Near-infrared luminescence from platinum(II) diimine compounds

Article abstract of DOI:10.1021/ic060399d

Square-planar Pt(II) complexes of the bis(mesitylimino)acenaphthene (mesBIAN) ligand are emissive from a MMLL'CT excited state in a dichloromethane solution at room temperature. Investigation of the nature of the frontier orbitals in these near-IR emitter

Full text of DOI:10.1021/ic060399d

Inorg. Chem. 2006, 45, 6105−6107
Near-Infrared Luminescence from Platinum(II) Diimine Compounds
Christopher J. Adams,*,† Natalie Fey,† and Julia A. Weinstein*,‡
School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, U.K., and Department of Chemistry,
UniVersity of Sheffield, Sheffield BS3 7HF, U.K.
Received March 9, 2006
Square-planar Pt(II) complexes of the bis(mesitylimino)acenaph-
thene (mesBIAN) ligand are emissive from a MMLL CT excited
The nature of the emitting triplet state depends on the
electronic structure of the NN and LL ligands. For dithiolate
complexes, where LL is a bidentate S donor ligand, it has
been described as a “mixed metal-ligand-to-ligand charge
transfer” (MMLL′CT), from a highest occupied molecular
orbital (HOMO) composed of orbitals from both the metal
and the dithiolate ligand to a lowest unoccupied MO
(LUMO), which is predominantly localized on the π* system
of the NN ligand.6 In complexes where L is a terminal
alkynyl ligand, it is generally thought to be more MLCT,
with a similar LUMO but a HOMO more exclusively based
on the Pt atom.1e
We report here a series of compounds that absorb strongly
across the visible region down to 600 nm and emit at
wavelengths longer than 750 nm because they contain the
nonheterocyclic bidentate diimine ligand bis(mesitylimino)-
acenaphthene (mesBIAN; Scheme 1). The extensively con-
jugated rigid π system of mesBIAN causes the LUMO of
the Pt(mesBIAN)(LL) complexes reported herein to be lower
in energy than that in previously reported Pt(NN)(LL)
complexes, with a correspondingly smaller HOMO-LUMO
gap leading to lower energy absorption and emission.
The reaction of mesBIAN with Zeise’s salt affords Pt-
(mesBIAN)Cl2 (1). The Cl ligands of 1 can then be readily
replaced with other ligands, by metathesis with Me2Sn(dto)
(dto ) 1,2-dithiooxalate) to form Pt(mesBIAN)(dto) (2)7 and
in a copper(I) iodide catalyzed reaction with a terminal
alkyne in the presence of triethylamine to form Pt(mes-
BIAN)(-CCC6H4CF3)2 (3).8
′
state in a dichloromethane solution at room temperature. Investiga-
tion of the nature of the frontier orbitals in these near-IR emitters
by a combination of emission spectroscopy, electron paramagnetic
resonance spectroscopy, and density functional theory calculations
suggests that emission is enabled by the presence of low-lying
ligand
π* orbitals on the mesBIAN.
The past decade has seen growing interest in the photo-
chemistry of Pt(II) compounds of the form Pt(NN)(LL),
where NN is a bidentate N-heterocyclic diiimine ligand such
as bipyridine or phenanthroline and LL is either one bidentate
ligand (for example, a dithiolate) or two monodentate (such
as alkynyl) ligands. The reason for this interest is the
luminescence in solution at room temperature exhibited by
these compounds and the fact that their excited-state proper-
ties can be tuned by peripheral modification of the ligands.
This property has been thoroughly investigated,1-3 and Pt-
(NN)(LL) compounds and derivatives thereof have been
incorporated into electroluminescent devices2a and dye-
sensitized solar cells4 and utilized for the sensitization of
lanthanide excited states.5
* To whom correspondence should be addressed. E-mail: chcja@bris.ac.uk
(C.J.A.), julia.weinstein@sheffield.ac.uk (J.A.W.).
† University of Bristol.
