The Solute Polarization and Structural Effects on the Nonlinear Optical Response of Based Chromone Molecules.

The solute polarization due to solvent is a an electrostatic quantum effect that impacts diverse molecular properties, including the nonlinear optical response of a material. An iterative procedure that allows updating the solute charge distribution in the presence of the solvent is combined with a sequential Monte Carlo/Quantum Mechanics methodology and Density Functional Theory methods to evaluate the nonlinear optical (NLO) response using the hyper Rayleigh scattering (HRS) of a series of chromones recently identified in Chamaecrista diphylla, an herbaceous plant abundant throughout the Americas and used in folk medicine. From this study, it is determined that from gas to solvent environment, the systems acquire low refractive index (n) and an improvement of the first hyperpolarizability (ßHRS ), signaling potential NLO uses. It is shown that the octupolar contributions (ßJ=3 ) superate the dipolar ones (ßJ=1 ) and dominate the second-order optical response in both gas and liquid phases, which indicate nontrivial optical materials. Moreover, the solvent environment and structural changes in the periphery can tune significantly the dipolar/octupolar balance, showing a key to control the decoupling between these contributions.

Role of the Solvent and Intramolecular Hydrogen Bonds in the Antioxidative Mechanism of Prenylisoflavone from Leaves of Vatairea guianensis.

This work discusses the electron structure, antioxidative properties, and solvent contribution of two new antioxidant molecules discovered, named S10 and S11, extracted from a medicinal plant called Vatairea guianensis, found in the Amazon rain-forest. To gain a better understanding, a study using density functional theory coupled with the polarizable-continuum model and the standard 6-311++G(d,p) basis set was conducted. The results indicate that S10 has a higher antioxidant potential than S11, confirming the experimental expectations. In the gas phase, the hydrogen atom transfer route dominates the hydrogen scavenging procedure. However, in the water solvents, the antioxidant mechanism prefers the sequential proton loss electron transfer mechanism. Furthermore, the solvent plays a fundamental role in the antioxidant mechanism. The formation of an intramolecular OH···OCH3 hydrogen bond is crucial for accurately describing the hydrogen scavenging phenomenon, better aligning with the experimental data. The results suggest that the two isoflavones investigated are promising for the pharmacologic and food industries.

Calculation of the one- and two-photon absorption spectra of water-soluble stilbene derivatives using a multiscale QM/MM approach.

We calculated the one- (OPA) and two-photon absorption (TPA) spectra of two large water-soluble stilbene derivatives presenting TPA cross sections of about 400 GM. However, the reported experimental TPA spectra present a spectral gap region, and a theoretical study of these promising molecules seems now timely and relevant. These molecules are composed of 200 or more atoms, becoming a challenge to obtain the TPA spectra even using density functional theory at the time-dependent quadratic response formalism. Thus, both OPA and TPA were also calculated using the INDO-S semi-empirical method. We used explicit solvent molecules using the sequential-quantum mechanics/molecular mechanics to include the solvent effects. Our results show that different transitions are participating in the OPA and TPA processes and that exchange-correlation functionals, including larger Hartree-Fock contributions, provide a better description of the OPA spectra; however, the opposite trend is observed on the TPA spectra. Alternatively, INDO-S/CISD, including contributions from single and double excitations, systematically describes both OPA and TPA bands with similar shifts and better reproduces the relative intensities of the two TPA bands compared to the experimental ones. The OPA spectra are characterized by a Highest Occupied Molecular Orbital-Lowest Unoccupied Molecular Orbital (HOMO-LUMO) excitation, while the low-energy TPA band is ascribed to a single transition encompassing the (HOMO-1)-LUMO and HOMO-(LUMO+1) excitations and the high-energy one is a combination of several transitions. Thus, although more studies are required to better assess the capability of the INDO-S/CISD method in describing the TPA spectra of large molecules, our results corroborate that it is a promising alternative.

On the population of triplet states of 2-seleno-thymine.

