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. In this study, tautomerization of curcumin in methanol, acetone and acetonitrile was investigated using time-resolved 1H nuclear magnetic resonance spectroscopy. Curcumin undergoes hydrogen-deuterium exchange with the solvents and the proton resonance peak corresponding to the hydrogen at the α-carbon position (Cα) decays as a function of time, signifying deuteration at this position. Because tautomerization is the rate limiting step in the deuteration of curcumin at the Cα position, the rate of tautomerization is inferred from the rate of deuteration. The rate constant of tautomerization of curcumin shows a temperature dependence and analysis using the Arrhenius equation revealed activation energies (Ea) of tautomerization of (80.1 ± 5.9), (64.1 ± 1.0) and (68.3 ± 5.5) kJ mol-1 in methanol, D2O/acetone and D2O/acetonitrile, respectively. Insight into the role of water in tautomerization of curcumin was further offered by density functional theory studies. The transition state of tautomerization was optimized in the presence of water molecules. The results show a hydrogen-bonded solvent bridge between the diketo moiety and Cα of curcumin. The Ea of tautomerization of curcumin shows a strong dependence on the number of water molecules in the solvent bridge, indicating the critical role played by the solvent bridge in catalyzing tautomerization of curcumin.

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. In this study, we characterize the effect of oxygen on 5,12-bis(triisopropylsilylethynyl) tetracene (TIPS-Tn) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) using transient absorption spectroscopy, and show that oxygen can significantly influence the population of excited states, in particular enhancing the polyacene triplet population. We additionally combine the time-resolved excited-state dynamics with photodegradation studies to determine the predominant mechanism of photooxidation, which has previously been unclear. We find that both molecules photodegrade predominantly via singlet oxygen; however, for TIPS-Tn, this occurs through the triplet state, whereas for TIPS-Pn, degradation occurs through the excited singlet. The photodegradation of TIPS-Tn is thus enhanced by faster rates of SF, whereas SF in TIPS-Pn increases the molecule's photostability. This work has implications both for the design of new materials for next-generation photovoltaics that can avoid photooxidation and for the study of their photophysics in real-world environments.

Enhanced Photocatalytic and Photovoltaic Performance Arising from Unconventionally Low Donor-Y6 Ratios.

Development of both organic photovoltaics (OPVs) and organic photocatalysts has focused on utilizing the bulk heterojunction (BHJ). The BHJ promotes charge separation and enhances the carrier lifetime, but may give rise to increased charge traps, hindering performance. Here, high photocatalytic and photovoltaic performance is displayed by electron donor-acceptor (D-A) nanoparticles (NPs) and films, using the nonfullerene acceptor Y6 and polymer donor PIDT-T8BT. In contrast to conventional D-A systems, the charge generation in PIDT-T8BT:Y6 NPs is mainly driven by Y6, allowing a high performance even at a low D:A mass ratio of 1:50. The high performance at the low mass ratio is attributed to the amorphous behavior of PIDT-T8BT. Low ratios are generally thought to yield lower efficiency than the more conventional ≈1:1 ratio. However, the OPVs exhibit peak performance at a D:A ratio of 1:5. Similarly the NPs used for photocatalytic hydrogen evolution show peak performance at the 1:6.7 D:A ratio. Interestingly, for the PIDT-T8BT:Y6 system, as the polymer proportion increases, a reduced photocatalytic and photovoltaic performance is observed. The unconventional D:A ratios provide lower recombination losses and increased charge-carrier lifetime with undisrupted ambipolar charge transport in bulk Y6, enabling better performance than conventional ratios. This work reports novel light-harvesting materials in which performance is reduced due to unfavorable morphology as D:A ratios move toward conventional ratios of 1:1.2-1:1.

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, d, to study the dependence of SF in TIPS-Pn on d. At large d values, where the bimolecular SF process should be unfavourable, a relatively high SF quantum yield (ΦSF) is still observed, which implies a deviation from a random distribution of TIPS-Pn throughout the NP. Here, using polarisation-sensitive femtosecond time-resolved spectroscopy and Monte Carlo simulations of exciton migration and SF, we quantify the level of clustering of TIPS-Pn in the host matrix, which is responsible for the higher than expected ΦSF. The experimental data indicate a preservation of polarisation correlation by SF, which is uncommon because energy transfer in amorphous materials tends to result in depolarisation. We show that the preservation of polarisation correlation is due to SF upon exciton migration. Although exciton migration decorrelates polarisation, SF acts to remove decorrelated excitons to give an overall preservation of polarisation correlation.

