Excited-State Lifetimes of DNA-Templated Cyanine Dimer, Trimer, and Tetramer Aggregates: The Role of Exciton Delocalization, Dye Separation, and DNA Heterogeneity.

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.