Strong two-photon absorption in new porphyrins with asymmetrical meso-substitution

Porphyrins and related molecules with strong two-photon absorption (TPA) are extremely called for because of several emerging applications, including 3D optical memory, high-resolution fluorescence microscopy and photodynamic therapy. In this paper we demonstrate for the first time that an asymmetric meso-substitution of porphyrin macrocycle with electron-donating diphenylamino-stilbene or bis-(diphenylamino)-stilbene groups results in a drastic enhancement of intrinsic TPA cross section in the near-IR region. The cross section value amounts to 500-900 GM depending on substituent group and link structure, which is-about 102 times the corresponding value for the unsubstituted parent molecule. Compared to symmetrical porphyrins, the TPA spectra of this series follow qualitatively the corresponding one-photon spectra. Therefore, we describe the observed TPA spectra and absolute cross section values by taking into account the change of permanent dipole moment upon excitation. A new zeroth-generation dendrimer, consisting of a porphyrin core, symmetrically tertakis-meso-substituted with strong TPA dendrons, reveals 7 times increase of the cross, section (740 vs 110 GM) as compared to its mono-meso-substituted analogue. We also demonstrated an efficient singlet oxygen generation upon two-photon excitation of these new molecules, which makes them particularly attractive for photodynamic therapy.