Fluorescent Probes and Sensors based on Quinolinium and Isoquinolinium Chromophores
Introduction
Luminescent chromophores (luminophores) are used for constructing fluorescent probes, fluorescent labels and fluorescent sensors. Basic requirements for a good chromophore are a high quantum yield, good stability and facile functionalization at multiple positions. We have synthesized 7-fluoroquinoline (1) and 7-fluoro-1-methyl-quinolinium iodide (2). Compound 2 specifically reacts with “soft nucleophiles” like amines by a nucleophilic aromatic substitution reaction and produces highly fluorescent 7-amino-1-methyl-quinolinium salts (3), see Scheme 1.1 This reaction enables the synthesis of a wide variety of fluorescent probes, sensors and polymers, all with excellent properties. Recently we have also synthesized isoquinolinium based probes by a similar method.
Scheme 1. Derivativization of 7-fluoroquinoline (1).
Mobility Sensitive Probes, Color Shifting
We have demonstrated that 7-dimethylamiono-1-methyl-quinolinium tetrafluoroborate (R1=R2=R3=CH3, X=BF4, 3a) is an excellent color shifting mobility sensitive probe.2 The emission wavelength of this molecule is sensitive to the mobility of the probes’ environment. In Figure 1, the emission of 3a in copolymers of ethyl methacrylate (EMA) and methyl methacrylate (MMA) as a function of temperature is displayed. These copolymers are chosen because the glass transition temperature can be tuned at will between 70 and 115°C by increasing the MMA contents. Figure 1 demonstrates that upon cooling clear changes in gradient are observed at the corresponding glass transition temperatures. Due to the excellent stability of 3a, this probe has been used for highly demanding applications.3
Figure 1. Emission wavelength of 3a a function of temperature in two random PMMA-co-PEMA copolymers. Datapoints were taken during cooling.
pH Sensors of the “Light-up” Type
pH sensitive “light-up” probes are obtained by adding an external amine to 3, either at R1, R2 or R3, see Scheme 2 and Figures 2 and 3. Figure 3 clearly demonstrates that upon protonation the absorption of the fluorescent pH sensor shifts to the blue, whereas the emission increases by a factor of 200! The blue shift in absorption (and emission by the way) is due to a decreased donor strength upon protonation. The strong increase in emission occurs because quenching by electron transfer is prevented upon protonation.4 Figure 4 shows that for the series 3c-3h, the dissociation constant pKA ranges from 3.5 to 10. Obviously, the electron donating capability of the R substituent(s) determines the value of the dissociation constant pKA to a large extent. By proper substitution, any value of pKA between 2 and 10 can be obtained. Incorporation of pH sensors in a polymer network by attaching a methacrylate to R1 has recently been achieved.
Scheme 2. Synthesis of the fluorescent pH sensors 3c-3h.
Figure 2. Absorption and emission spectra of a fluorescent pH sensor in water, as a function of the pH.
Figure 3. Emission intensity as a function of the pH for 3c-3h in water.
Research Projects
Currently, we have research projects for master and bachelor students on the following subjects:
- Synthesis of novel fluorescent platforms, novel sensors, fluorescent responsive polymers, and 'molecular logic elements'
- Mechanistic research on fluorescence quenching and energy transport in multichromophoric arrays
- Probing mobility in (confined) polymer systems (in cooperation with the Nano-Structured Materials group)
References
1: van den Berg, O.; Jager, W.F.; Picken, S.J.
7-Dialkylamino-1-alkylquinolinium Salts; Highly Versatile and Stable Fluorescent Probes.
J.Org. Chem.2006, 71, 2666-2676. [doi]
2: Novel Color-Shifting Mobility Sensitive Fluorescent Probes for Polymer Characterization.
Jager, W.F.; van den Berg, O.; Picken, S.J.
Macromol. Symp. 2005, 230, 11-19. [doi]
3: A Wavelength Shifting Fluorescent Probe for Investigating Physical Aging.
van den Berg, O.; Jager, W.F.; Cangialosi, D.; van Turnhout, J.; Verheijen, P.J.T.; Wübbenhorst, M.; Picken, S.J.
Macromolecules2006, 39. 224-231. [doi]
4: Signaling Recognition Events with Fluorescent Sensors and Switches.
Silva, A. P. de.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev.1997, 97, 1515. [doi]
