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1. A twist in the non-slanted H-mers to control π-conjugation in 2-dimensions and optical properties

   https://doi.org/10.1039/D0MA00424C  

   Das, Mrinal Kanti; Hameed, Fatima; Lillis, Ryan; Gavvalapalli, Nagarjuna (November 2020, Materials Advances)

   null (Ed.)

   Understanding the structural parameters that determine the extension of π-conjugation in 2-dimensions is key for controlling the optical, photophysical, and electronic properties of 2D-π-conjugated materials. In this article, three non-slanted H-mers including a donor–acceptor H-mer (H-mer-3) with an increase in dihedral angle (twist) between the strands and rungs are synthesized and studied. These non-slanted H-mers represent the repeat units of 2D-π-conjugated materials. H-mer-3, containing donor-strands and an acceptor-rung, is an unexplored donor–acceptor architecture in both slanted and non-slanted H-mers. The H-mers displayed both acid and base dependent optical properties. While the rungs have a little impact on the H-mer absorption spectra they play a key role in the emission and fluorescence lifetime. H-mer-3 ( i.e. , donor–acceptor H-mer) shows a higher Stokes shift and fluorescence lifetime than the other two H-mers. The twist and the presence of an electron deficient rung in H-mer-3 facilitated an intramolecular charge transfer in the excited state from the strands to the electron deficient rung, and therefore control over the H-mer emission properties. The lack of insulating pendant chains, reduced π–π interactions in thinfilms, and longer fluorescence lifetimes make these H-mers interesting candidates for various electronic and optoelectronic applications. 

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2. Synthesis and Properties of Fluorenone‐Containing Cycloparaphenylenes and Their Late‐Stage Transformation

   https://doi.org/10.1002/chem.202303490  

   Bliksted_Roug_Pedersen, Viktor; Price, Tavis_W; Kofod, Nicolaj; Zakharov, Lev_N; Laursen, Bo_W; Jasti, Ramesh; Brøndsted_Nielsen, Mogens (December 2023, Chemistry – A European Journal)

   Abstract Cycloparaphenylenes (CPPs) are the smallest possible armchair carbon nanotubes, the properties of which strongly depend on their ring size. They can be further tuned by either peripheral functionalization or by replacing phenylene rings for other aromatic units. Here we show how four novel donor–acceptor chromophores were obtained by incorporating fluorenone or 2‐(9H‐fluoren‐9‐ylidene)malononitrile into the loops of two differently sized CPPs. Synthetically, we managed to perform late‐stage functionalization of the fluorenone‐based rings by high‐yielding Knoevenagel condensations. The structures were confirmed by X‐ray crystallographic analyses, which revealed that replacing a phenylene for a fused‐ring‐system acceptor introduces additional strain. The donor–acceptor characters of the CPPs were supported by absorption and fluorescence spectroscopic studies, electrochemical studies (displaying the CPPs as multi‐redox systems undergoing reversible or quasi‐reversible redox events), as well as by computations. The oligophenylene parts were found to comprise the electron donor units of the macrocycles and the fluorenone parts the acceptor units. 

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3. Pyridine-terminated low gap π-conjugated oligomers: design, synthesis, and photophysical response to protonation and metalation

   https://doi.org/10.1039/c8qo00963e  

   Weldeab, Asmerom O.; Li, Lei; Cekli, Seda; Abboud, Khalil A.; Schanze, Kirk S.; Castellano, Ronald K. (October 2018, Organic Chemistry Frontiers)

   null (Ed.)

