The controlled assembly of randomly dispersed colloidal particles can provide access to materials with advanced optical and electronic properties while providing fundamental insights into self-assembly processes in nature and nanotechnology. Typically, self-assembled nanoparticles are prepared by exploiting electrostatic interactions, lithographic techniques, and covalently linked molecular scaffolds. This results in static morphologies that cannot be disassembled easily. On the other hand, having access to systems that can be assembled or disassembled in a controlled manner could allow for in-depth understanding of the nanoparticles as well as rational control over the morphology and fundamental properties of the resulting constructs. If the changes in aggregation are induced by a specific external chemical stimulus, it could also permit the development of new chemosensors. Here we demonstrate the reversible assembly and disassembly of gold nanoparticles achieved by modulating the noncovalent interactions between surface-bound calix[4]pyrroles and added bis-imidazolium cations. We also demonstrate the use of these nanoparticles in the selective sensing of anions.

The subporphyrazine (SubPz) dimer diboron(III) [μ-1′,4′-benzenediolato][bis-(1,2,6,7,11,12-hexapropylsubporphyrazinato)] (2) and trimer triboron(III)[μ-1′,3′,5′-benzenetriolato][tris-(1,2,6,7,11,12-hexapropylsubporphyrazinato)] (4), consisting of SubPz units assembled through their axial positions by hydroquinone- and phloroglucinol-linkers, respectively, have been prepared. Selective ruthenium-π-coordination to the linking aromatic rings afforded SubPz arrays 8 and 9, respectively. These latter systems displayed different degrees of electronic communication between the macrocycles relative to 2 and 4 as inferred from analyses of the ground- and excited-state features. For instance, as revealed by time-resolved pump probe transient absorption spectroscopy, the excited singlet states of dimer 2 and trimer 4 undergo rather rapid deactivation. In contrast, the presence of a ruthenoarene linker, as in 8 and 9, serves to increase the SubPz singlet excited state lifetimes; these now reach values similar to those seen in the case of monomeric SubPz compounds that either do or do not contain ruthenoarene subunits (i.e., 7 and 6, respectively). These findings are ascribed to the redox active nature of the hydroquinone and phloroglucinol linkers in 2 and 4 and a change in the electronics that results from organometallic functionalization.

A new approach to the construction of self-assembled structures is reported that is based on ion-pair recognition. Towards this end, the calix[4]pyrrole naphthocrown-4 hybrid structures 2 and 3 were prepared. These multitopic receptors contain recognition sites for both anions and cations. On the basis of solution-phase ¹H NMR spectroscopic analysis and solid-state single-crystal X-ray diffraction structural studies, it was established that receptors 2 and 3 are able to bind specific ion pairs with high selectivity via different binding modes. In the case of CsF and CsCl, the ion-pair complexes formed from receptors 2 and 3 were found to self-assemble to produce either linear supramolecular polymeric crystalline solids or nanotube-like cyclic hexamers depending on the specific choice of ion pairs and crystallization solvents. Proton NMR studies provided evidence for solution-phase self-association in organic media.

A new theranostic strategy is described. It is based on the use of an “all in one” prodrug, namely the biotinylated piperazine-rhodol conjugate 4 a. This conjugate, which incorporates the anticancer drug SN-38, undergoes self-immolative cleavage when exposed to biological thiols. This leads to the tumor-targeted release of the active SN-38 payload along with fluorophore 1 a. This release is made selective as the result of the biotin functionality. Fluorophore 1 a is 32-fold more fluorescent than prodrug 4 a. It permits the delivery and release of the SN-38 payload to be monitored easily in vitro and in vivo, as inferred from cell studies and ex vivo analyses of mice xenografts derived from HeLa cells, respectively. Prodrug 4 a also displays anticancer activity in the HeLa cell murine xenograft tumor model. On the basis of these findings we suggest that the present strategy, which combines within a single agent the key functions of targeting, release, imaging, and treatment, may have a role to play in cancer diagnosis and therapy.

A new theranostic strategy is described. It is based on the use of an “all in one” prodrug, namely the biotinylated piperazine-rhodol conjugate 4 a. This conjugate, which incorporates the anticancer drug SN-38, undergoes self-immolative cleavage when exposed to biological thiols. This leads to the tumor-targeted release of the active SN-38 payload along with fluorophore 1 a. This release is made selective as the result of the biotin functionality. Fluorophore 1 a is 32-fold more fluorescent than prodrug 4 a. It permits the delivery and release of the SN-38 payload to be monitored easily in vitro and in vivo, as inferred from cell studies and ex vivo analyses of mice xenografts derived from HeLa cells, respectively. Prodrug 4 a also displays anticancer activity in the HeLa cell murine xenograft tumor model. On the basis of these findings we suggest that the present strategy, which combines within a single agent the key functions of targeting, release, imaging, and treatment, may have a role to play in cancer diagnosis and therapy.

