Time resolved spectroscopy on Pigment Yellow 101 in solid state

The photochemistry of the fluorescent Pigment Yellow 101 (P.Y.101) in the crystalline phase is investigated combining time resolved vibrational and electronic spectroscopy. The experiments reveal a complex reaction dynamics spanning several orders of magnitude in time and including excited state intramolecular proton transfer (ESIPT) from the initial trans-diol conformer. Following photoexcitation and the subsequent wavepacket motion out of the Franck–Condon region two tautomers, an excited trans-diol and trans-keto state are formed. The cis–trans isomerization of the keto form, which was experimentally observed and theoretically confirmed in DFT calculations in studies on P.Y.101 in solution is obstructed in the solid state, consequently the formation of the cis conformer is not directly observed. In addition, a long lived photoproduct with red shifted vibrational frequencies is identified. The life time of this intermediate is determined to be 50 μs, although an unambiguous assignment to a specific molecular configuration cannot be given at present.     

TDDFT Study of the Electronic Structure,Absorption and Emission Spectra of the Light Emitters of the Amazing Firefly Bioluminescence and Solvation Effects on the Spectra

The ground and excited state properties of luciferin (LH₂) and oxyluciferin (OxyLH₂), the bioluminescent chemicals in the firefly, have been characterized using density functional theory (DFT) and time dependent DFT (TDDFT) methods. The effects of solvation on the electronic absorption and emission spectra of luciferin and oxyluciferin were predicted with a self‐consistent isodensity polarized continuum model of the solvent using TDDFT. The S₀→S₁ vertical excitation energies in the gas phase and in water were obtained. Optimizations of the excited state geometries permitted the first predictions of the fluorescence spectra for these biologically important molecules. Shifts in both of the absorption and emission spectra on proceeding from the gas phase to aqueous solution were also predicted.     

Electronic transitions involved in the absorption spectrum and dual luminescence of tetranuclear cubane [Cu4I4(pyridine)4] cluster: a density functional theory/time-dependent density functional theory investigation

We present a combined density functional theory (DFT)/time-dependent density functional theory (TDDFT) study of the geometry, electronic structure, and absorption and emission properties of the tetranuclear "cubane" Cu4I4py4 (py = pyridine) system. The geometry of the singlet ground state and of the two lowest triplet states of the title complex were optimized, followed by TDDFT excited-state calculations. This procedure allowed us to characterize the nature of the excited states involved in the absorption spectrum and those responsible for the dual emission bands observed for this complex. In agreement with earlier experimental proposals, we find that while in absorption the halide-to-pyridine charge-transfer excited state (XLCT*) has a lower energy than the cluster-centered excited state (CC*), a strong geometrical relaxation on the triplet cluster-centered state surface leads to a reverse order of the excited states in emission.     

Ab initio analysis of the conformational changes of the prolyl-proline model peptide

For- -Pro- -Pro-NH₂ is an ab initio model of the prolyl-proline sequence unit present in numerous peptides and proteins. Cis–trans isomerization of the peptide linkage is a crucial step in accessing the active conformation of several proline containing macromolecules.

The present study focuses on the flexibility of the five-membered pyrrolidine ring, which is considered to help other conformational changes as well as cis–trans isomerization. Ring flexibility is characterized by the pseudorotational amplitude, A, and the phase angle, P. Calculations are carried out at the RHF/6-31+G(d) level of theory. The choice of method and level of theory is further supported by single point DFT calculations.

In the course of NMR structure determination of peptides or proteins, proline residues present in the sequences need special attention. Because of the lack of an amide hydrogen, sequential assignment of proline is rather complicated. Furthermore, in solution state, peptide cis–trans isomers are almost always present. Ab initio study on the For- -Pro- -Pro-NH₂ model is a useful tool to discover the structural characteristics of the prolyl-proline sequence unit.     

