Triplet photosensitizers (PSs) are the compounds showing strong absorption of visible light, efficient intersystem crossing (ISC) thus high triplet state yield and good photostability.¹ Triplet PSs are widely used in photocatalysis, photodynamic therapy (PDT), and recently the triplet-triplet annihilation upconversion (TTA UC).² However, the conventional triplet photosensitizers are limited to the porphyrin complexes and Ru(II), Pt(II) complexes, and some halogenated xanthane dyes, such as Eosin Y. It is still a major challenge to design a new triplet photosensitizer which show predictable ISC, especially for those contain no heavy atoms because the intersystem crossing (ISC) of metal-free organic chromophores is usually unpredictable.

TTA UC has attracted much attention, due to the strong absorption of the photoexcitation energy, high upconversion quantum yields, and low photoexcitation threshold (excitation light power density can be as low as 5 mW cm^-2. Solar light is sufficient). However, most of the conventional triplet photosensitizers are Pt(II)/Pd(II) porphyrin complexes.¹ These complexes are difficult to prepare and to purify, and the molecular structures are unable to be modified feasibly. We proposed a few new approaches to prepare triplet PSs, such as to use Bodipy as the chromophore in heavy atom-containing compounds, or with C₆₀ as the electron spin converter heavy atom-free organic photosensitizers.³

Scheme 1. A strategy to prepare heavy-atom-free organic triplet photosensitizer.

Reference

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2. (a) Zhao, J.; Ji, S.; Guo, H. RSC Adv., 2011, 1, 937. (b) Zhao, J.; Ji, S.; Wu, W.; et al., RSC Adv., 2012, 2, 1712.
3. (a) Zhang, C.; Zhao, J.; Wu, S.; et al, J. Am. Chem. Soc. 2013, 135, 10566; (b) Wu, W.; Zhao, J., et al., J. Org. Chem. 2012, 77, 5305−5312; (c) Wu, W.; Zhao, J., et al., J. Org. Chem., 2011, 76, 7056–7064; (d) Guo, S.; Zhao, J., et al., Chem. Sci., 2015, 6, 3724–3737. (e) Chen, Y., Zhao, J., et al., RSC Adv., 2012, 2, 3942–3953.