Hexasubstituted Donor–Acceptor Benzenes as Nonlinear Optically Active Molecules with Multiple Charge‐Transfer Transitions

The synthesis of three novel nonlinear optical (NLO) chromophores with threefold symmetry, namely 1,3,5-tris(4-N,N-diethylaminophenyl)-2,4,6-tris(4-nitrophenyl)benzene (3), 1,3,5-tris(4-N,N-dihexylaminophenylbutadiynyl)-2,4,6-tris(4-nitrophenyl)benzene (13) and 1,3,5-tris(4-N,N-dihexylaminophenylethynyl)-2,4,6-tris(4-nitrophenylethynyl)benzene (4b), is reported. We used the [Co 2 (CO) 8 ]-catalysed trimerisation of 4-N,N-diethylamino-4'-nitro-tolane (5) to prepare 3. The trimerisation experiment carried out with 1-(4-N,N-diethylaminophenyl)-6-(4-N,N-nitrophenyl)hexatriyne (6) and [Rh-(PPh 3 ) 3 Cl] afforded 13. A stepwise approach was used to prepare 4b. 1,3,5-Trichloro-2,4,6-triiodobenzene (8b) was coupled with 4-nitrophenyl-acetylene (14) under Pd° catalysis to yield 1,3,5-trichloro-2,4,6-tris(4-nitrophenylethynyl)benzene (15). The coupling reaction of 15 with 4-N,N-dihexylaminophenylethynyltributylstannane (21) led to 4b. X-ray investigations on 3, 4b and 13 confirmed the structural assignments and revealed that the peripheral aryl rings in 4b are less twisted around the connecting bonds than in 3 and 13. A large second-order polarisability (β) of 4b relative to 3 and 13 was determined by hyper-Rayleigh scattering (HRS). Compound 4b represents an NLO chromophore with second-order polarisabily among the highest obtained so far for two-dimensional nondipolar NLO chromophores.