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1. Quantum Simulations of Charge Separation at a Model Donor-Acceptor Interface: Role of Delocalization and Local Packing

   https://doi.org/10.1155/2018/6834908  

   Kelley, Allen ; Patel, Kush ; Bittner, Eric R. ( January 2018 , Advances in Condensed Matter Physics)

   Organic Polymer-based photovoltaic systems offer a viable alternative to more standard solid-state devices for light-harvesting applications. In this study, we investigate the electronic dynamics of a model organic photovoltaic (OPV) heterojunction consisting of polyphenylene vinylene (PPV) oligomers and a [ 6,6 ] -phenyl C61-butyric acid methyl ester (PCBM) blend. Our approach treats the classical molecular dynamics of the atoms within an Ehrenfest mean-field treatment of the π - π ⁎ singly excited states spanning a subset of donor and acceptor molecules near the phase boundary of the blend. Our results indicate that interfacial electronic states are modulated by C=C bond stretching motions and that such motions induce avoided crossings between nearby excited states thereby facilitating transitions from localized excitonic configurations to delocalized charge-separated configurations on an ultrafast time-scale. The lowest few excited states of the model interface rapidly mix allowing low frequency C-C out-of-plane torsions to modulate the potential energy surface such that the system can sample both intermolecular charge-transfer and charge-separated electronic configurations on sub-100 fs time scales. Our simulations support an emerging picture of carrier generation in OPV systems in which interfacial electronic states can rapidly decay into charge-separated and current producing states via coupling to vibronic degrees of freedom. 

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2. BF3–Catalyzed Diels–Alder Reaction between Butadiene and Methyl Acrylate in Aqueous Solution—An URVA and Local Vibrational Mode Study

   https://doi.org/10.3390/catal12040415  

   Freindorf, Marek ; Kraka, Elfi ( April 2022 , Catalysts)

   In this study we investigate the Diels–Alder reaction between methyl acrylate and butadiene, which is catalyzed by BF3 Lewis acid in explicit water solution, using URVA and Local Mode Analysis as major tools complemented with NBO, electron density and ring puckering analyses. We considered four different starting orientations of methyl acrylate and butadiene, which led to 16 DA reactions in total. In order to isolate the catalytic effects of the BF3 catalyst and those of the water environment and exploring how these effects are synchronized, we systematically compared the non-catalyzed reaction in gas phase and aqueous solution with the catalyzed reaction in gas phase and aqueous solution. Gas phase studies were performed at the B3LYP/6-311+G(2d,p) level of theory and studies in aqueous solution were performed utilizing a QM/MM approach at the B3LYP/6-311+G(2d,p)/AMBER level of theory. The URVA results revealed reaction path curvature profiles with an overall similar pattern for all 16 reactions showing the same sequence of CC single bond formation for all of them. In contrast to the parent DA reaction with symmetric substrates causing a synchronous bond formation process, here, first the new CC single bond on the CH2 side of methyl acrylate is formed followed by the CC bond at the ester side. As for the parent DA reaction, both bond formation events occur after the TS, i.e., they do not contribute to the energy barrier. What determines the barrier is the preparation process for CC bond formation, including the approach diene and dienophile, CC bond length changes and, in particular, rehybridization of the carbon atoms involved in the formation of the cyclohexene ring. This process is modified by both the BF3 catalyst and the water environment, where both work in a hand-in-hand fashion leading to the lowest energy barrier of 9.06 kcal/mol found for the catalyzed reaction R1 in aqueous solution compared to the highest energy barrier of 20.68 kcal/mol found for the non-catalyzed reaction R1 in the gas phase. The major effect of the BF3 catalyst is the increased mutual polarization and the increased charge transfer between methyl acrylate and butadiene, facilitating the approach of diene and dienophile and the pyramidalization of the CC atoms involved in the ring formation, which leads to a lowering of the activation energy. The catalytic effect of water solution is threefold. The polar environment leads also to increased polarization and charge transfer between the reacting species, similar as in the case of the BF3 catalyst, although to a smaller extend. More important is the formation of hydrogen bonds with the reaction complex, which are stronger for the TS than for the reactant, thus stabilizing the TS which leads to a further reduction of the activation energy. As shown by the ring puckering analysis, the third effect of water is space confinement of the reacting partners, conserving the boat form of the six-member ring from the entrance to the exit reaction channel. In summary, URVA combined with LMA has led to a clearer picture on how both BF3 catalyst and aqueous environment in a synchronized effort lower the reaction barrier. These new insights will serve to further fine-tune the DA reaction of methyl acrylate and butadiene and DA reactions in general. 

