Probing the excited state properties of the highly phosphorescent Pt(dpyb)Cl compound by high-resolution optical spectroscopy

Article abstract of DOI:10.1021/ic901523u

Detailed photophysical studies of the emitting triplet state of the highly phosphorescent compound Pt(dpyb)CI based on high-resolution optical spectroscopy at cryogenic temperatures are presented {dpyb = N∧C²∧N- coordinated 1,3di(pyridylbenzene

Full text of DOI:10.1021/ic901523u

Inorg. Chem. 2009, 48, 11407–11414 11407
DOI: 10.1021/ic901523u
Probing the Excited State Properties of the Highly Phosphorescent Pt(dpyb)Cl
Compound by High-Resolution Optical Spectroscopy
†
‡
,‡
,†
Andreas F. Rausch, Lisa Murphy, J. A. Gareth Williams,* and Hartmut Yersin*
†
Universit a€ t Regensburg, Institut f u€ r Physikalische und Theoretische Chemie, 93053 Regensburg, Germany, and
Department of Chemistry, University of Durham, Durham, DH1 3LE, U.K.
‡
Received July 31, 2009
Detailed photophysical studies of the emitting triplet state of the highly phosphorescent compound Pt(dpyb)Cl based on
high-resolution optical spectroscopy at cryogenic temperatures are presented {dpyb = N C N-coordinated 1,3-
∧
2∧
-
1
di(pyridylbenzene)}. The results reveal a total zero-field splitting of the emitting triplet state T of 10 cm and relatively
1
short individual decay times for the two higher lying T substates II and III, while the decay time of the lowest substate I is
1
distinctly longer. Further evidence for the assignment of the T substates is gained by emission measurements under high
1
magnetic fields. Distinct differences are observed in the vibrational satellite structures of the emissions from the substates I
and II, which are dominated by Herzberg-Teller and Franck-Condon activity, respectively. At T=1.2 K, the individual
spectra of these two substates can be separated by time-resolved spectroscopy. For the most prominent Franck-Condon
active modes, Huang-Rhys parameters of S ≈ 0.1 can be determined, which are characteristic of very small geometry
rearrangements between the singlet ground state and the triplet state T . The similar geometries are ascribed to the high
1
∧
∧
rigidity of the Pt(N C N) system which, unlike complexes incorporating bidentate phenylpyridine-type ligands and
exhibiting similar metal-to-ligand charge transfer admixtures, cannot readily distort from planarity. The results provide new
insight into strategies for optimizing the performance of platinum-based emitters for applications such as organic light-
emitting diode (OLED) technology and imaging.
1
. Introduction
Phosphorescent transition metal complexes that emit effi-
spin-forbidden with a very low probability (radiative rate
-1
constants k of only around 1 s ), limiting the maximum
r
achievable internal efficiencies to 25%. In contrast, in third-
ciently from triplet excited states are the subject of a great
deal of attention. Much of this interest over the past decade
has been driven by research into the new generation of
display screen technology, organic light-emitting diodes
row transition metal complexes, strong spin-orbit coupling
(SOC) induced by the metal ion can promote the radiative
decay of the triplet state; for example, k may be increased to
r
6
-1
the order of 10 s . Intersystem crossing from higher-lying
singlet states is also promoted, so that the singlet states are
utilized too, and thus, all excitons are potentially converted
into light. On the other hand, the resulting phosphorescence
lifetimes of the triplet excited states of such complexes, of the
order of microseconds, are sufficiently long compared to
fluorescence lifetimes of organic molecules (typically 1-
(
OLEDs), which can benefit from the incorporation of such
materials to induce emission from otherwise wasted triplet
states.
electroluminescent device leads to triplet and singlet excitons
in a ratio as high as 3:1.
1-5
The recombination of charge carriers within an
6-8
In a device comprised of a purely
organic emitting material, emission from the triplet states is
10 ns) to allow them to be employed in sensing and imaging
*
To whom correspondence should be addressed. E-mail: hartmut.yersin@
chemie.uni-regensburg.de (H.Y.); j.a.g.williams@durham.ac.uk (J.A.G.
W.).
applications that make use of time-resolved detection
9-11
procedures.
These techniques offer not only a means of
(
1) Yersin, H., Ed. Highly Efficient OLEDs with Phosphorescent Materi-
als; Wiley-VCH: Weinheim, 2008.
2) Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. Rev. 2006,
50, 2093.
discriminating from short-lived background emission to
improve sensitivity, but also the possibility of using lifetimes
(
2
(
(
3) Polikarpov, E.; Thompson, M. E. Mater. Matters 2007, 2(3), 21.
4) Williams, J. A. G.; Develay, S.; Rochester, D. L.; Murphy, L. Coord.
(9) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;
Chem. Rev. 2008, 252, 2596.
(
(
5) Chou, P. T.; Chi, Y. Chem.;Eur. J. 2007, 13, 380.
Springer: New York, 2006; Chapters 20 and 22.
6) For an introduction to the principles of OLEDs and their operation,
(10) Botchway, S. W.; Charnley, M.; Haycock, J. W.; Parker, A. W.;
Rochester, D. L.; Weinstein, J. A.; Williams, J. A. G. Proc. Natl. Acad. Sci.
U.S.A. 2008, 105, 16071.
see, for example: Hung, L. S.; Chen, C. H. Mater. Sci. Eng. R 2002, 39, 143.
7) Baldo, M. A.; O’Brien, D. F.; Thompson, M. E.; Forrest, S. R. Phys
Rev. B 1999, 60, 14422.
8) Yersin, H. Top. Curr. Chem. 2004, 241, 1.
(
(11) Mak, C. S. K.; Pentlehner, D.; Stich, M.; Wolfbeis, O. S.; Chan,
W. K.; Yersin, H. Chem. Mater. 2009, 21, 2173.
(
r 2009 American Chemical Society
Published on Web 10/30/2009
pubs.acs.org/IC

1
1408 Inorganic Chemistry, Vol. 48, No. 23, 2009
Rausch et al.
weight, charge-neutrality, and lipophilic character facilitate
rapid diffusion-controlled passage through the cell mem-
brane, while the bright emission and lifetimes of the order
of a microsecond, even in the presence of oxygen, allow time-
gated images to be readily acquired. Other applications of
22
these compounds have included oxygen sensors; metal ion
sensors in which emission is modulated in response to ion
Figure 1. Structure of Pt(dpyb)Cl.
