Aromatic Excimers: Ab Initio and TD-DFT Study

Abstract

Excited dimers (excimers) formed by aromatic molecules are important in biological systems as well as in chemical sensing. The structure of many biological systems is governed by excimer formation. Since theoretical studies of such systems provide important information about mutual arrangement of aromatic molecules in structural biology, we carried out extensive calculations on the benzene excimer using EOM-CCSD, RI-CC2, CASPT2, and TD-DFT approaches. For the benzene excimer, we evaluate the reliability of the TD-DFT method based on the B3LYP, PBE, PBE0, and ωPBEh functionals. We extended the calculations to naphthalene, anthracene, and pyrene excimers. We find that nearly parallel stacked forms are the minimum energy structure. On the basis of the benzene to pyrene excimers, we might roughly estimate the equilibrium layer-to-layer distance for bilayer-long arenes in the first singlet excited state, which is predicted to be bound.

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

“…The dissociation of the (Bz) * 2 upon excitation at the 6 1 0 1 1 0 band 18 agrees well with our calculations. Combining these results with previous calculations of the S 1 and S 2 excited states at the M1 and S8 geometries, [28][29][30][31][32] we show that the S 0 → S 1 and S 0 → S 2 transitions are optically forbidden for many ground-state structures that are populated in the jet-cooled (Bz) 2 ground state, specifically for VRT levels near S3 and M1. The only other structure with a reasonable transition-dipole moment is the C ′ s symmetric T-shaped dimer S3a, which has a moderately intense S 0 → S 2 and a weak S 0 → S 1 transition.…”

“…This strong bias of the S 1 and S 2 transitiondipole moment surfaces of (Bz) 2 against geometries that are not T-shaped, such as M1 and S8, is the main reason why all vibronic spectroscopic experiments on jet-cooled (Bz) 2 to date have indicated a T-shaped structure, 8,9,[12][13][14][15]17,18 although theory predicts that the parallel-displaced M1 structure is a local minimum. [27][28][29][30][31][32] The electronic spectrum of jet-cooled (Bz) 2 can only be observed through an experimental "keyhole" that strongly favors observation of the T-shaped geometry and does not allow to observe the other low-energy minima on the S 0 state surface. An analogous bias exists against the microwave spectroscopic observation 2,5-7 of VRT ground-state levels near the M1 parallel displaced structure, because the M1 structure has no permanent dipole moment.…”

“…For this reason, Bz 2 and especially the relative energies of the T-shaped, tilted T-shaped, stacked-parallel, and parallel displaced geometries have been used as a benchmark system for high-level electronic structure calculations. [27][28][29][30][31][32][33] Szalewicz and co-workers, who calculated the six-dimensional intermolecular PES of Bz 2 using the symmetry-adapted perturbation theory (density functional theory) (SAPT(DFT)) method, located T-shaped and stacked minima as well as many index-1 saddle points. 3 They later complemented this work by performing CCSD(T): coupled-cluster with singles, doubles and perturbative triples excitations calculations at three minima and nine index-1 stationary points.…”

“…[27][28][29][30][31] Note that the interplanar distance of S8-at which the vertical electronic excitations are calculated-is R ∼ 3.7 Å, longer than the optimum distance R e = 3.05 Å in the S 1 (B 1g ) state, thus the true B 1g minimum is much deeper than shown in Figure 10. [27][28][29][30][31][32] Several theory groups have studied the 1 B 1g and 1 B 2u excimer states of the S8 and M1 benzene dimer, usually as a function of the interplanar distance R while maintaining the D 6h and D 2h symmetries, respectively. [27][28][29][30][31][32] The calculated B 1g excimer well depths range between D e = 0.46 eV (3570 cm −1 ) 27 and D e = 0.70 eV (5650 cm −1 ) 29 at the TD-DFT level and D e = 0.43 eV (3470 cm −1 ) at the CASPT2 level.…”

“…[27][28][29][30][31][32] Several theory groups have studied the 1 B 1g and 1 B 2u excimer states of the S8 and M1 benzene dimer, usually as a function of the interplanar distance R while maintaining the D 6h and D 2h symmetries, respectively. [27][28][29][30][31][32] The calculated B 1g excimer well depths range between D e = 0.46 eV (3570 cm −1 ) 27 and D e = 0.70 eV (5650 cm −1 ) 29 at the TD-DFT level and D e = 0.43 eV (3470 cm −1 ) at the CASPT2 level. 28 The S 0 → S 1 and S 0 → S 2 transitions of D 6h -symmetric S8 are both electric-dipole forbidden.…”