Two-Photon Absorption in Quadrupolar Bis(acceptor)-Terminated Chromophores with Electron-Rich Bis(heterocycle)vinylene Bridges Shijun Zheng, ² Amalia Leclercq, ² Jie Fu, ‡ Luca Beverina, ² Lazaro A. Padilha, ‡,§ Egbert Zojer, ², | Karin Schmidt, ² Stephen Barlow, ² Jingdong Luo, ⊥ Sei-Hum Jiang, ⊥ Alex K.-Y. Jen, ⊥ Yuanping Yi, # Zhigang Shuai, # Eric W. Van Stryland, ‡,¶ David J. Hagan, ‡,¶ Jean-Luc Bre ´das, ² and Seth R. Marder* ,² School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, CREOL, The College of Optics and Photonics, UniVersity of Central Florida, Orlando, Florida 32816-2700, Institute of Solid-State Physics, Graz UniVersity of Technology, Petergasse 16, A-8010 Graz, Austria, Materials Science and Engineering Department, UniVersity of Washington, Seattle, Washington 98195, and Key Laboratory of Organic Solids, Institute of Chemistry, The Chinese Academy of Sciences, 100080 Beijing, P. R. China ReceiVed July 19, 2006. ReVised Manuscript ReceiVed NoVember 6, 2006 Two-photon absorption spectra for a range of bis(acceptor)-substituted bis(dibutoxythienyl)ethene and bis(N-hexylpyrrolyl)ethene chromophores have been recorded using Z-scan and white-light-continuum pump-probe techniques. All the chromophores studied show strong near-infrared two-photon absorption with cross sections in the range of 2400-5900 GM (1 GM ) 1 × 10 -50 cm 4 s/photon) at photon wavelengths between 1.0 and 1.3 µm; cross sections as high as 10000 GM can be accessed close to the one-photon absorption edge. Quantum-chemical calculations reproduce the experimentally observed variations of the two-photon properties with the chemical structure. Introduction Two-photon absorption (2PA) processes are currently of great interest for applications including three-dimensional (3D) fluorescence imaging, 1,2 photodynamic therapy, 3 optical pulse supression, 4 and 3D microfabrication. 5-7 In order to realize the full potential of these applications, substantial efforts have been directed at understanding the molecular design principles necessary to develop chromophores with large 2PA cross sections, δ, at a range of visible and near- infrared (NIR) wavelengths. Dipolar, 8,9 quadrupolar, 10,11 and octupolar molecules 12,13 have been investigated both experi- mentally and theoretically. More complex systems investi- gated include dendritic species 14,15 and porphyrin oligomers and polymers. 16 In quadrupolar conjugated systems, the low- energy peak two-photon cross section, δ max , seems to be related to symmetric charge transfer associated with the one- photon excitation. 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