Pseudo cage effect in double energy transfer

In some systems, the donor of a triplet—triplet energy transfer can be sensitized in its singlet state through a singlet—singlet energy transfer (Dexter mechanism), where the donor is the acceptor of the triplet transfer itself. As a consequence an extra acceptor molecule in the triplet energy transfer is present in the vicinity of the donor, thus enhancing the efficiency of the transfer process. Experiments show clearly this effect and a diffusional model gives semi-quantitative agreement with the experimental data.     

Tailoring Excited State Properties and Energy Levels Arrangement via Subtle Structural Design on D‐π‐A Materials

The donor‐π‐conjugated‐acceptor (D‐π‐A) structure is an important design for the luminescent materials because of its diversity in the selections of donor, π‐bridge and acceptor groups. Herein, we demonstrate two examples of D‐π‐A structures capable to finely modulate the excited state properties and arrangement of energy levels, TPA‐AN‐BP and CZP‐AN‐BP , which possess the same acceptor and π‐bridge but different donor. The investigation of their photophysical properties and DFT calculation revealed that the D‐π‐A structure with proper donor, π‐bridge and acceptor can result in separation of frontier molecular orbitals on the corresponding donor and acceptor with an obvious overlap on the π‐bridge, resulting in a hybridized local and charge‐transfer (HLCT ) excited state with high photoluminescent (PL ) efficiencies. Meanwhile, their singlet and triplet states are arranged on corresponding moieties with large energy gap between T₂ and T₁ , and a small energy gap between S₁ and T₂ , which favor the reverse intersystem crossing (RISC ) from high‐lying triplet levels to singlet levels. As a result, the sky‐blue emission non‐doped OLED based on the TPA‐AN‐BP reached maximum external quantum efficiency (EQE ) of 4.39% and a high exciton utilization efficiency (EUE ) of 77%. This study demonstrates a new strategy to construct highly efficient OLED materials.     

Molecular Design Strategy of Thermally Activated Delayed Fluorescent Emitters Using CN‐Substituted Imidazopyrazine as a New Electron‐Accepting Unit

Thermally activated delayed fluorescence (TADF)‐based organic light‐emitting diodes (OLEDs) have attracted enormous attention recently due to their capability to replace conventional phosphorescent organic light‐emitting diodes for practical applications. In this work, a newly designed CN‐substituted imidazopyrazine moiety was utilized as an electron‐accepting unit in a TADF emitter. Two TADF emitters, 8‐(3‐cyano‐4‐(9,9‐dimethylacridin‐10(9H)‐yl)phenyl)‐2‐phenylimidazo[1,2‐a]pyrazine‐3‐carbonitrile (Ac‐CNImPyr) and 8‐(3‐cyano‐4‐(10H‐phenoxazin‐10‐yl)phenyl)‐2‐phenylimidazo[1,2‐a]pyrazine‐3‐carbonitrile (PXZ‐CNImPyr), were developed based on the CN‐substituted imidazopyrazine acceptor combined with acridine and phenoxazine donor, respectively. A CN‐substituted phenyl spacer was introduced between the donor and acceptor for a sufficiently small singlet‐triplet energy gap (ΔE_ST) and molecular orbital management. Small ΔE_ST of 0.07 eV was achieved for the phenoxazine donor‐based PXZ‐CNImPyr emitter. As a result, an organic light‐emitting diode based on the PXZ‐CNImPyr emitter exhibited a high external quantum efficiency of up to 12.7 %, which surpassed the EQE limit of common fluorescent emitters. Hence, the CN‐modified imidazopyrazine unit can be introduced as a new acceptor for further modifications to develop efficient TADF‐based OLEDs.     

Delayed Fluorescent Emission from Pyrene Doped Phenanthrene Nanoparticles Based on Triplet-triplet Energy Transfer

Doped nanoparticles were prepared from pyrene and phenanthrene using a facile reprecipitation method. The doped nanoparticles presented unique delayed fluorescent emissions of pyrene under the unprotected condition. The ratio of the intensity of delayed fluorescence (I_DF) to that of phosphorescence (I_P) is about 4:1, which almost keeps unchanged with the decrease of pyrene content at room temperature. The intensity of the delayed fluorescence emissions is dependent on the relative content of pyrene, as well as the aggregation degree of nanoparticles. The delayed emissions are contributed to efficient triplet‐triplet energy transfer from phenanthrene (donor) to pyrene (acceptor). Steady fluorescence measurement have proved that the singlet‐singlet energy transfer process was also existent dominated by the radiation energy transfer mechanism.     

