1. Introduction
Emissive transition metal complexes (TMCs) with dual or multiband emissions have attracted much attention because of their intriguing luminescence properties [1,2]. A number of dual-emissive mononuclear TMCs have been developed including ruthenium [3,4,5,6,7], iridium [8,9,10], platinum [11], and other metal complexes [12,13,14]. Due to the presence of two or multiple distinct emission bands, these kinds of materials have been widely used in the ratiometric sensing and imaging of metal ions [15], O2 [16,17,18,19,20], and biomacromolecules (protein and DNA) [21,22,23], with improved sensing accuracy and reliability.
One strategy to construct dual-emissive TMCs is by tuning the ligand electronic structures of mononuclear complexes such as introducing additional organic luminophores [17,18,24,25] or new ligand-localized charge-transfer excited states [19,26,27]. Another strategy is the synthesis of dinuclear or multinuclear bridged complexes with the same or different metal ions, which may exhibit interesting photophysical properties by tuning the electron/energy transfer among individual metal components [28,29,30]. Examples of dual-emissive dinuclear TMCs based on ruthenium [31] and platinum [32,33] complexes have been documented recently, stimulating the design and synthesis of new bridged complexes with intriguing emission properties.
In a previous work, we reported a bis-bidentate ligand dapz (dapz = 2,5-di(N-methyl-N′-(pyrid-2-yl)amino)pyrazine; Figure 1) with a larger bite angle and its application in the synthesis of a dual-emissive mononuclear ruthenium complex [34]. To extend our research interest in the development of dual-emissive TMCs, an IrIII-PtII heterodimetallic complex [(ppy)2Ir(dapz)PtCl2]Cl (4) and the corresponding monometallic complexes [(ppy)2Ir(dapz)]Cl (3) and [(dapz)PtCl2] (2) were designed and prepared. The photophysical properties of these complexes are presented herein. Moreover, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed and are discussed.
2. Experimental Section 2.1. Spectroscopic Analysis A TU-1810DSPC spectrometer of Beijing Purkinje General Instrument Co. Ltd. (Beijing, China) was used to measure the absorption spectra at ambient conditions. A F-380 spectrofluorometer of Tianjin Gangdong Sci. & Tech. Development Co. Ltd. (Tianjin, China) was used to measure the emission spectra. Samples for absorption and emission measurements were prepared within quartz cuvettes of a 1 cm path length. Luminescence quantum yields were calculated by using quinine sulfate in 1.0 M aq H2SO4 (Φ = 55%) as the standard. The uncertainty was about ±10% or better. A Bruker Advance 300 or 400 MHz spectrometer (Bruker, Billerica, MA, USA) was used to obtain the 1H and 13C NMR spectra. A Waters MALDI micro MX mass spectrometer (Bruker, Karlsruhe, BW, Germany) was used to record the MALDI-TOF positive ion data in a reflection mode. A Flash EA 1112 or Carlo Erba 1106 analyzer (Thermo Fisher Scientific, Waltham, MA, USA) was used to record elemental analysis results. Thermal analysis was carried out using a PerkinElmer TGA 8000 under nitrogen (PerkinElmer Inc., Waltham, MA, USA). 2.2. Emission Lifetime Analysis The luminescence decays were analyzed on a Hamamatsu Quantaurus-Tau Fluorescence lifetime spectrometer C11367 (Hamamatsu Photonics K.K., Shizuoka, Japan) and a nanosecond flash photolysis setup (Edinburgh FLS920 spectrometer) equipped with a picosecond pulsed diode laser (Edinburgh Instruments, Edinburgh, UK). 2.3. DFT and TDDFT Calculations Gaussian 09 package (revision 09, Gaussian Inc., Wallingford, CT, USA) and the B3LYP exchange correlation functional were used to carry out the DFT calculations. The electronic structures were optimized with the LANL2DZ basis set for Ir, Pt, and 6-31G* for other atoms. All calculations were performed taking into account the solvation effects in CH3CN. No symmetry constraints were used for all of the calculations. In order to ensure the optimized geometries to be local minima, frequency calculations were carried out at the same level of theory. All orbitals were computed by setting an isovalue of 0.02 e bohr−3. With the same level of theory, the DFT-optimized structures were used for the subsequent TDDFT calculations. 2.4. X-ray Crystallography
A Rigaku Saturn 724 diffractometer on a rotating anode (Mo Kα radiation, 0.71073 Å) was used to collect the X-ray diffraction data at 173 K. The structure solution and refinement were performed on SHELXS-97 and Olex2 software (version Olex2.refine, Durham University, UK). Olex2 software was used to generate the structure graphics shown in Figure 2.
