  In a recent paper Segal et al. [1] attempted to explain the dynamics of spin
1/2 photoluminescence detected magnetic resonance (PLDMR) in films of a
pi-conjugated polymer, namely a soluble de-rivative of poly(phenylene-vinylene)
[MEH-PPV] using a model (dubbed TPQ), in which the PLDMR is due to spin
dependent triplet-polaron interactions that reduce the polarons density and
consequent quenching of singlet excitons. We studied the full PLDMR and
photoinduced ab-sorption (PA) dynamics of MEH-PPV films as a function of
microwave power at various tempera-tures. We show, firstly, that the TPQ model
is incompatible with the full frequency dependent spin 1/2 PLDMR response;
secondly, it is not in agreement with the spin-1 PLDMR temperature dependence;
thirdly, it predicts a much shorter triplet exciton lifetime than that obtained
experimentally; and fourthly, that is in contradiction with the temperature
dependencies of spin 1/2 PLDMR and triplet exciton PA. In contrast, an
alternative model, namely the spin dependent re-combination of polarons, is
capable of explaining the whole body of experimental results, and in particular
the PLDMR dynamics.
