Characterization1H (400 MHz) and 13C{1H} (100 MHz) NMR spectra were recorded on a JEOL JNM-AL400 spectrometer. In 1H and 13C{1H} NMR spectra, tetramethylsilane (TMS) and/or residual solvent peaks were used as an internal standard. 27Al{1H} NMR (130 MHz) spectra were recorded on a JEOL JNM-ECZ500 spectrometer and referenced to 1.1 M Al(NO3)3 in D2O (0 ppm) as an external standard. The peak in 27Al{1H} NMR spectra at around 60 ppm was attributed to the signal from an NMR tube. HRMS was performed at the Technical Support Office (Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University), and the HRMS spectra were obtained on a Thermo Fisher Scientific EXACTIVE spectrometer for electrospray ionization (ESI) and for direct analysis in real time (DART). Single-crystal X-ray diffraction data were collected using a Rigaku R-AXIS RAPID-F. Data were collected at 93 K with graphite-monochromated Mo Kα radiation diffractometer and an imaging plate. Equivalent reflections were merged, and a symmetry related absorption correction was carried out with the program ABSCOR49. The structures were solved with SHELXT 201450 and refined on F2 with SHELXL51 on Yadokari-XG52 or ShelXle53. The program ORTEP-354 was used to generate the X-ray structural diagram. For crystallographic data, see Supplementary Table 1. Elemental analysis was performed at the Microanalytical Center of Kyoto University.Photophysical measurementsUV–vis absorption spectra were recorded on a SHIMADZU UV-3600 spectrophotometer. Fluorescence and phosphorescence emission spectra and phosphorescence decay were measured with a HORIBA JOBIN YVON Fluorolog-3 spectrofluorometer and an Oxford Optistat DN for temperature control. Absolute photoluminescence quantum yields were measured with a Hamamatsu Photonics Quantaurus-QY Plus C13534-01 and a sample holder for low temperature, A11238-05, was used for the measurements at 77 K. Picosecond photoluminescence (PL) lifetimes were measured with the second harmonic generation of a Ti:sapphire pulsed laser (wavelength: 400 nm, pulse width: 100 fs, time resolution: 40 ps) and a streak camera (Hamamatsu Photonics C4334-01).MaterialsAll reactions were performed under argon atmosphere using modified Schlenk line techniques and an MBRAUN glove box system, UNIlab, unless otherwise noted. Analytical thin layer chromatography was performed with silica gel 60 Merck F254 plates. Column chromatography was performed with Wakogel C-200 SiO2. n-BuLi (1.55 M in hexane, Kanto Chemical Co., Inc), AlMe3 (ca. 1.4 M in hexane, Tokyo Chemical Industry Co., Ltd.; TCI), AlEt3 (ca. 1.0 M in hexane, TCI), AlBr3 (99.999% trace metal basis, Sigma-Aldrich Co., LLC.; Aldrich), iodine (FUJIJILM Wako Pure Chemical Corporation; Wako), deoxygenated toluene (Wako), and deoxygenated hexane (Wako) were purchased from commercial sources and used as received. AlCl3 (99.999% trace metal basis, Aldrich) was purified by sublimation under inert atmosphere before use. LH37 and LAlCl22 were synthesized according to the reported literatures. LAlMe35, LAlBr36, and LAlI36 were synthesized by using modified procedures of the literature for the related compounds.Synthesis of LAlMeAlMe3 (1.3 mL, ca. 1.4 M in hexane, 1.8 mmol) was added dropwise to a solution of LH (1.0 g, 1.8 mmol) in 5 mL of toluene at 0 °C. The yellow transparent solution was stirred at 0 °C for 5 min, allowed to warm to 100 °C, and stirred for 16 h. All volatiles were removed under reduced pressure. The crude product was dissolved in ca. 80 mL of hot hexane under air. After filtration with a warmed apparatus, the filtrate was slowly cooled to room temperature to give bright yellow crystals suitable for X-ray analysis (0.96 g, 87% yield). 