Patent ID: 8277695

Claim:
A reversible thermochromic system comprising a) a compound of formula (IA) or a tautomer thereof, or a compound of formula (IB) 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; phenyl, phenyl substituted by 1, 2 or 3 C 1 -C 4 alkyl; or C 7 -C 12 -phenylalkyl; R 2 is hydrogen or a group —SO 2 —R; R 3 and R 4 independently of one another are hydrogen, 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; phenyl, phenyl substituted by 1, 2 or 3 C 1 -C 4 alkyl; or C 7 -C 12 -phenylalkyl; and R 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; phenyl, phenyl substituted by 1, 2 or 3 C 1 -C 4 alkyl; or C 7 -C 12 phenylalkyl; and b) a compound of formula (C-1-a), (C-1-b), (C-1-c), (C-1d) or (C-6-a) and wherein the weight ratio of component a) to component b) is 1:1 to 1:20.