Synthesis and characterization
To assess the effects of substituents on aluminum on the photophysical properties of β-diketiminate complexes, we synthesized five compounds with different substituents (Fig. 1a). The isopropyl groups on the aromatic groups of the ligand (LH) were employed for the kinetic stabilization of the complex.16,39–41 The dialkylaluminum complexes, LAlMe and LAlEt, were successfully synthesized by reacting trialkylaluminum with LH in toluene at 100°C according to the literatures on the syntheses of the related complexes.13,39,42 The dihaloaluminum complexes, LAlCl, LAlBr, and LAlI, were synthesized with the modified procedure of the related compounds through the reactions of LH and n-butyllithium followed by the treatment with the corresponding aluminum trihalides.39,40 These molecular structures were characterized with 1H, 13C{1H} and 27Al{1H} NMR spectroscopies, HRMS spectrometry, elemental analysis, and single crystal X-ray diffraction analysis (Fig. 1b–e).
Photophysical properties at room temperature
There are two significant effects from the substituents on the photoluminescent properties (Fig. 2a): (i) The dihaloaluminum complexes emit light in solid states at room temperature, while the dialkyl ones hardly show emission under the same condition; (ii) the emission color of the dihaloaluminum complexes depends on the type of halogen (LAlCl, blue; LAlBr, bluish white; LAlI, green). Meanwhile, all compounds exhibited almost no luminescence in the solution states at room temperature in the similar manner as shown in the previous reports.22–24,43 The previous studies demonstrated that the conventional β-diketiminate complexes have the CIE property with blue emission in crystalline states at room temperature when neither electron-accepting nor donating substituent was introduced on the ligands. Therefore, the non-emissive nature of the alkylaluminum complexes and the halogen-dependent emission color are the peculiar features among this class of luminophores.
We conducted spectroscopic studies under nitrogen at room temperature to elucidate the electronic structures of the complexes (Fig. 2b–d and Table 1). Firstly, UV–vis absorption spectra were recorded in the 2-methylpentane (2MP)/toluene solutions (99/1, v/v, 1 × 10–5 M) at room temperature (Fig. 2b). All compounds showed similar absorption spectra with slight difference in the position of absorption maximum. The shapes and positions of the longest-wavelength absorption bands were similar to the typical β-diketiminate complexes and assignable mainly to the π–π* (S0–S1) transition of the ligand moiety, suggesting that the electronic character of the ground-state structure of the complexes should not be affected by the substituents on the aluminum atom. Nevertheless, the substituents apparently change the S0–S1 transition energy probably due to their different contribution to the frontier orbitals.44 Secondly, photoluminescent (PL) spectra were measured for the same solutions and their crystalline powders (Fig. 2c and d). As we presumed, quite weak and broad emission spectra were obtained from all solutions. Their absolute quantum yields (ΦPL) were lower than 0.01. In the crystalline state, LAlMe and LAlEt hardly exhibited emission enhancement (ΦPL < 0.01), while LAlCl, LAlBr, and LAlI showed significant emission spectra in the visible region. ΦPL values were determined to be 0.48 for LAlCl, 0.44 for LAlBr, and 0.58 for LAlI. Importantly, the PL spectra of LAlBr and LAlI composed two distinct bands in the blue and green regions. PL lifetime (τPL) measurements revealed that the shorter-wavelength bands possessed nanosecond-order τPL, while the longer-wavelength ones had microsecond-order τPL (Fig. 2g and h, Table 2). Finally, their PL spectra recorded after several milliseconds after photoexcitation showed only longer-wavelength component (Figure S8). Consequently, the blue- and green-emission bands were assignable to fluorescence and phosphorescence, respectively. These data mean that the heavy atom effect of bromine and iodine should lead to the RTP properties of LAlBr and LAlI. To the best of our knowledge, the estimated phosphorescence quantum yield (ΦP) value of LAlI (0.54) is the highest one among the aluminum complexes. It is also of interest to note that the related boron complexes with iodine on the peripheral aromatic rings hardly show apparent RTP.25 The heavy atoms directly attached on the central element might efficiently accelerate intersystem crossing and phosphorescence processes.
Table 1
Results of photophysical measurements at room temperaturea
| | λabs / nm | ε / 104 M–1 cm–1 | λFluo / nm | λPhos / nm | ΦF | ΦP |
LAlMe | solution | 403 | 2.5 | 460 | n.d. | < 0.01 | – |
crystal | – | – | 473 | n.d. | < 0.01 | – |
LAlEt | solution | 410 | 2.0 | 458 | n.d. | < 0.01 | – |
crystal | – | – | 480 | n.d. | < 0.01 | – |
LAlCl | solution | 386 | 2.8 | 436 | n.d. | < 0.01 | < 0.01 |
crystal | – | – | 442 | n.d. | 0.48 | – |
LAlBr | solution | 385 | 2.9 | 441 | n.d. | < 0.01 | < 0.01 |
crystal | – | – | 447 | 511 | 0.25 | 0.19 |
LAlI | solution | 384 | 2.4 | 450 | 528 | < 0.01 | < 0.01 |
crystal | – | – | 454 | 515 | 0.04 | 0.54 |
a Photoluminescence properties were recorded with photoexcitation at the absorption maximum wavelength in solution state at room temperature. Solution, 1 × 10–5 M in 2-methylpentane/toluene (99/1, v/v); crystal, recrystallized from hexane; –, not determined. Phosphorescence spectra were recorded with pulsed excitation. Quantum yields of fluorescence and phosphorescence were estimated by absolute quantum yields and deconvoluted photoluminescence spectra with multi-component Gaussian functions.
