Showing papers on "Excited state published in 2005"

Two-dimensional spectroscopy of electronic couplings in photosynthesis

Abstract: Time-resolved optical spectroscopy is widely used to study vibrational and electronic dynamics by monitoring transient changes in excited state populations on a femtosecond timescale1. Yet the fundamental cause of electronic and vibrational dynamics—the coupling between the different energy levels involved—is usually inferred only indirectly. Two-dimensional femtosecond infrared spectroscopy based on the heterodyne detection of three-pulse photon echoes2,3,4,5,6,7 has recently allowed the direct mapping of vibrational couplings, yielding transient structural information. Here we extend the approach to the visible range3,8 and directly measure electronic couplings in a molecular complex, the Fenna–Matthews–Olson photosynthetic light-harvesting protein9,10. As in all photosynthetic systems, the conversion of light into chemical energy is driven by electronic couplings that ensure the efficient transport of energy from light-capturing antenna pigments to the reaction centre11. We monitor this process as a function of time and frequency and show that excitation energy does not simply cascade stepwise down the energy ladder. We find instead distinct energy transport pathways that depend sensitively on the detailed spatial properties of the delocalized excited-state wavefunctions of the whole pigment–protein complex.

New relativistic ANO basis sets for transition metal atoms.

Abstract: New basis sets of the atomic natural orbital (ANO) type have been developed for the first, second, and third row transition metal atoms. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive and negative ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies, electron affinities, and excitation energies for all atoms and polarizabilities for spherically symmetric atoms. These calculations include spin-orbit coupling using a variation-perturbation approach. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The accuracy of computed electron affinities is the same except in cases where the experimental values are smaller than 0.5 eV. Accurate results are obtained for the polarizabilities of atoms with spherical symmetry. Multiplet levels are presented for some of the third row transition metals.

Energy-consistent pseudopotentials for group 11 and 12 atoms: adjustment to multi-configuration Dirac–Hartree–Fock data

Abstract: Two-component relativistic pseudopotentials (i.e., scalar-relativistic and spin–orbit (SO) potentials) of the energy-consistent variety have been adjusted for the group 11 and 12 atoms Cu, Zn; Ag, Cd; Au, Hg, replacing the 1s–2p; 1s–3d; and 1s–4f cores, respectively. The adjustment has been done for the valence-energy spectrum of (near-)neutral atoms, to reference data from numerical all-electron four-component multi-configuration Dirac–Hartree–Fock (MCDHF) calculations, including the two-electron Breit interaction. For use in molecular calculations, the potentials have been supplemented by energy-optimized (12s12p9d3f2g)/[6s6p4d3f2g] valence basis sets. First benchmark applications of the potentials and basis sets are presented for atomic excitation energies and SO splittings at a correlated level, and for ground and excited state spectroscopic properties of group 11 monohalides and group 12 dimers.

Elementary steps in excited-state proton transfer.

Abstract: The absorption of a photon by a hydroxy-aromatic photoacid triggers a cascade of events contributing to the overall phenomenon of intermolecular excited-state proton transfer. The fundamental steps involved were studied over the last 20 years using a combination of theoretical and experimental techniques. They are surveyed in this sequel in sequential order, from fast to slow. The excitation triggers an intramolecular charge transfer to the ring system, which is more prominent for the anionic base than the acid. The charge redistribution, in turn, triggers changes in hydrogen-bond strengths that set the stage for the proton-transfer step itself. This step is strongly influenced by the solvent, resulting in unusual dependence of the dissociation rate coefficient on water content, temperature, and isotopic substitution. The photolyzed proton can diffuse in the aqueous solution in a mechanism that involves collective changes in hydrogen-bonding. On longer times, it may recombine adiabatically with the excited base or quench it. The theory for these diffusion-influenced geminate reactions has been developed, showing nice agreement with experiment. Finally, the effect of inert salts, bases, and acids on these reactions is analyzed.

