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Water-soluble distyrylbenzene-based aldehydes are excellent fluorescence turn-on indicators for amine detection, since they are themselves non-fluorescent in water. Here, the fluorescence quenching mechanisms of aldehydes with systematically increasing π -systems are investigated in an aqueous environment using quantum chemical methods. Although the aldehydes are structurally related, with each extension of the π -system the fluorescence quenching mechanism changes from excited-state hydrogen transfer to trans→cis isomerization and n π ∗ state-mediated intersystem crossing, until the aldehydes become fluorescent in water. For distyrylbenzene aldehyde, n π ∗ mediated ISC has been identified as major fluorescence quenching channel, which becomes closed upon reaction with an amine explaining its fluorescence turn-on sensitivity.

The nuclear dynamics of the s-trans- 1 , 3 -butadiene cation following a fast ionization process is studied theoretically for the first time, using a fully quantal approach. The three lowest coupled X 2 B g , A 2 A u and B 2 A g states are taken into account and up to six nuclear degrees of freedom, including out-of-plane dihedral angles, are included. The underlying potential energy surfaces have been computed at RS2C level, a CASPT2 variant, and widely different CAS spaces have been evaluated beforehand. In the dynamics simulation, a small population transfer from the X 2 B g to the A 2 A u and B 2 A g states has been observed on a time scale 30–40 fs. The vibronic structure of the first band of the simulated photoelectron spectrum is in very good agreement with the experimental one. Our calculation reveals the vibrational mode corresponding to the symmetric stretching of the terminal C C bond to dominate the vibrational progression of the first band of the photoelectron spectrum of s-trans- 1 , 3 -butadiene.

The photodynamics of s-trans-butadiene in the 6 eV excitation energy range is investigated by ab initio quantum dynamical methods, paying particular attention to the nonadiabatic coupling between the 1Bu and 2Ag singlet excited states. The existence of a conical intersection between their potential energy surfaces is confirmed. Key parameters of the system, like the energy gap between the interacting states and their coupling strength, are critically assessed. Up to eight nuclear degrees of freedom are considered in the dynamical treatment and are shown to lead to a more realistic description of the interactions. The gas phase (jet) UV absorption spectrum is well reproduced. The related ultrafast nonradiative population transfer from 1Bu to 2Ag is the initial processes leading to fluorescence quenching of trans-butadiene.

Spin-orbit coupling (SOC) effects are of great importance for understanding photochemical and -physical relaxation processes. Mean-field approaches have been shown to allow for the efficient calculation of SOC elements without significantly comprising accuracy. We have combined an atomic mean-field approach with the algebraic diagrammatic construction scheme for the polarization propagator, an ab initio excited state method based on perturbation theory. In addition to describing the details of our implementation, we show results from test calculations on the organic molecules thiophene and 1,2-dithiin and compare our computed SOC values at ADC level to known literature values.

Summary While Sandmeyer-type brominations/iodinations and Meerwein-type arylations are useful methodologies, they are not without their drawbacks. Here, we report an alternative catalyst- and additive-free access to Sandmeyer-type brominations/iodinations and to a diverse set of Meerwein-type arylations. The key for the highly selective transformations is the excitation of slowly and thermally formed aryldiazonium-derived aryl cations by blue LEDs, conditions that differ from the known photodissociation pathway. An excited singlet state (diradical character) of the cation is shown to be the key intermediate, which interacts with the solvent MeOH to eventually form the sp2-centered chain-initiating radical. Our results demonstrate a facile, mild, atom-economic, sustainable, and economically attractive route for a broad spectrum of aryl radical arylations, with methanol as the solvent and blue LED irradiation being the only restrictions. In addition, this methodology can be directly pursued from the abundant feedstock, the anilines.

