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`2
`
`

`

`J. Am. Chern. Soc. 2000, 122, 8561-8562
`
`8561
`
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`
`P·NEt3' abs p NE •
`j t, fir
`
`8
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`
`Amplified Fluorescence Quenching in a
`Poly(p-phenylene)-Based Cationic Polyelectrolyte
`
`Benjamin S. Harrison, Michael B. Ramey,
`John R. Reynolds, and Kirk S. Schanze*
`
`Department of Chemistry and Center for
`Macromolecular Science and Engineering
`University of Florida, Gainesville, Florida 32611-7200
`
`Received March 7, 2000
`Revised Manuscript Received June 21, 2000
`
`Application of fluorescent conjugated polymers to "amplified"
`sensing of chemical and biological analytes has received consider(cid:173)
`able recent attention. 1- 8 Fluorescence sensing is amplified by
`conjugated polymers because of the "molecular wire effect", 1•2
`which causes a polymer to be quenched by a considerably lower
`analyte concentration than its monomer analogue. Most sensor
`work has been carried out to date using polymers dissolved in an
`organic solvent; 1·2•5- 7 however, several recent reports feature
`fluorescent thin film sensors that operate by coming into contact
`with a liquid- or vapor-phase analyte.3.4 Although these studies
`demonstrate amplified quenching to allow trace detection of
`analytes, the systems are limited because the polymers only
`dissolve in organic solvents. A sensor would be more useful if it
`operates in an aqueous environment. This issue was addressed
`by a recent report that demonstrated fluorescence quenching of a
`water soluble sulfonatoalkoxy poly-(phenylene vinylene) polya(cid:173)
`nion by methyl viologen at nM concentrations.8
`In the present contribution we describe fluorescence quenching
`of the water soluble, poly(p-phenylene )-based polycation, P-NEt3 +
`dibromide by several anionic quenchers, including Ru(phen'h4-
`and Fe(CN)64- in aqueous solution (phen' = 4,7-bis(4-sulfophen(cid:173)
`yl)-1,10-phenanthroline). P-NEt3+ fluorescence is quenched by
`these anions with considerably higher efficiency compared to a
`terphenyl model compound, M-NEt3 + dibromide. Amplified
`quenching of P-NEt3 + arises because (1) ion-pairing enhances
`the concentration of the anionic quencher in the vicinity of the
`polyelectrolyte, and (2) the high mobility of the 1:rc,:rc• exciton
`rapidly brings it into contact with the ion-paired quencher.9 We
`4- occurs
`also establish that quenching of P-NEt3 +by Ru(phen')3
`via energy transfer by observing the metal complexes' photolu(cid:173)
`minescence when the excitation light is absorbed mainly by the
`polymer.
`
`Polycation P-NEt3 +,prepared by Suzuki coupling, 10 was fully
`characterized by NMR, elemental analysis and GPC which
`indicated Mn = 12.4 kD (X. = 28, corresponding to over 50
`phenylene rings) and PDI = 1.16. 10 As illustrated in Figure 1a,
`in aqueous solution P-NEt3 + features an absorption at Amax =
`(I) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593-12602.
`(2) Swager, T. M. Ace. Chem. Res. 1998, 31, 201-207.
`(3) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321-5322.
`
`Ru(phen')," & P·NEt3'
`
`,....,......
`
`300
`
`350
`
`400
`
`450
`
`500
`
`550
`
`600
`
`Wavelength / nm
`Figure 1. (a) UV-Visible absorption spectra of P-NEt3+ (-) and
`Ru(phen')34- (---)in HzO. Fluorescence of P-NEt3+ in H 20 (- ••
`-
`•• ). (b) Emission excitation spectrum with detector set at emission
`wavelength corresponding to Ru(phen')34- MLCT emission (Aem = 610
`nm). (solid line): Ru(phen'h4- only, c = lf.tM; (dashed line): P-NEt3+
`and Ru(phen')3,4 - both at c = I f.tM .
