Structure and photophysics of near-infrared emissive ytterbium(iii) monoporphyrinate acetate complexes having neutral bidentate ligands

Article abstract of DOI:10.1039/b909243a

Substitution reactions between [Yb(TPP)(OOCCH₃)(CH ₃OH)₂] (1) and neutral bidentate ligands NN led to the formation of monoporphyrinate ytterbium(iii) complexes [Yb(TPP)(OOCCH ₃)(NN)] (TPP = 5,10,15,20-tetraphen

Full text of DOI:10.1039/b909243a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Structure and photophysics of near-infrared emissive ytterbium(III)
monoporphyrinate acetate complexes having neutral bidentate ligands†
Hongshan He,*a Andrew G. Sykes,b P. Stanley Mayb and Guishan Hec
Received 11th May 2009, Accepted 16th June 2009
First published as an Advance Article on the web 27th July 2009
DOI: 10.1039/b909243a
Substitution reactions between [Yb(TPP)(OOCCH3)(CH3OH)2] (1) and neutral bidentate ligands NN
led to the formation of monoporphyrinate ytterbium(III) complexes [Yb(TPP)(OOCCH3)(NN)]
(TPP = 5,10,15,20-tetraphenylporphyrinate anion; NN = 4-methyl-1,10-phenanthroline (2), 1,10-phe-
nanthroline (3), 4,7-dimethyl-1,10-phenanthroline (4), 5,6-epoxy-5,6-dihydroxy-1,10-phenanthroline (5)
and 2,2¢-dipyridylamine (6)). Single-crystal X-ray diffraction analysis revealed that ytterbium(III) ions
-
in 1 and 6 were seven-coordinate with OOCCH3 in monodentate coordination, whereas those in 2, 3, 4
-
and 5 were eight-coordinate with OOCCH3 in bidentate coordination. The visible emission (650 and
720 nm) from the porphyrin and near-infrared (NIR) emission (980 and 1003 nm) from ytterbium(III)
ion were observed for all complexes. The eight-coordinate complexes exhibited stronger NIR emission
and longer lifetimes in toluene solution than the seven-coordinate complexes. The NIR emission of
complexes with decreased lifetimes was also observed when they were blended into organic polymer
PMMA.
lanthanide complexes has focused mainly on the acetylacetonate
Introduction
adducts, which were synthesized from reactions between porphyrin
Monoporphyrinate lanthanide complexes have one porphyrinate
free bases and lanthanide acetylacetonate hydrates in 1,2,4-
trichlorobenzene (TCB) at 215 ◦C as shown in eqn (1):1,3
dianion binding directly to the central metal ion with other ancil-
lary ligands occupying the remaining coordination sites.1–26 They
are stable in air, soluble in most organic solvents, and have strong
absorption in the UV-visible region. Ytterbium(III), erbium(III)
2
H2Por + Ln(acac)3 → [Ln(Por)(h -acac)(Solv)] + 2Hacac (1)
where Ln is the trivalent lanthanide ion, Por is the porphyrinate
and neodymium(III) complexes also exhibit characteristic emission
dianion, acac is the acetylacetonate anion and Solv is the solvent.
