Topochemical Polymerization at Diacetylene Metal-Organic Framework Thin Films for Tuning Nonlinear Optics

Developing the topochemical polymerization of metal-organic frameworks (MOFs) is of pronounced significance for expanding their functionalities but is still a challenge on third-order nonlinear optics (NLO). Here, we report diacetylene MOF (CAS-1-3) films prepared using a stepwise deposition method and film structural transformation approach, featuring dynamic structural diversity. The MOF structures were determined by the three-dimensional electron diffraction (3D ED) method from nanocrystals collected from the films, which provides a reliable strategy for determining the precise structure of unknown MOF films. We demonstrate the well-aligned diacetylene groups in the MOFs can promote topological polymerization to produce a highly conjugated system under thermal stimulation. As a result, the three MOF films have distinct NLO properties: the CAS-1 film exhibits saturable absorption (SA) while CAS-2 and CAS-3 films exhibit reverse saturable absorption (RSA). Interestingly, due to the topochemical polymerization of the MOF films, a transition from SA to RSA response was observed with increasing temperatures, and the optical limiting effect was significantly enhanced (∼46 times). This study provides a new strategy for preparing NLO materials and thermally regulation of nonlinear optics.

Temperature-Controlled Pathway Complexity in Self-Assembly of Perylene Diimide-Polydiacetylene Supramolecule

Self-assembly process represents one of the most powerful and efficient methods for designing functional nanomaterials. For generating optimal functional materials, understanding the pathway complexity during self-assembly is essential, which involves the aggregation of molecules into thermodynamically or kinetically favored pathways. Herein, a functional perylene diimide (PDI) derivative by introducing diacetylene (DA) chains (PDI-DA) is designed. Temperature control pathway complexity with the evolution of distinct morphology for the kinetic and thermodynamic product of PDI-DA is investigated in detail. A facile strategy of UV-induced polymerization is adopted to trap and capture metastable kinetic intermediates to understand the self-assembly mechanism. PDI-DA showed two kinetic intermediates having the morphology of nanosheets and nanoparticles before transforming into the thermodynamic product having fibrous morphology. Spectroscopic studies revealed the existence of distinct H- and J-aggregates for kinetic and thermodynamic products respectively. The polymerized fibrous PDI-DA displayed reversible switching between J-aggregate and H-aggregate.

One-Step Interfacial Integration of Graphene Oxide and Organic Chromophores into Multicomponent Nanohybrids with Photoelectric Properties

A one-step protocol for interfacial self-assembly of graphene oxide (GO), glutamine-substituted perylene diimide (PDI-glu), 10,12-pentacosadiynoic acid (PCDA), and zinc acetate into three- and four-component hybrid nanofilms through hydrogen and coordination bonding was developed. The hybrids deposited onto solid supports were studied after polymerization of PCDA by UV-vis absorption, fluorescence, and Raman spectroscopies, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results of spectroscopic studies suggest that the hybrids assembled through H-bonds can maintain the light-induced Förster energy transfer from the PDI-glu chromophore to the conjugated polymer and then to GO leading to fluorescence quenching. In the hybrids assembled through coordination bonding with zinc clusters, the energy transfer proceeds from PDI-glu to the PDA polymer, whereas the transfer from PDA to GO is quenched completely. Another important characteristic of these ultrathin hybrids is their stability with respect to photobleaching of chromophores due to the acceptor properties of GO. The as-assembled hybrid nanofilms were integrated with conventional photovoltaic planar architectures to study their photoelectric properties. The zinc-containing hybrids integrated with a hole transport layer exhibited photovoltaic properties. The cell with the integrated four-component hybrid comprising both PDI-glu and PDA showed a photocurrent/dark current ratio almost an order higher than that of the three-component hybrid assembled with PDA only. The supramolecular method based on the interfacial self-assembly can be extended to a wide variety of organic chromophores and polymerizable surfactants for integrating them into multicomponent functional GO-based nanohybrids with targeted properties for organic electronics.