A series of tetrathiafulvalene (TTF)-annulated porphyrins, and their corresponding Zn^II complexes, have been synthesized. Detailed electrochemical, photophysical, and theoretical studies reveal the effects of intramolecular charge-transfer transitions that originate from the TTF fragments to the macrocyclic core. The incremental synthetic addition of TTF moieties to the porphyrin core makes the species more susceptible to these charge-transfer (CT) effects as evidenced by spectroscopic studies. On the other hand, regular positive shifts in the reduction signals are seen in the square-wave voltammograms as the number of TTF subunits increases. Structural studies that involve the tetrakis-substituted TTF–porphyrin (both free-base and Zn^II complex) reveal only modest deviations from planarity. The effect of TTF substitution is thus ascribed to electronic overlap between annulated TTF subunits rather than steric effects. The directly linked thiafulvalene subunits function as both π acceptors as well as σ donors. Whereas σ donation accounts for the substituent-dependent charge-transfer transitions, it is the π-acceptor nature of the appended tetrathiafulvalene groups that dominates the redox chemistry. Interactions between the subunits are also reflected in the square-wave voltammograms. In the case of the free-base derivatives that bear multiple TTF subunits, the neighboring TTF units, as well as the TTF^⋅+ generated through one-electron oxidation, can interact with each other; this gives rise to multiple signals in the square-wave voltammograms. On the other hand, after metalation, the electronic communication between the separate TTF moieties becomes restricted and they act as separate redox centers under conditions of oxidation. Thus only two signals, which correspond to TTF^.+ and TTF²⁺, are observed. The reduction potentials are also seen to shift towards more negative values after metalation, a finding that is considered to reflect an increased HOMO–LUMO gap. To probe the excited-state dynamics and internal CT character, transient absorption spectral studies were performed. These analyses revealed that all the TTF–porphyrins of this study display relatively short excited-state lifetimes, which range from 1 to 20 ps. This reflects a very fast decay to the ground state and is consistent with the proposed intramolecular charge-transfer effects inferred from the ground-state studies. Complementary DFT calculations provide a mechanistic rationale for the electron flow within the TTF–porphyrins and support the proposed intramolecular charge-transfer interactions and π-acceptor effects.

The synthesis, characterization, photophysical properties, and theoretical analysis of a series of tetraaza porphyrin analogues (HPn: n=1–4) containing a dipyrrin subunit and an embedded 1,10-phenanthroline subunit are described. The meso-phenyl-substituted derivative (HP1) interacts with a Mg²⁺ salt (e.g., MgCl₂, MgBr₂, MgI₂, Mg(ClO₄)₂, and Mg(OAc)₂) in MeCN solution, thereby giving rise to a cation-dependent red-shift in both the absorbance- and emission maxima. In this system, as well as in the other HPn porphyrin analogues used in this study, the four nitrogen atoms of the ligand interact with the bound magnesium cation to form Mg²⁺–dipyrrin–phenanthroline complexes of the general structure MgXPn (X=counteranion). Both single-crystal X-ray diffraction analysis of the corresponding zinc-chloride derivative (ZnClP1) and fluorescence spectroscopy of the Mg-adducts that are formed from various metal salts provide support for the conclusion that, in complexes such as MgClP1, a distorted square-pyramidal geometry persists about the metal cation wherein a chloride anion acts as an axial counteranion. Several analogues (HPn) that contain electron-donating and/or electron-withdrawing dipyrrin moieties were prepared in an effort to understand the structure–property relationships and the photophysical attributes of these Mg–dipyrrin complexes. Analysis of various MgXPn (X=anion) systems revealed significant substitution effects on their chemical, electrochemical, and photophysical properties, as well as on the Mg²⁺-cation affinities. The fluorescence properties of MgClPn reflected the effect of donor-excited photoinduced electron transfer (d-PET) processes from the dipyrrin subunit (as a donor site) to the 1,10-phenanthroline acceptor subunit. The proposed d-PET process was analyzed by electron paramagnetic resonance (EPR) spectroscopy and by femtosecond transient absorption (TA) spectroscopy, as well as by theoretical DFT calculations. Taken together, these studies provide support for the suggestion that a radical species is produced as the result of an intramolecular charge-transfer process, following photoexcitation. These photophysical effects, combined with a mixed dipyrrin–phenanthroline structure that is capable of effective Mg²⁺-cation complexation, lead us to suggest that porphyrin-inspired systems, such as HPn, have a role to play as magnesium-cation sensors.

Reaction of bromanil with N,N′-dimesitylformamidine followed by deprotonation with NaN(SiMe₃)₂ afforded 1,1′,3,3′-tetramesitylquinobis(imidazolylidene) (1), a bis(N-heterocyclic carbene) (NHC) with two NHC moieties connected by a redox active p-quinone residue, in 72 % yield of isolated compound. Bimetallic complexes of 1 were prepared by coupling to FcN₃ (2) or FcNCS (3; Fc=ferrocenyl) or coordination to [M(cod)Cl] (4 a or 4 b, where M=Rh or Ir, respectively; cod=1,5-cyclooctadiene). Treatment of 4 a and 4 b with excess CO(g) afforded the corresponding [M(CO)₂Cl] complexes 5 a and 5 b, respectively. Analysis of 2–5 by NMR spectroscopy and X-ray diffraction indicated that the electron-deficient quinone did not significantly affect the inherent spectral properties or coordination chemistry of the flanking imidazolylidene units, as compared to analogous NHCs. Infrared spectroscopy and cyclic voltammetry revealed that decreasing the electron density at ML_n afforded an increase in the stretching energy and a decrease in the reduction potential of the quinone, indicative of metal–quinone electronic interaction. Differential pulse voltammetry and chronoamperometry of the metal-centered oxidations in 2–4 revealed two single, one-electron peaks. Thus, the metal atoms bound to 1 are oxidized at indistinguishable potentials and do not appear electronically coupled. However, the metal–quinone interaction was used to increase the electron density at coordinated metal atoms. Infrared spectroelectrochemistry revealed that the average ν_CO values for 5 a and 5 b decreased by 14 and 15 cm⁻¹, respectively, upon reduction of the quinone embedded within 1. These shifts correspond to 10 and 12 cm⁻¹ decreases in the Tolman electronic parameter of this ditopic ligand.