Luminescent, three-coordinate azadipyrromethene complexes of d10 copper, silver, and gold

Article abstract of DOI:10.1021/ic700776t

Boron azadipyrromethenes are red-light-absorbing dyes with chromophoric capabilities deriving from a conjugated, chelating framework. Reported here are tricoordinate copper(l), silver(l), and gold(l) complexes of a tetraphenylazadipyrromethene ligand. The new complexes are characterized by optical absorption and emission spectroscopy, multinuclear NMR, mass spectrometry, elemental analysis, and X-ray diffraction crystallography. Time-dependent density functional theory calculations indicate that the principal absorption features in azadipyrromethene complexes result from optically allowed intraligand transitions that undergo configuration interaction.

Full text of DOI:10.1021/ic700776t

Inorg. Chem. 2007, 46, 6218−6220
Luminescent, Three-Coordinate Azadipyrromethene Complexes of d10
Copper, Silver, and Gold
Thomas S. Teets,† David V. Partyka,† Arthur J. Esswein,‡ James B. Updegraff, III,† Matthias Zeller,§
Allen D. Hunter,§ and Thomas G. Gray*,†
Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106, Department
of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and
Department of Chemistry, Youngstown State UniVersity, Youngstown, Ohio 44555
Received April 23, 2007
Scheme 1. Syntheses of 2-4a
Boron azadipyrromethenes are red-light-absorbing dyes with
chromophoric capabilities deriving from a conjugated, chelating
framework. Reported here are tricoordinate copper(I), silver(I), and
gold(I) complexes of a tetraphenylazadipyrromethene ligand. The
new complexes are characterized by optical absorption and
emission spectroscopy, multinuclear NMR, mass spectrometry,
elemental analysis, and X-ray diffraction crystallography. Time-
dependent density functional theory calculations indicate that the
principal absorption features in azadipyrromethene complexes result
from optically allowed intraligand transitions that undergo config-
uration interaction.
a Isolated yields are in parentheses.
Recently, mercury(II) ion sensing was demonstrated with a
BF2-azadipyrromethene flanked with o-pyridyl donors.6
The optical properties of boron azadipyrromethenes derive
+
from the delocalized π network; the bound BF2 moiety is
Boron azadipyrromethenes are long known,1 but their
optical properties have only lately provoked study.2 These
chromophores absorb visible light intensely, with absorption
maximizing between 640 and 680 nm. Quantum yields of
the ensuing fluorescence range from 0.23 to 0.36. These
values increase upon rigidification of the chromophoric
skeleton.3 Their photoresponsivity toward red light com-
mends azadipyrromethenes for applications in vivo, including
drug discovery, assays, sensing, tumor imaging, and photo-
dynamic therapy.4,5 Tetraarylazadipyrromethene chelates of
BF2+ exhibit triplet excited-state properties upon bromination
at carbon. Oxygen sensitization was observed, as was in vitro
photokilling of human cervical carcinoma (HeLa) cells.2
saturated and not itself photoactive. The full potential of
azadipyrromethenes as chelating ligands is unrealized.
Presented here are synthetic, structural, and preliminary
optical studies of (Ph3P)MI complexes (M ) Cu, Ag, Au)
of a tetraphenyl-substituted azadipyrromethene. All metal
centers are three-coordinate in the solid state, and the gold
complex is an unusual example of trigonally coordinated
gold(I).7,8 The complexes substantially retain the absorption
+
characteristics of the free ligand and its BF2 chelate.
Reaction of (triphenylphosphine)copper(I) triflate [pre-
pared in situ from (CuOTf)2‚toluene] or (triphenylphosphine)-
silver(I) triflate9 with 3,5-diphenyl-1H-pyrrol-2-yl-3,5-
diphenylpyrrol-2-ylideneamine (tetraphenylazadipyrromethene,
1)2b in the presence of diisopropylethylamine in tetrahydro-
furan (THF) affords the corresponding (Ph3P)MI azadipyr-
romethene complexes in good yields (Scheme 1). Attempted
generation of a (phosphine)gold(I) azadipyrromethene com-
* To whom correspondence should be addressed. E-mail: tgray@case.edu.
