Synthesis of Diketopyrrolopyrrole Containing Copolymers: A Study of Their Optical and Photovoltaic Properties

The diketopyrrolopyrrole-based copolymers PDPP-BBT and TDPP-BBT were synthesized and used as a donor for bulk heterojunction photovoltaic devices. The photophysical properties of these polymers showed absorption in the range 500−600 nm with a maximum peak around 563 nm, while TDPP-BBT showed broadband absorption in the range 620 − 800 nm with a peak around 656 nm. The power conversion efficiencies (PCE) of the polymer solar cells based on these copolymers and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were 0.68% (as cast PDPP-BBT:PCBM), 1.51% (annealed PDPP-BBT:PCBM), 1.57% (as cast TDPP-BBT:PCBM), and 2.78% (annealed TDPP-BBT:PCBM), under illumination of AM 1.5 (100 mW/cm2). The higher PCE for TDPP-BBT-based polymer solar cells has been attributed to the low band gap of this copolymer as compared to PDPP-BBT, which increases the numbers of photogenerated excitons and corresponding photocurrent of the device. These results indicate that PDPP-BBT and TDPP-BBT act as excellent electron donors for bulk heterojunction devices

Influence of Side-Chain on Structural Order and Photophysical Properties in Thiophene Based Diketopyrrolopyrroles: A Systematic Study

In this work, we have synthesized a series of TDPP derivatives with different alkyl groups such as <i>n</i>-hexyl (−C₆H₁₃) <b>3a</b>, 2-ethylhexyl (-(2-C₂H₅)­C₆H₁₂) <b>3b</b>, triethylene glycol mono methyl ether (-(CH₂CH₂O)_<b>3c</b>H₃, TEG) <b>3c</b>, and octadodecyl (-(8-C₈H₁₇)­C₁₂H₂₂) <b>3d</b>. <i>N</i>,<i>N</i> dialkylation of thiophene-diketopyrrolopyrrole (TDPP, <b>1</b>) strongly influences its solubility, solid state packing, and structural order. These materials allow us to explicitly study the influence of alkyl chain on solid state packing and photophysical properties. TDPP moiety containing two different alkyl groups <b>3e</b> (TEG and 2-ethylhexyl) and <b>3f</b> (TEG and <i>n</i>-hexyl) were synthesized for the first time. The absorption spectra of all derivatives exhibited a red shift in solid state when compared to their solution spectra. The type of alkyl chains leads to change in the optical band gaps in solid state. The fluorescence study reveals that TDPP derivatives have strong π–π interaction in the solid state and the extent of bathochromic shift is due to combination of intramolecular interaction and formation of aggregates in solid state. This behavior strongly depends on the nature of alkyl chain. The presence of strong C–H···O inter chain interactions and CH−π interactions in solid state exhibits strong influence on the photophysical properties of TDPP chromophore

Heterogeneity in Dye–TiO₂ Interactions Dictate Charge Transfer Efficiencies for Diketopyrrolopyrrole-Based Polymer Sensitized Solar Cells

Power conversion efficiency of a solar cell is a complex parameter which usually hides the molecular details of the charge generation process. For rationally tailoring the overall device efficiency of the dye-sensitized solar cell, detailed molecular understanding of photoinduced reactions at the dye-TiO₂ interface has to be achieved. Recently, near-IR absorbing diketopyrrolopyrrole-based (DPP) low bandgap polymeric dyes with enhanced photostabilities have been used for TiO₂ sensitization with moderate efficiencies. To improve the reported device performances, a critical analysis of the polymer–TiO₂ interaction and electron transfer dynamics is imperative. Employing a combination of time-resolved optical measurements complemented by low temperature EPR and steady-state Raman spectroscopy on polymer–TiO₂ conjugates, we provide direct evidence for photoinduced electron injection from the TDPP-BBT polymer singlet state into TiO₂ through the CO group of the DPP-core. A detailed excited state description of the electron transfer process in films reveals instrument response function (IRF) limited (<110 fs) charge injection from a minor polymer fraction followed by a picosecond recombination. The major fraction of photoexcited polymers, however, does not show injection indicating pronounced ground state heterogeneity induced due to nonspecific polymer–TiO₂ interactions. Our work therefore underscores the importance of gathering molecular-level insight into the competitive pathways of ultrafast charge generation along with probing the chemical heterogeneity at the nanoscale within the polymer–TiO₂ films for optimizing photovoltaic device efficiencies

Photoimageable Organic Coating Bearing Cyclic Dithiocarbonate for a Multifunctional Surface

