Silicon incorporation in polymethine dyes

Abstract

Methods to red-shift fluorophores have garnered considerable interest due to the broad utility of low energy light. The incorporation of silicon into xanthene and coumarin scaffolds has resulted in an array of visible and near-infrared fluorophores. Here, we extend this approach to polymethine dyes, another popular fluorophore class, performing experimental and computational analyses. We found that when oxygen was replaced with SiMe₂, bathochromic shifts of up to 121 nm and fluorophores with emission above 900 nm were achieved.

Figure 1.

Fluorophore scaffolds. A) Xanthene scaffold, the first fluorophore where silicon was incorporated. B) Coumarin scaffold. C) Fluorescein scaffold. D) Rhodamine scaffold. E) Cyanine scaffold: a classic polymethine dye. F) This work on silicon polymethines and their oxygen counterparts.

Scheme 1.

Synthesis of silicon and oxygen fluorophores studied herein. A) Heterocycle synthesis. B) Polymethine dye synthesis. For 7, conditions (a). For 8, conditions (b). For 10, conditions (c). For 11, conditions (d).

Figure 2.

Photophysical data of silicon fluorophores. A) Normalized absorbance (solid) and emission (dotted) in dichloromethane (DCM). B) Photophysical characterization of 3, 7, 10 in DCM. ^aError and experimental in detail in Figure S8.

Figure 3.

A/B) Schematic of symmetric state (delocalized charge, A) and asymmetric state (dipolar, charge is localized on one heterocycle, B). C) Absorbance of 10 (normalized to λ_max) in a range of nonpolar and polar solvents. D) Normalized absorption spectra (Abs, black) and excitation spectra (Ex, grey, excitation from 515–1025 nm and collection 1050 nm) of 10 in DCM. E) Absorption spectra of 10 normalized at 663 nm in a solution of tetrahydrofuran (THF, 0.01 mM) supplemented with increasing amounts of H₂O. F) Absorption spectra of 10 normalized at 938 nm in a solution of THF (0.01 mM) supplemented with increasing amounts of CHCl₃.

Figure 4.

A) Conformer 10-H₂O-B shows desymmetrization of 0.17e charge transfer toward the right heterocycle represented schematically and by electron density map. B) Frontier molecular orbitals of Wigner sampled structures of 10 computed with CIS(D)/cc-pvdz-RI correspond to λ_max,abs of 790 nm, show complete electron delocalization. C) HOMO and LUMO of sampled structures of 10 with predicted λ_max,abs of 575 nm show asymmetric character in their electronic structure.

Figure 5.

Comparison of silicon and oxygen xanthene polymethines. A/B) λ_max of silicon- (blue square) and oxygen- (green circle) containing polymethine fluorophores in DCM. The values depicted on the plot indicate the degree of red-shift imparted by silicon. A) Absorbance. B) Emission. C) Normalized absorbance spectra of 7 and 11 in DCM. D) Percent absorbance remaining at λ_max,abs of 7 and 11 with continuous irradiation at 730 nm (146 mW/cm²) over 6 hours. Error is the standard deviation of three experiments.