Friday 24 February 2017

Ultrafast Energy Transfer

There is another paper with some contributions from myself that just appeared: "Ultrafast Electronic Energy Transfer in an Orthogonal Molecular Dyad" in J. Phys. Chem. Lett. In this paper the question is discussed how it is possible to have energy transfer in a molecular dyad that occurs on the time scale of 100 fs. Clearly, no equilibrium Föster theory type approach is possible here but you need explicit nonadiabatic dynamics simulations, in this case using Newton-X.

The value of the dynamics simulations performed is not only to support the experimental measurements. It also gives new insight into the mechanism: The ultrafast energy transfer is mediated by a state with partial charge transfer character. Or in other words, the electron and hole are not transferred at the same time. As seen in the presented example trajectory: the electron goes first and pulls the hole behind itself.


Shun said...

Dear Felix,

Due to my inadequate understanding of quantum chemistry, I am not positive that my interpretation of your paper is correct. I greatly appreciate it if you could point out my errors in my understanding. I am not a theoretical/quantum chemist.

If I understand correctly, EET is based on the two-electron Hamiltonian term about resonance and exchange interaction between two excited states, each where the excitation is localized on one side of the molecule. I also believe that Forster and Dexter theory is based on the crude-adiabatic approximation of this term.

Is your paper about following the trajectory of S2 to S1 relaxation and what constitutes the transition? The way I interpreted was that you incorporated actual nuclear configuration change of the transition, which includes nonadiabatic processes at intersection of S2 and S1 potential, and that you found that the intermediate state had a CT character.

I am often confused about the general picture of EET and what level of approximations are being considered when one talks about it. Would it be right to say that any ET process consists, to a varying degree, of two-electron correlation, excitonic interaction, and/or nonadiabatic processes?

Thank you,

Felix said...

Hi, we are using surface hopping dynamics. In this case, we propagate the actual nuclear configuration similar to standard molecular dynamics simulations. The S2/S1 transition is performed according to the surface hopping method. We did find some charge-transfer character in this paper. But actually, in a subsequent study we found that the transfer also happens when we exclude CT states:

I would not say that two-electron correlation plays a role in CT. Excitonic coupling and nonadiabatic coupling do play a role, but they are actually two sides of the same coin. Excitonic coupling is also a type of potential coupling and nonadiabatic coupling is also called kinetic coupling.

The main problem here is that there are a number of different ways to talk about the same thing. And that's what makes all this look more complicated than it actually is.