Auxiliary Donor Research Articles

Constructing Charge‐Transfer Excited States Based on Frontier Molecular Orbital Engineering: Narrowband Green Electroluminescence with High Color Purity and Efficiency

AbstractThe design and synthesis of organic materials with a narrow emission band in the longer wavelength region beyond 510 nm remain a great challenge. For constructing narrowband green emitters, we propose a unique molecular design strategy based on frontier molecular orbital engineering (FMOE), which can integrate the advantages of a twisted donor–acceptor (D‐A) structure and a multiple resonance (MR) delayed fluorescence skeleton. Attaching an auxiliary donor to a MR skeleton leads to a novel molecule with twisted D‐A and MR structure characteristics. Importantly, a remarkable red‐shift of the emission maximum and a narrowband spectrum are achieved simultaneously. The target molecule has been employed as an emitter to fabricate green organic light‐emitting diodes (OLEDs) with Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.69) and a maximum external quantum efficiency (EQE) of 27.0 %.

Constructing Charge-Transfer Excited States Based on Frontier Molecular Orbital Engineering: Narrowband Green Electroluminescence with High Color Purity and Efficiency.

The design and synthesis of organic materials with a narrow emission band in the longer wavelength region beyond 510 nm remain a great challenge. For constructing narrowband green emitters, we propose a unique molecular design strategy based on frontier molecular orbital engineering (FMOE), which can integrate the advantages of a twisted donor-acceptor (D-A) structure and a multiple resonance (MR) delayed fluorescence skeleton. Attaching an auxiliary donor to a MR skeleton leads to a novel molecule with twisted D-A and MR structure characteristics. Importantly, a remarkable red-shift of the emission maximum and a narrowband spectrum are achieved simultaneously. The target molecule has been employed as an emitter to fabricate green organic light-emitting diodes (OLEDs) with Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.69) and a maximum external quantum efficiency (EQE) of 27.0 %.

Twisted chromophore assist to tetrathiafulvalene-spiropyran hybrid driving four-state molecular switch

A series of novel molecular hybrids, in which tetrathiafulvalene (TTF) unit and spiropyran (SP) core are covalently linked to various typical π-conjugated bridges, has been designed based on auxiliary donors and acceptors model and twisted intramolecular charge transfer (TICT) strategy. And the second-order nonlinear optical (NLO) responses of these molecular hybrids have been investigated by using density functional theory (DFT) calculations. The results show that grafting of the TTF unit and SP core to the twisted chromophore (TC) can effectively enhances second-order NLO responses. The promoting effect of enhancing NLO response can be attributed to a strongly favorable charge transfer in TTF-TC-SP hybrid. More importantly, we have found that this TTF-TC-SP hybrid with donor-π-conjugated bridge-acceptor structure can efficiently switch the first hyperpolarizability across four states via redox and photochemical stimuli. Meanwhile, time-dependent (TD) DFT calculations were carried out to probe the origin of the second-order NLO response of TTF-TC-SP molecular hybrid. All these findings provide a clear and well understanding of the relationship between the tunable first hyperpolarizability and electronic structure. We hope these findings would be very useful to guide the search for potential polymorphic molecular switch.

Synthesis and characterization of multi-substituted carbazole derivatives exhibiting aggregation-induced emission for OLED applications

