Unraveling Photoinduced Desorption Dynamics of π-Conjugated Ligands in ZnS Nanocrystals via Pump-Push-Probe Spectroscopy.

The photoinduced ligand desorption from nanocrystal (NC) surfaces plays a critical role in the diverse functionalities of NCs. However, this reaction is inherently complex because photophysical and photochemical reactions are involved, and many aspects remain elusive. In this study, using ZnS NCs coordinated with perylenebisimide (PBI) ligands as a model system, we revealed that pump-push-probe spectroscopy provides detailed insights into the transient species following photoinduced ligand desorption reactions.

Time-resolved keto-enol tautomerization of the medicinal pigment curcumin.

Curcumin is a medicinal agent that exhibits anti-cancer and anti-Alzheimer's disease properties. It has a keto-enol moiety that gives rise to many of its chemical properties including metal complexation and acid-base equilibria. A previous study has shown that keto-enol tautomerization at this moiety is implicated in the anti-Alzheimer's disease effect of curcumin, highlighting the importance of this process.

Role of Singlet and Triplet Excited States in the Oxygen-Mediated Photophysics and Photodegradation of Polyacenes.

Polyacenes, such as tetracene and pentacene, are common model systems for the study of photophysical phenomena such as singlet fission (SF) and triplet fusion, processes which may lead to increased photovoltaic efficiencies. While they exhibit desirable photophysical properties, these materials are not photostable and convert to unwanted endoperoxides in the presence of oxygen and light, limiting their use in real-world applications. Not only does oxygen degrade polyacenes but also it can affect their photophysics, leading to both the sensitization and quenching of different excited states.

Singlet fission preserves polarisation correlation of excitons.

Singlet fission (SF) holds the promise to circumvent the photovoltaic efficiency limit to reach a power-conversion efficiency above 34%. SF of TIPS-pentacene (TIPS-Pn) has been investigated but its mechanism is yet to be well elucidated. Recently, we developed a nanoparticle (NP) system, in which doping of TIPS-Pn in a host matrix yields a range of average intermolecular distances, , to study the dependence of SF in TIPS-Pn on .

Experimental and computational characterisation of an artificial light harvesting complex.

Photosynthesis has been shown to be a highly efficient process for energy transfer in plants and bacteria. Like natural photosynthetic systems, the artificial light harvesting complex (LHC) BODIPY pillar[5]arene exhibits Förster resonance energy transfer (FRET). However, extensive characterisation of the BODIPY pillar[5]arene LHC to determine its suitability as an artificial LHC has yet to occur.

Pitfalls of quantifying intersystem crossing rates in singlet-fission chromophore solutions.

Singlet fission (SF), a process that produces two triplet excitons from one singlet exciton, has attracted recent interest for its potential to circumvent the detailed-balance efficiency limit of single-junction solar cells. For the potential of SF to be fully realized, accurate assignment and quantification of SF is necessary. Intersystem crossing (ISC) is another process of singlet to triplet conversion that is important to distinguish from SF to avoid either over- or under-estimation of SF triplet production.

Multiphoton Phosphorescence of Simple Ketones by Visible-light Excitation and Its Consideration for Active Sensing in Space.

Acetone and butanone were seen to emit blue light around 450 nm when excited in the green by a high intensity pulsed laser. The pathway of this anti-Stokes emission is believed to be multiphoton absorption followed by phosphorescence, with emission being observed in the samples at cryogenic temperatures below their melting point and not seen from either ketone in their cold liquid state. Given the widespread nature of these simple ketones in off-world bodies and their potential importance as an organic resource for Space Resource Utilization, signals which enable the identification and tracing of these materials are of use in applications from remote sensing and mapping to monitoring during extraction processes.

Characterization of the ultrafast spectral diffusion and vibronic coherence of TIPS-pentacene using 2D electronic spectroscopy.

TIPS-pentacene is a small-molecule organic semiconductor that is widely used in optoelectronic devices. It has been studied intensely owing to its ability to undergo singlet fission. In this study, we aim to develop further understanding of the coupling between the electronic and nuclear degrees of freedom of TIPS-pentacene (TIPS-Pn).

