(1 of 12) 1605779 structure, and the perfectly reproducible synthetic procedures that largely avoid the variability between different batches seen in polymerizations. Recently, successful small-molecule donor systems have been reported that reach high power conversion efficiencies (PCEs) in organic solar cells. [2] The combination of a judicious molecular design with the optimal material processing conditions and device engineering remains a critical point for the success in the photoenergy conversion process. Rational molecular design is of great importance for further improving the photovoltaic performance of small-molecule bulk-heterojunction organic solar cells. However, achieving a high performance employing newly designed small-molecule materials remains a challenge because it is difficult to predict in sufficient detail how mole cules assemble in thin films and how this affects photovoltaic performance.When small-molecule chromophores assemble in the solid state, they often form H-type or J-type aggregates, depending on the relative alignment of the transition dipole moments on adjacent molecules. In an H-aggregate, molecules stack predominantly face-to-face, while J-aggregates form when molecules primarily stack in a head-to-tail arrangement. The formation of such aggregates has important consequences for the energies of the excited states and the oscillator strengths of the transitions to these states from the ground state. Consequently, H-and J-aggregation can strongly modify optical absorption and the photoluminescence spectra. To contribute to our understanding of the effect of molecular packing on photovoltaic performance, it is of interest to assemble one type of molecule in different packing modes.In this contribution, we do that by using molecules based on diketopyrrolopyrrole (DPP) flanked by two 5-(thiophen-2-yl) pyridin-2-yl units in which solubilizing hexyl side chains are introduced on the free positions of the peripheral thiophene units (Scheme 1). The DPP fragment has been extensively explored for organic semiconductors, because of its strong optical absorption and excellent charge transport properties, [3] especially for the design of materials for organic photovoltaic applications. [4] The electron density of the DPP core can be modified by employing different (hetero)aromatic flanking groups. The combination of DPP with two flanking pyridin-2-yl moieties leads to the simultaneous decrease of the frontier energy levels due to the electron-withdrawing nature of

A novel class of π-conjugated systems, which combine the indolo[3,2-b]carbazole unit with the formation of four-coordinate boron complexes, is presented. The resulting conjugated compounds have a double-laddered structure that provides interesting optical and electrochemical properties. The wide absorption range, covering most of the visible spectrum, along with the narrowing of the HOMO-LUMO energy gap, due to the presence of diphenylboryl centers, reinforces the potential of these molecules within the area of organic electronics.

The rational design of a rigid π-extended ligand, suitable for the formation of four-coordinate boron complexes, has led to the synthesis of the fused hexacyclic structure of carbazolo­[2,1-c]­phenanthridine. The photophysical characterization of the novel fluorophore revealed a significant Stokes shift whose intramolecular charge transfer origin has been corroborated by computational calculations. The usefulness of the reported N,N-difluoroboryl complex as fluorescent probe with large Stokes shift has been demonstrated for cancer cells imaging.

The application of organoboron compounds as light-absorbing or light-emitting species in areas as relevant as organic electronics or biomedicine has motivated the search for new materials which contribute to the progress of those applications. This article reports the synthesis of four-coordinate boron complexes based on the unexplored 7-(azaheteroaryl)indole ligands. An easy synthetic approach has enabled the fine-tuning of the electronic structure of the organoboron species by modifying a heteroaromatic component in the conjugated system. Furthermore, a comprehensive characterization by X-ray diffraction, absorption and emission spectroscopy, both in solution and in the solid state, cyclic voltammetry, and computational methods has evidenced the utility of this simple strategy. Large Stokes shifts have been achieved in solid thin-films which show a range of emitted light from blue to orange. The synthesized compounds have been used as biocompatible fluorophores in cell bioimaging.

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