Two-photon absorption cross section of excited phthalocyanines by a femtosecond Ti-sapphire laser

Abstract

In the past few years, photodynamic therapy (PDT) has become a major treatment for neovascular age-related macular degeneration (AMD) in which there is abnormal growth of choroidal neovasculature (CNV) that eventually obscures central vision, leading to blindness. However, one of the main limitations of current PDT is the relatively low specificity of the photosensitizer (PS) and light for pathological tissue which may induce damage to adjacent healthy tissue. An alternative approach to circumvent the specificity limitation is to improve the irradiation process. In particular two photon (2-γ) excitation promises a more precise illumination of the target tissue. PS are activated by the simultaneous absorption of 2-γ delivered by ultra-fast pulses of near infrared light. In order to evaluate the efficiency of phthalocyanine (Pc) dyes for 2-γ absorption we measured 2-γ absorption cross sections (σ₂) of a number of metalated Pc (MPc) dyes at λ_ex = 800 nm using a femtosecond laser. The studied Pc molecules vary by the type of the central metal ion (Al or Zn) and the number of peripheral sulfo substituents (MPcS). Each MPc dye of our series shows an improved 2-γ absorption σ₂ as compared to that obtained for Photofrin (3.1 ± 0.1 GM, with 1 GM = 10⁻⁵⁰ cm⁴ s photon⁻¹ mol⁻¹), the PS currently approved for 1-γ PDT. Our data show an 2.5-fold enhancement for AlPcCl, AlPcS_2adj and ZnPcS₃C₉, up to 10-fold (28.6 ± 0.72 GM) for the ZnPcS₄ dye relative to Photofrin. These findings confirm the efficiency of Pc for 2-γ absorption processes and represent the first detailed comparison study of 2-γ absorption σ₂ between Photofrin and Pc dyes.