Coupling carbon nanomaterials with photochromic molecules for the generation of optically responsive materials

Abstract

Multifunctional carbon-based nanomaterials offer routes towards the realization of smart and high-performing (opto)electronic (nano)devices, sensors and logic gates. Meanwhile photochromic molecules exhibit reversible transformation between two forms, induced by the absorption of electromagnetic radiation. By combining carbon-based nanomaterials with photochromic molecules, one can achieve reversible changes in geometrical structure, electronic properties and nanoscale mechanics triggering by light. This thus enables a reversible modulation of numerous physical and chemical properties of the carbon-based nanomaterials towards the fabrication of cognitive devices. This review examines the state of the art with respect to these responsive materials, and seeks to identify future directions for investigation.

Figure 1. Chemical structures of the most widely used photochromic molecules.

Azobenzenes, stilbenes and spiropyrans can be reverted either photochemically or thermally, while diarylethenes can be switched between open and closed form photochemically or electrochemically.

Figure 2. Functionalization of carbon-based nanomaterials with photochromic molecules.

The functionalization can be performed through either covalent or non-covalent approaches. Non-covalent modification includes π–π stacking, hydrophobic interaction or electrostatic interaction, which only mildly perturbs the sp² structure of the carbon allotrope. While covalent functionalization can be done via cycloaddtion, condensation reaction or radical polymerization and so on, offering strong and robust bonding. 0D, zero dimensional; 1D, one dimensional; 2D, two dimensional.

Figure 3. Applications of photochromic carbon-based nanomaterials in molecular junctions (upper images) and transistors (lower images).

Amide formation is used (a) to bridge two CNTs with a diarylethene molecule. Adapted from ref. (Copyright 2007 American Chemical Society) and (b) to connect covalently two graphene point contacts with an azobenzene molecule. Adapted from ref. (Copyright 2013 John Wiley & Sons, Ltd.). (c) Spiropyrans derivatized with either alkane or pyrene groups were physisorbed on CNTs. Adapted from ref. (Copyright 2005 American Chemical Society). (d) A graphene-based field-effect transistor functionalized with pyrene-substituted spiropyrans. Adapted from ref. (Copyright 2012 American Chemical Society).

Figure 4. Applications of photochromic carbon-based nanomaterials in solar thermal storage and memory devices.

(a) Mechanism of solar thermal fuels based on azobenzene covalently linked to CNTs. Reproduced from ref. (Copyright 2011 American Chemical Society). (b) Scheme for photochemical and photochemical/thermal cycling of azobenzene covalently attached to CNTs used for solar thermal fuels. Figure 4b is drawn according to ref. (Copyright 2014 Nature Publishing Group). (c) Voltage-controlled non-volatile molecular memory devices by using an azobenzene monolayer as the active layer sandwiched between two rGO electrodes via a solution process, and memory-retention performances of the ON state and the OFF state. Reproduced from ref. (Copyright 2013 John Wiley & Sons, Ltd.).

Figure 5. Applications of photochromic carbon-based nanomaterials in sensing (upper images) and biology (lower image).

(a) CNTs modified with azobenzenes were used as a color detector. Reproduced from ref. (Copyright 2009 American Chemical Society, http://pubs.acs.org/doi/pdf/10.1021/nl8032922). (b) Spiropyrans covalently attached to CNTs were used to regulate horseradish peroxidase (HRP) activity via light irradiation. Reproduced from ref. (Copyright 2011 Royal Society of Chemistry). (c) rGO/hyaluronic acid-spiropyran used for in vivo fluorescence imaging. Adapted from ref. (Copyright 2013 American Chemical Society).

Figure 6. Potential interesting photochromic molecules.

These molecules have not been combined so far with carbon-based nanomaterials, which can be used for specific applications. Intramolecular transfer of phenoxyquinone for sensing (ref. 130). Photo-induced radical formation of biindenylidenediones (ref. 131) or photo-induced valence tautomerization of cobalt complexes (ref. 132) for spintronics. The three examples proposed represent basic chemical structures, which can be possibly redesigned with different substituents.