Research Projects

Chromophores confined in nanoscale environments such as molecular cages, metal-organic or covalent-organic frameworks, or even biological hosts, display interesting excited-state properties compared to their free counterparts. Confinement can affect the chromophore by altering its electronic structure, suppressing non-radiative decay by restricting motion, enhancing (often selectively) its photochemical pathways, and even stabilizing otherwise unstable states of the molecule. Such effects find use in photoluminescence-based sensing and imaging, organic LEDs and photovoltaics, photocatalysis, molecular switches and information storage, and drug delivery. Hence, there is wide interest in host-guest complexes involving organic chromophores. Organic hosts that mimic the hydrophobic interior of proteins and vesicles are of fundamental interest as they provide a model system to study the behaviour of chromophores important in biological function. Octa acid, a supramolecular assembly based on resorcinarene, consists of a macrocyclic platform with four carboxylic groups at its rims. This system forms dimers, or capsuleplexes, in the presence of hydrophobic guests, with the hydrophobic effect as the primary driving force for encapsulation, supported by π...π interactions, CH...π interactions, and van der Waals forces. Notably, the octa acid capsule is soluble in aqueous solutions under basic conditions (pH ~9) due to its eight carboxylic groups, forming inclusion complexes with host/guest ratios of 2:1 or 2:2. Despite the aqueous environment, the capsule’s interior remains hydrophobic. The system’s stability and the confined environment of the capsule offer unique insights into excited state dynamics, minimizing solvent effects and diffusion limitations. This makes it a valuable tool for studying the dynamics of excited states, as polarization effects inside the capsule are less pronounced compared to free solution. When encapsulated in OA, thiocoumarins such as TC-1 and TC-3 (see Fig. 1) show a significant enhancement in phosphorescence at the expense of fluorescence, making them particularly suitable for room-temperature phosphorescence (RTP) applications. However, the 7-amino group’s role in charge transfer and the ability of some thiocoumarins to undergo bond cleavage in their excited states introduce variability in their behavior. For instance, TC-2, despite showing both fluorescence and phosphorescence at low temperatures, does not exhibit RTP in the presence of OA due to fragmentation pathways that override the effects of encapsulation. This highlights the complex interplay between molecular structure, solvent environment, and host interactions in controlling the photophysical properties of thiocoumarins, motivating further studies to optimize their use in various practical applications. In this project, using highly accurate and state-of-the-art density-functional theoretical techniques, we plan to explore the intriguing photophysics of octa-acid confined thiocoumarins, the mechanism of photocleavage reactions that could enable targeted drug delivery, and the mechanism of electron transfer between a caged guest and an external acceptor.