Friday, May 28, 2010

Key questions about organometallic LED materials

The figure below, taken from a JACS paper, is a possible schematic for photo-physics of the excited states of Ru(bpy)3 which is a model compound for attempting to understand materials used in phosphorescent organic LEDs.

My 5 biggest questions concerning this class of materials are:

1. What is the character of the triplet emitting state?
To what extent is it a metal-to-ligand charge transfer state (MLCT) and to what extent is it ligand centred (LC)?

2. What is the non-radiative decay path from the emitting state?
What are the relevant vibrational co-ordinates?

3. What is the physical mechanism for the ultra-fast (tens of fsec) transition from the singlet to the triplet MLCT state?
Is there are conical intersection associated with this intersystem crossing?

4. Are the excited states delocalised over all the ligands or localised on single ligands?
For example, the equation below (from the same JACS) suggests that the singlet state is delocalised and the triplet is localised on a single ligand.

5. Is there a metal-centred (MC) state that is relevant to the non-radiative decay, as suggested by the above figure?
The idea of a t2g->eg state being relevant has recently been proposed, and has some support from quantum chemistry calculations, described here. Hopefully, I will blog about this later.

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