Jeremy K. Klosterman

Engineering discrete stacks of aromatic molecules

Intrigued by transannular interactions occurring in stacked aromatic molecules, chemists have long endeavored to engineer discrete stacks of specific lengths and orientation. The maturation of self-assembly methodologies has shifted the focus away from utilizing covalent scaffolds to harnessing non-covalent interactions such as ionic interactions, hydrogen bonds, metal-ligand interactions, and aromatic interactions. Aromatic molecules often assemble into ill-defined, infinite aggregates and thus multiple self-assembly techniques must be combined to achieve the desired stack size and conformations. This critical review briefly highlights covalent scaffolds of stack aromatics before focusing on modern self-assembly based strategies for engineering discrete stacks of aromatic molecules (149 references).

ON/OFF red emission from azaporphine in a coordination cage in water.

Red-emissive molecules have important applications as chemosensors, biological tags, and new light source materials but are severely limited by solubility and the tendency to form nonemissive aggregrates. Here we present an unusual example where a coordination cage sequesters and maintains the red-emissive properties of tetraazaporphine (TAP) in aqueous solution. Additionally, encapsulation within the highly cationic host lowers the pK(a) of the TAP interior protons, leading to the formation of an acid/base ON/OFF fluorescent switch.

Energy Transfer in a Mechanically Trapped Exciplex

Host-guest complexes involving M(6)L(4) coordination cages can display unusual photoreactivity, and enclathration of the very large fluorophore bisanthracene resulted in an emissive, mechanically trapped intramolecular exciplex. Mechanically linked intramolecular exciplexes are important for understanding the dependence of energy transfer on donor-acceptor distance, orientation, and electronic coupling but are relatively unexplored. Steady-state and picosecond time-resolved fluorescence measurements have revealed that selective excitation of the encapsulated guest fluorophore results in efficient energy transfer from the excited guest to an emissive host-guest exciplex state.

Solution-state photophysics of N-carbazolyl benzoate esters: dual emission and order of states in twisted push–pull chromophores

The stepwise photoinduced charge transfer in a series of N-carbazolyl benzoate ester push-pull chromophores has been studied in solution. Dual emission from the locally excited (LE, the lowest-energy singlet excited state of 1Lb nature localized on the carbazole donor) and the highly polarized, intramolecular charge-transfer states of (pre)-twisted type (TICT states) is observed in non-polar and polar solvents. Ultrafast transient spectroscopy reveals that the excitation into the 1Lb LE state is followed by rapid (∼ps) charge separation into an emissive TICT state. Excitation into the second singlet excited state localized on the carbazole (S2) with 1La nature results in sub-100 fs population of both 1Lb and TICT states.

Photophysics of manisyl-substituted 2-pyridin-2-yl-1,10-phenanthrolines. Dual emission dependent on structure and solvent

Systematic variation of the substitution pattern of 2-pyridin-2-yl-1,10-phenanthrolines with 4-methoxy-2,6-dimethylphenyl (manisyl) groups alpha, beta, and gamma to the nitrogen atoms results in a 3 x 3 array of pyridyl-phenanthrolines displaying low to high quantum yields (Phi(f) = 0.03-0.60). Photophysical studies elucidated a duality of emissive states: a weakly emissive, locally excited state, similar to the (1)pi,pi state of phenanthroline; and a strongly emissive, charge-transfer state, dependent on manisyl regiochemistry and solvent polarity. Ab initio calculations underscore the similarities in the electronic structures of phenanthroline and pyridyl-phenanthrolines rather than between terpyridine and pyridyl-phenanthroline.

Conformations of large macrocycles and ring-in-ring complexes

A kinetically directed, stepwise approach towards molecular Borromean links enabled the isolation and structural characterization of synthetic intermediates along the way. Here we report the synthesis and crystal structures of three flexible macrocyclic intermediates and a new ring-in-ring complex, anchored together through ruthenium(II) centers, which contains open terpyridine caps in the inner Ring II. Terpyridines circumvent the conformational cis/trans limitations of bipyridines and the new ring-in-ring complex forms tetrametallic complexes with Zn(II), Pt(II) and Ru(III) metal ions. Analysis of the four macrocyclic structures provides a good foundation for the conformational flexibility in these complexes and demonstrates the robust applicability of the terpyridine design elements towards the engineered synthesis of ring-in-ring topologies.

Towards the molecular Borromean link with three unequal rings: double-threaded ruthenium(II) ring-in-ring complexes†

This study describes synthetic efforts towards the molecular Borromean link consisting of three unequal rings. The design and strategy involve step by step construction of a ruthenium(II) templated ring-in-ring complex doubly threaded with endocyclic ligands ready for the macrocyclization. The control over the topology is achieved by using specially designed directional building blocks based on 2,2′:6′,2′′-terpyridine. Preliminary macrocyclization attempts utilizing the copper-mediated Eglington reaction provide the mass spectrometric evidence consistent with the ruthenium(II) complex of the molecular Borromean link.

Crystal structure of poly[[[μ4-5-(9H-carbazol-9-yl)isophthalato][μ3-5-(9H-carbazol-9-yl)isophthalato]bis­(di­methyl­formamide)(methanol)dizinc] di­methyl­formamide monosolvate]

The structure of the polymeric title compound, {[Zn2(C20H11NO4)2(C3H7NO)2(CH3OH)]·C3H7NO}n, comprises carbazolylisophthalate moieties connecting dimetallic tetracarboxylate zinc secondary building units (SBUs) parallel to [100] and [010], leading to a layer-like arrangement parallel to (001). Each SBU consists of two Zn atoms in slightly distorted tetrahedral and octahedral coordination environments [Zn⋯Zn = 3.5953 (6) Å]. Three carboxylate groups bridge the two Zn atoms in a μ2-O:O′ mode, whereas the fourth coordinates through a single carboxylate O atom (μ1-O). The O atoms of two dimethylformamide (DMF) and one methanol molecule complete the Zn coordination spheres. The methanol ligand interacts with the noncoordinating DMF molecule via an O—H⋯O hydrogen bond of medium strength. Carbazoles between the layers interdigitate through weak C—H⋯.π interactions to form a laminar solid stacked along [010]. Two kinds of C—H⋯π interactions are present, both with a distance of 2.64 Å, between the H atoms and the centroids, and a third C—H⋯π interaction, where the aromatic H atom is located above the carbazole N-atom lone pair (H⋯N = 2.89 Å). Several C—H⋯O interactions occur between the coordinating DMF molecule, the DMF solvent molecule, and ligating carboxylate O atoms.