A compound of formula (I) is provided:
wherein:
R
1
is an optionally substituted 2-(1-azathiaxanthone);
each —R
2
is independently of the formula —CH
2
—C(═O)—R
4
, wherein R
4
is an amino acid or a salt thereof, attached to the remainder of R
2
through the nitrogen atom of the amino group; and
R
3
is hydrogen or a C
1-6
alkyl group; or
wherein:
R
1
is an optionally substituted 2-(1-azaxanthone);
each R
2
is independently an optionally substituted glutaric or succinic acid, or a salt or ester thereof; and
R
3
is hydrogen or a C
1-6
alkyl group.