Π-conjugated Oligomers Research Articles

Near-Infrared Emissive π-Conjugated Oligomer Nanoparticles for Three- and Four-Photon Deep-Brain Microscopic Imaging Beyond 1700 nm Excitation.

High-resolution visualization of the deep brain is still a challenging and very significant issue. Multiphoton microscopy (MPM) holds great promise for high-spatiotemporal deep-tissue imaging under NIR-III and NIR-IV excitation. However, thus far, their applications have been seriously restricted by the scarcity of efficient organic probes. Herein, we designed and synthesized two donor-acceptor-donor-type conjugated small molecules (TNT and TNS) for in vivo mouse deep-brain imaging with three- and four-photon microscopy under 1700 and 2200 nm excitation. With a selenium (Se) substitution, we synthesized two conjugated small molecules to promote their emission into the deep near-infrared region with high quantum yields of 55% and 20% in THF solvent, respectively, and their water-dispersive nanoparticles have relatively large absorption cross-sections in the 1700 and 2200 nm windows, respectively, with good biosafety. With these superiorities, these organic NPs achieve high-resolution deep-brain imaging via three-photon and four-photon microscopy with excitation at 1700 and 2200 nm windows, and 1620 μm deep in the brain vasculature can be visualized in vivo. This study demonstrates the efficiency of NIR-emissive conjugated small molecules for high-performance MPM imaging in the NIR-III and NIR-IV window and provides a route for the future design of organic MPM probes.

Multistate Parabolic Model: A Unified Framework for Understanding Charge Distribution in π-Conjugated Oligomers

Multistate Parabolic Model: A Unified Framework for Understanding Charge Distribution in π-Conjugated Oligomers

The third-order nonlinear optical responses of zinc porphyrin oligomers: Cycles vs linear chains

Porphyrins are widely used as potential nonlinear optical (NLO) materials because of their highly delocalized π electrons and feasible synthesis and functionalization with broad biological applications. A variety of linear and cyclic porphyrin derivatives have been synthesized, and the correlation between their structures and NLO properties awaits being disclosed. In this work, the electronic structures and third-order NLO properties of linear and cyclic butadiyne-linked zinc porphyrin oligomers have been studied by quantum chemical methods and sum-over-states model. The static second hyperpolarizability (<γ0>) increases exponentially with the number of zinc porphyrin units ([<γ0>n] = 0.67[<γ0>1]n2.63, n = 2 ∼ 6) in linear π-conjugated oligomers, and the <γ0> of the linear hexamer is about 74 times that of the monomer. Such enhancement of <γ0> in linear oligomers originates from closely-lying frontier molecular orbitals available for low energy electron excitations and strong charge transfer-based excitations across porphyrins. The <γ0>s of cyclic porphyrins are lower than that of the linear hexamer, though the interaction between the ring and the ligand enhances the <γ0> of some cyclic zinc porphyrin complexes. The large two-photon absorption cross sections confer on these zinc porphyrin derivatives excellent candidates for two-photon absorption applications.

Programmed Heterostructures for Enhanced Electrical Conductivity.

Interfacial electron transport in multicomponent systems plays a crucial role in controlling electrical conductivity. Organic-inorganic heterostructures electronic devices where all the entities are covalently bonded to each other can reduce interfacial electrical resistance, thus suitable for low-power consumption electronic operations. Programmed heterostructures of covalently bonded interfaces between ITO-ethynylbenzene (EB) and EB-zinc ferrite (ZF) nanoparticles, a programmed structure showing 67 978-fold enhancement of electrical current as compared to pristine NPs-based two terminal devices are created. An electrochemical approach is adopted to prepare nearly π-conjugated EB oligomer films of thickness ≈26nm on ITO-electrode on which ZF NPs are chemically attached. A "flip-chip" method is employed to combine two EB-ZnFe2O4 NPs-ITO to probe electrical conductivity and charge conduction mechanism. The EB-ZnFe2O4 NPs exhibit strong electronic coupling at ITO-EB and EB-NPs with an energy barrier of 0.13eV between the ITO Fermi level and the LUMO of EB-ZF NPs for efficient charge transport. Both the DC and AC-based electrical measurements manifest a low resistance at ITO-EB and EB-ZF NPs, revealing enhanced electrical current at ± 1.5V. The programmed heterostructure devices can meet a strategy to create well-controlled molecular layers for electronic applications toward miniaturized components that shorten charge carrier distance, and interfacial resistance.

