Structural dynamics and reactivity of a cationic mono(acridyl)bis(arginyl)porphyrin: A spectroscopic study down to femtoseconds

The geometric and electronic structure in solution of a cationic bichromophore, mono(acridyl)bis(arginyl)porphyrin (MABAP), was examined for the first time by investigating its photophysical properties using steady state as well as time resolved absorption and fluorescence spectroscopies down to femtosecond resolution. The study was extended to the tetrakis(N-methyl-4-pyridyl)porphyrin (H2TMPyP) and to the acridine and porphyrin moieties of MABAP named the acridine arm and bis(arginyl)porphyrin (BAP). The comparison between the absorption, emission and excitation spectra, fluorescence lifetimes and femtosecond transient absorption measurements of the different molecules investigated allows us to propose the existence of three distinct conformations of the cationic bi-chromophore: a stacking complex (P-B, 10%) which can be stabilized by either a single acridine-porphyrin intramolecular hydrogen bond (P-aH, 70%) or additional hydrogen bonds with the solvent (P-nH, 20%). The P-aH conformation is unstable in its singlet excited state and evolves toward the P-B conformation within a few picoseconds. Hydrogen bond formation between the solvent and either porphyrin H2TMPyP or BAP has also been characterized in this study.