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1. Charge-transfer biexciton annihilation in a donor–acceptor co-crystal yields high-energy long-lived charge carriers

   https://doi.org/10.1039/D0SC03301D  

   Schlesinger, Itai ; Powers-Riggs, Natalia E. ; Logsdon, Jenna L. ; Qi, Yue ; Miller, Stephen A. ; Tempelaar, Roel ; Young, Ryan M. ; Wasielewski, Michael R. ( September 2020 , Chemical Science)

   null (Ed.)

   Organic donor–acceptor (D–A) co-crystals have attracted much interest due to their important optical and electronic properties. Co-crystals having ⋯DADA⋯ π-stacked morphologies are especially interesting because photoexcitation produces a charge-transfer (CT) exciton, D˙ + –A˙ − , between adjacent D–A molecules. Although several studies have reported on the steady-state optical properties of this type of CT exciton, very few have measured the dynamics of its formation and decay in a single D–A co-crystal. We have co-crystallized a peri -xanthenoxanthene ( PXX ) donor with a N , N -bis(3-pentyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide) ( Ph4PDI ) acceptor to give an orthorhombic PXX – Ph4PDI ⋯DADA⋯ π-stacked co-crystal with a CT transition dipole moment that is perpendicular to the transition moments for S n ← S 0 excitation of PXX and Ph4PDI . Using polarized, broadband, femtosecond pump–probe microscopy, we have determined that selective photoexcitation of Ph4PDI in the single co-crystal results in CT exciton formation within the 300 fs instrument response time. At early times (0.3 ≤ t ≤ 500 ps), the CT excitons decay with a t −1/2 dependence, which is attributed to CT biexciton annihilation within the one-dimensional ⋯DADA⋯ π-stacks producing high-energy, long-lived (>8 ns) electron–hole pairs in the crystal. These energetic charge carriers may prove useful in applications ranging from photovoltaics and opto-electronics to photocatalysis. 

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2. Organic single crystals of charge-transfer complexes: model systems for the study of donor/acceptor interactions

   https://doi.org/10.1039/D1MH01214B  

   Goetz, Katelyn P. ; Iqbal, Hamna F. ; Bittle, Emily G. ; Hacker, Christina A. ; Pookpanratana, Sujitra ; Jurchescu, Oana D. ( January 2022 , Materials Horizons)

   The charge-transfer (CT) state arising as a hybrid electronic state at the interface between charge donor and charge acceptor molecular units is important to a wide variety of physical processes in organic semiconductor devices. The exact nature of this state depends heavily on the nature and co-facial overlap between the donor and acceptor; however, altering this overlap is usually accompanied by extensive confounding variations in properties due to extrinsic factors, such as microstructure. As a consequence, establishing reliable relationships between donor/acceptor molecular structures, their molecular overlap, degree of charge transfer and physical properties, is challenging. Herein, we examine the electronic structure of a polymorphic system based on the donor dibenzotetrathiafulvalene (DBTTF) and the acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) in the form of high-quality single crystals varying in the donor–acceptor overlap. Using angle-resolved photoemission spectroscopy, we resolve the highest occupied molecular orbital states of the CT crystals. Analysis based on field-effect transistors allows us to probe the sub-gap states impacting hole and electron transport. Our results expand the understanding on the impact of donor and acceptor interactions on electronic structure and charge transport. 

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3. Charge‐Transport Properties of F 6 TNAP‐Based Charge‐Transfer Cocrystals

   https://doi.org/10.1002/adfm.201904858  

   Dasari, Raghunath R. ; Wang, Xu ; Wiscons, Ren A. ; Haneef, Hamna F. ; Ashokan, Ajith ; Zhang, Yadong ; Fonari, Marina S. ; Barlow, Stephen ; Coropceanu, Veaceslav ; Timofeeva, Tatiana V. ; et al ( October 2019 , Advanced Functional Materials)

   The crystal structures of the charge‐transfer (CT) cocrystals formed by the π‐electron acceptor 1,3,4,5,7,8‐hexafluoro‐11,11,12,12‐tetracyanonaphtho‐2,6‐quinodimethane (F₆TNAP) with the planar π‐electron‐donor molecules triphenylene (TP), benzo[b]benzo[4,5]thieno[2,3‐d]thiophene (BTBT), benzo[1,2‐b:4,5‐b′]dithiophene (BDT), pyrene (PY), anthracene (ANT), and carbazole (CBZ) have been determined using single‐crystal X‐ray diffraction (SCXRD), along with those of two polymorphs of F₆TNAP. All six cocrystals exhibit 1:1 donor/acceptor stoichiometry and adopt mixed‐stacking motifs. Cocrystals based on BTBT and CBZ π‐electron donor molecules exhibit brickwork packing, while the other four CT cocrystals show herringbone‐type crystal packing. Infrared spectroscopy, molecular geometries determined by SCXRD, and electronic structure calculations indicate that the extent of ground‐state CT in each cocrystal is small. Density functional theory calculations predict large conduction bandwidths and, consequently, low effective masses for electrons for all six CT cocrystals, while the TP‐, BDT‐, and PY‐based cocrystals are also predicted to have large valence bandwidths and low effective masses for holes. Charge‐carrier mobility values are obtained from space‐charge limited current (SCLC) measurements and field‐effect transistor measurements, with values exceeding 1 cm²V⁻¹s¹being estimated from SCLC measurements for BTBT:F₆TNAP and CBZ:F₆TNAP cocrystals.

