Dual Binding Site Assisted Chromogenic and Fluorogenic
Recognition and Discrimination of Fluoride and Cyanide by a Peripherally
Borylated Metalloporphyrin: Overcoming Anion Interference in Organoboron
Based Sensors
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Abstract
Peripherally
triarylborane decorated porphyrin (<b>2</b>)
and its Zn(II) complex (<b>3</b>) have been synthesized. Compound <b>3</b> contains of two different Lewis acidic binding sites (Zn(II)
and boron center). Unlike all previously known triarylborane based
sensors, the optical responses of <b>3</b> toward fluoride and
cyanide are distinctively different, thus enabling the discrimination
of these two interfering anions. Metalloporphyrin <b>3</b> shows
a multiple channel fluorogenic response toward fluoride and cyanide
and also a selective visual colorimetric response toward cyanide.
By comparison with model systems and from detailed photophysical studies
on <b>2</b> and <b>3</b>, we conclude that the preferential
binding of fluoride occurs at the peripheral borane moieties resulting
in the cessation of the EET (electronic energy transfer) process from
borane to porphyrin core and with negligible negetive cooperative
effects. On the other hand, cyanide binding occurs at the Zn(II) core
leading to drastic changes in its absorption behavior which can be
followed by the naked eye. Such changes are not observed when the
boryl substituent is absent (e.g., Zn-TPP and TPP). Compounds <b>2</b> and <b>3</b> were also found to be capable of extracting
fluoride from aqueous medium