Excited S1 Research Articles

To Boldly Look Where No One Has Looked Before: Identifying the Primary Photoproducts of Acetylacetone.

We investigate the gas-phase photochemistry of the enolone tautomer of acetylacetone (pentane-2,4-dione) following S2(ππ*) ← S0 excitation at λ = 266 and 248 nm, using three complementary time-resolved spectroscopic methods. Contrary to earlier reports, which claimed to study one-photon excitation of acetylacetone and found OH and CH3 as the only important gas-phase products, we detect 15 unique primary photoproducts and demonstrate that five of them, including OH and CH3, arise solely by multiphoton excitation. We assign the one-photon products to six photochemical channels and show that the most significant pathway is phototautomerization to the diketone form, which is likely an intermediate in several of the other product channels. Furthermore, we measure the equilibrium constant of the tautomerization of the enolone to diketone on S0 from 320 to 600 K and extract Δ H = 4.1 ± 0.3 kcal·mol-1 and Δ S = 6.8 ± 0.5 cal·mol-1·K-1 using a van't Hoff analysis. We correct the C-OH bond dissociation energy in acetylacetone, previously determined as 90 kcal·mol-1 by theory and experiment, to a new value of 121.7 kcal·mol-1. Our experiments and electronic structure calculations provide evidence that some of the product channels, including phototautomerization, occur on S0, while others likely occur on excited triplet surfaces. Although the large oscillator strength of the S2 ← S0 transition results from the (ππ*) excitation of the C═C-C═O backbone, similar to conjugated polyenes, the participation of triplets in the dissociation pathways of acetylacetone appears to have more in common with ketone photochemistry.

Proton-Transfer-Induced Fluorescence in Self-Assembled Short Peptides.

We employ molecular dynamics (MD) and time-dependent density functional theory (TDDFT) to explore the fluorescence of hydrogen-bonded dimer and trimer structures of cyclic FF (Phe-Phe) molecules. We show that in some of these configurations a photon can induce either an intra-molecular proton transfer, or an inter-molecular proton transfer that can occur in the excited S1 and S2 states. This proton transfer, taking place within the hydrogen bond, leads to a significant red-shift that can explain the experimentally observed visible fluorescence in biological and bioinspired peptide nanostructures with a β-sheet biomolecular arrangement. Finally, we also show that such proton transfer is highly sensitive to the geometrical bonding of the dimers and trimers and that it occurs only in specific configurations allowed by the formation of hydrogen bonds.

Site-Resolved Observation of Vibrational Energy Transfer Using a Genetically Encoded Ultrafast Heater.

Allosteric information transfer in proteins has been linked to distinct vibrational energy transfer (VET) pathways in a number of theoretical studies. Experimental evidence for such pathways, however, is sparse because site-selective injection of vibrational energy into a protein, that is, localized heating, is required for their investigation. Here, we solved this problem by the site-specific incorporation of the non-canonical amino acid β-(1-azulenyl)-l-alanine (AzAla) through genetic code expansion. As an exception to Kasha's rule, AzAla undergoes ultrafast internal conversion and heating after S1 excitation while upon S2 excitation, it serves as a fluorescent label. We equipped PDZ3, a protein interaction domain of postsynaptic density protein 95, with this ultrafast heater at two distinct positions. We indeed observed VET from the incorporation sites in the protein to a bound peptide ligand on the picosecond timescale by ultrafast IR spectroscopy. This approach based on genetically encoded AzAla paves the way for detailed studies of VET and its role in a wide range of proteins.

Photophysical Constants of the Tetraoxa[8]Circulene Molecule

The rate constants of photophysical processes (radiation rate constant, internal conversion rate constant, and quantum fluorescence yield) have been calculated for the tetraoxa[8]circulene molecule by the INDO/S, TDDFT, and CC2 methods. It is confirmed that the doubly degenerate triplet level Т2 and the level Т1 are located below the first singlet excited level S1. According to the selection rules for the angular momentum, the spin-orbit interaction between the levels S1 and T1 (S1 and T2) is equal to zero. Therefore, the internal conversion is the only nonradiative channel in this molecule. The results of calculations also demonstrate that values of the internal conversion rate constant and the quantum fluorescence yield obtained by the INDO/S method are incorrect.

