A compound represented by formula wherein R1 is COOH or COO-; X is an organic or inorganic anion carrying one or more negative charges; Y and Y' are H, or an alkyl; R2 is NH 2, NHBoc, or H; and Z is S or Se R comprises a group comprising an unsaturated bond in β-position to the sulfonium center, being either a carbon-carbon double bond, carbon-oxygen double bond, carbon-sulfur double bond, carbon-nitrogen double bond, a carbon-carbon triple bond, carbon-nitrogen triple bond, an aromatic carbocyclic or heterocyclic system, bound to a group comprising at least one member selected from fluorophores, fluorescence quenchers, affinity tags, crosslinking agents, nucleic acid cleaving reagents, spin labels, heavy atoms or heavy atom clusters suitable for phasing of X-ray diffraction data, radioactive or stable rare isotopes, chromophores, proteins, peptides or amino acids which may optionally be modified, nucleotides, nucleosides, nucleic acids which may optionally be modified, carbohydrates, lipids, transfection reagents, intercalating agents, nanoparticles, beads, and a functional group, the functional group being selected from an amino group, a thiol group, a 1,2-diol group, a hydrazino group, a hydroxyamino group, a haloacetamide group, a maleimide group, an aldehyde group, a ketone group, an 1,2-aminothiol group, an azido group, an alkyne group, a 1,3-diene function, a dienophilic function such as activated carbon-carbon double bond, an arylhalide group, a terminal alkyne group, an arylboronic acid group, a terminal haloalkyne group, a terminal silylalkyne group and a protected amino, thiol, 1,2-diol, hydrazino, hydroxyamino, aldehyde, ketone and 1,2-aminothiol group. R1 is COOH or COO-; X is an organic or inorganic anion carrying one or more negative charges; Y and Y' are H, or an alkyl; R2 is NH 2, NHBoc, or H; and Z is S or Se R comprises a group comprising an unsaturated bond in β-position to the sulfonium center, being either a carbon-carbon double bond, carbon-oxygen double bond, carbon-sulfur double bond, carbon-nitrogen double bond, a carbon-carbon triple bond, carbon-nitrogen triple bond, an aromatic carbocyclic or heterocyclic system, bound to a group comprising at least one member selected from fluorophores, fluorescence quenchers, affinity tags, crosslinking agents, nucleic acid cleaving reagents, spin labels, heavy atoms or heavy atom clusters suitable for phasing of X-ray diffraction data, radioactive or stable rare isotopes, chromophores, proteins, peptides or amino acids which may optionally be modified, nucleotides, nucleosides, nucleic acids which may optionally be modified, carbohydrates, lipids, transfection reagents, intercalating agents, nanoparticles, beads, and a functional group, the functional group being selected from an amino group, a thiol group, a 1,2-diol group, a hydrazino group, a hydroxyamino group, a haloacetamide group, a maleimide group, an aldehyde group, a ketone group, an 1,2-aminothiol group, an azido group, an alkyne group, a 1,3-diene function, a dienophilic function such as activated carbon-carbon double bond, an arylhalide group, a terminal alkyne group, an arylboronic acid group, a terminal haloalkyne group, a terminal silylalkyne group and a protected amino, thiol, 1,2-diol, hydrazino, hydroxyamino, aldehyde, ketone and 1,2-aminothiol group.