Plasma formation and temperature measurement during single-bubble cavitation

Abstract: Single-bubble sonoluminescence (SBSL) results from the extreme temperatures and pressures achieved during bubble compression; calculations have predicted the existence of a hot, optically opaque plasma core with consequent bremsstrahlung radiation. Recent controversial reports claim the observation of neutrons from deuterium-deuterium fusion during acoustic cavitation. However, there has been previously no strong experimental evidence for the existence of a plasma during single- or multi-bubble sonoluminescence. SBSL typically produces featureless emission spectra that reveal little about the intra-cavity physical conditions or chemical processes. Here we report observations of atomic (Ar) emission and extensive molecular (SO) and ionic (O2+) progressions in SBSL spectra from concentrated aqueous H2SO4 solutions. Both the Ar and SO emission permit spectroscopic temperature determinations, as accomplished for multi-bubble sonoluminescence with other emitters. The emissive excited states observed from both Ar and O2+ are inconsistent with any thermal process. The Ar excited states involved are extremely high in energy (>13 eV) and cannot be thermally populated at the measured Ar emission temperatures (4,000-15,000 K); the ionization energy of O2 is more than twice its bond dissociation energy, so O2+ likewise cannot be thermally produced. We therefore conclude that these emitting species must originate from collisions with high-energy electrons, ions or particles from a hot plasma core.

Thermal quenching of Eu2+ 5d-4f luminescence in inorganic compounds

Abstract: The thermal quenching of Eu2+ 5d–4f emission on Ba, Sr, or Ca sites in compounds is often attributed to a large displacement between the ground state and excited state in the configuration coordinate diagram. This work will demonstrate that the ionization of the 5d electron to conduction band states is the genuine quenching mechanism. A model is proposed to explain why in some types of compounds the quenching temperature decreases when going from the Ba variant via the Sr variant to the Ca variant and in other types of compounds the reverse behaviour occurs. The nature of the bottom of the conduction band plays an important role in this.

Base stacking controls excited-state dynamics in A·T DNA

Abstract: Solar ultraviolet light creates excited electronic states in DNA that can decay to mutagenic photoproducts. This vulnerability is compensated for in all organisms by enzymatic repair of photodamaged DNA. As repair is energetically costly, DNA is intrinsically photostable. Single bases eliminate electronic energy non-radiatively on a subpicosecond timescale1, but base stacking and base pairing mediate the decay of excess electronic energy in the double helix in poorly understood ways. In the past, considerable attention has been paid to excited base pairs2. Recent reports have suggested that light-triggered motion of a proton in one of the hydrogen bonds of an isolated base pair initiates non-radiative decay to the electronic ground state3,4. Here we show that vertical base stacking, and not base pairing, determines the fate of excited singlet electronic states in single- and double-stranded oligonucleotides composed of adenine (A) and thymine (T) bases. Intrastrand excimer states with lifetimes of 50–150 ps are formed in high yields whenever A is stacked with itself or with T. Excimers limit excitation energy to one strand at a time in the B-form double helix, enabling repair using the undamaged strand as a template.

Excited states dynamics of DNA and RNA bases: characterization of a stepwise deactivation pathway in the gas phase.

Abstract: Radiationless deactivation pathways of excited gas phase nucleobases were investigated using mass-selected femtosecond resolved pump-probe resonant ionization. By comparison between nucleobases and methylated species, in which tautomerism cannot occur, we can access intrinsic mechanisms at a time resolution never reported so far (80 fs). At this time resolution, and using appropriate substitution, real nuclear motion corresponding to active vibrational modes along deactivation coordinates can actually be probed. We provide evidence for the existence of a two-step decay mechanism, following a 267 nm excitation of the nucleobases. The time resolution achieved together with a careful zero time-delay calibration between lasers allow us to show that the first step does correspond to intrinsic dynamics rather than to a laser cross correlation. For adenine and 9-methyladenine a first decay component of about 100 fs has been measured. This first step is radically increased to 200 fs when the amino group hydrogen atoms of adenine are substituted by methyl groups. Our results could be rationalized according to the effect of the highly localized nature of the excitation combined to the presence of efficient deactivation pathway along both pyrimidine ring and amino group out-of-plane vibrational modes. These nuclear motions play a key role in the vibronic coupling between the initially excited pipi(*) and the dark npi(*) states. This seems to be the common mechanism that opens up the earlier phase of the internal conversion pathway which then, in consideration of the rather fast relaxation times observed, would probably proceed via conical intersection between the npi(*) relay state and high vibrational levels of the ground state.

