Friday, July 2, 2010

Deconstructing excited state dynamics in conjugated polymers

Joseph Shinar (Ames Lab, Iowa State) gave a nice talk today about using optically detected magnetic resonance (ODMR) to elucidate the dynamics of optically excited states in conjugated polymers.

First, two "human interest" asides.
1. The results of this research led to share prices of some companies going up and down.
2. Shinar said he build his first ODMR spectrometer using an ESR spectrometer he got from a dumpster outside the chemistry building at the Technion in Israel.

Basically what one does in this experiment is to monitor an optical property (such as photoluminescence) at the same time that one applies a microwave field and magnetic field. When the microwave field frequency is on resonance with that required to induce transitions between different Zeeman levels on sees changes in the optical property.
Seeing any detectable change may be surprising because one does not necessarily expect optical properties to be so spin dependent. It turns out understanding why one gets a signal at all and the physical mechanism took almost 20 years.

There are many possible competing processes following photoexcitation of a singlet (exciton) state.

S1 --> P+ + P-
or T1 + T1
where P+ is a positive polaron (this is just a cation with a significant bond relaxation
in the neighbourhood of the charge)
and T1 denotes a triplet state.
The polarons have unpaired spins and will produce an ESR signal. But how does flipping these spins enhance the photoluminescence?

Some of the key results to understanding what goes on are in this PRL.
Shinar considered 2 scenarios [note the method of multiple hypotheses].
The one that is consistent with experiment is

Enhanced spin-dependent annihilation of Triplet excitons by polarons
T1 + P+ --> P+ (note this conserves charge and spin)
TPQ [triplet polaron quenching] model
(This is discussed in the classic book by Pope and Swenberg)

Shinar claims TPQ is one of the most important interactions in organic photonic materials and devices.

The picture that emerges of the photoexcitation dynamics is that in the "steady state" the number of triplet excitons and the number of polarons is about 10,000 times larger than the number of singlet excitons.
Eventually the polaron pairs recombine into a singlet exciton which decays radiatively.

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