A rationale for the development of heterogeneous singlet oxygen photosensitizers

YAIR E. LITMAN; HERNÁN B. RODRÍGUEZ; SILVIA E. BRASLAVSKY; ENRIQUE SAN ROMÁN

Liège

Congreso; 15th Congress of the European Society for Photobiology; 2013

European Society for Photobiology

As a general rule, dyes with a high triplet quantum yield are required for the development of photosensitizers based on the production of reactive oxygen species suited for photodynamic therapy and related applications. Though molecular photosensitizers are currently used, some applications ? e.g., bacterial inactivation ? can be favored by the use of heterogeneous systems like nanoparticle suspensions, microparticle beads or immobilized films. For these systems to be effective, high light absorption rates, achievable at high dye concentrations, are mandatory. Unfortunately, at the required dye concentrations, molecular aggregation and formation of statistical traps usually lead to singlet state deactivation with the consequent loss of triplet yield.                 During the last years we have developed various methods to account quantitatively for the photophysics of dyes embedded into light scattering solid materials. In particular, recently we determined triplet quantum yields of dyes in model systems as a function of concentration using Laser Induced Optoacoustic Spectroscopy (LIOAS). Whereas phenazinium dyes showed a rapid decrease of triplet formation with concentration, various xanthene dyes yielded practically constant triplet quantum yields up to equivalent bulk concentrations in the order of 10-3 M.                 A radical pair recombination mechanism based on the charge transfer quenching of the singlet state by neighboring dye molecules may account for this behavior. The triplet state quantum yield decreases in parallel to the fluorescence quantum yield for dyes in which the energy of the charge transfer state lays below the energy of the dye pair triplet state, whereas fluorescence concentration quenching with triplet formation takes place in the opposite case. As excitonic interactions depend on the square of the transition moment, they are negligible for triplet dye pairs, which behave as isolated, monomeric triplets.                 These results provide a strong rationale for the design of heterogeneous photosensitizers with high triplet quantum yields at large dye concentrations.