Photo-Spectro-Electrochemical Properties of Phospholes – Promising Molecules for Organic Electronics
Authors: Kolacna Lucie , Tobrman Tomáš , Liška Alan , Klíma Jiří , Ludvik Jiri 
Published: July 2022

Pentasubstituted phospholes with extended p-conjugated arms in positions 1,2 and 5 were synthesized using a new procedure1 and characterized electrochemically and by UV-vis and EPR spectra. Quantum chemical calculations of redox potentials and HOMO-LUMO energies were performed and experimental and theoretical data were successfully correlated. Tuning of redox properties can be achieved by combination of induction and resonance effects of substituents, extension / diminution of the p- delocalized system and by steric changes which affect intramolecular electron communication.
These non planar five-membered heterocycles have tri-coordinated phosphorus atom with a non-bonding electron pair. In the series of analogous heterocycles like pyrrol, thiophene and furan, phosphole exhibits lowest aromaticity and therefore it behaves rather like a diene 2,3. Thanks to specific sterical and "push-pull" properties resulting from combination of electron withdrawing and electron donating substituents, phospholes are considered as promising material for organic electronics with application e.g. in solar cells, organic light-emitting diodes (OLEDs) or as fluorescent probes.4,5

The studied derivatives are substituted asymmetrically (two para-extended aryl substituents in positions 1,2 and a different one in position 5), whereas in the positions 3 and 4 are methyl groups. The aryl in the position 1 is perfectly perpendicular to the plane of the heterocycle, therefore this substituent is completely electronically isolated. On the other hand, the two aromates in the positions 2 and 5 are partly twisted, therefore some π-overlap with the heterocycle is preserved.
In this contribution6 we focus on the first oxidation and the first reduction potentials and their difference, which we call "electrochemical gap" (as an analogy to HOMO-LUMO gap). This value is related to electron delocalization and also to the push-pull abilities of substituents and, in this way, electrochemistry can be interconnected with photochemistry and light absorption or emission.
All first reduction and oxidation potentials are reversible or quasi-reversible, therefore they are thermodynamically relevant and thus suitable for correlation with quantum chemical calculations.
Based on their structure and fundamental electrochemical, spectroscopic and computational characterization, the new compounds could be arranged into several individual homologous „series“, where influence of substituents and their position on a) reduction / oxidation potentials; b) changes and extent of electron delocalization; c) absorption / emission of light, was followed and evaluated. The understanding of the structure – properties relationship enables tuning of photo-redox qualities of the title molecules.

Acknowledgement: The authors are grateful to the grant support from GAČR No. 18-12150 S and to the institutional support RVO 61388955.

References

Polák P., Čejka J., Tobrman T., Org. Lett., 22, 2187 (2020).

Szűcs R., Bouit P.-A., Hissler M., Nyulászi L., Struct. Chem. 26, 1351 (2015).

Charrier C., Bonnard H., De Lauzon G., Mathey F., J. Am. Chem. Soc. 105, 6871 (1983).

Shameem M. A., Orthaber A., Chem. Eur. J.
22, 10718 2016).

Duffy M. P., Delaunay W., Bouit P. A., Hissler M., Chem. Soc. Rev. 45, 5296 (2016).

to be published