2.5. Synthesis All reagents were purchased from commercial resources and used as received. Standard Schlenk techniques were employed to run all reactions under N2.
2.5.1. [(dapz)PtCl2] (2)
A suspension of K2PtCl4 (41.5 mg, 0.1 mmol) and dapz (1) (29.2 mg, 0.1 mmol) [34] in MeOH/H2O (5 mL/5 mL) was refluxed for 12 h. After cooling to room temperature, the solvent was evaporated under reduced pressure. Silica gel chromatography (eluent: CH2Cl2/MeOH = 100/5) was used to purify the crude product to provide 15 mg of complex 2 as a yellow solid in 27% yield. 1H NMR (400 MHz, CD2Cl2): δ 3.59 (s, 3H), 3.66 (s, 3H), 7.04 (t, J = 6.4 Hz, 1H), 7.09 (t, J = 6.8 Hz, 1H), 7.16 (t, J = 7.6 Hz, 2H), 7.72 (t, J = 8.4 Hz, 1H), 7.91 (t, J = 8.0 Hz, 1H), 8.16 (s, 1H), 8.35 (d, J = 4.8 Hz, 1H), 9.00 (d, J = 6.0 Hz, 1H), 9.25 (s, 1H). MALDI-MS (m/z): 523.2 for [M–Cl]+, 486.2 for [M – 2Cl – 1]2+. Anal. Calcd. for C16H16Cl2N6Pt⋅H2O: C, 33.34; H, 3.15; N, 14.58. Found: C, 33.57; H, 3.01; N, 14.18.
2.5.2. [(. ppy)2Ir(dapz)]Cl (3)
A mixture of dapz (21 mg, 0.07 mmol) and [(ppy)2IrCl]2 (32 mg, 0.03 mmol) was added to the mixed solvent of 5 mL of CH2Cl2 and 5 mL of MeOH. The mixture was heated under reflux for 18 h. The solvent was then removed under reduced pressure. The crude product was purified by flash chromatography using silica gel (eluent: CH2Cl2/MeOH = 100/8) to give 45 mg of complex 3 as a yellow solid in 91% yield. 1H NMR (400 MHz, CD2Cl2): δ 3.41 (s, 3H), 3.65 (s, 3H), 6.12 (d, J = 7.6 Hz, 1H), 6.16 (d, J = 7.6 Hz, 1H), 6.71 (t, J = 7.6 Hz, 1H), 6.80–7.00 (m, 6H), 7.18–7.24 (m, 2H), 7.54–7.59 (m, 3H), 7.64 (d, J = 7.6 Hz, 1H), 7.74 (d, J = 5.2 Hz, 1H), 7.87–7.97 (m, 7H), 8.09 (d, J = 6.0 Hz, 1H), 8.26 (d, J = 6.0 Hz, 1H), 8.59 (s, 1H). 13C NMR (75 MHz, CD3CN): δ 35.6, 40.7, 114.3, 116.6, 119.4, 120.3, 120.7, 121.0, 123.0, 123.1, 123.3, 123.6, 125.2, 125.5, 130.5, 130.7, 132.1, 132.2, 135.2, 137.4, 139.2, 140.9, 144.5, 144.6, 144.8, 148.6, 149.7, 150.3, 150.7, 151.0, 151.5, 156.4, 157.6, 167.9. MALDI-MS (m/z): 793.5 for [M – Cl]+. MALDI-HRMS (m/z) calcd. for C38H32N8Ir: 793.2375. Found: 793.2371. Anal. Calcd. for C38H32ClN8Ir⋅2H2O: C, 52.80; H, 4.20; N, 12.96. Found: C, 52.75; H, 4.24; N, 12.63.
