Electronic excitations of the free-base porphine–Ar van der Waals complex

In this paper we report on the excited-state energetics of the two lowest-lying intravalence electronic excitations S0 → S1x and S0 → S1yof a large van der Waals complex, consisting of Ar bound to free-base porphine (H2P). H2P•Arn complexes were synthesized and interrogated by laser-induced fluorescence in seeded supersonic expansions of Ar and He/Ar. Diagnostic methods, which were based on the dependence of the intensity of the spectral features on the stagnation pressure and on spectroscopy in He(99%)/Ar(1%) mixtures, were utilized for the identification of the vibrationless electronic excitations of H2P•Ar1. The spectroscopic data were supplemented by model calculations of the potential surface, which demonstrate that the single equilibrium configuration of H2P•R1 (R = Ne, Ar, Kr, and Xe) corresponds to the R atom being located at the twofold symmetry axis perpendicular to the porphyrin ring. While the intense S0 → S1y transition of H2P•Ar, exhibits a red, dispersive, microscopic solvent shift (δv = −24 cm−1), the S0 → S1x transition is characterized by a blue microscopic spectral shift (δv = +8 cm−1), which provides a unique example for excited-state destabilization, originating from intramolecular configurational changes induced by van der Waals binding in a large complex.