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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. 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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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. Electron‐Deficient Conjugated Materials via p–π* Conjugation with Boron: Extending Monomers to Oligomers, Macrocycles, and Polymers

   https://doi.org/10.1002/chem.202003481  

   Yin, Xiaodong; Liu, Jun; Jäkle, Frieder (December 2020, Chemistry – A European Journal)

   Abstract The extension of conjugated organoboranes from monomeric species to oligomers, macrocycles, and polymers offers access to a plethora of fascinating new materials. The p–π* conjugation between empty orbitals on boron and the conjugated linkers not only affects the electronic structure and optical properties, but also enables mutual interactions between electron‐deficient boron centers. The unique properties of these electron‐deficient π‐conjugated systems are exploited in highly luminescent materials, organic optoelectronic devices, and sensing applications. 

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5. Highly fluorescent purine-containing conjugated copolymers with tailored optoelectronic properties

   https://doi.org/10.1039/D2PY00545J  

   O'Connell, C. Elizabeth; Sabury, Sina; Jenkins, J. Elias; Collier, Graham S.; Sumpter, Bobby G.; Long, Brian K.; Kilbey, S. Michael (August 2022, Polymer Chemistry)

   Conjugated copolymers containing electron donor and acceptor units in their main chain have emerged as promising materials for organic electronic devices due to their tunable optoelectronic properties. Herein, we describe the use of direct arylation polymerization to create a series of fully π-conjugated copolymers containing the highly tailorable purine scaffold as a key design element. To create efficient coupling sites, dihalopurines are flanked by alkylthiophenes to create a monomer that is readily copolymerized with a variety of conjugated comonomers, ranging from electron-donating 3,4-dihydro-2 H -thieno[3,4- b ][1,4]dioxepine to electron-accepting 4,7-bis(5-bromo-3-hexylthiophen-2-yl)benzo[ c ][1,2,5]thiadiazole. The comonomer choice and electronic nature of the purine scaffold allow the photophysical properties of the purine-containing copolymers to be widely varied, with optical bandgaps ranging from 1.96–2.46 eV, and photoluminescent quantum yields as high as ϕ = 0.61. Frontier orbital energy levels determined for the various copolymers using density functional theory tight binding calculations track with experimental results, and the geometric structures of the alkylthiophene-flanked purine monomer and its copolymer are found to be nearly planar. The utility of direct arylation polymerization and intrinsic tailorability of the purine scaffold highlight the potential of these fully conjugated polymers to establish structure–property relationships based on connectivity pattern and comonomer type, which may broadly inform efforts to advance purine-containing conjugated copolymers for various applications. 

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