Room temperature phosphorescence from a platinum(II) diimine bis(pyrenylacetylide) complex

Article abstract of DOI:10.1021/ic0261631

Room temperature phosphorescence has been observed in a synthetically facile Pt(II) complex, Pt(dbbpy)(C≡C-pyrene)₂ (dbbpy = 4,4′-di(tert-butyl)-2,2′-bipyridine; C≡C-pyrene = 1-ethynylpyrene), in fluid solution. The static and time-resolved absorption and luminescence data are consistent with phosphorescence emerging from the appended C≡C-pyrenyl units following excitation into the low energy dπ Pt → π* dbbpy metal-to-ligand charge transfer absorption bands.

Full text of DOI:10.1021/ic0261631

Inorg. Chem. 2003, 42, 1394−1396
Room Temperature Phosphorescence from a Platinum(II) Diimine
Bis(pyrenylacetylide) Complex
Irina E. Pomestchenko,† Charles R. Luman,† Muriel Hissler,‡ Raymond Ziessel,‡ and
,†
Felix N. Castellano*
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43403, and Laboratoire de Chimie Mole´culaire, ECPM, 25 rue Becquerel,
67087 Strasbourg Cedex 02, France
Received November 7, 2002
Room temperature phosphorescence has been observed in a
synthetically facile Pt(II) complex, Pt(dbbpy)(CtC-pyrene)2 (dbbpy
) 4,4′-di(tert-butyl)-2,2′-bipyridine; CtC-pyrene ) 1-ethynyl-
pyrene), in fluid solution. The static and time-resolved absorption
and luminescence data are consistent with phosphorescence
emerging from the appended CtC-pyrenyl units following excitation
into the low energy dπ Pt f π* dbbpy metal-to-ligand charge
transfer absorption bands.
select organic chromophore(s) to the diimine ligand structure
such that the ligand-localized (intraligand, IL) excited states
are strategically positioned below the MLCT manifold. The
relative energy differences between these two states dictate
whether the MLCT emission is controlled by thermal
equilibrium or slow back energy transfer processes.3-6
Extended lifetimes have also been observed in systems where
the excited states have been proposed to consist of mixed
3π-π* and intraligand charge transfer (ILCT) character.7,8
Although rare, direct excitation of the MLCT excited state
can lead to IL-centered long-lived 3π-π* phosphorescence
in room temperature (RT) solutions.9
There has been considerable recent interest in transition
metal complexes possessing metal-to-ligand charge transfer
(MLCT) excited states that display lifetimes well beyond
those imposed by the energy gap law.1-9 Visible-absorbing
luminescence probes that possess long decay times are
becoming increasingly important in lifetime-based chemical
sensing, biophysics, and clinical chemistry.10 Substantial
lifetime lengthening can be accomplished by introducing a
* To whom correspondence should be addressed. E-mail: castell@
bgnet.bgsu.edu.
† Bowling Green State University.
‡ Laboratoire de Chimie Mole´culaire, ECPM.
(1) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1992, 96, 389-394.
(2) Wilson, G. J.; Launikonis, A.; Sasse, W. H. F.; Mau, A. W.-H. J.
Phys. Chem. A 1997, 101, 4860-4866.
Over the past few years, Pt(II) diimine bis(acetylide)
complexes have emerged as chromophores that feature visible
MLCT-based absorption with corresponding high quantum
yield photoluminescence.11-13 The detailed study recently
completed by Schanze and co-workers demonstrated that the
3IL states of the acetylide ligands can markedly influence
excited state decay.12 We therefore postulated that one could
potentially observe significantly extended lifetimes in these
synthetically facile Pt(II) systems if the appropriate chro-
(3) 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.
(4) (a) Tyson, D. S.; Castellano, F. N. J. Phys. Chem. A 1999, 103, 10955-
10960. (b) Tyson, D. S.; Bialecki, J.; Castellano, F. N. Chem. Commun.
2000, 2355-2356. (c) Tyson, D. S.; Luman, C. R.; Zhou, X.;
Castellano, F. N. Inorg. Chem. 2001, 40, 4063-4071. (d) Tyson, D.
S.; Henbest, K. B.; Bialecki, J.; Castellano, F. N. J. Phys. Chem. A
2001, 105, 8154-8161.
(5) (a) Harriman, A.; Hissler, M.; Khatyr, A.; Ziessel, R. Chem. Commun.
1999, 735-736. (b) Hissler, M.; Harriman, A.; Khatyr, A.; Ziessel,
R. Chem. Eur. J. 1999, 11, 3366-3381.
(6) McClenaghan, Barigelletti, F.; Maubert, B.; Campagna, S. Chem.
Commun. 2002, 602-603.
(10) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer
Academic/Plenum Publishers: New York, 1999.
(7) (a) Michalec, J. F.; Bejune, S. A.; McMillin, D. R. Inorg. Chem. 2000,
39, 2708-2709. (b) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.;
Summerton, G. C.; Gertenbach, J. A.; Field, J. S.; Haines, R. J.;
McMillin, D. R. Inorg. Chem. 2001, 40, 2193-2200.
(8) Del Guerzo, A.; Leroy, S.; Fages, F.; Schmehl, R. H. Inorg. Chem.
2002, 41, 359-366.
(11) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447-457.
(12) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg.
Chem. 2001, 40, 4053-4062.
(13) Yang, Q.-Z.; Wu, L. Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg.
Chem. 2002, 41, 5653-5655.
(9) Shaw, J. R.; Schmehl, R. H. J. Am. Chem. Soc. 1991, 113, 389-394.
1394 Inorganic Chemistry, Vol. 42, No. 5, 2003
10.1021/ic0261631 CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/08/2003