The population and depopulation mechanisms leading to the lowest-lying triplet states of 2-Se-Thymine were studied at the MS-CASPT2/cc-pVDZ level of theory. Several critical points on different potential energy hypersurfaces were optimized, including minima, conical intersections, and singlet-triplet crossings. The accessibility of all relevant regions on the potential energy hypersurfaces was investigated by means of minimum energy paths and linear interpolation in internal coordinates techniques. Our analysis indicates that, after the population of the bright S2 state in the Franck-Condon region, the first photochemical event is a barrierless evolution towards one of its two minima. After that, three viable photophysical deactivation paths can take place. In one of them, the population in the S2 state is transferred to the T2 state via intersystem crossing and subsequently to the T1 state by internal conversion. Alternatively, the S1 state could be accessed by internal conversion through two distinct conical intersections with S2 state followed by singlet-triplet crossing with the T2 state. The absence of a second minimum on the T1 state and a small energy barrier on pathway along the potential energy surface towards the ground state from the lowest triplet state are attributed as potential reasons to explain why the lifetime of the triplet state of 2-Se-Thymine might be reduced in comparison with its thio-analogue.

Preferential solvation and optical properties of eumelanin building blocks in binary mixture of methanol and water.

Employing a sequential quantum mechanical/molecular mechanical approach for polar protic solvents, we study the absorption spectrum of eumelanin building blocks including monomers, dimers, and tetramers in pure water and methanol and three water-methanol binary mixtures having water molar fractions (Xw = 0.25, 0.50, and 0.75). The binary mixture of solvents is a common situation in experiments, but theoretical studies are limited to the use of continuum models. Here, we use explicit solvent molecules, and specific solute-solvent interaction is analyzed and seen to play an important role. Effects of the electronic polarization of solute by the environment were included using a reliable iterative scheme. The results illustrate that the monomers, dimers, and tetramers are preferably solvated by methanol, but the composition of the mixture in the vicinity of the solute molecules is different from the bulk composition with a preferential microsolvation (hydrogen bonds) in water for most species considered. It is observed that the short-range electrostatic polarization effects of the hydrogen bonds lead to a slight blue shift of the excitation energies when the concentration of water in the mixture is enhanced. For the same species, there is an enhancement of the higher-energy absorption intensity caused by long-range electrostatic interactions with the environment and that the behavior of the experimental spectrum, which is characterized by a nearly monotonic decay from the ultraviolet to the infrared, is qualitatively reproduced by the superposition of the absorption spectra of monomers, dimers, and tetramers in the liquid phase.

Unraveling the Electric Field-Induced Second Harmonic Generation Responses of Stilbazolium Ion Pairs Complexes in Solution Using a Multiscale Simulation Method.

The electric field-induced second harmonic generation (EFISHG) response has been largely used to describe the first ß and the second γ hyperpolarizabilities in solution. Although the EFISHG technique cannot be applied to charged compounds (due to the external static electric field), it can be used to describe ion pairs as neutral complexes. A multiscale computational approach is required to generate representative geometrical configurations of such kinds of complexes (using classical force fields), to compute the electronic structure of each configuration (using quantum mechanics methods), and to perform statistical analyses describing the behavior of the nonlinear optical properties. In this work, we target solvated neutral ion pairs complexes, of which the cation is an organic chromophore, and we estimate their EFISHG and hyper-Rayleigh scattering responses. It is shown that the anion-cation relative spatial distribution determines the permanent dipole moment of the complexes, and therefore the relative distance controls the EFISHG response. On the other hand, the ß tensor is independent of the dipole moment and it shows a weak linear correlation with the π-electron conjugation length of the cations. The γ contributions in the global EFISHG response range from 5% to 15%, which is mostly due to the variations of amplitude of the µß∥ contribution, which results from differences in the µ and ß vectors' orientations. The applied multiscale approach provides reasonable results compared with experimental ones, although additional efforts are still required to improve such comparison mainly to consider the possible dissociation effects.

DICE: A Monte Carlo Code for Molecular Simulation Including the Configurational Bias Monte Carlo Method.

