Process of manufacturing a compound of the formula from a compound of the formula by photooxidation wherein R 1 and R 2 independently from each other represent either H or a C 1-4 alkyl group or a halogenated C 1-4 alkyl group, a OR 9 group or a -CH 2 -OR 9 group; and wherein R 3 and R 4 either independently from each other represent H or a C 1-4 alkyl group or a halogenated C 1-4 alkyl group or a OR 9 group or a -CH 2 -OR 9 group, particularly H or CH 3; or form together a group of the formula wherein R 5 and R 6 and R 7 independently from each other represent either H or OH or a C 1-4 alkyl group or a OR 9 group; with the proviso that at least two of the residues R 5 and R 6 and R 7 are different from OH; the dotted line represents the bond by which the substituent of the formula (is bound to the rest of the compound of formula or formula; wherein R 9 is a C 1-4 alkyl group, particularly CH 3, using oxygen and a photosensitizer of the formula wherein R 8, R 8', R 8" and R 8′′′ independently from each other represent either a H, or a C 1-4 alkyl group; or wherein R 8 and R 8' and/or R 8" and R 8′′′ form together with N a five or six membered ring; and X- represents an anion; with the proviso that R 4 is different from H if R 1 =R 2 =R 3 =CH 3; and R 3 is different from H if R 1 =R 2 =R 4 =CH 3; and at least one of the residues R 1, R 2, R 3 and R 4 is different from H and at least one of the residues R 8, R 8', R 8" and R 8′′′ is different from H; in a solvent mixture of water and at least one C 1-8 alkanol or at least one C 2-4 alkylene diol; and using light which has a peak wavelength (λ max) in its spectrum in the range of between 580 and 780 nm. wherein R 1 and R 2 independently from each other represent either H or a C 1-4 alkyl group or a halogenated C 1-4 alkyl group, a OR 9 group or a -CH 2 -OR 9 group; and wherein R 3 and R 4 either independently from each other represent H or a C 1-4 alkyl group or a halogenated C 1-4 alkyl group or a OR 9 group or a -CH 2 -OR 9 group, particularly H or CH 3; or form together a group of the formula wherein R 5 and R 6 and R 7 independently from each other represent either H or OH or a C 1-4 alkyl group or a OR 9 group; with the proviso that at least two of the residues R 5 and R 6 and R 7 are different from OH; the dotted line represents the bond by which the substituent of the formula (is bound to the rest of the compound of formula or formula; either independently from each other represent H or a C 1-4 alkyl group or a halogenated C 1-4 alkyl group or a OR 9 group or a -CH 2 -OR 9 group, particularly H or CH 3; or form together a group of the formula wherein R 5 and R 6 and R 7 independently from each other represent either H or OH or a C 1-4 alkyl group or a OR 9 group; with the proviso that at least two of the residues R 5 and R 6 and R 7 are different from OH; the dotted line represents the bond by which the substituent of the formula (is bound to the rest of the compound of formula or formula; wherein R 5 and R 6 and R 7 independently from each other represent either H or OH or a C 1-4 alkyl group or a OR 9 group; with the proviso that at least two of the residues R 5 and R 6 and R 7 are different from OH; the dotted line represents the bond by which the substituent of the formula (is bound to the rest of the compound of formula or formula; wherein R 9 is a C 1-4 alkyl group, particularly CH 3, using oxygen and a photosensitizer of the formula wherein R 8, R 8', R 8" and R 8′′′ independently from each other represent either a H, or a C 1-4 alkyl group; or wherein R 8 and R 8' and/or R 8" and R 8′′′ form together with N a five or six membered ring; and X- represents an anion; wherein R 8, R 8', R 8" and R 8′′′ independently from each other represent either a H, or a C 1-4 alkyl group; or wherein R 8 and R 8' and/or R 8" and R 8′′′ form together with N a five or six membered ring; and X- represents an anion; with the proviso that R 4 is different from H if R 1 =R 2 =R 3 =CH 3; and R 3 is different from H if R 1 =R 2 =R 4 =CH 3; and at least one of the residues R 1, R 2, R 3 and R 4 is different from H and at least one of the residues R 8, R 8', R 8" and R 8′′′ is different from H; R 4 is different from H if R 1 =R 2 =R 3 =CH 3; and R 3 is different from H if R 1 =R 2 =R 4 =CH 3; and at least one of the residues R 1, R 2, R 3 and R 4 is different from H and at least one of the residues R 8, R 8', R 8" and R 8′′′ is different from H; in a solvent mixture of water and at least one C 1-8 alkanol or at least one C 2-4 alkylene diol; and using light which has a peak wavelength (λ max) in its spectrum in the range of between 580 and 780 nm.