A highly sensitive short‐wave infrared (SWIR, λ > 1000 nm) organic photodiode (OPD) is described based on a well‐organized nanocrystalline bulk‐heterojunction (BHJ) active layer composed of a dicyanovinyl‐functionalized squaraine dye (SQ‐H) donor material in combination with PC61BM. Through thermal annealing, dipolar SQ‐H chromophores self‐assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full‐width at half‐maximum of only 85 nm (815 cm−1) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J‐type aggregates. Photoplethysmography application for heart‐rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short‐range coupled dye aggregates for low‐power operating wearable applications.

Ultranarrowband organic photodiodes (OPDs) are demonstrated for thin film solid state materials composed of tightly packed dipolar merocyanine dyes. For these dyes the packing arrangement can be controlled by the bulkiness of the donor substituent, leading to either strong H‐ or strong J‐type exciton coupling in the interesting blue (H‐aggregate) and NIR (J‐aggregate) spectral ranges. Both bands are shown to arise from one single exciton band according to fluorescence measurements and are not just a mere consequence of different polymorphs within the same thin film. By fabrication of organic thin‐film transistors, these dyes are demonstrated to exhibit hole transport behavior in spin‐coated thin films. Moreover, when used as organic photodiodes in planar heterojunctions with C60 fullerene, they show wavelength‐selective photocurrents in the solid state with maximum external quantum efficiencies of up to 11% and ultranarrow bandwidths down to 30 nm. Thereby, narrowing the linewidths of optoelectronic functional materials by exciton coupling provides a powerful approach to produce ultranarrowband organic photodiodes.

Fused S,N-heterohexacene 4 was synthesized by applying Pd-catalyzed tandem Buchwald-Hartwig coupling and further functionalized to corresponding acceptor-capped derivatives 5 and 6 showing bond length equalization in the π-conjugated backbone and intense optical transitions. Organic thin film transistors (OTFTs) based on a vacuum-deposited film of 6 exhibit p-channel charge-carrier mobilities as high as 0.021 cm(2) V(-1) s(-1) and current on/off ratios of 10(5).

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.

Strongly emissive solid‐state materials are mandatory components for many emerging optoelectronic technologies, but fluorescence is often quenched in the solid state owing to strong intermolecular interactions. The design of new organic pigments, which retain their optical properties despite their high tendency to crystallize, could overcome such limitations. Herein, we show a new material with monomer‐like absorption and emission profiles as well as fluorescence quantum yields over 90 % in its crystalline solid state. The material was synthesized by attaching two bulky tris(4‐tert‐butylphenyl)phenoxy substituents at the perylene bisimide bay positions. These substituents direct a packing arrangement with full enwrapping of the chromophore and unidirectional chromophore alignment within the crystal lattice to afford optical properties that resemble those of their natural pigment counterparts, in which chromophores are rigidly embedded in protein environments.