`330 nm and a strong blue fluorescence with Amax = 408 nm ( r ~
`600 ps). These features are very similar to those of other PPP(cid:173)
`type polymers indicating that the nominal photophysics of P-NEt3 +
`is not strongly influenced by the presence of the quaternary
`ammonium side-groups. 11 - 14 The fluorescence of P-NEt3+ in
`aqueous solution is quenched by a variety of anions at a very
`low concentration. Examples of this effect are illustrated in the
`Stem-Volmer (SV) plots shown in Figure 2a and 2b, which
`illustrate quenching ofP-NEt3+ by Ru(phen'h4- and Fe(CNV- .
`The SV plots exhibit upward curvature and the quenching
`efficiencies depend strongly on the polymer concentration.15 Both
`of these features indicate that the quencher anions preassociate
`with P-NEt3 + (i.e., quenching is static). 16- 18 Ru(phen'h4- and
`Fe(CN)64- quench the fluorescence of the monomer dication
`terphenyl model (M-NEt3 +), also by a static quenching mecha(cid:173)
`nism. However, the quenching of the model is much less
`efficient-a large stoichiometric excess of the quencher is needed
`to significantly quench the fluorescence of N-NEt3 +.19 The fact
`that the anions quench M-NEt3 + less efficiently than P-NEt3 +
`indicates that the polymer chain amplifies the quenching.
`
`(4) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864-11873.
`(5) Wang, B.; Wasielewski, M. R. J. Am. Chem. Soc. 1997, 119, 12-21.
`(6) Jiang, B. ; Sahay, S.; Jones, W. E., Jr. MRS Symp. Proc. 1998 488
`671-676.
`.
`.
`(7) Kimura, M.; Horai, T.; Hanabusa, K.; Shirai, H. Adv. Mater. 1998 10
`459-462.
`.
`.
`(8) Chen, L.; McBranch, D. W.; Wang, H.-L.; Helgeson, R.; Wudl, F.;
`Whitten, D. G. Proc. Nat. Acad. Sci. U.S.A. 1999, 96, 12287- 12292.
`(9) Levitsky, I. A.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 1999 121
`1466-1472.
`'
`'
`(10) Balanda, P. B.; Ramey, M. B.; Reynolds, J. R. Macromolecules 1999
`32, 3970-3978.
`.
`(11) Grem, G.; Leditzky, G.; Ullrich, B.; Leising, G. Adv. Mater. 1992 4
`36-37.
`. '
`(12) Rice, M. J.; Gartstein, Y. N. Phys. Rev. Lett .. 1994, 73, 2504-2507.
`(13) Remmers, M.; Schulze, M.; Wegner, G. Macromol. Rapid Commun.
`1996, 17, 239-252.
`(14) Kim, S.; Jackiw, J.; Robinson, E.; Schanze, K. S.; Reynolds, J. R.;
`Baur, J.; Rubner, M. F.; Boils, D. Macromolecules 1998, 31 , 964-974.
`(15) SV quenching efficiencies obtained by "best fit" linear least-squares
`are as follows: [P-NEt3+] = 1.0 ,uM, Ksv(Ru(phen'h4- ) = 1.4 x JOS M- 1
`and Ksv(Fe(CN)64-) = 9.3 x 107 M- 1; [P-NEt3 +] = 10 ,uM, Ksv(Ru(phen'h4-)
`= 8.0 x lOS M-1 and Ksv(Fe(CN)64-) = 5.4 x lOS M- 1.
`(16) Keizer, J. J. Am. Chem. Soc. 1983, 88, 1494-1498.
`(17) Delaire, J. A.; Rodgers, M.A. J.; Webber, S. E. J. Phys. Chem. 1984
`88, 6219-6227.
`.
`(18) Static quenching is confirmed by the fact that the fluorescence lifetime
`of P-NEt3+ does not vary with the quencher concentration.
`
`10.1021/ja000819c CCC: $19.00 © 2000 American Chemical Society
`Published on Web 08/16/2000
`
`3
`
`

`

`8562 J. Am. Chern. Soc., Vol. 122, No. 35, 2000
`
`Communications to the Editor
`
`not observed, indicating that quenching is instantaneous on the
`time scale accessible with our instrumentation (200 -ps). We
`conclude that exciton diffusion and quenching occurs with a rate
`in excess of 1010 s- 1.
