Two-Photon Absorption in Tetraazachlorin and its Benzo- and 2,3-naphto-fused Derivatives: Effective symmetry of  -conjugation pathway Mikhail Drobizhev a, Nikolay S. Makarov a, Aleksander Rebane a, Elena A. Makarova b, Evgeny A. Luk’yanets b a Physics Department, Montana State University-Bozeman, USA b Organic Intermediates and Dyes Institute, Moscow, Russia Acknowledgments: This work was supported by AFOSR and MBRCT 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 (H 2 TAC), tribenzo-tetraazachlorin (H 2 TBTAC), and trinaphto-tetraazachlorin (H 2 TNTAC). 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  -electron conjugation pathway in the stable tautomer form of tetraazachlorin and tribenzo-tetraazachlorin. Possible Explanation 1.In stable tautomers of H 2 TAC and H 2 TBTAC, the aromatic pathway is much more similar to that of tetraazaporphyrin (and Pc), than to that of chlorin. 2.According to quantum chemical calculations of  -electron ring currents [2], in tetraazaporphyrins and phthalocyanines the main current pathway does not encounter the outer parts of two pyrrolenine rings, see figure: Two-Photon Absorption Spectra (room temperature, in CH 2 Cl 2 ) Photo-Tautomerization and Temperature Stability of T 2 of H 2 TBTAC Conclusions: 1.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 H 2 TAC and H 2 TBTAC resemble those of symmetric tetraazaporphyrins and phthalocyanines. They show narrow and strong 2PA peak near nm, which is not reproduced in 1PA. Peak 2PA cross sections are much larger in H 2 TAC and H 2 TBTAC, than in H 2 TAP and H 2 Pc, 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 H 2 TAC and H 2 TBTAC by an effective centro-symmetry of electronic wavefunctions in them. 4.H 2 TNTAC 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. H 2 TACH 2 TBTAC and H 2 TNTAC Chemical Structures Thermal stability of photoproduct (T 2 )Dark stability of T 2 at 77K Photochemical switching between two tautomers in PVB at 77K T 1  T 2 T 2  T 1 H 2 TAC H 2 TBTAC H 2 TNTAC Main Results of 2PA Spectroscopy and Open Issues 1.Strong and narrow 2PA peak is observed in H 2 TAC and H 2 TBTAC spectra in the region between Q- and B-bands, near nm. 2.Despite the absence of centrosymmetry, there is no one- photon counterpart of this peak. Question (1): Why? 3. 2PA peak cross section of H 2 TBTAC is 5 times larger that of H 2 TAC. This can be explained by more extensive conjugation system in the former. 4. H 2 TNTAC 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 H 2 TNTAC in this region is similar to that of H 2 TAC, regardless much larger conjugation system of the former. Question (3): Why? Are these molecules closer to chlorin or tetraazaporphyrin (and Pc, Nc) in terms of optical and photochemical properties? T 1/2 = 82K is close to an average between chlorin (50 K) [1] and Pc (120K) [1] values! References 1.I. Renge, H. Wolleb, H. Spahni, U.P. Wild, J. Phys. Chem., A, 101 (1997) Y.B. Vysotsky, V.A. Kuzmitsky, K.N. Solovyov, Theor. Chim. Acta, 59 (1981) M. Drobizhev, N.S. Makarov, Y. Stepanenko, A. Rebane, J. Chem. Phys., 124 (2006) M. Drobizhev, A. Karotki, M. Kruk, N.Zh. Mamardashvili, A. Rebane, Chem. Phys. Lett., 361 (2002) Therefore, the  -conjugation pathway an electronic wavefunction remain centro-symmetric, like in symmetric tetraazaporphyrins and phthalocyanines. 4. In H 2 TNTAC, 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. H 2 TAC and H 2 TBTAC:H 2 TNTAC: Two-photon absorption spectrum of H 2 TBTAC (left) reminds that of symmetrical metal-free phthalocyanine [3], H 2 Pc, right: Both show rather narrow peak at nm. For H 2 Pc it was assigned to A g -> A g transition [3]. An order of magnitude increase of peak cross section in H 2 TBTAC can be explained by better resonance conditions for 2PA and other factors. About the same enhancement is observed for H 2 TAC, compared to H 2 TAP [4]. h h S0S0 SiSi  i0 SfSf  fi ff