23
binding; derivatives with long alkyl chains in which the
as a reporter parameter, in contrast to intensity or wave-
length.
We have been exploring the chemistry of platinum(II)
complexes with terdentate cyclometalating ligands based on
24
emission is a function of the liquid crystallinity; and
mechanochromic complexes in which the color of solid-state
25
emission is influenced by grinding.
The high emission efficiency of these complexes must arise
from a combination of minimal nonradiative decay and
1
,3-di(2-pyridyl)benzene (dpybH), which offer the metal ion
∧
∧
12,13
an N C N coordination environment (Figure 1).
These compounds are among the brightest known Pt-
14,26,27
spin-orbit coupling favoring the triplet radiative decay.
14
Typically, SOC is efficient in complexes with emitting states of
based emitters in solution at room temperature; e.g. for
Pt(dpyb)Cl, ΦPL = 0.60 and τ = 7.2 μs in deoxygenated
dichloromethane. The complexes are thermally stable and
sublimable, rendering them suitable for vacuum deposition
into OLED structures. High device efficiencies have been
8,26,27
metal-to-ligand charge transfer (MLCT) character,
owing
to the greater participation of the metal than in ligand-centered
26-29
(LC) states.
However, the emission of Pt(dpyb)Cl and
most of its derivatives, at least at first sight, appears to display
features more typical of LC transitions (see below). Moreover, it
has been shown that SOC depends on a compound’s coordina-
tion geometry and is frequently more efficient in 6-coordinate
15,16
obtained in this way,
and the color can be tuned widely
according to ligand substituents. Moreover, the exci-
mers and/or aggregates formed by these complexes emit
intensely in the red-to-near-infrared (NIR) region of the
than in comparable 4-coordinate complexes {e.g. Ir(III) vs
26,27,29-31
12,13,17
spectrum.
Pt(II)}.
Clearly, the nature of the emitting states of
The use of a thin film of the pure compounds
these complexes merits a more detailed investigation. In this
contribution, we probe the emitting triplet state of Pt(dpyb)Cl
by means of high-resolution emission and excitation spectros-
copy at cryogenic temperatures and in the presence of external
as an emissive layer in an OLED structure leads to NIR-
emitting devices with external quantum efficiencies as high as
1
8
1
0.7% photons/electron. The balance of emission from the
monomeric and aggregate emission bands in a doped emis-
sive layer can be tuned, according to doping concentration, to
give white-light-emitting devices (e.g., at 15% doping by
mass, CIE = 0.40, 0.43 and efficiency = 15.5% photons/
magnetic fields. The zero-field splitting of the T state, the
1
individual deactivation rates of the three triplet substates, and
the vibrational satellite structures in emission are investigated to
provide further insight into the nature of the emitting state,
which governs the intense luminescence of this class of com-
plexes with diverse applications.
19,20
electron).
Such systems also offer promise as efficient
growth lights in horticulture, owing to the close match of the
bimodal-like emission profile with the absorption spectra of
21
green plants.
Very recently, the same class of complexes has also been
2
. Experimental Section
,3-Di(2-pyridyl)benzene (dpybH) was prepared by palla-
dium-catalyzed reaction of 2-(tributylstannyl)-pyridine with
,3-dibromobenzene. Its platinum(II) complex was formed
upon reaction of the ligand with K PtCl in acetic acid,
followed by extraction into dichloromethane, as described
10
1
used for time-resolved imaging in live cells. The features
that make the complexes suitable for use in OLEDs also
prove to be attractive for this purpose: their low molecular
1
2
4
12
previously. Emission spectra at 300 and 77 K were mea-
sured using a Jobin Yvon FluoroMax-2 spectrofluorimeter,
fitted with a red-sensitive Hamamatsu R928 photomultiplier
tube. Samples were degassed by a minimum of three free-
(
12) Williams, J. A. G.; Beeby, A.; Davies, E. S.; Weinstein, J. A.; Wilson,
C. Inorg. Chem. 2003, 42, 8609.
13) Farley, S. J.; Rochester, D. L.; Thompson, A. L.; Howard, J. A. K.;
Williams, J. A. G. Inorg. Chem. 2005, 44, 9690.
14) For a review of luminescent Pt(II) complexes: Williams, J. A. G. Top.
Curr. Chem. 2007, 281, 205.
15) Sotoyama, W.; Satoh, T.; Sawatari, N.; Inoue, H. Appl. Phys. Lett.
005, 86, 153505.
16) (a) Cocchi, M.; Virgili, D.; Fattori, V.; Rochester, D. L.; Williams, J.
(
(
-2
ze-pump-thaw cycles to base pressures of <5 ꢀ 10 mbar
at 77 K. For investigations at low temperature, Pt(dpyb)Cl
(
2
(
(
22) Evans, R. C.; Douglas, P.; Williams, J. A. G.; Rochester, D. L. J.
Fluorescence 2006, 16, 201.
23) Rochester, D. L.; Develay, S.; Z ꢀa li ꢁs , S.; Williams, J. A. G. Dalton
Trans. 2009, 1728.
24) Kozhevnikov, V. N.; Donnio, B.; Bruce, D. W. Angew. Chem., Int.
Ed. 2008, 47, 6286.
25) Abe, T.; Itakura, T.; Ikeda, N.; Shinozaki, K. Dalton Trans. 2009,
11.
26) Rausch, A. F.; Homeier, H. H. H.; Yersin, H. In Topics in Organo-
A. G. Adv. Funct. Mater. 2007, 17, 285. (b) Kalinowski, J.; Cocchi, M.; Virgili,
D.; Fattori, V.; Williams, J. A. G. Chem. Phys. Lett. 2006, 432, 110. (c) Virgili,
D.; Cocchi, M.; Fattori, V.; Sabatini, C.; Kalinowski, J.; Williams, J. A. G. Chem.
Phys. Lett. 2006, 433, 145.
(
(
(17) Develay, S.; Williams, J. A. G. Dalton Trans. 2008, 4562.
