Varying Line-Broadening\nSources by Tuning the Composition\nof DNA-Assembled Cyanine Tetramer Aggregates

Molecular (dye) aggregates are a materials platform that\nfeature\ncollective excitations, known as excitons, with applications in light\nharvesting, organic optoelectronics, and nanoscale computing. Several\nrecent works have explored the possibility of using exciton–exciton\nsuperpositions as a basic unit in quantum information science (QIS).\nFor their successful use in QIS, it is necessary to maximize both\nthe beating frequency, ν<sub>h</sub>, and the decoherence time,\nτ<sub>d</sub>, of the superposition. Although direct measurements\nof these parameters are challenging, it has been shown that excitonic\nτ<sub>d</sub> is similar to the decoherence time of the associated\noptical transitions, that is, optical τ<sub>d</sub>. Further,\noptical τ<sub>d</sub> is related to line broadening, which can\nreadily be measured via absorption spectroscopy. In this work, we\ncharacterize line broadening in Cy5 and Cy5.5 monomers and homo- and\nheterotetramers, tethered to and assembled with Deoxyribonucleic acid\n(DNA), using steady-state absorption spectroscopy and two-dimensional\nelectronic spectroscopy (2D ES). We also characterize the line broadening\nof Cy5 free in solution. We find that the width of the primary feature\nin the tetramer steady-state absorption spectra decreases with increasing\nCy5 content. Additionally, the line width of the Cy5 tetramer is smaller\nthan the Cy5 monomer, suggesting that collective excitonic effects\nmay act to reduce the line width. Using 2D ES, we find that homogeneous\nbroadening of all monomers is similar up to time scales of 10 ps;\nhowever, after 10 ps DNA appears to cause additional homogeneous broadening,\npossibly due to large-scale DNA fluctuations. We find using 2D ES\nthat the relative homogeneous and inhomogeneous contributions to spectral\nbroadening in the tetramers are influenced by their composition and\nexcitonic coupling strength. In addition to revealing how aggregate\ncomposition influences spectral broadening, this work highlights structural\nfeatures of dyes that may serve to reduce spectral broadening, which,\nin turn, may increase optical and excitonic τ<sub>d</sub>.