Aromatic Pathways of Porphins, Chlorins, and Bacteriochlorins

Magnetically induced current densities have been calculated for free-base porphynoids using the gauge including magnetically induce current (GIMIC) method. Numerical integration of the current density passing selected chemical bonds yields current pathways and the degree of aromaticity according to the magnetic criterion. The ring-current strengths of the porphins, chlorins, and bacteriochlorins are 1.5–2.5 times stronger than for benzene. The calculations show that the 18π [16]­annulene inner cross is not the correct picture of the aromatic pathway for porphyrins. All conjugated chemical bonds participate in the current transport independently of the formal number of π electrons. The ring current branches at the pyrrolic rings taking both the outer and the inner route. The NH unit of the pyrrolic rings has a larger resistance and a weaker current strength than the pyrroles without inner hydrogens. The traditional 18π [18]­annulene with inactive NH bridges is not how the ring-current flows around the macroring. The porphins have the strongest ring current of ca. 27 nA/T among the investigated porphynoids. The current strengths of the chlorins and bacteriochlorins are 19–24 nA/T depending on whether the ring current is forced to pass an NH unit or not. The current strengths of the 3-fold and 4-fold β-saturated porphynoids are 13–17 nA/T, showing that the inner-cross 18π [16]­annulene pathway is not a preferred current route

Computational Studies of Nonstoichiometric Sodium Auride Clusters

The molecular structures of low-lying isomers of anionic and neutral sodium auride clusters have been studied computationally at the second-order Møller–Plesset perturbation theory level using quadruple-ζ basis sets augmented with a double set of polarization functions. The first vertical detachment energies were calculated at the Møller–Plesset level as the energy difference between the cluster anion and the corresponding neutral cluster. The photodetachment energies of higher-lying ionization channels were calculated by adding electronic excitation energies of the neutral clusters to the first vertical detachment energy. The excitation energies were calculated at the linear response approximate coupled-cluster singles and doubles level using the anionic cluster structures. The obtained ionization energies for NaAu^–, NaAu₂^–, NaAu₃^–, NaAu₄^–, Na₂Au₂^–, Na₂Au₃^–, Na₃Au₃^–, and Na₂Au₄^– were compared to values deduced from experimental photoelectron spectra. Comparison of the calculated photoelectron spectra for a few energetically low-lying isomers shows that the energetically lowest cluster structures obtained in the calculations do not always correspond to the clusters produced experimentally. Spin-component-scaled second-order Møller–Plesset perturbation theory calculations shift the order of the isomers such that the observed clusters more often correspond to the energetically lowest structure, whereas the spin-component-scaled approach does not improve the photodetachment energies of the sodium aurides. The potential energy surface of the sodium aurides is very soft, with several low-lying isomers requiring an accurate electron correlation treatment. The calculations show that merely the energetic criterion is not a reliable means to identify the structures of the observed sodium auride clusters; other experimental information is needed to ensure a correct assignment of the cluster structures. The cluster structures of nonstoichiometric anionic sodium aurides have been determined by comparing calculated ionization energies for low-lying structures of the anionic clusters with experimental data

Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules

Triplet–triplet annihilation (TTA) leads to a reduced efficiency of organic light-emitting diodes (OLEDs) at high current densities. Spacial confinement of the triplet excitons, which is mainly dependent on triplet energy differences, can reduce the TTA rate. Therefore, a deliberate choice of the organic semiconductor materials with particular attention to their triplet energies can help to considerably increase the device efficiency. Organic solid-state lasers are, on the other hand, efficiently quenched by singlet–triplet annihilation (STA), which is closely related to the triplet–triplet absorption of the organic semiconductors. To establish a useful set of parameters related to the processes in organic semiconducting devices, we provide theoretical estimates for the triplet energy of 31 organic semiconductor molecules using state-of-the art ab initio quantum chemical methods. For a subset of 22 molecules, the triplet–triplet absorption spectra were calculated as well. We also discuss related features like localizations of excitations to molecular fragments, driven by the structural changes of the molecules in the excited triplet state. The calculated excited-state properties can assist experimentalists and serve as input parameters in simulations of organic electronics

Computational Studies of the Electronic Absorption Spectrum of [(2,2′;6′,2″-Terpyridine)–Pt(II)–OH] [7,7,8,8-Tetracyanoquinodimethane] Complex

