Inorganic Chemistry
Article
Table 2. Calculated Excitation Energy (E), Wavelength (λ), Oscillator Strength (f), Main Orbital Contribution, and Charge
a
Characters of the T1 and S1 States of Pt(II) Complexes
complex
state
E [eV]
λ [nm]
f
orbital contribution (>10%)
assignment
Pt(ppy-1)
T1
S1
2.472
2.716
2.472
2.669
2.249
2.519
2.196
502
457
502
465
551
492
565
0
HOMO → LUMO (87%)
HOMO → LUMO (96%)
HOMO → LUMO (80%)
HOMO → LUMO (97%)
HOMO → LUMO (72%)
HOMO → LUMO (97%)
HOMO → LUMO (57%)
HOMO → LUMO+1 (21%)
HOMO → LUMO (94%)
3MLCT(πppyCOOdPt → πPy*)
1
0.0196
1LC(πppy → πppy*), MLCT(πppyCOOdPt → πPy*)
Pt(bp-6)
Pt(bp-7)
Pt(bp-8)
T1
S1
0
3MLCT(πbpdPt → πPy*)
1
0.0212
1LC(πbp → πbp*), MLCT(πbpdPt → πPy*)
T1
S1
0
3MLCT (πbpdPt → πPy*)
1
0.0073
0
1LC(πCzPh → πCzPh*), MLCT(πbpdPt → πPy*)
T1
3MLCT(πbpdPt → πACz*)
1
S1
2.407
515
0.0338
1LC(πCzPh → πCzPh*), MLCT(πbpdPt → πACz*)
a
Calculated by the TD-B3LYP method with a basis set of SV for H, SVP for C, F, and N atoms, and a def2-TZVP basis set for the Pt atom based on
optimized S0 geometries.
complexes (1.6−2.6°) (Table S2). Notably, the coordination
bonds Pt−N1 in Pt(bp-7) (2.232 Å) and Pt(bp-8) (2.237 Å)
are significantly longer than those in Pt(ppy-1) (2.047 Å),
Pt(bp-6) (2.187 Å), and the previously reported Pt(bp-1),
Pt(bp-2), and Pt(bp-3) (2.182−2.186 Å)61 (Figure 1); this is
attributed to that the rigid Cz segment in Pt(bp-7) and Pt(bp-
8) makes its 9-position pyridine away from the central Pt(II)
ion. This is also supported by the X-ray analyses that the bond
length of Pt−N1 in Pt(bp-7) (2.1460(19) Å) is significantly
longer than that of Pt(bp-6) (2.115(2) Å), and the Ac moiety
in Pt(bp-6) is a seriously twisted chair configuration. In
contrast, the Cz moiety in Pt(bp-7) is a planar configuration
H···π interactions are observed in Pt(bp-6) between the methyl
group and the pyridine ring (Figure 2a), and intermolecular
π−π interactions are observed in Pt(bp-7) between the two
pyridine rings with a distance of 3.396 Å (Figure 2b).
Pt(bp-6), Pt(bp-7), and Pt(bp-8) show similar highest
occupied molecular orbital (HOMO) distributions predom-
inantly on the πAcPh, πCzPh orbitals and the dPt centers, and the
lowest unoccupied molecular orbital (LUMO) distributions
dominantly occupy on the two πPy for Pt(bp-6) and Pt(bp-7),
and exclusively on πACz for Pt(bp-8) (Figure S1), indicative of
a stronger electron-withdrawing ability of the ACz compared
to Py. Pt(ppy-1) has significantly different HOMO and LUMO
distributions. The HOMO is mainly on the πAc-pPt‑O-πOC
orbitals, and the LOMO is dominantly on the electron-
deficient two πPy moieties (Figure S1). Additionally, Pt(ppy-1)
exhibits stabilized HOMO and LUMO levels (−5.40 and
−1.89 eV, respectively) compared to the bp-based Pt(II)
complexes, which are −4.67, −4.65, and −4.61 eV for HOMO
levels, and −1.28, −1.42, and −1.61 eV for HOMO levels of
Pt(bp-6), Pt(bp-7), and Pt(bp-8), respectively (Figure S1),
because the electron-withdrawing ability of the ppy is much
stronger than that of the bp moiety. This result can be also
supported by the electrochemical study that Pt(ppy-1) has a
greatly larger oxidation potential of 0.51 eV than those of the
bp-based Pt(II) complexes of 0.20−0.27 eV, and less negative
reduction potential of −2.10 eV compared to the other Pt(II)
complexes of −2.61 to −2.34 eV (Table 1). These significant
differences reveal that the redox processes in ppy-based Pt(ppy-
1) are different with those in the bp-based Pt(II) complexes.
