Π-conjugated Organic Molecules Research Articles

Controlled Assembly of Conjugated Ladder Molecules with Different Bridging Structures toward Optoelectronic Application.

Structural design of organic π-conjugated small molecules allows the energy band structure and electronic properties of the molecules to be tuned as needed, which provides a feasible strategy for enhancing the performance of optoelectronic devices. The introduction of bridging structures is a common structural modification method to adjust the rigidity and coplanarity of the molecular backbone, thus affecting the molecular packing. However, patterning of organic single-crystalline microstructures based on conjugated ladder molecules with different bridging structures still remains challenging for large-area integration of optoelectronic devices. In this paper, a controlled dewetting process is applied to obtain organic single-crystalline arrays with precise positioning and a regular morphology based on two isomers with silicon-oxygen bridging and their two carbon-oxygen-bridged analogues. Molecules with different bridging structures show disparate packing models due to the difference of dihedral angles and ring tensions. A microwire-array ultraviolet photodetector based on the oxygen-silicon-bridging ladder molecule exhibits a high light on/off ratio of 24 and a responsivity of 0.63 mA W-1 owing to the effective π-π stacking governed by the molecular planarity. This work not only provides a universal method for the integration of organic optoelectronic devices but also explains the effect of bridging structure engineering on molecular assembly and optoelectronic performance.

Elucidating Inner Workings of Naturally Sourced Organic Optoelectronic Materials with Ultrafast Spectroscopy.

Recent advances in sustainable optoelectronics including photovoltaics, light-emitting diodes, transistors, and semiconductors have been enabled by π-conjugated organic molecules. A fundamental understanding of light-matter interactions involving these materials can be realized by time-resolved electronic and vibrational spectroscopies. In this Minireview, the photoinduced mechanisms including charge/energy transfer, electronic (de)localization, and excited-state proton transfer are correlated with functional properties encompassing optical absorption, fluorescence quantum yield, conductivity, and photostability. Four naturally derived molecules (xylindein, dimethylxylindein, alizarin, indigo) with ultrafast spectral insights showcase efficient energy dissipation involving H-bonding networks and proton motions, which yield high photostability. Rational design principles derived from such investigations could increase the efficiency for light harvesting, triplet formation, and photosensitivity for improved and versatile optoelectronic performance.

Application of a Modified Porphyrin in a Polymer Electrolyte with Superior Properties for All-Solid-State Lithium Batteries.

Porphyrins and their derivatives are a unique class of multifunctional and modifiable π-conjugated heterocyclic organic molecules, which have been widely applied in the fields of optoelectronic devices and catalysis. However, the application of porphyrins in polymer electrolytes for all-solid-state lithium-ion batteries (ASSLIBs) has rarely been reported. Herein, porphyrin molecules modified by polyether chains are used for composite solid-state polymer electrolytes (CSPEs) for the first time. The introduction of a modified porphyrin in an electrolyte can not only promote the electrochemical properties by constructing ordered ion channels via the intermolecular interaction between π-conjugated heterocyclic porphyrins, but also significantly improve the mechanical strength and interface contact between the electrolyte membrane and the lithium metal anode. Consequently, the all-solid-state batteries assembled by the modified porphyrin composite polymer electrolyte, LiFePO4 cathodes, and Li anodes deliver a higher discharge capacity of 158.2 mA h g-1 at 60 °C, 0.2 C, which remains at 153.6 mA h g-1 after 120 cycles with an average coulombic efficiency of ∼99.60%. Furthermore, the flexible porphyrin-based composite polymer electrolyte can also enable a Li || LiCoO2 battery to exhibit a maximum discharge capacity of 108.6 mA h g-1 at 60 °C, 0.1 C with an active material loading of 2-3 mg cm-2, which is unable to realize for the corresponding batteries with a pure PEO-based polymer electrolyte. This work not only broadens the application scope of porphyrins, but also proposes a novel method to fabricate CSPEs with improved electrochemical and mechanical properties, which may shed new light on the development of CSPEs for next-generation high-energy-density lithium-ion batteries.

