AROMATICITY AND PHOTOPHYSICAL PROPERTIES OF TETRASILA- AND TETRAGERMA[8]CIRCULENES AS NEW REPRESENTATIVES OF HETERO[8]CIRCULENES FAMILY

Baryshnikov G.V., PhD, docent

Division of Theoretical Chemistry and Biology, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, 10691, Stockholm, Sweden

AROMATICITY AND PHOTOPHYSICAL PROPERTIES OF TETRASILA- AND TETRAGERMA[8]CIRCULENES AS NEW REPRESENTATIVES OF HETERO[8]CIRCULENES FAMILY

Among the numerous representatives of hetero[8]circulenes the heteroannelated derivatives of tetraphenylene constitute a main and most studied part. Some of them (azaoxa[8]circulenes and tetraoxa[8]circulenes) demonstrates the intensive blue fluorescence that is useful for the fabrication of blue fluorescent and white exciplex-based extremely stable organic light emitting diodes (OLEDs) [1, 2]. At the same time, tetraoxa[8]circulenes and other heterocirculenes like tetraaza[8]circulenes and tetrathia[8]circulenes demonstrates an ability to form the continuous infinite one- and two-dimensional ribbons and sheets. It was shown that these graphene-like materials demonstrate semiconductive properties and high dynamic stability that makes these materials promising organic semiconductors for organic electronics applications [3, 4]. Very recently, Miyake et al. have synthesized novel representatives of heterocirculenes family called tetrasilatetrathia-[8]circulene and tetragermatetrathia[8]circulene by the palladium-catalyzed silylation/germylation with subsequent rhodium-catalyzed intramolecular dehydrogenative cyclization [5, 6]. These novel synthesized compounds are still a challenge for the computational chemistry in order to explain their aromaticity and photophysical properties.
According to gauge-including magnetically induced currents (GIMIC) calculations it was found that both tetrasilatetrathia-[8]circulene and tetragermatetrathia[8]circulene represent nonaromatic compounds. Their molecules demonstrate a specific bifacial aromaticity unusual for the most of hetero[8]circulenes [4, 7]: inner eight-membered core is characterized by the presence of paratropic (“antiaromatic”) ring currents, whereas the outer macrocycle possesses the magnetically-induced diatropic (“aromatic”) ring current. But the overall magnetically-induced ring current for both studied circulenes is close to zero because of the strong local diatropic currents in every thiophene ring compensate the paratropic component of the inner cyclooctatetraene core (Fig. 1).


Fig. 1. The current strengths and current pathways for the tetrasilatetrathia[8]circulene and tetragermatetrathia[8]circulene.

The electronic absorption and emission spectra for both studied heterocirculenes demonstrate a clear visible vibronic progression. The 0-0 band is a most active in the absorption spectra, while in fluorescence spectra 0-1 band formed by the several normal vibrations is more intensive comparing with the 0-0 band in excellent agreement with experiment [5, 6]. Analysis of photophysical constants for both compound including spin-orbit coupling effects demonstrates: 1) clear manifestation of internal heavy atom effect on the inter-system crossing efficiency; 2) one to two orders domination of non-radiative rates over the fluorescence rates; 3) S1~S0 internal conversion is extremely slow and cannot compete with the fluorescence and inter-system crossing pathways which means that S1~T1 quenching is a main deactivation channel of S1 excited state (Fig. 2). These results provide a new insights into the electronic structure and photophysics of tetrasilatetrathia[8]circulene and tetragermatetrathia[8]circulene as novel standalone representatives of hetero[8]circulenes.


Fig. 2. Photophysical properties of the tetrasilatetrathia[8]circulene and tetragermatetrathia[8]circulene.
REFERENSES

1. C. B. Nielsen, T. Brock-Nannestad, T. K. Reenberg, P. Hammershøj, J. B. Christensen, J. W. Stouwdam, M. Pittelkow, Chem. Eur. J., 2010, 16, 13030.
2. K. B. Ivaniuk, G. V. Baryshnikov, P. Y. Stakhira, S. K. Pedersen, M. Pittelkow, A. Lazauskas, D. Volyniuk, J. V. Grazulevicius, B. F. Minaev, H. Ågren, J. Mater. Chem. C, 2017, 5, 4123.
3. N. N. Karaush, G. V. Baryshnikov, V. A. Minaevа, H. Ågren, B. F. Minaev, Mol. Phys., 2017, 115, 2218.
4. Electronic structure and spectral properties of heterocirculenes: monograph / B. F. Minaev, N. M. Каraush-Karmazin, G. V. Baryshnikov, V. A. Міnaeva. – Cherkasy: Published from Chabanenko Yu. А., 2018, 300 p.
5. Y. Serizawa, S. Akahori, S. Kato, H. Sakai, T. Hasobe, Y. Miyake, H. Shinokubo, Chem. Eur. J., 2017, 23, 6948.
6. S. Akahori, H. Sakai, T. Hasobe, H. Shinokubo, Y. Miyake, Org. Lett., 2018, 20, 304.
7. G. V. Baryshnikov, B. F. Minaev, V. A. Minaeva, Russ. Chem. Rev., 2015, 84, 455.