Reduktion von 1,2‐bis[(Z)‐(2‐nitrophenyl)‐NNO‐azoxy]benzol: Synthese von Cyclotrisazobenzol ( = (5E,6aZ,11E,12aZ,17E,18aZ)‐5,6,11,12,17,18‐Hexaazatribenzo[aei][1,3,5,7,9,11]cyclododeca‐hexaen)

Abstract

325 Reduktion von 1,2-Bis((Z)-(2-nitrophenyl)-NNO -azoxy]benzol'): Synthese von Cyclotrisazobenzol(Na,S reduction of 1,2-bis[(Z)-(2-nitrophenyl)-NNO-azoxy]benzene (2) yielded 3 deoxygenated products: the (known) red 2,2'-((E,E)-1,2-phenylenbisazo)dianiline (3, 23 %), the orange 2-[2-((E)-2-aminophenylazo)phenyl]-2H-benzotriazol(4,55 %) and the colorless 2,2'-(1,2-phenylene)di-2H-benzotriazol(5, 13 %). The constitutions of >5 and of 6, the N-acetyl derivative of 4, were deduced from their 'H-NMR spectra (chemical shifts, couplings, and symmetry properties), and the configurations of 3,4, and 6 at their N,N-double bonds are assumed to be the same as in 2. Oxidation of 3 with 2 mol-equiv. of Pb(OAc), afforded 5 (47%) and a novel, highly symmetrical macrocycle, called cyclotrisazobenzene (7, 24%). The constitution of 7 as a tribenzo-hexaaza[ 12lannulene and its (E)-configuration at the N,N-bonds was confirmed by X-ray analysis. 'The molecular symmetry expressed by the 'H-, I3C-and "N-NMR spectra of 7 reveals a rapid torsional motion around the six N,C bonds. This implies that the N,N-double bonds in the cyclic 12x-electron system (or 24n-electron system if the benzene rings are included) of 7 are highly localized. (C,,H,,N,; Schema 2)'). Die Strukturen von .F5 folgen vor allem aus den 'H-NMR-Spektren (in der Tabelle rnit 2 verglichen) und den darin zum Ausdruck kommenden Symmetrieeigenschaften, wobei die Konfiguration der Azo-Gruppierungen in 3 und 4 bzw. 6 (Herstellung s. unten) entsprechend derjenigen der Azoxy-Gruppierung in 2 als ( E ) angenommen wird. Das am weitesten reduzierte Produkt 3 war schon fruher aus den Arbeiten von Zollinger und Skrabal et al. [6] [7] bekannt'). Einleitung. ~ Vor kurzem haben wir mittels RontgenstrukturanalyseEinige Aspekte der 'H-NMR-Spektren fur 3-5 sind in der Tabelle zusammengefasst. Dort, wo die 6-Werte nach [8] durch Standardinkremente berechnet werden konnten, gibt es geniigend Ubereinstirnmung mit den durch Entkopplung festgelegten Nachbarschaftsbeziehungen (s. Exper. Teil). Die 6-Werte der Protonen an den beiden (2H-Benzotriazol-2-yl)-Gruppen von 5 stimmen rnit denen von 2-Ivlethyl-2H-benzotriazol[9] gut ' ) 4,Aus o-Nitroazo-und o-Nitroazoxybenzolen sind rnit Sulfid-Ionen in einigen Fallen 2-Phenyl-2H-benzotriazol-1-oxide erhalten worden [5]. Mit 2 haben wir diese Reaktion nicht beobachtet. Die Atomnumerierungen in 4 4 cntsprechen nicht ihren systematischen IUPAC-Namen, sondern wurdenim Interesse der spektroskopischen Vergleichbarkeit ~ derjenigen von 2 und 3 angeglichen. Die Numerierung in 7 ist ebenfalls willkurlich. Systematische Namen im Exper. T d .Wir danken Herrn Dr. P. Skrabal vom Technisch-Chemischen Laboratorium, ETH Zurich, fur die Uberlassung einer Vergleichsprobe von 3, rnit der unsere Probe identisch war. 5,

