Electronic Communication in Linear Oligo(azobenzene) Radical Anions

Multiple Azoarenes Based Systems - Photoswitching, Supramolecular Chemistry and Application Prospects

In the recent decades, the investigations on photoresponsive molecular systems with multiple azoarenes are quite popular in diverse perspectives ranging from fundamental understanding of multiple photoswitches, supramolecular chemistry, and various application prospects. In fact, several insightful and conceptual designs of such systems were investigated with architectural distinctions. In particular, the demonstration of applications such as data storage with the help of multistate or orthogonal photoswitches, light modulation of catalysis via cooperative switching, sensors using supramolecular host-guest interactions, and materials such as liquid crystals, grating, actuators, etc. are some of the milestones in this area. Herein, we cover the recent advancements in the research areas of multiazoarenes containing systems that have been classified into Type-1 {linear, non-linear, and core-based (A)}, Type-2 {tripodal C₃ -symmetric (C3)} and Type-3 {macrocyclic (M)} structural motifs.

Tuning of Bistability, Thermal Stability of the Metastable States, and Application Prospects in the C3 -Symmetric Designs of Multiple Azo(hetero)arenes Systems

Light-responsive molecular systems with multiple photoswitches in C₃ -symmetric designs have enormous application potential. The design part of such molecular systems is critical due to its influence in several properties associated with the photoswitches. In order to tune, and in the evaluation of the design-property relationship, we synthesized 18 tripodal systems with variations in the core, linkers, connectivity, and azo(hetero)arene photoswitches. Through extensive spectroscopic and computational studies, we envisaged the factors controlling near-quantitative photoisomerization in both the directions (bistability) and the thermal stability of the metastable states. Furthermore, we also evaluated the impact of designs in obtaining reversible photo-responsive sol-gel phase transitions, solvatochromism, photo- and thermochromism.

Stable, yet "naked", azo radical anion ArNNAr- and dianion ArNNAr2- (Ar = 4-CN-2,6-iPr2-C6H2) with selective CO2 activation

Azo radical anion 1˙- and dianion 12- have been isolated by one- and two-electron reduction of the azo compound 1 (ArNNAr, Ar = 4-CN-2,6-iPr2-C6H2) with alkali metals, respectively. The reduced species have been characterized by single-crystal X-ray analysis, EPR, UV and FT-IR spectroscopy, as well as SQUID measurements. The filling of one and two electrons in the π* orbital of the N-N double bond of 1 leads to a half-double N-N bond in 1˙- and a single N-N bond in 12-. The uncoordinated nature of these reduced species enables them to activate CO2. The exposure of 1˙- solution to CO2 led to the formation of oxalate anion C2O42-, while that of 12- solution to CO2 afforded the hydrazine dicarboxylate dianion [1-2CO2]2-, which reversibly dissociated back to 1 and CO2 upon oxidation.

Computational design of a molecular triple photoswitch for wavelength-selective control

meta-Trisazobenzenes allow for wavelength-selective photo-switching of the individual azobenzene branches.

A small single molecule with multiple photoswitchable subunits, selectively and independently controllable by light of different wavelengths, is highly attractive for applications in multi-responsive materials and biological sciences. Herein, triple photoswitches are presented consisting of three independent azobenzene (AB) subunits that share a common central phenyl ring: the meta-trisazobenzenes (MTA). It is the unique meta-connectivity pattern leading to decoupling of all azo-subunits although they do overlap spatially. Based on this pattern, we design a triple MTA photoswitch, as proof-of-principle, with three different, electronically independent AB branches on the computer, which can be individually photo-excited to trigger ultra-fast E → Z isomerization at the selected AB branch.

Multiphotochromic molecular systems

We review molecular compounds encompassing several photochromic units with a focus on their functionalities.

ortho-Fluoroazobenzenes: visible light switches with very long-Lived Z isomers

Improving the photochemical properties of molecular photoswitches is crucial for the development of light-responsive systems in materials and life sciences. ortho-Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ-electron-withdrawing F atoms ortho to the NN unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para-electron-withdrawing groups (EWGs) work in concert with ortho-F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho-fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.