Luminescence mechanism analysis of a TADF molecule showing peculiar thermal behavior.

In recent years, much attention has been paid to the development of thermally activated delayed fluorescence (TADF) materials with short delayed-fluorescence lifetimes to improve the device performances of OLEDs. In principle, by reducing the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) overlap, while the energy difference between S1-T1 (ΔEST) and activation energy (Ea) can be reduced, and the reverse intersystem crossing rate constant (kRISC) can be accelerated, a decrease in the radiative rate constant happens, necessitating an advanced molecular design. Furthermore, a molecule based on heptazine as a parent skeleton has recently been found to have a peculiar temperature dependence of luminescence decay, suggesting a negative gap (NG) material. In this report, we show that 9-[1,4]benzoxaborino[2,3,4-kl]phenoxaborine-7-yl-1,3,6,8-tetramethyl-9H-carbazole (TMCz-BO), a donor-acceptor linked TADF molecule with a very short delay lifetime of 750 ns, exhibits a peculiar thermal behavior similar to that of NG materials based on the temperature dependence of its luminescence decay in solution.

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.

A Red-Light-Driven CO-Releasing Complex: Photoreactivities and Excited-State Dynamics of Highly Distorted Tricarbonyl Rhenium Phthalocyanines.

A complex comprising one [Re(CO)3 ]+ unit and a phthalocyanine (Pc) ligand (Re1 Pc) is shown to function as a photo-induced CO-releasing molecule (photoCORM) in the presence of O2 and a coordinative solvent under irradiation with red light, which can deeply penetrate living tissues. Transient absorption spectroscopic measurements indicate very short excited-state lifetimes and ultrafast intersystem crossing for Re1 Pc and Re2 Pc, which contains two [Re(CO)3 ]+ units. The excited-state properties are ascribed to efficient spin-orbit coupling and large Franck-Condon factors originating from the complexes' distorted structures, that is, unsymmetric coordination of [Re(CO)3 ]+ unit(s), one of which was confirmed by single-crystal X-ray analysis of a symmetrically substituted Pc with two [Re(CO)3 ]+ units. Re1 Pc represents a promising red-light-driven photoCORM that can be applied in biological environments or therapeutic applications.

Vapor-Induced Conversion of a Centrosymmetric Organic-Inorganic Hybrid Crystal into a Proton-Conducting Second-Harmonic-Generation-Active Material.

Chemical responsivity in materials is essential to build systems with switchable functionalities. However, polarity-switchable materials are still rare because inducing a symmetry breaking of the crystal structure by adsorbing chemical species is difficult. In this study, we demonstrate that a molecular organic-inorganic hybrid crystal of (NEt4)2[MnN(CN)4] (1) undergoes polarity switching induced by water vapor and transforms into a rare example of proton-conducting second-harmonic-generation-active material. Centrosymmetric 1 transforms into noncentrosymmetric polar 1·3H2O and 1·MeOH by accommodating water and methanol molecules, respectively. However, only water vapor causes a spontaneous single-crystal-to-single-crystal transition. Moreover, 1·3H2O shows proton conduction with 2.3 × 10-6 S/cm at 298 K and a relative humidity of 80%.

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).

Guest-Tunable Excited States in a Cyanide-Bridged Luminescent Coordination Polymer.

The excited-state energy was tuned successfully by guest molecules in a cyanide-bridged luminescent coordination polymer (CP). Methanol or ethanol vapor reversibly and significantly changed the luminescent color of the CP between green and yellow (Δλem = 32 nm). These vapors did not significantly affect the environment around the luminophore in the ground state of the CP, whereas they modulated the excited states for the resulting bathochromic shift. The time-resolved photoluminescent spectra of the CP systems showed that solvent adsorption enhanced the energetic relaxation in the excited states. Furthermore, time-resolved infrared spectroscopy indicated that cyanide bridging in the CP became more flexible in the excited states than that in the ground state, highlighting the sensitivity of the excited states to external stimuli, such as the guest vapor. Overall, guest-tunable excited states will allow the more straightforward design of sensing materials by characterizing the transient excited states.