   Reported here is the design and synthesis of among the first pyridine terminated acceptor–donor–acceptor–donor–acceptor (A–D–A–D–A) based π-conjugated oligomers, EH_DPP_2T_Pyr ( 1 ), EH_II_2T_Pyr ( 2 ), and EH_II_1T_Pyr ( 3 ). The molecules incorporate thiophenes as electron donors, isoindigo/diketopyrrolopyrrole as electron acceptors, and are capped with pyridine, a weak electron acceptor, on both ends. All target oligomers show attractive photophysical properties, broad absorption in the visible region ( λ max = 636 nm, 575 nm, and 555 nm, for 1 , 2 , and 3 , respectively) and emission which extends to the IR region (emission λ max = 734 nm and 724 for 1 and 2 , respectively). Given the pyridine nitrogens, the optoelectronic properties of the compounds can be further tuned by protonation/metalation. All compounds show a bathochromic shift in visible absorption and fluorescence quenching upon addition of trifluoroacetic acid (TFA). Similar phenomena are observed upon addition of metals with a particularly strong response to Cu 2+ and Pd 2+ as indicated by Stern–Volmer analysis ( e.g. , for Pd 2+ ; K sv = 7.2 × 10 4 M −1 ( λ = 673 nm), 8.5 × 10 4 M −1 ( λ = 500 nm), and 1.1 × 10 5 ( λ = 425 nm) for 1 , 2 , and 3 , respectively). The selective association of the molecules to Cu 2+ and Pd 2+ is further evidenced by a color change easily observed by eye and under UV light, important for potential use in colorimetric sensing. 

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4. Exploration and development of gold- and silver-catalyzed cross dehydrogenative coupling toward donor–acceptor π-conjugated polymer synthesis

   https://doi.org/10.1039/C8PY01162A  

   Kang, Lauren J.; Xing, Liwen; Luscombe, Christine K. (January 2019, Polymer Chemistry)

   π-Conjugated polymers are materials of interest for use in organic electronics. Within these polymers, donor–acceptor polymers are favorable for solar cell applications due to improved charge mobility, better absorption in the low energy region of the solar spectrum, and tunable band gaps. One of the barriers to commercializing these donor–acceptor materials is that their synthetic pathways are complex because of the alternating repeat units in the polymer. To address this, the application of cross dehydrogenative coupling (also called oxidative CH/CH cross-coupling) toward the synthesis of donor–acceptor polymers was explored. In this work, the roles of specific reagents in a one-pot gold- and silver-catalyzed cross dehydrogenative coupling and the factors that contribute to selectivity for cross-coupling rather than homo-coupling are analyzed. Based on our results, we postulate that the percentage of alternating repeat units in the final polymer is affected by the increased reactivity of the dimer that forms in the initial stages of the polymerization compared to the monomer, which ultimately may be exploited to control the ratio of electron-rich to electron-poor monomers. 

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5. The magic of biaryl linkers: the electronic coupling through them defines the propensity for excited-state symmetry breaking in quadrupolar acceptor–donor–acceptor fluorophores

   https://doi.org/10.1039/D3SC03812B  

   Clark, John A.; Kusy, Damian; Vakuliuk, Olena; Krzeszewski, Maciej; Kochanowski, Krzysztof J.; Koszarna, Beata; O'Mari, Omar; Jacquemin, Denis; Gryko, Daniel T.; Vullev, Valentine I. (January 2023, Chemical Science)

   Intermediate donor–acceptor electronic coupling leads to a brilliant fluorescence behaviour. Charge transfer (CT) is key for molecular photonics, governing the optical properties of chromophores comprising electron-rich and electron-deficient components. In photoexcited dyes with an acceptor– donor–acceptor or donor–acceptor–donor architecture, CT breaks their quadrupolar symmetry and yields dipolar structures manifesting pronounced solvatochromism. Herein, we explore the effects of electronic coupling through biaryl linkers on the excited-state symmetry breaking of such hybrid dyes composed of an electron-rich core, i.e., 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP), and pyrene substituents that can act as electron acceptors. Experimental and theoretical studies reveal that strengthening the donor–acceptor electronic coupling decreases the CT rates and the propensity for symmetry breaking. We ascribe this unexpected result to effects of electronic coupling on the CT thermodynamics, which in its turn affects the CT kinetics. In cases of intermediate electronic coupling, the pyrene-DHPP conjugates produce fluorescence spectra, spreading over the whole visible range, that in addition to the broad CT emission, show bands from the radiative deactivation of the locally excited states of the donor and the acceptors. Because the radiative deactivation of the low-lying CT states is distinctly slow, fluorescence from upper locally excited states emerge leading to the observed anti- Kasha behaviour. As a result, these dyes exhibit white fluorescence. In addition to demonstrating the multifaceted nature of the effects of electronic coupling on CT dynamics, these chromophores can act as broad-band light sources with practical importance for imaging and photonics. 

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