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Present calculations provide a theoretical understanding of the correlation of the conformations of tetramethyltetraphenylcyclotetrasiloxane with the properties of the ground states (S0-state) and excited states (S1-/T1state), as well as with the formation of the excimers. The difference between the energy-lowest conformation and the energy-highest conformation is only 0.51 kcal/mol. The isoenergetic conformers have similar electronic structures and absorption spectra. The difference between the vertical transition and the adiabatic transition in energy is relatively large for S1-states, but negligible for T1-states. The singlet-triplet gap for the adiabatic transition is very small (0.52 eV). The excited transition of the singlet electron may cause the geometrical deformation of S0-states, and the distorted phenyl ring is the excimer-forming site. The type of the excitation transition is associated with the conformation. The cooperation of the S0-states to the S1-states through the π-π or π-H interaction is a nonspontaneous-endothermic process. The formation and stability of the excimers are dependent upon the conformation.

The present study performed a DFT study on the conformation dependence of the properties of excited states and the excimer's formation for the tetramethyl tetraphenyl cyclotetrasiloxane. The conformations have little effect on the electronic structure of the ground state. The deformed phenyl group in the geometry of the excited state is the excimerforming site. The energetically matched HOMO of the excited state with the LUMO of the ground state should cause an attraction to form an excimer. Most conformations are likely to form the excimer but have different contributions to the excimer formation. The interaction of the ground state with the excited state in the excimer has been described quantitatively, which shows that the most stable intermolecular arrangement is the stacked-eclipsed mode. The excimers display a broad excitation spectrum. The local excitation type is dominant during the excitation, and the charge transfer contributes to the excimer's stability.