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Photophysical properties are the set of quantities that describe how a molecule or a material takes up light and disposes of the excitation energy, including absorption and emission spectra, fluorescence and phosphorescence quantum yields, excited-state lifetimes, and the rate constants of internal conversion, intersystem crossing and energy transfer. The quantum yield is the ratio of emitted to absorbed photons; it is usually measured against a standard or, for scattering and weakly emitting samples, in an integrating sphere, where reabsorption has to be corrected ^[1]. The luminescence of the sphere wall itself also biases the result unless it is subtracted ^[2]. Typical fluorescence lifetimes fall in the nanosecond range, and a yield below unity means that non-radiative channels compete with emission; which channel dominates depends on the gap between the excited states, the spin-orbit coupling and the rigidity of the environment ^[3]. Intersystem crossing into the triplet manifold can be quantified by quantum-chemical calculation of the spin-orbit matrix elements and competes with fluorescence on the picosecond-to-nanosecond scale ^[4]. Time-resolved methods, femtosecond transient absorption among them, follow the excited population directly and resolve the individual decay channels ^[5]. Reported values for one compound still scatter between laboratories, mostly because of reabsorption, oxygen quenching and the choice of standard ^[2].