Why are singlet states usually more stable than triplet states in ordinary organic compounds?

In most organic compounds, the singlet state is more stable than the triplet state because the ground state of the majority of molecules consists of electrons paired in orbitals with opposing spins. According to the Pauli exclusion principle, each orbital can contain only one electron of each spin; when all occupied orbitals hold these paired electrons, the molecule exists in a singlet state, which represents the lowest energy configuration for most organic structures.12

In contrast, triplet states occur when electrons have parallel spins, which typically requires them to occupy different orbitals. Because these unpaired electrons interact differently, triplet states in ordinary organic compounds are generally higher-energy, excited states. While carbenes are a notable exception—where the triplet state can be of comparable stability to or even lower than the singlet state depending on the substituents—the singlet state remains the standard ground state for the vast majority of organic chemistry.12