Abstract-The. picosecond time-resolved fluorescence decays I ( t )and spectra Z(h, t

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As it is recognized that photoradiation therapy using hematoporphyrin derivative (HpD) as a photosensitizer is effective to treatment of human cancers, its photophysical, photochemical and biological behaviors are being widely investigated [1]. The mechanisms enabling the selective accumulation of HpD in cancerous cells and of the subsequent energy relaxations and photochemical reactions are not well understood. For the purpose of elucidation of the mechanisms, the fluorescence decay kinetics from the first excited singlet state has been investigated in the time region of nanoseconds by Milan's group [2] and picoseconds by us [3] and Rochester's group [4]. Consequently, it has been found that a fluorescence decaying curve of Hpd in a phosphate buffer saline aqueous (PBS) solution shows two components of fast (less than a few hundreds picoseconds) and slow (more than several nanoseconds) decays. The fluorescence decay is lengthened with decreasing the HpD concentration. It is thought generally that the fast and slow decay components are due to the aggregates (including dimers) and monomers, respectively. On the other hand, from biological studies on the relation between HpD components and treatment efficacy [5,6], the aggregates are essential to treatment. The selective accumulation of HpD is thought to involve the aggregational properties of HpD components. However, the direct spectroscopic information on the accumulation properties and the energy relaxation from the excited state through the interaction between the aggregates and the monomers is not yet obtained.