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. In this paper we experimentally and computationally investigate the photophysical properties of the LHC by comparing the light absorption of the BODIPY LHC to individual BODIPY chromophores. Our results show evidence for quantum coherence, with oscillation frequencies of 100 cm-1 and 600 cm-1, which are attributable to vibronic, or exciton-phonon type coupling. Computational analysis suggests strong couplings of the molecular orbitals of the LHC resulting from the stacking of neighbouring BODIPY chromophore units. Interestingly, we find a 40% reduction in the absorbance of light for the BODIPY LHC compared to the individual chromophores which we attribute to electronic interactions between the conjugated π-systems of the BODIPY chromophores and the pillar[5]arene backbone.

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. Here, we quantify an upper bound on the rate of ISC in two commonly studied SF chromophores, TIPS-pentacene and TIPS-tetracene, by using transient absorption spectroscopy of solutions of varying concentrations in toluene. We show that SF in solutions of these acenes has previously been misidentified as ISC, and vice versa. By determining a bimolecular SF rate constant in concentrated solutions in which SF dominates over ISC, we distinguish triplet formation due to SF from triplet formation due to ISC and show that the characteristic time scale of ISC must be longer than 325 ns in TIPS-pentacene, while it must be longer than 118 ns in TIPS-tetracene. We additionally note that no excimer formation is observed in the relatively dilute (up to 8 mM) solutions studied here, indicating that previous excimer formation observed at much higher concentrations may be partially due to aggregate formation. This work highlights that an accurate quantification of ISC is crucial as it leads to accurate determination of SF rate constants and yields.

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. While the excitation process has a low efficiency, the ability to use visible light for sensing of these targets has advantages over UV sources, such as the wider availability of high-powered lasers which could be utilized.

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). We measured and analyzed the 2D electronic spectra of TIPS-Pn in solutions. Using center line slope (CLS) analysis, we characterized the frequency-fluctuation correlation function of the 0-0 vibronic transition. Strong oscillations in the CLS values were observed for up to 5 ps with a frequency of 264 cm-1, which are attributable to a large vibronic coupling with the TIPS-Pn ring-breathing vibrational mode. In addition, detailed analysis of the CLS values allowed us to retrieve two spectral diffusion lifetimes, which are attributed to the inertial and diffusive dynamics of solvent molecules. Amplitude beating analysis also uncovered couplings with another vibrational mode at 1173 cm-1. The experimental results can be described using the displaced harmonic oscillator model. By comparing the CLS values of the simulated data with the experimental CLS values, we estimated a Huang-Rhys factor of 0.1 for the ring-breathing vibrational mode. The results demonstrated how CLS analysis can be a useful method for characterizing the strength of vibronic coupling.

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. Second, the calibration methods required to achieve adequate timing precision are also reported. Compared to existing designs using translating wedges, the rotating optical flats can achieve equivalent optical delay with a shorter path length in glass, reducing errors due to spectral dispersion of the broadband laser pulses used in 2DES. Our approach presents a simple, low-cost technique for multidimensional optical spectroscopy that is capable of resolving complex light-induced dynamics.

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. These nanoparticles are amorphous when initially formed but crystalline after addition of PVA. Surface characterization indicates that a diffuse PVA layer surrounds the nanoparticles. We propose that a periodic interaction between the hydroxyl groups of PVA and TIPS groups of TIPS-Pn on the nanoparticle surface induces a large-scale structural rearrangement to yield crystalline TIPS-Pn. Such reorganization in preformed organic nanoparticles is unprecedented, and we believe that this is the first report of such an effect induced by polymer adsorption. Transient absorption spectroscopic results reveal that SF within these nanoparticles is accelerated by an order of magnitude upon structural rearrangement.

The optical properties of Cs4PbBr6-CsPbBr3 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). The PL spectra at different temperatures simultaneously contain two sub-peaks (520 nm and 550 nm), which are ascribed to the emissions of the band-edge and the defect trapped exciton of CsPbBr3. This finding will help to understand the controversial photoluminescence currently observed in different 0D Cs4PbBr6 perovskites.

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. Here we show, using 5,12-bis((triisopropylsilyl)ethynyl)tetracene in solution as a prototypical example, that, rather than acting as an intermediate, the excimer serves to trap excited states to the detriment of singlet-fission yield. We clearly demonstrate that singlet fission and its conjugate process, triplet-triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet-fission material must be designed to avoid excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion.

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. In this work, calculations have been performed on P3HT analogues using nonlinear response time-dependent density functional theory to show that an increase in the oligomer length correlates with the dominance of the S1 → S3 transition. Furthermore, the predicted transition energy shows an excellent agreement with experiment. The calculations also yielded results on intramolecular charge transfer in P3HT due to the S1 → S3 transition, providing insight into the mechanism of exciton dissociation to form charge carriers.

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

The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) 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. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.

