Andrzej L. Sobolewski

Excited-state hydrogen detachment and hydrogen transfer driven by repulsive 1πσ* states: A new paradigm for nonradiative decay in aromatic biomolecules

The combined results of ab initio electronic-structure calculations and spectroscopic investigations of jet-cooled molecules and clusters provide strong evidence of a surprisingly simple and general mechanistic picture of the nonradiative decay of biomolecules such as nucleic bases and aromatic amino acids. The key role in this picture is played by excited singlet states of πσ* character, which have repulsive potential-energy functions with respect to the stretching of OH or NH bonds. The 1πσ* potential-energy functions intersect not only the bound potential-energy functions of the 1ππ* excited states, but also that of the electronic ground state. Via predissociation of the 1ππ* states and a conical intersection with the ground state, the 1πσ* states trigger an ultrafast internal-conversion process, which is essential for the photostability of biomolecules. In protic solvents, the 1πσ* states promote a hydrogen-transfer process from the chromophore to the solvent. Calculations for chromophore–water clusters have shown that a spontaneous charge-separation process takes place in the solvent shell, yielding a microsolvated hydronium cation and a microsolvated electron. These results suggest that the basic mechanisms of the complex photochemistry of biomolecules in liquid water can be revealed by experimental and theoretical investigations of relatively small chromophore–water clusters.

Efficient deactivation of a model base pair via excited-state hydrogen transfer

We present experimental and theoretical evidence for an excited-state deactivation mechanism specific to hydrogen-bonded aromatic dimers, which may account, in part, for the photostability of the Watson-Crick base pairs in DNA. Femtosecond time-resolved mass spectroscopy of 2-aminopyridine clusters reveals an excited-state lifetime of 65 ± 10 picoseconds for the near-planar hydrogen-bonded dimer, which is significantly shorter than the lifetime of either the monomer or the 3- and 4-membered nonplanar clusters. Ab initio calculations of reaction pathways and potential-energy profiles identify the mechanism of the enhanced excited-state decay of the dimer: Conical intersections connect the locally excited 1ππ* state and the electronic ground state with a 1ππ* charge-transfer state that is strongly stabilized by the transfer of a proton.

Ab initio studies on the photophysics of the guanine–cytosine base pair

The low-lying excited singlet states of the Watson–Crick form of the guanine–cytosine base pair have been investigated with multi-reference ab initio methods (complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory based on the CASSCF reference (CASPT2)). The reaction paths and energy profiles for single proton transfer from guanine to cytosine in the 1ππ* guanine-to-cytosine charge-transfer state and for twisting of the CC double bond of the cytosine ring in the locally excited 1ππ* state of cytosine have been explored by excited-state geometry optimization using the configuration-interaction-with-singles (CIS) method and single-point energy calculations at the CASPT2 level. Avoided crossings of the 1ππ* potential-energy functions with the electronic ground-state potential-energy function have been identified along both reaction paths. The results suggest the existence of low-lying conical intersections of the 1ππ* potential-energy surface with the S0 surface which become accessible by possibly barrierless single proton transfer as well as out-of-plane deformation of cytosine and may trigger an ultrafast radiationless decay to the ground state. The relevance of these results for the rationalization of the photostability of the genetic code is briefly discussed.

Ab initio investigations on the photophysics of indole

Abstract Reaction paths and potential-energy profiles for detachment of the hydrogen atom of the NH group in excited singlet states of indole have been investigated using the CIS, CASSCF and CASPT2 ab initio methods. The potential-energy profile of the lowest π σ ∗ excited singlet state is found to be essentially repulsive. It crosses the potential-energy functions of the 1 L b and 1 L a excited states of ππ ∗ character as well as those of the ground state. The resulting multiple conical intersections can provide the mechanism for efficient internal conversion to the ground state. The polarities of the excited states are remarkably different, indicating a complex interplay of internal conversion and solvation dynamics of photoexcited indole in polar solvents.

Conical intersections induced by repulsive 1πσ* states in planar organic molecules: malonaldehyde, pyrrole and chlorobenzene as photochemical model systems

Abstract Ab initio calculations of reaction paths and potential-energy profiles of excited states are reported for the model systems malonaldehyde, pyrrole and chlorobenzene. In all three cases, optically dark singlet states of πσ * character have been found, which are repulsive with respect to an appropriate reaction coordinate. The resulting surface crossings with 1 ππ * states and/or the electronic ground state, which are symmetry allowed for the planar systems, are converted into conical intersections by out-of-plane modes of appropriate symmetry. It is argued that conical intersections of this type are a common phenomenon in planar organic molecules with heteroatoms and may dominate the photochemistry of these systems.

