A compound of Formula (1) or a salt thereof: wherein: X is N or C(CN); Z is optionally substituted naphthyl; Ar is an aryl group carrying at least one substituent selected from the group consisting of an acetyl group, an acetate ester group, an amide group, a sulfonamide group, a sulfoxide group, a sulfone group, a phosphonate group, a nitrile group, an isonitrile group, a quaternary amine, a carbonyl group (other than a carboxylic acid), a polyhaloalkyl group or a halogen atom; and Ar may optionally be substituted with one or more further substituents.