    

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4. Panchromatic Light‐Capturing Bis‐styryl BODIPY‐Perylenediimide Donor‐Acceptor Constructs: Occurrence of Sequential Energy Transfer Followed by Electron Transfer

   https://doi.org/10.1002/chem.202301686  

   Ileperuma, Chamari V. ; Garcés‐Garcés, José ; Shao, Shuai ; Fernández‐Lázaro, Fernando ; Sastre‐Santos, Ángela ; Karr, Paul A. ; D'Souza, Francis ( August 2023 , Chemistry – A European Journal)

   Two wide‐band‐capturing donor‐acceptor conjugates featuring bis‐styrylBODIPY and perylenediimide (PDI) have been newly synthesized, and the occurrence of ultrafast excitation transfer from the¹PDI* to BODIPY, and a subsequent electron transfer from the¹BODIPY* to PDI have been demonstrated. Optical absorption studies revealed panchromatic light capture but offered no evidence of ground‐state interactions between the donor and acceptor entities. Steady‐state fluorescence and excitation spectral recordings provided evidence of singlet‐singlet energy transfer in these dyads, and quenched fluorescence of bis‐styrylBODIPY emission in the dyads suggested additional photo‐events. The facile oxidation of bis‐styrylBODIPY and facile reduction of PDI, establishing their relative roles of electron donor and acceptor, were borne out by electrochemical studies. The electrostatic potential surfaces of the S₁and S₂states, derived from time‐dependent DFT calculations, supported excited charge transfer in these dyads. Spectro‐electrochemical studies on one‐electron‐oxidized and one‐electron‐reduced dyads and the monomeric precursor compounds were also performed in a thin‐layer optical cell under corresponding applied potentials. From this study, both bis‐styrylBODIPY⋅⁺and PDI⋅⁻could be spectrally characterizes and were subsequently used in characterizing the electron‐transfer products. Finally, pump–probe spectral studies were performed in dichlorobenzene under selective PDI and bis‐styrylBODIPY excitation to secure energy and electron‐transfer evidence. The measured rate constants for energy transfer,k_ENT, were in the range of 10¹¹ s⁻¹, while the electron transfer rate constants,k_ET, were in the range of 10¹⁰ s⁻¹, thus highlighting their potential use in solar energy harvesting and optoelectronic applications.

    

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5. Assembled triphenylamine bis -urea macrocycles: exploring photodriven electron transfer from host to guests

   https://doi.org/10.1039/D1CP03000K  

   Islam, Md Faizul ; Sindt, Ammon J. ; Hossain, Muhammad Saddam ; Ayare, Pooja J. ; Smith, Mark D. ; Vannucci, Aaron K. ; Garashchuk, Sophya ; Shimizu, Linda S. ( October 2021 , Physical Chemistry Chemical Physics)

   Absorption of electronic acceptors in the accessible channels of an assembled triphenylamine (TPA) bis -urea macrocycle 1 enabled the study of electron transfer from the walls of the TPA framework to the encapsulated guests. The TPA host is isoskeletal in all host–guest structures analyzed with guests 2,1,3-benzothiadiazole, 2,5-dichlorobenzoquinone and I 2 loading in single-crystal-to-single-crystal transformations. Analysis of the crystal structures highlights how the spatial proximity and orientation of the TPA host and the entrapped guests influence their resulting photophysical properties and allow direct comparison of the different donor–acceptor complexes. Diffuse reflectance spectroscopy shows that upon complex formation 1·2,5-dichlorobenzoquinone exhibits a charge transfer (CT) transition. Whereas, the 1·2,1,3-benzothiadiazole complex undergoes a photoinduced electron transfer (PET) upon irradiation with 365 nm LEDs. The CT absorptions were also identified with the aid of time dependent density functional theory (TD-DFT) calculations. Cyclic voltammetry experiments show that 2,1,3-benzothiadiazole undergoes reversible reduction within the host–guest complex. Moreover, the optical band gaps of the host 1·2,5-dichlorobenzoquinone (1.66 eV), and host 1·2,1,3-benzothiadiazole (2.15 eV) complexes are significantly smaller as compared to the free host 1 material (3.19 eV). Overall, understanding this supramolecular electron transfer strategy should pave the way towards designing lower band gap inclusion complexes. 

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