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The trans-urocanic acid, a UV chromophore in the epidermis of human skin, was found to exhibit a wavelength dependent isomerization property. The isomerization quantum yield to cis-urocanic is greatest when being excited to the S1 state, whereas exciting the molecule to the S2 state causes almost no isomerization. The comparative photochemical behavior of the trans-urocanic on the S1 and S2 states continues to be the subject of intense research effort. This study is concerned with the unique photo-behavior of this interesting molecule on the S2 state. Combining the on-the-fly surface hopping dynamics simulations and static electronic structure calculations, three decay channels were observed following excitation to the S2 state. An overwhelming majority of the molecules decay to the S1 state through a planar or pucker characterized minimum energy conical intersection (MECI), and then decay to the ground state along a relaxation coordinate driven by a pucker deformation of the ring. A very small fraction of molecules decay to the S1 state by a MECI characterized by a twisting motion around the CC double bond, which continues to drive the molecule to deactivate to the ground state. The latter channel is related with the photoisomerization process, whereas the former one will only generate the original trans-form products. The present work provides a novel S2 state decay mechanism of this molecule, which offers useful information to explain the wavelength dependent isomerization behavior. [ABSTRACT FROM AUTHOR]

We present a full quantum-mechanical study of the laser control of the radiationless decay between the B3u(nπ*) and B2u(ππ*) states of pyrazine using the dynamic Stark effect. In contrast to our previous study [Sala et al., J. Chem. Phys. 140,194309 (2014)], where a four-dimensional model was used, all the 24 degrees of freedom are now included in order to test the robustness of the strategy of control. Using a vibronic coupling Hamiltonian model in a diabatic representation, the multilayer version of the multi-configuration time-dependent Hartree method is exploited to propagate the corresponding wave packets. We still observe a trapping of the wavepacket on the B2u(ππ*) potential energy surface due to the Stark effect for a longer time than the "non-resonant field-free" B2u(ππ*) lifetime. [ABSTRACT FROM AUTHOR]

Abstract Radiationless deactivation channels of 2-thiocytosine in aqueous environment are revisited by means of quantum-chemical simulations of excited-state absorption spectra, and investigations of potential energy surfaces of the chromophore clustered with two water molecules using the algebraic diagrammatic construction method to the second-order (ADC(2)), and multireference configuration interaction with single and double excitations (MR-CISD) methods. We argue that interactions of explicit water molecules with thiocarbonyl group might enable water-chromophore electron transfer (WCET) which leads to formation of intersystem crossing that was not considered previously. This is the first example of a WCET process occurring in the triplet manifold of electronic states. This phenomenon might explain nonradiative decay of the triplet state population observed in thiopyrimidines in the absence of molecular oxygen. According to our calculations this WCET process might also entail a subsequent, virtually barrierless, electron-driven proton transfer (EDPT) resulting in the formation of hydroxyl radical which could further participate in photohydration or deamination reactions. [ABSTRACT FROM AUTHOR]

Two tetraphenylethylene (TPE) bridged tetraimidazolium salts, [H4 L-Et](PF6)4 and [H4 L-Bu](PF6)4, were used as precursors for the synthesis of the dinuclear AgI and AuI tetracarbene complexes [Ag2( L-Et)](PF6)2, [Ag2( L-Bu)](PF6)2, [Au2( L-Et)](PF6)2, and [Au2( L-Bu)](PF6)2. The tetraimidazolium salts show almost no fluorescence ( ΦF<1 %) in dilute solution while their NHC complexes display fluorescence 'turn-on' ( ΦF up to 47 %). This can be ascribed to rigidification mediated by the restriction of intramolecular rotation within the TPE moiety upon complexation. DFT calculations confirm that the metals are not involved in the lowest excited singlet and triplet states, thus explaining the lack of phosphorescence and fast intersystem crossing as a result of heavy atom effects. The rigidification upon complexation for fluorescence turn-on constitutes an alternative to the known aggregation-induced emission (AIE). [ABSTRACT FROM AUTHOR]

