FHBC, a Hexa‐\u3cem\u3eperi\u3c/em\u3e‐hexabenzocoronene–Fluorene Hybrid: A Platform for Highly Soluble, Easily Functionalizable HBCs with an Expanded Graphitic Core

Materials based upon hexa‐peri‐hexabenzocoronenes (HBCs) show significant promise in a variety of photovoltaic applications. There remains the need, however, for a soluble, versatile, HBC‐based platform, which can be tailored by incorporation of electroactive groups or groups that can prompt self‐assembly. The synthesis of a HBC–fluorene hybrid is presented that contains an expanded graphitic core that is highly soluble, resists aggregation, and can be readily functionalized at its vertices. This new HBC platform can be tailored to incorporate six electroactive groups at its vertices, as exemplified by a facile synthesis of a representative hexaaryl derivative of FHBC. Synthesis of new FHBC derivatives, containing electroactive functional groups that can allow controlled self‐assembly, may serve as potential long‐range charge‐transfer materials for photovoltaic applications

From Intramolecular (Circular) in an Isolated Molecule to Intermolecular Hole Delocalization in a Two‐Dimensional Solid‐State Assembly: The Case of Pillarene

To achieve long‐range charge transport/separation and, in turn, bolster the efficiency of modern photovoltaic devices, new molecular scaffolds are needed that can self‐assemble in two‐dimensional (2D) arrays while maintaining both intra‐ and intermolecular electronic coupling. In an isolated molecule of pillarene, a single hole delocalizes intramolecularly via hopping amongst the circularly arrayed hydroquinone ether rings. The crystallization of pillarene cation radical produces a 2D self‐assembly with three intermolecular dimeric (sandwich‐like) contacts. Surprisingly, each pillarene in the crystal lattice bears a fractional formal charge of +1.5. This unusual stoichiometry of oxidized pillarene in crystals arises from effective charge distribution within the 2D array via an interplay of intra‐ and intermolecular electronic couplings. This important finding is expected to help advance the rational design of efficient solid‐state materials for long‐range charge transfer

X-ray Structural Characterization of Charge Delocalization onto the Three Equivalent Benzenoid Rings in Hexamethoxytriptycene Cation Radical

Definitive X-ray crystallographic evidence is obtained for a single hole (or a polaron) to be uniformly distributed on the three equivalent 1,2-dimethoxybenzenoid (or veratrole) rings in the hexamethoxytriptycene cation radical. This conclusion is further supported by electrochemical analysis and by the observation of an intense near-IR transition in its electronic spectrum, as well as by comparison of the spectral and electrochemical characteristics with the model compounds containing one and two dimethoxybenzene rings

Charge-Transfer or Excimeric State? Exploring the Nature of The Excited State in Cofacially Arrayed Polyfluorene Derivatives

It is well known that upon electronic excitation various π-stacked dimers readily exhibit excimer formation, facilitated by a perfect sandwich-like arrangement between the chromophores. However, it is unclear whether such a dimer is also capable of electron transfer upon excitation, if a strong electron-donating group is covalently attached. In this work, we probe the nature of the excited state in a series of cofacially arrayed polyfluorene derivatives with electron-rich aromatic donor attached via a methylene linker. Our studies show that in all cases excimer formation is energetically favorable, and promotion of a charge-transfer state in such systems is possible but requires a free energy for electron transfer far exceeding 1 V. These findings shed light on important design principles for molecular scaffolds capable of stabilizing both excimeric and charge-transfer states upon their excitation

Charge Delocalization in Self-Assembled Mixed-Valence Aromatic Cation Radicals

The spontaneous assembly of aromatic cation radicals (D+•) with their neutral counterpart (D) affords dimer cation radicals (D2+•). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography

Interplay between Entropy and Enthalpy in (Intramolecular) Cyclophane-Like Folding versus (Intermolecular) Dimerization of Diarylalkane Cation Radicals

