The Effect of Substituent‐Dependent Photoinduced Intramolecular Charge Transfer on the Photochromism of Hemithioindigos

Abstract

A series of N,N′‐diarylamine‐modified hemithioindigos (HTIs) were synthesized and characterized. Photochemical Z/E isomerization around the C=C double bond occurred for all novel HTIs in various solvents. Due to a strong donor–acceptor π‐conjugated structure, the novel HTIs exhibited a strong red shift and clear solvatochromism in steady‐state absorption and fluorescence spectroscopic measurements, respectively. An intramolecular charge transfer (ICT) from the diarylamine‐substituted stilbene component (donor) to the thioindigo part (acceptor) strongly affects the isomerization behavior. HTI is the dominating photochromic unit in this system but the substitution pattern on the triarylamine (TAA) moiety causes unusually strong changes in the photostationary states (PSS) and thermal stabilities. Electron‐withdrawing substituents (such as cyano groups) strongly improve thermal stabilities and enrich the E‐isomer content in the photostationary state, however electron‐donating substituents play a converse effect, for example, in the solvents hexane and tetrachloroethene. The low E ratio in the PSS in more polar solvents, such as acetonitrile, can be explained by a very fast thermal back reaction and formation of a new decay state in OMe‐substituted compounds.

Hemithioindigos for Cellular Photopharmacology: Desymmetrised Molecular Switch Scaffolds Enabling Design Control over the Isomer‐Dependency of Potent Antimitotic Bioactivity

“…Determining PSS by spectrophotometry required HOTub solution concentrations well above their aqueous solubility limit (as do NMR measurements), so we constrained our measurements to water‐cosolvent mixtures. We do not assume that the average cellular PSS or relaxation rates for HTIs will be identical to those determined in cuvette measurements, as in cellulo HTIs will be in heterogenous environments depending on cellular biolocalisation (e.g., into lipid membranes or aqueous cytosol) and the absorption band intensities and positions, quantum yields of photoisomerisations, and the thermal E to Z relaxation rate are all known to be solvent dependent . However, we expected that the trends of PSS wavelength dependency as measured in UV/Vis would reasonably correspond to those in the cellular context.…”

Electronic and Geometric Characterization of TICT Formation in Hemithioindigo Photoswitches by Picosecond Infrared Spectroscopy

“…We have recently established TICT formation in hemithioindigo (HTI) chromophores (Figure )an emerging class of photoswitches , and used this excited-state pathway for controlling the type of light-induced molecular motion by simply changing the solvent . A similar charge transfer behavior in the excited state was also suggested for N , N -diarylamine-substituted HTIs . With the additional TICT state, it is possible to either favor double-bond isomerization (DBI) in apolar solvents or single-bond twisting (SBT) in strongly polar solvents using the same molecule (Figure b).…”

“…21 A similar charge transfer behavior in the excited state was also suggested for N,N-diarylamine-substituted HTIs. 28 With the additional TICT state, it is possible to either favor double-bond isomerization (DBI) in apolar solvents or single-bond twisting (SBT) in strongly polar solvents using the same molecule (Figure 1b). The reason for this behavior is the inherently most polar character of the TICT state as compared to other excited states (Figure 1c).…”

Recent progress on the chemistry of thieno[3,2‐b]quinoline derivatives (part III)

“…Wang et al [ 22 ] reported the reaction of thiophenols 44b with chloroacetic acid 68 in the presence of sodium hydroxide to form the 2‐(arylthio)acetic acid 69 , which cyclized to the benzo[b]thiophen‐3‐ols 70 upon heating with thionyl chloride followed by heating the formed acid chloride in 1, 2‐dichloroethane in the presence of AlCl 3 . Condensation of compound 70 with 2‐nitrobezaldehyde 71 in ethanol formed the nitroarylidene 72 , which reduced in anhydrous acetic acid in the presence of tin chloride to achieve the aminoarylidine 73 , which undergoes intramolecular cyclization under irradiation with UV light to the benzothieno[3,2‐ b ]quinoline 74 (Scheme 13).…”