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1. Deciphering the unusual fluorescence in weakly coupled bis-nitro-pyrrolo[3,2-b]pyrroles

   https://doi.org/10.1038/s42004-020-00434-6  

   Poronik, Yevgen M.; Baryshnikov, Glib V.; Deperasińska, Irena; Espinoza, Eli M.; Clark, John A.; Ågren, Hans; Gryko, Daniel T.; Vullev, Valentine I. (December 2020, Communications Chemistry)

   Abstract Electron-deficient π-conjugated functional dyes lie at the heart of organic optoelectronics. Adding nitro groups to aromatic compounds usually quenches their fluorescence via inter-system crossing (ISC) or internal conversion (IC). While strong electronic coupling of the nitro groups with the dyes ensures the benefits from these electron-withdrawing substituents, it also leads to fluorescence quenching. Here, we demonstrate how such electronic coupling affects the photophysics of acceptor–donor–acceptor fluorescent dyes, with nitrophenyl acceptors and a pyrrolo[3,2-b]pyrrole donor. The position of the nitro groups and the donor-acceptor distance strongly affect the fluorescence properties of thebis-nitrotetraphenylpyrrolopyrroles. Concurrently, increasing solvent polarity quenches the emission that recovers upon solidifying the media. Intramolecular charge transfer (CT) and molecular dynamics, therefore, govern the fluorescence of these nitro-aromatics. While balanced donor-acceptor coupling ensures fast radiative deactivation and slow ISC essential for large fluorescence quantum yields, vibronic borrowing accounts for medium dependent IC via back CT. These mechanistic paradigms set important design principles for molecular photonics and electronics. 

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2. Advancements in π-conjugated polymers: harnessing cycloalkyl straps for high-performance π-conjugated materials

   https://doi.org/10.1039/D4CC03799E  

   Ochonma, Charles; Francis, Victor S; Biswas, Sayan Kumar; Gavvalapalli, Nagarjuna (November 2024, Chemical Communications)

   Pendant alkyl chains are widely used to successfully obtain a wide variety of soluble linear 1D π-conjugated polymers. Over the past several decades, a wide variety of π-conjugated polymers have been synthesized to realize the desired properties and improve the performance of organic electronic devices. However, this strategy is not suitable for generating soluble 2D-π-conjugated materials, including ladder polymers, nanoribbons, and 2D-π-conjugated polymers, due to strong van der Waals interactions between the ribbons and sheets. The drive to synthesize higher dimensional polymers and to enhance polymers' properties has spurred the exploration of a novel direction in materials chemistry—the synthesis of unconventional monomers and polymers. The Gavvalapalli research group has developed and used cycloalkyl straps containing aryl building blocks for the synthesis of conjugated polymers. These cycloalkyl straps, positioned either above or below the π-conjugation plane, have been shown to directly control the π–π interactions between the polymer chains. We have demonstrated that π-face masking cycloalkyl straps hinder interchain π–π interactions. The first part of this review article highlights the use of cycloalkyl straps for the synthesis of higher dimensional π-conjugated polymers. In this section, we discuss the synthesis of 2D-H-mers, dispersible hyperbranched π-conjugated polymers, and conjugated porous polymers without the pendant solubilizing chains. The second part of the feature article highlights how the cycloalkyl straps can be used to gain control over polymer–acceptor interactions, including the interaction strength and the location of the acceptor along the polymer backbone. We conclude the article with the future outlook on cycloalkyl strap-containing building blocks in the world of conjugated polymers. 

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3. 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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4. 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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5. Donor acceptor fluorophores: synthesis, optical properties, TD-DFT and cytotoxicity studies

   https://doi.org/10.1039/D0OB02313B  

   Essam, Zahraa M.; Ozmen, Guliz Ersoy; Setiawan, Dani; Hamid, Riri Rizkianty; Abd El-Aal, Reda M.; Aneja, Ritu; Hamelberg, Donald; Henary, Maged (March 2021, Organic & Biomolecular Chemistry)

   null (Ed.)

   Donor–π-acceptor (D–π-A) fluorophores consisting of a donor unit, a π linker, and an acceptor moiety have attracted attention in the last decade. In this study, we report the synthesis, characterization, optical properties, TD-DFT, and cytotoxicity studies of 17 near infrared (NIR) D–π-A analogs which have not been reported so far to the best of our knowledge. These fluorophores have chloroacrylic acid as the acceptor unit and various donor units such as indole, benzothiazole, benzo[ e ]indole, and quinoline. The fluorophores showed strong absorption in the NIR (700–970 nm) region due to their enhanced intramolecular charge transfer (ICT) between chloroacrylic acid and the donor moieties connected with the Vilsmeier–Haack linker. The emission wavelength maxima of the fluorophores were in between 798 and 870 nm. Compound 20 with a 4-quinoline donor moiety showed an emission wavelength above 1000 nm in the NIR II window. The synthesized fluorophores were characterized by 1 H NMR and 13 C NMR, and their optical properties were studied. Time dependent density functional theory (TD-DFT) calculations showed that the charge transfer occurs from the donor groups (indole, benzothiazole, benzo[ e ]indole, and quinoline) to the acceptor chloroacrylic acid moiety. Fluorophores with [HOMO] to [LUMO+1] transitions were shown to possess a charge separation character. The cytotoxicity of selected fluorophores, 4 , 7 , 10 and 12 was investigated against breast cancer cell lines and they showed better activity than the anti-cancer agent docetaxel. 

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