Diarylpropane cation radicals are known to exist as folded cyclophane-like structures, as evidenced by the appearance of intervalence transitions in their optical spectra. Despite the expected enthalpic stabilization of cyclophane-like cation radicals of diarylpropanes by ∼350 mV, we demonstrate that only partial folding (∼50%) occurs due to the entropic penalty associated with restriction of conformational flexibility via the freezing of multiple free C–C bond rotors together with the strain in the folded cyclophane-like structure. This important demonstration of the interplay between enthalpy and entropy is deduced via a systematic study of various diarylalkane cation radicals with two- to five-methylene spacers using electrochemistry, optical spectroscopy, X-ray crystallography, and DFT calculations. We also show that diarylalkane cation radicals with greater than three methylene spacers cannot fold into cyclophane-like structures, as the entropic penalty for freezing increasing number of C–C bond rotors and associated strain in the folded cyclophane-like structures far outweighs the enthalpic gain of ∼350 mV. We also designed and synthesized a derivative of diarylpropane with a bulky alkyl group at the second carbon of three-methylene spacer, which undergoes quantitative folding due to a reduction in the entropic penalty by hindering the C–C bond rotors. Unlike diarylpropane cation radicals, diarylethane cation radicals undergo ready intermolecular self-association due to the favorable enthalpic gain (∼700 mV) from two pairs of sandwiched aryl groups from two molecules of diarylethane cation radical. This demonstration of the role of enthalpy and entropy in intramolecular folding of diarylpropane cation radicals will open new avenues for designing next-generation cofacially arrayed structures for modern photovoltaic applications

Design of Tunable Multicomponent Polymers as Modular Vehicles To Solubilize Highly Lipophilic Drugs

Synthetic and natural polymers hold tremendous potential to improve therapeutic potency, bioavailability, stability, and safety through aiding the solubility of lipophilic drug candidates that may otherwise be clinically inaccessible. For the leading pharmaceutical delivery method (oral administration), one such approach involves maintaining drugs in an amorphous, nonequilibrium state using spray-dried dispersions (SDDs). However, few well-understood vehicles exist, and available formulations employ Edisonian approaches without regard to examining chemical, thermodynamic, and kinetic phenomena. Herein, we present a rational approach to study polymer–drug interactions with a multicomponent polymer platform, inspired by hydroxypropyl methylcellulose acetate succinate (an excipient increasingly utilized as a delivery vehicle). The controlled syntheses of these modular analogs were strategically defined with (i) hydroxypropyl, (ii) methoxy, (iii) acetyl, (iv) succinoyl, and (v) glucose groups to tune the amphiphilicity balance (i–v), ionization near gastrointestinal pH levels (iv), hydrogen bonding (i, iii, iv, v), and glass transition temperature (v). We examined how polymer architecture produces amorphous SDDs with a highly hydrophobic drug model (probucol, log <i>P</i> = 8.9). Dissolution experiments revealed dramatic differences in bioavailability as a function of polymeric chemical specificity. We identify chemically driven interactions as crucial ingredients for facilitating amorphous phase behavior and supersaturation maintenance. In particular, increasing the fraction of ionizable carboxylic acid moieties and selective deprotection of glucose acetates into hydroxyls established stabilizing ionic character and polar interactions. Our results show the utility of rationally designed polymer platforms, which we can precisely tune via monomer selection and functionality, as direct handles for elucidating important structure–property relationships in oral delivery

Ask Not How Many, But Where They Are: Substituents Control Energetic Ordering of Frontier Orbitals/Electronic Structures in Isomeric Methoxy-Substituted Dibenzochrysenes

Redox properties of polycyclic aromatic hydrocarbons (PAHs) can be modulated by substitution with electron-rich groups. Here we show, using the example of dibenzo­[g,p]­chrysene (DBC), that substitution <i>position</i> (i.e., <i>meta</i> vs <i>para</i>) alters the energetic ordering of frontier molecular orbitals (FMOs), leading to cation radicals with altered electronic structures and thereby redox/optical properties. In particular, incorporation of four methoxy groups in parent DBC at <i>meta</i> positions similarly impacts the energies of phenanthrene-like HOMO and biphenyl-like HOMO-1, while their incorporation at <i>para</i> position swaps energetic ordering of HOMO and HOMO-1. We demonstrate that a straightforward analysis of FMOs provides valuable insight toward the rational design of novel PAHs with tailored redox properties

Ask Not How Many, But Where They Are: Substituents Control Energetic Ordering of Frontier Orbitals/Electronic Structures in Isomeric Methoxy-Substituted Dibenzochrysenes

Redox properties of polycyclic aromatic hydrocarbons (PAHs) can be modulated by substitution with electron-rich groups. Here we show, using the example of dibenzo­[g,p]­chrysene (DBC), that substitution <i>position</i> (i.e., <i>meta</i> vs <i>para</i>) alters the energetic ordering of frontier molecular orbitals (FMOs), leading to cation radicals with altered electronic structures and thereby redox/optical properties. In particular, incorporation of four methoxy groups in parent DBC at <i>meta</i> positions similarly impacts the energies of phenanthrene-like HOMO and biphenyl-like HOMO-1, while their incorporation at <i>para</i> position swaps energetic ordering of HOMO and HOMO-1. We demonstrate that a straightforward analysis of FMOs provides valuable insight toward the rational design of novel PAHs with tailored redox properties