*This is the second post of the Helicene series*
An unexpected discovery…that would be wonderful… But sometimes one research project could lead to another unexpectedly…. This was how we started the carbon-sulfur helicene project. So, when carrying out research projects, in addition to keeping our eyes wide open, we keep our mind open too.
Here is the story. In the mid 90s, one of the two major research projects in our lab was the synthesis and study of chiral conjugated π-systems with high configurational stability. Our focus was on derivatives of tetraphenylene.
Actually, tetraphenylene is an achiral, saddle-shape molecule derived from four phenylenes that are ortho-annelated to form an eight-membered ring in the center of the molecule. And, oh, I should mention that we gave it nickname as “horse”. You see, the symmetry of the tetraphenylene molecule may be broken by substitution or ring annelation to provide chiral π-conjugated systems with extraordinary high barriers for racemization. So, for a while tetraphenylene was a workhorse in our group… Visit this site for more information.
Anyway, we can’t keep doing the same things for a long, long, time, it’s boring, right?
So, in 1999 we started to explore conjugated π-systems of thiophenes. At that time, α-oligothiophenes were already quite popular in organic materials research field.
We first looked into various modes of annelation of thiophenes using simple computational models. In α-oligothiophenes, the thiophene rings are connected at α-positions, forming a linearly extended, conjugated π-system. Ring annelations of such oligothiophenes would lead to a planar, rigid structure of quasi-linear oligomers (thienoacenes). We noticed that there were only a few reports of β-oligothiophenes and tetrathienylenes, which are thiophene-analogues of tetraphenylenes, aha…, our horses. Those structures in our consideration were achiral β-tetrathiophenes and its annelated analogue, an approximately planar cyclic β-octathiophene or circulene. Then we moved on to consider analogous “defective” structures of circulene missing one sulfur (seven annelated thiophene rings), surprise! surprise!, we got to see a on a computer screen a beautiful structure of helical β-heptathiophene. The space-fill was breathtaking! We were even more elated when we realized that such structure could be extended to all-thiophene, or carbon-sulfur, [n]helicenes, which could be viewed as fragments of a (C2S)n helical polymer. Take a look…(click the picture to see 3D model)
It was exciting to come up with the unique conjugated π-systems of electroactive oligothiophenes helical structures. As we know, [n]helicenes possess some of the strongest intrinsic chiral properties. So, these electroactive carbon-sulfur [n]helicenes may provide both extraordinary chiral properties and optical transparency, which are desirable properties for organic optoelectronic materials and devices.
OK, that was great! We began to focus our attention on the carbon-sulfur [n]helicenes.
But could we synthesize such molecules?
The third post of the Helicene series, Carbon-Sulfur [n]Helicenes: Syntheses, Structures and Properties, will follow….
- The longest oligothienoacenes, octathienoacene (eight fused thiphene rings), was reported by the research group at Nagoya U. in 2007. Chem Euro J., 2007, 13, 548. 10.1002/chem.200601064.
- The carbon-sulfur circulene was claimed by the research group at Moscow State U. in 2006. The title of the article was “Sulflower”: A New Form of Carbon Sulfide, and it was highlighted as the Angewandte Chemie journal cover. (Angewandte Chemie International Edition, 2006, 45, 7367; doi:10.1002/anie.200602190).We would love to congratulate the authors for their work but there was a concern. First, the reported red color was a surprise; the cross-conjugated π-systems, circulene, should be colorless. Second x-ray powder diffraction was reported, not single crystal x-ray structure…. Interestingly, a few days after this article appeared, an e-mail, the subject “wilted sulflower?, arrived, so we were not alone!
My personal note (that is, come after me, not the RajcaLab):
- Wanna publish in Angewandte? Hype up with some fancy figures and catch words, ah the Angewandte’s songbook…And hey, a doll was published there, which seemed to catch Excimer off guard!