Luminescent dipyrrinato complexes of trivalent group 13 metal ions

Article abstract of DOI:10.1021/ic061581h

Although free dipyrrins (dipyrromethenes) do not strongly luminesce, certain dipyrrinato complexes of BF₂ and zinc(II) are known to be intensely luminescent species. Two new dipyrrinato fluorophores, based on complexes with gallium(III) and indium(III), are described. Using a previously described meso-mesityl-substituted dipyrrin, namely 5-mesityldipyrrin (mesdpm), the complexes [Ga(mesdpm)₃] and [In(mesdpm)₃] were prepared and structurally characterized. The complexes display the expected octahedral geometry about the metal ions. In some solvents, such as hexanes, the complexes emit green light upon excitation with UV light at room temperature, with quantum yields of 2.4% ([Ga(mesdpm)₃]) and 7.4% ([In(mesdpm)₃]) and lifetimes in the low nanosecond range. Observations are consistent with assignment to ligand-localized transitions, and this interpretation is further confirmed by density functional calculations described herein. The new complexes are important additions to the widely used family of dipyrrin-based fluorescent species and show that dipyrrinato complexes containing metals other than BF₂ and zinc(II) may be useful fluorophores.

Full text of DOI:10.1021/ic061581h

Inorg. Chem. 2006, 45, 10688−10697
Luminescent Dipyrrinato Complexes of Trivalent Group 13 Metal Ions
Van S. Thoi, Jay R. Stork, Douglas Magde,* and Seth M. Cohen*
Department of Chemistry and Biochemistry, UniVersity of California, San Diego, La Jolla,
California 92093-0358
Received August 21, 2006
Although free dipyrrins (dipyrromethenes) do not strongly luminesce, certain dipyrrinato complexes of BF2 and
zinc(II) are known to be intensely luminescent species. Two new dipyrrinato fluorophores, based on complexes
with gallium(III) and indium(III), are described. Using a previously described meso-mesityl-substituted dipyrrin, namely
5-mesityldipyrrin (mesdpm), the complexes [Ga(mesdpm)3] and [In(mesdpm)3] were prepared and structurally
characterized. The complexes display the expected octahedral geometry about the metal ions. In some solvents,
such as hexanes, the complexes emit green light upon excitation with UV light at room temperature, with quantum
yields of 2.4% ([Ga(mesdpm)3]) and 7.4% ([In(mesdpm)3]) and lifetimes in the low nanosecond range. Observations
are consistent with assignment to ligand-localized transitions, and this interpretation is further confirmed by density
functional calculations described herein. The new complexes are important additions to the widely used family of
dipyrrin-based fluorescent species and show that dipyrrinato complexes containing metals other than BF2 and
zinc(II) may be useful fluorophores.
Introduction
of the analysis of several other zinc(II) dipyrrinato deriva-
tives, it was determined that restricted rotation of the meso-
mesityl ring relative to the plane of the dipyrrin chromophore
was crucial for obtaining good emission from these com-
plexes. This hypothesis was further confirmed in an inde-
pendent study, where the formation of a rigid, dimeric
zinc(II) dipyrrinato structure ([Zn2Cl2(2-pyrdpm)2]) also
showed enhanced luminescence (Φf ) 0.057).9
Recently, we have utilized dipyrrinato coordination com-
plexes as metalloligands in the preparation of metal-organic
frameworks (MOFs).10,11 MOFs hold promise as a new
family of materials with potential applications in molecular
storage, sensors, and catalysis.12-14 On the basis of the
aforementioned reports, we were intrigued by the possibility
of preparing fluorescent MOFs derived from dipyrrinato
metal complexes; however, the metalloligands utilized in our
earlier studies are based on 3-fold symmetric tris(dipyrrinato)
complexes of iron(III) and cobalt(III), as opposed to the
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (difluorobo-
rondipyrromethene or BODIPY) dyes represent a group of
organic fluorophores widely used in chemical and biochemi-
cal investigations (Chart 1).1-7 The widespread interest in
BODIPY dyes can be attributed to, among other traits, their
intense absorption/emission features and photochemical
stability. BODIPY dyes were the only fluorescent dipyrrin
(dipyrromethene) species known for some time until recently,
when Lindsey and co-workers showed that a zinc(II) dipyr-
rinato complex, namely [Zn(mesdpm)2] (mesdpm ) 5-mesi-
tyldipyrrin), was highly emissive (Φf ) 0.36).8 On the basis
* To whom correspondence should be addressed. Phone: (858) 822-
5596 (S.M.C.). Fax: (858) 822-5598 (S.M.C.). E-mail: scohen@ucsd.edu
(S.M.C.).
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S. J.; Boyle, R. W. Chem. Commun. 2004, 1328-1329.
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Chem. Commun. 2005, 5506-5508.
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10688 Inorganic Chemistry, Vol. 45, No. 26, 2006
10.1021/ic061581h CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/09/2006

Group 13 Dipyrrinato Complexes
Chart 1. Fluorescent Dipyrrin Compounds.
2-fold symmetric bis(dipyrrinato) complexes described with
zinc(II). With these ideas in mind, we sought to identify a
new class of dipyrrin fluorophores based on trivalent cations
that would support an octahedral geometry and hence
generate a 3-fold symmetric, luminescent metalloligand. We
reasoned that the closed-shell d10 electron configuration of
zinc(II) was likely important to avoid nonradiative deactiva-
tion pathways for the dipyrrinato excited state. This assump-
tion, combined with the success of the boron-based BODIPY
dyes, suggested to us that closed-shell, trivalent Group 13
ions might generate the desired compounds.
The hypothesis that Group 13 cations such as gallium-
(III) and indium(III) might form emissive tris(dipyrrinato)
complexes was further supported by literature reports that
these ions can generate luminescent porphyrin species.
Several studies have reported on the luminescence and
photochemistry of porphyrin and phthalocyanine complexes
with Group 13 metals.15-20 Group 13 tetraphenylporphyrin
(TPP) complexes have been reported with quantum yields
ranging from 0.0037 to 0.11 in a variety of organic solvents,15
and phthalocyanine complexes have shown quantum yields
up to 0.58.20 Interestingly, in these planar systems, quantum
yields follow the trend aluminum(III) > gallium(III) >
indium(III). As synthetic progenitors of porphyrins, dipyr-
rinato ligands were anticipated to form Group 13 complexes
with interesting photophysical characteristics. Indeed, a prior
patent application does make reference to aluminum(III)
dipyrrinato complexes as fluorophores,21 although this docu-
ment lacks any detailed description of these compounds.
