Contrasting Photo-Switching Rates in Azobenzene Derivatives: How the Nature of the Substituent Plays a Role

Abstract

A molecular design approach was used to create asymmetrical visible light-triggered azo-derivatives that can be good candidates for polymer functionalization. The specific electron–donor substituted molecules were characterized and studied by means of NMR analyses and UV-visible spectroscopy, comparing the results with Time Dependent Density Functional (TD-DFT) calculations. A slow rate of isomerization (ki = 1.5 × 10−4 s−1) was discovered for 4-((2-hydroxy-5methylphenyl) diazenyl)-3-methoxybenzoic acid (AZO1). By methylating this moiety, it was possible to unlock the isomerization mechanism for the second molecule, methyl 3-methoxy-4-((2-methoxy-5-methylphenyl) diazenyl)benzoate (AZO2), reaching promising isomerization rates with visible light irradiation in different solvents. It was discovered that this rate was heightened by one order of magnitude (ki = 3.1 × 10−3 s−1) for AZO2. A computational analysis using density functional (DFT/PBE0) and wavefunction (QD-NEVPT2) methodologies provided insight into the photodynamics of these systems. Both molecules require excitation to the second (S2) excited state situated in the visible region to initiate the isomerization. Two classic mechanisms were considered, namely rotation and inversion, with the former being energetically more favorable. These azo-derivatives show potential that paves the way for future applications as building blocks of functional polymers. Likewise, they could be really effective for the modification of existing commercial polymers, thus transferring their stimuli responsive properties to polymeric bulky structures, converting them into smart materials.

Ferroelectricity of Hydrogen-Bonded Azobenzene Derivatives

“…Similar time-dependent spectral changes induced by UV irradiation were confirmed in the cases of 1a and 2 in CH 2 Cl 2 , and first-order trans–cis isomerization occurred with k = 2.89 × 10 –2 and 7.73 × 10 –2 s –1 (Figures S5c and S6c), respectively. k for the trans–cis isomerization of 1a , 2 , and 3a had the same magnitude as those for other azobenzene derivatives. , …”

“…k for the trans−cis isomerization of 1a, 2, and 3a had the same magnitude as those for other azobenzene derivatives. 60,61 Trans−cis isomerization after UV irradiation was also verified by the 1 H NMR spectra of 3a in CDCl 3 . After photoirradiation, significant upper-field shifts from δ = 7.98, 7.91, and 6.15 ppm to δ = 7.63, 6.85, and 6.00 ppm were observed at the ortho-and meta-H atoms of the azobenzene πcore, which were accordance with the production of the cisisomer (Figures S2−S4).…”

Self-aggregation, H-bonding, and photoresponse in film and solution states of azobenzene containing polyurea

“…Spectroscopic techniques like UV-visible absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and nuclear magnetic resonance (NMR) are routinely employed in the experimental studies of photochemistry of azobenzene polymers. [28][29][30] A complementary approach to experimental studies is the quantum chemical calculations using density functional theory (DFT) and its time-dependent extension (TD-DFT), 31,32 which can elucidate various physical, chemical, optical and thermal properties of materials. Such approaches are particularly efficient for providing insights into electronic properties, and therefore, for particularly understanding the photodynamics of azobenzene systems.…”

“…Such approaches are particularly efficient for providing insights into electronic properties, and therefore, for particularly understanding the photodynamics of azobenzene systems. 29,33 Researchers utilised TD-DFT approaches for fast and accurate interpretation of the UV-visible near infrared spectra of polymeric macromolecules 34 and investigated the thermal cis to trans isomerization in azopyridine derivatives. Their experimental findings were augmented with DFT computations which provided insights into electronic structures, density of states, and the mechanism of the isomerization.…”

Oriented External Electric Field Tuning of Unsubstituted Azoheteroarene Thermal Isomerization Half-Lives

“…Molecular switches are defined as molecular systems that can reversibly shift between at least two different states . Azobenzene (AB) has been studied extensively as a prototypical photoswitch molecule, − and its applications range from memory devices , to molecular motors and actuators. − AB exhibits a photoinduced isomerization process, converting from its trans isomer to cis isomer, after which either a thermal relaxation or absorption of another photon can allow reversion back to the trans isomer. Although AB-based photoswitches can be switched through photon absorption alone, in practice, incomplete photoswitching occurs due to overlapping absorbance of the two isomers .…”