Patent ID: 8119778

Claim:
A reversible thermochromic system comprising a) a compound of formula (I) or a tautomer thereof wherein R 1 is C 1 -C 18 alkyl, C 2 -C 18 alkenyl, C 3 -C 12 cycloalkyl, C 3 -C 12 cycloalkyl substituted by 1, 2 or 3 C 1 -C 4 alkyl; or C 7 -C 12 -phenylalkyl; R 2 and R 3 are hydrogen or R 2 and R 3 together form a group —CH═CH—CH═CH—; and R 4 is hydrogen, —NO 2 , —SO 2 —R 1 or (4-nitrophenyl)azo; and b) a compound containing a group of formula