2.5.3. [(. ppy)2Ir(dapz)PtCl2]Cl (4)
A suspension of complex 3 (149 mg, 0.18 mmol) and K2PtCl4 (83 mg, 0.2 mmol) in MeOH/H2O (10 mL/10 mL) was refluxed under N2 for 1 h. After removing the solvent under reduced pressure, the crude product was purified by silica gel chromatography (eluent: CH2Cl2/MeOH = 100/15) to afford 140 mg of complex 4 as an orange solid in 71% yield. 1H NMR (400 MHz, CD3CN): δ 2.94 (s, 3H), 3.56 (s, 3H), 6.16 (d, J = 7.6 Hz, 1H), 6.22 (d, J = 7.6 Hz, 1H), 6.86 (t, J = 7.2 Hz, 1H), 6.94 (t, J = 8.0 Hz, 2H), 7.02 (q, J = 8.4 Hz, 2H), 7.09–7.17 (m, 3H), 7.22 (t, J = 6.4 Hz, 1H), 7.54–7.60 (m, 2H), 7.68 (s, 1H), 7.74 (d, J = 7.6 Hz, 1H), 7.81 (d, J = 7.6 Hz, 1H), 7.93–7.98 (m, 4H), 8.05 (d, J = 7.6 Hz, 1H), 8.12–8.16 (m, 2H), 8.20 (d, J = 5.6 Hz, 1H), 8.82 (d, J = 5.6 Hz, 1H), 9.22 (s, 1H). MALDI-MS (m/z): 1058.1 for [M – Cl]+. MALDI-HRMS (m/z) calcd. for C38H32Cl2IrN8Pt: 1058.1390. Found: 1058.1406. Anal. Calcd. for C38H32Cl3IrN8Pt⋅3H2O: C, 39.74; H, 3.34; N, 9.76. Found: C, 39.45; H, 3.16; N, 9.83. 3. Results and Discussion 3.1. Synthesis and Single Crystal X-ray Analysis
The compounds that were studied in this work are displayed in Figure 1. The bridging ligand dapz (1) was prepared according to the previously reported procedure [34]. The treatment of K2PtCl4 with 1 in a mixture of methanol and water afforded the mononuclear PtII complex 2 in 27% yield. The yield of this reaction was not optimized. However, when this reaction was performed in acetic acid [35,36] or a mixture of water and acetonitrile [37,38], no desired product could be isolated, and the reason is not clear at this stage. By the treatment of ligand 1 with the IrIII dimer [(ppy)2IrCl]2 (ppy = 2-phenylpyridine) in a mixture of methanol and dichloromethane [39], the mononuclear IrIII complex 3 was isolated in a high yield of 91% after purification by flash column chromatography. When complex 3 was further treated with one equiv of K2PtCl4 in a mixture of methanol and water, the IrIII-PtII heterodimetallic complex 4 was obtained in 71% yield after flash column chromatography. All new compounds were thoroughly characterized by mass and NMR spectrometry, as well as elemental analysis. The comparison of 1H NMR spectra of 1–4 is shown in Figure S1. After coordination with a [PtCl2] or [(ppy)2Ir] unit, the methyl protons of ligand 1 were split into two peaks with some degree of shifts in complexes 2–4, suggestive of the asymmetric structures of these compounds. In addition, compared to those of ligand 1, one singlet pyrazine proton and one doublet α-pyridine proton of the platinum complexes 2 and 4 shift distinctly to the low field. HPLC analysis shows high purity of over 99% for both dapz and 4 (Figure S2). The 1H and 13C NMR spectra of new complexes are provided in the Supporting Information (Figures S3–S6). The thermogravimetric analysis indicated that complexes 3 and 4 show thermal decomposition temperatures of 178 and 281 °C at a 5% weight loss, respectively (Figure S7).
