Solvent effect on the singlet excited-state dynamics of 5-fluorouracil in acetonitrile as compared with water

The excited-state dynamics of 5-fluorouracil in acetonitrile has been investigated by femtosecond fluorescence upconversion spectroscopy in combination with quantum chemistry TD-DFT calculations ((PCM/TD-PBE0). Experimentally, it was found that when going from water to acetonitrile solution the fluorescence decay of 5FU becomes much faster. The calculations show that this is related to the opening of an additional decay channel in acetonitrile solution since the dark n/pi* excited state becomes near degenerate with the bright pi/pi* state, forming a conical intersection close to the Franck-Condon region. In both solvents, a S1-S0 conical intersection, governed by the out-of-plane motion of the fluorine atom, is active, allowing an ultrafast internal conversion to the ground state.     

Theoretical study on the singlet excited state of pterin and its deactivation pathway

The excited-state properties and related photophysical processes of the acidic and basic forms of pterin have been investigated by the density functional theory and ab initio methodologies. The solvent effects on the low-lying states have been estimated by the polarized continuum model and combined QM/MM calculations. Calculations reveal that the observed two strong absorptions arise from the strong pi --> pi* transitions to 1(pipi*L(a)) and 1(pipi*L(b)) in the acidic and basic forms of pterin. The first 1(pipi*L(a)) excited state is exclusively responsible for the experimental emission band. The vertical 1(n(N)pi*) state with a small oscillator strength, slightly higher in energy than the 1(pipi*L(a)) state, is less accessible by the direct electronic transition. The 1(n(N)pi*) state may be involved in the photophysical process of the excited pterin via the 1(pipi*L(a)/n(N)pi*) conical intersection. The radiationless decay of the excited PT to the ground state experiences a barrier of 13.8 kcal/mol for the acidic form to reach the (S(1)/S(0)) conical intersection. Such internal conversion can be enhanced with the increase in excitation energy, which will reduce the fluorescence intensity as observed experimentally.     

Ab initio study on deactivation pathways of excited 9H-guanine

The complete active space with second-order perturbation theory/complete active space self-consistent-field method was used to explore the nonradiative decay mechanism for excited 9H-guanine. On the 1pipi* (1L(a)) surface we determined a conical intersection (CI), labeled (S0pipi*)(CI), between the 1pipi* (1L(a)) excited state and the ground state, and a minimum, labeled (pipi*)min. For the 1pipi* (1L(a)) state, its probable deactivation path is to undergo a spontaneous relaxation to (pipi*)min first and then decay to the ground state through (S0pipi*)(CI), during which a small activation energy is required. On the 1n(N)pi* surface a CI between the 1n(N)pi* and 1pipi* (1L(a)) states was located, which suggests that the 1n(N)pi* excited state could transform to the 1pipi* (1L(a)) excited state first and then follow the deactivation path of the 1pipi* (1L(a)) state. This CI was also possibly involved in the nonradiative decay path of the second lowest 1pipi* (1L(b)) state. On the 1n(O)pi* surface a minimum was determined. The deactivation of the 1n(O)pi* state to the ground state was estimated to be energetically unfavorable. On the 1pisigma* surface, the dissociation of the N-H bond of the six-membered ring is difficult to occur due to a significant barrier.     

Direct versus indirect H atom elimination from photoexcited phenol molecules

The active role of the optically dark pi sigma* state, following UV absorption, has been implicated in the photochemistry of a number of biomolecules. This work focuses on the role of the pi sigma* state in the photochemistry of phenol upon excitation at 200 nm. By probing the neutral hydrogen following UV excitation, we show that hydrogen elimination along the dissociative pi sigma* potential energy surface occurs within 103 +/- 30 fs, indicating efficient coupling at the S1/S2 and S0/S2 conical intersections, with no identifiable role of statistical unimolecular decay of vibronically excited (S0) phenol in the timeframe of our measurements.     

