A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans-1,3-butadiene, and transtrans-1,3,5-hexatriene. The method has the aim of being accurate to better than 0.5 eV for excitation energies and is expected to provide structural and physical data for the excited states with good reliability. The approach is based on the complete active space (CAS) SCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second order perturbation theory with the CASSCF wave function as the reference state. The approach is here tested in a calculation of the valence and Rydberg excited singlet and triplet states of the title molecules, using extended atomic natural orbital (ANO) basis sets. The ethene calculations comprised the two valence states plus all singlet and triplet Rydberg states of 3s, 3p, and 3d character, with errors in computed excitation energies smaller than 0.13 eV in all cases except the V state, for which the vertical excitation energy was about 0.4 eV too large. The two lowest triplet states and nine singlet states were studied in butadiene. The largest error (0.37 eV) was found for the 2 'B, state. The two lowest triplet and seven lowest singlet states in hexatriene had excitation energies in error with less than 0.17 eV.

No abstract

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.

In this paper we use ab initio multiconfigurational second-order perturbation theory to establish the intrinsic photoisomerization path model of retinal chromophores. This is accomplished by computing the ground state (S0) and the first two singlet excited-state (S1, S2) energies along the rigorously determined photoisomerization coordinate of the rhodopsin chromophore model 4-cis-␥-methylnona-2,4,6,8-tetraeniminium cation and the bacteriorhodopsin chromophore model all-trans-hepta-2,4,6-trieniminium cation in isolated conditions. The computed S2 and S1 energy profiles do not show any avoided crossing feature along the S1 reaction path and maintain an energy gap >20 kcal⅐mol ؊1 . In addition, the analysis of the charge distribution shows that there is no qualitative change in the S2 and S1 electronic structure along the path. Thus, the S1 state maintains a prevalent ionic (hole-pair) character whereas the S2 state maintains a covalent (dot-dot) character. These results, together with the analysis of the S1 reaction coordinate, support a two-state, two-mode model of the photoisomerization that constitutes a substantial revision of the previously proposed models.T he photoisomerization of the retinal chromophore triggers the conformational changes underlying the activity of rhodopsin proteins (1). In rhodopsin itself (the human retina visual pigment) the retinal molecule is embedded in a cavity where it is covalently bound to a lysine residue via a protonated Schiff base (PSB) linkage. The absorption of a photon of light causes the isomerization (see equation below) of the 11-cis isomer of the retinal PSB (PSB11) to its all-trans isomer (PSBT).Similarly in the bacterial proton-pump bacteriorhodopsin, the photoexcitation causes the isomerization of the PSBT to its 13-cis isomer (PSB13). The photoisomerization of PSB11 and PSBT in the protein environment are among the fastest chemical reactions observed so far. Thus, the photoexcitation of PSB11 in rhodopsin yields a fluorescent transient with a lifetime of ca. 150 fs (2). After this state is left, ground-state PSBT is formed within 200 fs (3). Similarly, irradiation of PSBT in bacteriorhodopsin leads to formation of a 200-fs (4-7) transient and production of the PSB13 within 500 fs. In contrast, the photochemistry of free PSB11 in solution (8) is more than 1 order of magnitude slower: in methanol, PSB11 has a ca. 3-ps fluorescence lifetime and PSBT is formed in 10 ps (9). Similar lifetimes have been reported for PSBT (10-12) and PSB13 (10) in solution.The decrease in excited-state lifetime and the increase in reaction rate of chromophores bound within the protein with respect to the corresponding free forms in solution is a central problem of photobiology. The first step in the quest for a solution to this problem is the detailed understanding of the chromophore photoisomerization path. Recently, Anfinrud and coworkers (13,14) have summarized the experimental evidence in support of a three-electronic state (S 0 , S 1 , and S 2 ) model-the three-state model-of the p...

Distinct photophysical behavior of nucleobase adenine and its constitutional isomer, 2-aminopurine, has been studied by using quantum chemical methods, in particular an accurate ab initio multiconfigurational second-order perturbation theory. After light irradiation, the efficient, ultrafast energy dissipation observed for nonfluorescent 9H-adenine is explained here by the nonradiative internal conversion process taking place along a barrierless reaction path from the initially populated 1 (* La) excited state toward a low-lying conical intersection (CI) connected with the ground state. In contrast, the strong fluorescence recorded for 2-aminopurine at 4.0 eV with large decay lifetime is interpreted by the presence of a minimum in the 1 (* La) hypersurface lying below the lowest CI and the subsequent potential energy barrier required to reach the funnel to the ground state. Secondary deactivation channels were found in the two systems related to additional CIs involving the 1 (* Lb) and 1 (n*) states. Although in 9H-adenine a population switch between both states is proposed, in 7H-adenine this may be perturbed by a relatively larger barrier to access the 1 (n*) state, and, therefore, the 1 (* Lb) state becomes responsible for the weak fluorescence measured in aqueous adenine at Ϸ4.5 eV. In contrast to previous models that explained fluorescence quenching in adenine, unlike in 2-aminopurine, on the basis of the vibronic coupling of the nearby 1 (*) and 1 (n*) states, the present results indicate that the 1 (n*) state does not contribute to the leading photophysical event and establish the prevalence of a model based on the CI concept in modern photochemistry.conical intersections ͉ DNA photophysics ͉ fluorescence quenching ͉ quantum chemistry ͉ ultrafast decay S tudies on the photostability of DNA and RNA bases after absorption of UV light have flourished since the early 1970s, when quenched DNA fluorescence at room temperature was first reported (1). Knowledge about the mechanisms that control nonradiative decay is fundamental, not only because of the extreme importance of photodamage in systems that form the genetic code but also because they have been used to establish the paradigm of the essentials of radiationless decay processes in nonadiabatic photochemistry (2). Adenine (6-aminopurine), the most studied nucleobase, maintains photostability by efficiently quenching its fluorescence, whereas a close constitutional isomer, 2-aminopurine, displays strong emission, and it is commonly used to substitute adenine in DNA as a fluorescent probe to detect protein-induced local conformational changes (3-7). Former proposals to explain low quantum yields of fluorescence and excited singlet deactivation in nucleobases by means of excited-state photoreactions or phototautomerisms were basically ruled out because of the absence of photoproducts and deuterium isotope effects in different solvents (2). Apparently more successful was the hypothesis known as proximity effect (8, 9), which explained ultrafast internal conver...

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.

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.

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.