Donor-acceptor Compounds Research Articles

Early Events in the Photoinduced Electron Transfer between Carbazole and Divinylbenzene in a Silylene-Bridged Donor–Acceptor Compound

Meeting the challenge of designing new light-emitting materials, we synthesized the compound N-isopropylcarbazole (CBL)–SiMe2–divinylbenzene (DVB), which represents the general idea of building att...

Investigation of Thermally Activated Delayed Fluorescence from a Donor-Acceptor Compound with Time-Resolved Fluorescence and Density Functional Theory Applying an Optimally Tuned Range-Separated Hybrid Functional.

Emitters showing thermally activated delayed fluorescence (TADF) in electroluminescent devices rely on efficient reverse intersystem crossing (rISC) arising from small thermal activation barriers between the lowest excited triplet and singlet manifolds. A small donor-acceptor compound consisting of a demethylacridine donor and a methylbenzoate acceptor group is used as a model TADF emitter. The spectroscopic signatures of this system are characterized using a combination of photoluminescence and photoluminescence excitation, and the photoluminescence decay dynamics are recorded between delays of 2 ns and 20 ms. Above T = 200 K, our data provide convincing evidence for TADF at intermediate delays in the microsecond range, whereas triplet-triplet annihilation and slow triplet decay at later times can be observed over the entire temperature range from T = 80 K to room temperature. Moreover, close to room temperature, we find a second and faster up-conversion mechanism, tentatively assigned to reverse internal conversion between different triplet configurations. An interpretation of these experimental findings requires a calculation of the deformation patterns and potential minima of several electronic configurations. This task is performed with a range-separated hybrid functional, outperforming standard density functionals or global hybrids. In particular, the systematic underestimation of the energy of charge transfer (CT) states with respect to local excitations within the constituting chromophores is replaced by more reliable transition energies for both kinds of excitations. Hence, several absorption and emission features can be assigned unambiguously, and the observed activation barriers for rISC and reverse internal conversion correspond to calculated energy differences between the potential surfaces in different electronic configurations.

Vibrational Spectroscopy Reveals Effects of Electron Push-Pull and Solvent Polarity on Electron Delocalization in Radical Anions of Donor-Acceptor Compounds.

The nature of excess electrons has been studied in donor-acceptor (D-A) compounds based on substituted triarylamines and a nitrile-functionalized fluorene by changing the substituents on the triarylamines and also the solvent polarity. We observed that both electron push-pull capability at the distant location in the amine donor unit and solvation in solvents of varying polarity significantly affect the nitrile ν(C≡N) vibrations of the fluorene acceptor unit in radical anions of these D-A compounds. Quantum calculations show that the push-pull capability translates the position of an excess electron while keeping its width relatively constant. On the other hand, solvation changes both, making an electron more compact in polar media. The current study points to the idea that solvation plays a more significant role in controlling the nature of excess electrons, while synthetic modification that influences electron push-pull capability enables further tuning.

Mix and (Mis)match: further studies of the electronic structure and mixed-valence characteristics of 1,4-diethynylbenzene-bridged bimetallic complexes.

The 1,4-diethynylbenzene motif is commonly employed as a bridging ligand in bimetallic molecular systems intended to show pronounced intramolecular electronic interactions, delocalized electronic structures and 'wire-like' properties between the metal fragments at the ligand termini. In contrast to these expectations, the donor-acceptor compounds [{Cp'(CO)xM'}(μ-C[triple bond, length as m-dash]CC6H4C[triple bond, length as m-dash]C){M(PP)Cp'}]n+ [n = 0, 1; M'(CO)xCp' = Fe(CO)2Cp, W(CO)3Cp*; M(PP)Cp' = Fe(dppe)Cp, Fe(dppe)Cp*, Ru(PPh3)2Cp, Ru(dppe)Cp, Ru(dppe)Cp*] display remarkably little bridge-mediated electronic interaction between the electron-rich {M(PP)Cp'} and electron-poor {M'(CO)xCp'} fragments in the ground state. However, a relatively high-energy (26 000-30 000 cm-1) M-to-M' charge transfer can be identified. One-electron oxidation is largely localized on the {M(C[triple bond, length as m-dash]CR)(PP)Cp'} fragment and gives rise to a new charge transfer band with bridging-ligand-to-{M(PP)Cp'}+ (M'(CO)xCp' = Fe(CO)2Cp) or M'-to-M(+) (M(CO)xCp' = W(CO)3Cp*) character. The localized electronic ground state of these complexes is better revealed through analysis of the IR spectra, taking advantage of the well-resolved ν(C[triple bond, length as m-dash]C) and ν(CO) bands and IR spectroelectrochemical methods, than through the more classical analysis based on the concepts of Marcus-Hush theory and analysis of the putative IVCT electronic transition. The conclusions are supported by DFT calculations using the BLYP35 functional.