The fluorescence mechanism of 5-methyl-2-pyrimidinone: an ab initio study of a fluorescent pyrimidine analog

The photophysically important potential energy surfaces of the fluorescent pyrimidine analog 5-methyl-2-pyrimidinone have been explored using multireference configuration-interaction ab initio methods at three levels of dynamical correlation, all of which support a fluorescence mechanism. At vertical excitation S1 (dark, n(N)pi*) and S2 (bright, pipi*) are almost degenerate at 4.4 eV, with S3 (dark, n(O)pi*) at 5.1 eV. The excited system can follow the S1-S2 seam of conical intersections, accessible from the Franck-Condon region, to its minimum and then evolve from this conical intersection on the S1 (pipi*) surface to a global minimum. At lower levels of correlation, the S1 surface shows two minima separated by a barrier of up to 0.18 eV. The secondary minimum found at the lower levels of correlation becomes the global minimum with higher correlation. The S1 population at this minimum can be trapped from accessing the lowest energy S0-S1 (pipi*/gs) conical intersection by an energy gap at least 0.3-0.4 eV higher than the S1 minimum. The calculated emission energy from this minimum is 2.80 eV. Gradient pathways connecting important S1 geometries are presented, as well as other excited state conical intersections.     

Quantum chemical investigation of the electronic spectra of the keto, enol, and keto-imine tautomers of cytosine

The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi --> pi and n --> pi excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitation energy of the pi --> pi state (4.06 eV including zero-point vibrational energy corrections) supports the resonant two-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys. 2002, 5, 4780). On its S1 potential energy surface, a conical intersection between the 1pipi state and the electronic ground state has been identified. The barrier height of the reaction along a constrained minimum energy path amounts to merely 0.2 eV above the origin and explains the break-off of the R2PI spectrum. The 1pipi minimum of the enol tautomer is found at considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect to the ground state, long vibrational progressions are expected, in accord with experimental observations. For the keto-imine tautomer, a crossing of the 1pipi potential energy surface with the ground-state surface has been found, too. Its n --> pi minimum (3.27 eV) is located well below the conical intersection between the pi --> pi and S0 states, but it will be difficult to observe because of its small transition moment. The identified conical intersections of the pi --> pi excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.     

A new pathway for the rapid decay of electronically excited adenine

Combined density functional and multireference configuration interaction methods have been used to calculate the electronic spectrum of 9H-adenine, the most stable tautomer of 6-aminopurine. In addition, constrained minimum energy paths on excited potential energy hypersurfaces have been determined along several relaxation coordinates. The minimum of the first (1)[n-->pi*] state has been located at an energy of 4.54 eV for a nuclear arrangement in which the amino group is pyramidal whereas the ring system remains planar. Close by, another minimum on the S(1) potential energy hypersurface has been detected in which the C(2) center is deflected out of the molecular plane and the electronic character of S(1) corresponds to a nearly equal mixture of (1)[pi-->pi*] and (1)[n-->pi*] configurations. The adiabatic excitation energy of this minimum amounts to 4.47 eV. Vertical and adiabatic excitation energies of the lowest n-->pi* and pi-->pi* transitions as well as transition moments and their directions are in very good agreement with experimental data and lend confidence to the present quantum chemical treatment. On the S(1) potential energy hypersurface, an energetically favorable path from the singlet n-->pi* minimum toward a conical intersection with the electronic ground state has been identified. Close to the conical intersection, the six-membered ring of adenine is strongly puckered and the electronic structure of the S(1) state corresponds to a pi-->pi* excitation. The energetic accessibility of this relaxation path at about 0.1 eV above the singlet n-->pi* minimum is presumably responsible for the ultrafast decay of 9H-adenine after photoexcitation and explains why sharp vibronic peaks can only be observed in a rather narrow wavelength range above the origin. The detected mechanism should be equally applicable to adenosine and 9-methyladenine because it involves primarily geometry changes in the six-membered ring whereas the nuclear arrangement of the five-membered ring (including the N(9) center) is largely preserved.     

