Explaining the temperature dependence of spirilloxanthin's S* signal by an inhomogeneous ground state model

The nature of carotenoid S* state and its role in the nonphotochemical quenching of plants

In plants, light-harvesting complexes serve as antennas to collect and transfer the absorbed energy to reaction centers, but also regulate energy transport by dissipating the excitation energy of chlorophylls. This process, known as nonphotochemical quenching, seems to be activated by conformational changes within the light-harvesting complex, but the quenching mechanisms remain elusive. Recent spectroscopic measurements suggest the carotenoid S* dark state as the quencher of chlorophylls' excitation. By investigating lutein embedded in different conformations of CP29 (a minor antenna in plants) via nonadiabatic excited state dynamics simulations, we reveal that different conformations of the complex differently stabilize the lutein s-trans conformer with respect to the dominant s-cis one. We show that the s-trans conformer presents the spectroscopic signatures of the S* state and rationalize its ability to accept energy from the closest excited chlorophylls, providing thus a relationship between the complex's conformation and the nonphotochemical quenching.

Understanding Carotenoid Dynamics via the Vibronic Energy Relaxation Approach

Carotenoids are an integral part of natural photosynthetic complexes, with tasks ranging from light harvesting to photoprotection. Their underlying energy deactivation network of optically dark and bright excited states is extremely efficient: after excitation of light with up to 2.5 eV of photon energy, the system relaxes back to ground state on a time scale of a few picoseconds. In this article, we summarize how a model based on the vibrational energy relaxation approach (VERA) explains the main characteristics of relaxation dynamics after one-photon excitation with special emphasis on the so-called S* state. Lineshapes after two-photon excitation are beyond the current model of VERA. We outline this future line of research in our article. In terms of experimental method development, we discuss which techniques are needed to better describe energy dissipation effects in carotenoids and within the first solvation shell.

Photogeneration of Long-Lived Triplet States through Singlet Fission in Lycopene H-Aggregates

Molecular triplet excitons produced through singlet fission (SF) usually have shorter triplet lifetimes due to exciton-exciton recombination and relaxation pathways, thereby resulting in complex device architectures for SF-boosted solar cells. Using broadband transient absorption spectroscopy, we here show that the photoexcitation of nanostructured lycopene H-aggregates at room temperature produces free triplets with an unprecedented 35-fold enhancement in the lifetime compared to those localized on the monomer backbone. The observed rise of a spectrally blue-shifted correlated T-T pair state in ∼19 ps with distinct vibronic features provides the basis for SF-induced triplet generation.

Effects of tunable excitation in carotenoids explained by the vibrational energy relaxation approach

Carotenoids are fundamental building blocks of natural light harvesters with convoluted and ultrafast energy deactivation networks. In order to disentangle such complex relaxation dynamics, several studies focused on transient absorption measurements and their dependence on the pump wavelength. However, such findings are inconclusive and sometimes contradictory. In this study, we compare internal conversion dynamics in [Formula: see text]-carotene, pumped at the first, second, and third vibronic progression peak. Instead of employing data fitting algorithms based on global analysis of the transient absorption spectra, we apply a fully quantum mechanical model to treat the high-frequency symmetric carbon-carbon (C=C and C-C) stretching modes explicitly. This model successfully describes observed population dynamics as well as spectral line shapes in their time-dependence and allows us to reach two conclusions: Firstly, the broadening of the induced absorption upon excess excitation is an effect of vibrational cooling in the first excited state ([Formula: see text]). Secondly, the internal conversion rate between the second excited state ([Formula: see text]) and [Formula: see text] crucially depends on the relative curve displacement. The latter point serves as a new perspective on solvent- and excitation wavelength-dependent experiments and lifts contradictions between several studies found in literature.

Origin of the S* Excited State Feature of Carotenoids in Light-Harvesting Complex 1 from Purple Photosynthetic Bacteria

This spectroscopic study investigates the origin of the transient feature of the S* excited state of carotenoids bound in LH1 complexes from purple bacteria. The studies were performed on two RC-LH1 complexes from Rba. sphaeroides strains that bound carotenoids with different carbon-carbon double bond conjugation N, neurosporene (N = 9) and spirilloxanthin (N = 13). The S* transient spectral feature, originally associated with an elusive and optically silent excited state of spirilloxanthin in the LH1 complex, may be successfully explained and mimicked without involving any unknown electronic state. The spectral and temporal characteristics of the S* feature suggest that it is associated with triplet-triplet annihilation of carotenoid triplets formed after direct excitation of the molecule via a singlet fission mechanism. Depending on pigment homogeneity and carotenoid assembly in the LH1 complex, the spectro-temporal component associated with triplet-triplet annihilation may simply resolve a pure T-S spectrum of a carotenoid. In some cases (like spirilloxanthin), the T-S feature will also be accompanied by a carotenoid Stark spectrum and/or residual transient absorption of minor carotenoid species bound into LH1 antenna complex.

A Unified Picture of S* in Carotenoids

In π-conjugated chain molecules such as carotenoids, coupling between electronic and vibrational degrees of freedom is of central importance. It governs both dynamic and static properties, such as the time scales of excited state relaxation as well as absorption spectra. In this work, we treat vibronic dynamics in carotenoids on four electronic states (|S0⟩, |S1⟩, |S2⟩, and |Sn⟩) in a physically rigorous framework. This model explains all features previously associated with the intensely debated S* state. Besides successfully fitting transient absorption data of a zeaxanthin homologue, this model also accounts for previous results from global target analysis and chain length-dependent studies. Additionally, we are able to incorporate findings from pump-deplete-probe experiments, which were incompatible to any pre-existing model. Thus, we present the first comprehensive and unified interpretation of S*-related features, explaining them by vibronic transitions on either S1, S0, or both, depending on the chain length of the investigated carotenoid.

