Stannyl Derivatives of Naphthalene Diimides and Their Use in Oligomer Synthesis

The Excited-State Lifetime of Poly(NDI2OD-T2) Is Intrinsically Short

Conjugated polymers composed of alternating electron donor and acceptor segments have come to dominate the materials being considered for organic photoelectrodes and solar cells, in large part because of their favorable near-infrared absorption. The prototypical electron-transporting push-pull polymer poly(NDI2OD-T2) (N2200) is one such material. While reasonably efficient organic solar cells can be fabricated with N2200 as the acceptor, it generally fails to contribute as much photocurrent from its absorption bands as the donor with which it is paired. Moreover, transient absorption studies have shown N2200 to have a consistently short excited-state lifetime (∼100 ps) that is dominated by a ground-state recovery. In this paper, we investigate whether these characteristics are intrinsic to the backbone structure of this polymer or if these are extrinsic effects from ubiquitous solution-phase and thin-film aggregates. We compare the solution-phase photophysics of N2200 with those of a pair of model compounds composed of alternating bithiophene (T2) donor and naphthalene diimide (NDI) acceptor units, NDI-T2-NDI and T2-NDI-T2, in a dilute solution. We find that the model compounds have even faster ground-state recovery dynamics (τ = 45, 27 ps) than the polymer (τ = 133 ps), despite remaining molecularly isolated in solution. In these molecules, as in the case of the N2200 polymer, the lowest excited state has a T2 to NDI charge-transfer (CT) character. Electronic-structure calculations indicate that the short lifetime of this state is due to fast nonradiative decay to the ground state (GS) promoted by strong CT-GS electronic coupling and strong electron-vibrational coupling with high-frequency (quantum) normal modes.

A Distannylated Monomer of a Strong Electron-Accepting Organoboron Building Block: Enabling Acceptor-Acceptor-Type Conjugated Polymers for n-Type Thermoelectric Applications

Acceptor-acceptor (A-A) copolymerization is an effective strategy to develop high-performance n-type conjugated polymers. However, the development of A-A type conjugated polymers is challenging due to the synthetic difficulty. Herein, a distannylated monomer of strong electron-deficient double B←N bridged bipyridine (BNBP) unit is readily synthesized and used to develop A-A type conjugated polymers by Stille polycondensation. The resulting polymers show ultralow LUMO energy levels of -4.4 eV, which is among the lowest value reported for organoboron polymers. After n-doping, the resulting polymers exhibit electric conductivity of 7.8 S cm^-1 and power factor of 24.8 μW m^-1  K^-2 . This performance is among the best for n-type polymer thermoelectric materials. These results demonstrate the great potential of A-A type organoboron polymers for high-performance n-type thermoelectrics.

Dodecatwistarene Imides with Zigzag-Twisted Conformation for Organic Electronics

1D nonplanar graphene nanoribbons generally have three possible conformers: helical, zigzag, and mixed conformations. Now, a kind of 1D nonplanar graphene nanoribbon, namely dodecatwistarene imides featuring twelve linearly fused benzene rings, was obtained by bottom-up synthesis of palladium-catalyzed Stille coupling and C-H activation. Single-crystal X-ray diffraction analyses revealed that it displays a zigzag-twisted conformation caused by steric hindrance between imide groups and neighboring annulated benzene rings with the pendulum angle of 53°. This conformation is very stable and could not convert into other conformations even when heated up to 250 °C for 6 h. Despite of the highly twisted topology, organic field-effect transistor based on it exhibits electron mobility up to 1.5 cm²  V^-1  s^-1 after annealing.

Buchwald-Hartwig Coupling at the Naphthalenediimide Core: Access to Dendritic, Panchromatic NIR Absorbers with Exceptionally Low Band Gap

The first successful Buchwald-Hartwig reaction at the naphthalenediimide core is reported, leading to the coupling of diverse secondary aromatic amines including dendritic donors. The G1-dendrimer-based donor exhibit blackish color, providing access to black absorbing systems. λ_onset values up to 1070 nm was achieved, which is the maximum from a single NDI scaffold. These dyes also manifest multielectron reservoir properties. A total of eight-redox states with a band gap of ∼0.95 eV was accomplished.

