Simultaneous two-photon absorption of tetrapyrrolic molecules : from femtosecond coherence experiments to photodynamic therapy

Abstract

Simultaneous two-photon absorption (TPA) in tetrapyrrolic molecules is studied and its applications to two-photon coherence gratings and singlet oxygen generation for photodynamic therapy are demonstrated in this thesis. First ever comprehensive study of TPA properties of tetrapyrrolic molecules is conducted in this work. Two-photon transitions in two key spectral regions, red to green and blue to near-UV (transition wavelengths) are investigated. Physical mechanisms leading to enhancement of TPA cross section in tetrapyrroles are elucidated. Porphyrin molecules with greatly enhanced two-photon cross sections are obtained. Spectral coherence interference gratings are created by means of two-photon excitation with pairs of phase-locked femtosecond pulses in tetrapyrrolic molecules. First, gratings are detected by means of persistent spectral hole burning, which constitutes the first ever demonstration of spectral hole burning by simultaneous absorption of two photons. Next, the gratings are detected in fluorescence spectrum, which we use to study zero-phonon lines and phonon sidebands in two-photon transitions. Application of tetrapyrrolic molecules to two-photon photosensitization of singlet molecular oxygen is investigated. First, TPA properties of some known one-photon photosensitizers are investigated. Then, a new class of TPA based photosensitizers with greatly enhanced two-photon cross sections is developed. The generation of singlet molecular oxygen upon two-photon excitation of the new photosensitizers demonstrated for the first time, which opens up new perspectives for two-photon photodynamic therapy