Exciton Delocalization in a DNA-Templated Organic Semiconductor Dimer Assembly.
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ABSTRACT:
SUBMITTER: Wang X
PROVIDER: S-EPMC8793135 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:A bacteriochlorophyll a (Bchla) dimer is a basic functional unit in the LH1 and LH2 photosynthetic pigment-protein antenna complexes of purple bacteria, where an ordered, close arrangement of Bchla pigments-secured by noncovalent bonding to a protein template-enables exciton delocalization at room temperature. Stable and tunable synthetic analogs of this key photosynthetic subunit could lead to facile engineering of exciton-based systems such as in artificial photosynthesis, organic optoelectronics, and molecular quantum computing. Here, using a combination of synthesis and theory, we demonstrate that exciton delocalization can be achieved in a dimer of a synthetic bacteriochlorin (BC) featuring stability, high structural modularity, and spectral properties advantageous for exciton-based devices. The BC dimer was covalently templated by DNA, a stable and highly programmable scaffold. To achieve exciton delocalization in the absence of pigment-protein interactions critical for the Bchla dimer, we relied on the strong transition dipole moment in BC enabled by two auxochromes along the Qy transition, and omitting the central metal and isocyclic ring. The spectral properties of the synthetic "free" BC closely resembled those of Bchla in an organic solvent. Applying spectroscopic modeling, the exciton delocalization in the DNA-templated BC dimer was evaluated by extracting the excitonic hopping parameter, J to be 214 cm-1 (26.6 meV). For comparison, the same method applied to the natural protein-templated Bchla dimer yielded J of 286 cm-1 (35.5 meV). The smaller value of J in the BC dimer likely arose from the partial bacteriochlorin intercalation and the difference in medium effect between DNA and protein.
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Project description:DNA-templated molecular (dye) aggregates are a novel class of materials that have garnered attention in a broad range of areas including light harvesting, sensing, and computing. Using DNA to template dye aggregation is attractive due to the relative ease with which DNA nanostructures can be assembled in solution, the diverse array of nanostructures that can be assembled, and the ability to precisely position dyes to within a few Angstroms of one another. These factors, combined with the programmability of DNA, raise the prospect of designer materials custom tailored for specific applications. Although considerable progress has been made in characterizing the optical properties and associated electronic structures of these materials, less is known about their excited-state dynamics. For example, little is known about how the excited-state lifetime, a parameter essential to many applications, is influenced by structural factors, such as the number of dyes within the aggregate and their spatial arrangement. In this work, we use a combination of transient absorption spectroscopy and global target analysis to measure excited-state lifetimes in a series of DNA-templated cyanine dye aggregates. Specifically, we investigate six distinct dimer, trimer, and tetramer aggregates-based on the ubiquitous cyanine dye Cy5-templated using both duplex and Holliday junction DNA nanostructures. We find that these DNA-templated Cy5 aggregates all exhibit significantly reduced excited-state lifetimes, some by more than 2 orders of magnitude, and observe considerable variation among the lifetimes. We attribute the reduced excited-state lifetimes to enhanced nonradiative decay and proceed to discuss various structural factors, including exciton delocalization, dye separation, and DNA heterogeneity, that may contribute to the observed reduction and variability of excited-state lifetimes. Guided by insights from structural modeling, we find that the reduced lifetimes and enhanced nonradiative decay are most strongly correlated with the distance between the dyes. These results inform potential tradeoffs between dye separation, excitonic coupling strength, and excited-state lifetime that motivate deeper mechanistic understanding, potentially via further dye and dye template design.
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Project description:Coherent exciton delocalization in dye aggregate systems gives rise to a variety of intriguing optical phenomena, including J- and H-aggregate behavior and Davydov splitting. Systems that exhibit coherent exciton delocalization at room temperature are of interest for the development of artificial light-harvesting devices, colorimetric detection schemes, and quantum computers. Here, we report on a simple dye system templated by DNA that exhibits tunable optical properties. At low salt and DNA concentrations, a DNA duplex with two internally functionalized Cy5 dyes (i.e., dimer) persists and displays predominantly J-aggregate behavior. Increasing the salt and/or DNA concentrations was found to promote coupling between two of the DNA duplexes via branch migration, thus forming a four-armed junction (i.e., tetramer) with H-aggregate behavior. This H-tetramer aggregate exhibits a surprisingly large Davydov splitting in its absorbance spectrum that produces a visible color change of the solution from cyan to violet and gives clear evidence of coherent exciton delocalization.
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Project description:Molecular excitons play a foundational role in chromophore aggregates found in light-harvesting systems and offer potential applications in engineered excitonic systems. Controlled aggregation of chromophores to promote exciton delocalization has been achieved by covalently tethering chromophores to deoxyribonucleic acid (DNA) scaffolds. Although many studies have documented changes in the optical properties of chromophores upon aggregation using DNA scaffolds, more limited work has investigated how structural modifications of DNA via bridged nucleotides and chromophore covalent attachment impact scaffold stability as well as the configuration and optical behavior of attached aggregates. Here we investigated the impact of two types of bridged nucleotides, LNA and BNA, as a structural modification of duplex DNA-templated cyanine (Cy5) aggregates. The bridged nucleotides were incorporated in the domain of one to four Cy5 chromophores attached between adjacent bases of a DNA duplex. We found that bridged nucleotides increase the stability of DNA scaffolds carrying Cy5 aggregates in comparison with natural nucleotides in analogous constructs. Exciton coupling strength and delocalization in Cy5 aggregates were evaluated via steady-state absorption, circular dichroism, and theoretical modeling. Replacing natural nucleotides with bridged nucleotides resulted in a noticeable increase in the coupling strength (≥10 meV) between chromophores and increased H-like stacking behavior (i.e., more face-to-face stacking). Our results suggest that bridged nucleotides may be useful for increasing scaffold stability and coupling between DNA templated chromophores.
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Project description:Exciton delocalization in dye aggregate systems is a phenomenon that is revealed by spectral features, such as Davydov splitting, J- and H-aggregate behavior, and fluorescence suppression. Using DNA as an architectural template to assemble dye aggregates enables specific control of the aggregate size and dye type, proximal and precise positioning of the dyes within the aggregates, and a method for constructing large, modular two- and three-dimensional arrays. Here, we report on dye aggregates, organized via an immobile Holliday junction DNA template, that exhibit large Davydov splitting of the absorbance spectrum (125 nm, 397.5 meV), J- and H-aggregate behavior, and near-complete suppression of the fluorescence emission (?97.6% suppression). Because of the unique optical properties of the aggregates, we have demonstrated that our dye aggregate system is a viable candidate as a sensitive absorbance and fluorescence optical reporter. DNA-templated aggregates exhibiting exciton delocalization may find application in optical detection and imaging, light-harvesting, photovoltaics, optical information processing, and quantum computing.
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