Multiple ligand-based emissions from a platinum(II) terpyridine complex attached to pyrene

Article abstract of DOI:10.1021/ic000304m

Attaching an electron-rich aryl substituent at the 4' Position of the trpy ligand in Pt(trpy)Cl⁺ endows the low-lying excited state with intraligand charge-transfer (ILCT) character, enhances the emission intensity, extends the excited-state lifetime, and inhibits exciplex quenching by Lewis bases. When pyrene is the substituent, the complex exhibits singlet- and triplet-state emission and the lifetime extends to 64 μs in room-temperature dichloromethane solution.

Full text of DOI:10.1021/ic000304m

2708
Inorg. Chem. 2000, 39, 2708-2709
Multiple Ligand-Based Emissions from a Platinum(II) Terpyridine Complex Attached to Pyrene
Joseph F. Michalec, Stephanie A. Bejune, and David R. McMillin*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393
ReceiVed March 20, 2000
The Pt(trpy)Cl+ (trpy ) 2,2′:6′,2′′-terpyridine) complex has
long been of interest in regard to binding studies with biological
macromolecules.1-3 Despite the existence of a low-lying metal-
to-ligand charge-transfer (MLCT) absorption, the complex is
virtually nonluminescent in fluid solution,4 one possible explana-
tion being facile deactivation via a thermally accessible d-d
excited state. However, recent studies have shown that complexes
of the type Pt(4′-X-T)Cl+, where 4′-X-T denotes a 4′-substituted
trpy derivative and X is an electron-donating substituent like the
dimethylamino group (NMe2), can have a triplet emission with a
lifetime on the order of a microsecond in room-temperature
dichloromethane (DCM).5 In such systems the emitting state takes
on intraligand charge-transfer (ILCT) character and the emission
lifetime increases even as the excited state shifts to lower energy.
The present studies of a series of 4′-Ar-T derivatives, where Ar
denotes an aryl substituent, demonstrate how one can prepare
systems that have even longer excited-state lifetimes. With pyrene
Figure 1. Spectral data in dichloromethane at 293 K showing the
as a substituent, the ILCT character of the absorption becomes
absorbance of [Pt(4′-Ph-T)Cl]+ (A), [Pt(4′-pMeOPh-T)Cl]+ (B, thick
dominant, the lifetime extends to 64 µs, and emission from a short-
curve), and [Pt(4′-Pyre-T)Cl]+ (C) and the emission of the 4′-Pyre-T
complex in air (D) and under N2 (E). The excitation wavelength was
480 nm in each case.
lived singlet (1ILCT) state becomes quite prominent.
The electronic absorption spectra include a series of intraligand
π-π* transitions below 350 nm and CT absorptions at longer
wavelengths. In the compounds of interest Ar ranges from phenyl
(Ph) to p-methoxyphenyl (pMeOPh) to 1-pyrenyl (Pyre); see
Figure 1 and Table 1 for spectra data.6 Despite the fact that the
reduction potentials of the complexes are all very similar (Table
1), the CT absorption energies and intensities vary widely. The
aryl substituent can influence the dipole length of the CT
transition,7,8 and the Ar f trpy CT character of the transition
becomes increasingly important as the reduction potential of the
substituent couple (ArH+/0) drops from 2.30 V9 (benzene) to 1.76
V10 (anisole) to 1.16 V9 (pyrene).
The aryl substituents have even greater effects on the emission.
Thus, even though the nonemissive trpy complex has a higher
energy MLCT state, the Pt(4′-Ph-T)Cl+ complex exhibits a
structured emission in DCM solution with a lifetime of 85 ns.
3
Due in significant part to the ILCT character of their emitting
