The development of ambient stable organic n-channel semiconductor molecules for thin-film transistors has experienced a tremendous impetus in the last decade to close the gap in performance in comparison to that of their p-channel counterparts. Especially naphthalene and perylene tetracarboxylic diimides (NDI and PDI) have shown to be the most valuable building blocks to achieve this challenging goal and to gain insight into the molecular structure-transistor performance relationship. Remaining challenges and new emerging research fields for these n-type semiconductors are the optimization of their deposition on flexible substrates, the control of their long term ambient stability and their implementation in complementary transistor circuits, display and sensor devices.

Organic dyes and pigments constitute a large class of industrial products. The utilization of these compounds in the field of organic electronics is reviewed with particular emphasis on organic field-effect transistors. It is shown that for most major classes of industrial dyes and pigments, i.e., phthalocyanines, perylene and naphthalene diimides, diketopyrrolopyrroles, indigos and isoindigos, squaraines, and merocyanines, charge-carrier mobilities exceeding 1 cm(2) V(-1) s(-1) have been achieved. The most widely investigated molecules due to their n-channel operation are perylene and naphthalene diimides, for which even values close to 10 cm(2) V(-1) s(-1) have been demonstrated. The fact that all of these π-conjugated colorants contain polar substituents leading to strongly quadrupolar or even dipolar molecules suggests that indeed a much larger structural space shows promise for the design of organic semiconductor molecules than was considered in this field traditionally. In particular, because many of these dye and pigment chromophores demonstrate excellent thermal and (photo-)chemical stability in their original applications in dyeing and printing, and are accessible by straightforward synthetic protocols, they bear a particularly high potential for commercial applications in the area of organic electronics.

Research on small-molecule-based organic semiconductors has undoubtedly been strongly influenced by xerographic photoconductors like triarylamines, the first important organic electronic materials in market products.[1] Their development was strongly influenced by the Bässler model, which provided a rationale for the design of amorphous organic photo-and semiconductors.[2] According to this model, only compounds that lack dipole moments are considered promising for charge-carrier transport because the increased energetic disorder associated with dipole moments is thought to impede charge hopping. Recently, we questioned this paradigm in the field of organic photovoltaics (OPV) and successfully implemented highly dipolar merocyanine dyes as active components for light harvesting as well as exciton and hole transport in solution-cast bulk heterojunction (BHJ) solar cells.[3] The rationale behind our concept [4] was that highly dipolar donor-acceptor (D-A) substituted p systems (also called push-pull dyes) self-assemble into centrosymmetric dimers, [5] thus effectively eliminating molecular dipole moments on the supramolecular and material levels.[6] Two drawbacks of our BHJ materials, however, limited the acceptance of our concept so far. Firstly, the best solar cells were obtained for merocyanine dyes whose molecular scaffolds were equipped with rather bulky substituents that interfere with close face-to-face antiparallel dimerization.[3] Secondly, the power-conversion efficiencies (h) under standard AM1.5, 100 mW cm À2 simulated solar illumination conditions for solution-cast BHJ cells with fullerenes-although significantly advanced by more sophisticated vacuum processing [7] -could not be improved beyond 2.6 %, which is significantly lower than the best solutionprocessed small-molecule-based BHJ devices fabricated with A-D-A and D-A-D chromophores, for example, acceptorsubstituted oligothiophenes (up to 3.7 %) [8] and triarylamines (up to 4.3 %), [9] diketopyrrolopyrroles (up to 4.4 %), [10] and squaraines (up to 5.2 %).[11] Herein, we introduce dipolar D-A dyes with flat structures that undoubtedly form centrosymmetric dimers [5] with perfectly cancelled dipole moments in the solid state. Solution-processed BHJ solar cells derived thereof exhibit power-conversion efficiencies up to 4.5-5.1 % (dependent on light intensity), clearly placing D-A dyes now among the top-performing small molecules in the field of organic photovoltaics.Scheme 1 outlines the synthetic route that follows our earlier work on merocyanine dyes for photorefractive materials [12] and the simple access to 5-dialkylamino-thiophene-2-carbaldehydes by Hartmann.[13] Detailed synthetic procedures and characterization data are described in the Supporting Information.The optical properties of the synthesized dyes were investigated by UV/Vis and electro-optical absorption spectroscopy.[14] Furthermore, cyclic voltammetry was performed for each dye to obtain information about their highest occupied molecular orbital (HOMO) and lowest unoccupied molecu...

This article discusses recent progress by a combination of spectroscopy and quantum-chemical calculations in classifying and characterizing organic mixed-valence systems in terms of their localized vs. delocalized character. A recently developed quantum-chemical protocol based on non-standard hybrid functionals and continuum solvent models is evaluated for an extended set of mixed-valence bis-triarylamine radical cations, augmented by unsymmetrical neutral triarylamine-perchlorotriphenylmethyl radicals. It turns out that the protocol is able to provide a successful assignment to class II or class III Robin-Day behavior and gives quite accurate ground-and excited-state properties for the radical cations. The limits of the protocol are probed by the anthracene-bridged system 8, where it is suspected that specific solute-solvent interactions are important and not covered by the continuum solvent model. Intervalence charge-transfer excitation energies for the neutral unsymmetrical radicals are systematically overestimated, but dipole moments and a number of other properties are obtained accurately by the protocol.

Four well-defined π-stacks of perylene bisimide (PBI) dyes were obtained in solution by covalent linkage of two chromophores with spacer units of different length and sterical demand. Structural elucidation of the folda-dimers by in-depth nuclear magnetic resonance studies and geometry optimization at the level of density functional theory suggest different, but highly defined molecular arrangements of the two chromophores in the folded state enforced by the various spacer moieties. Remarkably, the dye stacks exhibit considerably different optical properties as investigated by UV/vis absorption and fluorescence spectroscopy, despite only slightly different chromophore arrangements. The distinct absorption properties can be rationalized by an interplay of long- and short-range exciton coupling resulting in optical signatures ranging from conventional H-type to monomer like absorption features with low and appreciably high fluorescence quantum yields, respectively. To the best of our knowledge, we present the first experimental proof of a PBI-based "null-aggregate", in which long- and short-range exciton coupling fully compensate each other, giving rise to monomer-like absorption features for a stack of two PBI chromophores. Hence, our insights pinpoint the importance of charge-transfer mediated short-range coupling that can significantly influence the optical properties of PBI π-stacks.