A Supramolecular Nanocomposite as a Near-Infrared-Transmitting Optical Filter for Security and Forensic Applications

Abstract

Visibly opaque but near-infrared (NIR)-transparent materials are an essential component for night-vision photography, security imaging, and forensic applications. Herein, the development of a novel supramolecular black dye from a diketopyrrolopyrrole (DPP)-based low-molecular-weight organogelator is described. In the solution state, the monomer of DPP–Amide exhibits a deep green color with a broad absorption in the visible region due to firm intramolecular charge transfer from the donor to the acceptor unit. Interestingly, due to the synergistic effect of H-bonding and π-stacking, DPP–Amide can form a black organogel in toluene with complete spectral coverage from 300 to 800 nm, and transmits beyond 850 nm. In the gel state, complete visible-spectrum coverage is achieved due to the simultaneous formation of both H- and J-type aggregates, which is confirmed via absorption studies. To create a free-standing NIR-transmitting elastomeric black filter, nanoscopic molecular aggregates of DPP–Amide (0.15 wt%) are embedded into a poly(dimethylsiloxane) matrix. This nanocomposite possesses high NIR transparency with good thermal and photostability for practical applications. Finally, the use of the developed material for NIR photography, security, and forensic-related applications is demonstrated.

A π-conjugated diketopyrrolopyrrole (DPP–Amide) forms a black supramolecular organogel with a broad spectral absorption from 300 to 800 nm, and transmits beyond 850 nm. A free-standing near-infrared-filter lens fabricated by incorporating the nanoscopic molecular aggregates of DPP–Amide into a poly(dimethylsiloxane) (PDMS) matrix is found to be useful for NIR photography, security, and forensic-related applications.

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