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.     

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.     

"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.     

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.     

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.     

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.     

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.     

Quenching of tryptophan (1)(pi,pi*) fluorescence induced by intramolecular hydrogen abstraction via an aborted decarboxylation mechanism

CASSCF computations show that the hydrogen-transfer-induced fluorescence quenching of the (1)(pi,pi*) excited state of zwitterionic tryptophan occurs in three steps: (1) formation of an intramolecular excited-state complex, (2) hydrogen transfer from the amino acid side chain to the indole chromophore, and (3) radiationless decay through a conical intersection, where the reaction path bifurcates to a photodecarboxylation and a phototautomerization route. We present a general model for fluorescence quenching by hydrogen donors, where the radiationless decay occurs at a conical intersection (real state crossing). At the intersection, the reaction responsible for the quenching is aborted, because the reaction path bifurcates and can proceed forward to the products or backward to the reactants. The position of the intersection along the quenching coordinate depends on the nature of the states and, in turn, affects the formation of photoproducts during the quenching. For a (1)(n,pi*) model system reported earlier (Sinicropi, A.; Pogni, R.; Basosi, R.; Robb, M. A.; Gramlich, G.; Nau, W. M.; Olivucci, M. Angew. Chem., Int. Ed. 2001, 40, 4185-4189), the ground and the excited state of the chromophore are hydrogen acceptors, and the excited-state hydrogen transfer is nonadiabatic and leads directly to the intersection point. There, the hydrogen transfer is aborted, and the reaction can return to the reactant pair or proceed further to the hydrogen-transfer products. In the tryptophan case, the ground state is not a hydrogen acceptor, and the excited-state hydrogen transfer is an adiabatic, sequential proton and electron transfer. The decay to the ground state occurs along a second reaction coordinate associated with decarboxylation of the amino acid side chain and the corresponding aborted conical intersection. The results show that, for (1)(pi,pi*) states, the hydrogen transfer alone is not sufficient to induce the quenching, and explain why fluorescence quenching induced by hydrogen donors is less general for (1)(pi,pi*) than for (1)(n,pi*) states.     

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.     

Radiationless decay mechanism of cytosine: an ab initio study with comparisons to the fluorescent analogue 5-methyl-2-pyrimidinone

The ultrafast radiationless decay mechanism of photoexcited cytosine has been theoretically supported by exploring the important potential energy surfaces using multireference configuration-interaction ab initio methods for the gas-phase keto-tautomer free base. At vertical excitation, the bright state is S1 (pipi*) at 5.14 eV, with S2 (nNpi*) and S3 (nOpi*) being dark states at 5.29 and 5.93 eV, respectively. Minimum energy paths connect the Franck-Condon region to a shallow minimum on the pipi* surface at 4.31 eV. Two different energetically accessible conical intersections with the ground state surface are shown to be connected to this minimum. One pathway involves N3 distorting out of plane in a sofa conformation, and the other pathway involves a dihedral twist about the C5-C6 bond. Each of these pathways from the minimum contains a low barrier of 0.14 eV, easily accessed by low vibronic levels. The path involving the N3 sofa distortion leads to a conical intersection with the ground state at 4.27 eV. The other pathway leads to an intersection with the ground state at 3.98 eV, lower than the minimum by about 0.3 eV. Comparisons with our previously reported study of the fluorescent cytosine analogue 5-methyl-2-pyrimidinone (5M2P) reveal remarkably similar conformational distortions throughout the decay pathways of both bases. The different photophysical behavior between the two molecules is attributed to energetic differences. Vertical excitation in cytosine occurs at a much higher energy initially, creating more vibrational energy than 5M2P in the Franck-Condon region, and the minimum S1 energy for 5M2P is too low to access an intersection with the ground state, causing population trapping and fluorescence. Calculations of vertical excitation energies of 5-amino-2-pyrimidinone and 2-pyrimidinone reveal that the higher excitation energy of cytosine is likely due to the presence of the amino group at the 4-position.     

