Theoretical study toward understanding ultrafast internal conversion of excited 9H-adenine

The CASPT2/CASSCF method with the 6-311G basis set and an active space up to (14, 11) was used to explore the ultrafast internal conversion mechanism for excited 9H-adenine. Three minima, two transition states, and seven conical intersections were obtained to build up the two deactivation pathways for the internal conversion mechanism. Special efforts were made to explore the excited-state potential energy surfaces near the Franck-Condon region and determine the various barriers in the processes of deactivation. The barrier required from the 1pipi (1La) state to deactivate nonradiatively is found to be lower than that required from the 1pipi (1Lb) state. On 250 nm excitation, the 1pipi (1La) state is populated, and the transition from 1pipi (1La) to the lowest 1npi state involves very low barriers, which may account for the observed short (<50 fs) lifetime of the 1pipi excited state. The deactivation of the lowest 1npi state is required to overcome a barrier of 3.15 kcal/mol, which should be responsible for the 750 fs lifetime of the npi excited state. On 267 nm excitation, the vibrationally active 1pipi (1Lb) state is populated. Excitation at 277 nm prepares the 1pipi (1Lb) state without much excessive vibrational energy, which may be responsible for the observed >2 ps lifetime.     

On the electronically excited states of uracil

Vertical excitation energies in uracil in the gas phase and in water solution are investigated by the equation-of-motion coupled-cluster and multireference configuration interaction methods. Basis set effects are found to be important for converged results. The analysis of electronic wave functions reveals that the lowest singlet states are predominantly of a singly excited character and are therefore well described by single-reference equation-of-motion methods augmented by a perturbative triples correction to account for dynamical correlation.Our best estimates for the vertical excitation energies for the lowest singlet n --> pi* and pi --> pi* are 5.0 +/- 0.1 eV and 5.3 +/- 0.1 eV, respectively. The solvent effects for these states are estimated to be +0.5 eV and +/- 0.1 eV, respectively. We attribute the difference between the computed vertical excitations and the maximum of the experimental absorption to strong vibronic interaction between the lowest A" and A' states leading to intensity borrowing by the forbidden transition.     

On the internal rotations in p-cresol in its ground and first electronically excited states

The overall rotation and internal rotation of p-cresol (4-methyl-phenol) has been studied by comparison of the microwave spectrum with accurate ab initio calculations using the principal axis method in the electronic ground state. Both internal rotations, the torsions of the methyl and the hydroxyl groups relative to the aromatic ring, have been investigated. The internal rotation of the hydroxyl group can be approximately described as the motion of a symmetrical rotor on an asymmetric frame. For the methyl group it has been found that the potential barrier hindering its internal rotation is very small with the first two nonvanishing Fourier coefficients of the potential V(3) and V(6) in the same order of magnitude. Different splittings of b-type transitions for the A and E species of the methyl torsion indicate a top-top interaction between both internal rotors through the benzene ring. An effective coupling potential for the top-top interaction could be estimated. The hindering barriers of the hydroxyl and methyl rotation have been calculated using second-order Moller-Plesset perturbation theory and the approximate coupled-cluster singles-and-doubles model (CC2) in the ground state and using CC2 and the algebraic diagrammatic construction through second order in the first electronically excited state. The results are in excellent agreement with the experimental values.     

Time-resolved photoelectron imaging of the chloranil radical anion: ultrafast relaxation of electronically excited electron acceptor states

The spectroscopy and dynamics of near-threshold excited states of the isolated chloranil radical anion are investigated using photoelectron imaging. The photoelectron images taken at 480 nm clearly indicate resonance-enhanced photodetachment via a bound electronic excited state. Time-resolved photoelectron imaging reveals that the excited state rapidly decays on a timescale of 130 fs via internal conversion. The ultrafast relaxation dynamics of excited states near threshold are pertinent to common electron acceptor molecules based on the quinone moiety and may serve as doorway states that enable efficient electron transfer in the highly exergonic inverted regime, despite the presence of large free energy barriers.     