Ab initio potential-energy functions for excited state intramolecular proton transfer: a comparative study of o-hydroxybenzaldehyde, salicylic acid and 7-hydroxy-1-indanone

Potential-energy profiles along the minimum-energy reaction path for intramolecular proton transfer in the 1ππ* excited state have been calculated for the title compounds. The CASSCF and CIS electronic-structure methods have been employed for excited-state geometry optimization. Single-point energy calculations along the reaction path have been performed using the CASPT2 and TDDFT methods. The TDDFT method has been tested against accurate CASSCF and CASPT2 data for malonaldehyde. CASPT2 yields transition energies for photon absorption and emission which are in excellent agreement with experimental data (within 0.2 eV). The CASPT2 potential energy functions exhibit, however, artifactual kinks (on a scale of a single kcal mol-1) which reflect inherent limitations of the CASSCF-based perturbation approach. TDDFT yields potential-energy functions which are essentially parallel to the CASPT2 functions and free of artifacts. Transition energies for absorption and emission are systematically overestimated, however, by about 0.5 eV in TDDFT. For all three title compounds, a barrierless 1ππ* potential-energy function is predicted. The location of the 1ππ* minimum varies from near-enol in salicylic acid to near-keto in 7-hydroxy-1-indanone.

Characterization of the S1–S2 conical intersection in pyrazine using ab initio multiconfiguration self‐consistent‐field and multireference configuration‐interaction methods

Potential‐energy surfaces of the three lowest singlet states of pyrazine have been calculated as a function of ab initio determined ground‐state normal coordinates, using complete‐active‐space self‐consistent‐field (CASSCF) and multireference configuration interaction (MRCI) techniques. The conical intersection of the S1 and S2 adiabatic potential‐energy surfaces has been mapped out in selected subspaces spanned by the most relevant vibrational coordinates. A unitary transformation from the adiabatic to a quasidiabatic electronic representation is performed, which eliminates the rapid variations of the wave functions responsible for the singularity of the nonadiabatic coupling element. Transition‐dipole‐moment functions have been obtained in the adiabatic and in the diabatic representation. The leading coefficients of the Taylor expansion of the diabatic potential‐energy and transition‐dipole‐moment surfaces in terms of ground‐state normal coordinates at the reference geometry have been obtained at the CA...

Charge transfer in aminobenzonitriles: do they twist?

Abstract Ab initio electronic structure calculations have been performed to characterize the charge-transfer process in benzonitrile, 4-aminobenzonitrile and 4-dimethylaminobenzonitrile. The HF, CIS, CASSCF and CASPT2 methods have been employed. Geometry optimization of the charge-transfer state predicts a planar configuration with bent CN group. The results suggest that bending of the cyano group rather than twisting of the amino group is the intramolecular motion which is responsible for the stabilization of the charge-transfer state in aminobenzonitriles.

Ab initio study of the excited-state coupled electron–proton-transfer process in the 2-aminopyridine dimer

Abstract The low-lying 1 ππ * excited states of the 2-aminopyridine dimer have been investigated with multi-reference ab initio methods (CASSCF and MRMP2). The 2-aminopyridine dimer can be considered as a mimetic model of Watson–Crick DNA base pairs. The reaction path and the energy profile for single proton transfer in the lowest 1 ππ * inter-monomer charge-transfer state have been obtained. A weakly avoided crossing of the 1 ππ * surface with the electronic ground-state surface has been found near the single-proton-transfer minimum of the 1 ππ * surface. From the splitting of the adiabatic surfaces at the avoided crossing, an internal-conversion lifetime of the excited state of

Ab initio investigation of potential‐energy surfaces involved in the photophysics of benzene and pyrazine

Potential‐energy surfaces of the lowest singlet and triplet excited states of benzene and pyrazine have been calculated using complete‐active‐space self‐consistent‐field and multireference configuration interaction (MRCI) techniques. We have focused our attention on the saddle points and surface intersections associated with the reaction path to a biradical form called prefulvene. The barrier heights separating the prefulvenic minimum from the minimum of the planar aromatic form on the ππ* excited singlet surface and on the ground‐state surface have been estimated by large‐scale MRCI calculations. The conical intersection of the lowest ππ* excited singlet surface with the S0 surface has been mapped out in two dimensions, the reaction coordinate to prefulvene and the coordinate of maximum coupling perpendicular to it. The relevance of these ab initio potential‐energy data for the interpretation of photophysical relaxation pathways in benzene and pyrazine (‘‘channel‐three’’ effect) is discussed.