Supersonic jet spectroscopy of parent hemiporphycene: Structural assignment and vibrational analysis for S 0 and S 1 electronic states
Résumé
Hemiporphycene (HPc), a constitutional isomer of porphyrin, is studied under supersonic expansion conditions by means of laser-induced fluorescence (LIF), visible-visible hole-burning experiments, single vibronic level fluorescence (SVLF) techniques, and quantum chemical calculations. Only one trans form of jet-cooled HPc is observed, in contrast with solutions studies that evidence a mixture of two trans tautomeric forms separated in energy by ca. 1 kcal/mol. Reliable structural assignment is provided by simulating absorption and emission patterns at the DFT and TD-DFT levels of theory. The vibronic spectra are nicely reproduced for both electronic ground and lowest excited singlet states for the most stable trans form. In contrast with another porphyrin isomer, porphycene (Pc), no tunneling or photo-induced hydrogen transfer are detected. The lower symmetry of HPc compared with Pc and the concomitant non-equivalent positions of the inner-cavity nitrogen atoms result in a non-symmetric double minimum potential for tautomerization, larger energy barrier, and a longer tunneling distance, with the average intramolecular hydrogen bonds length larger in HPc than in Pc. HPc readily forms hydrates that show red-shifted absorption relative to the bare molecule.
Domaines
Chimie
Origine : Fichiers produits par l'(les) auteur(s)
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