Experimental and Theoretical Investigation of Excited-State Intramolecular Proton Transfer Processes of Benzothiazole Derivatives in Amino-polydimethylsiloxanes before and after Cross-Linking by CO₂

Abstract

The changes in the ability of three fluorescent derivatives of 2-(2′-hydroxyphenyl)benzothiazole to undergo excited-state intramolecular proton transfer (ESIPT) processes have been correlated with the rheological properties of four amino-polydimethylsiloxanes with different molar masses and containing different amounts of monomer units with amino pendant groups, in the presence and absence of a crosslinking molecule, CO 2 . The changes lead to a variety of species (keto, enol, and enolate forms) in both the ground and excited states. Calculations using the density-functional theory/time-dependent density-functional theory method at the CAM-B3LYP/6-311++G(d,p) level helped to identify how ESIPT is involved in the formation of the intermediates. The results demonstrate that proton transfer in 2-(2′-hydroxyphenyl)benzothiazoles is a powerful tool to identify local changes in the viscosity and micropolarity of the environment that are attributed to the structural differences of the aminopolydimethylsiloxanes and their cross-linking.

Aryl-Phenanthro[9,10-d]imidazole: A Versatile Scaffold for the Design of Optical-Based Sensors

“…Regarding ground-state properties, it was observed that compound P43 (1 μM) in aqueous acetonitrile (50%, v/v) presented an absorption maximum at 365 nm with a high-energy shoulder at 310 nm. Upon changing the pH from 7 to 1, the absorption peak was blue-shifted by 15 nm with a minor decrease in molar absorption; however, under alkaline conditions (pH 12), the absorption band centered at 365 nm was diminished with a new red-shifted absorption maximum appearing at 409 nm, as expected with the deprotonation of the ESIPT compounds . In the excited state, the authors reported that the imidazole nitrogen was protonated in an acidic medium (pH 1), interrupting ESIPT, which led to a hypsochromic effect with an emission at 445 nm.…”

Interaction Study between ESIPT Fluorescent Lipophile-Based Benzazoles and BSA

“…For 16-17, both the emission maxima (542 nm) and Stokes shifts (~8800 cm −1 ) were observed at higher wavelengths. Thus, the emission produced by compound 15 is probably related to its ionized species [42]. Compounds 16-17 exhibit proton transfer in the excited state, and the emission is a result of this process.…”

“…This feature could be a result of scattering and may be an indication of aggregation of these compounds in PBS (pH 7.2), due to the presence of the long alkyl chains in these compounds, which are extremely hydrophobic. Significantly, Molecules 2021, 26, 6728 3 of 15 the similarity in shape and maxima wavelength compared to those of organic solvents, as well as the absence of additional red-shifted bands in PBS, indicates the absence of ionized species in the ground state for derivatives in this media [42,43].…”

“…[ 42,43]. Finally, the UV-Vis absorption spectra were used to calculate the experimental extinction coefficients ε from the oscillator strengths fe using the Strickler-Berg relationship represented by Equation (1) [44].…”

Evaluation of the acidic strengths on electronic ground and excited states of proton transfer dye using Excitation-Emission fluorescence matrix (EEM) and Multivariate Curve Resolution with Alternating Least Squares (MCR-ALS)

“…2-(2′-hydroxyphenyl)benzazole derivatives are a widespread class of emitters that figures great relevance due to their photoacidity coupled to the phototautomerization processes of Excited State Intramolecular Proton Transfer (ESIPT) [5,[10][11][12][13][14]. Both photoreactions, i.e., proton transfer on the excited state inter or intramolecularly (ESPT and ESIPT), have being applied as fluorescent sensors [2,[15][16][17][18][19][20].…”