Phenylene Ethynylene Research Articles

Laser-controlled electronic symmetry breaking in a phenylene ethynylene dimer: Simulation by the hierarchical equations of motion and optimal control

The ability to prepare a specific superposition of electronic excited states leading to a transitory symmetry breaking of the electronic density in complex systems remains a challenging concern. We investigate how an initial coherence can be controlled by laser fields. The selected molecular system is a symmetric dimer of phenylene ethynylene presenting different interesting properties: Two bright nearly degenerate excited states coupled through a conical intersection are addressable by orthogonal transition dipole moments and are well separated from neighboring states. Creating a superposed state with equal weights corresponds to a right or left electronic localization as in the double-well system followed by a transitory oscillation between the two wells. To ensure a spectral bandwidth typically smaller than 0.25 eV, the pulse duration is in the tens of femtoseconds range, so nuclear motion cannot be neglected. Optimal control theory (OCT) is applied with guess fields that effectively create the target coherence in the absence of dephasing due to the vibrational baths. We analyze the field reshaping proposed by the control and we further fit a sequence of pulses on the optimal field. The overall result is efficient and robust disymmetry control over reasonable timescales of few tens of femtoseconds, exceeding the pulse duration. The monotonically convergent algorithm is combined with the hierarchical equations of motion (HEOM) able to treat strongly coupled non-Markovian dynamics. We also check the implementation of the combined OCT-HEOM approach in the tensor-train representation with propagation using the time-dependent variational method.

Switching in Nanoscale Molecular Junctions due to Contact Reconfiguration

Switching effects are key elements in the design and characterization of nanoscale molecular electronics systems. They are used to achieve functionality through the transition between different conducting states. In this study, we analyze the presence of switching events in reference molecular systems, which are not designed to have switching behavior, such as oligo(phenylene ethynylene)s and alkanes, using the mechanically controllable break junction technique. These events can be classified in two groups, depending on whether the breaking trace shows exponential decay or plateau-like features before the switch happens. We argue that the former correspond to junctions forming after rupture of the gold atomic point contact, while the latter can be related to a change in the contact geometry of the junction. These results highlight how a proper choice of anchoring group and careful comparison with reference compounds are essential to understanding the origin of switching in molecular break junctions.

Uniform π‐Conjugated‐Co‐Oligomer‐Based Nanofibers of Controlled Length with Near‐Infrared Emission, Photodynamic and Photothermal Activities

Abstractπ‐Conjugated‐polymer‐based nanofibers (CPNFs) have attracted considerable interest in the community of nanomedicine owing to their high‐aspect‐ratio architecture and inherent photo‐imaging, photothermal, photodynamic, and photoacoustic properties. However, the preparation of uniform CPNFs of controlled length and composition with near‐infrared emission (NIR), photothermal and photodynamic activity remains in its infancy. A novel block copolymer consisting of a donor‐acceptor‐donor (D‐A‐D) π‐conjugated co‐oligomer with a diketopyrrolopyrrole (DPP) unit as electron‐deficient acceptor flanked by oligo(p‐ phenylene ethynylene) (OPE) as an electron‐rich donor and a poly(2‐vinylpyridine) (P2VP) block (OPE4‐DPP‐OPE4‐b‐P2VP44, the subscripts describe the extra and average number of repeat units for π‐conjugated OPE segment and P2VP block, respectively) is designed and synthesized. By using the self‐seeding approach of living crystallization‐drive self‐assembly (CDSA), uniform CPNFs containing a D‐A‐D OPE4‐DPP‐OPE4 core and a P2VP corona with tunable length from 31 to 611 nm are generated. The resulting CPNFs exhibit NIR emission with a peak at 837 nm. The CPNFs also can serve as a photosensitizer to efficiently produce single oxygen and show a photothermal effect upon 660 nm irradiation. To the author's best knowledge, it is the first report on the generation of uniform CPNFs of controlled length with NIR emission, and photodynamic and photothermal activities by living CDSA.

Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges.

