Achieving Accurate Reduction Potential Predictions for Anthraquinones in Water and Aprotic Solvents: Effects of Inter- and Intramolecular H‑Bonding and Ion Pairing

In this combined computational and experimental study, specific chemical interactions affecting the prediction of one-electron and two-electron reduction potentials for anthraquinone derivatives are investigated. For 19 redox reactions in acidic aqueous solution, where AQ is reduced to hydroanthraquinone, density functional theory (DFT) with the polarizable continuum model (PCM) gives a mean absolute deviation (MAD) of 0.037 V for 16 species. DFT­(PCM), however, highly overestimates three redox couples with a MAD of 0.194 V, which is almost 5 times that of the remaining 16. These three molecules have ether groups positioned for intramolecular hydrogen bonding that are not balanced with the intermolecular H-bonding of the solvent. This imbalanced description is corrected by quantum mechanics/molecular mechanics (QM/MM) simulations, which include explicit water molecules. The best theoretical estimations result in a good correlation with experiments, <i>V</i>(Theory) = 0.903<i>V</i>(Expt) + 0.007 with an <i>R</i>² value of 0.835 and an MAD of 0.033 V. In addition to the aqueous test set, 221 anthraquinone redox couples in aprotic solvent were studied. Five anthraquinone derivatives spanning a range of redox potentials were selected from this library, and their reduction potentials were measured by cyclic voltammetry. DFT­(PCM) calculations predict the first reduction potential with high accuracy giving the linear relation, <i>V</i>(Theory) = 0.960<i>V</i>(Expt) – 0.049 with an <i>R</i>² value of 0.937 and an MAD of 0.051 V. This approach, however, significantly underestimates the second reduction potential, with an MAD of 0.329 V. It is shown herein that treatment of explicit ion-pair interactions between the anthraquinone derivatives and the cation of the supporting electrolyte is required for the accurate prediction of the second reduction potential. After the correction, <i>V</i>(Theory) = 1.045<i>V</i>(Expt) – 0.088 with an <i>R</i>² value 0.910 and an MAD value reduced by more than half to 0.145 V. Finally, molecular design principles are discussed that go beyond simple electron-donating and electron-withdrawing effects to lead to predictable and controllable reduction potentials

Synthesis and Enhanced Linear and Nonlinear Optical Properties of Chromophore–Au Metal Cluster Oligomers

Using nanoclusters as building blocks for supracrystals can offer unique optical and electronic properties of metal nanoclusters for designing new materials. The advantage of building nanocluster crystals through molecular linkers is that the linker can be used as a functional group for the nanoclusters (e.g., dye–nanocluster systems). In this study, we employed the ligand exchange reaction to synthesize chromophore–Au₂₅ nanocluster oligomers and investigated their linear and nonlinear optical properties. The chromophore–Au₂₅ nanocluster oligomers product mixture was separated into four bands by polyacrylamide gel electrophoresis and characterized by matrix-assisted laser desorption ionization mass spectrometry and scanning transmission electron microscopy imaging. The linear optical properties of the systems were investigated by steady state UV–vis absorption and fluorescence spectroscopy. The chromophore–Au₂₅ nanocluster oligomers showed increased oscillator strength and transition dipole moment compared to single Au₂₅ nanoclusters. Energy transfer from the chromophore 4,4′-thiodibenzenethiol (TBT) to the metal cluster was observed in the chromophore–Au₂₅ nanocluster dimer system. The excited state and fluorescence dynamics were investigated by transient absorption spectroscopy, time-resolved fluorescence upconversion, and time-correlated single photon counting. The chromophore–Au₂₅ nanocluster oligomers have a long-lived surface state due to the contribution of energy transfer by two nanocluster cores. The two-photon absorption cross sections of the chromophore–Au₂₅ nanocluster oligomers showed an increasing enhancement trend with increasing oligomer length. An enhancement factor of up to 68 times was found compared to single Au₂₅ nanoclusters. Finally, we performed a structure–property correlation analysis to explain the observed optical properties of these systems

Density Functional Physicality in Electronic Coupling Estimation: Benchmarks and Error Analysis