Donor-acceptor interaction and intramolecular proton transfer in aminopolyenes

The influence of donor and acceptor substituents at chain termini on the geometry of the chain and charge distribution on atoms was studied for the ground and lower triplet electronically excited state of model ω-dimethylaminopolyene molecules (CH₃)₂N(CH=CH) _n CH=C(CN)₂, n = 1–3. Calculations were performed by the B3LYP/6-31+G** method. The influence of substituents on bond lengths and the amplitude of deviations from the equilibrium carbon-carbon bond length in unsubstituted polyenes increased as the conjugation chain grew longer. The deviations of the effects of both donor and acceptor groups from additivity, however, decreased. In the lower triplet electronically excited state of the molecule, the effect of substituents on changes in C-C bond lengths along the chain was not damped. The section of the potential energy surface for intramolecular proton shift from the donor amino to the acceptor nitrile group in “cyclic” (cis) conformers of the H₂N-CH=CH-CN and H₂N-CH=CH-CH=CH-CN molecules was analyzed. The structure of the reaction transition state and the height of the barrier to proton transfer were calculated.     

Long-Lived Triplet Excited State Accessed with Spin–Orbit Charge Transfer Intersystem Crossing in Red Light-Absorbing Phenoxazine-Styryl BODIPY Electron Donor/Acceptor Dyads

Orthogonal phenoxazine-styryl BODIPY compact electron donor/acceptor dyads were prepared as heavy atom-free triplet photosensitizers (PSs) with strong red light absorption (ϵ=1.33×10⁵ M⁻¹ cm⁻¹ at 630 nm), whereas the previously reported triplet photosensitizers based on the spin-orbit charge transfer intersystem crossing (SOCT-ISC) mechanism show absorption in a shorter wavelength range (<500 nm). More importantly, a long-lived triplet state (τ_T=333 μs) was observed for the new dyads. In comparison, the triplet state lifetime of the same chromophore accessed with the conventional heavy atom effect (HAE) is much shorter (τ_T=1.8 μs). Long triplet state lifetime is beneficial to enhance electron or energy transfer, the primary photophysical processes in the application of triplet PSs. Our approach is based on SOCT-ISC, without invoking of the HAE, which may shorten the triplet state lifetime. We used bisstyrylBodipy both as the electron acceptor and the visible light-harvesting chromophore, which shows red-light absorption. Femtosecond transient absorption spectra indicated the charge separation (109 ps) and SOCT-ISC (charge recombination, CR; 2.3 ns) for BDP-1 . ISC efficiency of BDP-1 was determined as Φ_T=25 % (in toluene). The dyad BDP-3 was used as triplet PS for triplet-triplet annihilation upconversion (upconversion quantum yield Φ_UC=1.5 %; anti-Stokes shift is 5900 cm⁻¹).     

Triplet–Triplet Energy Transfer from a UV‐A Absorber Butylmethoxydibenzoylmethane to UV‐B Absorbers

The phosphorescence decay of a UV‐A absorber, 4‐tert‐butyl‐4′‐methoxydibenzolymethane (BMDBM) has been observed following a 355 nm laser excitation in the absence and presence of UV‐B absorbers, 2‐ethylhexyl 4‐methoxycinnamate (octyl methoxycinnamate, OMC) and octocrylene (OCR) in ethanol at 77 K. The lifetime of the lowest excited triplet (T₁) state of BMDBM is significantly reduced in the presence of OMC and OCR. The observed quenching of BMDBM triplet by OMC and OCR suggests that the intermolecular triplet–triplet energy transfer occurs from BMDBM to OMC and OCR. The T₁ state of OCR is nonphosphorescent or very weakly phosphorescent. However, we have shown that the energy level of the T₁ state of OCR is lower than that of the enol form of BMDBM. Our methodology of energy‐donor phosphorescence decay measurements can be applied to the study of the triplet–triplet energy transfer between UV absorbers even if the energy acceptor is nonphosphorescent. In addition, the delayed fluorescence of BMDBM due to triplet–triplet annihilation was observed in the BMDBM–OMC and BMDBM–OCR mixtures in ethanol at 77 K. Delayed fluorescence is one of the deactivation processes of the excited states of BMDBM under our experimental conditions.     

