A thiocarbonyl-containing small molecule for optoelectronics

We report the synthesis and characterization of a novel thiocarbonyl iso-DPP derivative, namely 1,3,4,6-tetraphenylpyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dithione. Even without solubilising alkyl chains, the small molecule could be processed from organic solvents such as dichloromethane, chloroform or dichlorobenzene, and it was found that the optical properties of neat thin films were strongly dependent on the solvent used. Field effect hole mobilities were of the order 10 cm V s, with mobilities measured in a diode configuration solvent dependent and at least an order of magnitude lower. Importantly, blends of the iso-DPP derivative with PCBM, a typically used electron acceptor in bulk heterojunction solar cells, were found to possess hole mobilities of up to 10 cm V s in a diode configuration, which was an order of magnitude larger than the electron mobility. Finally, simple bulk heterojunction solar cells were fabricated with maximum power conversion efficiencies of 2.3%

Electrochemical polymerization of 1,3,4,6-tetraarylpyrrolo[3,2-b]pyrrole-2,5-dione (isoDPP) derivatives

New pi-conjugated polymers containing 1,3,4,6-tetraarylpyrrolo[3,2-b] pyrrole-2,5-dione (isoDPP) units in the main chain were prepared by electrochemical polymerization of isoDPP-based monomers M1-M3. Monomers M1 and M2 were previously obtained upon palladium-catalyzed Suzuki cross-coupling of dibromo-isoDPP derivatives 3,6-bis(5-bromothiophen-2-yl)-1,4-bis(4-tert-butylphenyl) pyrrolo-[3,2-b]pyrrole-2,5-dione 1a, or 3,6-bis(5-bromothiophen-2-yl)-1,4-bis(4-hexylphenyl)-pyrrolo-[3,2-b] pyrrole2,5-dione 1b and 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl)-3-hexylthiophene. Monomer M3 was synthesized by Stille coupling of 1a and 2-(tributylstannyl)-4-octylthiophene. The yields were about 88%. M1-M3 were obtained as dark red powders with excellent solubility in organic solvents. The electrochemical polymerization was carried out under potentiodynamic conditions, i. e., the potential was repeatedly cycled between 0.0 (or -0.2) V and 1.2 V vs. ferrocene (FOC). The polymers P1-P3 precipitated as stable, insoluble, well adhering thin films on ITO-coated glass or glassy carbon electrodes. P1-P3 exhibit broad absorption spectra with maxima between 526 and 629 nm. They are non-fluorescent. Cyclovoltammetric studies indicate reversible oxidation and reduction behavior. Anodic cycling is accompanied with reversible colour changes of polymer films. The oxidation potentials are between 0.27 and 0.71 V vs. FOC, and the reduction potentials are between -1.21 and -1.69 V. The band gaps of the polymers are between 1.51 and 1.62 eV. Due to their optical and electrochemical properties the polymers might be useful as active materials in electrochromic displays

Deeply Colored Polymers Containing 1,3,4,6-Tetraarylpyrrolo[3,2-<i>b</i>]pyrrole-2,5-dione (IsoDPP) Units in the Main Chain

Synthesis and characteristic properties of polymers <b>P1</b>–<b>P4</b> containing 1,3,4,6-tetraarylated pyrrolo­[3,2-<i>b</i>]­pyrrole-2,5-dione (isoDPP) units in the main chain are described. <b>P1</b> and <b>P2</b> were prepared upon palladium-catalyzed polycondensation of 3,6-bis­(4-bromophenyl)-1,4-bis­(4-<i>tert</i>-butylphenyl)­pyrrolo­[3,2-<i>b</i>]­pyrrole-2,5-dione (<b>M1</b>) or 1,4-bis­(4-bromophenyl)-3,6-bis­(4-<i>tert</i>-butylphenyl)­pyrrolo­[3,2-<i>b</i>]­pyrrole-2,5-dione (<b>M2</b>) and 2,2′-(9,9-dihexyl-9<i>H</i>-fluoren-2,7-diyl)­bis­(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (<b>M4</b>), while <b>P3</b> and <b>P4</b> were synthesized upon polycondensation of 3,6-bis­(5-bromothien-2-yl)-1,4-bis­(4-dodecylphenyl)­pyrrolo­[3,2-<i>b</i>]­pyrrole-2,5-dione (<b>M3</b>) and 2,5-bis­(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)­thiophene (<b>M5</b>) or 9-(2-ethylhexyl)-2,7-bis­(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)­carbazole (<b>M6</b>). Deeply colored polymers with molecular weights between 3.5 and 22 kDa were obtained. The polymers were soluble in common organic solvents such as dichloromethane, chloroform, and tetrahydrofuran. All polymers exhibit broad absorption bands with high extinction coefficients (ϵ > 2 × 10⁴ L mol^–1 cm^–1) but weak fluorescence, the quantum yields being below 1%. Although <b>P1</b> and <b>P2</b> are isomers, their optical properties are rather different. <b>P1</b> with polyconjugated backbone exhibits an absorption maximum at 409 nm, while <b>P2</b> has a maximum at 360 nm due to interruption of π-conjugation at the lactam N atoms. The presence of thienyl-isoDPP units in the backbone causes a red-shift of the absorption to 489 nm (<b>P3</b>) and 435 nm (<b>P4</b>). All polymers exhibit nearly irreversible oxidation and reduction behavior. Bandgaps of the polymers with phenyl-substituted isoDPP units (<b>P1</b> and <b>P2</b>) are at about 2 eV, while those of polymers with thienyl-substituted isoDPP (<b>P3</b> and <b>P4</b>) are at about 1.5 eV

