Complexing of porphyrins in supersonic jets

Abstract

The complexing of porphyrins' is a subject of considerable interest in the areas of physical chemistry and biophysical diagnosis. The attachment of various ligands (L) to metal porphyrins (MP) in s ~ l u t i o n ~ * ~ and in lowtemperature solids4 is manifested in the appearance of new spectral features of the MP-L complexes. We would like to draw attention to the potential of supersonic expans i o n ~ ~ for the production of internally cold, collision-free, large, molecular complexes. We have applied the techniques of laser spectroscopy in seeded supersonic expans i o n ~ ~ for the synthesis, identification, and exploration of excited-state energetics of isolated, ultracold complexes of porphyrins with medium-sized molecules (L = water, methanol, acetonitrile, benzene, and pyridine), providing a new approach for the elucidation of solvent perturbat i o n ~ ~ on porphyrins, as explored from the microscopic point of view. We have studied the laser-induced fluorescence (LIF) excitation spectra of the complexes of zinc octaethylporphyrin (ZnOEP) with medium-sized molecules. The ZnOEP-L complexes were synthesized in pulsed supersonic jets of He seeded with the two molecules which form the complex. A solid sample of ZnOEP was heated in the nozzle chamber to 350 C to yield a vapor pressure of 0.1 torr and seeded into a mixture of He + L. The partial pressure of L was pL = 0.1-1 torr, while the stagnation pressure of He was p = 1200-2000 tom. The He + ZnOEP + L mixture was expanded through a circular pulsed nozzle (diameter D = 600 pm)*. A nitrogen-pumped dye laser (spectral resolution 0.3 cm-') crossed with supersonic expansion at the distance X = 15 mm (X/D = 25) down the nozzle. The LIF spectra were measured as previously described.6 Figure 1 shows the LIF spectrum of ZnOEP in a jet of He. The spectral features, whose energies and relative intensities are independent of the stagnation pressure in the range p = 1200-2500 torr, are assigned to the electronic-vibrational excitation of the So S1 transition (the Q band)g of the bare ZnOEP. No spectral features are exhibited at wavelengths above 5600 A. The lowest-energy spectral features consist of a triplet (Figure 1) exhibiting