Abstract: 3-Substituted isoquinoline containing squaric acids of formula (1) are described: wherein Ar1 is a 3-substituted isoquinolin-1-yl group; L2 is a covalent bond or a linker atom or group; Ar2 is an optionally substituted aromatic or heteroaromatic chain; Alk is a chain in which R is a carboxylic acid (—CO2H) or a derivative or biostere thereof; R1 is a hydrogen atom or a C1-6alkyl group; L1 is a covalent bond or a linker atom or group; Alk1 is an optionally substituted aliphatic chain; n is zero or the integer 1; R2 is a hydrogen atom or an optionally substituted heteroaliphatic, cycloaliphatic, heterocycloaliphatic, polycycloalphatic, heteropolycycloaliphatic, aromatic or heteroaromatic group; and the salts, solvates, hydrates and N-oxides thereof.