in the near-infrared (NIR) region upon “sensitization”, which
This method has been used widely in the past, and in the present,
makes them promising in medical diagnostics and photonics.27–30
for the preparation of their analogues.11,33–42 The major problem is
In medical diagnostics, they could be used as stable luminescence
that the acac anion is very difficult to replace with other ancillary
labels for bioassays, where transparency of NIR light to biological
ligands for further functionalization. In addition, the acac anion
fluids provides an opportunity for high sensitivity techniques in
could be cleaved through thermal pyrolysis resulting in acetate or
early detection of tumor markers.31,32 In photonics, erbium(III) and
propionate-coordinate complexes.43,44 One solvent molecule (water
neodymium(III) are commonly used in optical fiber amplifiers due
or methanol) also coordinates to lanthanide ion leading to weak
to their intra-4f transitions at 1550 and 1340 nm, respectively,
NIR emission due to the O–H oscillation.45 In 1999, Wong46
which are two standard telecommunication wavelengths. By
developed another method as shown in eqn (2):
blending or doping these ions in optical fibers, the input signals
can be amplified through laser principles for long-distance signal
transmission.31
H2Por + Ln[N(SiMe3)2]3·x[LiCl(THF)3]
→ [Ln(Por)(H2O)3]Cl
(2)
Development of new lanthanide complexes that emit efficiently
in the NIR region is critical for diagnostic and photonic applica-
tions. During last several decades, research on monoporphyrinate
The interaction of porphyrin free base with lanthanide amide,
followed by hydrolysis in the air, led to the formation of aqua-
coordinated monoporphyrinate complexes [Ln(Por)(H2O)3]Cl
(Ln = Yb3+, Er3+). The three water molecules were labile and
could be replaced by anionic tripodal ligands forming stable
neutral pseudo-“sandwich” complexes with a porphyrin ring and
a tripodal ligand on each side.19 The aqua complexes can also
interact with each other and form dimers under neutral, acidic or
basic conditions.17 In 2003, a similar method for the preparation
of pseudo-“sandwich” complexes was also reported by Reynolds
et al.47 The same authors also fabricated NIR OLEDs using
these complexes and demonstrated their potential in photonics
applications.22 However, they lack other active groups for further
functionalization and their preparation required an oxygen-free
environment, which limited its applications. In addition, there
aCenter for Advanced Photovoltaics, Department of Electrical Engineering,
South Dakota State University, Brookings, SD 57007, USA. E-mail:
hongshan.he@sdstate.edu; Fax: 01 6056884401; Tel: 01-6056886962
bDepartment of Chemistry, University of South Dakota, Vermillion, SD
560069, USA. E-mail: asykes@usd.edu, Stanley.May@usd.edu; Fax: 01
6056776397; Tel: 01-6056775487
cClinical Laboratory, Tumor Hospital of Gansu Province, Lanzhou, 730050,
P. R. China
† Electronic supplementary information (ESI) available: Fig. 1S: The decay
curves of complex 4 in CD3OD and CH3OH. The excitation wavelength
was 510 nm and decay was monitored at 978 nm. CCDC reference numbers
731407 (4), 731408 (5) and 731409 (6). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/b909243a
7454 | Dalton Trans., 2009, 7454–7461
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Scheme 1 Ytterbium(III) monoporphyrinate complexes.
are only a few tripodal ligands appropriate for the formation of
pseudo-“sandwich” complexes
described previously.43 Elemental compositions were determined
on a Perkin Elmer Series II-2400 CHNS analyzer.
Recently we developed a method48 to replace two water
molecules in [Yb(TPP)(H2O)3]Cl by protic bidentate ligands, but
the remaining water still quenches the NIR emission. In 2001,
Garc´ıa-Sa´nchez et al.49 reported a monoporphyrinate lanthanide
acetate from reaction (3):
Preparation of [Yb(TPP)(OOCCH3)(NN)] (NN = Phen, 3;
4,7-Me-Phen, 4; EPO-Phen, 5; DPA, 6)
All complexes were prepared by a similar procedure. A typical
2
procedure is given for [Yb(TPP)(h -OOCCH3)(Phen)] (3). To a
H2Por + Ln(OOCCH3)3·xH2O
toluene solution (20 mL) of 1 (24.2 mg, 0.027 mmol) was added an
excess of 1,10-phenanthroline (20.2 mg, 0.11 mmol). The solution
was magnetically stirred at room temperature for 12 h, and solvent
was removed under reduced pressure. The crude product was then
dissolved in dichloromethane and loaded on a column (silica gel)
for purification. The column was flushed with dichloromethane
and the first major band collected. After filtration, the volume of
the solvent was reduced to ~1 mL and methanol (5 mL) was
added. The purple solid was collected by filtration and dried
under vacuum for 24 h. Yield: 23.1 mg, 86%. Calc. (found) for
C58H41N6O3Yb (3·CH3OH): C, 66.79 (66.72); H, 3.96 (3.85); N,
8.06 (8.11%).
→ [Ln(Por)(OOCCH3)(Solv)2]
(3)
It was proposed that the lanthanide ions were eight-coordinate
with four nitrogen atoms from the porphyrinate dianion, two
oxygen atoms from the chelating acetate anion, and two other
atoms from solvent molecules. In a previous communication,
we reported for the first time that the resulting complex
[Yb(TPP)(OOCCH3)(CH3OH)2] was seven-coordinate with the
OOCCH3- anion in a monodentate coordination mode.43 We also
demonstrated the potential of this complex as a precursor for
complexes with higher coordination numbers by its reaction with
4-methyl-1,10-phenanthroline.43 In this report, we describe the
details of this reaction, as shown in Scheme 1, structural analysis
and photophysical properties of resulting complexes in solution
and in an organic polymer matrix.
