Spiro-Controlled Triplet-State Dynamics in Boron β-Diketiminate Complexes

Triplet excited states are important in molecular photochemistry because they support phosphorescence, photosensitization, photoredox activity, and energy-transfer processes. Their use depends on efficient conversion from a singlet to a triplet state, a process that is usually slow in organic chromophores because of spin-selection rules. Heavy elements can accelerate this conversion, but researchers are also seeking ways to achieve it without heavy metals. One way to improve intersystem crossing is to control the orbital character of the excited states. According to the El-Sayed framework, this process becomes more favorable when the singlet and triplet states differ in orbital angular momentum. Twisted donor–acceptor molecules can meet this condition because their charge-transfer and locally excited states have different spatial and electronic characters. The arrangement of the donor and acceptor fragments affects more than molecular shape. It can influence which excited states form, how they interact, and whether the molecule emits light or loses energy through structural changes and nonradiative decay. β-Diketiminate complexes of group 13 elements provide an unusual setting in which to examine these relationships. Their ligand-centred electronic structure can support both fluorescence and phosphorescence, especially under conditions that suppress molecular motion. Earlier complexes commonly displayed phosphorescence at low temperature, and room-temperature emission required heavy halogen substituents on the central element. This raised a specific design question: could a boron β-diketiminate molecular system be reorganized so that its own excited-state manifold promotes intersystem crossing without depending on a conventional heavy-atom effect?

In a recently published research paper in Dalton Transactions, Keisuke Suwa, Assistant Professor Shunichiro Ito, and Professor Kazuo Tanaka from Kyoto University synthesized a series of tetracoordinate boron β-diketiminate complexes in which biphenylene or diolate donor units are connected to the chromophore through a perpendicular spiro geometry. The biphenylene design introduces higher triplet states with local and interligand charge-transfer character that support intersystem crossing and room-temperature phosphorescence without requiring a heavy central atom. The diolate analogues demonstrate that donor strength can instead place a weakly allowed interligand charge-transfer state at the lowest singlet level. The main technical advance is using substituents at the spiro boron centre to control the order of the excited states and access to triplet states across the β-diketiminate complexes. The biphenylene derivatives included mesityl-, phenyl-, and brominated phenyl-substituted compounds, whereas the diolate series contained catecholate and naphthalenediolate donors. Structural analysis showed no apparent π–π stacking in the crystals, consistent with the steric congestion around the complexes.

The authors found that the biphenylene complexes displayed clear crystallization-induced emission and were almost non-emissive in dilute solution at room temperature, yet their crystalline forms produced greenish-yellow light. Lifetime measurements assigned the dominant emission to fluorescence, accompanied by a weaker, red-shifted delayed component. For Mes_FL, this delayed emission persisted on the millisecond timescale, supporting its assignment as room-temperature phosphorescence.

In their experiments, the brominated complex gave the strongest emission, but its phosphorescence was similar to the other compounds. The increase came mainly from stronger fluorescence, suggesting that bromine had little effect on intersystem crossing. The team cooled the biphenylene complexes to 77 K to restrict molecular motion and to allow both fluorescence and phosphorescence to be observed more clearly. Fluorescence remained the dominant pathway, with phosphorescence appearing as a weaker delayed band at longer wavelength. This behaviour differed from the more evenly divided fluorescence–phosphorescence response reported for many related group 13 β-diketiminate complexes under similar conditions.

The team observed the two diolate derivatives, the catecholate complex Mes_cat and the naphthalenediolate complex Mes_naph, followed distinct photophysical patterns. Neither showed detectable emission at room temperature in solution or in the crystalline state. At 77 K, Mes_cat produced weak fluorescence together with phosphorescence, whereas Mes_naph emitted almost entirely from a long-lived phosphorescent state. The longer-lived emission and higher phosphorescence efficiency of Mes_naph indicated more effective access to the triplet manifold.

The mesityl-substituted biphenylene complex, Mes_FL, underwent the greatest loss of excitation energy through nonradiative decay. Its crystal structure contained solvent-associated voids, giving the molecules more freedom to change shape after excitation and adopt a folded geometry that emits only weakly. By contrast, the brominated phenyl-substituted biphenylene complex, Ph_FLBr, was more constrained in the crystal and showed much less nonradiative decay. Tighter crystal packing therefore helped preserve luminescence by limiting excited-state deformation. Electronic-structure calculations clarified the mechanistic origin of the contrast between the two series. In the biphenylene complexes, the lowest singlet state was largely localized on the β-diketiminate unit and retained appreciable oscillator strength. Nearby triplet states included borafluorene-centred excitation together with intra- and interligand charge-transfer configurations. Because the interacting fragments are arranged at substantial dihedral angles, transitions between these states involve a change in orbital angular momentum and support stronger spin–orbit coupling.

In the diolate complexes, the lowest singlet state involved charge transfer from the diolate donor to the β-diketiminate unit. Because the orbitals overlapped only slightly, this transition was weak and the complexes emitted little light. Mes_naph also had more nearby triplet states with different orbital characters than Mes_cat, which allowed intersystem crossing to occur more readily and led to stronger phosphorescence.

The molecular design developed by Suwa, Ito, and Tanaka addresses the challenge of generating triplet excited states without platinum, iridium, or other heavy elements and offers a practical way to control light emission through coordination geometry and the electronic properties of the substituents. Organic light-emitting technology is one possible application because the crystalline biphenylene complexes produced both prompt fluorescence and delayed phosphorescence from singlet and triplet states. For materials engineering, this suggests that the relative contributions of these channels may be adjusted by changing the donor attached to the spiro boron centre and by controlling the rigidity of the surrounding solid. The findings also identify local rigidity as a relevant design parameter for preserving radiative excited-state decay in the solid state. The new findings are also applicable to luminescent probes and solid-state sensing materials. Their sensitivity to molecular restriction could be used in systems where aggregation, solidification, local rigidity, or environmental confinement is converted into an optical response.

Photoredox catalysis is another possible application for the new study of Professor Kazuo Tanaka and colleagues because the spiro boron centre can be modified to control triplet states that support electron or energy transfer. The diolate complexes also show that donor choice matters because it affects charge transfer, light emission, and access to triplet states. The same spiro design could also be used in functional dyes that require control over excited-state energy, molecular relaxation, and the conversion between singlet and triplet states.