High-temperature reversible emission in a simple Schiff base: activated quenching, exciton-phonon renormalization,...

Thermally robust solid-state emission remains rare among simple organic chromophores because elevated temperature usually enhances nonradiative decay, structural relaxation, and irreversible degradation. Here we investigate the thermal photophysics of the Schiff base para-phenylenediamine salicylaldehyde (SA-p-PD) bringing a unified kinetic and thermodynamic understanding. The material preserves measurable photoluminescence up to 200 °C and shows near-complete recovery on cooling, establishing reversible high-temperature luminescence. Across 300-473 K, the emission maximum shifts from approximately 580 to 600 nm, corresponding to a redshift coefficient of about 0.12 nm K^-1. The intensity decay follows a Mott-Seitz description with an activation barrier near 0.22 eV, while lifetime shortening yields a comparable nonradiative barrier in the 0.18 eV range. A Bose-Einstein analysis links the continuous redshift to exciton-phonon coupling with an effective phonon energy of about 38 meV. Density-functional modeling and temperature-dependent spectral evolution both indicate that the high-temperature shoulder near 620 nm is associated with keto-like tautomeric emission. A thermodynamic population analysis further shows that a simple ground-state Boltzmann redistribution cannot, by itself, explain the strength of that shoulder: for a 0.20 eV enol-to-keto free-energy offset, the predicted keto population at 473 K remains below 1%. The data are therefore more consistent with a mixed mechanism consisting of a phonon-driven redshift of an enol-like emissive state together with a thermally activated keto-like emissive branch. This framing turns SA-p-PD from a single thermochromic emitter into a compact model system for studying coupled quenching, lattice-assisted spectral renormalization, and excited-state tautomerism. Contrasting the temperature evolution of peak energy, integrated intensity, lifetime, and low-energy shoulder growth further shows that gap renormalization precedes the dominance of nonradiative loss and that a simple ground-state Boltzmann picture is insufficient to explain the hot-state 620 nm feature.