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1.
J Chem Phys ; 160(3)2024 Jan 21.
Article in English | MEDLINE | ID: mdl-38230810

ABSTRACT

Scaffolded molecular networks are important building blocks in biological pigment-protein complexes, and DNA nanotechnology allows analogous systems to be designed and synthesized. System-environment interactions in these systems are responsible for important processes, such as the dissipation of heat and quantum information. This study investigates the role of nanoscale molecular parameters in tuning these vibronic system-environment dynamics. Here, genetic algorithm methods are used to obtain nanoscale parameters for a DNA-scaffolded chromophore network based on comparisons between its calculated and measured optical spectra. These parameters include the positions, orientations, and energy level characteristics within the network. This information is then used to compute the dynamics, including the vibronic population dynamics and system-environment heat currents, using the hierarchical equations of motion. The dissipation of quantum information is identified by the system's transient change in entropy, which is proportional to the heat currents according to the second law of thermodynamics. These results indicate that the dissipation of quantum information is highly dependent on the particular nanoscale characteristics of the molecular network, which is a necessary first step before gleaning the systematic optimization rules. Subsequently, the I-concurrence dynamics are calculated to understand the evolution of the vibronic system's quantum entanglement, which are found to be long-lived compared to these system-bath dissipation processes.

2.
Phys Chem Chem Phys ; 25(5): 3651-3665, 2023 Feb 01.
Article in English | MEDLINE | ID: mdl-36648290

ABSTRACT

Nature uses chromophore networks, with highly optimized structural and energetic characteristics, to perform important chemical functions. Due to its modularity, predictable aggregation characteristics, and established synthetic protocols, structural DNA nanotechnology is a promising medium for arranging chromophore networks with analogous structural and energetic controls. However, this high level of control creates a greater need to know how to optimize the systems precisely. This study uses the system's modularity to produce variations of a coupled 14-Site chromophore network. It uses machine-learning algorithms and spectroscopy measurements to reveal the energy-transport roles of these Sites, paying particular attention to the cooperative and inhibitive effects they impose on each other for transport across the network. The physical significance of these patterns is contextualized, using molecular dynamics simulations and energy-transport modeling. This analysis yields insights about how energy transfers across the Donor-Relay and Relay-Acceptor interfaces, as well as the energy-transport pathways through the homogeneous Relay segment. Overall, this report establishes an approach that uses machine-learning methods to understand, in fine detail, the role that each Site plays in an optoelectronic molecular network.

3.
J Phys Chem C Nanomater Interfaces ; 126(40): 17164-17175, 2022 Oct 13.
Article in English | MEDLINE | ID: mdl-36268205

ABSTRACT

Molecular excitons are useful for applications in light harvesting, organic optoelectronics, and nanoscale computing. Electronic energy transfer (EET) is a process central to the function of devices based on molecular excitons. Achieving EET with a high quantum efficiency is a common obstacle to excitonic devices, often owing to the lack of donor and acceptor molecules that exhibit favorable spectral overlap. EET quantum efficiencies may be substantially improved through the use of heteroaggregates-aggregates of chemically distinct dyes-rather than individual dyes as energy relay units. However, controlling the assembly of heteroaggregates remains a significant challenge. Here, we use DNA Holliday junctions to assemble homo- and heterotetramer aggregates of the prototypical cyanine dyes Cy5 and Cy5.5. In addition to permitting control over the number of dyes within an aggregate, DNA-templated assembly confers control over aggregate composition, i.e., the ratio of constituent Cy5 and Cy5.5 dyes. By varying the ratio of Cy5 and Cy5.5, we show that the most intense absorption feature of the resulting tetramer can be shifted in energy over a range of almost 200 meV (1600 cm-1). All tetramers pack in the form of H-aggregates and exhibit quenched emission and drastically reduced excited-state lifetimes compared to the monomeric dyes. We apply a purely electronic exciton theory model to describe the observed progression of the absorption spectra. This model agrees with both the measured data and a more sophisticated vibronic model of the absorption and circular dichroism spectra, indicating that Cy5 and Cy5.5 heteroaggregates are largely described by molecular exciton theory. Finally, we extend the purely electronic exciton model to describe an idealized J-aggregate based on Förster resonance energy transfer (FRET) and discuss the potential advantages of such a device over traditional FRET relays.