The structure of complex 4 was also confirmed by single-crystal X-ray analysis. The ORTEP and crystal packing of 4 are shown in Figure 2. Corresponding crystallographic data are summarized in Tables S1 and S2. The single crystal of 4 was obtained by the slow diffusion of diethyl ether into a solution of 4 in acetonitrile. The Ir atom adopts a distorted octahedral configuration surrounded by dapz and two ppy ligands, whereas the Pt atom possesses a square planar coordination geometry. The N-Ir-N bite angle with the dapz ligand (85.94(14)°) is slightly larger with respect to the C-Ir-N bite angles associated with two ppy ligands (80.44(15) and 80.63(16)° respectively). The N-Pt-N bite angle with the dapz ligand (87.95(16)°) is also larger than those of most platinum complexes with N∧N bidentate ligands, which fall in the range of 77.5(2)–79.02(14)° [40,41,42]. The Ir-N bond lengths with dapz (2.167(4) and 2.174(3) Å) are slightly longer than the Ir-C bond lengths with ppy (2.063(3) and 2.055(3) Å). The Pt-N bond lengths with dapz (2.007(4) and 2.032(5) Å) are similar to those of known platinum complexes [38,40,41,42]. As suggested by Figure 2b, the intermolecular Cl∙∙∙Cl distance is 4.248 Å, indicating the absence of Cl∙∙∙Cl interaction. The intermolecular aromatic plane∙∙∙plane and Pt∙∙∙Pt distances of complex 4 are 3.886 and 3.839 Å, respectively. The distance is slightly larger than, but close to, the Pt∙∙∙Pt interaction distance [43,44] and conventional π–π stacking [45]. This suggests that the intermolecular π–π and Pt∙∙∙Pt interactions, if present, are rather weak in the crystal of complex 4.
3.2. DFT Calculations
In order to understand the electronic structure of these complexes, DFT calculations were performed on 2, 3+, and 4+ (counteranions were not considered) with the B3LYP/LANL2DZ/6-31G*/CPCM method. Figure 3a shows the calculated energy diagram of these complexes. The frontier energy gaps of 2 and 3+ were calculated to be 3.73 and 3.65 eV, respectively. Complex 4+ has a calculated energy gap of 2.9 eV, which is much narrower than those of 2 and 3+. This is mainly ascribed to the stabilization of the lowest unoccupied molecular orbital (LUMO) level of 4+. The isodensity plots of some representative orbitals of these complexes are shown in Figure S8 (2), Figure S9 (3+), and Figure 3b (4+). The highest occupied molecular orbital (HOMO) of complex 2 is dominated by the ligand dapz, while the HOMO–1, HOMO–2, and HOMO–3 of 2 are associated with the platinum component (Figure S8). The LUMO of 2 has a major contribution from the central pyrazine unit of dapz and the [PtCl2] unit and the pyridine segments of dapz make bigger contributions to the higher unoccupied orbitals. The HOMO and HOMO–1 of complex 3 are associated with the iridium component and ligand dapz, while its HOMO–2 and HOMO–3 are dominated by the iridium component (Figure S9). The LUMO of 3+ is also dominated by the central pyrazine unit of dapz and its higher unoccupied orbitals are associated with ppy ligands. The HOMO, HOMO–1, and HOMO–3 of 4+ are dominated by the iridium component, while its HOMO–2 has significant contributions from the ligand dapz, with little contributions from the platinum component (Figure 3b). The LUMO of 4+ has the same character as those of 2 and 3+, associated with the central pyrazine unit of dapz. The LUMO+1 of 4+ is dominated by the [PtCl2] unit. Other higher unoccupied orbitals of 4+ are associated with the dapz and ppy ligands.
3.3. Absorption Studies and TDDFT Calculations
The electronic absorption spectra of compounds 1–4 are displayed in Figure 4a. Corresponding data, including absorption maxima and molar absorption coefficients, are delineated in Table 1. Transitions in the UV region are largely caused by intraligand π–π* excitations, while the absorption bands in the visible region are attributed to charge transfer (CT) transitions. The lower-energy side of the visible absorptions of the dimetallic complex 4 extends to the region with wavelengths longer than 500 nm, which are distinctly red-shifted with respect to that of 1–3. This is in agreement with the above DFT calculations showing that complex 4 has the narrowest energy gap.
In order to assist in the assignment of the electronic absorptions, the nature of the low energy transitions was investigated by TDDFT calculations. The predicted transitions of complexes 2, 3+, and 4+ are shown in Figure S10 (2 and 3+) and Figure 4b (4+). Table 2 collects major predicted transitions with the excitation energy, oscillator strength (f), dominant configuration contribution and assignment. The mononuclear platinum complex 2 shows a broad band centered at 402 nm. The TDDFT results indicate that the S1 state (f = 0.0648) is responsible for this band. This state is mainly assigned to the HOMO → LUMO excitation, with a character of intraligand charge transfer (ILCT) from the aminopyridine units to the central pyrazine unit of dapz. The mononuclear iridium complex 3 displays an absorption band centered at 370 nm. According to the TDDFT results, this band is ascribed to the metal to ligand charge transfer (MLCT) from the Ir component to ligand dapz.