(18) (a) Cocchi, M.; Virgili, D.; Fattori, V.; Williams, J. A. G.;
(
Kalinowski, J. Appl. Phys. Lett. 2007, 90, 023506. (b) Cocchi, M.; Kalinowski,
J.; Virgili, D.; Williams, J. A. G. Appl. Phys. Lett. 2008, 92, 113302.
7
(
(
19) (a) Cocchi, M.; Kalinowski, J.; Virgili, D.; Fattori, V.; Develay, S.;
metallic Chemistry - Photophysics of Organometallics; Lees, A. J., Ed.;
Springer: Berlin/Heidelberg, 29, 2010
Williams, J. A. G. Appl. Phys. Lett. 2007, 90, 163508. (b) Kalinowski, J.;
Cocchi, M.; Virgili, D.; Fattori, V.; Williams, J. A. G. Adv. Mater. 2007, 19, 4000.
(
27) Yersin, H.; Finkenzeller, W. J. In Highly Efficient OLEDs with
Phosphorescent Materials; Yersin, H., Ed.; Wiley-VCH: Weinheim, 2008, 1.
28) Yersin, H.; Donges, D. Top. Curr. Chem. 2001, 214, 81.
(
20) Other examples of white light-emitting diodes using monomer and
∧
aggregate emission from N C-coordinated platinum complexes have been
described previously: (a) Adamovich, V.; Brooks, J.; Tamayo, A.;
Alexander, A. M.; Djurovich, P. I.; D’Andrade, B. W.; Adachi, C.; Forrest,
S. R.; Thompson, M. E. New J. Chem. 2002, 26, 1171. (b) Williams, E. L.;
Haavisto, K.; Li, J.; Jabbour, G. E. Adv. Mater. 2007, 19, 197.
(
(29) Rausch, A. F.; Homeier, H. H. H.; Djurovich, P. I.; Thompson, M.
E.; Yersin, H. Proc. SPIE 2007, 66550F.
(30) Wilson, M. H.; Ledwaba, L. P.; Field, J. S.; McMillin, D. R. Dalton
Trans. 2005, 2754.
(31) Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K. Inorg. Chem.
2003, 42, 6366.
(
21) Fattori, V.; Williams, J. A. G.; Murphy, L.; Cocchi, M.; Kalinowski,
J. Photon. Nanostruct. Fundam. Appl. 2008, 6, 225.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11409
a
Table 1. Photophysical Data of Pt(dpyb)Cl at 300 K in CH
2
Cl
2
Except Where Stated Otherwise
absorbance λmax/nm (ε Μ- cm-1
1
)
b
332 (6510), 380 (8690), 401 (7010) , 454 (270), 485 (240)
c
emission λmax/nm
491, 524, 562
0.60 (0.039)
7.2 (0.5)
7.0
d
ΦPL degassed (aerated)
e
τ
0
degassed (aerated)/μs
f
τ at 77 K/μs
a
b
c
Data from ref 12. Lowest energy singlet band. Lowest energy triplet band. Obtained using fluorescein, quinine sulfate, and Ru(bpy)
d
3
Cl
2
as three
is the lifetime at infinite dilution obtained from the concentration dependence of the decay rate constant; estimated
e
independent standards.
uncertainty (10%. In diethyl ether/isopentane/ethanol (2:2:1 by volume).
τ
0
f
state transition. Two very weak but better resolved bands
at lower energy (450-495 nm) are attributed to direct
absorptions of the T state, facilitated by SOC associated
1
with the Pt(II) ion. These bands are much less solvato-
chromic than the S f S band.
1
2,32
The emission spec-
0
1
trum is structured. At room temperature, the highest
energy emission peak and the lowest energy absorption
peak exhibit a Stokes shift of 250 cm . As for S f T in
-
1
0
1
absorption, the emission bands display only weak nega-
tive solvatochromism.
Upon the basis of the structured profile of the emission
spectrum and the small Stokes shift at T = 300 K, we
previously assigned the emission from this complex to a
2 2
Figure 2. Absorption and emission spectra of Pt(dpyb)Cl in CH Cl
3
3
12
solution at 300 K (λexc = 400 nm). The low-energy portion of the
absorption spectrum is shown on an expanded scale (ꢀ50) to highlight
state of predominant LC ( ππ*) character. Certainly,
the form of the spectrum is very different from the broad,
largely featureless profiles typically observed for classic
0 1
the S f T transition and the small Stokes shift.
-
5
27,33
was dissolved in n-octane at a concentration of ≈10 mol/L.
ThemeasurementswerecarriedoutinaHecryostat(Cryovac
KontiCryostatIT) inwhichtheHegasflow, Hepressure, and
heating were controlled. For magnetic field experiments, an
Oxford Instruments MD10 cryostat equipped with a 12 T
magnet was used. A pulsed Nd:YAG laser (IB Laser Inc.,
DiNY pQ 02) with a pulse width of about 7 ns was applied as
the excitation source for emission spectra and lifetime mea-
MLCT emitters such as Ir(ppy)
3
,
(pic), Ir(4,6-dFppy) (acac), or for neutral iridium
Ir(4,6-dFppy) -
2
3
4
35
2
3
6
complexes containing the dpyb ligand. On the other
hand, there must be a considerable contribution from the
metal orbitals to the excited state, in order to induce the
SOC necessary to promote the triplet radiative decay
constant to give a lifetime as short as 7 μs. Such an
interpretation is supported by the DFT calculations of
Sotoyama et al., who modeled Pt(dpyb)Cl in the gas
-
1
surements, using the third harmonic at 355 nm (28170 cm ).
For measurements of excitation spectra, a pulsed dye laser
Lambdaphysik Scanmate 2C) with Coumarin 102 was ope-
rated. The spectra were recorded with an intensified CCD
camera (Princeton PIMAX) or a cooled photomultiplier
RCA C7164R) attached to a triple spectrograph (S&I Tri-
3
2
(
phase. They estimated that the T and S states have
1 1
25% and 32% dπ* character, respectively. Interestingly,
these theoretical results on the molecular orbitals in-
volved also help to account for the different degrees of
solvatochromism between the triplet and singlet transi-
tions, since the latter involves a larger redistribution of
charge along the Cl-Pt-C axis, strongly perturbing the
dipole moment, while the former involves movement of
charge “outwards” from this axis, with lesser influence on
the dipole moment. Related calculations by Z ꢀa li ꢁs , which
take into account the influence of the solvent, have also
revealed significant metal participation, not only in the
(
vista TR 555). Time-resolved emission spectra were recorded
with a gated photon counter (Stanford Research Systems,
Model SR 400). Decay times were registered using a FAST
Comtec multichannel scaler PCI card with a time resolution
of 250 ps.