Transient lens spectroscopy of ultrafast internal conversion processes in citranaxanthin

The ultrafast internal conversion (IC) dynamics of the apocarotenoid citranaxanthin have been studied for the first time by means of two-color transient lens (TL) pump-probe spectroscopy. After excitation into the high-energy edge of the S2 band by a pump pulse at 400 nm, the subsequent intramolecular processes were probed at 800 nm. Experiments were performed in a variety of solvents at room temperature. Upper limits for the S2 lifetime tau2 on the order of 100-200 fs are estimated. The S1 lifetime tau1 varies only slightly between solvents (10-12 ps), and the only clear decrease is observed for methanol (8.5 ps). The findings are consistent with earlier results from transient absorption studies of other apocarotenoids and carotenoid ketones and transient lens experiments of C40 carbonyl carotenoids. Possible reasons for the observed weak solvent dependence of tau1 for citranaxanthin are discussed.     

Complex dynamics at conical intersections: vibronic spectra and ultrafast decay of electronically excited trifluoroacetonitrile radical cation

An ab initio quantum dynamical study is performed here to examine the complex nuclear motion underlying the first two photoelectron bands of trifluoroacetonitrile. The highly overlapping structures of the latter are found to originate from transitions to the five lowest electronic states (viz., X(2)E, A(2)A1, B(2)A2, C(2)A1, and D(2)E) of the trifluoroacetonitrile radical cation. The Jahn-Teller (JT) instability of the doubly degenerate X and, D and their pseudo-Jahn-Teller (PJT) interactions with the nondegenerate A, B, and C electronic states along the degenerate vibrational modes lead to multiple multidimensional conical intersections and complex nuclear trajectories through them. It is found that the JT splitting is very weak in the X and relatively stronger in the D state. However, the PJT couplings play the pivotal role in the detailed shape of the vibronic bands of the radical cation. Ultrafast nonradiative decay of electronically excited radical cation has been examined. The findings of this paper are compared with the experimental data and are also discussed in relation to those observed for the methyl cyanide radical cation.     

Ab initio based surface-hopping dynamics study on ultrafast internal conversion in cyclopropanone

Cyclopropanone exhibits an intriguing phenomenon that the fluorescence from the S(1) state disappears below 365 nm. This is ascribed to the ultrafast S(1) → S(0) internal conversion process via conical intersection, which deprives opportunity of the fluorescence emission. In this work, we have used ab initio based surface hopping dynamics method to study vibrational-mode-dependent S(1) → S(0) internal conversion of cyclopropanone. A new conical intersection between the S(1) and S(0) states is determined by the state-averaged CASSCF/cc-pVDZ calculations, which is confirmed to play a critical role in the ultrafast S(1) → S(0) internal conversion by the nonadiabatic dynamics simulations. It is found that the internal conversion occurs more efficiently when the initial kinetic energies are distributed in the four vibrational modes related to the C═O group, especially in the C-O stretching and the O-C-C-C out-of-plane torsional modes. Meanwhile, the S(1) lifetime and the time scale of the S(1) → S(0) internal conversion are estimated by the ab initio based dynamics simulations, which is consistent with the ultrafast S(1) → S(0) internal conversion and provides further evidence that the ultrafast internal conversion is responsible for the fluorescence disappearance of cyclopropanone.     

