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

• Published in: Chemistry : a European journal 2004-03, Vol.10 (5), p.1227-1238
• Main Authors: Traber, Boris, Wolff, J. Jens, Rominger, Frank, Oeser, Thomas, Gleiter, Rolf, Goebel, Mark, Wortmann, Rüdiger
• Format: Article
• Language: English
• Subjects: alkynes; cross-coupling; cyclotrimerization; nonlinear optics; UV/Vis spectroscopy
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200305458

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 (4 b), is reported. We used the [Co2(CO)8]‐catalysed trimerisation of 4‐N,N‐diethylamino‐4′‐nitrotolane (5) to prepare 3. The trimerisation experiment carried out with 1‐(4‐N,N‐diethylaminophenyl)‐6‐(4‐N,N‐nitrophenyl)hexatriyne (6) and [Rh(PPh3)3Cl] afforded 13. A stepwise approach was used to prepare 4 b. 1,3,5‐Trichloro‐2,4,6‐triiodobenzene (8 b) was coupled with 4‐nitrophenyl‐acetylene (14) under Pd0 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 4 b. X‐ray investigations on 3, 4 b and 13 confirmed the structural assignments and revealed that the peripheral aryl rings in 4 b are less twisted around the connecting bonds than in 3 and 13. A large second‐order polarisability (β) of 4 b relative to 3 and 13 was determined by hyper‐Rayleigh scattering (HRS). Compound 4 b represents an NLO chromophore with second‐order polarisabily among the highest obtained so far for two‐dimensional nondipolar NLO chromophores. Studies on three new hexasubstituted donor–acceptor benzenes with threefold symmetry demonstrate that molecular structure and linear/nonlinear optical (NLO) properties strongly correlate with each other. The increasing length of the π system causes higher βzzz values from 1 to 2 and from 2 to 3 owing to reduced steric hindrance between donor/acceptor phenyl groups. Compound 3 is a new type of NLO chromophore with the highest second‐order polarisability among the two‐dimensional nonpolar NLO chromophores obtained so far.