Acetylene-based materials in organic photovoltaics

Solvent‐Free Synthesis of Core‐Functionalised Naphthalene Diimides by Using a Vibratory Ball Mill: Suzuki, Sonogashira and Buchwald–Hartwig Reactions

Solvent‐free synthesis by using a vibratory ball mill (VBM) offers the chance to access new chemical reactivity, whilst reducing solvent waste and minimising reaction times. Herein, we report the core functionalisation of N,N’‐bis(2‐ethylhexyl)‐2,6‐dibromo‐1,4,5,8‐naphthalenetetracarboxylic acid (Br₂‐NDI) by using Suzuki, Sonogashira and Buchwald–Hartwig coupling reactions. The products of these reactions are important building blocks in many areas of organic electronics including organic light‐emitting diodes (OLEDs), organic field‐effect transistors (OFETs) and organic photovoltaic cells (OPVCs). The reactions proceed in as little as 1 h, use commercially available palladium sources (frequently Pd(OAc)₂) and are tolerant to air and atmospheric moisture. Furthermore, the real‐world potential of this green VBM protocol is demonstrated by the double Suzuki coupling of a monobromo(NDI) residue to a bis(thiophene) pinacol ester. The resulting dimeric NDI species has been demonstrated to behave as an electron acceptor in functioning OPVCs.

Attosecond spectroscopy of size-resolved water clusters

Electron dynamics in water are of fundamental importance for a broad range of phenomena^1-3, but their real-time study faces numerous conceptual and methodological challenges^4-6. Here, we introduce attosecond size-resolved cluster spectroscopy and build up a molecular-level understanding of the attosecond electron dynamics in water. We measure the effect that the addition of single water molecules has on the photoionization time delays^7-9 of water clusters. We find a continuous increase of the delay for clusters containing up to 4-5 molecules and little change towards larger clusters. We show that these delays are proportional to the spatial extension of the created electron hole, which first increases with cluster size and then partially localizes through the onset of structural disorder that is characteristic of large clusters and bulk liquid water. These results suggest a previously unknown sensitivity of photoionization delays to electron-hole delocalization and indicate a direct link between electronic structure and attosecond photoionization dynamics. Our results offer novel perspectives for studying electron/hole delocalization and its attosecond dynamics.

Autotandem Catalysis: Inexpensive and Green Access to Functionalized Ketones by Intermolecular Iron-Catalyzed Amidoalkynylation/Hydration Cascade Reaction via N-Acyliminium Ion Chemistry

Iron-based catalysts were applied in cascade-type reactions for the synthesis of different carbonyl compounds. The reactions proceeded by a new iron-catalyzed cascade of alkynylation/hydration by using both the σ- and π-Lewis acid properties of iron salts. The alkynylation reactions of several endo and exocyclic acetoxylactams were achieved with three different catalysts including FeCl 3 .6H 2 O, FeCl 3 , and Fe(OTf) 3 showing the efficiency of σ-Lewis acidity of iron (III) salts in catalyzing the alkynylation reaction. We also demonstrated that the reaction sequence could be shortened by the direct use of hydroxylactams, leading to an environmentally friendly protocol, avoiding the need to perform unnecessary lengthy steps. A combination of the hard/soft iron Lewis acid properties was then used to implement an unprecedented tandem intermolecular alkynylation/intramolecular hydration sequence allowing expedient access to a new carbonyl structures from trivial materials.

A review of oligo(arylene ethynylene) derivatives in molecular junctions

Oligo(arylene ethynylene) (OAE) derivatives are the "workhorse" molecules of molecular electronics. Their ease of synthesis and flexibility of functionalisation mean that a diverse array of OAE molecular wires have been designed, synthesised and studied theoretically and experimentally in molecular junctions using both single-molecule and ensemble methods. This review summarises the breadth of molecular designs that have been investigated with emphasis on structure-property relationships with respect to the electronic conductance of OAEs. The factors considered include molecular length, connectivity, conjugation, (anti)aromaticity, heteroatom effects and quantum interference (QI). Growing interest in the thermoelectric properties of OAE derivatives, which are expected to be at the forefront of research into organic thermoelectric devices, is also explored.

