Mechanistic Insights into the Photophysics of Ortho-hydroxyl GFP Core Chromophores†

Herein we have employed the MS-CASPT2//CASSCF method to study the S1 excited-state intramolecular proton transfers (ESIPTs) of recently synthesized ortho-hydroxyl GFP core chromophores, i.e. OHIM, CHBDI, and MHBID, and their excited-state relaxation pathways. We have found that in OHIM and CHBDI, the ESIPT process is associated with small barriers of 3.4 and 4.2 kcal/mol; while, in MHBDI, it becomes essentially barrierless. Moreover, we have found two main S1 excited-state radiationless channels. In the first one, the enol S1 species decays to the S0 state via the enol S1/S0 conical intersection after overcoming considerable barriers of 7.0 and 7.7 kcal/mol in OHIM and CHBDI (however, in MHBDI, it is nearly barrierless). In the second one, the keto S1 species is first generated through the ESIPT event; then, it is de-excited into the S0 state in the vicinity of the keto S1/S0 conical intersection. These energetically allowed excited-state decay channels rationalize experimentally observed ultralow fluores...