Not yet reviewed by Pith; the record is open.

The organic spacer cation plays a crucial role in determining the exciton fine structure in two-dimensional (2D) perovskites. Here, we use low-temperature magneto-optical spectroscopy to gain insight into the influence of the organic spacer on dark excitons in Ruddlesden-Popper (RP) perovskites. We show that by using modest magnetic field strengths (<1.5 T), the spin-forbidden dark exciton state can be identified and its emission properties significantly modulated through the application of in-plane magnetic fields, up to temperatures of 15 K. At low temperatures, an increase in collected photoluminescence efficiency of >30% is demonstrated, signifying the critical role of the dark exciton state for light-emitting applications of 2D perovskites. The exciton fine structure and the degree of magnetic-field-induced mixing are impacted by the choice of organic spacer cation, with 4-methoxyphenethylammonium (MeO-PEA) showing the largest effect due to larger bright-dark exciton splitting. Our results suggest that dark excitons preferentially form biexcitons depending on the choice of spacer. We distinguish between interior (bulk) and surface dark-exciton emission, showing that bright-dark exciton splitting differs between the interior and surface. Our results emphasize the significance of the organic spacer cation in controlling the exciton fine structure in 2D perovskites and have important implications for the development of optoelectronic technology based on 2D perovskites.