4.
J Phys Chem A ; 125(44): 9632-9644, 2021 Nov 11.
Article in English | MEDLINE | ID: mdl-34709821

ABSTRACT

Structural DNA nanotechnology is a promising approach to create chromophore networks with modular structures and Hamiltonians to control the material's functions. The functional behaviors of these systems depend on the interactions of the chromophores' vibronic states, as well as interactions with their environment. To optimize their functions, it is necessary to characterize the chromophore network's structural and energetic properties, including the electronic delocalization in some cases. In this study, parameters of interest are deduced in DNA-scaffolded Cyanine 3 and Cyanine 5 dimers. The methods include steady-state optical measurements, physical modeling, and a genetic algorithm approach. The parameters include the chromophore network's vibronic Hamiltonian, molecular positions, transition dipole orientations, and environmentally induced energy broadening. Additionally, the study uses temperature-dependent optical measurements to characterize the spectral broadening further. These combined results reveal the quantum mechanical delocalization, which is important for functions like coherent energy transport and quantum information applications.


Subject(s)
DNA , Quantum Theory
5.
J Phys Chem B ; 125(11): 2812-2820, 2021 03 25.
Article in English | MEDLINE | ID: mdl-33728918

ABSTRACT

Optical signals come from coherences between quantum states, with spectral line widths determined by the coherences' dephasing dynamics. Using a 2D electronic spectrometer, we observe weak coherence- and rephasing-time-domain signals persisting to 1 ps in the Fenna-Matthews-Olson complex at 77 K. These are coherences between the ground and excited states prepared after the complex interacts once or three times with light, rather than zero-quantum coherences that are more frequently investigated following two interactions. Here, we use these small but persistent signal components to isolate spectral contributions with narrowed peaks and reveal the system's eigenenergies.


Subject(s)
Light-Harvesting Protein Complexes , Photosynthetic Reaction Center Complex Proteins , Quantum Theory , Spectrum Analysis
6.
J Phys Chem Lett ; 9(1): 89-95, 2018 Jan 04.
Article in English | MEDLINE | ID: mdl-29236502

ABSTRACT

Pigment-protein complexes in photosynthetic antennae can suffer oxidative damage from reactive oxygen species generated during solar light harvesting. How the redox environment of a pigment-protein complex affects energy transport on the ultrafast light-harvesting time scale remains poorly understood. Using two-dimensional electronic spectroscopy, we observe differences in femtosecond energy-transfer processes in the Fenna-Matthews-Olson (FMO) antenna complex under different redox conditions. We attribute these differences in the ultrafast dynamics to changes to the system-bath coupling around specific chromophores, and we identify a highly conserved tyrosine/tryptophan chain near the chromophores showing the largest changes. We discuss how the mechanism of tyrosine/tryptophan chain oxidation may contribute to these differences in ultrafast dynamics that can moderate energy transfer to downstream complexes where reactive oxygen species are formed. These results highlight the importance of redox conditions on the ultrafast transport of energy in photosynthesis. Tailoring the redox environment may enable energy transport engineering in synthetic light-harvesting systems.


Subject(s)
Light-Harvesting Protein Complexes/chemistry , Photosynthesis , Photosynthetic Reaction Center Complex Proteins/chemistry , Energy Transfer , Light , Oxidation-Reduction , Spectrum Analysis
9.
J Phys Chem A ; 120(9): 1479-87, 2016 Mar 10.
Article in English | MEDLINE | ID: mdl-26630123

ABSTRACT

The bacterial reaction center is capable of both efficiently collecting and quickly transferring energy within the complex; therefore, the reaction center serves as a convenient model for both energy transfer and charge separation. To spectroscopically probe the interactions between the electronic excited states on the chromophores and their intricate relationship with vibrational motions in their environment, we examine coherences between the excited states. Here, we investigate this question by introducing a series of point mutations within 12 Å of the special pair of bacteriochlorophylls in the Rhodobacter sphaeroides reaction center. Using two-dimensional spectroscopy, we find that the time scales of energy transfer dynamics remain unperturbed by these mutations. However, within these spectra, we detect changes in the mixed vibrational-electronic coherences in these reaction centers. Our results indicate that resonance between bacteriochlorophyll vibrational modes and excitonic energy gaps promote electronic coherences and support current vibronic models of photosynthetic energy transfer.