Electronic coupling estimates from constrained density functional theory configuration interaction (CDFT-CI) depend critically on choice of density functional. In this Letter, the orbital multielectron self-interaction error (OMSIE), vertical electron affinity (VEA), and vertical ionization potential (VIP) are shown to be the key indicators inherited from the density functional that determine the accuracy of electronic coupling estimates. An error metric η is derived to connect the three properties, based on the linear proportionality between electronic coupling and overlap integral, and the hypothesis that the slope of this line is a function of VEA/VIP, η = (1/<i>N</i>_testset)­Σ_<i>i</i>^testset|−VE^Ref × OMSIE + ΔVE – ΔVE × OMSIE|_<i>i</i>. Based on η, BH&HLYP and LRC-ωPBEh are suggested as the best functionals for electron and hole transfer, respectively. Error metric η is therefore a useful predictor of errors in CDFT-CI electronic coupling, showing that the physical correctness of the density functional has a direct effect on the accuracy of the electronic coupling

Organic Macromolecular High Dielectric Constant Materials: Synthesis, Characterization, and Applications

Hyperbranched and dendritic architectures have been targeted for various applications such as sensing, drug delivery, optical limiting, and light harvesting. One interesting development in this area has focused on utilizing the existence of long-range delocalization in hyperbranched structures to achieve high dielectric constants. In this Feature Article, we will review the creation and development of this concept, and we highlight our recent research progress in this aspect. In particular, we discuss (1) synthetic methods for a particular group of hyperbranched polymers; (2) detailed optical and electronic characterization of this group of hyperbranched polymers, revealing the design criteria for achieving a good combination of high dielectric constant and minimum loss in such materials; and (3) the importance and potential applications of these materials

New Approaches for Energy Storage with Hyperbranched Polymers

New hyperbranched polymers have been investigated to provide new structure–function relationships necessary for electrical and optical applications. We can take advantage of long-range delocalization in these structures for energy storage applications. This is accomplished by obtaining high dielectric constants. In this Article, we demonstrate the need for developing high dielectric hyperbranched polymers by first investigating a ceramic/polymer hybrid system and then studying the design criteria for these hyperbranched systems using detailed optical and electronic characterization techniques. Provided in this contribution are the energy storage results with ion-doped polyaniline (PANI) polymers. An enhancement in the dielectric constant emerged from strong long-range polaron delocalization and the mechanism of a hyperelectronic polarization in these polymer systems. A copper phthalocyanine (CuPc) core was selected to build novel hyperbranched polymers to investigate their energy storage and optical properties. We report the results of these hyperbranched polymers, which exhibited high dielectric constants and low dielectric losses. Detailed structure–function relationships were carried out to probe the polaron delocalization mechanism. An outstanding result of a new hyperbranched polymer showed the greatest energy storage capacity of 7.97 J cm^–3. These results provide new insights into the design of new organic macromolecules for energy storage

Two-Photon and Time-Resolved Fluorescence Spectroscopy as Probes for Structural Determination in Amyloid‑β Peptides and Aggregates

The development of new sensitive methods for the detailed collection of conformational and morphological information about amyloids is crucial for the elucidation of critical questions regarding aggregation processes in neurodegenerative diseases. The combined approach of two-photon and time-resolved fluorescence spectroscopy described in this report interrogates the early conformational dynamics seen in soluble oligomers of amyloid-β(1–42). Concentration-dependent aggregation studies using two-photon absorption show enhanced sensitivity toward conformational changes taking place in the secondary structure of the amyloid peptide as aggregation proceeds. Fluorescence lifetimes and changes in anisotropy values indicate Förster-type energy transfer occurring as a function of aggregation state. The sensitivity of our two-photon methodology is compared to that of circular dichroism (CD) spectroscopy, and the results indicate that the two-photon absorption cross-section method exhibits superior sensitivity. A theoretical model is developed that, together with electronic structure calculations, explains the change in cross section as a function of aggregation in terms of interacting transition dipoles for aggregates showing stacked or parallel structures. This suggests that the two-photon method provides a sensitive alternative to CD spectroscopy while avoiding many of the inherent challenges particular to CD data collection. The implication of this finding is significant, as it indicates that a two-photon-based technique used in conjunction with time-resolved fluorescence might be able to reveal answers to conformational questions about amyloid-β(1–42) that are presently inaccessible with other techniques