† Case Western Reserve University.
‡ Massachusetts Institute of Technology.
§ Youngstown State University.
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6218 Inorganic Chemistry, Vol. 46, No. 16, 2007
10.1021/ic700776t CCC: $37.00
© 2007 American Chemical Society
Published on Web 07/14/2007

COMMUNICATION
Figure 2. Absorption (blue) and emission (550-nm excitation, red) spectra
of gold complex 4 in deoxygenated CHCl3 (298 ( 2 K).
Backbone C-Nmeso-C angles in metallaazadipyrromethenes
are distended compared to those in difluoroboron complexes.
O’Shea and co-workers2a,b report crystal structures of
two BF2-azadipyrromethenes. In these, the C-Nmeso-C
angles are 119.7(3)° and 119.5(3)°. In the group 11 com-
plexes reported here, this angle has widened: the range is
127.46(12)° in the gold complex 4 to 130.1(2)° in one
independent molecule of 3. The azadipyrromethene chro-
mophore is sufficiently flexible that chelation distorts the
remote nitrogen from idealized sp2 hybridization.
Metal ions within structurally characterized azadipyr-
romethene complexes are trigonal planar, but the coordination
plane is canted relative to the plane of the azadipyrromethene
complex. Here the canting angle Θ is defined as the angle
between the best-fit plane of the ligand (excluding substituted
phenyl groups) and the plane of the two chelating nitrogens
and metal(I). For 4, Θ is 29.8°. This is the largest such tilt
encountered here; the smallest is 18.3° for one crystallo-
graphically independent molecule of 3. Ligand bite angles,
defined as N-M-N, are 94.49(6)° for 2, 86.30(7)°,
82.89(7)°, and 82.20(6)° for the three independent molecules
of 3, and 81.51(4)° for 4.
Figure 1. (a) Thermal ellipsoid projection of 4. (b) Packing diagram of 4
along b. An intermolecular π-stacking interaction between distal phenyl
groups of neighboring molecules is indicated. Data were collected at 100
( 2 K; ellipsoids are shown at 50% probability. Hydrogen atoms are omitted
for clarity.
plex by dehalogenation of (phosphine)gold(I) halides gave
no evidence of binding. However, the reaction of sodium
tert-butoxide with azadipyrromethene 1 and (triphenylphos-
phine)gold(I) chloride in a 1:10 (v:v) THF/toluene mixture
yields gold(I) azadipyrromethene 4. 1H NMR measurements
in CDCl3 show that azadipyrromethene resonances are largely
insensitive to metal-ion complexation. The most conspicuous
change is a 0.3-ppm upfield shift of the ortho and meta
protons of the proximal azadipyrromethene phenyl substit-
uents (i.e., the phenyl groups deriving from acetophenone).2a,b
31P{1H} NMR exhibits singlet resonances at δ 3.8 (2), 14.4
(3), and 30.7 (4) ppm. All three products are deep blue in
solution; in the solid state, they appear violet, black, or (for
4) gold.
Vapor diffusion of pentane into concentrated THF solu-
tions affords diffraction-quality platelike crystals.10 Figure
1a depicts a thermal ellipsoid plot of gold complex 4.
Structures of 2 and 3 appear as Supporting Information.
Trigonal-planar coordination of gold is evident. Au-N bond
lengths are 2.2349(12) and 2.2448(12) Å. The near-C2V
microsymmetry of gold in 4 contrasts with the asymmetric
structure of [(Ph3P)Au(bpy)](PF6), where Au-N distances
are 2.166 and 2.406 Å (estimated standard deviations are
said to range from 0.001 to 0.003).11 At 2.2088(3) Å, the
Au-P bond length is unremarkable.12-14 Structural com-
parisons with copper(I) and silver(I) bipyridine and phenan-
throline complexes suggest that metal-ligand bond metrics
in 2 and 3 (Supporting Information) are normal.15,16 Aza-
dipyrromethene complexes undergo intermolecular π stack-
ing in the crystalline state. For example, symmetry-related
molecules of 4 approach within 3.69 Å (Figure 1b).
