Furan-Containing Chiral Spiro-Fused Polycyclic Aromatic Compounds: Synthesis and Photophysical Properties

Spiro-fused polycyclic aromatic compounds (PACs) have received growing interest as rigid chiral scaffolds. However, furan-containing spiro-fused PACs have been quite limited. Here, we design spiro[indeno[1,2-b][1]benzofuran-10,10′-indeno[1,2-b][1]benzothiophene] as a new family of spiro-fused PACs that contains a furan unit. The compound was successfully synthesized in enantiopure form and also transformed to its S,S-dioxide derivative and the pyrrole-containing analog via aromatic metamorphosis. The absorption and emission properties of the obtained furan-containing chiral spiro-fused PACs are apparently different from those of their thiophene analogs that have been reported, owing to the increased electron-richness of furan compared to thiophene. All of the furan-containing chiral spiro-fused PACs were found to be circularly polarized luminescent materials.

Ethyl 1-Butyl-2-(2-hydroxy-4-methoxyphenyl)-1H-benzo[d]imidazole-5-carboxylate

Ethyl 4-(butylamino)-3-nitrobenzoate upon “one-pot” nitro-reductive cyclization using sodium dithionite and substituted aldehyde in dimethyl sulphoxide affords ethyl 1-butyl-2-(2-hydroxy-4-methoxyphenyl)-1H-benzo[d]imidazole-5-carboxylate in an 87% yield. The structural characterization was determined by Fourier-transfer infrared spectroscopy (FT-IR), Proton nuclear magnetic resonance (1H-NMR), Carbon-13 nuclear magnetic resonance (13C-NMR), mass spectrometry, Ultraviolet-visible(UV-Vis), photoluminescence (PL), thin-film solid emission spectra, cyclic voltammetry (CV) and thermogravimetric (TGA) analysis. Molecular electrostatic potential (MEP) was studied to determine the reactive sites of the molecule.

The impact of thermal treatment on the performance of benzo[1,2-b:4,5-b']difuran-based organic solar cells

The new low bandgap benzo[1,2-b:4,5-b']difuran (BDF)-based organic small molecule, namely B1, was synthesized by Stille coupling polymerization reactions. B1 was found to be soluble in common organic solvents such as chloroform, toluene and chlorobenzene with excellent film forming properties. The structure of B1 was verified by 1H NMR, GC-MS and elemental analysis. The B1 films exhibit broad absorption bands from 300 to 750 nm. The hole mobility of B1 : PC61BM (1 : 1, w/w) blend film reached up to 7.7 × 10-2 cm V-1 s-1 after thermal annealing by the space-charge-limited current method. BHJ organic solar cells (OSCs) were fabricated with a device structure of ITO/PEDOT : PSS/B1 : C61BM/LiF/Al. When the active layer was thermally annealed at 120 °C, B1 showed the best photovoltaic performance, with a PCE up to 5.0%. We also studied the connection between the morphologies of the active layers and the photovoltaic performance by AFM, PL, etc. Our observation will guide future design for even better small molecules for highly efficient OSCs.

Rigidly Fused Spiro-Conjugated π-Systems

Spiro-fused π-systems have gained considerable attention for their application as semiconductors in molecular electronics. Here, a synopsis regarding recent breakthroughs in ladderized spirobifluorenes and indeno-spirobifluorenes, along with further spiro-condensed heteroatomic hydrocarbons with donor-acceptor moieties, is provided. Additionally, an extended range of rigid spirobifluorene polymers and specific doubly linked spiro-systems with partial chiral character is discussed. The diverse applications of the aforementioned structures are thoroughly evaluated.

