Fully Bridged Triphenylamines Comprising Five- and Seven-Membered Rings

Abstract

A family of fully bridged triphenylamines with embedded 5- and 7-membered rings is presented. The compounds are potent electron donors capable to undergo donor/acceptor interactions with strong cyano-based acceptors both in the solid state and solution. These interactions were evaluated by IR and UV/vis spectroscopy as well as X-ray crystallography. The vinylene-bridged compound was oxidized to the corresponding 1,2-diketone which readily underwent acid-catalyzed condensation with selected 1,2-phenylenediamines. The resulting π-extended quinoxaline derivatives represent valuable building blocks for the development of functional chromophores upon appropriate functionalization.

Keywords: N-heterocycle; bridged triarylamines; charge transfer; co-crystallization; electron donor.

Scheme 1

Known doubly bridged TPA 1. Synthesis of fully bridged TPAs 2 and 3. SB=single bond, DB=double bond.

Figure 1

X‐ray crystal structures and nucleus‐independent chemical shifts (NICS(1)) of 2 and 3 (50 % probability level, H‐atoms omitted). Color code: grey=carbon, blue=nitrogen. (a) Selected bond lengths of 2 (two independent molecules in the unit cell) [Å] c=1.552(4)/1.556(4); bond angles [°] α=108.9(2)/108.7(2); β=108.5(2)/108.1(2); γ=140.5(2)/141.0(2). Rounded torsion angles a‐b‐c 40°/40° and e‐d‐c 51°/53°. (b) Selected bond lengths of 3 [Å] c=1.362(3); bond angles [°] α=109.11(18); β=108.92(19); γ=141.9(2); rounded torsion angles a‐b‐c 2° and e‐d‐c 2°. Relative aromaticity (red) and antiaromaticity (blue) of 2 and 3 is schematically displayed by the size of the solid dots.

Figure 2

UV/vis spectra of compounds 1, 2, and 3 recorded in CH₂Cl₂ at room temperature.

Figure 3

X‐ray crystal structures of 3 with electron acceptors TCNE (a), TCNQ (b), and F₄TCNQ (c) with selected distances (50 % probability level, H‐atoms and solvent molecules omitted). Color code: grey=carbon, blue=nitrogen, green=fluorine, orange=plane through all non‐H‐atoms of 3, red dot=centroid through all atoms of respective molecule except H‐atoms.

Figure 4

(a) Further transformations of 3 towards quinoxalines 5 and 6. (b) X‐ray crystal structure at 50 % probability ellipsoids of 6 with P2₁/c space group. Hydrogen atoms and solvent molecules are omitted for clarity. Color code: grey=carbon, blue=nitrogen. Selected binding angles [°] α=109.6(4); β=108.8(4); γ=141.3(4). From upper left to lower right: side view, top view, two front views at different viewing angles.