Arguably the leading feature of azadipyrromethenes is
their absorption of orange or red light, at wavelengths to
which human tissue is partly transparent. Figure 2 depicts
(10) Crystallographic data for 4: violet irregular chunk, crystal dimensions
0.44 × 0.42 × 0.18 mm3, space group P21/n, a ) 12.3616(2) Å, b )
12.9923(2) Å, c ) 23.7047(5) Å, â ) 90.6780(10)°, V ) 3806.84-
(12) Å3, Z ) 4, Fcalc ) 1.584 g m-3, µ(Mo KR) ) 3.947 mm-1, data
measured on a Bruker AXS SMART APEX II CCD-based diffrac-
tometer (Mo KR, λ ) 0.710 73 Å) at 100(2) K; structure solved by
direct methods, 119 551 reflections collected, 10 670 independent
reflections (R(int) ) 0.0232), data/restraints/parameters 10 670/0/496,
final R indices [I > 2σ(I)] R1 ) 0.0157 and wR2 ) 0.0401, R indices
(all data) R1 ) 0.0180 and wR2 ) 0.0413, largest difference peak
and hole +1.177 and -0.560 e Å-3. Data for other complexes appear
as Supporting Information.
(11) Clegg, W. Acta Crystallogr. 1976, B32, 2712-2714.
(12) Partyka, D. V.; Zeller, M.; Hunter, A. D.; Gray, T. G. Angew. Chem.,
Int. Ed. 2006, 45, 8188-8191.
(13) Partyka, D. V.; Updegraff, J. B.; Zeller, M.; Hunter, A. D.; Gray, T.
G. Organometallics 2007, 26, 183-186.
(14) Schmidbaur, H., Ed. Gold: Progress in Chemistry, Biochemistry and
Technology; John Wiley & Sons: Chichester, U.K., 1999.
(15) (a) Zhang, L.; Chen, C.; Zhang, Q.; Zhang, H.; Kang, B. Acta
Crystallogr. 2003, E59, m536-m537. (b) Khalaji, A. D.; Amirnasr,
M.; Aoki, K. Anal. Sci. 2006, 32, x87-x88.
(16) (a) Hamilton, C. W.; Laitar, D. S.; Sadighi, J. P. Chem. Commun.
2004, 1628-1629. (b) Cunningham, C. T.; Moore, J. J.; Cunningham,
K. L. H.; Fanwick, P. E.; McMillin, D. E. Inorg. Chem. 2000, 39,
3638-3644.
Inorganic Chemistry, Vol. 46, No. 16, 2007 6219

COMMUNICATION
absorption and emission spectra of the representative com-
plex 4. Spectra of all complexes appear as Supporting
Information. An intense absorption at 597 nm (ꢀ ∼ 30 000
M-1 cm-1; CHCl3) is characteristic of the free azadipyr-
romethene. This absorption red shifts slightly (1-19 nm)
upon metalation. It also intensifies markedly: for silver
complex 3, the extinction coefficient at λmax more than
doubles (ꢀ ) 65 000 M-1 cm-1; CHCl3). Resolution is
improved at 77 K in 2-methyltetrahydrofuran (2-MeTHF)
glasses. At low temperature, distinct absorption maxima
emerge for 4 at 576 and 617 nm; a third transition appears
as a shoulder near 637 nm. Broadly similar observations
pertain to copper and silver analogues 2 and 3.
Compounds 2-4 are luminescent in fluid solution at room
temperature and at 77 K in 2-MeTHF. Figure 2 reproduces
the 298 K emission spectrum of 4 in chloroform; spectra of
1-3 are deposited as Supporting Information. The mean
Stokes shift for the four compounds is 41 nm, and an
approximate mirror-image symmetry relates the emission
profile and the absorption features near 600 nm. Room-
temperature emission quantum yields ((10%) for the three
complexes are 0.0025 (2), 0.0039 (3), and 0.0024 (4); these
are comparable to a 0.0014 emission quantum yield of ligand
1. Structured luminescence appears in 2-MeTHF glass at 77
K (Supporting Information) for 2-4. The average peak-to-
peak separations for 2 and 4 are 657 and 651 cm-1,
respectively, consistent with vibronic activation of azadipyr-
romethene skeletal modes.17 For silver complex 3, two 77
K emissions are resolved at 642 and 693 nm, with the longer-
wavelength emission showing vibronic progression (mean
peak-to-peak spacing of 610 cm-1).
