Increasing Conjugation Length Research Articles

Perfect Polar Alignment of Parallel Beloamphiphile Layers: Improved Structural Design Bias Realized in Ferroelectric Crystals of the Novel "Methoxyphenyl Series of Acetophenone Azines".

An improved design is described for ferroelectric crystals and implemented with the "methoxyphenyl series" of acetophenone azines, (MeO-Ph, Y)-azines with Y=F (1), Cl (2), Br (3), or I (4). The crystal structures of these azines exhibit polar stacking of parallel beloamphiphile monolayers (PBAMs). Azines 1, 3, and 4 form true racemates whereas chloroazine 2 crystallizes as a kryptoracemate. Azines 1-4 are helical because of the N-N bond conformation. In true racemates the molecules of opposite helicity (M and P) are enantiomers A(M) and A*(P) while in kryptoracemates they are diastereomers A(M) and B*(P). The stacking mode of PBAMs is influenced by halogen bonding, with 2-4 showcasing a kink due to directional interlayer halogen bonding, whereas fluoroazine 1 demonstrates ideal polar stacking by avoiding it. Notably, (MeO-Ph, Y)-azines display a stronger bias for dipole parallel alignment, attributed to the linearity of the biphenyl moiety as compared to the phenoxy series of (PhO, Y)-azines with their non-linear Ph-O-Ph moiety. The crystals of 1-4 all feature planar biphenyls and this synthon facilitates their crystallization through potent triple T-contacts and enhances their nonlinear optical (NLO) performance by increasing conjugation length and affecting favorable chromophore conformations in the solids.

Electrochemical synthesis of conjugated copolymer poly(3-hexylthiophene-co-dithenopyrrole): Stepwise approach to enhance electro-optical properties

Electrochemical polymerisation of 3-hexylthiophene (3HT) and electrochemical copolymerisation of 3HT and N-benzoyl dithienopyrrole (BDP) were performed to prepare polymer and copolymer films using a linear sweep voltammetry potentiostatic technique with a voltage range from 1.7-1.8 V and a scan rate of 0.1 mV/s. Cyclic voltammetry (CV) curves, Fourier-transform infrared (FTIR) spectra, scanning electron microscope (SEM) images, and UV-Vis absorption spectra of the polymer and copolymer were recorded for analysis and comparison of the electropolymerisation process and properties of the resultant films. The SEM photographs of the copolymer base on 3HT and BDP exhibited a rigid structure with large spherical aggregates with an average size around 200 nm. The copolymer of 3HT and BDP exhibited the band gaps of 2.25 eV which was determined via electrochemical measurement that is higher than of band gaps of homopolymer poly(3-hexylthiophene) (P3HT). The copolymer’s energy bandgap HOMO-LUMO was enhanced compared to polymer P3HT. Moreover, the increased conjugation length significantly improved the morphology structure as well as the electrochemical stability of the conjugated copolymer.

Influence of the π‐bridge and heteroatom on fluorene‐based donor‐acceptor polymers: Structure and optoelectronic properties

AbstractConjugated copolymers possess intriguing electronic characteristics that can be precisely modulated by manipulating their chemical structures. In this study, we explored two approaches for fine‐tuning the energy levels of these copolymers: the incorporation of heterocyclic moieties between the donor and acceptor units, and the substitution of a chalcogen atom in the acceptor unit. Specifically, we investigated the insertion of a thiophene along the polymer backbone and the replacement of sulfur with selenium in the acceptor unit. A series of copolymers were synthesized and thoroughly characterized using both experimental and theoretical methods. Partial density distribution of states analysis revealed that the inclusion of a thiophene significantly altered the HOMO level. Furthermore, the resulting Eg exhibited a decrease of approximately 0.8–0.9 eV compared to the copolymer without thiophene, indicating an enhanced planarization of the chain and increased conjugation length. Experimental data obtained from cyclic voltammetry demonstrated that both the conjugation length and the chalcogen atom in the acceptor unit played crucial roles in controlling the energy levels of the HOMO and LUMO. These findings establish a clear correlation between the chemical structure and the properties of the copolymers, demonstrating the potential for precise control and tailoring of their electronic characteristics.

