Novel tri-carbazole modified fluorene host material for highly efficient solution-processed blue and green electrophosphorescent devices

A series of novel solution-processable small-molecule host materials: 2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-TCz comprising a fluorene monomer as the rigid core and tri-carbazole as the periphery have been designed and synthesized, and their optical, electrochemical, and thermal properties have been fully characterized. The host materials exhibit high glass-transition temperatures (231–310 °C) and high triplet energy levels (2.61–2.73 eV). High-quality amorphous thin films can be obtained by spin-coating the host materials from solutions. It is found that the HOMO level of the host materials can be tuned by linking the tri-carbazole unit to the 2,7 positions of the fluorine core, resulting in appropriate HOMO energy levels (−5.36 to −5.23 eV) for improved hole-injection in the device. Solution-processed blue and green electrophosphorescent devices bases on the developed host materials exhibit high efficiencies of 21.2 and 34.8 cd A⁻¹, respectively.     

Broadband Visible Light-Absorbing [70]Fullerene-BODIPY-Triphenylamine Triad: Synthesis and Application as Heavy Atom-Free Organic Triplet Photosensitizer for Photooxidation

A broadband visible light-absorbing [70]fullerene-BODIPY-triphenylamine triad (C₇₀-B-T) has been synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxidation. By attaching two triphenylmethyl amine units (TPAs) to the π-core of BODIPY via ethynyl linkers, the absorption range of the antenna is extended to 700 nm with a peak at 600 nm. Thus, the absorption spectrum of C₇₀-B-T almost covers the entire UV–visible region (270–700 nm). The photophysical processes are investigated by means of steady-state and transient spectroscopies. Upon photoexcitation at 339 nm, an efficient energy transfer (ET) from TPA to BODIPY occurs both in C₇₀-B-T and B-T, resulting in the appearance of the BODIPY emission at 664 nm. Direct or indirect (via ET) excitation of the BODIPY-part of C₇₀-B-T is followed by photoinduced ET from the antenna to C₇₀, thus the singlet excited state of C₇₀ (¹C₇₀^*) is populated. Subsequently, the triplet excited state of C₇₀ (³C₇₀^*) is produced via the intrinsic intersystem crossing of C₇₀. The photooxidation ability of C₇₀-B-T was studied using 1,5-dihydroxy naphthalene (DHN) as a chemical sensor. The photooxidation efficiency of C₇₀-B-T is higher than that of the individual components of C₇₀-1 and B-T, and even higher than that of methylene blue (MB). The photooxidation rate constant of C₇₀-B-T is 1.47 and 1.51 times as that of C₇₀-1 and MB, respectively. The results indicate that the C₇₀-antenna systems can be used as another structure motif for a heavy atom-free organic triplet photosensitizer.     

A computational quest for the effects of fused rings on the stability of Hammick carbenes type remote N‐heterocyclic carbenes

Substituent effects of fused six, and five‐membered aromatic rings are investigated on the stability, aromaticity, charge distribution, nucleophilic (N), and electrophilic (ω) characters of 20 singlet (s) and triplet (t) Hammick carbenes, at B3LYP/AUG‐cc‐pVTZ and M06‐2X/AUG‐cc‐pVTZ. Results display: (a) The higher thermodynamic and kinetic stability is revealed by carbenes situated between two nitrogen and/or two oxygen heteroatoms of two substituted rings, in a “W” arrangement toward the carbenic center; (b) Regardless of the arrangement, the order of thermodynamical and kinetic stabilization for fused rings is pyrrole > furan > thiophene > phosphole. (c) The substituted Hammick carbenes with two fused heterocyclics, in a given arrangement, show more stability than unsubstituted Hammick carbene; (d) While two five‐membered heterocyclic rings stabilize their corresponding substituted carbenes, two benzene rings destabilize Hammick carbene; (e) In all structures, s species emerges as ground state, exhibiting more stability than its t state; (f) The scrutinized s carbenes show higher N and lower ω than their corresponding t states.     

The Impact of C2 Insertion into a Carbazole Donor on the Physicochemical Properties of Dibenzo[a,j]phenazine-Cored Donor–Acceptor–Donor Triads

Novel electron donor–acceptor–donor (D-A-D) compounds comprising dibenzo[a,j]phenazine as the central acceptor core and two 7-membered diarylamines (iminodibenzyl and iminostilbene) as the donors have been designed and synthesized. Investigation of their physicochemical properties revealed the impact of C₂ insertion into well-known carbazole electron donors on the properties of previously reported twisted dibenzo[a,j]phenazine-core D-A-D triads. Slight structural modification caused a drastic change in conformational preference, allowing unique photophysical behavior of dual emission derived from room-temperature phosphorescence and triplet–triplet annihilation. Furthermore, electrochemical analysis suggested sigma-dimer formation and electrochemical polymerization on the electrode. Quantum chemical calculations also rationalized the experimental results.     