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they overlap the MLCT bands. The first reduction waves
measured for 1 and 2, obtained with cyclic voltammetry in
CH2Cl2, occur at the same potential within experimental error
(-1.35 V vs Ag/AgCl). Eisenberg and co-workers have
already assigned this process to a dbbpy ligand-based
reduction in 2.11 Reductive electrochemistry performed on
pyrene-CtCsH yielded an irreversible process that initiates
at -1.73 V. It is therefore reasonable to assume that the
lowest acceptor ligand orbital is located on dbbpy in 1.
Consistent with other related Pt(II) diimine bis(acetylide)
complexes, 1 and 2 displayed irreversible metal-based
oxidations at +0.95 V and +1.11,11-13 respectively. The fact
that the former is shifted to a more negative potential reflects
significant σ-donation of electron density from the CtC-
pyrenyl units to the Pt(II) center, consistent with the red shift
observed in the absorption spectrum.
Figure 1. Absorption (left) and emission (right) spectra of compound 1
(s) and 2 (- - -) in deaerated CH2Cl2 at room temperature. Emission spectra
were recorded using 480 ( 2 nm excitation.
mophores are appended as acetylide ligands. There is an
interesting consequence to using a square planar geometry
The RT emission spectrum of 2 was easily measured in
aerated CH2Cl2 whereas the corresponding spectrum for 1
is almost quantitatively quenched by dioxygen, necessitating
its removal, Figure 1. Excitation at 480 nm produces a
distinct emission spectrum in each case. The luminescence
in 2 has already been shown to originate from an MLCT
manifold,11,12 whereas the emission from 1 is red-shifted
(over 100 nm), structured, and relatively sharp in comparison.
The quantum yield for 1, measured relative to [Ru(bpy)3]2+,
was determined to be 0.011. The corrected excitation
spectrum of 1 in CH2Cl2 was superimposable with its
absorption spectrum at all wavelengths above 300 nm and
was completely invariant to the monitoring wavelength. The
RT luminescence lifetimes of 1 and 2, measured with a
broadband N2-pumped dye laser (460 ( 2 nm, 500 ps fwhm)
in deaerated CH2Cl2, were also strikingly different, 48.5 and
1.25 µs, respectively. Using 360 nm excitation, the lifetime
of the phosphorescence of 3 (Figure 2) in deaerated CH2Cl2
was 68 µs, similar to that measured for 1 following visible
MLCT excitation. The photophysical properties of 1 are
clearly not attributed to an excited state composed of MLCT
parentage. Since Pt(II) chromophores of this type are known
to undergo excited state self-quenching reactions,11,12,16-18
all luminescence experiments were performed at or below
10 µM, where the measured lifetimes were invariant to
concentration.
3
in that the lowest expected MLCT (dπ Pt f π* diimine)
3
and IL (acetylide π f π*) excited states are located on
opposite ends of the molecule.
In the present work, we report a new Pt(II) diimine
complex, Pt(dbbpy)(CtC-pyrene)2 (1), where dbbpy is 4,4′-
di(tert-butyl)-2,2′-bipyridine and CtC-pyrene is 1-ethy-
nylpyrene. Excitation into the low energy MLCT bands of
1 produces long-lived structured phosphorescence at RT in
fluid solution, emanating from the appended CtC-pyrenyl
units.
Compound 1 was prepared by reaction of Pt(dbbpy)Cl2
and pyrene-CtCsH in CH2Cl2/diisopropylamine in the
presence of a CuI catalyst.11,12 Crude 1 was purified by flash
chromatography on silica and structurally characterized by
1H NMR, MS, and elemental analysis. Pt(dbbpy)(CtCsPh)2
(2)11 and trans-Pt(PBu3)2(CtC-pyrene)2 (3)5b serve as models
for 1. Compound 3 is vital to this study since it does not
possess any MLCT excited states but contains the Pt(II)
center (to impart the heavy atom effect) and two pyrenyl-
acetylide units in a trans-geometry (to ensure the absence
of intramolecular ground-state interactions). The absorption
spectra of compounds 1 and 2 in CH2Cl2 are displayed in
Figure 1. The pyrenylacetylide chromophores in 1 are largely
responsible for the structured π-π* transitions between 350
and 400 nm. These absorptions are substantially red-shifted
relative to those of the free pyrene-CtCsH species (see
Supporting Information), suggesting large σ-donation of the
CtC-pyrenyl electron density to the Pt(II) center. The visible
absorption bands near 450 nm are assigned to dπ Pt f π*
dbbpy MLCT transitions and, although red-shifted, are
qualitatively similar in shape and extinction coefficient
(10 300 M-1 cm-1 at 450 nm) to 2.11 The MLCT assignment
is consistent with the solvatochromic shifts observed for the
low energy absorption bands in 1 (and 2).11,14,15 It should be
noted that the sharp absorption features between 350 and
400 nm also exhibit solvatochromism, suggesting either that
these bands possess some charge transfer character or that
The emission spectra of 1 and 3 obtained at RT and 77 K
in 4:1 EtOH/MeOH are compared in Figure 2. The difference
in the RT and 77 K emission energies, or thermally induced
Stokes shift (∆Es), is extremely small for both compounds,
∼ 70 cm-1. Under the same conditions, the luminescence
from 2 displayed a substantial ∆Es value of 2800 cm-1 (0.35
eV). Large values of ∆Es are correlated with significant
changes in dipole moment between ground and excited states,
such as those attributed to MLCT excited states.12,15 The
small value of ∆Es in the case of 1 indicates that the emission
does not originate from an MLCT excited state. The emission
(16) Connick, W. B.; Geiger, D.; Eisenberg, R. Inorg. Chem. 1999, 38,
3264-3265.
(17) Kunkely, H.; Vogler, A. J. Am. Chem. Soc. 1990, 112, 5625-5626.
(18) Wan, K.-T.; Che, C.-M.; Cho, K.-C. J. Chem. Soc., Dalton Trans.
1991, 1077-1080.
(14) Kober, E. M.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23,
2098-2104.
(15) Chen, P.; Meyer, T. J. Chem. ReV. 1998, 98, 1439-1477.
Inorganic Chemistry, Vol. 42, No. 5, 2003 1395