3
. Results and Discussion
.1. Photophysical Properties at Room Temperature
3
and at 77 K. Key photophysical properties of Pt(dpyb)Cl
at 300 K are summarized in Table 1, and the corres-
ponding absorption and emission spectra are shown in
Figure 2.
23
HOMO but also in the LUMO.
At 77 K, the emission spectrum is very similar to that at
room temperature. Likewise, the lifetime measured at
77 K in a diethyl ether/isopentane/ethanol (2:2:1) glass
The intense high-energy absorption bands below ≈300
is 7.0 μs, the same, within the uncertainty of the mea-
surement, as the value extrapolated to infinite dilution in
CH Cl at 300 K. The lack of significant changes in the
nm can also be found in the absorption spectrum of the
3
2
free dpybH ligand and thus can be assigned to corre-
spond largely to transitions to ligand-centered (LC) sing-
let states. The absorption envelope in the region 350-440
nm apparently comprises at least three absorption bands,
2
2
spectra and lifetimes over this temperature range of more
than 200 K highlights the extraordinary rigidity of the
the lowest energy of which, presumed to be the S state,
1
1
2
(33) Finkenzeller, W. J.; Yersin, H. Chem. Phys. Lett. 2003, 377, 299.
exhibits pronounced negative solvatochromic behavior.
This would suggest a significant degree of charge-transfer
character (though not necessarily MLCT) to this singlet
(
34) Rausch, A. F.; Thompson, M. E.; Yersin, H. Inorg. Chem. 2009, 48,
1928.
(35) Rausch, A. F.; Thompson, M. E.; Yersin, H. J. Phys. Chem A. 2009,
1
13, 5927.
(36) (a) Wilkinson, A. J.; Puschmann, H.; Howard, J. A. K.; Foster, C. E.;
(
32) Sotoyama, W.; Satoh, T.; Sato, H.; Matsuura, A.; Sawatari, N.
Williams, J. A. G. Inorg. Chem. 2006, 45, 8685. (b) Williams, J. A. G.;
Wilkinson, A. J.; Whittle, V. L. Dalton Trans. 2008, 2081.
J. Phys. Chem. A 2005, 109, 9760.

1
1410 Inorganic Chemistry, Vol. 48, No. 23, 2009
Rausch et al.
Figure 3. Comparison of the emission spectra of Pt(dpyb)Cl in n-octane
at T = 300 K and at T = 4.2 K.
molecule, minimizing nonradiative decay, and the influ-
ence of the cyclometalation in displacing potentially
deactivating metal-centered dd* states to high energies,
out of reach at room temperature.
Figure 4. Emission and excitation spectra of Pt(dpyb)Cl in n-octane at
selected temperatures. For the excitation spectrum, the emission was
detected at an energy of 19 570 cm , which corresponds to a vibrational
3
.2. High-Resolution Spectroscopy at Cryogenic Tem-
-1
satellite ofthe transition IIf 0 (684cm- vibration). The emissionspectra
1
peratures. The emission spectra at room temperature and
at 77 K allow only a crude characterization of the emitting
triplet state. Therefore, the compound was dissolved in
n-octane at low concentration and cooled to liquid helium
temperatures. In the polycrystalline alkane matrix, the
guest molecules (dopants) substitute host molecules and
were recorded under UV excitation (λexc = 355 nm).
2
8
thus lie at defined positions. In suitable cases, the
37
application of this so-called Shpol’skii technique results
in highly resolved spectra with line widths of only a few
The obtained
-
1
cm due to a residual inhomogeneity.
28,38
line widths are a factor of 100 smaller than those usually
obtained with amorphous or glassy host materials. Fre-
quently, the obtained spectra in Shpol’skii matrices re-
present superpositions of spectra stemming from several
sites. In these cases, the investigation of just one specific
site is possible by selective excitation with a tunable dye
laser. However, in the case of Pt(dpyb)Cl, a very favor-
able situation is found, since, even under unselective UV
excitation, almost all dopant molecules occupy just one
site. In Figure 3, the emission spectra of the compound
dissolved in n-octane at temperatures of 300 and 4.2 K are
compared to demonstrate the impressive line narrowing
effect achieved by the Shpol’skii technique.
Figure 5. Emission spectra of Pt(dpyb)Cl in n-octane at different mag-
netic field strengths after UV excitation at 355 nm. The temperature of 14
K was chosen to provide a sufficient thermal population of all three
substates, being separated by 23 cm at B = 12 T. The assignment of the
three 0-0 transitions at B = 0 T results from the temperature-dependent
emission and excitation measurements depicted in Figure 4.
-1
The ambient temperature spectrum consists, as dis-
cussed above, of overlapping and relatively broad bands
with halfwidths of about 500 cm , while the emission
-
1
spectrum at T = 4.2 K exhibits narrow lines with half-
out at different temperatures (Figure 4) and under appli-
cation of external magnetic fields (Figure 5). These ex-
periments allow us to identify the three T substates,
-
1
widths of only 4 cm . It will be shown below that the
most intense line in the highly resolved spectrum depicted
in Figure 3, which has its maximum at 20254 cm , mainly
1
-
1
denoted as I, II, and III. At zero magnetic field, the lines
stemming from the high-energy substates II and III, lying
represents the electronic 0-0 transition from one specific
-
1
substate of T , denoted as substate II, to the singlet
ground state S . The lines of minor intensity correspond
at 20254 cm and 20258 , respectively, can be observed in
excitation and are in resonance (no Stokes shift) with
corresponding emission peaks (Figure 4).
1
0
to vibrational satellites stemming from ground state
modes. A detailed investigation of the vibrational satellite
structure will be carried out in section 3.5.
An intensity ratio for the excitation peaks of Int(0 f
II)/Int(0 f III)=2.3 is found, which displays the ratio of
the oscillator strengths of the corresponding transitions.
