On the Vibronic Interactions in Aromatic Hydrocarbon Radicals and Radical Cations

Abstract

The study of the fate of electronically excited radical and radical cation of aromatic hydrocarbons is an emerging topic in modern chemical dynamics. Observations like low quantum yield of fluorescence and photostability are of immediate concern to unravel the mechanism of ultrafast nonradiative internal conversion dynamics in such systems. The radical cations of polycyclic aromatic hydrocarbons (PAHs) have received considerable attention in this context and invited critical measurements of their optical spectroscopy in a laboratory, in striving to understand the enigmatic diffuse interstellar bands (DIBs).

The Born–Oppenheimer (BO) approximation breaks down owing to the feasibility of crossings of electronic states of polyatomic molecules. These crossings lead to conical intersections of electronic potential energy surfaces (PESs), which are proved to be the bottleneck in the photophysical/chemical processes in those systems. Understandably, a concurrent treatment of electronic and nuclear motions is required to explore the excited state dynamics of polyatomic systems. Motivated by the new experimental measurements, we recently carried out ab initio quantum dynamical studies on phenyl radical (Ph^●) and phenylacetylene radical cation (PA^●+) and established nonadiabatic interactions in their low-lying electronic states. These are the derivatives of the Jahn–Teller active benzene molecule, and are precursors of formation of PAHs. Employing a general vibronic coupling scheme, the ultrafast decay of their electronic states through successive conical intersections was studied by us recently.More specifically, the electronic ground $\widetilde{X}^2 A_1$ state of Ph^● is energetically well separated from its excited $\widetilde{A}^2 B_1$ and $\widetilde{B}^1 A_2$ states, and the nuclear dynamics in this state followthe adiabatic BO mechanism. In contrast, the $\widetilde{A}^2 B_1$ and $\widetilde{B}^2 A_2$ states are very close in energy (~0:57 eV spaced vertically at the equilibrium configuration of the reference phenide anion) and low-lying conical intersections are discovered which drive the nuclear dynamics via nonadiabatic paths. An ultrafast nonradiative decay rate of ~30 fs of the $\widetilde{B}$ state is estimated. In PA^●+ both the long-lived and short-lived electronic states are discovered. The resolved structures of the vibronic bands are compared with the experimental photoelectron, mass analyzed threshold ionization and photoinduced Rydberg ionization spectroscopy data. The diffused structure of vibronic band for the $\widetilde{A}$ state of the radical cation is attributed to an ultrafast decay (~20 fs) to the electronic ground state. Benchmark ab initio quantum dynamical studies are carried out for the prototypical naphthalene and anthracene radical cations of the PAH family aiming to understand the vibronic interactions and ultrafast decay of their low-lying electronic states. The broadening of vibronic bands and ultrafast internal conversion through conical intersections in the D ₀ − D ₁ − D ₂ electronic states of these species is examined in conjunction with the experimental results. The results demonstrate the crucial role of electronic nonadiabatic interactions to understand their low quantum yield of fluorescence and photostability and adds to the understanding of DIBs.