Figure 3. Partial Kohn-Sham orbital energy-level diagram of 3′, the H3-
PAgI complex of ligand 1. Plots of selected orbitals are inset (contour level
0.03 au). Irreducible representations of orbitals in idealized C2V symmetry
appear at the right.
r HOMO-2, is the primary component of a second allowed
B2 excitation, calculated at 525 nm, which undergoes
configuration interaction with the 1B2 state that is the lowest
singlet excited state. For 3′, an absorption transition calcu-
lated at 541 nm also contributes absorption intensity (LUMO
1
r HOMO-1; A1 state). The calculations indicate that
optically allowed intraligand transitions govern the electronic
spectra of azadipyrromethene chelates. Low-lying singlet
excited states undergo configuration interaction with oscil-
lator strengths determined by constitutive transition dipole
vectors.
Scalar-relativistic density functional theory (DFT) and
time-dependent DFT calculations18 on 3′, a truncated model
of 3, were undertaken. Here, PH3 ligands replace PPh3 for
computational tractability; all computations preserve the full
tetraphenylazadipyrromethene. The calculations find the
highest occupied Kohn-Sham orbital (HOMO) and the
lowest unoccupied Kohn-Sham orbital (LUMO) to be
centered on the azadipyrromethene and similarly for the
HOMO-1 and HOMO-2. These frontier orbitals bear
minimal silver character, each being derived from a corre-
sponding a2 (HOMO and HOMO-2) and b1 (LUMO and
HOMO-1) orbital of the free ligand. Figure 3 depicts a
partial Kohn-Sham orbital energy level diagram of the
(phosphine)silver(I) adduct of 3′. Vertical excitation energies
were calculated for optimized geometries. Time-dependent
DFT calculations on 3′ predict an intense absorption near
591 nm having two constituent excitations. These are the
LUMO r HOMO and LUMO r HOMO-2 one-electron
transitions. In idealized C2V symmetry, both are b1 r a2
excitations, leading to B2 states that engage in configuration
interaction. The transition dipole vector associated with the
LUMO r HOMO transition dominates the lowest-energy
vertical excitation. The other one-electron transition, LUMO
In conclusion, copper(I), silver(I), and gold(I) complexes
of azadipyrromethene ligands have been prepared and
structurally characterized. Metal chelates of azadipyr-
+
romethenes share the qualitative absorption features of BF2
complexes. The new complexes are weakly luminescent in
solution at room temperature. At 77 K, a vibronic structure
appears in the emission profile. Time-dependent DFT
calculations indicate allowed transitions to 1B2 states (in C2V
symmetry) that undergo configuration interaction. Further
experiments, including cellular uptake studies, are ongoing.
Acknowledgment. The authors thank Case Western
Reserve University (CWRU) and the donors of the Petroleum
Research Fund, administered by the American Chemical
Society (Grant 42312-G3 to T.G.G.), for support. The
diffractometer at CWRU was funded by NSF Grant
CHE0541766 and that at Youngstown State University
(YSU) by NSF Grant 0087210, by the Ohio Board of Regents
Grant CAP-491, and by YSU. T.G.G. thanks Professor D.
G. Nocera (Massachusetts Institute of Technology) for access
to instrumentation.
Supporting Information Available: Full experimental proce-
dures and crystallographic (CIF) and computational data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(17) Li, X.-Y.; Czernuszewicz, R. S.; Kinkaid, J. R.; Su, O.; Spiro, T. G.
J. Phys. Chem. 1990, 94, 31-47.
(18) Full computational details appear as Supporting Information.
IC700776T
6220 Inorganic Chemistry, Vol. 46, No. 16, 2007