Interrupted Energy Transfer

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					Interrupted Energy Transfer: Highly
Selective Detection of Cyclic Ketones
                  in the Vapor Phase
 Jason R. Cox, Peter Muller, and Timothy M. Swager
   Recent worldwide events have highlighted the need for a portable,
    highly sensitive explosives sensing device that is reliable in various

       Explosives that contain 1,3,5-trinitro-1,3,5-triazacyclohexanone (RDX) are
        difficult to detect
           Equilibrium vapor pressure: 6 ppt at 25⁰C
           Unfavorable reduction potential
   Making a sensor for RDX would be far too difficult

   Instead, focus on detecting cyclohexanone
       Used in the recrystallization of RDX
       Equilibrium vapor pressure of 4470 ppm

   PROBLEM (again):
       Prevalence of ketones in natural environment requires a sensor to be highly
   Detection Method
       Stable, highly emissive conjugated polymer used as a light-harvester
        to amplify the emission of squaraine dye embedded in a thin film by
        electronic energy transfer (EET)
           Inspection of absorbance and photoluminescence data shows that energy
            transfer occurs by electron exchange from the excited polymer to the
            squaraine, which then goes through relaxation and emission

Synthesis of squaraine reporter dye 1. (i) 3,5-Bis-(trifluoromethyl)phenyl   Molecular structure of PPE 2
isocyanate, CH2Cl2, rt, 12 h. (ii) Squaric acid,toluene/n-butanol (3:1),     (R=C14H29).
reflux, 6 h.
  Detection Method

Absorbance (solid line) and photoluminescence (circles)spectra of PPE 2 (blue) and
squaraine 1 (red). PPE 2 photoluminescence spectra are from a thin film; all other
measurements taken inCHCl3 (2 λex = 380 nm, 1 λex = 660 nm). The red shift of the emission
maximum of 1 in thin films is likely due to stabilization of the excited state of 1 in the
polymer matrix.
Detection Method
   Exposure to H-bond acceptors like cyclohexanone results in a
    strong response
       Not seen when exposed to saturated acyclic ketones like acetone, 3-
        pentanone, and diisopropyl ketone
       Exposure to other cyclic ketones like cyclopentanone resulted in similar
        responses to cyclohexanone
Why so selective?
   Structure of squaraine includes small binding cavity for oxygen
       Perfect for cyclic ketones’ pinned back alkyl substituents
What about substituents?
   Selectivity related to steric bulk surrounding ketone
       Sensor should behave differently when exposed to 2-
        methylcyclopentanone than when exposed to cyclopentanone

   Response much smaller
       Receptor pocket of squaraine requires sterically unhindered
        ketone functionality to function properly

   Light harvesting ability of conjugated polymers used to
    amplify emission from embedded squaraine

   Resulting film displays high selectivity for cyclic ketones,
    including cyclohexanone – component of plastic
       Due to binding pocket of receptor
       Limit of detection = 4.76 ppm
Conclusion/What next?
   We have now found a technique for detecting cyclic ketones that is
    very selective
       Use conjugated polymer with squaraine dye

   The next step is to make a portable and durable detector
       Main application of this work is explosives detection in the field, specifically
        in the military
       Portability – Soldiers already carry heavy loads
       Durability – Needs to work in various (sometimes extreme) environments

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