1H NMR (400 MHz, C6D6): δ 7.14–7.00 (m, 10H), 6.82–6.79 (m, 6H), 5.72 (s, 1H), 3.57 (sept, 4H, 3JH–H = 6.7 Hz), 1.35 (d, 12H, 3JH–H = 6.7 Hz), 0.94 (d, 12H, 3JH–H = 6.7 Hz), −0.22 (s, 6H); 13C{1H} NMR (100 MHz, C6D6): δ 170.8 (C=N), 144.4, 141.3, 139.8, 129.2, 128.8, 127.6, 127.1, 124.5, 103.2, 28.6, 26.4, 23.9, −9.98; 27Al{1H} NMR (130 MHz, C6D6): δ 148.4 (br); HRMS (ESI, m/z): [M + H]+ calcd. for C41H51AlN2 + H+, 599.3940; found, 599.3930; analysis (calcd., found for C41H51AlN2): C (82.23, 82.38), H (8.58, 8.43), N (4.68, 4.67).Synthesis of LAlEtAlEt3 (0.92 mL, ca. 1.0 M in hexane, 0.92 mmol) was added dropwise to a solution of LH (0.5 g, 0.92 mmol) in 5 mL of toluene at room temperature. The yellow transparent solution was allowed to warm to 100 °C, and stirred for 16 h. After filtration with a pre-warmed glass filter, the filtrate was concentrated to give a crude crystal. The product was recrystallized from hexane under air to afford a pure compound as a bright yellow crystal (0.38 g, 65% yield). 1H NMR (400 MHz, C6D6): δ 7.17–7.02 (m, 10H), 6.83–6.81 (m, 6H), 5.64 (s, 1H), 3.60 (sept, 4H, 3JH–H = 6.7 Hz), 1.39 (d, 12H, 3JH–H = 6.7 Hz), 1.37 (t, 6H, 3JH–H = 8.3 Hz), 0.95 (d, 12H, 3JH–H = 6.7 Hz), 0.34 (q, 4H, 3JH–H = 8.1 Hz); HRMS (ESI, m/z): [M + H]+ calcd. for C43H55AlN2 + H+, 627.4253; found, 627.4260.Synthesis of LAlClTo a slurry of LH (0.50 g, 0.91 mmol) and hexane (9 mL) was added n-BuLi (1.55 M in hexane, 0.65 mL, 1.0 mmol) dropwise at −78 °C. The mixture was stirred for 10 min at −78 °C, then allowed to slowly warm to r.t. and stirred for 5 h. The slurry was added to a suspension of freshly sublimed AlCl3 (0.24 g, 1.82 mmol) in hexane (5 mL) at −78 °C. The mixture was warmed to 50 °C and stirred for 19 h at the same temperature. The product was extracted with hexane then filtered with Merck Millipore 0.20 μm hydrophobic PTFE filter repeatedly. The filtrate was concentrated under reduced pressure. Analytically pure product was obtained by repeated crystallization from hexane with slow evaporation method. The crystals were collected and washed with hexane to give the spectroscopically pure and X-ray quality product (87 mg, 18% yield). 1H NMR (400 MHz, C6D6): δ 7.08–6.99 (m, 10H), 6.81–6.74 (m, 6H, Ar), 5.84 (s, 1H), 3.65 (sept, 4H, 3JH–H = 6.7 Hz), 1.47 (d, 12H, 3JH–H = 6.7 Hz), 0.94 (d, 12H, 3JH–H = 6.7 Hz); 13C{1H} NMR (100 MHz, CDCl3): δ 172.2, 144.6, 138.1, 137.9, 129.5, 129.0, 127.7, 127.6, 124.4, 103.6, 28.7, 26.1, 23.6; 27Al{1H} NMR (130 MHz, C6D6): δ 100.7; HRMS (DART, m/z): [M + H]+ calcd. for C39H45AlCl2N2 + H+, 639.2848; found, 639.2828; analysis (calcd., found for C39H45AlCl2N2): C (73.23, 73.27), H (7.09, 7.19), N (4.38, 4.33), Cl (11.08, 10.78).Synthesis of LAlBrTo a slurry of LH (0.50 g, 0.91 mmol) and hexane (9 mL) was added n-BuLi (1.55 M in hexane, 0.65 mL, 1.0 mmol) dropwise at −78 °C. The mixture was stirred for 10 min at −78 °C, then allowed to slowly warm to r.t. and stirred for 5 h. The yellow slurry was concentrated under reduced pressure. The yellow powder was washed with hexane and dried under reduced