Chemically Triggered Release of Singlet Oxygen from Bisphenalenyl Endoperoxides with a Brønsted Acid.

Aromatic endoperoxides have emerged as intriguing stimulus-responsive materials for molecular oxygen (O2) storage and delivery but are currently limited in their application because they require heat to trigger O2 release. Here we present the first example of acid-triggered singlet oxygen (1O2) release that does not require external heating by treating bisphenalenyl endoperoxides (EPOs) with trifluoroacetic acid. Mechanistic studies reveal that diprotonation of EPOs leads to a >10-fold increase in cycloreversion rates by lowering the energy of activation (ΔEa) by as much as 71.1 kJ mol-1. Remarkably, acid-catalyzed 1O2 release is even demonstrated at room temperature. Chemical trapping experiments indicate that reactive 1O2 is present during acid-triggered release, which is promising for the development of these molecular materials for metal-free, on-demand 1O2 delivery.

Self-Sensitized and Reversible O2 Reactivity with Bisphenalenyls for Simple, Tunable, and Multicycle Colorimetric Oxygen-Sensing Films.

Monitoring the levels of molecular oxygen (O2) is critical for numerous applications, but there is still a long-standing challenge to develop robust and cost-effective colorimetric sensors that enable detection by changes in color. Current technologies employ chromophores that require additional additives, which inherently increase the cost and complexity. Here, we report that bisphenalenyls (PQPLs) function as the single active component for colorimetric O2 sensing through their quantitative conversion into aromatic endoperoxides (EPOs). PQPLs display self-sensitizing reactivity: they are capable of generating singlet oxygen and binding it without the need for external photosensitizers. The rates of PQPL photooxygenation depend on the electron-donating ability of substituents, which highlights a simple strategy for tuning O2 sensitivity. EPOs are stable under ambient conditions but can be thermally stimulated to convert back to PQPLs and concomitantly release O2. Polymer-supported (PTMSP) films of PQPLs (2 wt %) reproduce these reactivity trends with a rapid red-to-colorless transition that is visible to the naked eye within 1 h of exposure and show a very low limit of detection (<5 ppm O2). Films are chemically and thermally robust and maintain up to >99% of their original colorimetric response when reused and subjected to multiple cycles of photooxygenation and O2 release. The simplicity and solution processability of these materials highlight their potential as "intelligent" inks for printable colorimetric sensors.

Spin multiplicity effects in doublet versus singlet emission: the photophysical consequences of a single electron.

Ambient-stable fluorescent radicals have recently emerged as promising luminescent materials; however, tailoring their properties has been difficult due to the limited photophysical understanding of open-shell organic systems. Here we report the experimental and computational analysis of a redox pair of π-conjugated fluorescent molecules that differ by one electron. A π-dication (DC) and π-radical cation (RC) demonstrate different absorption spectra, but similar red emission (λ emiss,max = ∼630 nm), excitation maxima (λ exc,max = ∼530 nm), fluorescence lifetimes (1-10 ns), and even excited-state (non-emissive) lifetimes when measured by transient absorption spectroscopy. Despite their experimental similarities, time-dependent density functional theory (TDDFT) studies reveal that DC and RC emission mechanisms are distinct and rely on different electronic transitions. Excited-state reorganization occurs by hole relaxation in singlet DC, while doublet RC undergoes a Jahn-Teller distortion by bending its π-backbone in order to facilitate spin-pairing between singly occupied molecular orbitals. This relationship between the excited-state dynamics of RC and its π-backbone geometry illustrates a potential strategy for developing π-conjugated radicals with new emission properties. Additionally, by comparing TDDFT and CIS (configuration interaction singles) excitations, we show that unrestricted TDDFT accurately reproduces experimental absorption spectra and provides an opportunity to examine the relaxed excited-state properties of large open-shell molecules like RC.

Open-Shell Effects on Optoelectronic Properties: Antiambipolar Charge Transport and Anti-Kasha Doublet Emission from a N-Substituted Bisphenalenyl.