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Figure 3. Transient absorption difference spectrum of 1 in deaerated CH2-
Cl2 measured 500 ns after a 355 nm laser pulse.
in their relative CtC-pyrenyl absorption band positions (see
Supporting Information). On the basis of these data, the
absorptions are assigned to ground-state bleaching (λ < 410
nm) and to the triplet-to-triplet absorption (410-700 nm)
of the -CtC-pyrenyl moieties. The bleach recovery and
the absorption decays followed similar single exponential
kinetics that exhibit concentration-dependent lifetimes on the
order of tens of microseconds. The absorption kinetics
paralleled the time-resolved emission decays in all cases,
illustrating that the absorption transients are likely of the
same origin as the phosphorescence. Future planned ultrafast
transient absorption measurements will undoubtedly shed
light on energy migration processes within this chromophore.
The acetylide linkages in 1 may also permit time-resolved
IR spectroscopic investigations.12
Figure 2. (a) Emission spectrum (λex ) 480 ( 2 nm) measured for 1 in
deaerated 4:1 EtOH/MeOH at room temperature (s) and at 77 K (- - -).
(b) Emission spectrum (λex ) 400 ( 2 nm) measured for 3 in deaerated
4:1 EtOH/MeOH at room temperature (s) and at 77 K (- - -).
spectra obtained for 1 and 3 at both temperatures are nearly
identical in shape and vibronic spacing, illustrating that the
emitting state in 1 is 3IL phosphorescence from the appended
CtC-pyrenyl units. Surprisingly, the spectra are identical
even though the pyrenyl fragments are in different molecular
geometries around the Pt(II) center (cis in 1 and trans in 3).
The emission results strongly suggest that the two pyrenyl
units do not interact (intramolecularly) at all in 1.
Excitation into the low energy MLCT absorption bands
in 1 produces sensitized 3IL phosphorescence in the appended
3
CtC-pyrenyl moieties at RT. The relatively strong IL
phosphorescence is a consequence to the presence of the
internal Pt heavy atom. Compound 1 represents an alternative
molecular design that imparts long lifetime red photolumi-
nescence into a synthetically facile metal-organic system.
Such molecules have important potential applications in
emerging luminescence-based technologies and excited state
chemistry schemes and serve to further enhance our knowl-
edge regarding manipulation of molecular excited states.
Additional support for the sensitization of the CtC-
3
pyrenyl-based IL excited states in 1 comes from transient
absorption measurements. Figure 3 displays the absorption
transients obtained for 1 in CH2Cl2 following a 355 nm
nanosecond laser flash. Since the concentrations required for
flash photolysis resulted in self-quenching, no detailed
quantitative kinetic analyses of these data were performed.
The absorption features observed for 2 are consistent with
an MLCT excited state (see Supporting Information),12
whereas the transients in 1 are completely different. The
spectrum observed for 1 appeared promptly within the 15
ns instrument response so no information could be obtained
on the evolution of the transients. Flash photolysis performed
on 3 generated a similar transient spectrum throughout the
visible; however, the ground state bleaching of the pyrenyl
units occurs at lower energy in 3, consistent with differences
Acknowledgment. F.N.C. acknowledges support from the
NSF (CAREER Award CHE-0134782) and the ACS (ACS-
PRF 36156-G6). C.R.L. is supported by a McMaster Fel-
lowship from BGSU.
Supporting Information Available: Synthesis, characterization
details, and additional spectroscopic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC0261631
1396 Inorganic Chemistry, Vol. 42, No. 5, 2003