This report discusses the synthesis, structure, and photo-
physical properties of tris(5-mesityldipyrrinato)gallium(III)
and tris(5-mesityldipyrrinato)indium(III) ([M(mesdpm)3], M
) Ga3+, In3+). Although not as intense as their boron
difluoride and zinc(II) analogues,8 these metal ions do
activate the dipyrrin fluorophore to produce a visible green
emission. These complexes represent the only known dipyr-
rinato fluorophores aside from the boron and zinc(II) adducts.
In their present formulation, these metal complexes lack the
pendant donor groups to form MOFs, but they demonstrate
that such metalloligands can be prepared and appropriately
derivatized to obtain a building block with the desired
symmetry and fluorescent properties. The complexes de-
scribed here may find use in self-assembling antenna
pigments,8,22 light-harvesting complexes,1 and serve to lay
the foundation for the synthesis of luminescent MOFs based
on a new family of dipyrrinato fluorophores.
Experimental Section
General. Unless otherwise noted, starting materials were ob-
tained from commercial suppliers and used without further purifica-
tion. 5-Mesityldipyrromethane and 4-cyanophenyldipyrromethane
were synthesized according to literature procedures.11,23 Mass
spectrometry was performed at the University of California, San
Diego Mass Spectrometry Facility in the Department of Chemistry
and Biochemistry. A ThermoFinnigan LCQ-DECA mass spectrom-
eter was used for ESI or APCI analysis, and the data were analyzed
using the Xcalibur software suite. A ThermoFinnigan MAT 900XL
mass spectrometer with a fast-atom bombardment (FAB) source
was used to acquire the data for the high-resolution mass spectra
(HRMS). Infrared spectra were collected on a Mattson Research
Series FT-IR instrument (using KBr pellets or NaCl plates,
laboratory of Prof. Clifford P. Kubiak) at the Department of
Chemistry and Biochemistry, University of California, San Diego.
1H/13C NMR spectra were recorded on Varian FT-NMR spectrom-
eters at the Department of Chemistry and Biochemistry, University
of California, San Diego. UV-visible spectra were recorded in
hexanes using a Perkin-Elmer Lambda 25 spectrophotometer with
the UVWinLab 4.2.0.0230 software package. Absorbance spectra
are reported as λmax/nm (ꢀ/M-1 cm-1).
(15) Ohno, O.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1985, 82, 1779-
1787.
(16) Ebeid, E. M.; Habib, A. M.; Abdel-Kader, M. H.; Yousef, A. B.;
Guilard, R. Spectrosc. Acta 1988, 44A, 127-130.
(17) Kadish, K. M.; Maiya, G. B.; Xu, Q. Y. Inorg. Chem. 1989, 28, 2518-
2523.
(18) Ishii, K.; Abiko, S.; Kobayashi, N. Inorg. Chem. 2000, 39, 468-472.
(19) Harvey, P. D.; Proulx, N.; Martin, G.; Drouin, M.; Nurco, D. J.; Smith,
K. M.; Bolze, F.; Gros, C. P.; Guilard, R. Inorg. Chem. 2001, 40,
4134-4142.
(20) Brannon, J. H.; Magde, D. J. Am. Chem. Soc. 1980, 102, 62-65.
(21) Toguchi, I.; Ishikawa, H.; Morioka, Y.; Oda, A. Organic electrolu-
minescent component; 2000.
5-Mesityldipyrrin. A solution of 5-mesityldipyrromethane23 (231
mg, 0.88 mmol) in CH2Cl2 (50 mL) was oxidized with 2,3-dicyano-
5,6-dichloroparabenzoquinone (DDQ, 198 mg, 0.88 mmol) in
benzene (25 mL) for 15 min at 0 °C. Once oxidation was complete
(22) Yu, L. H.; Muthukumaran, K.; Sazanovich, I. V.; Kirmaier, C.; Hindin,
E.; Diers, J. R.; Boyle, P. D.; Bocian, D. F.; Holten, D.; Lindsey, J.
S. Inorg. Chem. 2003, 42, 6629-6647.
(23) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.; Lindsey,
J. S. Org. Proc. Res. DeV. 2003, 7, 799-812.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10689

Thoi et al.
(as gauged by TLC), [Cu(acac)2] (acac ) acetylacetonate, 114 mg,
0.44 mmol) was added to the solution and the mixture was stirred
for ∼4 h at room temperature. The solvent was removed using a
rotary evaporator, and the resulting residue was purified by flash
column chromatography (SiO2, CH2Cl2) to afford bis(5-mesityl-
dipyrrinato)copper(II) as a red solid after removal of solvent (166
mg, 0.28 mmol). Bis(5-mesityldipyrrinato)copper(II) was dissolved
in THF (50 mL) and stirred overnight with KCN (187 mg, 2.8
mmol) in H2O (20 mL). After the mixture was stirred overnight,
the THF was removed using a rotary evaporator, and the free
5-mesityldipyrrin was extracted with 3 × 50 mL CH2Cl2 to yield
the product as a dark yellow solid (167 mg). Yield: 47% (based
on isolated bis(5-mesityldipyrrinato)copper(II)). mp 106-109 °C.
1H NMR (300 MHz, CDCl3): δ 7.62 (s, 2H), 6.93 (s, 2H), 6.40
(d, J ) 4.4 Hz, 2H), 6.33 (d, J ) 4.4 Hz, 2H), 2.36 (s, 3H), 2.10
(s, 6H). APCI-MS: m/z 263.14 [M]+, 264.14 [M + H]+. HR-EIMS
Calcd for C18H18N2: 262.1465. Found: 262.1464. λabs (hexanes):
285, 424 nm.
(30 mL) was added to a solution of 5-(4′-cyanophenyl)dipyrrin (186
mg, 0.75 mmol) in CH3CN (100 mL), and the solution was heated
to reflux overnight. After the removal of the solvent on a rotary
evaporator, the remaining solid was triturated with hexanes and an
orange precipitate was isolated by vacuum filtration (68 mg).
Yield: 33%. 1H NMR (300 MHz, CDCl3): δ 7.37 (d, J ) 8.2 Hz,
6H), 7.55 (d, J ) 8.2 Hz, 6H), 6.86 (d, J ) 3.6 Hz, 6H), 6.50 (d,
J ) 4.4 Hz, 6H), 6.29 (d, J ) 3.9 Hz, 6H). FAB-MS: m/z 852.1
[M]+, 853.1 [M + H]+. λbs(hexanes): 448, 496 nm.