Efficient Electron Mobility in an All-Acceptor Napthalenediimide-Bithiazole Polymer Semiconductor with Large Backbone Torsion

An all-acceptor napthalenediimide-bithiazole-based co-polymer, P(NDI2OD-BiTz), was synthesized and characterized for application in thin-film transistors. Density functional theory calculations point to an optimal perpendicular dihedral angle of 90° between acceptor units along isolated polymer chains; yet optimized transistors yield electron mobility of 0.11 cm²/(V s) with the use of a zwitterionic naphthalene diimide interlayer. Grazing incidence X-ray diffraction measurements of annealed films reveal that P(NDI2OD-BiTz) adopts a highly ordered edge-on orientation, exactly opposite to similar bithiophene analogs. This report highlights an NDI and thiazole all-acceptor polymer and demonstrates high electron mobility despite its nonplanar backbone conformation.

Carbocyclization approaches to electron-deficient nanographenes and their analogues

Versatile π-aromatic building blocks and selective coupling transformations enable rapid assembly of complex electron-deficient molecules, useful as n-type organic semiconductors.

X-Shaped Oligomeric Pyromellitimide Polyradicals

The synthesis of stable organic polyradicals is important for the development of magnetic materials. Herein we report the synthesis, isolation, and characterization of a series of X-shaped pyromellitimide (PI) oligomers (X_n-R, n = 2-4, R = Hex or Ph) linked together by single C-C bonds between their benzenoid cores. We hypothesize that these oligomers might form high-spin states in their reduced forms because of the nearly orthogonal conformations adopted by their PI units. ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopies confirmed the isolation of the dimeric, trimeric, and tetrameric homologues. X-ray crystallography shows that X₂-Ph crystallizes into a densely packed superstructure, despite the criss-crossed conformations adopted by the molecules. Electrochemical experiments, carried out on the oligomers X_n-Hex, reveal that the reductions of the PI units occur at multiple distinct potentials, highlighting the weak electronic coupling between the adjacent redox centers. Finally, the chemically generated radical anion and polyanion states, X_n-Hex^•- and X_n-Hex^n(•-), respectively, were probed extensively by UV-vis-NIR absorption, EPR, and electron nuclear double resonance (ENDOR) spectroscopies. The ENDOR spectra of the radical monoanions X_n-Hex^•- reveal that the unpaired electron is largely localized on a single PI unit. Further reductions of X_n-Hex^•- yield EPR signals (in frozen solutions) that can be assigned to spin-spin interactions in X₂-Hex^2(•-), X₃-Hex^3(•-), and X₄-Hex^4(•-). Taken together, these findings demonstrate that directly linking the benzene rings of PIs with a single C-C bond is a viable method for generating stabilized high-spin organic anionic polyradicals.

A Class of Electron-Transporting Vinylogous Tetrathiafulvalenes Constructed by the Dimerization of Core-Expanded Naphthalenediimides

The combination of the (1,3-dithiol-2-ylidene)malononitrile (DTYM) and/or (1,3-dithiol-2-ylidene)acetonitrile (DTYA) moieties with naphthalenediimide (NDI) core affords two singly linked NDI-based dimers, (DTYM-NDI-DTYA)₂ (D1) and (NDI-DTYA)₂ (D2), which both contain a dicyano-substituted vinylogous tetrathiafulvalene (TTF) unit. The synthesis, thermal/optical/electrochemical properties of D1 and D2, and their primary applications in n-channel organic thin film transistors are studied. The results demonstrate that these NDI-fused vinylogous TTFs are excellent electron acceptors, and their further applications are promising.