‡ University of Sheffield.
(1) (a) McGarrah, J. E.; Eisenberg, R. Inorg. Chem. 2003, 42, 4355-
4365. (b) Wadas, T. J.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem.
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Schanze, K. S. Inorg. Chem. 2001, 40, 4053-4062.
1-3 are air-stable crystalline solids and dissolve in
solvents such as CH2Cl2 and tetrahydrofuran (THF) to form
stable solutions. 1-3 are darkly colored, with relatively
strong absorption bands between 500 and 600 nm. The value
of the extinction coefficient (Table 1) and the solvato-
chromism1c (e.g., a shift from 498 nm in CH3CN to 552 nm
in toluene for 1; see the Supporting Information for details)
(5) Shavaleev, N. M.; Accorsi, G.; Virgili, D.; Bell, Z. R.; Lazarides, T.;
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10.1021/ic060399d CCC: $33.50
Published on Web 07/04/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 16, 2006 6105

COMMUNICATION
Scheme 1. Synthesis of Compounds 1-3
weak low-energy absorption band. A good match between
the excitation spectrum and the corresponding absorption
spectrum is consistent with emission originating from the
parent compounds. The emission lifetimes of 1-3 are
considerable for near-IR emitters based on transition-metal
complexes.9
To further understand the frontier orbitals of 1-3 and thus
their absorption and emission spectra, we have investigated
their electro- and spectroelectrochemical behavior (Table 2).
All of the compounds display at least two well-defined
electrochemical processes, consisting of a fully reversible
reduction at around -0.7 V, a quasi-reversible reduction at
around -1.3 V, and in the case of 3 an irreversible oxidation
at +1.5 V (all values are versus SCE). Performing a single-
electron reduction populates the LUMO of the neutral
species, and the nature of the resulting singly occupied MO
(SOMO) has been probed by electron spin resonance (ESR)
spectroscopy.
As a representative example, the experimental and simu-
lated ESR spectra at 110 K of a sample of 1 chemically
reduced with cobaltocene are shown in Figure 2. The
parameters used for the simulation are given in Table 2;
assignment of the g values and couplings follows the
methodology of McInnes and co-workers for reduced Pt-
(bipy)Cl2 compounds.10 Using their approach, we can
calculate the contribution of orbitals from the metal to the
SOMO, which for 1 gives values for the 5dyz unpaired
electron density of 9.6% and for the 6pz unpaired electron
density of 4.6%. Thus, we may conclude that the SOMO of
the anion 1- (and, by extension, the LUMO of 1) is
approximately 14% metal-based. The spectra of 1--3- show
a much better resolution of the N hyperfine coupling on the
high-field component than their bipyridyl10 and diazabuta-
diene11 equivalents.
Table 1. Optical Data for 1-3 in CH2Cl2 at Ambient Temperature
absorptiona
emission
λ
max/nm (ꢀ/dm3 mol-1 cm-1
)
λ
max/nm φ/10-4 τ/ns
1
2
3
250 (33 910), 358 (17 499), 518 (6910),
670 (170), 740 (120)
383 (11 672), 486 (10 615), 530 (10 464),
675 (470), 760 (145)
299 (43 790), 379 (11 490), 514 (11 809),
720 (280)
777
820
800
3
20
17
30
0.4
8
a In degassed solutions; over 10-300 µmol concentration range,
throughout which the absorptions obey Beer’s law.
Conclusions about the LUMO drawn from the ESR data
are supported by density functional theory studies. These
studies consisted of in vacuo geometry optimizations, fol-
lowed by the application of an ꢀ ) 8.93 continuum to model
a CH2Cl2 solvation field for determination of the MOs (see
the Supporting Information for details).