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3. Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state

   https://doi.org/10.1039/c9cp04556b  

   Blodgett, Karl N. ; Sun, Dewei ; Fischer, Joshua L. ; Sibert, Edwin L. ; Zwier, Timothy S. ( October 2019 , Physical Chemistry Chemical Physics)

   Laser-induced fluorescence (LIF) excitation, dispersed fluorescence (DFL), UV–UV-hole burning, and UV-depletion spectra have been collected on methyl anthranilate (MA, methyl 2-aminobenzoate) and its water-containing complex (MA–H 2 O), under jet-cooled conditions in the gas phase. As a close structural analog of a sunscreen agent, MA has a strong absorption due to the S 0 –S 1 transition that begins in the UV-A region, with the electronic origin at 28 852 cm −1 (346.6 nm). Unlike most sunscreens that have fast non-radiative pathways back to the ground state, MA fluoresces efficiently, with an excited state lifetime of 27 ns. Relative to methyl benzoate, inter-system crossing to the triplet manifold is shut off in MA by the strong intramolecular NH⋯OC H-bond, which shifts the 3 nπ* state well above the 1 ππ* S 1 state. Single vibronic level DFL spectra are used to obtain a near-complete assignment of the vibronic structure in the excited state. Much of the vibrational structure in the excitation spectrum is Franck–Condon activity due to three in-plane vibrations that modulate the distance between the NH 2 and CO 2 Me groups, ν 33 (421 cm −1 ), ν 34 (366 cm −1 ), and ν 36 (179 cm −1 ). Based on the close correspondence between experiment and theory at the TD-DFT B3LYP-D3BJ/def2TZVP level of theory, the major structural changes associated with electronic excitation are evaluated, leading to the conclusion that the major motion is a reorientation and constriction of the 6-membered H-bonded ring closed by the intramolecular NH⋯OC H-bond. This leads to a shortening of the NH⋯OC H-bond distance from 1.926 Å to 1.723 Å, equivalent to about a 25% reduction in the H⋯O distance compared to full H-atom transfer. As a result, the excited state process near the S 1 origin is a hydrogen atom dislocation that is brought about primarily by heavy atom motion, since the shortened H-bond distance results from extensive heavy-atom motion, with only a 0.03 Å increase in the NH bond length relative to its ground state value. 

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4. The relationship between the crystal habit and the energy framework pattern: a case study involving halogen bonding on the edge of a covalent bond

   https://doi.org/10.1039/d3ce00316g  

   Torubaev, Yury V. ; Howe, Devin ; Leitus, Gregory ; Rosokha, Sergiy V. ( June 2023 , CrystEngComm)

