Field effects induce bathochromic shifts in xanthene dyes

Abstract

There is ongoing interest in near-infrared (NIR) absorbing and emitting dyes for a variety of biomedical and materials applications. Simple and efficient synthetic procedures enable the judicious tuning of through-space polar (field) effects as well as low barrier hydrogen bonding to modulate the HOMO-LUMO gap in xanthene dyes. This affords unique NIR-absorbing xanthene chromophores.

Figure 1

Structures of benzannulated xanthenes.

Figure 2

Absorption and fluorescence spectra of fully annulated 1b, 2b, and semiannulated 3b and 4b in MeOH. (a) Absorption. Inset: Room light Photograph of solutions in MeOH (left to right: 3b, 4b, 1b and 2b). (b) Normalized fluorescence excitation spectra. (c) Normalized fluorescence emission spectra. Formal transposition of R₂ (Figure 1) modulates the HOMO-LUMO gap and shifts the absorption and fluorescence in both the symmetric (1 and 2) and asymmetric (3 and 4) pairs. Additional spectra in MeOH and DMSO are provided (SI, Figures S31–S33).

Figure 3

¹H NMR spectra in DMSO-d₆ of compounds 1b, 2b, 3b, and 4b showing the different chemical shifts observed for the hydroxyl protons.

Figure 4

Hydrogen bond network of 2 and energy-minimized structure of 2b showing how this enhances coplanarity of the fused ring system.

Figure 5

Calculated molecular surfaces of the HOMO and LUMO of 1c and 2c (basis set DFT B3LYP/6-31G). Additional molecular surfaces for other compounds are provided in the supporting information.

Figure 6

Overlapping of the molecular orbitals of the oxygens at C-1 and C-13 in compound 2c.

Figure 7

Known architectures positioning oxygen atoms relatively close in space.

Figure 8

Fluorescence emission spectra of 3b, 3c and methyl ether 4d in 10:90 DMSO:aqueous. Solutions were prepared such that the absorbances at the 500 nm excitation wavelength were equal for all solutions. Inset: Fluorescence emission in a darkened room of 7.5 μM solutions (3b, 4d, and 3c from left to right) excited from below with 3-watt megaMAX 505 nm ALS system. Neutral compound 3b is in pH 6 phosphate buffer, 12.5 mM while anion 3c is in pH 12 NaOH. Compound 4d is in pH 7.4 phosphate buffer, 12.5 mM.