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Table 1. Absorption and photoluminescence data of Gd-1–4 and Yb-1–2.
[b]
Compound
UV/Vis absorption lmax [nm] (loge [Mꢀ1 cmꢀ1])
Emission lmax [nm] (t [ms])[a]
Fp
Soret band Q bands
Gd-1
Gd-2
Gd-3
Gd-4
Yb-1
Yb-2
422 (5.80)
424 (5.64)
428 (5.48)
428 (5.46)
422 (5.77)
424 (5.55)
553 (4.45), 588 (3.75)
566 (4.18), 610 (4.71)
539 (3.90), 583 (4.13), 635 (4.66)
543 (3.99), 614 (4.19), 665 (5.21)
553 (4.44), 588 (3.77)
728 (111.1), 828, 920
817 (51.7), 924, 1050
896 (19.1), 995
995 (15.7), 1164
916, 949, 970 (26.2), 995, 1018, 1044
913, 949, 970 (30.0), 996, 1017, 1048
0.039
0.025
0.005
0.006
/
566 (4.12), 611 (4.66)
/
[a] Determined in degassed CH2Cl2 solutions at room temperature. For Gd-1–4, the emissions refer to ligand-centered phosphorescence; for Yb-1–2, the
emissions refer to metal-centered luminescence. [b] Referenced to the ligand-centered phosphorescence emission and determined in degassed CH2Cl2 sol-
utions at room temperature by using ZnTPP in toluene (F=0.033) as reference.
Zn, Pt, and Pd analogs,[11d,e] and suggests the capability for ef-
fective excitation in the deep-red to NIR region.
replacement, whereas the first reduction potentials (ꢀ1.19 to
ꢀ0.63 V) were highly related and the ease of reduction follows
the trend Gd-4>Gd-3>Gd-2>Gd-1. Thus, the electrochemi-
cally determined HOMO–LUMO gaps decreased from Gd-1 to
Gd-4, which is consistent with the redshifted Q band absorp-
tions (Figure 2).
Upon excitation at approximately 425 nm (B band maxima)
in degassed CH2Cl2 at room temperature, Gd-1–4 exhibited vi-
brationally structured emission bands at about 700–1300 nm
(Figure 2b). According to previous spectroscopic studies on
GdIII porphyrinates[7a,b] and PtII or PdII-based porpholactona-
tes,[11e,13] such low-energy emission bands for Gd-1–4 are ten-
tatively assigned as the triplet states of intraligand (IL, p!p*)
transitions. Simultaneously, emission at 580–700 nm, originat-
ing from the singlet states of 1–4 is strongly reduced (by ca.
99%) for both the Gd and Yb complexes, which could be at-
tributed to the spin-orbital coupling as a result of the heavy
atom effect of the Ln3+ ions. The measured lifetimes (111.1,
51.7, 19.1, and 15.7 ms for Gd-1–4, respectively) are in the mi-
crosecond range and the air-sensitive emissions confirmed the
observation of phosphorescence (Table 1 and Figures S2–5 in
the Supporting Information). Compared with Gd-1, Gd-2–4
showed the 0–0 phosphorescence band was redshifted by ap-
proximately 89, 168, and 267 nm respectively. The measured
lifetime and quantum yield decreased with the triplet state
energy, suggesting that the emission decay for this family of
Gd complexes follows the energy gap law.[21] For comparison,
Yb-1–2 display only air-insensitive, characteristic NIR emission
Singlet oxygen sensitization
The excited triplet states of Gd-1–4 were probed by nanosec-
ond transient absorption (TA) measurements in degassed or
oxygen-saturated (O2 bubble) CH2Cl2 at room temperature by
using 355 nm nanosecond pulsed Nd:YAG laser excitation. TA
spectra displayed major triplet absorption bands for Gd-1–4 at
approximately 370 and 460 nm. Mono-exponential decay life-
times of the triplet state were measured as approximately 79.7,
43.5, 15.2, 13.2 ms for Gd-1–4, respectively (Figures S7–10 in
the Supporting Information), which were generally consistent
with the emission decay lifetimes (Table 1). In the presence of
O2, the lifetimes of all GdIII complexes dramatically decreased
to approximately 1.6, 0.5, 0.2, 0.2 ms for Gd-1–4, respectively,
strongly suggesting the efficiency of O2 quenching on the ex-
cited triplet states of GdIII porphyrinates or porpholactonates.
However, Yb-1–2 showed air-insensitive TA spectra (Figur-
es S11–12 in the Supporting Information), which is ascribed to
more efficient intramolecular energy transfer from the triplet
excited states of the porphyrinoid ligands to Yb3+ than inter-
molecular energy transfer to ground-state O2.
2
2
of Yb3+ at 900–1100 nm derived from F5/2! F7/2 transitions
(Figure S1 in the Supporting Information). The lack of ligand-
centered phosphorescence for YbIII complexes indicated that
the IL state of porphyrinoids could be quenched efficiently by
the excited-state of Yb3+ (2F5/2, ca. 10200 cmꢀ1) through intra-
molecular photoinduced energy transfer, which strongly sug-
gested the importance of the choice of Ln ions on the design
of Ln photosensitizers.
As oxygen efficiently quenches the phosphorescence of GdIII
complexes, it is necessary to investigate whether and how
1
much O2 could be produced upon light irradiation. Qualitative
The redox properties of the GdIII complexes were measured
by cyclic voltammograms (CV) (Table S2 and Figure S6 in the
Supporting Information) in acetonitrile (CH3CN) solutions in the
presence of 0.1m tetra-n-butylammonium hexafluorophos-
phate (NBu4PF6) at room temperature. In general, Gd-1–4 dis-
played redox couples that are tentatively assigned as the re-
duction and oxidation of porphyrinoid ligands with reference
to the electrochemical studies on the related Zn, Pd, and Pt
complexes.[11d,e] Importantly, the first oxidation potentials (+
1.08 to +1.14 V) versus Ag/AgCl electrode for Gd-1–4 were in-
sensitive to the extent and orientation of b-oxazolone moiety
assessment was performed by electronic paramagnetic reso-
nance (EPR) using 2,2,6,6,-tetramethyl-4-piperidine (TMPD) as
a O2 quencher.[22] Upon reaction with O2, TMPD can give rise
to a detectable spin-active species 2,2,6,6-tetramethylpiperidin-
1-yl)oxyl (TEMPO).[23] As shown in Figure 3a, the EPR signal of
TEMPO significantly increased after addition of Gd-2 upon
light irradiation (l>400 nm) (Figure 3b) and can be inhibited
1
1
1
by sodium azide, a known O2 scavenger, confirming the pro-
1
duction of O2. For Yb-2, the EPR signal of TEMPO was very
weak under the same reaction conditions (Figure 3b and Fig-
ure S13 in the Supporting Information). These results con-
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Chem. Eur. J. 2016, 22, 1 – 12
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