Triangulene sample 2

To circumvent the issue of oxidation in transit, the reduction of 4,8-dioxo-4H,8H-

dibenzo[cd,mn]pyrene 340 was completed at IBM in Switzerland and placed in the

AFM/STM vacuum chamber immediately.

Figure 84: STM images on a Cu(111) surface where the dihydride group of 12a can be clearly seen as a very bright spot on the images.321

Dihydro-triangulene12awas dehydrogenated by pulsing a current of specific energy

over the bright dihydride region with the STM tip. After the hydrogen is cleaved, the

same is completed on the second dihydride (Figure 84). Thus, triangulene 10 is

formed (Figure 85). In addition to the observed dihydro-triangulene isomers, some

oxidised products were also visualised (Figure 85). This illustrates how unstable the

precursor dihydride12ais given that it was prepared with little exposure to air.

i )

Figure 85: AFM images of molecules observed on NaCl (100) surface; (a-b) dihydro-triangulene isomers. (c-e) oxidised species including 4,8-dioxo-4H,8H-dibenzo[cd,mn]pyrene (f) with its STM image (e). Images on Cu (111) surface of triangulene (g-h) and the corresponding STM (i).

The STM image relates to the AFM image in the same way, where the bright area

contains more electron density then the darker areas. For example, the STM image of

triangulene is triangular, though the dark patches of the ketones appear to have

indented either side of the image for 4,8-dioxo-4H,8H-dibenzo[cd,mn]pyrene 340

(Figure 85). Triangulene was produced on a xenon surface at 5 K, as this crystal

formation is inert and will create the least perturbation of the structure, thus forming

a realistic image of triangulene.

Furthermore, STS and STM orbital imaging were performed on triangulene using the

same method as compound310(see Chapter 3, Figure 74). Using NIR and PIR, both

orbital images were identical, thus proving the triplet state of triangulene (Figure 86).

Figure 86: a) STM image of the PIR (-1.4 V), b) STM image of the NIR (1.85 V) of triangulene 12a. DFT calculation of c) SOMO 1, d) SOMO 2 and e) SOMO 1 & 2 combined. f) the simulated

image of (e) under the STM tip, where the actual images can be seen in (a) & (b). All data and calculations provided by IBM Zürich.319, 412

4.4

Summary

After the initial setback in the synthetic route, the formation of the very unstable

triplet-state of triangulene, by the combination of synthesis and STM/AFM was

successfully completed. It has been over 74 years since Clar first mentioned

triangulene, including many remarks stating that it cannot exist due to its non-Kekulé

nature. Nevertheless, STM/AFM is a powerful tool in producing and characterising

5 Chapter 5: Conclusions and future work

Successful synthesis of the 6H-benzo[cd]pyrene radical and the imaging of the

olympicene molecule by the use of AFM/STM was accomplished. Optimisation of

the synthetic route led to the discovery that 6H-benzo[cd]pyrene is produced first but

oxidises in the reaction conditions to 6-oxo-6H-benzo[cd]pyrene. From this,

different arene starting materials were tested in glycidol and sulfuric acid

(naphthalene, anthracene and perylene) and the discovery of further reactions were

observed. Furthermore, it is suggested thatperi-condensation reactions occur on the

starting materials until the products formed precipitate out of the solution due to their

insolubility. Carbon labelled studies have supported this view. The mechanism has

also been proposed for ring addition, as a sequential two-stage polar and radical

reaction onto each starting material with agreeable DFT calculations. AFM/STM

proved to be the best technique to identify most of the products formed in the

reactions, including the larger PAHs which were created due to multiple ring

additions (black solid) where previous literature did not analyse this.

Another target was also accomplished in that of triangulene, where an optimised

method of an older synthesis was utilised. The unstable diradical was isolated and

imaged for the first time since its proposal in 1941 by the use of AFM/STM. It has

become apparent that PAHs are difficult compounds to study, due to the simple

structure they possess and therefore, many have found them difficult to characterise.

Here, AFM/STM has served as an invaluable technique to fully characterise PAHs

and also for the production of reactive non-Kekulé structures. In the future, this

technique is sure to become increasingly popular to the wider scientific field, for

but also challenging to identify and difficult to prepare by common techniques in

order to alleviate any doubt about their structure.

Future work would include more analysis and calculations on the mechanism of peri-

condensation ring addition with the suggested polar and radical pathways.

The production of structures with different side chains to be observed under the

STM/AFM to help distinguish side-groups of compounds. Furthermore, imaging

different molecules with heteroatoms to gain a better understanding of how different

elements look under the microscopy techniques, such as poly-thiazoles.

Synthesise a library of difficult but interesting PAHs such as other triplet state

compounds of interest and possibly those in the triangulene series.

Finally, to consider how graphene could be synthesised via this peri-condensation

6 Experimental