ESIPT-induced fluorescent o-hydroxycinnamate: a self-monitoring phototrigger for prompt image-guided uncaging of alcohols

Abstract

o-Hydroxycinnamate derivatives are well-known phototriggers for fast and direct release of alcohols and amines without proceeding through the cleavage of carbonate or carbamate linkages. Despite these unique features, o-hydroxycinnamates lack extensive applications in biological systems mainly because of their non-fluorescent nature. To overcome this limitation, we have attached a 2-(2'-hydroxyphenyl) benzothiazole (HBT) moiety, capable of rapid excited-state intramolecular proton transfer (ESIPT) to the o-hydroxycinnamate group. The ESIPT effect induced two major advantages to the o-hydroxycinnamate group: (i) large Stokes' shifted fluorescence (orange colour) properties and (ii) distinct fluorescence colour change upon photorelease. In vitro studies exhibited an image guided, photoregulated release of bioactive molecules by the o-hydroxycinnamate-benzothiazole-methyl salicylate conjugate and real-time monitoring of the release action.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

“…The 2-hydroxy­cinnamyl moiety ( 134 , Scheme ) was first utilized as a PPG by Porter and co-workers to achieve photochemical activation of thrombin. , This system undergoes an initial photoinduced isomerization followed by cyclization to form a coumarin derivative, causing the release of caged substrates such as alcohols. − The cyclization rates of 2-hydroxy­cinnamyl esters or amides approached those of the trimethyl lock system ( k = 0.03–50 × 10 5 s –1 ). ,,,− The absorption maxima of 134 could be further bathochromically shifted (up to λ max abs = 394 nm) by introducing electron-donating substituents on the phenyl ring ,, or extending the system’s π-conjugation. , A small set of 2-hydroxy­cinnamic derivatives was synthesized to study and improve the 2P-absorption-mediated release process. ,, …”

“…723,728,729,751−754 The absorption maxima of 134 could be further bathochromically shifted (up to λ max abs = 394 nm) by introducing electron-donating substituents on the phenyl ring 743,755,756 or extending the system's π-conjugation. 750,757 A small set of 2-hydroxycinnamic derivatives was synthesized to study and improve the 2P-absorption-mediated release process. 755,756,758 Dougherty and co-workers incorporated conformationally locked Z-cinnamyl esters (with a cis-alkenyl lock 759 ) into amino-substituted 1,4-benzoquinones 135 and 136, and 1,4naphthoquinone 137 (Figure 13), enabling visible-light mediated quinone photoreduction to serve as an intramolecular cyclization initiator and leaving-group release trigger (Scheme 40).…”

Monitoring of uncaging processes by designing photolytical reactions

“…32 Two examples of the use of ohydroxycinnamates were recently reported by Pradeep Singh and co-workers. 33,34 The first one is based on excited-state intramolecular proton transfer (ESIPT) inducing fluorescent ohydroxycinnamates formation. 33 In this study, an ESIPT is followed by a Z-E isomerization followed by a rearrangement yielding to a fluorescent benzothiazole-coumarin by-product, allowing the uncaging event monitoring in real time.…”

“…33,34 The first one is based on excited-state intramolecular proton transfer (ESIPT) inducing fluorescent ohydroxycinnamates formation. 33 In this study, an ESIPT is followed by a Z-E isomerization followed by a rearrangement yielding to a fluorescent benzothiazole-coumarin by-product, allowing the uncaging event monitoring in real time. The ESIPT phenomena leads to a huge stokes shift only for the PPG leading to a distinct fluorescence color change upon photorelease.…”

pH-Dependent Photoinduced Interconversion of Furocoumaric and Furocoumarinic Acids

“…Photochemical control of material functions using cinnamate photoswitches was reported for hydro/nanogels, micelle solutions, and polymer networks [ 31 , 32 , 33 , 34 ]. o -Hydroxycinnamate phototriggers were used for image-guided photo-regulated uncaging of biologically relevant and cosmeceutical agents [ 35 , 36 ].…”