In accordance with this observation, emission from sub-
state II dominates at higher temperatures, while the
transition III f 0 only occurs as a shoulder. The 0-0
transition between the lowest triplet substate I and the
ground state is very weak, i.e. it is largely forbidden, but
it can be detected at low temperatures in excitation (very
3
.3. Electronic 0-0 Transitions. In order to gain a
deeper insight into the properties of the emitting triplet
state, emission and excitation measurements were carried
(
(
37) Shpol’skii, E. V. Sov. Phys. Usp. (Engl. Transl.) 1960, 3, 372.
38) Rausch, A. F.; Thompson, M. E.; Yersin, H. Chem. Phys. Lett. 2009,
4
68, 46.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11411
Figure 6. Emission decay curve of Pt(dpyb)Cl in n-octane at T = 1.2 K
after UV excitation at 355 nm. The emission was detected at the electronic
-
1
0
-0 transition II f 0 (20254 cm ).
Figure 7. Thermalized emission decay of Pt(dpyb)Cl in n-octane versus
temperature after pulsed UV excitation at 355 nm. The emission was
detected at the electronic 0-0 transition II f 0 (20 254 cm ). The solid
line represents a fit of eq 1 to the experimental data.
-
1
weak) and in emission at 20248 cm . At T=1.2 K, an
-1
additional line at lower energy is observed, which is
-
1
assigned to correspond to a 7 cm local phonon mode
low energy vibration of the dopant in its matrix cage)
2
8,39,40
spin-lattice relaxation (SLR)
below.
is further discussed
(
occurring only in the emission of substate I. It will be
shown below that the relatively high intensity of the
emisson line II f 0 at 1.2 K is not ascribed to thermal
population according to a Boltzmann distribution, but is
a consequence of relatively slow processes of spin-lattice
relaxation between the substates II and I at that tempera-
ture. The spectra depicted in Figure 4 allow the zero-field
splitting parameters of the emitting triplet to be deter-
mined as ΔEII-I=5.7 cm and ΔEIII-I=9.9 cm
For a further verification of the assignment of the
observed lines as representing the electronic origins of
the T substates of one single site, emission measurements
under the influence of high external magnetic fields have
been carried out (Figure 5).
Due to the Zeeman effect, the wave functions of the
three T1 sublevels significantly mix with increasing
B-fields, leading to distinct changes in the emission
The individual emission decay times of all three substates
to the ground state can be determined from the temperature
dependence of the thermalized emission decay. In Figure 7,
the measured values are plotted versus temperature for the
range 1.2 e T e 50 K. For a system of three thermally
equilibrated excited states, the rate constant for the depopu-
lation ktherm, representing the inverse of the measured decay
33,40-42
-
1
-1
time τtherm, is given by the expression
.
1
ktherm
¼
¼
τtherm
1
ꢀ
ꢀ
ꢁ
ꢁ
ꢀ
ꢁ
-ΔEII -I
kBT
-ΔEIII -I
k þ k exp
þ kIII exp
I
II
kBT
ꢀ
ꢁ
-
ΔEII -I
kBT
-ΔEIII -I
kBT
1 þ exp
þ exp
-
1
spectra. The total splitting, amounting to 10 cm at
ð1Þ
-
1
B=0 T, grows to 23 cm at B=12 T. Moreover, the
radiative allowedness is redistributed between the sub-
states. With increasing B-field strength, the emission from
the lowest B-field disturbed substate I(B) becomes domi-
nant while the transition II(B) f 0, mainly governing the
emission spectrum at zero field, distinctly loses intensity.
This behavior shows that the magnetic field drastically
alters the properties of the respective 0-0 transitions.
Further, the observed Zeeman splitting pattern is a proof
that the lines ascribed as substates I, II, and III in Figure 4
wherein k = 1/τ (i=I, II, III) is the rate constant for the
i
i
depopulation of substate i to the ground state. ΔEII-I and
ΔEIII-I represent the energy separations between the re-
spective substates. For a fit of eq 1 to the experimental data,
-
1
the energy separations of ΔE =5.7 cm and ΔEIII-I
9
3
=
II-I
-
1
.9 cm that result from highly resolved spectra (see section
.3) were kept constant. The fitting procedure gives the
individual decay times of the T substates of τ =51.4 μs, τ =
1
I
II
1
.9 μs, and τ =4.5 μs. The value obtained for τ is slightly
III I
longer than the measured decay time at 1.2 K (Figure 6),
since the substates I and II are already slightly thermalized
even at this low temperature. Interestingly, the obtained rate
constant ratio k /k of 2.4 is in good agreement with the
II III
ratio of 2.3 as determined in section 3.3 from the electronic
really belong to the T state of one single site.
1
3
.4. Thermalized Emission Decay and Energy Level
Diagram. The different oscillator strengths of the transi-
tions I T 0, II T 0, and III T 0 manifest themselves also in
the emission decay times of the respective T substates.
1
0
-0 transitions in the excitation spectrum. This indicates
The emission decay curve for a detection energy of 20 254
-
1
comparable radiative and nonradiative deactivation me-
chanisms for both substates.
cm (electronic 0-0 transition II f 0) measured at T=
.2 K is shown in Figure 6. A biexponential curve with
1
At T=1.2 K, the spin-lattice relaxation between the
substates I and II of Pt(dpyb)Cl in n-octane is domi-
nated by the direct process, describing a resonant one-
phonon transition. For a determination of the
SLR time, one has to take into account that substate II is
time constants of 300 ns and 49.4 μs is observed. The long
component represents the thermalized emission decay
from the three T substates (mainly of substate I), while
1
the short component stems from substate II, determined
mainly by its relaxation to substate I. This so-called
2
8,39,40,43
(
41) Harrigan, R. W.; Crosby, G. A. J. Chem. Phys. 1973, 59, 3468.
(42) Azumi, T.; O’Donnell, C. M.; McGlynn, S. P. J. Chem. Phys. 1966,
45, 2735.
(43) Scott, P. L.; Jeffries, C. D. Phys. Rev. 1962, 127, 32.
(
39) Strasser, J.; Homeier, H. H. H.; Yersin, H. Chem. Phys. 2000, 255,
01.
