Whenever double bonds, such as C=N, C=C, N=N are presented, dyes may have photochromic properties and can be confirmed by light irradiation. By identifying another long-lived/reversible peak in UV spectrometer, people were used to assigned the new grown peak as cis form of the dye (trans -> cis), triggering by photoisomerization.
But in this special case, salicylaldehyde-based dyes with imine bond (shown below), the geometry assignments after light irradiation are inconsistent.
Some say long-lived state is coming from trans->cis isomerzation from original double bond (ref.1), but some say ESIPT is there, and the lifetime is long enough for steady state measurement. (ref. 2)
I am personally on the side of simple trans->cis photoisomerzation cause' i dont think ESIPT can be simply measured by steady-state techniques. Please share your ideas or which mechanism do you consider more trustable.
ref
1. Photochem. Photobiol. Sci., 2012,11, 1389-1400
http://pubs.rsc.org/en/Content/ArticleLanding/2012/PP/c2pp25081k#!divAbstract
2. Chemistry. 2017 Apr 19;23(22):5317-5329.
Article Halogen-Bond Effects on the Thermo- and Photochromic Behavio...
Check out the following ref.
Photoenolization via excited state proton transfer and ion sensing
studies of hydroxy imidazole derivatives
Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 190–199
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