A mechanistic understanding of how the molecular structure governs the photoluminescence quantum yield (PLQY) in non-fullerene acceptors (NFAs) remains elusive, hindering further progress in organic solar cell materials. Here, we report a crucial yet previously overlooked nonradiative recombination pathway-the twisted intramolecular charge transfer (TICT) state-in low-bandgap NFAs. Through a combination of spectroscopy, quantum calculations, and exciton decay analysis in nearly 100 organic semiconductors-primarily NFAs-we show that TICT, which is formed via conformation variation after photoexcitation, acts as a dark state on the first excited-state potential energy surface. It leads to suppressed PLQYs and multi-exponential decay behavior, providing a unified explanation for the wide variation in PLQYs observed across the investigated materials. We further demonstrate that blocking intramolecular motion, for instance, by employing a polystyrene matrix or at low temperatures, suppresses TICT formation and thereby enhances PLQYs. This work provides deeper insight into excited-state species and recombination mechanisms in organic semiconductors and further underscores the highly emissive characteristics of traditional low-bandgap NFAs, highlighting their considerable potential for emission applications.