2
[Yb(TPP)(h -OOCCH3)(4,7-Me-Phen)] (4): 4,7-dimethyl-1,10-
phenanthroline (20.5 mg, 0.10 mmol) and 1 (18.0 mg, 0.02 mmol)
were used. Yield: 16.7 mg, 80%. Calc. (found) for C61H47N6O3Yb
(4·CH3OH): C, 67.52 (67.88); H, 4.37 (4.35); N, 7.74 (7.74%).
2
[Yb(TPP)(h -OOCCH3)(EPO-Phen)]
(5):
5,6¢-epoxy-5,6-
Experimental
dihydroxy-1,10-phenanthroline (24.5 mg, 0.12 mmol) and 1
(24.6 mg, 0.027 mmol) were used. Yield: 25.1 mg, 89%. Calc.
(found) for C59H43N6O5Yb (5·CH3OH·5H2O): C, 65.07 (64.99);
H, 3.98 (4.09); N, 7.72 (7.62%).
[Yb(TPP)(OOCCH3)(DPA)] (6): 2,2¢-dipyridylamine (38.6 mg,
0.22 mmol) and 1 (23.0 mg, 0.025 mmol) were used. Yield:
22.0 mg, 85%. Calc. (found) for C57H44N7O3Yb (6·CH3OH): C,
65.32 (65.67); H, 4.23 (4.14); N, 9.36 (9.34%).
Reagents and general procedures
5,10,15,20-Tetraphenylporphyrin (H2TPP) was prepared ac-
cording to the literature method.50 Reagent grade yt-
terbium(III) acetate trihydrate, 1,10-phenanthroline (Phen),
4-methyl-1,10-phenanthroline (4-Me-Phen), 4,7-dimethyl-1,10-
phenanthroline (4,7-Me-Phen) and 5,6-epoxy-5,6-dihydroxy-1,10-
phenanthroline (EPO-Phen), 2,2¢-dipyridylamine (DPA) and
poly(methylmethaceylate) (PMMA) (MW = 123000) were ob-
tained commercially and used without further purification. 1,2,4-
Trichlorobenzene (TCB) was dried over molecular sieves overnight
prior to use. All other chemicals and solvents for synthesis and pu-
rification were analytical grade and used as received. The solvents
for photophysical studies were ACS grade, ultradry in Aeroseal
containers and used as received. CD3OD was from Cambridge
Isotope Laboratories with 99.8% D in ampoule and used with-
out further purification. [Yb(TPP)(OOCCH3)(CH3OH)2] (1) and
X-Ray crystallography
Single crystals were obtained by slow evaporation of
dichloromethane–methanol solutions of the complexes at room
temperature. The crystals were mounted on glass fibers before
data collection. Diffraction measurements were made on a Bruker
APEC II CCD detector at 125(2) K. The frames were collected
with a scan width 0.3◦ in w and integrated with Bruker SAINT
software package using a narrow-frame integration algorithm.
