Fundamental Impacts of Enforced Planarity in Triarylboranes

Electronic and redox characterization of the planarized species revealed surprising results. The absorption bands of 40 were of slightly shorter wavelength 

40 featured a much larger Stokes shift with the longest emission maximum even exceeding that of 14 nm, A axa 4 = 400nm). Additionally, 40 

Investigation of the reduced state of planarized triarylborane 40 was conducted by chemically generating the radical anion salt 41 by treatment with potassium metal and [2.2.2] cryptand in THF [44]. Electron spin resonance 

X-ray crystal structures of the bowl-shaped fluoroborate anions (counterions omitted). Reproduced with permission from [24]. Copyright 2012 American Chemical Society. 

A related study on planarized borataanthracene radical anion 42 [45] showed that it maintained a planar geometry in the reduced state, reflected in its structural and photophysical properties. Unlike the unconstrained 9-phenyl-9-borataanthracene 31 [35], 42 did not show any interactions with counterions owing to broad delocalization of the negative charge between the borataanthracene component and the planarized phenyl ring. Electronic spectra of 42 were red-shifted with enhanced vibronic structure compared to 9-Mes-9-borataanthracene. While the fluorescence behavior had not previously been observed in unconstrained borataanthracene derivatives, 42 exhibited intense emission = 584 nm, O = 0.45). 

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