Origin of the coloration and structure of azobenzene chromogen

• Published in: Russian Journal of Physical Chemistry A Vol. 89; no. 11; pp. 2036 - 2050
• Main Authors: Mikheev, Yu. A., Guseva, L. N., Ershov, Yu. A.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01-11-2015
• Subjects: Chemistry; Chemistry and Materials Science; Commemorating the Journal’s 85th Anniversary; Physical Chemistry; photoisomerization; azobenzene; coloration; nonpolar and polar electronic tautomers
• ISSN: 0036-0244; 1531-863X
• DOI: 10.1134/S0036024415110138

Enhancement of the visible (VIS) absorption band intensity of
trans
-azobenzene (
t
-AB) in solutions containing water and hydrogen ions is established. This contradicts the current belief that it is part of the
n
→ π* transition. At the same time, it qualitatively reflects the properties of the π → π* bands of protonated azobenzene (ABH
+
). It is concluded that
t
-AB molecules display an autopolarization property and exist in the form of two individual electronic (e) tautomers. One of these is nonpolar and has the canonical chemical structure; its content considerably exceeds that of the polar
e
-tautomer. The polar
e
-tautomer forms as a result of the reversible transfer of an electron from the nonbonding donor
sp
2
orbital of nitrogen to the nonbonding acceptor Rydberg’s 3
S
orbital (
R
3
S
) of the local N=N chromophore within the molecule. The positively charged chromogen corresponding to it displays a π → π* transition in the visible spectral region, and the π→π* transition (not the traditionally postulated
n
→ π* transition) is clearly responsible for the orange color of AB. A model of transient e-configurations with the participation of
R
3
S
and explaining the previous poorly understood experimental results from optical absorption, fluorescent, raman spectroscopic, and photoionization femtosecond kinetic studies is considered. It is shown that famous ideas about the violation of Kasha’s rule in
t
-AB fluorescence and photoisomerization processes are incorrect. The reasons for the increased intensity of the VIS band of
cis
-azobenzene (
c
-AB) are explained. It is concluded that there is an equilibrium between nonpolar and polarized
e
-tautomers in
cis
-azobenzene as well, but it is shifted more toward the polar tautomer in
c
-AB due to its structural features, making the VIS band more intense.