Three-Dimensional Spirothienoquinoline-Based Small Molecules for Organic Photovoltaic and Organic Resistive Memory Applications

A new electron-rich spirothienoquinoline unit, ^tBuSAF-Th, has been developed via incorporation of a thienyl unit instead of a phenyl unit into the six-membered ring of the spiroacridine (SAF) and utilized for the first time as a building block for constructing small-molecule electron donors in organic solar cells (OSCs) and as active layers in organic resistive memory devices. The resulting three-dimensional spirothienoquinoline-containing 1-4 exhibit high-lying highest occupied molecular orbital (HOMO) energy levels. By the introduction of electron-deficient benzochalcogenodiazole linkers, with the chalcogen atoms being varied from O to S and Se, a progressively lower lowest unoccupied molecular orbital (LUMO) energy level has been achieved while keeping the HOMO energy levels similar. This strategy has allowed an enhanced light-harvesting ability without compromising open-circuit voltage (V_oc) in vacuum-deposited bulk heterojunction OSCs using 1-4 as donors and C₇₀ as the acceptor. Good photovoltaic performances with power conversion efficiencies (PCEs) of up to 3.86% and high short-circuit current densities (J_sc) of up to 10.84 mA cm^-2 have been achieved. In addition, organic resistive memory devices fabricated with these donor-acceptor small molecules exhibit binary logic memory behaviors with long retention times and high on/off current ratios. This work indicates that the spirothienoquinoline moiety is a potential building block for constructing multifunctional organic electronic materials.

Designing a near-infrared circularly polarized luminescent dye by dissymmetric spiro-fusion

A novel spiro-fused terrylene dimer (SDT) was designed and synthesized by a dissymmetric spiro-fusion strategy. The spiro-conjugation effect caused a distinct red-shift and enhancement of the absorption spectrum. Two chiral enantiomers of SDT have been absolutely resolved and identified in combination with theoretical calculations. Circularly polarized luminescence (CPL) measurement revealed its potential as a near-infrared chiral luminescent material.

Prediction of the Synthesis of Spiro Derivatives by Double Intramolecular Aromatic Electrophilic Substitution Using Reactivity Indices

The synthesis of heterocyclic spirobifluorene (SBF) analogs generally requires long and complicated synthetic pathways. Despite this synthetic effort, such structural modification allows the (opto)electronic properties of this remarkable three-dimensional node to be tuned especially for molecular electronic applications. For this reason, the development of a simple, robust, and efficient synthetic methodology to introduce various heterocycles in place of classical phenyl rings in the spirofluorene motif is highly and timely desirable. In this context, we describe herein our efforts to develop a straightforward and efficient synthesis leading to replacement of 2 phenyl rings by various heterocycles in spiro compounds from 2,2'-dibromobenzophenone. As the same procedure to form fully heterocyclic compounds failed, an original theoretical approach based mainly on the uncommon Fukui dual function was developed in order to determine clearly the limitation of this strategy and provide an efficient predictive tool. Indeed, such calculation allows prediction of the thermodynamic and kinetic aspects of the synthesis of spiro derivatives using a double aromatic electrophilic substitution. If this procedure reproduces well our experimental results focused on (heterocyclic) SBF compounds, it can be certainly adapted and generalized to other intramolecular substitutions.

Morphology of a Bulk Heterojunction Photovoltaic Cell with Low Donor Concentration

Atomistic nonequilibrium molecular dynamics simulations have been used to model the morphology of small-molecule bulk heterojunction films formed by vapor deposition as used in organic photovoltaics. Films comprising C₆₀ and 1, 5, 10, and 50 wt % of 1,1-bis[4-bis(4-methylphenyl)aminophenyl]cyclohexane (TAPC) were compared to films of neat C₆₀. The simulations suggest that if holes can hop between donor molecules separated by as little as 1.2-1.5 nm, then a TAPC concentration of 5 wt % is sufficient to form a percolating donor network and facilitate charge extraction. The results provide an explanation for why low donor content organic photovoltaics can still have high efficiencies. In addition, the roughness, porosity, and crystallinity of the films were found to decrease with increasing TAPC content.

