| 1. |
- Escayola, Silvia, et al.
(författare)
-
Guidelines for Tuning the Excited State Huckel-Baird Hybrid Aromatic Character of Pro-Aromatic Quinoidal Compounds**
- 2021
-
Ingår i: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 60:18, s. 10255-10265
-
Tidskriftsartikel (refereegranskat)abstract
- Pro-aromatic molecules have higher-energy diradicaloid states that are significantly influenced by resonance structures in which conjugated rings take on Huckel-aromatic character. Recently, it has been argued that there are also pro-aromatic molecules that adopt central units with 4n pi-electron Baird-aromatic character in the T-1 state, although detailed analysis suggests that these compounds are better labelled as T-1 Huckel-Baird hybrid molecules where Huckel-aromaticity dominates. Herein, we consider a series of symmetrically substituted conjugated rings with potential Baird aromaticity in the lowest excited triplet and singlet states. Our computational results allow us to establish general guidelines for the rational design of molecules with excited state Huckel/Baird aromaticity in pro-aromatic quinoidal compounds. We found two main strategies to promote high Baird aromatic character: 1) anionic and small conjugated rings with electron donating groups as substituents and small exocyclic groups with electron withdrawing substituents, or 2) electron deficient conjugated rings with exocyclic electron-donor substitution.
|
|
| 2. |
- Gillett, Alexander J., et al.
(författare)
-
Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors
- 2021
-
Ingår i: Nature Communications. - : Nature Portfolio. - 2041-1723. ; 12:1
-
Tidskriftsartikel (refereegranskat)abstract
- Engineering a low singlet-triplet energy gap (Delta E-ST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 x 10(5) cm(-1)) and a relatively large Delta E-ST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 mu s), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a similar to 24 ns timescale and have an average electron-hole separation of >= 1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low Delta E-ST in organic DF emitters.
|
|
