Cascading energy transfer to extend the photochromic activation range of a donor-acceptor Stenhouse adduct

Donor-acceptor Stenhouse adducts (DASA) are a class of compounds that have been of great interest in the last decade, particularly due to their efficient negative photochromic properties, their short-step synthesis and their very high molar absorption coefficients in the visible. These photochromic dyes have numerous potential applications, but their narrow absorption band remains a challenge. Furthermore, when incorporated into polymer matrices, their photochromic properties are significantly altered. Often, their discoloration efficiencies are reduced, and their recoloring kinetics are slowed down. In this study, we demonstrate that doping a polycarbonate film with three dyes (one DASA accompanied by two well-chosen fluorophores, pyrene and coumarin) significantly expands its photochromic properties. This is made possible by a sequential double Förster-type energy transfer effect between pyrene, coumarin, and the DASA counterpart. This dual effect extends DASA activation over a wide range of the visible spectrum and into the UV. Furthermore, a systematic study of the recoloring process reveals that it occurs after irradiation at various wavelengths, starting to demonstrate the protective effect of pyrene and coumarin on the DASA dye.