Photophysical properties of molecules: From biological imaging to new materials

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Computational study allows the interpretation of modern experimental measurements. Importantly, models can be developed to generalize observed behaviour, understand related systems, and design improved molecules or materials. Our research develops and uses theoretical and computational methods for understanding and predicting molecular structure and properties. The over-arching emphasis is on excited electronic states, which are connected to experimental observables (e.g., 1- or multi-photon absorption, light emission, photochemistry...). However, the problems and applications studied are diverse: (1) In biological imaging, the focus is on both fluorescent proteins and fluorescent nucleobase analogues, which mimic the structure and function of the natural nucleobase building blocks of DNA and RNA. Fluorescent proteins are useful probes of biological mechanisms, functions, and structures in molecular and cell biology. Meanwhile, fluorescent nucleobase analogues are needed because their natural counterparts are non-fluorescent, and thus cannot be used to observe DNA and RNA structure, function, and dynamics directly. For both classes of probes, there is rapidly developing research moving beyond one-photon absorption to two- or three-photon absorption, as the longer wavelengths used permit deeper penetration and allow better focus, which are critical needs for live cell or organism imaging. However, corresponding computational study is lagging behind experimental development. The proposed research aims to fill that gap. Overall, a diversity of multi-photon processes in biological probes will be examined with the goals of a better understanding of the underlying photophysics, rationalizing the connections between structure and photophysical properties, and assisting with the design of new improved probes specifically for multi-photon applications. (2) A number of new phosphorescent compounds containing tellurium and bismuth, rather than expensive transition metals, have recently been synthesized and characterized. Phosphorescence is the basis for many technologies, including light-emitting devices (LEDs). While a basic understanding of the photophysics has been obtained computationally, including qualitative ideas on why compounds are phosphorescent versus non-phosphorescent, a detailed understanding requires the use of more sophisticated quantum chemistry approaches than have been used to-date. Moreover, to understand the relative quantum yields, new tools will be developed and used to map the relaxation pathways following photoexcitation, which will lead to an improved understanding of the factors leading to higher (or lower) quantum yields, i.e., better or worse materials. Through these simulations of the properties of known compounds, we can rationally design new materials with improved spectral characteristics. (3) We will develop and apply methods for studying quantum dynamics in small molecules for atmospheric chemistry and astrochemistry.