Transient and stationary spectroscopy of cytochrome c: ultrafast internal conversion controls photoreduction

Photoreduction of cytochrome c (Cyt c) has been reinvestigated using femtosecond-to-nanosecond transient absorption and stationary spectroscopy. Femtosecond spectra of oxidized Cyt c, recorded in the probe range 270-1000 nm, demonstrate similar evolution upon 266 or 403 nm excitation: an ultrafast 0.3 ps internal conversion followed by a 4 ps vibrational cooling. Late transient spectra after 20 ps, from the cold ground-state chromophore, reveal a small but measurable signal from reduced Cyt c. The yield phi for Fe3+-->Fe2+ photoreduction is measured to be phi(403) = 0.016 and phi(266) = 0.08 for 403 and 266 nm excitation. These yields lead to a guess of the barrier E(f)(A) = 55 kJ mol(-1) for thermal ground-state electron transfer (ET). Nanosecond spectra initially show the typical absorption from reduced Cyt c and then exhibit temperature-dependent sub-microsecond decays (0.5 micros at 297 K), corresponding to a barrier E(A)(b) = 33 kJ mol(-1) for the back ET reaction and a reaction energy DeltaE = 22 kJ mol(-1). The nanosecond transients do not decay to zero on a second time scale, demonstrating the stability of some of the reduced Cyt c. The yields calculated from this stable reduced form agree with quasistationary reduction yields. Modest heating of Cyt c leads to its efficient thermal reduction as demonstrated by differential stationary absorption spectroscopy. In summary, our results point to ultrafast internal conversion of oxidized Cyt c upon UV or visible excitation, followed by Fe-porphyrin reduction due to thermal ground-state ET as the prevailing mechanism.     

N-dealkylation of electronically excited coumarins by nitrobenzenes

When excited (monochromatically or with λ ⩾ 280 nm) in benzene solution in the presence of nitroaromatic acceptors, several 7-dialkylamino-coumarins undergo moderately efficient mono-N-dealkylation to form 7-alkylaminocoumarins, accompanied by the reduction of the nitro function in the acceptor. For 7-(N,N-diethylamino)-4-methylcoumarin (1a) a linear plot of ø_P⁻¹ versus the reciprocal of the concentration of the starting material suggests that two molecules of 1a are involved in the dealkylation of irradiated 1a by ground state 3-chloronitrobenzene. When the concentration of 1a is kept constant but that of the acceptor is varied, ø_P goes through a maximum at intermediate concentrations. Furthermore, ø_P is found to be larger for weaker acceptors (more negative reduction potential) than for stronger acceptors (less negative reduction potential). The rigid dye Coumarin 102 does not show any analogous decomposition. All results are interpreted in terms of the interference of the nitroaromatic with the self-quenching mechanism of coumarin N-dealkylation put forward recently by Jones and coworkers.     

Ultrafast internal conversion of excited cytosine via the lowest pipi electronic singlet state

Computational evidence at the CASPT2 level supports that the lowest excited state pipi* contributes to the S1/S0 crossing responsible for the ultrafast decay of singlet excited cytosine. The computed radiative lifetime, 33 ns, is consistent with the experimentally derived value, 40 ns. The nOpi* state does not play a direct role in the rapid repopulation of the ground state; it is involved in a S2/S1 crossing. Alternative mechanisms through excited states pisigma* or nNpi* are not competitive in cytosine.     

Unified model for the ultrafast decay of pyrimidine nucleobases

Ultrafast decay processes detected after absorption of UV radiation in gas-phase pyrimidine nucleobases uracil, thymine, and cytosine are ascribed to the barrierless character of the pathway along the low-lying 1(pipi*) hypersurface connecting the Franck-Condon region with an out-of-plane distorted ethene-like conical intersection with the ground state. Longer lifetime decays and low quantum yield emission are on the other hand related to the presence of a 1(pipi*) state planar minimum on the S1 surface and the barriers to access other conical intersections. A unified model for the three systems is established on the basis of accurate multiconfigurational CASPT2 calculations, whereas the effect of the different levels of theory on the results is carefully analyzed.     

Coherent oscillation and ultrafast internal conversion of tetrafluoroethene after excitation at 197 nm.