Conformational and NBO analysis on cis and trans isomers of methyl-1-(4-hydroxy-3-methoxyphenyl)-1,2,3,4-tetrahydro-9H-β-carboline-3-carboxylate

The cis and trans-methyl-1-(4-hydroxy-3-methoxyphenyl)-1,2,3,4-tetrahydro-9H-β-carboline-3-carboxylates were prepared and ¹H and ¹³C NMR spectra were recorded for both isomers. Conformational and NBO analysis were carried out for the cis and trans isomers. Conformer structures were pre-optimized using the hybrid method B3LYP along with the 6-311+G(d) basis set. Frequency calculations were employed to confirm the structures as minimum points. Potential energy surfaces (PES) were built at the same level of theory. Geometries obtained from DFT calculations were used to perform NBO analysis by the NBO 3.1 module in GAUSSIAN 03. The results obtained through theoretical calculations revealed that the shielding observed at C1 for the trans isomer can be attributed to carbomethoxy γ-effect, together with the hyperconjugative effect, while only hyperconjugative effects were found to explain the shielding of C3. The higher chemical shift value of C3 of the cis isomer was attributed to the carbonyl substituent, which plays an important role by capturing part of the electronic density in C3.     

N,N'-二-[3-羟基-4-(2-苯并噻唑)苯基]脲的光谱实验与密度泛函理论研究

利用实验观测与密度泛函理论(DFT)计算方法考察了新化合物N, N'-二-[3-羟基-4-(2-苯并噻唑)苯基]脲(4-DHBTU)的红外、核磁与紫外吸收光谱性质.与单体2-(4-氨基-2-羟苯基)苯并噻唑(4-AHBT)相比, 4-DHBTU的实验紫外吸收强度显著增强,最大吸收峰发生了明显红移,并呈现出双吸收峰特征.结合实验光谱数据与密度泛函理论计算分析表明, 4-DHBTU分子最稳定的基态异构体为cis-C₁₁和trans-C₁₁,而导致上述紫外光谱差异的主要原因是4-DHBTU样品中cis-C₁₁, trans-C₁₁, cis-C₂₂, trans-C₂₂等多种异构体共存.此外, 4-DHBTU与溶剂二甲基亚砜(DMSO)间氢键作用使得核磁实验中4-DHBTU的15H、16H氢谱化学位移显著增大.     

有机染料敏化剂JK16和JK17的几何结构、电子结构及相关性质

运用密度泛函理论中的杂化泛函B3LYP研究了高效太阳能电池新型染料敏化剂JK16和JK17的几何结构、电子结构、极化率和超极化率, 并用含时密度泛函理论(TDDFT)研究了电子吸收谱. 基于含时密度泛函理论计算结果和实验结果的定性符合, 指认了在可见和近紫外区的吸收属于π→π*跃迁. 计算结果还表明JK16和JK17激发能最低的三个跃迁都与光诱导电荷转移过程有关, 而且二-二甲基芴氨基苯并噻吩基团对光电转换过程的敏化起主要作用, 发生于染料敏化剂JK16、JK17和TiO2界面之间的电荷转移是由染料分子激发态向半导体导带的电子注入过程. 此外, 通过对JK16和JK17的比较, 分析了亚乙烯基对几何结构、电子结构和谱学特性的影响.     

Photodynamics of azobenzene in a hindering environment

We have run trajectory surface hopping simulations of the trans → cis photoisomerization of azobenzene, subject to a pulling force. The model mimics two situations: a trans-azobenzene derivative with bulky substituents that may not be easily displaced, and a recent experiment by Gaub’s group [T. Hugel, N.B. Holland, A. Cattani, L. Moroder, M. Seitz, H.E. Gaub, Science 296 (2002) 1103; N.B. Holland, T. Hugel, G. Neuert, A. Cattani-Scholz, C. Renner, D. Oesterhelt, L. Moroder, M. Seitz, H.E. Gaub, Macromolecules 36 (2003) 2015; G. Neuert, T. Hugel, R.R. Netz, H.E. Gaub, Macromolecules 39 (2005) 789], in which a polymer with azobenzene units was stretched in an atomic force microscope. In both cases, the shortening of the azobenzene moiety in going from the trans to the cis form is opposed by a pulling force. Our simulations show that the trans → cis photoconversion is only partially suppressed by considerably large forces (≈500 pN or more). However, the cis isomer reverts to trans in the ground state, with the help of the pulling force and using the vibrational energy that is available in the first 1–2 ps. The lowering of the quantum yields is therefore the combined result of hindering of the excited state process and of the hot ground state back reaction.     

