Irene Conti

The Different Photoisomerization Efficiency of Azobenzene in the Lowest nπ* and ππ* 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.

Adenine deactivation in DNA resolved at the CASPT2//CASSCF/AMBER level

We have employed hybrid CASPT2//CASSCF/AMBER calculations to map the (1)L(a)(1pipi*) deactivation path of a single quantum mechanical adenine in a d(A)(10).d(T)(10) double strand in water that is treated at the molecular mechanics level. We find that (a) the L(a) relaxation route is flatter in DNA than in vacuo and (b) the L(a) relaxation energy in DNA is much larger than the stabilization energy of the corresponding L(a) excimer. An intra-monomer relaxation process is found to be compatible with the multiexponential decay recorded in DNA, possibly including the longer (4100 ps) lifetime component.

Deciphering low energy deactivation channels in adenine.

The radiationless decay paths of 9H-adenine in its lowest excited states (1)npi*, (1)L(b)((1)pipi*), and (1)L(a)((1)pipi*) and in dissociative (1)pisigma* states have been mapped in vacuo at the CASPT2//CASSCF resolution. The minimum energy path (MEP) of the (1)L(a) state, which shows the strongest absorption below 5 eV, is found to decrease monotonically along the puckering coordinate from the vertical excitation to a S(0)/(1)L(a) conical intersection (CI). The vertically excited (1)npi* and (1)L(b) states are found to relax to the respective minima and to require some energy to reach CIs with S(0). This picture suggests that (1)L(a) alone is responsible of both components of the ultrafast biexponential decay (with tau(1) < 0.1 ps and tau(2) < 1 ps) recently observed in time-resolved pump-probe resonant ionization and fluorescence spectroscopy, and that the (1)npi* and (1)L(b) states do not act as important intermediates in the (1)L(a) decay process. We find that the (1)L(a)-->(1)pisigma(N9H)* internal conversion can be followed by N(9)-H photocleavage, albeit with tiny quantum yield. The amino N(10)-H bond photocleavage is hindered by the high barrier encountered along the N(10)-H bond-breaking path in the (1)pisigma(N10H)* state.

Deciphering the photochemical mechanisms describing the UV-induced processes occurring in solvated guanine monophosphate.

The photophysics and photochemistry of water-solvated guanine monophosphate (GMP) are here characterized by means of a multireference quantum-chemical/molecular mechanics theoretical approach (CASPT2//CASSCF/AMBER) in order to elucidate the main photo-processes occurring upon UV-light irradiation. The effect of the solvent and of the phosphate group on the energetics and structural features of this system are evaluated for the first time employing high-level ab initio methods and thoroughly compared to those in vacuo previously reported in the literature and to the experimental evidence to assess to which extent they influence the photoinduced mechanisms. Solvated electronic excitation energies of solvated GMP at the Franck-Condon (FC) region show a red shift for the ππ* La and Lb states, whereas the energy of the oxygen lone-pair nπ* state is blue-shifted. The main photoinduced decay route is promoted through a ring-puckering motion along the bright lowest-lying La state toward a conical intersection (CI) with the ground state, involving a very shallow stationary point along the minimum energy pathway in contrast to the barrierless profile found in gas-phase, the point being placed at the end of the minimum energy path (MEP) thus endorsing its ultrafast deactivation in accordance with time-resolved transient and photoelectron spectroscopy experiments. The role of the nπ* state in the solvated system is severely diminished as the crossings with the initially populated La state and also with the Lb state are placed too high energetically to partake prominently in the deactivation photo-process. The proposed mechanism present in solvated and in vacuo DNA/RNA chromophores validates the intrinsic photostability mechanism through CI-mediated non-radiative processes accompanying the bright excited-state population toward the ground state and subsequent relaxation back to the FC region.

Probing deactivation pathways of DNA nucleobases by two-dimensional electronic spectroscopy: first principles simulations

The SOS//QM/MM [Rivalta et al., Int. J. Quant. Chem., 2014, 114, 85] method consists of an arsenal of computational tools allowing accurate simulation of one-dimensional (1D) and bi-dimensional (2D) electronic spectra of monomeric and dimeric systems with unprecedented details and accuracy. Prominent features like doubly excited local and excimer states, accessible in multi-photon processes, as well as charge-transfer states arise naturally through the fully quantum-mechanical description of the aggregates. In this contribution the SOS//QM/MM approach is extended to simulate time-resolved 2D spectra that can be used to characterize ultrafast excited state relaxation dynamics with atomistic details. We demonstrate how critical structures on the excited state potential energy surface, obtained through state-of-the-art quantum chemical computations, can be used as snapshots of the excited state relaxation dynamics to generate spectral fingerprints for different de-excitation channels. The approach is based on high-level multi-configurational wavefunction methods combined with non-linear response theory and incorporates the effects of the solvent/environment through hybrid quantum mechanics/molecular mechanics (QM/MM) techniques. Specifically, the protocol makes use of the second-order Perturbation Theory (CASPT2) on top of Complete Active Space Self Consistent Field (CASSCF) strategy to compute the high-lying excited states that can be accessed in different 2D experimental setups. As an example, the photophysics of the stacked adenine-adenine dimer in a double-stranded DNA is modeled through 2D near-ultraviolet (NUV) spectroscopy.