Challenges facing an understanding of the nature of low-energy excited states in photosynthesis

While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.

Ultra-broadband 2D electronic spectroscopy of carotenoid-bacteriochlorophyll interactions in the LH1 complex of a purple bacterium

We investigate the excitation energy transfer (EET) pathways in the photosynthetic light harvesting 1 (LH1) complex of purple bacterium Rhodospirillum rubrum with ultra-broadband two-dimensional electronic spectroscopy (2DES). We employ a 2DES apparatus in the partially collinear geometry, using a passive birefringent interferometer to generate the phase-locked pump pulse pair. This scheme easily lends itself to two-color operation, by coupling a sub-10 fs visible pulse with a sub-15-fs near-infrared pulse. This unique pulse combination allows us to simultaneously track with extremely high temporal resolution both the dynamics of the photoexcited carotenoid spirilloxanthin (Spx) in the visible range and the EET between the Spx and the B890 bacterio-chlorophyll (BChl), whose Qx and Qy transitions peak at 585 and 881 nm, respectively, in the near-infrared. Global analysis of the one-color and two-color 2DES maps unravels different relaxation mechanisms in the LH1 complex: (i) the initial events of the internal conversion process within the Spx, (ii) the parallel EET from the first bright state S2 of the Spx towards the Qx state of the B890, and (iii) the internal conversion from Qx to Qy within the B890.

Ultrafast Dynamics of Long Homologues of Carotenoid Zeaxanthin

Three zeaxanthin homologues with conjugation lengths N of 15, 19, and 23 denoted as Z15, Z19, and Z23 were studied by femtosecond transient absorption spectroscopy, and the results were compared to those obtained for zeaxanthin (Z11). The energies of S2 decrease from 20 450 cm(-1) (Z11) to 18 280 cm(-1) (Z15), 17 095 cm(-1) (Z19), and 16 560 cm(-1) (Z23). Fitting the N dependence of the S2 energies allowed the estimation of [Formula: see text], the S2 energy of a hypothetical infinite zeaxanthin, to be ∼14 000 cm(-1). Exciting the 0-0 band of the S2 state produces characteristic S1-Sn spectral profiles in transient absorption spectra with maxima at 556 nm (Z11), 630 nm (Z15), 690 nm (Z19), and 740 nm (Z23). The red shift of the S1-Sn transition with increasing conjugation length is caused by a decrease in the S1 state energy, resulting in S1 lifetimes of 9 ps (Z11), 0.9 ps (Z15), 0.35 ps (Z19), and 0.19 ps (Z23). Essentially the same lifetimes were obtained after excess energy excitation at 400 nm, but S1-Sn becomes broader, indicating a larger conformation disorder in the S1 state after 400 nm excitation compared to excitation into the 0-0 band of the S2 state. An S* signal was observed in all samples, but only for Z15, Z19, and Z23 does the S* signal decay with a lifetime different from that of the S1 state. The S* lifetimes are 2.9 and 1.6 ps for Z15 and Z19, respectively. In Z23 the S* signal needs two decay components yielding lifetimes of 0.24 and 2.3 ps. The S* signal is more pronounced after 400 nm excitation.

Carotenoids and light-harvesting: from DFT/MRCI to the Tamm-Dancoff approximation

Carotenoids are known to play a fundamental role in photosynthetic light-harvesting (LH) complexes; however, an accurate quantum-mechanical description of that is still missing. This is due to the multideterminant nature of the involved electronic states combined with an extended conjugation which limits the applicability of many of the most advanced approaches. In this study, we apply a multireference configuration interaction extension of density functional theory (DFT/MRCI) to describe transition energies and densities as well as the corresponding excitonic couplings, for the three lowest singlet excited states of nine carotenoids present in three different LH complexes of algae and plants. These benchmark results are used to find an approximated computational approach, which could be used to quantitatively reproduce the key quantities at a reduced computational cost. To this end, we tested the Tamm-Dancoff approximation (TDA) to time-dependent density functional theory in combination with different functionals. By analyzing the errors with respect to DFT/MRCI-TDA results for the full set of electronic properties, we conclude that TDA-TPSS with small basis sets indeed represents an effective approach to investigate LH processes that involve carotenoids.

Vibronic energy relaxation approach highlighting deactivation pathways in carotenoids

Energy relaxation between two electronic states of a molecule is mediated by a set of relevant vibrational states.

Collisional relaxation of apocarotenals: identifying the S* state with vibrationally excited molecules in the ground electronic state S0*

The S* signal of carotenoids corresponds to vibrationally hot molecules in the ground electronic state S₀*.

On the photophysics of carotenoids: a multireference DFT study of peridinin

We present a quantum-mechanical investigation of the photophysics of a specific carotenoid, peridinin, which is present in light-harvesting complexes. The fundamental role played by the geometry in determining the position and character of its low-lying singlet electronic states is investigated using a multireference DFT approach in combination with a continuum solvation model. The main photophysical properties of peridinin appear to be governed by the lowest two singlet excited states, as no evidence points to an intermediate S* state and the energies of the upper excited states are too high to allow their population with excitation in the visible range. These two excited states (S1, 2(1)A(g)(-) and S2, 1(1)B(u)(+)) are highly connected through the conjugation path here characterized by the value of the bond length alternation (BLA). The S1 and S2 states present distinct natures for small BLA values, whereas for larger ones they become more similar in terms of both brightness and dipolar character and their energies become closer. The geometrical issue is thus of fundamental importance for a correct interpretation of the spectroscopic signatures of peridinin.