4 ethynylaniline

Aryl diazonium salt D-NO 2 was prepared by the method published previously by our group. 35 2 g of 4-nitroaniline (≥99%, Sigma-Aldrich) and 10 mL of acetonitrile (≥99.9%, Sigma-Aldrich) were sequentially added to 12 mL of tetrafluoroboric acid (48% in H 2 O, Sigma-Aldrich). Then the mixed solution was transferred to an ice water bath and cooled down to 2 °C under continuous stirring. Afterwards, 2 mL of tert-butyl nitrite (90%, Sigma-Aldrich) was added to the reaction solution drop by drop. The solution was incubated for 30 min under stirring. After 3 times washes by diethyl ether (≥99.7%, Sigma-Aldrich), the resulting diazonium salt D-NO 2 was dissolved in acetone (≥99.9%, Sigma-Aldrich) and kept at room tempera-ture for drying. For the preparation of D-CCH and D-CN, 4-ethynylaniline (97%, Sigma-Aldrich) and 4-aminobenzonitrile (98%, Sigma-Aldrich) were used as corresponding precursors based on the same protocol.

4-ethynylaniline (HC ---CC 6 H 4 NH 2 ), 4-azidoaniline hydrochloride (N 3 C 6 H 4 NH 2 • HCl), sodium nitrite (NaNO 2 ), tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA), tetrakis(acetonitrile)copper(I) hexafluorophosphate (TCH), (+)-sodium L-ascorbate (SA), dimethyl sulfoxide (DMSO), acetonitrile (ACN), iron(III) chloride hexahydrate (FeCl 3 • 6 H 2 O), potassium chloride (KCl), hydrochloric acid (HCl, 37 %), isopropyl alcohol (IPA) and acetone were purchased from Sigma Aldrich (Belgium). All reagents were used without further purification. Alkyne-and azide-modified oligonucleotides (25mer) were purchased from IDT (Belgium) (sequences in Supporting Information).