Reversibly thermochromic, fluorescent ultrathin films with a supramolecular architecture.

Tunable luminescent materials that respond to different external stimuli have attracted great attention during the last few years, owing to their potential applications in fluorescent switches, sensors, and optical recording devices. Mechanisms responsible for the change in luminescence include chemical reactions and alteration of the molecular packing mode. Since solid-state chemical reactions frequently have low conversion efficiency, recent attention has focused on controlling and tuning the molecular packing mode as a strategy for the design and preparation of intelligent luminescent materials. To date, although several types of responsive luminescent materials have been developed (e.g., piezochromic, deformation-induced chromic, photochromic, thermochromic, and humidity-related colorimetric luminescent systems), challenges remain. For instance, to meet the requirements of luminescent devices or sensors, it is important to be able to assemble ordered thin films with regular molecular orientation and intermolecular packing mode on two-dimensional surfaces. Switching of solid-state luminescence based on such ordered thin films is rather rare, however. Therefore, it is of crucial importance to develop new ways to fabricate ordered film systems with fluorescent properties which respond to environmental stimuli. Furthermore, fast response, facile reversibility, and stable repeatability are all necessary from the viewpoint of practical application of such materials in sensors. Therefore, there is an urgent demand to develop new types of solid-state responsive materials as well as sensors with high efficiency, stability, and reproducibility. Herein we present a supramolecular ultrathin film (UTF) system with thermochromic luminescence (TCL) based on the layer-by-layer (LBL) assembly of anionic bis(2-sulfonatostyryl)biphenyl (BSB, Scheme 1a) and positively charged layered double hydroxide (LDH, Scheme 1b) nanosheets as basic building blocks. BSB is generally used as a fluorescent brightener in the chemical industry; moreover, the phenylenevinylene unit in BSB also has attracted considerable interest because of its excellent optical and electronic properties. LDHs are a class of layered anionic clays which have been widely used in the fields of catalysis, biology, and optical materials. LDHs can be exfoliated into positively charged nanosheets which can be fabricated into functional ultrathin films (UTFs) by LBL deposition of alternate layers of the LDH and polymers or metal complexes. However, LBL assembly of LDH nanosheets with small anions has rarely been reported because of the tendency of the small anions to escape from the nanosheets. Herein we describe fabrication of ordered BSB/LDH UTFs (Scheme 1) with a fast and reversible TCL response at ambient temperature. The TCL behavior of the assembled BSB anions, which is absent for their pristine form, originates from the host–guest interactions within the UTF system. Molecular dynamics (MD) and periodic density functional theoretical (PDFT) studies demonstrate that the TCL process of the BSB/LDH system is related to the changes in the orientation and aggregation of BSB anions between LDH monolayers. Moreover, coassembly of BSB with other luminescent anions into a UTF allows fabrication of responsive TCL film systems, in which the ratio of the luminescence at two different wavelengths can be reversibly transformed by varying the temperature, thus altering the luminescent color of the film. Therefore, this work provides a feasible route for designing and constructing luminescent supramolecular structures with potential applications in sensors and switching. The multilayer assembly process of the BSB/LDH UTFs was monitored by UV/Vis absorption spectra, which showed Scheme 1. Assembly process for the (BSB/LDH)n UTF. a) Chemical structure and schematic representation of BSB. b) Schematic illustrations of one Mg–Al–LDH monolayer. c) Quartz glass substrate.