Intramolecular charge transfer dynamics in the excited states of diphenylamine substituted 1,3,4-oxadiazole derivatives

The excited state dynamics processes of two diphenylamine substituted symmetric 1,3,4-oxadiazole derivatives in different solvents were studied through femtosecond transient absorption spectroscopy. It was revealed that in cyclohexane, the locally excited (LE) state relaxes to the intramolecular charge transfer (ICT) state within 2 ps timescale, then decays to the ground state. In tetrahydrofuran and acetonitrile, the ICT state can be stabilized via solvation, so besides the LE → ICT conversion within 1 ps, another ICT → solvent stabilized ICT (SSICT) conversion in the 1–200 ps time scale could also be observed, and eventually decay to the ground state. Whereas, ICT → SSICT process in acetonitrile is slower than that in tetrahydrofuran, which leads to radiationless deactivation dominates the ICT state deactivation process and low fluorescence quantum efficiency. These results offer a guidance to understand the relationship of low fluorescence quantum efficiency and excited state deactivation mechanism of organic π-conjugated molecules, which would be very helpful for designing new advanced opto-electronic materials.

Pyrene-cyanostyrene-pyridine triad: Multi-stimuli responsive fluorescent emitter and mitochondrial imaging

Tuning the fluorescence of π-conjugated small organic molecules through the structural modifications and in the presence of external stimuli have found immense applications ranging from sensing, switching, optoelectronics to bioimaging. Herein, we demonstrated a simple design strategy for the synthesis of a pyrene-cyanostyrene-pyridine molecular triad (PYBNP) using a cost-effective, one-step Knoevenagel condensation between 1-pyrenecarboxaldehyde and 2-(4-(pyridin-4-yl)phenyl)acetonitrile. PYBNP exhibited strong solid-state emission having a quantum yield of ∼ 36 ± 3% due to the deactivation of nonradiative decay channels through the restriction of intramolecular rotations and reduced π-π stacking interactions as revealed from the crystal structure analysis. Similarly, PYBNP was found to be highly emissive in the constrained environments like viscous medium and aggregated state. The optical properties of PYBNP could be tuned by the UV light-mediated dynamic Z/E-isomerization as elucidated through fluorescence spectroscopy and NMR analysis. Additionally, the pyridinic nitrogen centre allowed the fabrication of methylated analog (PYBNPM; a new cationic photoresponsive species). The cationic PYBNPM showed a better solvatochromic behavior contrary to PYBNP due to the efficient intramolecular charge transfer (ICT). The different crystal packing of PYBNP and that of hexafluorophosphate salts of PYBNPM led to distinct fluorescent properties in the solid-state. As a preliminary exploration, the cationic PYBNPM(PF6) was employed for the specific imaging of the mitochondria through multiple fluorescence channels in an aqueous medium. The cost-effective and viable design strategy demonstrated for the fabrication of a simple molecular triad holds the promise for the emergence of new functional fluorescent molecules to molecular materials for task-specific applications.

Dynamic Spin-Spin Interaction Observed as Interconversion of Chemical Bonds in Stepwise Two-Photon Induced Photochromic Reaction.

Biradicaloids in π-conjugated organic molecules have been extensively studied in recent years because of the fundamental insights into the chemical bonds and unique optical, electrical, and magnetic properties. Several studies have reported that the spin-spin interactions of biradicaloids with flexible molecular frameworks dynamically evolve correlating with molecular structural changes. Although these dynamic behaviors will provide important insights into the relationship between molecular structures and spin properties, studies on such behaviors have been limited to two-spin systems. Here, we investigated the stepwise photochromic properties of biphotochromic molecules involving multiple spin interactions by double-pulse laser flash photolysis. The one-photon photochromic reaction generates the o-biradical form as the open-closed form, which thermally isomerizes to the o-quinoidal form and reaches the thermal equilibrium state between them. The additional absorption of a photon by the open-closed form leads to the photochromic reaction of the other photochromic unit, resulting in the generation of unpaired spins at the p-position of the central aromatic bridge of the biradical or quinoidal form. Under the situation, while the interaction between the unpaired spins and the o-biradical preferentially produces the p-quinoidal form in which the antiferromagnetic interaction at the p-position is dominant, that between the spins and the o-quinoidal form kinetically produces the bis(o-quinoidal) form followed by the thermal isomerization to the thermodynamically stable p-quinoidal form. These dynamic spin-spin interactions along with the rearrangement of chemical bonds will give a deeper understanding of the singlet biradicaloids and that to bridge organic multiradicals in molecular systems to cooperative spin behaviors in bulk materials.