Mode-Specific Vibrational Analysis of Exciton Delocalization and Structural Dynamics in Conjugated Oligomers

Exciton delocalization in organic semiconducting polymers, affected by structures at a molecular level, plays a crucial role in modulating relaxation pathways, such as charge generation and singlet fission, which can boost device efficiency. However, the structural diversity of polymers and broad signals from typical electronic spectroscopy have their limits when it comes to revealing the interplay between local structures and exciton delocalization. To tackle these problems, we apply femtosecond stimulated Raman spectroscopy in archetypical conjugated oligothiophenes with different chain lengths. We observed Raman frequency dispersions of symmetric bond stretching modes and mode-specific kinetics in the S₁ Raman spectra, which underpins the subtle and complex interplay between exciton delocalization and bond length alternation along the conjugation coordinate. Our results provide a more general picture of exciton delocalization in the context of molecular structures for conjugated materials.

Cross-Dehydrogenative Alkynylation: A Powerful Tool for the Synthesis of Internal Alkynes

Alkynes are among the most fundamentally important organic compounds and are widely used in synthetic chemistry, biochemistry, and materials science. Thus, the development of an efficient and sustainable method for the preparation of alkynes has been a central concern in organic synthesis. Cross-dehydrogenative coupling utilizing E-H and Z-H bonds in two different molecules can avoid the need for prefunctionalization of starting materials and has become one of the most straightforward methods for the construction of E-Z chemical bonds. This Review summarizes recent progress in the preparation of internal alkynes by cross-dehydrogenative coupling with terminal alkynes.

X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems

Customizable and technology-friendly functional materials are one of the mainstays of emerging organic electronics and optoelectronics. We show that recombination exciplexes of simple substituted diphenylacetylenes with tertiary amines can be a convenient source of tunable deep-blue emission with possible applications in organic electroluminescent systems. The optically inaccessible exciplexes were produced via recombination of radiation-generated radical ion pairs in alkane solution, which mimics charge transport and recombination in the active layer of practical organic light-emitting diodes in a simple solution-based experiment. Despite varying and rather poor intrinsic emission properties, diphenylacetylene and its prototypical methoxy (donor) or trifluoromethyl (acceptor) monosubstituted derivatives readily form recombination exciplexes with N,N-dimethylaniline and other tertiary amines that produce emission with maxima ranging from 385 to 435 nm. The position of emission band maximum linearly correlates with readily calculated gas-phase electron affinity of the corresponding diphenylacetylene, which can be used for fast computational prescreening of the candidate molecules, and various substituted diphenylacetylenes can be synthesized via relatively simple and universal cross-coupling reactions of Sonogashira and Castro. Together, the simple solution-based experiment, computationally cheap prescreening method, and universal synthetic strategy may open a very broad and chemically convenient class of compounds to obtain OLEDs and OLED-based multifunctional devices with tunable emission spectrum and high conversion efficiency that has yet not been seriously considered for these purposes.

Vinylene spacer effects of benzothiadiazole–quarterthiophene based conjugated polymers on transistor mobilities

Opposite crystallinity and charge carrier mobility variation tendency with increasing vinyl ratio.

The synthesis and properties of linear A-π-D-π-A type organic small molecule containing diketopyrrolopyrrole terminal units

A novel linear A-π-D-π-A-type organic small molecule Ph2(PDPP)₂ consisting diketopyrrolopyrrole (DPP) as acceptor unit, biphenylene as donor unit and acetylene unit as π-linkage has been successfully designed and synthesized. Its corresponding thermal, photophysical and electrochemical properties as well as the photoinduced charge-separation process were investigated. Ph2(PDPP)₂ exhibits high thermal stability and it can be soluble in common organic solvents such as chloroform and tetrahydrofuran. The photophysical properties show that DPP₂Ph₂ harvests sunlight over the entire visible spectrum range in the thin-film state (300-800nm). DPP₂Ph₂ has lower band gaps and appropriate energy levels to satisfy the requirement of solution-processable organic solar cells. The efficient photoinduced charge separation process was clearly observed between DPP₂Ph₂ with PC₆₁BM and the K_sv value was found to be as high as 2.13×10⁴M^-1. Therefore, these excellent properties demonstrate that the designed A-π-D-π-A-type small molecule Ph2(PDPP)₂ is the prospective candidate as donor material for organic photovoltaic material.

