CONFORMATIONAL ISOMERIZATION OF TETRABENZO[8]CIRCULENE DERIVATIVES FOR THE OPTICAL TUNING PURPOSES

CONFORMATIONAL ISOMERIZATION OF TETRABENZO[8]CIRCULENE DERIVATIVES FOR THE OPTICAL TUNING PURPOSES

In the present work we have analyzed the electronic structure and UV–Vis spectra for a series of functionalized tetrabenzo[8]circulenes [1-3] (Figure 1). We have found that these compounds can exist in the forms of two conformational isomers due the twisting of a circulenoid “saddle” through a pseudorotation pathway. Despite the fact that these isomers are energetically closely similar, they are characterized by principally different electronic structures: the global minimum isomer (TB8C-GM, Figure 1) is less electronically stable (antiaromatic) than the local minimum TB8C-LM nonaromatic molecule [2,3].



Figure 1. The structure of TB8C derivatives (a) and conformations of local (b) and global (c) minima for the simplest TB8C (R=H) molecule.

The second difference between these molecules is that the global minimum tetrabenzo[8]circulene derivatives possess an absorption at longer wavelengths (blue-green region) comparing with the local minimum structure active in the near-UV-visible region [3]. Accounting for the experimental absorption spectra and measured ionization potentials we can conclude that in solution media all the tetrabenzo[8]circulene derivatives exist in the form of global minimum isomers. In order to utilize tetrabenzo[8]circulene derivatives in organic electronics they should be functionalized by additional π-extension or inclusion of π-conjugated substituents into the outer perimeter in order to enhance the fluorescence intensity. The comparison with experimental data indicates that current TDDFT based methods are capable of predicting subtle geometrically induced spectral features, such as those due to changes in isomerization and aromaticity that could provide a useful concept for further tuning the performance of all-organic, circulene based, OLEDs. We have successfully applied the concept of conformational-dependent fluorescence for the design of novel emissive tetrabenzo[8]circulenes substituted by commonly used electron withdrawing groups.



Figure 2. Spectroscopic characterization and MOs representation for the S0-S1 transition of TB8C (MN = malononitrile, CAA = cyanoacrylic acid).

These new species demonstrate the fluorescence switching on/off upon conformational isomerization that could be very useful for OLED, bioimaging and sensor applications.

REFERENCES
1. Miller R.W. General Method for the Synthesis of Functionalized Tetrabenzo[8]circulenes / R. W. Miller, S. E. Averill, S. J. Van Wyck, A. C. Whalley // J. Org. Chem. – 2016. – Vol. 81. – P.12001-12005.
2. Sakamoto Y. Tetrabenzo[8]circulene: Aromatic Saddles from Negatively Curved Graphene // Sakamoto Y, Suzuki T. J Am Chem Soc 2013;135:14074-7.
3. Baryshnikov G. V. Optical tuning of tetrabenzo[8]circulene derivatives through pseudorotational conformational isomerization / G. V. Baryshnikov, R. R. Valiev, B. F. Minaev, H. Ågren // Dyes Pigm. – 2018. – Vol. 151. – P. 372–379.