The photoabsorption spectra of C 4 H 4 O and C 4 D 4 O have been measured between ∼5.5 and 17.7 eV using a synchrotron radiation-based Fourier transform spectrometer. In addition to several broad bands due to transitions into valence states, the spectra exhibit numerous sharp bands associated with Rydberg states belonging to series converging onto the X 2 A 2 or the A 2 B 1 state limits. Vertical excitation energies and oscillator strengths have been computed using the second- and third-order algebraic-diagrammic construction polarisation propagator methods (ADC(2) and ADC(3)), and the equation-of-motion coupled-cluster method at the level of singles and doubles model (EOM-CCSD). Adiabatic excitation energies have been estimated using previously computed corrections. The theoretical predictions have allowed assignments to be proposed for the Rydberg series observed in the present single-photon absorption spectra and for some additional series, mainly of A 2 symmetry, reported in previous multiphoton excitation studies. The assignments of some of the Rydberg series converging onto the A 2 B 1 state limit have been revised and, guided by our calculations, the principal series is ascribed to the 2b 1 → n da 2 1 B 2 and 2b 1 → n db 1 1 A 1 transitions. f-type Rydberg series, previously observed only in the multiphoton absorption spectrum of furan, have been observed and assigned. Such f-type series, converging onto either the X 2 A 2 or the A 2 B 1 state thresholds, contribute significantly to the single-photon absorption spectrum. Many of the absorption bands associated with Rydberg states display vibrational progressions which resemble those in the corresponding photoelectron band. It appears that some of the structure associated with the 1a 2 → 3pb 2 1 B 1 and 1a 2 → 3pb 1 1 B 2 transitions involves excitation of non-totally symmetric vibrational modes.

The inverse electron-demand Diels–Alder reaction of trans -cyclooctene and sym -tetrazine is a bioorthogonal reaction which has found widespread applications during the last decade, e.g. for fluorescent labeling of living cells. However, the use of this reaction is still limited due to the rapid isomerization of trans -cyclooctene to its unreactive cis -conformer. Using computational methods, we have derived two optimized derivatives of trans -cyclooctene, which pave the road to more efficient in vivo labeling of arbitrary proteins.

Pigment Yellow 101 (PY101) exhibits a rich photochemistry in its S-1 state as it undergoes excited state intramolecular proton transfer and trans-cis isomerizations upon photoexcitation. Relaxed scans of its potential energy surface are thus computed along the reaction paths connecting the six most stable conformers using time-dependent density functional theory. The influence of solvation onto the energetics is estimated. PY101 has been chosen, since its photochemistry is not dominated by non-adiabatic effects but can be interpreted like hot ground state chemistry, such that a kinetic rate model may be applicable for its simulation. For the latter an adapted rate model taking the dissipation of the initial excess energy into account has been used, and the obtained results for the excited state processes are in good agreement with available time-resolved experimental data. (C) 2014 Elsevier B.V. All rights reserved.

1-Butene was investigated by rotational femtosecond degenerate four-wave mixing spectroscopy (fs DFWM) under supersonic expansion conditions as well as by quantum chemical calculations. Fs DFWM is a time-resolved rotational Raman spectroscopy, which allows for precise determination of rotational constants. The experimental fs DFWM spectrum was successfully reproduced by a fitted simulation using a single structure ascribed to the gauche conformer of 1-butene. The obtained rotational constants A = 22.6 ± 1.7 GHz, B = 4.1554 ± 0.0004 GHz and C = 4.0550 ± 0.0004 GHz are in excellent agreement with the ones from microwave spectroscopy. The cis conformer was not observed in the fs DFWM spectrum recorded under the low-temperature conditions of a supersonic expansion. The absence of the cis form along with simulations of a fs DFWM spectrum indicate that the cis conformer is at most equally but most likely less stable than the gauche rotamer. The experimental findings were supplemented by high level quantum chemical calculations, which predict the gauche form to have lower total energy than the cis structure.

A symmetric carbon monoxide dimer (CO dimer) serves as one of the simplest possible models to investigate exciton localization in excitonically coupled chromophores with high-level quantum chemical methods. It is demonstrated that distance dependent excitonic coupling tunes the energy splitting of vibronically coupled dimer states. With decreasing excitonic coupling strength a pseudo Jahn–Teller distortion emerges resulting in exciton localization on one of both monomers. Connection between excitonic coupling and Jahn–Teller theory is made. Furthermore it is shown that an external electric field can direct the localization of the exciton towards one particular monomer via a charge transfer mediated localization mechanism.

