Spirobifluorene-Based Conjugated Polymers for Polymer Solar Cells with High Open-Circuit Voltage

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.

Spirobifluorene-Based o-Carboranyl Compounds: Insights into the Rotational Effect of Carborane Cages on Photoluminescence

9,9'-Spirobifluorene-based closo-o-carboranyl (SFC1 and SFC2) compounds and their nido-derivatives (nido-SFC1 and nido-SFC2) were prepared and characterized. The two closo-compounds displayed major absorption bands assignable to π-π* transitions involving the spirobifluorene group, as well as weak intramolecular charge-transfer (ICT) transitions between the o-carboranes and their spirobifluorene moieties. The nido-compounds exhibited slightly blueshifted absorption bands resulting from the absence of the ICT transitions corresponding to the o-carborane moieties due to the anionic character of the nido-o-carboranes. While SFC1 exhibited only high-energy emissions in THF at 298 K (only from locally excited (LE) states assignable to π-π* transitions on the spirobifluorene group), remarkable emissions in the low-energy region were observed in the rigid state such as in THF at 77 K and in the film state. SFC2 displayed intense emissions in the low-energy region in all states. The fact that neither of the nido-derivatives of SFC1 and SFC2 exhibited low-energy emissions and the TD-DFT calculation results of each closo-compound clearly verified that the low-energy emission was based on ICT-based radiative decay. The conformational barriers from each relative energy calculation upon changing the dihedral angles around the o-carborane cages for both compounds confirmed that the rotation of the o-carborane cages and terminal phenyl rings for SFC1 is freer than that for SFC2.

Photophysical Properties of Spirobifluorene-Based o-Carboranyl Compounds Altered by Structurally Rotating the Carborane Cages

9,9′-Spirobifluorene-based o-carboranyl compounds C1 and C2 were prepared and fully characterized by multinuclear nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The solid-state structure of C1 was also determined by single-crystal X-ray diffractometry. The two carboranyl compounds display major absorption bands that are assigned to π−π* transitions involving their spirobifluorene groups, as well as weak intramolecular charge-transfer (ICT) transitions between the o-carboranes and their spirobifluorene groups. While C1 only exhibited high-energy emissions (λ_em = ca. 350 nm) in THF at 298 K due to locally excited (LE) states assignable to π−π* transitions involving the spirobifluorene group alone, a remarkable emission in the low-energy region was observed in the rigid state, such as in THF at 77 K or the film state. Furthermore, C2 displays intense dual emissive patterns in both high- and low-energy regions in all states. Electronic transitions that were calculated by time-dependent-DFT (TD-DFT) for each compound based on ground (S₀) and first-excited (S₁) state optimized structures clearly verify that the low-energy emissions are due to ICT-based radiative decays. Calculated energy barriers that are based on the relative energies associated with changes in the dihedral angle around the o-carborane cages in C1 and C2 clearly reveal that the o-carborane cage in C1 rotates more freely than that in C2. All of the molecular features indicate that ICT-based radiative decay is only available to the rigid state in the absence of structural fluctuations, in particular the free-rotation of the o-carborane cage.

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.

Effect of carbon hybridization in 9H-fluorene unit on the photovoltaic properties of different fluorene-based conjugated polymers

To investigate the effect of different carbon hybridization in 9H-fluorene on the resultant polymers, a series of donor–acceptor conjugated polymers have been synthesized by copolymerizing substituted 9H-fluorenes with triazoloquinoxaline. All polymers exhibit good solution-processability and broad absorption in 350–1000 nm region with narrow optical bandgaps ranging from 1.27 eV to 1.55 eV. The results indicate that fluorene functionalization via different carbon hybridization on 9-position could be an effective strategy to fine-tune the absorption, energy levels, and photovoltaic performance of the polymers. 9-Position sp2-hybridized carbon in fluorene could enhance absorption of ultraviolet-visible and form good morphology of blending layers, through its planar rigid structure. All polymer solar cells devices exhibited moderate performance with the best power conversion efficiency of 3.02% achieved based on P2. Compared to ladder-type multifused fluorene, carbon-hybridized 9H-fluorene units can be a very useful building block for constructing narrow bandgap polymers with facile synthesis, and even interesting optoelectronic properties.

