Conjugated Polymers Based on 4,10-Bis(thiophen-2-yl)anthanthrone: Synthesis, Characterization, and Fluoride-Promoted Photoinduced Electron Transfer

Effect of substituting donors on the hole mobility of hole transporting materials in perovskite solar cells: a DFT study

Several hole-transporting materials (HTMs) have been designed by incorporating different types of π-conjugation group such as long chain aliphatic alkenes and condensed aromatic rings of benzene and thiophene and their derivatives on both sides between the planar core and donor of a reference HTM. Various electronic, optical, and dynamic properties have been calculated by using DFT, TDDFT, and Marcus theory. In this study, all the designed HTMs show a lower HOMO energy level and match well with the perovskite absorbers. Inserting condensed rings results in better hole mobility compared to aliphatic double bonds. It is found that the charge transfer integral is the dominant factor which mainly influences the hole mobility in our studied HTMs. Other factors such as hole reorganization energy, hole hopping rate, and centroid distance have a minor effect on hole mobility. Thus, this study is expected to provide guidance for the design and synthesis of new HTMs with increased hole mobility.

Dibenzannulated peri-acenoacenes from anthanthrene derivatives

A series of dibenzannulated phenyl-annulated [4,2]peri-acenoacenes have been synthesized in three straightforward steps from 4,10-dibromoanthanthrone (vat orange 3). The phenyl bisannulation of [4,2]peri-acenoacene provides extra stability by increasing the overall aromatic character of the molecules, and allows for a 45-80% increase of the molar extinction coefficient (ε) compared to their [5,2]peri-acenoacene isomers. Depending on the substituents attached to the π-conjugated core, some derivatives exhibit strong aggregation in the solid state with association constant (Ka) up to 255 M-1, resulting in a significant broadening of the absorption spectrum and a substantial decrease of the bandgap value (more than 0.3 V) from solution to the solid state. One [4,2]peri-acenoacene derivative was doubly reduced using cesium and the crystal structure of the resulting salt has been obtained. Field-effect transistors showing a temperature-dependent hole mobility have been tested.

Discriminative Behavior of a Donor-Acceptor-Donor Triad toward Cyanide and Fluoride: Insights into the Mechanism of Naphthalene Diimide Reduction by Cyanide and Fluoride

A new molecular donor-acceptor-donor (D-A-D) triad, comprised of an electron deficient 1,4,5,8-naphthalene tetracarboxylic diimide (NDI) unit covalently connected to two flanking photosensitizers, i.e., a bis-heteroleptic Ru(II) complex of 1,10-phenanthroline and pyridine triazole hybrid ligand, is described. The single crystal X-ray structure of the perchlorate salt of the triad demonstrates that the electron deficient NDI unit can act as a host for anions via anion-π interaction. Detailed solution-state studies indicate that fluoride selectively interacts with the D-A-D triad to form a dianionic NDI, NDI^2-, via a radical anion, NDI^•-. On the contrary, cyanide reduces the NDI moiety to NDI^•-, as confirmed by UV-vis, NMR, and EPR spectroscopy. Further, femtosecond transient absorption spectroscopic studies reveal a low luminescence quantum yield of the D-A-D triad attributable to the photoinduced electron transfer (PET) process from the photoactive Ru(II) center to the NDI unit. Interestingly, the triad displays "OFF-ON" luminescence behavior in the presence of fluoride by restoring the Ru(II) to phenanthroline/pyridine-triazole-based MLCT emission, whereas cyanide fails to show a similar property due to a different redox process operational in the latter. The reduction of NDI in the presence of fluoride and cyanide in different polar solvents indicates that involvement of such deprotonated solvents in the electron transfer mechanism may not be operative in our present system. Low-temperature kinetic studies support the formation of a charge transfer associative transient species, which likely allows overcoming the thermodynamically uphill barrier for the direct electron transfer mechanism.

Amphiphilic anthanthrene trimers that exfoliate graphite and individualize single wall carbon nanotubes

A phosphodiester-linked dialkynyl substituted anthanthrene trimer (1) has been designed and synthesized. Its graphene ribbon like structure is expected to facilitate interactions with nanographene (NG) and single wall carbon nanotubes (SWCNT) to yield novel and stable carbon-based nanomaterials. Interactions with trimer 1 lead to exfoliation of NG and to the individualization of SWCNTs. Phosphate groups, in general, and their negative charges, in particular, render the resulting nanomaterials soluble in ethanol, which is ecologically favourable over DMF required for the processing of pristine NG or SWCNTs. The newly formed nanomaterials were probed by complementary spectroscopic and microscopic techniques. Of particular importance were transient absorption and fluorescence excitation measurements, which revealed an efficient energy transfer within the carbon-based nanomaterials.