Design of Diketopyrrolopyrrole (DPP)-Based Small Molecules for Organic-Solar-Cell Applications

After the first report in 2008, diketopyrrolopyrrole (DPP)-based small-molecule photovoltaic materials have been intensively explored. The power conversion efficiencies (PCEs) for the DPP-based small-molecule donors have been improved up to 8%. Furthermore, through judicious structure modification, DPP-based small molecules can also be converted into electron-acceptor materials, and, recently, some exciting progress has been achieved. The development of DPP-based photovoltaic small molecules is summarized here, and the photovoltaic performance is discussed in relation to structural modifications, such as the variations of donor-acceptor building blocks, alkyl substitutions, and the type of conjugated bridges, as well as end-capped groups. It is expected that the discussion will provide a guideline in the exploration of novel and promising DPP-containing photovoltaic small molecules.

2,1,3-Benzooxadiazole, thiophene and benzodithiophene based random copolymers for organic photovoltaics: thiophene versus thieno[3,2-b]thiophene as π-conjugated linkers

Influence of spacers on the optoelectronic properties of polymers.

Unexpected multiple activated steps in the excited state decay of some bis(phenylethynyl)-fluorenes and -anthracenes

The temperature effect on the photophysical parameters of four acetylene-derivatives [bis(phenylethynyl)-anthracenes and -fluorenes with substituents of different electron acceptor efficiencies] has been investigated by absorption and emission spectroscopy, using stationary and pulsed (ns/fs resolution) techniques. The nature of the central nucleus (anthracene or fluorene) and the peripheral electron-withdrawing group (nitro or formyl) strongly affect the deactivation of the excited states of these push-pull molecules. In some cases the study evidenced an interesting role of two activated steps in the deactivation of the excited singlet state, namely an activated inter-system crossing to an upper triplet state of n,π* nature (previously hypothesized on the basis of TD-DFT calculations) and a sort of activated internal conversion, discussed also on the basis of maximum entropy method analysis of the fluorescence decay data. Nicely, an efficient ISC was found for the fluorene-derivatives where small energy gaps between S1 (π,π*) and Tn (n,π*) states had been calculated while no activated ISC was evidenced in the case of anthryl-derivatives where higher S1-Tn energy gaps are expected. A peculiar temperature effect for a fluorene-derivative was pointed out and also explained on the basis of quantum-mechanical calculations at the DFT level taking into account the solvation effects by means of the conductor-like polarizable continuum model CPCM. The presence of dual emission, at first evidenced by a shoulder in the emission spectrum of the fluorene-derivative featuring a peripheral formyl group in dichloromethane at low temperatures, was nicely confirmed by femtosecond up-conversion measurements at room temperature.

Modulating carrier transfer ability—linker effect on thieno[3,4-c]pyrrole-4,6-dione based conjugated polymers

Introduction of a vinyl linkage in a thieno[3,4-c]pyrrole-4,6-dione based conjugated polymer results in an electron mobility increase while the ethynylene-linked polymer shows ambipolar performances in air condition.

Synthesis, photophysical and thin-film self-assembly properties of novel fluorescent molecules with carbon-carbon triple bonds

Three novel fluorescent molecules with carbon-carbon triple bonds 2TBEA, 2TBDA and TEPEB are successfully designed and synthesized. Their thermal, photophysical, electrochemical, electronic and thin-film self-assembly properties were characterized. Three dyes showed typical photoluminescence (PL) emission behaviors, the PL intensities firstly increased and then decreased with gradually decreasing concentration. The appealing fluorescence properties indicated that three dyes could be used as good fluorescent materials. Additionally, the thin-film self-assembly behaviors of three dyes were also investigated. The microstructures of their optical microscopy (OM) images exhibited high flexibility. Furthermore, SEM and AFM surface morphology of these self-assembly nanostructures revealed that three well-defined long-range order of rod-like and tube-like self-assembly systems exhibited interesting morphology properties. Therefore, three compounds may be of great interest for the development of organic thin-film materials.

