Host–Guest Complexation-Mediated Supramolecular Photon Upconversion

The
phenomenon of photon upconversion, in which a system absorbs
two or more photons of lower energy and emits a photon of higher energy,
has been used in numerous applications, including non-destructive
bioimaging, deep-penetrating photodynamic therapy, catalysis, and
photovoltaic devices. To date, photon upconversion has been observed
typically in inorganic nanocrystals, nanoparticles, metal–organic
frameworks, supra­molecular assemblies, and organic dyads. Herein,
we demonstrate a new strategy for harnessing photon upconversionsupra­molecular
upconversionbased on host–guest chemistry. We have
identified a box-like fluorescent tetra­cationic host incorporating
a thiazolo­thiazole emitter, which can accommodate a guest-sensitizer,
5,15-diphenyl­porphyrin, inside its cavity, and demonstrated
that the host–guest inclusion complex displays triplet-fusion
upconversion when the guest is excited with low-energy light. The
strategy of supra­molecular upconversion has been employed successfully
in two other host–guest complexeswith hosts comprised
of anthracene emitters and a 5,15-diphenyl­porphyrin guestcorroborating
the fact that this strategy is a general one and can be applied to
the design of a new family of host–guest complexes for photon
upconversion. More importantly, supra­molecular upconversion
is accessible in solution under dilute conditions (μM) compared
to most of the existing approaches that require significantly higher
concentrations (mM) of emitters and/or sensitizers. Transient absorption
spectroscopy and density functional theory have been employed in order
to confirm a triplet-fusion upconversion mechanism. Host–guest
complexation-mediated supra­molecular photon upconversion eliminates
multiple issues in the existing systems related to high working concentrations,
high incident laser power, and low optical penetration depths.