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Magnetic Circular Dichroism of 5,10,15,20‐Tetraphenylsapphyrin
Author(s) -
Lament Bożena,
Rachlewicz Krystyna,
LatosGrażyński Lechosław,
Waluk Jacek
Publication year - 2002
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/1439-7641(20021018)3:10<849::aid-cphc849>3.0.co;2-9
Subject(s) - homo/lumo , chemistry , magnetic circular dichroism , chromophore , protonation , circular dichroism , crystallography , atomic orbital , spectral line , molecular orbital , singlet state , stereochemistry , photochemistry , physics , atomic physics , molecule , excited state , electron , organic chemistry , astronomy , ion , quantum mechanics
Magnetic circular dichroism (MCD) spectra were obtained for the inverted and “normal” forms of doubly protonated 5,10,15,20‐tetraphenylsapphyrin and for the inverted neutral form. In all these species, the same pattern of the signs of Faraday B terms, {+,−,+, −}, is revealed for the four lowest singlet electronic transitions. This sequence is opposite to that observed in sapphyrins that are unsubstituted at the meso positions. The results can be accounted for by the reversed patterns of the orbital energy splittings in the two highest occupied π orbitals (ΔHOMO) and the two lowest unoccupied π orbitals (ΔLUMO). Indeed, the application of the perimeter model leads to the prediction that in the parent sapphyrin , ΔHOMO<ΔLUMO, whereas the opposite becomes true after fourfold phenyl substitution at the meso positions. These predictions are corroborated by the results of quantum‐chemical calculations. The calculated values of ΔHOMO−ΔLUMO are quite small, which places sapphyrin close to the case of the so‐called “soft chromophore”, for which ΔHOMO=ΔLUMO. Titration of solutions containing the neutral chromophore with bases and acids leads to parallel changes in the electronic absorption and MCD spectra that indicate presence of the anionic and singly protonated species.