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Phonon-mediated path-interference in electronic energy transfer

We present a formalism to quantify the contribution of path-interference in phonon-mediated electronic energy transfer. The transfer rate between two molecules is computed by considering the quantum mechanical amplitudes associated with pathways connecting the initial and final sites. This includes contributions from classical pathways, but also terms arising from interference of different pathways. We treat the vibrational modes coupled to the molecules as a non-Markovian harmonic oscillator bath, and investigate the correction to transfer rates due to the lowest-order interference contribution. We show that depending on the structure of the harmonic bath, the correction due to path-interference may have a dominant vibrational or electronic character, and can make a notable contribution to the transfer rate in the steady state.     

Quantitative investigations of quantum coherence for a light-harvesting protein at conditions simulating photosynthesis

Recent measurements using two-dimensional electronic spectroscopy (2D ES) have shown that the initial dynamic response of photosynthetic proteins can involve quantum coherence. We show how electronic coherence can be differentiated from vibrational coherence in 2D ES. On that basis we conclude that both electronic and vibrational coherences are observed in the phycobiliprotein light-harvesting complex PC645 from Chroomonas sp. CCMP270 at ambient temperature. These light-harvesting antenna proteins of the cryptophyte algae are suspended in the lumen, where the pH drops significantly under sustained illumination by sunlight. Here we measured 2D ES of PC645 at increasing levels of acidity to determine if the change in pH affects the quantum coherence; quantitative analysis reveals that the dynamics are insensitive to the pH change.     

Chromophores in phenylenevinylene-based conjugated polymers: role of conformational kinks and chemical defects

The influence of chemical defects and conformational kinks on the nature of the lowest electronic excitations in phenylenevinylene-based polymers is assessed at the semiempirical quantum-chemical level. The amount of excited-state localization and the amplitude of through-space (Coulomb-like) versus through-bond (charge-transfer-like) interactions have been quantified by comparing the results provided by excitonic and supermolecular models. While excitation delocalization among conjugated segments delineated by the defects occurs in the acceptor configuration, self-confinement on individual chromophores follows from geometric relaxation in the excited-state donor configuration. The extent of excited-state localization is found to be sensitive to both the nature of the defect and the length of the conjugated chains. Implications for resonant energy transfer along conjugated polymer chains are discussed.     

EPR-ENDOR of the Cu(I)NO complex of nitrite reductase

With limited reductant and nitrite under anaerobic conditions, copper-containing nitrite reductase (NiR) of Rhodobacter sphaeroides yielded endogenous NO and the Cu(I)NO derivative of NiR. (14)N- and (15)N-nitrite substrates gave rise to characteristic (14)NO and (15)NO EPR hyperfine features indicating NO involvement, and enrichment of NiR with (63)Cu isotope caused an EPR line shape change showing copper involvement. A markedly similar Cu(I)NONiR complex was made by anaerobically adding a little endogenous NO gas to reduced protein and immediately freezing. The Cu(I)NONiR signal accounted for 60-90% of the integrated EPR intensity formerly associated with the Type 2 catalytic copper. Analysis of NO and Cu hyperfine couplings and comparison to couplings of inorganic Cu(I)NO model systems indicated approximately 50% spin on the N of NO and approximately 17% spin on Cu. ENDOR revealed weak nitrogen hyperfine coupling to one or more likely histidine ligands of copper. Although previous crystallography of the conservative I289V mutant had shown no structural change beyond the 289 position, this mutation, which eliminates the Cdelta1 methyl of I289, caused the Cu(I)NONiR EPR spectrum to change and proton ENDOR features to be significantly altered. The proton hyperfine coupling that was significantly altered was consistent with a dipolar interaction between the Cdelta1 protons of I289 and electron spin on the NO, where the NO would be located 3.0-3.7 A from these protons. Such a distance positions the NO of Cu(I)NO as an axial ligand to Type 2 Cu(I).     

Mechanism and origin of exciton spin relaxation in CdSe nanorods

The dynamics of exciton spin relaxation in CdSe nanorods of various sizes and shapes are measured by an ultrafast transient polarization grating technique. The measurement of the third-order transient grating (3-TG) signal utilizing linear cross-polarized pump pulses enables us to monitor the history of spin relaxation among the bright exciton states with a total angular momentum of F = +/-1. From the measured exciton spin relaxation dynamics, it is found that the effective mechanism of exciton spin relaxation is sensitive to the size of the nanorod. Most of the measured cross-polarized 3-TG signals show single-exponential spin relaxation dynamics, while biexponential spin relaxation dynamics are observed in the nanorod of the largest diameter. This analysis suggests that a direct exciton spin flip process between the bright exciton states with F = +/-1 is the dominant spin relaxation mechanism in small nanocrystals, and an indirect spin flip via the dark states with F = +/-2 contributes as the size of the nanocrystal increases. This idea is examined by simulations of 3-TG signals with a kinetic model for exciton spin relaxation considering the states in the exciton fine structure. Also, it is revealed that the rate of exciton spin relaxation has a strong correlation with the diameter, d, of the nanorod, scaled by the power law of 1/d4, rather than other shape parameters such as length, volume, or aspect ratio.     

