BOIMPY: Fluorescent Boron Complexes with Tunable and Environment-Responsive Light-Emitting Properties.

A series of air-stable boron complexes 1-5 were prepared by using N-aryl iminopyrrolide ligands. Designed as minimalist structural mimics of the privileged BODIPY motif, these new BOIMPY (BOron complexes of IMinoPYrrolide ligands) fluorophores feature low molecular symmetry that promotes emission from CT-type excited states with large Stokes shifts and little self-quenching. Through comparative studies on the homologous set of compounds 1-4, we have confirmed that a delicate interplay between conformational twisting and donor-acceptor interaction dictates the mechanism of de-excitation, which responds sensitively to solvent polarity as well as protonation states. Over a wide visible spectral range, the structure-dependent light-emitting properties of BOIMPY molecules are well manifested, even in the solid-state. In order to exploit the environment-sensitive nature of CT-type emission, the BOIMPY motif was elaborated further into a bioprobe molecule 5. Live-cell fluorescence imaging studies have established that 5 is localized exclusively at lipid droplets to produce well-resolved staining patterns without affecting cell viability. These findings promise future elaboration of BOIMPY-based functional molecules for applications in biological imaging, chemical sensing, and molecular switching.

Interdigitated hydrogen bonds: electrophile activation for covalent capture and fluorescence turn-on detection of cyanide.

Hydrogen-bonding promoted covalent modifications are finding useful applications in small-molecule chemical synthesis and detection. We have designed a xanthene-based fluorescent probe 1, in which tightly held acylguanidine and aldehyde groups engage in multiple intramolecular hydrogen bonds within the concave side of the molecule. Such an interdigitated hydrogen bond donor-acceptor (HBD-HBA) array imposes significant energy barriers (ΔG(‡) = 10-16 kcal mol(-1)) for internal bond rotations to assist structural preorganization and effectively polarizes the electrophilic carbonyl group toward a nucleophilic attack by CN(-) in aqueous environment. This covalent modification redirects the de-excitation pathways of the cyanohydrin adduct 2 to elicit a large (>7-fold) enhancement in the fluorescence intensity at λmax = 440 nm. A remarkably faster (> 60-fold) response kinetics of 1, relative to its N-substituted (and therefore "loosely held") analogue 9, provided compelling experimental evidence for the functional role of HBD-HBA interactions in the "remote" control of chemical reactivity, the electronic and steric origins of which were investigated by DFT computational and X-ray crystallographic studies.

Three-stage binary switching of azoaromatic polybase.

An OFF-ON-OFF-type three-stage binary switching was realized with an azoaniline-based polybase 1. The optical properties of 1 and [1·2H](2+) are essentially indistinguishable to the naked eye but distinctively different from those of [1·H](+) to produce an unusual bell-shaped response as a function of protonation state; the underlying molecular mechanism was unraveled by a combination of experimental and DFT computational studies.

Reactivity-based detection of copper(II) ion in water: oxidative cyclization of azoaromatics as fluorescence turn-on signaling mechanism.

An oxidative cyclization reaction transforms nonemissive azoanilines into highly fluorescent benzotriazoles. We have found that introduction of multiple electron-donating amino groups onto a simple o-(phenylazo)aniline platform dramatically accelerates its conversion to the emissive polycyclic product. Notably, this chemistry can be effected by µM-level concentrations of copper(II) ion in water (pH = 6-8) at room temperature to elicit >80-fold enhancement in the green emission at λ(em) = 530 nm. Comparative kinetic and electrochemical studies on a series of structural analogues have established that the accelerated reaction rates correlate directly with a systematic cathodic shift in the oxidation onset potential of the azo precursors. In addition, single-crystal X-ray crystallographic analysis on the most reactive derivative revealed the presence of a five-membered ring intramolecular hydrogen-bonding network. An enhanced contribution of the quinoid-type resonance in such conformation apparently facilitates the mechanistically required proton transfer step, which, in conjunction with electron transfer at lower oxidation potential, contributes to a rapid cyclization reaction triggered by copper(II) ion in water.

Fluorescence switching of imidazo[1,5-a]pyridinium ions: pH-sensors with dual emission pathways.

Imidazo[1,5-a]pyridinium ions are identified as highly emissive and water-soluble fluorophores accessed by an efficient three-component coupling reaction. Synthetic modifications of groups conjugated to the polyheterocyclic core are shown to profoundly impact the emission properties of these molecules. Notably, two structural isomers of functionalized imidazo[1,5-a]pyridinium ions were found to exhibit distinct de-excitation pathways, which are responsible for either a fluorescence turn-on or ratiometric response to pH change.

The art of stacking: structural folding and self-assembly of branched π-conjugation assisted by O-H···O and C-H···F hydrogen bonds.

An intimate interplay of O-H···O/C-H···F hydrogen bonds and π-π stacking interactions allows a phenyleneethynylene-based dendritic molecule to fold and self-assemble into two distinctively different molecular crystals as pseudopolymorphs.

Accurate interaction energies at density functional theory level by means of an efficient dispersion correction.

This paper presents an approach for obtaining accurate interaction energies at the density functional theory level for systems where dispersion interactions are important. This approach combines Becke and Johnson's [J. Chem. Phys. 127, 154108 (2007)] method for the evaluation of dispersion energy corrections and a Hirshfeld method for partitioning of molecular polarizability tensors into atomic contributions. Due to the availability of atomic polarizability tensors, the method is extended to incorporate anisotropic contributions, which prove to be important for complexes of lower symmetry. The method is validated for a set of 18 complexes, for which interaction energies were obtained with the B3LYP, PBE, and TPSS functionals combined with the aug-cc-pVTZ basis set and compared with the values obtained at the CCSD(T) level extrapolated to a complete basis set limit. It is shown that very good quality interaction energies can be obtained by the proposed method for each of the examined functionals, the overall performance of the TPSS functional being the best, which with a slope of 1.00 in the linear regression equation and a constant term of only 0.1 kcal/mol allows to obtain accurate interaction energies without any need of a damping function for complexes close to their exact equilibrium geometry.

The use of atomic intrinsic polarizabilities in the evaluation of the dispersion energy.

The recent approach presented by Becke and Johnson [J. Chem. Phys. 122, 154104 (2005); 123, 024101 (2005); 123, 154101 (2005); 124, 174104 (2006); 124, 014104 (2006)] for the evaluation of dispersion interactions based on the properties of the exchange-hole dipole moment is combined with a Hirshfeld-type partitioning for the molecular polarizabilities into atomic contributions, recently presented by some of the present authors [A. Krishtal et al., J. Chem. Phys. 125, 034312 (2006)]. The results on a series of nine dimers, involving neon, methane, ethene, acetylene, benzene, and CO(2), taken at their equilibrium geometry, indicate that when the C(6), C(8), and C(10) terms are taken into account, the resulting dispersion energies can be obtained deviating 3% or 8% from high level literature data [E. R. Johnson and A. D. Becke, J. Chem. Phys. 124, 174104 (2006)], without the use of a damping function, the only outlier being the parallel face-to-face benzene dimer.