Structural Control of the Photodynamics of Boron-Dipyrrin Complexes

Abstract: Boron−dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-tert-butylphenyl) group show low fluorescence yields (∼0.06) and short excited-singlet-state lifetimes (∼500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (∼0.9) and long excited-state lifetimes (∼6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of ∼60° with respect to the dipyrrin framework whereas the angle is ∼80° for mesityl or o-tolyl groups. The calculated potential energy surface for the phenyl-substituted complex indicates that the excited state has a second, lower energy minimum in which the nonhindered aryl...

TiO2-based Photocatalysis: Surface Defects, Oxygen and Charge Transfer

Abstract: A detailed discussion of the photochemistry of TiO2 surfaces is presented, covering important work from the literature as well as more recent studies. The production and characterization of surface defects is discussed, and studies of the adsorption of molecular oxygen on these defects is presented. In addition, both chemical and physical methods for detection and measurement of defect sites on TiO2 are reviewed. The role of nitrogen doping on shifting the photothreshold energy of TiO2 is discussed and the active chemical state of the nitrogen is described, based on XPS N(1s) binding energies. Observations of charge transfer between excited TiO2 and adsorbates are presented, and it is shown that the electronegativity of the attachment atom which forms the surface bond is important in governing charge transfer

Strongly coupled optical phonons in the ultrafast dynamics of the electronic energy and current relaxation in graphite

Abstract: Ultrafast charge carrier dynamics in graphite has been investigated by time-resolved terahertz spectroscopy Analysis of the transient dielectric function and model calculations show that more than 90% of the initially deposited excitation energy is transferred to a few strongly coupled lattice vibrations within 500 fs These hot optical phonons also substantially contribute to the striking increase of the Drude relaxation rate observed during the first picosecond after photoexcitation The subsequent cooling of the hot phonons yields a lifetime estimate of 7 ps for these modes Graphite has attracted continuous attention in research over the past decades [1] Recently, a strong ambipolar electric field effect was found in thin films of this semimetal, thereby demonstrating its potential for future electronics [2] Moreover, graphite is closely related to carbon nanotubes, which are the focus of nanotechnology research [3] The latest experimental and theoretical efforts revealed that optical phonons in graphite strongly interact with the electrons [4,5] These strongly coupled optical phonons (SCOPs) have high quantum energies of up to 02 eV and can be excited only by electrons of elevated energy The scattering of electrons by SCOPsis believed to increase the dc resistivity of carbon nanotubes when high electric fields are applied [3] In view of possible applications in electronics, further investigations are required on how SCOPs influence the transport and energy relaxation of electrons Time-resolved terahertz spectroscopy (TRTS) is a promising experimental approach to this question Avisible pump pulse excites charge carriers in the sample and thus enables them to generate optical phonons A subsequent terahertz (THz) pulse probes the low-energy response of the system

Effects of (Multi)branching of Dipolar Chromophores on Photophysical Properties and Two-Photon Absorption

Abstract: To investigate the effect of branching on linear and nonlinear optical properties, a specific series of chromophores, epitome of (multi)branched dipoles, has been thoroughly explored by a combined theoretical and experimental approach. Excited-state structure calculations based on quantum-chemical techniques (time-dependent density functional theory) as well as a Frenkel exciton model nicely complement experimental photoluminescence and one- and two-photon absorption findings and contribute to their interpretation. This allowed us to get a deep insight into the nature of fundamental excited-state dynamics and the nonlinear optical (NLO) response involved. Both experiment and theory reveal that a multidimensional intramolecular charge transfer takes place from the donating moiety to the periphery of the branched molecules upon excitation, while fluorescence stems from an excited state localized on one of the dipolar branches. Branching is also observed to lead to cooperative enhancement of two-photon absorption (TPA) while maintaining high fluorescence quantum yield, thanks to localization of the emitting state. The comparison between results obtained in the Frenkel exciton scheme and ab initio results suggests the coherent coupling between branches as one of the possible mechanisms for the observed enhancement. New strategies for the rational design of NLO molecular assemblies are thus inferred on the basis of the acquired insights.

