Adjustable Multimodality Fluorescence Emissions and Photochromism of Functional Molecular Device Containing Double Chromophores with Intramolecular FRET Process

Advanced biomimetic materials with tunable luminescence and changeable color have received immense attention for their great potential in various photonic applications. Herein, we fabricate two naphthalimide (NA)-functionalized spiropyran (SP) photoswitch molecular materials containing intramolecular energy donor-acceptor combinations (SP-N1, SP-N2), which have the adjustable multimodality fluorescence emissions and gradient photochromism. The intramolecular fluorescence resonance energy transfer within the excited states can be realized via designing the proximal distance between spiropyran and naphthalimide chromophore groups and their structure conversion, whereby the multimodality fluorescence emission can be generated. The color and intensity of fluorescence emissions are regulated by designing energy donor and acceptor system. The detailed spectroscopic and time-dependent density functional theory (TD-DFT) studies demonstrate that the effective photoconversion between SP-NA and the corresponding open structure merocyanine (MC) isomers (MC-NA) is facilitated by the large free volume induced by the rigid conjugation planes of the molecules. The PET composite films containing functional molecules have adjustable multimodal fluorescence emissions and variable photoinduced color. They are applied in multi-level anti-counterfeiting, and the patterns with the multi-signal output mode greatly enhance the level of anti-counterfeiting. The biomimetic materials with tunable luminescence and variable colors have potential applications in complex information encryption, advanced anti-counterfeiting, and optical devices.