Triplet–Triplet Coupling in Chromophore Dimers: Theory and Experiment

Knowledge of triplet state energies and triplet–triplet (T–T) interactions in aggregated organic molecules is essential for understanding photochemistry and dynamics of many natural and artificial systems. In this work, we combine direct phosphorescence measurements of triplet state energies, which are challenging due to the spin-forbidden nature of respective transitions and applicable to only a limited number of systems, with quantum chemical computational tools that can provide valuable qualitative and quantitative information about triplet states of interacting molecules. Using hexatriene, protoporphyrin, pheophorbide, and chlorophyll dimers as model systems, we demonstrate a complicated dependence of T–T coupling on a relative orientation of chromophores, governed by a nodal structure of overlapping electronic wave functions, that modulates interpigment interactions by orders of magnitude. It is also shown that geometrical relaxation of the triplet state is one of the critical factors for predictive modeling of T–T interactions in molecular aggregates

Does the Singlet Minus Triplet Spectrum with Major Photobleaching Band Near 680–682 nm Represent an Intact Reaction Center of Photosystem II?

We use both frequency- and time-domain low-temperature (5–20 K) spectroscopies to further elucidate the shape and spectral position of singlet minus triplet (triplet-bottleneck) spectra in the reaction centers (RCs) of Photosystem II (PSII) isolated from wild-type Chlamydomonas reinhardtii and spinach. It is shown that the shape of the nonresonant transient hole-burned spectrum in destabilized RCs from C. reinhardtii is very similar to that typically observed for spinach. This suggests that the previously observed difference in transient spectra between RCs from C. reinhardtii and spinach is not due to the sample origin but most likely due to a partial destabilization of the D1 and D2 polypeptides. This supports our previous assignments that destabilized RCs (referred to as RC680) (Acharya, K. et al. <i>J. Phys. Chem. B</i> <b>2012</b>, <i>116</i>, 4860–4870), with a major photobleaching band near 680–682 nm and the absence of a photobleaching band near 673 nm, do not represent the intact RC residing within the PSII core complex. Time-resolved absorption difference spectra obtained for partially destabilized RCs of C. reinhardtii and for typical spinach RCs support the above conclusions. The absence of clear photobleaching bands near 673 and 684 nm (where the P_D1 chlorophyll and the active pheophytin (Pheo_D1) contribute, respectively) in picosecond transient absorption spectra in both RCs studied in this work indicates that the cation can move from the primary electron donor (Chl_D1) to P_D1 (i.e., P_D1Chl_D1⁺Pheo_D1^– → P_D1⁺Chl_D1Pheo_D1^–). Therefore, we suggest that Chl_D1 is the major electron donor in usually studied destabilized RCs (with a major photobleaching near 680–682 nm), although the P_D1 path (where P_D1 serves as the primary electron donor) is likely present in intact RCs, as discussed in Acharya, K. et al. <i>J. Phys. Chem. B</i> <b>2012</b>, <i>116</i>, 4860–4870

The Fate of the Triplet Excitations in the Fenna–Matthews–Olson Complex

The fate of triplet excited states in the Fenna–Matthew–Olson (FMO) pigment–protein complex is studied by means of time-resolved nanosecond spectroscopy and exciton model simulations. Experiments reveal microsecond triplet excited-state energy transfer between the bacteriochlorophyll (BChl) pigments, but show no evidence of triplet energy transfer to molecular oxygen, which is known to produce highly reactive singlet oxygen and is the leading cause of photo damage in photosynthetic proteins. The FMO complex is exceptionally photo stable despite the fact it contains no carotenoids, which could effectively quench triplet excited states of (bacterio)­chlorophylls and are usually found within pigment–protein complexes. It is inferred that the triplet excitation is transferred to the lowest energy pigment, BChl 3, within the FMO complex, whose triplet state energy is shifted by pigment–protein interactions below that of the singlet oxygen excitation. Thus, the energy transfer to molecular oxygen is blocked and the FMO does not need carotenoids for photo protection

A Map of Dielectric Heterogeneity in a Membrane Protein: the Hetero-Oligomeric Cytochrome <i>b</i>₆<i>f</i> Complex

