A compound of general formula,, or wherein either R n and R m independently represent a C 7 -C 17 aliphatic group, or R n and R m represent an aliphatic group and the number of carbon atoms of R n and R m, as herein represented by the couple (n, m), is chosen from (7,19), (7,21), (7,23), (7,25), (7,27), (9,19), (9,21), (9,23), (9,25), (9,27), (11,19), (11,21), (11,23), (11,25), (11,27), (13,19), (13,21), (13,23), (13,25), (13,27), (15,19), (15,21), (15,23), (15,25), (15,27), (17,19), (17,21), (17,23), (17,25), (17,27), (19,19), (19,21), (19,23), (19,25), (19,27), (21,21), (21,23), (21,25), (21,27), (23,23), (23,25), (23,27), (25,25), (25,27) and (27,27) couples, R 1 to R 6 and R 9 to R 14, which may be the same or different at each occurrence, represent hydrogen or a C 1 -C 8 alkyl group or wherein a pair of substituents selected from R 1, R 2 and R 3 in formula or or from R 4, R 5 and R 6 in formula or from R 9, R 10 and R 11 in formula or from R 12, R 13 and R 14 in formula may form a divalent hydrocarbon radical with 2 to 6 carbon atoms thereby forming a ring structure with the nitrogen atom to which they are attached, R 7, R 8 and R 15, which may be the same or different at each occurrence, are hydrogen or a C 1 -C 8 alkyl group or R 7 and R 8 in formula may form a divalent hydrocarbon radical with 2 to 6 carbon atoms thereby forming a ring structure with the nitrogen atom to which they are attached, and X 1 and X 2, which may be the same or different at each occurrence, represent a linear or branched divalent hydrocarbon radical with 1 to 24 carbon atoms which can be optionally substituted and/or interrupted by one or more heteroatoms or heteroatom containing groups. wherein either R n and R m independently represent a C 7 -C 17 aliphatic group, or R n and R m represent an aliphatic group and the number of carbon atoms of R n and R m, as herein represented by the couple (n, m), is chosen from (7,19), (7,21), (7,23), (7,25), (7,27), (9,19), (9,21), (9,23), (9,25), (9,27), (11,19), (11,21), (11,23), (11,25), (11,27), (13,19), (13,21), (13,23), (13,25), (13,27), (15,19), (15,21), (15,23), (15,25), (15,27), (17,19), (17,21), (17,23), (17,25), (17,27), (19,19), (19,21), (19,23), (19,25), (19,27), (21,21), (21,23), (21,25), (21,27), (23,23), (23,25), (23,27), (25,25), (25,27) and (27,27) couples, R 1 to R 6 and R 9 to R 14, which may be the same or different at each occurrence, represent hydrogen or a C 1 -C 8 alkyl group or wherein a pair of substituents selected from R 1, R 2 and R 3 in formula or or from R 4, R 5 and R 6 in formula or from R 9, R 10 and R 11 in formula or from R 12, R 13 and R 14 in formula may form a divalent hydrocarbon radical with 2 to 6 carbon atoms thereby forming a ring structure with the nitrogen atom to which they are attached, R 7, R 8 and R 15, which may be the same or different at each occurrence, are hydrogen or a C 1 -C 8 alkyl group or R 7 and R 8 in formula may form a divalent hydrocarbon radical with 2 to 6 carbon atoms thereby forming a ring structure with the nitrogen atom to which they are attached, and X 1 and X 2, which may be the same or different at each occurrence, represent a linear or branched divalent hydrocarbon radical with 1 to 24 carbon atoms which can be optionally substituted and/or interrupted by one or more heteroatoms or heteroatom containing groups.