Nonlinear Absorption and Nonlinear Refraction of Self-Assembled Porphyrins Zhi-Bo Liu, ² Yi-Zhou Zhu, ‡ Yan Zhu, ‡ Shu-Qi Chen, ² Jian-Yu Zheng, ‡ and Jian-Guo Tian* ,² The Key Laboratory of AdVanced Technique and Fabrication for Weak-Light Nonlinear Photonics Materials, Ministry of Education and TEDA Applied Physical School, Nankai UniVersity, Tianjin 300457, China, and State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai UniVersity, Tianjin, 300071, China ReceiVed: March 23, 2006; In Final Form: June 7, 2006 Nonlinear refraction and nonlinear absorption of self-assembled porphyrins in the nanosecond and picosecond regimes were studied at 532 nm by the Z-scan technique. First, a marked difference in nonlinear refraction was observed between self-assembled zinc porphyrins and free base porphyrins; however, the effects of self- assembly and metallization on nonlinear absorption are small. Second, an enhancement of nonlinear absorption was observed for the monomeric components of self-assembled structures by adding pyridine, while their nonlinear refractions remained almost unchanged as pyridine was added. It is expected that the metallization and addition of ligand can provide more convenient routes to alter the optical nonlinearities of porphyrins than the modifications of molecular structures of traditional covalent-bond organic materials. Introduction Organic compounds and organometallic complexes with π-electron delocalization have received significant attention in the last two decades because of their large and fast nonlinear optical response and potential applications in optical com- munication, data processing, optical switching, and so forth. 1,2 Porphyrins are promising candidates for such nonlinear optical (NLO) materials in view of not only large π-conjugated systems but also versatile modifications of structures and various possibilities of the central metal ion. 3,4 Most reports about optical nonlinearities of porphyrins, up to now, have been focused on reverse saturable absorption (RSA) between Soret and Q absorption bands, 5-15 off-resonant third-order nonlinearities, and two-photon absorption (TPA). 16-27 For example, under off- resonant conditions, a linear porphyrins array linked by butadiyne has been shown to exhibit among the largest  (3) values in any organic materials. 17 Anderson et al. 12 reported triply linked porphyrin with the extended region of RSA from the visible to near-infrared range. Recently, a large number of porphyrins with different molecular structures were reported on their RSA, 13-15 off-resonant third-order nonlinearities, and TPA. 21-26 However, the reports on nonlinear refraction of porphyrins in the region of RSA are few. With the development of porphyrins, 28 a supramolecular porphyrins system with large third-order optical nonlinearities and strong TPA has been obtained by using the complementary coordination of imidazolyl to the zinc of imidazolylporphyri- natozinc(II) in the noncoordinating solvent chloroform. 19,20 The self-assembly of porphyrins greatly enhanced the off-resonant third-order nonlinearities and TPA for per-porphyrin unit. In the region of RSA, to our knowledge, the effects of self- assembly and ligand on optical nonlinearities of supramolecular porphyrins are not reported yet. In this paper, we studied both nonlinear refraction and nonlinear absorption of self-assembled porphyrins by using a Z-scan method 29 at 532 nm in nanosecond and picosecond regimes. Compared with free base porphyrins, self-assembled zinc porphyrins have a marked difference of nonlinear refraction and but little difference of nonlinear absorption. Furthermore, the effects of the coordination solvent pyridine on optical nonlinearities of self-assembled zinc porphyrins were also observed. An enhancement of RSA was obtained by the addition of pyridine. However, the addition of pyridine cannot affect the nonlinear refraction of zinc porphyrins. We used a five- level model to simulate experimental data. Experimental Section Materials. Molecular structures of porphyrins studied in this work are shown in Scheme 1. Since zinc porphyrins appended with nitrogen ligands can interact with each other to form complementary dimers or multi-composites, porphyrins 1b and 2b to which 2-imidazolyl groups are directly attached were self- assembled to afford complementary dimers 3 and 4. 19 Since the coordination bond can be cleaved in coordinating solvent, self- assembled porphyrin dimers can be dissociated to monomers by the addition of pyridine, and porphyrin monomers 5 and 6 can be obtained. 28 Measurements. Electronic absorption spectra were recorded on a UV spectrophotometer (Cary 300). Nonlinear refraction and reverse saturable absorption were measured by using the closed- and open-aperture Z-scan. 29 A Q-switched Nd:YAG laser (Continuum Surelite-II) and a mode-locked Nd:YAG laser (Continuum model PY61) were used to generate 5-ns pulses and 30-ps pulses with a repetition rate of 10 Hz at 532 nm, respectively. The spatial profiles of optical pulses were nearly Gaussian obtained by spatial filtering. The beam waist was 18 µm and 20 µm for 5-ns pulses and 30-ps pulses, respectively. The incident and transmitted pulse energies were measured simultaneously with two energy detectors (Molectron J3S-10). At every position, an average of 50 incident pulses which are not beyond (5% fluctuation was taken by a computer. The sample solutions poured in a 1-mm quartz cuvette were used with the same concentration of 2 × 10 -4 mol/L. The on-axis * Author to whom correspondence should be addressed. E-mail: jjtian@ nankai.edu.cn. ² TEDA Applied Physical School, Nankai University. ‡ College of Chemistry, Nankai University. 15140 J. Phys. Chem. B 2006, 110, 15140-15145 10.1021/jp061799p CCC: $33.50 © 2006 American Chemical Society Published on Web 07/14/2006