Femtosecond Raman Microscopy Reveals Structural Dynamics Leading to Triplet Separation in Rubrene Singlet Fission

Singlet fission generates multiple excitons from a single photon, which in theory can result in solar cell efficiencies with values above the Shockley–Queisser limit. Understanding the molecular structural dynamics during singlet fission will help to fabricate efficient organic photovoltaic devices. Here we use femtosecond stimulated Raman spectroscopy to reveal the structural evolution during the triplet separation in rubrene. We observe vibrational signatures of the correlated triplet pair, as well as shifting of the vibrational frequencies of the 1430 and 1542 cm^–1 excited state modes, which increase by more than 25 cm^–1 in 5 ps. Our results indicate that the correlated pair separation into two individual triplets occurs concurrently with the loss of electron density from the tetracene backbone in rubrene. This study provides new insights into the triplet separation process and proves the utility of structurally sensitive ultrafast vibrational techniques to understand the mechanism of singlet fission

Cooperative Catalysis Approach to Intramolecular Hydroacylation

Prior examples of hydroacylation to form six- and seven-membered ring ketones require either embedded chelating groups or other substrate design strategies to circumvent competitive aldehyde decarbonylation. A cooperative catalysis strategy enabled intramolecular hydroacylation of disubstituted alkenes to form seven- and six-membered rings without requiring substrate-embedded chelating groups

Palladium and Lewis-Acid-Catalyzed Intramolecular Aminocyanation of Alkenes: Scope, Mechanism, and Stereoselective Alkene Difunctionalizations

An expansion of methodologies aimed at the formation of versatile organonitriles, via the intramolecular aminocyanation of unactivated alkenes, is herein reported. Importantly, the need for a rigid tether in these reactions has been obviated. The ease-of-synthesis and viability of substrates bearing flexible backbones has permitted for diastereoselective variants as well. We demonstrated the utility of this methodology with the formation of pyrrolidones, piperidinones, isoindolinones, and sultams. Furthermore, subsequent transformation of these motifs into medicinally relevant molecules is also demonstrated. A double crossover ¹³C-labeling experiment is consistent with a fully intramolecular cyclization mechanism. Deuterium labeling experiments support a mechanism involving <i>syn</i>-addition across the alkene

Intramolecular Oxyacylation of Alkenes Using a Hydroxyl Directing Group

Alkene oxyacylation is a new strategy for the preparation of β-oxygenated ketones. Now, with Ir catalysis and low-cost salicylate esters, alkene oxyacylation can be promoted by simple and versatile hydroxyl directing groups. This paper discusses catalyst optimization, substituent effects, mechanistic experiments, and the challenges associated with asymmetric catalysis. Crossover experiments point to several key steps of the mechanism being reversible, including the most likely enantiodetermining steps. The oxyacylation products are also prone to racemization without catalyst when heated alone; however, crossover is not observed without catalyst. These observations account for the low levels of enantioinduction in alkene oxyacylation. The versatility of the hydroxyl directing group is highlighted by demonstrating further transformations of the products

Diarylindenotetracenes via a Selective Cross-Coupling/C–H Functionalization: Electron Donors for Organic Photovoltaic Cells

A direct synthesis of new donor materials for organic photovoltaic cells is reported. Diaryindenotetracenes were synthesized utilizing a Kumada–Tamao–Corriu cross-coupling of <i>peri-</i>substituted tetrachlorotetracene with spontaneous indene annulation via C–H activation. Vacuum deposited planar heterojunction organic photovoltaic cells incorporating these molecules as electron donors exhibit power conversion efficiencies exceeding 1.5% with open-circuit voltages ranging from 0.7 to 1.1 V when coupled with C₆₀ as an electron acceptor

