An unsymmetrical guanidine-cyclopropenimine proton sponge DAGUN andt he relatedB F 2-chelate DAGBO are reported. Insight into the structural, electronic, bonding and photophysical properties of these two molecules are presented.J oint experimental and theoretical studies reveal the protonated form of DAGUN possessesa ni ntramolecular N•••HÀNh ydrogen bond which affords ah igh experimental pK BH + of 26.6 (computed = 26.3). Photophysical studies show that in solution DAGUN displays ag reen emission at 534 nm, with al arge Stokess hift of 235 nm (14,718 cm À1). In contrast, the conjugate acid DAGUN-H + is only weakly emis-sive due to attenuated intramolecular charge transfer.X-ray diffraction studies revealt hat DAGBO contains as table tetracoordinate boronium cation, reminiscent of the well-established BODIPY family of dyes. In solution, DAGBOe xhibits a strong blue emission at 450 nm coupled with al arge Stokes shift (Dl = 158 nm, Dn = 11,957 cm À1)a nd quantum yield of 62 %, upon excitation at 293 nm. DAGBO sets the stage as the first entry into an ew class of boron-difluoride diaminonaphthalenes (BOFDANs)t hat represent highlyf luorescent and tunable next-generation dyes with future promise for biosensing and bioimaging applications.

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-NMe 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.

The discovery of fluorescence two centuries ago ushered in, what is today, an illuminating field of science rooted in the rational design of photochromic molecules for task-specific bio-, material-, and medical-driven applications. Today, this includes applications in bioimaging and diagnosis, photodynamic therapy regimes, in addition to photovoltaic devices and solar cells, among a vast multitude of other usages. In furthering this indispensable area of daily life and modern-day scientific research, we report herein the synthesis of a class of trisaminocyclopropenium fluorophores along with a systematic investigation of their unique molecular and electronic dependent photophysical properties. Among these fluorophores, tris[N(naphthalen-2-ylmethyl)phenylamino] cyclopropenium chloride (TNTPC) displayed a strong photophysical profile including a 0.92 quantum yield ascribed to intramolecular charge transfer and intramolecular through-space conjugation. Moreover, this cyclopropenium-based fluorophore functions as a competent imaging agent for DNA visualization and nuclear counterstaining in cell culture. To facilitate the broader use of these compounds, design principles supported by density functional theory calculations for engineering analogs of this class of fluorophores are offered. Collectively, this study adds to the burgeoning interest in cyclopropenium compounds and their unique properties as fluorophores with uses in bioimaging applications.

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Pyranylation and glycosylation are pivotal for accessing a myriad of natural products, pharmaceuticals, and drug candidates. Catalytic approaches for enabling these transformations are of utmost importance and integral to advancing this area of synthesis. In exploring this chemical space, a combined experimental and computational mechanistic study of pyranylation and 2-deoxygalactosylation catalyzed by a cationic thiourea organocatalyst is reported. To this end, a thiourea−cyclopropenium organocatalyst was employed as a model system in combination with an arsenal of mechanistic techniques, including 13 C kinetic isotope effect experiments, deuterated labeling studies, variable-temperature 1 H NMR spectroscopy, and density functional theory calculations. From these studies, two distinct reaction pathways were identified for this transformation corresponding to either dual hydrogen bond (Hbond) activation or Brønsted acid catalysis. The former involving thiourea orchestrated bifurcated hydrogen bonding proceeded in an asynchronous concerted fashion. In contrast, the latter stepwise mechanism involving Brønsted acid catalysis hinged upon the formation of an oxocarbenium intermediate accompanied by subsequent alcohol addition.