A novel spectroscopic window on conical intersections in biomolecules

Abstract

In PNAS, Keefer et al. propose an experimental approach which allows visualizing in detail the dynamics of a molecule as it passes a conical intersection (CI), that is, a real crossing point between two electronic states (1). In order to describe the photoinduced dynamics of molecules one needs to solve the time-dependent Schrodinger equation for a complex wavefunction which depends on many electronic and nuclear degrees of freedom. To simplify the problem, one typically adopts the Born–Oppenheimer approximation, which consists of separating the electronic and the nuclear coordinates in the wavefunction. This approximation, which is justified by the much higher speed of the electrons with respect to the nuclei, consists of solving the Schrodinger equation for the electrons at given fixed nuclear positions and then obtaining the electronic energies as a function of those fixed nuclear coordinates, the so-called potential energy surfaces (PES), in what is often referred to as the adiabatic approximation. In many molecules, however, there are regions of the potential energy landscape, such as CIs, where the electronic and nuclear degrees of freedom become strongly mixed and the Born–Oppenheimer (adiabatic) approximation breaks down. CIs are ubiquitous features in the photophysics and photochemistry of molecules and can be considered as “doorways” through which the photoexcited wavepacket (WP) is efficiently funneled to a lower-energy electronic state (2, 3), thus accounting for efficient nonradiative relaxation (also called internal conversion). CIs are topologies of the PES for which two or more electronic states become isoenergetic, forming a multidimensional “seam” (Fig. 1). Depending on the topography around the seam, CIs can be classified as “sloped” or “peaked.” In a peaked CI (Fig. 1, Left ) the WP is guided to the intersection seam regardless of the initial approach direction, resulting in a more efficient conversion and triggering photochemistry. In a sloped … [↵][1]1To whom correspondence may be addressed. Email: giulio.cerullo{at}polimi.it. [1]: #xref-corresp-1-1