Intramolecular charge transfer and the function of vibronic excitons in photosynthetic light harvesting.

A widely discussed explanation for the prevalence of pairs or clusters of closely spaced electronic chromophores in photosynthetic light-harvesting proteins is the presence of ultrafast and highly directional excitation energy transfer pathways mediated by vibronic excitons, the delocalized optical excitations derived from mixing of the electronic and vibrational states of the chromophores. We discuss herein the hypothesis that internal conversion processes between exciton states on the <100 fs timescale are possible when the excitonic potential energy surfaces are controlled by the vibrational modes that induce charge transfer character in a strongly coupled system of chromophores. We discuss two examples, the peridinin-chlorophyll protein from marine dinoflagellates and the intact phycobilisome from cyanobacteria, in which the intramolecular charge-transfer (ICT) character arising from out-of-plane distortion of the conjugation of carotenoid or bilin chromophores also results in localization of the initially delocalized optical excitation on the vibrational timescale. Tuning of the ground state conformations of the chromophores to manipulate their ICT character provides a natural photoregulatory mechanism, which would control the overall quantum yield of excitation energy transfer by turning on and off the delocalized character of the optical excitations.

Origin of Vibronic Coherences During Carrier Cooling in Colloidal Quantum Dots.

Recent two-dimensional electronic spectroscopy experiments [Tilluck et al. J. Phys. Chem. Lett. 2021, 12 (39), 9677-9683] indicate the creation of coherent vibronic wavepackets in the first femtoseconds of hot carrier cooling in hexadecylamine-passivated CdSe quantum dots. Here we present a quantum chemical study of the origin of these coherences in a CdSe nanocrystal. We find that coherent wavepacket motions along vibrational coordinates with alkylamine character promote nonradiative relaxation through conical intersections between the exciton states of the inorganic core. Electronic excitations in the core are found to pass energy to the vibrations of the ligands via two distinct mechanisms: excitation of core phonon modes that are coupled to the ligand vibrations and direct excitation of ligand vibrations by delocalization of the exciton onto the ligands, both of which naturally arise within a photochemical framework based on many-electron potential energy surfaces. If these findings are demonstrated to be general, vibronic coherences may be leveraged to control photophysical outcomes in colloidal quantum dots.

Photoactivation of the orange carotenoid protein requires two light-driven reactions mediated by a metastable monomeric intermediate.

The orange carotenoid protein (OCP) functions as a sensor of the ambient light intensity and as a quencher of bilin excitons when it binds to the core of the cyanobacterial phycobilisome. We show herein that the photoactivation mechanism that converts the resting, orange-colored state, OCPO, to the active red-colored state, OCPR, requires a sequence of two reactions, each requiring absorption of a single photon by an intrinsic ketocarotenoid chromophore. Global analysis of absorption spectra recorded during continuous illumination of OCPO preparations from Synechocystis sp. PCC 6803 detects the reversible formation of a metastable intermediate, OCPI, in which the ketocarotenoid canthaxanthin exhibits an absorption spectrum with a partial red shift and a broadened vibronic structure compared to that of the OCPO state. While the dark recovery from OCPR to OCPI is a first-order, unimolecular reaction, the subsequent conversion of OCPI to the resting OCPO state is bimolecular, involving association of two OCPO monomers to form the dark-stable OCPO dimer aggregate. These results indicate that photodissociation of the OCPO dimer to form the monomeric OCPO intermediate is the first step in the photoactivation mechanism. Formation of the OCPO monomer from the dimer increases the mean value and broadens the distribution of the solvent-accessible surface area of the canthaxanthin chromophore measured in molecular dynamics trajectories at 300 K. The second step in the photoactivation mechanism is initiated by absorption of a second photon, by canthaxanthin in the OCPO monomer, which obtains the fully red-shifted and broadened absorption spectrum detected in the OCPR product state owing to displacement of the C-terminal domain and the translocation of canthaxanthin more than 12 Å into the N-terminal domain. Both steps in the photoactivation reaction of OCP are likely to involve changes in the structure of the C-terminal domain elicited by excited-state conformational motions of the ketocarotenoid.

Excitation energy transfer and vibronic coherence in intact phycobilisomes.

