All novel compounds were characterized by NMR spectroscopy, HRMS, and FT-IR spectroscopy. The closed helical structures of ()-1a-C and ()-1b-C were confirmed by 1 H, C-HMBC NMR spectroscopy showing 3 JH-C couplings between the protons of the 1,14-dimethyl groups and bridging carbon atoms (Figures and). Unambiguous structural proof was obtained by singlecrystal X-ray structures for ()-1a-C and ()-1b-C (Figure ) which both exhibit a C-C distance of 1.57 Å between the bridging quarternary carbon atoms. The helical pitch angles along the inner sp 2 -hybridized C-C-C-C helices are 6.15°(()-1a-C)) and 14.09°(()-1b-C)), in conjunction with a nearly axial displacement of the methyl groups. The new compounds 1(a,b)-C clearly show a quinoidal structure at the terminal benzannulated rings with pronounced alternating bond lengths when compared with the parent aromatic [5]helicene scaffold (Figure ). UV/Vis Spectroscopy. Intrigued by the fact that we could only isolate the closed form molecules ()-1a-C and ()-1b-C as earlier observed for dimethylcethrene, we assumed a photochemical 6 retroelectrocyclization had led to a biradicaloid open form. We first investigated the optical absorption of these two compounds and found a hypsochromic shift of ()-1a-C to max = 342 nm and a bathochromic shift of ()-1b-C to max = 414 nm compared to the open [5]helicene precursor (±)-3 (max = 362 nm) (Figure ). Initial irradiation of ()-1a-C or ()-1b-C in MeCN with various wavelengths (LED light sources) at 295 K and above resulted in no observable change of the reaction mixture (see Figures ). Assuming a fast thermal backreaction under these conditions, we turned to irradiation at cryogenic temperatures. Cooling to 77 K and irradiation of ()-1a-C or ()-1b-C in a glassy matrix of 2methyltetrahydrofuran (2-MTHF) with a 375 nm or 405 nm LED, respectively, resulted in significant changes of the UV/Vis absorption (Figure ). In both cases the photostationary state is retained for at least one hour at 77 K in the dark with no observable spectral changes (Figures ). While the photochemical back reaction of ()-1a-O and ()-1b-C at various tested wavelengths remained unsuccessful (77 K), heating above a critical temperature resulted in the complete disappearance of the newly formed absorption bands and the spectra of the closed form was recovered (Figures ). Detailed variable temperature (VT-)UV/Vis absorption spectroscopy of the PSS405nm mixture with ()-1b revealed a rapid change of absorption bands at 92 K, regenerating the spectrum of the closed form ()-1b-C (Figures ). For the PSS375nm mixture of ()-1a the absorption remained constant until the 2-MTHF matrix started melting at 127 K (Figure ) and no defined critical temperature for the thermal ring closure of ()-1a-O to ()-1a-C could be determined due to dominating scattering effects of the melting matrix. At temperatures of 140 K, beyond the melting process of 2-MTHF, the thermal back reaction dominates again to form the closed form ()-1a-C and irradiation (405 nm) at that temperature generated no ()-1a-O. EPR Spectroscopy. Successful switching encouraged us to investigate the EPR activity of ()-1a and ()-1b before and after irradiation in 2-MTHF at 77 K. Before irradiation, the closed forms ()-1a-C and ()-1b-C are EPR silent under the experimental conditions, but after irradiation for 10 min with 375 nm and 405 nm (LED), respectively, an intense EPR signal can be observed at g ≈ 2.0 (Figure and Supporting Information Figure ). This process can be reversed upon heating the irradiated samples to the temperatures determined for the thermal back reaction from our previous VT-UV/Vis spectroscopy. Heating the PSS375nm mixture of diketone ()-1a-C/O to room temperature immediately converted it to the pure closed form ()-1a-C as confirmed by its EPR inactivity (77 K, second dark blue graph in Figure ). Repetition of such a cycle including irradiation at 77 K and subsequent heating was performed up to three times with no observable degradation in the HPLC traces or EPR signal loss (Figure and the Supporting Information Figure ). The PSS405nm mixture of bis(dicyano) ()-1b-C/O rapidly converted back to the pure closed form ()-1b-C upon heating to 93 K with the loss of the EPR signal, in agreement with our VT-UV/Vis study (Figure ). From our experimental findings we conclude that diketone ()-1a-C and bis(dicyanomethylidene) ()-1b-C can be photochemically switched into a stable EPR-active form and rapidly converted back to an EPR-silent form with a distinct thermal stimulus. In general, the singlet and triplet states in biradicaloids are thermally distributed with a distinct singlet-triplet energy gap (EST). Thus, we performed VT-EPR spectroscopy of the irradiated open forms of ()-1a and ()-1b and determined the spin multiplicity of their ground states. The EPR signal intensity of dioxo helicene ()-1a-O measured under the same experimental conditions, decreases with increasing temperature from 13 K to 77 K representative of a triplet ground state (Figure ). This ground state was further confirmed by our computational analysis (CASSCF active space (8,8), vide infra). Non-linear curve fitting using the Bleaney-Bowers equation gave an experimental ETS of 5.07 × 10 -2 kcal mol -1 (see the Supporting Information Figure ). Under equal experimental conditions, the EPR intensity change with increasing temperatures of irradiated bis(dicyanomethylidene) ()-1b-O is much smaller than that observed for the dioxo derivative ()-1a-O and fitting to the Bleaney-Bowers equation is not possible (see the Supporting Information Figure ). Theoretical Analysis. Bond breaking and making in this helicene switch proceeds through a formal 6 electrocyclization reaction in a conrotatory fashion that is photochemically allowed but thermally forbidden. Juríček and co-workers have encountered a structurally similar thermal electrocyclization mechanism that is formally forbidden according to Woodward-Hoffman rules suggesting a diradicaloid mechanism. An analysis of the aromaticity based on resonance structures of the closed and open form allows to estimate whether an open-or closed-shell electronic configuration is favored. In the closed form 1-C, no diradicaloid character is expected due to the presence of two aromatic Clar's sextets and exocyclic double bonds to the quinoidal 4,11substituents (CCS, Scheme 2). In contrast, the open form has a major diradicaloid resonance contribution because of the presence of three Clar's sextets and localization of unpaired electrons at the exocyclic substituents (OOS). The closed-shell resonance structure has an unfavorable all-quinoidal structure without localized aromatic Clar's sextets (OCS). Thus, it becomes obvious from this simple analysis that the open form-after irradiationhas a pronounced open-shell electronic configuration.