A three-state model for the photophysics of guanine

The nonadiabatic photochemistry of the guanine molecule (2-amino-6-oxopurine) and some of its tautomers has been studied by means of the high-level theoretical ab initio quantum chemistry methods CASSCF and CASPT2. Accurate computations, based by the first time on minimum energy reaction paths, states minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of the molecules lead to interpret the photochemistry of guanine and derivatives within a three-state model. As in the other purine DNA nucleobase, adenine, the ultrafast subpicosecond fluorescence decay measured in guanine is attributed to the barrierless character of the path leading from the initially populated 1(pi pi* L(a)) spectroscopic state of the molecule toward the low-lying methanamine-like conical intersection (gs/pi pi* L(a))CI. On the contrary, other tautomers are shown to have a reaction energy barrier along the main relaxation profile. A second, slower decay is attributed to a path involving switches toward two other states, 1(pi pi* L(b)) and, in particular, 1(n(O) pi*), ultimately leading to conical intersections with the ground state. A common framework for the ultrafast relaxation of the natural nucleobases is obtained in which the predominant role of a pi pi*-type state is confirmed.     

Time-dependent quantum wave-packet description of the 1pi sigma* photochemistry of phenol

The photoinduced hydrogen elimination reaction in phenol via the conical intersections of the dissociative 1pi sigma* state with the 1pi pi* state and the electronic ground state has been investigated by time-dependent quantum wave-packet calculations. A model including three intersecting electronic potential-energy surfaces (S0, 1pi sigma*, and 1pi pi*) and two nuclear degrees of freedom (OH stretching and OH torsion) has been constructed on the basis of accurate ab initio multireference electronic-structure data. The electronic population transfer processes at the conical intersections, the branching ratio between the two dissociation channels, and their dependence on the initial vibrational levels have been investigated by photoexciting phenol from different vibrational levels of its ground electronic state. The nonadiabatic transitions between the excited states and the ground state occur on a time scale of a few tens of femtoseconds if the 1pi pi*-1pi sigma* conical intersection is directly accessible, which requires the excitation of at least one quantum of the OH stretching mode in the 1pi pi* state. It is shown that the node structure, which is imposed on the nuclear wave packet by the initial preparation as well as by the transition through the first conical intersection (1pi pi*-1pi sigma*), has a profound effect on the nonadiabatic dynamics at the second conical intersection (1pi sigma*-S0). These findings suggest that laser control of the photodissociation of phenol via IR mode-specific excitation of vibrational levels in the electronic ground state should be possible.     

Solvent effect on the singlet excited-state lifetimes of nucleic acid bases: A computational study of 5-fluorouracil and uracil in acetonitrile and water

The first comprehensive quantum mechanical study of solvent effects on the behavior of the two lowest energy excited states of uracil derivatives is presented. The absorption and emission spectra of uracil and 5-fluorouracil in acetonitrile and aqueous solution have been computed at the time-dependent density-functional theory level, using the polarizable continuum model (PCM) to take into account bulk solvent effects. The computed spectra and the solvent shifts provided by our method are close to their experimental counterpart. The S0/S1 conical intersection, located in the presence of hydrogen-bonded solvent molecules by CASSCF (8/8) calculations, indicates that the mechanism of ground-state recovery, involving out-of-plane motion of the 5 substituent, does not depend on the nature of the solvent. Extensive explorations of the excited-state surfaces in the Franck-Condon (FC) region show that solvent can modulate the accessibility of an additional decay channel, involving a dark n/pi* excited state. This finding provides the first unifying explanation for the experimental trend of 5-fluorouracil excited-state lifetime in different solvents. The microscopic mechanisms underlying solvent effects on the excited-state behavior of nucleobases are discussed.     

