Structure and nonlinear electrical properties of squaric acid derivatives: a theoretical study of the conformation and deprotonation effects

Third-order nonlinear optical property contrast as self-assembly recognition for nanorings⊃C60

High third-order NLO contrasts tuned by self-assembly can be applied for the recognition of host–guest nanorings⊃C60.

Hydrogen-bond landscapes, geometry and energetics of squaric acid and its mono- and dianions: a Cambridge Structural Database, IsoStar and computational study

As part of a programme of work to extend central-group coverage in the Cambridge Crystallographic Data Centre's (CCDC) IsoStar knowledge base of intermolecular interactions, we have studied the hydrogen-bonding abilities of squaric acid (H2SQ) and its mono- and dianions (HSQ(-) and SQ(2-)) using the Cambridge Structural Database (CSD) along with dispersion-corrected density functional theory (DFT-D) calculations for a range of hydrogen-bonded dimers. The -OH and -C=O groups of H2SQ, HSQ(-) and SQ(2-) are potent donors and acceptors, as indicated by their hydrogen-bond geometries in available crystal structures in the CSD, and by the attractive energies calculated for their dimers with acetone and methanol, which were used as model acceptors and donors. The two anions have sufficient examples in the CSD for their addition as new central groups in IsoStar. It is also shown that charge- and resonance-assisted hydrogen bonds involving H2SQ and HSQ(-) are similar in strength to those made by carboxylate COO(-) acceptors, while hydrogen bonds made by the dianion SQ(2-) are somewhat stronger. The study reinforces the value of squaric acid and its anions as cocrystal formers and their actual and potential importance as isosteric replacements for carboxylic acid and carboxylate functions.

Quantum chemical study of benzimidazole derivatives to tune the second-order nonlinear optical molecular switching by proton abstraction

A novel sequence for reversible second-order nonlinear optical (NLO) molecular switching with protonation/deprotonation has been achieved and tuned as well. The NLO switching with first hyperpolarizabilities (beta(0)) as low as 14 x 10(-30) esu (Off-phase) and as large as 1189 x 10(-30) esu (On-phase) have been computed by using density functional theory (DFT). Remarkably large differences between the beta(0) values of benzimidazole containing chromophores and their deprotonated anions have shown their significant potential for a new type of NLO molecular switching, as (1-(4-methoxyphenyl)-2-(2-thienyl)pyrrolyl)-1,3-benzimidazole anion (1(-)) has a beta(0) value computed to be 61 x 10(-30) esu, which is 4 times larger than its neutral molecule 1. This beta(0) value has been tuned up to 2028 x 10(-30) esu by effective substitutions in the derivatives of 1(-) (1a(-), 1b(-), 1c(-), and 1d(-)). Interestingly, the substituted compounds have illustrated robustly large off-on NLO switching with a difference in beta(0) values of 7, 63, 85 and 75 times larger than their neutral counterparts, respectively. TD-DFT calculations along with natural bond orbital (NBO), frontier molecular orbitals (FMOs) and molecular electrostatic potential (MEP) analyses show that the abstraction of an imido proton brings about a change in push-pull configurations resulting in a red shift for both absorption and emission spectra which subsequently leads to a high performance second-order NLO molecular switching. A similar trend of NLO switching in F(-) compounds of these chromophores has also been observed with significantly large beta(0) values having analogous electro-optical properties like deprotonated anions. Furthermore, gas-phase acidity (GPA) calculations for the neutral molecule 1 and its derivatives (1a, 1b, 1c, and 1d) have also revealed that these are rationally potent nitrogen acids and can easily be dissociated to produce stable deprotonated anions.

Structural and spectroscopic analysis of hydrogensquarates of glycine-containing tripeptides

The hydrogensquarates of glycine-containing tripeptides glycylglycylglycine (H-Gly-Gly-Gly-OH), glycylglycylmethionine (H-Gly-Gly-Met-OH), and methionylglycylglycine (H-Met-Gly-Gly-OH) have been characterized structurally. Quantum chemical ab initio calculations, solid-state linear-dichroic infrared (IR-LD) spectroscopy, 1H and 13C NMR data, ESI-MS, HPLC-MS/MS, TGV, and DSC methods were employed. The structures consist in a positively charged peptide moiety and a negative hydrogensquarate anion (HSq), stabilized by strong intermolecular hydrogen bonds.

Energy of charged states in the acetanilide crystal: trapping of charge-transfer states at vacancies as a possible mechanism for optical damage

Calculations for the acetanilide crystal yield the effective polarizability (16.6 A(3)), local electric field tensor, effective dipole moment (5.41 D), and dipole-dipole energy (-12.8 kJ/mol). Fourier-transform techniques are used to calculate the polarization energy P for a single charge in the perfect crystal (-1.16 eV); the charge-dipole energy W(D) is zero if the crystal carries no bulk dipole moment. Polarization energies for charge-transfer (CT) pairs combine with the Coulomb energy E(C) to give the screened Coulomb energy E(scr); screening is nearly isotropic, with E(scr) approximately E(C)/2.7. For CT pairs W(D) reduces to a term deltaW(D) arising from the interaction of the charge on each ion with the change in dipole moment on the other ion relative to the neutral molecule. The dipole moments calculated by density-functional theory methods with the B3LYP functional at the 6-311++G(**) level are 3.62 D for the neutral molecule, changing to 7.13 D and 4.38 D for the anion and cation, relative to the center of mass. Because of the large change in the anion, deltaW(D) reaches -0.9 eV and modifies the sequence of CT energies markedly from that of E(scr), giving the lowest two CT pairs at -1.98 eV and -1.41 eV. The changes in P and W(D) near a vacancy are calculated; W(D) changes for the individual charges because the vacancy removes a dipole moment and modifies the crystal dielectric response, but deltaW(D) and E(C) do not change. A vacancy yields a positive change DeltaP that scatters a charge or CT pair, but the change DeltaW(D) can be negative and large enough to outweigh DeltaP, yielding traps with depths that can exceed 150 meV for single charges and for CT pairs. Divacancies yield traps with depths nearly equal to the sum of those produced by the separate vacancies and so they can exceed 300 meV. These results are consistent with a mechanism of optical damage in which vacancies trap optically generated CT pairs that recombine and release energy; this can disrupt the lattice around the vacancy, thereby favoring trapping and recombination of CT pairs generated by subsequent photon absorption, leading to further lattice disruption. Revisions to previous calculations on trapping of CT pairs in anthracene are reported.