40) Yersin, H.; Strasser, J. Coord. Chem. Rev. 2000, 208, 331.
3
(

1
1412 Inorganic Chemistry, Vol. 48, No. 23, 2009
Rausch et al.
Figure 9. Time-integrated emission spectra of Pt(dpyb)Cl in n-octane at
T = 1.2and4.2K afterUVexcitationat 355nm. Notethe scalingfactorin
the 4.2 K spectrum. The region of the electronic 0-0 transitions is
additionally depicted on an enlarged scale.
1
Figure 8. Energy level diagram and decay times of the T substates of
Pt(dpyb)Cl in n-octane. The value of τSLR = 360 ns applies to T = 1.2 K.
depopulated by relaxation to substate I, given by the SLR
rate kSLR=(τSLR
relaxation to the ground state 0, given by the usual decay
the dopant in its matrix cage, which represent so-called
local phonon modes. It should be noticed that a mixing
of internal low-energy vibrations of the complex
with such local phonon modes is still relevant up to
-
1
) , and by a radiative and nonradiative
-
1
rate k = (τ ) . Thus, the experimentally determined
II
II
-
1 44
decay rate (short decay component) can be expressed
by
≈150 cm
.
Overlapping with this energy range up to
2
8,38-40
-1
500/600 cm , metal-ligand (M-L) vibrations are found,
-1
while fundamentals higher than ≈600 cm can usually
2
8
be assigned as internal ligand modes. As already sub-
stantiated in the discussion of the electronic 0-0 transi-
tions in section 3.3, the spectrum at 4.2 K is dominated by
1
exp
II
exp
II
k
¼ τ
¼ kSLR þ kII
ð2Þ
exp
With τII =300 ns (Figure 6) and k =(τ )-1=(1.9 μs)-1
a value of τSLR=360 ns is found for T=1.2 K.
The results discussed above are summarized in an
emission from T substate II. Due to the short decay time
1
;
II II
(relatively high allowedness) of the transition to the
singlet ground state and the appearance of (weak) over-
tone satellites for the most intense fundamentals (e.g.,
energy level diagram for the T substates as shown in
1
-
1
77, 684, 1135, and 1326 cm ), many of the vibrational
2
satellites observed at 4.2 K can be assigned to corres-
Figure 8.
-
1
The energy separation of ΔEIII-I ≈ 10 cm , that is the
total zero-field splitting, can be used to classify the
emitting triplet state. This value characterizes the amount
of MLCT character and the importance of spin-orbit
coupling for the emitting triplet state via the involved d-
orbital admixtures. According to an empirical ordering
pond to totally symmetric Franck-Condon (FC) active
modes.
2
8,45-47
For those modes showing second members of vibra-
tional progressions, one can determine the so-called
Huang-Rhys parameter S, which is specific for each
vibrational mode and depends on the shift ΔQ of the
equilibrium positions of the involved electronic states for
8
,26-28,40
scheme,
a triplet state which exhibits a value of
-
1
ΔE(ZFS) ≈ 10 cm can be assigned as being largely of
28,45-47
3
3
the corresponding vibrational coordinate.
A
ligand-centered ( LC, ππ*) character, though with mod-
erate MLCT (5dπ*) perturbations. The extremely weak
allowedness observed for the transition 0 f I (Figure 4)
Huang-Rhys parameter of zero corresponds to equal
geometries of the excited state and the ground state. In
this case, the whole emission intensity is carried by the
electronic 0-0 transition. With an increase of the dis-
placement ΔQ, and thus an increase of S, the intensity of
the electronic 0-0 line decreases and the vibrational
satellites of the respective Franck-Condon progression
gain intensity. In the low-temperature limit, the
Huang-Rhys parameter for a specific mode does not
and the relatively long decay time of τ =51.4 μs indicate
I
that substate I exhibits almost pure triplet character,
while the substates II and III with τ =1.9 μs, τIII=4.5
II
μs, and much higher allowednesses for the respective
absorptions from the ground state (Figure 4) contain
1
significantly larger admixtures of higher-lying MLCT
states.
4
5
depend on the vibrational energy and can be determined
3.5. Vibrational Satellite Structures. Beside the electro-
2
8,47,48
by using the expression
nic 0-0 transitions, the vibrational satellite structures in
the highly resolved emission spectra of Pt(dpyb)Cl in
n-octane are also very informative with respect to the
different properties of the triplet substates. In particular,
the vibrational satellite structure in the emission of sub-
state I differs strongly from the satellite structure in the
emission of the substates II and III. In Figure 9, the
electronic 0-0 transitions and the vibrational satellites
corresponding to ground state modes are depicted for the
temperatures of 1.2 and 4.2 K.
Iv
S ¼ v I
ð3Þ
v -1
(44) Becker, D.; Yersin, H.; von Zelewsky, A. Chem. Phys. Lett. 1995,
35, 490.
(45) Henderson, B.; Imbusch, G. F. Optical Spectroscopy of Inorganic
2
Solids; Clarendon: Oxford, 1989.
(46) Ballhausen, C. J. Molecular Electronic Structures of Transition Metal
Complexes; McGraw-Hill: New York, 1979.
Many of the satellites observable in Figure 9 corre-
spond to fundamentals. Low energy modes (with energies
(
(
47) Solomon, E. I. Comments Inorg. Chem. 1984, 3, 300.
48) Seiler, R.; Kensy, U.; Dick, B. Phys. Chem. Chem. Phys. 2001, 3,
-
1
up to ≈100 cm ) are largely determined by vibrations of
5373.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11413
wherein v is the vibrational quantum number and I is the
v
intensity of the respective member of the Franck-
Condon progression. In the case of Pt(dpyb)Cl in
n-octane at 4.2 K, most of the emission intensity is carried
by the electronic 0-0 transition II f 0, while the satellites
stemming from vibrational modes are very weak. For the
FC modes with fundamental energies of 277, 684, 1135,
-
1
and 1326 cm (see above), a Huang-Rhys parameter of
S ≈ 0.1 is found. A value of that magnitude shows that
only very small geometry changes occur between the
singlet ground state and the emitting T state, at least in
1
a rigid n-octane matrix at cryogenic temperatures.