   The relationship between solid-state supramolecular interactions and crystal habits is highlighted based on experimental and computational analysis of the crystal structure of strong halogen-bonded (HaB) associations between iodine-containing dihalogens (ICl, IBr) with 1,4-diazabicyclo[2,2,2]octane (DABCO) as well as with substituted pyridines and phenazine. The pattern of the energy frameworks and the interplay of the attractive and repulsive interactions in the solid-state associations involving these HaB donors and acceptors directly correlated with their crystal habits. This correlation suggests that analysis of the energy framework serves as a useful tool (complementary to the earlier developed methods) to rationalize and predict the crystal habit. The X-ray structural analysis also revealed that the I⋯N distances in the complexes were in the 2.24–2.54 Å range, i.e. they were much closer to the I⋯N covalent bond length than to the van der Waals separation. The computational analysis of the nature of halogen bonding in these complexes showed delocalization of their molecular orbitals' between donor and acceptors resulting in a substantial charge transfer from the nucleophiles to dihalogens and elongation of the I⋯X bond. As a result, both I⋯N and I⋯X bonds in the strongest complexes ( e.g. , ICl with DABCO or 4-dimethylaminopyridine) are characterized by the comparable Mayer bonds orders of about 0.6, along with the electron and energy densities at their bond critical points of about 0.1 a.u. and −0.02 a.u., respectively. These data as well as the density overlap regions indicator (DORI) point to the covalency of the I⋯N bonding and suggest that the interaction within the IX complexes can be described as (unsymmetrical) hypervalent 3c/4e N⋯I⋯X bonding akin to that in trihalide or halonium ions. 

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5. Shortwave infrared luminescent Pt-nanowires: a mechanistic study of emission in solution and in the solid state

   https://doi.org/10.1039/c7dt02317k  

   Cheadle, Carl ; Ratcliff, Jessica ; Berezin, Mikhail ; Pal'shin, Vadim ; Nemykin, Victor N. ; Gerasimchuk, Nikolay N. ( January 2017 , Dalton Trans.)

   Several complexes of “PtL 2 ” composition containing two cyanoxime anions – 2-oximino-2-cyano- N -piperidineacetamide (PiPCO − ) and 2-oximino-2-cyano- N -morpholylacetamide (MCO − ) – have been obtained and characterized both in solution and in the solid state. Complexes exist as two distinct polymorphs: monomeric yellow complexes and dark-green [PtL 2 ] n 1D polymers, while for the MCO − anion a red, solvent containing dimeric [Pt(MCO) 2 ·DMSO] 2 complex has also been isolated. The interconversion of polymorphs was investigated. The monomeric PtL 2 units are arranged into anisotropic extended solid [PtL 2 ] n polymers with the help of Pt⋯Pt metallophilic interactions. Crystal structures of monomeric PtL 2 (L = PiPCO − , MCO − ) and red dimeric [Pt(MCO) 2 ·DMSO] 2 complexes were determined and revealed the cis -arrangement of cyanoxime anions. The Pt–Pt distance in the “head-to-tail” red dimer was found to be 3.133 Å. The structure of the polymeric [Pt(PiPCO) 2 ] n compound was elucidated using the EXAFS method and evidenced the formation of Pt-wires with ∼3.15 Å intermetallic separation. The EPR spectra of both 1D polymers at variable temperatures indicate the absence of Pt( iii ) species. Both pure dark-green [PtL 2 ] n polymers showed a considerable room temperature electrical conductivity of 20–30 S cm −1 , which evidences the formation of a mixed valence Pt( ii )/Pt( iv ) system. We discovered that these 1D polymeric [PtL 2 ] n complexes show an intense NIR fluorescence beyond 1000 nm, while yellow monomeric PtL 2 complexes are not emissive at all. The room temperature excitation spectra of 1D polymeric [PtL 2 ] n complexes demonstrated their strong emission beyond 1000 nm regardless of the used excitation wavelength between 350 and 800 nm, which is typical of systems with delocalized charge carriers. For the first time the formation of mixed valence “metal wires” held together by metallophilic interactions is directly linked both with an intense fluorescence in the NIR region of the spectrum and with the electrical conductivity. The effect of the concentration of [PtL 2 ] n complexes dispersed in the dielectric salt matrix on the photoluminescence wavelength and intensity was investigated. Both polymers show a quantum yield that is remarkably high for this region of the spectrum, reaching ∼2%. Variable temperature emission of polymeric [PtL 2 ] n in the −190–+60 °C range was studied as well. 

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