Quantum-coherence and correlations in π-conjugated molecules and multichromophoric systems

Chemistry is intrinsically founded on quantum mechanical principles and examples of quantum-mechanical phenomena abound on a range of energy and length scales. In this article some examples of quantum-mechanical phenomena that can be probed by optical spectroscopy are discussed. Recent experimental studies of quantumcoherence in electronic energy transfer in π-conjugated polymers are reported as examples of weak correlations. The nature of the electron-hole binding energy for excitons in organic systems is investigated as a case of intermediate correlations. Possible experimental probes of strong correlations involved in chemical reactions are critically examined in the final section of the paper.     

Photosynthetic light-harvesting is tuned by the heterogeneous polarizable environment of the protein

In photosynthesis, special antenna proteins that contain multiple light-absorbing molecules (chromophores) are able to capture sunlight and transfer the excitation energy to reaction centers with almost 100% quantum efficiencies. The critical role of the protein scaffold in holding the appropriate arrangement of the chromophores is well established and can be intuitively understood given the need to keep optimal dipole-dipole interactions between the energy-transferring chromophores, as described by Fo?rster theory more than 60 years ago. However, the question whether the protein structure can also play an active role by tuning such dipole-dipole interactions has not been answered so far, its effect being rather crudely described by simple screening factors related to the refractive index properties of the system. Here, we present a combined quantum chemical/molecular mechanical approach to compute electronic couplings that accounts for the heterogeneous dielectric nature of the protein-solvent environment in atomic detail. We apply the method to study the effect of dielectric heterogeneity in the energy migration properties of the PE545 principal light-harvesting antenna of the cryptomonad Rhodomonas CS24. We find that dielectric heterogeneity can profoundly tune by a factor up to ～4 the energy migration rates between chromophore sites compared to the average continuum dielectric view that has historically been assumed. Our results indicate that engineering of the local dielectric environment can potentially be used to optimize artificial light-harvesting antenna systems.     

Preparative size-exclusion chromatography for purification and characterization of colloidal quantum dots bound by chromophore-labeled polymers and low-molecular-weight chromophores

We explore the use of preparative size-exclusion chromatography (SEC) and high-performance liquid chromatography (HPLC) to purify quantum dots (QDs) after surface modification. In one example, in which Bio-Beads (S-X1) were used as the packing material for the preparative SEC column, CdSe QDs treated with a functional coumarin dye could be separated from the excess free dye by using tetrahydrofuran (THF) as the mobile phase. This column was unable to separate polymer-coated QDs from free polymer (M ∼ 8000) because of the relatively low cutoff mass of the column. Here a preparative HPLC column packed with TOYOPEARL gel allowed the effective separation of polymer-bound QDs from the excess free polymer by using N-methyl-2-pyrrolidinone (NMP) as the mobile phase. When other solvents such as absolute ethanol, acetonitrile, THF, and THF–triethylamine mixtures were used as the eluent, QDs stuck to the column. While NMP was an effective medium to remove excess free polymer from the QDs, it was difficult to transfer the purified QDs to more volatile solvents and maintain colloidal stability.     

Plasticization of ultra-thin polysulfone membranes by carbon dioxide

Plasticization of gas separation membranes by carbon dioxide permanently alters their performance and increases the possibility of membrane failure. This is amplified in ultra-thin composite membranes, where the active polymeric layer is less than 2 μm. Here, the plasticization influence of CO₂ is measured on ultra-thin polysulfone composite membranes for a range of active layer thicknesses, at four temperatures. The resulting permeability–pressure isotherms demonstrate plasticization occurs for all thicknesses at pressures lower than has been reported for dense membranes. These isotherms were quantitatively fitted with an expanded dual-sorption model that takes into account plasticization of the membrane. The plasticization potential of CO₂ for polysulfone was found to increase with reduced active layer thickness. Similarly, the plasticization potential of CO₂ was found to decrease with temperature. These results are consistent with similar research that shows that thin films behave differently to dense membranes.