A series of carbazole based derivatives differing in the number of dicyanovinyl acceptors and donors ( N -alkyl carbazole or N -phenyl carbazole) are designed, synthesized and characterized. The effect of donors on optical, electrochemical and thermal properties of carbazole-dicyanovinyl dyads is critically analysed. The dyes containing C3-linked carbazole donor showed red-shifted absorption and larger molar extinction coefficients when compared to the dyes those containing N -phenylcarbazole attributable to the extended conjugation of C3-linked N -butylcarbazole. All the compounds showed positive solvatochromism in fluorescence attributable to intramolecular charge transfer in excited state. The higher thermal stability of the dyes suggests the rigidity of the carbazole derivatives. Interestingly, all the dyes containing N -phenylcarbazole showed aggregation-induced emission properties in THF-H 2 O mixtures attributable to increased distortion in molecular configuration and restricted rotation in the solid state. Furthermore, a green OLED device fabricated with the dye containing N -butyl carbazole donor as dopant emitter (3%) in CBP host achieved good performance with high external quantum efficiency (4.2%), current efficiency (13.1 cd A −1 ) and maximum brightness (L max ) of 6180 cd m −2 . • Carbazole derivatives possessing two or three auxiliary donors and dicyanovinyl acceptor are synthesized and characterized. • They exhibited aggregation induced emission dependent on the nature and orientation of the auxiliary chromophore with the central core. • A green OLED device of a dye containing N -butylcarbazole donor exhibited high external quantum efficiency (4.2%).

Enhancing Charge Transfer and Photoelectric Characteristics for Organic Solar Cells

The main purpose of this work is to analyze the effect of steric hindrance on the photoelectric performance of three different donor sensitizers (ZHG5, ZHG6, and ZHG7) by molecular theory simulation engineering. Photoelectric physical and photoelectric chemical parameters are investigated by means of frontier molecular orbital, global reactivity descriptors, optical absorption properties, fluorescent lifetime, charge density difference, and influence of external electric field. The results showed that the performance of the quinoxaline sensitizer was deteriorated by gradually increasing the steric hindrance to auxiliary donors. The optical properties of the hybridization of cir-coronene graphene quantum dot (GR) with the three dyes have been revealed, and the results show that graphene quantum dots can indeed improve the optical properties of solar cells. In addition, nine new molecules were designed by inserting six functional groups; it is found that inserting -CN in the acceptor part of the molecular structure is beneficial to the performance of the sensitizer.

Chemometric modeling of power conversion efficiency of organic dyes in dye sensitized solar cells for the future renewable energy

The present study reports chemometric modeling of power conversion efficiency (PCE) of dye sensitized solar cells (DSSCs) using the biggest available data set till date which comprises around 1200 dyes covering 7 chemical classes. To extract the best structural features required for higher PCE, we have developed multiple partial least squares (PLS) quantitative structure-property relationship (QSPR) models for the Triphenylamine, Phenothiazine, Indoline, Porphyrin, Coumarin, Carbazole and Diphenylamine chemical classes using descriptors derived from the best subset selection method followed by selection of best five models in each dataset based on the Mean Absolute Error (MAE) values. The models were validated both internally and externally followed by the consensus predictions employing “Intelligent Consensus Predictor” tool to examine whether the quality of predictions can be improved with the “intelligent” selection of multiple PLS models. The quality of predictions for the respective external sets showed that the consensus models (CM) are better than the individual models (IM) in most of the cases. From the insights of the developed models, we concluded that attributes like a packed structure toward higher conductivity of electrons, auxiliary donor fragment of aromatic tertiary amines, number of thiophenes inducing the bathochromic shift and augmenting the absorption, presence of additional electron donors, enhancement of electron-donating abilities, number of non-aromatic conjugated C(sp2) which helps as conjugation extension units to broaden the absorption and highly conjugated π-systems exert positive contributions to the PCE. On the contrary, features negatively contributing to PCE are the followings: fragments which lower the tendency of localized π–π* transition, fragments related to larger volume and surface area of dyes along with hydrophobicity resulting in poor adhesion, fragment RC = N causing dye hydrolysis, steric hindrance for π electronic mobility, fragments enhancing polarity, etc. The identified features from the best QSPR model of the coumarin dataset was employed in designing of ten more efficient coumarin dyes (predicted %PCE ranging from 8.93 to 10.62) than the existing ones.

