Excited State Hydrogen Transfer Research Articles

Ab initio electronic structure theory as an aid to understanding excited state hydrogen transfer in moderate to large systems

Hypocrellin and hypericin are naturally occurring polycyclic perylene quinones, and they have both been found to exhibit photoactivated antiviral and anticancer activity. One mode of action proposed involves excited-state hydrogen transfer. Consequently these compounds have been widely studied using spectroscopic methods, and are found to both absorb and emit in the visible region. Recently an analog dihydroxy perylene quinone was synthesized in order to examine its antiviral activity in relation to the naturally occurring compounds. Its UV-visible absorption and emission spectra are quite different to those of hypocrellin and hypericin, with very weak absorption and no visibility of emission at all. The ab inito excited-state methods, configuration interaction singles (CIS), state-averaged complete active space self-consistent field (SA–CASSCF), and SA-multireference perturbation theory (SA–MRMP2) are used to examine the origin of this different absorption and emission behavior. Owing to the size of these systems (between 24 and 40 heavy atoms) extensive use of parallel processor algorithms was made, especially a parallel atomic orbital-based CIS energy and gradient code developed at the ABCC. The performance of these methods, and possible ,as well as future directions and prospects are discussed.

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 1pipi* state and the electronic ground state with a 1pipi* charge-transfer state that is strongly stabilized by the transfer of a proton.

Substituent Effects on Competitive Release of Phenols and 1,3‐Rearrangement in α‐Keto Amide Photochemistry.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Substituent effects on competitive release of phenols and 1,3-rearrangement in alpha-keto amide photochemistry.

[reaction: see text] Photolysis of alpha-keto amides bearing 4-YC(6)H(4)O leaving groups at the position alpha to the keto group efficiently produces high yields of phenols when Y is an electron-withdrawing group or H. The photoelimination likely involves cleavage of zwitterionic intermediates produced via excited-state hydrogen transfer. When Y is an electron-donating group, competing excited-state ArO-C(alpha) bond scission to radicals occurs, followed by recombination to give 1,3-photorearrangment products.

Photochemistry of phenol–(NH3)n clusters: Solvent effect on a radical cleavage of an OH bond in an electronically excited state and intracluster reactions in the product NH4(NH3)n−1 (n⩽5)

The potential energy surfaces of PhOH–(NH3)0,1 and NH4(NH3)1–4 have been investigated theoretically by ab initio methods. Intermolecular stretching in PhOH–NH3 assists in the radical cleavage of an OH bond occurring through a ππ*/πσ* potential crossing. Thus, excited state hydrogen transfer (ESHT) is expected to take place by a solvent-assisted mechanism even in the larger PhOH–(NH3)n. Because sufficient energy is obtained by ESHT from PhOH–(NH3)n (ππ*) to PhO–NH4(NH3)n−1 (πσ*) (n⩽5), hydrogen relocation and/or ammonia migration in the product NH4(NH3)n−1 can readily follow ESHT, which is responsible for observing isomer bands in the absorption spectra of the photoinduced reaction products of PhOH–(NH3)n.

Ultrafast vibrational relaxation processes induced by intramolecular excited state hydrogen transfer

Vibrational spectra between 1000 and 1700 cm −1 are studied after ultrafast transfer of a hydrogen atom in the excited state of 2-(2 ′-hydroxyphenyl)benzothiazole. Femtosecond pump–probe experiments reveal new vibrational bands of the keto-S 1 state, including the carbonyl stretching band formed by hydrogen transfer. Such bands display a negligible spectral reshaping but blue-shift by up to 7 cm −1 following biexponential kinetics with time constants of 700 fs and 15 ps. The blue-shift is attributed to the anharmonic coupling of the fingerprint vibrations to Raman-active low-frequency modes that are excited upon electronic excitation and depopulated by intramolecular redistribution and cooling to the solvent.