The unit cell was determined and refined by least squares upon
2
2
[Yb(TPP)(h -OOCCH3)(h -4-Me-Phen)] (2) were prepared as we
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Table 1 X-Ray crystallographic data for 4, 5 and 6
4
5
6
Empirical formula
C60H43N6O2Yb
1053.04
125(2)
0.71073
Triclinic
¯
P1
10.2453(6)
13.4713(7)
18.5620(10)
74.6050(10)
89.8300(10)
68.7340(10)
2289.3(2)
2
1.528
2.097
1062
1.69–25.39
23222/8417
0.0460
C59H41N6O4Yb
1071.02
125(2)
0.71073
Triclinic
¯
P1
C58H48N7O4Yb
1080.07
125(2)
0.71073
Triclinic
¯
P1
10.0590(12)
13.2450(15)
19.689(3)
102.797(2)
92.282(2)
111.7990(10)
2353.5(5)
2
1.524
2.045
1094
1.71–25.52
23790/8637
0.0377
Mr
T/K
˚
l/A
Crystal system
Space group
˚
a/A
9.766(2)
˚
b/A
13.559(3)
17.444(3)
82.934(3)
89.885(3)
88.619(3)
2291.7(8)
2
1.552
2.099
1078
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Dc/g cm-3
m(Mo-Ka)/mm-1
F(000)
q Range for data collection/◦
Refl. collected/independent
Rint
1.80–25.70
8487/8487
0.0000
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
8417/0/622
1.045
R1 = 0.0374
wR2 = 0.0871
R1 = 0.0502
wR2 = 0.0921
8487/0/631
1.078
R1 = 0.0363
wR2 = 0.1029
R1 = 0.0424
wR2 = 0.1061
8637/3/645
1.107
R1 = 0.0385
wR2 = 0.1058
R1 = 0.0439
wR2 = 0.1082
R indices (all data)
the refinement of XYZ-centroids of reflections above 20s(I). The
data were corrected for absorption using the SADABS program.51
The structures were refined on F2 using the Bruker SHELXTL
single photon counting technique. The intensities were monitored
using a Hamamatsu H5773–02 detector. A pulse diode laser (EPL-
375, Edinburgh Instruments, Inc) with excitation wavelength at
375 nm used as the light source and lifetimes were obtained by
exponential fitting of deconvoluted data. The NIR decay curves
were acquired using an optical parametric oscillator (OPOTEK
Opolette) as an excitation source. NIR emission was detected
using a 0.3 m flat-field monochromator (Jobin Yvon TRIAX 320)
equipped with a NIR-sensitive photomultiplier tube (Hamamatsu
R2658P) in a cooled housing (Products for Research). All spectra
were corrected for instrumental function. The output from the
photomultiplier was pre-amplified (Stanford Research SR 445A)
and fed to a multichannel scaler (Stanford Research SR 430)
for time-resolved photon counting. The entire system was PC
controlled using LabView software.
(version 5.1) software package.52 A large residual density 0.55 A
˚
from H60C of 4 was observed and was unable to be defined. The
structural parameters are listed in Table 1.
Preparation of complex-blended polymer film
PMMA (0.5 g) was dissolved in chloroform (5 mL) and the
solution was kept at room temperature overnight. Then to 0.5 mL
of this solution was added 100 mL of 4 or 6 in toluene with
concentration of 2.0 ¥ 10-3 mol L-1. The mixture was magnetically
stirred for 2 h at room temperature, then dip-coated on microscopic
glass slides (25 mm ¥ 25 mm) with full coverage. The slides
were placed on the bench, covered with plastic dishes, and left
overnight. The films were then dried under vacuum for 24 h prior
to measurements. The concentration of samples in the resulting
PMMA films were 4 ¥ 10-4 mmol per 100 mg of PMMA.
Results and discussion
Preparation and structure of [Yb(TPP)(OOCCH3)(NN)]
The precursor [Yb(TPP)(OOCCH3)(CH3OH)2] (1) for the prepa-
ration of [Yb(TPP)(OOCCH3)(NN)] was synthesized by direct
reaction between ytterbium(III) acetate hydrate and H2TPP in
Photophysical measurements
Electronic absorption spectra in the UV-VIS region were recorded
on a HP Agilent 5439 UV-Visible spectrophotometer using 1.0 cm
quartz cell. Steady state luminescence spectra were obtained on
a FS920 spectrometer (Edinburgh Instruments, Inc) with an arc
lamp (Xe900) as light source. The Hamamatsu R928P and TE-
cooled Hamamatsu G8605–23 detectors were used for UV-visible
and NIR emission measurements, respectively. The instruments
were PC-controlled through the F900 software, and the spectra
were corrected from reference spectra of detectors. Decay curves
of samples in the visible region were acquired on a LifeSpec II
spectrometer (Edinburgh Instrument, Inc) using a time-correlated
◦
TCB at 210 C for 48 h under N2, followed by solvent exchange
in methanol as we described previously.43 In this precursor, the
ytterbium(III) ion was seven-coordinate, with four nitrogen atoms
from TPP2-, two oxygen atoms from two CH3OH molecules and
-
one oxygen atom from the monodentate OOCCH3 anion. The
two methanol molecules were labile and were substituted by
1,10-phenanthroline derivatives and 2,2¢-dipyridylamine, forming
new ytterbium(III) complexes 2–6 with high yields. The resulting
complexes were very stable in the air, had a bright purple color, and
were soluble in dichloromethane, chloroform, toluene and slightly
7456 | Dalton Trans., 2009, 7454–7461
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soluble in methanol. Compositions of [Yb(TPP)(OOCCH3)(NN)]
complexes were confirmed by elemental analysis and single-crystal
X-ray diffraction analysis.
