where ∆EST is the relative energy of the ( 3 T1) triplet above the ground state singlet minimum, while ∆EQT is the relative energy of the triplet above the energy minimum of the quintet, and ζ is the spin-orbit coupling constant. The thermally averaged Franck-Condon factor quantifies the overlaps between the wave functions of the final χn and initial χm vibrational states): (5) We have used the frequencies of the breathing mode-like vibrations for the HS (ωQ) and LS (ω) states to calculate the F (T ) according to Equation where the ∆E 0 HL stands for the energy difference of the lowest vibrational levels of the HS and LS states.