Large femtosecond third-order nonlinear optical response in a novel donor-acceptor copolymer consisting of ethynylfluorene and tetraphenyldiaminobiphenyl units

A novel donor-acceptor conjugated copolymer consisting of the electron-acce pting moiety 2,7-diethynylfluorene and the electron-donating moiety tetraph enyldiaminobiphenyl (TPD), i.e., poly{(2,7-diethynyl-9,9-di-2-ethylhexylflu orene)-alt-co-[N,N'-diphenyl-N,N'-bis(4-phenyl)-1,1'-biphenyl-4,4'-diamine] } (TPD-PFE), and its cousins without the alkyne or TPD segments, namely, po ly{(9,9-di-2-ethylhexyl-2,7-fluorene)-alt-co-[N,N'-diphenyl-N,N'-bis(4-phen yl)-1,1'- biphenyl-4,4'-diamine]) (TPD-PF) and poly(2,7-9,9-di-2-ethylhexyl fluorenyleneethynylene) (PFE), were synthesized via Pd-catalyzed coupling r eactions. The third-order nonlinear optical (NLO) properties of these polym ers were characterized using a femtosecond time-resolved optical Kerr effec t technique. The ultrafast second-order hyperpolarizability of TPD-PFE was estimated to be as large as 4.5 x 10(-30) esu in the nonresonant region, la rger than those of its counterparts. The electronic transitions of the fluo rene-based polymers can be readily tuned by varying the nature of the co-un its in the main chain, and the intrachain charge transfer between the elect -ron-deficient and electron-excessive units can enhance the NLO properties of the polymers.