Table 2
PL lifetime and estimated rate constants at room temperaturea
| | <τF> / ns | <τP> / ms | krS / 107 s–1 | knrS / 107 s–1 | kISCS / 107 s–1 | krT / 102 s–1 | knrT / 102 s–1 |
LAlCl | solution | 0.03 | – | 3.3 | 3300 | 3.3 | – | – |
| crystal | 2.1 | – | 34 | 31 | 6.4 | – | – |
LAlBr | solution | 0.04 | – | 2.5 | 2500 | 25 | – | – |
| crystal | 2.0 | 4.0 | 13 | 16 | 23 | 1.1 | 1.4 |
LAlI | solution | 0.04 | 0.10 | 1.0 | 2200 | 250 | 0.4 | 100 |
| crystal | 0.14 | 0.29 | 29 | 10 | 680 | 20 | 15 |
a <τF > and < τP>, average fluorescence and phosphorescence lifetimes, respectively; krS, radiative decay rate constant from singlet state (fluorescence); knrS, nonradiative decay rate constant from singlet state (internal conversion); kISCS, intersystem crossing rate constant from singlet state; krT, radiative decay rate constant from triplet state (phosphorescence); knrT, nonradiative decay rate constant from triplet state.
From the fluorescence and phosphorescence quantum yields and PL lifetime measurements with the dihaloaluminum complexes at room temperature, rate constants of each photophysical processes were estimated (Table 2, see the Supplementary Information). For all three complexes in the solution states, quite large nonradiative decay rate constants for singlet states (knrS ~ 1010 s–1) were obtained, suggesting that most excited molecules would be quenched nonradiatively through the internal conversion from S1 to S0 probably because this process could occur through conical intersections.45 Only in the case of LAlI, the rapid intersystem crossing process derived from the strong heavy atom effect of iodine could occur to some extent (kISC/knrS ~ 0.1). On the other hand, the crystals of these complexes exhibited at least 100 times smaller knrS values than their solutions, leading to their CIE properties. In the cases of LAlBr and LAlI, the suppression of the internal conversion should open the intersystem crossing processes as well as fluorescence. It is worth noting that the radiative rate constant from singlet states (krS) and kISCS of some complexes were enhanced by the crystallization, which might originate from the intermolecular interactions and could contribute to their CIE properties.
Photophysical properties at 77 K
To gain further information about the photophysical processes, we recorded PL spectra of the solutions and crystalline powders at 77 K with a cryostat under nitrogen atmosphere (Fig. 2e and Table 3). Importantly, all compounds clearly exhibited phosphorescence in the frozen solution state, probably because the nonradiative decay processes could be closed at the low temperature. Indeed, the hypsochromic shifts of the emission band were observed, indicating that structural changes in the excited state should be hampered under the frozen environment. In other words, it is suggested that there are significant structural relaxations in the excited state that cause nonradiative decay of the singlet excited states in room-temperature solutions. Interestingly, LAlMe and LAlCl exhibited phosphorescence at 77 K, despite the absence of heavy atoms, implying the intrinsic triplet-forming properties of these series of compounds.43 In the crystalline states at 77 K, the slight hypsochromic shifts of emission bands were observed except for LAlMe. These shifts might originate from the tight packing of the crystals. On the other hand, the bathochromic shift for the crystal of LAlMe might be attributed to the weakening of the 0–0 band. Significantly, the apparent crystallization-induced phosphorescence enhancement was observed from LAlI. The estimated ΦPhos in crystal was 2.2 times higher than that in the frozen solution, possibly because of the acceleration of intersystem crossing and phosphorescence processes and because the restriction of nonradiative decay from excited triplet states.
Table 3
Results of photoluminescence measurements at 77 Ka
| | λFluo / nm | λPhos / nm | ΦFluo | ΦPhos |
LAlMe | solution | 445 | 541 | 0.96 | 0.04 |
crystal | 474 | 569 | 0.25 | n.d. |
LAlEt | solution | 429 | 530 | n.d. | n.d. |
crystal | 429 | n.d. | n.d. | n.d. |
LAlCl | solution | 428 | 510 | 0.93 | 0.03 |
crystal | 422 | 535 | 0.96 | 0.04 |
LAlBr | solution | 435 | 514 | 0.56 | 0.35 |
crystal | 422 | 506 | 0.56 | 0.26 |
LAlI | solution | 426 | 515 | 0.13 | 0.33 |
crystal | 425 | 507 | 0.03 | 0.74 |
a Excited at the absorption maximum wavelength in solution state at room temperature. Solution, 1 × 10–5 M in 2-methylpentane/toluene (99/1, v/v); crystal, recrystallized from hexane; n.d., not determined due to negligible phosphorescence. Phosphorescence spectra were recorded with pulsed excitation. Quantum yields of fluorescence and phosphorescence were estimated by absolute quantum yields and deconvoluted photoluminescence spectra with multi-component Gaussian functions.