Diketopyrrolopyrrole-based acceptors with multi-arms for organic solar cells

Three diketopyrrolopyrrole-based small molecule acceptors consisting of a spirobifluorene (SBF) core unit and diketopyrrolopyrrole dicyanovinyl (DPPDCV) units as the arms have been synthesized for organic solar cells.

Synthesis, structure and optoelectronic properties of rigid 3D tetraindeno-fused spirofluorene with thioxanthene or dioxothioxanthene substitutes

This work reports the synthesis and characterization of two blue emitters. Their 3D rigid structures are the appealing feature.

Bifunctional Heterocyclic Spiro Derivatives for Organic Optoelectronic Devices

A series of heterocyclic spiro derivatives has been successfully synthesized and characterized by photophysical and electrochemical studies. Taking advantage of their excellent hole-transporting properties, highly efficient small-molecular organic photovoltaic devices based on these heterocyclic compounds as donors with very low dopant concentrations have been prepared; particularly, a high open-circuit voltage of up to 1.10 V and a power conversion efficiency of up to 5.12% have been realized. In addition, most of these heterocyclic spiro derivatives are found to be highly emissive in solutions with photoluminescence quantum yields of up to 0.91, and high-performance deep-blue-emitting organic light-emitting diodes (OLEDs) have been achieved. Such devices exhibit a stable deep blue emission with CIE coordinates of (0.16, 0.04) and high external quantum efficiencies of up to 4.7%, which is one of the best values among the reported OLEDs with CIEy < 0.08.

A 9,9′-spirobi[9H-fluorene]-cored perylenediimide derivative and its application in organic solar cells as a non-fullerene acceptor

A structurally orthogonal molecule (SBF-PDI4) with a 9,9′-spirobi[9H-fluorene] (SBF) core and four perylenediimide (PDI) at periphery was developed for use in polymer solar cells. Proper LUMO energy level (−4.1 eV) and good light absorption ability over 450–550 nm make it an excellent non-fullerene acceptor.

A bifunctional copolymer additive to utilize photoenergy transfer and to improve hole mobility for organic ternary bulk-heterojunction solar cell

To realize the high efficiency organic photovoltaics (OPVs), two critical requirements have to be fulfilled: (1) increasing the photon energy absorption range of the active layer, and (2) improving charge separation and transport in the active layer. This study reports the utilization of THC8, a novel fluorescence-based polymer containing propeller-shaped di-triarylamine and fluorene moieties in the active layer consisting of poly-3-hexylthiophene and [6,6]-phenyl-C61-butyric acid methyl ester to form a ternary bulk heterojunction. The results showed that the high absorbance and strong fluorescence of THC8 at 420 and 510 nm, respectively, broadened the spectral absorption of the OPV, possibly through Förster resonance energy transfer. In addition, the morphology of the device active layer was improved with the addition of a suitable amount of THC8. Consequently, the charge transport property of the active layer was improved. The best power conversion efficiency (PCE) of the device with THC8 was 3.88%, a 25% increase compared to the PCE of a pristine OPV.

Monodisperse Low-Bandgap Macromolecule-Based 5,5'-Bibenzo[c][1,2,5]thiadiazole Swivel Cruciform for Organic Solar Cells

A novel low bandgap star-like macromolecule was synthesized and applied as electron donor material in the bulk heterojunction solar cells, in which the 5,5'-bibenzo[c][1,2,5]thiadiazole was used as the central node, in conjunction with four conjugated donor-acceptor arms. Compared with the corresponding small molecule with first generation arms, the macromolecule with second generation branches exhibited significantly enhanced photovoltaic device performances (blended with PC71BM as the active layer) due to dramatically improved short-circuit current density (J_sc) and fill factor (FF). The improvement in J_sc and FF can be attributed to the more broad absorption and the more favorable phase separation when comparing a monodisperse macromolecule with the second generation arms (SFTBT) with a small molecule with first generation branches (DFTBT).