Excited-state intramolecular proton transfer (ESIPT) reactions belong to the fastest chemical reactions known. Moreover, the resulting ultrafast radiationless electronic deactivation guarantees record UV photostabilities. ESIPT molecules thus offer huge advantages in numerous fields, e.g. as photostabilizers in sunscreens or, of interest here, for novel photochromic molecular switches.1We report on a study of the stepwise, ultrafast formation of the proton transfer product of the bistable ESIPT photoswitch N-(3-pyridinyl)-2-pyridine-carboxamide (NPPCA; Figure 1) in solution in dry acetonitrile by femtosecond time-resolved fluorescence up-conversion and transient absorption spectroscopy.The measurements indicate that the initial proton transfer from tautomer I to tautomer II in the excited state takes place within t1 ~ 100 to t2 ~ 500 fs. The latter value also corresponds to the time for the transition through the conical intersection connecting the excited state of II to structure III in the electronic ground state. Subsequent slower proton transfer on the time scale of t3 ~ 20 ps leads to the final product IV, which was observed by transient absorption at ? = 247 nm. The interpretation of the experimental results was supported by ab initio excited-state calculations at the MP2/cc-pVDZ, CC2/cc-pVDZ and ADC(2)/cc-pVDZ levels of theory.1L. Lapinski, M. J. Nowak, J. Nowacki, M. F. Rode and A. L. Sobolewski, ChemPhysChem 2009, 10  , 2290.

Ultrafast dynamics of a bistable excited-state intramolecular proton transfer photoswitch

248th National Meeting (2014)

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