6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

▪ Abstract Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several are...

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Proton-Coupled Electron Transfer

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.

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

The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H 3 O + ) and Zundel (H 5 O 2 + ) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment.

https://science.sciencemag.org/content/sci/308/5729/1765.full.pdf

Spectral signatures of hydrated proton vibrations in water clusters.

Solute−solvent intermolecular photoinduced electron transfer (ET) reaction was proposed to account for the drastic fluorescence quenching behaviors of oxazine 750 (OX750) chromophore in protic alcoholic solvents. According to our theoretical calculations for the hydrogen-bonded OX750−(alcohol)n complexes using the time-dependent density functional theory (TDDFT) method, we demonstrated that the ET reaction takes place from the alcoholic solvents to the chromophore and the intermolecular ET passing through the site-specific intermolecular hydrogen bonds exhibits an unambiguous site selectivity. In our motivated experiments of femtosecond time-resolved stimulated emission pumping fluorescence depletion spectroscopy (FS TR SEP FD), it could be noted that the ultrafast ET reaction takes place as fast as 200 fs. This ultrafast intermolecular photoinduced ET is much faster than the diffusive solvation process, and even significantly faster than the intramolecular vibrational redistribution (IVR) process of the ...

Site-selective photoinduced electron transfer from alcoholic solvents to the chromophore facilitated by hydrogen bonding : A new fluorescence quenching mechanism

Ab initio (RHF, MP2, CASSCF, and CASPT2) calculations have been performed for the electronic ground and lowest excited singlet states of phenol, the complexes of phenol with water and ammonia, and the corresponding cations. In agreement with recent experiments it is found that proton transfer is a barrierless process in the phenol−(H2O)3 and phenol−NH3 cations, whereas no proton transfer occurs in the phenol−H2O cation. Novel aspects of the reaction dynamics in the excited-state manifold of the neutral clusters are revealed by the calculations. Predissociation of the S1(ππ*) state by a low-lying 1πσ* state leads to a concerted electron and proton-transfer reaction from the chromophore to the solvent. The excited-state reaction is endothermic in phenol−H2O and phenol−(H2O)3 clusters but exothermic (though activated) in the phenol−NH3 complex. These results substantiate recent reinterpretations of spectroscopic and kinetic data on hydrogen-transfer reactions in phenol−ammonia clusters. The close relationshi...

Photoinduced Electron and Proton Transfer in Phenol and Its Clusters with Water and Ammonia

The role of electron- and proton-transfer processes in the photophysics of hydrogen-bonded molecular systems has been investigated with ab initio electronic-structure calculations. Adopting indole, pyridine, and ammonia as molecular building blocks, we discuss generic mechanisms of the photophysics of isolated aromatic chromophores (indole), complexes of π systems with solvent molecules (indole−ammonia, pyridine−ammonia), hydrogen-bonded aromatic pairs (indole−pyridine), and intramolecularly hydrogen-bonded π systems (7-(2‘-pyridyl)indole). The reaction mechanisms are discussed in terms of excited-state minimum-energy paths, conical intersections, and the properties of frontier orbitals. A common feature of the photochemistry of the various systems is the electron-driven proton-transfer (EDPT) mechanism:  highly polar charge-transfer states of 1ππ*, 1nπ*, or 1πσ* character drive the proton transfer, which leads, in most cases, to a conical intersection of the S1 and S0 surfaces and thus ultrafast internal...

Computational Studies of the Photophysics of Hydrogen-Bonded Molecular Systems

Ab initio (MP2, CASSCF, CASPT2) and DFT/B3LYP calculations have been performed to explore the structures and the electronic and vibrational spectra of hydronium−water clusters as well as the corresponding cluster cations. Minimum-energy and transition-state structures have been optimized at the MP2 and DFT/B3LYP levels. Whereas protonated water clusters can exist both in Eigen-type structures, H3O+(H2O)n, as well as in Zundel-type structures, H5O2+(H2O)n, the neutral radical clusters are found to prefer Eigen-type structures, H3O(H2O)n. While H3O(H2O)3, like the hydronium radical, is a metastable species, it exhibits a significantly higher barrier for hydrogen detachment (7 kcal/mol), indicating the possibility of kinetic stability of larger H3O(H2O)n clusters at low temperatures. Remarkably, hydronium−water clusters are charge-separated species, consisting of a hydronium cation and a localized electron cloud, which are connected by a water network. The vertical electronic excitation energies of the vario...

Ab initio investigation of the structure and spectroscopy of hydronium-water clusters

The equilibrium geometries and vibrational spectra of anthranilic acid (AA) and salicylic acid (SA) in the S1(ππ*) excited state have been determined with time-dependent density functional theory (TDDFT). A single minimum of the excited-state potential-energy surface, rather than separate amino/imino (enol/keto) minima, is found for AA and SA. A significant strengthening of the intramolecular hydrogen bond upon electronic excitation is predicted for both systems. The excited-state minimum corresponds to a shortening of the intramolecular hydrogen bond by 0.226 A (0.322 A) in AA (SA). The calculated 0−0 excitation energies are within 0.1 eV of the experimental values. An exceptionally large red shift of 1400 cm-1 and a strong mixing with H-chelate ring bending and stretching motions is predicted for the phenolic OH stretching vibration in the S1 state of salicylic acid. In anthranilic acid, a red shift of 500 cm-1 is obtained for the H-bonded amino NH stretching vibration, in excellent agreement with a rec...

Intramolecular Hydrogen Bonding in the S1(ππ*) Excited State of Anthranilic Acid and Salicylic Acid: TDDFT Calculation of Excited-State Geometries and Infrared Spectra

We present here a detailed analysis of the mechanism of photoinduced electron and proton transfer in the planar pyrrole-pyridine hydrogen-bonded system, a model for the photochemistry of hydrogen bonds in DNA base pairs. Two different crossings, an avoided crossing and a conical intersection, are the key steps for forward and backward electron and proton transfer providing to the system photostability against UV radiation by restoring the system in its initial electronic and geometric structure.

and Andrzej L. Sobolewski

Papers

Photoinduced Electron and Proton Transfer in Phenol and Its Clusters with Water and Ammonia

Computational Studies of the Photophysics of Hydrogen-Bonded Molecular Systems

Ab initio investigation of the structure and spectroscopy of hydronium-water clusters

Intramolecular Hydrogen Bonding in the S1(ππ*) Excited State of Anthranilic Acid and Salicylic Acid: TDDFT Calculation of Excited-State Geometries and Infrared Spectra

Photoinduced electron and proton transfer in the hydrogen-bonded pyridine-pyrrole system.