We investigate excitonic properties of 2D layered perovskites with a focus on controlling exciton fine structure. Ultra-high-field magneto-optical spectroscopy (up to 140 T) yields scaling laws linking exciton binding energy to band gap and diamagnetic coefficient. In-plane magnetic field resolves exciton fine-structure, which—via an effective-mass model—is correlated with structural parameters and reveals strong steric control from organic spacers. Lattice distortions tune the dark–bright exciton energy splitting, highlighting 2D perovskites as promising platforms for efficient light emitters and quantum technologies.

We test the magnetic-field robustness of a common luminescent thermometer, [Tb₀.₉₃Eu₀.₀₇(bpy)₂(NO₃)₃], up to 58 T. The standard Tb/Eu intensity-ratio method breaks down even in weak fields. By identifying transitions with minimal magnetic correlation, the thermometer operates across the full temperature range to 20 T and at higher fields for T > 120 K. The results show that performance is highly material-dependent and outline a route to robust thermometry in strong-field environments.

Layered halide perovskites host strongly bound excitons with spin–orbit and exchange couplings that create a dark-ground/bright-exciton landscape. Combining low-temperature magneto-optical spectroscopy with many-body theory, we track dark–bright thermalization and explain the unexpectedly strong bright emission: a pronounced phonon bottleneck slows relaxation into dark states. Tuning the bright–dark splitting and optical phonon energies modulates this bottleneck, providing design rules to control exciton emission for perovskite optoelectronics.