2-(2′-Hydroxyphenyl)benzoxazole-Containing Two-Photon-Absorbing Chromophores as Sensors for Zinc and Hydroxide Ions Yanqing Tian, †,§ Ching-Yi Chen, † Chang-Chung Yang, †,‡ A. Cody Young, † Sei-Hum Jang, †,§ Wen-Chang Chen, ‡,| and Alex K.-Y. Jen* ,†,§ Department of Materials Science and Engineering, Box 352120, UniVersity of Washington, Seattle, Washington 98195-2120; Department of Chemical Engineering, National Taiwan UniVersity, Taipei, Taiwan 106; Institute of AdVanced Materials and Technology, UniVersity of Washington, Seattle, Washington 98195; and Institute of Polymer Science and Engineering, National Taiwan UniVersity, Taipei, Taiwan 106 ReceiVed September 4, 2007. ReVised Manuscript ReceiVed NoVember 30, 2007 Three new two-photon-absorbing (2PA) chromophores derived from 2-(2′-hydroxyphenyl)benzoxazole (HPBO), having two-photon absorption cross sections up to 530 GM (δ, expressed in GM; 1 GM ) 1 × 10 -50 cm 4 s photon -1 molecule -1 ), were synthesized and investigated as metal ion and pH sensors. These chromophores were designed with an increasing number (0, 1, and 2) of HPBO(s) groups as binding sites for metal ions. Such variations provide a systematic approach to correlate structures with their linear photophysical, sensing, and two-photon absorption properties. The HPBO-containing sensors show unique response for zinc ion. Significant zinc ion stimulated photophysical properties with evident enhancements in fluorescence intensities from both one- and two-photon processes were detected. Moreover, one chromophore having HPBO groups in both terminal ends exhibits significant modulation of fluorescence intensities in response to the phenol group deprotonation in the HPBO moiety. A very large two-photon absorption cross section of 4120 GM, the highest reported for bisphenolate-containing 2PA materials, was measured for the chromophore under basic conditions. The molecular structures have a significant influence on the 2PA properties, which follows a trend of the substitution groups on the HPBO moiety in the order of -O - >-OZn >-OCH 2 R. The present study provides a protocol for elucidating the relationship between the photophysical properties and molecular structures. 1. Introduction The development of novel two-photon-absorbing (2PA) materials, which can be excited by near-infrared (NIR) light (700-1100 nm), is an area of intensive research because of their potential applications for frequency up-conversion lasing, high-density optical storage, holographic data storage, 3-D microfabrication, two-photon fluorescence microscopy, and photodynamic therapy. 1 Environmental sensing materials based on 2PA materials for detecting various metal ions, the fluoride ion, and pH values have been reported recently. 2–6 2PA sensing materials with two-photon fluorescence (2PF) have many advantages over the commonly used one-photon fluorescence (1PF) materials (excited with the light in the range 300-600 nm in wavelength). These include exclusive confinement of the excitation to the focal volume with high 3D resolution and reduced photobleaching and phototoxicity by virtue of the low-energy NIR excitation with greater depths of penetration in living tissues for biological applica- tions. 7 Although the advantages of using 2PA sensing materials are clear, only a few 2PA metal ion sensors have been reported. 2,3,6 The selectivity and sensitivity demon- strated in these sensing materials still need to be improved. The selective detection of zinc ion is extremely important in environmental or biological systems. 8 Various 1PF-based sensing materials for Zn 2+ , including fluorescein, quinoline, and 2-(2′-hydroxyphenyl)benzoxazole (HPBO) derivatives, have been developed for this purpose. 9,10 However, inves- tigation on the 2PF-based zinc sensor is quite limited. 6 The HPBO-based functional materials can undergo excited- state intramolecular proton transfer (ESIPT, as shown in * To whom all correspondence should be addressed. Tel: (206)543-2626; Fax: (206)543-3100; E-mail: ajen@u.washington.edu. † Department of Materials Science and Engineering, University of Washington. ‡ Department of Chemical Engineering, National Taiwan University. § Institute of Advanced Materials and Technology, University of Washington. | Institute of Polymer Science and Engineering, National Taiwan University. (1) See a recent review: Lin, T. C.; Chung, S. J.; Kim, K. S.; Wang, X. P.; He, G. S.; Swiatkiewica, J.; Pudavar, H. E.; Prasad, P. N. In AdVances in Polymer Science; Lee K. S., Ed.; Springer: Berlin, 2003; Vol. 161; p 157. (2) Pond, S. J. K.; Tsutsumi, O.; Rumi, M.; Kwon, O.; Zojer, E.; Brédas, J. L.; Marder, S. R.; Perry, J. W. J. Am. Chem. Soc. 2004, 126, 9291. (3) (a) Kim, K. M.; Jeong, M. Y.; Ahn, H. C.; Jeon, S. J.; Cho, B. R. J. Org. Chem. 2004, 69, 5749. (b) Ahn, H. C.; Yang, S. K.; Kim, H. M.; Li, S. J.; Jeon, S. J.; Cho, B. R. Chem. Phys. Lett. 2005, 410, 312. (c) Kim, J. S.; Kim, H. J.; Kim, H. M.; Kim, S. H.; Lee, J. W.; Kim, S. K.; Cho, B. R. J. Org. Chem. 2006, 71, 8016. (d) Kim, H. M.; Yang, P. Y.; Seo, M. S.; Yi, J.-S.; Hong, J. H.; Jeon, S.-J.; Ko, Y.-G.; Lee, K. J.; Cho, B. C. J. Org. Chem. 2007, 72, 2088. (4) Werts, M. H. V.; Gmouh, S.; Mongin, O.; Pons, T.; Blanchard-Desce, M. J. Am. Chem. Soc. 2004, 126, 16294. (5) (a) Liu, Z. Q.; Shi, M.; Li, F. Y.; Fang, Q.; Chen, Z. H.; Yi, T.; Huang, C. Org. Lett. 2005, 7, 5481. (b) Zhang, M.; Li, M.; Zhao, Q.; Li, F.; Zhang, D.; Zhang, J.; Yia, T.; Huang, C. Tetrahedron Lett. 2007, 48, 2329. (6) Bozio, R.; Cecchetto, E.; Fabbrini, G.; Ferrante, C.; Maggini, M.; Menna, E.; Pedron, D.; Ricco, R.; Signorini, R.; Zerbetto, M. J. Phys. Chem. A 2006, 110, 6459. (7) Cahalan, M. D.; Parker, I.; Wei, S. H.; Miller, M. J. Nat. ReV. Immunol. 2002, 2, 872. 1977 Chem. Mater. 2008, 20, 1977–1987 10.1021/cm702527m CCC: $40.75  2008 American Chemical Society Published on Web 01/25/2008