Photophysical dynamics of excitons in polyaniline nanotubes with enhanced resistance against exciton quenching for applications in flexible high-efficiency PLEDs

The development of a high-efficiency polymer light-emitting diode is strongly hindered by injected charge carriers in the emissive polymer layer. But real-time identification of the interactions that quench the photoluminescence is extremely difficult owing to the complex photophysics of polymers involved in it. Here, we have synthesized the polyaniline nanotubes to study interactions at the nanoscale within the emissive layer. Acid doping of PAni helped us to quantify the processes that suppress the emission, e.g., Förster resonance energy transfer (FRET) and charge transfer (CT). We have developed a material design perspective by demonstrating how an effective increase in conjugation length of PAni nanochain through 1D nanostructure growth can avoid the undesired emission quenching due to exciton quenching by the polarons. We have demonstrated how this approach helped us to preserve up to 95 % of the emission, which would otherwise be lost by exciton-polaron quenching interaction. Exploration of this new strategy could be a way forward for both fundamental and application aspects to enhance the emission efficiency of the conducting conjugate polymers and emissive layers in flexible polymer light-emitting diodes (PLEDs).

Energy Storage Application of Conducting Polymers Featuring Dual Acceptors: Exploring Conjugation and Flexible Chain Length Effects.

Solution-processable conducting polymers (CPs) are a compelling alternative to inorganic counterparts because of their potential for tuning chemical properties and creating flexible organic electronics. CPs, which typically comprise either only an electron donor (D) or its alternative combinations with an electron acceptor (A), exhibit charge transfer behavior between the units, resulting in an electrical conductivity suitable for utilization in electronic devices and for energy storage applications. However, the energy storage behavior of CPs with a sequence of electron acceptors (A-A), has rarely been investigated, despite their promising lower band gap and higher charge carrier mobility. Utilizing the aforesaid concept herein, four CPs featuring benzodithiophenedione (BDD), and diketopyrrolepyrrole (DPP) are synthesized. Among them, the BDDTH-DPPEH polymer exhibited the highest specific capacitance of 126.5 F g-1 at a current density of 0.5 A g-1 in an organic electrolyte over a wide potential window of -0.6-1.4V. Notably, the supercapacitor properties of the polymeric electrode materials improved with increasing conjugation length by adding thiophene donor units and shortening the alkyl chain lengths. Furthermore, a symmetric supercapacitor device fabricated using BDDTH-DPPEH exhibited a high-power density of 4000W kg-1 and an energy density of 31.66Wh kg-1 .

Doping with Niobium Nanoparticles as an Approach to Increase the Power Conversion Efficiency of P3HT:PCBM Polymer Solar Cells.

Metal additive processing in polymer: fullerene bulk heterojunction systems is recognized as a viable way for improving polymer photovoltage performance. In this study, the effect of niobium (Nb) metal nanoparticles at concentrations of 2, 4, 6, and 8 mg/mL on poly(3-hexylthiophene-2,5-diyl) (P3HT)-6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends was analyzed. The effect of Nb volume concentration on polymer crystallinity, optical properties, and surface structure of P3HT and PCBM, as well as the enhancement of the performance of P3HT:PC61BM solar cells, are investigated. Absorption of the P3HT:PC61BM mix is seen to have a high intensity and a red shift at 500 nm. The reduction in PL intensity with increasing Nb doping concentrations indicates an increase in PL quenching, suggesting that the domain size of P3HT or conjugation length increases. With a high Nb concentration, crystallinity, material composition, surface roughness, and phase separation are enhanced. Nb enhances PCBM's solubility in P3HT and decreases the size of amorphous P3HT domains. Based on the J-V characteristics and the optoelectronic study of the thin films, the improvement results from a decreased recombination current, changes in morphology and crystallinity, and an increase in the effective exciton lifespan. At high doping concentrations of Nb nanoparticles, the development of the short-circuit current (JSC) is associated with alterations in the crystalline structure of P3HT. The highest-performing glass/ITO/PEDOT:PSS/P3HT:PCBM:Nb/MoO3/Au structures have short-circuit current densities (JSC) of 16.86 mA/cm2, open-circuit voltages (VOC) of 466 mV, fill factors (FF) of 65.73%, and power conversion efficiency (µ) of 5.16%.

