Varying Line-Broadening Sources by Tuning the Composition of DNA-Assembled Cyanine Tetramer Aggregates

Molecular (dye) aggregates are a materials platform that feature collective excitations, known as excitons, with applications in light harvesting, organic optoelectronics, and nanoscale computing. Several recent works have explored the possibility of using exciton-exciton superpositions as a basic unit in quantum information science (QIS). For their successful use in QIS, it is necessary to maximize both the beating frequency, ν_h, and the decoherence time, τ_d, of the superposition. Although direct measurements of these parameters are challenging, it has been shown that excitonic τ_d is similar to the decoherence time of the associated optical transitions, that is, optical τ_d. Further, optical τ_d is related to line broadening, which can readily be measured via absorption spectroscopy. In this work, we characterize line broadening in Cy5 and Cy5.5 monomers and homo- and heterotetramers, tethered to and assembled with Deoxyribonucleic acid (DNA), using steady-state absorption spectroscopy and two-dimensional electronic spectroscopy (2D ES). We also characterize the line broadening of Cy5 free in solution. We find that the width of the primary feature in the tetramer steady-state absorption spectra decreases with increasing Cy5 content. Additionally, the line width of the Cy5 tetramer is smaller than the Cy5 monomer, suggesting that collective excitonic effects may act to reduce the line width. Using 2D ES, we find that homogeneous broadening of all monomers is similar up to time scales of 10 ps; however, after 10 ps DNA appears to cause additional homogeneous broadening, possibly due to large-scale DNA fluctuations. We find using 2D ES that the relative homogeneous and inhomogeneous contributions to spectral broadening in the tetramers are influenced by their composition and excitonic coupling strength.