Identification of descriptors for structure-activity relationship in ruthenium (II) mixed compounds with antiparasitic activity

Herein is presented the synthesis, characterization, electrochemical studies, DFT calculations and in vitro evaluation of amoebicidal activity in trophozoites of Entamoeba histolytica of twenty ruthenium (II) mixed compounds with general formulae: [Ru(pdto)(E-E)]Clx (E-E bidentate, either neutral or negatively charged ligands). For compounds under study, O–O, N–O and N–N auxiliary donor ligands demonstrate to have a crucial impact on the electronic properties and that it is possible to modulate the antiparasitic activity. Among analyzed complexes, only four present a better performance compared to typically used metronidazole drug (IC50 < 6.80 μmol/L) to treat amebiasis disease. For studied compounds, structure-activity relationships are strongly determined by either the redox potential (E1/2) of RuII/RuIII and calculated molar volume (V) of the complexes.

Microwave assisted synthesis of imidazolyl fluorescent dyes as antimicrobial agents

In this article, we report, the synthesis and characterization of series of novel fluorescent imidazolyl dyes (5a-d)via highly efficient and cost-effective microwave assisted protocol as a potential candidate to overcome the problem of microbial resistance. By utilizing the green microwave protocol the reactions are completed in a short span of time without using the harsh conditions. The incorporation of imidazole nucleus is an important strategy in drug discovery. While designing desired fluorescent imidazole dyes 5a-d, with a suitable auxiliary donor such as aromatic rings and −OCH3 group on one end of the imidazolyl moiety and electron acceptors such as -NO2 and −COOH on other end of the compounds was achieved to get a promising fluorescent dyes for antimicrobial. The optoelectronic properties and antimicrobial studies of the synthesized materials indicated their exploration as a promising candidate as antimicrobial agents.

Effect of Auxiliary Donors on 3,8-Phenothiazine Dyes for Dye-Sensitized Solar Cells.

Phenothiazines are one of the more common dye scaffolds for dye-sensitized solar cells. However, these sensitizers are exclusively based on a 3,7-substitution pattern. Herein, we have synthesized and characterized novel 3,8-substituted phenothiazine dyes in order to evaluate the effect of auxiliary donor groups on the performance of this new dye class. The power conversion efficiency increased by 7%–10% upon insertion of an auxiliary donor in position 8 of the phenothiazine, but the structure of the auxiliary donor (phenyl, naphthyl, pyrene) had a low impact when electrodes were stained with chenodeoxycholic acid (CDCA) additive. In the absence of CDCA, the highest power conversion efficiency was seen for the phenyl-based sensitizer attributed to a higher quality dye-monolayer. By comparing the novel dyes to their previously reported 3,7- analogues, only subtle differences were seen in photophysical, electrochemical, and performance measurements. The most notable difference between the two geometries is a lowering of the oxidation potentials of the 3,8-dyes by 40–50 mV compared to the 3,7-analogues. The best auxiliary donor for the 3,8-phenothiazine dyes was found to be pyrenyl, with the best device delivering a power conversion efficiency of 6.23% (99 mW cm−2, 10 eq. CDCA, JSC = 10.20 mA cm−2, VOC = 791 mV, and FF = 0.765).

New cyanopyridine based conjugated polymers carrying auxiliary electron donors: From molecular design to blue emissive PLEDs

Three new D-A (Donor-Acceptor) configured conjugated polymers, i.e.PPy1-3, centered on strong electron accepting cyanopyridine scaffold carrying varied auxiliary donors, viz. phenylene (PPy1), biphenyl (PPy2), and fluorene (PPy3) were designed and synthesized as blue emitters for PLEDs. The new polymers were subjected to spectral, thermal, photophysical and electrochemical characterization. Also, computational studies (DFT) were performed on the repeating units of polymer using Turbomole 7.2 V software package at the B3LYP/TZVP hybrid levels. Further, their weight average molecular masses were found to be 38.8 kDa, 38.9 kDa and 57.7 kDa, respectively as determined by GPC technique. Furthermore, the new polymers PPy1-3, were shown to be stable thermally up to 308–374 °C. Evidently, they exhibited good photophysical behavior with their optical energy band gaps of 2.53–2.64 eV. Finally, the polymers PPy1-3 were employed as an active emissive layer in standard ITO/PEDOT:PSS/Polymer/Al configured PLEDs. Interestingly, at 12 V all the newly fabricated devices exhibit a stable blue characteristic electroluminescence with low threshold voltages of 3.40–5.20 V, confirming an efficient injection of electrons in the diodes. From the results, it is clear that, the polymers PPy1-3, can be considered as prospective blue light emitters for PLED application.

