A series of photochromic four-coordinate spirocyclic boron(iii) compounds that display thermally activated upconversion processes from the lower-lying unreactive excited state to the higher-lying photoreactive excited state is reported.

A series of cyclometalated platinum(II) complexes with a dithienylethene-containing N^C and an ancillary O^O ligand have been synthesized and characterized. All the complexes, [Pt(thpy-DTE)(dtpd)] (1), [Pt(CF 3 -thpy-DTE)(dtpd)] ( 2) and [Pt(Methpy-DTE)(dtpd)] (3), exhibit photochromism that can be triggered by photoexcitation at 420 nm into the metal-to-ligand charge transfer (MLCT)/intraligand charge transfer (ILCT) band at the visible region, which is accompanied with colour changes from yellow to green, as well as the quenching of red phosphorescence originated from the emissive triplet intraligand ( 3 IL) excited state. The photophysical and photochromic properties can be readily tuned by varying the ligands. Moreover, the fatigue resistance and thermal irreversibility of this series of complexes have also been demonstrated by the representative complex 1 with persistence of the photochromic activity after six photoswitching cycles and a thermal decay half-live of 4.8 days even at 308 K.