For weaker satellites, second members of progressions
cannot be observed; presumably, they are hidden in
the noise of the spectrum due to small Huang-Rhys
parameters.
Figure 10. Time-resolved emission spectra of Pt(dpyb)Cl in n-octane at
T = 1.2 K after UV excitation at 355 nm. The short-time spectrum was
recorded immediately after the exciting laser pulse in a time window of
1
μs. For the delayed spectrum, a time delay of 25 μs and a time window of
A temperature increase to, e.g., 10 K does not lead to
significant changes in the emission spectrum (not shown).
Thus, in particular, the same vibrational satellites as
found in the emission of substate II occur also in the
emission of substate III. This behavior supports the
results as presented already in section 3.4 concerning
the similar deactivation mechanisms of the two substates.
At T=1.2 K, the emission mainly stems from the lowest
triplet substate I, which carries a much lower allowedness
with respect to the electronic 0-0 transition to the ground
state as compared to substate II. Thus, substate I can be
deactivated by a different vibrational mechanism, and a
completely different situation is found. Many intense
vibrational satellites observed in the emission of substate
I are absent in the emission spectrum of substate II at
500 μs were chosen. Note the scaling factor in the short-time spectrum.
The region of the electronic 0-0 transitions is additionally depicted on an
enlarged scale.
of HT active vibrational fundamentals represent so-called
“false origins”. Indeed, this behavior is observed for the
T substate I of Pt(dpyb)Cl in n-octane.
1
Moreover, the substates I and II exhibit distinctly
different emission decay times (see section 3.4). There-
fore, a separation of the individual spectra of the sub-
states on the time domain is possible and provides deeper
insight. Figure 10 shows emission spectra at T = 1.2 K
which were recorded with different delay times after the
exciting laser pulse and in different time windows for
detection.
The spectrum obtained immediately after excitation in
a time window of 1 μs (compare the fast decay component
displayed in Figure 6) is very similar to the time-inte-
grated spectrum obtained at 4.2 K. No other electronic
4
.2 K. Moreover, one of these satellites is even more
intense than the corresponding electronic 0-0 transition I
-
1
f 0 (442 cm mode). Obviously, the radiative deactiva-
tion at the purely electronic transition is less efficient than
the radiative deactivation involving vibrational modes.
Further, for these fundamentals, no progressions occur,
while some of the most intense Franck-Condon modes
0-0 transition beside II f 0 is observable. Further, the
vibrational satellite structure stems almost exclusively
from the fast decaying T substate II. On the other hand,
1
after a delay time of 25 μs, the obtained spectrum is
comparable to the time-integrated emission at 1.2 K
and is dominated by the emission of substate I. However,
substate II also emits in the delayed spectrum, but less
intensely than in the time-integrated spectrum (compare
the intensity ratios of the electronic 0-0 transitions in
Figures 9 and 10). Since processes of spin-lattice relaxa-
(
identified in the emission of substate II) form combina-
-
1
tions with the intense 442 cm mode observed in the
-
1
emission spectrum of the substate I (e.g., 442 þ 684 cm
,
-
1
4
tions carried out with Pt(2-thpy) ,
42 þ 1326 cm ). In analogy to the extensive investiga-
2
7,28,49
Pt(4,6-dFppy)-
it can be concluded that
2
2
6,29
2þ 50
(
acac),
and [Os(bpy)3]
,
the observed vibrational satellite structure stemming
from substate I is mainly induced by processes of
vibronic coupling of substate I to higher lying states via
spin-vibronic mechanisms (details are discussed in refs 27
and 28). This type of coupling, frequently called
tion (with τ
=360 ns) can be neglected after a delay
SLR
time of 25 μs and the initial population of substate II has
completely decayed, it can be concluded that the emission
from this substate in the delayed spectrum stems only
from thermal repopulation according to a Boltzmann
distribution.
4
6,51,52
Herzberg-Teller (HT) coupling,
is of particular
importance, if the electronic 0-0 transition is largely
spin-forbidden and carries only low allowedness. Spin-
vibronic coupling becomes effective by modulation of
spin-orbit coupling through an adequate vibration along
a specific normal coordinate Q of a complex. As a
consequence, intensity is induced to the corresponding
4. Assignments and Conclusions
This study shows that high-resolution optical spectroscopy
at cryogenic temperatures in an n-octane matrix allows a
detailed characterization of the emitting triplet state of the
highly luminescent compound Pt(dpyb)Cl. A total zero-field
vibrational satellite with the vibrational energy νh , but
Q
2
8,46,52
-1
not to the purely electronic transition.
The satellites
splitting of the emitting triplet state of ΔE(ZFS) = 9.9 cm
is observed (see Figure 8). In general, ligand-centered triplet
3
(
49) Wiedenhofer, H.; Sch u€ tzenmeier, S.; von Zelewsky, A.; Yersin, H.
J. Phys. Chem. 1995, 99, 13385.
50) Yersin, H.; Kratzer, C. Coord. Chem. Rev. 2002, 229, 75.
51) Herzberg, G.; Teller, E. Z. Z. Phys. Chem. 1933, B21, 410.
52) Fischer, G. Vibronic Coupling; Academic Press: London, 1984.
states ( LC states) in organo-transition metal compounds
-
1
exhibit ΔE(ZFS) values smaller than 1 cm , while for
(
(
(
MLCT emitters the splitting can reach a value up to
-
1 8,27,28,40,50
200 cm
.
Thus, it is indicated that the lowest

1
1414 Inorganic Chemistry, Vol. 48, No. 23, 2009
Rausch et al.
triplet of Pt(dpyb)Cl can be described as being largely ligand-
centered with moderate metal-to-ligand charge transfer char-
acter mixed in. This is in accordance with the relatively short
At ambient temperature in fluid solution, larger values of the
Huang-Rhys parameters are observed. In this situation of a
“soft” environment, the displacements ΔQ of the potential
energy surfaces of the excited electronic state can be assumed to
be larger than in a rigid (poly)crystalline host cage at 4.2 K. The
temperature dependence of S can be neglected in this respect,
since, for vibrational fundamentals with energies between 1000
individual decay times of the higher-lying T substates II and
1
III, which mainly govern the emission properties at ambient
3
temperature. For compounds with emitting states of LC
character, direct spin-orbit coupling to higher-lying singlet
and triplet MLCT states is weak. However, the admixture of
such states, which is necessary to obtain a considerable
magnitude of ZFS and relatively short substate decay times,
can proceed via a two-step mechanism involving configura-
-
1
and 1500 cm , the low-temperature approximation of the
6
1
Huang-Rhys parameter is still valid at 300 K. Interestingly,
the trends for the S values found from highly resolved spectra
at cryogenic temperatures are still maintained in fluid solution
under ambient conditions. For Pt(dpyb)Cl, the 300 K electro-
nic origin band is dominant and significantly more intense than
the vibrational satellite bands which stem from (overlapping)
intraligand fundamentals, combinations, and/or progressions.