Incorporation of spirobifluorene regioisomers in electron-donating molecular systems for organic solar cells

We report herein the synthesis and characterizations of new small molecules for organic photovoltaics including diketopyrrolopyrrole and spirobifluorene.

Organosoluble and optically active polyamides based on axially asymmetric 9,9′-spirobifluorene moieties

Optically active polyamides containing axially asymmetric 9,9′-spirobifluorene units in the main chain were synthesized for the first time by low-temperature solution polycondensation of ( R)-2,2′-bis(4-amino-2-trifluoromethylphenoxy)-9,9′-spirobifluorene with various diacyl chlorides. All these polymers were obtained in quantitative yields with inherent viscosities of 0.28–0.43 dL g−1. These polyamides were amorphous and readily soluble in many organic solvents such as N-methyl-2-pyrolidone, N, N-dimethylacetamide (DMAc), dimethyl sulfoxide, and tetrahydrofuran. The glass transition temperatures of these polyamides were recorded in the range of 199–261°C by differential scanning calorimetry, and the 10% weight loss temperatures were over 410°C both under nitrogen and in air atmospheres. The specific rotations of these chiral polyamides oscillated between 65.0° and 127.8° depending on the structures of the diacid chlorides. All the optically active polymers exhibited high chiral stability in solid state or in DMAc solvent at high temperature due to incorporation of chiral spirobifluorone unit in polymer backbones. The ultraviolet–visible and circular dichroism spectroscopic properties of these chiral polyamides in solutions were also studied.

9-arylidene-9H-fluorene-containing polymers for high efficiency polymer solar cells

9-Arylidene-9H-fluorene containing donor-acceptor (D-A) alternating polymers P1 and P2 were synthsized and used for the fabrication of polymer solar cells (PSCs). High and low molecular weight P1 (HMW-P1 and LMW-P1) and high molecular weight P2 were prepared to study the influence of molecular weight and the position of alkoxy chains on the photovoltaic performance of PSCs. HMW-P1:PC71BM-based PSCs fabricated from 1,2-dichlorobenzene (DCB) solutions showed a power conversion efficiency (PCE) of 6.26%, while LMW-P1:PC71BM-based PSCs showed poor photovoltaic performance with a PCE of only 2.75%. PCE of HMW-P1:PC71BM-based PSCs was further increased to 6.52% with the addition of 1,8-diiodooctane (DIO) as the additive. Meanwhile, PCE of only 2.51% was obtained for P2:PC71BM-based PSCs. The results indicated that the position of alkoxy substituents on the 9-arylidene-9H-fluorene unit and the molecular weight of polymers are very crucial to the photovoltaic performance of PSCs.

Indolo[3,2-b]carbazole and benzofurazan based narrow band-gap polymers for photovoltaic cells

Two indolo[3,2-b]carbazole and 5,6-bis(octyloxy)benzofurazan based narrow band-gap polymersPICzODTBFandPICzHDDTBFgive maximum power conversion efficiencies of 3.32% and 3.13% with open circuit voltages of 0.80 V and 0.95 V, respectively.

Low Band Gap Polymers Incorporating a Dicarboxylic Imide-Derived Acceptor Moiety for Efficient Polymer Solar Cells

Two novel copolymers incorporating N-alkyl-4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dicarboxylic imide (DI) and benzo[1,2-b:4,5-b']dithiophene (BDT) units have been designed, synthesized, and characterized. By the incorporation of the DI unit, both polymers show a bathochromically shifted absorption with a deep lying highest occupied molecular orbital (HOMO) energy level. The polymer based on thienyl group substituted BDT exhibits an intense absorption in the longer-wavelength region, a deeper lying HOMO energy level, and a higher carrier mobility, all of which contribute to the resulting polymer solar cells with a higher power conversion efficiency (PCE) of 5.19% and an increased V_oc of 0.91 V.