Conjugated Imine Polymer Synthesized via Step‐Growth Metathesis for Highly Stable Silicon Nanoparticle Anodes in Lithium‐Ion Batteries

AbstractThis work reports a new method to synthesize polyphenylmethanimine (polyPMI) as a linear or a hyperbranched, conjugated polymer using an aldehyde‐imine metathesis reaction. This work details the reaction mechanisms of this polymerization by characterizing a red‐shift in its absorption spectrum as polymer conjugation length increases and verifies that this optical shift results from extended π‐condensation using density functional theory. This new synthetic approach provides a polymer that can potentially be depolymerized for facile recyclability and is compatible with air‐ and water‐sensitive chemistries. As an example of the utility of this new approach, this work demonstrates that this polymer can be directly grown on silicon nanoparticles to create silicon anodes for lithium‐ion batteries with a high degree of electrochemical interfacial passivation. These silicon anodes exhibit Coulombic efficiencies above 99.9% and can accommodate silicon nanoparticle expansion and contraction during lithiation and delithiation as demonstrated by stable reversible capacities for 500 cycles. Finally, this work demonstrates that polyPMI facilitates the formation of a lithium fluoride rich solid electrolyte interphase that remains chemically and mechanically stable after long term cycling.

Fabrication and Characterization of Free-Standing and Flexible Polyaniline Membranes: Role of Graphene Nanoscrolls

Wearable technologies can contribute to the early and accurate detection of chronic diseases which can be achieved by the integration of biosensors into wearable technologies. However, the challenges associated with the performance of current electrode materials—i.e., flexibility, conductivity, and mechanical stability, made from conducting polymers are preventing their widespread usage. Herein, we report a freestanding and flexible electrode synthesized from polyaniline (PANI) and graphene nanoscrolls (GNS). The PANI-GNS nanohybrid membranes were synthesized via chemical oxidative polymerization and characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), nanoindentation (NI), and four-point probe techniques. FTIR results showed an increase in conjugation length of the PANI after the addition of GNS into the mixture which can be indicative of an enhancement of electrical properties. Nanoindentation studies showed an elastic modulus and hardness of 2.6 GPa and 0.17 GPa, respectively, for PANI-GNS-5 nanocomposite, compared to 1.9 GPa and 0.08 GPa, for pure PANI. This was later confirmed by the four-point probe technique as the addition of GNS increased the conductivity of electrodes up to 9 S/cm at a 5% weight ratio. Moreover, SEM results of the PANI-GNS showed an open porous morphology of the polymer matrix in comparison with pure PANI samples which would readily translate into higher amounts of enzyme immobilization on the surface.

Deciphering Photoinduced Charge Transfer Dynamics in a Cross-Linked Graphene-Dye Nanohybrid.