The thermal ellipsoid diagrams of complexes 4, 5 and 6 are
shown in Fig. 1–3 and selected bond lengths and angles are
listed in Table 2. In complexes 4 and 5, Yb3+ is eight-coordinate,
with four N atoms from the porphyrinate dianion, two N atoms
from the 1,10-phenanthroline derivatives and two O atoms from
acetate anion occupying the coordination sphere. The Yb3+ ions
are located above the porphyrin ring with a distance to the mean
˚
N1/N2/N3/N4 plane of 1.1729(19) and 1.1691(19) A for 4 and 5,
respectively. The phenanthroline ligands lie above two neighboring
phenyl groups of TPP2- with torsion angles between the two
pyridyl groups of 14.75(0.34)◦ in 4 and 25.04 (0.19)◦ in 5. The
˚
bite distances (N5 ◊ ◊ ◊ N6) are 2.662 and 2.652 A for 4 and 5,
respectively. Complex 6 is seven-coordinate with acetate acting
as a monodentate ligand and the Yb3+ ion is 1.1217 A above
˚
2
Fig. 3 Top: The ORTEP diagram of [Yb(TPP)(OOCCH3)(h -DPA)]·
2CH3OH (6) with 50% thermal ellipsoid probability. The hydrogen atoms
˚
˚
are omitted for clarity; O2 ◊ ◊ ◊ O4 = 2.711 A, N7 ◊ ◊ ◊ O4 = 2.913 A. Bottom:
Packing diagram of hydrogen-bonding interaction between porphyrins
viewed from top and side positions. Colour code: N, blue; O, red; Yb,
purple; C, gray. The distance between O3 and O4 that are H-bonded in a
2
2
Fig. 1 The ORTEP diagram of [Yb(TPP)(h -OOCCH3)(h -4,7-Me-
Phen)] (4) with 50% thermal ellipsoid probability. The hydrogen atoms
are omitted for clarity.
˚
chain structure is 2.738 A.
the porphyrin ring. The torsion angle between the two pyridyl
groups in DPA is 50.65(22)◦, which is much higher than those
observed in 2, 4 and 5. The bite distance (N5 ◊ ◊ ◊ N6) in 6 is
˚
˚
2.825 A, which is 0.16 A larger than those in 4 and 5. This increase
leads to a larger N5–Yb1–N6 angle and results in a monodentate
coordination mode of the acetate anion in 6. Hydrogen bonding
interactions are observed in 5 and 6. In 5, one O atom of the acetate
anion is hydrogen-bonded to one O atom of methanol solvent.
In complex 6, two methanol molecules are hydrogen-bonded to
acetate oxygen (O2) and amino nitrogen (N7), respectively. They
also form hydrogen bonds to each other by translation along the
[1 1 0] direction forming one-dimensional chains as depicted in
Fig. 3.
The geometry of eight-coordinate complexes 2, 3, 4 and 5 can
be described as square antiprismatic. Four N atoms (N1, N2,
N3 and N4) from the porphyrin fall in one quadrilateral plane
with the four inner angles close to 90◦, whereas two acetate
oxygen atoms (O1 and O2) and two phenanthroline nitrogen atoms
(N5 and N6) lie in a trapezoidal plane due to the different bite
2
2
Fig. 2 The ORTEP diagram of [Yb(TPP)(h -OOCCH3)(h -EPO-Phen)]
(5) with 50% thermal ellipsoid probability. The hydrogen atoms are omitted
for clarity; O2 ◊ ◊ ◊ O3 = 2.786 A.
˚
distances in the acetate (O1 ◊ ◊ ◊ O2 ª 2.2 A) and the phenanthroline
˚
˚
(N5 ◊ ◊ ◊ N6 ª 2.7 A). The two planes are nearly parallel to each
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Table 2 Selected bond lengths and angles of 4, 5 and 6
crossing to its triplet state. All complexes also exhibited character-
istic Yb3+ emission in the near-infrared region upon excitation, as
shown in Fig. 4. In seven-coordinate complexes (1 and 6), a peak
centered at approximately 975 nm with three weak shoulders at
950, 1000 and 1025 nm were observed. This feature is consistent
with other seven-coordinate Yb3+ complexes we have described
previously.17–21,43,45,48,53 In eight-coordinate complexes (2–5) two
strong peaks at 975 and 1005 nm, and a shoulder at 946 nm
were observed. The substituents on 1,10-phenanthroline did not
have a significant effect on emission efficiency, as shown in Fig. 5.
