In 2D perovskites, as in all semiconductors, the degeneracy of the dark and bright exciton states is lifted by the exchange interaction between the electron and hole. This exciton fine structure is essential to understand the interaction of matter with light, and reflects the underlying symmetry of the system. The bright-dark splitting is also of paramount importance for light emitters which rely on the radiative recombination of excitons, since the excitons usually relax to the lowest lying dark state, which is detrimental for the device efficiency. It is expected that the enhanced Coulomb interaction in the 2D limit strongly increases the splitting of dark and bright excitonic states, however, such a splitting has never been measured directly in 2D perovskites. The only available report, based on photoluminescence (PL) studies, provides only a rough estimate of the splitting, since PL can be affected by trap states, and the complex exciton dynamics. For the particular case of the soft perovskites lattice, a direct comparison of the bright-dark splitting with the energy of phonons is crucial to understand the thermal mixing of the two excitonic states which drives radiative recombination in this material system. For the future development of 2D perovskites it is therefore of paramount importance to understand the exciton fine structure in 2D perovskites. I will discuss our optical spectroscopy measurements with an applied in-plane magnetic
field to mix the bright and dark excitonic states of (PEA)2PbI4, providing the first direct measurement of the bright-dark splitting. The induced brightening of the dark state allows us
to directly observe an enhancement of the absorption at the low-energy side of the spectrum related to the dark state. With the signature of dark state in the optical spectra we are able precisely determine the bright-dark exciton state splitting of 8 meV for (PEA)2PbI4at B= 0 T. The brightening of the dark state was also visible in photoluminescence, dominating the emission already at a moderate magnetic field of 6T. The evolution of the PL signal in the magnetic field, suggests that at low temperatures (4.5K) the exciton population is not fully thermalized due to the existence of a phonon bottle-neck, which occurs due to the specific nature of the exciton-phonon coupling in soft perovskite materials.