Novel Blue Phosphorescent Group 15 Compounds MR3 (M = P, Sb, Bi; R = p-(N-7-Azaindolyl)phenyl)

Dual Fluorescence and Phosphorescence Emissions from Dye-Modified (NCN)-Bismuth Pincer Thiolate Complexes

We report the synthesis, characterization, and photophysical properties of four new dye-modified (NCN)Bi pincer complexes with two mercaptocoumarin or mercaptopyrene ligands. Their photophysical properties were probed by UV/vis spectroscopy, photoluminescence (PL) studies, and time-dependent density functional theory (TD-DFT) calculations. Absorption spectra of the complexes are dominated by mixed pyrene or coumarin π → π*/n(pS) → pyrene or coumarin π* transitions. While unstable toward reductive elimination of the corresponding disulfide under irradiation at room temperature, the complexes provide stable emissions at 77 K. Under these conditions, coumarin complexes 2 and 4 exhibit exclusively green phosphorescence at 508 nm. In contrast, the emissive properties of pyrene complexes 1 and 3 depend on the excitation wavelength and on sample concentration. Irradiation into the lowest-energy absorption band exclusively triggers red phosphorescence from the pyrenyl residues at 640 nm. At concentrations c < 1 μM, excitation into higher excited electronic states results in blue pyrene fluorescence. With increasing c (1-100 μM), the emission profile changes to dual fluorescence and phosphorescence emission, with a steady increase of the phosphorescence intensity, until at c ≥ 1 mM only red phosphorescence ensues. Progressive red-shifts and broadening of steady-state excitation spectra with increasing sample concentration suggest the presence of static excimers, as we observe it for concentrated solutions of pyrene. Crystalline and powdered samples of 1 indeed show intermolecular association through π-stacking. TD-DFT calculations on model dimers and a tetramer of 1 support the idea of aggregation-induced intersystem crossing (AI-ISC) as the underlying reason for this behavior.

Phosphorescence Properties of Boron/Bismuth Hybrid Conjugated Materials

By introducing main-group elements such as boron and bismuth to π-conjugated systems, it is possible to modify the optical properties of π-conjugated materials through orbital interactions between the orbital on the elements and π/π*-orbitals, and the heavy atom effect. Moreover, bismuth, which is the heaviest stable element, induces a significant heavy atom effect, making organobismuth compounds promising for applications as phosphorescent materials. In this study, we synthesized new room-temperature phosphorescent materials by incorporating bismuth into thiophene units. The phosphorescence properties of these materials, such as emission lifetime and wavelength, could be further controlled by combining tricoordinate boron with the thienylbismuth structures. The synthesized bismuth- and boron-containing thiophene compounds exhibited phosphorescence at room temperature in both solution and solid states. Furthermore, the introduction of boron raised the energy of the triplet state in the π-conjugated system, resulting in a blue shift of the phosphorescence wavelength. The analysis of photoluminescence properties and TD-DFT calculations revealed that the introduction of bismuth enhances phosphorescence properties, whereas the introduction of boron further promotes intersystem crossing.

Excited state energy landscape of phosphorescent group 14 complexes

Great progress has been achieved on phosphorescent or photoactive complexes of the Earth-abundant transition metals, while examples for phosphorescent heavy main group element complexes are rare, in particular for group 14 complexes in the oxidation state +II. The known compounds often show only weak phosphorescence with fast non-radiative deactivation. The underlying photophysical processes and the nature of the phosphorescent electronic states have remained essentially unexplored. The present combined photophysical and theoretical study on tin(ii) and lead(ii) complexes E(bpep) with the dianionic tridentate ligand bpep^2- (E = Sn, Pb; H₂bpep = 2-[1,1-bis(1H-pyrrol-2-yl)ethyl]pyridine) provides unprecedented insight in the excited state energy landscape of tetrel(ii) complexes. The tin complex shows green intraligand charge transfer (ILCT) phosphorescence both in solution and in the solid state. In spite of its larger heavy-atom effect, the lead complex only shows very weak red phosphorescence from a strongly distorted ligand-to-metal charge transfer (LMCT) state at low temperatures in the solid state. Detailed (TD-)DFT calculations explain these observations and delineate the major path of non-radiative deactivation via distorted LMCT states. These novel insights provide rational design principles for tetrel(ii) complexes with long-lived phosphorescence.

Synthesis, structure and properties of trivalent and pentavalent tricarbabismatranes

The first trivalent and pentavalent tricarbabismatranes were synthesized by the reaction of N(CH₂{2-LiC₆H₄})₃ with BiCl₃ and subsequent reaction with XeF₂, respectively. The trivalent bismatrane was easily oxidized by air, while the pentavalent bismatrane difluoride was relatively stable to air. A similar pentavalent bismatrance dichloride was prone to C-Cl bond reductive elimination even at room temperature.

