Yam also explored photoswitching of triarylborane-containing DTEs (9-12). Compounds 9 and 10 are inactive to photocyclization, but become photoswitchable upon fluorination (11 and 12). The authors relate the lack of photoreactivity in 9 and 10 to the existence of a highly emissive excited state. Even disregarding the presence of a competing radiative decay path, the LUMO in the nonfluorinated molecules notably localizes towards the backbone (away from the pro-reactive carbon atoms), which is predicted to hinder photocyclization. On the other hand, fluorination pushes the LUMO towards the reactive region (Figure ). Hence, we conclude that fluorination in 11 and 12 helps to localize the excited state in the pro-reactive region, facilitating the ring-closure. Compound 13 is discussed in the SI (Figure ). .