To elucidate the excited-state species involved in -PLLpyrs in more detail, we measured the fluorescence lifetimes in PBS upon hydrostatic pressurization. In Figure ,b, the fluorescence decay profile at 400 nm (around the monomer) or 500 nm (around the excimer) in PBS exhibited multiple components and could be fitted reasonably well with a sum of three exponential functions (fitting results shown in Figure ). Interestingly, in the monomer emissive region, we observed three distinct excited species with lifetimes of 0.2 ns (τ1), 9.1 ns (τ2), and 27 ns (τ3) at atmospheric pressure (0.1 MPa), as listed in Table . Comparing with pyr-ref, the longest τ3 species was attributed to the pyrene monomeric species. The shortest species of 0.2 ns was also observed in pyr-ref, and its origin was not previously concluded in the literature for similar chromophores. This short lifetime may be explained by a proton transfer involving the NH group in the amide moiety. Additionally, the middle species with a lifetime of τ2 can be attributed to the ground-state stacked conformer (GSC), which generally exhibits a shorter lifetime compared to the monomeric species, as reported in our previous study. This observation is further supported by the hydrostatic pressure-excitation spectra, which do not overlap with the UV/vis spectrum (refer to Figure in the SI). Based on these findings, we can identify the ground-state conformers and excited-state species on the polymer backbone, as shown in Figure . Increasing hydrostatic pressure results in shortened lifetimes and decreasing relative abundances (A factors) of the main monomer emissive species, leading to a decrease in fluorescence intensity in the monomer emission region. This behavior is likely attributed to the flexibility of the polymer backbone. A detailed discussion on this aspect will be provided later by comparing with ε-PLLpyrs. Similarly, three lifetimes of 4.2 (4), 24 (5), and 54 ns (6) were observed in the excimer region at 0.1 MPa; however, their origin was unclear. The most plausible explanation for τ4 is that it may correspond to a different type of ground-state stacked conformation (GSC). In this GSC2, the pyr stacking energy is more stable than in GSC1, resulting in emission at a longer wavelength. Similarly to pyr chromophores in a polymer matrix, the two longer lifetimes, τ5 and τ6, can be attributed to a second (partially overlapped) excimer and a sandwich (fully overlapped) excimer, respectively. Applying hydrostatic pressure leads to an increase in the A factor of the sandwich excimer. These observations in fluorescence lifetime measurements are the underlying cause of the hydrostatic pressureinduced ratiometric responses in the polymer chemosensor. Next, we investigated the hydrostatic pressure signaling of -PLLpyrs, in which the polymer backbone is believed to be more susceptible to a hydrostatic pressure stimulus owing to the flexible methylene chain in the repeating unit. Figure demonstrates that increasing the DS results in the stepwise development of excimer emission in DMSO, similar to the behavior observed in α-PLLpyrs. However, the excimer emission of ε-PLLpyrhigh in PBS shows a slightly distorted spectral shape (see Figure ). This distortion may be attributed to the presence of aggregated patterns in the excited state, which are plausible in hydrophilic and high-ionic-strength solutions such as PBS owing to the flexible chain. Notably, ε-PLLpyrmid exhibits remarkable ratiometric signaling upon hydrostatic pressurization in Figure , with the monomer emission intensity increasing and the excimer emission intensity decreasing with increasing pressure. Interestingly, this behavior is opposite to -PLLpyrhigh. To elucidate the excited-state dynamics of -PLLpyrmid, the hydrostatic pressure lifetime decay was measured. The fluorescence decay profiles measured at 400 (monomer region) and 500 (excimer region) nm were reasonably fitted to give a sum of three functions (see Figure and Table in SI). These six excited species correspond to τ1 and τ3 originating from the pyr chromophore, τ2 and τ4 as GSC1 and GSC2, τ5 as the second excimer, and τ6 as the sandwich excimer, respectively. The opposite ratiometry, with decreasing excimer emission and increasing monomer emission, can be attributed to the decreasing A factor of τ6. While the A factors in the monomer species also decrease, they are not the main determining factor for the increase in fluorescence intensity. Generally, the fluorescence intensity increases with increasing hydrostatic pressure owing to the increasing viscosity of the solution, suppressing collisional deactivation by a solvent attack. In the monomer region, this mechanism is preferable, eventually leading to an increase in intensity.