Exploring the photoisomerization of benzoylated fluorene-based phosphaalkenes and their gold complexes.

Carbon-phosphorus double bonds are promising but underexplored motifs for molecular photoswitching. Here, we report a benzoylated fluorene-based phosphaalkene and its Au(I) complexes to explore how asymmetry and heavy-atom coordination affect CP photoisomerization. The free phosphaalkene underwent clean, reversible photoisomerization under UV irradiation. Femtosecond transient absorption spectroscopy showed rapid singlet excited-state relaxation without evidence for productive triplet involvement. In contrast, gold coordination red-shifted the absorption and promoted triplet-state formation, but also led to photodecomposition. Together with computations, these experimental findings establish that the isomerization of fluorene-based CP switches proceeds predominantly on the singlet surface. Metal coordination can redirect excited-state dynamics toward a population of a long-lived state consistent with a triplet character, with the complexes displaying decreased photochemical stability.