Detailed analysis of the vibronic structure of phenetole in its first excited state and ionic ground state.

The vibronic structure of the first electronically excited state S1 and ionic ground state D0 of phenetole has been investigated by means of resonance enhanced multi photon ionization (REMPI) and mass analyzed threshold ionization (MATI) spectroscopy. The vibronic levels were assigned with the aid of quantum chemical calculations at the (TD)DFT level of theory and a multidimensional Franck-Condon approach. The S1 excitation energy of phenetole has been determined to be 36370 ± 4 cm-1 (4.5093 ± 0.0005 eV). The adiabatic ionization energy was determined to be 65665 ± 7 cm-1 (8.1415 ± 0.0008 eV). The vibronic structure has been analyzed whereby the in-plane bending vibration νbend shows high activity in the first excited state but is more pronounced in the ionic ground state. Moreover, a strong Duschinsky rotation effect can be observed for several D0←S1 transitions that causes violations of the Δv = 0 propensity rule.

Controlled intramolecular H-transfer in malonaldehyde in the electronic ground state mediated through the conical intersection of 1nπ* and 1ππ* excited electronic states.

We report photo-isomerization of malonaldehyde in its electronic ground state (S0), mediated by coupled 1nπ*(S1)-1ππ*(S2) excited electronic states, accomplished with the aid of optimally designed ultraviolet (UV)-laser pulses. In particular, control of H-transfer from a configuration predominantly located in the left well (say, reactant) to that in the right well (say, product) of the electronic ground S0 potential energy surface is achieved by a pump-dump mechanism including the nonadiabatic interactions between the excited S1 and S2 states. An interplay between the nonadiabatic coupling due to the conical intersection of the S1 and S2 states and the laser-molecule interaction is found to be imprinted in the time-dependent electronic population. The latter is also examined by employing optimal fields of varying intensities and frequencies of the UV laser pulses. For the purpose of the present study, we constructed a three-state and two-mode coupled diabatic Hamiltonian with the help of adiabatic electronic energies and transition dipole moments calculated by ab initio quantum chemistry methods. The electronic diabatic model is developed using the calculated adiabatic energies of the two excited electronic states (S1 and S2) in order to carry out the dynamics study. The optimal fields for achieving the controlled isomerization are designed within the framework of optimal control theory employing the optimization technique of a multitarget functional using the genetic algorithm. The laser-driven dynamics of the system is treated by numerically solving the time-dependent Schrödinger equation within the dipole approximation. A time-averaged yield of the target product of ∼40% is achieved in the present treatment of dynamics with optimal laser pulses.

4s to 5s and 4p photoexcitation dynamics of K atoms from the surface of helium nanodroplets: a theoretical study

We study the photodissociation of the potassium atom from a superfluid helium nanodroplet upon 5s 2S or 4p 2P excitation using the time-dependent helium density functional method (He-TDDFT). The importance of quantum effects is assessed by comparing the absorption spectrum obtained for a classical or a quantum description of the K atom. In the case of the 5s 2S ← 4s 2S excitation the difference is rather large, and we use a quantum description for the ensuing direct dissociation dynamics. In the case of the 4p 2P ← 4s 2S absorption spectrum, the difference is much smaller, hence a classical description of K is used to describe 4p 2P excitation dynamics. Excitation to the 4p 2Σ1/2 state leads to the direct dissociation of the K atom, while the 4p 2Π3/2 state initially leads to the formation of an exciplex and the 4p 2Π1/2 state to a bouncing atom above the droplet surface. Remarkably, electronic relaxation can be observed for the latter two states, leading to spin-orbit relaxation and the binding of the initially departing one-atom excimer as a ring excimer for the 2P3/2 state and to the formation of a bound, ring excimer for the 2Π1/2 state.

