The elusive S2 state, the S1/S2 splitting, and the excimer states of the benzene dimer

Abstract

We observe the weak S 0 → S 2 transitions of the T-shaped benzene dimers (Bz) 2 and (Bz-d 6 ) 2 about 250 cm −1 and 220 cm −1 above their respective S 0 → S 1 electronic origins using two-color resonant two-photon ionization spectroscopy. Spin-component scaled (SCS) second-order approximate coupled-cluster (CC2) calculations predict that for the tipped T-shaped geometry, the S 0 → S 2 electronic oscillator strength f el (S 2 ) is ∼10 times smaller than f el (S 1 ) and the S 2 state lies ∼240 cm −1 above S 1 , in excellent agreement with experiment. The S 0 → S 1 (ππ * ) transition is mainly localized on the "stem" benzene, with a minor stem → cap charge-transfer contribution; the S 0 → S 2 transition is mainly localized on the "cap" benzene. The orbitals, electronic oscillator strengths f el (S 1 ) and f el (S 2 ), and transition frequencies depend strongly on the tipping angle ω between the two Bz moieties. The SCS-CC2 calculated S 1 and S 2 excitation energies at different T-shaped, stacked-parallel and parallel-displaced stationary points of the (Bz) 2 ground-state surface allow to construct approximate S 1 and S 2 potential energy surfaces and reveal their relation to the "excimer" states at the stacked-parallel geometry. The f el (S 1 ) and f el (S 2 ) transition dipole moments at the C 2v -symmetric T-shape, parallel-displaced and stacked-parallel geometries are either zero or ∼10 times smaller than at the tipped T-shaped geometry. This unusual property of the S 0 → S 1 and S 0 → S 2 transition-dipole moment surfaces of (Bz) 2 restricts its observation by electronic spectroscopy to the tipped and tilted T-shaped geometries; the other ground-state geometries are impossible or extremely difficult to observe. The S 0 → S 1 /S 2 spectra of (Bz) 2 are compared to those of imidazole · (Bz) 2 , which has a rigid triangular structure with a tilted (Bz) 2 subunit. The S 0 → S 1 / S 2 transitions of imidazole-(benzene) 2 lie at similar energies as those of (Bz) 2 , confirming our assignment of the (Bz) 2 S 0 → S 2 transition. C 2015 AIP Publishing LLC. [http://dx

Multireference Approaches for Excited States of Molecules

“…973 Lately, the TDDFT method has been used to investigate the benzene, [974][975][976] and larger polycyclic aromatic dimers. 974,[977][978][979][980][981][982][983] Alternatively to the TDDFT method, the properties of the benzene dimer have been calculated using the linear response coupled cluster 46,984,985 and EOM-CCSD methods. 986 Multireference methods have been used not only to account for the character of the wavefunction but also to benchmark the methods utilizing the single-reference approach.…”

Dynamics of benzene excimer formation from the parallel-displaced dimer

“…14 More recently, it has been argued that resonance fluorescence experiments are biased towards detecting T-shaped structures due to differences in oscillator strengths between those structures and parallel-stacked structures. 17 Excimer formation in liquid benzene has been shown to happen quickly due to an abundance of close-to-parallel benzene pairs. 23 The goal of this work is to characterize the full-dimensional nonadiabatic dynamics 45 of ultrafast excimer formation in a prototypical system for aromatic interactions, the benzene dimer in the gas phase.…”

Molecular Actuators in Action: Electron-Transfer-Induced Conformation Transformation in Cofacially Arrayed Polyfluorenes

“…Yet upon ionization, configurations that consist of displaced fully π-stacked (F) n clusters are observed (Figure ), suggesting the increasing role of the cofacial arrangement in the cation radical state as compared to the neutral state . Indeed, it is well-known for the simplest model system, the benzene dimer, that in the neutral state a non-π-stacked conformation is preferred (i.e., T-shaped), while at the cation radical (and also at the excited state) a sandwich-like π-stacked arrangement is the global minimum. ,− In the same vein, many conformationally mobile bichromophoric actuators at neutral state exist in the (noncofacial) open conformation, yet upon ionization reorganize into a (cofacial) closed conformation. − …”