Challenges in the simulation of dye-sensitized ZnO solar cells: quantum confinement, alignment of energy levels and excited state nature at the dye/semiconductor interface

We report a first principles density functional theory/time-dependent density functional theory (DFT/TDDFT) computational investigation on a prototypical perylene dye anchored to realistic ZnO nanostructures, approaching the size of the ZnO nanowires used in dye-sensitized solar cells devices. DFT calculations were performed on (ZnO)(n) clusters of increasing size, with n up to 222, of 1.3 × 1.5 × 3.4 nm dimensions, and for the related dye-sensitized models. We show that quantum confinement in the ZnO nanostructures substantially affects the dye/semiconductor alignment of energy levels, with smaller ZnO models providing unfavourable electron injection. An increasing broadening of the dye LUMO is found moving to larger substrates, substantially contributing to the interfacial electronic coupling. TDDFT excited state calculations for the investigated dye@(ZnO)(222) system are fully consistent with experimental data, quantitatively reproducing the red-shift and broadening of the visible absorption spectrum observed for the ZnO-anchored dye compared to the dye in solution. TDDFT calculations on the fully interacting system also introduce a contribution to the dye/semiconductor admixture, due to configurational excited state mixing. Our results highlight the importance of quantum confinement in dye-sensitized ZnO interfaces, and provide the fundamental insight lying at the heart of the associated DSC devices.     

Molecular modeling of the olefin metathesis by tungsten(0) carbene complexes

Density functional and second-order Moller–Plesset theory were used to model W(0) carbene mediated homogeneous metathesis reaction of propylene. The calculations show that the rate determining step of the metathesis is the initiation. After the initiation has been completed the rate determining step becomes dissociation of olefin–metallocarbene complex. The low stereoselectivity of the olefin metathesis reaction is due to the close matching of activation energies for cis and trans isomer formation and the fast cis–trans isomerization caused by the catalysts. The non-productive olefin metathesis reaction always dominates the reaction mixture owing to its very low activation energy. The electronic structure of metal carbene olefin complexes can be described as a combination of donor–acceptor interactions between HOMO of the olefin and LUMO of metal carbene located at carbene carbon on the one hand, and the Dewar, Chatt and Duncanson back donation scheme on the other.     

Interaction between naphthyl-indolyl-ethenes and aliphatic amines: Effects on fluorescence and trans→cis photoisomerization

The fluorescence of two trans naphthyl-indolyl-ethenes in solvents of different polarity and H-bonding ability is quenched by aliphatic amines. The H-bonded complexes formed both in the ground- and in the first excited singlet state are responsible for the quenching in non-hydroxylated media. In methanol the interaction, which occurs only in the excited state, leads to the deprotonation of the indolyl-moiety. The consequences of the quenching on the trans→cis photoisomerization yield are reported.     

