Diaminophenothiazinium derivatives of formula: in which: - one of the R 1, R 2, R 3 and R 4 groups represents -A 1 -OR 7, with A 1 representing a linear or branched alkylene chain comprising from 2 to 12 carbons, the oxygen and nitrogen atoms being separated by at least two consecutive carbon atoms, and R 7 representing a phosphorus-comprising group capable of reacting with a free hydroxyl under oligonucleotide synthesis conditions, said group forming, with OA 1 to which it is bonded, a phosphoramidite, phosphodiester or hydrogen phosphonate group, - the other R 1, R 2, R 3 and R 4 groups, which may be identical or different, represent, independently of one another: o an -A 2 -OR 8 group, with A 2 representing a linear or branched alkylene chain comprising from 2 to 12 carbons, the oxygen and nitrogen atoms being separated by at least two consecutive carbon atoms, and R 8 representing a group chosen from trityl, 4-O-monomethoxytrityl, 4,4'-O-dimethoxytrityl, tert-butyldimethylsilyl, acetyl, trifluoroacetyl, 9-phenylxanthen-9-yl and fluorenylmethyloxycarbonyl groups, ○ an alkyl group having from 2 to 12 carbon atoms, ○ or else R 1 and R 2 or R 3 and R 4 are bonded to one another to form, with the nitrogen atom to which they are bonded, a piperidinyl or pyrrolidinyl group, - R 5 and R 6, which may be identical or different, represent, independently of one another, a hydrogen, chlorine, bromine, iodine or fluorine atom, or an alkyl group having from 1 to 12 carbon atoms, an alkenyl group having from 2 to 12 carbon atoms, an alkynyl group having from 2 to 12 carbon atoms, an acyl group or a phenyl group, - X - represents an anion, preferably chosen from Cl -, I -, ClO 4 - and NO 3 -. - one of the R 1, R 2, R 3 and R 4 groups represents -A 1 -OR 7, with A 1 representing a linear or branched alkylene chain comprising from 2 to 12 carbons, the oxygen and nitrogen atoms being separated by at least two consecutive carbon atoms, and R 7 representing a phosphorus-comprising group capable of reacting with a free hydroxyl under oligonucleotide synthesis conditions, said group forming, with OA 1 to which it is bonded, a phosphoramidite, phosphodiester or hydrogen phosphonate group, - the other R 1, R 2, R 3 and R 4 groups, which may be identical or different, represent, independently of one another: o an -A 2 -OR 8 group, with A 2 representing a linear or branched alkylene chain comprising from 2 to 12 carbons, the oxygen and nitrogen atoms being separated by at least two consecutive carbon atoms, and R 8 representing a group chosen from trityl, 4-O-monomethoxytrityl, 4,4'-O-dimethoxytrityl, tert-butyldimethylsilyl, acetyl, trifluoroacetyl, 9-phenylxanthen-9-yl and fluorenylmethyloxycarbonyl groups, ○ an alkyl group having from 2 to 12 carbon atoms, ○ or else R 1 and R 2 or R 3 and R 4 are bonded to one another to form, with the nitrogen atom to which they are bonded, a piperidinyl or pyrrolidinyl group, o an -A 2 -OR 8 group, with A 2 representing a linear or branched alkylene chain comprising from 2 to 12 carbons, the oxygen and nitrogen atoms being separated by at least two consecutive carbon atoms, and R 8 representing a group chosen from trityl, 4-O-monomethoxytrityl, 4,4'-O-dimethoxytrityl, tert-butyldimethylsilyl, acetyl, trifluoroacetyl, 9-phenylxanthen-9-yl and fluorenylmethyloxycarbonyl groups, ○ an alkyl group having from 2 to 12 carbon atoms, ○ or else R 1 and R 2 or R 3 and R 4 are bonded to one another to form, with the nitrogen atom to which they are bonded, a piperidinyl or pyrrolidinyl group, - R 5 and R 6, which may be identical or different, represent, independently of one another, a hydrogen, chlorine, bromine, iodine or fluorine atom, or an alkyl group having from 1 to 12 carbon atoms, an alkenyl group having from 2 to 12 carbon atoms, an alkynyl group having from 2 to 12 carbon atoms, an acyl group or a phenyl group, - X - represents an anion, preferably chosen from Cl -, I -, ClO 4 - and NO 3 -.