A Theoretical Study of the Formation of Benzene Excimer: Effects of Geometry Relaxation and Spin-state Dependence

Abstract

Geometry relaxation effects on the formation of benzene excimer were investigated by means of ab initio calculation at SOS-CIS(D 0 )/aug-cc-pVDZ level. In the case of T-shaped dimer configuration, intermolecular interactions in the excited states are found to be nearly the same as those in the ground state and structural deformations are limited within a single molecule; the geometry relaxation effects are then negligible and singlet-triplet energy gap remains constant. As for face-to-face eclipsed dimer, on the other hand, both molecules undergo structural change. As a result, intermolecular interactions in the excited states are significantly different than those in the ground state. Although the intermolecular distances obtained from potential energy curve calculation with frozen molecular structures are in qualitative agreement, the excitedstate binding energies are notably overestimated with respect to those at optimized structures. In particular, the effects are calculated to be larger in T 1 state and hence singlet-triplet energy gap, which reduces markedly in this configuration, is underestimated without relaxation.

A Theoretical Study of Benzene Dimers in the Excited States: Wavefunction Delocalization, Charge‐Transfer Admixture, and Electronic Coupling

“…As mentioned above, the excitation-induced geometric relaxation occurs where the exciton for the state resides. 32 To confirm this finding, the NTO analyses were also carried out, and the hole-particle wavefunction pairs of the NTOs are depicted for the S 1 and T 1 states of the Hb, F2F-E, and F2F-S dimers in Figures 2 and 3, respectively (see also Figures S1 and S2, Supporting Information for the PD and T dimers, respectively). As can clearly be seen, the pairs of the hole-particle wavefunctions for the edge-on dimers are localized within single molecule that undergoes structural changes in the excited state, while those pairs are evenly held by both monomers in the F2F-E dimer configuration.…”

“…0.37 and 0.18 eV for the F2F-E and F2F-S dimers, respectively), giving rise to a significant reduction in the ΔE ST value. 32,37 It should be emphasized that although the cofacial benzene dimers exhibit stronger interactions in their excited states, the edge-on dimers are known to be more stable in the ground state. 45 In this study, the computed −ΔE values for the T and Hb ground-state dimers are ca.…”

“…It was previously unraveled that upon electronic excitation, the C-C bond length of a benzene molecule changes, while its C-H bonds remain unaltered. 32 In addition, it is the molecules associated with the excitation that relax their geometries. In this regard, the C-C bond lengths are a tentative indicator of the excitation sites.…”

“…Lim and coworkers studied the excimers of various acene molecules, [19][20][21][22][23][24][25][26][27][28] and studies employing more sophisticated theoretical methodologies followed to improve the general understanding. 15,[29][30][31][32][33][34][35][36][37][38] Even for these rather simple molecules, however, most theoretical studies have been devoted to the eclipsed-cofacial dimer configurations, focusing on their binding energies and geometric features in their singlet excited states; only a few studies have dealt with them in other configurations 26,29,34 and/or different spin states, i.e., triplet states. 15,19,26,28,30,32,34,37,39 The excitation energy transfer (EET) is also considered to be one of the essential processes in devices such as OPVs 40 and OLEDs.…”

“…15,[29][30][31][32][33][34][35][36][37][38] Even for these rather simple molecules, however, most theoretical studies have been devoted to the eclipsed-cofacial dimer configurations, focusing on their binding energies and geometric features in their singlet excited states; only a few studies have dealt with them in other configurations 26,29,34 and/or different spin states, i.e., triplet states. 15,19,26,28,30,32,34,37,39 The excitation energy transfer (EET) is also considered to be one of the essential processes in devices such as OPVs 40 and OLEDs. [41][42][43] In contrast to that of charge transport, however, much less attention has been paid to the electronic coupling for EET among acene molecules.…”