It seemed that introducing an electron withdrawing group, such
as an epoxy group in 5, was unfavorable for NIR emission. The
photophysical data are listed in Table 3.
4
Yb1–N1
Yb1–N2
Yb1–N3
Yb1–N4
Yb1–N5
Yb1–N6
Yb1–O1
Yb1–O2
2.352(4)
2.376(4)
2.367(4)
2.355(4)
2.528(4)
2.488(4)
2.369(3)
2.418(3)
N1–Yb1–N2
N2–Yb1–N3
N3–Yb1–N4
N1–Yb1–N4
N5–Yb1–N6
O1–Yb1–O2
O1–C59–O2
75.77(12)
75.03(13)
76.17(13)
75.95(13)
64.10(12)
54.68(11)
121.0(5)
5
Yb1–N1
Yb1–N2
Yb1–N3
Yb1–N4
Yb1–N5
Yb1–N6
Yb1–O1
Yb1–O2
2.346(4)
2.338(4)
2.358(4)
2.359(4)
2.551(4)
2.523(4)
2.351(3)
2.437(3)
N1–Yb1–N2
N2–Yb1–N3
N3–Yb1–N4
N1–Yb1–N4
N5–Yb1–N6
O1–Yb1–O2
O1–C57–O2
76.29(13)
75.72(13)
75.47(13)
75.21(13)
63.02(13)
54.45(12)
120.3(5)
6
Yb1–N1
Yb1–N2
Yb1–N3
Yb1–N4
Yb1–N5
Yb1–N6
Yb1–O1
2.346(5)
2.351(4)
2.324(4)
2.342(4)
2.471(5)
2.508(5)
2.207(4)
N1–Yb1–N2
N2–Yb1–N3
N3–Yb1–N4
N1–Yb1–N4
N5–Yb1–N6
O1–C55–O2
76.98(16)
76.63(15)
76.32(15)
76.96(16)
69.12(15)
123.7(6)
other with dihedral angles of 0.61(0.01), 0.90(0.20) and 0.28(0.19)◦
for 2, 4 and 5, respectively. The geometry of the seven-coordinate
complex 6 is capped trigonal prismatic with O1 (acetate O) and N6
(phenanthroline N) in the cap positions, which are 1.8517(27) and
Fig. 4 Visible (black) and NIR (red) emission of 2 in toluene solution
at room temperature (excitation wavelength = 375 nm; the intensity is
normalized).
˚
1.977(53) A above the mean square planes of N1/N4/O5/O4 and
N1/N4/N5/O1, respectively. The planarity of the porphyrin rings
in these complexes is also quite different. The porphyrin rings of
the synthesized complexes do not retain their planar geometry
as observed in the H2TPP free base and their Zn2+ and Cu2+
complexes. For eight-coordinate complexes, two torsion angles
between the mean planes of the pyrrole rings on opposite sides of
the porphyrin are different with one larger than another (29.04(26)
vs. 2.97(48)◦ and 34.96(29) vs. 3.31(45)◦ for 4 and 5, respectively),
as a result, the porphyrin rings adopt an arched structure. In the
seven-coordinate complexes, two pairs of pyrroles on opposite
sides of the porphyrin bend almost equally away from the mean
plane of N1N2N3N4 with torsion angles close to each other
(14.90(46) vs. 11.48(44)◦ for 6), resulting in a domed structure
as observed in other seven-coordinate monoporphyrinate acety-
lacetonate lanthanide complexes and tripodal ligand-coordinated
monoporphyrinate lanthanide complexes.17–21
Fig. 5 Near-infrared emission spectra of complexes 1–6 in toluene at
room temperature (excitation wavelength = 375 nm; concentration = 7.7 ¥
10-5 mol L-1).
