On the Electronic Character of Localized Singlet 2,2-Dimethoxycyclopentane-1,3-diyl Diradicals: Substituent Effects on the Lifetime Manabu Abe,* ,† Waldemar Adam, ‡ Michihiro Hara, § Masanori Hattori, † Tetsuro Majima, § Masatomo Nojima, † Kei Tachibana, † and Sachiko Tojo § Department of Materials Chemistry, Graduate School of Engineering, Osaka UniVersity, Suita 565-0871, Osaka, Japan, Institut fu ¨ r Organische Chemie der UniVersita ¨ t Wu ¨ rzburg, Am Hubland, D-97074 Wu ¨ rzburg, Germany, and Institute of Scientific and Industrial Research, Osaka UniVersity, Ibaraki 567-0047, Osaka, Japan Received March 24, 2002 Our present day understanding of the electronic nature of localized triplet diradicals is well-documented by experimental as well as theoretical studies; 1 however, this is not the case for the corresponding singlet diradicals. The major reason resides in the fact that these species are quite short-lived and difficult to handle experimentally. 2 A breakthrough was the spectral detection and characterization of the first localized singlet diradical, namely the 2,2-difluoro-substituted 1,3-diradical 1 (X ) F; Y, Z ) H; λ max 530 nm; τ 293 K ) 80 ns in n-pentane), 3 whose singlet ground state was predicted by Borden’s computational work 4 for the parent cyclopentane-1,3-diyl (1,3-DR,X ) F). More recently, we have found that an alkoxy group (X ) OR in 1,3-DR) stabilizes sufficiently such transient species to place the singlet below the triplet state, as confirmed by the long-lived singlet diradical 2 (X ) OEt; Y, Z ) H; λ max 550 nm; τ 293K ) 880 ns in benzene). 5 In this study, we have examined the electronic substituent effects on the lifetime of the singlet diradicals 3a-h (X ) OMe), for which electron-donating (Y, Z ) Me, OMe) and/or electron-withdrawing groups (Y, Z ) Cl, CN) were introduced at the para position of the phenyl rings. The experimental results have provided valuable information on the electronic character of localized singlet diradi- cals. For the generation of the diradicals 3, we have chosen the photodenitrogenation of the azoalkanes 4a-h (λ max ∼360 nm, ǫ ∼ 100) by photolysis (>320 nm) in benzene, which afforded quantitatively the housanes 5a-h (>95%, Scheme 1) by intramo- lecular cyclization of the singlet diradicals 3a-h. The transient absorption spectra and decay traces of the intermediary singlet diradicals 3 were measured in benzene by means of laser-flash photolysis (λ exc ) 355 nm, 5 ns pulse). Strong absorption was observed in the visible region (ca. 600 nm), which decayed with clean first-order kinetics, as exemplified for the diazene 4a (Y, Z ) OMe) in Figure 1 (spectrum a and time profile d). A similar absorption band was observed in a methylcyclopentane (MCP) matrix at 77 K (Figure 1, spectrum b). The transient species were assigned to the diradicals 3a-h on the following experimental evidence: (i) the absorption maxima (around 600 nm) are similar to those of the singlet diradicals 1 and 2; (ii) the persisting species 3a,c at 77 K are EPR silent; (iii) the lifetimes (Table 1) are not affected by the presence of molecular oxygen; 6 and (iv) the large (log A ) 11.2) preexponential Arrhenius factor for 3c (Y ) H, E a ) 25.9 kJ/mol) suggests spin-allowed ring closure to housane 5. Additionally, significant fluorescence (λ max 740 nm at λ exc 590 nm) was observed for the localized singlet diradical 3a in the MCP matrix at 77 K (Figure 2c). As may be clearly seen from the lifetime (τ 293K ) data of the singlet diradicals 3 in Table 1, both electron- donating and electron-withdrawing groups stabilize the singlet * Corresponding author. E-mail: abe@ap.chem.eng.osaka-u.ac.jp. † Department of Materials Chemistry, Osaka University. ‡ Institut fu ¨r Organische Chemie der Universita ¨t Wu ¨rzburg. § Institute of Scientific and Industrial Research, Osaka University. Scheme 1 Figure 1. (a) Transient absorption spectrum for the singlet diradical 3a measured immediately after the laser pulse (λexc 355 nm); (b) absorption spectrum in a MCP matrix at 77 K; (c) fluorescence spectrum (λmax 740 nm) in the MCP matrix at 77 K (λexc 590 nm); (d) transient decay trace at 580 nm and 20 °C. Table 1. Substituent Effects on the Lifetime of the Singlet Diradicals 3 (X ) OMe) a entry 3 τ293K (ns) b 1 3a (Y, Z ) OMe) 1050 ( 80 2 3b (Y, Z ) Me) 460 ( 30 3 3c (Y, Z ) H) 320 ( 15 4 3d (Y, Z ) Cl) 490 ( 30 5 3e (Y, Z ) CN) 625 ( 25 6 3f (Y ) OMe, Z ) H) 600 ( 35 7 3g (Y ) CN, Z ) H) 470 ( 30 8 3h (Y ) OMe, Z ) CN) 740 ( 60 a The diradicals 3a-h were generated during laser flash photolysis (355 nm, 5-ns pulse width) of the diazenes 4a-h. b In benzene solution at 20 °C; errors are standard deviations of the mean (5 data points). Published on Web 05/15/2002 6540 9 J. AM. CHEM. SOC. 2002, 124, 6540-6541 10.1021/ja026301l CCC: $22.00 © 2002 American Chemical Society