# why are triplet excitons more tightly bound than singlet?

In organic light emitting diodes, the Jablonski diagrams always show he T1 states to be lower in energy to the S1 states. They are also declared to be more tightly bound as a result. I don't understand why this is the case. I would almost think the opposite true since the singlet orbital wavefunction must be symmetric, the electron and hole might be closer to one another.