Fluorescent Liquid Tetrazines

Abstract

Tetrazines with branched alkoxy substituents are liquids at ambient temperature that despite the high chromophore density retain the bright orange fluorescence that is characteristic of this exceptional fluorophore. Here, we study the photophysical properties of a series of alkoxy-tetrazines in solution and as neat liquids. We also correlate the size of the alkoxy substituents with the viscosity of the liquids. We show using time-resolved spectroscopy that intersystem crossing is an important decay pathway competing with fluorescence, and that its rate is higher for 3,6-dialkoxy derivatives than for 3-chloro-6-alkoxytetrazines, explaining the higher fluorescence quantum yields for the latter. Quantum chemical calculations suggest that the difference in rate is due to the activation energy required to distort the tetrazine core such that the nπ*S1 and the higher-lying ππ*T2 states cross, at which point the spin-orbit coupling exceeding 10 cm-1 allows for efficient intersystem crossing to occur. Femtosecond time-resolved anisotropy studies in solution allow us to measure a positive relationship between the alkoxy chain lengths and their rotational correlation times, and studies in the neat liquids show a fast decay of the anisotropy consistent with fast exciton migration in the neat liquid films.

Keywords: excited states; fluorescence; inter system crossing; rheology; viscosity.

Scheme 1

Compounds studied.

Figure 1

Absorption and emission spectra of $${\mathrm{C}}_{16}{\mathrm{C}}_{16}$$ in four different solvents. The absorption spectra were normalized relative to the $${n\pi }^{*}$$ absorption. The emission intensities were normalized relative to the maximum emission in dioxane, corrected for the differences in absorption of the samples, and multiplied by the solvent refractive index squared so that the maximum intensities reflect relative $${\varphi }_f$$.

Figure 2

Nanosecond time-resolved spectra of (a) $$\mathrm{C}\mathrm{l}{\mathrm{C}}_{16}$$ and (b) $${\mathrm{C}}_{16}{\mathrm{C}}_{16}$$ at a few selected delays, and their respective transient matrices (c,d).

Figure 3

Picosecond time-resolved absorption and anisotropy of $$\mathrm{C}\mathrm{l}{\mathrm{C}}_{16}$$ in DCM. (a) ps time-resolved absorption of $$\mathrm{C}\mathrm{l}{\mathrm{C}}_{16}$$ in DCM measured with parallel and perpendicular relative polarization of the pump and probe. (b) The difference absorption integrated over the absorption band ($$\mathsf{\Sigma }\Delta A$$) vs delay time a parallel and perpendicular pump/probe configuration. (c) The decay of the anisotropy fitted to a single exponential decay. (d) Residuals of the fit.

Figure 4

$$S_1/T_2$$ minimum energy crossing points of $$C_1{C}_1$$ (a) and of $$Cl{C}_1$$ (b).

Figure 5

The decay of the anisotropy in the dichloroalkoxy neat films, plotted against the curves predicted by the Huber model.