A family of three ruthenium bipyridyl rigid-rod compounds of the general form [Ru(bpy)2(LL)](PF6)2 were anchored to mesoporous thin films of tin-doped indium oxide (ITO) nanocrystals. Here, LL is a 4-substituted 2,2-bipyridine (bpy) ligand with varying numbers of conjugated phenylenethynylene bridge units between the bipyridine ring and anchoring group consisting of a bis-carboxylated isophthalic group. The visible absorption spectra and the formal potentials, Eo(RuIII/II), of the surface anchored rigid-rods were insensitive to the presence of the phenylene ethynylene bridge units in 0.1 M tetrabutyl ammonium perchlorate acetonitrile solutions (TBAClO4/CH3CN). The conductive nature of the ITO enabled potentiostatic control of the Fermi level and hence a means to tune the Gibbs free energy change, -ΔG°, for electron transfer from the ITO to the rigid-rods. Pseudo-rate constants for this electron transfer reaction increased as the number of bridge units decreased at a fixed -ΔG°. With the assumption that the reorganization energy, λ, and the electronic coupling matrix element, Hab, were independent of the applied potential, rate constants measured as a function of -ΔG° and analyzed through Marcus-Gerischer theory provided estimates of Hab and λ. In rough accordance with the dielectric continuum theory, λ was found to increase from 0.61 to 0.80 eV as the number of bridge units was increased. In contrast, Hab decreased markedly with distance from 0.54 to 0.11 cm-1, consistent with non-adiabatic electron transfer. Comparative analysis with previously published studies of bridges with an sp3-hybridized carbon indicated that the phenylene ethynylene bridge does not enhance electronic coupling between the oxide and the rigid-rod acceptor. The implications of these findings for practical applications in solar energy conversion are specifically discussed.

Dynamic Fluorescent Anti-Counterfeiting Labels Based on Conjugated Polymers Confined in Submicron Fibrous Membranes.

Developing a new anti-counterfeiting strategy is of great significance to combating the global counterfeiting problem. Here we report the construction of a dynamic fluorescence response system for anti-counterfeiting by combining the photochromism induced by the ring-opening of spiropyran (SP) to merocyanine (MC) with the fluorescence resonance energy transfer (FRET) between the conjugated polymer and MC. After elucidating the design principle, a new conjugated polymer, PPETE-SP, consisting of a poly[p-(phenylene ethynylene)-alt-(thienylene-ethynylene)] (PPETE) backbone with pendant SP, was synthesized and characterized. With poly(methyl methacrylate) (PMMA) as the matrix, the PPETE-SP/PMMA fibrous membrane was prepared via electrospinning. Under the irradiation of UV light, the fluorescent color of the membrane dynamically changed from green to light green, then light pink, and finally pink, and this process was reversible under visible light. The fluorescence emission switch was examined for 10 cycles and proved to have good repeatability, indicating that the membrane can be directly used as an anti-counterfeiting label for multiple verifications. The FRET efficiency was found to be about 61% based on the FRET study with confocal laser scanning microscopy. The covalent bonding between PPETE backbone and SP, the confinement of PPETE-SP chains in the fibrous membrane, as well as employing PMMA as the matrix were demonstrated to be crucial in realizing the photochromism and the FRET. Different anti-counterfeiting modes were proposed, providing rich selections for operation of verification. Such facile-to-operate and hard-to-imitate dynamic fluorescent responsive materials are very promising for use in practical anti-counterfeiting applications.

Functionalization of Water-Soluble Conjugated Polymers for Bioapplications.

