  This paper presents the development and characterization of photon
upconverters fabricated with ionic liquids (ILs), which are novel fluids that
are recently drawing attention due to their unique properties, such as
negligible vapor pressures and high thermal stabilities. The upconverters in
this study are based on triplet-triplet annihilation (TTA) between excited
polycyclic aromatic molecules, and TTA requires a fluidic media to allow for
the molecules to collide with each other for energy transfer. This process,
TTA-based photon upconversion (TTA-UC), was therefore mainly accomplished with
organic solvents previously. It is found that the molecules used for TTA-UC,
which are non-polar or weakly polar, are stably solvated in a certain class of
ILs. The mechanism of the observed solvation is proposed and discussed. The
upconversion quantum yields (UC-QYs) measured by continuous wave (CW) light
excitation reach as high as 10 % with moderate excitation intensity (~ 6
W/cm2), which is considerably higher than those in previous TTA-UC studies
performed with organic solvents (up to ~ 4 % with CW light excitations). It is
found that the value of UC-QY starts to saturate as the excitation power
increases for all the cases, even within the moderate CW excitation power range
in this study. An analytical model that describes the UC-QY vs. excitation
power relationship is derived and compared with the experimental results. The
agreement between them suggests that the donor-acceptor energy transfer in this
system is highly efficient. Based on these experimental and analytical
findings, it is found that efficient energy transfer between the molecules is
possible in ILs and therefore ILs are not actually viscous media for the
purpose of TTA-UC.
