Getting to the (Square) Root of the Problem: How to Make Noncoherent Pumped Upconversion Linear

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⁺ 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_ex dictates the observed incident power dependence. Finally, pure linear behavior was derived from noncoherent excitation for the first time at higher sensitizer concentrations