Femtosecond Third-Order Optical Nonlinearity of Conjugated Polymers Consisting of Fluorene and Tetraphenyldiaminobiphenyl Units: Structure-Property Relationships Xiaowei Zhan, ² Yunqi Liu,* ,² Daoben Zhu,* ,² Wentao Huang, ‡ and Qihuang Gong ‡ Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Mesoscopic Physics Laboratory & Department of Physics, Peking UniVersity, Beijing 100871, China ReceiVed: September 10, 2001; In Final Form: NoVember 13, 2001 Femtosecond time-resolved optical Kerr effect technique has been used to investigate the third-order nonlinear optical (NLO) properties of a series of conjugated polymers consisting of fluorene and/or tetraphenyldiami- nobiphenyl (TPD) units designed to elucidate structure-property relationships for the microscopic second- order hyperpolarizability γ in polymeric materials. The γ per repeated unit of the series of polymers has off-resonant values at 830 nm in the range of 2.0 × 10 -33 -2.4 × 10 -31 esu, demonstrating a large modulation of nonlinear optical response by simple structural variations. The γ values of alternative copolymers containing fluorene and TPD moieties are 2 orders of magnitude higher than the γ value of the homopolymer polyfluorene, revealing the vital role of the strong electron donor TPD in the NLO enhancement. The fluorene segment was found to result in 4-fold enhancement of γ in TPD-containing copolymers compared to p-phenylenevinylene segment, indicating that the planar rigid ring of fluorene is an efficient third-order NLO chromophore. No relationship between the magnitude of γ and the optical band gap was found. The large variation of γ value with molecular structure of these polymers can be explained by molecular exciton theory other than the band theory. Introduction Since the advent of nonlinear optics, there has been a long and sustained interest in organic and polymeric materials with significant nonlinear optical (NLO) properties. The interest is driven by the need for devices for the shifting (doubling and tripling) of frequencies into domains where inexpensive and efficient light sources are unavailable 1 and also by the need for devices for optical signal processing, all-optical switching, optical computing, bistable elements, logic devices, and sensor protection. 2-4 The criteria for practical applications and device fabrication include high nonlinear susceptibility, fast response times, adequate optical transparency, excellent environmental stability, and good processability for multilayer integration into large-area devices. Although conjugated quasi one-dimensional polymers with delocalized π-electron backbone have the largest nonresonant third-order NLO susceptibility  (3) with ultrafast response times (<1 ps) among all classes of materials, the measured off-resonant  (3) values in the range of 10 -12 -10 -9 esu are still orders of magnitude less than the projected requirements for practical photonic applications. 5-8 An under- standing of the structure-function relationships that underlie the third-order NLO properties of conjugated polymers is currently lacking but is required to guide the rational design of polymers with enhanced optical nonlinearity. 9 During our search for highly efficient third-order NLO materials, 10-18 we were attracted by the structural feature of the well conjugated, planar, and rigid ring of fluorene with ease to substitute with solubilizing groups. 19 It is well-known that increasing planarity and rigidity helps to enhance third-order susceptibilities as it optimizes the overlap of π orbitals thus enhancing electron delocalization. 20 On the other hand, we were also interested in the strong electron donating ability of tetraphenyldiaminobiphenyl (TPD) with high charge carrier mobility 21 since the third-order optical nonlinearity can be enhanced by increasing π delocalized electron density and the carrier transport. 22 Thus, we synthesized a series of systemati- cally designed polymers containing fluorene and/or TPD units, 11,23,24 and found that an alternative copolymer consisting of ethynylfluorene and TPD exhibited a large nonresonant second-order molecule hyperpolarizability γ up to 4.5 × 10 -30 esu. 11 In this paper, we investigated the third-order nonlinear optical properties of a series of conjugated polymers containing fluorene and/or TPD moieties using a femtosecond time-resolved optical Kerr effect (OKE) technique. We probed structure-property relationships for the microscopic second-order hyperpolariz- ability γ in these polymers, and found the important role of the strong electron donor TPD and the planar rigid ring of fluorene in the NLO enhancement. Experimental Section Materials. The detailed synthesis and characterization of the polymers investigated here (TPD-PFE, TPD-PF, TPD-PPV, and PF) have been reported elsewhere. 11,23,24 Characterization of the molecular and electronic structures of the polymers was done by 1 H NMR and FTIR spectroscopies, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV- vis absorption spectroscopy, photoluminescence spectroscopy, and cyclic voltammetry. 11,23,24 The chemical structures of these polymers are illustrated in Figure 1. Absorption and Fluorescence Spectra. UV-vis absorption spectra of the polymers in chloroform solution were recorded * To whom correspondence should be addressed. Fax: +86-10- 62559373. E-mail: liuyq@infoc3.icas.ac.cn. ² Center for Molecular Science. ‡ Mesoscopic Physics Laboratory & Department of Physics. 1884 J. Phys. Chem. B 2002, 106, 1884-1888 10.1021/jp013472o CCC: $22.00 © 2002 American Chemical Society Published on Web 02/05/2002