Intersystem crossing vibrational energy transfer

If the lower energy state intersects at a point (Figure 5.1) above the zero vibrational level of the transferring state, a temperature dependent factor e w/kT may be involved in the rate constant for intersystem crossing. The energy term W corresponds to the activation energy needed to raise the molecule from the zero point to the point of intersection. 

Intersystem crossing (ISC) is the intramolecular crossing from one state to another of different multiplicity without the emission of radiation. In Fig. 3.9 (ISC), shows the transfer from the first excited singlet state S, to the first excited triplet state T,. Since the process is horizontal, the total energy remains the same and the molecule initially is produced in upper vibrational and rotational levels of T, from which it is deactivated as shown by the vertical wavy line. Similarly, (ISC)2 shows the intersystem crossing from T, to upper vibrational and rotational states of the ground state S(l, from which vibrational deactivation to v" = 0 then occurs. 

Electron transfer takes place prior to any significant redistribution of vibrational energy in the dye s excited state. This mechanism means that injection takes place from the LCT state which is populated by the initial absorption process and that intersystem crossing to the lower-lying 3MLCT state normally observed in solution is not competitive for the adsorbed species (see Figure 6.12 above). 

---