Initially, 1a which synthesis was recently established in micelles, [38] was subjected to an aqueous solution of simple sodium dodecyl sulfate (SDS) micelles using 456 nm irradiation, and to our delight converted smoothly into the desired photoproduct (Table , entry 1). The activation mechanism through direct excitation was investigated by examining the absorption characteristics of compound 1a in various solvents (Figure ). The maximum absorption for all tested solvents is around 310 315 nm and the slight red shift in MeOH, HFIP and in the micellar medium can be associated to hydrogen bonding interactions between 1a and the solvents. Noteworthy, no notable overlap of the absorbance band of 1a and the emission of the LED (λmax = 456 nm) is present. Since these data do not explain the observed photochemical reactivity, absorption spectra were recorded at reaction concentration (35 mM). These measurements expose the tail end of the absorption band at around 425 nm for all reaction media and indicates a small absorption overlap with the LED emission (Figure , Figure ). After optimization the combination of 1.0 equivalent of 1a with 6.0 eq. 2a in 4 wt% SDS in water under blue light irradiation (λmax = 456 nm) was found to successfully deliver the desired cyclobutane (3a) in 68% isolated yield (Table , entry 1) and in a isomeric ratio (i.r.) of 71:20:5:2:1:1, where the major isomer is the expected trans-trans isomer (see SI). We attribute this unusual distribution to the long excited state lifetimes observed (vide infra). It is noteworthy that no efforts were made to remove oxygen and the applied SDS concentration (0.14 M) is well above the critical micelle concentration (0.8 mM). Control experiments revealed that the reaction does not proceed efficiently in the absence of SDS (entry 2) and is shut down without light (entry 3), excluding a thermal background reaction and confirming the direct excitation pathway. Upon the removal of oxygen by freeze-pump-thaw, no significant difference in yield was observed (entry 4). It is proposed that molecular oxygen is solubilised in the micellar core, while the substrate is in proximity to the hydrophilic head-groups of the micelle. Consequently, molecular oxygen, which normally causes undesirable triplet quenching, is in a different compartment of the micellar environment, thereby reducing unproductive triplet quenching. The reaction does not proceed in organic solvents, which are typically used for similar transformations (entries 5-9) as previously reported for Ir-photocatalyzed [2+2] cycloadditions using 1a. Cetyltrimethylammonium chloride (CTAC, entry 11), a cationic surfactant and Triton X-100 (TX-100, entry 12), a non-ionic surfactant, were tested to assess the dependence on the surfactant ionic character. A slight decrease in yield could be observed. The investigated [2+2] photocycloaddition via direct excitation under aerobic conditions could be effortlessly scaled up to 1 mmol (entry 13) by extending the reaction time to 24 h, demonstrating the ease of application of micellar photocatalysis. The difference in reactivity between organic solvents and micellar media cannot be explained only by the absorption spectra. Therefore, we set out to investigate whether the micellar environment forms the excited state of aggregates responsible for the photochemical reactivity. Table . Control experiments and investigations of the influence of the surfactant.