The phycobilisome is an oligomeric chromoprotein complex that serves as the principal mid-visible light-harvesting system in cyanobacteria. Here we report the observation of excitation-energy-transfer pathways involving delocalized optical excitations of the bilin (linear tetrapyrrole) chromophores in intact phycobilisomes isolated from Fremyella diplosiphon. By using broadband multidimensional electronic spectroscopy with 6.7-fs laser pulses, we are able to follow the progress of excitation energy from the phycoerythrin disks at the ends of the phycobilisome's rods to the C-phycocyanin disks along their length in <600 fs. Oscillation maps show that coherent wavepacket motions prominently involving the hydrogen out-of-plane vibrations of the bilins mediate non-adiabatic relaxation of a manifold of vibronic exciton states. However, the charge-transfer character of the bilins in the allophycocyanin-containing segments localizes the excitations in the core of the phycobilisome, yielding a kinetic bottleneck that enables photoregulatory mechanisms to operate efficiently on the >10-ps timescale.

Fluorescence of carotenoids: Probing binding site interactions and conformational motion in carotenoproteins.

The function of carotenoids in carotenoproteins is optimized by the electrostatic and steric interactions between the carotenoid and its surrounding binding site. Binding to the protein distorts the conformation of the carotenoid and induces a charge-transfer character. This chapter shows how the line shape of the fluorescence spectrum, the fluorescence quantum yield, and the fluorescence anisotropy of the second excited singlet state of a carotenoid, S2, can be used to probe the structure and dynamics of carotenoids in carotenoproteins. The experimental approach and a brief introduction to the theory we used to detect hydrogen bonding interactions with the active ketocarotenoids in the orange carotenoid protein are discussed here as an example. Fluorescence anisotropy is then introduced as a probe of the conformational motions that follow optical excitation of a carotenoid using results from a study of ß-carotene in solution over a range of temperatures.

Linear and Nonlinear Optical Processes Controlling S2 and S1 Dual Fluorescence in Cyanine Dyes.

We report on the changes in the dual fluorescence of two cyanine dyes IR144 and IR140 as a function of viscosity and probe their internal conversion dynamics from S2 to S1 via their dependence on a femtosecond laser pulse chirp. Steady-state and time-resolved measurements performed in methanol, ethanol, propanol, ethylene glycol, and glycerol solutions are presented. Quantum calculations reveal the presence of three excited states responsible for the experimental observations. Above the first excited state, we find an excited state, which we designate as S1', that relaxes to the S1 minimum, and we find that the S2 state has two stable configurations. Chirp-dependence measurements, aided by numerical simulations, reveal how internal conversion from S2 to S1 depends on solvent viscosity and pulse duration. By combining solvent viscosity, transform-limited pulses, and chirped pulses, we obtain an overall change in the S2/S1 population ratio of a factor of 86 and 55 for IR144 and IR140, respectively. The increase in the S2/S1 ratio is explained by a two-photon transition to a higher excited state. The ability to maximize the population of higher excited states by delaying or bypassing nonradiative relaxation may lead to the increased efficiency of photochemical processes.

Vibronic Excitons and Conical Intersections in Semiconductor Quantum Dots.

Surface defects and organic surface-capping ligands affect the photoluminescence properties of semiconductor quantum dots (QDs) by altering the rates of competing nonradiative relaxation processes. In this study, broadband two-dimensional electronic spectroscopy reveals that absorption of light by QDs prepares vibronic excitons, excited states derived from quantum coherent mixing of the core electronic and ligand vibrational states. Rapidly damped coherent wavepacket motions of the ligands are observed during hot-carrier cooling, with vibronic coherence transferred to the photoluminescent state. These findings suggest a many-electron, molecular theory for the electronic structure of QDs, which is supported by calculations of the structures of conical intersections between the exciton potential surfaces of a small ammonia-passivated model CdSe nanoparticle.

Broadband 2DES detection of vibrational coherence in the Sx state of canthaxanthin.

The nonadiabatic mechanism that mediates nonradiative decay of the bright S2 state to the dark S1 state of carotenoids involves population of a bridging intermediate state, Sx, in several examples. The nature of Sx remains to be determined definitively, but it has been recently suggested that Sx corresponds to conformationally distorted molecules evolving along out-of-plane coordinates of the isoprenoid backbone near a low barrier between planar and distorted conformations on the S2 potential surface. In this study, the electronic and vibrational dynamics accompanying the formation of Sx in toluene solutions of the ketocarotenoid canthaxanthin (CAN) are characterized with broadband two-dimensional electronic spectroscopy (2DES) with 7.8 fs excitation pulses and detection of the linear polarization components of the third-order nonlinear optical signal. A stimulated-emission cross peak in the 2DES spectrum accompanies the formation of Sx in <20 fs following excitation of the main absorption band. Sx is prepared instantaneously, however, with excitation of hot-band transitions associated with distorted conformations of CAN's isoprenoid backbone in the low frequency onset of the main absorption band. Vibrational coherence oscillation maps and modulated anisotropy transients show that Sx undergoes displacements from the Franck-Condon S2 state along out-of-plane coordinates as it passes to the S1 state. The results are consistent with the conclusion that CAN's carbonyl-substituted ß-ionone rings impart an intramolecular charge-transfer character that frictionally slows the passage from Sx to S1 compared to carotenoids lacking carbonyl substitution. Despite the longer lifetime, the S1 state of CAN is formed with retention of vibrational coherence after passing through a conical intersection seam with the Sx state.