Solid state fluorescence of Pigment Yellow 101 and derivatives: a conserved property of the individual molecules

The optical properties of the bisazomethine pigments Pigment Yellow 101 (P.Y.101) and three derivatives are investigated employing density functional theory (DFT) and time-dependent DFT (TDDFT). P.Y.101 and one of its derivatives exhibit unusual solid state fluorescence, although both possess OH groups and the latter pigment has particularly small intermolecular distances in its crystal, which are both properties that contradict common empirical rules for fluorescent pigments. Here it is shown that the OH groups are indeed essential for molecular fluorescence of the pigments due to the necessary formation of an intramolecular hydrogen bond with the lone pairs of the bisazomethine nitrogens. Furthermore, the quenching mechanism of molecular fluorescence in the non-fluorescent derivatives is analyzed in detail and a CNN bending motion of the central bisazomethine substructure is identified to be the relevant reaction coordinate along which efficient fluorescence quenching occurs in the individual molecule as well as in the crystal. Electron transfer quenching, which usually is expected to be an important quenching mechanism in aggregated media (here the crystals), is ruled out for the studied bisazomethine pigments. The solid state fluorescence properties of the pigments can finally be understood as conserved molecular properties of the individual pigment molecules.     

Time-dependent Density Functional Theory Study on the Electronically Excited States of N-Methylformamide in Aqueous Solution

Excited-state hydrogen-bonding dynamics of N-methylformamide (NMF) in water has been investigated by time-dependent density functional theory (TDDFT) method. The ground-state geometry optimizations were calculated by density functional theory (DFT) method, while the electronic transition energies and corresponding oscillation strengths of the low-lying electronically excited states of isolated NMF, water monomers and the hydrogen-bonded NMF-H 2 O were calculated by TDDFT method. According to Zhao's rule on the excited-state hydrogen bonding dynamics, our results demonstrate that the intermolecular hydrogen bond C=O···O-H is strengthened and weakened in different electronically excited states. The hydrogen bond strengthening and weakening in the electronically excited state plays an important role in the photophysics of NMF in solutions.     

Computational Assessment of MLCT versus MC Stabilities in First-to-Third-Row d6 Pseudo-Octahedral Transition Metal Complexes

An accurate modeling of metal-to-ligand-charge-transfer (MLCT) and metal-centered (MC) excited state energies is key to predict the photoinduced response in transition metal complexes (TMCs). Herein, the importance of the ground state and excited state reference geometries is addressed for three-prototype d⁶ pseudo-octahedral TMCs, each displaying a different potential energy landscape of MLCT versus MC relative stabilities. Several functionals are used within the time-dependent density functional theory (TDDFT), as well as multireference wave-function theory (MS-CASPT2), applied to [Mn(im)(CO)₃(phen)]⁺, [Ru(im)₂(bpy)₂]²⁺, and [Re(im)(CO)₃(phen)]⁺, (im: imidazole, bpy: bypiridine, phen: phenantroline). The results revel that TDDFT is robust except when using B3LYP functional for first-row d⁶ TMCs. In contrast, MS-CASPT2 calculations are strongly biased in those cases with competitive MLCT/MC states. The results reinforce the reliability of B3LYP to describe the excited states in d⁶ TMCs, but question the validity of assessing the density functional theory (DFT)/TDDFT performance via direct comparison with MS-CASPT2 performed at the same DFT reference geometry as a standard strategy. © 2019 Wiley Periodicals, Inc.     

Time-Dependent Density Functional Theory Study on the Electronic Excited State of Hydrogen-Bonded Clusters Formed by 2-Hydroxybenzonitrile (<Emphasis Type="Italic">o</Emphasis>-Cyanophenol) and Carbon Monoxide

Time-dependent density functional theory (TDDFT) method has been carried out to investigate excited-state hydrogen-bonding dynamics between 2-hydroxybenzonitrile (o-cyanophenol) and carbon monoxide. We have demonstrated that intermolecular hydrogen bond between 2-hydroxybenzonitrile (o-cyanophenol) and C=O group are significantly strengthened in the electronically excited state by theoretically monitoring the changes of the bond lengths of hydrogen bonds and hydrogen-bonding groups in different electronic states. In this study, we firstly analyze frontier molecular orbitals (MOs). Our results are consistent with the intermolecular hydrogen bond strengthening in the electronically excited state of Coumarin 102 in alcoholic solvents, which has been demonstrated for the first time by Zhao and Han. Moreover, the calculated electronic excitation energies of the hydrogen bonding C=O and O–H groups are markedly red-shifted upon photoexcitation, which illustrates the hydrogen bonds strengthen in the electronically excited state again. And the geometric structures in both ground state and the S₁ state of this hydrogen-bonded complex are calculated using the density functional theory (DFT) and TDDFT methods, respectively.     