Water-soluble conjugated polymers (WS-CPs) have found widespread use in bioapplications ranging from in vitro optical sensing to in vivo phototherapy. Modification of WS-CPs with specific molecular functional units is necessary to enable them to interact with biological targets. These targets include proteins, nucleic acids, antibodies, cells, and intracellular components. WS-CPs have been modified with covalently linked sugars, peptides, nucleic acids, biotin, proteins, and other biorecognition elements. The objective of this article is to comprehensively review the various synthetic chemistries that have been used to covalently link biofunctional groups onto WS-CP platforms. These chemistries include amidation, nucleophilic substitution, Click reactions, and conjugate addition. Different types of WS-CP backbones have been used as platforms including poly(fluorene), poly(phenylene ethynylene), polythiophene, poly(phenylenevinylene), and others. Example applications of biofunctionalized WS-CPs are also reviewed. These include examples of protein sensing, flow cytometry labeling, and cancer therapy. The major challenges and future development of functionalized conjugated polymers are also discussed.

One- and Two-Photon Activated Release of Oxaliplatin from a Pt(IV)-Functionalized Poly(phenylene ethynylene).

We report a water-soluble poly(phenylene ethynylene) (PPE-Pt(IV)) that is functionalized with oxidized oxaliplatin Pt(IV) units and its use for photoactivated chemotherapy. The photoactivation strategy is based on photoinduced electron transfer from the PPE backbone to oxaliplatin Pt(IV) as an electron acceptor; this process triggers the release of oxaliplatin, which is a clinically used anticancer drug. Mechanistic studies carried out using steady-state and time-resolved fluorescence spectroscopy coupled with picosecond-nanosecond transient absorption support the hypothesis that electron transfer triggers the drug release. Photoactivation is effective, producing oxaliplatin with a good chemical yield in less than 1 h of photolysis (400 nm, 5 mW cm-2). Photorelease of oxaliplatin from PPE-Pt(IV) can also be effected with two-photon excitation by using 100 fs pulsed light at 725 nm. Cytotoxicity studies using SK-OV-3 human ovarian cancer cells demonstrate that without photoactivation PPE-Pt(IV) is not cytotoxic at concentrations up to 10 μM in polymer repeating unit (PRU) concentration. However, following a short period of 460 nm irradiation, oxaliplatin is released from PPE-Pt(IV), resulting in cytotoxicity at concentrations as low as 2.5 μM PRU.

Palladium(II) and Platinum(II) Porphyrin Donors for Organic Photovoltaics

Three new A-π-D-π-A structural porphyrin-based donors, C19-H2P, C19-PdP, and C19-PtP, are designed and synthesized, with porphyrin (D) moieties equipped with no metal, palladium (Pd), and platinum (Pt) as the core, respectively, and end-capped with electron-deficient 3-ethylrhodanine (A) via a π-bridge of phenylene ethynylene linker. C19-PdP and C19-PtP exhibit low-lying highest occupied molecular orbital levels than C19-H2P. The three porphyrin donors show highly intensified and broad absorptions at ca. 400–530 nm (Soret bands) and at ca. 550–800 nm (Q-bands) with an absorption valley at ca. 540 nm. Using PC71BM as an electron acceptor, the devices based on C19-PdP and C19-PtP donors achieve power conversion efficiencies (PCEs) of 8.09 and 7.31% under 1 Sun, respectively, whereas the C19-H2P device, that has no metal, has a very low PCE of 2.51%. Furthermore, at 300 lux LED source, the C19-PdP and C19-PtP devices deliver promising PCEs of 16.54 and 16.74%, respectively. The improved performance of C19-PdP- and C19-PtP-based devices is mainly due to more efficient charge collection, faster charge transport, more suppressed recombination behavior, and smoother surface morphology. These findings pave the way for the development of new metalloporphyrin-based donors for organic solar cells using simple structural engineering.

Cover Image

The cover image is based on the Research Article Absorption and fluorescence spectra of conjugated polymers poly(propylene oxide)–poly(phenylene ethynylene) interpreted by Franck–Condon simulation by Hao Tang, Chen Wen Wang, Feng Long Gu and Chaoyuan Zhu, https://doi.org/10.1002/poc.4261.

