Design and Synthesis of Monofunctionalized, Water-Soluble Conjugated Polymers for Biosensing and Imaging Applications

Water-soluble conjugated polymers with controlled molecular weight characteristics, absence of ionic groups, high emission quantum yields, and end groups capable of selective reactions of wide scope are desirable for improving their performance in various applications and, in particular, fluorescent biosensor schemes. The synthesis of such a structure is described herein. 2-Bromo-7-iodofluorene with octakis(ethylene glycol) monomethyl ether chains at the 9,9′-positions, i.e., compound <b>4</b>, was prepared as the reactive premonomer. A high-yielding synthesis of the organometallic initiator (dppe)Ni(Ph)Br (dppe = 1,2-bis(diphenylphosphino)ethane) was designed and implemented, and the resulting product was characterized by single-crystal X-ray diffraction techniques. Polymerization of <b>4</b> by (dppe)Ni(Ph)Br can be carried out in less than 30 s, affording excellent control over the average molecular weight and polydispersity of the product. Quenching of the polymerization with [2-(trimethylsilyl)ethynyl]magnesium bromide yields silylacetylene-terminated water-soluble poly(fluorene) with a photoluminescence quantum efficiency of 80%. Desilylation, followed by copper-catalyzed azide–alkyne cycloaddition reaction, yields a straightforward route to introduce a wide range of specific end group functionalities. Biotin was used as an example. The resulting biotinylated conjugated polymer binds to streptavidin and acts as a light-harvesting chromophore to optically amplify the emission of Alexa Fluor-488 chromophores bound onto the streptavidin. Furthermore, the biotin end group makes it possible to bind the polymer onto streptavidin-functionalized cross-linked agarose beads and thereby incorporate a large number of optically active segments

Impact of Regiochemistry and Isoelectronic Bridgehead Substitution on the Molecular Shape and Bulk Organization of Narrow Bandgap Chromophores

A comparison of two classes of small molecules relevant to the field of organic electronics is carried out at the molecular and supramolecular levels. First, two molecules that differ only in the position of a pyridyl N-atom within an acceptor fragment are compared and contrasted. X-ray investigation of single crystals reveals that positioning the pyridyl N-atoms <i>proximal</i> to the molecules center changes the molecular shape by bending the molecule into a banana shape. Second, we demonstrate that the banana shape of the molecule can be controlled by replacing a Si atom within the dithienosilole fragment with a C or Ge atom. Here, utilization of cyclopentadithiophene or dithienogermole as the internal electron-rich unit leads to a decrease or an increase in the bending of the conjugated backbone, respectively. Such molecular shape changes alter intermolecular packing and thus affect bulk properties, leading to large differences in the optical, thermal, and crystallization properties

Impact of Regiochemistry and Isoelectronic Bridgehead Substitution on the Molecular Shape and Bulk Organization of Narrow Bandgap Chromophores

A comparison of two classes of small molecules relevant to the field of organic electronics is carried out at the molecular and supramolecular levels. First, two molecules that differ only in the position of a pyridyl N-atom within an acceptor fragment are compared and contrasted. X-ray investigation of single crystals reveals that positioning the pyridyl N-atoms <i>proximal</i> to the molecules center changes the molecular shape by bending the molecule into a banana shape. Second, we demonstrate that the banana shape of the molecule can be controlled by replacing a Si atom within the dithienosilole fragment with a C or Ge atom. Here, utilization of cyclopentadithiophene or dithienogermole as the internal electron-rich unit leads to a decrease or an increase in the bending of the conjugated backbone, respectively. Such molecular shape changes alter intermolecular packing and thus affect bulk properties, leading to large differences in the optical, thermal, and crystallization properties

Impact of Regiochemistry and Isoelectronic Bridgehead Substitution on the Molecular Shape and Bulk Organization of Narrow Bandgap Chromophores

A comparison of two classes of small molecules relevant to the field of organic electronics is carried out at the molecular and supramolecular levels. First, two molecules that differ only in the position of a pyridyl N-atom within an acceptor fragment are compared and contrasted. X-ray investigation of single crystals reveals that positioning the pyridyl N-atoms <i>proximal</i> to the molecules center changes the molecular shape by bending the molecule into a banana shape. Second, we demonstrate that the banana shape of the molecule can be controlled by replacing a Si atom within the dithienosilole fragment with a C or Ge atom. Here, utilization of cyclopentadithiophene or dithienogermole as the internal electron-rich unit leads to a decrease or an increase in the bending of the conjugated backbone, respectively. Such molecular shape changes alter intermolecular packing and thus affect bulk properties, leading to large differences in the optical, thermal, and crystallization properties

A Combined Experimental and Theoretical Study of Conformational Preferences of Molecular Semiconductors

Structural modules used for assembling molecular semiconductors have typically been chosen to give desirable optical and electronic properties. Growing evidence shows that chemical functionalities should be considered for controlling molecular shape, which is important for function because of its influence on polymer secondary structure, lattice arrangements in crystals, and crystallization tendencies. Using density functional theory (DFT) calculations, followed by a natural bond orbital (NBO) analysis, we examine eight molecular semiconductors with resolved single crystal X-ray structures to understand the features that dominate molecular conformations and ultimately develop practical rules that govern these preferences. All molecules can be described by a D′–A–D–A–D′ architecture and have a 4,4-dimethyl-4<i>H</i>-silolo­[3,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (DTS) donor (D) core unit, with [1,2,5]­thiadiazolo­[3,4-<i>c</i>]­pyridine (PT), 5-fluorobenzo­[<i>c</i>]­[1,2,5]­thiadiazole (FBT), or benzo­[1,2,5]­thiadiazole (BT) electron acceptor (A) units, and either thiophene, 5-hexyl-2,2′-bithiophene, or benzofuran electron-donating end-caps (D′). The NBO analysis shows that the energy difference between the two alternative conformations, or rotamers, (Δ<i>E</i>_rot) is a delicate balance of multiple competing nonbonding interactions that are distributed among many atoms. These interactions include attractive “donor–acceptor” electron sharing, steric repulsion, and electrostatic stabilization or destabilization. A proper grouping of these interactions reveals two primary factors determining <i>Δ<i>E</i></i>_rot. The first concerns heteroatoms adjacent to the bonds connecting the structural units, wherein the asymmetric distribution of π-electron density across the link joining the units results in stabilization of one of two rotamers. The second factor arises from electrostatic interactions between close-contact atoms, which may also shift the <i>Δ<i>E</i></i>_rot of the two rotamers. When all these constituent interactions cooperate, the dihedral angle is “locked” in a planar conformation with a negligible population of alternative rotamers

Polymorphism of Crystalline Molecular Donors for Solution-Processed Organic Photovoltaics

Using ab initio calculations and classical molecular dynamics simulations coupled to complementary experimental characterization, four molecular semiconductors were investigated in vacuum, solution, and crystalline form. Independently, the molecules can be described as nearly isostructural, yet in crystalline form, two distinct crystal systems are observed with characteristic molecular geometries. The minor structural variations provide a platform to investigate the subtlety of simple substitutions, with particular focus on polymorphism and rotational isomerism. Resolved crystal structures offer an exact description of intermolecular ordering in the solid state. This enables evaluation of molecular binding energy in various crystallographic configurations to fully rationalize observed crystal packing on a basis of first-principle calculations of intermolecular interactions