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Organic thin film transistors (OTFTs) of a series of twenty dipolar donor-acceptor-substituted polymethine dyes (D-A dyes, dipole moments from 3-15 D) are investigated. The employed merocyanine dyes contain a dimethine bridge that is substituted with 1-alkyl-3,3-dimethylindolin-2-ylidene ('Fischer base'), 3-alkyl-2,3-dihydrobenzothiazol-2-ylidene or 1,3-benzodithiole-2-ylidene, respectively, as electron-donating unit and various acceptor heterocycles. These studies show that thin films formed by these D-A dyes upon deposition in high vacuum are all composed of antiparallel π-stacked dimers. However, they are either amorphous, discontinuous or highly crystalline due to the interplay between molecule-substrate and dimer-dimer interactions. With the help of single crystal X-ray analysis, out-of-plane X-ray studies (XRD), selected area electron diffraction (SAED), and atomic force microscopy (AFM), a correlation between the molecular structure, film ordering, and hole charge transport ability can be established. The mobility values are compared to Bässler's disorder charge transport theory and a film growth mechanism is proposed based on DFT calculations and single crystal structures. The results show that with carefully adjusted bulky substituents and high dipolarity an intimate centrosymmetric packing with a slipped, but tight π-stacking arrangement could be realized. This provides two-dimensional percolation pathways for holes and ultimately results in charge carrier mobilities up to 0.18 cm2 V−1 s−1. [ABSTRACT FROM AUTHOR]

Multivalent organic molecules possessing spherically well-arranged functionalities are promising building blocks for extended molecular networks. [60]Fullerene hexakisadducts are intriguing candidates allowing the organization of up to 12 functional groups within one molecule with high geometric precision. We have synthesized a hexakisadduct bearing 12 glycolic acid side-chains in good yield and with high regioisomeric purity and elucidated its solid-state structure from single crystals grown by the slow diffusion of dichloromethane into a methanol solution. Individual fullerene moieties are arranged in cubic close-packing mode and form an extended network by means of multiple supramolecular interactions. Each molecule forms 24 hydrogen bonds and is connected to six neighbouring fullerene derivatives. The crystal structure showcases the first example of a 3D arrangement of fullerene derivatives by directed hydrogen bonding in the solid state. These findings indicate the high potential of six-fold substituted fullerene derivatives as building blocks for crystalline frameworks. [ABSTRACT FROM AUTHOR]

A broad series of more than 20 acceptor-substituted squaraines was synthesized that feature different acceptor functionalities at the central squaraine four-membered ring. The influence of these acceptor units on the reactivity of semisquaraine precursors and stability of the respective squaraines were explored. Thereby the dicyanovinyl group was found to be the most versatile acceptor group that enabled various modifications at the donor moiety of the squaraine scaffold, leading to an extended series of dicyanovinyl-functionalized squaraines. The variation of donor units afforded a set of NIR fluorophores that cover a wavelength region from the visible at about 650 nm far into the NIR up to 920 nm with fluorescence quantum yields between 0.93 and 0.11 and outstanding optical brightness. This excellent optical property is related to a rigid molecular scaffold that is fixed in an all- cis configuration by the additional dicyanovinyl acceptor unit. The change of the molecular symmetry from C2 h to C2 v upon functionalization of the squaraine core with dicyanovinyl acceptor group has been confirmed in solution by electro-optical absorption (EOA) spectroscopy, revealing permanent ground-state dipole moments μg in the range between 4.3 and 6.4 D. These dipole moments direct an antiparallel packing of the molecules in the solid state according to single-crystal X-ray analyses achieved for four dicyanovinyl-functionalized squaraines. The structural properties, the EOA results, as well as the band shapes of the optical spectra indicate that these polymethine dyes are cyanine-type chromophores. It is worth noting that the orientation of the dipole moment vectors is orthogonal to the orientation of the transition dipole moment vectors, which is an uncommon but characteristic feature of this rather novel class of polymethine dyes. With regard to applications of these dyes in organic solar cells, their redox properties were also studied by cyclic voltammetry. [ABSTRACT FROM AUTHOR]