Comparison of 3D non-fullerene acceptors for organic photovoltaics based on naphthalene diimide and perylene diimide-substituted 9,9′-bifluorenylidene

Tetra(PDI) derivatives of non-planar cores have previously been used as acceptors in OPVs; here a tetra(NDI) is shown to be a viable alternative, although, for the pair of acceptors and the donor polymers used here, the PDI species performs better.

Polycyclic aromatic hydrocarbons with orthogonal tetraimides as n-type semiconductors

We report two novel n-type polycyclic aromatic hydrocarbons bearing orthogonal tetraimides with high electron mobilities.

Halochromic and hydrochromic squaric acid functionalized perylene bisimide

A squaric acid functionalized perylene bisimide senses solvent polarity, pH and humidity through the intramolecular charge transfer initiated by protonation/deprotonation.

Pyridine-bridged diketopyrrolopyrrole conjugated polymers for field-effect transistors and polymer solar cells

Five different pyridine-bridged diketopyrrolopyrrole-based polymers with variable energy levels were applied in organic field-effect transistors and polymer solar cells.

Tailor-made rylene arrays for high performance n-channel semiconductors

Rylene dyes, made up of naphthalene units linked in peri-positions, are emerging as promising key building blocks to create π-functional materials. Chemists have found uses for these ribbonlike structures in a wide range of applications of optoelectronic devices. Because their structure combines two sets of six-membered electron-withdrawing dicarboxylic imide rings, rylene diimides exhibit enhanced solubility, excellent chemical and thermal stabilities, high electron affinities, and remarkable electron-transporting properties. Among them, perylene diimide (PDI) and naphthalene diimide (NDI) derivatives are important representatives improving the performance of electron-transporting technologies, relative to their p-channel counterparts. Pioneering works by Müllen and Langhals have inspired chemists to extend the π-conjugation along the peri-positions of rylene diimides, which generally results in impressive bathochromic shifts and a nearly linear increase in the extinction coefficient. In addition, in the past years, researchers have focused on π-expansion of NDI or PDI systems through bay-functionalization with carbocyclic and heterocyclic rings annulated onto the skeleton. However, chemists have rarely investigated lateral expansion via both bay- and nonbay-functionalization to construct homologous series of rylene arrays with different electronic delocalization and fine-tuned flexible linkage. This is probably due to the lack of effective procedures for the (multi) carbon-carbon formation and annulation of electron-deficient rylene imide units. In this Account, we discuss our recent progress in the design and synthesis of laterally expanded rylene dyes based on homocoupling and cross-coupling reactions of core-functionalized PDIs and NDIs to achieve novel high performance n-channel organic semiconducting materials. These new achievements offer us opportunities to learn fundamental issues about how chemical and physical properties alter with incremental changes in structure. We highlight synthetic methodology of transition-metal mediated coupling reactions (and/or C-H transformation) for singly linked, doubly linked, and fully conjugated triply linked oligoPDIs, and further for the construction of hybrid rylene arrays via bay- and/or nonbay-functionalization. In addition, we summarize the informative correlations between the molecular structures and their optoelectronic properties, especially the modulation of progressively red-shifted absorption maxima and positive shifts in the redox potentials. This decreases the energy gaps and increases the electron-accepting abilities through expansion of π-system, which has direct impacts on the compounds' potential applications in optoelectronic devices. Finally, we introduce the promising applications of these laterally expanded rylene dyes as exceptional high performance n-channel semiconductors in organic field-effect transistors (OFETs) and competitive candidates for non-fullerene acceptors in high efficient organic photovoltaic devices (OPVs).