A three-state model for the photophysics of adenine

An ab initio theoretical study at the CASPT2 level is reported on minimum energy reaction paths, state minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of two tautomers of adenine: 9H- and 7H-adenine. The obtained results led to a complete interpretation of the photophysics of adenine and derivatives, both under jet-cooled conditions and in solution, within a three-state model. The ultrafast subpicosecond fluorescence decay measured in adenine is attributed to the low-lying conical intersection (gs/pipi* La)(CI), reached from the initially populated 1(pipi* La) state along a path which is found to be barrierless only in 9H-adenine, while for the 7H tautomer the presence of an intermediate plateau corresponding to an NH2-twisted conformation may explain the absence of ultrafast decay in 7-substituted compounds. A secondary picosecond decay is assigned to a path involving switches towards two other states, 1(pipi* Lb) and 1(npi*), ultimately leading to another conical intersection with the ground state, (gs/npi*), with a perpendicular disposition of the amino group. The topology of the hypersurfaces and the state properties explain the absence of secondary decay in 9-substituted adenines in water in terms of the higher position of the 1(npi*) state and also that the 1(pipi* Lb) state of 7H-adenine is responsible for the observed fluorescence in water. A detailed discussion comparing recent experimental and theoretical findings is given. As for other nucleobases, the predominant role of a pipi*-type state in the ultrafast deactivation of adenine is confirmed.     

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.     

The decay from the dark npi* excited state in uracil: an integrated CASPT2/CASSCF and PCM/TD-DFT study in the gas phase and in water

By integrating the results of MS-CASPT2/CASSCF and TD-PBE0 calculations, we propose a mechanism for the decay of the excited dark state in pyrimidine, fully consistent with all the available experimental results. An effective conical intersection (CI-npi) exists between the spectroscopic pi/pi* excited state (Spi) and a dark n/pi* state (Sn), and a fraction of the population decays to the minimum of Sn (Sn-min). The conical intersection between Sn and the ground-state is not involved in the decay mechanism, because of its high energy gap with respect to Sn-min. On the other hand, especially in hydrogen bonding solvents, the energy gap between Sn-min and CI-npi is rather small. After thermalization in Sn-min, the system can thus recross CI-npi and then quickly proceed on the Spi barrierless path toward the conical intersection with the ground state.     

Time-resolved photoelectron and photoion fragmentation spectroscopy study of 9-methyladenine and its hydrates: a contribution to the understanding of the ultrafast radiationless decay of excited DNA bases

The excited state dynamics of the purine base 9-methyladenine (9Me-Ade) has been investigated by time- and energy-resolved photoelectron imaging spectroscopy and mass-selected ion spectroscopy, in both vacuum and water-cluster environments. The specific probe processes used, namely a careful monitoring of time-resolved photoelectron energy distributions and of photoion fragmentation, together with the excellent temporal resolution achieved, enable us to derive additional information on the nature of the excited states (pipi*, npi*, pisigma*, triplet) involved in the electronic relaxation of adenine. The two-step pathway we propose to account for the double exponential decay observed agrees well with recent theoretical calculations. The near-UV photophysics of 9Me-Ade is dominated by the direct excitation of the pipi* ((1)L(b)) state (lifetime of 100 fs), followed by internal conversion to the npi* state (lifetime in the ps range) via conical intersection. No evidence for the involvement of a pisigma* or a triplet state was found. 9Me-Ade-(H(2)O)(n) clusters have been studied, focusing on the fragmentation of these species after the probe process. A careful analysis of the fragments allowed us to provide evidence for a double exponential decay profile for the hydrates. The very weak second component observed, however, led us to conclude that the photophysics were very different compared with the isolated base, assigned to a competition between (i) a direct one-step decay of the initially excited state (pipi* L(a) and/or L(b), stabilised by hydration) to the ground state and (ii) a modified two-step decay scheme, qualitatively comparable to that occurring in the isolated molecule.     

