641. Tautomerism and geometrical isomerism of arylazo-phenols and -naphthols. Part I. 4-phenylazo-1-naphthol

Abstract

By ERNST FISCHER and (MRs.) YAEL F. FREI.4-Phenylazo-l-naphthol (I) exists in solution as an equilibrium mixture with its tautomer, a-naphthaquinone phenylhydrazone (11). On cooling, this equilibrium shifts towards the latter (in methylcyclohexane) or towards the former (in alcohol). The alcoholic solution a t -140" contains practically only the former (I), which on irradiation with light a t various wavelengths is photoequilibrated with its cis-isomer, in analogy with other azo-compounds.Between -135" and -90" the unstable cis-isomer is thermally converted into the stable trans-isomer via the phenylhydrazone. Each step has an activation energy of about 11 kcal./mole, as compared with about 23 kcal./mole for the normal cis + trans conversion. No direct photoconversion involving the hydrazone seems to take place. Similar results were obtained with solutions in tetrahydrofuran-methyltetrahydrofuran. KUHN and B A E R ~were the first to point out that 4-phenylazo-l-naphthol (I) exists in solution as a tautomeric equilibrium mixture with the phenylhydrazone (11). Their results were confirmed and extended by several authors2One would expect tautomer (I) to exist in a cis-and a trans-form (Ia and b) in analogy with azobenzeneJ3 the a~onaphthalenes,~ and 4-hydro~yazobenzene,~*~ but attempts to achieve such isomerisations have hitherto been ~nsuccessful.~

Synthesis of molybdenocene(IV) and tungstenocene(IV) tropolonato complexes: Its derivative containing calix[4]arene moiety

“…Complexation with cationic molybdenocene resulted in a large change in the absorption spectrum. The band at 420 nm decreased considerably and shifted to 470 nm; this shift can be attributed to the formation of the hydrazone form [15]. The present result is compatible with the study of Kishimoto and coworkers [17], who reported that, in tautomerism of 4-(4 0 -substituted-phenylazo)-1-naphthol [azo form vs. hydrazone form], the hydrazone form predominates with electronwithdrawing substituents.…”

“…The spectrum of 6 is characterized by the presence of one absorption maximum at around 420 nm; this value is closely similar to published data on 4-arylazo-1-naphthols [15] and p-nitrophenylazo-substituted calix [4]arenes [16], suggesting that 6 exists as the azo form. Complexation with cationic molybdenocene resulted in a large change in the absorption spectrum.…”

2‐Hydroxyazobenzenes to Tailor pH Pulses and Oscillations with Light

“…Structural changes should be similarly expected to provide a road to finely tune the kinetics for thermal return from the photoactivated cis stereoisomer, which is known to be faster than the corresponding azobenzene series. [29][30][31] Azobenzenes are additionally well known for the robustness of their photochromism: [32,33] 2-hydroxyazobenzenes should exhibit no significant fatigue upon multiple photocycles that lead to proton pulses. Eventually, their ionization that accompanies proton photorelease during the trans-to-cis photoisomerization could further enlarge the broad field of relevance of the azobenzene moiety, which has been extensively used to photocontrol the behavior of small molecules as well as of natural and synthetic macromolecules.…”

Temperature dependent absorption spectroscopy of some tautomeric azo dyes and Schiff bases

“…The dependence of their tautomeric ratios on the solvent and substituent effects has been intensively studied quantitatively in the last ten years. 8-12 However, there are only few studies describing temperature effect, especially the changes observed at low temperatures, [13][14][15] and then only qualitatively.…”