Ab initio insights into the mechanism of excited-state intramolecular proton transfer triggered by the second excited singlet state of a fluorescent dye: an anti-Kasha behavior

An excited-state intramolecular proton transfer (ESIPT) reaction triggered by the second excited singlet S2 state is found by ab initio calculations to be an ultrafast process for a novel fluorophore.

Similar chemical structures, dissimilar triplet quantum yields: a CASPT2 model rationalizing the trend of triplet quantum yields in nitroaromatic systems.

The photophysics of nitroaromatic compounds is characterized by an ultrafast decay into the triplet manifold and by significant triplet quantum yields. The latter quantity changes drastically depending on the system, as shown for 2-nitronaphthalene, 1-nitronaphthalene, and 2-methyl-1-nitronaphthalene, whose triplet quantum yields have been previously measured to be 0.93 ± 0.15, 0.64 ± 0.12, and 0.33 ± 0.05, respectively (J. Phys. Chem. A, 2013, 117, 14100). In this study, we rationalize the reported trend of the triplet quantum yield on the basis of the different abilities of the excited S1 state to reach a previously unreported conical intersection with the ground state. This path is in competition with the path leading to the triplet state, which appears to be equally favorable in the three systems. The energy barriers from the S1 CASPT2//CASSCF minima to a CASPT2 minimum-energy-crossing-point of the S1/S0 conical intersection have been computed to follow the same trend as the triplet quantum yields of the nitroaromatic systems under analysis. The path has also been characterized for nitrobenzene; an energy barrier was obtained that nicely fits the derived model and is in agreement with its triplet quantum yield value (>0.8). The ability of the present model to not only rationalize the experimental data of a single molecule but also to reproduce a trend for four slightly different systems demonstrates its reliability.

Coupling of the First and Second Excited States of R6G Dye with the Fabry–Perot Cavity

Coupling of macroscopic ensembles of highly concentrated R6G molecules with aluminum and silver Fabry–Perot cavities of different sizes, in which one or two standing wave modes are in resonance with the absorption transition(s) of the dye molecules, have been studied. (i) It has been found that strong coupling of the cavity with one molecular eigenstate (S0→S1 transition) does not perturb the energies of the other molecular states. (ii) The S0→S2 absorption transition is too weak to strongly couple to the resonant cavity (“λ” standing wave mode). However, the resonant cavity causes a blue shift of the S0→S2 excitation band. (iii) The splitting of the dispersion curve is theoretically predicted and experimentally observed at strong coupling of the cavity resonance (“λ” standing wave mode) with the material resonances of Ag, including bound electron and “epsilon equal zero” transitions.

Communication: Becke's virial exciton model gives accurate charge-transfer excitation energies.

First singlet (S1) excitations are of primary importance in the photoluminescence spectra of organic chromophores. However, due to the multi-determinantal nature of the singlet excited states, standard Kohn-Sham density-functional theory (DFT) is not applicable. While linear-response time-dependent DFT is the method of choice for the computation of excitation energies, it fails severely for excitations with charge-transfer character. Becke's recent virial exciton model [A. D. Becke, J. Chem. Phys. 148, 044112 (2018)] offers a promising solution to employ standard DFT for calculation of the S1 excitation energy in molecular systems. Here, it is shown that the virial exciton model is free of charge-transfer error. It is equally reliable for S1 excitations with significant charge-transfer character as for other classes of transitions.

Unravelling overlaps and torsion-facilitated coupling using two-dimensional laser-induced fluorescence

Two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy is employed to identify contributions to fluorescence excitation spectra that arise from both overlapping bands and coupling between zero-order states (ZOSs). Evidence is found for the role of torsional motion in facilitating the coupling between vibrations that particularly involves the lowest-wavenumber out-of-plane vibrational modes. The experiments are carried out on jet-cooled p-fluorotoluene, where the molecules are initially in the lowest two torsional levels. Here we concentrate on the 390–420 cm−1 features in the S1 ← S0 excitation spectrum, assigning the features seen in the 2D-LIF spectrum, aided by separate dispersed fluorescence spectra. The 2D-LIF spectra allow the overlapping contributions to be cleanly separated, including some that arise from vibrational-torsional coupling. Various coupling routes open up because of the different symmetries of the lowest two torsional modes; these combine with the vibrational symmetry to provide new symmetry-allowed vibration-torsion (‘vibtor’) interactions, and the role of the excited m = 1 torsional level is found to be significant.