By stabilizing unpaired spin in the ground state, open-shell π-conjugated molecules can achieve optoelectronic properties that are inaccessible to closed-shell compounds. Here, we report the synthesis and characterization of a N-substituted, bisphenalenyl π-radical cation [3(OTf)] that shows antiambipolar charge transport and fluorescence via anti-Kasha doublet emission. 3(OTf) produces a red emission (634-659 nm) by radiative decay from ß-LUMO to ß-SOMO, based on density functional theory and configuration interaction singles calculations, and records one of the highest photostabilities (t1/2 = 9.5 × 104 s) among fluorescent radicals. Characterization of 3(OTf)-based field-effect transistors reveals that the observed electrical conductivity (σRT ≤ 1.3 × 10-2 S/cm) is enabled by hole and electron transport (µe/µh ≤ 5.70 × 10-5 cm2 V-1 s-1) that is most efficient in the absence of gating, which represents the first example of antiambipolarity in a molecular material.

A Concise Synthetic Strategy for Accessing Ambient Stable Bisphenalenyls toward Achieving Electroactive Open-Shell π-Conjugated Materials.

Open-shell, π-conjugated molecules represent exciting next-generation materials due to their unique optoelectronic and magnetic properties and their potential to exploit unpaired spin densities to engineer exceptionally close π-π interactions. However, prior syntheses of ambient stable, open-shell molecules required lengthy routes and displayed intermolecular spin-spin coupling with limited dimensionality. Here we report a general fragment-coupling strategy with phenalenone that enables the rapid construction of both biradicaloid (Ph2- s-IDPL, 1) and radical [10(OTf)] bisphenalenyls in ≤7 steps from commercial starting materials. Significantly, we have discovered an electronically stabilized π-radical cation [10(OTf)] that shows multiple intermolecular closer-than-vdW contacts (<3.4 Å) in its X-ray crystal structure. DFT simulations reveal that each of these close π-π interactions allows for intermolecular spin-spin coupling to occur and suggests that 10(OTf) achieves electrostatically enhanced intermolecular covalent-bonding interactions in two dimensions. Single crystal devices were fabricated from 10(OTf) and demonstrate average electrical conductivities of 1.31 × 10-2 S/cm. Overall, these studies highlight the practical synthesis and device application of a new π-conjugated material, based on a design principle that promises to facilitate spin and charge transport.

Enhanced solid-state order and field-effect hole mobility through control of nanoscale polymer aggregation.

Efficient charge carrier transport in organic field-effect transistors (OFETs) often requires thin films that display long-range order and close π-π packing that is oriented in-plane with the substrate. Although some polymers have achieved high field-effect mobility with such solid-state properties, there are currently few general strategies for controlling the orientation of π-stacking within polymer films. In order to probe structural effects on polymer-packing alignment, furan-containing diketopyrrolopyrrole (DPP) polymers with similar optoelectronic properties were synthesized with either linear hexadecyl or branched 2-butyloctyl side chains. Differences in polymer solubility were observed and attributed to variation in side-chain shape and polymer backbone curvature. Averaged field-effect hole mobilities of the polymers range from 0.19 to 1.82 cm(2)/V·s, where PDPP3F-C16 is the least soluble polymer and provides the highest maximum mobility of 2.25 cm(2)/V·s. Analysis of the films by AFM and GIXD reveal that less soluble polymers with linear side chains exhibit larger crystalline domains, pack considerably more closely, and align with a greater preference for in-plane π-π packing. Characterization of the polymer solutions prior to spin-coating shows a correlation between early onset nanoscale aggregation and the formation of films with highly oriented in-plane π-stacking. This effect is further observed when nonsolvent is added to PDPP3F-BO solutions to induce aggregation, which results in films with increased nanostructural order, in-plane π-π orientation, and field-effect hole mobilities. Since nearly all π-conjugated materials may be coaxed to aggregate, this strategy for enhancing solid-state properties and OFET performance has applicability to a wide variety of organic electronic materials.

