To gain further insight into the photophysical properties and reversibility of the switching process, five different 1H-2-benzo [c]oxocins with different phenyl-ring substituents were subjected to irradiation (Figure and Table , 1a/1b with R = H; 3a/3b with R = OMe; 7a/7b with R = Br; 9a/9b with R = Cl; 12a/12b with R = CF3). Importantly, photochemical isomerization of these compounds can be triggered with visible light, conducive to the development of molecular photoswitches with high structural integrity. The UV/Vis spectra of this series of 1H-2-benzo [c]oxocins were recorded in DCM at room temperature (Figure ). revealing absorption maxima (λmax) in the UV-A region at 350-360 nm with tailing into visible wavelengths. TD-DFT calculations suggest this transition (λ = 394 nm) is essentially a singlet-to-singlet HOMO→LUMO →* transition. Specifically, the transition is from the carboncarbon -bonding donor orbital (HOMO) of the enol ether (-OC(Ph)=CH-) to the carbon-carbon *-antibonding acceptor orbital (LUMO) of the acrylate (-C(COOR)=CH-), with both the HOMO and LUMO slightly delocalized into the adjacent aryl-groups see SI for details). While it is clear that this excited state leads to C-C bond formation via a 4-cyclisation upon irradiation with UV-A light, these photocyclizations can be also initiated with white light, fully consistent with the light yellow color of the substrates originating from the observed tailing of this absorption band into the visible region. The absorption onset (λonset) of the 1H-2-benzo [c]oxocins in Figure and Table , ranges from 442-427 nm (i.e., visible violet light), making the photoinduced isomerization accessible with blue LED light. Altering aryl-substituents does not perturb absorption maximum (λmax) or absorption onset (λonset), instead exerting a pronounced influence on molar absorptivity (ε at λmax), with the unsubstituted 8membered ring 1a exhibiting ε = 8357 L mol -1 cm -1 (Table ). Changing the substituents to halogen or electronwithdrawing groups slightly decrease the ε, while 3a (featuring the electron-donating -OMe substituent) has the lowest ε among these 8-membered rings. The quantum yield of the ring-closure process (RC) exhibits a dependence on the nature of the substituent, with the 1H-2-benzo [c]oxocin 12a (featuring an electron-withdrawing -CF3 substituent) exhibiting RC = 14%, strongly contrasting 3a RC = 44%. As such, a higher RC compensates for the lower ε of these compounds (and vice versa, Table ), revealing why conversions within the same time scale are similar for all these compounds. Additionally, the switching behaviour between 1H-2-benzo [c]oxocins and dihydro-4H-cyclobuta [c]isochromenes can be followed by UV/Vis or NMR spectroscopy (Figure and). Due to their high stabilities, the isomer ratio can be controlled simply by on-off irradiation (Figure ). Overall, these photophysical measurements demonstrate that the ringclosure isomerization reactions of the 1H-2benzo [c]oxocins with aromatic substituents at the enol ether moiety is very efficient, producing the corresponding dihydro-4H-cyclobuta [c]isochromenes in good yields, with a high RC and without any observable side-product formation.