Syntheses, structures, two-photon absorption cross-sections and computed second hyperpolarisabilities of quadrupolar A-pi-A systems containing E-dimesitylborylethenyl acceptors

A series of bis(E-dimesitylborylethenyl)-substituted arenes, namely arene = 1,4-benzene, 1,4-tetrafluorobenzene, 2,5-thiophene, 1,4-naphthalene, 9,10-anthracene, 4,4'-biphenyl, 2,7-fluorene, 4,4'-E-stilbene, 4,4'-tolan, 5,5'-(2,2'-bithiophene), 1,4-bis(4-phenylethynyl) benzene, 1,4-bis(4-phenylethynyl) tetrafluorobenzene and 5,5 ''-(2,2':5',2 ''-terthiophene), have been synthesised via hydroboration of the corresponding diethynylarenes with dimesitylborane. Their absorption and emission maxima, fluorescence lifetimes and quantum yields are reported along with the two-photon absorption (TPA) spectra and TPA cross-sections for the 5,5 '-bis(E-dimesitylborylethenyl)- 2,2'-bithiophene and 5,5'-bis(E-dimesitylborylethenyl)-2,2':5',2 ''-terthiophene derivatives. The TPA crosssection of the latter compound of ca. 1800 GM is the largest yet reported for a 3-coordinate boron compound and is in the range of the largest values measured for quadrupolar compounds with similar conjugation lengths. The X-ray crystal structures of 1,4-benzene, 2,5-thiophene, 4,4'-biphenyl and 5,500( 2,2':5',2 ''- terthiophene) derivatives indicate pi-conjugation along the BC=C-arene-C=CB chain. Theoretical studies show that the second molecular hyperpolarisabilities, gamma, in each series of compounds are generally related to the HOMO energy, which itself increases with increasing donor strength of the of the spacer. A strong enhancement of gamma is predicted as the number of thiophene rings in the spacer increases.