Evolution of the Dynamics of As-Deposited and Annealed Lead Halide Perovskites

The rapid rise of organolead trihalide perovskites as solar photovoltaic materials has been followed by promising developments in light-emitting devices and lasers due to their unique and promising optical properties. Evolution of the photophysical properties in as-deposited or annealed CH₃NH₃PbBr₃ and CH₃NH₃PbI₃ perovskite films processed through the interdiffusion method has been investigated. Absorption spectra showed broad band edge saturation in the as-deposited films in contrast to sharp excitonic absorption in the annealed films. Fluorescence emission of the perovskite films showed strong dependence on the halogen type with a very high quantum yield of ∼90% for the annealed CH₃NH₃PbBr₃ film. An explanation for this was provided based on its crystallinity and quantum confinement of the excitons. The emission showed weakly Stokes shifted bands. Time-resolved spectroscopic measurements were carried out to probe the ultrafast dynamics for the perovskites for the as-deposited or annealed films. We classified the evolution in the absorption features in the excited state of CH₃NH₃PbBr₃ perovskite films for the first time and compared them to CH₃NH₃PbI₃. We suggest a bleach feature below 400 nm as the charge transfer band, which results in the photoinduced absorption in the CH₃NH₃PbBr₃ perovskite film, a charge-separated band gap state, and the existence of intermediate excited-state species that regenerate the ground state

A New Design Strategy and Diagnostic to Tailor the DNA-Binding Mechanism of Small Organic Molecules and Drugs

The classical model for DNA groove binding states that groove binding molecules should adopt a crescent shape that closely matches the helical groove of DNA. Here, we present a new design strategy that does not obey this classical model. The DNA-binding mechanism of small organic molecules was investigated by synthesizing and examining a series of novel compounds that bind with DNA. This study has led to the emergence of structure–property relationships for DNA-binding molecules and/or drugs, which reveals that the structure can be designed to either intercalate or groove bind with calf thymus dsDNA by modifying the electron acceptor properties of the central heterocyclic core. This suggests that the electron accepting abilities of the central core play a key role in the DNA-binding mechanism. These small molecules were characterized by steady-state and ultrafast nonlinear spectroscopies. Bioimaging experiments were performed in live cells to evaluate cellular uptake and localization of the novel small molecules. This report paves a new route for the design and development of small organic molecules, such as therapeutics, targeted at DNA as their performance and specificity is dependent on the DNA-binding mechanism

Effect of Acceptor Strength on Optical and Electronic Properties in Conjugated Polymers for Solar Applications

