Electronic Structure and Excited-State Dynamics of DNA-Templated Monomers and Aggregates of Asymmetric Polymethine Dyes

Abstract

Aggregates of conjugated organic molecules (i.e., dyes) may exhibit relatively large one- and two-exciton interaction energies, which has motivated theoretical studies on their potential use in quantum information science (QIS). In practice, one way of realizing large one- and two-exciton interaction energies is by maximizing the transition dipole moment (μ) and difference static dipole moment (Δ<i>d</i>) of the constituent dyes. In this work, we characterized the electronic structure and excited-state dynamics of monomers and aggregates of four asymmetric polymethine dyes templated via DNA. Using steady-state and time-resolved absorption and fluorescence spectroscopy along with quantum-chemical calculations, we found the asymmetric polymethine dye monomers exhibited a large μ, an appreciable Δ<i>d</i>, and a long excited-state lifetime (τ_<i>p</i>). We formed dimers of all four dyes and observed that one dye, Dy 754, displayed the strongest propensity for aggregation and exciton delocalization. Motivated by these results, we undertook a more comprehensive survey of Dy 754 dimer and tetramer aggregates using steady-state absorption and circular dichroism spectroscopy. Modeling these spectra revealed an appreciable excitonic hopping parameter (<i>J</i>). Lastly, we used femtosecond transient absorption spectroscopy to characterize τ_<i>p</i> of the dimer and tetramer, which we observed to be exceedingly short. This work revealed that asymmetric polymethine dyes exhibited μ, Δ<i>d</i>, monomer τ_<i>p</i>, and <i>J</i> values promising for QIS; however, further work is needed to overcome excited-state quenching and achieve long aggregate τ_<i>p</i>.