Excimer Formation Dynamics of Dipyrenyldecane in Structurally Different Ionic Liquids

Highly Efficient Intramolecular Excimer Formation in a Disulfide-Linked Dipyrenyl Compound: Proton Recognition and Fluidity Assessment

Intramolecular excimer formation has been utilized extensively in chemical sciences, especially to probe solvation within complex media as well as to assess physicochemical properties of the solubilizing milieu. Pyrene has been employed extensively as a fluorescence probe for this purpose due to its favorable multidimensional fluorescence properties. Termini-capped dipyrenyl scaffolds possessing various functionalities comprise the majority of such compounds. A new both end-tagged dipyrenyl compound DTP is designed and synthesized, which exhibits significantly high intramolecular excimer formation efficiency in polar solvents. The presence of a -NH-(CO)- and/or -S-S- functionality on the chain linking the two pyrenyl groups facilitates intramolecular excimer formation. Excited-state emission intensity decay reveals that the excimer formation exclusively takes place in the excited state with only one excimer conformation. The rate constant of excimer formation is found to be higher for DTP as compared to a similar compound with an alkyl backbone. The dependence of the excimer formation on the solvent (protic versus aprotic) as well as on temperature reveals further insights into the excimer formation process. The excimer formation by DTP is found to be highly sensitive to the presence of H+: the relative excimer formation efficiency decreases drastically in the presence of a small amount of H+ (∼10-5 M). Further, the recognition of protons by DTP via intramolecular excimer formation is also observed to be highly selective in nature. Based on the observation that both the excimer formation efficiency and kinetics depend on the viscosity of the solubilizing milieu, fluidity assessment of the (dimethyl sulfoxide + acetonitrile) mixture was carried out using DTP. Further, DTP is found to be an effective probe for the assessment of the amount of water in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide as well as in a deep eutectic solvent composed of choline chloride and urea in a 1:2 mol ratio. Highly efficient and rapid intramolecular excimer formation not only establishes DTP as a useful and versatile probe but also offers strategic pathways for designing effective excimer-forming compounds.

Monomer and Excimer Emission in Electrogenerated Chemiluminescence of Pyrene and 2,7-Di-tert-butylpyrene Associated with Electron Transfer Distance

Electrogenerated chemiluminescence (ECL) is a light emission phenomenon caused by electrochemically generated radical anions (R•-) and cations (R•+), in which the ion annihilation results in the formation of a pair of excited (R*) and ground state (R) of a luminescent molecule. Here, the ECL properties of pyrene (Py) and 2,7-di-tert-butylpyrene (di-t-BuPy) are reported. It was found that at a commonly employed concentration (1 mM), the ECL spectra were time-dependent because of increasing the oligomer emission and increasing the concentration of R near R*, leading to an enhancement of the excimer emission. At a low concentration range (20-30 μM), the shape of the ECL spectra containing the monomer and excimer emission was determined by isolated pairs of R* and R, which were generated through ion annihilation of R•- and R•+. It was found that in the ECL of Py and di-t-BuPy originated from the isolated pairs of R•- and R•+, 58 and 48% of the excited states were the excimer states, respectively. Diffusion equation analysis indicates that the lower excimer formation in the case of di-t-BuPy is because of a farther initial separation distance between R* and R, i.e., a longer electron transfer distance between the radical ions. The Marcus model for the electron transfer kinetics suggests that the farther electron transfer distance is mainly caused by the larger molecular size, which resulted in a smaller reorganization energy of the solvent acetonitrile molecule. Taking advantage of the photophysical and electrochemical properties of Py and di-t-Bu Py, the monomer and excimer emission in ECL is discussed.

Emerging properties from mechanical tethering within a post-synthetically functionalised catenane scaffold

Maintaining close spatial proximity of functional moieties within molecular systems can result in fascinating emergent properties. Whilst much work has been done on covalent tethering of functional units for myriad applications, investigations into mechanically linked systems are relatively rare. Formation of the mechanical bond is usually the final step in the synthesis of interlocked molecules, placing limits on the throughput of functionalised architectures. Herein we present the synthesis of a bis-azide [2]catenane scaffold that can be post-synthetically modified using CuAAC 'click' chemistry. In this manner we have been able to access functionalised catenanes from a common precursor and study the properties of electrochemically active, emissive and photodimerisable units within the mechanically interlocked system in comparison to non-interlocked analogues. Our data demonstrates that the greater (co-)conformational flexibility that can be obtained with mechanically interlocked systems compared to traditional covalent tethers paves the way for developing new functional molecules with exciting properties.

