A compound of the formula IVa or IVb: wherein R 1 is selected from a linear or branched (C 1 -C 18 )alkyl, or (C 3 -C 7 )cycloalkyl; R 2 and R 3 each independently is selected from a branched (C 3 -C 18 )alkyl or (C 3 -C 7 )cycloalkyl, or R 2 and R 3 together with the carbon atom to which they are attached form a fused, spiro or bridged cyclic or polycyclic ring, R 4 is H, or a caging group selected from: and wherein Pep is a peptide moiety consisting of at least two amino acid residues and linked via a carboxylic group thereof; L is absent or is a linker of the formula L1, L2 or L3, optionally substituted at the aromatic ring with one or more substituents each independently selected from (C 1 -C 18 )alkyl or (C 3 -C 7 )cycloalkyl, wherein M is absent or is -O- or -NH-, and the asterisk represents the point of attachment to the group Y, provided that M is -O- or -NH- unless R 4 is 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl or -B(OH) 2 , and when R 4 is H, L is absent; Y is absent or is -O-, provided that Y is -O- unless R 4 is 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl or -B(OH) 2 , and L is absent; R 7 is H, or represents at least one electron acceptor group selected from halogen, - NO 2 , -CN, -COOR 10 , -C(=O)R 10 and -SO 2 R 10 , each independently attached either ortho or para to the -Y-L-R 4 group; R 10 each independently is H or -(C 1 -C 18 )alkyl; and A is a π* acceptor group of the formula -CH=CH-E, attached either ortho or para to the -Y-L-R 4 group, wherein E is -CN, -COOH, -COO(C 1 -C 18 )alkyl, 4-pyridinyl, methylpyridinium-4-yl, 3,3-dimethyl-3H-indolyl, or 1,3,3-trimethyl-3H-indol-1-ium-2-yl. R 1 is selected from a linear or branched (C 1 -C 18 )alkyl, or (C 3 -C 7 )cycloalkyl; R 2 and R 3 each independently is selected from a branched (C 3 -C 18 )alkyl or (C 3 -C 7 )cycloalkyl, or R 2 and R 3 together with the carbon atom to which they are attached form a fused, spiro or bridged cyclic or polycyclic ring, R 4 is H, or a caging group selected from: and wherein Pep is a peptide moiety consisting of at least two amino acid residues and linked via a carboxylic group thereof; L is absent or is a linker of the formula L1, L2 or L3, optionally substituted at the aromatic ring with one or more substituents each independently selected from (C 1 -C 18 )alkyl or (C 3 -C 7 )cycloalkyl, wherein M is absent or is -O- or -NH-, and the asterisk represents the point of attachment to the group Y, provided that M is -O- or -NH- unless R 4 is 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl or -B(OH) 2 , and when R 4 is H, L is absent; Y is absent or is -O-, provided that Y is -O- unless R 4 is 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl or -B(OH) 2 , and L is absent; R 7 is H, or represents at least one electron acceptor group selected from halogen, - NO 2 , -CN, -COOR 10 , -C(=O)R 10 and -SO 2 R 10 , each independently attached either ortho or para to the -Y-L-R 4 group; R 10 each independently is H or -(C 1 -C 18 )alkyl; and A is a π* acceptor group of the formula -CH=CH-E, attached either ortho or para to the -Y-L-R 4 group, wherein E is -CN, -COOH, -COO(C 1 -C 18 )alkyl, 4-pyridinyl, methylpyridinium-4-yl, 3,3-dimethyl-3H-indolyl, or 1,3,3-trimethyl-3H-indol-1-ium-2-yl. <EOS>