The Effect of Electron-Withdrawing Substituents on Two-Photon Absorption Properties of A3- corroles Aleksander Rebane a, Mikhail Drobizhev a, Nikolay S. Makarov a, Daniel T. Gryko b a Physics Department, Montana State University-Bozeman, USA b Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, Poland Acknowledgments: This work was supported by AFOSR and MBRCT grants. Abstract Two-photon absorption (2PA) of tetrapyrroles can find use in several applications, including photodynamic therapy, optical power limiting, and 3D memory. In this paper we study two-photon absorption (2PA) spectra of A3-corroles substituted with electron-withdrawing groups [1]. In non-centrosymmetrical tetrapyrroles, 2PA in Soret region can be described at least by two quantum pathways of optical excitation: (1) through intermediate (Q-) state in three-level system or (2) transition in two-level system with large change of permanent dipole moment (with ground and final levels playing the role of intermediate states) [2]. We found that the one-photon absorption (1PA) Soret peak is reproduced in 2PA spectrum, which could imply the importance of the second mechanism. On the other hand, the peak 2PA cross section (  2 ) is too large to be explained by that mechanism. (Very large, ~ 10D, change of permanent dipole upon excitation would be required). Therefore, we conclude that the first pathway dominates in A3-corroles. The 2PA peak is only very slightly shifted with respect to Soret band because of its spectral narowness. We also observe almost no changes in maximum 2PA cross section when the triphenyl-corrole is substituted with electron-withdrawing groups in meta- positions of phenyl rings. If the substitution is in para- positions, we there is a ~ 60% enhancement of  2. We expain this observation by better  -conjugation in para- position. Two- and One-Photon Absorption Properties of Corroles in Soret Region in THF Solution Results and Discussion 1.All four molecules show pronounced 2PA peak near ex = 840 nm, which virtually coincides in shape and position with one-photon Soret transition. 2.The strength of this 2PA transition is moderately-high (200 – 300 GM). Question: Since the 2PA coincides with 1PA Soret peak, can the 2PA properties be described within the two-level model with permanent dipole change exclusively, without considering resonance enhancement in three-level system? In two-level system: Using the measured parameters for COR-3,  1  = 2300 cm -1, and |  10 |= 10D, we estimate that |  10 |= 10D is necessary to provide  2 = 210 GM, which is very improbable because COR-3 does not carry any strong electron-donating or withdrawing groups. Therefore, we conclude, that the resonance enhancement mechanism in three- (or more) level system is most important in 2PA. The absence of the shift (due to resonance factor) of 2PA peak with respect to 1PA peak is probably due to narrowness of the transition. Two-Photon Absorption Spectra Chemical Structures Molecule 2PA m nm  2 m 10 3 GM 1PA m nm  m   M -1 cm -1] COR ± COR ± COR ± COR ± Conclusions: 1.We observed moderately-strong and pronounced 2PA peak near ex = 840 nm in all corroles studied. 2.This peak virtually coincides in shape and position with one-photon Soret transition. 3.The strength of this 2PA transition cannot be explained with the change of permanent dipole moment upon transition. And we assume that the resonance enhancement in three-level system is more important. 4. The peak 2PA cross section in Soret region of COR-1 and COR-4 is almost the same as that of COR-3. Only COR-2 demonstrate a 60% increase in  2. This can be explained by better  -conjugation in para- than in meta-phenyl positions. COR-1 COR-2 COR-3COR-4 Our goal: To study the role of electron-withdrawing substituents, of variable strength, and attached at different positions (para- versus meta-) on two-photon absorption properties of corroles. Namely, to compare COR- 1 COR-2, and COR-4 with “non-substituted” COR-3 h h S0S0 S1S1  10 00 11 3. Peak 2PA cross section in Soret region of COR-1 and COR-4 is almost the same as that of COR-3. Only COR-2 demonstrate ~ 60% increase in  2, which is slightly above the experimental error margins. Possible explanation: The effect of electron-withdrawing substitutuents is minimized in COR-1 and COR-4 because: (a) Electron-withdrawing substituents usually enhance 2PA in tetrapyrroles via enhancement of transition dipole moments between the ground and Q-state and Q-state and 2PA state [1,2]. For such an enhancement of dipole moments, there must be strong electronic conjugation between the substituent groups and tetrapyrrolic core. (b) Such  -conjugation is disrupted in COR-1 and COR-4 because of non-coplanar arrangement of phenyls and tetrapyrrolic macrocycle and because of meta-substitution. Thanks to para-substitution in COR-2, the conjugation between fluoromethyl groups and tetrapyrrole macrocycle could be still present (despite of phenyl twisting), resulting in an enhancement of 2PA. Since the strength of the 0  Q transition is virtually the same for COR-2 and COR-3, the 2PA increases, most probably, due to the second,  BQ dipole enhancement, as in the case of phthalocyanines [2]. h h 0 Q  Q0 B  BQ ff Three-level system: Refererences 1.D.T. Gryko, B. Koszarna, Org. Biomol. Chem. 2003, 1, M. Drobizhev, F. Meng, A. Rebane, Y. Stepanenko, E. Nickel, C.W. Spangler, J. Phys. Chem., 110 (2006) M. Drobizhev, A. Karotki, M. Kruk, N.Zh. Mamardashvili, A. Rebane, Chem. Phys. Lett., 361 (2002) M. Drobizhev, N.S. Makarov, Y. Stepanenko, A. Rebane, J. Chem. Phys., 124 (2006)