Solute-solvent systems are an important topic of study, as the effects of the solvent on the solute can drastically change its properties. Theoretical studies of these systems are done with ab initio methods, molecular simulations, or a combination of both. The simulations of molecular systems are usually performed with either molecular dynamics (MD) or Monte Carlo (MC) methods. Classical MD has evolved much in the last decades, both in algorithms and implementations, having several stable and efficient codes developed and available. Similarly, MC methods have also evolved, focusing mainly in creating and improving methods and implementations in available codes. In this paper, we provide some enhancements to a configurational bias Monte Carlo (CBMC) methodology to simulate flexible molecules using the molecular fragments concept. In our implementation the acceptance criterion of the CBMC method was simplified and a generalization was proposed to allow the simulation of molecules with any kind of fragments. We also introduce the new version of DICE, an MC code for molecular simulation (available at https://portal.if.usp.br/dice). This code was mainly developed to simulate solute-solvent systems in liquid and gas phases and in interfaces (gas-liquid and solid-liquid) that has been mostly used to generate configurations for a sequential quantum mechanics/molecular mechanics method (S-QM/MM). This new version introduces several improvements over the previous ones, with the ability of simulating flexible molecules with CBMC as one of them. Simulations of well-known molecules, such as n-octane and 1,2-dichloroethane in vacuum and in solution, are presented to validate the new implementations compared with MD simulations, experimental data, and other theoretical results. The efficiency of the conformational sampling was analyzed using the acceptance rates of different alkanes: n-octane, neopentane, and 4-ethylheptane. Furthermore, a very complex molecule, boron subphtalocyanine, was simulated in vacuum and in aqueous solution showing the versatility of the new implementation. We show that the CBMC is a very good method to perform conformation sampling of complex moderately sized molecules (up to 150 atoms) in solution following the Boltzmann thermodynamic equilibrium distribution.

Solvation Structures and Deactivation Pathways of Luminescent Isothiazole-Derived Nucleobases: tzA, tzG, and tzI.

The photophysical relaxation pathways of tzA, tzG, and tzI luminescent nucleobases were investigated with the MS-CASPT2 quantum-chemical method and double-ζ basis sets (cc-pVDZ) in gas and condensed phases (1,4-dioxane and water) with the sequential Monte Carlo/CASPT2 and free energy gradient (FEG) methods. Solvation shell structures, in the ground and excited states, were examined with the pairwise radial distribution function (G(r)) and solute-solvent hydrogen-bond networks. Site-specific hydrogen bonding analysis evidenced relevant changes between both electronic states. The three luminescent nucleobases share a common photophysical pattern, summarized as the lowest-lying 1(ππ*) bright state that is populated directly after the absorption of radiation and evolves barrierless to the minimum energy structure, from where the excess of energy is released by fluorescence. From the 1(ππ*)min region, the conical intersection with the ground state ((ππ*/GS)CI) is not accessible due to the presence of high energetic barriers. By combining the present results with those reported earlier by us for the pyrimidine fluorescent nucleobases, we present a comprehensive description of the photophysical properties of this important class of new fluorescent nucleosides.

A new interpretation of the absorption and the dual fluorescence of Prodan in solution.

Remarkable interest is associated with the interpretation of the Prodan fluorescent spectrum. A sequential hybrid Quantum Mechanics/Molecular Mechanics method was used to establish that the fluorescent emission occurs from two different excited states, resulting in a broad asymmetric emission spectrum. The absorption spectra in several solvents were measured and calculated using different theoretical models presenting excellent agreement. All theoretical models [semiempirical, time dependent density functional theory and and second-order multiconfigurational perturbation theory] agree that the first observed band at the absorption spectrum in solution is composed of three electronic excitations very close in energy. Then, the electronic excitation around 340 nm-360 nm may populate the first three excited states (π-π*Lb, n-π*, and π-π*La). The ground state S0 and the first three excited states were analyzed using multi-configurational calculations. The corresponding equilibrium geometries are all planar in vacuum. Considering the solvent effects in the electronic structure of the solute and in the solvent relaxation around the solute, it was identified that these three excited states can change the relative order depending on the solvent polarity, and following the minimum path energy, internal conversions may occur. A consistent explanation of the experimental data is obtained with the conclusive interpretation that the two bands observed in the fluorescent spectrum of Prodan, in several solvents, are due to the emission from two independent states. Our results indicate that these are the n-π* S2 state with a small dipole moment at a lower emission energy and the π-π*Lb S1 state with large dipole moment at a higher emission energy.

Electronic structure and absorption spectra of fluorescent nucleoside analogues.