A theoretical characterization of the photoisomerization channels of 1,2-cyclononadienes on both singlet and triplet potential-energy surfaces

The ground-, (1)(pipi*)-, and (3)(pipi*)-state potential-energy surfaces of 1,2-cyclononadiene and isomeric C(9)H(14) species, as well as 1-methyl-1,2-cyclononadiene and isomeric C(10)H(16) species were all mapped using CASSCF and the 6-31G(d) basis set. Theoretical results were found to be in good agreement with the available experimental observations for both 1,2-cyclononadiene and 1-methyl-1,2-cyclononadiene isomerization reactions under singlet and triplet direct or sensitized irradiation. Extremely efficient decay occurs from the first singlet excited state to the ground state through at least three different conical intersections (surface crossings). The first of these crossing points is accessed by a one-bond ring closure. From this conical intersection point (CI-A or CI-C), some possible subsequent ground-state reaction paths have been identified: 1) intramolecular C--H bond insertion to form the bicyclic photoproduct and 2) intramolecular C--H bond insertion to form tricyclic photoproducts. An excited state [1,3]-sigmatropic shift leads to the second conical intersection (CI-B or CI-E), which can give a three-bond cyclononyne species. Besides these, in the singlet photochemical reactions of 1-methyl-1,2-cyclononadiene, excited-state, one allenic C--H bond insertion leads to a third conical intersection (CI-D). Possible ground-state reaction pathways from this structure lead to the formation of a diene photoproduct or to transannular insertion photoproducts. Moreover, in the case of triplet 1,2-cyclononadiene and 1-methyl-1,2-cyclononadiene photoisomerization reactions, both chemical reactions will adopt a 1,3-biradical (T(1)/S(0)-1, T(1)/S(0)-2, and T(1)/S(0)-3), which may undergo intersystem crossings leading to the formation of tricyclic or bicyclic photoproducts. The results obtained allow a number of predictions to be made.     

Excited state hydrogen transfer dynamics in substituted phenols and their complexes with ammonia: ππ*-πσ* energy gap propensity and ortho-substitution effect

Lifetimes of the first electronic excited state (S(1)) of fluorine and methyl (o-, m-, and p-) substituted phenols and their complexes with one ammonia molecule have been measured for the 0(0) transition and for the intermolecular stretching σ(1) levels in complexes using picosecond pump-probe spectroscopy. Excitation energies to the S(1) (ππ*) and S(2) (πσ*) states are obtained by quantum chemical calculations at the MP2 and CC2 level using the aug-cc-pVDZ basis set for the ground-state and the S(1) optimized geometries. The observed lifetimes and the energy gaps between the ππ* and πσ* states show a good correlation, the lifetime being shorter for a smaller energy gap. This propensity suggests that the major dynamics in the excited state concerns an excited state hydrogen detachment or transfer (ESHD/T) promoted directly by a S(1)/S(2) conical intersection, rather than via internal conversion to the ground-state. A specific shortening of lifetime is found in the o-fluorophenol-ammonia complex and explained in terms of the vibronic coupling between the ππ* and πσ* states occurring through the out-of-plane distortion of the C-F bond.     

Photostability versus photodegradation in the excited-state intramolecular proton transfer of nitro enamines: competing reaction paths and conical intersections

The phototautomerization mechanism of a model nitro enamine (NEA) chromophore (incorporated in the structure of a highly photolabile pesticide, tetrahydro-2-(nitromethylene)-2H-1,3-thiazine) has been studied using complete active space self-consistent field reaction path computations. The optically accessible 1pipi* excited state of NEA involves separation of charge and correlates diabatically with the ground state of the tautomerized acinitro imine (ANI) form. For optimum photostabilization, the 1pipi* state of NEA should be S1: in this case, the tautomer would be efficiently formed via a diabatic intramolecular proton-transfer pathway passing through an S1/S0 conical intersection, followed by a facile thermal back proton-transfer reaction. However, in NEA itself the lowest excited states correspond to nitro group 1npi* states, and there are additional surface crossings that provide a mechanism for populating the 1npi* manifold. The above results indicate that the high photolability observed for the pesticide [Kleier, D.; Holden, I.; Casida, J. E.; Ruzo, L. O. J. Agric. Food Chem. 1985, 33, 998-1000] has to be ascribed to photochemistry originating on the 1npi* manifold of states, populated indirectly from the 1pipi* state.     