states, the Pt(4′-pMeOPh-T)Cl+ and Pt(4′-Pyre-T)Cl+ complexes
exhibit progressively lower energy emissions with even longer
lifetimes (Table 1). Thus, the longest wavelength emission comes
from the 4′-Pyre-T complex, which exhibits an emission maxi-
mum at 685 nm in DCM along with a shoulder at around 640
nm (Figure 1). The introduction of ILCT character also accounts
for a systematic moderation of exciplex quenching because the
excited state is less susceptible to adduct formation with Lewis
bases.11 The Pt(4′-Ph-T)Cl+ complex is at one extreme where the
quenching of the emission is complete in butyronitrile, while in
the case of the photoexcited 4′-Pyre-T complex, the basicity of
the solvent is of little consequence, and the excited-state lifetime
is 18 µs in butyronitrile.
* To whom correspondence should be addressed. E-mail:
mcmillin@purdue.edu. Fax: (765) 494-0239.
(1) Howe-Grant, M.; Lippard, S. J. Biochemistry 1979, 18, 5762-5769.
(2) Ratilla, E. M. A.; Brothers, H. M., II.; Kostic, N. M. J. Am. Chem. Soc.
1987, 109, 4592-4599.
(3) Peyratout, C. S.; Aldridge, T. K.; Crites, D. K.; McMillin, D. R. Inorg.
Chem. 1995, 34, 4484-4489.
(4) Aldridge, T. K.; Stacy, E. M.; McMilllin, D. R. Inorg. Chem. 1993, 33,
722-727.
(5) Crites, D. K.; Cunningham, C. T.; McMillin, D. R. Inorg. Chim. Acta
1998, 273, 346-353.
(6) Standard synthetic methods yielded the 4′-substituted terpyridines.
(Krohnke, F. Synthesis 1976, 1-24. Albano, G.; Balzani, V.; Constable,
E. C.; Maestri, M.; Smith, D. R. Inorg. Chim. Acta 1998, 277, 225-
231.) Preparation of the 4′-substituted platinum terpyridine complexes
also followed standard synthetic methods. (Bu¨chner, R.; Cunningham,
C. T.; Field, J. S.; Haines, R. J.; McMillin, D. R.; Summerton, G. C. J.
Chem. Soc., Dalton Trans. 1999, 711-717. Ref. 5.) The complexes all
gave good microanalyses as salts of the tetrakis[3,5-bis(trifluoromethyl)-
phenyl]borate anion (TFPB). The most difficult synthesis began with
the preparation of analytically pure 4′-Pyre-T via (1) aldol condensation
of 2-acetylpyridine with pyrene-1-carboxaldehyde, (2) KOtBu-catalyzed
Michael addition of 2-acetylpyridine in THF, (3) ring closure with [NH4]-
OAc as well as air oxidation in refluxing ethanol, and (4) recrystallization
from MeOH/CHCl3. Formation of [Pt(4′-Pyre-T)Cl]TFPB involved (1)
treatment of dibenzonitriledichloroplaninum(II) with 1 equiv of AgSbF6
in acetonitrile, (2) reaction with 4′-Pyre-T and precipitation as the crude
The quenching results with dioxygen are even more striking
for the 4′-Pyre-T complex. The key observation is that dioxygen
preferentially quenches the emission intensity at 685 nm while
the signal at 640 nm persists. Moreover, under aerobic conditions,
it is possible to measure two emission lifetimes in the nanosecond
regime. In accordance with the steady-state emission spectrum,
time-resolved emission measurements reveal that the major
-
SbF6 salt, (3) exposure to a small excess of [Bu4N]Cl in warm
acetonitrile, and (4) metathesis of the Cl- salt.
(9) Pysh, E. S.; Yang, N. C. J. Am. Chem. Soc. 1963, 85, 2124-2130.
(10) Vauthey, E.; Ho¨gemann, C.; Albros, X. J. Phys. Chem. A 1998, 102,
7362-7369.
(7) Phifer, C. C.; McMillin, D. R. Inorg. Chem. 1986, 25, 1329-1333.
(8) Damrauer, N. H.; Boussie, T. R.; Devenney, M.; McCusker, J. K. J.
Am. Chem. Soc. 1997, 119, 8253-8268.
(11) Crites, D. K.; McMillin, D. R. Coord. Chem. ReV., in press.
10.1021/ic000304m CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/07/2000

