Volume 26, Issue 39 p. 8608-8620
Full Paper

Development of an Unsymmetrical Cyclopropenimine-Guanidine Platform for Accessing Strongly Basic Proton Sponges and Boron-Difluoride Diaminonaphthalene Fluorophores

Matt Guest

Matt Guest

Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, ON, L2S 3A1 Canada

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Richard Le Sueur

Richard Le Sueur

Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, ON, L2S 3A1 Canada

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Prof. Melanie Pilkington

Prof. Melanie Pilkington

Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, ON, L2S 3A1 Canada

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Prof. Travis Dudding

Corresponding Author

Prof. Travis Dudding

Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, ON, L2S 3A1 Canada

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First published: 21 April 2020
Citations: 9

Graphical Abstract

Development of the historic proton sponge family resulted in a new asymmetric cyclopropenium/guanidine platform (DAGUN) that was found to be superbasic. Functionalizing DAGUN further by the coordination of a BF2 group to the peri-nitrogen atoms resulted in the inaugural derivative (DAGBO) of a new class of molecules we have coined, boron-difluoride diaminonaphthalene (BOFDAN) fluorophores (see figure).

Abstract

An unsymmetrical guanidine-cyclopropenimine proton sponge DAGUN and the related BF2-chelate DAGBO are reported. Insight into the structural, electronic, bonding and photophysical properties of these two molecules are presented. Joint experimental and theoretical studies reveal the protonated form of DAGUN possesses an intramolecular N⋅⋅⋅H−N hydrogen bond which affords a high experimental pKBH+ of 26.6 (computed=26.3). Photophysical studies show that in solution DAGUN displays a green emission at 534 nm, with a large Stokes shift of 235 nm (14,718 cm−1). In contrast, the conjugate acid DAGUN-H+ is only weakly emissive due to attenuated intramolecular charge transfer. X-ray diffraction studies reveal that DAGBO contains a stable tetracoordinate boronium cation, reminiscent of the well-established BODIPY family of dyes. In solution, DAGBO exhibits a strong blue emission at 450 nm coupled with a large Stokes shift (Δλ=158 nm, Δν=11,957 cm−1) and quantum yield of 62 %, upon excitation at 293 nm. DAGBO sets the stage as the first entry into a new class of boron-difluoride diaminonaphthalenes (BOFDANs) that represent highly fluorescent and tunable next-generation dyes with future promise for biosensing and bioimaging applications.

Conflict of interest

The authors declare no conflict of interest.