When a phosphorescence emission occurs, Singlet spin state is converted to triplet spin state and when it comes to ground state after phosphorescence emission of energy it again in singlet state-----What is the driving force for such spin inversion ?
As far as I know, the most prominent contribution will stem from spin-orbit coupling. However, the details strongly depend on your actual sample system...
In other words, a change in orbital angular momentum (involving different "types" of orbitals) is needed to compensate for the change in spin angular momentum. For an organic molecule like benzophenone, there is n (nonbonding) to pi* (antibonding). As for transition metal complexes with prominent triplet state formation and phosphorescence, there is MLCT (metal-to-ligand charge transfer).
The reason is the same as the fluorescence, but the mechanism is slightly different due to the spin forbidden process. The ground and the electronically excited states are stationary states, the electron density of a molecule is time-independent in these states (because they are the solution of the time independent Schrödinger equation). They should not emit(lost) light(energy), but they do. The guilty is the electromagnetic field that surrounds the molecules. The spontaneous emission (fluorescence and phosphorescence) arises from the interaction of molecules with the vacuum fluctuation of the quantized electromagnetic field. Triplet and singlet states are only assumptions. The real atomic/molecular states are spin-mixed states (real state = c1*triplet + c2*singlet). The degree of mixing (spin-orbit coupling) is more important in the case of heavy atoms, there are no pure triplet states. The transition between singlet and triplet states are feasible due to the spin-orbit coupling. So..there are two driving forces:
1. coupling of radiation field to the matter (interaction of vacuum fluctuation of the electromagnetic field with molecular states)
2. Spin-orbit coupling dissolves the forbidden nature of singlet triplet transitions, because it is mixing the different spin sates. The real excited "triplet" state is not a pure triplet state, it has some singlet character --> transition to singlet groundstate is no more forbidden.
Just a minor addition. Semantically one may say there is technically no 'spin-flip' as spin is no longer a good quantum number. One has to look at the spin-orbit coupled states, which you may 'map' back to states with singlet and triplet character in a perturbative spin-orbit coupling approach.