Compared to those of the Ir complex 3, the absorptions of the Ir-Pt complex 4 are slightly red shifted, showing a broad band centered at 377 nm and some weak absorptions in the region 450–550 nm. The TDDFT results of 4 suggest that the Pt based MLCT transition (S12 excitation, f = 0.0408) may contribute to the relatively higher-energy absorptions of the compound. The lower-energy absorptions in the region 450–550 nm are more likely attributed to an admixture of the iridium-based MLCT, ILCT and ligand to ligand charge transfer (LLCT) transitions (S1, S2, and S4 states with f of 0.011, 0.057, and 0.0301, respectively). 3.4. Emission Spectroscopic Studies
The above complexes were further investigated regarding their emission properties. Their emission spectra are shown in Figure 5; Figure 6, and the luminescence data are provided in Table 1. The ligand dapz 1 exhibited an emission band at 460 nm with a quantum yield of 43% and a lifetime of 10 ns in nitrogen-saturated CH3CN (Figure 5a) [34]. The mononuclear platinum complex 2 shows an emission band at 432 nm with a quantum yield of 3.2%, and a lifetime of 6 ns in nitrogen-saturated CH3CN. The shape of the emission spectrum is independent of the excitation wavelength and the emission lifetime is independent of the emission wavelength (Figure 5b,c). These data suggest that the emission of 2 has a Pt-perturbed 1ILCT character. The incorporation of the [PtCl2] component into ligand dapz leads to the significant decrease of the emission quantum yield by the heavy atom effect. The mononuclear iridium complex 3 displays an emission band at 454 nm, with a quantum yield of 5.8% in CH3CN under nitrogen-saturated conditions. As suggested by TDDFT calculations, we believe that the dapz-targeted 3MLCT transition is responsible for the observed emission at 454 nm. The excited-state lifetime was determined as 19 and 135 ns in air-equilibrated and nitrogen-saturated conditions, respectively. This is in accordance with the excited-state lifetime of the most reported mononuclear cyclometalated iridium complexes [46,47,48]. In addition, the emission lifetime of 3 is also basically independent of the decay wavelength (Figure 5e). These results suggest that the emissions of 2 and 3 were originated from a single singlet and triplet excited state, respectively.
In contrast, the heterodimetallic complex 4 exhibits dual emissive behavior dependent on the excitation wavelength (Figure 6). Upon excitation at 350 nm, complex 4 shows a major emission band at about 420 nm and a shoulder emission band at around 520 nm with a total Φ of 1.5% in dilute solution (5 × 10−6 M). The latter longer-wavelength emission becomes more distinct when a longer excitation wavelength is used. The emissions at 420 and 520 nm have a distinctly different excited-state lifetime (8 and 99 ns for 420 and 520 nm, respectively) (Figure 6b). As suggested by TDDFT calculations, the vertical S1 excitation with an admixture of MLCT and LLCT transitions may be responsible for the observed lower-energy emission. This suggests that the lower-energy emission at 520 of 4 has a similar 3MLCT character as that of the mono-Ir complex 3, while the higher-energy emission at 420 nm of 4 may have a similar Pt-perturbed 1ILCT character as that of the mono-Pt complex 2. This assignment is also in accordance with the excitation spectrum of the emission at 420 nm and 520 nm, displaying an excitation maximum at 348 and 380 nm, respectively (Figure 6c). The presence of dual emissions is likely a result of an inefficient intramolecular energy transfer process between the 1ILCT and 3MLCT state. A similar mechanism has been proposed for other heterodimetallic ReΙ-PtII complexes bridged by a nonconjugated aminopyridine ligand [49]. Concentration-dependent studies show that an emission band centered at 670 nm appears as the concentration was increased from 2.5 × 10−7 to 1.7 × 10−4 M (Figure 6d). This band is possibly caused by an excimeric emission due to weak intermolecular π–π stackings as suggested by the above single-crystal X-ray analysis [45].
