In DPPC liposomes, the high-energy absorption band has the same shift as in the organic solvent. By contrast, the lower-energy band in the visible region shifts slightly bathochromically to 430 nm (Figure ), indicating a stabilization of the respective LUMO (lowest unoccupied molecular orbital) or destabilization of the respective HOMO (highest occupied molecular orbital), in the non-polar membrane environment, in line with similar dyes in unpolar solvents. The weak tail in the red region of the liposome absorption spectrum is due to light scattering and the Tyndall effect. In the emission spectra, a small hypsochromic shift towards higher energy of the emission band by 5 nm to 535 nm was observed, suggesting that the lowest excited state of [1] 2+ resides within the hydrophobic membrane core. The emission lifetime within the membrane remained similar with τ = 5.7 ± 0.6 ns; which is a low environmental influence and was also observed for other singlet emitters in solvents vs. membrane. Upon incorporation into the DPPC liposomes, which are less polar compared to acetonitrile, the absorption band at lower energies at around 410 nm undergoes a bathochromic shift to 430 nm, while the higher energy band remains at 330 nm as in acetonitrile. To rationalize why one of the bands shifts to lower energies while the other is barely affected, we resort to time-dependent density functional theory calculations. We used the CAM-B3LYP functional and def2-SVP set, as implemented in Gaussian 16, whereby the amount of HF exchange is reduced to reproduce the experimental absorption energies. Further details are presented in the SI Section S4. The calculations evidence that the two absorption bands should be assigned to the S1 and S3 singlet electronic excitations, respectively, while the transition to the S2 is dark. The hole and electron of the excitation to the S1 closely resemble the shape of the HOMO and LUMO (Figure ). During the excitation to the S1 (Figure ), electron density is transferred from the five six-membered rings closest to the center of the molecule to the most central moiety. By contrast, in the S3 (Figure ), both the hole and the excited state electron are similarly delocalized over large parts of the molecule, making the S3 comparable in electronic properties to the ground state. Further discussion of the electronic structures in the ground and excited states can be found in the SI Section S5.