Using time-dependent density functional theory, the excited electronic states of the astaxanthin radical cation (AXT +) are investigated. While the optically allowed excited D1 and D3 states are typical ππ∗ excited states, the D2 and D4 states are nπ∗ states. Special emphasis is put onto the influence of the carbonyl groups onto the excited states. For this objective, the excited states of four hypothetical carotenoids and zeaxanthin have been computed. Addition of a carbonyl group to a conjugated carbon double bond system does essentially not change the vertical excitation energies of the optically allowed ππ∗ states due to two counter-acting effects: the excitation energy should increase due to the –M-effect of the carbonyl group and at the same time decrease owing to the elongation of the conjugated double bond system by the carbonyl group itself.

Femtosecond transient absorption spectroscopy in the visible and NIR and ultrafast fluorescence spectroscopy were used to examine the excited state dynamics of astaxanthin and its radical cation. For neutral astaxanthin, two kinetic components corresponding to time constants of 130 fs (decay of the S 2 excited state) and 5.2 ps (nonradiative decay of the S 1 excited state) were sufficient to describe the data. The dynamics of the radical cation proved to be more complex. The main absorption band was shifted to 880 nm (D 0 → D 3 transition), showing a weak additional band at 1320 nm (D 0 → D 1 transition). We found, that D 3 decays to the lower-lying D 2 within 100 fs, followed by a decay to D 1 with a time constant of 0.9 ps. The D 1 state itself exhibited a dual behavior, the majority of the population is transferred to the ground state in 4.9 ps, while a small population decays on a longer timescale of 40 ps. Both transitions from D 1 were found to be fluorescent.

Nonphotochemical quenching (NPQ) is a fundamental mechanism in photosynthesis by which plants protect themselves against excess excitation energy and which is thus of crucial importance for plant survival and fitness. Recently, carotenoid radical cation (Car(*+)) formation has been discovered to be a key step in the feedback deexcitation quenching component (qE) of NPQ, whose molecular mechanism and location remains elusive. A recent model for qE suggests that the replacement of violaxanthin (Vio) by zeaxanthin (Zea) in photosynthetic pigment binding pockets can in principle result in qE via the so-called "gear-shift" or electron transfer quenching mechanisms. We performed pump-probe measurements on individual antenna complexes of photosystem II (CP24, CP26 and CP29) upon excitation of the chlorophylls (Chl) into their first excited Q(y) state at 660 nm when either Vio or Zea was bound to those complexes. The Chl lifetime was then probed by measuring the decay kinetics of the Chl excited state absorption (ESA) at 900 nm. The charge-transfer quenching mechanism, which is characterized by a spectral signature of the transiently formed Zea radical cation (Zea(*+)) in the near-IR, has also been addressed, both in solution and in light-harvesting complexes of photosystem II (LHC-II). Applying resonant two-photon two-color ionization (R2P2CI) spectroscopy we show here the formation of beta-Car(*+) in solution, which occurs on a femtosecond time-scale by direct electron transfer to the solvent. The beta-Car(*+) maxima strongly depend on the solvent polarity. Moreover, our two-color two-photon spectroscopy on CP29 reveals the spectral position of Zea(*+) in the near-IR region at 980 nm.

Light harvesting complexes have been identified in all chlorophyll-based photosynthetic organisms. Their major function is the absorption of light and its transport to the reaction centers, however, they are also involved in excess energy quenching, the so-called non-photochemical quenching (NPQ). In particular, electron transfer and the resulting formation of carotenoid radical cations have recently been discovered to play an important role during NPQ in green plants. Here, the results of our theoretical investigations of carotenoid radical cation formation in the major light harvesting complex LHC-II of green plants are reported. The carotenoids violaxanthin, zeaxanthin and lutein are considered as potential quenchers. In agreement with experimental results, it is shown that zeaxanthin cannot quench isolated LHC-II complexes. Furthermore, subtle structural differences in the two lutein binding pockets lead to substantial differences in the excited state properties of the two luteins. In addition, the formation mechanism of carotenoid radical cations in light harvesting complexes LH2 and LH1 of purple bacteria is studied. Here, the energetic position of the S 1 state of the involved carotenoids neurosporene, spheroidene, spheroidenone and spirilloxanthin seems to determine the occurrence of radical cations in these LHCs upon photo-excitation. An elaborate pump-deplete-probe experiment is suggested to challenge the proposed mechanism.

