Benchmark study of ionization potentials and electron affinities of armchair single-walled carbon nanotubes using density functional theory

The novel non-fully-fused ring small molecule acceptors: End-capped modification investigation

End-capped modification is an efficacious strategy for developing high-performance acceptor materials. In this paper, the experimentally synthesized A-D-A'-D-A type non-fully-fused ring acceptor IDTBT-4F (R) was used as a reference molecule, and five small molecule acceptors for R1-R5 were investigated by changing R's terminal functional groups. By using DFT/B3PW91/6-31G (d,p) method, the ground-state structures of all molecules were studied. The absorption spectra of these acceptors were gained by the TD-DFT/MPW1PW91/6-31G (d,p) approach. Meanwhile, the charge density difference and transition density matrix were analyzed effectively. It can be observed that, compared to the R molecule, all developed molecules exhibited narrower energy gaps, larger absorption wavelengths, more red-shifted absorption spectra, lower excitation energies, higher dipole moment and greater electron-accepting capacity. The strategy of functional group substitution is superior to halogen substitution in improving the aforementioned parameters. Both terminal π-extension and end-group chlorination strategies can synergistically enhance molecular performance. In addition, we also calculated the electron mobility of the dimers constructed by all the molecules, among which R1 and R4 molecules designed with -COOCH3 functional group substitution and R2 molecule with terminal chlorination achieved superior electron mobility compared to R molecule due to their significant electronic coupling. Overall, the study shows that the designed molecules can be highly effective candidates for applications of organic solar cells.

Antioxidant capacity of simplified oxygen heterocycles and proposed derivatives by theoretical calculations

CONTEXT

Some structural properties can be involved in the antioxidant capacity of several polyphenol derivatives, among them their simplified structures. This study examines the contribution of simplified structure for the antioxidant capacity of some natural and synthetic antioxidants. The resonance structures were related to the π-type electron system of carbon-carbon double bonds between both phenyl rings. Trans-resveratrol, phenyl-benzofuran, phenyl-indenone, and benzylidene-benzofuranone are the best basic antioxidant templates among the simplified derivatives studied here. Additionally, the stilbene moiety was found on the molecules with the best antioxidant capacity. Furthermore, our investigation suggests that these compounds can be used as antioxidant scaffold for designing and developing of new promising derivatives.

METHODS

To investigate the structure-antioxidant capacity for sixteen simplified natural and proposed derivatives we have employed density functional theory and used Gaussian 09. Our DFT calculations were performed using the B3LYP functional and the 6-31+G(d,p) basis set. All electron transfer mechanisms were investigated by using values of HOMO, ionization potential, energy affinity, stabilization energies, and spin density distributions.

Electron-Affinity Time-Dependent Density Functional Theory: Formalism and Applications to Core-Excited States

The lack of particle-hole attraction and orbital relaxation within time-dependent density functional theory (TDDFT) lead to extreme errors in the prediction of K-edge X-ray absorption spectra (XAS). We derive a linear-response formalism that uses optimized orbitals of the n - 1-electron system as the reference, building orbital relaxation and a proper hole into the initial density. Our approach is an exact generalization of the static-exchange approximation that ameliorates the particle-hole interaction error associated with the adiabatic approximation and reduces errors in TDDFT XAS by orders of magnitude. With a statistical performance of just 0.5 eV root-mean-square error and the same computational scaling as TDDFT under the core-valence separation approximation, we anticipate that this approach will be of great utility in XAS calculations of large systems.

Quantum Coupling of Two Atomic Defects in a Carbon Nanotube Semiconductor

Chemical defects can create organic color centers in the graphitic lattice of single-walled carbon nanotubes. However, the underlying physics remains somewhat of a mystery. Here we show that two sp³ atomic defects can interact with each other in a way reminiscent of atoms bonding to form molecules. Each defect creates an atom-like mid-gap state within the band gap of the nanotube semiconductor. Two such defects, when brought close to each other, interact to form a split pair of orbitals akin to two hydrogen atoms covalently bonding to form a H₂ molecule. This unexpected finding may help in understanding the nature of atomic defects in solids and provide a fresh perspective to the engineering of these color centers.

Time-resolved relaxation and fragmentation of polycyclic aromatic hydrocarbons investigated in the ultrafast XUV-IR regime

Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10-100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH^*, PAH^+* and PAH^2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH²⁺ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.

Mono- and Di-Pyrene [60]Fullerene and [70]Fullerene Derivatives as Potential Components for Photovoltaic Devices

In the present work, we report the successful synthesis and characterization of six (two new) fullerene mono- and di-pyrene derivatives based on C₆₀ and C₇₀ fullerenes. The synthesized compounds were characterized by spectral methods (ESI-MS, ¹H-NMR, ¹³C-NMR, UV-Vis, FT-IR, photoluminescence and photocurrent spectroscopy). The energy of HOMO and LUMO levels and the band gaps were determined from cyclic voltammetry and compared with the theoretical values calculated according to the DFT/B3LYP/6-31G(d) and DFT/PBE/6-311G(d,p) approach for fully optimized molecular structures at the DFT/B3LYP/6-31G(d) level. Efficiency of solar cells made of PTB7: C₆₀ and C₇₀ fullerene pyrene derivatives were analyzed based on the determined energy levels of the HOMO and LUMO orbitals of the derivatives as well as the extensive spectral results of fullerene derivatives and their mixtures with PTB7. As a result, we found that the electronic and spectral properties, on which the efficiency of a photovoltaic cell is believed to depend, slightly changes with the number and type of pyrene substituents on the fullerene core. The efficiency of constructed solar cells largely depends on the homogeneity of the photovoltaic layer, which, in turn, is a derivative of the solubility of fullerene derivatives in the solvent used to apply these layers by spincoating.

pH Controlled Intersystem Crossing and Singlet Oxygen Generation of 8-Azaadenine in Aqueous Solution

Azabases are intriguing DNA and RNA analogues and have been used as effective antiviral and anticancer medicines. However, photosensitivity of these drugs has also been reported. Here, pH-controlled intersystem crossing (ISC) process of 9H 8-azaadenine (8-AA) in aqueous solution is reported. Broadband transient absorption measurements reveal that the hydrogen atom at N9 position can greatly affect ISC of 8-AA and ISC is more favorable when 8-AA is in its neutral form in aqueous solution. The initial excited ππ* (S₂ ) state evolves through ultrafast internal conversion (IC) (4.2 ps) to the lower-lying nπ* state (S₁ ), which further stands as a door way state for ISC with a time constant of 160 ps. The triplet state has a lifetime of 6.1 μs. On the other hand, deprotonation at N9 position promotes the IC from the ππ* (S₂ ) state to the ground state (S₀ ) and the lifetime of the S₂ state is determined to be 10 ps. The experimental results are further supported by time-dependent density functional theory (TDDFT) calculations. Singlet oxygen generation yield is measured to be 13.8 % for the neutral 8-AA while the deprotonated one exhibit much lower yield (<2 %), implying that this compound could be a potential pH-sensitized photodynamic therapy agent.