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.     

Photoisomerization of disperse red 1 studied with transient absorption spectroscopy and quantum chemical calculations

The photoisomerization of the push-pull substituted azo dye Disperse Red 1 is studied using femtosecond time-resolved absorption spectroscopy and other spectroscopic and computational techniques. In comparison with azobenzene, the pipi* state is more stabilized by the effects of push-pull substitution than the npi* state, but the latter is probably still the lowest in energy. This conclusion is based on the kinetics, anisotropy of the excited state absorption spectrum, the spectra of the ground states, and quantum chemical calculations. The S(1)(npi*) state is formed from the initially excited pipi* state in <0.2 ps, and decays to the ground state with time constants of 0.9 ps in toluene, 0.5 ps in acetonitrile, and 1.4 ps in ethylene glycol. Thermal isomerization transforms the Z isomer produced to the more stable E isomer with time constants of 29 s (toluene), 28 ms (acetonitrile), and 2.7 ms (ethylene glycol). The pathway of photoisomerization is likely to be rotation about the N=N bond. Quantum chemical calculations indicate that along the inversion pathway ground and excited state energy surfaces remain well separated, whereas rotation leads to a region where conical intersections can occur. For the ground-state Z to E isomerization, conclusive evidence is lacking, but inversion is more probably the favored pathway in the push-pull substituted systems than in the parent azobenzene.     

Characterization of the singlet and triplet excited states of 3-chloro-4-methylumbelliferone

An extensive photophysical characterization of 3-chloro-4-methylumbelliferone (3Cl4MU) in the ground-state, S(0), first excited singlet state, S(1), and lowest triplet state, T(1), was undertaken in water, neutral ethanol, acidified ethanol, and basified ethanol. Quantitative measurements of quantum yields (fluorescence, phosphorescence, intersystem crossing, internal conversion, and singlet oxygen formation) together with lifetimes were obtained at room and low temperature in water, dioxane/water mixtures, and alcohols. The different transient species were assigned and a general kinetic scheme is presented, summarizing the excited-state multiequilibria of 3Cl4MU. In water, the equilibrium is restricted to neutral (N*) and anionic (A*) species, both in the ground (pK(a) = 7.2) and first excited singlet states (pK(a)* = 0.5). In dioxane/water mixtures (pH ca. 6), substantial changes of the kinetics of the S(1) state were observed with the appearance of an additional tautomeric T* species. In low water content mixtures (mixture 9:1 v:v), only the neutral (N*) and tautomeric (T*) forms of 3Cl4MU are observed, whereas at higher water content mixtures (water mole fraction superior to 0.45), all three species N*, T*, and A* coexist in the excited state. In the triplet state, in the nonprotic and nonpolar solvent dioxane, the observed transient signals were assigned as the triplet-triplet transition of the neutral form, N*(T(1)) → N*(T(n)). In water, two transient species were observed and are assigned as the triplets of the neutral N*(T(1)) and the anionic form, A*(T(1)) (also obtained in basified ethanol). The phosphorescence spectra and decays of 3Cl4MU, in neutral, acidified, and basified solutions, demonstrate that only these two species N*(T(1)) and A*(T(1)) exist in the lowest lying triplet state, T(1). The radiative channel was found dominant for the deactivation of the anionic species, whereas with the neutral the S(1) ? S(0) internal conversion competes with fluorescence. For both N* and A* the intersystem crossing yield represents a minor deactivation channel for S(1).