Molecular engineering strategy and optoelectronic properties of diaminobenzene-thiazole π-conjugated small donor molecules for high-performance organic solar cells

In this current study, the π-conjugated oligomers based on Thiazole units have been designed to understand the interaction of molecular orbitals and the influence of the substituents on the energetic HOMO-LUMO gap to build highly π-conjugated nanostructures. Their strong π-conjugated insight was also evidenced in gas phase through the photovoltaic properties using B3LYP/6-31G(d,p) level of theory for potential applications in organic electronic devices. Our research indicates geometries optimized structures, FMOs and chemical parameters were determined to predict optical properties of all oligomers. The results demonstrate that thiazole unit’s addition improved absorption above 400 nm at first excited states. The open-cicruit voltage Voc have been decreased from 2.22 V to 1.26 V in M1-a to M1-d. Whereas, LHE values was increased from 0.404 to 0.986 in M2-a to M2-d. All designed molecules have shown the enhanced optoelectronic characteristic that is plausible reason for their potential photoactive use in Organic Photovoltaics (OPVs) devices efficiencies.

Characterization of Excited-State Electronic Structure in Diblock π-Conjugated Oligomers with Adjustable Linker Electronic Coupling.

Diblock conjugated oligomers are π-conjugated molecules that contain two segments having distinct frontier orbital energies and HOMO-LUMO gap offsets. These oligomers are of fundamental interest to understand how the distinct π-conjugated segments interact and modify their excited state properties. The current paper reports a study of two series of diblock oligomers that contain oligothiophene (Tn) and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) segments that are coupled by either ethynyl (-C≡C-) or trans-(-C≡C-)2Pt(II)(PBu3)2 acetylide linkers. In these structures, the Tn segment is electron rich (donor), and the TBT is electron poor (acceptor). The diblock oligomers are characterized by steady-state and time-resolved spectroscopy, including UV-visible absorption, fluorescence, fluorescence lifetimes, and ultrafast transient absorption spectroscopy. Studies are compared in several solvents of different polarity and with different excitation wavelengths. The results reveal that the (-C≡C-) linked oligomers feature a delocalized excited state that takes on a charge transfer (CT) character in more polar media. In the (-C≡C-)2Pt(II)(PBu3)2-linked oligomers, there is weak coupling between the Tn and TBT segments. Consequently, short wavelength excitation selectively excites the Tn segment, which then undergoes ultrafast energy transfer (~1 ps) to afford a TBT-localized excited state.

Aerobic Catalytic Cross-Dehydrogenative Coupling of Furans with Indoles Provides Access to Fluorophores with Large Stokes Shift.

Sustainability in chemical processes is a crucial aspect in contemporary chemistry with sustainable catalysis as a vital parameter of the same. There has been a renewed focus on utilizing earth-abundant metal catalysts to expand the repertoire of organic reactions. Furan is a versatile heterocycle of natural origin used for multiple applications. However, it has scarcely been used in cross-dehydrogenative coupling. In this work, we have explored the cross-dehydrogentive coupling of furans with indoles using commonly available, inexpensive FeCl3 ⋅ 6H2 O (<0.25 $/g) as catalyst in the presence of so called 'ultimate oxidant' - oxygen, without the need for any external ligand or additive. The reactions were found to be scalable and to work even under partially aqueous conditions. This makes the reaction highly economical, practical, operationally simple and sustainable. The methodology provides direct access to π-conjugated short oligomers consisting of furan, thiophene and indole. These compounds were found to show interesting fluorescence properties with remarkably large Stokes shift (up to 205 nm). Mechanistic investigations reveal that the reaction proceeds through chemoselective oxidation of indole by the metal catalyst followed by nucleophilic trapping by furan.

Thiophene- and bithiophene-based π-conjugated Schiff base oligomers containing binaphthalene moieties in the backbone. Properties and computational simulations

Schiff base oligomers based on a binaphthalene core were synthesized from [1,1′-binaphthalene]-4,4′-diamine with thiophene-2,5-dicarbaldehyde and [2,2′-bithiophene]-5,5′-dicarbaldehyde by a high-temperature polycondensation method.

New highly π-conjugated bisalkynyl-linked oligomers of heteroatom-substituted perylene diimides: Optical and electronic properties and performance in perovskite solar cells

A series of four new oligomeric derivatives of NFA, NFA(S), NFA(Se) and NFA(N) with terminal perylene diimide groups modified with heteroatoms of S, Se, and N, were successfully synthesized and fully characterized by a variety of physicochemical methods. Due to deep LUMO levels, all compounds have an electron-acceptor character, which allows them to perform as electron transport compounds in perovskite solar cells of p-i-n architecture. The highest efficiency of 17.23 % was delivered by the solar cells with NFA(Se) electron transporting layer.

