Quantum-chemical investigation of the electroabsorption spectra of directly meso-meso-linked porphyrin arrays: essential role of charge-transfer excited states accidentally overlapping with soret bands

The electroabsorption (EA) spectra of directly meso-meso-linked porphyrin arrays (Zn, n = 1-3) have been investigated by means of the sum-over-states (SOS) approach at the INDO/S-SCI level theory. The experimental EA spectra of Zn (n > or = 2) exhibit an unusual second-derivative line shape at the exciton split low-energy B(x) band in contrast to the first-derivative spectrum of Z1, which is readily ascribed to a quadratic Stark shift of the B (Soret) band. Although the second-derivative line shape is usually attributed to a difference in the permanent dipole moment (Deltamu) between the ground and excited states, it should be vanishing for Zn due to their essentially D(2)(d) or D(2)(h) symmetry. As pointed out in our previous studies, the interporphyrinic charge-transfer (CT) excited states are accidentally overlapping with the excitonic B bands and the present calculations reveal that the B(x) state is strongly coupled via a transition dipole moment with two such CT states. These situations give rise to a quadratic Stark effect on the B(x) band that is intermediate between Stark shift (first derivative) and Stark broadening (second derivative), and play a central role in establishing the anomalous second derivative nature of the EA spectrum. Moreover, based on the comparison between the theoretical and experimental spectra, there must be an additional factor that further enhances the second derivative nature of the EA spectrum of porphyrin arrays. Discussions on this issue including the preliminary investigations on the role of solvent (PMMA)-induced asymmetry are also presented.     

Quantum chemical simulation of excited states of chlorophylls, bacteriochlorophylls and their complexes

The present review describes the use of quantum chemical methods in estimation of structures and electronic transition energies of photosynthetic pigments in vacuum, in solution and imbedded in proteins. Monomeric Mg-porphyrins, chlorophylls and bacteriochlorophylls and their solvent 1:1 and 1:2 complexes were studied. Calculations were performed for Mg-porphyrin, Mg-chlorin, Mg-bacteriochlorin, mesochlorophyll a, chlorophylls a, b, c(1), c(2), c(3), d and bacteriochlorophylls a, b, c, d, e, f, g, h, plus several homologues. Geometries were optimised with PM3, PM3/CISD, PM5, ab initio HF (6-31G*/6-311G**) and density functional B3LYP (6-31G*/6-311G**) methods. Spectroscopic transition energies were calculated with ZINDO/S CIS, PM3 CIS, PM3 CISD, ab initio CIS, time-dependent HF and time-dependent B3LYP methods. Estimates for experimental transition energies were obtained from linear correlations of the calculated transition energies of 1:1 solvent complexes against experimentally recorded solution energies (scaling). According to the calculations in five-coordinated solvent complexes the magnesium atom lies out of the porphyrin plane, while in six-coordinated complexes the porphyrin is nearly planar. Charge densities on magnesium and nitrogen atoms were strongly dependent on the computational method deployed. Several dark states of low oscillator strength below the main Soret band were predicted for solvent complexes and chlorophylls and bacteriochlorophylls in protein environment. Such states, though not yet identified experimentally, might serve as intermediate states for excitation energy transfer in photosynthetic complexes. Q(y), Q(x) and Soret transition energies were found to depend on the orientation of the acetyl group and external pressure. A method to estimate site energies and dimeric interaction energies and to simulate absorption and CD spectra of photosynthetic complexes is described. Simulations for the light harvesting complexes Rhodospirillum molischianum, chlorosomes of Chlorobium tepidum and Chloroflexus aurantiacus, and LHC-II of Spinacia oleracea are presented as examples.     

2D IR spectroscopy of the C-D stretching vibration of the deuterated formic acid dimer

We report transient grating and 2D IR spectra of the C-D stretching vibration of deuterated formic acid dimer. The C-D stretching transition is perturbed by an accidental Fermi resonance interaction that gives rise to a second transition. The transient grating results show that the population lifetime of these states, which are in rapid equilibrium, is 11 ps. 2D IR spectroscopy reveals the energies of the eigenstates in the regions of one quantum and two quanta of C-D stretching excitation. Using these eigenstate energies, we construct a simplified model for the zeroth-order states that we then use to simulate the 2D IR spectrum. The results of this simulation suggest that the model captures the essential features of the vibrational spectroscopy in the region of the C-D stretching transition and compares well with previous gas-phase spectroscopy of the C-D stretch of deuterated formic acid dimer.     

