Generally in D-A (donor-acceptor) based TADF system, it has been shown the CT emission are dominant most of the time than their individual 1LE donor or 1LE acceptor emission. This is happen when the D-A system has strong charge transfer character. What does lead to produce strong charge transfer character in a D-A system that can overtaken localized singlet emission from the system?
In polar solvents, charge transfer state will dominate the LE state as polar environment stabilizes more polar CT state as compared to LE state.
It's obvious that the polar media will help the CT state to get stabilize but my point is that, at what circumstances maximum CT state stabilization will happen in solvent with increasing polarity?
Suppose there are two different type of D-A molecules One of them (say M1) with near orthogonal D-A dihedral angle having localize HOMO at D and LUMO at A units respectively) shows almost 260meV of CT energy shifting by increasing solvent polarity from 2.2 to 9.9. And the other (say M2) with D-A dihedral orientation less than 90 degree, shows only 58meV CT energy shift for change for the change of same polarity. Then basically what leads them to different amount of CT energy relaxation in this case?
You cannot compare two different D-A systems (even if they have same dihedral angle) as they have different strength. So they will be stabilized to different extent. Basically it is the separation of HOMO and LUMO as well as the strength of D and A. More the separation and strength of DA, more is the stabilization in polar solvents. CT state may or may not need rotation between D and A (dihedral angle). In some cases, the rotation takes place to achieve CT state (TICT) and no rotation in others. Extent of change in dihedral angle will also vary for different molecules depending upon the electronic as well as steric factors. The separation of HOMO-LUMO with the strength of DA induce the CT state and the polarity of solvent will decide its stabilization.
Dear Amrita,
As far as i know there are two factors affecting the transfer of electrons from the donor molecules to the acceptor molecules:
- The acceptor molecules must contact a donor molecule before the exciton spends its diffusion length. The best situation is that one to one correspondence of the donor and acceptor molecules. That is they hug each other.
- The other condition which is raised by Gopal which is the lumo level of the acceptor molecule must lie under that of the donor with sufficient energy to torn out the electron from its accompanying hole in the exciton and capture it. At the same time the acceptor homo level must also lie under that of the donor molecule to repel the hole away from the acceptor molecules.
At the end effect the electron will be held away from the attraction of the holes decreasing the probability of recombination. If any external electic field to collect them to the electrodes they will be ultimately transferred to electrodes before they recombine.
Best wishes
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