X-ray Crystallographic Analysis. Single crystals of each
compound suitable for X-ray diffraction structural determination
were mounted on nylon loops with Paratone oil and were cooled
in a nitrogen stream on the diffractometer. Data were collected on
either a Bruker AXS or a Bruker P4 diffractometer, each equipped
with area detectors. Peak integrations were performed with the
Siemens SAINT software package. Absorption corrections were
applied using the program SADABS.24 Space group determinations
were performed by the program XPREP. The structures were solved
and refined with the SHELXTL software package (Sheldrick, G.
M. SHELXTL Vers. 5.1 Software Reference Manual; Bruker AXS:
Madison, WI, 1997). [Ga(mesdpm)3] was solved by direct methods,
while [In(mesdpm)3] and [Ga(4-cydpm)3] were solved by Patterson
methods. All hydrogen atoms were fixed at calculated positions
with isotropic thermal parameters, and all non-hydrogen atoms were
refined anisotropically unless otherwise noted. X-ray crystal-
lographic data is available from the Cambridge Crystallographic
Data Centre (http://www.ccdc.cam.ac.uk). Refer to CCDC reference
numbers 617591, 617592, and 617593.
Tris(5-mesityldipyrrinato)gallium(III) ([Ga(mesdpm)3]). A
solution of purified 5-mesityldipyrrin (75 mg, 0.28 mmol) in CH3-
CN (5 mL) was stirred with a solution of Ga(NO3)3 (23.0 mg, 0.09
mmol) in MeOH (0.5 mL) at room temperature overnight. The
mixture was then refrigerated overnight, and an orange precipitate
was isolated by vacuum filtration (20 mg). Yield: 26%. mp >260
1
°C. H NMR (400 MHz, CDCl3): δ 6.92 (s, 6H), 6.91 (s, 6H),
6.50 (d, J ) 4.4 Hz, 6H), 6.17 (d, J ) 3.7 Hz, 6H), 2.36 (s, 9H),
2.08 (s, 18H). FAB-MS: m/z 852.1 [M]+, 853.1 [M+H]+. HR-
EIMS Calcd for C54H51N6Ga: 852.3426. Found: 852.3437. λabs
(hexanes): 448 (87 000), 496 (68 000). λem (hexanes): 528 nm,
Φf ) 0.024(4).
Steady-State Fluorescence and Quantum Yield Determina-
tions. Fluorescence spectra were recorded on a Perkin-Elmer LS-
55 luminescence spectrometer. Relative fluorescence quantum
yields, Φf, were determined in hexanes or toluene by the optically
dilute method from eq 1,25
Tris(5-mesityldipyrrinato)indium(III) ([In(mesdpm)3]). The
same procedure was followed as in the synthesis of tris-(5-
mesityldipyrrinato)gallium, utilizing purified 5-mesityldipyrrin (75
mg, 0.28 mmol) and InCl3 (20 mg, 0.09 mmol), giving the product
IR nf2 Df AR
1
as a yellow solid (15 mg). Yield: 19%. mp >260 °C. H NMR
Φf ) ΦR
(1)
2
(400 MHz, CDCl3): δ 7.15 (s, 6H), 6.90 (s, 6H), 6.53-6.52 (d, J
) 4.4 Hz, 6H), 6.25-6.24 (d, J ) 4.4 Hz, 6H), 2.36 (s, 9H), 2.04
(s, 18H). FAB-MS: m/z 898.8 [M]+. HR-EIMS Calcd for C54H51N6-
In: 898.3209. Found: 898.3217. λabs (hexanes): 444 (117 000),
496 (60 100). λem (hexanes): 522 nm, Φf ) 0.074(6).
(I )( )(D )(A )
f
nR
R
f
where I is excitation intensity, n is the refractive index of the
solution, D is the integrated emission intensity, and A is the
absorption at the excitation wavelength for the standard reference
(R) and sample (f), respectively. Fluorescein in a solution of ethanol
and 10-4 M triethylamine served as the reference (λR ) 0.91).26
Briefly, for each quantum yield determination, emission spectra of
the sample and reference solutions were recorded at the same
excitation wavelengths (λex ) 451 for [Ga(mesdpm)3]; λex ) 445
for [In(mesdpm)3]) using identical instrument configurations to
reduce error from instrumental artifacts. Solutions were prepared
by adjusting the concentrations of the reference and sample solutions
to obtain an absorbance of 0.5 at the desired excitation wavelength.
The solutions were then diluted 10-fold to an assumed absorbance
of 0.05 before recording emission spectra. At least three independent
measurements were made. The emission spectra were integrated
with Origin 7.0 software (OriginLab Corporation).27 Excitation
spectra were recorded at 530 nm so that there would be additional
spectral overlap with the emission spectra. Slits were set at 4.0
(excitation slit) and 4.2 nm (emission slit). The excitation mono-
chromator was scanned from 275 to 520 nm (the sweep width was
5-(4′-Cyanophenyl)dipyrrin (4-cydpm). A solution of 5-(4′-
cyanophenyl)dipyrromethane11,23 (254 mg, 1.03 mmol) in CH2Cl2
(250 mL) was oxidized with DDQ (234 mg, 1.03 mmol) in benzene
(200 mL) for 1.5 h at 0 °C. Once oxidation was complete (as gauged
by TLC), the volume was reduced by half on a rotary evaporator,
and [Cu(acac)2] (135 mg, 0.51 mmol) was added to the solution
and the mixture was stirred for ∼4 h at room temperature. The
solvent was removed using a rotary evaporator, and the resulting
residue was purified by flash column chromatography (SiO2, CH2-
Cl2) to afford bis(5-(4′-cyanophenyldipyrrinato))copper(II) as a red
solid after removal of solvent (229 mg, 0.42 mmol). Bis-(5-(4′-
cyanophenyl)dipyrrinato)copper(II) was dissolved in THF (50 mL)
and stirred overnight with KCN (375 mg, 5.7 mmol) in H2O (20
mL). After the mixture was stirred overnight, the THF was removed
using a rotary evaporator, and the free 5-(4′-cyanophenyl)dipyrrin
was extracted with 3 × 50 mL CH2Cl2 to yield a dark brown solid
(186 mg). Yield: 73% (based on isolated bis(5-(4′-cyanophenyl)-
dipyrrinato)copper(II)). 1H NMR (300 MHz, CDCl3): δ 7.75 (d, J
) 7.7 Hz, 2H), 7.67 (s, 2H), 7.61 (d, J ) 8.2 Hz, 2H), 6.47 (d, J
) 4.4 Hz, 2H), 6.41 (d, J ) 4.4 Hz, 6H). APCI-MS: m/z 246.2
[M + H]+.
