Reversibly thermochromic compositions

A reversibly thermochromic system comprising

• a) a 6,11-dihydroxy-naphthacene-5,12-dione and
• b) a base with a molecular weight of at least 300,
• wherein the base b) is an secondary or tertiary amine
• where the secondary or tertiary amine is of formula NHR26R27 or NHR26R27R28 where
• R26, R27 and R28 are independently C1-C30alkyl, C7-C12aralkyl, C2-C30alkenyl, C8-C12aralkenyl, C2-C30alkynyl or C8-C12aralkynyl, which are unsubstituted or substituted by one or more COO—R21, CONR21R22, OCO—R23, NR21CO—R23, NR23R21, O—R23, S—R23, SO—R23 and/or S(═O)2—R23;
• or the aralkyl, aralkenyl and aralkynyl are substituted by C1-C30alkyl, C2-C30alkenyl and/or C2-C30alkynyl,
• R23 is C1-C30alkyl, C2-C30alkenyl, C2-C30alkynyl, C6-C10aryl, C7-C12aralkyl, C8-C12aralkenyl or C8-C12aralkynyl and R21 and R22 are independently H or as defined for R23 or
• the secondary or tertiary amine is of formula III
• wherein G4, G5, G6 and G7 are independently methyl or ethyl,
• E is hydrogen, C1-C18 alkyl or C3-C18alkenyl and
• E10 is a carbon atom which is unsubstituted or substituted by OH, ═O or by one or two organic residues containing in total 1-500 carbon atoms,
• wherein said reversibly thermochromic system demonstrates a reversibly thermochromic property.