We report the fabrication of photocross-linkable and surface-functionalizable polymeric thin films using reactive cyclic dithiocarbonate (DTC)-containing copolymers. The chemical functionalities of these material surfaces were precisely defined with light illumination. The DTC copolymers, namely, poly­(dithiocarbonate methylene methacrylate–random-alkyl methacrylate)­s, were synthesized via reversible addition–fragmentation chain transfer polymerization, and the reaction kinetics was thoroughly analyzed. The copolymers were cross-linked into a coating using a bifunctional urethane cross-linker that contains a photolabile o-nitrobenzyl group and releases aniline upon exposure to light. The nucleophilic attack of the aromatic amine opens the DTC group, forming a carbamothioate bond and generating a reactive thiol group in the process. The surface concentrations of the unreacted DTC and thiol were effectively controlled by varying the amounts of the copolymer and the cross-linker. The use of methacrylate comonomers led to additional reactive surface functionality such as carboxylic acid via acid hydrolysis. The successful transformations of the resulting DTC, thiol, and carboxylic acid groups to different functionalities via sequential nucleophilic ring opening, thiol–ene, and carbodiimide coupling reactions under ambient conditions were confirmed quantitatively using X-ray photoelectron spectroscopy. The presented chemistries were readily adapted to the immobilization of complex molecules such as a fluorophore and a protein in lithographically defined regions, highlighting their potential in creating organic coatings that can have multiple functional groups under ambient conditions

Diketopyrrolopyrrole–Diketopyrrolopyrrole-Based Conjugated Copolymer for High-Mobility Organic Field-Effect Transistors

In this communication, we report the synthesis of a novel diketopyrrolopyrrole–diketopyrrolopyrrole (DPP–DPP)-based conjugated copolymer and its application in high-mobility organic field-effect transistors. Copolymerization of DPP with DPP yields a copolymer with exceptional properties such as extended absorption characteristics (up to ∼1100 nm) and field-effect electron mobility values of >1 cm2 V–1 s–1. The synthesis of this novel DPP–DPP copolymer in combination with the demonstration of transistors with extremely high electron mobility makes this work an important step toward a new family of DPP–DPP copolymers for application in the general area of organic optoelectronics

Removable Nonconjugated Polymers To Debundle and Disperse Carbon Nanotubes

In this study, we explore design rules for block copolymer (BCP)-based dispersants for carbon nanotubes (CNTs). We demonstrate the influence of polymer architecture on the dispersion, debundling, and stability of single-walled CNTs. These polymer dispersants based on pyrene-functionalized BCPs are tailored to perform multiple functions, namely, to (a) solubilize CNTs, (b) debundle CNTs, and (c) lift off CNTs following processing. BCPs were synthesized through an efficient ring-opening reaction of a poly 2-vinyl-4,4-dimethylazlactone (PVDMA) block. This chemistry provides greater flexibility to alter the polymer architecture, solubility, and degradability as well as to achieve a higher degree of incorporation of pyrene side groups. UV–vis–NIR absorption and photoluminescence emission studies indicate that a block-brush architecture consisting of polystyrene (PS) as the first block and mixed side chains of pyrene/PS or pyrene/polymethylmethacrylate grafted to the second PVDMA block gave the most stable CNT dispersions with high yields. Our studies show that a large number of pyrene side groups, as well as a large molecular weight solubilizing block, are required to disperse CNTs with a wide range of diameters from 0.7 to 1.7 nm. We further show that our design allows for thiol–thioester exchange chemistry to release the polymer wrappers from the CNT surface in an acid-free organic solvent medium. We envision this method to be generalizable for the dispersion of CNTs from small to large diameters

Controlling Conformations of Diketopyrrolopyrrole-Based Conjugated Polymers: Role of Torsional Angle

Transport of charge carriers through conjugated polymers is strongly influenced by the presence and distribution of structural disorders. In the present work, structural defects caused by the presence of torsional angle were investigated in a diketopyrrolopyrrole (<b>DPP</b>)-based conjugated polymer. Two new copolymers of <b>DPP</b> were synthesized with varying torsional angles to trace the role of structural disorder. The optical properties of these copolymers in solution and thin film reveal the strong influence of torsional angle on their photophysical properties. A strong influence was observed on carrier transport properties of polymers in organic field-effect transistors (OFET) device geometry. The polymers based on phenyl DPP with higher torsional angle (<b>PPTDPP</b>-OD-TEG) resulted in high threshold voltage with less charge carrier mobility as compared to the polymer based on thiophene DPP (<b>2DPP</b>-OD-TEG) bearing a lower torsional angle. Carrier mobility and the molecular orientation of the conjugated polymers were correlated on the basis of grazing incidence X-ray scattering measurements showing the strong role of torsional angle introduced in the form of structural disorder. The results presented in this Article provide a deep insight into the sensitivity of structural disorder and its impact on the device performance of DPP-based conjugated polymers

Effect of Dipolar Molecule Structure on the Mechanism of Graphene-Enhanced Raman Scattering

