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.