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-accepting moiety 2,7-diethynylfluorene and the electron-donating moiety tetraphenyldiaminobiphenyl (TPD), i.e., poly{(2,7-diethynyl-9,9-di-2-ethylhexylfluorene)-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, poly{(9,9-di-2-ethylhexyl-2,7-fluorene)-alt-co-[N,N'-diphenyl-N,N'-bis(4-phenyl)-1,1'- biphenyl-4,4'-diamine]) (TPD-PF) and poly(2,7-9,9-di-2-ethylhexylfluorenyleneethynylene) (PFE), were synthesized via Pd-catalyzed coupling reactions. The third-order nonlinear optical (NLO) properties of these polymers were characterized using a femtosecond time-resolved optical Kerr effect 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, larger than those of its counterparts. The electronic transitions of the fluorene-based polymers can be readily tuned by varying the nature of the co-units 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.