements were employed to support our proposal. With the addition of 1 mM chenodeoxycholic acid (CDCA) as the coadsorbent, a maximum power conversion efficiency of a DSSC based on HP was 7.74% ( J sc = 16.17 mA/cm 2, V oc = 0.69 V, FF = 0.694) under simulated AM 1.5 G solar irradiation (100 mW/cm 2). This work suggests that N, N-diphenylhydrazone and N-(2-ethylhexyl) substituted pyrrole moieties can be used as an alternative and effective donor and π-conjugated bridge, respectively, in the construction of efficient D–π–A organic dyes.

Synthesis of new N, N-diphenylhydrazone dyes for solar cells: Effects of thiophene-derived π-conjugated bridge

Four novel D– π–A hydrazone dyes (HT, HM, HE, and HO) with an N, N-diphenylhydrazone moiety as the electron donor, different thiophene-derived π-conjugated bridges and a cyanoacrylic acid moiety as the electron acceptor have been designed and synthesized for the application in dye-sensitized solar cells. The influences of thiophene-derived bridges on the photoelectrochemical and photovoltaic performance of these hydrazone dyes were investigated. Results demonstrate that the introduction of 3,4-dialkyloxythiophene could red-shift the dye’s absorption spectrum due to the enhancement of the electron-donating ability of π-conjugated bridges. Importantly, electrochemical impedance spectroscopy analysis reveal that 3,4-dialkyloxythiophene bridge could change the charge recombination resistance at the TiO 2/dye/electrolyte interface and as a result to improve the open-circuit photovoltage. Among the four dyes, HO exhibits the maximum power conversion efficiency of 5.83% ( V oc = 0.65 V, J sc = 12.69 mA/cm 2, FF = 0.707) under simulated AM 1.5 irradiation (100 mW/cm 2).

Imidazo-benzo-15-crown-5 ethers bearing arylthienyl and bithienyl moieties as novel fluorescent chemosensors for Pd 2+ and Cu 2+

Novel fluorescent ionophores bearing imidazo-arylthienyl or imidazo-bithienyl π-conjugated bridges functionalized with one or two fused benzo-15-crown-5 ethers as receptor units are reported. The sensing ability of the compounds in the presence of metallic cations (Li +, Na +, K +, Ca 2+, Zn 2+, Cu 2+, Ni 2+, Pd 2+, and Hg 2+) and fluoride ion was studied in MeCN/DMSO solutions by absorption and emission spectroscopy. The experimental results indicate that all compounds could act as selective fluorimetric sensors for Cu 2+ and Pd 2+ and also for the fluoride ion, in the case of the bis-substituted crown ether derivatives.

Mixed‐Valence Nanoclusters: Fast Electron Transfer in Mixed‐Valence Systems with a Gold Nanoparticle as the Bridge

Trinuclear ruthenium clusters attached to Au nanoparticles can be partially reduced to form mixed-valence nanoclusters. The nanoparticles are synthesized using a modified Brust method. IR-SEC shows dynamic coalescence of the stretching vibrations of the carbonyls, ν(CO), indicating the electron transfer between Ru clusters attached via a π-conjugated bridge to NPs occurs on the picosecond timescale. Simulation of IR spectra gives ket on the order of 1011 s−1.

Theoretical investigation of one- and two-photon spectra of pyrazabole chromophores

An extensive series of pyrazabole chromophores containing pseudo-conjugated systems have been theoretically constructed and investigated on the one-photon absorption (OPA) and two-photon absorption (TPA) properties by using density functional theory and Zerner’s intermediate neglect of differential overlap methods. The results indicated that all the pyrazabole chromophores show strong OPA at around 400 nm and intense TPA properties in the range of 500–600 nm with TPA cross sections (δ max) as large as 540–3,560 GM, which are excellent candidates for optical power limiting materials. It is noteworthy that the δ max values of the two constructed pyrazaboles, PA3 and PAF2, are 308.8 GM at 772.0 nm and 157.8 GM at 834.4 nm, respectively, which may be particularly attractive as probes for two-photon fluorescence imaging. The influence of incorporating electron acceptors in the central core, π-conjugated bridge and terminal groups on OPA and TPA properties was analyzed in detail to derive structure–property relationships and to lay the guidelines for both spectral tuning and amplification of molecular TPA in the target region. Meanwhile, the solvent effects on these properties were taken into account within the PCM model. The solvent has a significant impact on the TPA properties for chromophore PA3 and leads to the two-photon absorption spectra (λ max ) red-shift and δ max enhancing relative to those in gas phase. In addition, from the calculations of molecule AlA2, we can draw the conclusion that the compounds with the Al2N4 center behave similarly to pyrazabole chromophores in the linear optical and TPA properties and increase TPA cross sections to some extent.

Quantum mechanical modelling and calculation of two-photon absorption properties of new class ‘Λ’-shaped conjugated molecules

Organic material with high intensity of two-photon absorption (TPA)-induced fluorescence can be used as the up-converter material for photovoltaic devices. In this work, quantum mechanics modelling techniques were applied to theoretically investigate one- and two-photon absorption properties of new ‘Λ’-shaped conjugated molecules. Fluorene and diphenylmethylene analogues as the π centres were chosen for building the π-conjugated bridges to connect the two types of strong electron donors, N,N-diphenylamino and carbazole groups. In these molecular structures, cyano and ketone groups were also selected to modify the π centres as electron acceptors. The TPA cross sections of these derivatives were calculated using two-state, three-state and sum-over-state models, respectively. Geometrical structures of these molecules were optimised using Hartree–Fock theory, and properties of excited states of these molecules were obtained based on configuration interaction with single excitations method. The effects of donor, acceptor and π centre on the TPA behaviours of these designed molecules were investigated. Several of these molecules have attractive TPA properties, which may have potential for photovoltaic device applications.