Mechanism of Persistent Luminescence in Eu2+ and Dy3+ Codoped Aluminate and Silicate Compounds

Abstract: A mechanism of persistent luminescence that was proposed in 1996 for SrAl2O4:Eu2+;Dy3+ has been widely adopted to explain afterglow in many Eu2+ and Dy3+ codoped aluminates and silicates. The mechanism involves the thermally activated release of a hole from Eu2+ in its excited 5d state to the valence band which is subsequently trapped by Dy3+. In this work the location of the lanthanide energy levels relative to the valence and conduction band of various compounds is presented. It is shown that the mechanism of persistent luminescence cannot be correct. An alternative model that involves the ionization of the 5d electron to conduction band states and subsequent trapping by Dy3+ is proposed. The level schemes are consistent, both qualitatively and quantitatively, with many observations regarding persistent luminescence. They also provide insight into the mechanism of thermal quenching of Eu2+ 5d-4f emission.

Bright infrared emission from electrically induced excitons in carbon nanotubes.

Abstract: We used the high local electric fields at the junction between the suspended and supported parts of a single carbon nanotube molecule to produce unusually bright infrared emission under unipolar operation. Carriers were accelerated by band-bending at the suspension interface, and they created excitons that radiatively recombined. This excitation mechanism is ∼1000 times more efficient than recombination of independently injected electrons and holes, and it results from weak electron-phonon scattering and strong electron-hole binding caused by one-dimensional confinement. The ensuing high excitation density allows us to observe emission from higher excited states not seen by photoexcitation. The excitation mechanism of these states was analyzed.

Ab Initio Studies on the Radiationless Decay Mechanisms of the Lowest Excited Singlet States of 9H-Adenine

Abstract: The mechanisms that are responsible for the rapid deactivation of the 1nπ* and 1ππ* excited singlet states of the 9H isomer of adenine have been investigated with multireference ab initio methods (complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory based on the CASSCF reference (CASPT2)). Two novel photochemical pathways, which lead to conical intersections of the S1 excited potential-energy surface with the electronic ground-state surface, have been identified. They involve out-of-plane deformations of the six-membered aromatic ring via the twisting of the N3C2 and N1C6 bonds. These low-lying conical intersections are separated from the minimum energy of the lowest (1nπ*) excited state in the Franck−Condon region by very low energy barriers (of the order of 0.1 eV). These properties of the S1 and S0 potential-energy surfaces explain the unusual laser-induced fluorescence spectrum of jet-cooled 9H-adenine, showing sharp structures only in a narrow energy interv...

ULTRAFAST CHEMISTRY: Using Time-Resolved Vibrational Spectroscopy for Interrogation of Structural Dynamics

Abstract: Time-resolved infrared (IR) and Raman spectroscopy elucidates molecular structure evolution during ultrafast chemical reactions. Following vibrational marker modes in real time provides direct insight into the structural dynamics, as is evidenced in studies on intramolecular hydrogen transfer, bimolecular proton transfer, electron transfer, hydrogen bonding during solvation dynamics, bond fission in organometallic compounds and heme proteins, cis-trans isomerization in retinal proteins, and transformations in photochromic switch pairs. Femtosecond IR spectroscopy monitors the site-specific interactions in hydrogen bonds. Conversion between excited electronic states can be followed for intramolecular electron transfer by inspection of the fingerprint IR- or Raman-active vibrations in conjunction with quantum chemical calculations. Excess internal vibrational energy, generated either by optical excitation or by internal conversion from the electronic excited state to the ground state, is observable through transient frequency shifts of IR-active vibrations and through nonequilibrium populations as deduced by Raman resonances.

Tautomeric selectivity of the excited-state lifetime of guanine/cytosine base pairs: the role of electron-driven proton-transfer processes.