Spectral Fingerprint of Excited-State Energy Transfer in Dendrimers through Polarization-Sensitive Transient-Absorption Pump–Probe Signals: On-the-Fly Nonadiabatic Dynamics Simulations

The time-resolved polarization-sensitive transient-absorption (TA) pump-probe (PP) spectra are simulated using on-the-fly surface-hopping nonadiabatic dynamics and the doorway-window representation of nonlinear spectroscopy. A dendrimer model system composed of two linear phenylene ethynylene units (2-ring and 3-ring) is taken as an example. The ground-state bleach (GSB), stimulated emission (SE), and excited-state absorption (ESA) contributions as well as the total TA PP signals are obtained and carefully analyzed. It is shown that intramolecular excited-state energy transfer from the 2-ring unit to the 3-ring unit can be conveniently identified by employing pump and probe pulses with different polarizations. Our results demonstrate that time-resolved polarization-sensitive TA PP signals provide a powerful tool for the elucidation of excited-state energy-transfer pathways, notably in molecular systems possessing several optically bright nonadiabatically coupled electronic states with different orientations of transition dipole moments.

Promotion and suppression of single-molecule conductance by quantum interference in macrocyclic circuits

Promotion and suppression of single-molecule conductance by quantum interference in macrocyclic circuits

Back Cover: Head‐to‐Tail Oligomerization by Silylene‐Tethered Sonogashira Coupling on Ag(111) (Angew. Chem. Int. Ed. 36/2021)

Silylene-tethered Sonogashira cross-coupling enables the synthesis of partially fluorinated phenylene–ethynylene (PFPE) oligomers on Ag(111), as reported by Akihiro Orita, Shigeki Kawai, and co-workers in their Communication on page 19598. The desilylative Sonogashira reaction achieved a high chemoselectivity of up to 75 % while the competing Ullmann homocoupling and desilylative Glaser coupling were suppressed.

Absorption and fluorescence spectra of conjugated polymers poly(propylene oxide)–poly(phenylene ethynylene) interpreted by Franck–Condon simulation

AbstractAbsorption and fluorescence spectra of the poly(propyleneoxide)–poly(phenylene ethynylene) with long polymer chains in the experiment are investigated by the Franck–Condon simulation with small polymer chains in the theory. Electronic calculations with time‐dependent density functional (TDDFT) indicate that vibronic spectra come from S1(ππ*) excitation and de‐excitation. Good agreement between Franck–Condon simulated and experimentally measured absorption and fluorescence spectra in three kinds of solutions is achieved. However, vibronic spectral analysis reveals that vibronic motion mechanisms are quite different for simulated absorption and fluorescence spectra. For absorption spectra, vibronic spectra are dominated by one most active mode localized in triple bond carbon–carbon stretch motion on backbone of the poly (phenylene ethynylene), and simulated absorption spectra indicate that the long chains can be as stable as short chains on the electronic ground state in comparison with experimental spectra in all solvents. In the case of fluorescence spectra, this most active mode is spitting into several equally active modes with very close values in vibrational frequencies, and thus, the spitting modes and mode couplings among those spitting dominate vibronic spectra. Simulated fluorescence spectra indicate that the only short chains are stable on the electronic excited state in comparison with experimental spectra in all solvents. Nevertheless, the present simulation concludes that both absorption and fluorescence spectra are induced mostly by the active normal mode in triple bond carbon–carbon stretch motion on backbone of poly(phenylene ethynylene). The present work provides physical insight for analyzing observed vibronic spectra of similar conjugated polymers and for designing novel optoelectronic materials.

Head-to-Tail Oligomerization by Silylene-Tethered Sonogashira Coupling on Ag(111).

On-surface synthesis is a powerful method for the fabrication of π-conjugated nanomaterials. Herein, we demonstrate chemoselective Sonogashira coupling between (trimethylsilyl)ethynyl and chlorophenyl groups in silylethynyl- and chloro-substituted partially fluorinated phenylene ethynylenes (SiCPFPEs) on Ag(111). The desilylative Sonogashira coupling occurred with high chemoselectivity up to 75 %, while the competing Ullmann and desilylative Glaser homocoupling reactions were suppressed. A combination of bond-resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) and DFT calculations revealed that the oligomers were obtained by the formation of intermolecular silylene tethers (-Me2 Si-) through CH3 -Si bond activation at 130 °C and subsequent elimination of the tethers at an elevated temperature of 200 °C.

