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Femtosecond spectroscopy and high-level quantum chemical calculations are used for studying the ultrafast excited-state dynamics of the fluorescent pigment yellow 101 (P.Y.101) and the closely related 1,1'-naphthalazine. The photostability of this organic pigment is obtained by the interplay of different ultrafast nondestructive decay channels.

Density functional theory (DFT) and time-dependent DFT (TD-DFT) were used to study the lowest excited states S1 and S2 of the carotenoids violaxanthin, lutein and zeaxanthin. All-s-cis zeaxanthin and s-cis-lutein conformers possess lower total ground state energies than the corresponding s-trans conformers and it was demonstrated that the energies of these states change significantly if the carotenoid adopts the equilibrium geometry of the S1 state.

A study was conducted to estimate the spatial sizes of various spin states of Fe(super 2+). The result shows that the spatial size of the Fe(super 2+) ion decreases with increasing spin multiplicity.

Recently, two contradicting experimental investigations have been published on the fluorescence behavior of 4-fluoro-N,N-dimethylaniline (FDMA). Motivated by the discrepancies between these studies, we have examined the deactivation pathways of photoexcited FDMA using high-level quantum chemical methods such as time-dependent density functional theory (TDDFT), coupled cluster (CC2) theory, and the algebraic diagrammatic construction scheme (ADC(2), ADC(3)) simulating solvation using continuum solvation models. Our results show the initial population of the bright S2 (ππ*) state of FDMA relax to a linear, almost planar S1 state minimum from which emission occurs. Our study further reveals that TDDFT/B3LYP erroneously predicts a planar twisted intramolecular charge transfer (TICT) S1 state minimum that vanishes when the amount of nonlocal Hartree-Fock exchange in the functional is increased. [ABSTRACT FROM AUTHOR]

The excited states of fac-tris(phenylpyridinato)iridium[Ir(ppy)3] and the smaller model complex Ir(C3H4N)3are computed using a number of high-levelab initio methods, including the recently implemented algebraic diagrammaticconstruction method to third-order ADC(3). A detailed descriptionof the states is provided through advanced analysis methods, whichallow a quantification of different charge transfer and orbital relaxationeffects and give extended insight into the many-body wave functions.Compared to the ADC(3) benchmark an unexpected striking differenceof ADC(2) is found for Ir(C3H4N)3, which derives from an overstabilization of charge transfer effects.Time-dependent density functional theory (TDDFT) using the B3LYP functionalshows an analogous but less severe error for charge transfer states,whereas the ωB97 results are in good agreement with ADC(3).Multireference configuration interaction computations, which are inreasonable agreement with ADC(3), reveal that static correlation doesnot play a significant role. In the case of the larger Ir(ppy)3complex, results at the TDDFT/B3LYP and TDDFT/ωB97levels of theory are presented. Strong discrepancies between the twofunctionals, which are found with respect to the energies, characters,as well as the density of the low lying states, are discussed in detailand compared to experiment. [ABSTRACT FROM AUTHOR]

We present a non-Dyson fourth-order algebraic diagrammatic construction formulation of the electron propagator, featuring the distinct IP- and EA-ADC(4) schemes for the treatment of ionization and electron attachment processes. The algebraic expressions have been derived automatically using the intermediate state representation approach and implemented in the Q-Chem quantum-chemical program package. The performance of the novel methods is assessed with respect to high-level reference data for ionization potentials and electron affinities of closed- and open-shell systems. While only minor improvements over the corresponding third-order methods are observed for one-hole ionization and one-particle electron attachment processes from closed-shell systems (MAE IP-ADC(4) = 0.27 eV and MAE EA-ADC(4) = 0.05 eV), a significantly enhanced performance is found in case of open-shell reference states (MAE IP-ADC(4) = 0.11 eV and MAE EA-ADC(4) = 0.02 eV). A particularly appealing feature of the novel methods is their accurate treatment of satellite transitions. For closed-shell reference states, we obtain accuracies of MAE IP-ADC(4) = 0.81 eV and MAE EA-ADC(4) = 0.27 eV, while in case of open-shell reference states, mean absolute errors of MAE IP-ADC(4) = 0.15 eV and MAE EA-ADC(4) = 0.27 eV are found.

Algebraic diagrammatic construction (ADC) schemes represent a family of ab initio methods for the calculation of excited electronic states and electron-detached and -attached states. All ADC methods have been demonstrated to possess great potential for molecular applications, e.g., for the calculation of absorption or photoelectron spectra or electron attachment processes. ADC originates from Green's function or propagator theory; however, most recent ADC developments heavily rely on the intermediate state representation or effective Liouvillian formalisms, which comprise new ADC methods and computational schemes for high-order properties. The different approaches for the calculation of excitation energies, ionization potentials, and electron affinities are intimately related, and they provide a coherent description of these quantities at equivalent levels of theory and with comparable errors. Most quantum chemical program packages contain ADC methods; however, the most complete ADC suite of methods can be found in the recent release of Q-Chem.

