Comment on Frequency response and origin of the spin-1/2 photolumines-cence-detected magnetic resonance in a pi-conjugated polymer

TL;DR

This paper critically evaluates the TPQ model for spin-1/2 PLDMR in pi-conjugated polymers and demonstrates that an alternative spin-dependent polaron recombination model better explains the experimental data.

Contribution

The study provides a comprehensive comparison showing the inadequacy of the TPQ model and supports the spin-dependent polaron recombination as the correct mechanism for PLDMR.

Findings

TPQ model incompatible with frequency-dependent PLDMR response

TPQ model inconsistent with temperature dependence of spin-1 PLDMR

Spin-dependent polaron recombination explains experimental results effectively

Abstract

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.