Head‐to‐Tail Oligomerization by Silylene‐Tethered Sonogashira Coupling on Ag(111)

AbstractOn‐surface synthesis is a powerful method for the fabrication of π‐conjugated nanomaterials. Herein, we demonstrate chemoselective Sonogashira coupling between (trimethylsilyl)ethynyl and chlorophenyl groups in silylethynyl‐ and chloro‐substituted partially fluorinated phenylene ethynylenes (SiCPFPEs) on Ag(111). The desilylative Sonogashira coupling occurred with high chemoselectivity up to 75 %, while the competing Ullmann and desilylative Glaser homocoupling reactions were suppressed. A combination of bond‐resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) and DFT calculations revealed that the oligomers were obtained by the formation of intermolecular silylene tethers (‐Me2Si‐) through CH3−Si bond activation at 130 °C and subsequent elimination of the tethers at an elevated temperature of 200 °C.

Photoinduced Intramolecular Electron Transfer in Phenylene Ethynylene Naphthalimide Oligomers.

This paper reports a photophysical investigation of a series of phenylene ethynylene oligomers (OPE) that are end-substituted with a 1,8-naphthalene imide (NI) acceptor. The NI acceptor is attached to the terminus of the OPEs via an ethynylene (-C≡C-) unit that is linked at the 4-position of the NI unit. A series of three oligomers is investigated, OPE1-NI, OPE3-NI, and OPE5-NI, which contain 1, 3, and 5 phenylene ethynylene repeat units, respectively. The properties of the OPEn-NI series are compared to a corresponding set of unsubstituted OPEs, OPE3 and OPE5, which contain 3 and 5 phenylene ethynylene repeats, respectively. The photophysics of all the compounds are interrogated using a variety of techniques including steady-state absorption, steady-state fluorescence, two-photon absorption, time-resolved fluorescence, and transient absorption spectroscopy on femtosecond-to-microsecond time scales. The effect of solvent polarity on the properties of the oligomers is examined. The results show that the NI-substituted oligomers feature a lowest charge transfer (CT) excited state, where the OPE segment acts as the donor and the NI moiety is the acceptor (OPEn•+-NI•-). The absorption spectra in one-photon and two-photon exhibit a clear manifold of absorption features that can be attributed to direct CT absorption. In moderately polar solvents, the emission is dominated by a broad, solvatochromic band that is due to radiative decay from the CT excited state. Ultrafast transient absorption provides evidence for initial population of a locally excited state (LE) which in moderately polar solvents rapidly (∼1 ps) evolves into the CT excited state. The structure, spectroscopy, and dynamics of the CT state are qualitatively similar for OPE3-NI and OPE5-NI, suggesting that delocalization in the OPE segment does not have much effect on the structure or energetics of the CT excited state.

Iron(III) Sensors Based on the Fluorescence Quenching of Poly(phenylene ethynylene)s and Iron-Detecting PDMS Pads

The detection of Fe ions is critical to environmental monitoring and disease diagnosis. We herein report fluorescent conjugated polymers that exhibit fluorescence quenching upon the addition of Fe (III) ions with the total concentration of 2 ppm. The polymer backbone consists of poly(phenylene ethynylene) (PPE) with side-chains bearing triethylene glycols or alkyl groups. The quenching phenomenon was analyzed by the Stern-Volmer equation, and the resulting Stern-Volmer constants show that the alkyl functional PPE presents higher sensitivity than the triethylene glycol functional PPE, which was unexpected. The polymer sensors showed the selectivity in that the Stern-Volmer constants for Fe3+ are 2–3 times higher than those for Na+, K+, and Ca2+. Time-resolved photoluminescence spectroscopy revealed that the quenching is static, indicating that there may be coordination between Fe (III) and alkyne/phenyl groups in the PPE backbone. For practical applications, we produced a PPE-containing, fluorescent polydimethylsiloxane (PDMS) pad. Dropping Fe solution like an ink on the pad exhibits a dark stain, demonstrating the scheme of producing Fe-detecting flexible pads for future applications in wearable sensor technology.

