Compound having the structure according to formula wherein R 1, R 2 and R 3 are independently selected from C 1 -C 30 -alkyl, C 3 -C 30 -cycloalkyl, C 2 -C 30 -heteroalkyl, C 2 -C 30 -heteroaryl, C 1 -C 30 -alkoxy, C 3 -C 30 -cycloalkyloxy, C 6 -C 30 -aryloxy, at least one of R 1, R 2 and R 3 is a group represented by formula the dashed line means the single bond to the phosphorus atom, Y 1 and Y 3 are independently selected from H, phenyl, naphtyl, 1,1'-biphenylyl, quinolinyl, anthracenyl, phenanthrenyl, pyrenyl, P R 4 R 5; Y 2 and Y 4 are independently selected from H, phenyl, naphtyl, 1,1'-biphenylyl, quinolinyl, anthracenyl, phenanthrenyl, pyrenyl; R 4 and R 5 are independently selected from C 6 -C 30 aryl, C 3 -C 30 cycloalkyl and C 3 -C 30 heteroaryl, X 1 and X 2 are independently selected from C and N, with the proviso that i) both X 1 and X 2 are not N at the same time and if any of X 1 and X 2 is N, the adjacent Y is an electron lone pair ii) none of Y 1 and Y 3 is P R 4 R 5 if any of X 1 and X 2 is N, and characterized in that at least one of Y 1, Y 2, Y 3, Y 4 is 1,1'-biphenylyl or comprises at least two condensed aromatic rings. wherein R 1, R 2 and R 3 are independently selected from C 1 -C 30 -alkyl, C 3 -C 30 -cycloalkyl, C 2 -C 30 -heteroalkyl, C 2 -C 30 -heteroaryl, C 1 -C 30 -alkoxy, C 3 -C 30 -cycloalkyloxy, C 6 -C 30 -aryloxy, at least one of R 1, R 2 and R 3 is a group represented by formula the dashed line means the single bond to the phosphorus atom, Y 1 and Y 3 are independently selected from H, phenyl, naphtyl, 1,1'-biphenylyl, quinolinyl, anthracenyl, phenanthrenyl, pyrenyl, P R 4 R 5; Y 2 and Y 4 are independently selected from H, phenyl, naphtyl, 1,1'-biphenylyl, quinolinyl, anthracenyl, phenanthrenyl, pyrenyl; R 4 and R 5 are independently selected from C 6 -C 30 aryl, C 3 -C 30 cycloalkyl and C 3 -C 30 heteroaryl, X 1 and X 2 are independently selected from C and N, with the proviso that i) both X 1 and X 2 are not N at the same time and if any of X 1 and X 2 is N, the adjacent Y is an electron lone pair ii) none of Y 1 and Y 3 is P R 4 R 5 if any of X 1 and X 2 is N, and characterized in that at least one of Y 1, Y 2, Y 3, Y 4 is 1,1'-biphenylyl or comprises at least two condensed aromatic rings. Y 1 and Y 3 are independently selected from H, phenyl, naphtyl, 1,1'-biphenylyl, quinolinyl, anthracenyl, phenanthrenyl, pyrenyl, P R 4 R 5; Y 2 and Y 4 are independently selected from H, phenyl, naphtyl, 1,1'-biphenylyl, quinolinyl, anthracenyl, phenanthrenyl, pyrenyl; R 4 and R 5 are independently selected from C 6 -C 30 aryl, C 3 -C 30 cycloalkyl and C 3 -C 30 heteroaryl, X 1 and X 2 are independently selected from C and N, with the proviso that i) both X 1 and X 2 are not N at the same time and if any of X 1 and X 2 is N, the adjacent Y is an electron lone pair ii) none of Y 1 and Y 3 is P R 4 R 5 if any of X 1 and X 2 is N, and characterized in that at least one of Y 1, Y 2, Y 3, Y 4 is 1,1'-biphenylyl or comprises at least two condensed aromatic rings. R 4 and R 5 are independently selected from C 6 -C 30 aryl, C 3 -C 30 cycloalkyl and C 3 -C 30 heteroaryl, X 1 and X 2 are independently selected from C and N, with the proviso that i) both X 1 and X 2 are not N at the same time and if any of X 1 and X 2 is N, the adjacent Y is an electron lone pair ii) none of Y 1 and Y 3 is P R 4 R 5 if any of X 1 and X 2 is N, and characterized in that at least one of Y 1, Y 2, Y 3, Y 4 is 1,1'-biphenylyl or comprises at least two condensed aromatic rings. i) both X 1 and X 2 are not N at the same time and if any of X 1 and X 2 is N, the adjacent Y is an electron lone pair ii) none of Y 1 and Y 3 is P R 4 R 5 if any of X 1 and X 2 is N, and characterized in that at least one of Y 1, Y 2, Y 3, Y 4 is 1,1'-biphenylyl or comprises at least two condensed aromatic rings.