Synthesis and Photophysical Properties of Small Helical Carbon Nanohoops Combined with Computational Studies on Racemization Pathway.

We herein report the facile synthesis of two helical carbon nanorings with small ring sizes, cyclo[6]paraphenylene-1,5-naphthylene ([6]CPPNap1,5), and cyclo[6]paraphenylene-1,5-anthrylene ([6]CPPAn1,5). The structures were determined by NMR and HR-MS. X-ray single-crystal data of [6]CPPNap1,5 was also achieved. The strain energy and racemization processes were investigated by DFT calculations. The reduced ring sizes result in increased ring strain and elevated energy barriers. The photophysical properties were studied by UV-Vis absorption, fluorescence emission, and time-resolved fluorescence decay.

Dual-Emissive Monoruthenium Complexes of N(CH3)-Bridged Ligand: Synthesis, Characterization, and Substituent Effect.

Three monoruthenium complexes 1(PF6)2-3(PF6)2 bearing an N(CH3)-bridged ligand have been synthesized and characterized. These complexes have a general formula of [Ru(bpy)2(L)](PF6)2, where L is a 2,5-di(N-methyl-N'-(pyrid-2-yl)amino)pyrazine (dapz) derivative with various substituents, and bpy is 2,2'-bipyridine. The photophysical and electrochemical properties of these compounds have been examined. The solid-state structure of complex 3(PF6)2 is studied by single-crystal X-ray analysis. These complexes show two well-separated emission bands centered at 451 and 646 nm (Δλmax = 195 nm) for 1(PF6)2, 465 and 627 nm (Δλmax = 162 nm) for 2(PF6)2, and 455 and 608 nm (Δλmax = 153 nm) for 3(PF6)2 in dilute acetonitrile solution, respectively. The emission maxima of the higher-energy emission bands of these complexes are similar, while the lower-energy emission bands are dependent on the electronic nature of substituents. These complexes display two consecutive redox couples owing to the stepwise oxidation of the N(CH3)-bridged ligand and ruthenium component. Moreover, these experimental observations are analyzed by computational investigation.

Sum-frequency vibrational spectroscopy of centrosymmetric molecule at interfaces.

The centrosymmetric benzene molecule has zero first-order electric dipole hyperpolarizability, which results in no sum-frequency vibrational spectroscopy (SFVS) signal at interfaces, but it shows very strong SFVS experimentally. We perform a theoretical study on its SFVS, which is in good agreement with the experimental results. Its strong SFVS mainly comes from the interfacial electric quadrupole hyperpolarizability rather than the symmetry-breaking electric dipole, bulk electric quadrupole, and interfacial and bulk magnetic dipole hyperpolarizabilities, which provides a novel and completely unconventional point of view.

Sum-frequency vibrational spectroscopy of methanol at interfaces due to Fermi resonance.

Sum-frequency nonlinear spectroscopy is a powerful tool in investigating physical and chemical properties at gas/liquid, gas/solid, liquid/liquid and liquid/solid interfaces. Fermi resonance is a well-documented anharmonic phenomenon related to molecular vibrational coupling and the energy transfer phenomenon that exists within and between molecules. Such a phenomenon is widely used in the fields of materials, biology and chemistry. Combining density functional theory and molecular dynamics simulation, we present a method of studying sum-frequency vibrational spectroscopy for the CH3 group of methanol at interfaces due to Fermi resonance. The calculated spectroscopic data agree with the experiment and provide a novel and untraditional point of view with respect to traditional approaches.

Doubly Resonant Sum-Frequency Vibrational Spectroscopy of 1,1'-Bi-2-naphthol Chiral Solutions Due to the Nonadiabatic Effect.

Second-order nonlinear spectroscopy is a powerful tool in exploring significant physical and chemical characteristics at various interfaces and on chiral systems. We present a method of computing the nonadiabatic couplings between the different excited electronic states with time-dependent density functional theory and then study doubly resonant sum-frequency vibrational spectroscopy (SFVS) of chiral solutions due to the nonadiabatic, Franck-Condon, and Herzberg-Teller (HT) effects. The calculations for R-1,1'-bi-2-naphthol show that the theoretical spectra agree with experiment, and the nonadiabatic corrections are comparable with the HT terms or even larger for some vibrational modes, which is different from the mechanism of SFVS off electronic resonance. Doubly resonant SFVS may be a useful method of studying the nonradiative transition and nonadiabatic effect between the excited electronic states.