Synthesis, Structure, and Properties of Monodispersed and Highly Luminescent Organoborane Oligomers.

Organoborane oligomers with well-defined molecular structures and high luminescence are scarce, among which those with boron not used as bridging atoms are even more so. Here, a series of well-defined ethynyl-linked or butadiynyl-linked conjugated organoborane oligomers with high fluorescence quantum yield and extinction coefficient (i.e., high brightness) were prepared by coupling different building blocks featuring dithienooxadiborepine moieties. Single crystal structures of hexyl modified dithienooxadiborepine (1a-hex) and hexyl-modified butadiynyl-linked conjugated dimer (D2-hex) not only verified the identity of the molecular structures but also revealed that the introduction of the hexyl chains distorted the molecular structures due to steric hindrance. Optical measurements showed that the absorption and emission maxima of the six oligomeric molecules bathochromic shifted with increasing numbers of repeating units. Molecules without hexyl chains emit efficient fluorescence upon photoexcitation, and the fluorescence quantum efficiency of the ethynyl-linked conjugated dimer (D1) is close to unity. Theoretical calculation results using density functional theory methods are consistent with the single crystal data, allowing a better understanding of the spectral properties. Such results indicate that the method is efficient for expanding small organoborane molecules into π-conjugated oligomers, which can be used to modulate to emit different colors with high efficiency.

Excited state electronic structure and dynamics in diblock π-conjugated oligomers

The excited state electronic structure and ultrafast energy transfer in two diblock oligomers F3-TBT and F3-mP-TBT was explored (F3 = tri-(9,9-dioctylfluorene), TBT = 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole, mP = meta-phenylene). These diblock oligomers feature strongly coupled, π-conjugated F3 and TBT segments that have different bandgaps. F3-TBT is a fully π-conjugated chromophore in which the two conjugated segments are directly linked. By contrast, in F3-mP-TBT the chromophores are separated by a meta-linked phenylene unit. The objective of this study was to discern whether the Franck-Condon excited state produced by selective excitation of the F3 moiety in fully conjugated F3-TBT gives rise to a localized or fully delocalized excited state. The oligomers’ excited state dynamics were probed by steady-state UV–visible absorption, fluorescence and excitation wavelength dependent ultrafast time resolved transient absorption (TA) spectroscopy. Density functional theory (DFT) calculations were applied to provide insight concerning the electronic structure of the oligomers. The diblock oligomers feature distinct absorption bands due to transitions mainly localized on the F3 (high energy, 3.5 eV) and TBT (low energy, 2.75 eV) segments. Ultrafast TA spectroscopy with excitation corresponding to the low energy TBT unit reveals that the initial excitation is localized on TBT and does not vary with time. By contrast, in both oligomers excitation corresponding to high energy F3 produces an excited state that evolves rapidly (200–300 fs) into the lower energy state that is localized on the TBT chromophore. The ultrafast process is attributed to relaxation of an excited state that is predominantly localized on F3 into a lower energy state that is localized on TBT. The observation of an F3-localized excited state is unexpected for F3-TBT, given that the molecule is fully π-conjugated.

De novo design of highly efficient type-I photosensitizer based on π-conjugated oligomer for photodynamic eradication of multidrug-resistant bacterial infections

Traditional photosensitizers show limited singlet oxygen generation in hypoxic infection lesions, which greatly suppress their performance in antibacterial therapy. Meanwhile, there still is lack of feasible design strategy for developing hypoxia-overcoming photosensitizers agents. Herein, radical generation of π-conjugated small molecules is efficiently manipulated by an individual selenium (Se) substituent. With this strategy, the first proof-of-concept study of a Se-anchored oligo (thienyl ethynylene) (OT-Se) with high-performance superoxide radical (O2•−) and hydroxyl radical (•OH) generation capability is present, and achieves efficient antibacterial activities towards the clinically extracted multidrug-resistant bacteria methicillin-resistant S. aureus (MRSA) and carbapenem-resistant E. coli (CREC) at sub-micromolar concentration under a low white light irradiation (30 mW/cm2). The water-dispersible OT-Se shows a good bacteria-anchoring capability, biocompatibility, and complete elimination of multidrug-resistant bacteria wound infection in vivo. This work offers a strategy to boost type-I photodynamic therapy (PDT) performance for efficient antibacterial treatments, advancing the development of antibacterial agents.

Amplification of Dissymmetry for Circularly Polarized Photodetection by Cooperative Supramolecular Polymerization.

