Introduction 4546 2. General Information on the Structure of Photosynthetic Complexes and StructureÀFunction Relationships 4548 2.1. Photosystem I (PSI) and Photosystem II (PSII) 4548 2.2. Basic Aspects of Bacterial Photosynthesis 4549 3. Interpigment Interactions, Excitation Energy Transfer (EET), and Charge Separation (CS) Rates-General Considerations 4550 4. Fundamentals of Spectral Hole-Burning (SHB) and Fluorescence Line-Narrowing Spectroscopy (FLNS) and Single Photosynthetic Complex Spectroscopy (SPCS) 4552 4.1. Zero-Phonon Lines, Homogeneous and Inhomogeneous Broadening 4553 4.1.1. ZPLs and Phonon Sidebands (PSBs) 4554 4.1.2. ElectronÀPhonon Coupling and Homogeneous Line Shapes 4555 4.2. Nonphotochemical, Photochemical, and Transient SHB Spectroscopy 4557 4.3. Mechanism of Nonphotochemical Hole-Burning (NPHB) 4558 4.4. Kinetics of NPHB 4559 4.5. Zero-Phonon Action (ZPA) Spectroscopy: Site Distribution Function (SDF) 4560 4.6. Hole Shapes and FLN Line Shapes-Electron Phonon Coupling and ΔFLNS 4561 4.7. Ground and Excited State Vibrational Frequencies 4567 4.8. SHB in Excitonically Coupled Systems 4567 4.9. Basic Principles of SPCS 4568 4.10. Basic Principles of Two-Dimensional Electronic Spectroscopy (2D ES) 4569 5. Examples of Applications of NPHB, FLNS, SPCS, and 2D ES to Photosynthesis 4570 5.1. Light-Harvesting and EET in Antenna Complexes 4570 5.1.1. Peripheral Antenna Systems of Photosystem II (

Previously published and new spectral hole burning (SHB) data on the B800 band of LH2 light-harvesting antenna complex of Rps. acidophila are analyzed in light of recent single photosynthetic complex spectroscopy (SPCS) results (for a review, see Berlin et al. Phys. Life Rev. 2007, 4, 64.). It is demonstrated that, in general, SHB-related phenomena observed for the B800 band are in qualitative agreement with the SPCS data and the protein models involving multiwell multitier protein energy landscapes. Regarding the quantitative agreement, we argue that the single-molecule behavior associated with the fastest spectral diffusion (smallest barrier) tier of the protein energy landscape is inconsistent with the SHB data. The latter discrepancy can be attributed to SPCS probing not only the dynamics of of the protein complex per se, but also that of the surrounding amorphous host and/or of the host-protein interface. It is argued that SHB (once improved models are developed) should also be able to provide the average magnitudes and probability distributions of light-induced spectral shifts and could be used to determine whether SPCS probes a set of protein complexes that are both intact and statistically relevant. SHB results are consistent with the B800 --> B850 energy-transfer models including consideration of the whole B850 density of states.

Hole-burning and single photosynthetic complex spectroscopy were used to study the excitonic structure and excitation energy-transfer processes of cyanobacterial trimeric Photosystem I (PS I) complexes from Synechocystis PCC 6803 and Thermosynechococcus elongatus at low temperatures. It was shown that individual PS I complexes of Synechocystis PCC 6803 (which have two red antenna states, i.e., C706 and C714) reveal only a broad structureless fluorescence band with a maximum near 720 nm, indicating strong electron-phonon coupling for the lowest energy C714 red state. The absence of zero-phonon lines (ZPLs) belonging to the C706 red state in the emission spectra of individual PS I complexes from Synechocystis PCC 6803 suggests that the C706 and C714 red antenna states of Synechocystis PCC 6803 are connected by efficient energy transfer with a characteristic transfer time of approximately 5 ps. This finding is in agreement with spectral hole-burning data obtained for bulk samples of Synechocystis PCC 6803. The importance of comparing the results of ensemble (spectral hole burning) and single-complex measurements was demonstrated. The presence of narrow ZPLs near 710 nm in addition to the broad fluorescence band at approximately 730 nm in Thermosynechococcus elongatus (Jelezko et al. J. Phys. Chem. B 2000, 104, 8093-8096) has been confirmed. We also demonstrate that high-quality samples obtained by dissolving crystals of PS I of Thermosynechococcus elongatus exhibit stronger absorption in the red antenna region than any samples studied so far by us and other groups.