Synthesis and characterization of 1,3,4,6-tetraarylpyrrolo[3,2-b]-pyrrole-2,5-dione (isoDPP)-based donor-acceptor polymers with low band gap

The synthesis of new pi-conjugated donor-acceptor (D-A) polymers containing tetraaryl-diketopyrrolo[3,2-b]-pyrrole (isoDPP) as the building block is described. Polymers were prepared upon palladium-catalyzed Stille coupling of 3,6-bis(5-bromothien-2-yl)-1,4-diphenyl-pyrrolo[3,2-b]pyrrole-2,5-dione (M1) with 4,4'-bis(2-ethylhexyl)-5,5'-bis(trimethyltin)dithieno[3,2-b:2',3'-d]silole (M2) or (4,8-bis-(2-ethylhexyloxy)benzo [1,2-b:4,5-b]dithiophene-2,6-diyl)bis(trimethylstannane) (M3). The polymers exhibit low band gaps of 1.53 and 1.67 eV, broad absorption bands with maxima of 523 nm and 620 nm, and high extinction coefficients of 6.5 x 10(4) L mol(-1) cm(-1), and 2.6 x 10(4) L mol(-1) cm(-1), respectively. Low fluorescence quantum yields between 0.13% and 0.82% were detected. Quantum chemical calculations indicated a nearly planar backbone with highly delocalized HOMO orbitals and localization of the LUMO electrons at the thienyl-isoDPP units. The X-ray structure analysis of M1 indicated a dihedral angle of 14.1 degrees between the thienyl groups and the isoDPP core, and a 76.6 degrees dihedral angle of the lactam phenyl group and the isoDPP core, the latter preventing any pi-stacking of the monomers. The polymers show high glass transitions (T-g) and excellent thermal stability. Photoirradiation studies indicate that the polymers are extremely stable to UV and visible light in solution. Spectroelectrochemical studies indicated a reversible electrochromism with isosbestic point near 710 nm

Photovoltaic polymers based on difluoroqinoxaline units with deep HOMO levels

We report the synthesis of low bandgap polymers with a difluoroquinoxaline unit by Stille polymerization for use in polymer solar cells (PSCs). A new series of copolymers with 2,3-didodecyl-6,7-difluoro quinoxaline as the electron-deficient unit and alkyloxybenzo[1,2-b:4,5-b]dithiophene and thiophene as the electron-rich unit were synthesized. The photovoltaic properties of the devices based on the synthesized polymers revealed that the fluorine atoms at the quinoxaline units aid in decreasing the highest occupied molecular orbital (HOMO) energy levels; this in turn increased the open circuit voltage of the devices. The polymers with long alkyl chains exhibited good solubility that increased their molecular weight, but the power conversion efficiency was low. Efficient polymer solar cells were fabricated by blending the synthesized copolymers with PC71BM, and the PCE increased up to 5.11% under 100 mW cm(-2) AM 1.5 illumination. These results demonstrate that the importance of having a control polymer to be synthesized and characterized side by side with the fluorine analogues

Pyrrolo[3,2-b]pyrrole based small molecules as donor materials for OPVs

A new accepter unit, pyrrolo[3,2-b]pyrrole-2,5-dione, deserves much attention as the electron-deficient unit for the generation of electron donor material for organic photovoltaic cells (OPVs). Pyrrolo[3,2-b]pyrrole-2,5-dione unit, regioisomer of the known pyrrolo[3,4-c]pyrrole-1,4-dione, is originated from the structure of stable synthetic pigment. The organic low band gap molecules with pyrrolo[3,2-b]pyrrole-2,5-dione, thiophene and triphenyl amine units were synthesized using Suzuki polymerization to generate SM-B, SM-M and SM-H. The spectrum of SM-B as the solid thin film shows absorption band with maximum peaks at 356 and 517 nm, and the absorption onset at 667 nm, corresponding to band gap of 1.86 eV. The device comprising SM-B and PC71BM (1:4) showed a V-OC of 0.79 V. a J(SC) of 6.04 mA/cm(2), and a FF of 0.33, giving a power conversion efficiency of 1.56%.close4