Two π-Electrons Make the Difference: From BODIPY to BODIIM Switchable Fluorescent Dyes.

Hadi Dolati,Robin Grotjahn,Lars Denker,Lisa C Haufe,Andreas Lorbach,Gerald Hörner,René Frank

doi:10.1002/chem.201905344

Hadi Dolati, Robin Grotjahn + Show 5 more

Open Access

https://doi.org/10.1002/chem.201905344

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Abstract

(aza‐)BODIPY dyes (boron dipyrromethene dyes) are well‐established fluorophores due to their large quantum yields, stability, and diversity, which led to promising applications including imaging techniques, sensors, organic (opto)electronic materials, or biomedical applications. Although the control of the optical properties in (aza‐)BODIPY dyes by peripheral functional groups is well studied, we herein present a novel approach to modify the 12 π‐electron core of the dipyrromethene scaffold. The replacement of two carbon atoms in the β‐position of a BODIPY dye by two nitrogen atoms afforded a 14 π‐electron system, which was termed BODIIM (boron diimidazolylmethene) in systematic analogy to the BODIPY dyes. Remarkably, the BODIIM dye was obtained with a BH2‐rigidifying entity, which is currently elusive and highly sought after for the BODIPY dye class. DFT‐Calculations confirm the [12+2] π‐electron relationship between BODIPY and BODIIM and reveal a strong shape correlation between LUMO in the BODIPY and the HOMO of the BODIIM. The modification of the π‐system leads to a dramatic shift of the optical properties, of which the fluorescent emission is most noteworthy and occurs at much larger Stokes shift, that is, ≈500 cm−1 in BODIPY versus >4170 cm−1 in BODIIM system in all solvents investigated. Nucleophilic reactivity was found at the meso‐carbon atom in the formation of stable borane adducts with a significant shift of the fluorescent emission, and this behavior contrasts the reactivity of conventional BODIPY systems. In addition, the reverse decomplexation of the borane adducts was demonstrated in reactions with a representative N‐heterocyclic carbene to retain the strongly fluorescent BODIIM compound, which suggests applications as fully reversible fluorescent switch.

Highlights

BODIPY dyes constitute an important class of fluorophores, which can be considered as boron chelates with a dipyrrin entity[1] and have attracted broad interest as photoresponsive compounds, in particular as efficient fluorescent dyes (Scheme 1)
Given that BODIPY compounds are commonly synthesized from pre-formed ligand scaffolds by the complexation to the boron reagent being the last step of the synthetic protocol, we attempted such strategy for compound 6
The lithiation of 7 at low temperature and subsequent reaction with benzoyl chloride afforded bisimidazole compound 8, the complexation of which was attempted with various boron reagents including BX3, BHX2·SMe2 or BH2X·SMe2 (X = Cl, Br)

Summary

Introduction

The optical properties of organic p-systems can systematically be tuned by incorporation of hetero atoms into the scaffold. Successful application of this methodology has been demonstrated with the development of aza-BODIPY dyes 3, in which the meso-CH unit is replaced by a nitrogen atom. In contrast to BODIPY dyes 2, absorbance and fluorescence emission in 3 are significantly redshifted to at least 650 nm (visible red or near-IR region), which suggests the application of these systems in bio-imaging procedures.[13] Despite the intriguing results achieved with aza-BODIPY dyes 3, the further heteroatom incorporation was attempted but remained unachieved. Due to the systematic analogy of compound 6 to BODIPY dyes we introduce the term boron diimidazolylmethene (BODIIM) for this type of novel compounds

Results and Discussion

Conclusions

Conflict of interest