Theoretical study of electronic spectra of the DNA bases

Excited π-electronic states of cytosine and uracil are calculated by the CI method. The effects of a transition from the single-excited configuration set to the set involving all double-excited configurations are considered. The set expansion is shown to affect essentially the transition energies and oscillator strengths, in particular, an additional electron transition related to the first absorption band occurs in the singlet uracil spectrum. When doubly excitations are taken into account the triplet transition energies considerably increase and become practically insensitive to repulsion integral parametrization.     

Electronic structure of TCNQ,studied with HAM/3

The electronic structure of tetracyanoquinodimethane (TCNQ) is calculated using the new semiempirical method HAM /3. The calculated photoelectron spectrum is in reasonable agreement with the measured spectrum. The excitation energies are obtained directly in HAM as the differences of the energies of the unoccupied and the occupied orbitals. The calculated UV spectrum is in good agreement with the measurements. The weak band at 5.3 eV, which earlier had been assumed to correspond to a forbidden transition, is allowed according to HAM . The electron affinity is also in reasonable agreement with the measured value. An explanation has been given for the experimental observation of several resonance states (negative electron affinities). p-Quinodimethane has also been studied.     

Spectroscopy of excited states of carbon anions above the photodetachment threshold

Electronic transitions of C3- and C5- to states lying above the electron affinity of the neutral (EA) have been recorded in the gas phase by laser photodetachment spectroscopy. The excited states are identified by comparison with absorption spectra for the mass-selected ions deposited in neon matrices and with ab initio calculations. The C 2 sigma u (+)-X 2 pi g transition and two higher energy band systems are observed for C3-, corresponding to excitation energies more than 1.5 eV above the EA. In the case of C5- the strongest features, at about 0.6 eV above the EA, are attributed to close lying 2 delta g-X 2 pi u and 2 sigma g(-)-X 2 pi u transitions. The dominant configurations in these states identify them as long-lived Feshbach resonances. Lifetimes for these resonances in C3- are estimated to be between 200 fs and 3 ps from the band widths.     

The photodissociation of the water dimer in the A band: a twelve-dimensional quasiclassical study

The quasiclassical absorption spectrum of the water dimer in the A band was calculated taking into account motion in all degrees of freedom of the system. The ab initio excited state potentials employed were interpolated by the modified Shepard interpolation method using QMRCI energies and state-averaged MCSCF gradients and Hessians. The ground state vibrational wavefunction was variationally calculated using an adiabatic separation between the high and low frequency normal modes of the system. The calculated spectrum of water dimer shows a clear blueshift with respect to the monomer, but also a small red tail, in agreement with the prediction by Harvey et al. [J. Chem. Phys. 109, 8747 (1998)]. Previous three-dimensional model studies of the photodissociation of the water dimer by Valenzano et al. [J. Chem. Phys. 123, 034303 (2005)] did not show this red tail. A thorough analysis of the dependence of the spectrum on the modes coupled explicitly in the calculation of the spectrum shows that the red tail is due to coupling between the intramolecular stretch vibrations on different monomers.     

On low‐lying excited states of extended nanographenes

Low‐lying excited states of planarly extended nanographenes are investigated using the long‐range corrected (LC) density functional theory (DFT) and the spin‐flip (SF) time‐dependent density functional theory (TDDFT) by exploring the long‐range exchange and double‐excitation correlation effects on the excitation energies, band gaps, and exciton binding energies. Optimizing the geometries of the nanographenes indicates that the long‐range exchange interaction significantly improves the C C bond lengths and amplify their bond length alternations with overall shortening the bond lengths. The calculated TDDFT excitation energies show that long‐range exchange interaction is crucial to provide accurate excitation energies of small nanographenes and dominate the exciton binding energies in the excited states of nanographenes. It is, however, also found that the present long‐range correction may cause the overestimation of the excitation energy for the infinitely wide graphene due to the discrepancy between the calculated band gaps and vertical ionization potential (IP) minus electron affinity (EA) values. Contrasting to the long‐range exchange effects, the SF‐TDDFT calculations show that the double‐excitation correlation effects are negligible in the low‐lying excitations of nanographenes, although this effect is large in the lowest excitation of benzene molecule. It is, therefore, concluded that long‐range exchange interactions should be incorporated in TDDFT calculations to quantitatively investigate the excited states of graphenes, although TDDFT using a present LC functional may provide a considerable excitation energy for the infinitely wide graphene mainly due to the discrepancy between the calculated band gaps and IP–EA values. © 2017 Wiley Periodicals, Inc.     