Nonmonotonic dependence of intramolecular charge-transfer sidechain interactions for triazole containing phenylene-ethynylene grafted Co-Polyoxetane brushes

This research explores effects of introduction of the rigid phenylene ethynylene (PEy) moiety as a side chain in polyoxetanes. The synthesis of PEy-grafted polyoxetanes was carried out via Huisgen copper (I) catalyzed alkyne-azide cycloaddition. These polymers have a flexible main chain (low Tg polyoxetane) and rigid side chains (phenylene ethynylene). Compositions for copolyoxetanes with PEy and alkynyl side chains were determined by 1H-NMR spectroscopy. Mole percents PEy, designated P-%, were P-21, P-44, P-69 and P-100. UV-VIS spectra for P-21, P-44, P-69, P-100 at 330 nm showed differentiation based on assignments to intramolecular charge-transfer (ICT) interactions between triazole and phenylene ethylene moieties. Interestingly, instead of a monotonic increase, absorption intensity is in the order P-69 ~ P-44 > P-100 > P-21. Low substitution and mainchain flexibility and account for P-21 having the lowest UV-VIS absorption at 330 nm. P-44 and P-69 have similar strong absorptions at 330 nm with overlapping curves. Steric effects culminate in P-100, which has the highest PEy content but a lower absorption at 330 nm than P-69 but higher than P-21. A model for optical absorptions for PEy copolyoxetanes is described that includes the effect of the rigid PEy side chain in decreasing molecular mobility.

Effective suppression of conductance in multichannel molecular wires

Although quantum effects have introduced fascinating properties to molecular circuits, exploring new features for molecular circuits remains a challenge. In this work, effective suppression of conductance in the parallel intramolecular circuit is identified, going beyond the well-known superposition effect. By introducing oligo(phenylene ethynylene) (OPE) as a high-conductivity channel (HCC) and [cis-Pt(PEt3)2]2+ coordinated with m-phenylethynyl as a low-conductivity channel (LCC), the single-molecule conductance measured by the scanning tunneling microscopy break junction (STM-BJ) technique demonstrates that, in the multichannel molecular wires, the conductance decreases progressively with the addition of the LCC. A theoretical investigation indicates that all channels participate in electron transport. The conductance suppression in the multichannel structures is ascribable to the higher contribution of the LCC than of the HCC to the conductive orbitals. This finding improves the understanding of electron transport theory at the molecular scale and provides a strategy to design parallel intramolecular circuits.

The Effect of Anchor Group on the Phonon Thermal Conductance of Single Molecule Junctions

There is a worldwide race to convert waste heat to useful energy using thermoelectric materials. Molecules are attractive candidates for thermoelectricity because they can be synthesised with the atomic precision, and intriguing properties due to quantum effects such as quantum interference can be induced at room temperature. Molecules are also expected to show a low thermal conductance that is needed to enhance the performance of thermoelectric materials. Recently, the technological challenge of measuring the thermal conductance of single molecules was overcome. Therefore, it is timely to develop strategies to reduce their thermal conductance for high performance thermoelectricity. In this paper and for the first time, we exploit systematically the effect of anchor groups on the phonon thermal conductance of oligo (phenylene ethynylene) (OPE3) molecules connected to gold electrodes via pyridyl, thiol, methyl sulphide and carbodithioate anchor groups. We show that thermal conductance is affected significantly by the choice of anchor group. The lowest and highest thermal conductances were obtained in the OPE3 with methyl sulphide and carbodithioate anchor groups, respectively. The thermal conductance of OPE3 with thiol anchor was higher than that with methyl sulphide but lower than the OPE3 with pyridyl anchor group.