Transient Nonlinear Optics of Organometallic Fullerene: Research on Iron(III) and Ruthenium(III) Derivatives of C 60 Shufeng Wang, Wentao Huang, Ruisheng Liang, and Qihuang Gong* State Key Laboratory for Mesoscopic Physics & Department of Physics, Peking UniVersity, Beijing, 100871, China Hongbo Li, Huiying Chen, and Di Qiang College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing, 100871, China ReceiVed: June 19, 2001; In Final Form: September 4, 2001 We synthesized and investigated ultrafast third-order optical response of iron(III) (Fe(III)) and ruthenium(III) (Ru(III)) coordinated fullerene derivatives, which were based on our former reported C 60 (NH 2 CN) 5 series, at 830 nm. The Fe(III), which is electron deficient and blocks the intramolecular charge-transfer process, reduces the optical nonlinearity of the complexes. However, Ru(III), even it is also electron deficient, enhanced the optical nonlinearity for an order of magnitude. By analyzing the infrared spectrum, a possible effect of linked multi-fullerene molecule system is supposed. I. Introduction Fullerene and its derivatives have attracted lots of interest for their unique structures and their wide range applications. 1 The properties, including photoconductivity, 2 photolumines- cence, 3 optical limiting, 4-6 and optical nonlinearity, 7-8 have been studied recently. The structures of fullerene molecules are highly symmetric and cage-like. Their abundance of double bonds makes them possible for three-dimension chemical modification such as addition. Fullerene molecules, especially C 60 , are electron deficient. Therefore, charge transfer (CT) complexes are easily formed between fullerene and other electron donor groups. This is a kind of new and important material for nonlinear optics. 9 Many theoretical and experimental research studies on optical nonlinearity of fullerene and their derivatives have been reported. Xie has theoretically investigated the nonlinear optical properties of fullerenes, their derivates, and carbon nanotubes. 10-13 Experimentally, different measurement methods have been employed to determine the optical nonlin- earity, especially the third-order optical response, for these unique materials. The related results are listed in Table 1. From the table, we can find out that the chemical-modified fullerene, endohedral fullerene materials, and carbon nanotubes showed enhancement on their third-order optical nonlinearity compared to small host fullerene molecules. Also, this enhance- ment is greatly affected by the measuring wavelength, pulse duration, etc. At short wavelengths, 497 nm or 640 nm, e.g., the third-order optical nonlinear susceptibility,  (3) , or second- order hyperpolarizability, γ, were extremely large compared to those at infrared wavelengths. However, it should be noticed that at these short wavelengths, the optical nonlinearity for fullerene molecules, such as C 60 or C 70 , are also very strong due to the singlet resonant contribution. For practical applica- tions, nonresonant and large magnitudes of  (3) , or γ, are the most important attributes. In our previous work, we had tested kinds of fullerene materials 15 including C 60 , C 60/70 -poly- aminonitrile, C 60/70 (NH 2 CN) 5 , and C 60/70 ((NH 2 ) 2 CNCN) 5 at infrared wavelengths of 810-830 nm. We found that the charge- transfer enhancement exhibited great contributions to optical nonlinearity. The nonresonant γ of the above molecules is in the order of C 60 < C 60 -poly-aminonitrile < C 60 (NH 2 CN) 5 < C 60 ((NH 2 ) 2 CNCN) 5 . This sequence is the same for the C 70 series. This trend indicates that the enhancement on optical nonlinearity is proportional to the increase of charge-transfer strength, which is an important rule for the design and synthesis of new molecules for third-order optical nonlinear applications. 9 Organometallic fullerene is also a kind of important nonlinear optical material. The optical nonlinearities of C 60 -metal films have been investigated. 20,28 Qian et al. measured third-order optical nonlinearity of C 60 M 2 (M ) Pd, Pt, and Sm) while Mavritsky tested (Ph 3 P) 2 PtC 60 . In these reports, metal atoms were directly connected to C 60 and acted as electronic donors. 20 The effective enhancement was found and in agreement with the theoretical analysis of Wright 18 et al., who predicted that the anions of C 60 exhibited a large third-order optical response. In this paper, we carried out optical Kerr experiments on metal ions, iron(III) and ruthenium(III), coordinated fullerene deriva- tives. The metal atoms coordinated to nitrogen atoms of the side group instead of the C 60 framework. The third-order optical nonlinearity for organoiron fullerene was reduced while en- hancement was found for organoruthenium. II. Materials and Experiments A. Materials. The synthesis and purify of organometallic fullerene materials used in our experiments was based on fullerene derivatives, C 60 (NH 2 CN) 5 and C 60 (NH 2 CNCN) 5 , re- ported previously, 26 in which more than one electron donor group of -NH 2 CN and -NH 2 CNCN were added to a single C 60 molecule framework. The Fe(NO 3 ) 3 and C 60 (NH 2 CN) 5 were dissolved in water with a concentration of 10 -3 M solutions. The Fe(NO 3 ) 3 solution was mixed into the C 60 (NH 2 CN) 5 solution drop by drop until the Fe(NO 3 ) 3 was excessive. During the process, the reaction condition of pH < 2 was kept. The * Corresponding author. E-mail: qhgong@pku.edu.cn. 10784 J. Phys. Chem. B 2001, 105, 10784-10787 10.1021/jp012336g CCC: $20.00 © 2001 American Chemical Society Published on Web 10/06/2001