The photophysics of haematoporphyrin dimers or aggregates in aqueous solution

The triplet lifetime and decay rate law observed from haematoporphyrin in aqueous solution were found to depend on concentration with the triplet-state decay being very much slower in dilute solution than in more concentrated solutions. The wavelength of excitation also has an effect on the decay kinetics. In a 1 x 10(-4) mol dm-3 solution the decay resulting from flash photolysis at 527 nm is satisfactorily described by a single exponential having a decay constant of 1.5 x 10(4) S-1 and was ascribed to the triplet state of a haematoporphyrin dimer or aggregate. However, excitation of a haematoporphyrin solution of the same concentration at 351 nm results in a slower triplet-state decay which is indistinguishable, within experimental error, from that observed from a dilute haematoporphyrin solution excited at 527 nm and which is ascribed to the monomer triplet state. Although resolved Soret band absorption spectra indicate that in a 1 x 10(-4) mol dm-3 aqueous solution of haematoporphyrin most radiation at 351 nm is absorbed by a dimer, the decay following flash photolysis at 351 nm is dominated by the triplet state of the haematoporphyrin monomer. This implies that the excited singlet state of the dimer whose ground state absorbs the shorter-wavelength region of the Soret band undergoes efficient intersystem crossing to the monomer triplet state.(ABSTRACT TRUNCATED AT 250 WORDS)