Excited state dynamics of the astaxanthin radical cation

Relaxation dynamics of high-energy excited states of carotenoids studied by UV excitation and pump-repump-probe transient absorption spectroscopy

The excited states of carotenoids have been a subject of numerous studies. While a majority of these reports target the excited state dynamics initiated by the excitation of the S2 state, the upper excited state(s) absorbing in the UV spectral region (denoted as SUV) has been only scarcely studied. Moreover, the relation between the SUV and Sn, the final state of the well-known S1-Sn transition of carotenoids, remains unknown. To address this yet-unresolved issue, we compared the excited state dynamics of two carotenoids, namely, β-carotene and astaxanthin, after excitation of either the SUV or Sn state. The SUV state was excited directly by UV light, and the excitation of the Sn state was achieved via re-pumping the S1-Sn transition. The results indicated that direct SUV excitation produces an S1-Sn band that is significantly broader than that obtained after S2 excitation, most probably due to the generation of multiple S1 conformations produced by excess energy. No such broadening is observed if the Sn state is excited by the re-pump pulse. This shows that the Sn and SUV states are different, each initializing a specific relaxation pathway. We propose that the Sn state retains the coupled triplet pair character of the S1 state, while the SUV state is the higher state of Bu+ symmetry accessible by one-photon transition.

The excited-state dynamics of the radical anions of cyanoanthracenes

The radical anion of 9,10-dicyanoanthracene (DCA) has been suggested to be a promising chromophore for photoredox chemistry, due to its nanosecond excited-state lifetime determined from indirect measurements. Here, we investigate the excited-state dynamics of the radical anion of three cyanoanthracenes, including DCA˙-, produced by photoinduced electron transfer in liquid using both pump-probe and pump-pump probe transient electronic absorption spectroscopy. All three excited radical ions are characterised by a 3-5 ps lifetime, due to efficient non-radiative deactivation to the ground state. The decay pathway most probably involves D1/D0 conical intersection(s), whose presence is favoured by the enhanced flexibility of the radical anions relative to their neutral counterparts. The origin of the discrepancy with the nanosecond lifetime of DCA˙-* reported previously is discussed. These very short lifetimes limit, but do not preclude, photochemical applications of the cyanoanthracene anions.

Excited-state Dynamics of Radical Ions in Liquids

Thomas Bally has acquired international recognition for his work on the photochemistry of reactive intermediates, which include radical ions. Here, we present a brief overview of our investigations of the excited-state dynamics of radical ions in liquids at room temperature, which are still poorly documented. A better understanding of these dynamics is most relevant, as open-shell ions in the excited state are being increasingly used in redox photochemistry and have been proposed to play a key role in highly exergonic photoinduced electron transfer reactions.

Carotenoid radical ions: A laser flash photolysis study

Laser excitation of a single precursor, namely 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (HHEMP), has been used for generating the radical cations and radical anions of various carotenoids in methanol. In the presence of oxygen, laser excitation of HHEMP undergoes an efficient α-cleavage reaction (Norrish type I) to form acyl radicals, which react with O₂, in a nearly diffusion-controlled reaction, to form their corresponding strong oxidizing acylperoxyl radicals (RO₂^•) (E = ~1.1 V (v SHE)), which are capable of oxidizing almost all carotenoids. Under argon-saturated conditions and in the presence of strong base (0.01 M NaOH or tetrabutylammonium hydroxide (TBAOH)), the initially formed 2-hydroxy-2-propyl radical (ACH^•), generated after LFP of HHEMP, is deprotonated to form the strong reducing acetone ketyl radical (AC^•-) (E {acetone/ AC^•-} = -2.1 V (v SHE)), which is capable of reducing all carbonyl-containing carotenoids. To validate this new proposed approach, retinal and β-apo-8'-carotenal (APO), with known spectroscopic data, were investigated in methanol, acetonitrile and tetrahydrofuran (THF). In addition, the radical ions of newly investigated carotenoids, namely 4-oxo-β-apo-15'-carotenoic acid (4-oxo-15'), crocetindial, 4-oxo-β-apo-10'-carotenoic acid ethyl ester (4-oxo-10') and 4-oxo-β-apo-8'-carotenoic acid ethyl ester (4-oxo-8') have been reported. Moreover, the scope of this approach has been extended to investigate the radical ions of chlorophyll b.

