Understanding of the Off–On Response Mechanism in Caged Fluorophores Based on Quantum and Statistical Mechanics

For many years, numerous fluorescent probes have been synthesized and applied to visualize molecules and cells. The development of such probes has accelerated biological and medical investigations. As our interests have been focused on more complicated systems in recent years, the search for probes with sensitive environment off–on response becomes increasingly important. For the design of such sophisticated probes, theoretical analyses of the electronically excited state are inevitable. Especially, understanding of the nonradiative decay process is highly desirable, although this is a challenging task. In this study, we propose an approach to treat the solvent fluctuation based on the reference interaction site model. It was applied to selected bioimaging probes to understand the importance of solvent fluctuation for their off–on response. We revealed that the this switching process involves the nonradiative decay through the charge transfer state, where the solvent relaxation supported the transition between excited and charge transfer states. In addition, energetically favorable solvent relaxation paths were found due to the consideration of multiple solvent configurations. Our approach makes it possible to understand the nonradiative decay facilitated by a detailed analysis and enables the design of novel fluorescent switching probes considering the effect of solvent fluctuation

Understanding the On–Off Switching Mechanism in Cationic Tetravalent Group-V-Based Fluoride Molecular Sensors Using Orbital Analysis

The precise control of on–off switching is essential to the design of ideal molecular sensors. To understand the switching mechanism theoretically, we selected as representative example a 9-anthryltriphenylstibonium cation, which was reported as a fluoride ion sensor. In this molecule, the first excited singlet state exhibits two minimum geometries, where one of them is emissive and the other one dark. The excited state at the geometry with bright emission is of π–π* character, whereas it is of π–σ* character at the “dark” geometry. Geometry changes in the excited state were identified by geometry optimization and partial potential energy surface (PES) mapping. We also studied Group V homologues of this molecule. A barrierless relaxation pathway after vertical excitation to the “dark” geometry was found for the Sb-containing compound on the excited-states PES, whereas barriers appear in the case of P and As. Molecular orbital analysis suggests that the σ* orbital of the antimony compound is stabilized along such relaxation and that the excited state changes its nature correspondingly. Our results indicate that the size of the central atom is crucial for the design of fluoride sensors with this ligand framework