Circularly polarized photodetectors require chiral light absorption materials with high sensing efficiency and low costs. Here readily accessible point chirality has been introduced to dicyanostilbenes as the chiral source, facilitating remote chirality transfer to the π-aromatic core by cooperative supramolecular polymerization. The single-handed supramolecular polymers display powerful circularly polarized photodetection capability with a dissymmetry factor value as high as 0.83, superior to those of π-conjugated small molecules and oligomers. Strong chiral amplification occurs between the enantiopure sergeants and the achiral soldiers. The resulting supramolecular copolymers exhibit comparable photodetection efficiency to those of the homopolymeric ones, with a 90 % decrease in the enantiopure compound consumption. Therefore, cooperative supramolecular polymerization provides an effective yet economical avenue toward circularly polarized photodetection applications.

DFT calculations of photophysical properties of ethylen-dioxy-thiophen derivatives oligomers with optoelectronic functionalities

Materials with π-conjugated system can absorb sunlight, create photo-generated current or even produce light. The easiness of its manipulation combined with cheap process has made organic components provide a new attractive field for electronic materials research. This paper presents computational study of π-conjugated oligomers structure. In this regard, we used density functional theory (DFT) calculations to analyze the geometry of oligomers based of EDOT (3,4-ethylen dioxythiophen) and VC (vinyl-carbazol). The band gap with simulated spectra (UV–Vis spectra, emission spectra) and several photovoltaic properties like open-circuit photovoltage (Voc) and light harvesting efficiency (LHE) are predicted and discussed for giving theoretical knowledge of studied compound efficacity in photovoltaic applications.

Strain-Sensitive On-Surface Ladderization by Non-Dehydrogenative Heterocyclization.

On-surface cyclodehydrogenation recently became an important reaction to planarize π-conjugated molecules and oligomers. However, the high-activation barrier to cleave the C-H bond often requires high-temperature annealing, consequently restricting structures of precursor molecules and/or leading to random fusion at their edges. Here, we present a synthesis of pyrrolopyrrole-bridged ladder oligomers from 11,11,12,12-tetrabromo-1,4,5,8-tetraaza-9,10-anthraquinodimethane molecules on Ag(111) with bond-resolved scanning tunnelling microscopy. This non-dehydrogenative cyclization between pyrazine and ethynylene/cumulene groups has a low-activation barrier for forming intermediary dimeric oligomer containing dipyrazinopyrrolopyrrolopyrazine units, thus giving new insight into the strain-sensitive in ladder-oligomer formation.

De Novo Synthesis of α-Oligo(arylfuran)s and Its Application in OLED as Hole-Transporting Material.

Tuning the photophysical properties of π-conjugated oligomers by functionalization of skeleton, to achieve an optically and electronically advantageous building block for organic semiconductor materials is a vital yet challenging task. In this work, a series of structurally well-defined polyaryl-functionalized α-oligofurans, in which aryl groups are introduced precisely into each of the furan units, are rapidly and efficiently synthesized by de novo metal-free synthesis of α-bi(arylfuran) monomers for the first time. This new synthetic strategy nicely circumvents the cumbersome substituent introduction process in the later stage by the preinstallation of the desired aryl groups in the starting material. The characterization of α-oligo(arylfuran)s demonstrates that photoelectric properties of coplanar α-oligo(arylfuran)s can be tuned through varying aryl groups with different electrical properties. These novel α-oligo(arylfuran)s have good hole transport capacity and can function as hole-transporting layers in organic light-emitting diodes, which is indicative of significant breakthrough in the application of α-oligofurans materials in OLEDs. And our findings offer an avenue for the ingenious use of α-oligo(arylfuran)s as p-type organic semiconductors for OLEDs.

Photophysics and charge transfer in oligo(thiophene) based conjugated diblock oligomers.

