Simultaneous Observation of Triplet and Singlet Cyclopentane-1,3-diyl Diradicals in the Intersystem Crossing Process

Mizuno, Takemi; Abe, Manabu; Ikeda, Noriaki

doi:10.1071/ch15062

Cited by 5 publications

(7 citation statements)

References 13 publications

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“…The lifetime of the singlet diradical S‐ DR16 was found to be 620±12 ns at 293 K (Figure S5 in the Supporting Information), which is much shorter than that of S‐ DR2 g , h with the macrocyclic system (Table ). As judged by the large lg A values of about 12, the decay of 580 nm species is spin‐allowed process to give the corresponding singlet products (Scheme ) . The activation energies of the first‐order decay process of DR2 g , h were found to be about 52–55 kJ mol −1 , which are close to the computed values for the ring‐closing reaction to the cis isomer of the ring‐closure product cis ‐ CP2 g (Table ).…”

Section: Resultssupporting

confidence: 67%

“…As judged by the large lg A valueso f about 12, the decay of 580 nm speciesi ss pin-allowed process to give the corresponding singlet products (Scheme 2). [22] The activation energies of the first-order decay process of DR2 g,h were found to be about 52-55 kJ mol À1 ,w hich are close to the computed values for the ring-closing reaction to the cis isomer of the ring-closure product cis-CP2 g ( Table 2). The negligible substituent effect of the meta substituents on the thermodynamic stabilization of the diradical was revealed by the similar absorption maxima of S-DR2 e,S -DR2 g,a nd S-DR16,w hich were found to be 575, 575, and 576 nm.…”

Section: Figuresupporting

confidence: 78%

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Extremely Long Lived Localized Singlet Diradicals in a Macrocyclic Structure: A Case Study on the Stretch Effect

Harada

Wang

Kumashiro

et al. 2018

Chemistry A European J

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Localized singlet diradicals have attracted much attention, not only in the field of bond-homolysis chemistry, but also in nonlinear optical materials. In this study, an extremely long lived localized singlet diradical was obtained by using a new molecular design strategy in which it is kinetically stabilized by means of a macrocycle that increases the molecular strain of the corresponding σ-bonded compound. Notably, the lifetime of this diradical (14 μs) is two orders of magnitude longer than that of a standard singlet diradical without a macrocyclic structure (≈0.2 μs) at 293 K. The species is persistent below a temperature of 100 K. In addition to the kinetic stabilization of the singlet diradical, the spontaneous oxidation of its corresponding ring-closed compound at 298 K produced oxygenated products under atmospheric conditions. Apparently, the "stretch effect" induced by the macrocyclic structure plays a crucial role in extending the lifetime of localized singlet diradicals and increasing the reactivity of their corresponding σ-bonded compounds.

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Section: Resultssupporting

confidence: 67%

Section: Figuresupporting

confidence: 78%

Extremely Long Lived Localized Singlet Diradicals in a Macrocyclic Structure: A Case Study on the Stretch Effect

Harada

Wang

Kumashiro

et al. 2018

Chemistry A European J

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“…Planar diradical pl-1 DR1 was generated by LFP of a degassed acetonitrile solution of AZ1 (0.33 mM, Abs 355 = 0.03) and Xan (0.50 mM, Abs 355 = 0.13) (Figure 5a). 48,49 Transient absorption spectra showed that the quenching of 3 Xan* was concomitant with the formation of pl-1 DR1 (λ max = 570 nm) (Figure 5a). The quenching rate constant (k q = 5.4 × 10 9 M −1 s −1 ) at 293 K in degassed acetonitrile was determined from the Stern−Volmer plot (Figure S10), showing that the decay of 3 Xan* was dominated by energy transfer from 3 Xan* to AZ1.…”

Section: Photochemical Denitrogenation Mechanism Of Az1mentioning

confidence: 99%

Impact of Diradical Spin State (Singlet vs Triplet) and Structure (Puckered vs Planar) on the Photodenitrogenation Stereoselectivity of 2,3-Diazabicyclo[2.2.1]heptanes