Theoretical Study on the Mechanism of the Acylate Reaction of ß-Lactamase.

Using density functional theory and a cluster approach, we study the reaction potential surface and compute Gibbs free energies for the acylate reaction of ß-lactamase with penicillin G, where the solvent effect is important and taken into consideration. Two reaction paths are investigated: one is a multi-step process with a rate-limit energy barrier of 19.1 kcal/mol, which is relatively small, and the reaction can easily occur; the other is a one-step process with a barrier of 45.0 kcal/mol, which is large and thus makes the reaction hard to occur. The reason why the two paths have different barriers is explained.

Theoretical study on resonance Raman spectra of meso-tetrakis(3,5-di-tertiarybutylphenyl)-porphyrin due to the second-order Herzberg-Teller mechanism.

Using quantum chemistry computations TDDFT//B3LYP/6-31G(d), we study resonance Raman spectra of the Q band for meso-tetrakis(3,5-di-tertiarybutylphenyl)-porphyrin (H2TBPP) molecule due to the Franck-Condon and non-Condon mechanisms including the Herzberg-Teller and second-order Herzberg-Teller terms. Generally, the Herzberg-Teller terms are large. However, for some vibrational modes, the second-order Herzberg-Teller terms are the strongest and dominate resonance Raman spectra, which may also impact on fluorescence and absorption spectra. Hence, the Taylor expansion of the electric dipole transition moment with respect to the normal coordinates at the equilibrium structure of the ground electronic state may not converge for H2TBPP. A method to solve this problem is suggested.

Herzberg-Teller Effect Predominates in Sum-Frequency Vibrational Spectroscopy of Limonene Chiral Liquids.

We theoretically study the bulk sum-frequency vibrational spectroscopy of chiral liquids under the influence of the Franck-Condon, Herzberg-Teller, and nonadiabatic effects. With quantum chemistry computations we calculate the chiral spectra for the R-limonene molecule. When we compare the theoretical and experimental spectra, we find that the Herzberg-Teller effect under the Born-Oppenheimer approximation, instead of the nonadiabatic effect, predominates in the chiral spectra.

Sum-frequency vibrational spectroscopy of limonene chiral liquids due to the nonadiabatic effect.

Using quantum computations we study sum-frequency vibrational spectroscopy of limonene chiral liquids due to the nonadiabatic effect in the non-resonant case for the first time. The nonadiabatic effect has an important impact on non-resonant antisymmetric polarizability and chiral sum-frequency vibrational spectroscopy. The theoretical spectroscopy agrees with the experimental spectroscopy. However, the nonadiabatic effect only has a small influence on non-resonant Raman. Bulk sum-frequency vibrational spectroscopy may become a powerful method of investigating the nonadiabatic effect and the nonradiative transition between excited electronic states for chiral molecules.

Theoretical study of vibrational energy transfer of free OH groups at the water-air interface.

Recent experimental studies have shown that the vibrational dynamics of free OH groups at the water-air interface is significantly different from that in bulk water. In this work, by performing molecular dynamics simulations and mixed quantum/classical calculations, we investigate different vibrational energy transfer pathways of free OH groups at the water-air interface. The calculated intramolecular vibrational energy transfer rate constant and the free OH bond reorientation time scale agree well with the experiment. It is also found that, due to the small intermolecular vibrational couplings, the intermolecular vibrational energy transfer pathway that is very important in bulk water plays a much less significant role in the vibrational energy relaxation of the free OH groups at the water-air interface.

General Applicable Frequency Map for the Amide-I Mode in ß-Peptides.