The search for synthetic materials that mimic natural photosynthesis by converting solar energy into other more useful forms of energy is an ever-growing research endeavor. Graphene-based materials, with their exceptional electronic and optical properties, are exemplary candidates for high-efficiency solar energy harvesting devices. High photoactivity can be conveniently achieved by functionalizing graphene with small molecule organic semiconductors whose band-gaps can be tuned by structural modification, leading to interactions between the π-conjugated electronic systems in both the semiconductor and graphene. Here we investigate the ultrafast transient optical properties of a cross-linked graphene–dye (diphenyl-dithiophenediketopyrrolopyrrole) nanohybrid material, in which oligomers of the organic semiconductor dye are covalently bound to a random network of few-layer graphene flakes, and compare the results to those obtained for the reference dye monomer. Using a combination of ultrafast transient absorption and two-dimensional electronic spectroscopy, we provide substantial evidence for photoinduced charge transfer that occurs within 18 ps in the nanohybrid system. Notably, subpicosecond photoinduced torsional relaxation observed in the constituent dye monomer is absent in the cross-linked nanohybrid system. Through density functional theory calculations, we compare the competing effects of covalent bonding, increasing conjugation length, and the presence of multiple graphene flakes. We find evidence that the observed ultrafast charge transfer process occurs through a superexchange mechanism in which the oligomeric dye bridge provides virtual states enabling charge transfer between graphene–dye covalent bond sites.

Pot- and atom-economic synthesis of oligomeric non-fullerene acceptors via C–H direct arylation

Three long-chain oligomeric acceptors with a stepwise increase in conjugation length are obtained via three successive one-pot reactions and a systematic structure–property–performance relationship study was carried out.

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

Investigation on structural, morphological, optical, and current-voltage characteristics of polyfluorene with dissimilar composition spin coated on ITO

Different weight composition of polyfluorene (PFO) ((0.625 mg, 1.250 mg, and 2.500 mg) have been prepared and dissolved in toluene before spin coating on indium tin oxide (ITO) glass substrates. Post-deposition annealing was carried out to transform the wet film to solid film. High resolution X-ray diffraction analysis has detected the presence of broad diffraction peaks belonged to β-phase of PFO in all of the investigated samples. An onset of crystallization for the β-phase happened in the 2.500 mg PFO coated sample. Photoluminescence emission spectra of the β-phase was increased in intensity. Besides, a red shift happened due to an increase in energy transfer from glassy to β-phase, following an increase in conjugation length in the PFO backbone. The formation of crystalline β-phase and increased conjugation in the 2.500 mg PFO coated sample. Hence, the corresponding current-voltage characteristic showed that the highest current conduction has been perceived by the sample. A schematic diagram showing energy band diagram of the Al/PFO/ITO structure was presented and discussed in association with structural, morphological, and optical characteristics of the PFO films.

Tuning Photophysical Properties of Donor/Acceptor Hybrid Thin- Film via Addition of SiO2/TiO2 Nanocomposites.

The influence of SiO2/TiO2 nanocomposites (STNCs) content on non-radiative energy transfer (Förster-type) from poly (9,9′-dioctylfluorene-2,7-diyl) (PFO) to poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) using steady-state and time-resolved photoluminescence spectroscopies was investigated at room temperature. The improved energy transfer from PFO to MEH-PPV upon an increment of the STNCs was achieved by examining absorbance, emission (PL) and photoluminescence excitation (PLE) spectra. The shorter values of the quantum yield (φDA) and lifetime (τDA) of the PFO in the hybrid thin films compared with the pure PFO, indicating efficient energy transfer from PFO to MEH-PPV with the increment of STNCs in the hybrid. The energy transfer parameters can be tuned by increment of the STNCs in the hybrid of PFO/MEH-PPV. The Stern–Volmer value (kSV), quenching rate value (kq), Förster radius (R0), distance between the molecules of PFO and MEH-PPV (RDA), energy transfer lifetime (τET), energy transfer rate (kET), total decay rate of the donor (TDR), critical concentration (Ao), and conjugation length (Aπ) were calculated. The gradually increasing donor lifetime and decreasing acceptor lifetime, upon increasing the STNCs content, prove the increase in conjugation length and meanwhile enhance in the energy transfer.