Fragmentation of Valence and Carbon Core Excited and Ionized CH2FCF3 Molecule.

The photofragmentation dynamics of 1,1,1,2-tetrafluoroethane (R134a) with photon energies from 12 eV up to 320 eV, surrounding the C 1s edge is discussed. The ionic moieties were measured in coincidence with the ejected electrons (PEPICO mode), and detected as a function of the photon energy. Around the C K core edge, the fragmentation profiles are examined regarding the site specific excitation of the CH2FCF3 molecule. In the present case, site-selectivity is favored by the distinct chemical environments surrounding both C atoms. NEXAFS spectrum at the C 1s edge simulation has been obtained at the TDDFT level and excited state geometry optimization calculations have been performed at the inner-shell multiconfigurational self-consistent field level. Our observations indicate that the C(H2F) 1s excitation to a highly repulsive potential expels a fluorine atom leaving the heavier radical fragment C2F3H2* which relaxes to the fundamental state of the ion C2F3H2+. On the other hand, the excitation from the C(F3) 1s carbon to a repulsive state in the C-C bond, leads to a C-C bond cleavage, explaining the observed site specific fragmentation.

On the Intracenter Relaxation of Shallow Arsenic Donors in Stressed Germanium. Population Inversion under Optical Excitation

The relaxation rates of lower excited states 1s(T), 2p0, 2s, 3p0, and 2p± of arsenic donors in a germanium crystal are calculated upon the interaction with long-wavelength acoustic phonons depending on the uniaxial stress in the crystallographic direction [111]. The populations of states under optical excitation are estimated for calculated times. It is shown theoretically that optical excitation of the medium forms an inverse population of arsenic donor levels and leads to the possibility of the implementation of a four-level laser scheme with the radiative transition between 2p states and the 1s triplet state at zero strain. The estimated value of the expected gain in the medium under optical excitation conditions by CO2 laser radiation in the medium at a donor concentration of 2 × 1015 cm–3 is ~0.35 cm–1 at a frequency of 1.98 THz if the laser transition is 2p± → 1s(T) and 1.25 THz if the laser transition is 2p0 → 1s(T).

Atmospheric Chemistry of Perfluoro-3-methyl-2-butanone [CF3C(O)CF(CF3)2]: Photodissociation and Reaction with OH Radicals.

Perfluoro-3-methyl-2-butanone (CF3C(O)CF(CF3)2, abbreviated as C5) is a potentially excellent fire suppression alternative to halons and a promising dielectric gas for SF6 replacement. As a prototypical perfluorinated asymmetrical ketone, photodissociation and reaction with hydroxyl radicals of C5 have been investigated theoretically to gain insights into its atmospheric chemistry and environmental impact. C5 has a broad UV absorption band in the range 260-360 nm with a maximum at 302 nm and the maximal photolysis rate coefficient is 8.3 × 10-5 s-1. Photoexcitation from S0 through the perpendicular n → π* transition produces the excited S1 species, which can either dissociate straightforwardly via the bifurcated α-CC bond cleavage or be trickled down to T1 via the S1/T1 intersystem crossing (ISC) pathway. In the Franck-Condon region of the S1 surface, the long-lived S1 species exists and the slow ISC pathway is dominant, followed by the α-cleavage through T1 barriers to form perfluoroalkyl and perfluoroacetyl radicals. While the excitation energy exceeds 286 nm, the direct dissociation of C5 though the S1 barriers takes over before the ISC occurs. Several pathways for regeneration of the ground-state S0 from S1 and T1 via seams of crossing or internal conversion were revealed. The C5 + OH reaction occurs via direct carbonyl addition mechanism followed by the rapid displacement of one of the alkyl groups. Although it can be accelerated considerably by the H2O-mediated catalysis or the intercepted vibrationally excited quantum states in the hot S0*, the degradation of C5 by OH radicals is too slow to compete with the photolysis pathways.