In this work, a general applicable amide-I vibrational frequency map (GA map) for ß-peptides in a number of common solvents was constructed, based on a peptide derivative, N-ethylpropionamide (NEPA). The map utilizes force fields at the ab initio computational level to accurately describe molecular structure and solute-solvent interactions, and also force fields at the molecular mechanics level to take into account long-range solute-solvent interactions. The results indicate that the GA map works reasonably for mapping the vibrational frequencies of the amide-I local-modes for ß-peptides, holding promises for understanding the complicated infrared spectra of the amide-I mode in ß-polypeptides.

A theoretical study of sum-frequency generation for chiral solutions near electronic resonance.

We present a method of calculating sum-frequency generation (SFG) for chiral solutions near electronic resonance including the vibronic contributions, which give reasonable SFG intensities and show the Franck-Condon progressions for SFG. When studying R-1,1'-bi-2-naphthol (R-BN), we found that the calculated spectrum is in good agreement with the experimental one (Phys. Rev. Lett., 2001, 87, 113001). Then we apply this method to investigate SFG for chiral arabinose solutions. Theoretical results show that it may be difficult to observe the corresponding SFG even when the sum-frequency is exactly in resonance with the low-lying excited electronic states. Furthermore, we discuss the reason why SFG of chiral arabinose solutions is small.

Rational Design of Charge-Transfer Interactions in Halogen-Bonded Co-crystals toward Versatile Solid-State Optoelectronics.

Charge-transfer (CT) interactions between donor (D) and acceptor (A) groups, as well as CT exciton dynamics, play important roles in optoelectronic devices, such as organic solar cells, photodetectors, and light-emitting sources, which are not yet well understood. In this contribution, the self-assembly behavior, molecular stacking structure, CT interactions, density functional theory (DFT) calculations, and corresponding physicochemical properties of two similar halogen-bonded co-crystals are comprehensively investigated and compared, to construct an "assembly-structure-CT-property" relationship. Bpe-IFB wire-like crystals (where Bpe = 1,2-bis(4-pyridyl)ethylene and IFB = 1,3,5-trifluoro-2,4,6-triiodobenzene), packed in a segregated stacking form with CT ground and excited states, are measured to be quasi-one-dimensional (1D) semiconductors and show strong violet-blue photoluminescence (PL) from the lowest CT1 excitons (ΦPL = 26.1%), which can be confined and propagate oppositely along the 1D axial direction. In comparison, Bpe-F4DIB block-like crystals (F4DIB = 1,4-diiodotetrafluorobenzene), packed in a mixed stacking form without CT interactions, are determined to be insulators and exhibit unique white light emission and two-dimensional optical waveguide property. Surprisingly, it seems that the intrinsic spectroscopic states of Bpe and F4DIB do not change after co-crystallization, which is also confirmed by theoretical calculations, thus offering a new design principle for white light emitting materials. More importantly, we show that the CT interactions in co-crystals are related to their molecular packing and can be triggered or suppressed by crystal engineering, which eventually leads to distinct optoelectronic properties. These results help us to rationally control the CT interactions in organic D-A systems by tuning the molecular stacking, toward the development of a fantastic "optoelectronic world".

Revealing the charge-transfer interactions in self-assembled organic cocrystals: two-dimensional photonic applications.

A new crystal of a charge-transfer (CT) complex was prepared through supramolecular assembly and it has unique two-dimensional (2D) morphology. The CT nature of the ground and excited states of this new Bpe-TCNB cocrystal (BTC) were confirmed by electron spin resonance measurements, spectroscopic studies, and theoretical calculations, thus providing a comprehensive understanding of the CT interactions in organic donor-acceptor systems. And the lowest CT1 excitons are responsible for the efficient photoluminescence (Φ(PL)=19%), which can actively propagate in individual 2D BTCs without anisotropy, thus implying that the optical waveguide property of the crystal is not related to the molecular stacking structure. This unique 2D CT cocrystal exhibits potential for use in functional photonic devices in the next-generation optoelectronic communications.

Theoretical investigation of quadrupole contributions to surface sum-frequency vibrational spectroscopy.

