Bijitha Balan

Thienoisoindigo-based low-band gap polymers for organic electronic devices

We synthesized a series of new low-band gap donor–acceptor copolymers containing 4,4′-bis(alkyl)-[6,6′-bithieno[3,2-b]pyrrolylidene]-5,5′(4H,4′H)-dione. This acceptor unit, so-called dithienoketopyrrole (DTKP), is an analogue of isoindigo, the phenyl rings of which are replaced by thiophenes. Donor moieties such as benzodithiophene, cyclopentadithiophene, fluorene, and dithienothiophene are polymerized with DTKP in an alternating fashion by Stille or Suzuki–Miyaura coupling methods. Exceedingly low-band gaps (Eg = 1.0–1.6 eV) were achieved in these copolymers through internal charge transfer interactions between the donor and acceptor moieties. The structural, photophysical, and electrochemical properties of the resultant copolymers were characterized, and field-effect transistor (FET) mobilities were measured. The copolymers showed electronic absorption spectra extending to the near infrared region (600–1400 nm) with absorption maxima at 745–971 nm, along with a low-lying LUMO of −3.8 eV. Density functional theory (DFT) calculation indicated high planarity for the copolymer backbone when compared to that of its phenyl-isoindigo counterparts. FET hole mobilities on the order of 10−4 to 10−3 cm2 V−1 s−1 were obtained, demonstrating a feasibility to use them in organic photovoltaic cells.

Oligofluorene-based nanoparticles in aqueous medium: hydrogen bond assisted modulation of functional properties and color tunable FRET emission

Fluorene-based linear π-conjugated oligomers with different end functional groups having zero- (OF1), one- (OF2), two- (OF3) and three-point (OF4) hydrogen bonding sites were synthesized and characterized. By using a reprecipitation method, self-assembled nanoparticles were prepared in aqueous medium. The spherical shape and amorphous nature of nanoparticles were established by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Zeta potential measurements showed that nanoparticles of OF2–4 have good colloidal stability, whereas those of OF1 have only moderate stability indicating that the hydrogen bonding groups in OF2–4 interact with the polar water molecules providing stability to the assembly. However, the interior of the nanoparticles remained non-polar, thus providing a conducive medium for hydrogen bonding between the oligofluorene molecules. This leads to varying interchromophore interactions in OF1–4 in the nanoparticle state depending on the H-bonding strength of the end groups. Dynamic light scattering (DLS) studies revealed that under identical conditions, the size of the nanoparticles decreased with increasing number of hydrogen bonding sites in the molecule. The interchromophore interactions were evident from the UV-Vis absorption and fluorescence studies. Bright blue fluorescence of the molecules in solution undergoes quenching in the nanoparticle state. The fluorescence quenching significantly increases from OF1 to OF4 indicating enhanced interaction between chromophores with increasing number of hydrogen bonding sites in the molecules. The nanoparticles were used as a donor scaffold for fluorescence resonance energy transfer (FRET) by encapsulating varying amounts of an orange red emitting neutral dye (D1) thereby achieving colour tunable emission including white. FRET studies were also conducted with a cationic dye (D2) adsorbed on the negatively charged nanoparticle surface. The FRET efficiency with both dyes showed direct correlation with the number of hydrogen bonding sites in the molecules.

Optical and electrical properties of dithienothiophene based conjugated polymers: medium donor vs. weak, medium, and strong acceptors

Herein we report the systematic study of the structure–property relationship of a few dithienothiophene (DTT) based donor–acceptor conjugated polymers using various techniques such as UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry (CV), flash-photolysis time-resolved microwave conductivity (FP-TRMC), density functional theory (DFT), X-ray diffraction (XRD), and field-effect transistor (FET). A medium donor, DTT, was coupled in an alternating fashion with thiazole-based weak, medium, and strong acceptors. Though the optical properties showed good correlation with the donor–acceptor strength, the FET properties indicated significant deviation. The XRD analysis and DFT calculations revealed that the deviation is caused by the difference in structural ordering of the polymers in the film state. Since the FP-TRMC analysis reflects the properties of semiconducting organic materials at the molecular level such as the donor–acceptor strength and structural ordering in the film state, it showed good correlation with FET properties. Thus the present work illustrates that the study of charge carrier generation and mobility dynamics by FP-TRMC is a valuable addition to the conventional structure–property analysis methods, and is reliable to find the suitability of conjugated polymers for electronic device applications.

Thiophene-bithiazole based metal-free dye as DSSC sensitizer: Effect of co-adsorbents on photovoltaic efficiency

AbstractA new molecule consisting of a bithiazole chromophore sandwiched between two thiophenes, functionalized with benzothiophene unit at one end and cyanoacrylic acid at the other end (BT1) was synthesized, photophysical properties were studied and employed as a photosensitizer in dye-sensitized solar cells (DSSCs). The molecule exhibited an intense absorption in the UV-visible region with absorption extending up to 500 nm. The ground and excited state potentials of BT1 were calculated to be 1.29 and -0.65 V, respectively vs. NHE using cyclic voltammetry. The ground state energy level is more positive than the triiodide electrolyte and excited state energy level is considerably more negative than the TiO2 satisfying the energetic requirements for a photosensitizer in DSSC. The solar cells fabricated from BT1 exhibited an efficiency of 1.13%. The effect of various co-adsorbents (CDCA,TP1 andTP2) on the DSSC performance was investigated in detail. In the presence of CDCA, the photovoltaic efficiency was enhanced to 1.25%, whereas, in the presence of TP1 and TP2, the efficiency lowered to 0.20% and 0.59%, respectively. The increased efficiency in the presence of CDCA could be attributed to the prevention of the aggregation of dye molecules induced by CDCA. On the other hand, TP1 and TP2 were found to be not as effective as CDCA to prevent aggregation leading to the lowering of photoconversion efficiency. Graphical AbstractA benzothiophene-bithiazole based metal-free sensitizer end-functionalized with cyanoacrylic acid acceptor was synthesized, photophysical properties were studied and used for dye sensitized solar cell (DSSC) application. The effect of various co-adsorbents on the DSSC performance was investigated.

Self-assembly and photoinduced electron transfer in a donor- \(\upbeta \)-cyclodextrin-acceptor supramolecular system\(^{\S }\)

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p/n Switching of Ambipolar Bithiazole–Benzothiadiazole-Based Polymers in Photovoltaic Cells

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Gold Nanoparticle Assisted Self-Assembly and Enhancement of Charge Carrier Mobilities of a Conjugated Polymer

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