Acknowledgments: This work was supported by AFOSR and MBRCT

1. Sf mfi hn Si mi0 hn S0 Gf Two-Photon Absorption in Tetraazachlorin and its Benzo- and 2,3-naphto-fused Derivatives: Effective symmetry of p-conjugation pathwayMikhail Drobizheva, Nikolay S. Makarova, Aleksander Rebanea, Elena A. Makarovab, Evgeny A. Luk’yanetsbaPhysics Department, Montana State University-Bozeman, USAbOrganic Intermediates and Dyes Institute, Moscow, Russia • Abstract • Two-photon absorption (2PA) of non-symmetrical metal-free tetrapyrroles can find use in ultrahigh capacity (Terabit) re-writable 3D memory and other applications, such as deeper-penetrating photodynamic therapy and optical power limiting. In this work we study 2PA spectra and cross sections of a family of substituted free-base tetraazachlorin (H2TAC), tribenzo-tetraazachlorin (H2TBTAC), and trinaphto-tetraazachlorin (H2TNTAC). Because of non-symmetrical structure, all three molecules can exist in the form of two unequivalent tautomers with completely different absorption spectra. We sow, that both tautomers are metastable at temperatures < 100 K and can be interconverted by irradiation with light resonant with the corresponding absorption peak. At room temperature one of the form completely dominates (more stable). Free-base tetraazachlorin and tribenzo-tetraazachlorin with non-symmetrical structure demonstrate very narrow and strong 2PA-transition (~ 1000 GM for the second), which is not present in one-photon absorption spectrum and reminiscent to that of symmetrical phthalocyanines. We explain this fact by the conservation of quasi-symmetric p-electron conjugation pathway in the stable tautomer form of tetraazachlorin and tribenzo-tetraazachlorin. Two-Photon Absorption Spectra (room temperature, in CH2Cl2) • Possible Explanation • In stable tautomers of H2TAC and H2TBTAC, the aromatic pathway is much more similar to that of tetraazaporphyrin (and Pc), than to that of chlorin. • According to quantum chemical calculations of p-electron ring currents [2], in tetraazaporphyrins and phthalocyanines the main current pathway does not encounter the outer parts of two pyrrolenine rings, see figure: H2TAC H2TAC and H2TBTAC: H2TNTAC: 3. Therefore, the p-conjugation pathway an electronic wavefunction remain centro-symmetric, like in symmetric tetraazaporphyrins and phthalocyanines. 4. In H2TNTAC, the symmetry is lost either due to larger conjugation system, or because of different tautomer (with reduced bond in one pyrrolenine ring) is more stable. Therefore, in this case, one- and two-photon absorption have similar selection rules, and, since one photon absorption is almost absent in the spectral region 480 – 540 nm, 2PA is weak. H2TBTAC Chemical Structures H2TAC H2TBTAC and H2TNTAC Two-photon absorption spectrum of H2TBTAC (left) reminds that of symmetrical metal-free phthalocyanine [3], H2Pc, right: Both show rather narrow peak at 920-940 nm. For H2Pc it was assigned to Ag -> Ag transition [3]. An order of magnitude increase of peak cross section in H2TBTAC can be explained by better resonance conditions for 2PA and other factors. About the same enhancement is observed for H2TAC, compared to H2TAP [4]. H2TNTAC Are these molecules closer to chlorin or tetraazaporphyrin (and Pc, Nc) in terms of optical and photochemical properties? Photo-Tautomerization and Temperature Stability of T2 of H2TBTAC Photochemical switching between two tautomers in PVB at 77K T1 T2 T2 T1 • Conclusions: • Free-base chlorin analogues of tetraazaporphyrin, phthalocyanine, and naphthalocyanine demonstrate photo-tautomerization temperatures, intermediate between those of chlorin and corresponding porphyrazine molecule. • 2. 2PA spectra of H2TAC and H2TBTAC resemble those of symmetric tetraazaporphyrins and phthalocyanines. They show narrow and strong 2PA peak near 920-940 nm, which is not reproduced in 1PA. Peak 2PA cross sections are much larger in H2TAC and H2TBTAC, than in H2TAP and H2Pc, respectively, which can be due to resonance enhancement and stronger excited-state transition. • 3. We explain the dissimilarity of one- and two-photon absorption spectra in H2TAC and H2TBTACby an effective centro-symmetry of electronic wavefunctions in them. • 4. H2TNTAC does not show any distinct peak in the same region and has lower 2PA cross sections. This can be due to a breaking of centrosymmetry of p-conjugation in this molecule. • Main Results of 2PA Spectroscopy and Open Issues • Strong and narrow 2PA peak is observed in H2TAC and H2TBTAC spectra in the region between Q- and B-bands, near 900-950 nm. • Despite the absence of centrosymmetry, there is no one-photon counterpart of this peak. • Question (1): Why? • 3. 2PA peak cross section of H2TBTACis 5 times larger that of H2TAC. This can be explained by more extensive conjugation system in the former. • 4. H2TNTAC does not show any distinct peak in the same region, but rather increasing 2PA with photon frequency. • Question (2): Why? • 5. Average 2PA cross section of H2TNTAC in this region is similar to that of H2TAC, regardless much larger conjugation system of the former. • Question (3): Why? Thermal stability of photoproduct (T2) Dark stability of T2 at 77K • References • I. Renge, H. Wolleb, H. Spahni, U.P. Wild, J. Phys. Chem., A, 101 (1997) 6202. • Y.B. Vysotsky, V.A. Kuzmitsky, K.N. Solovyov, Theor. Chim. Acta, 59 (1981) 467. • M. Drobizhev, N.S. Makarov, Y. Stepanenko, A. Rebane, J. Chem. Phys., 124 (2006) 224701. • M. Drobizhev, A. Karotki, M. Kruk, N.Zh. Mamardashvili, A. Rebane, Chem. Phys. Lett., 361 (2002) 504. T1/2 = 82K is close to an average between chlorin (50 K) [1] and Pc (120K) [1] values! Acknowledgments: This work was supported by AFOSR and MBRCT