Two-Dimensional Electronic Spectroscopy Using Rotating Optical Flats.

In two-dimensional electronic spectroscopy (2DES), precise control of the arrival time of ultrashort laser pulses is critical to correlating the molecular states that are accessed in the experiment. In this work, we demonstrate a 2D electronic spectrometer design with an interferometric phase stability of ∼λ/250 at 600 nm. First, we present a new method for controlling pulse delay times based on transmission through pairs of optical flats rotated perpendicular to the beam propagation direction.

Organizing Crystalline Functionalized Pentacene Using Periodicity of Poly(Vinyl Alcohol).

Nanoparticles of acenes exhibit highly efficient intermolecular singlet fission (SF). Recent reports indicate that altering the morphology of 6,13-bis-(triisopropylsilylethynyl)pentacene (TIPS-Pn) nanoparticles has a profound influence on their SF dynamics. Here, we show that poly(vinyl alcohol) (PVA) induces a phase transition in preformed TIPS-Pn nanoparticles.

The optical properties of CsPbBr-CsPbBr perovskite composites.

Although the optoelectronic applications of metal halide perovskites have been intensively investigated in recent years, the fundamental carrier dynamics of zero-dimensional (0D) Cs4PbBr6 perovskites has been relatively underexplored; in particular, the nature of the green fluorescence is highly debated. Nevertheless, the unique photophysical properties are of immense interest for a variety of potential applications. In this work, the green emission of the CsPbBr3-Cs4PbBr6 perovskite composites is studied using temperature dependent photoluminescence (PL).

Endothermic singlet fission is hindered by excimer formation.

Singlet fission is a process whereby two triplet excitons can be produced from one photon, potentially increasing the efficiency of photovoltaic devices. Endothermic singlet fission is desired for a maximum energy-conversion efficiency, and such systems have been considered to form an excimer-like state with multiexcitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear.

Difference in hot carrier cooling rate between Langmuir-Blodgett and drop cast PbS QD films due to strong electron-phonon coupling.

The carrier dynamics of lead sulphide quantum dot (PbS QD) drop cast films and closely packed ordered Langmuir-Blodgett films are studied with ultra-fast femtosecond transient absorption spectroscopy. The photo-induced carrier temperature is extracted from transient absorption spectra and monitored as a function of time delay. The cooling dynamics of carriers in PbS QDs suggest a reduction of the carrier energy loss rate at longer time delays through the retardation of the longitudinal optical (LO) phonon decay due to partial heating of acoustic phonon modes.

Origin of the Excited-State Absorption Spectrum of Polythiophene.

The excited states of conjugated polymers play a central role in their applications in organic solar photovoltaics. The delocalized excited states of conjugated polymers are short-lived (τ < 40 fs) but are imperative in the photovoltaic properties of these materials. Photoexcitation of poly(3-hexylthiophene) (P3HT) induces an excited-state absorption band, but the transitions that are involved are not well understood.

Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites.

The hot-phonon bottleneck effect in lead-halide perovskites (APbX) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA/MA/Cs, X=I/Br) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts.

Excited-state dynamics of the medicinal pigment curcumin in a hydrogel.

Curcumin is a yellow polyphenol with multiple medicinal effects. These effects, however, are limited due to its poor aqueous stability and solubility. A hydrogel of 3% octadecyl randomly substituted polyacrylate (PAAC18) has been shown to provide high aqueous stability for curcumin under physiological conditions, offering a route for photodynamic therapy.

Nanoprecipitation and Spectroscopic Characterization of Curcumin-Encapsulated Polyester Nanoparticles.

Curcumin-encapsulated polyester nanoparticles (Cur-polyester NPs) of approximately 100 nm diameter with a negatively charged surface were prepared using a one-step nanoprecipitation method. The Cur-polyester NPs were prepared using polylactic acid, poly(D,L-lactic-co-glycolic acid) and poly(ϵ-caprolactone) without any emulsifier or surfactant. The encapsulation of curcumin in these polyester NPs greatly suppresses curcumin degradation in the aqueous environment due to its segregation from water.

