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15 Cards in this Set

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  • Back
What is the chief signature of photochemical damage in conjugated polymer films
A rapid decay of the stimulated emission and the appearance of photo induced absorption in the emission region; the photo induced absorption in the near infrared (~800 nm) is only slightly affected.
How many electronic states are responsible for emission in MEH-PPV films?
The entire emission comes from a single electronic state and the peaks represent vibronic structure from a mode that is strongly displaced upon excitation.
Where does the vibronic structure in the emission spectra come from?
Vibronic structure comes from a mode that is strongly displaced upon excitation. The observed splitting corresponds well to the energy of a C=C stretch, a motion that is expected to be strongly displaced when the benzoid-like polymer ground state is excited to the quinoid-like excited state.
Where do aggregates emit?
Roughly in the same place as the 0-2 emission band. Aggregate emission sometimes responsible for the red tail near the 0-2 emission band.
Why is the PL of a film red shifted relative to the solution from which it was cast?
Red-shifted film relative to solution is consistent with the gas-to-crystal effect. This effect results from the fact that the dipole of an MEH-PPV excited state interacts more strongly with its surroundings in the highly polarizable environment of a conjugated polymer film than in solution. The stronger polarization interaction with the environment leads to a red-shifted emission.
In terms of the vibronic structure of the emission spectra, what is a general trend that occurs upon increasing the polymer concentration of a film?
The red portion of the spectrum increases in intensity relative to the bluest portion
Why does increasing the amount of interchain interactions change the relative heights of all three peaks instead of just the reddest peak?
Two likely explanations:
1) That the emission from the aggregate has enough amplitude near the 0-1 band to change the relative heights of both of the red peaks relative to the 0-0.
2) That the emission of the intrachain eciton changes its vibronic structure in different film environments.
What would be the difference in the emission spectra between two films cast from the same solvent but one has a higher concentration of aggregates?
The film containing the higher amount of aggregates would have an emisison spectrum that is red shifted and the height of the 0-1 and 0-2 peaks would be higher in intensity relative to the 0-0 band and the emission quantum yield would be less relative to the film with fewer aggregates.
Why is film with more aggregate concentration red shifted?
A film with higher aggregate concentration will be red-shifted because aggregate sites are lower in energy than the singlet intrachain exciton and because of the increased polarizability of the environment.
Why is the quantum yield lower in films with higher amounts of aggregates?
Films with a high degree of aggregation have a lower quantum efficiency because of an increased number of weakly emissive aggregate species.
Why does a film with a larger amount of aggregates have an emission spectrum with different relative heights between the 0-0, 0-1 and 0-2 peaks relative to a film with fewer aggregates?
The relative change in height of the emission peaks can be assigned to changes in vibronic coupling: both the Frank-Condon emission envelope and the number of sequence and combination bands underlying the main vibronic structure are likely to change as the interaction between a chromophore and its neighbor is altered.
Why do films with more interchain species have a different emission spectrum?
It is believed that the changes in the emission spectrum of MEH-PPV films with increasing interchain interactions are predominantly the result of changes in the spectrum of the exciton, with increasing amounts of emissive interchain species playing an important but secondary role.
What does annealing do to the polymer chains of the film?
Annealing allows the MEH-PPV chains to slowly untangle and increase their degree of pi-electron contact, producing films that are nearly flat.
What is one effect of cumulative photooxidative damage?
Photooxidative damage creates an absorbing species in the region where stimulated emission occurs.
What does the degree of exciton-exciton annihilation depend on?
The degree of E-EA depends on the morphology of the film. The more aggregates in the film the higher the probability is for E-EA. The degree of E-EA depends on the overlap of two neighboring excitonic wave functions, which in turn depends on the relative position of the adjacent conjugated polymer chromophore.