Abstract
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.
Highlights
Near-infrared (NIR) region absorbing dyes are essential for various applications in fields as diverse as materials science and medicine [1]
Boron azadipyrromethene dyes are of particular interest, since the incorporation of an aza-nitrogen atom into the boron dipyrromethene (BODIPY) chromophore is one of the main strategies used to red shift the main spectral band into the NIR region [3]
When electron-donating aryl groups, such as dimethylaniline, are introduced at the 3,5-positions there is a large red-shift of the main spectral band, since there is a marked decrease in the HOMO–LUMO gap [10]
Summary
Near-infrared (NIR) region absorbing dyes are essential for various applications in fields as diverse as materials science and medicine [1]. Boron azadipyrromethene (azaBODIPY) dyes are of particular interest, since the incorporation of an aza-nitrogen atom into the boron dipyrromethene (BODIPY) chromophore is one of the main strategies used to red shift the main spectral band into the NIR region [3]. Different strategies have been used to develop dyes with highly red-shifted spectral bands, such as attaching electron-donating and withdrawing group at the 3,5- and 1,7-positions, respectively; replacing the phenyl groups with smaller five-membered heterocyclic rings [5,6,7,8]; and the addition of fused rings to the pyrrole rings of the azaBODIDY core [9]. The red-shifts that are observed when a five-membered ring system is incorporated into the structure have been ascribed to increased delocalization that arises from enhanced co-planarity with the azaBODIPY core and reduced torsion angles relative to structures that
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