Persistent nonphotochemical and photochemical hole burning of the S0→S1 origin absorption bands of chromophores in amorphous hosts such as glasses, polymers and proteins at low temperatures have been used to address a number of problems that range from structural disorder and configurational tunneling to excitation energy transfer and charge separation in photosynthetic complexes. Often the hole burned spectra are interfered by photoproduct (antihole) absorption. To date there has been no systematic approach to modeling hole burned spectra and the dispersive kinetics of zero-phonon hole growth that accounts for the antihole. A “master” equation that does so is presented. A key ingredient of the equation is a time-dependent, two-dimensional site excitation frequency distribution function (SDF) of the zero-phonon lines. Prior to hole burning (t=0) the SDF is that of the educt sites. For t>0 the SDF describes both educt and photoproduct sites and allows for burning of the latter that revert to the educt sites from which they originate (light-induced hole filling). Our model includes linear electron–phonon coupling and the three distributions that lead to dispersive hole growth kinetics, the most important of which is the distribution for the parameter λ associated with tunneling between the bistable configurations of the chromophore-host system that are interconverted by hole burning. The master equation is successfully applied to free base phthalocyanine (Pc) in hyperquenched glassy ortho-dichlorobenzene (DCB) at 5 K. The mechanism of hole burning is photochemical and involves tautomerization of the two protons at the center of the macrocycle (Pc) that occurs in the S1(Qx) and/or T1(Qx) state of Pc. A single set of parameter values (some of which are determined directly from the hole burned spectra) provides a satisfactory description of the dependence of the hole burned spectra and hole growth kinetics on the location of the burn frequency within the inhomogeneously broadened Qx absorption band. The hole growth kinetics are found to be quite highly dispersive, although to a lesser degree than the kinetics for free base phthalocyanine tetrasulphonate in hyperquenched glassy water [Reinot et al., J. Lumin. 98, 183 (2002)]. The dispersion is attributed to structural heterogeneity of solvent molecules in the inner shell that leads to a distribution of chromophore-host interactions that affect the height of the barrier separating the two tautomers. The new master equation should also prove useful with no additional assumptions or modifications for interpretation of nonphotochemical hole burned spectra.

Nonphotochemical hole burning (NPHB) at low temperatures of the electronic absorption bands of molecular chromophores imbedded in amorphous solids (glasses and polymers) and in proteins is a striking manifestation of configurational tunneling triggered by electronic excitation. The current mechanism of NPHB has it due to a hierarchy of relaxation events that begin in the outer shell and involve the intrinsic two-level systems (TLS int ) of the glass and terminate in the inner shell where the rate determining step involving the extrinsic TLS (TLS ext ) occurs. The TLS correspond to asymmetric intermolecular double well potentials. The TLS ext are associated with the chromophore and the inner shell of solvent molecules. The TLS int are intimately associated with the excess free volume of glasses. Their tunneling leads to diffusion of excess free volume. Results for Al-phthalocyanine tetrasulfonate (APT) in hyperquenched glassy water (HGW) and ethanol (HGE) films that provide strong support for the critical role of excess free volume are discussed. Hole spectra of APT/HGW obtained over eight decades of burn fluence reveal that the current mechanism needs to be modified to include multilevel extrinsic systems (MLS ext ) in order to explain why the antihole ("photoproduct" absorption) lies to the blue of the burn frequency for sufficiently high burn fluences, an intriguing up-conversion process. The spectra also reveal, for the first time, that the zero-phonon hole (ZPH) profile is non-Lorentzian. This is shown to be a natural consequence of the interplay between the three distributions that result in dispersive hole growth kinetics. They are associated with the tunnel parameter λ of the TLS ext , the angle R between the laser polarization and transition dipole, and off-resonant absorption of the zero-phonon line (the ω distribution). Theoretical simulations of hole growth data for APT/HGW obtained over six decades of burn fluence show that the λ distribution is of primary importance, describing well the first 80% of the saturated burn. The paper ends with an application of NPHB combined with high pressure and external electric (Stark) fields to the critically important "red" antenna states of photosystem I. The addition of pressure and Stark fields enhances the already impressive selectivity of NPHB. The results show that the linear pressure shift, permanent dipole moment change, and linear electron-phonon coupling are correlated. Of particular importance is that these properties can be used to identify states which involve interacting chlorophyll molecules that possess significant charge transfer character because of electron-exchange coupling. The results also show that the site distribution functions of the antenna states are largely uncorrelated, consistent with the findings for previously studied complexes. This is important because the absence of correlation means that the electronic energy gaps of donor and acceptor states are distributed which, in turn, means that the kinetics can be dispersive under certain ...