Carbohydrates are essential building blocks of life that assume a multitude of biological functions in all living organisms found on Earth. It was recently reported that ribose was identified in UV-irradiated interstellar ice analogs, which suggests that it can be found on comets and that it may have been transported to Earth via the impact of comets. Herein, we present computational results obtained with multiconfigurational ab initio quantum-chemical methods showing that various photochemical processes for radiationless deactivation are available for photoexcited ribose. These processes are driven by nσ* states and involve either O-H- or endocyclic or exocyclic C-O-bond elongation whereby a conical intersection with the electronic ground state becomes accessible. The local topography of the potential-energy surfaces around these conical intersections suggests that these intersections mediate efficient radiationless deactivation and favor regeneration of the initially photoexcited ground-state reactant. These findings indicate that ribose found in interstellar space can be expected to be highly photostable upon irradiation with UV starlight, which could be of relevance in the field of astrobiology. [ABSTRACT FROM AUTHOR]

We present a comparative computational CASPT2 and CC2 study of the deactivation mechanisms of electronically excited cytosine (Cyt) and 2,4-diaminopyrimidine (DAPy). A number of S 1 / S 0 conical intersections exhibiting N H bond elongation were optimized. These conical intersections are accessible from the Franck–Condon region along 1 πσ* excited-state reaction paths. We also focus on the phototautomerism of DAPy and propose the photoinduced dissociation-association (PIDA) mechanism for this process. Supplementary experimental results on the UV irradiation of DAPy in acetonitrile solution show tautomerization to imino tautomers. By analyzing differences in the photophysics of the nucleobase Cyt and its analogue DAPy, this study provides new insight into the varying degrees of photostability between nucleobases and their close analogues. [ABSTRACT FROM AUTHOR]

Nonadiabatic coupling terms (NACTs)between different electronicstates lead to fast radiationless decay in photoexcited molecules.Using molecular symmetry, i.e., symmetry with respect to permutationof identical nuclei and inversion of the molecule in space, the irreduciblerepresentations of the NACTs can be determined with a combinationof molecular symmetry arguments and quantization rules. Here, we extendthese symmetry rules for electronic states and coupling elements anddemonstrate the importance of molecular symmetry for nonadiabaticnuclear dynamics. As an example, we consider the NACTs related tothe torsion around the CN bond in C5H4NH. Wepresent the results of quantum dynamical simulations of the photoinducedlarge amplitude torsion on three coupled electronic states and showhow the interference between wavepackets leads to radiationless decay,which depends on the symmetry of the NACTs. Moreover, we show thatthe nuclear spin of the system determines the symmetry of the initialnuclear wave function and thus influences the torsional dynamics.This may open new possibilities for nuclear spin selective laser controlof nuclear dynamics. [ABSTRACT FROM AUTHOR]

The radiationless decay of the Xe N4, 5 levels is investigated theoretically. The strong influence of the giant resonance in the 5s5p5(1P)εf channel on the partial and total Auger widths via the interchannel mixing is revealed. The computed widths of the N4 and N5 levels, equal to 105.9 and 109.6 meV, respectively, are in excellent agreement with the measured values 104(3) and 111(3) meV (Jurvansuu et al 2001 Phys. Rev. A 64 012502). The shape of the computed N4, 5OO Auger spectrum is in good agreement with the measured one. Interchannel interaction reduces the probability of the 5p5p − 4dεg Auger decay by more than an order of magnitude. This conclusion is supported by the good agreement between computed and measured Auger electron angular distribution parameters. [ABSTRACT FROM AUTHOR]