Transition levels of defects in ZnO: Total energy and Janak's theorem methods

Transition levels of defects are commonly calculated using either methods based on total energies of defects in relevant charge states or energy band single particle eigenvalues. The former method requires calculation of total energies of charged, perfect bulk supercells, as well as charged defect supercells, to obtain defect formation energies for various charge states. The latter method depends on Janak's theorem to obtain differences in defect formation energies for various charge states. Transition levels of V(Zn), V(O), and V(ZnO) vacancy defects in ZnO are calculated using both methods. The mean absolute deviation in transition level calculated using either method is 0.3 eV. Relative computational costs and accuracies of the methods are discussed.     

Photophysics and spectroscopy of the higher electronic states of zinc metalloporphyrins: a theoretical and experimental study

The photophysics of the S2 and S1 excited states of zinc porphyrin (ZnP) and five of its derivatives (ZnOEP, ZnTBP, ZnTPP, ZnTFPP, ZnTCl8PP) have been investigated by measuring their steady-state absorption and fluorescence spectra, quantum yields and excited state lifetimes at room temperature in several solvents. The radiative and radiationless decay constants of the fluorescent excited states accessible in the visible and near UV regions of the spectrum have been obtained. Despite the similarities in the Soret spectra of these compounds, their S2 excited state radiationless decay rates differ markedly. Although the S2-S1 electronic energies of a given zinc porphyrin vary linearly with the Lippert (refractive index) function of the solvent, the S2 radiationless decay rates of the set of compounds do not follow the energy gap law of radiationless transition theory. Calculations, using time-dependent density functional theory (TDDFT), of the energies and symmetries of the complete set of excited states accessible by 1- or 2-photon absorption in the near UV-visible have also been carried out. Substitution on the porphyrin macrocycle framework affects the ground state geometry and alters the electron density distributions, the orbital energies and the relative order of the excited electronic states accessible in the near UV-blue regions of the spectrum. The results are used to help interpret both the nature of the electronic transitions in the Soret region, and the relative magnitudes of the radiationless transition rates of the excited states involved.     

First‐Principles Study on Doping of SnSe2 Monolayers

Doping is a vitally important technique that can be used to modulate the properties of two‐dimensional materials. In this work, by using first‐principles density functional calculations, we investigated the electrical properties of SnSe₂ monolayers by p‐type/n‐type and isoelectronic doping. Substitution at Sn/Se sites was found to be easy if the monolayer was grown under Sn‐/Se‐poor conditions. Substitutions at Sn sites with metallic atoms (e.g. Ga, Ge, In, Bi, Sb, Pb) resulted in positive substitution energies, which indicated that they were not effective doping candidates. For substitutions at Se sites with nonmetallic atoms, no promising candidates were found for p‐type doping (e.g., N, P, As). Among these, N and As showed positive substitution energies. Although P had a negative substitution energy under Sn‐rich conditions, it introduced trap states within the band gap. For n‐type doping (e.g., F, Cl, Br), all the calculated substitution energies were negative under both Sn‐ and Se‐rich conditions. Br was proven to be a promising candidate, because the impurity introduced a shallow donor level. Finally, for isoelectronic doping (e.g., O, S, Te), the intrinsic semiconducting features of the SnSe₂ monolayer did not change, and the contribution from the impurity to the states near the band edge increased with the atomic number.     

6-甲基-4-羟基嘧啶单体及二聚体质子转移过程的理论研究

应用密度泛函理论的B3LYP/6-311+G(d)方法研究了6-甲基-4-羟基嘧啶单体及二聚体质子转移的异构化反应.对反应势能面的研究发现,该化含物可能存在9种单体异构体,对其最稳定的单体构型进行分析.各单体间异构化反应的过渡态共有9种,反应的活化能最小为22.06 kJ/mol,最大为356.55 kJ/mol,最可能的反应路径在室温下即可进行. 研究了2种二聚体及其异构化反应的过渡态,发现二聚体均比其对应的单体稳定,而且质子转移所需要的活化能仅为20.13 kJ/mol,比单体低很多. 氢键在这种变化中起了主要作用,由单体和二聚体的总能量计算了氢键的键能.     