C2F4 was excited by using a 150 fs pulse in its longest-wavelength band to the Rydberg (3 s) state and then probed by photoionization techniques at 810 nm. The molecule relaxes in two consecutive steps (time constants 29 and 118 fs), probably via the pipi* state, which is lowered in energy by stretching and twisting the C=C bond. A coherent oscillation (350 fs) was found, which we assign to an overtone of the twist vibration (47.6 cm(-1)) in this state. we also conclude that dissociation to singlet and some triplet CF2 only takes place in the hot ground state of C2F4, from where also the C2F4 triplet state is populated. The potentials and their conical intersections are discussed with respect to relaxation and dissociation, including also some new considerations of thermal processes.     

Hydrogen bonding in electronically excited states of molecules

Theoretical Chemistry Accounts - CNDO/2 calculations show that hydrogen bonds in the electronically excited states of +H2O and +HOCH3 systems are slightly weaker than in the ground states. The...     

Supersonic expansion cooling of electronically excited OH radicals

Supersonic nozzle cooling of excited-state melecular radicals produced in a corona discharge is demonstrated and investigated using hydroxyl radicals. Rotational temperatures of 11 ± 2 K and vibrational temperatures of 3400 ± 300 K are typically observed in the OH A ²Σ⁺ state.     

Theoretical investigation of the electronically excited states of chlorine hydrate

As a step toward a first principles characterization of the optical properties of chlorine hydrate, we have calculated the electronic absorption spectrum of a chlorine molecule trapped in dodecahedral (H2O)20 and hexakaidodecahedral (H2O)24 cages. For comparison, spectra were also calculated for an isolated Cl2 molecule as well as for selected Cl2(H2O)n, n < or =8, clusters cut out of the Cl2(H2O)20 cluster, allowing us to follow the evolution of the low-lying excited states with increasing number of surrounding water molecules. Although encapsulation of a chlorine molecule within the water cages has relatively little effect on its low-lying valence transitions, it does result in a large number of solvent-to-solute charge-transfer transitions at energies starting near 48,000 cm(-1).     

Modeling the nonradiative decay rate of electronically excited thioflavin T

A computational model of nonradiative decay is developed and applied to explain the time-dependent emission spectrum of thioflavin T (ThT). The computational model is based on a previous model developed by Glasbeek and co-workers (van der Meer, M. J.; Zhang, H.; Glasbeek, M. J. Chem. Phys. 2000, 112, 2878) for auramine O, a molecule that, like ThT, exhibits a high nonradiative rate. The nonradiative rates of both auramine O and ThT are inversely proportional to the solvent viscosity. The Glasbeek model assumes that the excited state consists of an adiabatic potential surface constructed by adiabatic coupling of emissive and dark states. For ThT, the twist angle between the benzothiazole and the aniline is responsible for the extensive mixing of the two excited states. At a twist angle of 90°, the S(1) state assumes a charge-transfer-state character with very small oscillator strength, which causes the emission intensity to be very small as well. In the ground state, the twist angle of ThT is rather small. The photoexcitation leads first to a strongly emissive state (small twist angle). As time progresses, the twist angle increases and the oscillator strength decreases. The fit of the experimental results by the model calculations is good for times longer than 3 ps. When a two-coordinate model is invoked or a solvation spectral-shift component is added, the fit to the experimental results is good at all times.     

