Influence of Bridgehead Substitution and Ring Annelation on the Photophysical Properties of Polycyclic DBO-Type Azoalkanes

Abstract

The photophysical data for the polycyclic, bridgehead-substituted derivatives 1-10 of the photoreluctant diazabicyclo[2.2.2]oct-2-ene (DBO) are presented. Substitution on the bridgehead positions with radical-stabilizing substituents enhances the photoreactivity (Phi(r)) and decreases the fluorescence quantum yields (Phi(f)) and lifetimes (tau) compared to the parent DBO. The annelated rings have no influence on the photoreactivity, except when steric interactions with an alpha substituent hinder the optimal radical-stabilizing conformation. The fused rings and some of the bridgehead substituents reduce the solvent-induced quenching of the singlet-excited azo chromophore by steric shielding of the azo group and, thus, increase the fluorescence quantum yields and lifetimes.

Bridgehead carboxy-substituted 2,3-diazabicyclo[2.2.2]oct-2-enes: synthesis, fluorescent properties, and host-guest complexation

“…In fact, Adam 38 and others [39][40][41][42] already studied more complex polycyclic derivatives carrying bridgehead carboxyl substituents, e.g. derivative 6, which can be obtained under mild acid-catalyzed conditions from the reaction of 4,5-dihydropyridazines with electron-rich alkenes (the SauerHünig route to polycyclic azoalkanes).…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13]38,41,48,53,54 DBO and its derivatives are special examples, because many of them are photochemically "persistent" azoalkanes, which show fascinating photophysical properties instead. 3,38,52,55 Besides an unexpectedly short phosphorescence lifetime, 41 DBO exhibits a very broad fluorescence spectrum 52 and an exceedingly long fluorescence lifetime (up to 1 µs in the gas phase). 52 The latter feature has been observed for most DBO derivatives (1-3, as well as alkyl and dichloro-substituted derivatives) 22,53 and applies also to the novel acetic acid 5 (τ = 345 ns, Table 1).…”

“…Such short fluorescence lifetimes are already known for some DBO derivatives, namely those carrying carboxylic acid ester, phenyl, or vinyl groups in the bridgehead positions. 38,53 A specific example is the 1-phenyl derivative 14. It was concluded that these substituents decrease the activation energy and accelerate the rate of nitrogen extrusion by stabilizing the incipient carbon radical site.…”

“…It was concluded that these substituents decrease the activation energy and accelerate the rate of nitrogen extrusion by stabilizing the incipient carbon radical site. 38,48,53,56 This affects also the quantum yield of photodecomposition (φ r ), which we have consequently determined 57 in water as solvent with reference to the parent 1 (φ r = 0.003). 17 We found that the photodecomposition quantum yield was only slightly enhanced for 5 (φ r = 0.029), but much higher for 4 (φ r ca.…”

Fluorescence Quenching of Ester-Substituted DBO-Type Azoalkanes by Olefins and Arenes: Electronic and Steric Effects, Exciplex Formation and Hydrogen Abstraction

“…For example, the fluorescence (DBO) and its derivatives has been thoroughly studied in quantum yield (Φ f ) rises in the order 1 (0.0066) < 2 (0.040) the last three decades and their photophysical behavior was < 3 (0.090), accompanied by an increase of the fluorescence the topic of intensive experimental and theoretical investi-lifetime (τ) in the order 1 (9.2 ns) < 2 (139 ns) 3 (273 ns). [6] gations. [1] In contrast to the diazabicyclo[2.2.1]hept-2-ene Since this photophysical behavior has as yet not been fully (DBH), the photoreactivity of DBO is extraordinary low understood, it was of interest to gain mechanistic insight and, therefore, constitutes a photoreluctant or photopersist-into the fluorescence process of the azoalkanes 1؊3.…”