This paper reports an investigation of the electronic structure and photophysical properties of two "diblock" π-conjugated oligomers (T4-TBT and T8-TBT) that feature electron rich tetra(thiophene) (T4) or octa(thiophene) (T8) segments linked to an electron poor 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) moiety. Electrochemistry and UV-visible absorption spectroscopy reveals that the diblock oligomers display redox and absorption features that can be attributed to the Tn and TBT units. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations support the experimental electrochemistry and optical spectroscopy results, suggesting that the frontier orbitals on the diblock oligomers retain characteristics of the individual π-conjugated segments. However, low energy optical transitions are anticipated to arise from Tn to TBT charge transfer. Fluorescence spectroscopy on the diblock oligomers reveals that the oligomers feature strongly solvent dependent fluorescence. In non-polar solvents (hexane, toluene), the emission is structured with a moderate Stokes shift; however, in more polar solvents the emission becomes broader, and red-shifts significantly. Transient absorption spectroscopy on timescales from femtoseconds (fs) to microseconds (μs) reveals that in non-polar solvents excitation produces a singlet excited state (LE) that decays uniformly to the ground state in parallel with intersystem crossing to a triplet state. By contrast, in more polar solvents, excitation produces a very short-lived excited state (1-3 ps) which evolves rapidly into a second excited state that is attributed to the charge transfer (CT) state. The fast dynamics are associated with crossing from the LE state, which is populated initially by photoexcitation, into the CT state, which then decays to the ground state. The photophysical properties and dynamics of the LE and CT excited states are very similar for T4-TBT and T8-TBT, suggesting that the length of the oligo(thiophene) segment does not have a strong influence on the energy, structure or dynamics of the LE and CT excited states.

Structurally well-defined conjugated meso-aminoporphyrin oligomers analogous to polyanilines.

Polyaniline, which is formed by the oxidative polymerization of aniline, is a widely explored conducting polymer with several stable oxidation states, and can be applied in advanced materials, including sensing devices and electrochemical catalysts. The marriage of polyanilines with the diverse chemistry of porphyrins is expected to confer new properties, including a combination of electrical, optical, magnetic and chemical properties. Herein, we demonstrate that meso-aminodiarylporphyrin, a porphyrin analogue of aniline, undergoes oxidative oligomerization in an acidic solution under an oxygen atmosphere to yield stable oligomeric products that are analogous to fully oxidized polyanilines. The so-formed oligomers are composed of the same number of electron-rich porphyrinoid and electron-deficient quinoid moieties, and they exhibit a broad electronic absorption band in the near infrared (NIR) region, which is attributable to intramolecular charge transfer (ICT) transition from electron-rich porphyrinoid moieties to electron-deficient quinoid ones. The quinoid moieties in the oligomers could be reversibly reduced using sodium ascorbate to obtain all-porphyrinoid oligomers that resemble fully reduced polyanilines. The fully reduced oligomers do not exhibit the NIR ICT band. Furthermore, three types of partially reduced tetramers consisting of a single quinoid moiety were also obtained, among which two interconverted in solution. Their interconversion was significantly accelerated in the presence of a protic solvent. This result is consistent with the high electron conductivity of partially oxidized polyanilines following their protonation.

Carbazole and Diketopyrrolopyrrole-Based D-A π-Conjugated Oligomers Accessed via Direct C-H Arylation for Opto-Electronic Property and Performance Study.

Five carbazole and diketopyrrolopyrrole-based donor-acceptor (D-A) new π-conjugated oligomers (π-COs) with gradually elongated lengths are facilely synthesized via a single pot of direct C-H arylation with merits of atom- and step-economy. The structure-property-performance correlations of these π-COs and their parent polymer are studied in detail by opto-electronic characterizations and bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. It is found that the π-COs having longer lengths enable better performance in OPVs owing to the enhanced intermolecular interaction with the elongation of the conjugations. The above results not only highlight the powerful synthetic strategy here provided, but also reveal that π-COs with unique properties might find promising application in OPVs.

Bright Near-Infrared π-Conjugated Oligomer Nanoparticles for Deep-Brain Three-Photon Microscopy Excited at the 1700 nm Window in Vivo.

The development of three-photon fluorophores with 1700 nm excitation is pressingly desirable for in vivo imaging of tissue resided deep inside the brain. Herein, we report a designed and synthesized fluorescent molecule (OFET) for in vivo mouse brain imaging with three-photon microscopy at a record imaging depth. The OFET molecule has a relatively high fluorescence brightness and has a near-infrared (NIR) maximum emission at 820 nm after integrating as water-dispersible nanoparticles (OEFT NPs). Under 1720 nm excitation, OFET NPs show a large three-photon action cross-section of 1.06 × 10-82 cm6 s2/photon2, which is more than twice that of the commonly used sulforhodamine 101 (SR101) dye. Benefiting from the high tissue penetration depths for both the long excitation in the second NIR window of 1720 nm and the emission wavelength in the first NIR window of 820 nm, a high brightness, and a large action cross-section of three-photon, OFET NPs have good deep-brain imaging performance. Brain vasculatures of a mouse located at a depth of 1696 μm can be clearly resolved in vivo. With no observable cytotoxicity even in a high concentration, the present OFET NPs suggest that fluorescent π-conjugated oligomers are of great potential in high-resolution 3PM imaging of in vivo deep-tissue.