A bacteriochlorophyll derivative of the formula I: ** Formula ** wherein M represents 2H, a divalent metal atom selected from Pd, Pt, Co, Sn, Ni, Cu, Zn or Mn, or a selected trivalent metal atom among the group consisting of Fe, Mn, Co, Au, Al, Gd, Er, Yb and Cr； R1 is Y-R8, -NR9R'9 or -N + R9R'9R "9A-； Y is O or S； R2 is H, OH or COOR9； R3 is H, OH, C1-C12 alkyl or C1-C12 alkoxy ； R4 is -CH>； = CR9R'9, -CH>； = CR9Hal, -CH>； = CH-CH2-NR9R'9, -CH>； = CH-CH2- N + R9R'9R "9A-, - CHO, - CH>； = NR9, -CH>； = N + R9R'9 A-, -CH2-OR9, -CH2-SR9, -CH2-Hal, -CH2-R9, -CH2-NR9R'9, -CH2-N + R9R '9R "9 A-, -CH2-CH2R9, -CH2- CH2Hal, -CH2-CH2OR9, -CH2-CH2SR9, -CH2-CH2-NR9R'9, -CH2-CH2-N + R9R'9R" 9A-, -COCH3, C (CH3)>； = CR9R'9, - C (CH3)>； = CR9Hal, -C (CH3)>； = NR9, -CH (CH3)>； = N + R9R'9A-, -CH (CH3) -Hal, -CH (CH3) -OR9, -CH (CH3) -SR9, -CH (CH3) - NR9R'9, CH (CH3) -N + R9R'9 R "9A- or -C>； = CR9； R5 is>； = O,>； = S,>； = N-R9,>； = N + R9R'9A-,>； = CR9R'9 or>； = CR9-Hal； every R8, R9, R'9 and R "9 is independently: (a) H； (b) C1-C25 hydrocarbyl, preferably C1-C25 alkyl, more preferably C1-C10 or C1-C6 alkyl, unsubstituted or substituted with one or more radicals selected from: (i) a functional group selected from halogen, nitro, oxo , OR, SR, epoxy, epitium, aziridine, -CONRR ', -COR, COOR, -OSO3R, -SO3R, -SO2R, -NHSO2R, - SO2NRR'>； = N-OR,>； = N-NRR ', - ( CH2) nNR-COR ', - (CH2) n-CO-NRR', -O- (CH2) n-OR, -O- (CH2) nO- (CH2) nR, -PRR ', -OPO3RR', - PO2HR or -PO3RR ', where n is an integer from 1 to 6； (ii) positively charged groups selected from: a group onio； a cation derived from a group containing N selected from O ← N + (RR ') -,>； C>； = N + (RR'), - N + (RR'R "), - (R) N-N + (RR'R "), - (R) NC (>； = HN) -N + RR'R", -C (>； = NH) -N + (RR'R ")； a cation derived from a heteroaromatic compound containing one or more N atoms and optionally O or S atoms； (iii) a negatively charged group selected from COO-, COS-, -OSO3 -, -SO3 -, -OPO
