Synthesis and macrocyclization-induced emission enhancement of benzothiadiazole-based macrocycle

Organic luminescent materials with high quantum efficiencies are important for applications in sensors, bioimaging, laser displays, light-emitting diodes, and anti-counterfeiting. A key challenge is aggregation-caused quenching (ACQ) in the solid state due to formation of excimers and exciplexes, which reduces emission efficiency. Existing strategies to overcome ACQ include aggregation-induced emission (AIE), crystallization-induced emission (CIE), and supramolecular assembly-induced emission enhancement, often relying on restriction of intramolecular motions and control of twisted conformations. The authors propose a new strategy—macrocyclization-induced emission enhancement (MIEE)—where luminophores are macrocyclized via sp3 methylene linkers to spatially separate chromophores and restrict intramolecular motions, thereby enhancing emission. They focus on a benzothiadiazole-based macrocycle (BT-LC) synthesized from a donor-acceptor monomer (BT-M) and investigate whether macrocyclization improves solid-state fluorescence and device performance, while elucidating the photophysical mechanism.

The study situates itself within efforts to mitigate ACQ in organic luminophores. It references AIE, CIE, and supramolecular assembly approaches that enhance emission by restricting intramolecular motions (RIM) and controlling conformation. Prior work has also shown that spatial separation via host–guest systems and structural engineering can improve emission in both solution and solid state. Benzothiadiazole (BT) is highlighted as a widely used acceptor building block in luminescent materials and donor–acceptor (D–A) architectures are known to promote intramolecular charge transfer (ICT), influencing photophysics and solvatochromism. The authors leverage these insights to design BT-M and examine macrocyclization as a generalizable route to emission enhancement.

Synthesis: The BT-based monomer 4,7-bis(2,4-dimethoxyphenyl)-2,1,3-benzothiadiazole (BT-M) was prepared via Suzuki–Miyaura coupling of 4,7-dibromo-2,1,3-benzothiadiazole with 2,4-dimethoxybenzeneboronic acid. Macrocycle BT-LC was synthesized by one-step condensation of BT-M with paraformaldehyde under Lewis acid catalysis (BF3·Et2O), affording BT-LC in 52% yield. No higher cyclic oligomers (e.g., tetramer, pentamer) were observed. Structures were confirmed by 1H/13C NMR, HRMS, and single-crystal X-ray diffraction. Characterization and photophysics: UV–vis absorption spectra were recorded; photoluminescence (PL) spectra and quantum yields (ΦPL) were measured, including in solid state and solution. Time-resolved emission decays were collected to obtain fluorescence lifetimes. Solvent-dependent absorption/emission studies probed ICT effects; dual-state emission behavior was examined. Thermal stability by TGA was measured. Computations: Ground-state optimizations used PBE0/6-31G**; excited-state calculations used TDA-TDDFT at TDA-PBE0/6-31G**. Harmonic frequency analyses confirmed stationary points. Minimum energy crossing points (MECPS1/S0) were located at TDA-PBE0/PBE0/6-31G** using the Newton–Lagrange method (LookForMECP program). Molecular structure depictions were prepared by CYLView. Device evaluation: OLED-related measurements included current density–voltage–luminance characteristics, electroluminescent spectra, and CIEy coordinates using a PR655 spectrometer and Keithley 2400 source meter. Instrumentation for spectroscopy included Shimadzu UV-2501PC and Hamamatsu C9920-02 for quantum efficiency. NMR used Bruker Avance III (400/500/600 MHz). Single-crystal XRD used Bruker APEX II CCD and Bruker D8 Venture with Mo Kα radiation.

• Successful macrocyclization of BT-M afforded BT-LC (52% yield) without formation of larger cyclic oligomers.
• Photophysics: Relative to BT-M, BT-LC shows a red-shifted emission (λem = 562 nm vs 491 nm) and markedly enhanced solid-state fluorescence quantum yield (ΦPL = 99% for BT-LC vs 65% for BT-M). Fluorescence lifetimes in the solid state increased (11.25 ns for BT-LC vs 8.45 ns for BT-M).
• Solution behavior: BT-LC exhibits high ΦPL (83–89%) in solution and displays dual-state emission (DSE). Absorptions (λabs ~401–410 nm) show minimal solvent dependence, while emissions show bathochromic shifts with increasing solvent polarity, consistent with ICT in the D–A system.
• Mechanism: Calculations indicate BT-M can undergo efficient non-radiative relaxation through MECPS1/S0, reducing fluorescence efficiency. Macrocyclization imposes structural rigidity that suppresses this non-radiative pathway and promotes radiative decay, yielding intense emission. Single-crystal analysis supports increased rigidity and specific intra/intermolecular interactions in BT-LC.
• Devices: OLEDs incorporating BT-LC exhibit higher maximum brightness (Bmax) and maximum external quantum efficiency (EQEmax) than devices with BT-M.
• Generality: The MIEE strategy is applicable beyond BT-LC; several macrocycles based on different luminophores also show emission enhancement.

The work demonstrates that macrocyclization via methylene linkers effectively enhances emission by simultaneously spacing chromophores to mitigate concentration quenching and restricting intramolecular motions to suppress non-radiative decay. For the BT system, the monomer accesses an S1→S0 non-radiative pathway via MECPS1/S0, whereas the macrocycle’s rigidity raises the barrier or eliminates access to this crossing, thus favoring radiative relaxation. The substantial increase in solid-state ΦPL (to 99%) and longer lifetime corroborate the mechanistic picture. Solvatochromic behavior confirms ICT in the D–A framework, while dual-state emission underscores utility in both solution and solid phases. Device measurements show that photophysical gains translate into improved OLED performance. The observation that other macrocycles also exhibit emission enhancement suggests MIEE is a general strategy for designing high-efficiency luminophores.

Macrocyclization-induced emission enhancement (MIEE) is introduced as a general and effective approach to boost emission efficiency of organic luminophores. The benzothiadiazole-based macrocycle BT-LC, obtained by linking BT-M units with methylene bridges, exhibits near-unity solid-state quantum yield, red-shifted emission, longer lifetimes, and improved OLED performance relative to the monomer. Experimental and computational analyses attribute the enhancement to suppression of non-radiative relaxation via MECPS1/S0 due to macrocycle-imposed rigidity. Given that multiple luminophore-based macrocycles also show similar improvements, MIEE provides a versatile design principle for high-performance luminescent materials. Future work could expand MIEE to a broader set of chromophores and device architectures, and further quantify structure–property relationships governing suppression of non-radiative pathways.