et al. 2016

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Versatile transformations of azo compounds are utilized not only in synthetic organic chemistry but also in materials science. In this study, a hitherto unknown stereoselectivity was observed by low-temperature in situ NMR spectroscopy for the photochemical denitrogenation of a cyclic azoalkane (2,3-diazabicyclo[2.2.1]heptane) derivative. Direct (singlet) photodenitrogenation at 188 K afforded two products, the configurationally retained ring-closed compound (ret-CP) and the inverted compound (inv-CP), in a ratio of 82/18 (±3) (ret-CP/inv-CP), with an overall yield of >95%. Triplet-sensitized denitrogenation at 199 K using benzophenone ((3)BP*) or xanthone ((3)Xan*) selectively produced inv-CP, with a ret-CP/inv-CP ratio of 7/93 (±3). Thermal isomerization of inv-CP into ret-CP was observed by low-temperature NMR spectroscopy. Transient absorption spectroscopy revealed that two distinct singlet diradicals are involved in the formation of CP during direct photodenitrogenation, that is, puckered puc-(1)DR and planar pl-(1)DR diradicals. The former produces ret-CP, whereas the latter affords inv-CP. Kinetic analysis using the integrated profiles method was used to determine the molecular absorption coefficient of pl-(1)DR (ε560 = 4900 ± 250 M(-1) cm(-1)) for the first time. The involvement of the puckered singlet diradical resolves the mechanistic puzzle of stereoselective denitrogenation of diazabicycloheptane-type azoalkanes.

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“…In this region, the activation parameters were obtained as Δ H ⧧ = 21.4 kJ mol –1 , Δ S ⧧ = −145.3 J K –1 mol –1 , and Δ G ⧧ = 58.2 kJ mol –1 at 253 K and Δ G ⧧ = 72.7 kJ mol –1 at 353 K. The large entropy contribution in the low-temperature region (iii) is reasonable because 9 is formed from triplet diradical T- 7 via the intersystem crossing process (ISC) from S- 7 (Scheme ). The spin-forbidden ISC processes are known to provide entropy control, whereby, in general, large negative entropy and small log A values are observed. , In the high-temperature region (i), the Gibbs energy for the ISC process becomes larger than the energy for another reaction pathway from S- 7a to 9aa . Thus, a dramatic temperature effect was observed for the product selectivity in the photodenitrogenation of 6a .…”

Section: Resultsmentioning

confidence: 99%

1,2-Diazacyclopentane-3,5-diyl Diradicals: Electronic Structure and Reactivity

Yoshidomi

Abe

2019

J. Am. Chem. Soc.

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Localized singlet diradicals are key intermediates in bond homolysis. A thorough study of the reactive species is needed to clarify the mechanisms of the homolytic bond cleavage and formation processes. In general, the singlet diradicals are quite short-lived because of the fast radical–radical coupling reactions. The short-lived characteristic has retarded the thorough study on bond homolysis. In this study, a new series of long-lived singlet diradicals, viz., 1,2-diazacyclopentane-3,5-diyl, were identified, and their electronic structures and novel reactivities were thoroughly studied using laser-flash photolysis (LFP), product analysis, and computational studies. A direct observation of the thermal equilibration (fast process) between the singlet diradicals and the corresponding ring-closing compounds was undertaken on the submicrosecond time scale. The solvent and substituent effects on the equilibration constant and rate constants for the ring-closing reaction and ring-opening reaction clarify the novel nitrogen-atom effect on the localized singlet 1,3-diyl diradicals. Two types of alkoxy-migrated compounds, 9 and 10, were isolated with high yields as the final products. Crossover, spin-trapping, and LFP experiments for the formation of alkoxy-group migration products (i.e., 9 versus 10) revealed the unique temperature effect on the product ratio of the two types of alkoxy-migration products. The temperature-insensitive intersystem crossing process (slow process, millisecond time scale) was found to be a key step in the formation of 9, which is an entropy-controlled pathway. An intramolecular migration process was identified for the formation of 10 that was accelerated by a polar solvent in an enthalpy-controlled process. This unique heteroatom effect has opened up a new series of localized singlet diradicals that are crucial intermediates in bond homolysis.

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Simultaneous Observation of Triplet and Singlet Cyclopentane-1,3-diyl Diradicals in the Intersystem Crossing Process

Cited by 5 publications

References 13 publications

Extremely Long Lived Localized Singlet Diradicals in a Macrocyclic Structure: A Case Study on the Stretch Effect

Extremely Long Lived Localized Singlet Diradicals in a Macrocyclic Structure: A Case Study on the Stretch Effect

Impact of Diradical Spin State (Singlet vs Triplet) and Structure (Puckered vs Planar) on the Photodenitrogenation Stereoselectivity of 2,3-Diazabicyclo[2.2.1]heptanes

1,2-Diazacyclopentane-3,5-diyl Diradicals: Electronic Structure and Reactivity

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