To our surprise, 2, generated from 6 by 365 nm irradiation, did not display QMT reactivity even after several days in the dark; this strongly indicates that 2 is persistent under cryogenic conditions, making it the first persistent singlet alkyl carbene reported to date. Only after irradiating 2 with 627 nm light, we observed the disappearance of the bands of 2 and the simultaneous appearance of bands corresponding to homohypostrophene (9). Even though we are pleased to have spectroscopically characterized only the second alkyl carbene, its unexpected cryogenic stability and QMT inactivity also excludes it from experiments related to ICS. In moving forward, we had to first understand the QMT behavior of 1 and 2. Figure . Experimental UV/Vis spectra. A): The spectra of 5 (black) and 1 (red) are in good agreement with literature data. Carbene 1 vanishes when keeping the matrix in the dark for 18 h (blue). B): Analogous spectra were recorded before (black) and after (red) irradiation of 6. However, 2 is persistent and its fine-structured UV/Vis absorption (red) only vanishes (blue) when the matrix is irradiated at 627 nm.