Strategies for the synthesis of functional naphthalene diimides

Naphthalene diimides, which have for a long time been in the shadow of their higher homologues the perylene diimides, currently belong to the most investigated classes of organic compounds. This is primarily due to the initial synthetic studies on core functionalization that were carried out at the beginning of the last decade, which facilitated diverse structural modifications of the naphthalene scaffold. Compounds with greatly modified optical and electronic properties that can be easily and effectively modulated by appropriate functionalization were made accessible through relatively little synthetic effort. This resulted in diverse interesting applications. The electron-deficient character of these compounds makes them highly valuable, particularly in the field of organic electronics as air-stable n-type semiconductors, while absorption bands over the whole visible spectral range through the introduction of core substituents enabled interesting photosystems and photovoltaic applications. This Review provides an overview on different approaches towards core functionalization as well as on synthetic strategies for the core expansion of naphthalene diimides that have been developed mainly in the last five years.

Direct core functionalisation of naphthalenediimides by iridium catalysed C–H borylation

We report the first boron-substituted naphthalenediimides (NDIs), prepared by iridium catalysed C–H activation. Both mono- and diborylated products are available, which have been further elaborated by Suzuki–Miyaura coupling.

Charge transfer dynamics in squaraine-naphthalene diimide copolymers

The synthesis of an alternating squaraine-naphthalene diimide donor-acceptor low band gap polymer (1.14-1.40 eV) as well as its monomolecular analogue is presented. Spectroelectrochemistry experiments and transient absorption spectroscopy in the fs-time regime reveal an ultrafast population of a charge separated state for both polymer and monomer. Local excitation of the squaraine moiety is followed by population of intermediate states, presumably charge transfer states, followed by full charge separation, which occurs within a ca. 2 ps. Charge recombination takes place within 5.2 ps, probably because the system is close to the Marcus optimal region for barrierless ET. For the polymer, measurements of the transient absorption anisotropy show that neither charge nor does energy transfer take place within the lifetime of the charge separated state, indicating that this state is essentially confined within one donor-acceptor pair.

Recent progress of core-substituted naphthalenediimides: highlights from 2010

Core-substituted naphthalenediimides (cNDIs) are rapidly emerging as a powerful strategy to create functional nanomaterials and their implications in biological and supramolecular chemistry are significant. Recent developments in the synthesis of cNDIs have allowed several groups to probe the function of this interesting class of dye molecules in a molecular and supramolecular sense. Core-substitution of the NDI can be seen as an opportunity to extend the planar, rigid core and could be used to prepare novel structures for applications in organic, biosupramolecular chemistry, biomedicine, materials science and organic solar cells. In this Emerging Area, we provide up-to-date recent progress in the field of cNDIs. We begin with a general discussion and the applications of cNDIs in the field of supramolecular chemistry i.e. generation of nanostructures such as vesicles and nanotubes etc., and we also discuss advances in artificial photosynthesis. Following this is a section on their implications in the field of sensors, particularly DNA intercalation, anion sensing and NDI based pH sensors. Finally, we explore the recent development of cNDIs in organic solar cell applications. We conclude with our views on the prospects of cNDIs in future research.

Stable solution-processed molecular n-channel organic field-effect transistors

A new solution-processable small-molecule containing electron-poor naphthalene diimide and tetrazine moieties has been synthesized. The optimized spin-coated n-channel OFETs on glass substrate shows electron mobility value up to 0.15 cm(2) V(-1) s(-1) . Inkjet-printed OFETs are fabricated in ambient atmosphere on flexible plastic substrates, which exhibits an electron mobility value up to 0.17 cm(2) V(-1) s(-1) and also shows excellent environmental and operational stability.

2,6-Diacylnaphthalene-1,8:4,5-bis(dicarboximides): synthesis, reduction potentials, and core extension

2,6-Diacyl derivatives of naphthalene-1,8:4,5-bis(dicarboximide)s have been synthesized via Stille coupling reactions of the corresponding 2,6-distannyl derivative with acyl halides. Reaction of these diketones with hydrazine gave phthalazino[6,7,8,1-lmna]pyridazino[5,4,3-gh][3,8]phenanthroline-5,11(4H,10H)-dione fused-ring derivatives. The products were characterized by UV-vis absorption spectroscopy and electrochemistry, modeled using density functional theory calculations, and, in some cases, studied and compared using single-crystal X-ray diffraction.