This work presents a systematic investigation of the electronic and conformational properties of five new fluorescent nucleobases belonging to the alphabet based on the isothiazole[4,3-d]pyrimidine molecule, very recently synthesized. This is of particular importance in the characterization of the main electronic aspects of these fluorescent nucleosides. The solvent effects of 1,4-dioxane and water were included combining the Sequential Monte Carlo/CASPT2 and the Free Energy Gradient (FEG) methods. For comparison, the Polarizable Continuum method was also used. The geometries of all compounds were optimized in solvent with the largest effects observed in water using the average solvent electrostatic configuration (ASEC) and the FEG approaches. Statistical analysis of the solute-solvent hydrogen bonds is performed and their effect on the absorption spectra analyzed. The dipole moments were calculated and the value obtained from the ASEC-FEG method in water follows the same trend as the natural canonical bases (adenine → uracil → guanine → cytosine). The theoretical results for the absorption spectra obtained from CASPT2(18,13) calculations using the geometries obtained with the ASEC-FEG procedure are in very good agreement with the experimental data. A detailed elucidation of the main aspects of the absorption spectra of the five new fluorescent nucleoside analogues is successfully attempted.

Solvent Effects on the Dynamic Polarizability and Raman Response of Molecule-Metal Oxide Hybrid Clusters.

Currently, there is considerable interest in the properties of semiconducting metal oxide nanoparticle substrates because of their utility in surface-enhanced Raman scattering, dye-sensitized solar cells, and photocatalysis. While the enhancement of Raman activities of molecules adsorbed on these nanoparticles is due to a large increase in the polarizability, because of charge transfer from the molecule to the semiconducting nanoparticle, little is known about the factors responsible for modulating the polarizability, particularly the influence of the solvent. Consequently, we have carried out Monte Carlo simulations of several hybrids to study the solvent effect on the dynamic polarizabilities and electronic spectra. Our results indicate that the presence of the solvent induces a shift and an increase in the polarization response that is dependent on the identity of the hybrid. The observed enhancement can be attributed to both the resonant character of the excitation and the participation of the solvent in the charge redistribution. The methodology employed in this work could be very valuable in both identifying and developing metal oxides as novel molecular sensors.

A First-Principles Approach to the Dynamics and Electronic Properties of p-Nitroaniline in Water.

Born-Oppenheimer molecular dynamics of p-nitroaniline (PNA) in water was carried out and the electronic structure was investigated by time-dependent density functional theory. Hydrogen bonding involving the PNA nitro and amine groups and the water molecules leads to an ∼160 cm(-1) red shift of the ν(N-O) and ν(N-H) stretching frequencies relative to the gas phase species. Our estimate for the peak position of the charge transfer (CT) band in the absorption spectrum of PNA in water (∼3.5 eV) is in good agreement with experimental data (3.3 eV). We have investigated the specific role played by local hydrogen bonding and electrostatic interactions on the electronic absorption spectrum. It is shown that although electrostatic interactions play a major role for explaining the structure of the PNA CT band in water, the theoretical prediction of the observed red shift is improved by the explicit consideration of local hydrogen bonding of PNA to water. For isolated PNA, we predict that the dipole moment of the second excited state (S2) is 9.6 D greater than ground state (S0) dipole, which is in good agreement with experimental information (8.2-9.3 D). Calculation of charge transfer indexes for the two first excitations of PNA in water indicates that despite the feature that a small fraction of S1 states (<5%) may exhibit some CT character, CT states in solution are mainly associated with S2 ← S0 transitions.

Hydration effects on the electronic properties of eumelanin building blocks.

Theoretical results for the electronic properties of eumelanin building blocks in the gas phase and water are presented. The building blocks presently investigated include the monomeric species DHI (5,6-dihydroxyindole) or hydroquinone (HQ), DHICA (5,6-dihydroxyindole-2-carboxylic acid), indolequinone (IQ), quinone methide (MQ), two covalently bonded dimers [HM ≡ HQ + MQ and IM ≡ IQ + MQ], and two tetramers [HMIM ≡ HQ + IM, IMIM ≡ IM + IM]. The electronic properties in water were determined by carrying out sequential Monte Carlo/time dependent density functional theory calculations. The results illustrate the role played by hydrogen bonding and electrostatic interactions in the electronic properties of eumelanin building blocks in a polar environment. In water, the dipole moments of monomeric species are significantly increased ([54-79]%) relative to their gas phase values. Recently, it has been proposed that the observed enhancement of the higher-energy absorption intensity in eumelanin can be explained by excitonic coupling among eumelanin protomolecules [C.-T. Chen et al., Nat. Commun. 5, 3859 (2014)]. Here, we are providing evidence that for DHICA, IQ, and HMIM, the electronic absorption toward the higher-energy end of the spectrum ([180-220] nm) is enhanced by long-range Coulombic interactions with the water environment. It was verified that by superposing the absorption spectra of different eumelanin building blocks corresponding to the monomers, dimers, and tetramers in liquid water, the behaviour of the experimental spectrum, which is characterised by a nearly monotonic decay from the ultraviolet to the infrared, is qualitatively reproduced. This result is in keeping with a "chemical disorder model," where the broadband absorption of eumelanin pigments is determined by the superposition of the spectra associated with the monomeric and oligomeric building blocks.

