Compounds for labelling nucleic acid and uses thereof

A compound having a formula:

• or a salt thereof;
• wherein:
• R1 is H, haloalkyl, alkyl, alkenyl, alkynyl, cycloalkyl, aryl, heteroaryl, heterocyclyl, or an optionally substituted group thereof;
• R2, R2a, R2b, and R3 are independently H, hydroxy, halogen, haloalkyl, cyano, amino, thiol, alkyl, alkenyl, alkynyl, cycloalkyl, aryl, heteroaryl, heterocyclyl, alkoxy, sulfenyl, acyl, sulfinyl, sulfonyl, O-carboxy, ester, thiocarbonyl, or an optionally substituted group thereof;
• R4 is an optional group and is H, haloalkyl, alkyl, alkenyl, alkynyl, cycloalkyl, aryl, heteroaryl, heterocyclyl, or an optionally substituted group thereof;
• and
• Y is an optional linker;
• provided that the compound is not