Electronic transitions of the Soret band of reaction centers from Rhodobacter sphaeroides studied by femtosecond transient absorbance spectroscopy

The Soret band of reaction centers from Rhodobacter sphaeroides has been systematically studied using femtosecond transient absorption spectroscopy. When the excitation wavelength was scanned over the entire Soret band, the approximate absorption spectra of the bacteriochlorophyll dimer, the monomer bacteriochlorophylls, and the bacteriopheophytins within the Soret band were determined by analyzing the ground state bleaching with about 100 fs resolution. The main contribution of H is on the blue end of the spectrum, peaking near 350 nm, P absorbs mostly on the red side of the spectrum, but probably has multiple bands, and the main absorbance of B likely lies between H and P, overlapping with P on the red side (particularly near 390 nm). The energy transfer from B to P in the QY band takes about 300 fs when Soret-band excitation is used and the time constant of overall energy transfer from H to B to P in the QY band when H is specifically excited near 350 nm is about 500 fs. Internal conversion after Soret-band excitation is the rate-limiting step for the energy-transfer process. The time constant of internal conversion for B and P is less than 300 fs, and for H it is about 500 fs.     

Electronic and geometric structure analysis of neutral and anionic metal nitric chalcogens: The case of MNX series (M=Li,Na, Be and X=O,S, Se,Te)

Coupled cluster and multireference configuration approaches are employed to study the electronic and geometric structures of mono-coordinated complexes of lithium, sodium, and beryllium with nitric oxide and its isovalent NS, NSe, and NTe species. Ground and low-lying excited states were examined for both linear-bonded and side-bonded isomers. We show that the ionic M⁺NX⁻ (M=Li, Na, Be and X=O, S, Se, Te) picture is a more natural representation and can account for the symmetry of the low−lying electronic states as Σ⁻, Δ, and Σ⁺, the smaller excitation energies and the larger binding energies for heavier X. An additional electron binds to the positively charged Li and Na terminal creating stable anions. The electron affinity (EA) of LiNX and NaNX species is in the 0.5–0.8 eV range. Despite the negative EA of beryllium and the very small EA of NO, the BeNO molecule has an EA of ~1.0 eV, which is increased to ~1.5 eV for the heavier BeNX species. This is attributed to the fact that the additional electron goes to the beryllium end for BeNO but to a π(MN) π*(NX) orbital of the rest species. Our accurate results contradict previous findings and serve as a guide for future experimental studies. © 2019 Wiley Periodicals, Inc.     

Structural and electronic properties of ZrX2)and HfX2 (X=S and Se) from first principles calculations

Early transition metal dichalcogenides (TMDC), characterized by their quasi-two-dimensional layered structure, have attracted intensive interest due to their versatile chemical and physical properties, but a comprehensive understanding of their structural and electronic properties from a first-principles point of view is still lacking. In this work, four simple TMDC materials, MX(2) (M = Zr and Hf, X = S and Se), are investigated by the Kohn-Sham density functional theory (KS-DFT) with different local or semilocal exchange-correlation (xc) functionals and many-body perturbation theory in the GW approximation. Although the widely used Perdew-Burke-Ernzelhof (PBE) generalized gradient approximation (GGA) xc functional overestimates the interlayer distance dramatically, two newly developed GGA functionals, PBE-for-solids (PBEsol) and Wu-Cohen 2006 (WC06), can reproduce experimental crystal structures of these TMDC materials very well. The GW method, currently the most accurate first-principles approach for electronic band structures of extended systems, gives the fundamental band gaps of all these materials in good agreement with the experimental values obtained from optical absorption. The minimal direct gaps from GW are systematically larger than those measured from thermoreflectance by about 0.1-0.3 eV, implying that excitonic effects may be stronger than previously estimated. The calculated density of states from GW quasi-particle band energies agrees very well with photo-emission spectroscopy data. Ionization potentials of these materials are also computed by combining PBE calculations based on the slab model and GW quasi-particle corrections. The calculated absolute band energies with respect to the vacuum level indicate that that ZrS(2) and HfS(2), although having suitable band gaps for visible light absorption, cannot be used for overall water splitting as a result of mismatch of the conduction band minimum with the redox potential of H(+)/H(2).     