Intramolecular Oxyacylation of Alkenes Using a Hydroxyl Directing Group

Alkene oxyacylation is a new strategy for the preparation of β-oxygenated ketones. Now, with Ir catalysis and low-cost salicylate esters, alkene oxyacylation can be promoted by simple and versatile hydroxyl directing groups. This paper discusses catalyst optimization, substituent effects, mechanistic experiments, and the challenges associated with asymmetric catalysis. Crossover experiments point to several key steps of the mechanism being reversible, including the most likely enantiodetermining steps. The oxyacylation products are also prone to racemization without catalyst when heated alone; however, crossover is not observed without catalyst. These observations account for the low levels of enantioinduction in alkene oxyacylation. The versatility of the hydroxyl directing group is highlighted by demonstrating further transformations of the products

Synthesis and Characterization of Electron-Deficient Asymmetrically Substituted Diarylindenotetracenes

Electron-deficient asymmetrically substituted diarylindenotetracenes were prepared via a series of Friedel–Crafts acylations, aryl–aryl cross-couplings, and an intramolecular oxidative cyclization to form the indene ring. Single-crystal X-ray experiments showed good π–π overlap with π–π distances ranging from 3.26 to 3.76 Å. Both thermogravimetric analysis and differential scanning calorimetry indicated that asymmetrically substituted indenotetracenes (ASIs) are stable at elevated temperatures. From cyclic voltammetry experiments, HOMO/LUMO energy levels of ASI derivatives were determined to be near −5.4/–4.0 eV. UV/visible absorption spectra showed strong absorption of light between 400 and 650 nm with molar attenuation coefficients from 10⁴ to 10⁵ M^–1 cm^–1. ASIs were also found to have very low fluorescence quantum yields, less than 4%. Using the solid-state packing determined from the single-crystal X-ray experiments, computational modeling indicated that ASI molecules should favor electron transport

Mechanistic Model for Enantioselective Intramolecular Alkene Cyanoamidation via Palladium-Catalyzed C–CN Bond Activation

We studied key aspects of the mechanism of Pd-catalyzed C–CN bond activation and intramolecular enantioselective alkene cyanoamidation. An Abboud–Abraham–Kamlet–Taft (AAKT) linear solvation energy relationship (LSER) model for enantioselectivity was established. We investigated the impact of Lewis acid (BPh₃), Lewis base (DMPU), and no additives. BPh₃ additive led to diminished enantioselectivity and differing results in ¹³CN crossover experiments, initial rate kinetics, and natural abundance ¹²C/¹³C kinetic isotope effect measurements. We propose two catalytic mechanisms to account for our experimental results. We propose that the DMPU/nonadditive pathway passes through a κ²-phosphoramidite-stabilized Pd⁺ intermediate, resulting in high enantioselectivity. BPh₃ prevents the dissociation of CN^–, leading to a less rigid κ²-phosphoramidite-neutral Pd intermediate

Diarylindenotetracenes via a Selective Cross-Coupling/C–H Functionalization: Electron Donors for Organic Photovoltaic Cells

A direct synthesis of new donor materials for organic photovoltaic cells is reported. Diaryindenotetracenes were synthesized utilizing a Kumada–Tamao–Corriu cross-coupling of <i>peri-</i>substituted tetrachlorotetracene with spontaneous indene annulation via C–H activation. Vacuum deposited planar heterojunction organic photovoltaic cells incorporating these molecules as electron donors exhibit power conversion efficiencies exceeding 1.5% with open-circuit voltages ranging from 0.7 to 1.1 V when coupled with C₆₀ as an electron acceptor

Development and Mechanistic Study of Quinoline-Directed Acyl C–O Bond Activation and Alkene Oxyacylation Reactions

The intramolecular addition of both an alkoxy and acyl substituent across an alkene, oxyacylation of alkenes, using rhodium catalyzed C–O bond activation of an 8-quinolinyl ester is described. Our unsuccessful attempts at intramolecular carboacylation of ketones via C–C bond activation ultimately informed our choice to pursue and develop the intramolecular oxyacylation of alkenes via quinoline-directed C–O bond activation. We provide a full account of our catalyst discovery, substrate scope, and mechanistic experiments for quinoline-directed alkene oxyacylation