The control of radiative and nonradiative decay is important in the design of bioimaging molecules. Dimethylaminobenzonitrile (DMABN) is a suitable model molecule to study radiative and nonradiative decay processes and has been investigated by theoretical and experimental methods. However, an atomistic understanding of the nonradiative decay in solutions remains to be achieved. In this study, we investigated the potential-energy surfaces in excited states along the rotation of the dimethylamino group and found that the degeneration between S1 and T1 states is one of the key factors in the nonradiative decay in polar solvents. In addition, we found that the degeneration is precisely controlled by a fundamental physical property, exchange integral. Although DMABN is a simple molecule, the understanding of the nonradiative decay process on the basis of physical properties should be useful in the design of more complicated imaging molecules.

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.

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.

•Acid-catalyzed hydrolysis of cellobiose in ionic liquid ([mmim][Cl]) is studied.•RISM-SCF-SEDD method is applied to identify the energy profile of SN1-type mechanism.•The solvation effect plays an important role to stabilize reaction product greatly.•The calculated activation energy is in accord with corresponding experimental value.The SN1-type hydrolysis reaction of cellobiose in ionic liquids (ILs) was theoretically investigated. First principles and ab initio quantum chemical methods were used in conjunction with the ‘reference interaction site model self-consistent field with spatial electron density distribution’ (RISM-SCF-SEDD) method. Reaction mechanism pathways are discussed and compared to calculations in gas phase and in aqueous solution. Analysis of solvation effects indicates strong interaction between hydrogen atoms of glucose hydroxyl groups and the anions in ILs, contributing to large stabilization of the reaction product. The calculated activation energy in ILs (24.5 kcal/mol) agrees quantitatively with the experimental value (26.5 kcal/mol).Graphical abstract

•New fitting approach of electrostatic potential.•Stable quantum mechanical calculation with reference interaction site model.•Positive semi-definite condition of electron density matrix.We proposed a new fitting approach of electrostatic potential (ESP) for stable quantum mechanical (QM) calculations using the reference interaction site model. In this approach, the approximated density matrix is fitted so that the ESP computed from QM calculations can be reproduced. We introduced two conditions: conservation of the number of electrons and positive semi-definite condition of the electron density matrix. This approach was introduced in a linear response approximation of the reference interaction site model self-consistent field explicitly including the spatial electron density distribution. By considering the two conditions, we overcame the instability inherent in a previous approach.