Fluorescence of Cyclopropenium Ion Derivatives

Abstract

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.

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

“…Collectively,t hese differences in Stokes shift and fluorescencei ntensities are not too surprising in light of previous fluorescences tudies performed on proton sponge scaffolds. [11,17] In this respect, we attribute the decreased Stokes shift in 16 to the greaterd egree of co-planarity between the cyclopropenium andn aphthaleneb ackbone in the ground state;i nc ontrastt ot he more distorted free base DAGUN (6), where presumably al arger population of conformers are accessible,r esulting in greater Stokes shifts.…”

“…[18,36] This high basicity is notable, given the PA and pK BH + values for the previously reported mono-substituted cyclopropenimine sponge Janus (266.8 kcal mol À1 ,2 3.8) andt he disubstituted guanidine sponge TMGN (257.5 kcal mol À1 ,2 5.1), thus making DAGUN (6)o ne the most basic protons ponges reported in the chemical literature to-date. [18,37] Given the literature precedence to support the observation that protons ponge-based molecules tend to also exhibit exceptional fluorescence properties, [11,34] we looked to both experimentally and theoretically gain insight into the photophysical properties of DAGUN (6)a nd its conjugatea cid DAGUN-H + (16)( Figure6). Turning first to DAGUN (6), its UV/Vis spectrum in DCMa tr oom temperature shows two distinct maximaa t l abs = 299 nm and 346 nm.…”

“…[5] By the same token, proton sponges lend themselves to ab road range of applicationst hat include directing agents for the synthesis of crystalline microporous materials, [6] deprotonating matrices in mass spectrometry, [7] and catalysts or reactants for chemical transformations. [8,9] In our laboratory, proton sponges and relateds uperbases have been employed as phase-transfer catalysts, [10] small molecule fluorophores, [11] as well as fort he fundamentals tudy of intra-versus intermolecular hydrogen bonding interactions within molecules. [12] As for proton sponges and the factorsc ontributing to high basicity,t he widely accepted criteria for their classification include:( i) free base destabilization arising from proximal nitrogen lone pair-to-lonep air repulsion;( ii)favorable formation of as trong intramolecular hydrogen bond (IMHB)N •••H-N upon protonation;( iii)reliefo fs teric strain in transitioningf rom a non-planar base to as tabilized planar cation, and most recently (iv) the generation of aromaticity upon protonation.…”

“…By the same token, proton sponges lend themselves to a broad range of applications that include directing agents for the synthesis of crystalline microporous materials, deprotonating matrices in mass spectrometry, and catalysts or reactants for chemical transformations . In our laboratory, proton sponges and related superbases have been employed as phase‐transfer catalysts, small molecule fluorophores, as well as for the fundamental study of intra‐ versus intermolecular hydrogen bonding interactions within molecules …”

Self‐Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

“…[2][3][4] Among the differently substituted cyclopropenium cations,t he aminocyclopropenium ions first described by Yoshida in 1971, [5] have received special attention. [6] Most recent work has focused on their use as phase transfer, Lewis acid or organocatalysts, [7][8][9][10][11][12][13][14][15][16] electrophotocatalysts, [17] ligands for catalytic metal complexes, [18][19][20] ionic liquids, [21][22][23][24][25] persistent radical cations, [26] redox active polymers for redox flow batteries, [27][28][29][30][31][32] fluorescent materials, [33][34][35] aromatic cations in hybrid halide perovskites, [36] biologically active compounds such as transfection agents, [37][38][39] and nanoparticles. [40,41] Surprisingly, the self-assembly of cyclopropenium compounds into liquid crystalline phases has not been reported.…”