A R T I C L E S
Haskins-Glusac et al.
complexed. In effect, the quenching sphere of action of the
viologens is spread out over a large number (>20) of polymer
repeat units due to the exciton’s high degree of delocalization
and its high intrachain hopping rate.11,58
emission intensity and lifetime SV quenching constants for Pt-m
are in the same range as the association constants obtained for
quenching of O-m by the viologens (Ka ≈ KsvI - Ksv ≈ 104-
τ
105 M-1, see eq 1).
A second notable feature is that, although the SV plots for
quenching of O-p by MV2+ and MV4+ are nearly linear at very
low quencher concentration (<500 nM), they are curved upward
at higher quencher concentration. The onset of the curvature
occurs at lower concentration for MV4+ than for MV2+, and as
a result the tetravalent quencher is a considerably more effective
quencher. This effect has been observed in the quenching of
other conjugated polymers by polyvalent quencher ions.9,11 We
believe that it arises due to the ability of the polyvalent quencher
ions to induce aggregation of the polymer chains.11,59,60 Ag-
gregation of the polymer chains allows interchain diffusion of
the exciton, which effectively increases the sphere of action
within which the viologen is able to efficiently quench a
fluorescent exciton (i.e., because of polymer aggregation, a
quencher is able to quench excitons produced on more than one
polymer chain).13
Now, we turn to consider the quenching properties of the
Pt-acetylide-based materials. There are several significant dif-
ferences with respect to quenching of Pt-m and Pt-p as compared
to quenching of the organic materials. First, all of the viologens
quench Pt-m and Pt-p with close to the same efficiency. This
result clearly indicates that there is little amplification of the
quenching response for the polymer relative to that of the
monomer model. The second noteworthy feature is that the Ksv
values obtained for the Pt-p/MV+ and Pt-p/MV2+ systems by
intensity quenching (Io/I vs [Q]) are only slightly larger as
compared to those obtained by lifetime quenching (τo/τ vs [Q],
see Table 1). A similar trend is seen for Pt-m quenching by all
of the viologen quenchers. The close correspondence of the Ksv
values derived from intensity and lifetime quenching indicates
that dynamic quenching is the dominant mechanism for quench-
ing in the Pt-acetylide materials; that is, the mechanism involves
diffusion of the quencher to the excited-state complex (or exciton
in the case of Pt-p). Interestingly, by using the Ksv values
obtained by dynamic quenching and the emission lifetimes of
Pt-m and Pt-p, we compute that the quenching rate constants
(kq) are close to those expected for diffusion-controlled colli-
sional quenching between oppositely charged ions.61,62 (For Pt-
m, kq ≈ (1-2) × 1010 M-1 s-1; Pt-p, kq ≈ (3-4) × 1010 M-1
s-1. Diffusional quenching of Pt-p is expected to be more rapid
because of the higher charge on the polymer chain.)
At very low quencher concentration (<1 µM), the SV plot
for the Pt-p/MV4+ system is linear, and the emission intensity
and lifetime are quenched equally. However, at higher [MV4+],
the intensity quenching plot is curved upward, while the lifetime
quenching remains linear. This behavior signals that static
quenching becomes significant in the Pt-p/MV4+ system when
the quencher is present at relatively higher concentrations.
Nevertheless, despite the fact that static quenching of Pt-p occurs
with MV4+, the metal-organic polymer is still quenched
considerably less efficiently than O-p. For example, at [MV4+
]
) 1 µM, approximately 90% of the fluorescence from O-p is
quenched, whereas only 50% of Pt-p phosphorescence is
quenched. The significant static quenching observed in the
Pt-p/MV4+ system is believed to arise due to MV4+-induced
aggregation of the polymer. Thus, from this standpoint, the
Pt-p/MV4+ system is different from the Pt-p/MV+ and Pt-p/
MV2+ systems. The fact that quenching is more efficient with
the tetravalent quencher results from the fact that the triplet
excitons are quenched due to interchain aggregation induced
by the tetravalent quencher ion. Importantly, the strong quench-
ing of Pt-p by MV4+ is not due to amplified quenching by the
quencher ion.
Model To Explain Triplet Quenching in Pt-p. The Triplet
Exciton Is Localized and Diffuses Slowly. For MV+ and MV2+
quenching of Pt-p and Pt-m, there is some evidence for static
quenching, as indicated by the fact that intensity quenching is
slightly more efficient than lifetime quenching. Nevertheless,
static quenching does not play a very large role in the overall
quenching in the Pt-p/MV+ and /MV2+ systems, because, due
to the long lifetime of the triplet excited state, dynamic
quenching is quite efficient. More important is the fact that static
quenching of Pt-p by MV+ and MV2+ is not much more efficient
than static quenching of Pt-m. The lack of a significant increase
in the static quenching component for Pt-p clearly indicates that
the quencher interacts with the polymer in much the same was
as it does with the monomer. Specifically, the data suggest that
only excitons that are produced within the immediate vicinity
(i.e., a few repeat units) of the quencher binding site are
efficiently quenched. In addition, diffusion of excitons along
the polymer chain to a quencher binding site is not competitive
with normal diffusional quenching pathways (e.g., diffusion of
the quencher to the exciton). This conclusion is based on the
fact that the rate of diffusional quenching in the Pt-p/MV+ and
/MV2+ systems is close to the diffusion-controlled rate constant.
If intrachain diffusion were rapid as compared to the triplet
exciton decay rate, one would expect dynamic quenching to be
considerably more efficient in Pt-p as compared to Pt-m.
Importantly, the fact that diffusional quenching of Pt-m and
Pt-p is dominant does not mean that there is not ion-pair
complex formation between the cationic viologen quenchers and
the anionic Pt-acetylides. The reason that static quenching is
unimportant (even though ion-pairing occurs) is that, because
of the long lifetime of the Pt-acetylides, dynamic quenching is
so efficient that it masks the static quenching component (i.e.,
Taken together, the Pt-p quenching data are consistent with
a model in which (1) the triplet exciton is spatially confined;
(2) intrachain triplet exciton diffusion is relatively slow (vide
infra); and (3) interchain exciton diffusion is precluded by the
fact that the triplet is quenched by interchain aggregates. These
hypotheses are consistent with a growing body of evidence
indicating that the structure and properties of the triplet exciton
differ significantly from those of the singlet in conjugated
polymers.20,23,24 Thus, because the triplet is confined to a few
τ
eq 1, Ksv > Ka). In support of this notion is the fact that the
association constants calculated from the difference in the
(58) Kukula, H.; Veit, S.; Godt, A. Eur. J. Org. Chem. 1999, 277-286.
(59) Grønbech-Jensen, N.; Mashl, R. J.; Bruinsma, R. F.; Gelbart, W. M. Phys.
ReV. Lett. 1997, 78, 2477-2480.
(60) Khan, M. O.; Mel’nikov, S. M.; Jonsson, B. Macromolecules 1999, 32,
8836-8840.
(61) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.;
Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815-4824.
(62) The quenching rates are computed by the expression, kq ) Ksv/τo.
9
14970 J. AM. CHEM. SOC. VOL. 126, NO. 45, 2004