Revealing Excited State Interactions by Quantum-Chemical Modeling of Vibronic Activities: The R2PI Spectrum of Adenine

We present a computational study encompassing quantum-chemical calculations of the ground and low-lying excited states of 9H-adenine and modeling of vibronic activities associated with the S(0) --> L(b) and S(0) --> n pi* transitions. Minima on the ground and excited states and the saddle point on the n pi* potential energy surface are determined with CASSCF calculations. Vibrational frequencies are computed at the same level of theory on ground and excited states while transition dipole moments and oscillator strengths are estimated, at the optimized geometries, with CASPT2//CASSCF calculations. Modeling of vibronic activities includes both Franck-Condon and Herzberg-Teller induced contributions. While the adopted harmonic approximation is acceptable for the S(0) --> L(b) transition and allows the assignment of several observed bands in the R2PI spectrum of adenine, the computed anharmonic potential along the puckering coordinate in the n pi* state requires a different treatment. To this end the vibronic levels and intensities associated with vibronic transitions in the puckering coordinate are evaluated by numerical solution of the 2D potential including the anharmonic puckering coordinate. All the remaining vibrational coordinates are treated as harmonic. On the basis of the modeling, the four major bands in the R2PI spectrum of adenine are assigned, along with a number of minor bands in the spectra.

Substituent controlled spectroscopy and excited state topography of retinal chromophore models: fluorinated and methoxy-substituted protonated Schiff bases

Ab initio multireference second-order perturbation theory computations are used to explore the spectroscopic behaviour (i.e. absorption and emission) and the structure of two isolated (i.e. in vacuo) short chain retinal chromophore models (i.e. the 2-cis-penta-2,4-dieniminium and all-trans-epta-2,4,6-trieniminium cations), which have been modified using fluorine or methoxyl substituents as representative examples of electron withdrawing and electron releasing groups, respectively. A systematic analysis has been performed for the systems substituted in all positions along the chain. Significant effects on the excited state structure of the chromophore and its absorption and emission features are unveiled by comparison with previously reported results for the corresponding unsubstituted cations in vacuo. Indeed, it is demonstrated that (i) substituents may affect the chromophore absorption/emission energy depending on the substituted position along the chain and the specific electronic effect of the substituent (with fluorine and methoxyl inducing opposite effects), (ii) substituted odd- and even-numbered positions along the chain behave in a different way in agreement with the experiments, and (iii) the initially relaxed geometry in the excited state, addressing the excited state dynamics of the chromophore out of the Franck–Condon region, may change. A rationale for these effects is shown, which provides a crude basis for understanding experimental spectroscopic observations and allows one to speculate and draw conjectures about the intrinsic photochemical behaviour (namely the photoisomerization efficiency/selectivity) of substituted retinal protonated Schiff bases.

UV-Light-Induced Vibrational Coherences: The Key to Understand Kasha Rule Violation in trans-Azobenzene

We combine sub-20 fs transient absorption spectroscopy with state-of-the-art computations to study the ultrafast photoinduced dynamics of trans-azobenzene (AB). We are able to resolve the lifetime of the ππ* state, whose decay within ca. 50 fs is correlated to the buildup of the nπ* population and to the emergence of coherences in the dynamics, to date unobserved. Nonlinear spectroscopy simulations call for the CNN in-plane bendings as the active modes in the subps photoinduced coherent dynamics out of the ππ* state. Radiative to kinetic energy transfer into these modes drives the system to a high-energy planar nπ*/ground state conical intersection, inaccessible upon direct excitation of the nπ* state, that triggers an ultrafast (0.45 ps) nonproductive decay of the nπ* state and is thus responsible for the observed Kasha rule violation in UV excited trans-AB. On the other hand, cis-AB is built only after intramolecular vibrational energy redistribution and population of the NN torsional mode.

Evolution of the Excitonic State of DNA Stacked Thymines: Intrabase ππ* → S0 Decay Paths Account for Ultrafast (Subpicosecond) and Longer (>100 ps) Deactivations

Monomer-like ring puckering decay paths for two stacked quantum mechanical thymines inside a solvated DNA duplex described at the molecular mechanics level are mapped using a hybrid CASPT2//CASSCF/MM protocol that accounts for steric, electronic and electrostatic interactions within the nucleobases native environment. Asymmetric stacking between nucleobases open ups different intrabase ππ* decay paths accounting for distinctive excited state lifetimes, spanning the subps to subns time window.

An ab initio Study of Decay Mechanism of Adenine: the Facile Path of the Amino NH Bond Cleavage

A comprehensive study of the radiationless decay processes of the lowest excited singlet states in the isolated 9H‐Adenine has been performed at the CASPT2//CASSCF level. The minimum energy paths of the La, Lb and nπ* singlet states along different skeletal distortions have been computed and the Conical Intersections (CIs) involving these states have been determined. The fast deactivation path of La along a skeletal deformation, which leads to a S0/La CI, as previously discussed, is confirmed. Moreover, low‐lying CIs between S0 and πσ* singlet states have been characterized, where σ* is the antibonding orbital localized on a N‐H bond of the amino (πσNH2*) or of the azine group (πσN9H*). We have found that the repulsive πσNH2* state associated with an amino N‐H bond can be populated through a barrierless way. Therefore, the decay path shows a bifurcation leading to two possible ways of radiationless deactivation: on one hand a non‐photochemical decay through the S0/La or S0/nπ* CIs and on the other hand a ...