Graphene-enhanced Raman scattering (GERS) is a promising characterization technique which uses a single layer of graphene. As the electronic coupling of adsorbates with graphene leads to enhancement in the Raman signal, it is of immense interest to explore the factors that affect the coupling of the adsorbates with graphene. To probe this effect we have designed and synthesized a series of dipolar molecules with the general structure, <i>N</i>-ethyl-<i>N</i>-(2-ethyl­(1-pyrenebutyrate)-4-(4-R-phenylazo)­aniline) where the R-groups are varied from methoxy (−OCH₃), methyl (−CH₃), hydrogen (−H), nitrile (−CN), nitro (−NO₂) to tricyanofuran (TCF) groups. This systematically changes the dipole moments and electronic/optical band gap of the molecules. By noncovalently interfacing these molecules on graphene, the Raman signal is enhanced by a factor of 40–90 at the excitation wavelength of 532 nm. Measurements of the Raman enhancement factor and Raman cross section are complemented with DFT calculations to correlate the dipole moment and the energy level of the hybrid to the Raman scattering efficiency. These studies highlight the relevance of the dipolar nature of chromophores, which determines their dipole moment and the band gap, and the resulting electronic coupling to graphene which simultaneously alters the energy level of the orbitals in the molecule and the Fermi level in graphene, resulting in efficient Raman excitations and GERS

Isomeric Effect Enabled Thermally Driven Self-Assembly of Hydroxystyrene-Based Block Copolymers

We demonstrate through isomeric effect the modulation of thermal properties of poly­(hydroxystyrene) (PHS)-based block copolymers (BCPs). A minimal structural change of substituting 3HS for 4HS in the BCP results in a drastic decrease in <i>T</i>_g, which in turn enables the thin film assembly of the BCP via thermal annealing. We synthesized a series of poly­(3-hydroxystyrene-<i>b</i>-<i>tert</i>-butylstyrene) [P­(3HS-<i>b</i>-<i>t</i>BuSt)] and poly­(4-hydroxystyrene-<i>b</i>-<i>tert</i>-butylstyrene) [P­(4HS-<i>b</i>-<i>t</i>BuSt)] BCPs by sequential anionic polymerization of protected 3HS/4HS monomer and <i>t</i>BuSt followed by deprotection. Measured <i>T</i>_g of P­(3HS) was ∼20–30 °C lower than P­(4HS) of comparable molecular weights. As a result, thermally driven self-assembly of P­(3HS-<i>b</i>-tBuSt) BCPs in both bulk and thin film is demonstrated. For P­(4HS-<i>b</i>-tBuSt) thermal annealing in thin-film at high temperatures results in poorly developed morphology due to cross-linking reaction of the 4HS block. The smallest periodicity observed for P­(3HS-<i>b</i>-tBuSt) was 8.8 nm in lamellar and 11.5 nm in cylindrical morphologies. The functionality of the 3HS block was exploited to incorporate vapor phase metal oxide precursors to generate sub-10 nm alumina nanowires

Isolation of Pristine Electronics Grade Semiconducting Carbon Nanotubes by Switching the Rigidity of the Wrapping Polymer Backbone on Demand

Conjugated polymers are among the most selective carbon nanotube sorting agents discovered and enable the isolation of ultrahigh purity semiconducting singled-walled carbon nanotubes (s-SWCNTs) from heterogeneous mixtures that contain problematic metallic nanotubes. The strong selectivity though highly desirable for sorting, also leads to irreversible adsorption of the polymer on the s-SWCNTs, limiting their electronic and optoelectronic properties. We demonstrate how changes in polymer backbone rigidity can trigger its release from the nanotube surface. To do so, we choose a model polymer, namely poly[(9,9-dioctylfluorenyl-2,7-diyl)-<i>alt</i>-<i>co</i>-(6,60-(2,20-bipyridine))] (PFO-BPy), which provides ultrahigh selectivity for s-SWCNTs, which are useful specifically for FETs, and has the chemical functionality (BPy) to alter the rigidity using mild chemistry. Upon addition of Re(CO)₅Cl to the solution of PFO-BPy wrapped s-SWCNTs, selective chelation with the BPy unit in the copolymer leads to the unwrapping of PFO-BPy. UV–vis, XPS, and Raman spectroscopy studies show that binding of the metal ligand complex to BPy triggers up to 85% removal of the PFO-BPy from arc-discharge s-SWCNTs (diameter = 1.3–1.7 nm) and up to 72% from CoMoCAT s-SWCNTs (diameter = 0.7–0.8 nm). Importantly, Raman studies show that the electronic structure of the s-SWCNTs is preserved through this process. The generalizability of this method is demonstrated with two other transition metal salts. Molecular dynamics simulations support our experimental findings that the complexation of BPy with Re(CO)₅Cl in the PFO-BPy backbone induces a dramatic conformational change that leads to a dynamic unwrapping of the polymer off the nanotube yielding pristine s-SWCNTs