Synthesis,Redox and Optical Properties of Low‐Bandgap Platinum(II) Polyynes with 9‐Dicyanomethylene‐Substituted Fluorene Acceptors

In view of the strong electron‐withdrawing nature of the cyano substituent, a blue donor/acceptor‐type organometallic polymer (trans‐[—Pt(PBu₃)₂—C≡C—R—C≡C—]_n (R = 9‐dicyanomethylenefluorene‐2,7‐diyl)) was prepared in good yield by CuI‐catalyzed polymerization involving the dehydrohalogenating coupling of trans‐[Pt(PBu₃)₂Cl₂] and H—C≡C—R—C≡C—H. The thermal, redox and photoconducting properties of the polymer are reported. Electronic absorption studies indicate that it has a bandgap of 1.58 eV which is the lowest among any of the metal polyyne polymers reported in the literature. The derivatization of the polymer backbone with electron deficient dicyano‐substituted electron acceptor in the side chain is found to be effective to tune the bandgap of this class of materials while maintaining their solubility and processability.     

Theoretical investigation of charge transfer excitation and charge recombination in acenaphthylene–tetracyanoethylene complex

Ab initio calculations were performed to investigate the charge separation and charge recombination processes in the photoinduced electron transfer reaction between tetracyanoethylene and acenaphthylene. The excited states of the charge‐balanced electron donor–acceptor complex and the singlet state of ion pair complex were studied by employing configuration interaction singles method. The equilibrium geometry of electron donor–acceptor complex was obtained by the second‐order Møller–Plesset method, with the interaction energy corrected by the counterpoise method. The theoretical study of ground state and excited states of electron donor–acceptor complex in this work reveals that the S₁ and S₂ states of the electron donor–acceptor complexes are excited charge transfer states, and charge transfer absorptions that corresponds to the S₀ → S₁ and S₀ → S₂ transitions arise from π–π* excitations. The charge recombination in the ion pair complex will produce the charge‐balanced ground state or excited triplet state. According to the generalized Mulliken–Hush model, the electron coupling matrix elements of the charge separation process and the charge recombination process were obtained. Based on the continuum model, charge transfer absorption and charge transfer emission in the polar solvent of 1,2‐dichloroethane were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 23–35, 2003     

Design,synthesis, and properties of polystyrene-based through-space charge transfer polymers: Effect of triplet energy level of electron donor moiety on delayed fluorescence and electroluminescence performance

Two kinds of polystyrene-based through-space charge transfer (TSCT) polymers consisting of spatially-separated acridan donor moieties bearing phenyl or naphthyl substituents and triazine acceptor moieties are designed and synthesized. It is found that TSCT polymers containing phenyl-substituted acridan donors exhibit high-lying singlet (S₁) and triplet (T₁) states with small singlet-triplet energy splitting (∆E_ST) of 0.04–0.05 eV, resulting in thermally activated delayed fluorescence (TADF) with reverse intersystem crossing rate constants of 1.1–1.2 × 10⁶ s⁻¹. In contrast, polymers bearing naphthyl-substituted acridan donors, although still having TSCT emission, exhibit no TADF effect because of the large ∆E_ST of 0.30–0.33 eV induced by low-lying locally excited T₁ state of naphthyl donor moiety. Solution-processed organic light-emitting diodes using TSCT polymers containing phenyl-substituted acridan donors reveal sky-blue emission at 483 nm together with maximum external quantum efficiency (EQE) of 11.3%, which is about 30 times that of naphthyl-substituted counterpart with maximum EQE of 0.38%, shedding light on the importance of high triplet energy level of donor moiety on realizing TADF effect and high device efficiency for through-space charge transfer polymer.     

