The Leitner and Furstner groups reported (Furstner, A.; Koch, D., Langemann, K.; Leitner, W.; Six, C. Angew, Chem., Int. Ed. Engl 1997, 36, 2466) on the ring closing metathesis (RCM) of a 16-membered diene dissolved in supercritical CO2 (scCO(2)). The authors found that the cyclic product, indicative of an intramolecular RCM event, was formed in excellent yield when the CO2 density was high, but oligomers were formed by an acyclic diene metathesis (ADMET) pathway at lower CO2 densities. These results suggest that changes in the CO2 density lead to changes in the intra-vs intermolecular interactions between the 16-membered diene dissolved in ScCO2. To assess this issue in more detail, we have prepared 6-(1-pyrenyl)hexyl-11-(1-pyrenyl)-undecanoate [1-Py(CH2)(10)COO(CH2)(6)1-Py] in which we replaced the terminal alkenes of Letiner and Furstner's original diene with the fluorophore pyrene. We have studied the pyrene excimer formation of 1-Py(CH2)(10)COO(CH2)(6)1-Py when it is dissolved in five organic solvents (cyclohexane, dichloromethane, ethanol, acetonitrile, and dimethyl sulfoxide) and supercritical carbon dioxide (scCO(2)) to determine how the tail segments interact with each other. The result show that the excimer formation mechanism is completely different when 1-Py(CH2)(10)COO(CH2)(6)1-Py is dissolved in scCO(2) or organic liquids. In liquids, excimer formation is purely dynamic in nature, there are two formation pathways to the excimer, and all the rates can be understood with the help of Kamlet-Taft linear solvent energy relationships. In scCO(2), We found that the 1-Py(CH2)(10)COO(CH2)(61)-Py excimer-to-monomer intensity ratio (E/M) correlates directly with (1) the observed RCM yield for Leitner and Furstner's original 16-membered diene and (2) the solvent refractive index function. The steady-state and time-resolved fluorescence of 1-Py(CH2)(10)COO(CH2)(6)1-Py dissolved in scCO(2) show that there are two excimers that form in scCO(2) and their relative contributions change in a systematic way with changes in the CO2 pressure/density. Interestingly, the typical dynamically formed excimer species that emits at 470-480 nm (E1) forms within 2 ns of optical excitation; however, it is not the dominant species at low CO2 densities. E1 is equivalent to the species that goes on to form the RCM product in Leitner and Farstner's original reaction. The second excimer (E2) emits in the 410-440 nm region. E2 is associated with intermolecular preassociated forms of the pyrene residues within a collection of 1-Py(CH2)(10)COO(CH2)(6)1-Py molecules, and this species dominates at low CO2 densities. E2 is equivalent to the species that goes on to form the oligomeric product in the original Leitner and Furstner reaction. As the CO2 density increases, the El excimer contribution increases relative to the E2 excimer contribution. The combination of the fluorescence and reaction outcome results are used to explain Leitner and Furstner's previous density-dependent RCM yields.