Heat sensitive recording material

A heat sensitive recording material, comprising

• a) at least one colour forming compound, and
• b) at least one developer of the formula
• wherein
• R1 is phenyl or naphthyl, which can be unsubstituted or substituted by C1-C8alkyl, C1-C8-alkoxy or halogen,
• X is a group of the formula
• A is unsubstituted or substituted phenylene, naphthylene or C1-C12aikylene, or is an unsubstituted or substituted heterocyclic group,
• B is a linking group of formula —O—SO2—, —SO2—O—, —NH—SO2—, —SO2—NH—, —S—SO2—, —O—CO—NH—, —NH—CO—, —NH—CO—O—, —S—CO—NH—, —S—CS—NH—, —CO—NH—SO2—, —O—CO—NH—SO2—, —NH═CH—, —CO—NH—CO—, —S—, —CO—, —O—, —SO2—NH—CO—, —O—CO—O— and —O—PO—(OR2)2, and
• R2 is phenyl, which is unsubstituted or substituted by C1-C8alkyl, halogen-substituted C1-C8alkyl, C1-C8alkoxy-substituted C1-C8alkyl, C1-C8alkoxy, halogen-substituted C1-C8alkoxy or halogen;
• naphthyl and benzyl, which is substituted by C1-C4alkyl or halogen,
• with the proviso, that, if B is not a linking group of formula —O—SO2—, R2 is unsubstituted or substituted phenyl or naphthyl.