Abstract: The UV spectra of three different conformers of the guanine/cytosine base pair were recorded recently with UV-IR double-resonance techniques in a supersonic jet [Abo-Riziq, A, Grace, L, Nir, E, Kabelac, M, Hobza, P & de Vries, M S (2005) Proc Natl Acad Sci USA 102, 20–23] The spectra provide evidence for a very efficient excited-state deactivation mechanism that is specific for the Watson–Crick structure and may be essential for the photostability of DNA Here we report results of ab initio electronic-structure calculations for the excited electronic states of the three lowest-energy conformers of the guanine/cytosine base pair The calculations reveal that electron-driven interbase proton-transfer processes play an important role in the photochemistry of these systems The exceptionally short lifetime of the UV-absorbing states of the Watson–Crick conformer is tentatively explained by the existence of a barrierless reaction path that connects the spectroscopic 1π π * excited state with the electronic ground state via two electronic curve crossings For the non-Watson–Crick structures, the photochemically reactive state is located at higher energies, resulting in a barrier for proton transfer and, thus, a longer lifetime of the UV-absorbing 1π π * state The computational results support the conjecture that the photochemistry of hydrogen bonds plays a decisive role for the photostability of the molecular encoding of the genetic information in isolated DNA base pairs

Enhanced emission efficiency and excited state lifetime due to restricted intramolecular motion in silole aggregates.

Abstract: The aggregation-induced emission (AIE) properties of 1,1,2,3,4,5-hexaphenylsilole (HPS) and poly{11-[(1,2,3,4,5-pentaphenylsilolyl)oxy]-1-phenyl-1-undecyne} (PS9PA) were studied by time-resolved fluorescence technique. The enhanced fluorescence and long fluorescent lifetime were obtained for the sample in an aggregate state as compared to the sample in solution. The time-decay of fluorescence of HPS and PS9PA in high viscosity solvents and low-temperature glasses has also been measured in detail to further investigate the possible mechanism for AIE. Enhanced light emission and long fluorescence lifetime were detected for both HPS and PS9PA in the solution-thickening and -cooling experiments. These results provided direct evidence that the enhanced photoluminescence (PL) efficiency is due to restricted intramolecular motion, which ascribes AIE to the deactivation of nonradiative decay caused by restricted torsional motions of the molecules in the solid state or aggregate form.

Correlated multielectron systems in strong laser fields: A multiconfiguration time-dependent Hartree-Fock approach

Abstract: The multiconfiguration time-dependent Hartree-Fock approach for the description of correlated few-electron dynamics in the presence of strong laser fields is introduced and a comprehensive description of the method is given. Total ionization and electron spectra for the ground and first excited ionic channels are calculated for one-dimensional model systems with up to six active electrons. Strong correlation effects are found in the shape of photoelectron peaks and the dependence of ionization on molecule size.

Electronic excitation energies of molecules in solution: state specific and linear response methods for nonequilibrium continuum solvation models.

Abstract: We present a formal comparison between the two different approaches to the calculation of electronic excitation energies of molecules in solution within the continuum solvation model framework, taking also into account nonequilibrium effects. These two approaches, one based on the explicit evaluation of the excited state wave function of the solute and the other based on the linear response theory, are here proven to give formally different expressions for the excitation energies even when exact eigenstates are considered. Calculations performed for some illustrative examples show that this formal difference has sensible effects on absolute solvatochromic shifts (i.e., with respect to gas phase) while it has small effects on relative (i.e., nonpolar to polar solvent) solvatochromic shifts.

Density functional methods for excited states: equilibrium structure and electronic spectra

Abstract: This chapter discusses density functional methods for excited states. Density functional theory (DFT) is nowadays one of the most popular methods for ground state electronic structure calculations in quantum chemistry and solid state physics. A number of commercial programs are available, and DFT calculations of ground state energies, structures, and many other properties are routinely performed by nonexperts in (bio-)chemistry, physics, and materials sciences. The chapter focuses on the use of time-dependent density functional theory (TDDFT). Algorithms to compute spectra and excited state properties are also reviewed. The chapter describes the steps necessary in a TDDFT excited state calculation and some timing for typical applications are presented. Further the chapter also summarizes the performance of TDDFT excitation energies, transition moments, and excited state properties. Specific applications are surveyed that included compounds such as aromatic systems and fullerenes, porphyrins and related compounds, transition metal compounds, metal and semiconductor clusters, organic polymers, and biologically relevant systems.

Photophysical properties of borondipyrromethene analogues in solution.

