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.

This review describes how conical intersections affect measured molecular spectra and simple photofragmentation processes. We consider excitations that result in electron ejection, that is, photoionization or photodetachment, as well as photoinduced H-atom elimination. Section 1 presents a brief overview of the history of conical intersections and their rise from an arcane theoretical concept to a major paradigm in nonadiabatic chemistry. In Section 2, the generic properties of conical intersections are discussed, as well as their characterization with modern electronic-structure methods. Section 3 briefly discusses computational tools used to compute the nuclear motion involving conical intersections. Section 4 describes how the ideas of Sections 2 and 3 are combined to simulate molecular spectra impacted by conical intersections. Section 5 describes selected recent experimental and computational studies of photoelectron, photodetachment, and photofragment spectra. Rather than providing an encyclopedic bibliography of the previous and current literature, we illustrate significant problems currently being addressed and describe what can be accomplished with current computational techniques and how these results are achieved. Section 6 suggests future directions in this field.

The UV spectra of three different conformers of the guanine/cytosine base pair were recorded recently with UV-IR double-resonance techniques in a supersonic jet [Abo-Riziq, A., Grace, L., Nir, E., Kabelac, M., Hobza, P. & de Vries, M. S. (2005) Proc . Natl . Acad . Sci . USA 102, 20–23]. The spectra provide evidence for a very efficient excited-state deactivation mechanism that is specific for the Watson–Crick structure and may be essential for the photostability of DNA. Here we report results of ab initio electronic-structure calculations for the excited electronic states of the three lowest-energy conformers of the guanine/cytosine base pair. The calculations reveal that electron-driven interbase proton-transfer processes play an important role in the photochemistry of these systems. The exceptionally short lifetime of the UV-absorbing states of the Watson–Crick conformer is tentatively explained by the existence of a barrierless reaction path that connects the spectroscopic ¹ π π * excited state with the electronic ground state via two electronic curve crossings. For the non-Watson–Crick structures, the photochemically reactive state is located at higher energies, resulting in a barrier for proton transfer and, thus, a longer lifetime of the UV-absorbing ¹ π π * state. The computational results support the conjecture that the photochemistry of hydrogen bonds plays a decisive role for the photostability of the molecular encoding of the genetic information in isolated DNA base pairs.

Chemists have long sought to understand proton-transfer processes in aqueous solution at the molecular level. Studies of molecular clusters can provide important insights into these solution processes. In their Perspective, Domcke and Sobolewski highlight the report by Tanner et al., who have studied the photo-induced hydrogen transfer through a hydrogen-bonded molecular wire in a size-selected chromophore-solvent cluster. The reaction can be controlled by the excitation of ammonia-wire vibrations of the reactant and is detected via the fluorescence of the product. The results support a new paradigm of excited-state proton-transfer reactivity. In contrast to ground-state proton transfer, where an ion-pair is formed, the transfer of a hydrogen atom is the primary process in excited-state acid-base reactions.