Confinement-Driven Photophysics in Cages, Covalent−Organic Frameworks, Metal–Organic Frameworks, and DNA

Photophysics tunability through alteration of framework aperture (metal–organic framework (MOF) = variable; guest = constant) was probed for the first time in comparison with previously explored concepts (MOF = constant; guest = variable). In particular, analysis of the confinement effect on a photophysical response of integrated 5-(3-chlorobenzylidene)-2,3-dimethyl-3,5-dihydro-4H-imidazol-4-one (Cl-BI) chromophore allowed us to establish a photophysics–aperture relationship. To shed light on the observed correlation, the framework confined environment was replicated using a molecular cage, Pd₆(TPT)₄ (TPT = 2,4,6-tri­(pyridin-4-yl)-1,3,5-triazine), thus allowing for utilization of crystallography, spectroscopy, and theoretical simulations to reveal the effect a confined space has on the chromophore’s molecular conformation (including disruption of strong hydrogen bonding and novel conformer formation) and any associated changes on a photophysical response. Furthermore, the chosen Cl-oHBI@Pd₆(TPT)₄ (Cl-oHBI = 5-(5-chloro-2-hydroxybenzylidene)-2,3-dimethyl-3,5-dihydro-4H-imidazol-4-one, chromophore) system was applied as a tool for targeted cargo delivery of a chromophore to the confined space of DNA, which resulted in promotion of chromophore–DNA interactions through a well-established intercalation mechanism. Moreover, the developed principles were applied toward utilizing a HBI-based chromophore as a fluorescent probe on the example of macrophage cells. For the first time, suppression of non-radiative decay pathways of a chromophore was tested by anchoring the chromophore to a framework metal node, portending a potential avenue to develop an alternative to natural biomarkers. Overall, these studies are among the first attempts to demonstrate the unrevealed potential of a confined scaffold environment for tailoring a material’s photophysical response.