Picosecond Fluorescence Spectroscopy of Hematoporphyrin Derivative and Related Porphyrins

Abstract

Recently it has been demonstrated that tunable laser irradiation permits the treatment of malignant tumors with a light-activated dye [1]. The success of this technique is due to the ability of the dye, hematoporphyrin derivative (HpD), to accumulate to a higher degree ( ~ten times) in malignant tumors than in normal tissues. If HpD in a cancerous cell is exposed to red light from a laser, it absorbs enough energy to destroy the cells but allows the full recovery of neighboring normal cells containing a lower concentration of the dye. However, little is yet known about the dynamic mechanisms of photochemical reactions occurring in the treatment, including the reason that HpD selectively accumulates in the tumors. In order to make clear the dynamics it is important to investigate quantitatively relaxation processes from excited electronic states of HpD and related molecules. A few spectroscopic studies of metal-free porphyrins which have low-intensity fluorescences have been performed [2–4], but no direct measurements of the fluorescence decay have been reported [5]. In this paper, we report the direct measurement of picosecond fluorescence lifetimes τ_M of HpD, hematoporphyrin IX (HpIX), hematoporphyrin IX diacetate (HpIXDA), protoporphyrin IX disodium salt (PpIXDS), and protoporphyrin IX dimethyl ester (PpIXDE), and the effects of oxygen molecules and acid solutions on the lifetimes by a use of a synchroscan streak camera [6]. In addition, fluorescence quantum yields of the metal-free porphyrins are measured, and natural radiative lifetimes and radiationless transition rates from the first excited singlet state are determined.