Unprecedented External Electric Field Effects on <i>S</i>‑Nitrosothiols: Possible Mechanism of Biological Regulation?

Reactions of <i>S</i>-nitrosothiols (RSNOs), ubiquitous carriers of nitric oxide NO and its physiological activity, are tightly regulated in biological systems, but the mechanisms of this regulation are not well understood. Here, we computationally demonstrate that RSNO properties can be dramatically altered by biologically accessible external electric fields (EEFs) by modulation of the two minor antagonistic resonance structures of RSNOs, which have opposite formal charge distributions and bonding patterns. As these resonance contributions relate to the two competing modes of RSNO reactivity with nucleophiles, via N- or S-atom directed nucleophilic attack, EEFs are predicted to be efficient in controlling biologically important RSNO reactions with thiols. For instance, EEF catalysis might be one of the mechanisms behind the high selectivity of protein trans-<i>S</i>-nitrosation reactions, or putative nitroxyl HNO formation via RSNO <i>S</i>-thiolation reactions

Key Role of End-Capping Groups in Optoelectronic Properties of Poly‑<i>p</i>‑phenylene Cation Radicals

Poly-<i>p</i>-phenylenes (PPs) are prototype systems for understanding the charge transport in π-conjugated polymers. In a combined computational and experimental study, we demonstrate that the smooth evolution of redox and optoelectronic properties of PP cation radicals toward the polymeric limit can be significantly altered by electron-donating <i>iso</i>-alkyl and <i>iso</i>-alkoxy end-capping groups. A multiparabolic model (MPM) developed and validated here rationalizes this unexpected effect by interplay of the two modes of hole stabilization: due to the framework of equivalent <i>p</i>-phenylene units and due to the electron-donating end-capping groups. A symmetric, bell-shaped hole in unsubstituted PPs becomes either slightly skewed and shifted toward an end of the molecule in <i>iso</i>-alkyl-capped PPs or highly deformed and concentrated on a terminal unit in PPs with strongly electron-donating <i>iso</i>-alkoxy capping groups. The MPM shows that the observed linear 1/<i>n</i> evolution of the PP cation radical properties toward the polymer limit originates from the hole stabilization due to the growing chain of <i>p</i>-phenylene units, while shifting of the hole toward electron-donating end-capping groups leads to early breakdown of these 1/<i>n</i> dependencies. These insights, along with the readily applicable and flexible multistate parabolic model, can guide studies of complex donor–spacer–acceptor systems and doped molecular wires to aid the design of the next generation materials for long-range charge transport and photovoltaic applications

Energy Gap between the Poly‑<i>p</i>‑phenylene Bridge and Donor Groups Controls the Hole Delocalization in Donor–Bridge–Donor Wires