Subject(s)
Mutation , Photosynthetic Reaction Center Complex Proteins/genetics , Rhodobacter sphaeroides/genetics , Energy Transfer , Spectrum Analysis/methods
10.
J Phys Chem B ; 119(24): 7447-56, 2015 Jun 18.
Article in English | MEDLINE | ID: mdl-25620363

ABSTRACT

Using ultrafast polarization-controlled transient absorption (TA) measurements, dynamics of the initial exciton states were investigated on the time scale of tens of femtoseconds to about 80 ps in two different types of conjugated polymers extensively used in active layers of organic photovoltaic devices. These polymers are poly(3-fluorothienothiophenebenzodithiophene) (PTB7) and poly-3-hexylthiophene (P3HT), which are charge-transfer polymers and homopolymers, respectively. In PTB7, the initial excitons with excess vibrational energy display two observable ultrafast time constants, corresponding to coherent exciton diffusion before the vibrational relaxation, and followed by incoherent exciton diffusion processes to a neighboring local state after the vibrational relaxation. In contrast, P3HT shows only one exciton diffusion or conformational motion time constant of 34 ps, even though its exciton decay kinetics are multiexponential. Based on the experimental results, an exciton dynamics mechanism is conceived taking into account the excitation energy and structural dependence in coherent and incoherent exciton diffusion processes, as well as other possible deactivation processes including the formation of the pseudo-charge-transfer and charge separate states, as well as interchain exciton hopping or coherent diffusion.

11.
J Am Chem Soc ; 136(49): 17224-35, 2014 Dec 10.
Article in English | MEDLINE | ID: mdl-25369425

ABSTRACT

Organic charge transfer cocrystals are inexpensive, modular, and solution-processable materials that are able, in some instances, to exhibit properties such as optical nonlinearity, (semi)conductivity, ferroelectricity, and magnetism. Although the properties of these cocrystals have been investigated for decades, the principal challenge that researchers face currently is to devise an efficient approach which allows for the growth of high-quality crystalline materials, in anticipation of a host of different technological applications. The research reported here introduces an innovative design, termed LASO-lock-arm supramolecular ordering-in the form of a modular approach for the development of responsive organic cocrystals. The strategy relies on the use of aromatic electronic donor and acceptor building blocks, carrying complementary rigid and flexible arms, capable of forming hydrogen bonds to amplify the cocrystallization processes. The cooperativity of charge transfer and hydrogen-bonding interactions between the building blocks leads to binary cocrystals that have alternating donors and acceptors extending in one and two dimensions sustained by an intricate network of hydrogen bonds. A variety of air-stable, mechanically robust, centimeter-long, organic charge transfer cocrystals have been grown by liquid-liquid diffusion under ambient conditions inside 72 h. These cocrystals are of considerable interest because of their remarkable size and stability and the promise they hold when it comes to fabricating the next generation of innovative electronic and photonic devices.

12.
J Phys Chem Lett ; 5(1): 196-204, 2014 Jan 02.
Article in English | MEDLINE | ID: mdl-24719679

ABSTRACT

The creation and manipulation of quantum superpositions is a fundamental goal for the development of materials with novel optoelectronic properties. In this letter, we report persistent (~80 fs lifetime) quantum coherence between the 1S and 1P excitonic states in zinc-blende colloidal CdSe quantum dots at room temperature, measured using Two-Dimensional Electronic Spectroscopy. We demonstrate that this quantum coherence manifests as an intradot phenomenon, the frequency of which depends on the size of the dot excited within the ensemble of QDs. We model the lifetime of the coherence and demonstrate that correlated interexcitonic fluctuations preserve relative phase between excitonic states. These observations suggest an avenue for engineering long-lived interexcitonic quantum coherence in colloidal quantum dots.

13.
J Chem Phys ; 140(8): 084701, 2014 Feb 28.
Article in English | MEDLINE | ID: mdl-24588185

ABSTRACT

Development of optoelectronic technologies based on quantum dots depends on measuring, optimizing, and ultimately predicting charge carrier dynamics in the nanocrystal. In such systems, size inhomogeneity and the photoexcited population distribution among various excitonic states have distinct effects on electron and hole relaxation, which are difficult to distinguish spectroscopically. Two-dimensional electronic spectroscopy can help to untangle these effects by resolving excitation energy and subsequent nonlinear response in a single experiment. Using a filament-generated continuum as a pump and probe source, we collect two-dimensional spectra with sufficient spectral bandwidth to follow dynamics upon excitation of the lowest three optical transitions in a polydisperse ensemble of colloidal CdSe quantum dots. We first compare to prior transient absorption studies to confirm excitation-state-dependent dynamics such as increased surface-trapping upon excitation of hot electrons. Second, we demonstrate fast band-edge electron-hole pair solvation by ligand and phonon modes, as the ensemble relaxes to the photoluminescent state on a sub-picosecond time-scale. Third, we find that static disorder due to size polydispersity dominates the nonlinear response upon excitation into the hot electron manifold; this broadening mechanism stands in contrast to that of the band-edge exciton. Finally, we demonstrate excitation-energy dependent hot-carrier relaxation rates, and we describe how two-dimensional electronic spectroscopy can complement other transient nonlinear techniques.