Opto-electronic properties of stable blue photosensitisers on a TiO2 anatase-101 surface for efficient dye-sensitised solar cells

We study the electronic and optical properties of narrow energy gap blue dyes, R6, featuring a stronger electron-donating poly-aromatic hydrocarbon, 9,19-dihydrobenzo[1′,10′]phenanthro[3′,4′:4,5]thieno[3,2-b]-benzo[1,10]phenanthro[3,4-d]thiophene (BPT2), and R4, featuring 4H’,4′H-2,2′-bibenzo[1.10]phenanthro[4,3-b]thiophene (2BPT). BPT2 and 2BPT are tethered with an auxiliary diarylamine electron donor and an electron acceptor of 4-(7-ethynylbenzo[c][1,2,4]thiadiazol-4-yl)benzoic acid, respectively. We explore these dyes with varying acceptor moieties, along with their binding interaction, at a TiO2 anatase-101 surface. All isolated dye molecules absorbed light in 1.97–2.14 eV range, although their binding-to-anatase affinities were very similar; for all absorbed dyes, valance-band maxima and the conduction-band minima of the composites system are mainly localized on dye and the TiO2-slab, respectively.

Three-component one-pot reaction for molecular engineering of novel cost-effective highly rigid quinoxaline-based photosensitizers for highly efficient DSSCs application: Remarkable photovoltage

A simple three-component one-pot approach was developed to generate three novel highly rigid quinoxaline-based dyes, which were further investigated as sensitizers of dye-sensitized solar cells (DSSCs). The effect of different push-pull molecular motifs containing various donors and attachment of long alkyl chains on photophysical, electrochemical and photovoltaic performances, along with theoretical calculations were studied comprehensively. In particular, LY03-based DSSCs showed the highest efficiency of 7.04% with a short-circuit photocurrent density (JSC) of 14.32 mA cm−2, an open-circuit photovoltage (VOC) of 910 mV and a fill factor (FF) of 0.54% under AM 1.5 irradiation (100 mW cm−2). The results confirm that improving the molecular rigidity of sensitizer and incorporation of long alkyl chains into an auxiliary acceptor and donor is one of the most effective pathways to prevent “trade-off” effect, which translated into remarkable improvement in open-circuit photovoltage of 0.91 V, hence, enhancing the photovoltaic performance.

Metal-free organic dyes with di(1-benzothieno)[3,2-b:2′,3′-d]pyrrole as an auxiliary donor for efficient dye-sensitized solar cells: Effect of the molecular engineering on the photovoltaic performance

A series of novel di(1-benzothieno)[3,2-b:2′,3′-d]pyrrole (DBTP)-based organic dyes (WL5-10) with D-D-π-A or D-D-A-π-A configurations are designed and the relationship between the properties and molecular configurations of these organic dyes are studied systematically. WL5,7–8 with D-D-π-A configuration incorporating triphenylamine or phenothiazine as donor and DBTP as auxiliary donor and WL9-10 with D-D-A-π-A configuration incorporating benzothiadiazole as an auxiliary acceptor are synthesized to study the effect of the molecular engineering on the photovoltaic performance. WL5,7–8 exhibit similar absorption spectra and high molar extinction coefficient. Especially, the dye WL5 with phenothiazine as terminal donor displays a high PCE of 6.75%, which is much higher than that of the dye WL1a with D-π-A configuration. WL9-10 show the weakest performance, which is attributed to electron trap and mismatch of energy-level due to the strongest electron withdrawing group benzothiadiazole. Finally, a high power conversion efficiency of 7.94% with chenodeoxycholic acid as the co-absorbent for the DSSC based on WL5 is achieved. The results indicate that the molecular engineering of the dyes affects the photovoltaic performance greatly, and the DBTP-based organic dye with D-D-π-A configuration by incorporating an extra terminal donor might be an effective approach to enhance the photovoltaic performance.