`Emission excitation spectroscopy indicates that quenching
`involves energy transfer from the P-NEh+ 1n,n* exciton to the
`triplet metal-to-ligand charge transfer CMLCT) state of Ru(cid:173)
`(phen')34- . Thus, Figure 1b compares excitation spectra for MLCT
`emission at 610 nm from a solution containing l,uM Ru(phen')34-
`only, and for a solution ofRu(phen'h4- and P-NEt3+ where both
`the quencher and repeat unit concentration = 1 ,uM. The
`significant aspect is that the excitation spectrum of the mixture
`displays considerably enhanced excitation efficiency in the UV
`region where P-NEt3+ absorbs (..1. = 300-375 nm). This feature
`establishes that light absorbed by P-NEt3+ leads to emission from
`the MLCT state ofRu(phen')34- . In essence the Ru(phen'h4- that
`is ion-paired with the P-NEt3 + chain acts as a low-energy
`photoluminescent trap for the highly mobile 1n,n* exciton. While
`the exciton is expected to be very mobile, 1•9.22·23 long-range energy
`transfer may be facilitated by dipole-dipole (Forster) coupling
`between the P-NEt3+ donor and the Ru(phen'h4- acceptor. Indeed,
`a computation based on the spectra and photophysical properties
`of the two chromophores indicates that the Forster transfer
`distance (R0 ) is :::::: 40 A.24 Quenching by Fe(CN)6
`4- may also
`occur by energy transfer, however, since this complex does not
`photoluminesce it is not possible to confirm that the excited-state
`complex is produced by quenching.
`Although we expected that increasing the P-NEt3 + concentra(cid:173)
`tion would attenuate the efficiency by which the anions quench
`the polymer, the effect is larger than anticipated. Specifically,
`Ru(phen'h4- and Fe(CN)64- quench the polymer approximately
`100-fold less efficiently when [P-NEt3+] = lO ,uM compared to
`that for [P-NEh+] = 1 ,uM (compare Figure 2a and 2b). The
`decreased quenching efficiency may arise from aggregation of
`the polycation at higher concentration.
`To demonstrate amplified quenching in a solid-state sensor,
`quenching studies were carried out with P-NEt3 + thin films. 25·26
`Hydrophilic glass slides that had been immersed into an aqueous
`solution containing P-NEt3+ (3 mM in repeat units) for 15 min
`followed by a rinse in distilled water were examined by absorption
`and photoluminescence spectroscopy. This analysis indicated the
`presence of a thin adsorbed P-NEt3 +film (absorption, Amax = 339
`nm, Amax = 0.011 ; fluorescence, Amax = 410 nm).27 Photolumi(cid:173)
`nescence from the film is quenched strongly when exposed to
`dilute solutions ofRu(phen')34- or Fe(CN)64-. Figure 2c illustrates
`a plot of fluorescence intensity vs amount of quencher added to
`an aqueous solution that was in contact with an adsorbed film of
`P-NEt3 +. Detectable quenching is observed upon addition of less
`than 20 nM of either quencher.
`
`Acknowledgment. This work was supported by grants from the
`National Science Foundation (K.S.S. and BSH, CHE-9901861) and
`AFOSR (J.R.R. and MBR, F49620-97-l-0232 and F49620-00-l-0047).
`We also acknowledge enlightening discussions with Professor David G.
`Whitten.
`
`Supporting Infonnation Available: Description of method used to
`determine association constant between P-NEt+ and Ru(phen')34- (PDF).
`This material is available free of charge via the Internet at http://pubs.acs.org.
`
`JA000819C
`
`(22) Rothberg, L. J.; Yan, M.; Papadimitrakopoulos, F.; Galvin, M. E.;
`Kwock, E. W.; Miller, T. M. Synth. Met. 1996, 80, 41-58.
`(23) Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995, 99,
`4886-4893.
`(24) Tum), N. J. Modern Molecular Photochemistry; Benjamin/Cum(cid:173)
`mings: Menlo Park, CA, 1978.
`(25) Baur, J. W.; Rubner, M. F.; Reynolds, J. R.; Kim, S. Langmuir 1999,
`15, 6460-6469.
`(26) Liu, J. F.; Ducker, W. A. Langmuir 2000, 16, 3467-3473.