(24) Sheldrick, G. M. Acta Crystallogr., Sect. A 1995, A51, 33-38.
(25) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024.
(26) Magde, D.; Wong, R.; Seybold, P. G. Photochem. Photobiol. 2002,
75, 327-334.
(27) Origin 7.0 SR0, v7.0220 (B220); OriginLab corporation: Northampton,
MA, 1991-2002.
Tris(5-(4′-cyanophenyl)dipyrrinato)gallium(III) ([Ga(4-cy-
dpm)3]). A solution of Ga(NO3)3 (66 mg, 0.26 mmol) in MeOH
10690 Inorganic Chemistry, Vol. 45, No. 26, 2006

Group 13 Dipyrrinato Complexes
Scheme 1. Synthesis of [Ga(mesdpm)3] and [In(mesdpm)3].
in situ after oxidation of the dipyrromethane precursor.10,32-35
The transition metal complexes of these ligands are generally
very stable and can be purified by using conventional flash
chromatography. Attempts to apply in situ synthetic methods
to the Group 13 ions of gallium(III) and indium(III) failed
to generate the desired tris(dipyrrinato) complexes (Scheme
1). It became apparent that the gallium(III) and indium(III)
complexes, if forming in the oxidation reaction mixture, were
not stable to column chromatography. With this obstacle in
mind, a new synthetic route to the complexes was required.
Because chromatographic purification was not possible, the
complexes needed to be prepared from purified dipyrrins,
so that the complexes could be formed with minimal
impurities and isolated by precipitation or crystallization.
However, in our experience, free base dipyrrins are difficult
to isolate in a pure form. A literature report has described
isolation of the pure dipyrrin by decomposition of the copper-
(II), zinc(II), and palladium(II) complexes by using 1,4-
dithiothreitol (DTT) under neutral conditions.22 Although a
versatile procedure, this method proved somewhat cumber-
some in our hands and required column chromatography to
obtain the desired quantities of purified dipyrrin. On the basis
of procedures used to remove copper from phenanthroline
ligands,36,37 a route was devised using KCN to strip copper-
(II) from the readily purified bis(dipyrrinato)copper(II)
complexes. After removal of copper by KCN, the dipyrrin
truncated to avoid first and second order Rayleigh peaks) with a
scan speed of 200 nm/min.
Time-Resolved Measurements. Fluorescence lifetimes were
measured by time-correlated photon counting (TCPC). Details of
the experimental apparatus and design can be found in the
Supporting Information.
Computational Methods. All DFT calculations were performed
using the Gaussian 03 package28 on a Linux cluster at the W. M.
Keck Laboratory for Integrated Biology II, at the University of
California, San Diego. Molecular geometries were optimized in the
gas phase with the B3LYP hybrid functional, which incorporates
Becke’s three-parameter nonlocal exchange potential and the
nonlocal correlation functional of Lee, Yang, and Parr.29,30 The
geometries were initially optimized with the 3-21G* basis set,
starting from the crystallographically determined atomic positions
for [Ga(mesdpm)3], [In(mesdpm)3], and bis-(5-phenyldipyrrinato)-
zinc(II).22 The meso-aryl groups were replaced with hydrogen
atoms, thus exchanging the mesdpm ligands with unsubstituted
dipyrrin (dpm) groups to reduce the computational time. Full
optimizations were performed using the 6-31G* basis set for [Ga-
(dpm)3] and [Zn(dpm)2] and the LanL2DZ basis set for [In(dpm)3],
using effective core potentials (ECP) for indium. Molecular structure
and orbital drawings were generated with the MOLEKEL pro-
gram.31
Results
Synthesis and Characterization of Dipyrrinato Com-
plexes. Transition metal complexes of R,â-unsubstituted
dipyrrinato ligands are readily synthesized by complexation
(32) Bru¨ckner, C.; Karunaratne, V.; Rettig, S. J.; Dolphin, D. Can. J. Chem.
1996, 74, 2182-2193.
(33) Bru¨ckner, C.; Zhang, Y.; Rettig, S. J.; Dolphin, D. Inorg. Chim. Acta
1997, 263, 279-286.
(34) Cohen, S. M.; Halper, S. R. Inorg. Chim. Acta 2002, 341, 12-16.
(35) Halper, S. R.; Cohen, S. M. Chem.sEur. J. 2003, 9, 4661-4669.
(36) Chambron, J.-C.; Sauvage, J.-P. J. Am. Chem. Soc. 1997, 119, 9558-
9559.
(28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; et al. Gaussian 03,
reVision B.04; Gaussian, Inc.: Wallingford, CT, 2004.
(29) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(30) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B: Condens. Matter Mater.
Phys. 1988, 37, 785-789.
(31) Flukiger, P.; Luthi, H. P.; Portmann, S.; Weber, J. Molekel 4.3; Swiss
Center for Scientific Computing: Manno, Switzerland, 2000-2002.
(37) Sauvage, J.-P. Acc. Chem. Res. 1990, 23, 319-327.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10691

Thoi et al.
Table 1. X-ray Crystallographic Data for [Ga(4-cydpm)3], [Ga(mesdpm)3], and [In(mesdpm)3]
[Ga(4-cydpm)3]
[Ga(mesdpm)3]
[In(mesdpm)3]
C55H52.50InN6.50
empirical formula
cryst syst
C
48.83H30.83Cl2.49GaN9O0.25
triclinic
C55H52Cl3GaN6
trigonal
trigonal
space group
P1h
P3hc1
unit cell dimensions
P3hc1
a
b
c
14.696(4) Å
16.567(4) Å
20.069(5) Å
95.906(4)°
110.802(4)°
106.542(4)°
4264(2) Å3, 4
0.41 × 0.13 × 0.09 mm3
orange needle
100(2)
23.338(3) Å
23.338(3) Å
15.406(4) Å
90°
90°
120°
23.7036(12) Å
23.7036(12) Å
14.7117(15) Å
90°
R
â
90°
γ
120°
V, Z
7267(2) Å3, 6
0.42 × 0.15 × 0.10 mm3
orange needle
100(2)
7158.5(9) Å3, 6
cryst size
color and habit
T (K)
0.28 × 0.07 × 0.07 mm3
orange needle
100(2)
reflns collected
independent reflns
data/restraint/params
GOF on F2
36 836
59 678
61 573
17 265 [R(int) ) 0.0352]
17 265/0/1129
1.022
R1 ) 0.0462
wR2 ) 0.1093
R1 ) 0.0622
wR2 ) 0.1188
4948 [R(int) ) 0.0344]
4948/6/316
1.046
R1 ) 0.0448
wR2 ) 0.1089
R1 ) 0.0532
wR2 ) 0.1152
4891[R(int) ) 0.0681]
4891/21/317
1.033
R1 ) 0.0411
wR2 ) 0.0933
R1 ) 0.0798
wR2 ) 0.1099
final R indices I > 2σ(I)a
R indices (all data)a
2
2
4
a R1 ) ∑ ||Fo| - |Fc||/ ∑ |Fo| , wR2 ) { ∑ [w(Fo - Fc )2]/ ∑ [wFo ] }1/2Z.