A comprehensive experimental study of five fundamental phenothiazine geometries increasing the diversity of the phenothiazine dye class for dye-sensitized solar cells

Phenothiazine is a versatile scaffold frequently used in both pharmaceutical and photovoltaic applications. Still, the structural diversity within the class of phenothiazine sensitizers for dye-sensitized solar cells is minute. Substituents are found in 3, 7 and 10-positions, often all three. In this work, we report the synthesis and evaluation of sensitizers illuminating five geometries for the phenothiazine dye class, of which three are novel geometries. Eleven sensitizers were prepared, investigating auxiliary donor contributions, effect of π-spacer and also the position of the anchoring group. We have established that the π-spacer has to be connected para to the 10H nitrogen atom of phenothiazine, the 3-position. Also, thiophene is a far superior π-spacer than phenyl, but we were unable to find any significant photovoltaic performance differences between the 3,7 and 3,8 geometries. A higher dye loading for the 3,8 geometry indicates there is hidden potential in this geometry which could be harvested by further optimization. The best device of the study was fabricated with AFB-27 of the 3,8 geometry delivering a PCE of 5.88% (JSC = 10.28 mA cm−2, VOC = 773 mV, FF = 0.75) under 1 sun AM 1.5G illumination.

D-π-A-π-D coumarin hybrids derived from arylamine donors: DFT and Z-scan studies

Four new D-π-A-π-D hybrid dyes containing coumarin as a major donor (4a-b and 5a-b) together with carbazole and triphenylamine as auxiliary donor having dicyanovinylene as a common acceptor are synthesized and studied in terms of photophysical, charge transfer, and non-linear optical properties. Optical properties of hybrid dyes considerably shifted towards red to NIR region upon increasing the solvent polarity. Highest emission solvatochromism is observed for dye 5b (∼137 nm) from nonpolar to polar solvent. Charge transfer is well established using solvatochromism, natural bond orbital analysis, and HOMO-LUMO energy gap. Two photon absorption is calculated using the theoretical model and dye 4b shows high σ2PA in the range 186–464 GM. Non-linear optical properties were investigated using solvatochromic two-level model, computationally using two global hybrid functionls (B3LYP and BHHLYP) and four range separated hybrid functionals (CAM-B3LYP, ωB97, ωB97× and ωB97XD) and Z-scan technique. The computed values of the second order hyperpolarizability (γ0) and first order hyperpolarizability (β0) are high in the range of 993–5473 × 10−36 esu and 240-806 × 10−30 esu respectively for these dyes compared to urea. Z-scan results reveal saturable kind of behavior for 4a and 4b, whereas 5a and 5b show reverse saturable kind of behavior in acetone and ethanol and hence give optical limiting values. Thus, from the experimental and theoretical data, these dyes can be considered as promising candidates for NLO application in material science.

