To exclude the influence of solid-state solvation effects on the spectral properties, we measured the relative permittivity (εr) of all acceptor hosts (Fig. , Supplementary Information 4.4). Conjugated phosphine oxide-based materials exhibit higher εr values of 6 to 7 than the conventional organic semiconductors (εr of approximately 4), which could enable the longer-lived LPL in phosphine oxide hosts. The solid-state solvation effects were evaluated by doping DACT-II into host materials because TADF materials are known to exhibit significant solvent effects (Extended Data Fig. ). The trend in the spectral redshift of DACT-II in acceptor films was consistent with that in εr (Fig. ). Donor materials used in this study, such as AAPC, also exhibit solvation effects owing to their partial intramolecular CT nature (Extended Data Fig. , Supplementary Information 4.12.1). However, in contrast to the DACT-II case, the spectral redshift behavior of donors in acceptor films cannot be correlated with εr, and it is clear that the redshift is caused by factors other than the solid-state solvent effect (Extended Data Fig. ).