Organic Tri‐ and Tetraradicals with High‐Spin or Low‐Spin States

Nau, Werner M.

doi:10.1002/anie.199724451

Cited by 19 publications

(12 citation statements)

References 31 publications

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“…Molecular Design Elements. TMM-type spin-containing units have been used to explore exchange coupling through sp 3 -hybridized coupling fragments and aromatic rings; ,− however, high-spin molecules having a TMM-type exchange coupling unit are rare compared to those based on m -phenylene linkers. , The major advantage to using the TMM coupler is that the parent biradical has a larger singlet−triplet gap (14.5 kcal/mol) than m -xylylene (9.2 ± 0.2 kcal/mol). Thus, the TMM coupler is an inherently stronger exchange coupler.…”

Section: Discussionmentioning

confidence: 99%

Structure−Property Relationships in Trimethylenemethane-Type Biradicals. 2. Synthesis and EPR Spectral Characterization of Dinitroxide Biradicals

et al. 1999

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Several trimethylenemethane-type (TMM-type) dinitroxide biradicals have been prepared that differ in tert-butylaminoxylphenyl ring torsion angles by virtue of different steric demands of their "spinprotecting groups". In addition, the synthesis of a TMM-type dinitroxide having a planar π-system is reported. The synthetic route employed for the planar TMM-type biradical is general and should therefore expand the utility of TMM-type biradicals that are susceptible to bond torsions that attenuate exchange coupling. The compounds presented are designed to be effective to map out J-coupling/conformation space in TMM-type biradicals. EPR spectroscopic parameters (hfcc and D-values) were compared to determine if their values reflected differences in conformation in the series of biradicals. Interestingly, neither N-nor H-hfcc varied within our series of biradicals/ monoradicals in a regular fashion, indicating that there is no apparent relationship between N-or H-hfcc and conformation (as judged by molecular mechanics calculations). However, D-values, estimated from relative intensities of ∆m s ) 1 and ∆m s ) 2 transitions, are consistent with a varied degree of delocalization in the dinitroxides: smaller D-values for biradicals having smaller aryl torsions (and greater delocalization), and larger D-values for dinitroxides having greater aryl torsions (and less delocalization). All the biradicals studied, except 5 •• , exhibited linear Curie plots. The linear Curie plots are consistent with both triplet ground-states and singlet-triplet degeneracies. Interestingly, dinitroxide 6 •• exhibited a linear Curie plot, despite the lack of a π-coupling fragment. Biradical 5 •• , however, is a ground-state singlet species with the triplet lying about 140 cal/mol above the singlet.

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Section: Discussionmentioning

confidence: 99%

Structure−Property Relationships in Trimethylenemethane-Type Biradicals. 2. Synthesis and EPR Spectral Characterization of Dinitroxide Biradicals

et al. 1999

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“…[1][2][3][4][5][6][7][8][9][10][11][12][13]38,41,48,53,54 DBO and its derivatives are special examples, because many of them are photochemically "persistent" azoalkanes, which show fascinating photophysical properties instead. 3,38,52,55 Besides an unexpectedly short phosphorescence lifetime, 41 DBO exhibits a very broad fluorescence spectrum 52 and an exceedingly long fluorescence lifetime (up to 1 µs in the gas phase).…”

Section: Photophysical Characterizationmentioning

confidence: 99%

“…[1][2][3] They have remained key intermediates in the synthesis of strained compounds 1,4 and in the mechanistic investigation of the behavior of biradicals, [5][6][7][8][9][10][11] with some seemingly never-ending mechanistic disputes like the double-inversion peculiarity of 2,3-diazabicyclo[2.2.1]hept-2-enes. 12,13 Derivatives of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO, 1) have recently become valuable structural and dynamic probes 14 as well as sensors for supramolecular [15][16][17][18][19][20][21][22][23] and biomolecular assemblies.…”

Section: Introductionmentioning

confidence: 99%

Bridgehead carboxy-substituted 2,3-diazabicyclo[2.2.2]oct-2-enes: synthesis, fluorescent properties, and host-guest complexation

Hennig¹,

Schwarzlose²,

Nau³

2007

Arkivoc

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Two novel derivatives of 2,3-diazabicyclo[2.2.2]oct-2-ene were synthesized, carrying a carboxyl (4) and a methylcarboxyl (5) substituent at the bridgehead position. The photodecomposition quantum yields (51% for 4 and 2.9% for 5) and fluorescence lifetimes (29 ns for 4 and 345 ns for 5) in water were determined. The higher photoreactivity and fluorescence quenching for 4 was attributed to its higher propensity to undergo photochemical elimination of nitrogen as a consequence of the presence of the radical-stabilizing carboxyl group in the α-position. The absolute photodecomposition rate constants of 4 became faster upon protonation (e.g., at pH 2), which contrasted anticipated substituent effects on the C-N bonds strengths. ) which was attributed to reduced Coulomb attractions as a consequence of the 1,3-alternate conformation which this host adopts in water. The binding constants towards β-cyclodextrin were also low at pH 7.0 (< 120 M -1 ), which was attributed to the low hydrophobicity of the anionic form of the guests; in line with this interpretation, the binding constants with β-cyclodextrin increased at pH 2.0, by about one order of magnitude.

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“…Starting from 4-hydroxybiphenyl (5), the long known bromination to 6 [52] was easily achieved, as was the Omethylation to 7, providing a protected hydroxyl group compatible with all subsequent reactions. Of several possible metal-assisted aryl couplings [23,24,25] we selected the Scheme 1 well developed nickel route using haloaromatics and aromatic Grignard reagents (cf.…”

Section: 35-tris[(2-methoxy-35-diphenyl)phenyl]-benzene (3)mentioning

confidence: 99%