The oxidation process of Pt(ppy-1) is mainly on the Ac-Pt
moiety, and the reduction process is dominantly on the
electron-deficient two Py rings. Pt(bp-6) has the oxidation
process on the bp-Pt moiety, and reduction process on the Py
ring. Pt(bp-7) and Pt(bp-8) have the oxidation processes on
the CzPh-Pt moieties; however, the reduction processes are
much different, mainly on the Py ring and ACz moiety,
respectively. Moreover, the HOMO and LUMO levels
calculated from the redox values are in good agreement with
the trend from the DFT calculations (Table 1). All the Pt(II)
complexes exhibit irreversible redox processes except the
reduction process of Pt(ppy-1) (Figure S2). These results
demonstrated that the distribution and energy levels of the
frontier orbitals can be effectively regulated through rational
ligand design.
Photophysical and Excited-State Properties. The UV−
vis absorption spectra of the Pt(II) complexes and their
corresponding ligand in dichloromethane (DCM) solutions at
room temperature (RT) are illustrated in Figures 3 and S3. For
all the Pt(II) complexes, the intense absorption bands below
325 nm are assigned as spin-allowed ligand-centered (1LC) π
→ π* transitions from localized ppy in Pt(ppy-1), localized bp
in Pt(bp-6), and from localized CzPh in Pt(bp-7) and Pt(bp-8)
on the basis of the time-dependent density functional theory
(TD-DFT) calculations47,61 (Tables 2, S5−S8) and natural
transition orbital (NTO) analyses61,64 (Figure S4). Notably,
Pt(ppy-1) exhibits much weaker π → π* transitions than those
of the other Pt(II) complexes because of less aryl groups in
Pt(ppy-1) (Figure 3). The absorptions at the region of 340−
475 nm are attributed to spin-allowed metal-to-ligand charge-
transfer (1MLCT) transitions, which are involved with both
the center Pt(II) ion and the cyclometalating ligand (Figure 3).
3
Pt(ppy-1) has a very weak unstructured MLCT transition
involving πppyCOOdPt → πPy* (Figure S4). However, Pt(bp-6),
3
Pt(bp-7), and Pt(bp-8) show well-resolved MLCT transitions
at about 498, 550, and 555 nm, respectively, involving πbpdPt →
πPy* for Pt(bp-6) and Pt(bp-7), and πbpdPt → πACz* for Pt(bp-
8) (Figure S4); these 3MLCT transitions are in good
agreement with the T1 absorptions by the TD-DFT
calculations of 502, 551, and 565 nm, respectively (Table 2).
The emission spectra of the Pt(II) complexes in various
conditions are illustrated in Figures 4 and S5, and the data are
recorded in Table 3. Ligand modifications have a great
influence on the photophysical properties. At 77 K in 2-
methyltetrahydrofuran (2-MeTHF), all the Pt(II) complexes
show well-resolved vibronic emission spectra, indicating the
30,62
3
dominant LC characters in their T1 states.
Pt(ppy-1) and
Pt(bp-6), both of which have PyAc-containing ligands, show a
dominant emission peak in high energy region at 508 and 500
nm, respectively (Figure 4). By contrast, PyCz-based Pt(bp-7)
and Pt(bp-8) have a dramatic red shift of about 50 nm and
show a dominant emission peak at 553 and 557 nm,
E
Inorg. Chem. XXXX, XXX, XXX−XXX