Photochromic fine-tuning of naphthopyrans through rhenium(I) coordination: Insights into photochromism and emission switching

This study reports the synthesis, characterisation and photophysical investigation of a series of rhenium(I)-pyridyl-substituted naphthopyran complexes, fac-[Re(N^N)(CO)₃(NP)]PF₆, with a focus on how Re(I) coordination fine-tunes the photochromic properties of the naphthopyran (NP) ligands. Additionally, this study describes how the photochromic reaction notably enables reversible photoswitching of the emissive behaviour of the complexes. The Re(I) complexes were readily obtained via ligand substitution (yields = 14–83 %) and were fully characterised by FT-IR, HRMS and NMR. The absorption spectra revealed bathochromic shifts of the λ_max compared to the NP precursors, with λ_max values in the 356–386 nm range. Upon UV irradiation, the Re(I) complexes exhibited remarkably enhanced photochromic properties compared to the NP precursors, including much faster colour transitions from colourless/pale yellow to yellow/orange/red hues (λ_PSS = 422–500 nm), bathochromically shifted absorption bands (by 6–18 nm), significantly improved photocolourability (up to ∼10²-fold) and accelerated thermal bleaching kinetics. Photoluminescence studies revealed broad, diimine (N^N)-sensitive emission bands with peak maxima centred in the 442–577 nm range, along with reversible phosphorescence quenching of up to 79 % upon photochromic activation. These results highlight the potential of Re(I) coordination to finely tune the photochromic properties of naphthopyrans, offering exciting opportunities for the rational design of ultrafast, highly coloured, tunable photochromes with applications in ophthalmic lenses, molecular switches and smart materials. Additionally, Re(I) coordination enables dynamic control of phosphorescence, unlocking promising possibilities for light-responsive photonic devices.