Synthesis and Properties of Conjugated Polymers Based on a Ladderized Anthanthrene Unit

Polycyclic aromatic hydrocarbons (PAHs) are interesting building blocks for the preparation of conjugated polymers due to their extended π surface and planar conformation. However, their use as comonomer in conjugated polymers often leads to nonplanar main chains as a consequence of high steric hindrance at the linking point. Herein, we report the synthesis of a ladderized anthanthrene unit using an sp3 carbon bridge. Three conjugated copolymers with fluorene, isoindigo, and bithiophene derivatives have been synthesized and characterized to study the effect of such ladderization on the electronic properties. The dihedral angle between the ladderized anthanthrene and adjacent units has been significantly reduced by the formation of the sp3 carbon bridge, thus eliminating the steric hindrance with the proton at the peri position of the anthanthrene unit and red-shifting the absorption spectrum by 25 nm.

Anion-Induced Electron Transfer

As counterintuitive as it might seem, in aprotic media, electron transfer (ET) from strong Lewis basic anions, particularly F^-, OH^-, and CN^-, to certain π-acids (πA) is not only spectroscopically evident from the formation of paramagnetic πA^•- radical anions and πA^2- dianions, but also thermodynamically justified because these anions' highest occupied molecular orbitals (HOMOs) lie above the π-acids' lowest unoccupied molecular orbitals (LUMOs) creating negative free energy changes (Δ G°_ET < 0). Depending on the relative HOMO and LUMO energies of participating anions and π-acids, respectively, the anion-induced ET (AIET) events take place either in the ground state or upon photosensitization of the π-acids. The mild basic and charge-diffuse anions with lower HOMO levels fail to trigger ET, but they often form charge transfer (CT) and anion-π complexes. Owing to their high HOMO levels in aprotic environments, strong Lewis basic anions, such as F^- enjoy much greater ET driving force (Δ G°_ET) than mild and non-basic anions, such as iodide. In protic solvents, however, the former become more solvated and stabilized and lose their electron donating ability more than the latter, creating an illusion that F^- is a poor electron donor due to the high electronegativity of fluorine. However, UV-vis, EPR, and NMR studies consistently show that in aprotic environments, F^- reduces essentially any π-acid with LUMO levels of -3.8 eV or less, revealing that contrary to a common perception, the electron donating ability of F^- anion is not dictated by the electronegativity of fluorine atom but is a true reflection of high Lewis basicity of the anion itself. Thus, the neutral fluorine atoms with zero formal charge and F^- anion have little in common when it comes to their electronic properties. The F^- anion can also legitimately act as a Brønsted base when the proton source has a p K_a lower than that of its conjugate acid HF (15), not the other way around, and ET from F^- to a poor electron acceptor is not thermodynamically feasible. While there is no shortage of indisputable evidence and clear-cut thermodynamic justifications for ET from F^- and other Lewis basic anions to various π-acids in aprotic solvents, because of the aforesaid misconception, it had been posited that F^- perhaps formed diamagnetic Meisenheimer complexes via nucleophilic attack, deprotonated an aprotic solvent DMSO against an insurmountably high p K_a (35) leading to a π-acid reduction, or formed [F^-/πA^•+] complexes via a thermodynamically prohibited oxidation of π-acids. Unlike AIET, however, none of these hypotheses was thermodynamically viable nor supported by any experimental evidence. First, by defining the thermodynamic criteria of AIET pathways and all other alternate hypotheses and then evaluating the spectroscopic signals emanating from the interactions between different anions and π-acids and Lewis acids in the light of these criteria, this Account comes to a conclusion that AIET is the only viable mechanism that can rationalize the reduction of π-acids without violating any thermodynamic rules. The paradigm-shifting discovery of AIET not only exposed a common misconception about the electron donating ability of F^- but also enabled naked-eye detection of toxic anions, electrode-free silver plating, luminescent silver nanoparticle synthesis, light-harvesting, and conductivity enhancement of conjugated polymers, with more innovative applications still to come.

Poly[(arylene ethynylene)-alt-(arylene vinylene)]s Based on Anthanthrone and Its Derivatives: Synthesis and Photophysical, Electrochemical, Electroluminescent, and Photovoltaic Properties

Anthanthrone and its derivatives are large polycyclic aromatic compounds (PACs) that pose a number of challenges for incorporation into the structure of soluble conjugated polymers. For the first time, this group of PACs was employed as the building blocks for the synthesis of copolymers (P1-P5) based on poly[(arylene ethynylene)-alt-(arylene vinylene)]s backbone (-Ph-C≡C-Anth-C≡C-Ph-CH=CH-Ph-CH=CH-) _n . During the synthesis of P1-P5, different alkyloxy side chains were incorporated in order to tune the properties of the polymers. Of the copolymer series only P1 (containing anthanthrone and branched 2-ethylhexyloxy side chains on phenylenes), P2 and P3 (for which the anthanthrones containing carbonyl groups were converted to anthanthrene containing alkyloxy substituents) were soluble. The photophysical, electrochemical, electroluminescent and photovoltaic properties of P1-P3 are reported, compared and discussed with respect to the effects of side chains.