A compound of formula (I) is provided: wherein: R1 is an optionally substituted 2-(1-azathiaxanthone); each —R2 is independently of the formula —CH2—C(═O)—R4, wherein R4 is an amino acid or a salt thereof, attached to the remainder of R2 through the nitrogen atom of the amino group; and R3 is hydrogen or a C1-6 alkyl group; or wherein: R1 is an optionally substituted 2-(1-azaxanthone); each R2 is independently an optionally substituted glutaric or succinic acid, or a salt or ester thereof; and R3 is hydrogen or a C1-6 alkyl group.