`(27) On the basis of the absorption, we estimate that the filin coverage is
`approximately 2.4 x 10- 10 mol-cm-2 (repeat unit), which is approximately
`equal to the coverage expected for a single monolayer of the polymer.
`
`06
`
`12
`
`18
`
`24
`
`30
`
`0 .02
`
`60
`
`0.04
`
`12
`
`0.06
`
`18
`
`0 .08
`
`24
`
`t
`
`•
`
`•
`
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`
`0 .1 0
`
`30
`
`b
`
`10
`
`c
`
`0 .02
`
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`
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`
`0 .08
`
`0.10
`
`14
`
`12
`
`10
`
`::::
`
`- 0
`
`0
`0 .00
`
`0
`
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`
`12
`
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`
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`
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`
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`c:
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`c:
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`-= 0.6
`
`"-
`
`0 .5
`0.00
`
`[Quencher] I ~-tM
`Figure 2. Stern-Volrner quenching plots for P-NEt3+ in H20, (A)
`Ru(phen'h4- and(+) Fe(CN)64- . (a) [P-NEt3+] = 1 ,uM; (b) [P-NEt3+]
`= 10 ,uM. Italic numbers at the top of (a) and (b) indicate the ratio
`[quencher]/[polymer chain], where [polymer] = [repeat unit]IX •. (c)
`Relative fluorescence intensity of adsorbed film of P-NEt3 + as a function
`of added quencher to an aqueous solution that is in contact with film,
`(A) Ru(phen'h4- and (+) Fe(CN)64-.
`
`There are several noteworthy features with respect to the
`P-NEt3 + quenching data. First, given the short fluorescence
`lifetime of the polymer, the extraordinary quenching efficiencies
`clearly indicate that quenching involves an ion-pair between the
`small molecule tetra-anions and P-NEt3+.zo Ru(phen')J4- quenches
`more efficiently than Fe(CN)6,4-
`suggesting that the larger
`"amphiphilic" Ru-complex anion associates more strongly with
`P-NEt3+. Second, in the quenching studies carried out with
`[P-NEt3 +] = 1 ,uM (repeat unit concentration), greater than 90%
`quenching is observed at [quencher] :::::: 0.08 ,uM, which corre(cid:173)
`sponds to approximately 12 repeat units per quencher. As such,
`the quenchers are present at 2:1 [quencher]:[polymer chain] ratio
`(Figure 2a), and on average binding of one or two quenchers per
`P-NEt3+ chain effectively "turns off' the fluorescence of the entire
`polymer.21 This implies that in P-NEt3+, the 1n ,n* exciton diffuses
`along the polymer chain to the quencher on a time scale that is
`rapid compared with its lifetime (600 ps).9 Time-resolved
`fluorescence experiments were carried out to determine if the
`fluorescence decay of P-NEt3 + in the presence of Ru(phen')34-
`exhibits a "fast" decay component (r « 600 ps) arising from the
`polymer-quencher ion pair. However, a fast decay component is
`
`(19) SV quenching efficiencies obtained by "best fit" linear least-squares
`are as follows: [M-NEt3+] = 10 ,uM, Ksv(Ru(phen'h4-) = 24000 M- 1 and
`4-) = 8900 M- 1•
`Ksv(Fe(CN)6
`(20) The Ksv values obtained at [P-NEt3 +] = 1 ,uM imply second-order
`quenching rate constants that are in excess of the diffusion-controlled rate by
`a factor of 108.
`(21) The association constant for Ru(phen')34- bindin~ to P-NEt3+ ~as
`been deterrruned mdependently (Kb = 4.6 x
`lOS M-, see Supporting
`Information). Based on this Kb we estimate that for a solution containing
`[P-NEt3+] = 1 ,uM and [Ru(phen'h4- ] = 80 nM ( > 90% fluorescence
`quenching, see Figure Ia) the concentration of polymer-bound Ru(phen'h4-
`is "" 25 nM. Remarkal;tly,\ the concentration of polymer-bound Ru(phen'h4 -
`corresponds closely to the polymer chain concentration, i.e., ([repeat unit]/
`X.) = (1 ,uM/28) = 36 nM, which indicates that > 90% quenching occurs
`when approximately one quencher is bound per chain.
`
`4
`
`