Theoretical calculations
Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed with the Gaussian 16 package46 to study the electronic structures of the dimethylaluminum and dihaloaluminum complexes (Fig. 3). Geometry optimization was performed for both S0 and S1 states with the CAM-B3LYP functional and the Lanl2DZ basis set for I and the 6-31G(d,p) one for the other atoms, followed by single-point transition energy calculations at the same level of theory except for the basis set for the light atoms (6-311 + + G(d,p)). The significant structural relaxation at the S1 state was estimated only for LAlMe, leading to the narrow S0–S1 energy gap (1.58 eV) and the small oscillator strength (f = 0.0080). The most characteristic change in the relaxation was the elongation of one of the Al–C bonds from 1.98 Å (S0) to 2.30 Å (S1). At the S0 geometry, its Kohn–Sham highest occupied molecular orbital (HOMO) was significantly delocalized over the Al–C bond as well as the ligand moiety, while the Kohn–Sham lowest unoccupied molecular orbital (LUMO) possessed little contribution from this bond because of a nodal plane passing through it. Hence, the electron density between aluminum and carbon atoms should decrease upon the photoexcitation from S0 to S1, resulting in the weakening of the bond and the considerably large structural relaxation at the S1 state. Similar photoinduced bond weakening or activation of Al–C bond have been reported in other systems of photochemical reactions.48–51 For the resulted S1 geometry, the HOMO mainly located at the Al–C moiety, where the HOMO–LUMO overlap much less effectively than that for the S0 geometry, making the f value of the S1–S0 transition smaller. Therefore, it is suggested that the structural relaxation should be responsible for the nonradiative decay process of the dialkylaluminum complexes.
On the other hand, the dihaloaluminum complexes presented only smaller structural changes between the S0 and S1 states, probably because the contribution from the Al–halogen bond to their HOMO would be smaller. Natural bond orbital (NBO) analysis suggested that the orbital on the Al–C bond should be mainly composed of the NBO attributed to the 2p orbital of the carbon atom, which is located at the similar energy region with the HOMO of the β-diketiminate ligand. As the corresponding p orbitals of chlorine, bromine, and iodine should be located at the much lower energy region because of their large electronegativity than carbon, the Al–halogen bond would not strongly contribute to the HOMO of the dihaloaluminum complexes due to the weaker orbital interactions. As a result, the undesired structural relaxation causing the non-radiative quenching could hardly occur in the S1 state and the S1–S0 electronic transition would be no longer forbidden at its S1 geometry (e.g., f = 0.5070 for LAlCl, Fig. 3b). Consequently, it is suggested that the photophysical processes of β-diketiminate complexes could be drastically modulated by the substituents on the central element. In addition, it is worth noting that the electron-donating contribution from the Al–C bond destabilizes the HOMO level compared to the dihaloaluminum complexes, leading to the lower S1 state consistent with the observed redshift of the absorption band. Indeed, LAlEt, with the more strongly electron-donating ethyl group, showed the absorption band in the lowest energy region among the complexes.
We also calculated excited triplet state (Tn) energy and spin–orbit coupling (SOC) constants between Sm and Tn states, ξ(Sm–Tn), for the dihaloaluminum complexes with the Q-Chem 5 package47 to get deeper insights into their phosphorescent properties (Fig. 4). The S1 and T1 states of each complex were dominantly characterized by the locally excited (LE) state within the central N2C3 moiety. As the S1–T1 energy gap was calculated to be about 1.0 eV or larger, the ISC from S1 to T1 seemed to be less efficient. On the other hand, the Tn (n = 2–4) states were located in the similar energy region of the S1 state (± 100 meV). In addition, the SOC values were estimated to be large enough to accept the efficient ISC between S1 and Tn. For LAlCl and LAlBr, these large SOC values are attributable to the charge transfer (CT) character of these Tn states with twisted conformations between the donor (aromatic rings) and acceptor (N2C3 unit) as shown in Fig. 4b. As the transitions between the S1(LE) to the Tn(CT) occurs with the large change in orbital angular momentum derived from the twisted conformations, the electron-spin flipping is allowed with holding the angular momentum conservation.48 On the other hand, the T2 and T3 states of LAlI significantly consist of the transition from the nonbonding orbitals (lone pairs) of the iodine atoms (HOMO–1 and HOMO–2) to its LUMO. Consequently, the heavy-atom effect of iodine could efficiently accelerate the ISC between S1 to T2 and T3. Importantly, it was suggested that the SOC constants not only between S1 and Tn but also between S0 and T1 significantly increased as the atomic number of the halogen atoms become larger because of the heavy atom effect. Therefore, both of ISC and phosphorescence processes should be enhanced in LAlBr and LAlI compared to LAlCl.