Hongyan Xiao

No Straight Path: Roaming in Both Ground- and Excited-State Photolytic Channels of NO3 → NO + O2

Exclusive Roaming How do polyatomic molecules fall apart? The basic model, supported by centuries of chemical theory and experiment, invokes a series of internal rearrangements that lead to a fleeting high-energy transition-state geometry from which lower-energy products emerge. Over the past decade, several molecules have been shown to manifest a competing dissociation mechanism such that alongside trajectories that pass through the transition state, there are energetically accessible pathways that roam around it. Grubb et al. (p. 1075; see the Perspective by Jordan and Kable) showed that the light-induced reaction of NO3 to form NO and O2 proceeded exclusively by roaming. Although there are distinct pathways to the products in two different electronic states, neither one passes through a conventional transition state. A chemical reaction proceeds exclusively by mechanisms that do not pass through a conventional transition state. Roaming mechanisms have recently been observed in several chemical reactions alongside trajectories that pass through a traditional transition state. Here, we demonstrate that the visible light–induced reaction NO3 → NO + O2 proceeds exclusively by roaming. High-level ab initio calculations predict specific NO Λ doublet propensities (orientations of the unpaired electron with respect to the molecular rotation plane) for this mechanism, which we discern experimentally by ion imaging. The data provide direct evidence for roaming pathways in two different electronic states, corresponding to both previously documented photolysis channels that produce NO + O2. More broadly, the results raise intriguing questions about the overall prevalence of this unusual reaction mechanism.

Color-Tuning Mechanism of Firefly Investigated by Multi-Configurational Perturbation Method

This is the first report on a multiconfigurational reference second-order perturbation theory-molecular mechanics study of the color modulation of the observed bioluminescence of the oxyluciferin-luciferase complex of the Japanese genji-botaru firefly using structures according to recent X-ray data. Our theoretical results do not support the experimentally deduced conclusion that the color modulation of the emitted light primarily depends on the size of the compact luciferase protein cavity embedding the excited oxyluciferin molecule. Rather, we find, in agreement with recent experimental observations, that the wavelength of the emitted light depends on the polarity of the microenvironment at the phenol/phenolate terminal of the benzothiazole fragment in oxyluciferin.

Synthesis of novel nonlinear optical chromophore to achieve ultrahigh electro-optic activity

A new diene-conjugated chromophore WJ1 was synthesized with high yield of 36% through an H-bonding induced Vilsmeier reaction. By simple guest-host doping, a large electro-optic efficiency of 337 pm V(-1) at 1310 nm and excellent temporal stability at 75 °C have been achieved in poled films of WJ1/APC with a high loading density of 40 wt%.

Comparison of nonlinear optical chromophores containing different conjugated electron-bridges: the relationship between molecular structure-properties and macroscopic electro-optic activities of materials

In electro-optic (EO) materials, realization of large EO coefficients for organic EO materials requires the simultaneous optimization of chromophore first hyperpolarizability, acentric order, molecular shape etc. As these parameters are complicatedly inter-related, thorough analyses are required to understand the dependence of macroscopic EO activity upon chromophore structure and property. Herein, we presented the synthesis of three chromophores containing different conjugated electron-bridges by acidic and alkaline formylation. Electron-rich moieties thiophene and formyl-thiophene in the different positions of chromophores played the different roles of electron-bridge, site-isolator and electron-isolator, generating intriguing property variations of electron distribution of push–pull structure, intramolecular charge-transfer, solvatochromism, microscopic hyperpolarizability and related density functional theory calculation results. In addition these molecular structure–property relationships were rationally related to the EO activities to understand the impact of microscopic molecular property on macroscopic EO activities of materials.

Synthesis of novel nonlinear optical chromophores: achieving excellent electro-optic activity by introducing benzene derivative isolation groups into the bridge

Three novel second order nonlinear optical (NLO) chromophores based on julolidinyl donors and tricyanofuran (TCF) acceptors linked together via modified polyene π-conjugation with rigid benzene derivative steric hindrance groups (chromophore CL1 and CL2) or unmodified polyene π-conjugation (chromophores CL) moieties as the bridges have been synthesized in good overall yields and systematically characterized. Density functional theory (DFT) was used to calculate the HOMO–LUMO energy gaps and first-order hyperpolarizability (β) of these chromophores. Besides, to determine the redox properties of these chromophores, cyclic voltammetry (CV) experiments were performed. Compared with CL, after introducing benzene derivative steric hindrance groups into the bridge, chromophores CL1 and CL2 had good thermal stabilities with high thermal decomposition temperatures which were 32 °C and 24 °C higher than chromophore CL, respectively. Most importantly, the introduction of rigid steric hindrance groups can effectively reduce dipole–dipole interactions to translate their relatively small β values into bulk high EO activities. By doping chromophores CL, CL1 and CL2 with a high loading of 45 wt% in APC, EO coefficients (r33) of up to 121, 197 and 202 pm V−1 at 1310 nm can be achieved, respectively. The r33 values of new chromophores CL1 and CL2 were about 1.6 times of chromophore CL. The high r33 value, good thermal stability and high yield suggest the potential use of the new chromophores in an nonlinear optical area.

