Exciton Interactions, Excimer Formation, and [2π+2π] Photodimerization in Nonconjugated Curcuminoid‐BF 2 Dimers

Abstract : We describe the synthesis of a series of covalently linked dimers of quadrupolar curcuminoid‐BF2 dyes and the detailed investigation of their solvent‐dependent spectroscopic and photophysical properties. In solvents of low polarity, intramolecular folding induces the formation of aggregated chromophores, the UV/Vis absorption spectra of which display the optical signature characteristic of weakly‐coupled H‐aggregates. The extent of folding and, in turn, of ground‐state aggregation is strongly dependent on the nature of the flexible linker. Steady‐state and time‐resolved fluorescence emission spectroscopies show that the Frenkel exciton relaxes into a fluorescent symmetrical excimer state with a long lifetime. Furthermore, our in‐depth studies show that a weakly emitting excimer lies on the pathway toward a photocyclomer. Two‐dimensional 1H NMR spectroscopy and density functional theory (DFT) allowed the structure of the photoproduct to be established. To our knowledge, this represents the first example of a [2π+2π] photodimerization of the curcuminoid chromophore.