Substituent Effects on the Solubility and Electronic Properties of Dyes: Density Functional and Time Dependent Density Functional Theory Calculations

DNA nanotechnology enables the construction of nanodevices for excitonic applications by enabling the positional manipulation of dyes covalently attached to the DNA. Recently, dye aggregates templated with DNA scaffolds have been studied by a number of groups for exciton delocalization resulting from the dye’s dipole-dipole interactions The dynamics of excitons in an aggregate depend on the difference between the dye monomer’s ground and excited state dipole moments (Δd), the monomer’s transition dipole moment (μ), and the inter-dye distances [1]. To improve the viability of dye aggregates for excitonic applications, the enhancement of electronic properties (i.e. larger Δd and μ) and improvement of aggregation (i.e. reduction in inter-dye distances by altering solubility) is necessary. The addition of functional group substituents on dyes has been demonstrated to alter their photophysical, electronic, and aggregation properties. To quantify the effects of substituents on dyes, density functional theory (DFT) and time-dependent (TD-) DFT can be used to determine optimized structures and electronic properties of dye monomers [2].

To determine how electron donating (D) and electron withdrawing (W) substituents impact the solubility and electronic properties of dyes such as Cy5 (shown in Fig. 1), DFT and TD-DFT methods were employed for dyes solvated in water, pyridine, quinoline, and isoquinoline [3]. Pyridine, quinoline, and isoquinoline solvents, which are structurally similar to DNA bases, were used to roughly estimate how substituents affect dye intercalation into DNA structures. The solvation energy (ΔGsolv), which is correlated with solubility, Δd, and μ of the substituted dyes were calculated. It was found that ΔGsolv generally decreases (i.e. the dyes become more soluble) upon substitution, and dyes with pairs of W substituents have the most negative ΔGsolv. The values of μ are relatively unaffected upon substitution relative to the unsubstituted dye. However, the values of Δd generally increase upon substitution and asymmetry of the dyes. Pairs of weak and strong W substituents or pairs of D and W substituents yield relatively larger Δd values than pairs of only D substituents. Our computational results advance the understanding of the effects substituents have on key dye attributes. The results of this study can help guide dye synthesis for tailoring the dipole properties of DNA-templated dyes for excitonic applications.