l, and the radical represented by, CH2CH2#OCH2CH2#nOH wherein n is any integer between 1 and 20, with the proviso that at least one of Rl, R, R3 and R4 is/are LR5; or a salt thereof, or a composition including said anthraquinone compound or salt thereof, with predetermined conditions so as to reduce the anthraquinone compound or salt thereof to a reduced form oxidizable by oxygen; and (ii) exposin