Progress and trends in self-assembly driven fluorescent organic nanoparticles: A brief overview

Organic nanoparticles have recently attracted increasing attention in diverse field of applications in materials, sensing, biomedicine and others due to their extraordinary physico-optical properties and facile synthesis. These π-conjugated small organic molecules exhibit aggregation induced emission (AIE) property, photo/physical stability, good cytocompatibility as well as biodegradability and easy surface modulation ability. In this mini-review, we have summarized the progress and trends in the area of development of organic nanoparticles from π-conjugated small organic molecule. Some typical as well as few unconventional examples of AIE active systems are discussed and also focussed on their structure-property relationship to exhibit unique photophysical characteristics. To decipher the mechanistic aspect of AIE phenomenon, systematic structural design strategies are also discussed. Finally, development of optoelectronic devices, sensors, bioimaging probes, therapeutic agents, and AIE-active luminogens are exemplified with high tech innovations.

Light manipulation using organic semiconducting materials for enhanced photosynthesis

Photosynthetic microorganisms, such as algae, are sources of bioproducts. Because they require only sunlight, water, and carbon dioxide to grow, they have potential for future mitigation of CO2 emissions. However, inefficient growth of these organisms remains an issue for realizing these emission reductions, primarily in terms of photosynthetic efficiency, photoinhibition, and photolimitation. Here, we show how use of light filtration through semi-transparent films comprised of organic π-conjugated molecules and organic photovoltaic devices has the potential to improve the photosynthesis efficiency of algae and the total power generation of a combined organic photovoltaic/algal system. Experimental data are used to fit a photosynthetic model predicting algal photosynthetic growth given light intensity and light transmission through an organic photovoltaic device. This work demonstrates the feasibility of using an “agrivoltaic” system combining photosynthetic growth with electricity-producing organic photovoltaics and provides a template for exploring other blended applications of these technologies.

Hybrid Tetrameric Perylene Diimide Assemblies.

Organic photovoltaics have found utility as indoor light recycling devices providing an opportunity for the sustainable powering of IoT sensors and related smart electronics. In the report, two organic π-conjugated molecules consisting of four perylene diimide (PDI) chromophores each are presented and used as non-fullerene acceptors in indoor photovoltaic devices. The new materials consist of a dimeric N-annulated PDI core with single PDIs grafted onto the pyrrolic N-atom positions of the core. Compounds PDI4 e and PDI4 i are PDI tetramers and differ with PDI4 e having the terminal N-annulated PDI with pyrrolic N-atom distal to the core and PDI4 i having the terminal N-annulated PDI with pyrrolic N-atom proximal to the core. The structural and optoelectronic properties were investigated using NMR spectroscopy, optical absorption and emission spectroscopy, and cyclic voltammetry. The compounds exhibit typical optical signatures for PDIs but notable is that the addition of grafted PDI molecules prevents significant aggregation of the dimeric PDI core, as compared to a reference dimer. Use as non-fullerene acceptors in ternary bulk-heterojunction blends with the polymer FBT and fullerene PC61 BM lead to increased open-circuit voltages and power conversion efficiencies upwards of 13.7 % at 2000 lux light intensity.

