The Effects of Cyclic Terminal Groups in 4-Aminoazo-benzene and Related Azo Dyes: Part 2—pKa Values of Some Monoazo Dyes Derived from

Spectral Tuning of Azobenzene Photoswitches for Biological Applications

“…The presence of a six-membered ring (5 c, 5 e, 5 f) introduces steric interactions that lead to loss of sp 2 character on the N atom, whereas a five-membered ring (5 d), or no ring at all (5 a, 5 b) produces enhanced N delocalization. [15,16] In addition, compounds 5 e and 5 f have heteroatoms in the 2,2' Scheme 1. Synthesis of 4,4'-diacetamido azobenzenes bearing amino substituents in the 2,2'-positions.…”

The effects of cyclic terminal groups in aminoazobenzene dyes*

“…( 2 ) since increasing acid concentration leads to an increase in azonium ion concentration [31]. The tautomeric equilibrium is dependent on steric effects, especially those which lead to deconjugation of the donor group; the basicities of the p-azo and the terminal nitrogen atoms are largely determined by the extent of conjugation between the amino group and the rest of the molecule [30].…”

“…Prevention of conjugation by crowding substituents leads to the ready protonation of the relatively basic terminal nitrogen atom, as in the case of 4-dimethylamino-3,5-dimethylazobenzene [l], and the virtual absence of azonium tautomer. On the other hand, enhanced conjugation of the terminal nitrogen atom in the case of dyes derived from julolidine (5) results in the exclusive formation of the azonium ion [30].…”

“…95%) as u o n i u m cation whereas the corresponding piperidino dcriv'itives are present predominantly (approx. 857L) in the ammonium form [30]. The acyclic diethylamino cornpounds (1; R' = R2 = H) show much less bias towxds the azonium ions than do the pyrrolidinyl dyes The dycs obtained from N-phenylhexamethyleneimine and from .V-phenylheptamethyleneimine also exist almost entirely in the azonium form, reflecting efficient conjugation between the terminal lone pair of electrons and the ,ironlatic system 1291.…”

Synthesis, optical characterization and crystal and molecular X-ray structure of a phenylazojulolidine derivative

“…Spectroscopic data (UVeVis) [23,24], proton magnetic resonance spectra [25], and dipole moments measurements [26] of dipolar azodyes suggest that julolidine is a particularly powerful electron donor moiety, thanks to the nearly planar sp 2 conformation at the nitrogen atom, that allows a more efficient delocalization of the electron lone pair. Second harmonic generation efficiency confirmed the larger push-pull nature of dipolar NLO-phores containing julolidine moiety compared with no-cyclic arylamines derivatives [27].…”