40 | December 2021 www.facs.website The f luorescence spectrum (250 nm excitation) of a solution of trans-32 (1.5 × 10–5 M) in 3-methylpentane (3-MP) was measured at liquid nitrogen temperature (77 K). Interestingly, fluorescence corresponding to a phenyl-group-derived vibrational structure was observed around 280–360 nm (lifetime 8.7 ns, Figure 12a) and broad fluorescence with no vibrational structure was noticed at 460–660 nm (lifetime 7.9 ns, Figure 12b). The excitation spectra of these two types of emission (Figures 12a and 12b) were consistent with the absorption fluorescence emissions emission (a) t = 8.7 ns trans-32 emission (b) t = 7.9 ns MeO OMe 2-Naph 2-Naph Me Me 2-Naph 2-Naph trans-39 trans-40 Me Me Ph Ph trans-38 0.15 0.10 0.05 0.00 650 600 550 500 450 400 350 117 K 94 K PhH2CO OCH2Ph Ar Ar N N Ar Ar PhH2CO OCH2Ph 35 h –N2 max ~450 nm Ar = OMe MeO + Ar Ar OCH2Ph OCH2Ph max ~580 nm wavelength /nm absorbance 36 37 low-temperature matrix MTHF Ph Ph MeO OMe MeO OMe Ph Ph + MeO OMe Ph Ph –N2 sensitized h (triplet) direct h (singlet) 31 trans-32 cis-32 6 33 N N Ph Ph MeO OMe N N Ph Ph MeO OMe –N2 ISC 134 334 N N Ph Ph MeO OMe Ph OMe OMe 36 puckered diradical planar diradical Ph OMe OMe Scheme 7. Mechanism for the stereoselectivity in the denitrogenation of 31. Figure 11. Detection of puckered diradical 36 and planar diradical 37 in the photolysis of 35. Figure 12 Fluorescence spectra from trans-32 in 3-MP at 250 nm at 77 K. Short-wavelength emission (a) at around 320 nm and long-wavelength emission (b) at around 520 nm. MeO OMe Ph Ph ~250 nm ~320 nm ~450 nm ~515 nm ~580 nm trans-32* ~650 nm > 10 kcal mol–1 2 kcal mol–1 MeO OMe Ph Ph MeO OMe Ph Ph Ph OMe OMe 6* 33* MeO OMe Ph Ph Ph OMe OMe 6 33 total electronic energy at SA-CASSCF(10,10)/6-31G(d) trans-32 Figure 13. Ground state and excited state potential energy surfaces in trans-32, calculated at the SA-CASSCF(10,10)/631G(d) level of theory. spectrum of trans-32. These observations indicate that not only the fluorescence at shorter wavelengths but also the fluorescence with emission maxima around 515 nm originates from the electronically excited state of trans-32. Considering the results of quantum chemical calculations shown below, we conclude that the long-wavelength emission originates from the electronically excited state 33* of the Packard-type diradical, which is generated from the electronically excited state of trans-32 via an adiabatic σ-bond cleavage process. Similar long-wavelength fluorescence was not observed for trans-38, an analog of 32; only fluorescence from the phenyl group (260–355 nm, 9.2 ns) was observed. The substituent ef fect on the emission spectra observed in trans-32 and 38 was also observed in trans-39 and 40, wherein the phenyl group was replaced by a naphthyl group. To understand these emissions, the reaction potentials of the electronic excited states of trans-32 and 38 were calculated using the state-averaged complete active space sel f-consistent f ield
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