Direct synthesis of poly(p-phenyleneethynylene)s from calcium carbide

An efficient method for the preparation of poly(p-phenyleneethynylene)s (PPEs) from direct coupling reactions between aryl diiodides and inexpensive chemical feedstock calcium carbide is developed.

Pyromellitic Diimide-Ethynylene-Based Homopolymer Film as an N-Channel Organic Field-Effect Transistor Semiconductor

We report the synthesis and characterization of two solution-processable pyromellitic diimide (PyDI)-acetylene-based conjugated homopolymers. Adjacent PyDI cores were connected with triple bond linkages by reacting 3,6-dibromo-N,N'-dialkyl pyromellitic diimides with bis(tributylstannyl)acetylene under Stille coupling conditions. Cyclic voltammetry revealed that these polymers have sufficient electron affinity to accept electrons. Absorption spectra revealed that one polymer, with a simple octyl chain, has greater intermolecular interaction or conjugation after forming a thin film, and that film exhibited electron transport in top-gate bottom-contact mode organic field-effect transistor (OFET) devices. X-ray diffraction (XRD) and atomic force microscopy (AFM) results show that the octyl polymer is amorphous on the bulk scale. The polymer exhibited electron mobility of about 2 × 10^-4 cm² V^-1 s^-1 with on/off ratio of 10³ and is the simplest n-channel polymer yet reported. A 4-trifluoromethylphenethyl side chain did not result in measurable electron mobility. The octyl polymer exhibited negative Seebeck coefficient on the order of -40 μV/K in thermoelectric devices, further substantiating its n-channel activity. The demonstration of electron transport from such a simple polymer has opened a new path for obtaining n-channel semiconducting activity from polymer films.

Novel planar and star-shaped molecules: synthesis, electrochemical and photophysical properties

Three novel planar and star-shaped molecules containing thiophene-functionalized group and acetylenic spacers, 4-((5″-iodo-[2,2':5',2″-terthiophen]-5-yl)ethynyl)aniline (I3TEA), 4,4'-([2,2':5',2″-terthiophene]-5,5″-diylbis(ethyne-2,1-diyl))dianiline (3TDDA), 1,3,5-tris((5-((trimethylsilyl)ethynyl)thiophen-2-yl)ethynyl)benzene (TETEB) were synthesized through Sonogashira coupling reaction. Their photophysical and electrochemical properties were explored by UV-vis, photoluminescence (PL) emission and DFT calculation. Three compounds possess unusual phenomena of PL, with the concentration of gradually decreased, the intensity of PL firstly increased and then decreased, this may be due to the aggregate-enhanced emission (AEE) effect and aggregation-caused quenching (ACQ) effect. Their spectroscopic data demonstrate that D-π-A-π-D structured organic dye 3TDDA based on NH2 as donor units and 3T as acceptor units with acetylenic spacers π-conjugated chain between them exhibits good luminescent properties. Dye I3TEA with donor(D)-acceptor(A) structure shows good charge transfer because of amino functional group and iodine atom. Octupolar organic π-conjugated star-shaped molecule TETEB possesses excellent luminescent properties because of its unique structure applied in the optoelectronic field. The redox curves and results of DFT calculations suggest that three compounds have low ionization potential and low Eoxd. All the good properties demonstrate three compounds could be promising candidates for photovoltaic materials.

Theoretical study of phenylene-thiophene oligomers: structure-properties relationship

Theoretical results including geometrical characteristics, electronic structures, photophysical parameters (lowest excitation energies, electron affinities (EAs), ionization potentials (IPs), maximum of absorption and emission) and vibrational modes of some new oligomers, based on phenylene-thiophene motives, are investigated using Density Functional Theory DFT/B3LYP/6-31G(d) approach. The electronic and optical properties of phenylene-based derivatives can be tuned through the insertion of thiophene in the main oligomers backbone as well as the addition of alkoxy-substituent groups on 2 and 5 positions on phenylene groups. It can be noticed, that different conformational behaviors and steric effects take place. Then, an increase in conjugation length induces a decrease in the gaps energy and a bathochromic shift of absorption/emission spectra. Based on these computed results, which are consistent with the available experimental data, the correlation structure-properties is better understood, where these nanostructures show a great potential for opto-electronic devices.