Abstract: The geometry, energetics and dipole moment of the most stable conformers of cytosine in the ground state were calculated at different density functional methods, namely, B3LYP, M06-2X, ωB97-D and PEBPEB methods and the 6-311++G(3df,3pd) basis set. The most stable conformer, the keto-amino conformer is only 1 Kcal/mol more stable than the imino-enol form. The ultrafast radiationless decay mechanism has been theoretically investigated using Complete Active Space Multiconfiguration SCF calculation. The conical intersection seam was searched in the full dimensional space for the vibrational degrees of freedom. A new conical intersection has been identified, a semi-planar conical intersection (SPCI) with main deformations inside the cytosine ring and C=O bond. The g-vector and h-vector for the semi-planar conical intersection were calculated and discussed along with their geometrical parameters. A classical trajectory dynamic simulation has been performed to characterize and identify the evolution of geometry and energy changes of the SPCI with time. [Copyright &y& Elsevier]

Highlights: [•] The triplet states of 2-thiothymine, its riboside and deoxyriboside were examined. [•] The T1 states were formed on femtosecond time scale and with unit efficiency. [•] Lifetimes of the nucleosides’ T1 states differed considerably from that of the base. [•] The reason for the enhanced rate of nonradiative decay in the nucleosides is discussed. [ABSTRACT FROM AUTHOR]

The photochemistry and photophysics of the amino-oxo (AO) and imino-oxo (IO) tautomers of 1-methylcytosine are investigated with ab initio calculations, using the CASPT2//CASSCF approach. Our aim is to rationalize recent experimental results which show that the AO tautomer has an excited state lifetime of about 1 ps, similar to cytosine (J.-W. Ho, H.-C. Yen, W.-K. Chou, C.-N. Weng, L.-H. Cheng, H.-Q. Shi, S.-H. Lai and P.-Y. Cheng, J. Phys. Chem. A, 2011, 115, 8406–8418), and that irradiation with wavelengths shorter than 308 nm induces the AO→IO tautomerization (I. Reva, M. J. Nowak, L. Lapinski and R. Fausto, J. Phys. Chem. B, 2012, 116, 5703–5710). For the canonical AO tautomer we find two analogous decay mechanisms to those described previously for cytosine, involving a conical intersection of ethylenic type and one where the amino group is bent out of plane. Decay through these intersections provides an unreactive return path to the AO ground state species. More importantly, we have identified new decay paths that lead from the two intersections to the trans-IO tautomer, without a barrier. These paths provide the possibility of forming the IO tautomer, presumably in small yields, as a side product of the radiationless decay. Thus, we have established for the first time computationally the mechanism of the UV-induced tautomerization, which is compatible with the well-established decay mechanism for cytosine. For the IO tautomer, we also find a mechanism for the excited state interconversion of the cis and trans forms and for efficient radiationless decay through a conical intersection where the imino group is perpendicular to the ring and bent out of the plane. These results are likely to be valid also for cytosine. [ABSTRACT FROM AUTHOR]

This paper presents the results of a spectral (absorption and emission) and photophysical study of 6-aminocoumarin (6AC) in the solvents with which this molecule interacts only nonspecifically (n-alkanes, tetrachloromethane and 1-chloro-n-alkanes) and in nitriles. The strong effects of the solvents on the emission spectra, fluorescence quantum yield and lifetime of 6AC were observed. The results of the steady-state and time-resolved photophysical study suggest the presence of very fast nonradiative deactivation processes. It is concluded that besides fluorescence, the efficient S1-ICT → S0 internal conversion in nonpolar aprotic solvents arises from vibronic interactions between close-lying S1-ICT(π,π*) and S2(n,π*) states. Moreover, unexpectedly efficient triplet state formation occurs. In nitriles the intermolecular hydrogen-bonding interactions with solvent molecules also facilitate the nonradiative decay process involving the S1-exciplex. [ABSTRACT FROM AUTHOR]