Spectral Signatures of Canthaxanthin Translocation in the Orange Carotenoid Protein.

The orange carotenoid protein (OCP) is involved in the photoprotective processes in cyanobacteria via nonphotochemical quenching. Triggered by blue-green light absorption, the carotenoid chromophore undergoes translocation, displacing around 12 Å from the C-terminal domain (CTD) to the N-terminal domain (NTD). The detailed molecular rearrangements that occur within the carotenoid and the protein during this process remain largely elusive. By using a combination of molecular dynamics, well-tempered metadynamics, and hybrid quantum mechanical/molecular mechanical (QM/MM) calculations, we were able to mimic the translocation of the carotenoid from the inactive OCPO and obtain metastable red-shifted states in the photoactivation mechanism, replicating the λmax values of reference experimental spectra. In addition, our simulations give insight into the structure of the red-shifted form of the inactive state of OCP.

Fluorescence Anisotropy Detection of Barrier Crossing and Ultrafast Conformational Dynamics in the S2 State of ß-Carotene.

Carotenoids are usually only weakly fluorescent despite being very strong absorbers in the mid-visible region because their first two excited singlet states, S1 and S2, have very short lifetimes. To probe the structural mechanisms that promote the nonradiative decay of the S2 state to the S1 state, we have carried out a series of fluorescence lineshape and anisotropy measurements with a prototype carotenoid, ß-carotene, in four aprotic solvents. The anisotropy values observed in the fluorescence emission bands originating from the S2 and S1 states reveal that the large internal rotations of the emission transition dipole moment, as much as 50° relative to that of the absorption transition dipole moment, are initiated during ultrafast evolution on the S2 state potential energy surface and persist upon nonradiative decay to the S1 state. Electronic structure calculations of the orientation of the transition dipole moment account for the anisotropy results in terms of torsional and pyramidal distortions near the center of the isoprenoid backbone. The excitation wavelength dependence of the fluorescence anisotropy indicates that these out-of-plane conformational motions are initiated by passage over a low-activation energy barrier from the Franck-Condon S2 structure. This conclusion is consistent with detection over the 80-200 K range of a broad, red-shifted fluorescence band from a dynamic intermediate evolving on a steep gradient of the S2 state potential energy surface after crossing the activation barrier. The temperature dependence of the oscillator strength and anisotropy indicate that nonadiabatic passage from S2 through a conical intersection seam to S1 is promoted by the out-of-plane motions of the isoprenoid backbone with strong hindrance by solvent friction.

Structural Tuning of Quantum Decoherence and Coherent Energy Transfer in Photosynthetic Light Harvesting.

Photosynthetic organisms capture energy from solar photons by constructing light-harvesting proteins containing arrays of electronic chromophores. Collective excitations (excitons) arise when energy transfer between chromophores is coherent, or wavelike, in character. Here we demonstrate experimentally that coherent energy transfer to the lowest-energy excitons is principally controlled in a light-harvesting protein by the temporal persistence of quantum coherence rather than by the strength of vibronic coupling. In the peridinin-chlorophyll protein from marine dinoflagellates, broad-band two-dimensional electronic spectroscopy reveals that replacing the native chlorophyll a acceptor chromophores with chlorophyll b slows energy transfer from the carotenoid peridinin to chlorophyll despite narrowing the donor-acceptor energy gap. The formyl substituent on the chlorophyll b macrocycle hastens decoherence by sensing the surrounding electrostatic noise. These findings demonstrate how quantum coherence enhances the efficiency of energy transfer despite being very short lived in light-harvesting proteins at physiological temperatures.

Fluorescence and Excited-State Conformational Dynamics of the Orange Carotenoid Protein.