A Mitochondrion-Localized Two-Photon Photosensitizer Generating Carbon Radicals Against Hypoxic Tumors

The efficacy of photodynamic therapy is typically reliant on the local concentration and diffusion of oxygen. Due to the hypoxic microenvironment found in solid tumors, oxygen-independent photosensitizers are in great demand for cancer therapy. We herein report an iridium(III) anthraquinone complex as a mitochondrion-localized carbon-radical initiator. Its emission is turned on under hypoxic conditions after reduction by reductase. Furthermore, its two-photon excitation properties (λ_ex =730 nm) are highly desirable for imaging. Upon irradiation, the reduced form of the complex generates carbon radicals, leading to a loss of mitochondrial membrane potential and cell death (IC₅₀ ^light =2.1 μm, IC₅₀ ^dark =58.2 μm, PI=27.7). The efficacy of the complex as a PDT agent was also demonstrated under hypoxic conditions in vivo. To the best of our knowledge, it is the first metal-complex-based theranostic agent which can generate carbon radicals for oxygen-independent two-photon photodynamic therapy.

Ion-Pair Dynamics upon Photoinduced Electron Transfer Monitored by Pump-Pump-Probe Spectroscopy

The excited-state dynamics of the radical anion of perylene (Pe) generated upon bimolecular photoinduced electron transfer (PET) with a donor was investigated using broadband pump-pump-probe spectroscopy. It was found to depend on the age of the anion, that is, on the time interval between the first pump pulse that triggers PET and the second one that excites the ensuing Pe anion (Pe^•-). These differences, observed in acetonitrile but not in tetrahydrofuran, report on the evolution of the PET product from an ion pair to free ions. Two photoinduced charge recombination pathways of the ion pair to the neutral Pe*(S₁) + donor state were identified: one occurring in a few picoseconds from Pe^•-*(D₁) and one taking place within 100-200 fs from Pe^•-*(D _n>1). Both processes are sensitive to the interionic distance over different length scales and thus serve as molecular rulers.

Revisiting Vertical Models To Simulate the Line Shape of Electronic Spectra Adopting Cartesian and Internal Coordinates

Vertical models for the simulation of spectroscopic line shapes expand the potential energy surface (PES) of the final state around the equilibrium geometry of the initial state. These models provide, in principle, a better approximation of the region of the band maximum. At variance, adiabatic models expand each PES around its own minimum. In the harmonic approximation, when the minimum energy structures of the two electronic states are connected by large structural displacements, adiabatic models can breakdown and are outperformed by vertical models. However, the practical application of vertical models faces the issues related to the necessity to perform a frequency analysis at a nonstationary point. In this contribution we revisit vertical models in harmonic approximation adopting both Cartesian (x) and valence internal curvilinear coordinates (s). We show that when x coordinates are used, the vibrational analysis at nonstationary points leads to a deficient description of low-frequency modes, for which spurious imaginary frequencies may even appear. This issue is solved when s coordinates are adopted. It is however necessary to account for the second derivative of s with respect to x, which here we compute analytically. We compare the performance of the vertical model in the s-frame with respect to adiabatic models and previously proposed vertical models in x- or Q₁-frame, where Q₁ are the normal coordinates of the initial state computed as combination of Cartesian coordinates. We show that for rigid molecules the vertical approach in the s-frame provides a description of the final state very close to the adiabatic picture. For sizable displacements it is a solid alternative to adiabatic models, and it is not affected by the issues of vertical models in x- and Q₁-frames, which mainly arise when temperature effects are included. In principle the G matrix depends on s, and this creates nonorthogonality problems of the Duschinsky matrix connecting the normal modes of initial and final states in adiabatic approaches. We highlight that such a dependence of G on s is also an issue in vertical models, due to the necessity to approximate the kinetic term in the Hamiltonian when setting up the so-called GF problem. When large structural differences exist between the initial and the final-state minima, the changes in the G matrix can become too large to be disregarded.

Ultrafast primary processes of the stable neutral organic radical, 1,3,5-triphenylverdazyl, in liquid solution

Femtosecond pump–probe spectroscopy reveals ultrafast photochemical processes of a stable neutral organic radical in solution.