The temperature (T) dependence of hole growth kinetics (HGK) data that span more than four decades of burn fluence are reported for aluminum-phthalocyanine tetrasulfonate (APT) in fresh and annealed hyperquenched glassy water (HGW) for temperatures between 5 and 20 K. The highly dispersive HGK data are modeled by using the "master" equation based on the two level system (TLS) model described in 2000 by Reinot and Small [J. Chem. Phys. 2000, 113, 10207]. We have demonstrated that thermal line broadening is not enough to account for temperature-dependent HGK for temperatures greater than 10 K. To overcome the discrepancy, the hole growth model must account for thermal hole filling (THF) processes. For the first time, the "master" equation used for HGK simulations is modified to take into account both the temperature dependence of the (single site) absorption spectrum and THF processes, effectively turning off those TLS which do not participate in the hole burning process at higher temperatures. A single set of parameters, some of which were determined directly from the hole spectra, was found to provide satisfactory fits to the HGK data for APT in fresh and annealed HGW for holes burned in the 679.7-676.9 nm range from the high to low energy sides of the Qx absorption band. Furthermore, we propose that HGK modeling at high burn fluences requires that the TLS model be further modified to take into account the existence of extrinsic multiple level systems.

Zero-phonon hole (ZPH) growth kinetics data that span six decades of burn fluence are reported for Al-phthalocyanine tetrasulphonate (APT) in hyperquenched glassy water (HGW) at 5.0 K. The kinetics are highly dispersive. The hole growth equation used for analysis of the dispersion incorporates three distributions (λ, α, and ω) where λ is the tunnel parameter associated with nonphotochemical hole burning (NPHB), α is the angle between the transition dipole and the laser polarization and the ω-distribution stems from off-resonant absorption of the zero-phonon line (ZPL). The single site absorption profile used includes the phonon sideband as well as the ZPL. The homogeneous width of the ZPL and shape of the phonon sideband were determined from experiment. Eight models, which include the possible combinations of the above distributions, were used to fit the data. As in previous works the λ-distribution was taken to be a Gaussian peaked at λ=λ0 with a standard deviation of σλ. The results show that the contribution to the dispersive kinetics from the λ-distribution is of primary importance. It provides a good fit to the data over the first three decades of burn fluence (∼80% of the saturated ZPH depth). The intrinsic contributions from the α- and ω-distributions become important for the last ∼20% of the burn. These two distributions by themselves or in combination yielded poor fits to the data. The three distributions in combination (λαω-model) provided a good fit over the first five decades of burn fluence. Importantly, the λ0 and σλ values of 8.3 and 0.95 from the λ-distribution alone are nearly the same as those from the λαω-distribution. The above findings for APT/HGW should be widely applicable since previous studies of other NPHB systems led to σλ values ≳1. It is emphasized that APT/HGW is an ideal system for hole growth studies because of its very narrow ZPL and weak electron-phonon coupling (S∼0.2) and because it satisfies the homogeneity condition, i.e., all sites are burnable.

The results of infrared absorption (-OH) experiments and nonphotochemical hole-burning experiments of aluminum-phthalocyanine-tetrasulfonate (ATP) in hyperquenched glassy films of water (HGW) are reported. Films were produced by deposition of liquid water clusters (-2 pm), generated by a thermal spray nozzle source, onto either a sapphire or polycrystalline copper cryoplate. Deposition temperatures (TD) in the -5-150 K range were employed. TD = 5 K films were annealed at various temperatures (TA), up to 140 K. For each value of TA, the infrared and hole-burning properties (zero-phonon hole width and hole growth kinetics) of the film (annealed) are identical to those of unannealed HGW formed at TD = TA. Thus, HGW formed at a deposition temperature of TD' is kinetically accessible, by annealing of HGW formed at temperatures TD < To'. Dramatic irreversible manifestations of configurational relaxation in HGW are observed to onset at TA (TD) -90 K. This configurational relaxation progresses smoothly with temperature up to 150 K (highest TD and TA used). Zero-phonon hole widths were usually determined for a burning and reading temperature of 5 K. Hole growth kinetics were always monitored at a burning temperature of 5 K. It was found, for example, that HGW annealed or deposited at 140 K yields a zero-phonon hole width of 180 MHz, a factor of 3 times narrower than the hole of HGW formed at TD = 5 K. Decrease of the hole width with annealing onsets at TA -90 K. Both unannealed and annealed films yielded a power law for the dependence of the hole width on the burning temperature (5 10 K), proving that pure dephasinghpectral diffusion is governed by the electron-TLSint (intrinsic two-level systems) interaction. An interpretation of the aforementioned configuration relaxation, onsetting at -90 K, in terms of the TLSint model is given. ATP in HGW turns out to be the most efficient system for nonphotochemical hole burning yet discovered, with an average quantum yield as high as 0.18. (The SI lifetime of ATP is 4.8 ns.) Remarkably, the hole burning is essentially inoperative in cubic ice formed by warming of HGW. However, this cessation is consistent with the current mechanism for nonphotochemical hole burning.