Molecular‐Level Understanding of Ground‐ and Excited‐State O?H⋅⋅⋅O Hydrogen Bonding Involving the Tyrosine Side Chain: A Combined High‐Resolution Laser Spectroscopy and Quantum Chemistry Study

The present study combines both laser spectroscopy and ab initio calculations to investigate the intermolecular O? H???O hydrogen bonding of complexes of the tyrosine side chain model chromophore compounds phenol (PH) and para‐cresol (pCR) with H₂O, MeOH, PH and pCR in the ground (S₀) state as well as in the electronic excited (S₁) state. All the experimental and computational findings suggest that the H‐bond strength increases in the S₁ state and irrespective of the hydrogen bond acceptor used, the dispersion energy contribution to the total interaction energy is about 10–15 % higher in the S₁ state compared to that in the S₀ state. The alkyl‐substituted (methyl; +I effect) H‐bond acceptor forms a significantly stronger H bond both in the S₀ and the S₁ state compared to H₂O, whereas the aryl‐substituted (phenyl; ?R effect) H‐bond donor shows a minute change in energy compared to H₂O. The theoretical study emphasizes the significant role of the dispersive interactions in the case of the pCR and PH dimers, in particular the C? H???O and the C? H???π interactions between the donor and acceptor subunits in controlling the structure and the energetics of the aromatic dimers. The aromatic dimers do not follow the acid–base formalism, which states that the stronger the base, the more red‐shifted is the X? H stretching frequency, and consequently the stronger is the H‐bond strength. This is due to the significant contribution of the dispersion interaction to the total binding energy of these compounds.     

The Importance of Excited‐State Energy Alignment for Efficient Exciplex Systems Based on a Study of Phenylpyridinato Boron Derivatives

Understanding excited‐state dynamics is critical for improving the photoluminescence (PL) efficiency of exciplexes. A series of exciplexes based on conventional hole‐transporting materials as donor and newly developed phenylpyridinato boron derivatives as acceptor were investigated. High PL efficiencies were achieved in only some combinations, and a large difference in performance among combinations provided insight into nonradiative processes in exciplex systems. Furthermore, the triplet local excited states (³LE) of each donor and acceptor were found play an important role in triplet exciplex harvesting. Significant contributions from triplets were clearly observed when the charge‐transfer excited states (¹CT and ³CT) and ³LE were ideally aligned. We also demonstrated fine control of relative energy alignment via the concentration to improve the PL efficiency.     

Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine–triazine donor–acceptor molecule TpAT-tFFO

Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three of them, a pair of singlet and triplet charge transfer (CT) states (S₁ and T₁) and a locally excited (LE) triplet state (T₃), can be associated with the (Me → N) conformer, the other two CT-type states (S₂ and T₂) form the lowest excited singlet and triplet states of the (Me → Ph) conformer. The two conformers, which differ in essence by the shearing angle of the face-to-face aligned donor and acceptor moieties, are easily interconverted in the electronic ground state whereas the reorganization energy is substantial in the excited singlet state, thus explaining the two experimentally observed time constants of prompt fluorescence emission. Forward and reverse intersystem crossing between the singlet and triplet CT states is mediated by vibronic spin–orbit interactions involving the LE T₃ state. Low-frequency vibrational modes altering the distance and alignment of the donor and acceptor π-systems tune the S₁ and T₃ states (likewise S₂ and T₃) into and out of resonance. The enhancement of intersystem crossing due to the interplay of vibronic and spin–orbit coupling is considered a general feature of organic through-space charge-transfer thermally activated delayed fluorescence emitters.

DFT/MRCI quantum chemical studies suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO.     

