To be or not to be – nematic liquid crystals from shape-persistent V-shaped nematogens with the ‘magic angle’

<p>The tetrahedral bending angle in V-shaped nematogens was claimed to be the optimum for finding a biaxial nematic liquid crystal phase. The benzo[1,2-b:4,3-b’]dithiophene core, recently successfully applied as a tetrahedral bending unit in mesogens with lateral flexible chains, is here embedded in a scaffold with only terminal chains, which conventionally promotes the formation of nematic phases at low temperature. A series of new mesogens has been successfully prepared, realising hockey-stick, hockey-stick dimer and V-shaped molecular topologies. Only the hockey-stick mesogens assemble in uniaxial nematic phases over a broad temperature range. Single crystal structure analysis of a hockey-stick and V-shaped compound reveal remarkable similarities with the benzodithiophene core wrapped by aliphatic chains. A model explaining the absence of nematic mesophases in the family of V-shaped, shape-persistent mesogens with terminal aliphatic chains is presented and results in the proposal of a new design for biaxial nematogens.</p

To be or not to be – nematic liquid crystals from shape-persistent V-shaped nematogens with the ‘magic angle’

<p>The tetrahedral bending angle in V-shaped nematogens was claimed to be the optimum for finding a biaxial nematic liquid crystal phase. The benzo[1,2-b:4,3-b’]dithiophene core, recently successfully applied as a tetrahedral bending unit in mesogens with lateral flexible chains, is here embedded in a scaffold with only terminal chains, which conventionally promotes the formation of nematic phases at low temperature. A series of new mesogens has been successfully prepared, realising hockey-stick, hockey-stick dimer and V-shaped molecular topologies. Only the hockey-stick mesogens assemble in uniaxial nematic phases over a broad temperature range. Single crystal structure analysis of a hockey-stick and V-shaped compound reveal remarkable similarities with the benzodithiophene core wrapped by aliphatic chains. A model explaining the absence of nematic mesophases in the family of V-shaped, shape-persistent mesogens with terminal aliphatic chains is presented and results in the proposal of a new design for biaxial nematogens.</p

NIR-Absorbing Merocyanine Dyes for BHJ Solar Cells

We have synthesized a series of new, polymethine chain extended merocyanine dyes <b>1</b>–<b>4</b> bearing varied acceptor units and an aminothiophene donor moiety. The optical and electronic properties of these new merocyanines have been studied in comparison with their corresponding lower homologues <b>5</b>–<b>8</b>, which contain two methine groups less, by UV–vis and electro-optical absorption (EOA) spectroscopy and cyclic voltammetry. The absorption spectra of π-extended merocyanines are markedly red-shifted, and their extinction coefficients are significantly increased compared to those of their lower homologues. The photovoltaic characteristics of these dyes have been explored in devices using them as donor and PC₆₁BM fullerene as acceptor materials. Our detailed studies reveal that, despite more favorable absorption properties, the π-extended merocyanines exhibit lower short-circuit current densities (<i>J</i>_SC) as well as decreased open-circuit voltages (<i>V</i>_OC) and power conversion efficiencies (<i>PCE</i>) compared with those of their respective lower homologues. The unexpected decreased <i>J</i>_SC values could be explained in terms of looser packing features of π-extended chromophores in the solid state as revealed by single-crystal X-ray analysis of two pairs (<b>1</b>/<b>5</b> and <b>4</b>/<b>8</b>) of these dyes. By optimization of device setup <i>PCE</i> of 2.3% has been achieved with the π-extended donor material <b>4</b>

NIR-Absorbing Merocyanine Dyes for BHJ Solar Cells

We have synthesized a series of new, polymethine chain extended merocyanine dyes <b>1</b>–<b>4</b> bearing varied acceptor units and an aminothiophene donor moiety. The optical and electronic properties of these new merocyanines have been studied in comparison with their corresponding lower homologues <b>5</b>–<b>8</b>, which contain two methine groups less, by UV–vis and electro-optical absorption (EOA) spectroscopy and cyclic voltammetry. The absorption spectra of π-extended merocyanines are markedly red-shifted, and their extinction coefficients are significantly increased compared to those of their lower homologues. The photovoltaic characteristics of these dyes have been explored in devices using them as donor and PC₆₁BM fullerene as acceptor materials. Our detailed studies reveal that, despite more favorable absorption properties, the π-extended merocyanines exhibit lower short-circuit current densities (<i>J</i>_SC) as well as decreased open-circuit voltages (<i>V</i>_OC) and power conversion efficiencies (<i>PCE</i>) compared with those of their respective lower homologues. The unexpected decreased <i>J</i>_SC values could be explained in terms of looser packing features of π-extended chromophores in the solid state as revealed by single-crystal X-ray analysis of two pairs (<b>1</b>/<b>5</b> and <b>4</b>/<b>8</b>) of these dyes. By optimization of device setup <i>PCE</i> of 2.3% has been achieved with the π-extended donor material <b>4</b>

Exciton Coupling of Merocyanine Dyes from H- to J‑type in the Solid State by Crystal Engineering

A key issue for the application of π-conjugated organic molecules as thin film solid-state materials is the packing structure, which drastically affects optical and electronic properties due to intermolecular coupling. In this regard, merocyanine dyes usually pack in H-coupled antiparallel arrangements while structures with more interesting J-type coupling have been rarely reported. Here we show that for three highly dipolar merocyanine dyes, which exhibit the same π-scaffold and accordingly equal properties as monomers in solution, the solid-state packing can be changed by a simple variation of aliphatic substituents to afford narrow and intense absorption bands with huge hypsochromic (H) or bathochromic (J) shifts for their thin films and nanocrystals. Time-dependent density functional theory calculations show that the energetic offset of almost 1 eV magnitude results from distinct packing motifs within the crystal structures that comply with the archetype H- or J-aggregate structures as described by Kasha’s exciton theory