Intramolecular Charge Transfer and Ultrafast Nonradiative Decay in DNA-Tethered Asymmetric Nitro- and Dimethylamino-Substituted Squaraines

Abstract

Molecular (dye) aggregates are a materials platform of interest in light harvesting, organic optoelectronics, and nanoscale computing, including quantum information science (QIS). Strong excitonic interactions between dyes are key to their use in QIS; critically, properties of the individual dyes govern the extent of these interactions. In this work, the electronic structure and excited-state dynamics of a series of indolenine-based squaraine dyes incorporating dimethylamino (electron donating) and/or nitro (electron withdrawing) substituents, so-called asymmetric dyes, were characterized. The dyes were covalently tethered to DNA Holliday junctions to suppress aggregation and permit characterization of their monomer photophysics. A combination of density functional theory and steady-state absorption spectroscopy shows that the difference static dipole moment (Δd) successively increases with the addition of these substituents while simultaneously maintaining a large transition dipole moment (μ). Steady-state fluorescence and time-resolved absorption and fluorescence spectroscopies uncover a significant nonradiative decay pathway in the asymmetrically substituted dyes that drastically reduces their excited-state lifetime (τ). This work indicates that Δd can indeed be increased by functionalizing dyes with electron donating and withdrawing substituents and that, in certain classes of dyes such as these asymmetric squaraines, strategies may be needed to ensure long τ, e.g., by rigidifying the π-conjugated network.

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

“…We attribute the complex kinetics to a nonuniform distribution of nonradiative decay rates, leveraging the consensus that, in the case of many polymethine dyes, the primary contributor to nonradiative decay is torsional motions of the π-conjugated network. − A nonuniform distribution of nonradiative decay rates in the DNA-templated polymethine dyes may originate from (i) intrinsic disorder of the dye, i.e., which can lead to a distribution of ground-state conformers of the π-conjugated network; , (ii) binding of the dye to the DNA template, ,, which can change the intrinsic disorder of the dye and inhibit torsional motions of the π-conjugated network in its excited state; and/or (iii) conformational disorder of the DNA template. ,, Figure also shows the amplitude-weighted average time constant, which we assign to τ p of the lowest lying electronic excited state. Critically, τ p of ∼1–2 ns observed here for the Dy (i.e., asymmetric polymethine) monomers is much longer than the τ p of ∼5–40 ps previously reported for asymmetric squaraine monomers . In fact, the lifetime is much closer to the ∼1–10 ns lifetime expected when radiative decay is able to effectively compete with nonradiative decay .…”

“…Critically, τ p of ∼1−2 ns observed here for the Dy (i.e., asymmetric polymethine) monomers is much longer than the τ p of ∼5−40 ps previously reported for asymmetric squaraine monomers. 63 In fact, the lifetime is much closer to the ∼1−10 ns lifetime expected when radiative decay is able to effectively compete with nonradiative decay. 69 To evaluate the influence of tethering the dyes to DNA, we also characterized τ p of the "free" dyes not tethered to DNA (see, e.g., Section S4).…”

“…We chose to examine four polymethine dyes, i.e., Dy 634, 652, 754, and 765 (Figure A), because they represent a diverse set of asymmetric structures, are commercially available, and can be covalently tethered to DNA. Previously, we showed that structural (and electronic) asymmetry could be induced in a series of squaraine dyes functionalized with small-scale, strongly electron donating and withdrawing substituents on either or both indolenine end groups . Here, larger-scale electron donating and withdrawing moieties, specifically indolenine and benzopyrylium-based moieties, are located on either side of a polymethine bridge.…”

“…These values of Δd are appreciable, larger than those previously reported for asymmetric squaraines and cyanines that had a maximum calculated Δd of ∼2.2 and 3.4 D, respectively. 60,63 We additionally arrive at the following findings. First, comparing Dy 652 and 754 we find that increasing bridge length results in a large increase in Δd.…”

“…Many dyes have large μ values. For example, cyanines and squaraines can have μ values as large as ∼10–15 D. − In contrast to μ, Δ d is less commonly evaluated. In part, this could be because it is more difficult to characterize.…”