Poly-<i>p</i>-phenylene wires are critically important as charge-transfer materials in photovoltaics. A comparative analysis of a series of poly-<i>p</i>-phenylene (^{<i><b>R</b></i>}<b>PP</b>_{<i><b>n</b></i>}) wires, capped with isoalkyl (^{<i><b>iA</b></i>}<b>PP</b>_{<i><b>n</b></i>}), alkoxy (^{<i><b>RO</b></i>}<b>PP</b>_{<i><b>n</b></i>}), and dialkylamino (^{<i><b>R2N</b></i>}<b>PP</b>_{<i><b>n</b></i>}) groups, shows unexpected evolution of oxidation potentials, i.e., decrease (−260 mV) for ^{<i><b>iA</b></i>}<b>PP</b>_{<i><b>n</b></i>}, while increase for ^{<i><b>RO</b></i>}<b>PP</b>_{<i><b>n</b></i>} (+100 mV) and ^{<i><b>R2N</b></i>}<b>PP</b>_{<i><b>n</b></i>} (+350 mV) with increasing number of <i>p</i>-phenylenes. Moreover, redox/optical properties and DFT calculations of ^{<i><b>R2N</b></i>}<b>PP</b>_{<i><b>n</b></i>}/^{<i><b>R2N</b></i>}<b>PP</b>_{<i><b>n</b></i>}^+• further show that the symmetric bell-shaped hole distribution distorts and shifts toward one end of the molecule with only 4 <i>p</i>-phenylenes in ^{<i><b>R2N</b></i>}<b>PP</b>_{<i><b>n</b></i>}^+•, while shifting of the hole occurs with 6 and 8 <i>p</i>-phenylenes in ^{<i><b>RO</b></i>}<b>PP</b>_{<i><b>n</b></i>}^+• and ^{<i><b>iA</b></i>}<b>PP</b>_{<i><b>n</b></i>}^+•, respectively. Availability of accurate experimental data on highly electron-rich dialkylamino-capped ^{<i><b>R2N</b></i>}<b>PP</b>_{<i><b>n</b></i>} together with ^{<i><b>RO</b></i>}<b>PP</b>_{<i><b>n</b></i>} and ^{<i><b>iA</b></i>}<b>PP</b>_{<i><b>n</b></i>} allowed us to demonstrate, using our recently developed Marcus-based multistate model (MSM), that an increase of oxidation potentials in ^{<i><b>R2N</b></i>}<b>PP</b>_{<i><b>n</b></i>} arises due to an interplay between the electronic coupling (<i>H</i>_ab) and energy difference between the end-capped groups and bridging phenylenes (Δε). A comparison of the three series of ^{<i><b>R</b></i>}<b>PP</b>_{<i><b>n</b></i>} with varied Δε further demonstrates that decrease/increase/no change in oxidation energies of ^{<i><b>R</b></i>}<b>PP</b>_{<i><b>n</b></i>} can be predicted based on the energy gap Δε and coupling <i>H</i>_ab, i.e., decrease if Δε < <i>H</i>_ab (i.e., ^{<i><b>iA</b></i>}<b>PP</b>_{<i><b>n</b></i>}), increase if Δε > <i>H</i>_ab (i.e., ^{<i><b>R2N</b></i>}<b>PP</b>_{<i><b>n</b></i>}), and minimal change if Δε ≈ <i>H</i>_ab (i.e., ^{<i><b>RO</b></i>}<b>PP</b>_{<i><b>n</b></i>}). MSM also reproduces the switching of the nature of electronic transition in higher homologues of ^{<i><b>R2N</b></i>}<b>PP</b>_{<i><b>n</b></i>}^+• (<i>n</i> ≥ 4). These findings will aid in the development of improved models for charge-transfer dynamics in donor–bridge–acceptor systems

Ask Not How Many, But Where They Are: Substituents Control Energetic Ordering of Frontier Orbitals/Electronic Structures in Isomeric Methoxy-Substituted Dibenzochrysenes

Redox properties of polycyclic aromatic hydrocarbons (PAHs) can be modulated by substitution with electron-rich groups. Here we show, using the example of dibenzo­[g,p]­chrysene (DBC), that substitution <i>position</i> (i.e., <i>meta</i> vs <i>para</i>) alters the energetic ordering of frontier molecular orbitals (FMOs), leading to cation radicals with altered electronic structures and thereby redox/optical properties. In particular, incorporation of four methoxy groups in parent DBC at <i>meta</i> positions similarly impacts the energies of phenanthrene-like HOMO and biphenyl-like HOMO-1, while their incorporation at <i>para</i> position swaps energetic ordering of HOMO and HOMO-1. We demonstrate that a straightforward analysis of FMOs provides valuable insight toward the rational design of novel PAHs with tailored redox properties

Ask Not How Many, But Where They Are: Substituents Control Energetic Ordering of Frontier Orbitals/Electronic Structures in Isomeric Methoxy-Substituted Dibenzochrysenes

Redox properties of polycyclic aromatic hydrocarbons (PAHs) can be modulated by substitution with electron-rich groups. Here we show, using the example of dibenzo­[g,p]­chrysene (DBC), that substitution <i>position</i> (i.e., <i>meta</i> vs <i>para</i>) alters the energetic ordering of frontier molecular orbitals (FMOs), leading to cation radicals with altered electronic structures and thereby redox/optical properties. In particular, incorporation of four methoxy groups in parent DBC at <i>meta</i> positions similarly impacts the energies of phenanthrene-like HOMO and biphenyl-like HOMO-1, while their incorporation at <i>para</i> position swaps energetic ordering of HOMO and HOMO-1. We demonstrate that a straightforward analysis of FMOs provides valuable insight toward the rational design of novel PAHs with tailored redox properties