Figure 1. X-ray structures (50% probability ellipsoids) of [Ga(mesdpm)3] (left) and [In(mesdpm)3] (right). Hydrogen atoms and solvent molecules have
been omitted for clarity.
can be separated and isolated in high yield using a simple
extraction. As outlined in Scheme 1, this method is straight-
forward, and we believe will be applicable to a variety of
dipyrrins, providing the purified dipyrrin ligands without
requiring any additional chromatography.
is ongoing. Nevertheless, sufficient quantities of the com-
plexes could be isolated for further characterization and
study.
Structures of Complexes. The compounds [Ga(mes-
dpm)3], [In(mesdpm)3], and [Ga(4-cydpm)3] were crystallized
and examined by single-crystal X-ray crystallography.
Crystal data are given in Table 1. In all cases, the dipyrrin
ligand was similar to those in other dipyrrinato metal
complexes that lack pyrrole substituents.22,32-35,38-40 [Ga-
(mesdpm)3] crystallized as orange needles by slow evapora-
tion from a solution of the complex in deuterated chloroform
(Figure 1). The selected crystal was twinned by a 180°
rotation about the [2 1 0] reciprocal axis. The asymmetric
unit consists of two independent, partial complexes each
residing on a special position. One of the two partial
complexes includes a gallium atom and a single mesdpm
Using the purified 5-mesityldipyrrin, the gallium(III) and
indium(III) complexes were prepared by mixing 3 equiv of
the ligand with 1 equiv of the appropriate metal salt in a
MeOH/CH3CN solvent mixture. UV-visible spectroscopy
shows the reaction to be essentially quantitative; however,
isolation/recrystallization of the product proved to be chal-
lenging. The MeOH/CH3CN solvent mixture was selected
to maximize precipitation of the product from the reaction
mixture, which was vacuum filtered to obtain the pure
complexes in relatively low isolated yields (∼20-25%). The
solubility of the complexes depends on the meso substituent
on the dipyrrin ligand. Therefore, different precipitation/
isolation procedures will likely be required for other tris-
(dipyrrinato) complexes of this type (e.g., see synthesis of
[Ga(4-cydpm)3]). Optimization of these isolation procedures
(38) Halper, S. R.; Cohen, S. M. Angew. Chem., Int. Ed. 2004, 43, 2385-
2388.
(39) Gill, H. S.; Finger, I.; Bozidarevic, I.; Szydlo, F.; Scott, M. J. New J.
Chem. 2005, 29, 68-71.
(40) Do, L.; Halper, S. R.; Cohen, S. M. Chem. Commun. 2004, 2662-
2663.
10692 Inorganic Chemistry, Vol. 45, No. 26, 2006

Group 13 Dipyrrinato Complexes
ligand, while the other partial complex presents a gallium-
(III) ion and one-half of a mesdpm ligand. The full
complexes are generated by crystallographically imposed
symmetry. A half-occupied, deuterated chloroform solvent
molecule is also present and disordered over a special
position. Selected bond distances and angles are provided
in Figure S1.
is octahedrally coordinated to the pyrrolic nitrogen atoms
of three chelating mesdpm ligands. The In-N distances are
2.212(3) Å for one complex and 2.215(3) and 2.219(3) Å
for the other. These distances are generally intermediate
between the In-N distances found in indium porphyrin
complexes and complexes of indium with 2,2′-bipyridine.
In the former, In-N distances range from 2.125 to 2.212 Å,
while in the latter case, the In-N distances range from 2.233
to 2.385 Å.42 The N-In-N angles within the metallocycles
of [In(mesdpm)3] are 84.54(15)° and 84.08(10)°, slightly
more acute than the similar angles seen in [Ga(mesdpm)3],
but in keeping with the larger covalent radius of indium-
(III). The dipyrrin moieties are nearly planar with angles
between the least squares planes of the pyrroles of 2.8(3)°
and 4.0(3)°. As in [Ga(mesdpm)3], the mesityl groups are
nearly orthogonal to the dipyrrin moieties, with dihedral
angles of 79.9(3)° and 84.79(10)°. The closest non-hydrogen
distance between ligands of a single complex is 3.295 Å.
The gallium(III) ion in [Ga(mesdpm)3] is octahedrally
coordinated to the pyrrole nitrogen atoms of three chelating
mesdpm ligands. The Ga-N distances of 2.054(2) and 2.055-
(2) Å are quite similar to the Ga-N distances of 2.054, 2.056,
and 2.068 Å found in tris(2,2′-bipyridine)gallium(III) triio-
dide41 and are comparable to those of other six-membered
gallium(III) metallocycles (Ga-N distance of 2.02(7) Å)
identified by a search in the Cambridge Structural Database
(CSD).42 A separate CSD search for gallium(III) porphyrin
compounds uncovered a mean Ga-N distance of 2.04(2) Å.
The N-Ga-N angles within the Ga(mesdpm) metallocycles
are 89.04(12)° and 89.07(8)°. These angles are slightly less
acute than the 87.37° mean N-Ga-N angles of gallium-
(III) porphyrins. However, it should be noted that the metal
is displaced from the mean porphyrin plane in most
gallium(III) porphyrin compounds, thus reducing the bite
angle.17,19,43-50 The dipyrrin moieties are essentially planar,
with the largest deviation having an angle of 3.4(2)° between
the least squares planes of the pyrroles. As would be
expected, the mesityl groups are approximately orthogonal
to the planar dipyrrin moieties. The dihedral angles between
the mesityl groups and the metallocycles are 79.90(9)° and
86.02(8)°, respectively. The closest non-hydrogen distance
between ligands of a single complex is 3.040 Å between an
R-carbon atom of one dipyrrinato ligand and a nitrogen atom
of a neighboring ligand.
Diffraction-quality, orange needles of [Ga(4-cydpm)3]
were prepared by vapor diffusion of pentane into a chloro-
form solution of the compound. Two independent, full
complexes make up the asymmetric unit. The compound
crystallized with two partially occupied chloroform solvent
molecules and several partially occupied water molecules.