Triplet Pair States in Singlet Fission.

This account aims at providing an understanding of singlet fission, i.e., the photophysical process of a singlet state ( S1) splitting into two triplet states (2 × T1) in molecular chromophores. Since its discovery 50 years ago, the field of singlet fission has enjoyed rapid expansion in the past 8 years. However, there have been lingering confusion and debates on the nature of the all-important triplet pair intermediate states and the definition of singlet fission rates. Here we clarify the confusion from both theoretical and experimental perspectives. We distinguish the triplet pair state that maintains electronic coherence between the two constituent triplets, 1(TT), from one which does not, 1(T···T). Only the rate of formation of 1(T···T) is defined as that of singlet fission. We present distinct experimental evidence for 1(TT), whose formation may occur via incoherent and/or vibronic coherent mechanisms. We discuss the challenges in treating singlet fission beyond the dimer approximation, in understanding the often neglected roles of delocalization on singlet fission rates, and in realizing the much lauded goal of increasing solar energy conversion efficiencies with singlet fission chromophores.

Coordination Geometrical Effect on Ligand-to-Metal Charge Transfer-Dependent Energy Transfer Processes of Luminescent Eu(III) Complexes.

Photophysical properties of europium (Eu(III)) complexes are affected by ligand-to-metal charge transfer (LMCT) states. Two luminescent Eu(III) complexes with three tetramethylheptadionates (tmh) and pyridine (py), [Eu(tmh)3(py)1] (seven-coordinated monocapped-octahedral structure) and [Eu(tmh)3(py)2] (eight-coordinated square antiprismatic structure), were synthesized for geometrical-induced LMCT level control. Distances between Eu(III) and oxygen atoms of tmh ligands were estimated using single-crystal X-ray analyses. The contribution percentages of π-4f mixing in HOMO and LUMO at the optimized structure in the ground state were calculated using DFT (LC-BLYP). The Eu-O distances and their π-4f mixed orbitals affect the energy level of LMCT states in Eu(III) complexes. The LMCT energy level of an eight-coordinated Eu(III) complex was higher than that of a seven-coordinated Eu(III) complex. The energy transfer processes between LMCT and Eu(III) ion were investigated using temperature-dependent and time-resolved emission lifetime measurements of 5D0 → 7FJ transitions of Eu(III) ions. In this study, the LMCT-dependent energy transfer processes of seven- and eight-coordinated Eu(III) complexes are demonstrated for the first time.

Dual Energy Transfer Pathways from an Antenna Ligand to Lanthanide Ion in Trivalent Europium Complexes with Phosphine-Oxide Bridges.

Trivalent europium (Eu3+) complexes are attractive materials for luminescence applications if energy transfer from antenna ligands to the lanthanide ion is efficient. However, the microscopic mechanisms of the transfer remain elusive, and fundamental physical chemistry questions still require answers. We track the energy transfer processes in a luminescent complex Eu(hfa)3(DPPTO)2 (hfa, hexafluoroacetylacetonate; DPPTO, 2-diphenylphosphoryltriphenylene) using time-resolved photoluminescence spectroscopy. In addition to the conventional energy transfer pathway through the T1 state of the ligands, we discovered ultrafast energy transfer pathway directly from the singlet excited states of the ligands to the 5D1 state of Eu3+. The short time scale of the energy transfer (3 ns, 200 ns) results in its high photoluminescence quantum yield. The discovery of the distinct energy transfer pathways from a single chromophore is important for establishing design strategies of luminescent complexes.

Intramolecular-rotation driven triplet-to-singlet upconversion and fluctuation induced fluorescence activation in linearly connected donor-acceptor molecules.

