Benzannulation through Ruthenium(0)-Catalyzed Transfer Hydrogenative Cycloaddition: Precision Synthesis and Photophysical Characterization of Soluble Diindenoperylenes.

The first application of ruthenium(0)-catalyzed 1,2-dione-diyne [2+2+2] cycloaddition to PAH construction is achieved by the precision synthesis of soluble diindenoperylenes (DIPs), the electronic structures of which were investigated using steady-state absorption and emission, transient absorption, cyclic voltammetry and time-dependent density functional theory.

Alternating oligo(o,p-phenylenes) via ruthenium catalyzed diol-diene benzannulation: orthogonality to cross-coupling enables de novo nanographene and PAH construction.

Ruthenium(0) catalyzed diol-diene benzannulation is applied to the conversion of oligo(p-phenylene vinylenes) 2a-c, 5 and 6 to alternating oligo(o,p-phenylenes) 10a-c, 11-13. Orthogonality with respect to conventional palladium catalyzed biaryl cross-coupling permits construction of p-bromo-terminated alternating oligo(o,p-phenylenes) 10b, 11-13, which can be engaged in Suzuki cross-coupling and Scholl oxidation. In this way, structurally homogeneous nanographenes 16a-f are prepared. Nanographene 16a, which incorporates 14 fused benzene rings, was characterized by single crystal X-ray diffraction. In a similar fashion, p-bromo-terminated oligo(p-phenylene ethane diol) 9, which contains a 1,3,5-trisubstituted benzene core, is converted to the soluble, structurally homogeneous hexa-peri-hexabenzocoronene 18. A benzothiophene-terminated pentamer 10c was prepared and subjected to Scholl oxidation to furnish the helical bis(benzothiophene)-fused picene derivative 14. The steady-state absorption and emission properties of nanographenes 14, 16a,b,d,e,h and 18 were characterized. These studies illustrate how orthogonality of ruthenium(0) catalyzed diol-diene benzannulation with respect to classical biaryl cross-coupling streamlines oligophenylene and nanographene construction.

Surface States Mediate Triplet Energy Transfer in Nanocrystal-Acene Composite Systems.

Hybrid organic:inorganic materials composed of semiconductor nanocrystals functionalized with acene ligands have recently emerged as a promising platform for photon upconversion. Infrared light absorbed by a nanocrystal excites charge carriers that can pass to surface-bound acenes, forming triplet excitons capable of fusing to produce visible radiation. To fully realize this scheme, energy transfer between nanocrystals and acenes must occur with high efficiency, yet the mechanism of this process remains poorly understood. To improve our knowledge of the fundamental steps involved in nanoparticle:acene energy transfer, we used ultrafast transient absorption to investigate excited electronic dynamics of PbS nanocrystals chemically functionalized with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) ligands. We find photoexcitation of PbS does not lead to direct triplet energy transfer to surface-bound TIPS-pentacene molecules but rather to the formation of an intermediate state within 40 ps. This intermediate persists for ∼100 ns before evolving to produce TIPS-pentacene triplet excitons. Analysis of transient absorption lineshapes suggests this intermediate corresponds to charge carriers localized at the PbS nanocrystal surface. This hypothesis is supported by constrained DFT calculations that find a large number of spin-triplet states at PbS NC surfaces. Though some of these states can facilitate triplet transfer, others serve as traps that hinder it. Our results highlight that nanocrystal surfaces play an active role in mediating energy transfer to bound acene ligands and must be considered when optimizing composite NC-based materials for photon upconversion, photocatalysis, and other optoelectronic applications.

Lanthanide Texaphyrins as Photocatalysts.

Here, we report the use of gadolinium(III)-, lutetium(III)-, and lanthanum(III)-texaphyrins as bioinspired photocatalysts that promote a novel approach to the degradation of curcumin, a 1,3-diketo-containing natural product. Complexation of curcumin to the lanthanide centers of the texaphyrins yields stable species that display limited reactivity in the dark or under anaerobic conditions. However, upon exposure to mWatt intensity light (pocket flashlight) or simply under standard laboratory illumination in the presence of atmospheric oxygen, substrate oxidation occurs readily to generate curcumin-derived cleavage products. These latter species were identified on the basis of spectroscopic and mass spectrometric analyses. The mild nature of the activation conditions serves to highlight a potential new role for photoactive lanthanide complexes.