Synthesis and Isomerization Studies of Cyclotrisazobiphenyl

“…The smallest possible macrocycle containing more than two azobonds, that is, cyclotrisazobenzene 5, was first reported by Dreiding and co-workers (Scheme 3). [18] However, the authors did not comment on the photochemical properties. We have recently shown that cyclotrisazobenzene compounds are accessible in good yields through a different route.…”

Ultrafast Excited‐State Deactivation Dynamics of Cyclotrisazobenzene—A Novel Type of UV‐B Absorber

“…[8] (E)-AB has as trong pp*t ransition band at % 320 nm (260 nm in (Z)-AB) and aw eakly allowed np*t ransition band at % 450 nm (440 nm in (Z)-AB). [13a, 15] Thus, the question what geometricalr estrictions are required to successfully alter the energy landscape and therefore modify the quantum yields of the photoreactionso re ven completelyb lock the isomerization remains.S ufficiently high strain could turn AB from am olecular photoswitchi nto an efficient UV absorber that non-photochemicallyd issipates the excitation energy.I nt his context,w ea ddress here the photophysical properties of cyclotrisazobenzene (CTA) [16] -a highly constrained oligoazobenzene ring system.[a] Dr. It has been demonstrated that molecular interaction between individual AB units in multi-AB structures could give rise to numerousn ew and versatile properties.…”

“…[9] For example, the investigation of bis-ABs (bis[phenylazo]benzenes) has demonstrated the importance of the connectivity pattern between the photoswitch units, that is, subtle differences in the structural layout have an unexpectedly large influence on the optical properties of the bichromophores. [13a, 15] Thus, the question what geometricalr estrictions are required to successfully alter the energy landscape and therefore modify the quantum yields of the photoreactionso re ven completelyb lock the isomerization remains.S ufficiently high strain could turn AB from am olecular photoswitchi nto an efficient UV absorber that non-photochemicallyd issipates the excitation energy.I nt his context,w ea ddress here the photophysical properties of cyclotrisazobenzene (CTA) [16] -a highly constrained oligoazobenzene ring system. Furthermore, it was demonstrated for ar elated cyclic meta-bis-AB that the geometric restriction in the molecule leads to "reverse" stabilization,t hat is, the Z-isomer is the stable one and thermal E!Z isomerization occurs.…”

“…Furthermore, it was demonstrated for ar elated cyclic meta-bis-AB that the geometric restriction in the molecule leads to "reverse" stabilization,t hat is, the Z-isomer is the stable one and thermal E!Z isomerization occurs. [13a, 15] Thus, the question what geometricalr estrictions are required to successfully alter the energy landscape and therefore modify the quantum yields of the photoreactionso re ven completelyb lock the isomerization remains.S ufficiently high strain could turn AB from am olecular photoswitchi nto an efficient UV absorber that non-photochemicallyd issipates the excitation energy.I nt his context,w ea ddress here the photophysical properties of cyclotrisazobenzene (CTA) [16] -a highly constrained oligoazobenzene ring system. [13] Therefore, the significant differences in the photochromic properties (spectra, quantum yield, kinetic rates) of bis-ABs or even larger multi-ABc onstructs could be used in different applications.I nt his regard, cyclic oligo-AB systems have been examined with respectt ot he effect of the ring strain on the Z!E thermalb arrier,a nd their potentiala pplication as molecular solar thermal( MOST) energy storage.…”

“…[14] Numerous studies demonstrate that ABs retain their isomerization capabilities even in highly rigids ystems. [13a, 15] Thus, the question what geometricalr estrictions are required to successfully alter the energy landscape and therefore modify the quantum yields of the photoreactionso re ven completelyb lock the isomerization remains.S ufficiently high strain could turn AB from am olecular photoswitchi nto an efficient UV absorber that non-photochemicallyd issipates the excitation energy.I nt his context,w ea ddress here the photophysical properties of cyclotrisazobenzene (CTA) [16] -a highly constrained oligoazobenzene ring system. In the electronic ground state (GS), the CTA ( Figure 1) is essentially planar with the benzene rings slightly twisted out of the molecular plane (see Figure S1).…”