Described are colour developers of formula (I) Ar1—SO2—NH—C6H4—SO2—C6H4—NH—CO—NH—Ar2 (I), a heat-sensitive recording material, comprising a carrier substrate and also a heat-sensitive, colour-forming layer comprising at least one colour former and at least one phenol-free colour developer, the at least one colour developer being the compound of formula (I), and a method for producing this heat-sensitive recording material.