A systematic study of the structure–property relationship of a series of nonlinear optical (NLO) julolidinyl-based chromophores with a thieno[3,2-b]thiophene moiety

A series of nonlinear optical (NLO) chromophores a–d bearing thieno[3,2-b]thiophene (TT) as the conjugated bridge or the lateral moiety have been synthesized and investigated. These chromophores display the same julolidinyl-based electron donor, but different electron acceptors (i.e., 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran, TCF, or malononitrile). The solvatochromic behavior, thermal stability and electrochemical properties were evaluated to study the structure–property relationships. The solvent dependence of dipole moment (μ), static polarizability (α), hyperpolarizability (β) and bond length alternation (BLA) of all chromophores was demonstrated by density functional theory (DFT) calculations. Upon using the D–A–π–A structure, the blue-shifted phenomenon and substantially enhanced microscopic NLO properties of chromophore d were obtained. The electrooptic coefficient (r33) of chromophore a (94 pm V−1 at 1.31 μm) was four times higher than that of chromophore b (∼20 pm V−1), while the calculated hyperpolarizability (β) of chromophore b was five times larger with respect to chromophore a. All the results demonstrated that the TT unit is a highly efficient conjugated bridge, and it has some electronical and sterical effects on macroscopic electrooptic (EO) activity when it is used as the lateral moiety. Guidelines can be proposed for the design of a new series of guest–host polymers including julolidinyl-based chromophores with a TT moiety, which could be useful in organic EO device fabrication.

Auxiliary donor for tetrahydroquinoline-containing nonlinear optical chromophores: enhanced electro-optical activity and thermal stability

A series of chromophores FTC, L1 and L2 have been synthesized based on three different types of electron donors, including diethylaminophenyl, tetrahydroquinolinyl and N-(4-dimethylaminophenyl) tetrahydroquinolinyl groups respectively, with the same thiophene bridges and strong tricyanovinyldihydrofuran (TCF) acceptors. In particular, the donor part of the chromophore L2 was modified with an additional donor N-(4-dimethylaminophenyl) substituent, resulting in enhanced thermal stability and electro-optic activity. Cyclic voltammetry measurements showed that chromophore L2 had a smaller energy gap than chromophore L1 due to the additional donor. Moreover, density functional theory calculations suggested that the molecular quadratic hyperpolarizability (μβ) value of chromophore L2 is 29% and 44% larger than that of chromophores L1 and FTC, respectively. The doped film containing the chromophore L2 showed an r33 value of 100 pm V−1 at the concentration of 25 wt% which is much higher than the EO activity of the chromophore L1 (57 pm V−1) and two times higher than that of the traditional chromophore FTC (39 pm V−1).

CASPT2 Study of Photodissociation Pathways of Ketene

The mechanism of various photodissociation channels of ketene involving the three lowest singlet states (S0, S1, and S2) and the three lowest triplet states (T1, T2, and T3) was investigated by means of the (MS-)CAS(8e,8o)PT2/6-31+G* method. Stationary structures, i.e., global minima (GMs), local minima (LMs), transition states (TSs), minimum energy conical intersections (MECIs), and minima on seam of crossing (MSXs), were explored systematically by the global reaction route mapping (GRRM) strategy. On the basis of these structures, we discussed related dissociation channels starting from S2 that have been studied experimentally with 193-215 nm excitation wavelength. Five working pathways were found for relaxation to the low-lying S0, S1, and T1 potential energy surfaces (PESs) from the Franck-Condon region of S2, and the relaxation is expected to occur very quickly. On these low-lying states, five dissociation channels are open: three CH2 + CO channels for different CH2 electronic states, H + HCCO, and H2 + C2O. Pathways for all of these five channels were identified and discussed, including new minor paths leading to H2 + C2O.

Theoretical Study on the Photodissociation of Methylamine Involving S1, T1, and S0 States

Various photodissociation pathways of methylamine involving the three lowest electronic states, namely, singlet ground S0 state, singlet first excited S1 state, and triplet ground T1 state, were studied by the (MS-)CAS(8e,8o)PT2/6-31++G** method. All critical points, i.e., minima, transition states, minimum energy conical intersections, and minima on the seam of crossing, were explored systematically by the global reaction route mapping (GRRM) strategy utilizing the anharmonic downward distortion following (ADDF) and artificial force induced reaction (AFIR) methods. On the basis of obtained structures, we discuss the photodissociation mechanism of methylamine in the experimental excitation wavelength range 222-240 nm in detail. Especially, the T1 potential energy surface was explored systematically for the first time. The N-H bond rupture is a primary channel on the S1 state. Along the N-H dissociation path on S1, there is a low-energy conical intersection (CI), and through this CI the system can go back to the S0 state; from the CI the system can directly dissociate to CH3NH + H or reproduce the original CH3NH2 on S0. There is a seam of crossing between S0 and T1 in a partially dissociated CH3---NH2 geometry, and through this seam the system may go up to the T1. On the T1 state, a roaming-like pathway giving CH4 + NH (X(3)Σ(-)) products was found, which would explain the recently proposed intersystem crossing mediated roaming dynamics.

Global ab Initio Potential Energy Surfaces for Low-Lying Doublet States of NO3

We report analytical global potential energy surfaces (PESs) for three low-lying doublet states (D0, D1, and D2) of NO3. The fits are made on roughly 74000 MS-CAS(17e,13o)PT2/aug-cc-pVTZ calculations of electronic energies, where these PESs are invariant of permutations of oxygen atoms. The surfaces describe two roaming pathways for NO3 → NO2-----O → NO + O2 involving different electronic states discovered in the photolysis of NO3 [ Xiao, H. Y. et al. J. Phys. Chem. Lett.2011, 2, 934 ]. These pathways become accessible at excess energy of ∼210 kJ/mol above the ground-state global minimum of NO3. The ab initio data below 360 kJ/mol are reproduced very well by the fitted PESs with the fitting rms errors of less than 5.5 kJ/mol for all the three states. Moreover, key local minima and energy profiles along the roaming pathways on the fitted PESs are compared with those on the ab initio PESs. In addition, potential contour maps in the roaming region are also compared. These careful evaluations of the fitted PESs suggest that the present fitted PESs are well suited for future dynamics calculations of this system.