NIR Absorbing AzaBODIPY Dyes for pH Sensing

Two near-infrared (NIR) absorbing di(thien-2-nyl)-di(dimethylanilino)azaBODIPY dyes <b>2a</b> and <b>2b</b> were synthesized and characterized that differ depending on whether the dimethylaniline substituents are introduced at the 3,5- or 1,7-positions of the azaBODIPY core....

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Bibliographic Details
Main Authors: Gugu Kubheka, John Mack, Tebello Nyokong, Zhen Shen
Format: Article
Language:English
Published: MDPI AG 2020-08-01
Series:Molecules
Subjects:
Online Access:https://www.mdpi.com/1420-3049/25/16/3689
Description
Summary:Two near-infrared (NIR) absorbing di(thien-2-nyl)-di(dimethylanilino)azaBODIPY dyes <b>2a</b> and <b>2b</b> were synthesized and characterized that differ depending on whether the dimethylaniline substituents are introduced at the 3,5- or 1,7-positions of the azaBODIPY core. The main spectral bands lie at 824 and 790 nm, respectively, in CH<sub>2</sub>Cl<sub>2</sub>. The effect of substituent position on the photophysical and pH sensing properties was analyzed through a comparison of the optical properties with the results of time-dependent density functional theory (TD-DFT) calculations. Protonation of the dimethylamino nitrogen atoms eliminates the intramolecular charge transfer properties of these compounds, and this results in a marked blue-shift of the main absorption bands to 696 and 730 nm, respectively, in CH<sub>2</sub>Cl<sub>2</sub>, and a fluorescence “turn-on” effect in the NIR region. The pH dependence studies reveal that the pK<sub>a</sub> values of the non-protonated <b>2a</b> and <b>2b</b> molecules are ca. 6.9 (±0.05) and 7.3 (±0.05), respectively, while that of the monoprotonated species for both dyes is ca. 1.4 (±0.05) making them potentially suitable for use as colorimetric pH indicators under highly acidic conditions.
ISSN:1420-3049