Pigment Yellow 101 (P.Y.101) is a fluorescent yellow pigment which exhibits a surprisingly rich photochemistry of several competing reaction pathways as revealed by recent time-resolved femtosecond experiments. Our elaborate quantum chemical investigations employing density functional theory (DFT) and time-dependent DFT (TDDFT) show that the observed fluorescence competes with excited state intramolecular proton transfer and trans – cis isomerization processes. Moreover, the experimentally observed spectral features of the complicated excited state dynamics can be assigned to stable trans -diol, trans -keto and cis -diol, cis -keto isomers on the ground and excited state surfaces. Still, due to its molecular size P.Y.101 poses a challenge to electronic structure theory and many problems occur in particular with respect to the excited state calculations. Thus, P.Y.101 serves also as an educative example for which TDDFT yields a reasonable vertical electronic spectrum, but fails in the prediction of excited state structures, when standard GGA or hybrid functionals with low fractions of Hartree–Fock exchange are employed. This failure is attributed to the charge-transfer failure of TDDFT.

Doubly excited states play important roles in the low-energy region of the optical spectra of polyenes and their investigation has been subject of theoretical and experimental studies for more than 30 years now and still is in the focus of ongoing research. In this work, we address the question why doubly excited states play a role in the low-energy region of the optical spectrum of molecular systems at all, since from a naive point of view one would expect their excitation energy approximately twice as large as the one of the corresponding single excitation. Furthermore, we show that extended-ADC(2) is well suited for the balanced calculation of the low-lying excited 2 1 A g - , 1 1 B u - and 1 1 B u + states of long all-trans polyenes, which are known to possess substantial double excitation character. A careful re-investigation of the performance of TDDFT calculations for these states reveals that the previously reported good performance for the 2 1 A g - state relies heavily on fortuitous cancellation of errors. Finally, the title question is answered such that for short polyenes the lowest excited 2 1 A g - and 1 1 B u - states can clearly be classified as doubly excited, whereas the 1 1 A g - ground state is essentially represented by the (ground-state) HF determinant. For longer polyenes, in addition to increasing double excitation contributions in the 2 1 A g - and 1 1 B u - states, the ground state itself aquires substantial double excitation character (45% in C22H24), so that the transition from the ground state to these excited states should not be addressed as the excitation of two electrons relative to the 1 1 A g - ground state.

Recently, a new class of free stable gas-phase dianions with general formula Be n - 1 O n 2 - was discovered experimentally and, moreover, a CuBe 2 O 4 2 - dianion as well as an unidentified half-integral m/z signal at 53.5 tentatively assigned to MgBe 2 O 4 2 - were detected. Here, the structural and electronic properties of these dianions are investigated by means of high-level ab initio calculations. It is demonstrated that the Be n - 1 O n 2 - ( n ⩾ 4), MgBe 2 O 4 2 - and CuBe 2 O 4 2 - dianions exhibit linear geometries and represent electronically stable dianions in the gas phase. Relations of these dianions to other known free stable dianions are discussed.

Motivated by recent mass spectrometric observations of N 2 − , we have examined several high-spin sextet states of this fundamental ion. Following a general construction principle for long-lived electronic states of small molecules, we have found a long-lived 6 Σ u + state of N 2 − that is stable with respect to direct single electron loss, as well as with respect to dissociation. All possible decay channels are spin forbidden and, therefore, this 6 Σ u + state is long-lived and observable in a mass spectrometer. Comparison is made with CO − and general aspects of long-lived high-spin states of anions are stressed.

Hieringer and Gorling [W. Hieringer, A. Gorling, Chem. Phys. Lett. 419 (2006) 557] have identified failures of time-dependent density functional theory (TDDFT) with standard spatially local functionals for symmetric systems with non-overlapping entities where zero net charge-transfer occurs. We clarify that this particular failure is a direct consequence of the general problem of charge-transfer nevertheless, in the special case of an exact degeneracy. The role of exact and accidental degeneracies in large systems treated by present-day TDDFT is briefly discussed.