Photochromism of photoenolizable ketones in quinoline and 1,8-naphthyridine series studied by time-resolved absorption spectroscopy.
For the two photochromic molecules, 3-benzoyl-2-benzyl-1-methyl-1H-quinolin-4-one (QC1) and 3-benzoyl-1,2-dibenzyl-1H-1,8-naphthyridin-4-one (QC18a) as well as the nonphotochromic 3-benzoyl-1-benzyl-2-methyl-1H-1,8-naphthyridin-4-one (QC18b), the full photochemical mechanism, which is based on the photoenolization process, has been elucidated using stationary and time-resolved spectroscopy techniques. After photoexcitation, the S1(n,pi*)-T1(n,pi*) ISC process involving the exocyclic carbonyl chromophore is demonstrated to occur. Subsequently, gamma-hydrogen transfer proceeds very rapidly to give rise to the triplet photoenol with a probable 1,4-biradical structure. For all three molecules, the biradical is clearly detected and proved quantitatively to be the direct precursor of the colored form (photochromic compounds) or ground state starting material (nonphotochromic compound). Solvent effects for the three molecules studied may suggest the existence of intramolecular hydrogen bonding in both biradical and colored form species. Structural effects on the gamma-hydrogen transfer rate and biradical decay are related to the photochromic performances.
Aloïse
Stéphane
S
Laboratoire de Spectrochimie Infrarouge et Raman (UMR 8516 du CNRS), Université des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq Cedex, France. stephane.aloise@univ-lille1.fr
Réhault
Julien
J
Moine
Baptiste
B
Poizat
Olivier
O
Buntinx
Guy
G
Lokshin
Vladimir
V
Valès
Magalie
M
Samat
André
A