Local and Collective Reaction Coordinates in the Transport of the Aqueous Hydroxide Ion

We investigate local and collective reaction coordinates for the structural diffusion of the hydroxide ion in dilute aqueous NaOH solution using a multistate empirical valence bond (MS-EVB) simulation. We characterize a 15 fs time scale associated with shifting of the equally shared proton within a Zundel-like H₃O₂^– ion to form a water molecule, a 550 fs relaxation from this transition state largely guided by electrostatic fluctuations of the surrounding environment, and a 9.6 ps time scale that corresponds to the solvation of the water molecule formed by the proton transfer event. When individual proton transfer events are examined, we are unable to identify a unique transition state solely on the basis of a decrease in the hydroxide ion’s coordination number. Instead, we find that the collective electric field along the proton transfer direction is better suited to describe the creation and relaxation of Zundel-like transition states that allow structural diffusion of the hydroxide ion

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 <i>N</i>–<i>N</i>′-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

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

<i>p</i>-Bromo-terminated oligo­(<i>p</i>-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

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

<i>p</i>-Bromo-terminated oligo­(<i>p</i>-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

Exciton-Delocalizing Ligands Can Speed Up Energy Migration in Nanocrystal Solids

Researchers have long sought to use surface ligands to enhance energy migration in nanocrystal solids by decreasing the physical separation between nanocrystals and strengthening their electronic coupling. Exciton-delocalizing ligands, which possess frontier molecular orbitals that strongly mix with nanocrystal band-edge states, are well-suited for this role because they can facilitate carrier-wave function extension beyond the nanocrystal core, reducing barriers for energy transfer. This report details the use of the exciton-delocalizing ligand phenyldithiocarbamate (PDTC) to tune the transport rate and diffusion length of excitons in CdSe nanocrystal solids. A film composed of oleate-terminated CdSe nanocrystals is subjected to a solid-state ligand exchange to replace oleate with PDTC. Exciton migration in the films is subsequently investigated by femtosecond transient absorption. Our experiments indicate that the treatment of nanocrystal films with PDTC leads to rapid (∼400 fs) downhill energy migration (∼80 meV), while no such migration occurs in oleate-capped films. Kinetic Monte Carlo simulations allow us to extract both rates and length scales for exciton diffusion in PDTC-treated films. These simulations reproduce dynamics observed in transient absorption measurements over a range of temperatures and confirm excitons hop via a Miller–Abrahams mechanism. Importantly, our experiments and simulations show PDTC treatment increases the exciton hopping rate to 200 fs, an improvement of 5 orders of magnitude relative to oleate-capped films. This exciton hopping rate stands as one of the fastest determined for CdSe solids. The facile, room-temperature processing and improved transport properties offered by the solid-state exchange of exciton-delocalizing ligands show they offer promise for the construction of strongly coupled nanocrystal arrays

Can Exciton-Delocalizing Ligands Facilitate Hot Hole Transfer from Semiconductor Nanocrystals?

Exciton-delocalizing ligands (EDLs) are of interest to researchers due to their ability to allow charge carriers to spread into the ligand shell of semiconductor nanocrystals (NCs). By increasing charge carrier surface accessibility, EDLs may facilitate the extraction of highly photoexcited carriers from NCs prior to their relaxation to the band edge, a process that can boost the performance of NC-based photocatalysts and light harvesting systems. However, hot carrier extraction must compete with carrier cooling, which could be accelerated by the stronger interaction of charge carriers and EDLs. This report describes the influence of the EDL phenyl­dithiocarbamate (PTC) on the electron and hole cooling rates of CdSe NCs. Using state-resolved transient absorption spectroscopy, we find that PTC treatment accelerates hole cooling by a factor of 1.7. However, upon further treatment of these NCs with cadmium­(II) acetate, the hole cooling rate reverts to the value measured prior to PTC treatment, yet these NCs maintain a red-shifted absorption spectrum indicative of PTC bound to the NC surface. This result provides strong evidence for the existence of two distinct surface-bound PTC species: one that traps holes before they cool and can be removed by cadmium­(II) acetate, and a second species that facilitates exciton delocalization. This conclusion is supported by both DFT calculations and photoluminescence measurements. The outlook from our work is that EDLs do not necessarily lead to an acceleration of carrier cooling, suggesting that they may provide a path for hot carrier extraction

Aqueous Colloidal Acene Nanoparticles: A New Platform for Studying Singlet Fission

Singlet fission is a process that occurs in select molecular systems wherein a singlet excited state divides its energy to form two triplet excitations on neighboring chromophores. While singlet fission has been largely studied in molecular crystals, colloidal nanoparticles offer the ability to investigate fission using liquid suspensions, allowing questions regarding the importance of molecular arrangement and charge transfer to be assessed. Herein, we report the synthesis of aqueous colloidal nanoparticles of 5,12-diphenyltetracene (DPT), a material recently demonstrated to undergo singlet fission in disordered films. Upon synthesis, nanoparticles display absorption features that lie between those of monomeric DPT and disordered DPT films. These features evolve over a few days in a manner that suggests an increase in the degree of association between neighboring molecules within the nanoparticles. Transient absorption and time-resolved emission experiments indicate that photoexcited DPT nanoparticles undergo fission, but produce a lower triplet yield than disordered films

Using Heterodyne-Detected Electronic Sum Frequency Generation To Probe the Electronic Structure of Buried Interfaces

Organic semiconductors (OSCs) are attractive optoelectronic materials due to their high extinction coefficients, processing advantages, and ability to display unique phenomena such as singlet exciton fission. However, employing OSCs as active electronic components remains challenging, as this necessitates forming junctions between OSCs and other materials. Such junctions can distort the OSC’s electronic properties, complicating the transfer of energy and charge across them. To investigate these junctions, our group has employed an interface-selective technique, electronic sum frequency generation spectroscopy (ESFG), yet one complication in applying ESFG to thin OSC films is they necessarily have two interfaces that can each produce signals. In a conventional ESFG measurement, information regarding the phase of the ESFG signal is lost. However, this information can be recovered with heterodyne detection (HD) techniques. Here, we present experiments and model calculations that illustrate some key advantages offered by HD-ESFG over conventional ESFG measurements for the study of OSC films. Specifically, we report HD-ESFG spectra of <i>N</i>,<i>N</i>′-dimethyl-3,4,9,10-perylenedicarboximide (C1-PDI) thin films that have been grown on SiO₂. To implement these measurements, we have constructed an HD-ESFG spectrometer that uses common path optics to maintain a high degree of phase stability over multiple hours. We find that not only does HD-ESFG offer increased sensitivity to weak features in ESFG spectra, but the phase information included in these measurements aids in selectively isolating signals that arise from a specific film interface. Interestingly, we find that resonances in HD-ESFG spectra of C1-PDI are significantly shifted from those in linear absorption spectra of bulk C1-PDI films, suggesting that the intermolecular packing of molecules at film interfaces differs from the bulk

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 Å between 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