Raman Observation of Unusual Excited State Species in Axial Ligand Binding/Release Photoprocesses for NI(TMpy-P4) in Water

Abstract

Water-soluble cationic porphyrins attracts an increased attention because of their possible biomedical applications. It seems to be interesting to use such porphyrins for studying of specific interactions with DNA and other polunucleotides. In our earlier work the study of photoinduced behavior of water soluble Ni(TMpy-P4) (NiP) in ineraction with poly(dG-dC)₂ and poly(dA-dT)₂ was carried out [1]. Besides, we began to investigate NiP in a phosphate water buffer solution. In the present work, we performed a detailed investigation of the excited-state properties and light-initiated reactions for this object by using transient nanosecond resonance Raman (RR) spectroscopic technique. In water solution, where NiP exists as equilibrium mixture of four- and six-coordinate species in the ground electronic state, the excitation of either 4-coordinate nonligated form at 397 nm or 6-coordinate form with two axial water ligands at 458 nm resulted in appearance of transient Raman bands originated from the excited ³B_lg state of a 4-coordinate species. Analysis of transient RR spectra obtained with intermediate excitation at 423 nm revealed the simultaneous existence of opposite photoprocesses of attachment / release of water molecules to initially 4-coordinate species / from initially 6-coordinate species, with predomination in spectra of Raman lines originated from the electronic state ³B_1g(L)₂.