Ab initio studies on the photophysics of guanine tautomers: out-of-plane deformation and NH dissociation pathways to conical intersections

The radiationless decay mechanisms of the S1 excited states of the 7H-keto-amino, 7H-enol-amino, and 7H-keto-imino tautomers of guanine have been investigated with the CASPT2//CASSCF method. Out-of-plane deformation of the six-membered ring or the imino group as well as dissociation of NH bonds have been considered as photochemical pathways leading to conical intersections with the electronic ground state. It has been found that all three tautomers can reach S0-S1 conical intersections by out-of-plane deformation. However, only in the 7H-keto-amino tautomer the reaction path leading to the conical intersection is barrierless. This tautomer also has the lowest energy barrier for hydrogen detachment via the (1)pi sigma* state, whose potential energy surface intersects that of the (1)pi pi* state as well as that of the ground state. The other tautomers of guanine exhibit substantial energy barriers on their S1 potential energy surfaces with respect to both reaction mechanisms. These findings suggest that the 7H-keto-amino tautomer exhibits the shortest excited-state lifetime of the three tautomers due to particularly fast nonradiative deactivation processes through S0-S1 conical intersections. The computational results explain the remarkable observation that the energetically most stable 7H-keto-amino tautomer is missing in the resonant two-photon ionization spectrum of guanine in a supersonic jet. The results also explain that the energetically less stable 7H-enol-amino and 7H-keto-imino tautomers have longer excited-state lifetimes and are thus detectable by resonant two-photon ionization.     

"H/Vinyl" and "Alkyl/Vinyl" conical intersections leading to carbene formation from the excited states of cyclohexene and norbornene

The photochemical [1,2]-shifts leading to carbene intermediates from cyclohexene and norbornene have been studied using CASSCF calculations and a 6-31G* basis set. In each case, a S(1)/S(0) conical intersection hyperline was identified that extends from the region of the reactant excited state to the carbene product. It is traditionally thought that the Rydberg R(pi,3s) state is responsible for carbene formation on photolysis of cyclic alkenes, but these new results indicate an efficient mechanism for carbene formation following excitation to the (1)(pipi*) state. This pathway is essentially barrierless and involves internal conversion to the ground-state surface via conical intersections between the excited zwitterionic valence state and the ground-state surface, similar to those responsible for cis-trans isomerization in ethene and other acyclic alkenes. These results are in excellent agreement with recent experimental data obtained using femtosecond spectroscopy.     

Energetics of cytosine singlet excited-state decay paths--a difficult case for CASSCF and CASPT2

Three deactivation paths for singlet excited cytosine are calculated at the CASPT2//CASSCF (complete active space second-order perturbation//complete active space self-consistent field) level of theory, using extended active spaces that allow for a reliable characterization of the paths and their energies. The lowest energy path, with a barrier of approximately 0.1 eV, corresponds to torsion of the C5-C6 bond, and the decay takes place at a conical intersection analogous to the one found for ethylene and its derivatives. There is a further path with a low energy barrier of approximately 0.2 eV associated with the (n(N),pi*) state which could also be populated with a low energy excitation. The path associated with a conical intersection between the ground and (n(O),pi*) states is significantly higher in energy (> 1 eV). The presence of minima on the potential energy surface for the (n,pi*) states that could contribute to the biexponential decay found in the gas phase was investigated, but could not be established unequivocally.     

The different photoisomerization efficiency of azobenzene in the lowest n pi* and pi pi* singlets: the role of a phantom state

Azobenzene E<==>Z photoisomerization, following excitation to the bright S(pi pi*) state, is investigated by means of ab initio CASSCF optimizations and perturbative CASPT2 corrections. Specifically, by elucidating the S(pi pi*) deactivation paths, we explain the mechanism responsible for azobenzene photoisomerization, the lower isomerization quantum yields observed for the S(pi pi*) excitation than for the S1(n pi*) excitation in the isolated molecule, and the recovery of the Kasha rule observed in sterically hindered azobenzenes. We find that a doubly excited state is a photoreaction intermediate that plays a very important role in the decay of the bright S(pi pi*). We show that this doubly excited state, which is immediately populated by molecules excited to S(pi pi*), drives the photoisomerization along the torsion path and also induces a fast internal conversion to the S1(n pi*) at a variety of geometries, thus shaping (all the most important features of) the S(pi pi*) decay pathway and photoreactivity. We reach this conclusion by determining the critical structures, the minimum energy paths originating on the bright S(pi pi*) state and on other relevant excited states including S1(n pi*), and by characterizing the conical intersection seams that are important in deciding the photochemical outcome. The model is consistent with the most recent time-resolved spectroscopic and photochemical data.     