The cytochrome <i>b</i>₆<i>f</i> complex, a member of the cytochrome <i>bc</i> family that mediates energy transduction in photosynthetic and respiratory membranes, is a hetero-oligomeric complex that utilizes two pairs of <i>b</i>-hemes in a symmetric dimer to accomplish trans-membrane electron transfer, quinone oxidation–reduction, and generation of a proton electrochemical potential. Analysis of electron storage in this pathway, utilizing simultaneous measurement of heme reduction, and of circular dichroism (CD) spectra, to assay heme–heme interactions, implies a heterogeneous distribution of the dielectric constants that mediate electrostatic interactions between the four hemes in the complex. Crystallographic information was used to determine the identity of the interacting hemes. The Soret band CD signal is dominated by excitonic interaction between the intramonomer <i>b</i>-hemes, <i>b</i>_n and <i>b</i>_p, on the electrochemically negative and positive sides of the complex. Kinetic data imply that the most probable pathway for transfer of the two electrons needed for quinone oxidation–reduction utilizes this intramonomer heme pair, contradicting the expectation based on heme redox potentials and thermodynamics, that the two higher potential hemes <i>b</i>_n on different monomers would be preferentially reduced. Energetically preferred intramonomer electron storage of electrons on the intramonomer <i>b</i>-hemes is found to require heterogeneity of interheme dielectric constants. Relative to the medium separating the two higher potential hemes <i>b</i>_n, a relatively large dielectric constant must exist between the intramonomer <i>b</i>-hemes, allowing a smaller electrostatic repulsion between the reduced hemes. Heterogeneity of dielectric constants is an additional structure–function parameter of membrane protein complexes

Triplet Excited State Energies and Phosphorescence Spectra of (Bacterio)Chlorophylls

(Bacterio)­Chlorophyll ((B)­Chl) molecules play a major role in photosynthetic light-harvesting proteins, and the knowledge of their triplet state energies is essential to understand the mechanisms of photodamage and photoprotection, as the triplet excitation energy of (B)­Chl molecules can readily generate highly reactive singlet oxygen. The triplet state energies of 10 natural chlorophyll (Chl <i>a</i>, <i>b</i>, <i>c</i>₂, <i>d</i>) and bacteriochlorophyll (BChl <i>a</i>, <i>b</i>, <i>c</i>, <i>d</i>, <i>e</i>, <i>g</i>) molecules and one bacteriopheophytin (BPheo <i>g</i>) have been directly determined via their phosphorescence spectra. Phosphorescence of four molecules (Chl <i>c</i>₂, BChl <i>e</i> and <i>g</i>, BPheo <i>g</i>) was characterized for the first time. Additionally, the relative phosphorescence to fluorescence quantum yield for each molecule was determined. The measurements were performed at 77K using solvents providing a six-coordinate environment of the Mg²⁺ ion, which allows direct comparison of these (B)­Chls. Density functional calculations of the triplet state energies show good correlation with the experimentally determined energies. The correlation determined computationally was used to predict the triplet energies of three additional (B)­Chl molecules: Chl <i>c</i>₁, Chl <i>f</i>, and BChl <i>f</i>

Singlet Exciton Fission in Polycrystalline Thin Films of a Slip-Stacked Perylenediimide

The crystal structure of <i>N</i>,<i>N</i>-bis­(<i>n</i>-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis­(dicarboximide), <b>1</b>, obtained by X-ray diffraction reveals that <b>1</b> has a nearly planar perylene core and π–π stacks at a 3.5 Å interplanar distance in well-separated slip-stacked columns. Theory predicts that slip-stacked, π–π-stacked structures should enhance interchromophore electronic coupling and thus favor singlet exciton fission. Photoexcitation of vapor-deposited polycrystalline 188 nm thick films of <b>1</b> results in a 140 ± 20% yield of triplet excitons (^3*<b>1</b>) in τ_SF = 180 ± 10 ps. These results illustrate a design strategy for producing perylenediimide and related rylene derivatives that have the optimized interchromophore electronic interactions which promote high-yield singlet exciton fission for potentially enhancing organic solar cell performance and charge separation in systems for artificial photosynthesis

Singlet Exciton Fission in Polycrystalline Thin Films of a Slip-Stacked Perylenediimide

The crystal structure of <i>N</i>,<i>N</i>-bis­(<i>n</i>-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis­(dicarboximide), <b>1</b>, obtained by X-ray diffraction reveals that <b>1</b> has a nearly planar perylene core and π–π stacks at a 3.5 Å interplanar distance in well-separated slip-stacked columns. Theory predicts that slip-stacked, π–π-stacked structures should enhance interchromophore electronic coupling and thus favor singlet exciton fission. Photoexcitation of vapor-deposited polycrystalline 188 nm thick films of <b>1</b> results in a 140 ± 20% yield of triplet excitons (^3*<b>1</b>) in τ_SF = 180 ± 10 ps. These results illustrate a design strategy for producing perylenediimide and related rylene derivatives that have the optimized interchromophore electronic interactions which promote high-yield singlet exciton fission for potentially enhancing organic solar cell performance and charge separation in systems for artificial photosynthesis