Abstract
Organic pigments such as indigos, quinacridones, and phthalocyanines are widely produced industrially as colorants for everyday products as various as cosmetics and printing inks. Herein we introduce a general procedure to transform commercially available insoluble microcrystalline pigment powders into colloidal solutions of variously sized and shaped semiconductor micro- and nanocrystals. The synthesis is based on the transformation of the pigments into soluble dyes by introducing transient protecting groups on the secondary amine moieties, followed by controlled deprotection in solution. Three deprotection methods are demonstrated: thermal cleavage, acid-catalyzed deprotection, and amine-induced deprotection. During these processes, ligands are introduced to afford colloidal stability and to provide dedicated surface functionality and for size and shape control. The resulting micro- and nanocrystals exhibit a wide range of optical absorption and photoluminescence over spectral regions from the visible to the near-infrared. Due to excellent colloidal solubility offered by the ligands, the achieved organic nanocrystals are suitable for solution processing of (opto)electronic devices. As examples, phthalocyanine nanowire transistors as well as quinacridone nanocrystal photodetectors, with photoresponsivity values by far outperforming those of vacuum deposited reference samples, are demonstrated. The high responsivity is enabled by photoinduced charge transfer between the nanocrystals and the directly attached electron-accepting vitamin B2 ligands. The semiconducting nanocrystals described here offer a cheap, nontoxic, and environmentally friendly alternative to inorganic nanocrystals as well as a new paradigm for obtaining organic semiconductor materials from commercial colorants.
Highlights
Hydrogen-bonded organic pigments are industrially applied in the form of microcrystalline powders to colorize textiles, food, cosmetics, plastics, or cars.[1]
We found that other nanocrystal modifications of phthalocyanine showed almost no measurable gate modulation, only the long nanowires showed promising field-effect transistor (FET) performance
The demonstrated syntheses routes toward colloidal H-bonded organic pigment micronanocrystals via deprotection of latent pigment precursors can be applied to various materials and is likely not limited to the five archetypical organic pigments presented above
Summary
Hydrogen-bonded organic pigments are industrially applied in the form of microcrystalline powders to colorize textiles, food, cosmetics, plastics, or cars.[1]. The response of such a quinacridone nanocrystal/PCBM heterojunction, fabricated by drop casting of a blend with a 1/1 mass ratio on an interdigitated electrode structure with 20 μm spacing, reached up to 10 mA/W under a bias of 1 kV/cm (Figure S13) These values are on-par with those measured in the best binary blends of conjugated polymers and PCBM, which are used as photosensitive layers in state-of-the-art, solution-processed organic solar cells.[66] We have obtained here substantial improvements by replacing the nanocrystal-electron acceptor blend in the photoconducting film by direct attachment of the electron accepting species as a ligand to the nanocrystal surface. The riboflavin-coated quinacridone device exhibits a high responsivity and a high dark resistance of 100 GΩ, resulting (by assuming a Johnson noise-limited performance) in a high specific detectivity of 3 × 1013 Jones
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