The calculations show that for 1 the LUMO (Figure 3) is
an orbital of b1 symmetry (in the C2V point group) that is
mainly formed from the π* system of the mesBIAN ligand,
particularly the NdC-CdN backbone of the chelate ring,
with an approximately 11% contribution from the metal
orbitals. 1- is computed to have 12% of the unpaired electron
spin density on the Pt atom, contained in a virtually identical
orbital. Similar LUMOs are calculated for 2 and 3 (9 and
8% metal-based, respectively), and their anions give similar
Figure 1. Absorption and emission spectra of 1 in CH2Cl2.
observed for this absorption band indicate that the corre-
sponding electronic transition is at least partly CT in nature.
Each compound also has very weak absorption bands at
longer wavelength (>650 nm), which, as in the case of
previously reported Pt(II) chromophores,3 are attributed to
the direct population of triplet states, a process facilitated
by the high spin-orbit coupling associated with the Pt(II)
ion.
(9) Draper, S. M.; Gregg, D. J.; Schofield, E. R.; Browne, W. R.; Duati,
M.; Vos, J. G.; Passaniti, P. J. Am. Chem. Soc. 2004, 126, 8694-
8701. Bergman, S. D.; Gut, D.; Kol, M.; Sabatini, C.; Barbieri, A.;
Barigelletti, F. Inorg. Chem. 2005, 44, 7943-7950.
(10) (a) McInnes, E. J. L.; Farley, R. D.; Macgregor, S. A.; Taylor, K. J.;
Yellowlees, L. J.; Rowlands, C. C. J. Chem. Soc., Faraday Trans.
1998, 94, 2985-2991. (b) McInnes, E. J. L.; Farley, R. D.; Rowlands,
C. C.; Welch, A. J.; Rovatti, L.; Yellowlees, L. J. J. Chem. Soc., Dalton
Trans. 1999, 4203-4208.
(11) (a) Hasenzahl, S.; Hausen, H.-D.; Kaim, W. Chem.sEur. J. 1995, 1,
95-99. (b) Klein, A.; Hasenzahl, S.; Kaim, W. J. Chem. Soc., Perkin
Trans. 2 1997, 2573-2577.
Compounds 1-3 are luminescent in a CH2Cl2 solution at
room temperature. Their emission spectra are structureless,
and in each case, the emission maximum is close in energy
to the weak, lowest-energy absorption band (e.g., 777 and
740 nm, respectively, for 1; Figure 1 and Table 1). The small
Stokes shift supports further the triplet assignment of the
6106 Inorganic Chemistry, Vol. 45, No. 16, 2006

COMMUNICATION
Table 2. Electrochemical and ESR Data for 1-3 and 1--3- a
Eo′/V
oxidation
reduction
giso
gy
gx
gz
g
Aiso
Ay(Pt)
-72.6
Ax(Pt)
Az(Pt)
-48.3
A
Az(N)
1
2
3
-0.60
-1.85
-0.57
-1.48
-0.69
-1.74
1-
2-
3-
2.011
2.074
2.003
1.952
2.009
75.1
-112.2
-77.7
13.6
2.014
2.008
2.043
2.034
2.017
2.010
1.978
1.979
2.012
2.007
50.0
-55.2
-46.8
-72.5
-50.0
-22.1
-6.0
-49.9
-34.3
13.5
13.0
1.53
a Potentials vs SCE, in a CH2Cl2 solution with a 0.1 M [Bu4N][PF6] supporting electrolyte. ESR spectra were recorded at X band at 110 K in 2:1
THF/CH2Cl2. Hyperfine coupling constants are 10-4 cm-1
.