Four new low-bandgap electron-accepting polymerspoly­(4,10-bis­(2-butyl­octyl)-2-(2-(2-ethylhexyl)-1,1-dioxido-3-oxo-2,3-dihydrothieno­[3,4-<i>d</i>]­isothiazol-4-yl)­thieno­[2′,3′:5,6]­pyrido­[3,4-<i>g</i>]­thieno­[3,2-<i>c</i>]­isoquinoline-5,11­(4<i>H</i>,10<i>H</i>-dione) (PNSW); poly­(4,10-bis­(2-butyl­octyl)-2-(5-(2-ethylhexyl)-4,6-dioxo-5,6-dihydro-4<i>H</i>-thieno­[3,4-<i>c</i>]­pyrrol-1-yl)­thieno­[2′,3′:5,6]­pyrido­[3,4-<i>g</i>]­thieno­[3,2-<i>c</i>]­isoquinoline-5,11­(4<i>H</i>,10<i>H</i>)-dione) (PNTPD); poly­(5-(4,10-bis­(2-butyl­octyl)-5,11-dioxo-4,5,10,11-tetrahydrothieno­[2′,3′:5,6]­pyrido­[3,4-<i>g</i>]­thieno­[3,2-<i>c</i>]­isoquinolin-2-yl)-2,9-bis­(2-decyldodecyl)­anthra­[2,1,9-<i>def</i>:6,5,10-<i>d′e′f′</i>]­diisoquinoline-1,3,8,10­(2<i>H</i>,9<i>H</i>)-tetraone) (PNPDI); and poly­(9,9-bis­(2-butyl­octyl)-9<i>H</i>-fluorene-bis­((1,10:5,6)­2-(5,6-dihydro-4<i>H</i>-cyclopenta­[<i>b</i>]­thiophene-4-ylidene)­malonitrile)-2-(2,3-dihydrothieno­[3,4-<i>b</i>]­[1,4]­dioxine)) (PECN)containing thieno­[2′,3′:5′,6′]­pyrido­[3,4-<i>g</i>]­thieno­[3,2-<i>c</i>]­isoquinoline-5,11­(4<i>H</i>,10<i>H</i>)-dione and fluorenedicyclopentathiophene dimalononitrile, were investigated to probe their structure–function relationships for solar cell applications. PTB7 was also investigated for comparison with the new low-bandgap polymers. The steady-state, ultrafast dynamics and nonlinear optical properties of all the organic polymers were probed. All the polymers showed broad absorption in the visible region, with the absorption of PNPDI and PECN extending into the near-IR region. The polymers had HOMO levels ranging from −5.73 to −5.15 eV and low bandgaps of 1.47–2.45 eV. Fluorescence upconversion studies on the polymers showed long lifetimes of 1.6 and 2.4 ns for PNSW and PNTPD, respectively, while PNPDI and PECN showed very fast decays within 353 and 110 fs. PECN exhibited a very high two-photon absorption cross section. The electronic structure calculations of the repeating units of the polymers indicated the localization of the molecular orbitals in different co-monomers. As the difference between the electron affinities of the co-monomers in the repeating units decreases, the highest occupied and lowest unoccupied molecular orbitals become more distributed. All the measurements suggest that a large difference in the electron affinities of the co-monomers of the polymers contributes to the improvement of the photophysical properties necessary for highly efficient solar cell performance. PECN exhibited excellent photophysical properties, which makes it to be a good candidate for solar cell device applications

Beads on a Chain (BoC) Phenylsilsesquioxane (SQ) Polymers via F^– Catalyzed Rearrangements and ADMET or Reverse Heck Cross-coupling Reactions: Through Chain, Extended Conjugation in 3‑D with Potential for Dendronization

In this paper, we assess the utility of complementary routes to silsesquioxane based compounds using F^– catalyzed coupling to synthesize [vinylSiO_1.5]_<i>x</i>PhSiO_1.5]_{10‑<i>x</i>/12‑<i>x</i>} mixtures followed by copolymerization with divinylbenzene (via ADMET), or using reverse Heck coupling with 1,4-dibromobenzene and 4,4′-dibromo-stilbene to prepare lightly branched, nonlinear BOC systems. In another paper, we describe the use of Heck and Suzuki coupling to synthesize model conjugated <i>p</i>-R-stilbeneSQ BOCs starting from [<i>p</i>-IPh₈SiO_1.5]₈ and coupling with divinylbenzene (DVB) and 1,4-diethynylbenzene (DEB) finding extended 3-D conjugation in the DEB polymers. We find that the reverse Heck coupling (where the linker contains the bromo moieties) works best for these systems giving BoC oligomers with <i>M</i>_n of ∼6 kDa, in which extended excited state conjugation is observed for 1,4-dibromobenzene linked systems through ∼50+ nm red shifts in the emission spectra compared with DVB linked systems and model compounds. We compare and contrast the photophysical properties of the two sets of BOCs and the system where the conjugation length of the linker changes from divinylbenzene to divinylstilbene. We find that for a linker with a longer conjugation length, a red-shifted absorption and emission is observed; however, the difference in emission is much larger for the 1,4-dibromobenzene-linked system as compared to the model compounds, suggesting that a more rigid linker contributes to better orbital overlap with the cage and/or phenyl groups, increasing excited state conjugation interactions