Ultrafast internal conversion in ethylene. I. The excited state lifetime

Using a combined theoretical and experimental approach, we investigate the non-adiabatic dynamics of the prototypical ethylene (C(2)H(4)) molecule upon π → π? excitation. In this first part of a two part series, we focus on the lifetime of the excited electronic state. The femtosecond time-resolved photoelectron spectrum (TRPES) of ethylene is simulated based on our recent molecular dynamics simulation using the ab initio multiple spawning method with multi-state second order perturbation theory [H. Tao, B. G. Levine, and T. J. Martinez, J. Phys. Chem. A 113, 13656 (2009)]. We find excellent agreement between the TRPES calculation and the photoion signal observed in a pump-probe experiment using femtosecond vacuum ultraviolet (hν = 7.7 eV) pulses for both pump and probe. These results explain the apparent discrepancy over the excited state lifetime between theory and experiment that has existed for ten years, with experiments [e.g., P. Farmanara, V. Stert, and W. Radloff, Chem. Phys. Lett. 288, 518 (1998) and K. Kosma, S. A. Trushin, W. Fuss, and W. E. Schmid, J. Phys. Chem. A 112, 7514 (2008)] reporting much shorter lifetimes than predicted by theory. Investigation of the TRPES indicates that the fast decay of the photoion yield originates from both energetic and electronic factors, with the energetic factor playing a larger role in shaping the signal.     

Theoretical study of the photodimerization of pyrimidine bases

The experimental results are correctly interpreted as from the delocalized model of the transition state and the theory of perturbations. The surfaces of the energy of interaction between both molecules in function of different coordinates of reaction make it possible to discuss the concerted nature of photodimerization and permit the two conjugated systems to be added between carbon atoms 5 and 6 of both molecules.

---

Zusammenfassung Die experimentellen Resultate werden mit Hilfe eines delokalisierten Modells des Übergangszustandes und der Störungstheorie richtig interpretiert. Die Energiehyperfläche der Wechselwirkung zwischen den beiden Molekülen als Funktion verschiedener Reaktionskoordinaten erlaubt die Diskussion des konzertierten Ablaufs der Photodimerisation und der Addition der beiden konjugierten Systeme, die zwischen den Kohlenstoffatomen 5 und 6 der beiden Moleküle stattfindet.

---

Résumé Les résultats expérimentaux sont correctement interprétés à partir du modèle delocalisé de l'état de transition et de la théorie des perturbations. Les surfaces de l'énergie d'interaction entre les deux molécules en fonction aux différentes coordonnées de réaction permettent de discuter le caractère concerté de la photodimerisation et que l'addition des deux systèmes conjugués se réalise entre les carbones 5 et 6 des deux molécules.

     

DNA excited-state dynamics: ultrafast internal conversion and vibrational cooling in a series of nucleosides

To better understand DNA photodamage, several nucleosides were studied by femtosecond transient absorption spectroscopy. A 263-nm, 150-fs ultraviolet pump pulse excited each nucleoside in aqueous solution, and the subsequent dynamics were followed by transient absorption of a femtosecond continuum pulse at wavelengths between 270 and 700 nm. A transient absorption band with maximum amplitude near 600 nm was detected in protonated guanosine at pH 2. This band decayed in 191 +/- 4 ps in excellent agreement with the known fluorescence lifetime, indicating that it arises from absorption by the lowest excited singlet state. Excited state absorption for guanosine and the other nucleosides at pH 7 was observed in the same spectral region, but decayed on a subpicosecond time scale by internal conversion to the electronic ground state. The cross section for excited state absorption is very weak for all nucleosides studied, making some amount of two-photon ionization of the solvent unavoidable. The excited state lifetimes of Ado, Guo, Cyd, and Thd were determined to be 290, 460, 720, and 540 fs, respectively (uncertainties are +/-40 fs). The decay times are shorter for the purines than for the pyrimidine bases, consistent with their lower propensity for photochemical damage. Following internal conversion, vibrationally highly excited ground state molecules were detected in experiments on Ado and Cyd by hot ground state absorption at ultraviolet wavelengths. The decays are assigned to intermolecular vibrational energy transfer to the solvent. The longest time constant observed for Ado is approximately 2 ps, and we propose that solute-solvent H-bonds are responsible for this fast rate of vibrational cooling. The results show for the first time that excited singlet state dynamics of the DNA bases can be directly studied at room temperature. Like sunscreens that function by light absorption, the bases rapidly convert dangerous electronic energy into heat, and this property is likely to have played a critical role in life's early evolution on earth.