The electronic excitation spectrum of the [(2,2′;6′,2″-terpyridine)–platinum­(II)–OH] [7,7,8,8-tetracyanoquinodimethane] ([Pt­(trpy)­OH]­TCNQ) complex has been studied at the linear-response approximate coupled-cluster singles and doubles (CC2) level using triple-ζ basis sets augmented with polarization functions (TZVP). The calculated ultraviolet–visible (UV–vis) spectrum of the [Pt­(trpy)­OH]­TCNQ complex is compared with the UV–vis spectrum measured for [Pt­(tbtrpy)­OH]­TCNQ (tbtrpy = 4,4′,4″-^<i>t</i>Bu₃-2,2′;6′,2″-terpyridine) in dichloromethane (CH₂Cl₂) solution. The UV–vis spectrum is also compared with the calculated UV–vis spectra of [Pt­(trpy)­OH]⁺ and of the neutral and negatively charged TCNQ species. In contrast to previous interpretations, the CC2 calculations suggest that the [Pt­(trpy)­OH]­TCNQ complex is dissociated into [Pt­(trpy)­OH]⁺ and TCNQ^– when dissolved in CH₂Cl₂. The computed electronic excitation energies of [Pt­(trpy)­OH]⁺ provide information about the charge-transfer excitations between the Pt­(II) metal center and the ligands. The UV–vis spectra were also calculated at the linear-response time-dependent density functional theory (TDDFT) level using the B3LYP, BHLYP, and CAM-B3LYP functionals in combination with TZVP quality basis sets. For the TCNQ species, the TDDFT calculations yield slightly smaller excitation energies than obtained at the CC2 level, whereas for [Pt­(trpy)­OH]⁺ the CC2 excitation energies are slightly smaller than the TDDFT ones. For the [Pt­(trpy)­OH]­TCNQ complex, the B3LYP calculations yield spurious low-lying excited states rendering the spectral assignment using B3LYP data difficult. The low-energy part of the electronic excitation spectrum for the [Pt­(trpy)­OH]­TCNQ complex calculated at the BHLYP and CAM-B3LYP levels is reminiscent of the CC2 one because the larger amount of Hartree–Fock exchange and the long-range correction of the potential blue shifts the excitation energies

Importance of Vibronic Effects in the UV–Vis Spectrum of the 7,7,8,8-Tetracyanoquinodimethane Anion

We present a computational method for simulating vibronic absorption spectra in the ultraviolet–visible (UV–vis) range and apply it to the 7,7,8,8-tetracyanoquinodimethane anion (TCNQ^–), which has been used as a ligand in black absorbers. Gaussian broadening of vertical electronic excitation energies of TCNQ^– from linear-response time-dependent density functional theory produces only one band, which is qualitatively incorrect. Thus, the harmonic vibrational modes of the two lowest doublet states were computed, and the vibronic UV–vis spectrum was simulated using the displaced harmonic oscillator approximation, the frequency-shifted harmonic oscillator approximation, and the full Duschinsky formalism. An efficient real-time generating function method was implemented to avoid the exponential complexity of conventional Franck–Condon approaches to vibronic spectra. The obtained UV–vis spectra for TCNQ^– agree well with experiment; the Duschinsky rotation is found to have only a minor effect on the spectrum. Born–Oppenheimer molecular dynamics simulations combined with calculations of the electronic excitation energies for a large number of molecular structures were also used for simulating the UV–vis spectrum. The Born–Oppenheimer molecular dynamics simulations yield a broadening of the energetically lowest peak in the absorption spectrum, but additional vibrational bands present in the experimental and simulated quantum harmonic oscillator spectra are not observed in the molecular dynamics simulations. Our results underline the importance of vibronic effects for the UV–vis spectrum of TCNQ^–, and they establish an efficient method for obtaining vibronic spectra using a combination of linear-response time-dependent density functional theory and a real-time generating function approach

Antiaromatic Character of 16 π Electron Octaethylporphyrins: Magnetically Induced Ring Currents from DFT-GIMIC Calculations

The magnetically induced current density susceptibility, also called current density, has been calculated for a recently synthesized octaethylporphyrin (OEP) zinc­(II) dication with formally 16 π electrons. Numerical integration of the current density passing selected chemical bonds yields the current pathway around the porphyrinoid ring and the strength of the ring current. The current strengths show that the OEP-Zn­(II) dication is strongly antiaromatic, as also concluded experimentally. The calculation of the ring current pathway shows that all 24 π electrons participate in the transport of the ring current because the current splits into inner and outer branches of practically equal strengths at the four pyrrolic rings. The corresponding neutral octaethylporphyrinoid without Zn and inner hydrogens is found to be antiaromatic, sustaining a paratropic ring current along the inner pathway with 16 π electrons. The neutral OEP-Zn­(II) molecule with formally 18 π electrons is found to be almost as aromatic as free-base porphyrin. However, also in this case, all 26 π electrons contribute to the ring current, as for free-base porphyrin. A comparison of calculated and measured ¹H NMR chemical shifts is presented. The current strength susceptibility under experimental conditions has been estimated by assuming a linear relation between experimental shielding constants and calculated current strengths

Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules

Triplet–triplet annihilation (TTA) leads to a reduced efficiency of organic light-emitting diodes (OLEDs) at high current densities. Spacial confinement of the triplet excitons, which is mainly dependent on triplet energy differences, can reduce the TTA rate. Therefore, a deliberate choice of the organic semiconductor materials with particular attention to their triplet energies can help to considerably increase the device efficiency. Organic solid-state lasers are, on the other hand, efficiently quenched by singlet–triplet annihilation (STA), which is closely related to the triplet–triplet absorption of the organic semiconductors. To establish a useful set of parameters related to the processes in organic semiconducting devices, we provide theoretical estimates for the triplet energy of 31 organic semiconductor molecules using state-of-the art ab initio quantum chemical methods. For a subset of 22 molecules, the triplet–triplet absorption spectra were calculated as well. We also discuss related features like localizations of excitations to molecular fragments, driven by the structural changes of the molecules in the excited triplet state. The calculated excited-state properties can assist experimentalists and serve as input parameters in simulations of organic electronics

Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules

Triplet–triplet annihilation (TTA) leads to a reduced efficiency of organic light-emitting diodes (OLEDs) at high current densities. Spacial confinement of the triplet excitons, which is mainly dependent on triplet energy differences, can reduce the TTA rate. Therefore, a deliberate choice of the organic semiconductor materials with particular attention to their triplet energies can help to considerably increase the device efficiency. Organic solid-state lasers are, on the other hand, efficiently quenched by singlet–triplet annihilation (STA), which is closely related to the triplet–triplet absorption of the organic semiconductors. To establish a useful set of parameters related to the processes in organic semiconducting devices, we provide theoretical estimates for the triplet energy of 31 organic semiconductor molecules using state-of-the art ab initio quantum chemical methods. For a subset of 22 molecules, the triplet–triplet absorption spectra were calculated as well. We also discuss related features like localizations of excitations to molecular fragments, driven by the structural changes of the molecules in the excited triplet state. The calculated excited-state properties can assist experimentalists and serve as input parameters in simulations of organic electronics

Aromatic Pathways in Carbathiaporphyrins

Magnetically induced current densities and current pathways have been calculated for carbaporphyrins and carbathiaporphyrins using the gauge including magnetically induced current (GIMIC) method. The aromatic character and current pathways are obtained from the calculated current density susceptibilities. The current-density calculations show that five of the studied carbaporphyrinoids are aromatic, two are antiaromatic, and one is nonaromatic. The analysis of the current pathways of the investigated molecules reveals some general trends for the current flow in carbaporphyrinoids. Insertion of a CH₂ group into the all-carbon ring generally cuts or restricts the current flow, leading to a stronger current of the alternative pathway of the ring. No obvious trends regarding the current strengths and pathways of the thiophene and cyclopentadienyl rings were obtained. The present study shows that it is indeed difficult to predict the electron delocalization pathways of general carbaporphyrinoids. Thus, a careful analysis of the current density is necessary for determining their electron delocalization pathways

The Effect of Protein Environment on Photoexcitation Properties of Retinal

Retinal is the photon absorbing chromophore of rhodopsin and other visual pigments, enabling the vertebrate vision process. The effects of the protein environment on the primary photoexcitation process of retinal were studied by time-dependent density functional theory (TDDFT) and the algebraic diagrammatic construction through second order (ADC(2)) combined with our recently introduced reduction of virtual space (RVS) approximation method. The calculations were performed on large full quantum chemical cluster models of the bluecone (BC) and rhodopsin (Rh) pigments with 165–171 atoms. Absorption wavelengths of 441 and 491 nm were obtained at the B3LYP level of theory for the respective models, which agree well with the experimental values of 414 and 498 nm. Electrostatic rather than structural strain effects were shown to dominate the spectral tuning properties of the surrounding protein. The Schiff base retinal and a neighboring Glu-113 residue were found to have comparable proton affinities in the ground state of the BC model, whereas in the excited state, the proton affinity of the Schiff base is 5.9 kcal/mol (0.26 eV) higher. For the ground and excited states of the Rh model, the proton affinity of the Schiff base is 3.2 kcal/mol (0.14 eV) and 7.9 kcal/mol (0.34 eV) higher than for Glu-113, respectively. The protein environment was found to enhance the bond length alternation (BLA) of the retinyl chain and blueshift the first absorption maxima of the protonated Schiff base in the BC and Rh models relative to the chromophore in the gas phase. The protein environment was also found to decrease the intensity of the second excited state, thus improving the quantum yield of the photoexcitation process. Relaxation of the BC model on the excited state potential energy surface led to a vanishing BLA around the isomerization center of the conjugated retinyl chain, rendering the retinal accessible for <i>cis–trans</i> isomerization. The energy of the relaxed excited state was found to be 30 kcal/mol (1.3 eV) above the minimum ground state energy, and might be related to the transition state of the thermal activation process