Effects of intrachain disorder on photoexcitation in conjugated polymer chains

The luminescence efficiency of conjugated polymers has been a central topic in the study of light emitting. The effect of disorder plays an important role in generating excitons after the conjugated polymers have been excited by photons. In this paper, by using the Su-Schriffer-Heeger model, which has been modified to include intrachain disorder and electron correlation, we investigate the effects of disorder on the photoexcitation, especially on the yield of excitons in a conjugated polymer chain. We adopt the multi-configurational time-dependent Hartree–Fock method to solve the multi-electron time-dependent Schrödinger equation and the Newtonian equation of motion for the lattice vibration. The results show that after the photoexcitation relaxation process, the products of the disordered polymer chain are qualitatively distinct from those of the pure polymer chain. While the pairs of polarons can be generated directly after the photoexcitation in a pure polymer chain, the disorder favors excitons as the products of the photoexcitation, and the yield of excitons depends crucially on the kind and strength of the disorder. Furthermore, the influences of the electron correlation and the conjugation length on the yield of excitons are also discussed. Specifically, we find that in the case of diagonal disorder, when the conjugation length is short and the diagonal disorder is weak, the excitons are mainly generated by the recombination of two lattice defects with a high yield of excitons which will be reduced as the conjugation length increases. The excitons tend to be generated directly with a low yield of excitons which is enlarged as the disorder gets stronger when the conjugation length is long or the diagonal disorder is strong. The on-site Coulomb repulsion favors the generation of excitons as well. The case of off-diagonal disorder is similar to that of diagonal disorder except that the on-site Coulomb potential favors the generation of excitons in the weak disorder regime but depresses the generation of excitons in the strong disorder regime. When both diagonal and off-diagonal disorders are considered, the yield of excitons is dominated by the off-diagonal disorder. We hope that our investigations can provide useful guidance and help for designing organic photoelectric materials and devices.

Assessing the effects of increasing conjugation length on exciton diffusion: from small molecules to the polymeric limit.

Organic solar cells (OSC) generally contain long-chain π-conjugated polymers as donor materials, but, more recently, small-molecule donors have also attracted considerable attention. The nature of these compounds is of crucial importance concerning the various processes that determine device performance, among which singlet exciton diffusion is one of the most relevant. The efficiency of the diffusion mechanism depends on several aspects, from system morphology to electronic structure properties, which vary importantly with molecular size. In this work, we investigated the effects of conjugation length on the exciton diffusion length through electronic structure calculations and an exciton diffusion model. By applying extrapolation procedures to thiophene and phenylene vinylene oligomer series, we investigate their electronic and optical properties from the small-molecule point of view to the polymeric limit. Several properties are calculated as a function of oligomer size, including transition energies, absorption and emission spectra, reorganization energies, exciton coupling and Förster radii. Finally, an exciton diffusion model is used to estimate diffusion lengths as a function of oligomer size and for the polymeric limit showing agreement with experimental data. Results also show that longer conjugation lengths correlate with longer exciton diffusion lengths in spite of also being associated with shorter exciton lifetimes.

Effect of Different Ionic Surfactants on the Structural, Photophysical, and Morphological Properties of Water‐Based P3HT:PCBM Nanoparticle Dispersions and Films

AbstractAqueous suspensions of composite nanoparticles of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) are fabricated by miniemulsion method using three different ionic surfactants. The aim is to study how the length and conformation of the surfactants alkyl chains affect the properties of the nanoparticles. While the morphology and dimensions of the nanoparticles are similar, UV–vis spectroscopy evidences that the internal aggregation and ordering of the P3HT chains varies within the three nanoparticle formulations. The surfactant with branched alkyl chains promote the highest degree of ordering of P3HT chains in the nanoparticles (leading to increased conjugation length). In contrast, the lowest ordering is found for the nanoparticles with the surfactant having the shortest linear alkyl chain. The optical/structural properties of nanoparticles are partially retained in the films. Besides, the surfactant with branched alkyl chains favors the strongest coalescence of nanoparticles in the thin film, promoting a further ordering of the polymeric chains in the most external shell of the nanoparticles as evidenced by steady‐state and time‐resolved UV–vis spectroscopy and confocal fluorescence microscopy. These findings might guide the engineering of new surfactants for composite nanoparticles for optoelectronic applications.