In n-octane, the intensity of the most intense vibrational
satellite band amounts to ≈0.5 relative to the electronic origin
peak (see Figure 3), while for Pt(II) compounds with bidentate
phenylpyridine-type ligands, a much larger portion of the
3
tion interaction of the LC substates with the substates of
3
higher lying MLCT states. These latter states allow direct
1,3
spin-orbit coupling with other MLCT states. It is stressed
that only MLCT states which involve different metal d-
orbitals can mix via direct SOC. (For details see refs 26,
2
7, 29, 53, and 54.)
Another parameter to characterize the nature of the
emissive triplet state is given by the Huang-Rhys parameter
S. For compounds with ligand-centered T states, such as
emission intensity is found in the range of the vibrational
1
2
8
2þ 55
3þ 56
62-64
Pd(2-thpy)2, [Pt(bpy)2]
,
or [Rh(bpy) ]
(all of which
satellite bands.
For Pt(4,6-dFppy)(acac) in n-octane, for
3
-
1
exhibit ΔE(ZFS) values of <1 cm ), the maximum S values
are as large as ≈ 0.3, while for a typical MLCT emitter, such
example, the emission intensity of the most intense vibrational
satellite band relative to the intensity of the origin band is as
2
þ
-1 57
38
as [Os(bpy) ] (with ΔE(ZFS)=211 cm ), the maximum
large as ≈0.8.
3
Huang-Rhys parameter amounts to only 0.08. The occur-
rence of significantly smaller S parameters for compounds
with emitting triplet states of MLCT character than for
These presented results are highly important with respect
to color purity requirements for OLEDs. Especially in the
blue spectral region, Pt(N C N) systems can achieve higher
∧
∧
3
∧
compounds with emitting states of LC origin is ascribed to
emission color purity than related Pt(N C) compounds, since
the more distinct spatial extensions of the electronic wave
functions over the ligands and the metal with increasing
in the former ones the intensity contributions of the green-to-
yellow low-energy vibrational satellite bands are strongly
reduced. These aspects will, in combination with the higher
28,58,59
64
MLCT admixture.
In this situation, an electronic
transition results in a smaller charge density change per
ligand and thus has less influence on the specific bonding
emission quantum yields and relatively short emission decay
times of this class of Pt(II) complexes with terdentate ligands,
allow the development of OLEDs with further improved
device characteristics.
8,28
properties (small ΔQ values).
For the compounds Pt(4,6-dFppy)(acac) and Pt(ppy)-
26
6
0
(
acac), which exhibit similar zero-field splittings and thus
Note added in Proof: A recent related paper by Tong
and Che was published.
similar MLCT character in the emitting states as in Pt-
6
5
(
dpyb)Cl, Huang-Rhys parameters of S ≈ 0.2 were deter-
mined for prominent FC modes. The smaller S values for
Pt(dpyb)Cl of only ≈0.1, or the smaller geometry changes
between S and T , are ascribed to the higher rigidity of the
Pt(N C N) system with a terdentate ligand compared to the
related Pt(N C) compounds with bidentate phenylpyridine-
Acknowledgment. The Bundesministerium f u€ r Bildung
und Forschung (BMBF) is gratefully acknowledged for
providing the funding of this investigation. We thank the
British Council and the German Academic Exchange
Service (DAAD) for a bilateral ARC travel grant, the
COST project for financial support within the project
D35-0010-05, and the University of Durham for a doc-
toral fellowship to L.M.
0
1
∧
∧
∧
∧
type ligands. In the latter ones, the plane of the N C ligand
can potentially twist relative to the plane containing the other
two ligating atoms and, thus, can give rise to additional
distortions.
(
61) For a vibrational mode with an energy in the range of 1000-1500
(
53) Miki, H.; Shimada, M.; Azumi, T.; Brozik, J. A.; Crosby,
-1
cm , a low-temperature Huang-Rhys parameter of 0.1 increases only
marginally to ≈0.1001-0.101 at 300 K (compare ref 45, p 206).
G. A. J. Phys. Chem. 1993, 97, 11175.
(
(
(
(
(
54) Azumi, T.; Miki, H. Top. Curr. Chem. 1997, 191, 1.
(62) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.;
55) Humbs, W.; Yersin, H. Inorg. Chim. Acta 1997, 265, 139.
56) Humbs, W.; Yersin, H. Inorg. Chem. 1996, 35, 2220.
57) Yersin, H.; Humbs, W.; Strasser, J. Top. Curr. Chem. 1997, 191, 153.
58) Maestri, M.; Sandrini, D.; Balzani, V.; von Zelewsky, A.; Deuschel-
Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055.
(63) Ghedini, M.; Pugliese, T.; La Deda, M.; Godbert, N.; Aiello, I.;
Amati, M.; Belviso, S.; Lelj, F.; Accorsi, G.; Barigelletti, F. Dalton Trans.
2008, 32, 4303.
(64) Yang, X.; Wang, Z.; Makaduni, S.; Li, J.; Jabbour, G. E. Adv. Mater.
2008, 20, 2405.
(65) Tong, G. S.-M.; Che, C.-M. Chem. ;Eur. J. 2009, 15, 7225.
Cornioley, C.; Jolliet, P. Helv. Chim. Acta 1988, 71, 1053.
59) Moore, J. J.; Nash, J. J.; Fanwick, P. E.; McMillin, D. R. Inorg.
Chem. 2002, 41, 6387.
60) Rausch, A. F. Ph.D. thesis, Universit €a t Regensburg, in preparation
(
(