Reversibly thermochromic composition, reversibly thermochromic microcapsule pigment encapsulating reversibly thermochromic composition, and writing instrument using reversibly thermochromic microcapsule pigment

A reversibly thermochromic composition comprising:

• (a) a fluoran derivative represented by formula (Ia), as an electron-donating color-developing organic compound;
• (b) an electron-accepting compound;
• and
• (c) a reaction medium which reversibly induces an electron transfer reaction between the component (a) and the component (b) in a specific temperature range,
• wherein formula (Ia) is represented by
• wherein na is 0, 1, or 2, each Ya is independently a halogen atom or a linear or branched alkyl group having 1 to 4 carbon atoms, Za is a halogen atom provided that the substitution position of Za is a para position, and each Xa is independently a linear or branched alkyl group having 1 to 8 carbon atoms or a group represented by formula (ia) having 6 to 12 carbon atoms, or
• wherein na is 0, 1, or 2, each Ya is independently a halogen atom or a linear or branched alkyl group having 1 to 4 carbon atoms, Za is a halogen atom, a liner or blanched alkyl group having 1 to 4 carbon atoms, or a liner or blanched alkoxy group having 1 to 3 carbon atoms, provided that the substitution position of Za is a para position, one of the Xas is a linear or branched alkyl group having 1 to 8 carbon atoms, and the other of the Xas is the group represented by the formula (ia), wherein the formula (ia) is represented by
• wherein nia is 0 or 1 and Ria is a halogen atom, a nitro group, a cyano group, a hydroxy group, a carboxyl group, a linear or branched alkyl group having 1 to 6 carbon atoms, or a linear or branched alkoxy group having 1 to 5 carbon atoms.