Excited state quantum modulations in organic semiconductors

Organic semiconductors are formed by coupling organic molecules with π-conjugated structure through van der Waals interactions. Different from traditional inorganic semiconductors, such as silicon, the core element of organic semiconductors is carbon atom. Generally, in a π-conjugated organic molecule, carbon atoms are polymerized by sp or sp2 hybridization, forming small molecules or polymers due to the molecular size. Taking the polymer cis -polyacetylene as an example, each carbon atom has three 2sp2 hybrid orbitals in a plane and one 2p z orbital orthogonal to this plane. The 2sp2 orbitals give rise to three σ-bonds, two of which are formed with neighboring carbons and one with a hydrogen. The electron in the 2p z orbital of one carbon atom will be paired with the electron in the neighboring carbon’s 2p z orbital, such that this electron can be delocalized along the chain, denoted as π-electron. For such an organic molecule, according to the Peierls instability, it will be dimerized and form a single-double bond alternation lattice structure, behaving in a semiconductor character. Through doping, the conductivity of organic molecules can be greatly improved and become good conductors. This is the reason why the functional properties of organic molecules first attracted great attention in the 1970s, by which, Heeger, MacDiarmid and Shirakawa won the 2000 Nobel Prize in Chemistry. When organic molecules form solids, the packing of molecules will ultimately determine their functional properties and applications, such as organic polymer films and organic small molecule crystals. Over the past few decades, due to the flexibility and ease of processing, organic semiconductors have become very promising materials in functional devices, such as field-effect transistors, light-emitting diodes, solar cells, as well as organic spin valves. In general, the functional processes of these organic devices are closely related to the quantum dynamics of the formed excited states, including soliton, polaron, bipolaron, exciton, biexciton, charge transfer state, and trion, etc. These excited states are spatially localized due to the self-trapping effect of organic molecules, which arises from their strong electron-lattice interactions. Especially, these excited states behave in abundant and unusual charge-spin relations. For these reasons, the functional processes in organic devices are usually more unique and complex compared with conventional inorganic devices. To date, although many kinds of organic functional devices have been successfully developed, there are still many unsolved questions in their functional mechanisms. The excited state research is the key to solve these questions, and thus the core of organic semiconductor physics and device research. In view of this, through the combination of theoretical and experimental methods, we studied the quantum effects and modulation mechanisms of different excited states in organic semiconductors and their hetero structures. The main contents included are as follows: Quantum dynamics and field modulations for the photoexcitation, interfacial charge separation, and resonance energy transfer in organic photovoltaic systems; spin injection and transport in organic semiconductors, organic magnetoresistance effect, excited ferromagnetism, spin current and organic chiral spin effects. On the one hand, these studies greatly enrich the understanding for the physical phenomena and mechanisms in organic semiconductors; on the other hand, they should greatly promote the designs and applications of organic functional devices.

The optoelectronic properties of π-conjugated organic molecules based on terphenyl and pyrrole for BHJ solar cells: DFT / TD-DFT theoretical study

In the present paper, four π-conjugated materials, based on terphenyl and pyrrole, with A–D–A structure have been theoretically studied to propose new organic compounds to be used in the organic solar cell field. Moreover, the geometrical and optoelectronic properties of the designed molecules M1, M2, M3 and M4 have been computed after optimization in their fundamental states, using the quantum chemical method DFT / B3LYP/ 6−311G (d, p). Different parameters including HOMO and LUMO energy levels, bandgap energy, frontier molecular orbital (FMO), chemical reactivity indices, the density of states (DOS), Voc, electrostatic potential (ESP), and thermodynamic parameters at several temperatures in the range of 0-500 K have been determined. The absorption properties including the transition energy, the wavelengths (λmax), the excitation vertical energy, and the corresponding oscillator strengths of these molecules have been studied using the quantum chemical method TD−DFT / CAM–B3LYP / 6–311G (d, p). The obtained results of our studied compounds show that M3 (with 2H, 2'H-1, 1'-biisoindole moiety) as a donor group has special optoelectronic, absorption, and good photovoltaic characteristics. Thus, they can be utilized as an electron-donating in organic solar cells BHJ type.

Molecular Structure-Optical Property Relationship of Salicylidene Derivatives: A Study on the First-Order Hyperpolarizability.