The [Ga(4-cydpm)3] complexes are structurally quite similar
to [Ga(mesdpm)3]. Selected bond distances and angles are
provided in the Supporting Information (Figure S3). The
average Ga-N distance in [Ga(4-cydpm)3] is 2.053 Å. The
dihedral angles between the phenyl rings and the metallo-
cycles average 69.5° and range from 60.3° to 86.3°, reflecting
the loss of steric crowding that results from replacing the
mesityl groups with 4-cyanophenyl substituents.
Electronic Spectroscopy. The solution absorption spectra
of [Ga(mesdpm)3], [In(mesdpm)3], and [Ga(cydpm)3] are
very similar to each other. In addition to several complex,
weak, high-energy transitions, the primary spectral feature
in all three is an intense (ꢀ ≈ 70 000-120 000 M-1 cm-1),
split peak with maxima at about 450 and 500 nm (Figure
2). The apparently vibronic structure (ꢀ ≈ 2300 cm-1) in
this low-energy feature of all three spectra is considerably
more pronounced than that found in [Zn(mesdpm)2],8 the
absorption spectrum of which features a relatively weak,
high-energy shoulder (ꢀ ≈ 900 cm-1).
Slow diffusion of hexanes into an acetonitrile solution of
[In(mesdpm)3] produced orange needles of the complex as
the acetonitrile solvate (Figure 1). The asymmetric unit is
nearly identical to that of [Ga(mesdpm)3] and is comprised
of 1/3 of one [In(mesdpm)3] complex and 1/6 of another.
The structure also includes a partially occupied acetonitrile
molecule, which is disordered over a glide plane. Selected
bond distances and angles are provided in Figure S2. As in
[Ga(mesdpm)3], the central indium(III) ion of [In(mesdpm)3]
(41) Baker, R. J.; Jones, C.; Kloth, M.; Mills, D. P. New J. Chem. 2004,
28, 207-213.
(42) Allen, F. H. Acta Crystallogr., Sect. B: Struct. Sci. 2002, B58, 380-
388.
(43) Brancato-Buentello, K. E.; Coutsolelos, A. G.; Scheidt, W. R. Acta
Crystallogr., Sect. C: Cryst. Struct. Commun. 1996, 52, 2707-2710.
(44) Okamura, T.; Nishikawa, N.; Ueyama, N.; Nakamura, A. Chem. Lett.
1998, 199-200.
(45) Balch, A. L.; Hart, R. L.; Parkin, S. Inorg. Chim. Acta 1993, 205,
137-143.
(46) Parzuchowski, P. G.; Kampf, J. W.; Rozniecka, E.; Kondratenko, Y.;
Malinowska, E.; Meyerhoff, M. E. Inorg. Chim. Acta 2003, 355, 302-
313.
(47) Hsieh, Y.-Y.; Sheu, Y.-H.; Liu, I.-C.; Lin, C.-C.; Chen, J.-H.; Wang,
S.-S.; Lin, H.-J. J. Chem. Cryst. 1996, 26, 203-207.
(48) Coutsolelos, A. G.; Guilard, R.; Boukhris, A.; Lecomte, C. J. Chem.
Soc., Dalton Trans. 1986, 1779-1783.
Solutions of [Ga(cydpm)3] were not noticeably luminescent
when irradiated with a hand-held UV lamp (λex ) 365 nm)
or when examined using a spectrofluorimeter (λex ) 465 nm)
and were omitted from further luminescence studies. The
uncorrected, steady-state emission spectra of [Ga(mesdpm)3]
and [In(mesdpm)3] in hexanes or dichloromethane solutions,
presented in Figure 3, feature a single band that closely
mirrors the lower-energy feature of the absorption spectra.
The weak vibronic structure noted in the emission spectrum
of [Zn(mesdpm)2]8 is not apparent in the emission spectra
of [Ga(mesdpm)3] and [In(mesdpm)3]. However, the excita-
tion spectra when monitoring the emission at λem ) 451 nm
for [Ga(mesdpm)3] and 448 nm for [In(mesdpm)3] are
virtually identical to the absorption spectra; the emission
(49) Boukhris, A.; Lecomte, C.; Coutsolelos, A. G.; Guilard, R. J.
Organomet. Chem. 1986, 303, 151-165.
(50) Kadish, K. M.; Cornillon, J.-L.; Korp, J. D.; Guilard, R. J. Heterocycl.
Chem. 1989, 26, 1101-1104.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10693

Thoi et al.
Figure 4. Semilog plot of luminescence decays for [In(mesdpm)3] in mixed
hexanes, showing an overlay of normalized decay curves collected at 500,
530, 560, and 590 nm. All are identical and all are clean, single exponential
decays.
with [In(mesdpm)3], [Zn(mesdpm)2], and the free ligand
(mesdpm). Solvents explored included dichloro-, trichloro-,
tribromo-, and tetrachloromethane, acetonitrile, tetrahydro-
furan, toluene, methylcyclohexane, and mixed hexanes.
Fluorescence decays proved to be highly diagnostic for two
reasons. First, TCPC is immune to drift in most experimental
parameters and highly linear over many half-lives and thus
able to distinguish multiple decays. Second, we were using
an excitation wavelength near 400 nm, which is a minimum
in the excitation spectra of the complexes (Figure 4).
Normally, one might prefer to excite at an absorption
maximum to avoid complications by impurities or degrada-
tion products. However, excitation at an absorption minimum
is more useful for detecting impurities or degradation
products.
Figure 2. UV-visible spectra of [Ga(mesdpm)3] (dotted line) and [In-
(mesdpm)3] (solid line) in hexanes.
Solutions of the free ligand displayed a short lifetime
(about 16 ps) in all solvents, along with less than 1% of a
long-lived (3-3.5 ns) component that was not investigated
further. In all solvents investigated, except methylcyclohex-
ane and mixed hexanes, all the metal complexes likewise
showed a very similar, fast decay, varying in amplitude (10-
80%), along with two or more additional decay components
in the nanosecond range. This may be due to rapid solvolysis
that generates a mixture of metal species, together with the
free ligand. Among the solvents for which additional decay
components were observed, this behavior was least prominent
in toluene but was still quite evident from the data. In
addition to immediate solvolysis, there were further degrada-
tion processes that could be observed when the liquid
solutions were left in the dark for a week or more and re-
examined. These further changes resulted in more of the very
fast component, and some evolution of the slower decays
with no obvious pattern. As to variations of possible
solvolysis reactions for different metals, only in the case of
toluene was the reaction small enough to permit any
conclusion to be drawn. For toluene, [Zn(mesdpm)2] showed
more degradation than did the [Ga(mesdpm)3] or [In-
(mesdpm)3]. In fact, the [Zn(mesdpm)2] sample showed
degradation after one week even in hexanes and methylcy-
clohexane. By this measure, [Ga(mesdpm)3] and [In(mes-
dpm)3] are even more stable in noncoordinating solvents than
Figure 3. Normalized excitation (A) and emission (B) spectra of ∼1 µM
[Ga(mesdpm)3] (dotted line) and [In(mesdpm)3] (solid line) in hexanes. The
samples were excited at 451 nm for [Ga(mesdpm)3] and 448 nm for [In-
(mesdpm)3]. Excitation spectra were monitored at 530 nm.
spectrum of [In(mesdpm)3] is essentially unchanged whether
the compound is excited at 365, 448, or 496 nm (Figure S4).