Photophysical properties
Several mechanisms have been proposed for sensitized NIR
emission of ytterbium(III) complexes.31 One that was proposed
for monoporphyrinate lanthanide complexes was the Dexter
mechanism.31,54–57 In this mechanism porphyrin absorbs the light
and is excited to its singlet state (S1). The ligand then relaxes,
via intersystem crossing, to a lower-lying triplet state (T1), from
which energy is transferred to the lanthanide ion via an exchange
mechanism. The triplet state energy level of porphyrin is at
~15500 cm-1, which is very favorable for energy transfer. To
estimate the energy transfer rate, the emission of 4 in the presence
of O2 were recorded. Molecular oxygen is a quencher of the
The electronic absorption spectra of complexes 1–6 were charac-
teristic of regular metalloporphyrins with a Soret band at about
430 nm and two Q bands centered at about 550 and 600 nm. This
was in agreement with Gouterman’s four-orbital model,4 which
predicts that four Q bands of the porphyrin free base will be
reduced to two upon formation of a metalloporphyrin due to
an increase in symmetry.4 Upon excitation, all complexes give
typical excitation and emission spectra in the visible region with
a quantum efficiency (U) less than 0.005, which was significantly
lower than that of H2TPP (0.11) indicating the efficient intersystem
7458 | Dalton Trans., 2009, 7454–7461
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Table 3 Photophysical data of monoporphyrinate ytterbium(III) complexes [YbTPP(OOCCH3)(NN)]a
NIR regione
Vis region
c
c
NN
Absorption, lmax/nm (log e)
Emission, lemb/nm (t /ns)
Ud
Emission, lemb/nm (t /ns)
Uf
2CH3OH (1)
4-Me-Phen (2)
Phen (3)
4,7-Me-Phen (4)
EPO-Phen (5)
DPA (6)
424 (5.53), 554 (4.18)
429 (5.38), 558 (4.12)
429 (5.56), 557 (4.28)
430 (5.52), 558 (4.32)
427 (5.57), 557 (4.24)
424 (5.68), 553 (3.68)
651 (6.70), 719
655 (8.20), 722
653 (8.15), 722
651(10.14), 719
651 (10.04), 719
650 (9.14), 719
0.004
952, 975 (1.56), 1004, 1026,
949, 978 (17.29), 1005
947, 978 (18.48), 1006
948, 978 (17.82), 1005
948, 978 (14.12), 1006
950, 975 (11.60), 1006, 1023
1.30 ¥ 10-3
14.4 ¥ 10-3
15.4 ¥ 10-3
14.85 ¥ 10-3
11.7 ¥ 10-3
9.66 ¥ 10-3
0.0008
0.0011
0.0052
0.0049
0.0018
4 in PMMA
6 in PMMA
518, 558, 594
515, 555, 593
652, 722
651, 721
951, 977 (12.6), 1003, 1015, 1024
950, 978 (10.8), 1003
10.5 ¥ 10-3
8.75 ¥ 10-3
a All measurements were carried out in toluene at room temperature. b The excitation wavelength is 415 nm. c The excitation wavelength is 375 nm.
d Tetraphenylporphyrin (H2TPP) was used as reference standard (U = 0.11 at 419 nm in toluene) and decay was monitored at 650 nm. e The decay was
monitored at 978 nm. f Calculated according to UYb = tobs/t0, where t0 is 1200 ms.
porphyrin triplet state resulting in the production of singlet oxygen
that emits at ~1270 nm.58 As shown in Fig. 6, no emission at
~1270 nm was observed for 4 after bubbling oxygen through an
as-prepared toluene solution. In contrast a much stronger peak at
1270 nm was observed under similar condition for a solution of
ZnTPP, which is known to have a high triplet yield and can transfer
energy to molecular oxygen. This, together with very low emission
efficiency of the complexes in the visible region, indicated that
the energy transfer rate from the triplet state of the porphyrin to
ytterbium(III) was faster than that to molecular oxygen. Due to the
limitation of our instrument, it is difficult to obtain an accurate
energy transfer rate; however, the rate is larger than the energy
transfer rate to oxygen, i.e. 107 s-1. This estimation is also consistent
with other ytterbium(III) monoporphyrinate complexes.31
were much longer than those of 1 and 6. For eight-coordinate
complexes, no solvent molecule coordinates directly to metal
center, whereas in seven-coordinate ones, solvents are present in
the inner coordination sphere. In complex 1, two methanols bind
directly to the ytterbium(III) ion, resulting in dramatic quenching
of NIR emission through increased multiphonon relaxation of
the excited state of ytterbium(III). In complex 6, no methanol
coordinates directly to metal center; however, two methanol
-
molecules are H-bonded to DPA and OOCCH3 ; they are close to
metal center, but further away compared to methanol molecules
in complex 1, therefore the quenching effect is reduced. Consistent
with this observation was that the emission intensity of 5 with one
-
methanol H-bonded to OOCCH3 was weaker than that of 2 and
3 with no H-bonded methanol. It is also possible that 6 has a more
open structure for the access of solvent to the inner coordination
sphere. The results showed that methanol molecules in inner
coordination sphere quench the NIR emission significantly. This
can be attributed to the vibrational overtones of O–H in methanol
with the NIR frequency.15,59,60
The outer-sphere interaction also played an important role in
NIR emission. Fig. 7 showed the NIR emission of 4 in different
solvents. The emission intensity in CH3OH was four times weaker
than that in toluene, indicating the interaction of methanol with
Yb3+ in the complex. To identify whether this decrease comes from
Fig. 6 Comparison of production of singlet oxygen from a toluene
solution of 4 and ZnTPP upon excitation at 375 nm. The inset is the
emission from singlet oxygen.