Hydrogen transfer in photo-excited phenol/ammonia clusters by UV–IR–UV ion dip spectroscopy and ab initio molecular orbital calculations. II. Vibrational transitions

The vibrational spectra of phenol/ammonia clusters (1:2–5) in S0 and those of their photochemical reaction products, (NH3)n−1NH4 (n=2–5), which are generated by excited-state hydrogen transfer, have been measured by UV–IR–UV ion dip spectroscopy. The geometries, IR spectra and normal modes of phenol-(NH3)n (n=1–5) have been examined by ab initio molecular orbital calculations, at the second-order Møller–Plesset perturbation theory level with large basis sets. For the n=2 and 3 reaction products, similar vibrational analyses have been carried out. From the geometrical information of reactants and products, it has been suggested that the reaction products have memories of the reactant’s structure, which we call “memory effect.”

Hydrogen transfer in photoexcited phenol/ammonia clusters by UV–IR–UV ion dip spectroscopy and ab initio molecular orbital calculations. I. Electronic transitions

The electronic spectra of reaction products via photoexcited phenol/ammonia clusters (1:2–5) have been measured by UV-near-IR–UV ion dip spectroscopy. Compared with the electronic spectra of hydrogenated ammonia cluster radicals the reaction products have been proven to be (NH3)n−1NH4 (n=2–5), which are generated by excited-state hydrogen transfer in PhOH–(NH3)n. By comparing the experimental results with ab initio molecular orbital calculations at multireference single and double excitation configuration interaction level, it has been found that the reaction products (NH3)n−1NH4 (for n=3 and 4), contain some isomers.

Femtosecond mid-infrared spectroscopy of condensed phase hydrogen-bonded systems as a probe of structural dynamics.

We report the first time-resolved site-specific mid-infrared study of the photo-induced excited state hydrogen transfer reaction in 2-(2'-hydroxyphenyl)benzothiazole (HBT) with 130 fs time resolution. The transient absorption of the C=O stretching band marking the keto*-S1-state appears delayed on a time scale of 30-50 fs after electronic excitation to the enol*-S1-state. Its line center subsequently shifts up by about 3-5 cm(-1) after excitation, depending on the excitation wavelength tuned between 315 and 349 nm. This effect is attributed to intramolecular vibrational energy redistribution (IVR) and vibrational energy relaxation (VER) processes. We observe for the first time the coherent effects of anharmonic coupling of low frequency modes (approximately 60 cm(-1), approximately 120 cm(-1)), on the C=O mode marking the product state. We ascribe the 120 cm(-1) mode to a Raman-active in-plane deformation mode that is coherently excited by the UV-pump pulse. We tentatively explain the coherent excitation of the infrared active 60 cm(-1) out-of-plane deformation mode by nonradiative processes within the excited enol state after electronic excitation.

Quenching of tryptophan (1)(pi,pi*) fluorescence induced by intramolecular hydrogen abstraction via an aborted decarboxylation mechanism.

CASSCF computations show that the hydrogen-transfer-induced fluorescence quenching of the (1)(pi,pi*) excited state of zwitterionic tryptophan occurs in three steps: (1) formation of an intramolecular excited-state complex, (2) hydrogen transfer from the amino acid side chain to the indole chromophore, and (3) radiationless decay through a conical intersection, where the reaction path bifurcates to a photodecarboxylation and a phototautomerization route. We present a general model for fluorescence quenching by hydrogen donors, where the radiationless decay occurs at a conical intersection (real state crossing). At the intersection, the reaction responsible for the quenching is aborted, because the reaction path bifurcates and can proceed forward to the products or backward to the reactants. The position of the intersection along the quenching coordinate depends on the nature of the states and, in turn, affects the formation of photoproducts during the quenching. For a (1)(n,pi*) model system reported earlier (Sinicropi, A.; Pogni, R.; Basosi, R.; Robb, M. A.; Gramlich, G.; Nau, W. M.; Olivucci, M. Angew. Chem., Int. Ed. 2001, 40, 4185-4189), the ground and the excited state of the chromophore are hydrogen acceptors, and the excited-state hydrogen transfer is nonadiabatic and leads directly to the intersection point. There, the hydrogen transfer is aborted, and the reaction can return to the reactant pair or proceed further to the hydrogen-transfer products. In the tryptophan case, the ground state is not a hydrogen acceptor, and the excited-state hydrogen transfer is an adiabatic, sequential proton and electron transfer. The decay to the ground state occurs along a second reaction coordinate associated with decarboxylation of the amino acid side chain and the corresponding aborted conical intersection. The results show that, for (1)(pi,pi*) states, the hydrogen transfer alone is not sufficient to induce the quenching, and explain why fluorescence quenching induced by hydrogen donors is less general for (1)(pi,pi*) than for (1)(n,pi*) states.