Ab initio CI calculations on free-base porphin

Ab initio configuration interaction (CI ) calculations were carried out on low-lying singlet and triplet π–π* states and ionized states of free-base porphin. We take into account single and double excitations from σ and π electrons in the CI calculations. The composite natural orbitals were employed in order to reduce the size of orbital set to be used in the CI . The calculated excitation energies were in good agreement with experimental values. The use of split-valence-type basis and the inclusion of correlation effects of σ electrons were proved to be important to describe the low-lying π–π* states, especially the Soret band.     

On the electronic structures and electron affinities of the m-benzoquinone (BQ) diradical and the o-, p-BQ molecules: a synergetic photoelectron spectroscopic and theoretical study

Electron affinity (EA) is an important molecular property relevant to the electronic structure, chemical reactivity, and stability of a molecule. A detailed understanding of the electronic structures and EAs of benzoquinone (BQ) molecules can help rationalize their critical roles in a wide range of applications, from biological photosynthesis to energy conversion processes. In this Article, we report a systematic spectroscopic probe on the electronic structures and EAs of all three isomers-o-, m-, and p-BQ-employing photodetachment photoelectron spectroscopy (PES) and ab initio electronic structure calculations. The PES spectra of the three BQ(●-) radical anions were taken at several photon energies under low-temperature conditions. Similar spectral patterns were observed for both o- and p-BQ(●-), each revealing a broad ground-state feature and a large band gap followed by well-resolved excited states peaks. The EAs of o- and p-BQ were determined to be 1.90 and 1.85 eV with singlet-triplet band gaps of 1.68 and 2.32 eV, respectively. In contrast, the spectrum of m-BQ(●-) is distinctly different from its two congeners with no clear band gap and a much higher EA (2.89 eV). Accompanied theoretical study confirms the experimental EAs and band gaps. The calculations further unravel a triplet ground state for m-BQ in contrast to the singlet ground states for both o- and p-BQ. The diradical nature of m-BQ, which is consistent with its non-Kekule? structure, is primarily responsible for the observed high EA and helps explain its nonexistence in bulk materials.     

Tautomers and electronic states of jet-cooled 2-aminopurine investigated by double resonance spectroscopy and theory

We present resonant two-photon ionization (R2PI), IR-UV, and UV-UV double resonance spectra of jet-cooled 2-aminopurine (2AP) as well as Fourier transform infrared (FTIR) gas phase spectra. 2AP is a fluorescing isomer of the nucleobase adenine. The results show that there is only one tautomer of 2AP which absorbs in the wavelength range 32,300-34,500 cm(-1). The comparison with the calculated IR spectra of 9H- and 7H-2AP points to 9H-2AP as the dominating tautomer in the gas phase but the spectra are too similar to allow an unambiguous assignment to the respective tautomer. Hence, we determined vertical and adiabatic excitation energies of both tautomers employing combined density functional theory and multi-reference configuration interaction techniques. For the 0-0 band of the first 1pipi* transition of 9H-2AP we obtain a theoretical value of 32,328 cm(-1), in excellent agreement with the band origin of our R2PI spectrum at 32,371 cm(-1). The first singlet pipi* transition of the less stable 7H-2AP tautomer is predicted to be red-shifted by about 1700 cm(-1) with respect to the corresponding transition in 9H-2AP. From the absence of experimental bands in the energy region between 30,300 and 32,350 cm(-1) we conclude that 7H-2AP is not present to an appreciable extent in the molecular beam. Our calculations yield nearly equal energies for the 1npi* and 1pipi* minima of isolated 2AP, similar to the situation in adenine. The hitherto existing argument that the energetic order of states is responsible for the different spectroscopic properties of these isomers therefore does not hold. Rather, vibronic levels close to the origin of the 1pipi* transition cannot access the conical intersection between the 1pipi* and S(0) states along a puckering coordinate of the six-membered ring, in contrast to the situation in electronically excited 9H-adenine. As a consequence, a rich vibrational structure can be observed in the R2PI spectrum of 2AP whereas the spectrum of 9H-adenine breaks off at low energies.