Phyllochlorin analogues suitable for use in photodynamic therapy (PDT)

A compound of formula (I) or a complex of formula (II):

• or a pharmaceutically acceptable salt thereof, wherein:
• —R1 is selected from —CH2OR2, —CH2SR2, —CH2S(O)R2, —CH2S(O)2R2, —CH2N(R2)2 or —R2x;
• —R2, each independently, is selected from —H, —C(O)R4, —C(O)—OR4, —C(O)—SR4, —C(O)—N(R4)2, —C(S)—OR4, —C(S)—SR4, —C(S)—N(R4)2, —Rα—H, —Rβ, —Rα—Rβ, —Rα—OH, —Rα—ORβ, —Rα—SH, —Rα—SRβ, —Rα—S(O)Rβ, —Rα—S(O)2Rβ, —Rα—NH2, —Rα—NH(Rβ), —Rα—N(Rβ)2, —Rα—N3, —Rα—X, —Rα—[N(R5)3]Y, —Rα—[P(R5)3]Y, —Rα—[R6]Y, —Rα—[N(R5)2(R5′)], —Rα—[P(R5)2(R5′)] or —Rα—[R6′];
• —R2x is selected from —H, —Rα—H, —Rα—RP, —Rα—OH, —Rα—ORβ, —Rα—SH, —Rα—SRβ, —Rα—S(O)Rβ, —Rα—S(O)2Rβ, —Rα—NH2, —Rα—NH(Rβ), —Rα—N(RP)2, —Rα—N3, —Rα—X, —Rα—[N(R5)3]Y, —Rα—[P(R5)3]Y, —Rα—[R6]Y, —Rα—[N(R5)2(R5′)], —Rα—[P(R5)2(R5′)] or —Rα—[R6′];
• —R4, each independently, is selected from —H, —Rα—H, —Rβ, —Rα—Rβ, —Rα—OH, —Rα—ORβ, —Rα—SH, —Rα—SRβ, —Rα—S(O)Rβ, —Rα—S(O)2Rβ, —Rα—NH2, —Rα—NH(Rβ), —Rα—N(Rβ)2, —Rα—X, —Rα—[N(R5)3]Y, —Rα—[P(R5)3]Y, —Rα—[R6]Y, —Rα—[N(R5)2(R5′)], —Rα—[P(R5)2(R5′)] or —Rα—[R6′];
• —Rα—, each independently, is selected from a C1-C42 alkylene group, wherein the alkylene group may optionally be substituted with one or more C1-C4 alkyl, C1-C4 haloalkyl or halo groups, and wherein one or more carbon atoms in the backbone of the alkylene group may optionally be replaced by a heteroatom or group independently selected from O, S, NH or NMe;
• —Rβ, each independently, is a saturated or unsaturated hydrocarbyl group, wherein the hydrocarbyl group may be straight-chained or branched, or be or include cyclic groups, wherein the hydrocarbyl group may optionally be substituted, and wherein the hydrocarbyl group may optionally include one or more heteroatoms N, O, S, P or Se in its carbon skeleton;
• —R5, each independently, is selected from C1-C4 alkyl, C1-C4 haloalkyl, —(CH2CH2O)n—H, —(CH2CH2O)n—CH3, phenyl or C5-C6 heteroaryl, wherein the phenyl or C5-C6 heteroaryl may optionally be substituted with one or more C1-C6 alkyl, C1-C6 haloalkyl, —O(C1-C6 alkyl), —O(C1-C6 haloalkyl), halo, —CO2H, —CO2Z, —CO2NH2, —O—(CH2CH2O)n—H or —O—(CH2CH2O)n—CH3 groups;
• —R5′ is selected from C1-C4 alkyl, C1-C4 haloalkyl, —(CH2CH2O)n—H, —(CH2CH2O)n—CH3, phenyl or C5-C6 heteroaryl, each substituted with —CO2, wherein the phenyl or C5-C6 heteroaryl may optionally be further substituted with one or more C1-C6 alkyl, C1-C6 haloalkyl, —O(C1-C6 alkyl), —O(C1-C6 haloalkyl), halo, —CO2H, —CO2Z, —CO2NH2, —O—(CH2CH2O)n—H or —O—(CH2CH2O)n—CH3 groups;
• —R6 is —[NC5H5] optionally substituted with one or more C1-C6 alkyl, C1-C6 haloalkyl, —O(C1-C6 alkyl), —O(C1-C6 haloalkyl), halo, —CO2H, —CO2Z, —CO2NH2, —O—(CH2CH2O)n—H or —O—(CH2CH2O)n—CH3 groups;
• —R6′ is —[NC5H5] substituted with —CO2 and optionally further substituted with one or more C1-C6 alkyl, C1-C6 haloalkyl, —O(C1-C6 alkyl), —O(C1-C6 haloalkyl), halo, —CO2H, —CO2Z, —CO2NH2, —O—(CH2CH2O)n—H or —O—(CH2CH2O)n—CH3 groups;
• n is 1, 2, 3, 4, 5 or 6;
• X is a halo group;
• Y is a counter anion;
• Z is a counter cation;
• and
• M2+ is a metal cation.