Intramolecular Charge Transfer: Mechanism Behind Cyanide Anion Sensing of 3,5-Diformyl-borondipyrromethene

Anion Sensing by Novel Triarylboranes Containing Boraanthracene: DFT Functional Assessment, Selective Interactions, and Mechanism Demonstration

Analytical methods often involve expensive instrumentation and tedious sample pretreatment for an analyte detection. Being toxic and detrimental to human health, sensing of cyanide (CN-), fluoride (F-), chloride (Cl-), bromide (Br-), nitrate (NO3 -), acetate (CH3COO-), and bisulfate (HSO4 -) is performed by a boron-based molecular receptor, N,N,N,3,5-pentamethyl-4-{2-thia-9-boratricyclo[8.4.0.03,8]tetradeca-1(10),3(8),4,6,11,13-hexaen-9-yl}anili-nium (1), and the three newly designed receptors from it. Thermodynamics, electronic structure, and photophysical properties are computed by employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) to explore selective sensing of these anions and its mechanism. Free-energy changes (ΔG) and binding energies (ΔE) suggest that among these anions, only binding of CN- and F- is thermodynamically feasible with a very strong binding affinity with the receptors. Boron atoms containing positive natural charges act as the electrophilic centers to bind the anions involving a 2p-2p orbital overlap resulting in charge transfer. In the receptor-analyte complexes with CN- and F-, fluorescence is quenched due to the intramolecular charge transfer transitions (π-π* transitions in the case of the receptors lead to fluorescence), internal conversion, and associated configurational changes. Among the six tested functionals, CAM-B3LYP/6-31G(d) is found to be the most accurate one. The designed receptors are better fluorescent probes for F- and CN-, demonstrating their importance for the practical utility.

A theoretical study on anion sensing mechanism of multi-phosphonium triarylboranes: intramolecular charge transfer and configurational changes

The binding selectivity and recognition mechanism of a series of mono-, di- and triphosphonium substituted triarylboranes: (4-(dimesitylboryl)-3,5-dimethylphenyl)phosphonium ([Mes₂BAr^P]⁺, 1), 1,1'-mesitylboranediylbis(3,5-dimethylphenyl)phosphonium ([MesBAr^P₂]²⁺, 2) and 1,1',1''-boranetriyltris(3,5-dimethylphenyl)phosphonium ([BAr^P₃]³⁺, 3) where Ar^P = 4-(H₃P)-2,6-Me₂-C₆H₂, for various anions has been investigated by employing density functional theory (DFT) and time dependent-density functional theory (TD-DFT) methods. Natural population analysis indicates the electrophilic nature of the boron centers in 1-3 for the nucleophilic addition of anions. The calculated free energy changes (ΔG) reveal that out of CN^-, F^-, Cl^-, Br^-, NO₃^-, CH₃COO^- and HSO₄^- only the binding of CN^- and F^- with 1, 2 and 3 is thermodynamically feasible. In addition, the calculated binding energies reflect that CN^- shows lesser binding affinity than F^- with 1, 2 and 3. Frontier molecular orbital (FMO) analysis reveals that the first excited states (S₁) of 1-3 are the local excited states with a π → π* transition, whereas the third excited state (S₃), fifth excited state (S₅), fourth excited state (S₄) and fourth excited states (S₄) of [Mes₂BAr^P]⁺F (1F, the fluoro form of 1), [MesBAr^P₂]²⁺F (2F, the fluoro form of 2), [Mes₂BAr^P]⁺CN (1CN, the cyano form of 1) and [MesBAr^P₂]²⁺CN (2CN, the cyano form of 2), respectively, are charge separation states found to be responsible for the intramolecular charge transfer (ICT) process. The partial configuration changes and ICT induce fluorescence quenching in 1F, 2F, 1CN and 2CN synergistically after an internal conversion (IC) from their respective S₃, S₅, S₄ and S₄ to S₁.