Porphyrin-based fluorescent sensors

Fig. 10.43. Porphyrin-based fluorescent sensors of neutral molecules (P-1 Kijima H. et al. (1999) Chem. Commun. 2011. P-2 D Souza F. (1996) J. Am. Chem. Soc. 118, 923. P-3 D Souza F. (1997) Chem. Commun. 533).

A Zn2+ -responsive dual fluorescence and MRI probe (14) (Scheme 9) based on a porphyrin ligand platform was also designed. Compound 14 is a fluorescent sensor turned on by the presence of Zn +, while the Mn + complex functions as an MRI contrast agent. In the aqueous solution of Zn +, the relaxivity of Mn-14 is significantly altered. Both the metal-free and Mn + complex are efficiently taken up by live cells (HEK-293), and the intracellular zinc can be imaged by either fluorescence or MR. It is the first cell-permeable MRI sensor for the detection of intracellular zinc in both T - and r2-weighted images. 

The cucurbit [n]uril family (CB[n]) of molecular containers possess remarkable binding affinities and selectivities (Ka values up to 1012M-1, Krei values up to 106) which renders them useful as a component of molecular machines, sensors, and biomimetic systems (123-125). Recently, Wagner and coworkers have reported (126) that CB[10] - with its spacious 870A3 cavity - is capable of acting as a host for free base and metalated tetra (Af-methylpyridinium)porphyrins 19a-d (Fig. 17). Despite the large ellipsoidal deformation of CB[10] upon complexation, the complexed porphyrins retain their fundamental UV/VIS, fluorescence, and electrochemical properties. The CB[ 10] porphyrin... 

A different approach to dendritic sensors involves modification of a sensor core unit with dendritic substituents to confer beneficial solubility properties. An example of a sensor core unit is the porphyrin macrocycle, a heterocycle that has been employed extensively in prototypical photochemical sensor systems. Vinogradov and co-workers have exploited the versatile photoactive porphyrin sensor unit as a fluorescence-based pH indicator for use in biological assays [73], by attaching acid terminated polyamide-ether dendrons as substituents (Figure 8.12). The two imino nitrogen atoms present in the free-base porphyrin are susceptible to stepwise protonation to afford initially a cation and then a dication, respectively. Upon protonation, both the emission and absorption fluorescence spectroscopic characteristics of the porphyrin core are subject to dramatic hypochromic shifts. This spectroscopic phenomenon formed the basis for an accurate pH indicator with potential applications in proton gradient determination studies in biological systems. 

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