Acknowledgments: This work was supported by AFOSR, and MBRCT.

1. S1 hn m10 hn S0 m1 m0 Modulation of Two-Photon Absorption of Asymmetrically-Substituted Phthalocyanines by the Position of Inner Protons (Tautomerization) and Structure of Linking GroupAleksander Rebanea, Mikhail Drobizheva, Nikolay S. Makarova, Gema de la Torreb and Tomás TorresbaPhysics Department, Montana State University-Bozeman, USAb Departamento de Quimica Organica, Facultad de Ciencias, Universidad Autonoma de Madrid, Madrid, Spain Two-Photon Absorption Spectra (B-band Region) • Abstract • We have recently started a comprehensive study of two-photon absorption (2PA) properties of phthalocyanines (Pcs) in near IR region [1]. These investigations can lead to further development of several intriguing applications of Pcs, including optical power limiting, 3D memory, and deeper-penetrating photodynamic therapy. • Here we study 2PA properties of two metal-free push-pull nitrophenylethynyl- and nitrostyryl-substituted Pcs. We have found that in both molecules there are two unequivalent tautomeric forms (T1 and T2) simultaneously present at room temperature in comparable amounts. At low temperatures these forms can be reversibly photo-transformed, which is important property for re-writable 3D memory application. • By monitoring the fluorescence signal at a particular wavelength, corresponding to spectral maximum of each tautomer, we were able to obtain 2PA spectra of T1 and T2 separately. Intrinsic (femtosecond) maximum 2PA cross section, attained near 800 nm, vary from s2 ~ 1.5 x 103 to 4 x 103 GM (1GM = 10-50 cm4 s), depending on particular molecule and tautomer. Such large s2 values can be due to either (1) strong resonance enhancement in three-level system (with Q-state playing a role of real intermediate state) or (2) to a 2PA transition in two-level system with large change of permanent dipole moment (with ground and final levels working as intermediate states), or even to both, with possible quantum interference between the corresponding pathways [2]. While solvatochromic shifts imply large changes of permanent dipole moment, ranging from 15 to 30 D, the resonance enhancement effect dominates in excitation region from 800 to900 nm in both molecules and tautomers. Extraction of Permanent Dipole Moments in the Ground State NSPc NPEPc Four equations (1)-(4) contain 5 unknown parameters. To solve them, we add a fifth equation, based on independent measurement of <m0> in NSPc in chloroform [G. Rojo, G. de la Torre, J. Garcia-Ruiz, I. Ledoux, T. Torres, J. Zyss, F. Agullo-Lopez, Chem. Phys., 245 (1999) 27], which writes: Our additional measurements of the ratio of T1 and T2 Q(0-0) - bands in chloroform and octane shows that in chloroform, n1 = n2 = 0.5. Substituting these numbers in (5) and solving Eqs. (1)-(5) with the assumption that all dipole moments are either parallel or anti-parallel, we find the following molecular parameters for T1 and T2: The change of permanent dipole moment is found from solvatochromic shifts of CT B-bands in excitation spectra of T1 and T2: through equation: We find |Dm0B | = 27 D for T1 and 17 D for T2. Substitution of these values in Eq. (TLS) gives the “dipole” contribution to s2, see Table, last column. For both tautomers it is several times less than the total experimental s2. These spectra are obtained in octane by selectively monitoring fluorescence intensity either at the T1 (718 nm) and T2 (698 nm) fluorescence maxima. 2PA cross sections were calculated, by taking into account real number density of each tautomer in solution at room temperature. What Is The Role of Charge Transfer in 2PA Strength? Since the molecules are strongly non-centrosymmetrical (carry large permanent dipole moment), two-photon absorption should be allowed to the same states as one-photon absorption. If the low-frequency part of B-band is dominated by charge-transfer state, the 2PA to this state can acquire a particular strength if the change of permanent dipole moment is large. Question. Which mechanism dominates the 2PA cross section in this region: (1) A transition in two-level system with a large change of permanent dipole moment or (2) a transition in three-level system with Q-band playing a role of intermediate state? In the case (1), 2PA cross section is given by In order to obtain the magnitude of the permanent dipole moment change, we performed the solvatochromic analysis of excitation and fluorescence spectra of both T1 and T2 tautomers. Chemical Structures NSPc Absorption, Excitation and Fluorescence in Octane • Conclusions: • We found that at room temperature in solutions push-pull substituted phthalocyanines present in a dynamic equilibrium of two spectroscopically different tautomeric forms. • 2. By using selective fluorescence detection we obtained 2PA spectra of T1 and T2 separately. • 3. The absolute s2max values of both forms in the region of long-wavelength side of B-band is large, ~ 103 GM. • While large changes of permanent dipole moment contribute considerably to the resulting 2PA strength, the resonance enhancement effect dominates in both molecules and tautomers. • Nitrophenylethynyl- and nitrostyryl-substituted Pcsshow similar 2PA strength. NSPc NPEPc Solvatochromic Measurements for NSPc in Q-bands 1. Stokes shift as a function of solvent polarity: Results: Temperature dependence of absorption spectrum 2. Q1(0-0) absorption band shift as a function of solvent polarity: Results: Acknowledgments: This work was supported by AFOSR, and MBRCT.