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Tautomerization in 2,7,12,17‐Tetraphenylporphycene and 9‐Amino‐2,7,12,17‐tetraphenylporphycene: Influence of Asymmetry on the Direction of the Transition Moment
Author(s) -
Fita Piotr,
Pszona Maria,
Orzanowska Grażyna,
SánchezGarcía David,
ll Santi,
Vauthey Eric,
Waluk Jacek
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201201432
Subject(s) - tautomer , chemistry , intramolecular force , moment (physics) , anisotropy , ground state , excitation , absorption spectroscopy , atomic orbital , atomic physics , computational chemistry , physics , stereochemistry , quantum mechanics , electron
Femtosecond transient absorption anisotropy studies have been performed for two porphycenes of different symmetry. In 2,7,12,17‐tetraphenylporphycene, the chemical identity of two trans forms implies a change in the S 0 –S 1 transition‐moment direction upon tautomerization. Exploiting this phenomenon, the rates of double hydrogen transfer in both the S 0 and S 1 states (1.4×10 12 s −1 and 2.7×10 11 s −1 , respectively) have been determined by performing time‐resolved anisotropy studies. In the asymmetric 9‐amino‐2,7,12,17‐tetraphenylporphycene, tautomerization occurs with a similar rate in the ground state. In the S 1 state, the reaction is hindered in its vibrationally relaxed form, but the excitation spectra suggest that it may occur for an unrelaxed molecule. Unlike all porphycenes that have been studied so far, 9‐amino‐2,7,12,17‐tetraphenylporphycene does not reveal significant changes in anisotropy owing to intramolecular double hydrogen transfer; rather, the transition‐moment directions are similar in the two tautomeric forms. Analysis of the molecular orbitals allows for an explanation of the “locking” of the transition moments: it is due to a large splitting of the two HOMO orbitals, which retain the order of their energies in the two tautomers.