Action spectroscopy of spin forbidden states in the gas phase: A powerful probe for large non-luminescent molecules

Triplet action spectra of two similar copper porphyrins, copper tetraphenylporphyrin (CuTPP) and copper octaethylporphyrin (CuOEP), have been studied in the gas phase at low temperatures in the absence of external perturbations by using a resonant pump and a 193 nm probe, ionizing the ³ππ^* orbital localized on the porphyrin cycle. The molecules were prepared by laser desorption in a disk source, then cooled in a helium supersonic expansion, and finally excited in the Q band system (S₁ ← S₀). This type of experiment allows the spectroscopic characterization of large non-luminescent molecules in the absence of solvent perturbations. The two copper porphyrins exhibit a broad electronic origin Q₀₀ absorption spectrum, partly caused by the short lifetime of the excited (S₁) state. The two porphyrins differ strongly with a strong Q₀₀ band for CuOEP and a weak one for CuTPP, in agreement with the Gouterman four-orbital model. The two molecules exhibit different solvent shifts: CuOEP is blue shifted in non-polar solvents owing to its alkyl substituents, while CuTPP is red shifted as for regular transitions to ππ^* orbitals. The decay dynamics of the triplet state exhibit a collision-free lifetime of 70 ± 7 ns for CuTPP atop a microsecond decay. This non-exponential decay can be viewed as evidence of time evolution of two states combining the state with spin 1 borne by the porphyrin ring and that by the Cu atom 12. Therefore, this method allows solvent-free spectrodynamics of large molecules in a short microsecond time range.