4. Conclusions
In summary, three dapz-containing monometallic and heterodimetallic complexes were designed and prepared. The two monometallic complexes 2 and 3 display a single emission band in the visible region. Interestingly, the heterodimetallic complex 4 exhibits excitation wavelength-dependent dual emissions, which are assigned to the Pt-perturbed 1ILCT and Ir-based 3MLCT character, respectively. The inefficient energy transfer is believed for the observation of dual emissions. In contrast, most of the previously reported homo- or heterodinuclear complexes only exhibit a single emission band [50,51],consistent with Kasha’s rule [52]. This suggests the unique photophysical property of metal complexes with ligand dapz. It provides important information for the future design and synthesis of dual emission photofunctional materials, which are potentially useful in the time-gated or dual-wavelength ratiometric sensing and imaging of chemical or biological systems.
Enlarge this image.Figure 2.(a) ORTEP diagram of the single-crystal X-ray structure and (b) crystal packing of complex 4 with 50% probability of the thermal ellipsoids. For the reason of clarity, H atoms and solvents were omitted. The intermolecular aromatic plane∙∙∙plane, Pt∙∙∙Pt and Cl∙∙∙Cl distances are indicated in (b).
Enlarge this image.Figure 3.(a) DFT-calculated energy diagram of 2, 3+, and 4+. The HOMO-LUMO gap is indicated by an arrow. (b) Isodensity plots of frontier orbitals of 4+. Isovalue = 0.02 e bohr-3.
Enlarge this image.Figure 4.(a) UV/vis absorption spectra of 1-4 in acetonitrile at a concentration of 5.0 × 10-6 M. (b) Time-dependent DFT (TDDFT) results of complex 4+.
Enlarge this image.Figure 5.(a,b,d) Emission spectra of (a) 1, (b) 2, and (d) 3 excited at different wavelengths indicated (5 × 10-6 M in CH3CN). (c,e) Emission decay profiles of (c) 2 and (e) 3 at different emission wavelengths indicated (5 × 10-6 M in CH3CN). Excited at 360 nm.
Enlarge this image.Figure 6.(a) Emission spectra of 4 excited at different wavelengths indicated (5 × 10-6 M in CH3CN). (b) Emission decay profiles of 4 at different emission wavelengths indicated. Excited at 350 nm. (c) Excitation spectra of the emission at 400 and 520 nm of 4. (d) Emission spectra of 4 in CH3CN upon increasing the concentration from 2.5 × 10-7 to 1.7 × 10-4 M.
| Compound | λabs(max)/nm (ε/105 M−1cm−1) | λem(max)b/nm | τ (N2) c/ns | Φd |
|---|---|---|---|---|
| 1 | 309 (0.13), 369 (0.07) | 460 | 10 | 43% |
| 2 | 284 (0.24), 305 (0.22), 402 (0.07) | 432 | 6 (at 410 nm) | 3.2% |
| 3 | 252 (1.13), 303 (0.62), 370 (0.15) | 454 | 135 (at 480 nm) | 5.8% |
| 4 | 264 (0.96), 317 (0.43), 377 (0.21), 491 (0.02) | 420/520 | 8/99 | 1.5% |
a All spectra were measured in a 1.0 cm quartz cell (5 × 10−6 M in CH3CN). b The excitation wavelength is 340 nm for 1 and 2 and 360 nm for 3 and 4, respectively. c The data were simulated by a mono- (1, 2), bi- (3), and triexponential (4) decay. Recorded as the average lifetime τ in the case of multiexponential decay by τ = [A1(τ1)2 + A2(τ2)2 + ⋅⋅⋅ + An(τn)2]/(A1τ1 + A2τ2+ ⋅⋅⋅ + Anτn). d Relative quantum yield compared to that of quinine sulfate in 1.0 M aq H2SO4 (55%).