Geometry,Electronic Structure,and Related Properties of Dye Sensitizer： 3,4-bis[1- （carboxymethyl）-3-indolyl]- 1 H-pyrrole-2,5-dione

The geometry, electronic structure, polarizability and hyperpolarizability of dye sensitizer 3,4-bis[1-（carboxymethyl）-3-indolyl]-1H-pyrrole-2,5-dione （BIMCOOH） were studied using density functional theory （DFT） with hybrid functional B3LYP, and the electronic absorption spectra were investigated using semi-empirical quantum chemical method ZINDO-1 and time-dependent DFT （TDDFT）. The results of natural bond orbital suggest that the natural charges of the dione, indole, and acetic groups are about 0.15e, -0.29e, and 0.44e, respectively. The calculated isotropic polarizability, polarizability anisotropy invariant and hyperpolarizability are 305.4, 188.3, and 1155.4 a.u., respectively. The electronic absorption spectral features in visible and near-UV region were assigned to the π→π^＊ transition due to the qualitative agreement between the experiment and the TDDFT calculations, and the transitions of the excited states 9-11 related to photoinduced intramolecular charge transfer processes. The analysis of electronic structure and UV-Vis absorption indicates that the indole groups primarily contributed sensitization of photo-to-currency conversion processes, and the interracial electron transfer between semiconductor TiO2 electrode and dye sensitizer BIMCOOH are electron injection processes from excited states of the dyes to the semiconductor conduction band.     

A combined experimental and theoretical study on photoinduced intramolecular charge transfer in trans-ethyl p-(dimethylamino)cinamate

Intramolecular charge transfer (ICT) behavior of trans-ethyl p-(dimethylamino)cinamate (EDAC) in various solvents has been studied by steady-state absorption and emission, picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption experiments as well as time-dependent density functional theory (TDDFT). Large fluorescence spectral shift in more polar solvents indicates an efficient charge transfer from the donor site to the acceptor moiety in the excited state compared to the ground state. The energy for 0,0 transition (ν_0,0) for EDAC shows very good linear correlation with static solvent dielectric property. The relaxation dynamics of EDAC in the excited state can be effectively described by a “three state” model where, the locally excited (LE) state converts into the ICT state within 350 ± 100 fs. A combination of solvent reorganization and intramolecular vibrational relaxation within 0.5–6 ps populates the relaxed ICT state which undergoes fluorescence decay within few tens to hundreds of picoseconds.     

Excited-state properties and environmental effects for protonated schiff bases: a theoretical study.

Complete active space self-consistent field (CASSCF), multireference configuration interaction (MRCI), density functional theory (DFT), time dependent DFT (TDDFT) and the singles and doubles coupled-cluster (CC2) methodologies have been used to study the ground state and excited states of protonated and neutral Schiff bases (PSB and SB) as models for the retinal chromophore. Systems with two to four conjugated double bonds are investigated. Geometry relaxation effects are studied in the excited pipi* state using the aforementioned methods. Taking the MRCI results as reference we find that CASSCF results are quite reliable even though overshooting of geometry changes is observed. TDDFT does not reproduce bond alternation well in the pipi* state. CC2 takes an intermediate position. Environmental effects due to solvent or protein surroundings have been studied in the excited states of the PSBs and SBs using a water molecule and solvated formate as model cases. Particular emphasis is given to the proton transfer process from the PSB to its solvent partner in the excited state. It is found that its feasibility is significantly enhanced in the excited state as compared to the ground state, which means that a proton transfer could be initiated already at an early step in the photodynamics of PSBs.