The first-order hyperpolarizability of π-conjugated organic molecules is of particular interest for the fabrication of electro-optical modulators. Thus, we investigated the relationship between the molecular structure and the incoherent second-order nonlinear optical response (βHRS) of four salicylidene derivatives (salophen, [Zn(salophen)(OH2)], 3,4-benzophen, [Zn(3,4-benzophen)(OH2)]) dissolved in DMSO. For that, we employed the Hyper-Rayleigh Scattering technique with picosecond pulse trains. Our experimental results pointed out dynamic βHRS values between 32.0 ± 4.8 × 10-30 cm5/esu and 58.5 ± 8.0 × 10-30 cm5/esu at 1064 nm, depending on the molecular geometry of the salicylidene molecules. More specifically, the outcomes indicate a considerable increase of βHRS magnitude (∼30%) when in the ligands are incorporated the Zn(II) ion. We ascribed such results to the rise of the planarity of the π-conjugated backbone of the chromophores caused by the Zn(II). Furthermore, we observed an increase of ∼50% in dynamic βHRS when there is a replacement of one hydrogen atom (salophen molecule) by an acetophenone group (3,4-benzophen). This result is related to the increase of the effective π-electron number and the higher charge transfer induced at the excited state. All these findings were interpreted and supported in the light of time-dependent density functional theory (DFT) calculations. Solvent effects were considered in the quantum chemical calculations using the integral equation formalism variant of the polarizable continuum model.

Light Manipulation Using Organic Semiconducting Materials for Enhanced Photosynthesis

Photosynthetic microorganisms, such as algae, are sources of bioproducts and pharmaceuticals. As they require only sunlight and carbon dioxide to grow, they have potential for future mitigation of CO2 emissions. However, inefficiencies in the growth of these organisms remains an issue for realizing these emission reductions, primarily in terms of photosynthetic efficiency, photoinhibition, and photolimitation. Here, we show how the use of light filtration through semi-transparent films comprised of organic π-conjugated molecules and subsequent organic photovoltaic devices, has the potential to improve the photosynthetic efficiency of algae, and the total power generation of a combined organic photovoltaic/algae system. Experimental data is used to fit a photosynthetic model predicting algal photosynthetic growth given light intensity and light transmission through an organic photovoltaic device. This work demonstrates the feasibility of using a system combining photosynthetic growth with electricity-producing organic photovoltaics and provides a template for exploring other blended applications of these technologies.

Computational study of organic small molecules based on imidazolinone for photovoltaic applications

ABSTRACT Organic photovoltaic cells have received significant interest due to the low-cost manufacturing and compatibility to deposit on flexible substrate. However, this technology suffers from certain problems including lowest solar spectrum absorption by reason of the large band gap of donor materials. This could return to the lack of extensive studies concerning the effects of geometric structures and the energy levels associated with the synthesis of molecules. In this context, six organic π-conjugated molecules based on imidazolinone with low band gap have been studied theoretically using quantum methods namely density functional theory (DFT) and time dependent-density functional theory (TD-DFT). The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and chemicals descriptors of these molecules have been examined in this work. Spectroscopic and electronic properties have been reported in order to study the parameters which affect photovoltaic efficiency. The calculated results of these compounds show a lower HOMO energy, large absorption spectra, and good theoretical open circuit voltage. The findings of this work are helpful to enhance the performance of bulk hetero junction organic solar cells (BHJ).

Chiroptical Properties in Thin Films of π-Conjugated Systems.

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (gabs and glum) and on the application of ECD and CPL to derive structural information on aggregated states.

Efficient Synthesis of π-Conjugated Organic Molecules Utilizing Cross-Coupling Reactions and Application to Electronic Devices

The development of efficient synthetic methods of π-conjugated compounds by means of cross-coupling reactions are reported. Tetraarylethenes with four different substituents are synthetic targets. The Pt-catalyzed addition of diboron to alkynylsilanes to yield 1-aryl-1-silyl-2,2-diborylethenes with perfect stereoselectivity. Step by step chemoselective Suzuki-Miyaura couplings with aryl halides were performed to give a triarylsilylethene with perfect chemoselectivity. The silyl group was transformed to the aryl group with desilybromination, followed by Suzuki-Miyaura coupling, leading to the successful synthesis of the desired tetraarylated olefin. On the other hand, novel and versatile methods for the synthesis of various phenacene and thienophenacene derivatives by cross-coupling reactions were elucidated for application to organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). As one of representative phenacenes, picene is a polycyclic aromatic hydrocarbon consisting of five fused benzene rings with a W-shape structure, which exhibits high hole mobility. We successfully found the synthetic route of various substituted picenes. In addition, the synthesis of phenanthro [1,2-b : 8,7-b’] dithiophene (PDT) was achieved by replacing two terminal phenyl rings in picene with thiophene rings. The fabricated OFET devices based on C12-PDT-2 displayed a quite high mobility of up to 5.6 cm2 V−1 s−1. In regard to OPV devices, PDT-fluorinated benzothiadiazole (BT) polymers possessing alkyl side chains showed the highest power conversion efficiencies (PCE) of 9.0%.

