Room Temperature Reversible Z→E Photoisomerization of Azobenzene Appended to Anthraquinone‐Benzimidazole Based Photoswitches with Resolved n→π* Absorption Band

Abstract

Light‐triggered substituted azobenzene‐based molecular switches are well known for their potential applications as molecular machines. The Z‐isomer stability affected by heteroaromatic groups, extended aromaticity, different substituents, and varying the solvents based on their polarity. In this regards, five different anthraquinone/imidazole‐based azobenzene derivatives have been synthesized with different electronic push/pull substituents. The detailed study of photoswitching processes with kinetic data was appraised by using UV/Vis and 1H‐NMR spectroscopic techniques. The investigation of light‐induced hydroxyl group conversion to quinone‐hydrazone tautomerization has been observed during the isomerization. Most importantly, intermolecular hydrogen bonding interaction between tautomeric form and a higher polar solvent (DMSO) block the isomerization, but in a less polar solvent (DCM), light‐induced trans ↔ cis isomerization takes place by tautomerization pathway with well‐separated π→π* and n→π* transitions. Further, the detailed mechanistic study of solvent‐assisted light‐induced tautomerization with the role of hydrogen bonding and the following WRITE‐READ‐ERASE‐READ system have been derived by using molecular logic gates.

Light Triggered Reversible Aggregation/Dispersion of Hydroxy Azo‐benzenes During Photo Switching: Solvent, Ions Assisted Dispersion, and Induced Quenching Emission

“…After keeping it idle for two days, when the solution became homogeneous, the UV spectrum returned to its original trans form. The UV‐vis spectrum obtained here for compounds 1 and in polar solvents is very unusual, unlike any reports for free hydroxy azobenzene, where a fast photoswitching was always accompanied by a reduction and blue shift of π–π* transition [33–35] …”

“…The UV-vis spectrum obtained here for compounds 1 and in polar solvents is very unusual, unlike any reports for free hydroxy azobenzene, where a fast photoswitching was always accompanied by a reduction and blue shift of π-π* transition. [33][34][35] Clearly, the above results indicate that the presence of the À CH 2 OH and À F groups, as well as the presence of polar solvents, play an important role in the observed high stability of the isomerized compounds. An earlier study on p-hydroxyazobenzene found a regular switching behavior under MeOH-EtOH mixed solvent, [26] with a short cis lifetime.…”

“…Furthermore, in contrast to the prevalent mechanism of the hydroxy azobenzene, i. e., intramolecular or intermolecular proton transfer, which usually assists in protic solvents, converts the cis isomer into its hydrazone tautomer cis-keto-1-ck (having weak À NÀ N=C bond) through the rotation pathway, which causes fast cis-trans isomerization, because activation energy is low as it contains À NÀ C=CÀ bond, [34] tautomerization in our cases is less possible since switching is a time taking process, and thus we suggest that switching is followed through an inversion process through 1-inv (having strong À N=NÀ ) that has high activation energy and causes slow isomerization (Scheme 4).…”

Synthesis, crystal structure, photoisomerization, and DFT studies of novel azo compounds based on imidazole

“…The electron pushing effect of methyl –CH3, methoxy –OCH3 and 3‐(1H‐imidazol‐1‐yl)propyl substitutions might restrict the charge transfer, as well as hydrogen transfer to azo group by forming of dimers, by virtue of intermolecular hydrogen bonds involving the hydroxy group and the azo nitrogen, which further stabilizes and restricts the trans ↔ cis isomerization. [ 38,39 ]…”

“…The electron pushing effect of methyl -CH3, methoxy -OCH3 and 3-(1H-imidazol-1-yl)propyl substitutions might restrict the charge transfer, as well as hydrogen transfer to azo group by forming of dimers, by virtue of intermolecular hydrogen bonds involving the hydroxy group and the azo nitrogen, which further stabilizes and restricts the trans $ cis isomerization. [38,39] Full reversibility of the cis to trans isomerization process was observed when the UV light was switched off and the solution exposed to visible light for 2 h. The isosbestic point at 398 nm indicated that the azobenzene isomerization was the only process and was a reversible photoreaction. The π-π* and n-π* bands of the cis isomer were both red-shifted compared to trans isomer of azobenzene a.…”