Subject(s)
Cadmium Compounds/chemistry , Quantum Dots , Quantum Theory , Selenium Compounds/chemistry , Electrons , Particle Size , Spectrum Analysis , Surface Properties
14.
Appl Opt ; 53(9): 1909-17, 2014 Mar 20.
Article in English | MEDLINE | ID: mdl-24663470

ABSTRACT

Electronic dynamics span broad energy scales with ultrafast time constants in the condensed phase. Two-dimensional (2D) electronic spectroscopy permits the study of these dynamics with simultaneous resolution in both frequency and time. In practice, this technique is sensitive to changes in nonlinear dispersion in the laser pulses as time delays are varied during the experiment. We have developed a 2D spectrometer that uses broadband continuum generated in argon as the light source. Using this visible light in phase-sensitive optical experiments presents new challenges in implementation. We demonstrate all-reflective interferometric delays using angled stages. Upon selecting an ~180 nm window of the available bandwidth at ~10 fs compression, we probe the nonlinear response of broadly absorbing CdSe quantum dots and electronic transitions of Chlorophyll a.


Subject(s)
Amplifiers, Electronic , Lasers , Lenses , Spectrum Analysis/instrumentation , Equipment Design , Equipment Failure Analysis
15.
J Phys Chem Lett ; 5(11): 1856-63, 2014 Jun 05.
Article in English | MEDLINE | ID: mdl-26273865

ABSTRACT

Charge-transfer copolymers with local electron density gradients, systematically modified by quantity and position of fluorination, result in widely variable (2-8%) power conversion efficiencies (PCEs). Ultrafast, near-infrared, transient absorption spectroscopy on the corresponding films reveals the influence of exciton polarity on ultrafast populations and decay dynamics for the charge-separated and charge-transfer states as well as their strong correlation to device PCEs. By using an excitation energy-dependent, dynamic red shift in the transient absorption signal for the polymer cation, the exciton polarity induced by push-pull interactions within each polymer fragment is shown to enhance charge dissociation on time scales of tens to hundreds of picoseconds after excitation. These results suggest the important role played by the local electronic structure not only for exciton dissociation but also for device performance.

16.
J Chem Phys ; 140(3): 034903, 2014 Jan 21.
Article in English | MEDLINE | ID: mdl-25669410

ABSTRACT

We present the synthesis and characterization of a benzodithiophene/thiophene alternating copolymer decorated with rigid, singly branched pendant side chains. We characterize exciton migration and recombination dynamics in these molecules in tetrahydrofuran solution, using a combination of static and time-resolved spectroscopies. As control experiments, we also measure electronic relaxation dynamics in isolated molecular analogues of both the side chain and polymer moieties. We employ semi-empirical and time-dependent density functional theory calculations to show that photoexcitation of the decorated copolymer using 395 nm laser pulses results in excited states primarily localized on the pendant side chains. We use ultrafast transient absorption spectroscopy to show that excitations are transferred to the polymer backbone faster than the instrumental response function, ∼250 fs.


Subject(s)
Dendrimers/chemistry , Polymers/chemistry , Thiophenes/chemistry , Benzene Derivatives/chemistry , Energy Transfer , Light , Models, Molecular , Spectrophotometry
17.
Nature ; 488(7412): 485-9, 2012 Aug 23.
Article in English | MEDLINE | ID: mdl-22914165

ABSTRACT

Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures.


Subject(s)
Electricity , Electrons , Iron/chemistry , Organometallic Compounds/chemistry , Temperature , Anisotropy , Crystallization , Electron Transport , Hydrogen Bonding , Models, Molecular , Molecular Conformation , Surface Properties
18.
J Am Chem Soc ; 134(9): 4142-52, 2012 Mar 07.
Article in English | MEDLINE | ID: mdl-22309185

ABSTRACT

Record-setting organic photovoltaic cells with PTB polymers have recently achieved ~8% power conversion efficiencies (PCE). A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2-8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular ("pseudo") charge-transfer state in isolated PTBF polymers in solution, in the absence of the acceptor phenyl-C(61)-butyric acid methyl ester (PCBM) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole-electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed.