Helical Rod-like Phenylene Cages via Ruthenium Catalyzed Diol-Diene Benzannulation: A Cord of Three Strands.

p-Bromo-terminated oligo(p-phenylenevinylenes) emanating from a 1,3,5-benzene core are dihydroxylated and subjected to ruthenium catalyzed diol-diene benzannulation to form tripodal oligo(phenylenes). Copper- or nickel-mediated 3-fold reductive biaryl homocoupling delivers a series of triple-stranded phenylene cages of helical rod-like topology bearing 14, 17, and 20 benzene rings.

Singlet Fission Involves an Interplay between Energetic Driving Force and Electronic Coupling in Perylenediimide Films.

Due to its ability to offset thermalization losses in photoharvesting systems, singlet fission has become a topic of research interest. During singlet fission, a high energy spin-singlet state in an organic semiconductor divides its energy to form two lower energy spin-triplet excitations on neighboring chromophores. While key insights into mechanisms leading to singlet fission have been gained recently, developing photostable compounds that undergo quantitative singlet fission remains a key challenge. In this report, we explore triplet exciton production via singlet fission in films of perylenediimides, a class of compounds with a long history of use as industrial dyes and pigments due to their photostability. As singlet fission necessitates electron transfer between neighboring molecules, its rate and yield depend sensitively on their local arrangement. By adding different functional groups at their imide positions, we control how perylenediimides pack in the solid state. We find inducing a long axis displacement of ∼3 Šbetween neighboring perylenediimides gives a maximal triplet production yield of 178% with a fission rate of ∼245 ps despite the presence of an activation barrier of ∼190 meV. These findings disagree with Marcus theory predictions for the optimal perylenediimide geometry for singlet fission, but do agree with Redfield theory calculations that allow singlet fission to occur via a charge transfer-mediated superexchange mechanism. Unfortunately, triplets produced by singlet fission are found to decay over tens of nanoseconds. Our results highlight that singlet fission materials must be designed to not only produce triplet excitons but to also facilitate their extraction.

Defects Cause Subgap Luminescence from a Crystalline Tetracene Derivative.

We use steady-state and ultrafast nonlinear spectroscopies in combination with density functional theory calculations to explain light emission below the optical gap energy (Eo) of crystalline samples of 5,12-diphenyl tetracene (DPT). In particular, the properties of vibrational coherences imprinted on a probe pulse transmitted through a DPT single crystal indicate discrete electronic transitions below Eo of this organic semiconductor. Analysis of coherence spectra leads us to propose structural defect states give rise to these discrete transitions and subgap light emission. We use the polarization dependence of vibrational coherence spectra to tentatively assign these defects in our DPT samples. Our results provide fundamental insights into the properties of midgap states in organic materials important for their application in next-generation photonics and optoelectronics technologies.

Slow Singlet Fission Observed in a Polycrystalline Perylenediimide Thin Film.

Singlet exciton fission (SF) is a process wherein an exciton in an organic semiconductor divides its energy to form two excitations. This process can offset thermalization losses in light harvesting technologies, but requires photostable materials with high SF efficiency. We report ultrafast kinetics of polycrystalline films of N-N'-dioctyl-3,4,9,10-perylenedicarboximide (C8-PDI), a chromophore predicted to undergo SF on picosecond time scales. While transient absorption measurements display picosecond dynamics, such kinetics are absent from low-fluence time-resolved emission experiments, indicating they result from singlet-singlet exciton annihilation. A model that accounts for annihilation can reproduce both measurements and highlights that care must be taken when extracting SF rates from time-resolved data. Our model also reveals SF proceeds in C8-PDI over 3.8 ns. Despite this slow rate, SF occurs in high yield (51%) due to a lack of competing singlet deactivation pathways. Our results show perylenediimides are a promising class of SF materials that merit further study.

Extracting the Density of States of Copper Phthalocyanine at the SiO2 Interface with Electronic Sum Frequency Generation.

Organic semiconductors (OSCs) constitute an attractive platform for optoelectronics design due to the ease of their processability and chemically tunable properties. Incorporating OSCs into electrical circuits requires forming junctions between them and other materials, yet the change in dielectric properties about these junctions can strongly perturb the electronic structure of the OSC. Here we adapt an interface-selective optical technique, electronic sum frequency generation (ESFG), to the study of a model OSC thin-film system, copper phthalocyanine (CuPc) deposited on SiO2. We find that by modeling the thickness dependence of our measured spectra, we can identify changes in CuPc's electronic density of states at both its buried interface with SiO2 and air-exposed surface. Our work demonstrates that ESFG can be used to noninvasively probe the interfacial electronic structure of optically thick OSC films, indicating that it can be used for the study of OSC-based optoelectronics in situ.