New blue light emitting cyanopyridine based conjugated polymers: From molecular engineering to PLED applications

In this work, we report the design of three new cyanopyridine scaffold based polymers, viz. TPy1-3 as potential blue light emitters for PLED applications. The new polymer design comprise, a cyanopyridine core as an electron accepting entity, thiophene as an electron donating unit with different auxiliary donors, viz. phenylene (TPy1), biphenyl (TPy2), and fluorene (TPy3), and have been synthesized, following standard synthetic protocols including Suzuki-cross coupling polymerization reaction. Further, in order to assess all the prerequisites to act as an active emitter, the polymers TPy1-3, were subjected to structural, thermal, linear optical, electrochemical and computational studies. The results revealed that, all the polymers were thermally stable up to 300 ⁰C and their estimated optical band-gaps were found to be 2.59–2.80 eV. Finally, new polymer light emitting diode (PLED) were fabricated by employing the polymers TPy1-3, as active emissive material with a configuration of ITO/PEDOT: PSS/Polymer/Al. Interestingly, all the fabricated devices, exhibited an intense blue electroluminescence at 12 V with low threshold voltages of 4.2–4.8 V, signifying an effective injection of electron in the device.

Synthesis of novel colorants for DSSC to study effect of alkyl chain length alteration of auxiliary donor on light to current conversion efficiency

Five (MA1-MA5) hemicyanine based sensitizer having N, N-diethyl aniline as a primary donor and hydroxy or alkoxy as an auxiliary donor have been synthesized to establish a correlation between amphiphilic nature of the sensitizer and charge recombination. A strong electron withdrawing 3-(carboxymethyl)-2-methylbenzo[d]thiazol-3-ium bromide has been explored as an acceptor. All the dyes were characterized by 1H-NMR, 13C-NMR and CHN analysis. The photophysical properties of these dyes were recorded in seven different solvents which do not show any significant impact on absorption and emission maxima while molar absorptivity coefficient decreases with increase in alkyl chain length. These dyes show very poor emission in all the solvents. Nano-crystalline mesoporous TiO2 based dye-sensitized solar cells were fabricated using MA1 to MA5 sensitizers to evaluate their photovoltaic performance. MA5 having six carbon alkyl chain shows maximum efficiency of 4.97% while MA1 without any alkyl chain shows the lowest efficiency of 3.40%. As the length of alkyl chain increase efficiency increase due to increment in short circuit current (Jsc) and retardation in the recombination process. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were explored to obtain vertical excitation, HOMO-LUMO energy and electron density distribution.

Phenothiazine dye featuring encapsulated insulated molecular wire as auxiliary donor for high photovoltage of dye-sensitized solar cells by suppression of aggregation

Two efficient dye-sensitized solar cells have been fabricated by two novel D–D–π–A phenothiazine-based organic dyes (PH2 and PH3) with an encapsulated insulated molecular wire (EIMW) as an auxiliary donor. The cell sensitized by PH2 with EIMW as an auxiliary donor shows a much higher photovoltage (Voc) relative to the reference dye PH1 without EIMW, because the former dye can inhibit dye aggregation and suppress the charge recombination effectively. The results show that the cell sensitized by PH2 with co-adsorption of chenodeoxycholic acid obtains a high power conversion efficiency, even higher than that of the cell based on N719. Thus, an effective way to increase the photovoltage and efficiency of the DSSCs has been developed by introducing the EIMW into the organic dyes due to the effective inhibition of dye aggregation and charge recombination.

The Progress in the Field Auxiliary Donors and their Application in Novel Organic Second-Order Nonlinear Optical Chromophores

Conversion efficiency between electrical and optical signals is very important for the development of modern information technologies. Due to their advantages in half-wave voltage, bandwidth, cost and integration, as well as organic electro-optic (EO) parameters, these materials are widely studied and used in microwave photonic devices. Second order nonlinear optical (NLO) chromophores, as the core of organic EO materials have an increasing interest in this branch. Auxiliary donors present a new direction for the design and improvement of organic NLO chromophores. In this short review, the advantages, theoretical calculations and experimental results of auxiliary donors are reviewed and discussed in detail.

Auxiliary donors for phenothiazine sensitizers for dye-sensitized solar cells – how important are they really?

A comprehensive study into auxiliary donors for phenothiazine sensitizers in DSSCs and a critical look at a remarkably efficient literature dye.