Dual electron transfer pathways from the excited C60 radical anion: enhanced reactivities due to the photoexcitation of reaction intermediates

The excited C₆₀ radical anion showed enhanced electron transfer.

Oil bodies as a potential microencapsulation carrier for astaxanthin stabilisation and safe delivery

Astaxanthin (AST) is a valued molecule because of its high antioxidant properties. However, AST is extremely sensitive to oxidation, causing the loss of its bioactive properties. The purposes of this study were to define conditions for microencapsulating AST in oil bodies (OB) from Brassica napus to enhance its oxidative stability, and to test the bioactivity of the microencapsulated AST (AST-M) in cells. Conditions for maximising microencapsulation efficiency (ME) were determined using the Response Surface Methodology, obtaining a high ME (>99%). OB loaded with AST showed a strong electrostatic repulsion in a wide range of pH and ionic strengths. It was found that AST-M exposed to air and light was more stable than free AST. In addition, the protective effect of AST against intracellular ROS production was positively influenced by microencapsulation in OB. These results suggest that OB offer a novel option for stabilising and delivering AST.

Multispectral image analysis for robust prediction of astaxanthin coating

The aim of this study was to investigate the possibility of predicting the type and concentration level of astaxanthin coating of aquaculture feed pellets using multispectral image analysis. We used both natural and synthetic astaxanthin, and we used several different concentration levels of synthetic astaxanthin in combination with four different recipes of feed pellets. We used a VideometerLab with 20 spectral bands in the range of 385-1050 nm. We used linear discriminant analysis and sparse linear discriminant analysis for classification and variable selection. We used partial least squares regression (PLSR) for prediction of the concentration level. The results show that it is possible to predict the level of synthetic astaxanthin coating using PLSR on either the same recipe, or when calibrating on all recipes. The concentration prediction is adequate for screening for all recipes. Moreover, it shows that it is possible to predict the type of astaxanthin used in the coating using only ten spectral bands. Finally, the most selected spectral bands for astaxanthin prediction are in the visible range of the spectrum.

Ultrafast excited state dynamics and spectroscopy of 13,13'-diphenyl-β-carotene

Ultrafast transient broadband absorption spectroscopy based on the Pump-Supercontinuum Probe (PSCP) technique has been applied to characterize the excited state dynamics of the newly-synthesized artificial β-carotene derivative 13,13'-diphenyl-β-carotene in the wavelength range 340-770 nm with ca. 60 fs cross-correlation time after excitation to the S(2) state. The influence of phenyl substitution at the polyene backbone has been investigated in different solvents by comparing the dynamics of the internal conversion (IC) processes S(2)→ S(1) and S(1)→ S(0)* with results for β-carotene. Global analysis provides IC time constants and also time-dependent S(1) spectra demonstrating vibrational relaxation processes. Intramolecular vibrational redistribution processes are accelerated by phenyl substitution and are also solvent-dependent. DFT and TDDFT-TDA calculations suggest that both phenyl rings prefer an orientation where their ring planes are almost perpendicular to the plane of the carotene backbone, largely decoupling them electronically from the polyene system. This is consistent with several experimental observations: the up-field chemical shift of adjacent hydrogen atoms by a ring-current effect of the phenyl groups in the (1)H NMR spectrum, a small red-shift of the S(0)→ S(2)(0-0) transition energy in the steady-state absorption spectrum relative to β-carotene, and almost the same S(1)→ S(0)* IC time constant as in β-carotene, suggesting a similar S(1)-S(0) energy gap. The oscillator strength of the S(0)→ S(2) transition of the diphenyl derivative is reduced by ca. 20%. In addition, we observe a highly structured ground state bleach combined with excited state absorption at longer wavelengths, which is typical for an "S* state". Both features can be clearly assigned to absorption of vibrationally hot molecules in the ground electronic state S(0)* superimposed on the bleach of room temperature molecules S(0). The S(0)* population is formed by IC from S(1). These findings are discussed in detail with respect to alternative interpretations previously reported in the literature. Understanding the dynamics of this type of artificial phenyl-substituted carotene systems appears useful regarding their future structural optimization with respect to enhanced thermal stability while keeping the desired photophysical properties.