Synthesis and Two-Photon Spectrum of a Bis(Porphyrin)-Substituted Squaraine Susan A. Odom, † Scott Webster, ‡ Lazaro A. Padilha, ‡ Davorin Peceli, ‡ Honghua Hu, ‡ Gero Nootz, ‡ Sung-Jae Chung, † Shino Ohira, † Jonathan D. Matichak, † Olga V. Przhonska, § Alexei D. Kachkovski, | Stephen Barlow, † Jean-Luc Bre ´das, † Harry L. Anderson, ⊥ David J. Hagan, ‡ Eric W. Van Stryland, ‡ and Seth R. Marder* ,† School of Chemistry & Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, CREOL, The College of Optics and Photonics, UniVersity of Central Florida, Orlando, Florida 32826, Institute of Physics, National Academy of Sciences, KieV 03028, Ukraine, Institute of Organic Chemistry, National Academy of Sciences, KieV 03094, Ukraine, and Department of Chemistry, UniVersity of Oxford, South Parks Road, Oxford OX1 3TA, U.K. Received February 25, 2009; E-mail: seth.marder@chemistry.gatech.edu Organic materials with large two-photon absorption (2PA) cross sections, δ, are of interest for applications including fluorescence microscopy, 1 microfabrication, 2 and optical pulse suppression. 3 One of the design strategies that has been used to achieve large δ values is the use of π-conjugated molecules with donor-acceptor-donor (D-A-D) structural motifs. 4 Squaraines can be considered as examples of D-A-D chromophores and have been found to exhibit large values of δ at wavelengths close to their one-photon absorption (1PA) edges, 5 especially when bearing substituents with extended conjugation. 6 Dimers and oligomers of conjugated zinc porphyrins have also been shown to exhibit very strong 2PA, with δ max values up to 500 times those of monomeric analogues. 7 Moreover, linear absorption spectra of both squaraines and porphyrins exhibit bands characterized by large transition dipole moments, a key prerequisite for obtaining strong 2PA. In addition, the S 0 -S 1 transition energies of bis(indolinylidenemethyl) squaraines closely match those of porphyrins, suggesting the possibility of substantial excitonic electronic coupling in a π-conjugated hybrid of these two compo- nents. On the basis of these observations, we were interested in investigating whether the electronic coupling between the constitu- ent subunits of D-A-D bis(porphyrin)-substituted squaraines could be sufficiently strong to result in enhanced 2PA. Here we report on the synthesis and characterization of the new triad 1 (Scheme 1), in which a squaraine core is linked to zinc porphyrin donors using alkyne bridges. Compound 1 was synthesized using in situ deprotection of the trimethylsilyl-protected meso-ethynyl-substituted porphyrin 2 fol- lowed by Sonogashira coupling with the diiodosquaraine derivative 3a (Scheme 1). The in situ deprotection conditions were adopted because of the tendency of terminal alkyne derivatives of zinc porphyrins to undergo Glaser couplings [which would, in this case, give a bis(porphyrin)diacetylene], even without the addition of traditional alkyne-coupling reagents. 8 Compound 1 is soluble in solvents including dichloromethane, tetrahydrofuran, benzene, and glycerols. Compound 2 was synthesized in a fashion similar to that for previously reported porphyrin derivatives with alkynyl substituents in the meso position, 9 with the specific substituents being chosen to facilitate solubility and recrystallization of the intermediates. The diiodosquaraine building block 3a was synthesized by the conden- sation of squaric acid with 5-iodo-1-ethyl-3,3-dimethyl-2-methyl- eneindoline, which was obtained from its bromo analogue 5a,6b by bromine-lithium exchange with n-butyllithium followed by treat- ment with iodine. The noniodinated squaraine 3b 5a,6b was also synthesized as a model squaraine for comparison with 1. The structure of compound 1 was confirmed by 1 H, COSY, and 13 C NMR spectroscopy and mass spectrometry; the characterization data are shown in the Supporting Information (SI) along with full synthetic details for 1 and its precursors. The 1PA spectra for 1, 2, and 3b are shown in Figure 1a. The spectrum of 2 is typical for a porphyrin, showing a relatively weak low-energy Q band and a more intense B (Soret) band at shorter wavelength, the splitting of which can be attributed to the lowering of symmetry by the ethynyl substituent. The absorption spectrum of 3b shows a strong low-energy transition at 636 nm, which is typical for a squaraine of this type. The lowest-energy absorption band of 1 is intense and red-shifted by ∼100 nm relative to that of 3b, while the higher-energy features resemble the Q and B bands of 2. All three compounds are fluorescent with small Stokes shifts; fluorescence spectra are shown in the SI. Using density functional theory (DFT)-optimized (B3LYP/6- 31G**) geometries, we calculated the 1PA and 2PA properties of 1, following the INDO/MRD-CI methodology we successfully used previously in the case of large conjugated molecules 6b (see the SI). The frontier orbitals (Figure S3 in the SI) show significant delocalization across the whole molecular backbone. The 1PA calculations indicate the presence of three main peaks in the visible region, with the lowest-energy feature primarily due to a mixing of HOMO-to-LUMO and (to a lesser extent) HOMO-1-to- LUMO+1 transitions. † Georgia Institute of Technology. ‡ University of Central Florida. § Institute of Physics, Kiev. | Institute of Organic Chemistry, Kiev. ⊥ University of Oxford. Scheme 1. Synthesis of Compound 1 Published on Web 05/12/2009 10.1021/ja901244e CCC: $40.75  2009 American Chemical Society 7510 9 J. AM. CHEM. SOC. 2009, 131, 7510–7511