Excited‐State Intramolecular Proton Transfer in a Blue Fluorescence Chromophore Induces Dual Emission

Abstract

Compared with green fluorescence protein (GFP) chromophores, the recently synthesized blue fluorescence protein (BFP) chromophore variant presents intriguing photochemical properties, for example, dual fluorescence emission, enhanced fluorescence quantum yield, and ultra-slow excited-state intramolecular proton transfer (ESIPT; J. Phys. Chem. Lett., 2014, 5, 92); however, its photochemical mechanism is still elusive. Herein we have employed the CASSCF and CASPT2 methods to study the mechanistic photochemistry of a truncated BFP chromophore variant in the S0 and S1 states. Based on the optimized minima, conical intersections, and minimum-energy paths (ESIPT, photoisomerization, and deactivation), we have found that the system has two competitive S1 relaxation pathways from the Franck-Condon point of the BFP chromophore variant. One is the ESIPT path to generate an S1 tautomer that exhibits a large Stokes shift in experiments. The generated S1 tautomer can further evolve toward the nearby S1 /S0 conical intersection and then jumps down to the S0 state. The other is the photoisomerization path along the rotation of the central double bond. Along this path, the S1 system runs into an S1 /S0 conical intersection region and eventually hops to the S0 state. The two energetically allowed S1 excited-state deactivation pathways are responsible for the in-part loss of fluorescence quantum yield. The considerable S1 ESIPT barrier and the sizable barriers that separate the S1 tautomers from the S1 /S0 conical intersections make these two tautomers establish a kinetic equilibrium in the S1 state, which thus results in dual fluorescence emission.

Unveiling Controlling Factors of the S0/S1 Minimum Energy Conical Intersection: A Theoretical Study

“…With consideration for previous theoretical studies related to the MECI, ,− we optimized the S 0 EQ, S 1 EQ, and S 0 /S 1 MECI geometries on the basis of spin-restricted DFT, spin-restricted TDDFT, and spin-flip TDDFT, respectively. Single-point spin-restricted TDDFT calculations were then performed for all the obtained geometries to evaluate the excitation energy components.…”

Molecular Organization of Dipalmitoylphosphatidylcholine Bilayers Containing Bioactive Compounds 4-(5-Heptyl-1,3,4-thiadiazol-2-yl) Benzene-1,3-diol and 4-(5-Methyl-1,3,4-thiadiazol-2-yl) Benzene-1,3-diols

“…Other determinants of the appearance of dual fluorescence are excimer emissions appearing with an increasing concentration of the solution (ct.). To end the overview of the theoretical attempts at the elucidation of the occurrence of several fluorescence bands, it is worth mentioning that the excited-state intramolecular proton transfer is an equally interesting and popular explanation of this process. , Moreover, the very interesting mechanism involved in dual fluorescence, that is, the anti-Kasha mechanism, reported by Brancato et al in 2015 should be mentioned . However, molecular aggregation combined with the structure of the alkyl substituent may have the greatest impact on the fluorescence effects observed in the lipid environment in the case of the 1,3,4-thiadiazoles investigated in this study. , The analyses of the 1,3,4-thiadiazole analogues carried out with stationary and excited-state fluorescence spectroscopy demonstrated three fluorescence forms (as well as the dual fluorescence effect in some temperature and concentration ranges) induced by changes in the concentration of the compounds and temperature in the DPPC liposome medium.…”

Modern Theoretical Approaches to Modeling the Excited-State Intramolecular Proton Transfer: An Overview

“…Afterward, an analysis of ES relaxed properties is conducted, and the barrierless/barrier-restricted character of ESIPT is determined by excited-state geometry optimization of the relevant isomers, along with predictions for the energy and intensity of the Stokes-shifted fluorescence [ 38 , 46 ]. In the final step, adiabatic potential energy profiles (PEPs) [ 47 , 48 ] or PESs [ 3 , 33 ] may be calculated, if one or multiple reaction coordinates, respectively, need to be explicitly considered. In certain cases, the energy profiles of linearly interpolated reaction paths might also efficiently support the static ESIPT analysis [ 49 ].…”