The orange carotenoid protein (OCP) mediates nonphotochemical quenching (NPQ) mechanisms in cyanobacteria. A bound ketocarotenoid serves as a sensor of midvisible light intensity and as a quencher of phycocyanobilin excitons in the phycobilisome. The photochemical mechanism that triggers conversion of the protein from a resting, orange state (OCPO) to an active, red state (OCPR) after optical preparation of the S2 state of the carotenoid remains an open question. We report here that the fluorescence spectrum and quantum yield of the bound carotenoids in OCPO report important details of the motions that follow optical preparation of the S2 state. The fluorescence spectra from OCPO preparations containing 3'-hydroxyechinenone (3hECN) or canthaxanthin (CAN) are markedly mirror asymmetric with respect to the absorption line shape and more than an order of magnitude more intense than for carotenoids in solution. Further, 3hECN exhibits a narrower fluorescence line shape and a larger quantum yield than CAN because its excited-state motions are hindered by a hydrogen bonding interaction between the 3'-hydroxyl group on its ß2 ring and Leu37 in the N-terminal domain. These results show that large-amplitude motions of the carotenoid's ß2-cyclohexene ring and of the conjugated polyene backbone initiate photochemistry in OCPO.

Quantum Coherent Excitation Energy Transfer by Carotenoids in Photosynthetic Light Harvesting.

It remains an open question whether quantum coherence and molecular excitons created by delocalization of electronic excited states are essential features of the mechanisms that enable efficient light capture and excitation energy transfer to reaction centers in photosynthetic organisms. The peridinin-chlorophyll a protein from marine dinoflagellates is an example of a light-harvesting system with tightly clustered antenna chromophores in which quantum coherence has long been suspected, but unusually it features the carotenoid peridinin as the principal light absorber for mid-visible photons. We report that broad-band two-dimensional electronic spectroscopy indeed reveals the initial presence of exciton relaxation pathways that enable transfer of excitation from peridinin to chlorophyll a in <20 fs, but the quantum coherence that permits this is very short-lived. Strongly coupled excited-state vibrational distortions of the peridinins trigger a dynamic transition of the electronic structure of the system and a rapid conversion to incoherent energy transfer mechanisms.

Excitation Energy Transfer by Coherent and Incoherent Mechanisms in the Peridinin-Chlorophyll a Protein.

Excitation energy transfer from peridinin to chlorophyll (Chl) a is unusually efficient in the peridinin-chlorophyll a protein (PCP) from dinoflagellates. This enhanced performance is derived from the long intrinsic lifetime of 4.4 ps for the S2 (11Bu+) state of peridinin in PCP, which arises from the electron-withdrawing properties of its carbonyl substituent. Results from heterodyne transient grating spectroscopy indicate that S2 serves as the donor for two channels of energy transfer: a 30 fs process involving quantum coherence and delocalized peridinin-Chl states and an incoherent, 2.5 ps process initiated by dynamic exciton localization, which accompanies the formation of a conformationally distorted intermediate in 45 fs. The lifetime of the S2 state is lengthened in PCP by its intramolecular charge-transfer character, which increases the system-bath coupling and slows the torsional motions that promote nonradiative decay to the S1 (21Ag-) state.

Torsional Dynamics and Intramolecular Charge Transfer in the S2 (1(1)Bu(+)) Excited State of Peridinin: A Mechanism for Enhanced Mid-Visible Light Harvesting.

Of the carotenoids known in photosynthetic organisms, peridinin exhibits one of the highest quantum efficiencies for excitation energy transfer to chlorophyll (Chl) a acceptors. The mechanism for this enhanced performance involves an order-of-magnitude slowing of the S2 (1(1)Bu(+)) → S1 (2(1)Ag(-)) nonradiative decay pathway compared to carotenoids lacking carbonyl substitution. Using femtosecond transient grating spectroscopy with optical heterodyne detection, we have obtained the first evidence that the nonradiative decay of the S2 state of peridinin is promoted by large-amplitude torsional motions. The decay of an intermediate state termed Sx, which we assign to a twisted form of the S2 state, is substantially slowed by solvent friction in peridinin due to its intramolecular charge transfer (ICT) character.

Femtosecond Heterodyne Transient Grating Studies of Nonradiative Deactivation of the S2 (1(1)Bu(+)) State of Peridinin: Detection and Spectroscopic Assignment of an Intermediate in the Decay Pathway.

