Effects of Intermolecular Interactions on the Singlet-Triplet Energy Difference: A Theoretical Study of the Formation of Excimers in Acene Molecules

The effects of intermolecular interactions in the excited state of acene Molecules on the Singlet-triplet energy difference (Delta E-ST) were investigated by carrying out ab initio calculations at the SOS-CIS(D-0)/aug-cc-pVDZ level. Benzene, naphthalene, and anthracene molecules were employed, and their Delta E-ST values were compared with those of their respective cofacial excimers. Our theoretical results demonstrate that, upon the formation of excimer, the Delta E-ST values decrease significantly. By carrying out an excitation energy decomposition, we found that Delta E-ST, albeit also modulated by the changes in orbital energy difference and Coulomb energy, is dominated by the difference in exchange energy between the singlet and triplet states, with the exchange energy decreasing as the intermolecular interactions become stronger. The natural transition orbital analysis suggests that the decrease in the exchange energy may be caused by the different nature of the hole and electron wave functions of the excimers (antibonding vs bonding), which gives rise to theft spatial separation. Furthermore, it was found that the geometry relaxation effects depend on the spin state, thus leading to a further reduction of Delta E-ST.