A Thermally Populated, Perpendicularly Twisted Alkene Triplet Diradical

Abstract

Variable-temperature NMR and ESR spectroscopic studies reveal that bis(dibenzo[a,i]fluorenylidene) 1 possesses a singlet ground state, 1(S ), while the 90° twisted triplet 1(T ) is populated to a small extent already at room temperature. Analysis of the increasing amount of paramagnetic 1(T ) at temperatures between 300 and 500 K yields the exchange interaction J /h c=3351 cm and a singlet-triplet energy splitting of 9.6 kcal mol , which is in excellent agreement with calculations (9.3 kcal mol at the UKS BP86/B3LYP/revPBE level of theory). In contrast, the zero-field splitting parameter D is very small (calculated value -0.018 cm ) and unmeasurable.

Polarity‐Dependent Isomerization of an Unsymmetrical Overcrowded Ethylene Promoted by Zwitterionic Contribution in the Twisted Isomer

“…The solved structure is in the folded form, which was close to that of the folded isomer of 1 ( Figure S1). The C=Ob ond lengths of 2 and 1 were 1.242(3) and 1.2239 (13) ,r espectively.T he slightly longer bond length suggestst he zwitterionic character.T his difference in zwitterionic character could not be judged from the IR spectra.The antisymmetric stretching vibrations of the two C=Og roups of 1 was observed in 1668 cm À1 while the stretching vibration of the C=Og roup of 2 was observed in 1662 cm À1 ,a nd there was no significant differenceb etween them.…”

“…In this study,a nu nsymmetrical bianthrone (2), consisting of the electron-withdrawinga nthrone and electron-donating acridane, have been synthesized and shown to exhibit as olvent-polarity-dependent isomerization reaction between the folded and twisted isomers. [8][9][10][11][12][13] Several analogues of bianthrone were reportedi nw hich the energy difference between the two isomers was controlled by the introductiono ff unctionalg roups and/or the substitution of anthrone with different fused rings. The stabilization of the twistedi somer in polar solvents can be interpreted as proof of its relatively large zwitterionic character.T he DMF solution of 2 displayed paramagnetically-broadened NMR signals from the thermally populated triplet state resultingf rom rotation of the weakened ethylenic double bond of the twisted isomer.…”

“…[2][3][4][5][6][7][8][9][10] In responset oe xternal stimuli, such as photoirradiation, thermals timulation and mechanicalg rinding, 1 shows an isomerization between two isomers:t he yellow-colored folded isomer and the green-colored twisted isomer.This structural change reflects the nature of the overcrowdede thylene. [8][9][10][11][12][13] Several analogues of bianthrone were reportedi nw hich the energy difference between the two isomers was controlled by the introductiono ff unctionalg roups and/or the substitution of anthrone with different fused rings. In addition, much attention has been focusedo nt he twisted doubleb ond in that it gives ac ontribution of the open-shell electronic structure to the ground state, which is represented as the small HOMO-LUMOg ap and the high reactivityo ft he twisted isomer.…”

“…The structural strain due to twisting of the central double bond in the twisted isomer is larger than that due to bending of the anthrone parts in the folded isomer.T herefore, the twisted isomer of 1 is less stable than the folded isomer by 3.5 kcal mol À1 , [8] which does not allow its isolation. [8][9][10][11][12][13] Several analogues of bianthrone were reportedi nw hich the energy difference between the two isomers was controlled by the introductiono ff unctionalg roups and/or the substitution of anthrone with different fused rings. [8][9][10][11][12][13] Several analogues of bianthrone were reportedi nw hich the energy difference between the two isomers was controlled by the introductiono ff unctionalg roups and/or the substitution of anthrone with different fused rings.…”

“…In addition, much attention has been focusedo nt he twisted doubleb ond in that it gives ac ontribution of the open-shell electronic structure to the ground state, which is represented as the small HOMO-LUMOg ap and the high reactivityo ft he twisted isomer. [8][9][10][11][12][13] Several analogues of bianthrone were reportedi nw hich the energy difference between the two isomers was controlled by the introductiono ff unctionalg roups and/or the substitution of anthrone with different fused rings. [1,5,6] Dicyanomethylenesubstituted quinoidal compound is the first example of isolation and X-ray structurald etermination of both isomers.…”