Use of one or more compounds of the 4-bora-3a,4a-diaza-s-indacene family for the preparation of a securing element for a product, in particular a document, said securing element comprising a polymer and said compound or compounds being incorporated in said polymer and are chosen from those of formula I: wherein, R 1 is a phenyl substituted by one or several groups chosen from methyl, fluoro, hydroxy, acetyl and methacrylate, preferably from methyl, fluoro, hydroxy and acetyl, and more preferably from methyl or fluoro; R 2 and R 2' are independently chosen from hydrogen and C1 to C2 alkyl; R 3 and R 3 are independently chosen from hydrogen, aryl, heteroaryl, cycloalkyl, alkyl, alkenyl, alkynyl, said aryl, heteroaryl, cycloalkyl, alkyl, alkenyl and alkynyl being optionally substituted by one or several groups chosen from C1 to C4 alkyl, aryl, hydroxy and ferrocene, said aryl group being optionally substituted by one or several groups chosen from aryl, C1 to C2 alkyl, halogen, hydroxy, dimethylamino, nitro, said aryl being optionally substituted by a C1 to C2 alkyl group; R 4 and R 4' are independently chosen from aryl, heteroaryl, cycloalkyl, alkyl, alkenyl, said aryl, heteroaryl, cycloalkyl, alkyl and alkenyl being optionally substituted by one or several groups chosen from C1 to C3 alkyl, aryl, hydroxy and ferrocene, said aryl group being optionally substituted by one or several groups chosen from aryl, C1 to C2 alkyl, halogen, hydroxy, dimethylamino, nitro, said aryl being optionally substituted by a C1 to C2 alkyl group; R 5 is C1 to C4 alkyl or C2 to C4 alkenyl. R 6 and R 6' are independently chosen from halogens, C1 to C4 alkyl, C2 to C4 alkenyl or aryl, said aryl being optionally substituted by one or several groups chosen from C1 to C2 alkyl, hydroxy, R5COO- and halogen. R 1 is a phenyl substituted by one or several groups chosen from methyl, fluoro, hydroxy, acetyl and methacrylate, preferably from methyl, fluoro, hydroxy and acetyl, and more preferably from methyl or fluoro; R 2 and R 2' are independently chosen from hydrogen and C1 to C2 alkyl; R 3 and R 3 are independently chosen from hydrogen, aryl, heteroaryl, cycloalkyl, alkyl, alkenyl, alkynyl, said aryl, heteroaryl, cycloalkyl, alkyl, alkenyl and alkynyl being optionally substituted by one or several groups chosen from C1 to C4 alkyl, aryl, hydroxy and ferrocene, said aryl group being optionally substituted by one or several groups chosen from aryl, C1 to C2 alkyl, halogen, hydroxy, dimethylamino, nitro, said aryl being optionally substituted by a C1 to C2 alkyl group; R 4 and R 4' are independently chosen from aryl, heteroaryl, cycloalkyl, alkyl, alkenyl, said aryl, heteroaryl, cycloalkyl, alkyl and alkenyl being optionally substituted by one or several groups chosen from C1 to C3 alkyl, aryl, hydroxy and ferrocene, said aryl group being optionally substituted by one or several groups chosen from aryl, C1 to C2 alkyl, halogen, hydroxy, dimethylamino, nitro, said aryl being optionally substituted by a C1 to C2 alkyl group; R 5 is C1 to C4 alkyl or C2 to C4 alkenyl. R 6 and R 6' are independently chosen from halogens, C1 to C4 alkyl, C2 to C4 alkenyl or aryl, said aryl being optionally substituted by one or several groups chosen from C1 to C2 alkyl, hydroxy, R5COO- and halogen.