Singlet fission (SF) has the potential to dramatically increase solar cell efficiency by converting one singlet exciton to two free triplet excitons via a correlated triplet pair intermediate. Identification and characterization of excited states involved in SF are of great importance for understanding the fundamentals of SF. Despite their importance, it is still nontrivial to distinguish various species in transient absorption spectra due to their spectral overlaps and ultrashort lifetimes. Theoretical modeling of SF and its electronically excited state absorption (ESA) is generally challenging due to the multiexciton nature of the correlated triplet pair, which usually requires description by expensive high-level ab initio methods. In this work, taking the bis((triisopropylsilyl)ethynyl) (TIPS)-pentacene monomer and its covalently linked dimer as representative examples, we demonstrate the use of single-reference DFT-based approaches to simulate the ESA spectra during SF. In particular, the singlet and triplet ESA are evaluated by TDDFT, QR-TDDFT, SLR-TDDFT, SF-TDDFT, and UTDDFT, in combination with ten different exchange-correlation functionals. The correlated triplet pair and its ESA are characterized by broken-symmetry DFT and TDDFT, and the role of orbital relaxation is highlighted. With a rational choice of exchange-correlation functionals, we found the resulting spectra to show good agreement with transient absorption experiments and certain improvements over high-order CI methods.

We report the implementation and evaluation of a perturbative, density-based correction scheme for vertical excitation energies calculated in the framework of a polarizable continuum model (PCM). Because the proposed first-order correction terms depend solely on the zeroth-order excited-state density, a transfer of the approach to any configuration interaction-type excited-state method is straightforward. Employing the algebraic-diagrammatic construction (ADC) scheme of up to third order as well as time-dependent density-functional theory (TD-DFT), we demonstrate and evaluate the approach. For this purpose, we assembled a set of experimental benchmark data for solvatochromism in molecules (xBDSM) containing 44 gas-phase to solvent shifts for 17 molecules. These data are compared to solvent shifts calculated at the ADC(1), ADC(2), ADC(3/2), and TD-DFT/LRC-ωPBE levels of theory in combination with state-specific as well as linear-response type PCM-based correction schemes. Some unexpected trends and differences between TD-DFT, the levels of ADC, and variants of the PCM are observed and discussed. The most accurate combinations reproduce experimental solvent shifts resulting from the bulk electrostatic interaction with maximum errors in the order of 50 meV and a mean absolute deviation of 20-30 meV for the xBDSM set.

The mechanism of the photochemical conversion of spiropyran to merocyanine is investigated theoretically. Calculations were performed at TD-DFT/ωB97XD/cc-pVDZ level of theory, which shows good agreement with the reference RI-CC2 method. A two-dimensional scan of the potential energy surface has been performed along the C-O distance and the central torsion angle in the ground state and in the first excited state, where the reaction takes place. Starting at the Franck-Condon geometry, the energy of the first excited state decreases in the direction of the C-O dissociation while the ground-state energy increases. This leads to a barrierless C-O bond dissociation in the first excited state. While relaxing on the S1 PES toward longer C-O distances, the torsion angle hardly changes, but other coordinates start to vary, leading to a conical intersection of the ground state and the first excited state at a C-O distance of about 3.4 Å. Passing the conical intersection, the reaction continues on the ground-state PES. At these large C-O distances, either barrierless Cspiro-O rebinding occurs that quenches spiropyran isomerization or rotation around the central torsion angle occurs that leads to merocyanine. For the latter an energy barrier of 0.1 eV must be overcome explaining the low quantum yield of spiropyran to merocyanine photoswitching.

The photodecarboxylation mechanism of different structural isomers of nitrophenylacetate (NPA) has been investigated using quantum chemical calculations as well as time-resolved UV-pump VIS-probe spectroscopy. It is shown that for a proper theoretical description of the excited states of anionic NPA in aqueous solution a careful consideration of the influence of the solvent is indispensable. In this sense, NPA is an example that demonstrates how character and lifetime of the involved excited states affect the results of equilibrium and nonequilibrium solvation approaches. An ultrafast decay channel via a repulsive singlet state has been found to be responsible for observed ultrafast CO(2) release, while another very efficient but slower CO(2) release channel is found to proceed via intersystem crossing and subsequent decay via a repulsive triplet state. After all, differences and similarities in the observed excited state dynamics of the isomers are conclusively explained. Most notably, the much smaller quantum yield of CO(2) release from the ortho-isomer is due to an alternative excited-state hydrogen-transfer channel, which occurs along a triplet and singlet pathway. On the basis of theoretical and experimental evidence suggesting that the multiplicity of the route taken determines the photoproduct yield, we provide guidelines for the design of ortho-nitrobenzylic caging groups with improved uncaging yield.