Monitoring Hydrogen/Deuterium Tautomerization in Transient Isomers of Single Porphine by Highly Localized Plasmonic Field

Inner proton transfer between two trans isomers (tautomerization) in porphyrins plays a crucial role in many biological systems as well as molecular nanotechnology. Although the stepwise mechanism of tautomerization is well accepted, the involved intermediate cis-isomer has not been directly detected owing to its short lifetime and the extremely low intensities of corresponding hydrogen vibrations. Here, taking a single porphine as the prototype, we theoretically demonstrate that Raman intensities of the hydrogen vibrations become accessible under the highly localized plasmonic field because of the symmetry breaking effect. In addition, with the ultrafast incident excitations, we find that Raman signals of cis-porphine could be distinguished from the stable trans isomer, suggesting a general protocol for the direct characterization of transient isomers. Moreover, calculated results reveal that the position of inner hydrogen/deuterium can be unambiguously visualized from Raman images of the corresponding stretching modes, providing a unique optical means for the chemical monitoring of tautomerization in porphine and its derivatives.