Photoisomerization in different classes of azobenzene

Abstract

Azobenzene undergoes trans→cis isomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis→trans isomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Unprecedented high-temperature CO2 selectivity in N2-phobic nanoporous covalent organic polymers

“…The presence of chemical shifts in CP/MAS 13 C NMR spectra of azo-COPs, namely, azo-COP-1, azo-COP-2 and azo-COP-3 (also named as COP-68, -69, -70), (www.porouspolymers.com) located at 150.2, 144.7, 129.5, 123.4 and 54.9 ppm confirmed the formation of the azo-linked aromatic polymers 19,34 . Azobenzene moieties have a characteristic ultraviolet-visible spectra where an intense absorption band at 360 nm indicates the presence of transazobenzene unit 35 . The solid-state ultraviolet-visible analysis revealed 34 that the as-synthesized azo-COPs have predominantly trans configuration and feature low band gaps ( Supplementary Figs S3, S4).…”

Communication: Time-resolved fluorescence of highly single crystalline molecular wires of azobenzene

“…From the viewpoint of fundamental photochemistry, particularly, trans-cis photoisomerization mechanism and excited state decay dynamics of azobenzene are still a long standing issue to be much debated between theoretical calculations and experiments because of very fast isomerization process. [5][6][7][8][9][10][11] Recent ultrafast time-resolved spectroscopic results from probing short-lived azobenzene excited states propose that the isomerization pathways are rather complicated processes involving multiple relaxation channels. 12 In principle, azobenzene undergoes trans-cis photoisomerization by the excitation of the S 1 (n → π*) and the S 2 (π → π*) states in the UV and visible region.…”

“…According to previous works, the absorption band peaked at ∼450 nm can be easily assigned as the S 1 ← S 0 (n-π *) transition, corresponding to the tail of the first excited state. 5 In principle, since the fluorescence from the S 1 excited state in solution is extremely weak even in the time-integrated spectrum due to its very short lifetime and its optically forbidden nature, it is hardly observed. 22 On the other hand, we clearly observed the steady state fluorescence emission in molecular crystal wires at the intensity maximum of 576 nm.…”

“…12 In principle, azobenzene undergoes trans-cis photoisomerization by the excitation of the S 1 (n → π*) and the S 2 (π → π*) states in the UV and visible region. 5 Two pathways have been thus proposed to explain the intramolecular mechanism of the isomerization process: a twisting around the nitrogen-nitrogen double bond (rotational mechanism) and an increase in the C-N-N bending angles with an exchange in the position of the lone pair of one of the nitrogen atoms (inversion mechanism). 5 According to previous results, the average lifetime of the S 1 state of trans azobenzene and cis azobenzene were estimated by a few picoseconds (∼2.6 ps for trans and ∼0.2 ps for cis) in solution and decayed monomodally or bimodally depending on the solvent viscosity and excitation wavelength.…”

“…5 Two pathways have been thus proposed to explain the intramolecular mechanism of the isomerization process: a twisting around the nitrogen-nitrogen double bond (rotational mechanism) and an increase in the C-N-N bending angles with an exchange in the position of the lone pair of one of the nitrogen atoms (inversion mechanism). 5 According to previous results, the average lifetime of the S 1 state of trans azobenzene and cis azobenzene were estimated by a few picoseconds (∼2.6 ps for trans and ∼0.2 ps for cis) in solution and decayed monomodally or bimodally depending on the solvent viscosity and excitation wavelength. 12,13 As reported previously, azobenzene exhibits very weak fluorescence emission from the S 1 excited state with low quantum yield of ∼1 × 10 −6 due to deactivation of the first excited state by photoisomerization.…”