IR, UV-visible, NMR spectra and aromaticity of the covalent organic tetraoxa[8]circulene frameworks

Abstract

In this paper, we explore the aromaticity, electronic, vibrational and NMR spectra of the novel tetraoxa[8]circulene-based covalent organic frameworks which were first synthesized in 2022 [Angew. Chem. Int. Ed. 61 (2022) e202116527] being predicted by quantum chemical modeling in 2014 [Phys. Chem. Chem. Phys. 16 (2014) 6555–6559]. Both studied 2x2 and 3x3 tetraoxa[8]circulene (TOC) oligomers represent polyaromatic systems consisting of 108π (2x2) and 168π (3x3) electrons in the outer perimeter with a global diatropic (“aromatic”) current; they includes also two types of internal rings: the cyclooctatetraene cores with 8π electrons and larger C16(CH4)O4 cavities with 28π electrons. Both internal rings provide local paratropic (“antiaromatic”) ring currents.The elongated π-conjugated system of the 2x2 and 3x3 TOC oligomers is responsible for the unique nature of photoinduced charge transfer and vibrational spectra, which obey strict selection rules within the D4h symmetry point group. The nature of photoinduced charge transfer was interpreted using visualization methods including charge density difference and transition density matrix. It was found that strong optical absorption around 500 nm is for the doubly degenerated S3(4) (1Eu) states, in which one pair of oppositely located anthracene fragments has an alternating distribution of holes and electrons, while in the other pair of anthracene fragments, the holes and electrons are mainly localized on the central benzene fragments and the ethylene bridges of adjacent furan rings, respectively. For both 2x2 and 3x3 TOC oligomers, the optical absorption strength for S1 and S2 states is vanished. The density functional calculations of 2x2 and 3x3 TOC models have also allowed us to make the assignment of all the observed lines in the IR and Raman spectra of experimental pTOC samples. We believe that the TOC oligomers represent a new-generation of macrocyclic host molecules in heterocirculene and supramolecular chemistry.