Optoelectronic P DADdy

Figure 1. Synthesis of building block (1).

Speaking of Organic Electronics, I would like to share with you another recent article regarding some interesting organo-phosphorus conjugated compounds. [1]

In the paper, the researchers have developed synthetic routes towards novel phosphorus-containing π-conjugated compounds, by using an interesting phosphorus reagent known as (Tipp)P(SiMe₃)₂ (Figure 1). It was great to observe that the phosphorus compound (1) was not as air-sensitive as one might usually expect. Due to the presence of the two electron-withdrawing carbonyl groups, the yellow-colored compound (1) was stable enough to survive column chromatography.

Figure 2. Building up the conjugated pathway, as in (2).

On the other hand, the DAD (Donor-Acceptor-Donor) type compound (2) was synthesized by a Stille coupling at elevated temperature (Figure 2). It should be noted that the (mono or bis) tributylstannyl-thiophene is often a good candidate in these coupling reactions, especially when the thiophene units are need for organic electronics or functional materials. It is also interesting to note that the presence of the bulky TIPP group is essential for a successful coupling in this case.

X-ray crystallographic structures have been obtained for some of the compounds, and UV/vis spectra and cyclic voltammograms have also been acquired. When these compounds were compared with their corresponding nitrogen analogues, the LUMO energies of the organo-phosphorus compounds were found to be lower. The observations implied σ*-π* electronic couplings in these compounds. These phosphorus compounds were highly stable from TGA studies (likely due to the 2 carbonyl EWGs), which made them potential candidates for uses as organic materials.

Figure 3. Synthesis of the heavily conjugated organic molecule (3).

For the next stage, the compound (2) was used to synthesize the conjugated polymer (3) though sequential Stille coupling reactions (Figure 3). The researchers found that the optical bandgap of (3) was much narrower than its nitrogen counterpart. So, this polymer should be a great contender as a low bandgap conjugated polymer.

I like this paper because the novel organo-phosphorus compound looks interesting and useful to me. In the past, I have read about phospholes, and also some of the zirconium-based methods for their syntheses. A novel strategy has been used in this paper, and this approach is something I am not aware of. As a researcher working a lot with phosphines and phosphorus-based compounds, I find this article a rewarding one to read!

by Ed Law

27/9/2016

Reference:

1. Thieno[3,4-c]phosphole-4,6-dione: A Versatile Building Block for Phosphorus-Containing Functional π-Conjugated Systems 

Youhei Takeda, Kota Hatanaka, Takuya Nishida, and Satoshi Minakata

Chem. Eur. J., 2016

DOI: 10.1002/chem.201602392

Grassy Butterfly Dreams

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!

by Ed Law

26/9/2016

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