With a fluorescence quantum yield (Φf) of 0.074, the
emission of [In(mesdpm)3] in hexanes is more intense than
that of [Ga(mesdpm)3] in the same solvent (Φf ) 0.024).
By comparison, both Group 13 complexes are markedly less
luminescent than [Zn(mesdpm)2] (Φf ) 0.36 in toluene).8
The considerable Stokes shifts (1220 and 1113 cm-1 for [Ga-
(mesdpm)3] and [In(mesdpm)3], respectively) and high molar
absorptivity, as well as the closed-shell electronic configura-
tions of the central metal atoms, suggest that the most
prominent electronic transitions are ligand-centered.
Time-Resolved Fluorescence. Fluorescence decay mea-
surements by TCPC were employed to characterize two
completely independent preparations of [Ga(mesdpm)3] along
10694 Inorganic Chemistry, Vol. 45, No. 26, 2006

Group 13 Dipyrrinato Complexes
Table 2. Optical Properties of the Lowest Excited Electronic State of Dipyrrinato Metal Complexes in Mixed Hexanes Solvent
b
λa,max
nm
ꢀmax
M-1 cm-1
τ
rad (predicted)
ns
λf,max
nm
τf
ns
τrad
ns
metal
φ
Ga
In
Zn
496
496
487a
68 000
12
14
∼6
528
522
501
0.024 ( 0.004
0.074 ( 0.006
0.36a
3.75 ( 0.02
1.93 ( 0.02
3.51 ( 0.04
156
26
∼10
60 100
115 000a
a From ref 8. b For the lowest-energy peak.
Table 3. Average Structural Parameters (in Å and Degrees) for the Experimental and Calculated Structures
[Ga(dpm)3]
B3LYP/
[In(dpm)3]
B3LYP/
[Zn(dpm)2]
B3LYP/
6-31G*
exptla
6-31G*
exptla
LanL2DZ
exptlb
M-N
2.054
1.340
1.407
1.370
1.429
1.395
1.400
89.06
111.6
106.5
107.1
108.4
106.3
124.8
126.4
2.059
1.338
1.417
1.383
1.425
1.395
1.392
90.13
111.7
106.1
106.8
109.0
106.4
124.6
127.9
2.215
1.337
1.402
1.375
1.431
1.392
1.403
84.31
111.8
106.3
107.1
108.2
106.6
125.4
128.0
2.208
1.360
1.430
1.398
1.439
1.415
1.402
86.46
111.4
106.3
107.1
108.7
106.5
125.3
129.3
1.979
1.338
1.408
1.369
1.419
1.399
1.399
94.45
111.5
106.1
107.8
108.0
106.6
124.6
127.2
1.995
1.339
1.417
1.384
1.426
1.397
1.396
96.37
111.7
106.0
107.0
108.8
106.5
124.6
129.4
N-C1
C1-C2
C2-C3
C3-C4
C4-N
C4-C5
N-M-N
N-C1-C2
C1-C2-C3
C2-C3-C4
C3-C4-N
C4-N-C1
N-C4-C5
C4-C5-C4′
a Experimental values are derived from the crystal structures of [Ga(mesdpm)3] and [In(mesdpm)3] (this work). b Experimental values for [Zn(dpm)2] are
from the crystal structure of bis-(5-phenyldipyrrinato)-zinc(II).22
the previously reported [Zn(mesdpm)2]. The gallium(III) and
indium(III) complexes are quite stable in hexanes and
methylcyclohexane; for these complexes, only single expo-
nential decays (Figure 4) were observed even after solutions
were kept at ambient conditions, in low light, for several
weeks.
oxygenated at 1 atm dioxygen for 30 min. Both ground-
state bleaching and excited-state transient absorption could
be detected in all cases. The free ligand showed a long-lived
transient with lifetime in the range 6-10 ms, with little or
no dioxygen dependence. Small features decaying more
rapidly could be detected in the air and dioxygen samples,
but no such decays could be detected in the argon-purged
case. The metal complexes showed transients with lifetimes
of 300-320 ns in air, 56-62 ns in 1 atm dioxygen, and
45-80 µs in the argon-purged case, along with small
amounts of a much longer-lived component. The behavior
of [Zn(mesdpm)2] appeared similar to [Ga(mesdpm)3] and
[In(mesdpm)3].
DFT Calculations. The optimized geometries of [Ga-
(dpm)3], [In(dpm)3], and [Zn(dpm)3] (dpm ) dipyrromethene)
were in reasonably good agreement with the structures of
the parent compounds [Ga(mesdpm)3], [In(mesdpm)3], and
bis-(5-phenyldipyrrinato)zinc(II).22 A comparison of struc-
tural parameters is provided in Table 3. Most calculated bond
distances were within 0.01 Å of the corresponding distances
from the X-ray structures. Discrepancies in the C2-C3
distances may have resulted from the replacement of the
mesityl groups by hydrogen atoms in the computational
studies. The [Zn(dpm)2] complex was also optimized with
the LanL2DZ basis set; however, the bond distances so
obtained were consistently longer than the crystallographi-
cally determined distances by about 0.05 Å. It is probable
that a further geometry optimization of the [In(dpm)3]
complex by applying larger basis sets would improve the
All the measurements that yielded single-exponential fits
are reported in the Supporting Information (Table S1). These
include the solutions described above at four different
wavelengths each. For most samples, the liquid solutions
were kept for several weeks in the dark and re-evaluated
weekly; the results are listed in chronological order to convey
reproducibility. The lifetimes in hexanes may be compared
with lifetimes in methylcyclohexane. For [Ga(mesdpm)3],
the fluorescent lifetimes are τf ) 3.747 ( 0.009 ns in hexanes
and 3.758 ( 0.008 ns in methylcyclohexane. For [In-
(mesdpm)3], τf ) 1.929 ( 0.012 ns in hexanes and 1.815 (
0.019 ns in methylcyclohexane. In these four cases, the
uncertainties quoted represent 99% confidence intervals
calculated using Student’s t-factors from the data listed. It
appears likely that there is a small, but real, difference
between solvents. The lifetimes reported in Table 2 are also
means of the pertinent data in Table S1, although the
uncertainties in Table 2 are increased arbitrarily by a small
amount to be even more conservative.