Spin-Polarized Molecular Triplet States as Qubits: Phosphorus Hyperfine Coupling in the Triplet State of Benzoisophosphinoline.

Advances in quantum information science (QIS) require the development of new molecular materials to serve as microwave addressable qubits that can be read out optically. Laser photoexcitation of organic π-conjugated molecules often results in spin-polarized phosphorescent triplet states that can be readily observed and manipulated using time-resolved electron paramagnetic resonance (EPR) techniques. Photoexcitation of N-mesityl-1,8-naphthalimide (M-NMI) and its phosphorus analogues, 2-mesitylbenzoisophosphinoline (M-BIPD) and 2-mesitylbenzoisophosphinoline oxide (M-BIPDO) results in ultrafast spin-orbit charge-transfer intersystem crossing to form the corresponding phosphorescent triplet states M-3*NMI, M-3*BIPD and M-3*BIPDO. The ultrafast triplet formation dynamics, phosphorescence, and spin-polarized EPR spectra of these triplet states were examined. The most promising qubit candidate, M-3*BIPD, was examined using pulse-EPR to measure its spin relaxation times, and pulse electron-nuclear double resonance spectroscopy to perform a two-qubit CNOT gate using the phosphorus nuclear spin.

Identifying Atomic-Level Correlation between Geometric and Electronic Structure at a Metal–Organic Interface

The interface of π-conjugated organic molecules with metal surfaces plays a crucial role in determining the efficiency of organic-based electronic devices. Because the interface is buried under the...

Heteromolecular Bilayers on a Weakly Interacting Substrate: Physisorptive Bonding and Molecular Distortions of Copper-Hexadecafluorophthalocyanine.

Heteromolecular bilayers of π-conjugated organic molecules on metals, considered as model systems for more complex thin film heterostructures, are investigated with respect to their structural and electronic properties. By exploring the influence of the organic-metal interaction strength in bilayer systems, we determine the molecular arrangement in the physisorptive regime for copper-hexadecafluorophthalocyanine (F16CuPc) on Au(111) with intermediate layers of 5,7,12,14-pentacenetetrone and perylene-3,4,9,10-tetracarboxylic diimide. Using the X-ray standing wave technique to distinguish the different molecular layers, we show that these two bilayers are ordered following their deposition sequence. Surprisingly, F16CuPc as the second layer within the heterostructures exhibits an inverted intramolecular distortion compared to its monolayer structure.

3D global aromaticity in a fully conjugated diradicaloid cage at different oxidation states.

Aromaticity is a vital concept that governs the electronic properties of π-conjugated organic molecules and has long been restricted to 2D systems. The aromaticity in 3D π-conjugated molecules has been rarely studied. Here we report a fully conjugated diradicaloid molecular cage and its global aromaticity at different oxidation states. The neutral compound has an open-shell singlet ground state with a dominant 38π monocyclic conjugation pathway and follows the [4n + 2] Hückel aromaticity rule; the dication has a triplet ground state with a dominant 36π monocyclic conjugation pathway and satisfies [4n] Baird aromaticity; the tetracation is an open-shell singlet with 52 π-electrons that are delocalized along the 3D rigid framework, showing 3D global antiaromaticity; and the hexacation possesses D3 symmetry with 50 globally delocalized π-electrons, showing [6n + 2] 3D global aromaticity. Different types of aromaticity were therefore accessed in one molecular cage platform, depending on the symmetry, number of π-electrons and spin state.