Theoretical study on photophysical properties of 2,1,3-benzothiadiazole-based star-shaped molecules

Star-shaped molecules with tailoring functional groups in the core and the arms have great potential application in organic light-emitting devices, because it can be designed to realize low band gap, broad absorption, and excellent solubility for low-cost solution process. To gain an insight into the structure–property relationships, a set of four-arm star-shaped molecules with 2,1,3-benzothiadiazole as the core, different π-conjugated groups as the arm, and triphenylamine or 2-(pyridin-2-yl) pyridine as the end-group were designed. In this study, a systematic investigation into them was carried out using the density functional theory and time-dependent density functional theory methods. The calculated ionization potentials, electron affinities, and reorganization energies (λ) show that the properties of the π-conjugated bridge and the end-group significantly affect the carrier injection and transport characteristics of these molecules, especially for S-BTDP and S-EBTD. Among these molecules, S-BTDP exhibits better electron injection ability due to the introduction of 2-(pyridin-2-yl) pyridine as the end-group. However, S-EBTD, with ethylene as π-conjugated bridge, has excellent hole injection and carrier transport behaviors. We also calculated the singlet-to-triplet exciton-formation cross-section ratio (σS/σT), the exciton-formation fractions (χS), and the absorption and emission spectra of these molecules. We calculated that σS/σT ranges from 1.78 to 2.76 and that χS is ca. 0.37–0.48. These molecules have two absorption bands in the range of 340–410 nm and 500–613 nm, respectively. The calculated emission spectra range from 619 to 706 nm. It can be deduced that the studied 2,1,3-benzothiadiazole-based star-shaped molecules can serve as efficient red light-emitting electroluminescent materials.

Enhancement of electro-optic activity by introduction of a benzyloxy group to conventional donor-π-acceptor molecules

In this work, we investigated the enhancement of the electro-optic response by introducing benzyloxy group as an additional donor part on conventional donor-π-acceptor molecules. This new type donor exhibited a strong solvatochromic effect, indicating an extra donation to the π-conjugated bridge, which shifted the charge-transfer absorption of the chromophores to the lower energy region. Furthermore, the simple modification on the donor moiety resulted in about 2-fold improvement in the first hyperpolarizability and macroscopic electro-optic coefficient (r 33) over the benchmark 2-dicyanomethylen-3-cyano-4-{2-[E-(4-N,N-di(2-acetoxyethyl)-amino)-phenylene-(3,4-dibutyl)thien-5]-E-vinyl}-5,5-dimethyl-2,5-dihydrofuran (FTC) counterparts. An r 33 above 100 pm/V is ultimately achieved in conventional guest–host system.

Electrochemical and Optical Characterization of Triarylamine Functionalized Gold Nanoparticles

This paper describes the synthesis, structural analysis, and investigations of the optical and electrochemical properties of some gold nanoparticles (AuNPs) which consist of a triarylamine ligand shell attached to small gold cores (Au-Tara). The triarylamine chromophores were attached to small 4-bromobenzenethiol covered gold nanoparticles (ca. 2 nm in diameter) by Sonogashira reaction. This procedure yields triarylamine redox centers attached via π-conjugated bridging units of different length to the gold core. The AuNPs were analyzed with (1)H NMR spectroscopy, diffusion ordered NMR spectroscopy (DOSY), thermogravimetric analysis (TGA), and scanning transmission electron microscopy (STEM). Cyclic voltammetry (CV) technique was used to determine the composition of the redox active particles via the Randles-Sevcik equation. The optical and electrochemical properties of the Au-Tara nanoparticles and of their corresponding unbound ligands (Ref) were investigated with UV/vis/NIR absorption spectroscopy, Osteryoung square wave voltammetry (OSWV), and spectroelectrochemistry (SEC). These data show that the assembling of triarylamines in the vicinity of a gold nanoparticle can change the optical and electrochemical properties of the triarylamine redox chromophores depending on the kind and length of the bridging unit. This is due to gold core-chromophore and chromophore-chromophore interactions.

Synthesis and characterization of novel diazenes bearing pyrrole, thiophene and thiazole heterocycles as efficient photochromic and nonlinear optical (NLO) materials

Two series of novel thermally stable second-order nonlinear optical (NLO) and photochromic chromophores have been designed and synthesized. The two series of compounds were based on different combinations of donor groups (pyrrole or thienylpyrrole) which act simultaneously as π-conjugated bridges, together with diazoaryl or diazothiazolyl as acceptor moieties. Their photochromic and electrochemical behavior were characterized, while hyper-Rayleigh scattering (HRS) was employed to evaluate their second-order nonlinear optical properties. The results of these studies have been critically analyzed together with two other related compounds reported earlier from our laboratories in which the thienylpyrrole system was used as the donor group keeping the functionalized diazoaryl as acceptor moiety. The measured molecular first hyperpolarizabilities and the observed photochromic behavior showed strong variations for the different donor systems used (pyrrole or thienylpyrrole) and were also sensitive to the acceptor strength of the diazenehetero(aryl) moiety. The thienylpyrrole based compounds endowed with extended π-conjugated bridges and stronger donor auxiliary effects in comparison to the pyrrole compounds, when coupled to the stronger acceptor diazo(hetero)aryl groups gave rise to significantly larger hyperpolarizabilities ( β = 274–415 × 10 −30 esu) for an incident wavelength of 1064 nm. These compounds also displayed improved photochromic behavior with very fast response to the visible light stimulus (1.5 s) and fast thermal return to the original forms (2–3 s).