Modern information technology (IT) relies on the ability to store and process large amounts of data within a relatively short amount of time. This process is based on a controlled change of a particular physical property, such as the electrical, magnetic, or optical response of an information bit. That degree of control requires materials with at least two distinct states that can be altered by an external stimulus. In an ongoing demand for miniaturization of IT devices the search for the smallest possible information storing and processing unit has brought the attention towards single molecules. In this context, molecular organic photoswitches are an intensively studied field in which a particular property, such as basicity, catalytic or biological activity, mechanical and electric conducting properties, can be switched with high spatiotemporal resolution using light as an external stimulus. Of particular interest for potential applications in IT devices as qubits and contrast agents is the external control over the spin state of isolated molecules that leads to magnetic bistability. Controlling the spin state of discrete molecules has been demonstrated for metalorganic spin-crossover complexes or photochromic complexes containing first-row transition metals such as nickel or cobalt. Successful switching of spin states in all-organic molecules is rare. It can be realized based on several molecular mechanisms, but only two concepts that rely either on 1) a photoconformational trap (Figure , top) or on 2) a photochemical reaction (Figure , bottom) have the potential of achieving true magnetic bistability. An early example of a photo-induced paramagnetic diradicaloid form with simultaneous conformational change that gave rise to a paramagnetic state was reported by Toda and Tanaka, but no magnetic bistability was achieved (Figure ). Within a few hours, the thermal back-reaction converted the electron paramagnetic resonance (EPR)active form reversibly into the diamagnetic form. In contrast, magnetic bistability could be demonstrated based on a photoconformational change of an overcrowded aromatic ene by Kubo and co-workers (Figure ). The photo-generated paramagnetic form could be trapped in a least-strained orthogonal conformation with a thermal back-reaction occurring gradually at room temperature to revert the equilibrium into the diamagnetic form. In a subsequent optimization of Kubo's system, Feringa and coworkers introduced additional steric bulk in the fjordregion and thiaxanthylidene units that stabilize the diradical state, which eventually lead to absolute magnetic bistability at room temperature. However, the stabilization of spin states states that is dominantly based on conformational changes could be a conceptual limitation for the future demand of tuned bistability of molecular magnetic switches. Hence, switching mechanisms based on photochemical reactions that involve the making and breaking of chemical bonds is a motivating alternative concept to achieve amplified bistability among the magnetic states. In that context, the group of Oda has demonstrated the photochemical formal 6π retroelectrocyclization of a bis(quinone-extended) thioindigo that partially increases the population of a diradicaloid form with a paramagnetic spin state (Figure ). Yet, only a minor ratio of paramagnetic-to-diamagnetic states was achieved due to the permanent thermal population of the triplet state in the dark. Juríček and co-workers introduced a novel photoswitchable dimethylcethrene with the promise of generating a diradicaloid triplet state upon irradiation of the closed diamagnetic form (Figure ). In their approach, a photochemical 6π retroelectrocyclization gave rise to the open helical form containing a paraquinodimethane (p-QDM) motif exhibiting an open-shell singlet biradicaloid structure. However, no population of the triplet state was found at room temperature because of a too high singlet-triplet energy gap (ΔEST, calculated to ca. 10 kcal mol for the open form). Hence, neither form showed a paramagnetic resonance signal using EPR spectroscopy. Inspired by this approach of Juríček and co-workers, we herein present the first all-organic photochemical magnetic switch that shows bistability between two spin states upon breaking and making of chemical bonds (Figure ). We focused on a molecular design that stabilizes a diradicaloid form in the open [5] Synthesis. Dibromo [5]helicene ()-3 was synthesized in three reaction steps from commercially available starting materials following a previous report by Juríček and coworkers (see the Supporting Information Section S2). Palladium-catalyzed Miyaura borylation of dibromide ()-3 gave bis(boronic acid pinacol ester) ()-2 in good yields (74%, Scheme 1). Subsequent oxidative deborylation of ()-2 with an excess of hydrogen peroxide leads directly to diketone ()-1a-C. Only the closed diketone is observed in the synthesis that is presumably obtained through a diradical mechanism upon oxidation of a transient open diol. Closed bis(dicyanomethylidene) ()-1b-C was synthesized through a Pd-catalyzed Takahashi-coupling from dibromide ()-3 via dihydro intermediate ()-1b-O-H2, that underwent subsequent oxidation upon deprotonation and formal electrocyclization to give ()-1b-C. Furthermore, we were able to separate the (M)and (P)-enantiomers of boronic ester ()-2 via chiral high performance liquid chromatography (HPLC) and the absolute configuration was validated using a combination of time-dependent density functional theory (TD-DFT) calculations, optical rotatory dispersion (ORD), and electronic circular dichroism (ECD) spectroscopy (see the Supporting Information Section S3, S4). Unambiguous proof of the absolute configuration was obtained by a singlecrystal X-ray structure of enantiopure (+)-(P)-2 (Figure ). The pure enantiomers of bis(boronate) (P/M)-2 could be oxidized separately to give enantiopure stereoisomers (-)-(R,R)-1b-C and (+)-(S,S)-1b-C. That strategy enables rapid access to enantiopure dimethyl [5]helicene derivatives, which can be applied as switches with chiroptical properties in the future. Scheme 1. Synthesis of ()-1a-C and ()-1b-C a a Reagents and conditions: i) ()-3 (1.0 eq.), bis(pinacolato)diboron (2.2 eq.), [Pd(dppf)Cl2]⋅CH2Cl2 (10 mol%), KOAc (5.0 eq.), 1,4-dioxane, 80 °C, 3.5 h, 74%; ii) enantiomer separation by HPLC on a chiral stationary phase; iii) for (±)-1a-C: ()-2 (1.0 eq.), H2O2 (25.0 eq.), THF, 25 °C, 6 h, 78%; (+)-(S,S)-1b-C (70%) from (+)-(P)-2 and (-)-(R,R)-1b-C (63%) from (-)-(M)-2; iv) (1) malononitrile (6.0 eq.), NaH (10.0 eq.), THF, 0 °C, 0.5 h; (2) ()-3 (1.0 eq), [Pd(PPh3)2Cl2] (10 mol%), [Pd(PPh3)4] (10 mol%), THF, 80 °C, 16 h, not isolated; v) CHCl3, NEt3, air, 25 °C, 30%; dppf = 1,1'bis(diphenylphosphino)ferrocene, pin = pinacolato.