| Compound | Sn | E (ev) | λ (nm) | f | Dominant Transition(s) (Percentage Contribution b) | Assignment c |
|---|---|---|---|---|---|---|
| 2 | 1 | 3.10 | 399 | 0.0648 | HOMO → LUMO (77%) | ILdapzCT |
| 2 | 3.15 | 394 | 0.0245 | HOMO–1 → LUMO + 1 (34%) HOMO–2 → LUMO + 1 (28%) | MC | |
| 7 | 3.61 | 343 | 0.033 | HOMO → LUMO + 2 (50%) | ILdapzCT | |
| 8 | 3.68 | 337 | 0.0217 | HOMO → LUMO + 2 (35%) | ILdapzCT | |
| 10 | 3.90 | 318 | 0.122 | HOMO–3 → LUMO (70%) | MPtLdapzCT | |
| 3 | 1 | 2.99 | 415 | 0.0244 | HOMO → LUMO (97%) | MIrLdapzCT |
| 2 | 3.14 | 395 | 0.052 | HOMO → LUMO + 1 (89%) | MIrLppyCT/LdapzLppyCT | |
| 4 | 3.26 | 381 | 0.0565 | HOMO–1 → LUMO (87%) | MIrLdapzCT/LppyLdapzCT | |
| 5 | 3.52 | 353 | 0.0369 | HOMO → LUMO + 3 (85%) | MIrLdapzCT/LppyLdapzCT | |
| 11 | 3.83 | 324 | 0.1018 | HOMO–1 → LUMO + 3 (84%) | MIrLdapzCT/LppyLdapzCT | |
| 4 | 1 | 2.25 | 551 | 0.011 | HOMO → LUMO (99%) | MIrLdapzCT/LppyLdapzCT |
| 2 | 2.88 | 430 | 0.057 | HOMO–2 → LUMO (58%) HOMO–1 → LUMO (32%) | MIrLdapzCT/ILdapzCT | |
| 4 | 3.04 | 408 | 0.0301 | HOMO–3 → LUMO (61%) | MIrLdapzCT/LppyLdapzCT | |
| 9 | 3.20 | 388 | 0.062 | HOMO → LUMO+2 (91%) | MC | |
| 12 | 3.30 | 375 | 0.0408 | HOMO–5 → LUMO (17%) HOMO–5 → LUMO + 1 (19%) | MIrLdapzCT/MPtLdapzCT | |
| 14 | 3.40 | 364 | 0.0322 | HOMO–9 → LUMO (50%) | MPtLdapzCT |
a Computed at the B3LYP/LANL2DZ/6-31G*/CPCM level of theory. b Actual contributions [%] = (configuration coefficient)2 × 2 × 100. c ppy = phenylpyridine, dapz = 2,5-di(N-methyl-N′-(pyrid-2-yl)amino)pyrazine.
Supplementary Materials
The following are available online at https://www.mdpi.com/2073-4352/11/3/236/s1, Tables S1 and S2: crystallographic data of complex 4. Figure S1: The comparison of 1H NMR spectra of 1-4. Figure S2: Analytical HPLC of dapz and complex 4. Figures S3-S6: 1H- and 13C-NMR spectra of complex 2-4. Figure S7: Thermogravimetric traces of complexes 3-4. Figures S8-S9: Isodensity plots of selected frontier molecular orbitals of complex 2 and 3. Figure S10: TDDFT results of complex 2 and 3.
Author Contributions
Conceptualization, S.-H.W. and Y.-W.Z.; formal analysis, S.-H.W., D.-X.M., and Z.-L.G.; investigation, S.-H.W., Z.-L.G., J.-Y.S., J.M., and R.Y.; data curation, S.-H.W.; writing-original draft preparation, S.-H.W.; writing-review and editing, S.-H.W., J.-Y.S., and Y.-W.Z. All authors have written and revised the manuscript.
Funding
We thank the National Natural Science Foundation of China (22004041, 21925112, 21975264, and 21872154, 22090021), the Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University (ZQN-PY519), and Quanzhou City Science & Technology Program of China (2019C064R) for support with funding.
Conflicts of Interest
The authors declare no conflict of interest.
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Si-Hai Wu
1,*,
Dian-Xue Ma
2,
Zhong-Liang Gong
2,
Junjie Ma
1,
Jiang-Yang Shao
2,*,
Rong Yang
1 and
Yu-Wu Zhong
2,3,*
1School of Medicine, Huaqiao University, Quanzhou 362021, China
2Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
3School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
*Authors to whom correspondence should be addressed.
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