Selective Complexation of Cyanide and Fluoride Ions with Ammonium Boranes: A Theoretical Study on Sensing Mechanism Involving Intramolecular Charge Transfer and Configurational Changes

The anion binding selectivity and the recognition mechanism of two isomeric boranes, namely, 4-[bis­(2,4,6-trimethylphenyl)­boranyl]-<i>N</i>,<i>N</i>,<i>N</i>-trimethylaniline ([<i>p</i>-(Mes₂B)­C₆H₄(NMe₃)]⁺, <b>1</b>, where “Mes” represents mesitylene and “Me” represents methyl) and 2-[bis­(2,4,6-trimethylphenyl)­boranyl]-<i>N</i>,<i>N</i>,<i>N</i>-trimethylaniline ([<i>o</i>-(Mes₂B)­C₆H₄(NMe₃)]⁺, <b>2</b>) has been investigated using density functional theory (DFT) and time dependent-density functional theory (TD-DFT) methods. Natural population analysis indicates that the central boron atoms in <b>1</b> and <b>2</b> are the most active centers for nucleophilic addition of anions. The negative magnitude of free energy changes (Δ<i>G</i>) reveals that out of CN^–, F^–, Cl^–, Br^–, NO₃^–, and HSO₄^– only the binding of CN^– and F^– with <b>1</b> and <b>2</b> is thermodynamically feasible and spontaneous. In addition, the calculated binding energies reveal that the CN^– is showing lesser binding affinity than F^– both with <b>1</b> and <b>2</b>, while other ions, viz. NO₃^–, HSO₄^–, Br^–, and Cl^–, either do not bind at all or show very insignificant binding energy. The first excited states (S₁) of <b>1</b> and <b>2</b> are shown to be the local excited states with π → σ* transition by frontier molecular orbital analysis, whereas fourth excited states (S₄) of 4-[bis­(2,4,6-trimethylphenyl)­boranyl]-<i>N</i>,<i>N</i>,<i>N</i>-trimethylaniline cyanide ([<i>p</i>-(Mes₂B)­C₆H₄(NMe₃)] CN, <b>1CN</b>, the cyano form of <b>1</b>) and 4-[bis­(2,4,6-trimethylphenyl)­boranyl]-<i>N</i>,<i>N</i>,<i>N</i>-trimethylaniline fluoride ([<i>p</i>-(Mes₂B)­C₆H₄(NMe₃)] F, <b>1F</b>, the fluoro form of <b>1</b>) and fifth excited state (S₅) of 2-[bis­(2,4,6-trimethylphenyl)­boranyl]-<i>N</i>,<i>N</i>,<i>N</i>-trimethylaniline fluoride ([<i>o</i>-(Mes₂B)­C₆H₄(NMe₃)] F, <b>2F</b>, the fluoro form of <b>2</b>) are charge separation states that are found to be responsible for the intramolecular charge transfer (ICT) process. The synergistic effect of ICT and partial configuration changes induce fluorescence quenching in <b>1CN</b>, <b>1F</b>, and <b>2F</b> after a significant internal conversion (IC) from S₄ and S₅ to S<sub>1.</sub

Donor-acceptor type A2B2 porphyrins: synthesis, energy transfer, computational and electrochemical studies

A series of donor-acceptor type trans-A2B2 porphyrins and their Zn(II) and Pd(II) complexes 5-13 have been synthesized and characterized by various spectroscopic techniques. The effect of the donor moieties (e.g., N-butylcarbazole, N-butylphenothiazine, and triphenylamine) on the spectroscopic properties of the porphyrins has been studied. The structural changes indeed affected the optical and electrochemical properties of these porphyrins. Higher energy shifts of the Soret bands were observed for porphyrins upon varying the donor moieties. The electrochemical studies for all derivatives indicated increased interactions between the donor groups and porphyrin core, which in turn reflected in the anodic shifts in their reduction potentials. Both steady-state and time-resolved fluorescence studies revealed effective energy transfer (EET; up to 87%) from donor groups to porphyrin core in the porphyrins, 5-10. The palladium(II) porphyrin complexes, 11-13, showed characteristic phosphorescence in the near IR region. Density functional theory (DFT) studies support the presence of donor-acceptor interaction between the porphyrin core and the meso-substituents in the dyads. Density functional theory (DFT) and Time dependent-density functional theory (TD-DFT) studies showed that in 5, 8 and 11, the transitions are of π → π* type; where as in other molecules viz 6, 7, 9, 10, 12 and 13 intramolecular charge transfer is (ICT) involved in all the respective highest intensity absorption transitions.by Iti Gupta, Sudipta Das, Haamid R. Bhat, Naresh Balsukuri, Prakash C. Jha, Yutaka Hisamune, Masatoshi Ishida, Hiroyuki Furuta and Shigeki Mor