Thermally activated triplet-to-singlet upconversion is attractive from both fundamental science and exciton engineering, but controlling the process from molecular configuration is still unrevealed. In particular, the flexibility of the freedom of molecular geometry is of major importance to understand the kinetics of the phonon-induced upconversion. Here, we focus on two linearly connected donor-acceptor molecules, 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) and hexamethylazatriangulene-2,4,6-triphenyl-1,3,5-triazine (HMAT-TRZ), as the model system. While DMAC-TRZ possesses a rotational degree of freedom in the dihedral angle between the donor and acceptor moieties, i.e., C-N bond in tertiary amine, the rotation is structurally restricted in HMAT-TRZ. The rotationally flexible DMAC-TRZ showed significant triplet-to-singlet upconversion caused by thermal activation. On the other hand, the rotation-restricted HMAT-TRZ showed negligible thermal upconversion efficiency. We elaborate on the origin of the photophysical properties from the viewpoint of the geometries in the excited states using time-resolved infrared spectroscopy and quantum chemical calculations. We uncovered that the structural restriction of the intramolecular flexibility significantly affects the optimized geometry and phonon modes coupled to the spin conversion. As a result of the rotation restriction, the spin flipping in HMAT-TRZ was coupled to bending motion instead of the rotation. In contrast, the free rotation fluctuation in the DMAC-TRZ mixes local-excitation and charge-transfer characters, leading to successful activation of the delayed fluorescence as well as the reverse intersystem crossing. Our discovery sheds light on the mechanism of the triplet-to-singlet upconversion, providing a microscopic strategy to control the optoelectronic properties from a molecular viewpoint.

Liquid-like dielectric response is an origin of long polaron lifetime exceeding 10 µs in lead bromide perovskites.

Lead halide perovskites are promising materials for optoelectronic applications because of their exceptional performances in carrier lifetime and diffusion length; however, the microscopic origins of their unique characteristics remain elusive. The organic-inorganic hybrid perovskites show unique dielectric functions, i.e., ferroelectric-like phonon responses in the 0.1-10 THz region and liquid-like rotational relaxation in the 1-100 GHz range. To reveal the role of the dielectric responses is of primal importance because the dielectric screening is a key to understanding the optoelectronic properties governed by polarons in the perovskites. Here, we conducted comparative studies of broadband dielectric spectroscopy on both all-inorganic CsPbBr3 and organic-inorganic hybrid (CH3NH3)PbBr3 single crystals to uncover the origin of the liquid-like dielectric relaxation in the 1-100 GHz range. We confirmed the absence of the dielectric response in the range of 106-1010 Hz in CsPbBr3, which was clearly present in the hybrid (CH3NH3)PbBr3. This suggests that the response is almost purely due to the rotational motions of the organic dipoles in the hybrid perovskites. We evaluated the lifetimes of the polarons using surface-free transient photoluminescence. The lifetime in CsPbBr3 was up to 1.6 µs, while the lifetime in (CH3NH3)PbBr3 was 18 µs. The lifetime in the hybrid (CH3NH3)PbBr3 was significantly longer than in CsPbBr3, also confirmed by transient infrared spectroscopy. We concluded that the liquid-like dielectric response inhibits polaron recombination due to the efficient separation of opposite charges by the additional dynamic disorder.

Helical Nanoribbons for Ultra-Narrowband Photodetectors.

This Communication describes a new molecular design that yields ultranarrowband organic photodetectors. The design is based on a series of helically twisted molecular ribbons as the optoelectronic material. We fabricate charge collection narrowing photodetectors based on four different helical ribbons that differ in the wavelength of their response. The photodetectors made from these materials have narrow spectral response with full-width at half maxima of <20 nm. The devices reported here are superior by approximately a factor of 5 to those from traditional organic materials due to the narrowness of their response. Moreover, the active layers for the helical ribbon-based photodetectors are solution-cast but have performance that is comparable to the state-of-the-art narrowband photodetectors made from methylammonium lead trihalide perovskite single crystals. The ultranarrow bandwidth for detection results from the helical ribbons' high absorption coefficient, good electron mobility, and sharp absorption edges that are defined by the twisted molecular conformation.

Long, Atomically Precise Donor-Acceptor Cove-Edge Nanoribbons as Electron Acceptors.

