I have seen a number of publications state that organic radicals often exhibit quenched photo-luminescence, yet offer no explanation as to why. Can someone offer a physical explanation as to why this would be the case? This question is particularly puzzling to me as you can clearly see polaronic or excitonic absorption bands in an absorption spectrum corresponding to the excitation of the radical anion. Why then does it not luminesce when excited at these wavelengths?
Absorption does not necessarily guarantee photo luminescence since plasmon induced oscillations could also increase in intensity at specific resonance frequencies.This reduces the photo luminescence intensity, which is termed quenching.
All recombination mechanisms can be defined as radiative or non-radiative.
Photoluminescence measurements reveal radiative recombination processes. However, if they're intentionally or unintentionally added impurities or intrinsic defects they can form deep localized energy levels in the bandgap (gap of forbidden energies between conduction and valance bands or LUMO and HOMO). These deep traps act as efficient recombination centers according to Shockley-Read-Hall (SRH) statistics. As opposite to exciton or band-to-band bimolecular recombination, trap assisted recombination is non-radiative (usually becomes radiative only at very low temperatures in the form of additional PL peak at lower photon energy than bandgap). Since one additional non-radiative recombination path is added the photoluminescence signal is quenched because the net radiative recombination rate is reduced.
This paper is a nice example.
Article Creation and annealing of metastable defect states in CH3NH3...
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Condensed Matter Physics
- Solid State Physics