Including thermal disorder of hydrogen bonding to describe the vibrational circular dichroism spectrum of zwitterionic L-alanine in water.

The vibrational circular dichroism (VCD) spectrum of l-alanine amino acid in aqueous solution in ambient conditions has been studied. The emphasis has been placed on the inclusion of the thermal disorder of the solute-solvent hydrogen bonds that characterize the aqueous solution condition. A combined and sequential use of molecular mechanics and quantum mechanics was adopted. To calculate the average VCD spectrum, the DFT B3LYP/6-311++G(d,p) level of calculation was employed, over one-hundred configurations composed of the solute plus all water molecules making hydrogen bonds with the solute. Simplified considerations including only four explicit solvent molecules and the polarizable continuum model were also made for comparison. Considering the large number of vibration frequencies with only limited experimental results a direct comparison is presented, when possible, and in addition a statistical analysis of the calculated values was performed. The results are found to be in line with the experiment, leading to the conclusion that including thermal disorder may improve the agreement of the vibrational frequencies with experimental results, but the thermal effects may be of greater value in the calculations of the rotational strengths.

A first principles approach to the electronic properties of liquid and supercritical CO2.

The electronic absorption spectra of liquid and supercritical CO2 (scCO2) are investigated by coupling a many-body energy decomposition scheme to configurations generated by Born-Oppenheimer molecular dynamics. A Frenkel exciton Hamiltonian formalism was adopted and the excitation energies were calculated with time dependent density functional theory. A red-shift of ∼ 0.2 eV relative to the gas-phase monomer is observed for the first electronic absorption maximum in liquid and scCO2. The origin of this shift, which is not very dependent on deviations from the linearity of the CO2 molecule, is mainly related to polarization effects. However, the geometry changes of the CO2 monomer induced by thermal effects and intermolecular interactions in condensed phase lead to the appearance of an average monomeric electric dipole moment〈µã€‰= 0.26 ± 0.04 D that is practically the same at liquid and supercritical conditions. The predicted average quadrupole moment for both liquid and scCO2 is〈Θ〉= - 5.5 D Å, which is increased by ∼ -0.9 D Å relative to its gas-phase value. The importance of investigating the electronic properties for a better understanding of the role played by CO2 in supercritical solvation is stressed.

Experimental and theoretical investigation of the first-order hyperpolarizability of a class of triarylamine derivatives.

This paper reports on the static and dynamic first-order hyperpolarizabilities of a class of push-pull octupolar triarylamine derivatives dissolved in toluene. We have combined hyper-Rayleigh scattering experiment and the coupled perturbed Hartree-Fock method implemented at the Density Functional Theory (DFT) level of theory to determine the static and dynamic (at 1064 nm) first-order hyperpolarizability (ßHRS) of nine triarylamine derivatives with distinct electron-withdrawing groups. In four of these derivatives, an azoaromatic unit is inserted and a pronounceable increase of the first-order hyperpolarizability is reported. Based on the theoretical results, the dipolar/octupolar character of the derivatives is determined. By using a polarizable continuum model in combination with the DFT calculations, it was found that although solvated in an aprotic and low dielectric constant solvent, due to solvent-induced polarization and the frequency dispersion effect, the environment substantially affects the first-order hyperpolarizability of all derivatives investigated. This statement is supported due to the solvent effects to be essential for the better agreement between theoretical results and experimental data concerning the dynamic first-order hyperpolarizability of the derivatives. The first-order hyperpolarizability of the derivatives was also modeled using the two- and three-level models, where the relationship between static and dynamic first hyperpolarizabilities is given by a frequency dispersion model. Using this approach, it was verified that the dynamic first hyperpolarizability of the derivatives is satisfactorily reproduced by the two-level model and that, in the case of the derivatives with an azoaromatic unit, the use of a damped few-level model is essential for, considering also the molecular size of such derivatives, a good quantitative agreement between theoretical results and experimental data to be observed.

Monte Carlo-quantum mechanics study of magnetic properties of hydrogen peroxide in liquid water.