Theoretical characterization of photoisomerization channels of dimethylpyridines on the singlet and triplet potential energy surfaces

Photoexcitations and photoisomerizations due to low-lying n pi* and pi pi* excited states of dimethylpyridines are investigated by density functional theory, CASSCF, CASPT2 and MRCI methodologies. Mechanistic details for the formation of Dewar dimethylpyridines and the interconversions of dimethylpyridines are rationalized through the characterization of minima and transition states on the singlet and triplet potential energy surfaces of relevant intermediates. Our present theoretical schemes suggest that Mobius dimethylpyridine intermediate 14 and azabenzvalene intermediate 10 can serve as possible precursors to Dewar dimethylpyridines and singlet phototransposition products, respectively. The calculations suggest that an S1(pi pi*)/S0 conical intersection in dimethylpyridines 2 is involved in the formation of 14. An azabenzvalene 10 might be formed through S2(pi pi*)/S1(n pi*) interaction followed by an S1/S0 decay in dimethylpyridine 6. Calculated barriers of isomerizations from 14 to Dewar dimethylpyridine 7 and from 10 to 4 are 8.4 and 28.5 kcal mol(-1) at the B3LYP/6-311 G** level, respectively. In the suggested triplet multistage transposition mechanism, an out-of-plane distorted geometry 19 due to vibrational relaxation of the T1(3B1) excited state of 3,5-dimethylpyridine 6 is a precursor of the interconversion of 6 to 2.4-dimethylpyridine 4. The formation of a triplet azaprefulvene 21 with a barrier of 20.7 kcal mol(-1) is a key step during the triplet migration process leading to another out-of-plane distorted structure 27. Subsequent rearomatization of 27 completes the interconversion of 6 with 4. Present calculations provide some insight into the photochemistry of dimethylpyridines at 254 nm.     

Conical intersections in thymine

The mechanisms which are responsible for the radiationless deactivation of the npi* and pipi* excited singlet states of thymine have been investigated with multireference ab initio methods (the complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory with respect to the CASSCF reference (CASPT2)) as well as with the CC2 (approximated singles and doubles coupled-cluster) method. The vertical excitation energies, the equilibrium geometries of the 1npi*and 1pipi* states, as well as their adiabatic excitation energies have been determined. Three conical intersections of the S1 and S0 energy surfaces have been located. The energy profiles of the excited states and the ground state have been calculated with the CASSCF method along straight-line reaction paths leading from the ground-state equilibrium geometry to the conical intersections. All three conical intersections are characterized by strongly out-of-plane distorted geometries. The lowest-energy conical intersection (CI1) arises from a crossing of the lowest 1pipi* state with the electronic ground state. It is found to be accessible in a barrierless manner from the minimum of the 1pipi* state, providing a direct and fast pathway for the quenching of the population of the lowest optically allowed excited states of thymine. This result explains the complete diffuseness of the absorption spectrum of thymine in supersonic jets. The lowest vibronic levels of the optically nearly dark 1npi* state are predicted to lie below CI1, explaining the experimental observation of a long-lived population of dark excited states in gas-phase thymine.     

Theoretical insight into the spectroscopy and photochemistry of isoalloxazine, the flavin core ring

The electronic singlet-singlet and singlet-triplet electronic transitions of the isoalloxazine ring of the flavin core are studied using second-order perturbation theory within the framework of the CASPT2//CASSCF protocol. The main features of the absorption spectrum are computed at 3.09, 4.28, 4.69, 5.00, and 5.37 eV. The lowest singlet (S1) and triplet (T1) excited states are found to be both of pi character with a singlet-triplet splitting of 0.57 eV. On the basis of the analysis of the computed spin-orbit couplings and the potential energy hypersurfaces built for the relevant excited states, the intrinsic mechanism for photoinduced population of T1 is discussed. Upon light absorption, evolution of the lowest singlet excited state along the relaxation pathway leads ultimately to the population of the lowest triplet state, which is mediated by a singlet-triplet crossing with a state of npi* type. Subsequently a radiationless decay toward T1 through a conical intersection takes place. The intersystem crossing mechanism and the internal conversion processes documented here provide a plausible route to access the lowest triplet state, which has a key role in the photochemistry of the flavin core ring and is mainly responsible for the reactivity of the system.     