The overall NIR emission efficiency in solution was low. As
indicated in Table 3, estimated quantum yields in toluene were less
than 1% indicating the importance of the coordination environ-
ment on emission efficiency. The low emission efficiency is largely
a result of the solvent interaction in the inner coordination sphere
of ytterbium(III) ion. At the same concentration, the emission
intensity of seven-coordinate complexes was six times less than that
in eight-coordinate complexes as shown in Fig. 5. The emission
lifetimes of complexes 2–5 are between 14 and 20 ms, which
Fig. 7 Near-infrared emission spectra of 4 in different solvents at room
temperature (excitation wavelength = 375 nm; concentration = 4.0 ¥
10-5 mol L-1).
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direct coordination of methanol to ytterbium(III), the lifetimes of
single crystals of 4 that has no solvated methanol in the solid
were measured in anhydrous CH3OH and CD3OD. The number
of inner-sphere methanol molecules (q) was estimated from eqn
(4) (developed by Beeby et al.54):
the near-infrared region. In this report, we described structural and
photophysical properties of several seven- and eight-coordinate
ytterbium(III) tetraphenylporphyrinate complexes having diimines
as ancillary ligands. The eight-coordinate complexes exhibited
enhanced NIR emission and increased lifetime relative to the
seven-coordinate species. The energy transfer from the triplet
state of the porphyrin to the excited state of ytterbium(III) was
faster than that to molecular oxygen. The NIR emission of seven-
coordinate complexes is sensitive to solvent oscillators in both the
inner- and outer-coordination spheres, whereas the emission of
eight-coordinate complexes are only quenched by the less-effective
outer-sphere solvent.
qYb = 2(kCH
- kCD
- B)
(4)
OH
OD
3
3
where B is a corrective factor, and k is the decay rate in ms-1. From
the decay curves, as shown in Fig. S1 in the ESI,† the lifetimes at
978 nm in CH3OH and CD3OD were 7.2 and 17.2 ms, respectively.
The calculated q values were close to zero, if B is between 0.05 and
0.1. This indicates that oscillators in the outer coordination sphere
also quench the NIR emission in eight-coordinate complexes.61
To further confirm this outer-sphere quenching effect, complexes
4 and 6 were blended into poly(methylmethaceylate) (PMMA),
which is known to have a high percentage of C–H bonds.22,33,62–66
The spectra were very similar to those in solution, as shown in
Fig. 8. The lifetimes, as shown in the inset of Fig. 8, were 12.6 and
10.2 ms for 4 and 6 in PMMA, respectively. These values were lower
than those obtained in toluene, but higher than that in methanol.
The decrease relative to toluene may come from a clustering effect
or from C–H oscillation of the PMMA. The clustering effect is
common in lanthanide salt-doped polymers. Zheng et al.67 studied
the fluorescence of [Eu(DBM)(Phen)] (DBM = dibenzoylmethide)
in PMMA and no clustering effect was observed. In our system, the
ytterbium(III) was also encapsulated by a macrocyclic porphyrin
ring and the resulting films exhibited high homogeneity; therefore
it is unlikely that a clustering effect plays a significant role
in quenching the NIR emission in PMMA. On the contrary,
the decay curves fitted well with a single exponential function
indicating the quenching effect mostly came from outer-sphere
interaction. In this regard, the blending of these complexes into
deuteriated or fluorinated polymers will be very beneficial for
achieving high emission efficiency. Research in this area is ongoing.
Acknowledgements
This material is based upon work supported by NSF/EPSCoR-
0554609.
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