Photoexcitation and One-Electron Reduction Processes of a CO2 Photoreduction Dyad Catalyst Having a Zinc(II) Porphyrin Photosensitizer.

We have explored the photophysical properties and one-electron reduction process in the dyad photocatalyst for CO2 photoreduction, ZnP-phen=Re, in which the catalyst of fac-[Re(1,10-phenanthoroline)(CO)3Br] is directly connected with the photosensitizer of zinc(II) porphyrin (ZnP), using time-resolved infrared spectroscopy, transient absorption spectroscopy, and quantum chemical calculations. We revealed the following photophysical properties: (1) the intersystem crossing occurs with a time constant of ∼20 ps, which is much faster than that of a ZnP single unit, and (2) the charge density in the excited singlet and triplet states is mainly localized on ZnP, which means that the excited state is assignable to the π-π* transition in ZnP. The one-electron reduction by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole occurs via the triplet excited state with the time constant of ∼100 ns and directly from the ground state with the time constant of ∼3 µs. The charge in the one-electron reduction species spans ZnP and the phenanthroline ligand, and the dihedral angle between ZnP and the phenanthroline ligand is rotated by ∼24° with respect to that in the ground state, which presumably offers an advantage for proceeding to the next CO2 reduction process. These insights could guide the new design of dyad photocatalysts with porphyrin photosensitizers.

Optical Property Control by the Interligand Charge Transfer Excited State in Brominated Homoleptic and Heteroleptic Aluminum Dinuclear Triple-Stranded Helicates.

The utilization of aluminum, an abundant and inexpensive element, for the synthesis of novel functional complexes is extremely important, but the design and control of photofunctionality are still unexplored. In this study, we focused on our previously developed dinuclear triple-stranded helicates incorporating two aluminum ions (ALPHY) to synthesize both homoleptic and heteroleptic complexes with bromine atoms at the 3-position of the pyrrole moiety in the Schiff base ligands. The brominated Schiff base ligands were reacted with AlCl3 to synthesize homoleptic complexes, while different ligands were mixed to prepare heteroleptic complexes. Single-crystal X-ray structural analysis revealed the structures of these novel complexes. We found that increasing the degree of bromination resulted in a tunable emission color, shifting progressively from 550 (yellow) to 566 nm (orange). Optical resolution of the complexes facilitated the observation of mirror-image circular dichroism and circularly polarized luminescence. Furthermore, employing ultrafast spectroscopy techniques, we have elucidated that the optical properties are governed by the interligand charge transfer (ILCT) among the three ligands. The formation of heteroleptic complexes induces the ILCT state even in nonpolar environments, thereby accelerating nonradiative decay and intersystem crossing. These findings mark significant advancements in photofunctional materials based on multinuclear complexes.

Structural-transformation-induced Drastic Luminescence Changes in an Organic-Inorganic Hybrid [ReN(CN)4 ]2- Salt Triggered by Chemical Stimuli.

We synthesized a (1-propylpyridinium)2 [ReN(CN)4 ]-type organic-inorganic hybrid exhibiting water-vapor-induced drastic structural changes of the [ReN(CN)4 ]2- assemblies. Specifically, upon exposure to water vapor, dehydrated nitrido-bridged chains were converted to hydrated cyanido-bridged tetranuclear clusters via rearrangements of large molecular building units in the crystals. These switchable assembly forms display substantially different photo-physical properties, although in both cases the emission is caused by a metal-centered d-d transition. The nitrido-bridged chain exhibited a near-infrared (749 nm) emission, which blue-shifted as the temperature increased, while a visible (561 nm) emission and its red shift was demonstrated by the cyanido-bridged cluster.

Highly Fluorescent Bipyrrole-Based Tetra-BF2 Flag-Hinge Chromophores: Achieving Multicolor and Circularly Polarized Luminescence.

