 |
| Figure 1. Structure of the 'Butterfly' Compound. |
I have always been interested in the field of Organic
Electronics, and it is indeed a research field I would love to be involved in!
To me, organic compounds are not merely about biological stuff. It is often the
optical and electronic – optoelectronic – properties of the organic compounds
that make them useful contenders as novel functional and electronic materials.
Too often, it is through an understanding of the chemistry that we can
fine-tune the chemical properties of the target compound in question, and lead
to better functional materials for the future. I have recently read a paper
about an organic material, and it is exactly bearing the above philosophy in
mind. [1]
The compound has a potential as novel organic light-emitting
diode (OLED) material (Figure 1). The compound can display the property of thermally
activated delayed fluorescence (TADF), and for a good candidate in this field,
the compound should (a) have a small
energy gap between the lowest singlet and triplet states; (b) a high intrinsic
photoluminescence quantum yield (through orbital overlap) and (c) a relatively
short delayed lifetime. From the data, the compound seems to do pretty great
for all 3 requirements. Thus, this compound has a nice potential to serve as a
green fluorescent OLED.
The shape and design of the green-colored compound are
interesting. It resembles te shape of a butterfly, because the overall
structure is a donor- π system-acceptor- π system-donor type (D-π-A-π-D). The
donor group is phenoxazine (PXZ), while the acceptor group is a pyrimidine
derivative.
 |
| Figure 2. Buchwald-Hartwig Coupling approach towards the family of target compounds. |
The synthesis of the compound was through a double Pd-catalyzed
Buchwald-Hartwig amination of the phenoxazine nucelophile to a di-bromophenyl
pyrimidine derivative (Figure 2). An X-ray crystal structure could also be obtained, and
the molecular structure really resembled a butterfly! It is notable that the
twisting angle between the PXZ and the phenyl ring is large, very much due to
the steric hindrance provided from the PXZ component. This design is important
because by the incorporation of a larger steric demand, it can lead to a
spatial separation of the frontier molecular orbitals, lowering the
singlet-triplet energy gap as a result (Criterion a).
Cyclic voltammograms have been obtained to probe the HOMO / LUMO
energy levels of the butterfly compound. The absorption spectra of the compound
provides nice insights into the electronic characters of the compound. An
intense band with charge-transfer character signifies the transition from the
electron-donating PXZ group to the electron-withdrawing pyrimidine unit. It is
also noted that the absorption profiles overlap with that of a commonly used
host material, CBP, meaning that in a doping system, effective energy transfer
from CBP to this butterfly compound will be possible.
At the pyrimidine unit, there is a substituent at the
2-position, and this substituent can impact the property of the green-colored
compound. It is found that the delayed fluorescence (DF) can be reduced by a
bulkier 2-substituted group. This can suppress the non-radioactive decay and
also other undesirable quenching processes due to triplet excitons, thus improving
the performance at high luminance.
After playing with the chemical aspects, the researchers
then tested the novel compounds’ potential as TADFs in OLED devices. All the
compounds lead to green light emission in the systems, possess good thermal
stability, demonstrate very effective up-conversion (T1 > S1), achieve high
intrinsic photoluminescence quantum yield, and perform well at high luminance
conditions. The results are promising for the compounds involved.
Impressive work!
Reference:
1. Optimizing Optoelectronic Properties of Pyrimidine-Based
TADF Emitters by Changing the Substituent for Organic
Light-Emitting Diodes with External Quantum Efficiency Close to 25% and
Slow Efficiency Roll-Off
Kailong Wu, Tao Zhang, Lisi Zhan, Cheng Zhong, Shaolong Gong, Nan Jiang, Zheng-Hong Lu, and Chuluo Yang, Chem. Eur. J. 2016, 22, 1 – 8.
DOI: 10.1002/chem.201601686