This Communication describes a new molecular design for the efficient synthesis of donor-acceptor, cove-edge graphene nanoribbons and their properties in solar cells. These nanoribbons are long (∼5 nm), atomically precise, and soluble. The design is based on the fusion of electron deficient perylene diimide oligomers with an electron rich alkoxy pyrene subunit. This strategy of alternating electron rich and electron poor units facilitates a visible light fusion reaction in >95% yield, whereas the cove-edge nature of these nanoribbons results in a high degree of twisting along the long axis. The rigidity of the backbone yields a sharp longest wavelength absorption edge. These nanoribbons are exceptional electron acceptors, and organic photovoltaics fabricated with the ribbons show efficiencies of ∼8% without optimization.

Persistent Energetic Electrons in Methylammonium Lead Iodide Perovskite Thin Films.

In conventional semiconductor solar cells, carriers are extracted at the band edges and the excess electronic energy (E*) is lost as heat. If E* is harvested, power conversion efficiency can be as high as twice the Shockley-Queisser limit. To date, materials suitable for hot carrier solar cells have not been found due to efficient electron/optical-phonon scattering in most semiconductors, but our recent experiments revealed long-lived hot carriers in single-crystal hybrid lead bromide perovskites. Here we turn to polycrystalline methylammonium lead iodide perovskite, which has emerged as the material for highly efficient solar cells. We observe energetic electrons with excess energy ⟨E*⟩ ≈ 0.25 eV above the conduction band minimum and with lifetime as long as ∼100 ps, which is 2-3 orders of magnitude longer than those in conventional semiconductors. The energetic carriers also give rise to hot fluorescence emission with pseudo-electronic temperatures as high as 1900 K. These findings point to a suppression of hot carrier scattering with optical phonons in methylammonium lead iodide perovskite. We address mechanistic origins of this suppression and, in particular, the correlation of this suppression with dynamic disorder. We discuss potential harvesting of energetic carriers for solar energy conversion.

Ultrafast exciton dynamics in dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene thin films.

Ultrafast dynamics of excitons in organic semiconductors is essential for a deep understanding of the working mechanism of plastic opto-electronic devices. In this work, excited state dynamics in dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene thin films has been studied with femtosecond transient absorption and time-resolved photoluminescence spectroscopy. Upon the excitation with a femtosecond pulse at 400 nm, a broad positive absorption band at 1.5-2.4 eV is observed that contains two components: one decays with a time constant of a few ps and the other with 67 ± 7 ps. Because the decay curve of the latter coincides with that of photoluminescence, the slow decay component is ascribed to the lowest singlet exciton. The former fast decay component is ascribed to mixed states between charge transfer (CT) and Frenkel excitons, because it is accompanied by a feature due to the Stark effect caused by transient charged species: a combination of bleach and positive absorption at hνprobe > 2.4 eV which looks like derivative modulations of the ground state absorption spectrum. A pronounced polarization dependence is observed for the derivative-like features; this is due to anisotropic distributions of the dipole moments formed by the CT excitons. The derivative-like feature changes its shape after the decay of the mixed Frenkel-CT exciton and grows with a pump-probe delay time of up to 1 ns due to a thermal effect. The decay rate of the mixed Frenkel-CT exciton strongly depends on its density because of exciton-exciton annihilation at high density.

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.

Room-temperature quantum coherence of entangled multiexcitons in a metal-organic framework.

Singlet fission can generate an exchange-coupled quintet triplet pair state 5TT, which could lead to the realization of quantum computing and quantum sensing using entangled multiple qubits even at room temperature. However, the observation of the quantum coherence of 5TT has been limited to cryogenic temperatures, and the fundamental question is what kind of material design will enable its room-temperature quantum coherence. Here, we show that the quantum coherence of singlet fission-derived 5TT in a chromophore-integrated metal-organic framework can be over hundred nanoseconds at room temperature. The suppressed motion of the chromophores in ordered domains within the metal-organic framework leads to the enough fluctuation of the exchange interaction necessary for 5TT generation but, at the same time, does not cause severe 5TT decoherence. Furthermore, the phase and amplitude of quantum beating depend on the molecular motion, opening the way to room-temperature molecular quantum computing based on multiple quantum gate control.