Proton-Stabilized Photochemically Reversible E/ Z Isomerization of Spiropyrans

L Kortekaas; J Chen; D Jacquemin; W R Browne

doi:10.1021/acs.jpcb.8b03528

Proton-Stabilized Photochemically Reversible E/ Z Isomerization of Spiropyrans

J Phys Chem B. 2018 Jun 21;122(24):6423-6430. doi: 10.1021/acs.jpcb.8b03528. Epub 2018 Jun 11.

Authors

L Kortekaas¹, J Chen¹, D Jacquemin², W R Browne¹

Affiliations

¹ Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences , University of Groningen , Nijenborgh 4 , 9747AG Groningen , The Netherlands.
² Chimie Et Interdisciplinarité, Synthèse, Analyse, Modélisation (CEISAM) , UMR CNRS no. 6230, Université de Nantes , BP 92208, 2, Rue de la Houssinière , 44322 Nantes Cedex 3, France.

Abstract

Spiropyrans undergo C_spiro-O bond breaking to their ring-open protonated E-merocyanine form upon protonation and irradiation via an intermediate protonated Z-merocyanine isomer. We show that the extent of acid-induced ring opening is controlled by matching both the concentration and strength of the acid used and with strong acids full ring opening to the Z-merocyanine isomer occurs spontaneously allowing its characterization by ¹H NMR spectroscopy as well as UV/vis spectroscopy, and reversible switching between Z/ E-isomerization by irradiation with UV and visible light. Under sufficiently acidic conditions, both E- and Z-isomers are thermally stable. Judicious choice of acid such that its p K_a lies between that of the E- and Z-merocyanine forms enables thermally stable switching between spiropyran and E-merocyanine forms and hence pH gating between thermally irreversible and reversible photochromic switching.