Lactam-fused tropolones: a new tunable, environmentally sensitive fluorophore class

Abstract

Fluorescent small-molecules capable of altering their profiles in response to environmental changes are exceptionally valuable tool compounds throughout the scientific community. The following manuscriipt describes a new class of fluorescent small molecules based on lactam-fused tropolones that are responsive to a dynamic range of environmental changes. These molecules can be easily obtained through a rapid annulation procedure between appropriately functionalized tropolones and primary amines, which is often complete within minutes at room temperature. Molecules generated through this approach have been identified with fluoresence emission across the visible light spectra, and can be tuned based on either the tropolone or amine component. They are also highly responsive to changes in solvent, pH, and certain divalent metal ions. Tropolone-fused lactams thus represent a new class of tunable fluorescent small molecules that could find value throughout the scientific community.

Figure 1.. Tropolone Fluorescence at 10 μM in CH₂Cl₂.

(A) Images of solutions taken while undergoing irradiation with standard laboratory UV-lamps using long-wave setting (λ_ex ~ 365 nm). (B) Fluorescence emission spectra of select 10 μM solutions in CH₂Cl₂, normalized to their max fluorescence for visualization purposes. (C) Table of fluorescence of all lactam-fused tropolones.

Figure 2.. Fluorescence Emission Profiles at 10 mM in Different Solvents.

Emission spectra were read at with PMT voltage set at 800 V for A and B, 600 V for C, and 400 V for D. Inset fluorescent images are listed in, l-r, PhMe, CH₂Cl₂, MeCN, and iPrOH, and are taken upon irradiation at ~365 nm with hand-held UV lamp.

Figure 3.. pH-Dependent Fluorescence Emission.

(A) Absorbance and emission spectra of 25a at 10 μM in 100 mM potassium phosphate-based buffers. (B/C) Fluorescence intensity of 25a and 25b as a factor of pH, along with normalized absorbance spectral change and pKa values derived from these graphs. pKa₁ is the transition from cationic (ie, tropylium) to neutral, and is not observed in these studies. pKa₂ reflects the transition from neutral to mono-anionic, and pKa₃ the transition from monoanionic to dianionic. See supporting information for additional details.

Figure 4.. Tropolone response to metal additives.

(A) Change in fluorescence emission spectra when 10 μM tropolone solutions in water are excited at 420 nm in the presence of 10 mM of metal ions. (B) Vials containing solutions used for fluorescence emission spectra when viewed under long-wave irradiation with a handheld UV-Vis lamp, along with tropolone with MgCl2 and ZnCl2 for visual reference. (C) Fluorescence intensity measurements as a relationship to concentration of aqueous solutions containing either MgCl₂ or ZnCl₂, as measured using a fluorescence plate reader in 96 well plate format.

Scheme 1.

Examples of Environmental-Responsive Fluorophores

Scheme 2.

Tropolone and tropolone analogues of interest due to fluorescence

Scheme 3.. Strategy for Lactam-Fused Tropolone Synthesis.

(A) Representative example of isoindolinone synthesis. (B) Overview of route to the synthesis of 7-hydroxystipidalide and its potential application to towards lactam-fused tropolones

Scheme 4.. Reaction Optimization Studies.

TMBz = 1,3,5-trimethoxybenzene, used as internal standard. Yields calculated by ¹H NMR integration. Isolated yields following C18 chromatography. All yields based on tropolone as the limiting reagent.

Scheme 5.

Potential Mechanism for Formation of 19a/b and 20a/b.

Scheme 6.. Isolated Yields from Substrate Scope Studies.

Reactions were heated to 100 °C for 5 (24e) or 18 (19e) hrs.

Scheme 7.. Comparison of 19b to Related Non-Lactam Tropolones.

(A) Illustration of enhanced π-conjugation of planar lactam, along with model compound 26. (B-D) Excitation (solid, coloured) and Absorbance (black, dotted) of dimethoxytropolones. (E) Emission graphs at maximum excitation wavelengths of 16, 19b, and 26, along with tropolone (5).

Scheme 8.

Structural Similarities Between 3-Hydroxyflavone and Lactam-Fused Tropolones