Action spectroscopy of spin forbidden states in the gas phase: A powerful probe for large non-luminescent molecules
Résumé
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 absence of external perturbations by a resonant pump and a 193 nm probe ionizing the 3 * 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 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 M. Gouterman's 4 orbital model. Also, 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 * 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 an evidence of time evolution of two states combining: the state with spin 1 borne by the porphyrin ring and that by the Cu atom, ½. Therefore, this method allows solvent free spectrodynamics of large molecules in the short microsecond time range.
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