“…Excited-state intramolecular proton transfer (ESIPT) is a feature of organic molecules, containing strong proton-donating/accepting groups, which, due to the increased acidity/basicity in the excited state, exchange a proton over the short distance of pre-existing hydrogen bonding [1][2][3][4][5][6][7][8][9][10]. ESIPT-exhibiting molecules have become a field of active research in recent decades, due to their applications as light-emitting materials and laser dyes [11][12][13][14][15], optical sensors [16][17][18][19][20][21][22][23][24][25][26][27], organic light-emitting diodes [20,21,[28][29][30][31][32][33][34][35][36], optical storage devices [37] and photo stabilizers [38]. The same process underlies a special class of bistable photo switches [39], called proton cranes, where the ESIPT is accompanied by intramolecular rotation [40], leading to the transport of a movable proton from one side of the molecule to another.…”