Aggregation of polar merocyanine dyes has been identified as an important problem in the fabrication of organic materials for photonic applications. In this work, a series of merocyanine dyes is synthesized, and their aggregation is investigated by a combination of several experimental techniques to reveal structure-property relationships. These studies provide clear evidence for the formation of centrosymmetric dimers for all investigated merocyanines in concentrated solution and in the solid state. The thermodynamics of dimerization in liquid solution is studied by concentration-dependent permittivity measurements, UV-vis spectroscopy, and electrooptical absorption experiments. A centrosymmetric dimer structure with antiparallel ordering of the dipole moments is observed in solution by 2D NMR spectroscopy as well as in the solid state by X-ray crystallography and interpreted in terms of dipolar and pi-pi interactions. The optical properties of the dimer aggregates are satisfactorily explained by an excitonic coupling model. The effect of an external electric field on the dimerization equilibrium is considered and quantitatively determined by electrooptical absorption measurements. Implications of the observed findings on the design of nonlinear optical and photorefractive materials are discussed.

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One-dimensional linear oligo-and polythiophenes are among the most prominent organic semiconductors for applications in organic electronics owing to their outstanding optical, redox, self-organizing, and transport properties.[1] As an example, regioregular poly(3-hexylthiophene) [2] (P3HT)-based organic solar cells showed excellent performances and power conversion efficiencies.[3] More complicated molecular shapes and increased dimensionality, which have important consequences on the electronic properties, were realized in 2D-macrocyclic, [4] disk-and starlike, [5] as well as in 3D-cruciform, [6] catenated, [7] and branched dendritic [8] oligothiophenes.Conjugated dendrimers represent a new class of macromolecular materials with shape-persistent and defined, monodisperse structures.[9] Müllens giant polyphenylene dendrimers impressively demonstrated that by precise spatial arrangement of functional groups, nanoobjects with unique physical properties become available.[10] Decoration of phenylene-, [11] oligophenylene-, [12] and polyphenylene-based dendrimers [13] with oligothiophenes at the periphery resulted in a mixed type of branched structures, whereas all-thiophene dendrons and dendrimers up to a 15-and a 30-mer, respectively, were reported only recently by Advicula and co-workers.[8]The aim of our work was to develop an effective approach to larger 3D-dendritic oligothiophenes that can be further functionalized at the periphery with, for example, dyes, redoxactive, self-organizing, or biological groups and easily attached to cores with interesting geometric and electronic properties. As this type of novel semiconducting macromolecules is monodisperse, they offer the significant advantage of reproducible and unique physiochemical properties, which is particularly important for organic electronic devices.As a basic building block, trimethylsilyl (TMS)-functionalized terthiophene 1 (Figure 1), including one a-a and one branching a-b connection of the thiophene units, was used to directly allow selective reactions at the free a-position (marked with arrow) and to build up higher generational oligothiophenes. In contrast with Advinculas systems, [8] in our case the TMS groups are ideal for further transformations at the outer a-positions and can be converted to functional groups by electrophilic ipso reactions or be cleaved off to give the "pure" oligothiophenes. Thus, the first series of silylated and nonsubstituted dendritic oligothiophenes have been efficiently built up in an iterative divergent/convergent protocol. Monodisperse and highly soluble dendrons and dendrimers up to a fourth generation (G4) were obtained.Terthiophene building block 1 was synthesized by nickel-catalyzed crosscoupling of the Grignard reagent of 2-bromo-5-trimethylsilyl-thiophene and 2,3-dibromothiophene (79 % yield). Removal of the TMS group with tetrabutylammonium fluoride (TBAF) quantitatively gave branched terthiophene 2. Lithiation of 1 with n-butyllithium (nBuLi) and subsequent reaction with 2-isopropyloxy-4,4,5,5-tetrameth...