Abstract: The photophysical properties of seven new 8-(p-substituted)phenyl analogues of 4,4-difluoro-3,5-dimethyl-8-(aryl)-4-bora-3a,4a-diaza-s-indacene (derivatives of the well-known fluorophore BODIPY) in several solvents have been studied by means of absorption and steady-state and time-resolved fluorimetry For each compound, the fluorescence quantum yield and lifetime are lower in solvents with higher polarity owing to an increase in the rate of nonradiative deactivation Increasing the electron withdrawing strength of the p-substituent on the phenyl group in position 8 also leads to lower fluorescence quantum yields and lifetimes When the p-substituent on the phenyl group in position 8 is a tertiary amine [8-(4-piperidinophenyl), 8-(4-N,N-dimethylaminophenyl), and 8-(4-morpholinophenyl)], the low quantum yields of these compounds in more polar solvents can be rationalized by the inversion of the energy levels of an apolar, highly fluorescent and a polar, nonfluorescent excited state, where charge transfer f

Energy dissipation in multichromophoric single dendrimers

Abstract: Single-molecule spectroscopy of well-chosen dendritic multichromophoric systems allows investigation of fundamental photophysical processes such as energy or electron transfer in much greater detail than the respective ensemble measurements. In dendrimers with multiple chromophores, energy hopping and transfer to the chromophore with the energetically lowest S1 state was observed. If more than one chromophore is in an excited state in one molecule, annihilation, either singlet−triplet or singlet−singlet, can occur. In the latter case, a higher singlet state is populated opening new deactivation pathways. In the presence of an electron donor, reversible electron transfer could be observed, and the rate constants of forward and backward electron transfer were established. The value of these rate constants fluctuates time-correlated with the rotational motion of the dendrimer arms and the mobility of the embedding matrix.

Long-range interactions between alkali Rydberg atom pairs correlated to the ns–ns, np–np and nd–nd asymptotes

Abstract: We have calculated the long-range interaction potential curves of highly excited Rydberg atom pairs for the combinations Li–Li, Na–Na, K–K, Rb–Rb and Cs–Cs in a perturbative approach. The dispersion C-coefficients are determined for all symmetries of molecular states that correlate to the ns–ns, np–np and nd–nd asymptotes. Fitted parameters are given for the scaling of the C-coefficients as a function of the principal quantum number n for all homonuclear pairs of alkali metal atoms.

Emission properties of manganese-doped ZnS nanocrystals.

Abstract: We have performed steady-state and time-resolved fluorescence studies on undoped and Mn-doped ZnS nanocrystals with approximately 16 A diameter. While there is no band-edge emission, the intensity of the steady-state blue fluorescence from ZnS surface states decreases upon Mn incorporation, which gives rise to an orange emission. These results show that Mn incorporation competes very effectively with the donor-acceptor surface states for the energy transfer from the electron-hole pair excited across the band gap. In both undoped and doped samples, the time-resolved fluorescence studies establish the presence of a distribution of decay lifetimes possibly due to a number of emission centers in the nanocrystals. A faster short-time decay of the blue emission in the Mn-doped samples compared to that in the undoped sample suggests an additional decay channel for the surface states via an energy transfer from these states to the dopant levels.

Spontaneous emission of a cesium atom near a nanofiber: Efficient coupling of light to guided modes

Abstract: We study the spontaneous emission of a cesium atom in the vicinity of a subwavelength-diameter fiber. We show that the confinement of the guided modes and the degeneracy of the excited and ground states substantially affect the spontaneous emission process. We demonstrate that different magnetic sublevels have different decay rates. When the fiber radius is about $200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, a significant fraction (up to 28%) of spontaneous emission by the atom can be channeled into guided modes. Our results may find applications for developing nanoprobes for atoms and efficient couplers for subwavelength-diameter fibers.

A new pathway for the rapid decay of electronically excited adenine.

Abstract: 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 [n→π*]1 state has been located at an energy of 4.54eV for a nuclear arrangement in which the amino group is pyramidal whereas the ring system remains planar. Close by, another minimum on the S1 potential energy hypersurface has been detected in which the C2 center is deflected out of the molecular plane and the electronic character of S1 corresponds to a nearly equal mixture of [π→π*]1 and [n→π*]1 configurations. The adiabatic excitation energy of this minimum amounts to 4.47eV. Vertical and adiabatic excitation energies of the lowest n→π* and π→π* transitions as well as transition moments and their directions are in very good agreement with experim...