Shift of the Branching Point of the Side‐Chain in Naphthalenediimide (NDI)‐Based Polymer for Enhanced Electron Mobility and All‐Polymer Solar Cell Performance

The branching point of the side‐chain of naphthalenediimide (NDI)‐based conjugated polymers is systematically controlled by incorporating four different side‐chains, i.e., 2‐hexyloctyl (P(NDI1‐T)), 3‐hexylnonyl (P(NDI2‐T)), 4‐hexyldecyl (P(NDI3‐T)), and 5‐hexylundecyl (P(NDI4‐T)). When the branching point is located farther away from the conjugated backbones, steric hindrance around the backbone is relaxed and the intermolecular interactions between the polymer chains become stronger, which promotes the formation of crystalline structures in thin film state. In particular, thermally annealed films of P(NDI3‐T) and P(NDI4‐T), which have branching points far away from the backbone, possess more‐developed bimodal structure along both the face‐on and edge‐on orientations. Consequently, the field‐effect electron mobilities of P(NDIm‐T) polymers are monotonically increased from 0.03 cm² V⁻¹ s⁻¹ in P(NDI1‐T) to 0.22 cm² V⁻¹ s⁻¹ in P(NDI4‐T), accompanied by reduced activation energy and contact resistance of the thin films. In addition, when the series of P(NDIm‐T) polymers is applied in all‐polymer solar cells (all‐PSCs) as electron acceptor, remarkably high‐power conversion efficiency of 7.1% is achieved along with enhanced current density in P(NDI3‐T)‐based all‐PSCs, which is mainly attributed to red‐shifted light absorption and enhanced electron‐transporting ability.     

Side Chain Optimization of Naphthalenediimide–Bithiophene‐Based Polymers to Enhance the Electron Mobility and the Performance in All‐Polymer Solar Cells

Tuning the side chains of conjugated polymers is a simple, yet effective strategy for modulating their structural and electrical properties, but their impact on n‐type conjugated polymers has not been studied extensively, particularly in the area of all‐polymer solar cells (all‐PSCs). Herein, the effects of side chain engineering of P(NDI2OD‐T2) polymer (also known as Polyera Activink N2200) are investigated, which is the most widely used n‐type polymer in all‐PSCs and organic field‐effect transistors (OFETs), on their structural and electronic properties. A series of naphthalenediimide‐bithiophene‐based copolymers (P(NDIR‐T2)) is synthesized, with different side chains (R) of 2‐hexyldecyl (2‐HD), 2‐octyldodecyl (2‐OD), and 2‐decyltetradecyl (2‐DT). The P(NDI2HD‐T2) exhibits more noticeable crystalline behaviors than P(NDI2OD‐T2) and P(NDI2DT‐T2), thereby facilitating superior 3D charge transport. For example, the P(NDI2HD‐T2) shows the highest OFET electron mobility (1.90 cm² V^?1 s^?1). Also, a series of all‐PSCs is produced using different electron donors of PTB7‐Th, PTB7, and PPDT2FBT. The P(NDI2HD‐T2) based all‐PSCs produce much higher power conversion efficiency (PCE) irrespective of the electron donors. In particular, the PTB7‐Th:P(NDI2HD‐T2) forms highly ordered, strong face‐on interchain stackings, and has better intermixed bulk‐heterojunction morphology, producing the highest PCE of 6.11% that has been obtained by P(NDIR‐T2) based all‐PSCs to date.     

Imidazolyl‐Naphthalenediimide‐Based Threading Intercalators of DNA

Intercalation by threading is anticipated to feature in DNA‐binding molecules for developing DNA‐targeted diagnostics and therapeutics. We investigated the role of an imidazolyl moiety in threading intercalators of DNA by employing a number of imidazolyl–naphthalenediimide conjugates. Threading intercalation was studied by UV spectroscopy, competitive binding fluorescent dye displacement, circular dichroism, isothermal calorimetry, and computational analysis. NIm6 was found to be a strong candidate, with good half‐life, as revealed by dissociation kinetic analysis. Computational studies supported intercalation of the naphthalene core between base pairs and binding of the imidazolyl moieties in the adjacent grooves (threading mechanism) through electrostatic and hydrogen‐bonding interactions. The interaction of the positively charged imidazolium moieties with the negatively charged phosphate backbone of DNA contributed to the favorable enthalpy change, as revealed by the experimental and computational data. Threading intercalation by NIm6 caused significant retardation of DNA in an electrophoretic mobility shift assay. The biological significance of potent imidazolyl naphthalenediimide conjugates was demonstrated by the inhibition of topo‐ isomerase I activity and cytotoxicity against HeLa cells.     

Interactions Between Amino Acid‐Tagged Naphthalenediimide and Single Walled Carbon Nanotubes for the Design and Construction of New Bioimaging Probes

A new synthetic route to functionalized single walled carbon nanotubes (SWNTs) via supramolecular interactions using a specifically designed naphthalenediimide (NDI) nanoreceptor is demonstrated. The tendency of the NDI to spontaneously form composites with carbon nanomaterials leads to fluorescent amino acid tagged SWNTs, which are dispersible in widely accessible organic solvents (CHCl₃, DMSO) as well as in biocompatible cell medium (EMEM, Eagle's modified essential medium). The X‐ray crystal structure of the first iodine‐tagged and amino acid‐functionalized NDI molecule, designed especially to facilitate the high resolution transmission electron microscopy (HR TEM) imaging whilst retaining its ability to self‐assemble into a nanodimensional receptor in weakly polar solvents, is also described. A new hybrid material, NDI@SWNT, was prepared and characterized as dispersed in organic solvents and aqueous media and in the solid state by HR TEM, tapping mode atomic force microscopy (TM AFM), scanning electron microscopy (SEM), circular dichroism, Raman and fluorescence spectroscopies (steady‐state single and two‐photon techniques). Combined microscopy techniques, density functional theory (DFT) calculations using the Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) program and spectroscopic measurements in solution indicate that amino acid‐functionalized NDI interacts strongly with SWNTs and forms a donor‐acceptor complex. Density functional theory (DFT) calculations predicted the geometry and the binding energies of an NDI molecule loaded onto a SWNT strand and the possibility of charge transfer interactions within the hybrid. The NDI@SWNT composite translocates into cells (e.g. FEK‐4, HeLa, MCF‐7) as an intact object and localizes in the cells' cytoplasm and partially in the nucleus. The NDI coating enhances the biocompatibility of SWNTs and mediates its intracellular localization as shown by confocal fluorescence imaging and fluorescence lifetime imaging (FLIM) techniques. The excited state fluorescence lifetime of the probes in cells versus solution phase indicates that the probes remain unaffected by the change in their chemical environment within the experimental timescale (2 h).