Singlet excited-state dynamics of 5-fluorocytosine and Cytosine: an experimental and computational study

The photophysics of singlet excited 5-fluorocytosine (5FC) was studied in steady-state and time-resolved experiments and theoretically by quantum chemical calculations. Femtosecond transient absorption measurements show that replacement of the C5 hydrogen of cytosine by fluorine increases the excited-state lifetime by 2 orders of magnitude from 720 fs to 73 +/- 4 ps. Experimental evidence indicates that emission in both compounds originates from a single tautomeric form. The lifetime of 5FC is the same within experimental uncertainty in the solvents ethanol and dimethyl sulfoxide. The insensitivity of the S(1) lifetime to the protic nature of the solvent suggests that proton transfer is not the principal quenching mechanism for the excited state. Excited-state calculations were carried out for the amino-keto tautomer of 5FC, the dominant species in polar environments, in order to understand its longer excited-state lifetime. CASSCF and CAS-PT2 calculations of the excited states show that the minimum energy path connecting the minimum of the (1)pi,pi state with the conical intersection responsible for internal conversion has essentially the same energetics for cytosine and 5FC, suggesting that both bases decay nonradiatively by the same mechanism. The dramatic difference in lifetimes may be due to subtle changes along the decay coordinate. A possible reason may be differences in the intramolecular vibrational redistribution rate from the Franck-Condon active, in-plane modes to the out-of-plane modes that must be activated to reach the conical intersection region.     

Ultrafast decay of electronically excited singlet cytosine via a pi,pi* to n(O),pi* state switch

Singlet fluorescence lifetimes of adenosine, cytidine, guanosine, and thymidine, determined by femtosecond pump-probe spectroscopy (Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2000, 122, 9348. Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2001, 123, 10370), show that the excited states produced by 263 nm light in these nucleosides decay in the subpicosecond range (290-720 fs). Ultrafast radiationless decay to the ground state greatly reduces the probability of photochemical damage. In this work we present a theoretical study of isolated cytosine, the chromophore of cytidine. The experimental lifetime of 720 fs indicates that there must be an ultrafast decay channel for this species. We have documented the possible decay channels and approximate energetics, using a valence-bond derived analysis to rationalize the structural details of the paths. The mechanism favored by our calculations and the experimental data involves (1) a two-mode decay coordinate composed of initial bond length inversion followed by internal vibrational energy redistribution (IVR) to populate a carbon pyramidalization mode, (2) a state switch between the pi,pi* and nO,pi* (excitation from oxygen lone pair) excited states, and (3) decay to the ground state through a conical intersention. A second decay path through the nN,pi* state (excitation from the nitrogen lone pair), with a higher barrier, involves out-of-plane bending of the amino substituent.     

Nonradiative decay mechanisms of the biologically relevant tautomer of guanine

We present the excited-state potential energy profiles of the biologically relevant 9H-keto-amino tautomer of guanine with respect to the radiationless decay via the out-of-plane deformation of the six-membered ring as well as the dissociation of NH bonds. The CASPT2//CASSCF method is employed for the reaction-path calculations. The reaction path for the out-of-plane deformation in the (1)pi pi* state leads in a barrierless way to a conical intersection with the electronic ground state. For the NH dissociation via the (1)pi sigma* state, the 9H-keto-amino tautomer is shown to have lower energy barriers than the 7H tautomers which we have studied recently. These two radiationless decay mechanisms explain the unexpected missing of the biologically relevant form in the resonant two-photon ionization spectrum of guanine in a supersonic jet. It is suggested that these ultrafast deactivation processes provide the biologically relevant tautomer of guanine with a high degree of photostability.