Dramatic Effect of Solvent on the Rate of Photobleaching of Organic Pyrrole‐BF₂ (BOPHY) Dyes

The photochemical stability of two symmetrical pyrrole‐BF 2 (BOPHY) dyes has been assessed under continuous exposure to white light. The parent dye, lacking conjugated substituents, is highly stable when illuminated in a nonpolar film for prolonged periods but autocatalysis is observed such that the rate of degradation escalates with increasing exposure time. The rate of photofading is faster in solution and involves intermediate formation of a coloured product. The nature of the solvent exerts a small effect on the rate of bleaching. Under the same conditions, a BOPHY derivative equipped with both extended π‐conjugation and a high internal dipole moment displays a pronounced solvent effect. Here, the rate of photofading varies over a factor of 2 000‐fold on changing the solvent from cyclohexane to toluene. Most of the kinetic data can be explained in terms of polar and neutral resonance forms but, on the basis of NMR spectroscopy, it is concluded that toluene forms a weak complex with the dye that provides protection against photochemical damage.