No Longer a Complex, Not Yet a Molecule: A Challenging Case of Nitrosyl <i>O</i>‑Hydroxide, HOON

HOON might be an elusive intermediate of atmospheric photochemical reactions of HONO or recombination of the parent nitrene HN and molecular oxygen. However, no reliable data on HOON structure and stability are available, and the nature of the O–O bond is not well understood. In this study, we used high-level single- [CCSD­(T) and, CCSDTQ] and multireference [CASPT2, MR-AQCC] ab initio calculations to determine properties of HOON: geometry, harmonic and anharmonic vibrational frequencies, thermodynamic stability, and electronic structure. HOON has bonding minima only in the ¹A′ electronic state that correspond to cis- and trans-conformers; <i>trans</i>-HOON is more stable by 6.4–8.5 kJ/mol. The O–O bond in <i>trans</i>-HOON is unusually long, <i>R</i>(O–O) = 1.89 Å, and weak, <i>D</i>(O–O) = 33.3 kJ/mol; however, <i>trans</i>-HOON might be stable enough to be identified in cryogenic matrices. Though the electronic structure of the NO moiety in HOON most resembles nitric oxide, some nitrene character as well nitrosyl cation character are also important; therefore, the current name of HOON, hydroperoxynitrene, is misleading; instead, we propose the name “nitrosyl <i>O</i>-hydroxide” or “isonitrosyl hydroxide”

Hückel Theory + Reorganization Energy = Marcus–Hush Theory: Breakdown of the 1/<i>n</i> Trend in π‑Conjugated Poly‑<i>p</i>‑phenylene Cation Radicals Is Explained

Among the π-conjugated poly-<i>p</i>-phenylene wires, fluorene-based poly-<i>p</i>-phenylene (<b>FPP</b>_<i>n</i>) wires have been extensively explored for their potential as charge-transfer materials in functional photovoltaic devices. Herein, we undertake a systematic study of the redox and optical properties of a set of <b>FPP</b>_<i>n</i> (<i>n</i> = 2–16) wires. We find that, while their absorption maxima (ν_abs) follow a linear trend against cos­[π/(<i>n</i> + 1)] up to the polymeric limit, redox potentials (<i>E</i>_ox) show an abrupt breakdown from linearity beginning at <i>n</i> ∼ 8. These observations prompted the development of a generalized model to describe the unusual evolution of redox and optical properties of poly-<i>p</i>-phenylene wires. We show that the cos­[π/(<i>n</i> + 1)], commonly expressed as 1/<i>n</i>, dependence of the properties of various π-conjugated wires has its origin in Hückel molecular orbital (HMO) theory, which however, fails to predict the evolution of the redox potentials of these wires, as the oxidation-induced structural/solvent reorganization is unaccounted for in the original formulation of HMO theory. Accordingly, aided by DFT calculations, we introduce here a modified HMO theory that incorporates the reorganization energy (Δα) and coupling (β) and show that the modified theory provides an accurate description of the oxidized <b>FPP</b>_<i>n</i> wires, reproducing the breakdown in the linear cos­[π/(<i>n</i> + 1)] trend. A comparison with the Marcus-based multistate model (MSM), where reorganization (λ) and coupling (<i>H</i>_ab) are introduced by design with the aid of empirically adjusted parameters, further confirms that the structural/solvent reorganization limits hole delocalization to ∼8 <i>p</i>-phenylene units and leads to the breakdown in the linear evolution of the redox properties against cos­[π/(<i>n</i> + 1)]. The predictive power of the modified HMO theory and MSM offer new tools for rational design of the next-generation, long-range charge-transfer materials for photovoltaics and molecular electronics applications