Abstract: The radiationless deactivation mechanisms of photo-excited thiophene have been studied using the multi-reference second-order perturbation theory and linear response coupled cluster methods. The electronic spectrum has been established and various minimum energy structures and conical intersections involving the ground and lowest singlet excited states have been characterized. Simplified reaction paths connecting the optimized geometries have been calculated as well. Based on these investigations, several deactivation mechanisms have been identified leading from the lowest bright 1 ππ ∗ states back to the electronic ground state. The excited state depletion in each case is possible due to the existence of low-lying conical intersections formed by either cleavage of one of the CS bonds or out-of-plane deformations of the aromatic ring. The deactivation mechanisms suggested in this work should provide some very efficient decay channels after excitation into the first UV absorption band of thiophene, and are good candidates to explain why this compound is non-fluorescent. [Copyright &y& Elsevier]

Abstract: Possible mechanisms for the radiationless deactivation of photo-excited furan have been investigated using high-level electronic structure methods. Two different conical intersections between the S0 and S1 electronic states have been characterized, both involving various degrees of CO bond cleavage. One of these corresponds to a planar ring-opened structure and the other to an asymmetric ring-puckered structure. Calculations have been performed in order to establish the vertical electronic spectrum and to investigate the behaviour of the potential energy surfaces as the intersections are approached. The present results indicate that both crossings can be accessed through exothermic and barrierless processes after vertical excitation into the optically bright S2(ππ∗) state. These features make them good candidates to account for efficient radiationless deactivation in furan. The deactivation pathways considered in the present work are close analogues of those previously described for other five-membered heterocycles. [Copyright &y& Elsevier]

The effects of alkylation of the 4-amino group, polarity and viscosity of a solvent, and temperature on the fluorescence of N-(2,4-dimethylphenyl)imides of 4-amino-1,8-naphthalenedicarboxylic acid were studied. A decrease in the temperature and polarity of 4-dimethylaminonaphthalimide to decrease and have almost no effect on the rate of emissive deactivation of excited state. The abnormally low efficiency of fluorescence of 4-dimethylaminonaphthalimide in polar low-viscosity media at room temperature is associated with the formation of a nonfluorescent charge-transfer state. The increase in the rate of radiationless deactivation of 4-aminonaphthalimide in the alkylation of its amino group is associated with the increase in its electron-donor ability. [ABSTRACT FROM AUTHOR]

Here plasmonic effect specifically on electroluminescence (EL) is studied in terms of radiative and nonradiative decay rates for a dipole near a metal spherical nanoparticle (NP). Contribution from scattering is taken into account and is shown to play a decisive role in EL enhancement owing to pronounced size-dependent radiative decay enhancement and weak size effect on non-radiative counterpart. Unlike photoluminescence where local incident field factor mainly determines the enhancement possibility and level, EL enhancement is only possible by means of quantum yield rise, EL enhancement being feasible only for an intrinsic quantum yield Q0 < 1. The resulting plasmonic effect is independent of intrinsic emitter lifetime but is exclusively defined by the value of Q0, emission spectrum, NP diameter and emitter-metal spacing. For 0.1< Q0 < 0.25, Ag nanoparticles are shown to enhance LED/OLED intensity by several times over the whole visible whereas Au particles feature lower effect within the red-orange range only. Independently of positive effect on quantum yield, metal nanoparticles embedded in an electroluminescent device will improve its efficiency at high currents owing to enhanced overall recombination rate which will diminish manifestation of Auger processes. The latter are believed to be responsible for the known undesirable efficiency droop in semiconductor commercial quantum well based LEDs at higher current. For the same reason plasmonics can diminish quantum dot photodegradation from Auger process induced non-radiative recombination and photoionization thus opening a way to avoid negative Auger effects in emerging colloidal semiconductor LEDs. [ABSTRACT FROM AUTHOR]

The influence of the interchannel interaction on the radiationless decay of the Xe 4d9np resonances is found to be extremely large due to the potential barrier effect. Interaction between the 5s5p–4dεf and 5p5p–4dεg decay channels reduces the decay transition amplitude associated with the outgoing g wave by a factor of about 3. The effect is confirmed by the good agreement between computed and measured photoionization cross sections and polarization parameters of the Xe 5p4mp levels. [ABSTRACT FROM AUTHOR]