Long-Lived Photoluminescence of Molecular Group 14 Compounds through Thermally Activated Delayed Fluorescence

Photoluminescent molecules exploiting the sizable spin-orbit coupling constants of main group metals and metalloids to access long-lived triplet excited states are relatively rare compared to phosphorescent transition metal complexes. Here we report the synthesis of three air- and moisture-stable group 14 compounds E(^MePDP^Ph)₂, where E = Si, Ge, or Sn and [^MePDP^Ph]^2- is the doubly deprotonated form of 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine. In solution, all three molecules exhibit exceptionally long-lived triplet excited states with lifetimes in the millisecond range and show highly efficient photoluminescence (Φ ≤ 0.49) due to competing prompt fluorescence and thermally activated delayed fluorescence at and around room temperature. Temperature-dependent steady-state emission spectra and photoluminescent lifetime measurements provided conclusive evidence for the two distinct emission pathways. Picosecond transient absorption spectroscopy allowed further analysis of the intersystem crossing (ISC) between singlet and triplet manifolds (τ_ISC = 0.25-3.1 ns) and confirmed the expected trend of increased ISC rates for the heavier elements in otherwise isostructural compounds.

Open for Bismuth: Main Group Metal-to-Ligand Charge Transfer

The synthesis, characterization, and photophysical properties of 4- and 6-coordinate Bi³⁺ coordination complexes are reported. Bi(bzq)₃ (1) and [Bi(bzq)₂]Br (2) (bzq = benzo[h]quinoline) are synthesized by reaction of 9-Li-bzq with BiCl₃ and BiBr₃, respectively. Absorption spectroscopy, electrochemistry, and DFT studies suggest that 1 has 42% Bi 6s character in its highest-occupied molecular orbital (HOMO) as a result of six σ* interactions with the bzq ligands. Excitation of 1 at 450 nm results in a broad emission feature at 520 nm, which is rationalized as a metal-to-ligand charge transfer (MLCT) and phosphorescent emission resulting from bismuth-mediated intersystem crossing (ISC) to a triplet excited state. This excited state revealed a 35 μs lifetime and was quenched in the presence of oxygen. These results demonstrate that useful optoelectronic properties of Bi³⁺ can be accessed through hypercoordination with covalent organobismuth interactions that mimic the electronic structure of lead perovskites.

Four-Coordinate Organoboron Platforms for Efficient Red Phosphorescent Organic Light-Emitting Diodes

So far both three- and four-coordinate organoboron compounds have been widely applied in organic light-emitting diode (OLED) materials. However, the use of four-coordinate organoboron compounds as host materials is rarely reported. In this work, two new four-coordinate organoboron compounds, namely 8-(4-(9H-carbazol-9-yl)phenyl)-6,6-difluoro-6H-6λ⁴ ,7λ⁴ -benzo[4',5']imidazo[1',2':3,4][1,3,2]diazaborolo[1,5-a]pyridine (B1PCz) and 8-(3-(9H-carbazol-9-yl)phenyl)-6,6-difluoro-6H-6λ⁴ ,7λ⁴ -benzo[4',5']imidazo[1',2':3,4][1,3,2]diazaborolo[1,5-a]pyridine (B1MCz), were successfully designed, synthesized, and fully characterized. The red OLEDs using B1PCz and B1MCz as host materials achieved relatively high device performance with a maximum external quantum efficiency of 14.8 % and 11.8 %, respectively. These results will expand the scope of organoboron compounds for OLED materials and reveal the great potential of four-coordinate organoboron materials.

Synthesis, structure and density functional theory calculations of a novel photoluminescent trisarylborane-bismuth(III) complex

A novel trisarylborane-Bi(III) complex, tris(4-(dimesitylboryl)phenyl)bismuthine [Bi(PhBMes₂ )₃ ], in which (Ph = phenyl, and Mes = mesityl), was synthesized via the reaction of bismuth (III) chloride (BiCl₃ ) with three equivalents of lithiated (4-bromophenyl)- dimesitylborane [BrPhBMes₂ ]. The new trisarylbismuthine was characterized by elemental analysis, ultraviolet-visible (UV-vis) spectroscopy, and NMR (¹ H and ¹³ C) spectroscopy. The molecular structure of Bi(PhBMes₂ )₃ in the solid state was determined using single-crystal X-ray diffraction analysis, which showed short intermolecular C-H···H-C contact. The complex is a fluorescent emitter (λ_max  = 395 nm) at room temperature and a phosphorescent emitter (λ_max  = 423 nm) at 77 K, which displayed a long lifetime of 495 ms. The UV-vis transitions were investigated using density function theory (DFT) and time-dependent (TD)-DFT calculations. Natural bond orbital analysis showed that the bismuth (III) center was mainly Lewis acidic in nature.