Femtosecond heterodyne transient grating spectroscopy was employed to investigate the nonradiative decay pathway from the S2 (1(1)Bu(+)) state to the S1 (2(1)Ag(-)) state of peridinin in methanol solution. Just as previously observed by this laboratory for ß-carotene in benzonitrile, the real (absorption) and imaginary (dispersion) components of the transient grating signal obtained with Fourier transform spectral interferometry from peridinin exhibit ultrafast responses indicating that S2 state decays in 12 fs to produce an intermediate state, Sx. The excited state absorption spectrum from the Sx state of peridinin, however, is found to be markedly blue-shifted from that of ß-carotene because it makes a substantial contribution to the signal observed with 40 fs, 520 nm pulses. The results of a global target analysis and numerical simulations using nonlinear response functions and the multimode Brownian oscillator model support the assignment of Sx to a displaced conformation of the S2 state rather than to a vibrationally excited (or hot) S1 state. The Sx state in peridinin is assigned to a structure with a distorted conjugated polyene backbone moving past an activation-energy barrier between planar and twisted structures on the S2 potential surface. The lengthened lifetime of the Sx state of peridinin in methanol, 900 ± 100 fs, much longer than that typically observed for carotenoids lacking carbonyl substituents, ∼150 fs, can be attributed to the slowing of torsional motions by solvent friction. In peridinin, the system-bath coupling is significantly enhanced over that in ß-carotene solution most likely due to the intrinsic intramolecular charge transfer character it derives from the electron withdrawing nature of the carbonyl substituent. An important additional implication is that the Sx state, and the distorted structures reached subsequently along the torsional gradient on the S2 potential surface, may serve as the principal excitation energy transfer donors to chlorophyll a in the peridinin-chlorophyll a protein from dinoflagellates.

Controlling S2 Population in Cyanine Dyes Using Shaped Femtosecond Pulses.

Fast population transfer from higher to lower excited states occurs via internal conversion (IC) and is the basis of Kasha's rule, which states that spontaneous emission takes place from the lowest excited state of the same multiplicity. Photonic control over IC is of interest because it would allow direct influence over intramolecular nonradiative decay processes occurring in condensed phase. Here we tracked the S2 and S1 fluorescence yield for different cyanine dyes in solution as a function of linear chirp. For the cyanine dyes with polar solvation response IR144 and meso-piperidine substituted IR806, increased S2 emission was observed when using transform limited pulses, whereas chirped pulses led to increased S1 emission. The nonpolar solvated cyanine IR806, on the other hand, did not show S2 emission. A theoretical model, based on a nonperturbative solution of the equation of motion for the density matrix, is offered to explain and simulate the anomalous chirp dependence. Our findings, which depend on pulse properties beyond peak intensity, offer a photonic method to control S2 population thereby opening the door for the exploration of photochemical processes initiated from higher excited states.

Femtosecond Heterodyne Transient-Grating Studies of Nonradiative Decay of the S2 (1(1)Bu(+)) State of ß-Carotene: Contributions from Dark Intermediates and Double-Quantum Coherences.

Femtosecond transient-grating spectroscopy with heterodyne detection was employed to characterize the nonradiative decay pathway in ß-carotene from the S2 (1(1)Bu(+)) state to the S1 (2(1)Ag(-)) state in benzonitrile solution. The results indicate definitively that the S2 state populates an intermediate state, Sx, on an ultrafast time scale prior to nonradiative decay to the S1 state. Numerical simulations using the response function formalism and the multimode Brownian oscillator model were used to fit the absorption and dispersion components of the transient-grating signal with a common set of parameters for all of the relevant Feynman pathways, including double-quantum coherences. The requirement for inclusion of the Sx state in the nonradiative decay pathway is the observed fast rise time of the dispersion component, which is predominantly controlled by the decay of the stimulated emission signal from the optically prepared S2 state. The finding that the excited-state absorption spectrum from the Sx state is significantly red-shifted from that of S2 and S1 leads to a new assignment for the spectroscopic origin of the Sx state. Rather than assigning Sx to a discrete electronic state, such as the (1)Bu(-) state suggested in previous work, it is proposed that the Sx state corresponds to a transition-state-like structure on the S2 potential surface. In this hypothesis, the 12 fs time constant for the decay of the S2 state corresponds to a vibrational displacement of the C-C and C═C bond-length alternation coordinates of the conjugated polyene backbone from the optically prepared Franck-Condon structure to a potential energy barrier on the S2 surface that divides planar and torsionally displaced structures. The lifetime of the Sx state would be associated with a subsequent relaxation along torsional coordinates over a steep potential energy gradient toward a conical intersection with the S1 state. This hypothesis leads to the idea that twisted structures with intramolecular charge-transfer character along the S2 torsional gradient are active in excitation energy-transfer mechanisms to (bacterio)chlorophyll acceptors.