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Principles Of Fluorescence Spectroscopy

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

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

We present results from theoretical studies of aqueous ionic solvation of alkali halides aimed at developing a microscopic description of structure and dynamics at the interface between air and sal...

Ions at the Air/Water Interface

The absorption of a photon by a hydroxy-aromatic photoacid triggers a cascade of events contributing to the overall phenomenon of intermolecular excited-state proton transfer. The fundamental steps involved were studied over the last 20 years using a combination of theoretical and experimental techniques. They are surveyed in this sequel in sequential order, from fast to slow. The excitation triggers an intramolecular charge transfer to the ring system, which is more prominent for the anionic base than the acid. The charge redistribution, in turn, triggers changes in hydrogen-bond strengths that set the stage for the proton-transfer step itself. This step is strongly influenced by the solvent, resulting in unusual dependence of the dissociation rate coefficient on water content, temperature, and isotopic substitution. The photolyzed proton can diffuse in the aqueous solution in a mechanism that involves collective changes in hydrogen-bonding. On longer times, it may recombine adiabatically with the excited base or quench it. The theory for these diffusion-influenced geminate reactions has been developed, showing nice agreement with experiment. Finally, the effect of inert salts, bases, and acids on these reactions is analyzed.

Elementary steps in excited-state proton transfer.

Deactivation pathways of electronically excited states have been investigated in three protonated aromatic amino acids: tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe). The protonated amino acids were generated by electrospray and excited with a 266 nm femtosecond laser, the subsequent decay of the excited states being monitored through fragmentation of the ions induced and/or enhanced by another femtosecond pulse at 800 nm. The excited state of TrpH+ decays in 380 fs and gives rise to two channels: hydrogen atom dissociation or internal conversion (IC). In TyrH+, the decay is slowed down to 22.3 ps and the fragmentation efficiency of PheH+ is so low that the decay cannot be measured with the available laser. The variation of the excited state lifetime between TrpH+ and TyrH+ can be ascribed to energy differences between the dissociative πσ* state and the initially excited ππ* state.

Ultrafast deactivation mechanisms of protonated aromatic amino acids following UV excitation

We studied the photoionization of 2-pyridone and its tautomer, 2-hydroxypyridine by means of VUV synchrotron radiation coupled to a velocity map imaging electron/ion coincidence spectrometer. The photoionization efficiency (PIE) spectrum is composed of steps. The state energies of the [2-pyridone]+ cation in the ground and A excited electronic states, as well as of the [2-hydroxypyridine]+ cation in the electronic ground state, are determined. The slow photoelectron spectra (SPES) are dominated by the 000 transitions to the corresponding electronic states together with several weaker bands corresponding to the population of the pure or combination vibrational bands of the cations. These vibrationally-resolved spectra compare very well with state-of-the-art calculations. Close to the ionization thresholds, the photoionization of these molecules is found to be mainly dominated by a direct process whereas the indirect route (autoionization) may contribute at higher energies.

Photoionization of 2-pyridone and 2-hydroxypyridine

In recent experiments, the excited-state lifetimes of protonated aromatic amino acids (TrpH+ and TyrH+) have been recorded by means of pump-probe photodissociation technique. The lifetime of TyrH+ is much longer than that of TrpH+, which has been initially rationalized on the basis of a simple phenomenological model. Besides, specific photofragments including the formation of radical cation after hydrogen loss are observed for TrpH+ that are not found for TyrH+. The ab initio calculations reported here for TrpH+ and TyrH+ using a coupled-cluster method are meant to track the rich photochemistry of these protonated amino acids following UV excitation.

Ab initio study of the excited-state deactivation pathways of protonated tryptophan and tyrosine.

Charge Separation in Molecular Clusters: Dissolution of a Salt in a Salt-(Solvent)(n)() Cluster.

Small phenol–(NH3)n
 clusters have been studied through two-color two-photon and one-photon VUV ionization, in order
 to disentangle the contributions of various dissociation or reaction paths in the excited and ionic states.
 The most striking result of the two-color experiment is that (NH4)+(NH3)n=1,5 fragments are observed with
 large delays (up to a few hundred nanoseconds) between the excitation and ionization lasers, whereas
 these same fragments are not observed in the VUV one-photon ionization experiment. In order to account for these
 findings, a new deactivation channel in the excited state of phenol–(NH3)n clusters has to be
 introduced: the hydrogen atom transfer PhOH(S1)–(NH3)n→PhO•+(NH4)(NH3)n−1. In this case, the delayed (NH4)+(NH3)n−1
 signals
 correspond
 to direct
 ionization of the (NH4)(NH3)n−1 clusters produced in the excited state.

Gilles Grégoire

Papers

Ultrafast deactivation mechanisms of protonated aromatic amino acids following UV excitation

Photoionization of 2-pyridone and 2-hydroxypyridine

Ab initio study of the excited-state deactivation pathways of protonated tryptophan and tyrosine.

Charge Separation in Molecular Clusters: Dissolution of a Salt in a Salt-(Solvent)(n)() Cluster.

A forgotten channel in the excited state dynamics of phenol–(ammonia)n clusters: hydrogen transfer