Investigation of excited state electron-transfer reactions. A search for other associative processes

The present investigations were carried out to reveal the nature of the photoinduced electron-transfer (ET) process within the electron donors 1,2,3,4-tetrahydroquinoline (THQ) and 1-methyl-1,2,3,4-tetrahydroquinoline (THMe), and widely used acceptor tetracyanoquinodimethane (TCNQ) in the highly polar solvent acetonitrile (ACN) at 300 K. Observations of considerable overlapping between the emission spectrum of the donor molecules studied in the present investigation and the electronic absorption spectrum of the acceptor TCNQ, coupled to a high negative value of ΔG [the energy gap between the locally excited (LE) and radical ion pair (RIP) states] when one of the chromophores is excited, indicate the possibility of concurrent occurrence of the two processes, e.g. energy and electron transfer. Surprisingly even when the donor chromophore is photoexcited, no spectral manifestation of energy transfer was observed, though both steady state and time resolved (in the time domain of nanosecond order) spectroscopic measurements strongly suggest the occurrence of a highly exothermic ET reaction within the present donor—acceptor systems. Furthermore such ET reactions have been suggested to occur between donor and acceptor separated by a large distance ( ∼ 7 Å), and quenching of fluorescence emission of donor molecules is caused primarily due to outer sphere ET reactions with the acceptor. Measured electron transfer rates (k_ET) were found to be of much lower value ( ∼ 10⁷s⁻¹). It is demonstrated that loose structure of the transient geminate ion pair complex is formed due to the encounter between excited acceptor (or donor) and unexcited donor (or acceptor), and due to this structural property, a stable anionic species (TCNQ⁻ ion) is produced due to the rapid dissociation (probably in the picosecond time domain) of this excited complex. It is hinted that synthesis of biochromophoric systems in which the present donor and acceptor chromophore would be linked by a polymethylene type (σ-type) spacer might be useful in building good photoconducting materials.     

Synthesis of OMS Materials and Investigation of Their Acceptor–Donor Characteristics

Three ordered mesoporous siliceous (OMS) materials known as MCM41s—unmodified MCM-41C16 (“C16”), and two MCM41s with different surface functionalities: MCM-41C16-SH (“C16-SH”) and MCM-41C16-NH₂ (“C16-NH₂”)—were synthesized and studied by inverse gas chromatography in order to determine their acceptor–donor properties. The specific retention volumes of nonpolar and polar probes that were chromatographed on these ordered mesoporous silica adsorbents were evaluated under infinite dilution conditions. Two methods were employed to calculate the standard free energy of adsorption, ΔG _ads, of each chromatographed probe on the basis its specific retention volume. These ΔG _ads values were then employed to estimate the van der Waals contribution and the specific contribution of the free surface energy for each MCM41. DN values (donor numbers, based on the Gutmann scale) and AN* values (acceptor numbers, based on the Riddle–Fowkes scale) were employed to determine the values of parameters that characterize the ability of the MCM41s to act as electron acceptors (parameter: K _A) and donors (parameter: K _D). Considering the different compositions of the probes, each of which has different acceptor–donor properties, a new chromatographic test to supplement the Grob test is suggested.

     

Facile Generation of Thermally Activated Delayed Fluorescence and Fabrication of Highly Efficient Non-Doped OLEDs Based on Triazine Derivatives

A series of donor–acceptor–donor triazine-based molecules with thermally activated delayed fluorescence (TADF) properties were synthesized to obtain highly efficient blue-emitting OLEDs with non-doped emitting layers (EMLs). The targeted molecules use a triazine core as the electron acceptor, and a benzene ring as the conjugated linker with different electron donors to alternate the energy level of the HOMO to further tune the emission color. The introduction of long alkyl chains on the triazine core inhibits the unwanted intermolecular D –D/A–A-type π–π interactions, resulting in the intermolecular D–A charge transfer. The weak aggregation-caused quenching (ACQ) effect caused by the suppressed intermolecular D –D/A–A-type π–π interaction further enhances the emission. The crowded molecular structure allows the electron donor and acceptor to be nearly orthogonal, thereby reducing the energy gap between triplet and singlet excited states (ΔE_ST). As a result, blue-emitting devices with TH-2DMAC and TH-2DPAC non-doped EMLs showed satisfactory efficiencies of 12.8 % and 15.8 %, respectively, which is one of the highest external quantum efficiency (EQEs) reported for blue TADF emitters (λ_peak<475 nm), demonstrating that our tailored molecular designs are promising strategies to endow OLEDs with excellent electroluminescent performances.     