19.
J Phys Chem A ; 116(9): 1984-92, 2012 Mar 08.
Article in English | MEDLINE | ID: mdl-22292857

ABSTRACT

The metal-to-ligand-charge-transfer (MLCT) excited state of Cu(I) diimine complexes is known to undergo structural reorganization, transforming from a pseudotetrahedral D(2d) symmetry in the ground state to a flattened D(2) symmetry in the MLCT state, which allows ligation with a solvent molecule, forming an exciplex intermediate. Therefore, the structural factors that influence the coordination geometry change and the solvent accessibility to the copper center in the MLCT state could be used to control the excited state properties. In this study, we investigated an extreme case of the steric hindrance caused by attaching bulky tert-butyl groups in bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I), [Cu(I)(dtbp)(2)](+). The two bulky tert-butyl groups on the dtbp ligand lock the MLCT state into the pseudotetrahedral coordination geometry and completely block the solvent access to the copper center in the MLCT state of [Cu(I)(dtbp)(2)](+). Using ultrafast transient absorption spectroscopy and time-resolved emission spectroscopy, we investigated the MLCT state property changes due to the steric hindrance and demonstrated that [Cu(I)(dtbp)(2)](+) exhibited a long-lived emission but no subpicosecond component that was previously assigned as the flattening of the pseudotetrahedral coordination geometry. This suggests the retention of its pseudotetrahedral D(2d) symmetry and the blockage of the solvent accessibility. We made a comparison between the excited state dynamics of [Cu(I)(dtbp)(2)](+) with its mono-tert-butyl counterpart, bis(2-tert-butyl-1,10-phenanthroline)copper(I) [Cu(I)(tbp)(2)](+). The subpicosecond component assigned to the flattening of the D(2d) coordination geometry in the MLCT excited state was again present in the latter because the absence of a tert-butyl on the phenanthroline allows flattening to the pseudotetrahedral coordination geometry. Unlike the [Cu(I)(dtbp)(2)](+), [Cu(I)(tbp)(2)](+) exhibited no detectable emission at room temperature in solution. These results provide new insights into the manipulation of various excited state properties in Cu diimine complexes by certain key structural factors, enabling optimization of these systems for solar energy conversion applications.

20.
J Am Chem Soc ; 133(50): 20468-75, 2011 Dec 21.
Article in English | MEDLINE | ID: mdl-22077184

ABSTRACT

A new low band gap copolymer PBB3 containing [6,6']bi[thieno[3,4-b]thiophenyl]-2,2'-dicarboxylic acid bis-(2-butyloctyl) ester (BTT) and 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b']dithiophene (BDT) units was synthesized and tested for solar cell efficiency. PBB3 showed a broad absorbance in the near-IR region with a substantially red-shifted (by more than 100 nm) λ(max) at 790 nm as compared to the PTB series of polymers, which have been previously reported. The PBB3 polymer also showed both a favorable energy level match with PCBM (with a LUMO energy level of -3.29 eV) and a favorable film domain morphology as evidenced by TEM images. Despite these seemingly optimal parameters, a bulk heterojunction (BHJ) photovoltaic device fabricated from a blend of PBB3 and PC(71)BM showed an overall power conversion efficiency (PCE) of only 2.04% under AM 1.5G/100 mW cm(-2). The transient absorption spectra of PBB3 showed the absence of cationic and pseudo charge transfer states that were observed previously in the PTB series polymers, which were also composed of alternating thienothiophene (TT) and BDT units. We compared the spectral features and electronic density distribution of PBB3 with those of PTB2, PTB7, and PTBF2. While PTB2 and PTB7 have substantial charge transfer characteristics and also relatively large local internal dipoles through BDT to TT moieties, PTBF2 and PBB3 have minimized internal dipole moments due to the presence of two adjacent TT units (or two opposing fluorine atoms in PTBF2) with opposite orientations or internal dipoles. PBB3 showed a long-lived excitonic state and the slowest electron transfer dynamics of the series of polymers, as well as the fastest recombination rate of the charge-separated (CS) species, indicating that electrons and holes are more tightly bound in these species. Consequently, substantially lower degrees of charge separation were observed in both PBB3 and PTBF2. These results show that not only the energetics but also the internal dipole moment along the polymer chain may be critical in maintaining the pseudocharge transfer characteristics of these systems, which were shown to be partially responsible for the high PCE device made from the PTB series of low band gap copolymers.

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