Flash Photolysis. Flash photolysis measurements were
carried out for the free ligand, [Ga(mesdpm)3], [In(mes-
dpm)3], and [Zn(mesdpm)2] for samples prepared in an
ambient atmosphere, purged with argon for 30 min, or
Inorganic Chemistry, Vol. 45, No. 26, 2006 10695

Thoi et al.
absorption of the free ligand. This may involve excitonic
splitting of the lowest energy transition of the free ligand.
For [Zn(mesdpm)2], the spectrum is rather different and has
been considered a single state with a vibrational feature on
the blue end of the spectrum.22 The emission from [Ga-
(mesdpm)3] or [In(mesdpm)3] is a mirror image of the lower
of the two prominent absorption bands. The fluorescent state
has a lifetime of a few nanoseconds and a quantum yield of
several percent. All of this proves that the emission is spin-
allowed fluorescence from a state that behaves like an organic
moiety. A Strickler-Berg analysis51,52 including only the
lower absorption feature predicts radiative lifetimes as given
in Table 2. Nonradiative decay pathways reduce the state
lifetime below this value. The actual state lifetime divided
by the quantum yield gives a measured radiative lifetime,
also displayed in Table 2. Best agreement between prediction
and measurement is expected when the absorption is very
strong and the molecule is very rigid and undergoes no
change in geometry in the excited state. Although large
geometry changes were not expected in these molecules, if
the interpretation of excitonic coupling (between two or more
of the ligands bound to the metal center) is valid, then that
coupling is likely to be extremely sensitive to even small
changes. For reasons not yet clear, there is a large dis-
crepancy in the predicted and measured lifetimes for
[Ga(mesdpm)3].
Figure 5. Partial calculated energy level diagrams for [Zn(dpm)2], [Ga-
(dpm)3], and [In(dpm)3] (dpm ) dipyrrin).
The moderately low quantum yields for emission prove
that the dominant decay pathways are nonradiative. The flash
photolysis results reported above provide convincing evi-
dence that at least a portion of the decay is to a triplet, for
we measured transient features with appropriate lifetimes that
were strongly dependent on dioxygen concentration. The
transient features for [In(mesdpm)3] appeared slightly more
intense, consistent with a normal heavy-atom effect on
intersystem crossing to the triplet state. We have no
measurement of ground-state repopulation on the time scale
of emission, but it is likely that there is also internal
conversion to the ground state, as Lindsey and Holten and
their collaborators reported.22 This internal conversion may
be as much as 70% for the [Zn(mesdpm)2] complex. Lindsey
et al. further reported that, in a different, shorter-lived, weakly
fluorescent complex, internal conversion was substantially
higher.8 This is plausible, as the intersystem crossing rate
may well be more or less constant, while internal conversion
is the process that produces very short lifetimes. This is
consistent with our observation of very weak triplet features
in the free ligand. There is also a photochemical process that
produces a longer-lived product (several milliseconds), but
we do not conjecture any assignment at this time.
Figure 6. Drawings of the frontier orbitals calculated for [Ga(dpm)3].
agreement between the computed and experimental bond
parameters. A partial energy level diagram is presented in
Figure 5. The most likely low-energy electronic transitions
are ligand-centered, πfπ* excitations, as can be seen from
the frontier molecular orbitals calculated for [Ga(dpm)3]
shown in Figure 6. Frontier molecular orbitals calculated for
[In(dpm)3] and [Zn(dpm)2] are provided in the Supporting
Information (Figures S5 and S6).
Discussion
The three new complexes presented here are the first well-
characterized gallium(III) and indium(III) complexes of R,â-
unsubstituted dipyrrinato ligands. Notably, [Ga(mesdpm)3]
and [In(mesdpm)3] exhibit luminescence in the visible region
when excited by UV light. The optical properties of the
lowest excited electronic state in these complexes appear very
much as was expected for a ligand-localized transition in a
metal complex. The molar absorptivity values are too large
to be anything else, and this assertion is also supported by
DFT calculations, which indicate that the frontier orbitals
are exclusively ligand-centered. For [Ga(mesdpm)3] or [In-
(mesdpm)3], coordination to the metal results in an intensi-
fied, red-shifted, split feature compared to the lowest
The above description applies only in a noncoordinating
solvent and many of the solvents examined are not inert with
respect to these complexes. Mixed hexanes and methylcy-
clohexane are inert. Halogenated solvents, acetonitrile,
tetrahydrofuran, and even toluene are not. When compared
to [Zn(mesdpm)2], the new gallium(III) and indium(III)
(51) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814.
(52) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings,
Inc.: Menlo Park, CA, 1978.
10696 Inorganic Chemistry, Vol. 45, No. 26, 2006

Group 13 Dipyrrinato Complexes
complexes were slightly more stable. [Zn(mesdpm)2] was
found to be unstable over weeks even in noncoordinating
solvents. The combination of the well-behaved optical
properties reported here along with the stability of the
complexes suggest that further “tuning” of the structure might
well afford some improvement in the fluorescent quantum
yield, but even at the present level, one can visualize
applications for this new class of dipyrrin-based fluorophores.
the American Chemical Society Petroleum Research Fund,
and the National Science Foundation (CHE-0546531). This
research was supported in part by W. M. Keck Foundation
through computing resources at the W. M. Keck Laboratory
for Integrated Biology II. J.R.S was supported by an NIH
training grant (T32 HL007444-25) and V.S.T. was supported,
in part, by a U.C.S.D. Summer Research Fellowship. S.M.C.
is a Cottrell Scholar of the Research Corporation. D.M.
thanks the late Prof. K. Wilson and the late Prof. B. Zimm.
Acknowledgment. We thank Prof. Arnold L. Rheingold
(U.C.S.D.) and Dr. Yongxuan Su (U.C.S.D.) for assistance
with the X-ray and mass spectrometry data collection,
respectively. We thank Dr. Drew L. Murphy for synthetic
assistance with [Ga(4-cydpm)3]. This work was supported
by the University of California, San Diego, the donors of
Supporting Information Available: Figures S1-S6; Complete
ref 28; X-ray crystallographic data in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC061581H
Inorganic Chemistry, Vol. 45, No. 26, 2006 10697