Photodissociation of phenol in the adiabatic representation: Tunneling, motions of phenyl ring, and kinetic isotope effects

AbstractThe photodissociation of phenol is a prototype of the photoinduced hydrogen detachment reaction. The dissociation rates of phenol through the excited S1 state are calculated with the quantum instanton method in full dimensionality. The Arrhenius plot of the rates shows that the quantum tunneling dominates the OH bond dissociation at low temperatures. The degrees of freedom of phenyl ring (C6H5) play extremely important roles in the dissociation of phenol. Fixing the phenyl ring at the equilibrium geometry can only provide reliable rates between 400 K and 800 K. The motions of phenyl ring have an impact on enhancing the dissociation by lowering the free energy barrier. The larger the amplitudes of the phenyl ring motions are, the more the free energy barrier will be reduced. The dissociation rates of C6H5OH are much larger than those of C6H5OD, which is due to the zero‐point energy and entropy effects.

Ultrafast photoinduced charge transfer character in ofloxacin singlet decay

The mechanism of intramolecular charge transfer (CT) and the following radiationless dynamics of ofloxacin pharmaceutical product were investigated using femtosecond absorption spectroscopy combined with quantum chemical calculations. The CT and the intersystem crossing (ISC) processes have been established to be the major relaxation pathways responsible for the ultrafast nonradiation of the excited S1 state. The CT occurs within a time constant of ∼1.4 ps. The following ISC is determined to be ∼158 ps and triplet yield can be measured to keep ∼0.33. Simultaneous, the quantum yields of CT and triplet-triplet state absorption are contributed to be 0.29 and 0.38, respectively.

Trajectory surface hopping study of propane photodissociation dynamics at 157 nm.

The photodissociation dynamics of propane molecules has been studied using the quasiclassical trajectory surface hopping (TSH) method in conjunction with Tully's fewest switches algorithm. The trajectories are propagated on potential energy surfaces computed on-the-fly using the multiconfiguration and multireference ab initio method starting in the lowest excited singlet state (HOMO → 3s Rydberg state) of propane at 157 nm with the emphasis on the site specificity of atomic hydrogen elimination, molecular hydrogen elimination, and their product branching ratios. Our dynamics simulation revealed that there are three primary dissociation channels: the atomic hydrogen elimination, the molecular hydrogen elimination, and the C-C bond scission. The trajectories indicate that the H2 elimination from the internal carbon atom (2,2-H2 elimination) and terminal carbon atom (1,1-H2 elimination) is the major process and follows a three centred synchronous concerted mechanism. 1,2-H2 and 1,3-H2 eliminations on the other hand are minor processes and exclusively follow the roaming mediated nonadiabatic dynamics. The probability of elimination of the hydrogen atom from two terminal groups (terminal hydrogen elimination) is greater than that from the internal CH2 group (internal hydrogen elimination). Almost 83% of atomic hydrogen elimination occurs through the asynchronous concerted mechanism from the terminal carbon atom via triple dissociation leading to CH3 + C2H4 + H products. This finding is in good agreement with a recent experimental observation. The present TSH study indicates that approximately one-third of the trajectories those resulted in a triple dissociation channel, CH3 + C2H4 + H completed in the ground singlet state following a nonadiabatic path (hopping from the first excited singlet S1 to the ground state S0) via the C-C and C-H dissociation coordinate conical intersection S1/S0. The products CH3(1 2A2″) + C2H4(1Ag) + H, obtained are ground state methyl radicals and ground state ethylene. The trajectories those ended in a triple dissociation channel CH3 + C2H4 + H adiabatically in the S1 state lead to CH3(1 2A2″) + C2H4 (1 3B1) + H, where singlet methyl radicals and triplet ethylene are formed in their corresponding lowest electronic state via a spin conserving route. Two channels, CH4 + CH3CH and C2H6 + CH2, are found to have minor contributions. In the case of methane elimination, the trajectories that follow an adiabatic path lead to CH3CH(1 1A″) + CH4,(1 1A1), where ethylidene is in the excited state and methane is in the ground state. Methane elimination via nonadiabatic path leads to CH3CH(11A') + CH4(1 1A1), where both ethylidene and methane are in the ground electronic state. Ethane eliminations follow the adiabatic path leading to C2H6(1 1A1g) + CH2(1 1B1) where ethane is in the ground state and methylene is in the first excited state.

