erazinylalkyl, and the radical represented by, -CH2-CH2#OCH2CH2#nOH wherein n is as defined above, with the proviso that at least one of Rl, R, R3 and R4 is/are L-Rs ; 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) exp