Colour-Stable When Heated, Colour-Shifting When Excited: Deep-Blue TADF Hosts Need Vector, Not Scalar, Screening

Deep-blue thermally activated delayed fluorescence (TADF) materials are screened by a single figure of meritquantum yield, singlet–triplet gap, colour shift or activation energy—on the assumption that a material robust under one stress is robust under others. Here we show that this assumption fails. For three related phosphineoxide hosts, the host whose emission colour moves least on heating is the one whose colour moves most when the excitation wavelength is changed: an 8-to 17-fold reversal, read directly from instrument chromaticity coordinates. Solvent response, spectral dimensionality, quantum yield and delayed-fluorescence kinetics each add a different ordering, and no host is best on every axis. A thermal screen would therefore advance precisely the most excitationsensitive material. We replace the stability score with a four-axis descriptor vector—environment, excitation, temperature and time—that keeps each vulnerability visible and lets the intended application decide which tradeoff is acceptable, before a material is committed to devices.