Ultrafast Spectroscopy of Hydroxy‐Substituted Azobenzenes in Water

Abstract

Ultrafast UV/Vis pump/probe experiments on ortho-, meta- and para-hydroxy-substituted azobenzenes (HO-ABs), as well as for sulfasalazine, an AB-based drug, were performed in aqueous solution. For meta-HO-AB, AB-like isomerisation behaviour can be observed, whereas, for ortho-HO-AB, fast proton transfer occurs, resulting in an excited keto species. For para-HO-AB, considerable keto/enol tautomerism proceeds in the ground state, so after excitation the trans-keto species isomerises into the cis form. Aided by TD-DFT calculations, insight is provided into different deactivation pathways for HO-AB, and reveals the role of hydroxy groups in the photochemistry of ABs, as well as their acetylation regarding sulfasalazine. Hydroxy groups are position-specific substituents for AB, which allow tuning of the timescale of thermal relaxation, as well as the amount and contribution of the keto species to photochemical processes.

Molecular photoswitches in aqueous environments

“…172,183 The same behaviour was shown earlier for substituted parent azobenzenes, comparing the thermal half-lives of the Z isomers of 2-, 3-, and 4-hydroxyazobenzene. 209 While the Z isomers of ortho-and para-OH azobenzene rapidly isomerized through tautomerism back to the thermodynamically favoured E form, the meta-substituted analogue behaved like the parent azobenzene. 209 Not only the rate constant of the thermal Z -E reaction is affected when moving into aqueous environment, but also the photoisomerization QYs are solvent-dependent.…”

“…209 While the Z isomers of ortho-and para-OH azobenzene rapidly isomerized through tautomerism back to the thermodynamically favoured E form, the meta-substituted analogue behaved like the parent azobenzene. 209 Not only the rate constant of the thermal Z -E reaction is affected when moving into aqueous environment, but also the photoisomerization QYs are solvent-dependent. For parent azobenzene, F E-Z is higher in alcohols and water than in apolar solvents if the isomerization is triggered via the S 1 (n-p*) and S 2 (p-p*), while F Z-E (n-p*) is in general lower.…”

“… 209 While the Z isomers of ortho - and para -OH azobenzene rapidly isomerized through tautomerism back to the thermodynamically favoured E form, the meta -substituted analogue behaved like the parent azobenzene. 209 …”

Thermal, Photochromic and Dynamic Properties of Water-Soluble Spiropyrans

“…If applied correctly, light can be operated as a harmless and selective external trigger to control material properties or even biological processes if the target photoswitch is soluble in aqueous solution. In contrast to azobenzenes which undergo photoinduced cis / trans isomerization, spiropyrans, diarylethenes, fulgides and fulgimides exist in an open‐ring and closed isomeric form. Spiropyrans possess an indoline and a chromene moiety and can be switched reversibly between the closed‐ring, nonpolar spiropyran ( SP ) structure and the open‐ring merocyanine ( MC ) which adopts a zwitterionic or a quinoidal mesomeric resonance structure.…”

The ultrafast reactions in the photochromic cycle of water-soluble fulgimide photoswitches

“…To date, there is a limited number of studies of photoswitches in aqueous solutions, [9][10][11][12][13][14][15][16][17] of which only a few are focused on the investigation of the ultrafast photochemical reactions (e.g. spiropyran 13 and azobenzene 18 ). Thus, despite the importance of water as a solvent environment for chemical and biological processes and the excessive demand for their control and manipulation, the behaviour of photoswitches in aqueous surroundings remains largely uncharacterized.…”