Synthesis, assembly behaviors and optoelectronic properties of novel conjugated macrocycles

Abstract

With the expeditious development of synthetic technology, plentiful π-conjugated macrocycles with complex topologies and aesthetic structures have been ingeniously constructed during the last two decades. Characteristics of large inner cavity, constrained conformation, absence of terminal groups, π-conjugated macrocycles have attracted the attention in the fields of medicinal chemistry, supramolecular chemistry, and optoelectronic devices. However, the relationships of their structures-properties are still elusive due to the limited examples. In this thesis, modulating the connecting mode and introducing heteroatoms to tune their optoelectrical properties, we rationally designed and synthesized a series of conjugated macrocycles to understand the relationships of their structures-properties and explore their applications in supramolecular assembly, electronic and optical devices as followings: In Chapter 2, we constructed three strain-less macrocycles comprised of meta-linked aromatics, including planar dimer (2-1), twisted trimer (2-2) and flexible tetramer (2-3) via one-pot Suzuki coupling reaction. The conformation of flexible macrocycle 2-3 was successfully transformed from “figure-eight” into a boat-shaped and a belt-shaped geometry as encapsulating a planar guest (7,7,8,8-tetracyanoquinodimethane, TCNQ) and a spherical guest (C60), respectively, which were scrupulously revealed by single crystal structures of individual 2-3 and complex 2-3⊃TCNQ and 2-3⊃C60. Experiments and theoretical calculations were performed to gain more insights on the possible dynamic interconversion process and the origin of conformational regulation induced by guest molecules. In Chapter 3, two BN-doped cycloarenes 3-1 and 3-2 with expanded π-electron surface were synthesized via electrophilic borylation cyclization reaction. Both experimental results and theoretical calculations declared that 3-1 and 3-2 preserve the dominant local aromatic properties, same as kekulene. Benefitting from the high hole conductivity, suitable highest energy molecular orbital (HOMO) energy level and excellent hydrophilicity of 3-2, we applied it as a hole transport layer to fabricate inverted perovskite solar cells, in which the power conversion efficiency of 3-2 reached 14.4%. In Chapter 4, aiming to enhance the spin-orbit coupling of B-N doped thermally activated delayed fluorescence (TADF) materials, originally, we intended to synthesize a fully fused zigzag nanobelt. Due to the large tension, however, we obtained two partial fused BN-doped macrocycles 4-2 and 4-3. Unexpectedly, the BN-doped macrocycle 4-2 showed a narrow TADF with a full width at half maximum (~20 nm) and a small energy difference between the singlet and triplet excited states (ΔEST: ~0.14 eV). It is speculated that the cyclic geometry could suppress the molecular vibration mode and relaxation of excited states, affording a narrow ultrapure blue emission, small Stokes shift and high photoluminescence quantum yield. During my PhD studies, other efforts are also made on synthesizing a butterfly-shaped polycyclic conjugated hydrocarbon (PCH) 5-1a with four contiguous heptagons in Chapter 5. 5-1a possessed distinct antiaromatic property and narrow HOMO/lowest energy molecular orbital (LUMO) energy gap compared with its benzenoid structural isomer 5-1R. Moreover, 5-1a exhibited anti-Kasha emission from S3, which is highly related to the well alignment of two embedded azulene units, supported by experiments and detailed theoretical analysis.