Communications
Inorganic Chemistry, Vol. 39, No. 13, 2000 2709
Table 1. Physical Data for [Pt(4′-X-T)Cl]+ Systems in Solution at 293 K
emission dataa (two solvents)
E° d
dichloromethane
max,c nm
τ, µs
butyronitrile
max, nm
absorbanceb
DMF
complex
[Pt(trpy)Cl]+
[Pt(4′-Ph-T)Cl]+
λ
max, nm
λ
103φ
λ
V vs Fc+/0
305, 320, 340, 388, 405
310, 323, 336, 390sh, 414 (7100)
-1.24 (70, 1.1)
-1.23 (80, 1.2)
535, 570sh, 608sh
[538, 572sh, 610sh]
0.085 2.1
[Pt(4′-pMeOPh-T)Cl]+ 303sh, 322, 337, 366, 403sh, 426 (16150) 560, 590sh, 650sh
5.2
64
46
34
555, 585sh
672
-1.26 (90, 1)
[562, 600sh, 670sh]
∼640sh, 685 [700]
[Pt(4′-Pyre-T)Cl]+
310sh, 328sh, 340, 387, 405, 476 (8910)
-1.22 (70, 1.2)
a Estimated errors in lifetime (τ) and quantum yield (φ) are (10%. Deoxygenated solution. b Molar absorptivity (M-1 cm-1) in parentheses.
Solvent DCM. c Maxima from corrected spectra in brackets. d Ferrocene reference. Ep,c-Ep,a peak-to-peak separation in mV and ic/ia current ratio
in parentheses.
Scheme 1
component has a 1 ns lifetime and an emission maximum at
around 640 nm. On the other hand, with a CCD detector, one
can still observe the signal with a maximum at 685 nm, though
the lifetime has dropped to 910 ns. Both signals originate from
the complex because the excitation spectrum matches the absorp-
tion spectrum of Pt(4′-Pyre-T)Cl+ with or without the presence
of dioxygen. Except for changes in intensity, the emission
spectrum does not depend on concentration (3-40 µM in DCM).
The radiative rate constant associated with the long-lived (685
nm) emission is about 530 s-1 and is more than an order of
magnitude smaller than that expected for typical 3MLCT emission
(for Pt(4′-Ph-T)Cl+ kr ) 2.6 × 104 s-1). In contrast, with an
emission yield on the order of 10-3 in an aerated solution, the
640 nm component exhibits a radiative rate constant of ca. 106
s-1. This value agrees with those of the 1ILCT emissions reported
by Pilato and co-workers,12 and a similar assignment applies here.
The temperature dependence of the luminescence confirms that
two distinct excited states contribute to the emission. Thus, in
the absense of dioxygen the intensity at the 685 nm maximum
increases but the intensity of the high-energy shoulder decreases
at progressively lower temperatures. These results indicate that
there is an equilibrium involving two excited states and that the
shoulder arises from the thermal population of the higher lying
of the two emitting states. Examination of the spectra in Figure
1 strongly suggests that the latter component stems from the 1ILCT
state.13 Since the singlet-triplet splitting is relatively small for
CT states, the 685 nm component logically corresponds to the
excitation, the emitting states undoubtedly have an admixture of
MLCT character as indicated in Scheme 1. Indeed, d-orbital
participation is necessary to explain the emission yield of the 685
nm component because the radiative process would be strongly
spin-forbidden in a pure intraligand triplet state.17
In summary, these results demonstrate the ease with which one
can alter the orbital parentage of the lowest energy excited state
of the platinum(II) terpyridine system. By a simple change of a
substituent, it is possible to adjust the energy and the intensity of
the absorbance and at the same time tune the energy, lifetime,
emission yield, and reactivity of the luminescent excited state
over a wide range.
Acknowledgment. The authors thank Hartmut Hedderich for
help with the CCD measurements and Grant Summerton and John
Field for valuable discussions. The National Science Foundation
funded the research through Grant CHE-9726435.
3
associated ILCT state; however, pyrene itself has a low-lying
3π-π state at about this energy.14 The simplest three-state model
that can explain the results appears in Scheme 1 along with
estimated rate constants and plausible, mixed-parentage state
assignments. This model assumes the participation of only one
triplet state, but a more complicated equilibrium that includes
close-lying 3ILCT and 3π-π(Pyre) states is also feasible, analo-
gous to results with ruthenium polypyridine complexes bearing
a covalently attached pyrene group.15,16 Although the emission
from the 4′-Pyre-T complex primarily represents intraligand
Supporting Information Available: Figures showing the time-
resolved emission spectrum of Pt(4′-Pyre-T)Cl+ at 293 K in deoxygenated
DCM and the temperature-dependent emission spectrum of Pt(4′-Pyre-
T)Cl+ in deoxygenated DCM. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC000304M
(12) Kaiwar, S. P.; Vodacek, A.; Blough, N. V.; Pilato, R. S. J. Am. Chem.
Soc. 1997, 119, 3311-3316.
(13) Note that the 640 nm emission component (Figure 1D, which also tracks
the shoulder on the signal from the deoxygenated sample) tails into the
1ILCT absorption band in accordance with Stokes shifted emission from
the same absorbing state.
(15) Simon, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.; Piotrowiak,
P.; Jin, X. Q.; Thummel, R. P. J. Am. Chem. Soc. 1997, 119, 11012-
11022.
(16) Harriman, A.; Hissler, M.; Khatyr, A.; Ziessel, R. Chem. Commun. 1999,
735-736.
(14) The highest energy vibronic component of the 77 K emission from the
free 4′-Pyre-T ligand appears at 608 nm.
(17) Colombo, M. G.; Hauser, A.; Gu¨del, H. U. Inorg. Chem. 1993, 32, 3088-
3092.