Synthesis of Perfluoroalkyl‐Substituted γ‐Lactones and 4,5‐Dihydropyridazin‐3(2H)‐ones via Donor?Acceptor Cyclopropanes

Rh₂(OAc)₄‐Catalyzed decomposition of diazo esters in the presence of perfluoroalkyl‐ or perfluoroaryl‐substituted silyl enol ethers smoothly provided the corresponding alkyl 2‐siloxycyclopropanecarboxylates in very good yields. The generated donor? acceptor cyclopropanes are equivalents of γ‐oxo esters, which we demonstrated by their one‐pot transformations to yield fluorine‐containing heterocycles. A reductive procedure selectively afforded perfluoroalkyl‐substituted γ‐hydroxy esters or γ‐lactones. The treatment of the donor? acceptor cyclopropanes with hydrazine or phenylhydrazine afforded a series of perfluoroalkyl‐ and perfluoroaryl‐substituted 4,5‐dihydropyridazin‐3(2H)‐ones.     

Spiro‐Configured Bipolar Host Materials for Highly Efficient Electrophosphorescent Devices

In this study, we synthesized and characterized a series of spirobifluorene‐based bipolar compounds (D2 ACN, DNPACN, DNTACN, and DCzACN) in which a dicyano‐substituted biphenyl branch, linked orthogonally to a donor biphenyl branch bearing various diarylamines, acted as an acceptor unit allowing fine‐tuning of the morphological stability, triplet energy, bipolar transport behavior, and the HOMO and LUMO energy levels. The promising physical properties of these new compounds, together with their ability to transport electrons and holes with balanced mobilities, made them suitable for use as host materials in highly efficient phosphorescent organic light‐emitting diodes (PhOLEDs) with green iridium‐based‐ or red osmium‐based phosphors as the emitting layer (EML). We adopted a multilayer structure to efficiently confine holes and electrons within the EML, thus preventing exciton diffusion and improving device efficiency. The device incorporating D2 ACN doped with the red emitter [Os(bpftz)₂(PPhMe₂)₂] (bpftz=3‐(trifluoromethyl)‐5‐(4‐tert‐butylpyridyl)‐1,2,4‐triazolate) gave a saturated red electrophosphorescence with CIE coordinates of (0.65, 0.35) and remarkably high efficiencies of 20.3 % (21 cd A^?1) and 13.5 Lm W^?1 at a practical brightness of 1000 cd m^?2.     

Biindanylidenes: role of central bond torsion in nonvertical triplet excitation transfer to the stilbenes

The stilbenes were proposed to function as nonvertical triplet excitation (NVET) acceptors for energy-deficient donors because rotation about the central bond diminishes the energy gap between ground and triplet energy surfaces. Recently, the role of central bond torsion in facilitating NVET to cis-stilbene (c-St) was questioned because the behavior of 2,3-diphenylnorbornene as a triplet energy acceptor is similar to that of cis-stilbene. On the basis of the assumption that the rigidity of the norbornene skeleton precludes torsional displacement of the phenyl rings in the triplet state, an alternative mechanism was proposed involving phenyl-vinyl torsion as the key reaction coordinate for NVET to c-St. However, this proposal is inconsistent with theory, which predicts that the triplet state energy minimum corresponds to a geometry with significant displacement of the phenyl rings of 2,3-diphenylnorbornene from a common plane. We now provide experimental evidence demonstrating that central bond torsion is the key coordinate for NVET to stilbenes. Comparison of the activation parameters for the two rigid stilbene analogues, cis- and trans-1,1'-biindanylidene (c-Bi and t-Bi) to those for the stilbenes, shows that the excitation transfer processes remain nonvertical despite the strong structural inhibition of phenyl-vinyl torsion; the relatively small preexponential factors of the respective isomers are almost identical. Their magnitude is a measure of the attenuation introduced by Franck-Condon overlap factors which decrease as the torsional state quantum number corresponding to the transition state increases. These results and results from theoretical calculations are consistent with central bond torsion as the key reaction coordinate in NVET to the biindanylidenes and the stilbenes. The crystal structure of t-Bi shows it to be strictly planar, eliminating phenyl-vinyl torsion toward planarity as a crucial NVET reaction coordinate.