Getting to the (Square)
Root of the Problem:
How to Make Noncoherent Pumped Upconversion Linear
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Abstract
We present experimental data illustrating that photochemical
upconversion
based on sensitized triplet–triplet annihilation can exhibit
anti-Stokes emissions whose intensities with respect to the excitation
power can vary between quadratic and linear using a noncoherent polychromatic
light source. The benchmark upconverting composition consisting of
Pd(II) octaethylporphyrin (PdOEP) sensitizers and 9,10-diphenylanthracene
(DPA) acceptors/annihilators in toluene was selected to generate quadratic,
intermediate, and linear behavior under both coherent and noncoherent
excitation conditions. Each of these power laws was traversed in a
single sample in one contiguous experiment through selective pumping
of the sensitizer using an Ar<sup>+</sup> laser. Wavelength-dependent
responses ranging from quadratic to pseudolinear were also recorded
from the identical sample composition when excited by Xe lamp/monochromator
output in a conventional fluorimeter, where the optical density at
λ<sub>ex</sub> dictates the observed incident power dependence.
Finally, pure linear behavior was derived from noncoherent excitation
for the first time at higher sensitizer concentrations