Rationally Investigating the Influence of T1 Location on Electroluminescence Performance of Aryl Amine Modified Phosphine Oxide Materials

The correspondence between triplet location effect and host‐localized triplet–triplet annihilation and triplet–polaron quenching effects was performed on the basis of a series of naphthyldiphenylamine (DPNA)‐modified phosphine oxide hosts. The number and ratio of DPNA and diphenylphosphine oxide was adjusted to afford symmetrical and unsymmetrical molecular structures and different electronic environments. As designed, the first triplet (T₁) states were successfully localized on the specific DPNA chromophores. Owing to the meso‐ and multi‐insulating linkages, identical optical properties and comparable electrical performance was observed, including the same first singlet (S₁) and T₁ energy levels to support the similar singlet and triplet energy transfer and the close frontier molecular orbital energy levels. This established the basis of rational investigation on T₁ location effect without interference from other optoelectronic factors.     

Long-Lived Charge-Transfer State Induced by Spin-Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in a Compact Spiro Electron Donor/Acceptor Dyad

We prepared conceptually novel, fully rigid, spiro compact electron donor (Rhodamine B, lactam form, RB)/acceptor (naphthalimide; NI) orthogonal dyad to attain the long-lived triplet charge-transfer (³CT) state, based on the electron spin control using spin-orbit charge transfer intersystem crossing (SOCT-ISC). Transient absorption (TA) spectra indicate the first charge separation (CS) takes place within 2.5 ps, subsequent SOCT-ISC takes 8 ns to produce the ³NI* state. Then the slow secondary CS (125 ns) gives the long-lived ³CT state (0.94 μs in deaerated n-hexane) with high energy level (ca. 2.12 eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as ¹NI*→¹CT→³NI*→³CT. With time-resolved electron paramagnetic resonance (TREPR) spectra, an EEEAAA electron-spin polarization pattern was observed for the naphthalimide-localized triplet state. Our spiro compact dyad structure and the electron spin-control approach is different to previous methods for which invoking transition-metal coordination or chromophores with intrinsic ISC ability is mandatory.     

Engineering the Charge-Transfer State to Facilitate Spin–Orbit Charge Transfer Intersystem Crossing in Spirobis[anthracene]diones

Spiro conjugation has been proposed to dictate the efficiency of charge transfer, which could directly affect the spin–orbit charge transfer intersystem crossing (SOCT-ISC) process. However, this process has yet to be exemplified. Herein, we prepared three spirobis[anthracene]diones, in which two benzophenone moieties are locked in close proximity and differentially functionalized to fine-tune the charge transfer state. Its feasibility for SOCT-ISC was theoretically predicted, then experimentally evaluated. Through fine-tuning the spiro conjugation coupling and varying the solvent dielectric constants, ISC rate constants were engineered to vary in a dynamic range of three orders of magnitude, from 7.8×10⁸ s⁻¹ to 1.0×10¹¹ s⁻¹, which is the highest ISC rate reported for SOCT-ISC system to our knowledge. Our findings substantiate the key factors for effective SOCT-ISC and offer a new avenue for the rational design of heavy atom free triplet sensitizers.     

Ferromagnetically Coupled S=1 Chains in Crystals of Verdazyl-Nitronyl Nitroxide Diradicals

Thermally stable organic diradicals with a triplet ground state along with large singlet-triplet energy gap have significant potential for advanced technological applications. A series of phenylene-bridged diradicals with oxoverdazyl and nitronyl nitroxide units were synthesized via a palladium-catalyzed cross-coupling reaction of iodoverdazyls with a nitronyl nitroxide-2-ide gold(I) complex with high yields. The diradicals exhibit high stability and do not decompose in an inert atmosphere up to 180 °C. For the diradicals, both substantial AF (ΔE_ST≈−64 cm⁻¹) and FM (ΔE_ST≥25 and 100 cm⁻¹) intramolecular exchange interactions were observed. The sign of the exchange interaction is determined both by the bridging moiety (para- or meta-phenylene) and by the type of oxoverdazyl block (C-linked or N-linked). Upon crystallization, diradicals with the triplet ground state form unique one-dimensional exchange-coupled chains with strong intra- and weak inter-diradical ferromagnetic coupling.     

Semiconductor Quantum Dots as Components of Photoactive Supramolecular Architectures

Luminescent quantum dots (QDs) are colloidal semiconductor nanocrystals consisting of an inorganic core covered by a molecular layer of organic surfactants. Although QDs have been known for more than thirty years, they are still attracting the interest of researchers because of their unique size-tunable optical and electrical properties arising from quantum confinement. Moreover, the controlled decoration of the QD surface with suitable molecular species enables the rational design of inorganic-organic multicomponent architectures that can show a vast array of functionalities. This minireview highlights the recent progress in the use of surface-modified QDs – in particular, those based on cadmium chalcogenides – as supramolecular platforms for light-related applications such as optical sensing, triplet photosensitization, photocatalysis and phototherapy.