Contrasting ground- and excited-state intramolecular aggregation in choline chloride-based deep eutectic solvents versus a liquid polymer

Although the pyrenyl (Py) groups in several dipyrenyl (or bispyrenyl) compounds do not dimerize in the ground-state, they are known to intramolecularly aggregate exclusively in the excited-state to form excimers in common organic solvents. We present contrasting intramolecular aggregation behaviour of such a prototypical compound, 1,3-bis(1-pyrenyl)propane [1Py(3)1Py], dissolved in judiciously selected liquids having relatively high dynamic viscosities (η). Specifically, the intramolecular aggregation of 1Py(3)1Py is investigated in a liquid polymer polydimethylsiloxane with number average MW 2000, PDMS2000 (η^293.15K = 21.4 mPa s), and is compared with aggregation of 1Py(3)1Py in deep eutectic solvents (DESs) constituted of the H-bond acceptor (HBA) choline chloride (ChCl) and H-bond donors (HBDs) urea and glycerol in a 1 : 2 mole ratio of ChCl : urea (η^293.15K = 1372.0 mPa s) and ChCl : Gly (η^293.15K = 473.0 mPa s), respectively, in 293.15 to 363.15 K temperature range. The HBD constituent of ChCl : Gly, glycerol (Gly) (η^293.15K = 1412.0 mPa s) is also investigated for comparison purposes. It is found that while in PDMS2000, 1Py(3)1Py intramolecularly aggregates exclusively in the excited-state, thus forming a classical excimer, ground-state heterogeneity is clearly evident in both the DESs and Gly. High viscosity, a consequence of the extensive H-bonding in DESs/Gly, appears to induce the two Py units, both in the ground-state, to exist partly in a configuration where the interaction, albeit a weak one, takes place between the two. This ground-state interaction is not present in relatively low viscosity media PDMS2000 as observed in the common organic solvents. The role of viscosity/H-bonding of the solubilizing milieu on differential intramolecular aggregation in ground- and excited-states is highlighted.

Modular Synthesis of Pentagonal and Hexagonal Ring-Fused NBN-Phenalenes Leading to an Excited-State Aromatization-Induced Structural Planarization Molecular Library

Although polycyclic aromatic hydrocarbons (PAHs) with a nitrogen-boron-nitrogen (NBN) moiety have recently attracted tremendous interest due to their intriguing electronic and optoelectronic properties, all of the NBN-fused π-systems reported to date are called NBN-dibenzophenalenes and were synthesized by electrophilic aromatic substitution. The synthesis of NBN-phenalenes remains challenging, and transition-metal catalysis has never been utilized to construct NBN-embedded π-scaffolds. Herein, a palladium-catalyzed cyclization/bicyclization strategy was developed for the synthesis of diverse pentagonal and hexagonal ring-fused NBN-phenalenes and half-NBN-phenalenes. All of the NBN-embedded π-scaffolds presented in our paper are fluorescent in both solution and the solid state. Further investigations showed that the five-membered NBN rings exhibit the properties of traditional luminogens, while those with a six-membered NBN ring generally undergo photoinduced structural planarization (PISP) and exhibit different colors and quantum yields of fluorescence with different concentrations in solution. Time-resolved spectroscopy and TD-DFT calculations revealed that excited-state aromatization is the driving force for PISP in hexagonal ring-fused NBN-π systems, leading to the formation of excimers. Notably, the scope of PISP compounds is still quite limited, and PISP has never been observed in NBN-π systems before. These hexagonal ring-fused NBN-π systems constitute a novel PISP molecular library and appear to be a new class of aggregation-induced excimer emission (AIEE) materials. Finally, the AIEE behavior of these six-membered NBN rings was applied to the detection of nitro explosives, achieving excellent sensitivity. In general, this work provides a new viewpoint for synthesizing NBN-fused π-systems and understanding the excited-state motion of luminogens.

Fluorescence Quenching of Dipyrenylalkanes by an Electron/Charge Acceptor

Compounds possessing two fluorophoric moieties may exhibit dual fluorescence, one characterizing the monomeric fluorophore unit and the other characterizing the intramolecular aggregate. Fluorescence quenching of two dipyrenylalkanes, 1,3-bis(1-pyrenyl)propane [1Py(3)1Py] and 1,10-bis(1-pyrenyl)decane [1Py(10)1Py] having different alkyl chains separating the two termini pyrenyl groups that are capable of forming an intramolecular excimer, by an electron/charge-accepting quencher, nitromethane, is investigated in four different solvents-nonpolar (cyclohexane (CH)), polar-aprotic (acetonitrile (ACN)), polar-protic (ethanol (EtOH)), and chlorinated (dichloromethane (DCM))-under ambient conditions. For a given probe in a solvent, fluorescence from the monomer and the intramolecular excimer are quenched with similar efficiencies; the efficiency of quenching is higher for the probe with the longer alkyl chain separating the two fluorophores. Quenching efficiency is significantly higher in chlorinated solvent DCM. The bimolecular quenching rate constants for intramolecular excimer, however, are either comparable or lower for the longer alkyl chain compound. It is suggested that, while the donor electronic excited-state energetics is more favorable for the long alkyl chain compound, the approach of the quencher to the intramolecular excimer appears to be hindered by the presence of a longer alkyl chain.