Guest Molecule Induced <scp>FRET</scp> Enhancement of Photochromic Pillar[5]arene‐Based Phenothiazine Derivatives

Comprehensive Summary In recent years, the study of the photochromic behavior of phenothiazine derivatives has not only enriched the variety of color‐changing materials but also provided new donor molecules for the construction of Förster Resonance Energy Transfer (FRET). This advancement broadens the application potential of photochromic materials and offers fresh perspective for FRET research. Herein, pillar[5]arene‐linked biphenothiazine derivative ( DPP5 ) was synthesized, while p ‐dibenzyl‐linked biphenothiazine derivative ( DPDB ) and butyl‐linked biphenothiazine derivative ( DPB ) were designed for comparative study. The photochromic behavior was demonstrated by UV‐vis spectra, electron paramagnetic resonance (EPR) and chemical oxidation method, showing the transformation of DPP5 molecule into the radical cation DPP5 •+ and subsequently into the dication DPP5 2+ . Furthermore, a FRET system was constructed using dication species DPP5 2+ as the energy donor and Nile red (NiR) as the energy acceptor. The introduction of guest molecules, 1,6‐dibromohexane ( 1,6‐DBH ) and 1,10‐dibromodecane ( 1,10‐DBD ), into the above FRET system enhanced the energy transfer efficiency by increasing the aggregation degree of FRET system by utilizing the cavity of pillar[5]arene through host‐guest interaction. The application of the photochromic behavior of phenothiazine derivatives into FRET system, along with the strategy of using guest molecule to enhance FRET properties, will contribute to the development of novel photochromic materials.