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

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.

Fluorescence of Cyclopropenium Ion Derivatives.

The synthesis of cyclopropenium-substituted amino compounds and analysis of their photophysical properties is described. Systematic structural modifications of these derivatives lead to measurable and predictable changes in molar extinction coefficients, quantum yields, and Stokes shifts. Using time-dependent density functional theory (TD-DFT) calculations, the origin of these trends was traced to internal charge transfer (ICT) coupled with ensuing structural reorganization for select naphthalene functionalized derivatives. Associated with this structural reorganization was an inward gearing of the cyclopropenium ring and twisting of the peri-NMe2 group into coplanarity with the naphthalene ring system. Further, reinforcement of an intramolecular H-bond (IMHB) in the excited state of these derivatives alludes to the importance of photoinduced H-bonding in this new class of cyclopropenium based fluorophores.