The author has been in charge of writing this chapter since Photochemistry vol. 37 of 2009.¹  For about 12 years, he has read many papers and introduced them to readers.^1–7  During this time, as a Japanese, the author experienced the Great East Japan Earthquake in 2011. For some time, the author thought that such a serious problem would not happen again, but two years ago, when the author started writing the chapter of Photochemistry vol. 48,⁷  the outbreak of COVID-19 spread to the world.⁸  The author remembers that it became difficult to write at the university and the deadline for the manuscript became a problem. The author never thought that COVID-19 would continue for two years. Nevertheless, our colleagues continued to publish a large number of excellent papers between 2020 and 2021 even under these circumstances. Moreover, we are witnessing the serious situation in Ukraine. The author hopes that it will be resolved as soon as possible and that our colleagues will return to a normal peaceful life.