Coordination Environment Prevents Access to Intraligand Charge-Transfer States through Remote Substitution in Rhenium(I) Terpyridinedicarbonyl Complexes.

Six rhenium(I) κ3N-dicarbonyl complexes with 4'-(4-substituted phenyl)terpyridine ligands were evaluated in their ground and excited states. These complexes, bearing substituents of different electron-donating strengths-from CN to NMe2-were studied by a combination of transient IR (TRIR), electrochemistry, and IR spectroelectrochemistry, as well as time-dependent density functional theory (TD-DFT). They exhibit panchromatic absorption and can act as stronger photoreductants than their tricarbonyl counterparts. The ground- and excited-state potentials, absorption maxima, and lifetimes (250-750 ps) of these complexes correlate well with the Hammett σp substituent constants, showing the systematic effect of remote substitution in the ligand framework. TRIR spectroscopy allowed us to assign the lowest singlet and triplet excited states to a metal-to-ligand charge-transfer (MLCT) character. This result contrasts our previous report on analogous κ2N-tricarbonyl complexes, where remote substitution switched the character from MLCT to intraligand charge transfer. With the help of TD-DFT calculations, we dissect the geometric and electronic effects of coordination of the third pyridine, local symmetries, and increasing conjugation length. These results give valuable insights for the design of complexes with long-lived triplet excited states and enhanced absorption throughout the visible spectrum, while showcasing the boundaries of the excited-state switching strategy via remote substitution.

Design and synthesis of small molecules with difluoroquinoxaline units for OSCs

We designed and synthesized three novel small molecule donors (SM1, SM2 and SM3) that consist of thiophene as the electron-donating end group and 6,​7-​difluoroquinoxaline moiety as a novel electron-withdrawing core group. The organic low band gap molecules with 6,​7-​difluoroquinoxaline and thiophene units were synthesized using Stille coupling to generate SM1, SM2 and SM3. The absorption of SM2 and SM3 in solution was red shifted due to increased conjugation length of added thiophene units. In case of SM2 and SM3, introduction of hexyl chain in terminal thiophene units improve solubility in organic solvent. The maximum absorption peaks of SM1, SM2 and SM3 in solid thin films were at 482, 505 and 518 nm, respectively. SM3 was red shifted as compare to SM2 due to increased π-π stacking of electron donor materials.

On the design of new europium heteroaromatic carboxylates for OLED application

The approach to the directed synthesis of lanthanide aromatic carboxylates – precursors to the electroluminescent materials, - was proposed, namely the conjugation length increase and heteroatom introduction in the appropriate position in combination with the neutral ligand introduction. This resulted in the isolation of a series of new lanthanide complexes, among which the highest electroluminescence efficiency was obtained for mixed-ligand europium benzothiazole-2-carboxylate with bathophenanthroline in a solution-processed OLED. The peculiarities of energy transfer processes allowed obtaining luminescence thermometer materials based on this system, which demontrated the sensitivity of 2.8%/K in the physiological range.

Increase of polymeric chain conjugation length in UV irradiated monolayer of hydroxyalkadiynyl N-arylcarbamate derivative

Photopolymerization of Langmuir–Schaefer monolayers of 12-hydroxy-7,9-dodecadiyn-1-yl N-(4-methoxyphenyl)carbamate molecules under long-term UV irradiation provided the new polymer material. The observed blue phase polymer absorption band shift (∼30 nm) to the longer wavelengths during the sample UV irradiation corresponds to a continuous increase of the polymeric chain conjugation length. The morphology of mono-layers was determined by the scanning electron microscopy.