Inorganic-Cored Photoactive Assemblies: Synthesis, Structure, and Photochemical Investigations on Stannoxane-Supported Multifluorene Compounds

Organostannoxanes were used as inert supports for the preparation of multichromophore assemblies. The synthesis involves a single-step procedure and allows the preparation of compounds in which the number of chromophore units can be varied from one to six. Thus, the reactions of LCOOH (1-fluorenecarboxylic acid) or L'COOH (9-fluorenecarboxylic acid) with various organostannoxane precursors afforded the fluorenyl derivatives [Ph(3)SnO(2)CL] (1), [Ph(3)SnO(2)CL'] (2), [{nBu(3)SnO(2)CL''}(n)] (3), [{nBu(3)SnO(2)CL'}(n)] (4), [{tBu(2)Sn(OH)O(2)CL}(2)] (5), [{tBu(2)Sn(OH)O(2)CL'}(2)] (6), [{[nBu(2)SnO(2)CL](2)O}(2)] (7) [{[nBu(2)SnO(2)CL'](2)O}(2)] (8), [{nBuSn(O)O(2)CL}(6)] (9), and [{nBuSn(O)O(2)CL'}(6)] (10). Interestingly, the formation of 3 is accompanied by an unusual oxo-transfer reaction. The ligand L is oxidized at the 9-position. Compounds 1, 3, 5, 7, and 8 were characterized by X-ray crystallography. The solid-state structures of these compounds reveal rich supramolecular structures owing to multiple intermolecular interactions between the various supramolecular synthons present in these molecules. The optical behavior of 1-10 is primarily dictated by the fluorenyl periphery. These compounds display strong blue fluorescent emission in solution and blue-green fluorescent emission in the solid state. Fluorescence lifetimes of all of these compounds are on the nanosecond timescale, and this suggests that the emission originates from the singlet excited state to the ground state. Intermolecular interactions in the solid state lead to considerable broadening of the emission bands.

Blue luminescent rigid molecular rods bearing N-7-azaindolyl and 2,2'-dipyridylamino and their Zn(II) and Ag(I) complexes

To investigate the luminescent and thermal properties of organic compounds with rigid entities, a series of new blue/purple luminescent bridging ligands, 4,4'-(N-7-azaindolyl)diphenylacetylene (5), 4-(N-7-azaindolyl)-4'-(2,2'-dipyridylamino)diphenylacetylene (6), 4,4'-(2,2'-dipyridylamino)diphenylacetylene (7), and 4,4'-(dipyridylamino)diphenylbutadiyne (8) have been synthesized through Pd-mediated Sonogashira coupling. The structures of compounds 6 and 8 were determined by single-crystal X-ray diffraction analyses. Compounds 5-8 are luminescent in solution at room temperature, with emission lambda(max) = 361, 382, 386, and 405 nm, respectively. At 77 K, compounds 5-8 exhibit both fluorescent and phosphorescent emission. The 2,2'-dipyridylamino symmetrically substituted ligand 7 forms a linear dinuclear complex 9 with zinc(II) ions, in which the two pyridyl rings of the dipyridylamino portion are chelated to the metal center. However, with Ag(I) ions, ligand 7 forms a dinuclear complex (10), which displays a macrocyclic structure with only one of the two pyridyl rings from each dipyridylamino portion being coordinated to the silver atom. Both 9 and 10 exhibit luminescence in the near-UV region in CH(2)Cl(2) at room temperature with lambda(max) = 385 and 384 nm, respectively. The fluorescence of 7 can be partially quenched by either Zn(2+) or H(+). The behavior of ligands 8 toward Ag(I) and Zn(II) ions is similar to that of 7.

Blue luminescent three-coordinate organoboron compounds with a 2,2'-dipyridylamino functional group

Two new three-coordinate organoboron compounds tris[p-(2,2'dipyridylamino) phenylduryl]borane (1) and tris[p-(2,2'dipyridylamino)biphenylduryl]borane (2) have been synthesized in good yields by using Pd-catalyzed Suzuki-Miyauya coupling reactions between tri(p-iododuryl)borane and the corresponding boronic acid. Both compounds display bright blue luminescence when irradiated by UV light. The emission band of both compounds is highly solvent-dependent, indicating the presence of a highly polarized excited state. These new boron compounds are stable under air in solution and the solid state, and are capable of binding to metal ions such as Zn(II) via the 2,2'-dipyridylamino chelating sites. The crystal structures of 1 and 2 have been determined.