Synthesis, Characterization, and Photophysical Properties of Bodipy-Spirooxazine and -Spiropyran Conjugates: Modulation of Fluorescence Resonance Energy Transfer Behavior via Acidochromic and Photochromic Switching

Two series of Bodipy-containing photochromic spirooxazine and spiropyran derivatives have been designed, synthesized and characterized by ¹H NMR, ESI mass spectrometry and elemental analysis. Their electrochemical and photochromic properties were investigated. The photophysical, ultrafast transient absorption, and fluorescence resonance energy transfer (FRET) properties from Bodipy (donor) to the ring-opened merocyanine (acceptor) were also studied. Upon photoexcitation, all the photochromic spirooxazine- and spiropyran-containing compounds exhibited reversible photochromism. Computational studies have been performed to provide further insights into the nature of the electronic transitions for the two classes of compounds. The rate constants and activation parameters for thermal bleaching reactions of compounds SO, SP-alkyne, <b>1</b>–<b>3</b>, and <b>8</b>–<b>10</b> were determined through kinetic studies in acetonitrile. The thermal bleaching reaction rate of the spiropyran-containing compounds is found to be much slower than that of the spirooxazine-containing counterparts