Solvation effects on electronic excited states of methanol: A study of neat and mixed methanol/water clusters

Femtosecond pump-probe spectroscopic studies on mixed methanol/water clusters are presented. Two-photon absorption of light was employed to generate methanol molecules in the 3p Rydberg excited state, with transfer to the 3s Rydberg state during the ensuing dynamics. The experiments provide evidence that excited state hydrogen transfer occurs between a methanol molecule, which undergoes hydroxyl dissociation following electronic excitation, and a water molecule, leading to the involvement of a metastable H3O species.

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.

The Ground and Excited State Hydrogen Transfer Potential Energy Surface in 7-Azaindole

Multireference wave functions, augmented by second-order perturbation theory, are used to examine the hydrogen transfer process in the ground and first excited states of 7-azaindole and in the 1:1 7-azaindole:water complex. The presence of one water molecule dramatically reduces the barrier to proton transfer in both electronic states. In the excited state the order of the two tautomers is reversed, and the barrier for the hydrogen transfer from the (now higher energy) normal structure to the tautomer in the presence of one water is estimated to be ≤6 kcal/mol.

Theoretical investigation of potential energy surfaces relevant for excited-state hydrogen transfer in o-hydroxybenzaldehyde

The potential energy functions of the electronic ground state as well as the lowest n π * and ππ * excited single states of o-hydroxybenzaldedhyde (OHBA) have been theoretically characterized along the proton transfer (PT) reaction coridnate as well as the reaction coordinate leading to the prefulvenic form. The calculations have been performed with the ab initio complete-active-space self-consistent-field (CASSCF) method and with second-order perturbation theory, employing the CASSCF wave function as the reference state. It is found that the 1ππ * state is almost isoenergetic with the 1nπ * state after vertical excitation of OHBA. The 1ππ * potential energy function is found to be barrierless along the PT reaction coordinate, while the 1nπ * potential energy exhibits a significant barrier. An efficient non-radiative pathway has been identified resulting from strong non-adiabatic interactions between the ground and the excited singlet states along the reaction coordinate to the prefulvenic form of OHBA. It is argued that the consideration of multi-dimensional vibronic interactions is essential for the understanding of the excited state dynamics of OHBA.

Evidence for the need of a non-Born—Oppenheimer description of excited-state hydrogen transfer

The HOCHCHCHNH (enol from) to OCHCHCHNH 2 (keto form) excited-state isomerization of 1-amino-3-propenal is considered as a model system for photo-induced intramolecular hydrogen (or proton) transfer. The potential energy surfaces of the electronic ground state as well as of the lowest nπ* and ππ* excited singlet states have been characterized by ab initio compelte-active-space self-consistent-field and multireference configuration-interaction calculations. It is found that the nπ* potential energy function exhibits a significant barrier along the proton-transfer reaction coordinate, while the ππ* potential energy function is almost barrierless and intersects the nπ* surface, providing a pathway for ultrafast photo-induced proton transfer. It is argued that consideration of nπ*ππ* vibronic coupling is essential in understanding the photo-induced proton-transfer reaction.

The mechanism of excited-state hydrogen transfer in 2-hydroxypyridine

The excited-state isomerization of 2-hydroxypyridine to 2(1H)-pyridone is considered as a model for UV photo-induced intramolecular hydrogen (or proton) transfer in systems without an intramolecular hydrogen bond. The potential energy functions of the electronic ground as well as of the lowest nπ* and ππ* excited singlet states have been characterized by ab initio complete-active-space self-consistent-field calculations. The results suggest that photo-induced proton transfer in an isolated molecule occurs via successive dissociation and association of the mobile hydrogen atom.