the absorption spectrum is of MMLL′CT character. However,
in contrast to 1-3, they reported that the Pt(iPrDAB)(R)2
complexes were not luminescent from a MLCT excited state
in a fluid solution.12c
In Pt(II) complexes, emission from a CT state is often
quenched by the presence of a deactivating d-d state close
in energy. The strategy commonly adopted for the synthesis
of luminescent Pt(II) compounds typically employs high-
field ligands such as alkynyl, dithiolate, or cyclometalated
phenylpyridine type ligands in order to raise the energy of
the deactivating d-d excited state. While 2 and 3 contain
strong-field ligands, the luminescence from a CT state of
the low-field ligand complex 1 and the results of the
calculations suggest that the mesBIAN π* system lies well
below the d orbitals in all three cases reported herein, largely
forming the LUMO and thereby allowing the generation of
a MMLL′CT excited state that is sufficiently below any d-d
states to avoid becoming deactivated. The shift of the lowest
singlet absorption maximum from 390 to 470 to 518 nm in
Pt(NN)Cl2 complexes along the sequence of NN ligands bipy
Figure 2. Simulated and experimental X-band ESR spectra of 1-.
i
to Pr-DAB to mesBIAN also supports this interpretation.
To summarize, employing a highly conjugated rigid
diimine ligand with a low-lying LUMO allows Pt(II)
compounds even with weak-field ligands to possess lowest
CT excited states, which exhibit relatively long-lived red
luminescence in a fluid solution at room temperature. To
the best of our knowledge, 1 is the first example of a
platinum diimine dichloride complex that emits in a fluid
solution at room temperature. Further studies on the sub-
classes of dithiolate, alkynyl, halide, and pseudohalide
complexes originating from the parent mesBIAN complex,
to explore near-IR emission tuneability, are ongoing.
Figure 3. Computed HOMO (left) and LUMO (right) of 1.
ESR spectra (Table 2) but with reduced Pt hyperfine coupling
consistent with a smaller metal-derived component.
The calculations also show that the HOMOs of 1-3 are
the antibonding combination of the metal dxz orbital with
the pz or π orbitals of the ligands LL. The precise distribution
of the orbital over the metal and ligands varies between
compounds, being 44, 32, and 26% metal-based for 1-3,
respectively, with contributions of 20, 27, and 8% from the
bonded atoms of the ligands L. The total contribution from
each alkynyl ligand in 3 is 35%, and that from the
dithiooxalate ligand in 2 is 62%. The calculated orbitals
suggest that the complexes’ HOMOs are mixed metal-ligand
in nature and the LUMOs are mainly ligand-based, and thus
the excited-state responsible for the observed lowest-energy
absorptions and the emission of 1-3 is best regarded as
MMLL′CT.
Acknowledgment. We thank the University of Bristol
(C.J.A. and N.F.) and the EPSRC (J.A.W.) for funding, Prof.
M. D. Ward for access to the Perkin-Elmer LS50B, and
Johnson-Matthey for a loan of Pt salts. We also thank Drs.
Eric McInnes and Joris van Slageren for helpful discussions.
Supporting Information Available: Details of synthetic pro-
cedures for 1-3, of computational studies, and of optical measure-
ments. This material is available free of charge via the Internet at
http://pubs.acs.org.
It is worth comparing our results to those of Klein and
co-workers, who have studied Pt(iPrDAB)(R)2 compounds,
where R was a variety of saturated and unsaturated organic
ligands and iPrDAB is N,N′-diisopropyl-1,4-diazabutadiene,
an analogue of mesBIAN without the rigid aromatic back-
bone.12 When R was a π donor such as acetylide, they found
frontier orbitals of character and symmetry similar to those
of 1 and also concluded that the lowest-energy transition in
IC060399D
(12) (a) Klein, A.; Slageren, J. v.; Za´lisˇ, S. Inorg. Chem. 2002, 41, 5216-
5225. (b) Klein, A.; Schurr, T.; Za´lisˇ, S. Z. Anorg. Allg. Chem. 2005,
631, 2669-2676. (c) Klein, A.; Slageren, J. v.; Za´lisˇ, S. Eur. J. Inorg.
Chem. 2003, 1917-1928. (d) Kaim, W.; Klein, A.; Hasenzahl, S.;
Stoll, H.; Za´lisˇ, S.; Fiedler, J. Organometallics 1998, 17, 237-247.
Inorganic Chemistry, Vol. 45, No. 16, 2006 6107