Efficient small molecule bulk heterojunction solar cells with high fill factors via pyrene-directed molecular self-assembly.

Efficient organic photovoltaic (OPV) materials are constructed by attaching completely planar, symmetric end-groups to donor-acceptor electroactive small molecules. Appending C2-pyrene as the small molecule end-group to a diketopyrrolopyrrole core leads to materials with a tight, aligned crystal packing and favorable morphology dictated by π-π interactions, resulting in high power conversion efficiencies and high fill factors. The use of end-groups to direct molecular self-assembly is an effective strategy for designing high-performance small molecule OPV devices.

Combined effects on selectivity in Fe-catalyzed methylene oxidation.

Methylene C-H bonds are among the most difficult chemical bonds to selectively functionalize because of their abundance in organic structures and inertness to most chemical reagents. Their selective oxidations in biosynthetic pathways underscore the power of such reactions for streamlining the synthesis of molecules with complex oxygenation patterns. We report that an iron catalyst can achieve methylene C-H bond oxidations in diverse natural-product settings with predictable and high chemo-, site-, and even diastereoselectivities. Electronic, steric, and stereoelectronic factors, which individually promote selectivity with this catalyst, are demonstrated to be powerful control elements when operating in combination in complex molecules. This small-molecule catalyst displays site selectivities complementary to those attained through enzymatic catalysis.

A predictably selective aliphatic C-H oxidation reaction for complex molecule synthesis.

Realizing the extraordinary potential of unactivated sp3 C-H bond oxidation in organic synthesis requires the discovery of catalysts that are both highly reactive and predictably selective. We report an iron (Fe)-based small molecule catalyst that uses hydrogen peroxide (H2O2) to oxidize a broad range of substrates. Predictable selectivity is achieved solely on the basis of the electronic and steric properties of the C-H bonds, without the need for directing groups. Additionally, carboxylate directing groups may be used to furnish five-membered ring lactone products. We demonstrate that these three modes of selectivity enable the predictable oxidation of complex natural products and their derivatives at specific C-H bonds with preparatively useful yields. This type of general and predictable reactivity stands to enable aliphatic C-H oxidation as a method for streamlining complex molecule synthesis.

Serial ligand catalysis: a highly selective allylic C-H oxidation.

We are reporting a mild, chemo-, and highly regioselective Pd(II)-catalyzed allylic oxidation of alpha-olefins to furnish branched allylic esters that proceeds via a novel serial ligand catalysis mechanism in which two different ligands (i.e., vinyl sulfoxide 2 and BQ) interact sequentially with the metal to promote distinct steps of the catalytic cycle (i.e., C-H cleavage and pi-allyl functionalization, respectively).

A sulfoxide-promoted, catalytic method for the regioselective synthesis of allylic acetates from monosubstituted olefins via C-H oxidation.

Sulfoxide ligation to Pd(II) salts is shown to selectively promote C-H oxidation versus Wacker oxidation chemistry and to control the regioselectivity in the C-H oxidation products. A catalytic method for the direct C-H oxidation of monosubstituted olefins to linear (E)-allylic acetates in high regio- and stereoselectivities and preparatively useful yields is described. The method using benzoquinone as the stoichiometric oxidant and 10 mol % of Pd(OAc)2 or Pd(O2CCF3)2 as the catalyst in a DMSO/AcOH (1:1) solution was found to be compatible with a wide range of functionality (e.g., amides, carbamates, esters, and ethers, see Table 2). Addition of DMSO was found to be critical for promoting the C-H oxidation pathway, with AcOH alone or in combination with a diverse range of dielectric media, leading to mixtures favoring Wacker-type oxidation products (Tables 1, S3). To explore the role of DMSO as a ligand, the bis-sulfoxide Pd(OAc)2 complex 1 was formed and found to be an effective C-H oxidation catalyst in the absence of DMSO (eqs 2, 3). Moreover, catalyst 1 effects a reversal of regioselectivity, favoring the formation of branched allylic acetates.