The present invention provides a compound of formula I;

R²₂N—CH₂COCH₂—CH₂CO—OR¹  (I)

wherein R¹ represents (a) an optionally substituted branched C_6-30 alkyl group, comprising a straight chain C_4-29 alkyl group, branched by substitution with one or more C_1-6 alkyl groups, wherein said site of substitution is at C2 or a higher C atom, (b) a non-heteroaromatic aryl substituted alkyl group, wherein said aryl group is substituted, or (c) an alkoxy substituted alkyl group, wherein said alkoxy group is substituted by a methoxy group or an alkoxy group substituted with an alkoxy group, wherein in (a) and (b) said substituents are selected from hydroxy, alkoxy, acyloxy, alkoxycarbonyloxy, amino, aryl, nitro, oxo or fluoro groups, and said alkyl group is optionally interrupted or terminated by one or more —O—, NR³—, —S— or PR³— groups; R², each of which may be the same or different, represents a hydrogen atom or a group R¹, wherein R¹ represents (a) an optionally substituted branched C_5-30 alkyl group, comprising a straight chain C_4-29 alkyl group, branched by substitution with one or more C_1-6 alkyl groups, wherein said site of substitution is at C2 or a higher C atom, (b) a non-heteroaromatic aryl substituted alkyl group, wherein said aryl group is substituted, or (c) an alkoxy substituted alkyl group, wherein said alkoxy group is substituted by a methoxy group or an alkoxy group substituted with an alkoxy group, wherein in (a) and (b) said substituents are selected from hydroxy, alkoxy, acyloxy, alkoxycarbonyloxy, amino, aryl, nitro, oxo or fluoro groups, and said alkyl group is optionally interrupted or terminated by one or more —O—, NR³—, —S— or PR³— groups; and R³ represents a hydrogen or C_1-6 alkyl group, and salts thereof for use in photochemotherapy or diagnosis. Pharmaceutical compositions comprising said compound, methods of diagnosis or photochemotherapy using said compound and related compounds and compositions, and products and kits comprising said compounds are also provided.