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. In this study, the excited-state photophysics of curcumin in the PAAC18 hydrogel is investigated using a combination of femtosecond transient absorption and fluorescence upconversion spectroscopy. The transient absorption results reveal a multiexponential decay in the excited-state kinetics with fast (1 ps & 15 ps) and slow (110 ps & ≈5 ns) components. The fast decay component exhibits a deuterium isotope effect with D2O in the hydrogel, indicating that the 15 ps decay component is attributable to excited-state intramolecular hydrogen atom transfer of curcumin in the PAAC18 hydrogel. In addition, solvent reorganisation of excited-state curcumin is investigated using multiwavelength femtosecond fluorescence upconversion spectroscopy. The results show that the dominant solvation response (τ = 0.08 ps) is a fast inertial motion owing to the presence of bulk-like water in the vicinity of the hydrophobic octadecyl substituents of the PAAC18 hydrogel. The results also show an additional response with longer time constants of 1 and 6 ps, which is attributable to translational diffusion of confined water molecules in the three-dimensional, cross-linking network of the octadecyl substituents of PAAC18. Overall, we show that excited-state intramolecular hydrogen atom transfer and solvent reorganisation are major photophysical events for curcumin in the PAAC18 hydrogel.

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. In addition, the fluorescence of curcumin in polyester NPs has a quantum yield of 4 to 5%, which is higher than that of curcumin in micellar systems and comparable to those in organic solvents, further supporting the idea that the polyester NPs are capable of excluding water from curcumin. Furthermore, the results from femtosecond fluorescence upconversion spectroscopy reveal that there is a decrease in the signal amplitude corresponding to solvent reorganization of excited state curcumin in the polyester NPs compared with curcumin in micellar systems. The Cur-polyester NPs also show a lack of deuterium isotope effect in the fluorescence lifetime. These results indicate that the interaction between curcumin and water in the polyester NPs is significantly weaker than that in micelles. Therefore, the aqueous stability of curcumin is greatly improved due to highly effective segregation from water. The overall outcome suggests that the polyester NPs prepared using the method reported herein are an attractive system for encapsulating and stabilizing curcumin in the aqueous environment.

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. Both 66γCD2su-curcumin and 66γCD2ur-curcumin complexes in water show only an excited-state absorption (ESA) band at 530 nm without any stimulated emission (SE) signals, indicating non-radiative decays as the major relaxation pathways. The ESA dynamics of 66γCD2su-curcumin are similar to those of 66γCD2ur-curcumin, consisting of a rapid growth component and three decay components. The growth component, which has a time constant of 0.25-0.41 ps, is assigned to solvent reorganization. The relatively fast decay components with time constants of 9.3-21.8 ps show significant deuterium isotope effect, consistent with the presence of excited-state intramolecular hydrogen atom transfer (ESIHT) of curcumin. The small-amplitude and slow decay components may be attributed to the dynamics of complexed curcumin and molecular motions due to flexibility of 66γCD2su and 66γCD2ur. In addition, transient absorption anisotropy measurements reveal slow rotational motions of 66γCD2su-curcumin and 66γCD2ur-curcumin complexes. The overall results show that complexation in 66γCD2su and 66γCD2ur has pronounced effects on the photophysics of curcumin.

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. We report dynamics maps showing the pathways of charge transfer that clearly expose the significance of hot electron transfer. During this ultrafast electron transfer, vibrational coherence is directly transferred from the P3HT exciton to the P3HT hole polaron in the crystalline domain. This result reveals that the exciton converts to a hole with a similar spatial extent on a timescale far exceeding other photophysical dynamics including vibrational relaxation.

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 7.4 and 37 °C such that its degradation half-life is increased by 1600-2000-fold. The suppression of degradation is attributed to hydrophobic interactions between curcumin and the octadecyl substituents of PAAC18 within the micelle-like aggregates and the hydrogel, as indicated by 2D NOESY (1)H NMR spectroscopy. UV-visible absorption titration results are consistent with the interaction of curcumin with five octadecyl substituents on average, which appears to substantially exclude water and greatly decrease the curcumin degradation rate. Dynamic light scattering and zeta potential measurements show the average hydrodynamic diameters of the PAAC18 aggregates to be 0.86-1.15 µm with a negative surface charge. In contrast to the octadecyl substitution, the 3% dodecyl randomly substituted polyacrylate, PAAC12, shows a negligible effect on slowing the degradation of curcumin, consistent with the dodecyl substituents being insufficiently long to capture curcumin in a adequately hydrophobic environment. These observations indicate the potential for PAAC18 to act as a model drug delivery system.

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 (0.16 ps) and picosecond (2.4 ps) time constants. These dynamics are consistent with P3HT torsional relaxation, with the 0.16 ps time constant assigned to ultrafast inertial torsional relaxation. Additionally, the signal exhibits an incomplete recovery, indicating dissociation of high-energy excitons to form charge carriers due to excitation by the push pulse. Our analysis indicates that charge carriers are formed with a yield of 11%.