pressure to give the solid of the lithium complex (0.50 mg, 99%). To a precooled solution of the lithium complex (0.29 g, 0.53 mmol) in toluene (6 mL) was added a suspension of AlBr3 (0.14 g, 0.53 mmol) dropwise at −78 °C. The mixture was allowed to slowly warm to r.t. and stirred for 14 h. The turbid orange solution was filtered with a Merck Millipore 0.20 μm hydrophobic PTFE filter. The filtrate was concentrated under reduced pressure. Analytically pure product was obtained by repeated crystallization from mixed solvent of toluene and hexane (ca. 1/10, vol/vol). The crystals were collected and washed with hexane to give the spectroscopically pure and X-ray quality product (0.13 g, 33%). 1H NMR (400 MHz, C6D6): δ 7.08–7.00 (m, 10H), 6.79–6.73 (m, 6H), 5.90 (s, 1H), 3.72 (sept, 4H, 3JH–H = 6.7 Hz), 1.47 (d, 12H, 3JH–H = 6.6 Hz), 0.92 (d, 12H, 3JH–H = 6.7 Hz); 13C{1H} NMR (100 MHz, C6D6): δ 172.7, 145.0, 138.9, 138.4, 129.6, 129.2, 128.3, 127.8, 124.9, 104.9, 29.2, 26.9, 23.8; HRMS (DART, m/z): [M + H]+ calcd. for C39H45AlBr2N2 + H+, 727.1838; found, 727.1831; [M–Br]+ calcd. for C39H45AlBrN2+, 647.2576; found, 647.2570.Synthesis of LAlITo a solution of LAlMe (1.5 g, 2.5 mmol) in toluene (65 mL) was added a solution of I2 (1.6 g, 6.3 mmol) in toluene (14 mL) dropwise at 0 °C. The mixture was allowed to slowly warm to r.t. and stirred for 3 days. The dark red solution was filtered with a Merck Millipore 0.20 μm hydrophobic PTFE filter. The filtrate was concentrated under reduced pressure. Analytically pure product was obtained by repeated crystallization from mixed solvent of toluene and hexane. The yellow crystals were collected and washed with hexane to give the spectroscopically pure and X-ray quality product (0.82 g, 40% yield). 1H NMR (400 MHz, C6D6): δ 7.08–7.00 (m, 10H), 6.77–6.75 (m, 6H), 6.02 (s, 1H), 3.78 (br, 4H), 1.47 (d, 12H, 3JH–H = 6.6 Hz), 0.90 (br, 12H); 13C{1H} NMR (100 MHz, C6D6): δ 172.8, 145.1, 139.2, 138.5, 129.6, 129.1, 128.5, 128.4, 127.8, 125.0, 105.9, 29.5, 23.7; HRMS (DART, m/z): [M + H]+ calcd. for C39H45AlI2N2 + H+, 823.1544; found, 823.1560.Theoretical calculationsAll calculations were carried out using Gaussian 16 Revision C.03 at the (TD-)CAM-B3LYP/6-31G(d,p) and (TD-)CAM-B3LYP/6-311++G(d,p) levels of theory for geometry optimizations and for time-dependent single-point calculations, respectively. All optimized structures were confirmed to be at local minimum using frequency calculations. The crystal structures were employed as the initial geometries of the optimizations at S0 states. The initial geometries for the optimization at S1 states were the corresponding optimized structures at the S0 states. Calculated low frequencies are listed in Supplementary Tables 7–14. Molecular orbital components which have large absolute values of orbital coefficients are shown in Supplementary Tables 15–18. Supplementary Table 19 shows the calculated frontier orbital energies and the electronic transitions for LAlMe and LAlCl. NBO analyses were carried out using the CAM-B3LYP functional and the 6-31+G(d,p) basis set. In the case of the calculations using the 6-311++G(d,p) basis set, the corresponding NBOs could not be saved in the checkpoint files due to the large number of basis functions (1333 for LAlMe and 1307 for LAlCl). Selected natural atomic orbital occupancies and NBO coefficients are listed in Supplementary Tables 20–23.