Femtosecond Pump-Push-Probe and Pump-Dump-Probe Spectroscopy of Conjugated Polymers: New Insight and Opportunities.

Conjugated polymers are an important class of soft materials that exhibit a wide range of applications. The excited states of conjugated polymers, often referred to as excitons, can either deactivate to yield the ground state or dissociate in the presence of an electron acceptor to form charge carriers. These interesting properties give rise to their luminescence and the photovoltaic effect.

Vibrational coherence probes the mechanism of ultrafast electron transfer in polymer-fullerene blends.

The conversion of photoexcitations into charge carriers in organic solar cells is facilitated by the dissociation of excitons at the donor/acceptor interface. The ultrafast timescale of charge separation demands sophisticated theoretical models and raises questions about the role of coherence in the charge-transfer mechanism. Here, we apply two-dimensional electronic spectroscopy to study the electron transfer process in poly(3-hexylthiophene)/PCBM (P3HT/PCBM) blends.

Femtosecond transient absorption spectroscopy of the medicinal agent curcumin in diamide linked γ-cyclodextrin dimers.

Curcumin is a biologically active polyphenol and a yellow pigment extracted from turmeric. Our previous study has shown effective encapsulation of curcumin using diamide linked γ-cyclodextrin dimers, namely 66γCD2su and 66γCD2ur, through cooperative 1:1 host-guest complexation. In this study, the excited-state dynamics of curcumin complexed with either 66γCD2su or 66γCD2ur in water are investigated using femtosecond transient absorption spectroscopy.

The capture and stabilization of curcumin using hydrophobically modified polyacrylate aggregates and hydrogels.

Hydrophobically modified polyacrylates are shown to suppress the degradation of the medicinal pigment curcumin under physiological conditions. In aqueous solution, the 3% octadecyl randomly substituted polyacrylate, PAAC18, forms micelle-like aggregates at a concentration of <1 wt % and a hydrogel at >1 wt %. Under both conditions, PAAC18 shows a remarkable ability to suppress the degradation of curcumin at pH 7.

Optical Pumping of Poly(3-hexylthiophene) Singlet Excitons Induces Charge Carrier Generation.

The dynamics of high-energy excitons of poly(3-hexylthiophene) (P3HT) are shown to consist of torsional relaxation and exciton dissociation to form free carriers. In this work, we use pump-push-probe femtosecond transient absorption spectroscopy to study the highly excited states of P3HT in solution. P3HT excitons are generated using a pump pulse (400 nm) and allowed to relax to the lowest-lying excited state before re-excitation using a push pulse (900 or 1200 nm), producing high-energy excitons that decay back to the original excited state with both subpicosecond (0.

Diamide linked γ-cyclodextrin dimers as molecular-scale delivery systems for the medicinal pigment curcumin to prostate cancer cells.

Diamide linked γ-cyclodextrin (γ-CD) dimers are proposed as molecular-scale delivery agents for the anticancer agent curcumin. N,N'-Bis(6(A)-deoxy-γ-cyclodextrin-6(A)-yl)succinamide (66γCD2su) and N,N'-bis(6(A)-deoxy-γ-cyclodextrin-6(A)-yl)urea (66γCD2ur) markedly suppress the degradation of curcumin by forming a strong 1:1 cooperative binding complexes. The results presented in this study describe the potential efficacy of 66γCD2su and 66γCD2ur for intracellular curcumin delivery to cancer cells.

Molecular basis of binding and stability of curcumin in diamide-linked γ-cyclodextrin dimers.

Curcumin is a naturally occurring molecule with medicinal properties that is unstable in water, whose efficacy as a drug can potentially be enhanced by encapsulation inside a host molecule. In this work, the thermodynamics and mechanism of binding of curcumin to succinamide- and urea-linked γ-cyclodextrin (γ-CD) dimers in water are investigated by molecular dynamics simulations. The simulated binding constants of curcumin to succinamide- and urea-linked γ-CD dimers at 310 K are 11.