This study reports the facile syntheses of tetra-boron difluoride (tetra-BF2 ) complexes, flag-hinge-like molecules that exhibit intense green-to-orange luminescence in solution and yellow-to-red emission in the solid states. Single-crystal structure analysis and density functional theory calculations suggested a bent structure of this series of compounds. The complexes also exhibited excellent optical properties, with quantum yields reaching 100 % and a large Stokes shift. These properties were attributed to the altered bending angle of the molecule in the S1 excited state. As the rotational motion was suppressed around the 2,2'-bipyrrole axis, atropisomers with axial chirality were formed, which are optically resolvable into (R) and (S)-enantiomers through a chiral column. The atropisomers thus function as circularly polarized luminescent (CPL) materials, in which the color (green, green-yellow, and yellow) can be varied by controlling the aggregation state. This rational design of multi-BF2 complexes can potentially realize novel photofunctional materials.

Determining Excited-State Structures and Photophysical Properties in Phenylphosphine Rhenium(I) Diimine Biscarbonyl Complexes Using Time-Resolved Infrared and X-ray Absorption Spectroscopies.

We have explored the structural factors on the photophysical properties in two rhenium(I) diimine complexes in acetonitrile solution, cis,trans-[Re(dmb)(CO)2(PPh2Et)2]+ (Et(2,2)) and cis,trans-[Re(dmb)(CO)2(PPh3)2]+ ((3,3)) (dmb = 4,4'-dimethyl-2,2'-bipyridine, Ph = phenyl, Et = ethyl) using the combination method of time-resolved infrared spectroscopy, time-resolved extended X-ray absorption fine structure, and quantum chemical calculations. The difference between these complexes is the number of phenyl groups in the phosphine ligand, and this only indirectly affects the central Re(I). Despite this minor difference, the complexes exhibit large differences in emission wavelength and excited-state lifetime. Upon photoexcitation, the bond length of Re-P and angle of P-Re-P are significantly changed in both complexes, while the phenyl groups are largely rotated by ∼20° only in (3,3). In contrast, there is little change in charge distribution on the phenyl groups when Re to dmb charge transfer occurs upon photoexcitation. We concluded that the instability from steric effects of phenyl groups and diimine leads to a smaller Stokes shift of the lowest excited triplet state (T1) in (3,3). The large structural change between the ground and excited states causes the longer lifetime of T1 in (3,3).

Fast, efficient, narrowband room-temperature phosphorescence from metal-free 1,2-diketones: rational design and the mechanism.

We report metal-free organic 1,2-diketones that exhibit fast and highly efficient room-temperature phosphorescence (RTP) with high colour purity under various conditions, including solutions. RTP quantum yields reached 38.2% in solution under Ar, 54% in a polymer matrix in air, and 50% in crystalline solids in air. Moreover, the narrowband RTP consistently dominated the steady-state emission, regardless of the molecular environment. Detailed mechanistic studies using ultrafast spectroscopy, single-crystal X-ray structure analysis, and theoretical calculations revealed picosecond intersystem crossing (ISC) followed by RTP from a planar conformation. Notably, the phosphorescence rate constant k p was unambiguously established as ∼5000 s-1, which is comparable to that of platinum porphyrins (representative heavy-metal phosphor). This inherently large k p enabled the high-efficiency RTP across diverse molecular environments, thus complementing the streamlined persistent RTP approach. The mechanism behind the photofunction has been elucidated as follows: (1) the large k p is due to efficient intensity borrowing of the T1 state from the bright S3 state, (2) the rapid ISC occurs from the S1 to the T3 state because these states are nearly isoenergetic and have a considerable spin-orbit coupling, and (3) the narrowband emission results from the minimal geometry change between the T1 and S0 states. Such mechanistic understanding based on molecular orbitals, as well as the structure-RTP property relationship study, highlighted design principles embodied by the diketone planar conformer. The fast RTP strategy enables development of organic phosphors with emissions independent of environmental conditions, thereby offering alternatives to precious-metal based phosphors.