A theoretical study of magnetic properties of hydrogen peroxide in water has been carried out by means of Monte Carlo simulation and quantum mechanics calculations. The solvent effects were evaluated in supermolecular structures generated by simulations in the NPT ensemble. The solute-solvent structure was analyzed in terms of radial distribution functions, and the solute-solvent hydrogen bonds were identified with geometric and energetic criteria. Approximately three water molecules are hydrogen bonded to H2O2 (0.6 and 0.8 in each hydrogen and oxygen atom, respectively, of the H2O2). Although, on average, both hydroxyls of the peroxide are equivalent, the distribution of hydrogen-bonded water molecules is highly asymmetric. Analyzing the statistics of the hydrogen bonds, we identify that only 34% of the configurations give symmetric distributions around the two hydroxyls of the H2O2 simultaneously. The magnetic shieldings and the indirect spin-spin coupling constants were calculated at the B3LYP/aug-cc-pVTZ and aug-cc-pVTZ-J computational level. We find that the solvent shields the oxygen and unshields the hydrogen atoms of the peroxide (+5.5 and -2.9 ppm, respectively), with large fluctuation from configuration to configuration in the oxygen case, an effect largely accounted for in terms of a single hydrogen bond with H2O2 as the proton donor. The most sensitive coupling in the presence of the solvent is observed to be the one-bond J(O,H).

Communication: Transient anion states of phenol (H2O)n (n = 1, 2) complexes: search for microsolvation signatures.

We report on the shape resonance spectra of phenol-water clusters, as obtained from elastic electron scattering calculations. Our results, along with virtual orbital analysis, indicate that the well-known indirect mechanism for hydrogen elimination in the gas phase is significantly impacted on by microsolvation, due to the competition between vibronic couplings on the solute and solvent molecules. This fact suggests how relevant the solvation effects could be for the electron-driven damage of biomolecules and the biomass delignification [E. M. de Oliveira et al., Phys. Rev. A 86, 020701(R) (2012)]. We also discuss microsolvation signatures in the differential cross sections that could help to identify the solvated complexes and access the composition of gaseous admixtures of these species.

Influence of Solvents and Halogenation on ESIPT of Benzimidazole Derivatives for Designing Turn-on Fluorescence Probes.

This work reports a theoretical investigation of the solvent polarity as well as the halogenation of benzimidazole derivatives during excited state intramolecular proton transfer (ESIPT). It details how the environment and halogen substitution may contribute to the efficiency of ESIPT upon keto-enol tautomerism and exploits this effect to design fluorescence sensing. For this purpose, we first examine the conformational equilibrium of benzimidazole derivatives containing different halogen atoms, which results in intramolecular proton transfer, using density-functional theory (DFT) combined with the polarizable continuum model (PCM). Then we evaluate the fluorescence of the benzimidazole derivatives in different dielectric constants within time-dependent DFT (TD-DFT) approaches. Our results quantitatively allow the determination of large Stokes shifts in nonpolar solvents around 100 nm. These theoretical results are in agreement with experimental solvatochromism studies of benzimidazoles. The effect of halogenation, with fluorine, chlorine, and bromine, is less important than solvent polarization when ESIPT takes place. Thus, halogenation can be properly chosen depending on the interest of the synthesis of benzimidazole-based turn-on fluorescence in appropriate solvents.

Solvent effect on the Stokes shift and on the nonfluorescent decay of the daidzein molecular system.

The flavonoids have been the target of several experimental works due to its influence in the human health as antioxidant elements. The fluorescence properties of these compounds have been widely studied due to the large Stokes shifts experimentally observed and the variety of processes that lead to the fluorescence. In the present work the role of the solvent in the large Stokes shift experimentally observed in the daidzein molecular system in water is theoretically studied. Also studied is the nonfluorescent decay mechanism in a polar aprotic solvent like acetonitrile. The solvent effect in the ground and in the low-lying excited electronic states is taken into account by using the sequential-QM/MM methodology. Excited state properties like equilibrium geometries and transition energies were studied by using multiconfigurational calculations, CASSCF and CASPT2. The excited electronic state responsible for the fluorescence spectrum in water was identified, and the large Stokes shift seems to be the result of the large interaction of the system in this electronic state with the solvent. On the other hand, spin-orbit coupling calculations, between the singlet and triplet electronic states, indicate favorable conditions for intersystem crossing, in agreement with the experimental result of nonfluorescence observation.