We study the effect of electric transition quadrupole moments on surface sum-frequency vibrational spectroscopy (SFVS). The SSP, PPP and SPS effective sum-frequency susceptibilities are derived from the nonzero macroscopic susceptibility tensors and related to molecular quadrupole polarizabilities. Using time-dependent density functional theory, we calculate the quadrupole susceptibilities of R-limonene liquids for the first time. The results indicate that quadrupole contributions have a significant influence on SFVS transmitted signals in the SPS polarization combination. We also suggest that the SPS spectra may be used as a general technique for detecting electric transition quadrupole moments.

Metal chelation-assisted amine-amine electronic coupling through the 4,4'-positions of 2,2'-bipyridine.

A redox-active diamine ligand, 4,4'-bis(di-p-anisylamino)-2,2'-bipyridine (NNbpy), has been prepared. Electrochemical and spectroscopic studies suggest that little electronic coupling is present between two amine groups in NNbpy. After chelation with Ru(bpy)2 (bpy is 2,2'-bipyridine), the resulting complex displays two N(•+/0) processes at +1.02 and +1.16 V versus Ag/AgCl. In the mixed-valent state, rich near-infrared absorptions have been observed, which are believed to consist of multiple metal-to-ligand charge transfer and intervalence charge transfer transitions in the low-energy region. These results suggest that the amine-amine electronic coupling has been enhanced by chelation with Ru(bpy)2. In contrast, no efficient electronic coupling can be realized by chelation with Ir(ppy)2 (ppy is 2'-phenylpyridine) or Re(CO)3Cl. A ruthenium ion-mediated electron transfer mechanism, instead of through-space coupling, has been proposed to explain this phenomenon. For the purpose of comparison, a monoamine-substituted bpy ligand and corresponding Ru(bpy)2 complex have been synthesized and studied. In addition, EPR, DFT, and TDDFT studies have been performed to complement the experimental results.

Theoretical study of the resonance Raman spectra for meso-tetrakis(3,5-di-tertiarybutylphenyl)-porphyrin.

Applying time-dependent density functional theory (TDDFT), we study the resonance Raman spectra for the Q and B bands of the meso-tetrakis(3,5-di-tertiarybutylphenyl)-porphyrin (H2TBPP) molecule including both Raman A term (Franck-Condon term) and Raman B term (Herzberg-Teller term) contributions. It is found that Raman B term can be one order of magnitude larger than Raman A term and dominates resonance Raman for the Q band resonance. In comparison with the recent experimental Raman spectra of H2TBPP with incident light frequency 532nm, we predict the absence of 1580cm(-1) band in the resonance Raman spectra which agrees well with the experimental results, whereas the previous theoretical calculation using non-resonance strategy failed to do so.

Theoretical study of sum-frequency vibrational spectroscopy on limonene surface.

By combining molecule dynamics (MD) simulation and quantum chemistry computation, we calculate the surface sum-frequency vibrational spectroscopy (SFVS) of R-limonene molecules at the gas-liquid interface for SSP, PPP, and SPS polarization combinations. The distributions of the Euler angles are obtained using MD simulation, the ψ-distribution is between isotropic and Gaussian. Instead of the MD distributions, different analytical distributions such as the δ-function, Gaussian and isotropic distributions are applied to simulate surface SFVS. We find that different distributions significantly affect the absolute SFVS intensity and also influence on relative SFVS intensity, and the δ-function distribution should be used with caution when the orientation distribution is broad. Furthermore, the reason that the SPS signal is weak in reflected arrangement is discussed.

Ozone dissociation to oxygen affected by Criegee intermediate.

The detailed potential energy surfaces for the reactions of Criegee intermediate (CI, H2COO) and formaldehyde (H2CO) with ozone (O3) have been investigated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311++G(2d,2p) level of theory, respectively. New alternative reaction mechanisms, to the one previously proposed (J. Phys. Chem. Lett. 2013, 4, 2525) have been found. The lower barrier of the new mechanism shows that it is easy for H2COO + O3 to dissociate to formaldehyde and oxygen. For the reactions of H2CO with O3 to produce H2COO and O2, we find relatively high energy barriers, which makes the ozone dissociation to oxygen unlikely to be catalyzed by CI.