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Open AccessArticle

NIR Absorbing AzaBODIPY Dyes for pH Sensing

1
Institute for Nanotechnology Innovation, Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
2
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210046, China
*
Authors to whom correspondence should be addressed.
Academic Editor: M. Salomé Rodríguez-Morgade
Molecules 2020, 25(16), 3689; https://doi.org/10.3390/molecules25163689
Received: 23 July 2020 / Revised: 5 August 2020 / Accepted: 12 August 2020 / Published: 13 August 2020
(This article belongs to the Special Issue Phthalocyanines and Porphyrins)
Two near-infrared (NIR) absorbing di(thien-2-nyl)-di(dimethylanilino)azaBODIPY dyes 2a and 2b 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 CH2Cl2. 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 CH2Cl2, and a fluorescence “turn-on” effect in the NIR region. The pH dependence studies reveal that the pKa values of the non-protonated 2a and 2b 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. View Full-Text
Keywords: aza-BODIPY dye; pH sensing; intramolecular charge transfer; photophysical properties; pKa values aza-BODIPY dye; pH sensing; intramolecular charge transfer; photophysical properties; pKa values
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MDPI and ACS Style

Kubheka, G.; Mack, J.; Nyokong, T.; Shen, Z. NIR Absorbing AzaBODIPY Dyes for pH Sensing. Molecules 2020, 25, 3689.

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