Computational study of the cooperative effects of nitrogen and silicon atoms on the singlet-triplet energy spacing in 1,3-diradicals and the reactivity of their singlet states

Photoisomerization of a phosphorus-based biradicaloid: ultrafast dynamics through a conical intersection

As previously reported, photoisomerization of the open-shell singlet biradicaloid [TerNP]₂CNDmp (2) yields its closed-shell housane-type isomer (3). In the present study, pump-probe spectroscopy was applied to investigate the excited-state dynamics of the photoisomerization, indicating ultrafast de-excitation of the S₁ state through a conical intersection, in agreement with computational predictions. The structural and electronic changes during the isomerization process are discussed to gain an understanding of the reaction pathway and the transformation of the biradicaloid to a closed-shell species.

Unexpected σ bond rupture during the reaction of N-methyl-1,2,4-triazoline-3,5-dione with acenaphthylene and indene

The reaction of N-methyl-1,2,4-triazoline-3,5-dione (MeTAD) with acenaphthylene and indene leads not only to the formation of the expected [2 + 2] diazetidine cycloadducts but also to unexpected 2:1 adducts of MeTAD with substrate. The structures of the products derived from acenaphthylene were confirmed by X-ray crystallography. A similar distribution of products was afforded from indene. The 2:1 adducts appear to derive from a diradical intermediate, the radical centers of which are strongly stabilized by the bridging urazoyl ring and benzylic delocalization. The triplet states of these diradical intermediates may be trapped via exposure to molecular oxygen to afford oxygen-containing adducts. Computational studies at the (U)B3LYP/6-31G* level provide additional support for the conclusions of our experimental work.

Substituent effect on the energy barrier for σ-bond formation from π-single-bonded species, singlet 2,2-dialkoxycyclopentane-1,3-diyls

Background: Localized singlet diradicals are in general quite short-lived intermediates in processes involving homolytic bond-cleavage and formation reactions. In the past decade, long-lived singlet diradicals have been reported in cyclic systems such as cyclobutane-1,3-diyls and cyclopentane-1,3-diyls. Experimental investigation of the chemistry of singlet diradicals has become possible. The present study explores the substituents and the effect of their substitution pattern at the C(1)–C(3) positions on the lifetime of singlet octahydropentalene-1,3-diyls to understand the role of the substituents on the reactivity of the localized singlet diradicals.

Results: A series of singlet 2,2-dialkoxy-1,3-diaryloctahydropentalene-1,3-diyls DR were generated in the photochemical denitrogenation of the corresponding azoalkanes AZ. The ring-closed products CP, i.e., 3,3-dialkoxy-2,4-diphenyltricyclo[3.3.0.0^2,4]octanes, were quantitatively obtained in the denitrogenation reaction. The first-order decay process (k = 1/τ) was observed for the fate of the singlet diradicals DR (λ_max ≈ 580–590 nm). The activation parameters, ΔH^‡ and ΔS^‡, for the ring-closing reaction (σ-bond formation process) were determined by the temperature-dependent change of the lifetime. The energy barrier was found to be largely dependent upon the substituents Ar and Ar’. The singlet diradical DRf (Ar = 3,5-dimethoxyphenyl, OCH₂Ar’ = OCH₂(3,5-dimethoxyphenyl)) was the longest-lived, τ₂₉₃ = 5394 ± 59 ns, among the diradicals studied here. The lifetime of the parent diradical DR (Ar = Ph, OCH₂Ar’ = OCH₃) was 299 ± 2 ns at 293 K.

Conclusion: The lifetimes of the singlet 1,3-diyls are found to be largely dependent on the substituent pattern of Ar and Ar’ at the C(1)–C(3) positions. Both the enthalpy and entropy effect were found to play crucial roles in increasing the lifetime.