A compound of the general formula (I) and mixtures thereof, wherein X is independently on each occurrence selected from groups of the formula -A-(NAr 2 ), wherein A is independently on each occurrence a chemical bond or phenylene which is unsubstituted or substituted by 1, 2 or 3 substituents independently selected from the group consisting of C 1 -C 6 alkyl and C 1 -C 6 -alkoxy; Ar is independently on each occurrence selected from in each case unsubstituted or substituted aryl, wherein two groups Ar bound to the same nitrogen atom may together with the nitrogen atom also form a fused ring system having 3 or more than 3 unsubstituted or substituted rings; Y is independently on each occurrence selected from hydrogen, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 3 -C 8 -cycloalkyl, C 3 -C 8 -cycloalkoxy, phenyl and phenyloxy, wherein each of the cyclic rings in the four last-mentioned groups are unsubstituted or substituted by 1, 2 or 3 C 1 -C 6 -alkyl groups and wherein the phenyl ring bound by a single bond to the phenylindane moiety bears at least one Y group in one of the ortho-positions on the phenyl ring relative to the phenylindane moiety that is hydrogen; k is 1 or 2; I is 1 or 2; m is 2 or 3, wherein 0, 1, 2 or 3 of the m Y groups are different from hydrogen; n is 3 or 4, wherein 0, 1, 2 or 3 of the n Y groups are different from hydrogen; the sum of k and m is 4 and the sum of l and n is 5. X is independently on each occurrence selected from groups of the formula -A-(NAr 2 ), wherein A is independently on each occurrence a chemical bond or phenylene which is unsubstituted or substituted by 1, 2 or 3 substituents independently selected from the group consisting of C 1 -C 6 alkyl and C 1 -C 6 -alkoxy; Ar is independently on each occurrence selected from in each case unsubstituted or substituted aryl, wherein two groups Ar bound to the same nitrogen atom may together with the nitrogen atom also form a fused ring system having 3 or more than 3 unsubstituted or substituted rings; A is independently on each occurrence a chemical bond or phenylene which is unsubstituted or substituted by 1, 2 or 3 substituents independently selected from the group consisting of C 1 -C 6 alkyl and C 1 -C 6 -alkoxy; Ar is independently on each occurrence selected from in each case unsubstituted or substituted aryl, wherein two groups Ar bound to the same nitrogen atom may together with the nitrogen atom also form a fused ring system having 3 or more than 3 unsubstituted or substituted rings; Y is independently on each occurrence selected from hydrogen, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 3 -C 8 -cycloalkyl, C 3 -C 8 -cycloalkoxy, phenyl and phenyloxy, wherein each of the cyclic rings in the four last-mentioned groups are unsubstituted or substituted by 1, 2 or 3 C 1 -C 6 -alkyl groups and wherein the phenyl ring bound by a single bond to the phenylindane moiety bears at least one Y group in one of the ortho-positions on the phenyl ring relative to the phenylindane moiety that is hydrogen; k is 1 or 2; I is 1 or 2; m is 2 or 3, wherein 0, 1, 2 or 3 of the m Y groups are different from hydrogen; n is 3 or 4, wherein 0, 1, 2 or 3 of the n Y groups are different from hydrogen; <EOS>