Deep ultraviolet initiated excited state dynamics of riboflavin and flavin mononucleotide

AbstractFlavins are cofactors in several light‐activated enzymes and therefore their excited states are found to involve in many photobiological processes. Excited state dynamics of flavin compounds corresponding to their first singlet state (S1) has been studied using a plethora of techniques, whereas studies related to highly absorbing ultraviolet excited states are lacking. Here, we study the ultrafast excited state dynamics of riboflavin and flavin mononucleotide using resonance Raman intensity analysis upon photoexcitation into their most intense absorption band centered at 266 nm. Resonance Raman cross sections of each flavin band are quantitatively measured across the 266‐nm absorption band (257–280 nm), and Raman excitation profiles are constructed. We have used Lee and Heller's time‐dependent wave packet theory to simulate the experimental Raman cross sections in a self‐consistent manner. The simulation results in instantaneous structural changes along with solvation dynamics, through linewidth broadening within tens of femtoseconds following photoexcitation. Major structural changes were observed through contraction and elongation of several ring stretching coordinates, affecting at a different site when compared with the S1 excitation. The value of the total reorganization energy was determined to be 1,665 cm−1 (and 1,602 cm−1 for flavin mononucleotide) with a contribution of 1,310 cm−1 from the inertial response of water. We find upon excitation, the first solvation shell inertially responds with an ultrafast timescale of <30 fs for both the molecules. Our results can be useful to determine the structure and dynamics of flavoenzymes by using flavin as a probe following excitation within their 266‐nm absorption band.

Intermolecular dissociation energies of 1-naphthol·n-alkane complexes.

Using the stimulated-emission-pumping/resonant 2-photon ionization (SEP-R2PI) method, we have determined accurate intermolecular dissociation energies D0 of supersonic jet-cooled intermolecular complexes of 1-naphthol (1NpOH) with alkanes, 1NpOH·S, with S = methane, ethane, propane, and n-butane. Experimentally, the smaller alkanes form a single minimum-energy structure, while 1-naphthol·n-butane forms three different isomers. The ground-state dissociation energies D0(S0) for the complexes with propane and n-butane (isomers A and B) were bracketed within ±0.5%, being 16.71 ± 0.08 kJ/mol for S = propane and 20.5 ± 0.1 kJ/mol for isomer A and 20.2 ± 0.1 kJ/mol for isomer B of n-butane. All 1NpOH·S complexes measured previously exhibit a clear dissociation threshold in their hot-band detected SEP-R2PI spectra, but weak SEP-R2PI bands are observed above the putative dissociation onset for the methane and ethane complexes. We attribute these bands to long-lived complexes that retain energy in rotation-type intermolecular vibrations, which couple only weakly to the dissociation coordinates. Accounting for this, we find dissociation energies of D0(S0) = 7.98 ± 0.55 kJ/mol (±7%) for S = methane and 14.5 ± 0.28 kJ/mol (±2%) for S = ethane. The D0 values increase by only 1% upon S0 → S1 excitation of 1-naphthol. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3, and ωB97X-D predict that the n-alkanes bind dispersively to the naphthalene "Face." The assignment of the complexes to Face structures is supported by the small spectral shifts of the S0 → S1 electronic origins, which range from +0.5 to -15 cm-1. Agreement with the calculated dissociation energies D0(S0) is quite uneven, the B97-D3 values agree within 5% for propane and n-butane, but differ by up to 20% for methane and ethane. The ωB97X-D method shows good agreement for methane and ethane but overestimates the D0(S0) values for the larger n-alkanes by up to 20%. The agreement of the B3LYP-D3 D0 values is intermediate between the other two methods.