Finding the optimal exchange-correlation functional to describe the excited state properties of push-pull organic dyes designed for thermally activated delayed fluorescence.

To gauge the suitability of an organic dye for thermally activated delayed fluorescence (TADF), its excited state properties are often calculated using density functional theory. For this purpose, the choice of the exchange-correlation (XC) functional is crucial as it heavily influences the quality of the obtained results. In this work, 19 different XC functionals with various amounts of Hartree-Fock (HF) exchange and/or long-range correction parameters are benchmarked versus resolution-of-the-identity second-order coupled cluster (riCC2) calculations for a set of 10 prototype intramolecular donor-acceptor compounds. For the time-dependent density functional theory (TD-DFT) calculations, LC-BLYP(ω = 0.20) and M06-2X are the better performing XC functionals when looking at singlet and triplet excitation energies, respectively. For the singlet-triplet energy gap, LC-BLYP(ω = 0.17), LC-ωPBE(ω = 0.17) and a hybrid LC-BLYP(ω = 0.20)/M06-2X method give the smallest mean average errors (MAEs). Using the Tamm-Dancoff approximation (TD-DFT/TDA), the MAEs are further reduced for the triplet vertical excitation energies and the singlet-triplet energy gaps.

Facile structure-modification of xanthenone based OLED emitters exhibiting both aggregation induced emission enhancement and thermally activated delayed fluorescence

Four new donor-acceptor compounds were designed, synthesized and investigated by theoretical and experimental approaches aiming to estimate effect of the structure of a donor on the properties of potential OLED emitters. Because of the different electron-donating abilities of the nitrogen-containing heterocycles, derivatives of xanthenone containing di-tert-butyl-carbazolyl, di-tert-butyl-acridanyl, di-tert-butyl-phenothiazinyl and penoxazinyl moieties exhibited different photophysical behavior. Because of big dihedral angles between the donors and acceptor as well as because of possibility of rotation around N–C bond, the designed compounds were characterized by thermally activated delayed fluorescence and aggregation induced emission enhancement effect. Twice higher photoluminesce quantum yields reaching 38% in doped films were obtained for compounds containing di-tert-butyl-carbazolyl and di-tert-butyl-acridanyl moieties as compared to those observed for compounds with the donors containing S and O heteroatoms. Strong effect of the donor substituents on charge injection (ionization potentials were in the range of 5.67–5.96 eV) and charge-transporting properties (hole and electron mobilities were in a wide range from 6.3 × 10−8 to 6.3 × 10−4 cm2V−1s−1 at electric field of 2.5 × 105 V cm−1) was detected. The differently substituted compounds were utilized as emitters in OLEDs. Higher maximum values of external quantum efficiency (up to 3.5%) were observed for OLEDs based on emitters with nitrogen containing donors relative to estimated for OLEDs based on emitters containing di-tert-butyl-phenothiazinyl and penoxazinyl moieties.

Shortcuts for Electron-Transfer through the Secondary Structure of Helical Oligo-1,2-Naphthylenes.

Atropisomeric 1,2-naphthylene scaffolds provide access to donor-acceptor compounds with helical oligomer-based bridges, and transient absorption studies revealed a highly unusual dependence of the electron-transfer rate on oligomer length, which is due to their well-defined secondary structure. Close noncovalent intramolecular contacts enable shortcuts for electron transfer that would otherwise have to occur over longer distances along covalent pathways, reminiscent of the behavior seen for certain proteins. The simplistic picture of tube-like electron transfer can describe this superposition of different pathways including both the covalent helical backbone, as well as noncovalent contacts, contrasting the wire-like behavior reported many times before for more conventional molecular bridges. The exquisite control over the molecular architecture, achievable with the configurationally stable and topologically defined 1,2-naphthylene-based scaffolds, is of key importance for the tube-like electron transfer behavior. Our insights are relevant for the emerging field of multidimensional electron transfer and for possible future applications in molecular electronics.

Synthesis and optoelectronic properties of benzoquinone-based donor-acceptor compounds.

Herein, we report a mild and efficient palladium-catalyzed C–H functionalization method to synthesize a series of benzoquinone (BQ)-based charge-transfer (CT) derivatives in good yields. The optoelectronic properties of these compounds were explored both theoretically and experimentally and correlations to their structures were identified as a function of the nature and position of the donor group (meta and para) attached to the benzoquinone acceptor. Compound 3, where benzoquinone is para-conjugated to the diphenylamine donor group, exhibited thermally activated delayed fluorescence (TADF) with a biexponential lifetime characterized by a prompt ns component and a delayed component of 353 μs.

Frontispiece: Triarylborane‐Based Helical Donor–Acceptor Compounds: Synthesis, Photophysical, and Electronic Properties

Frontispiece: Triarylborane‐Based Helical Donor–Acceptor Compounds: Synthesis, Photophysical, and Electronic Properties

Electrochemical evaluation and DFT calculations of aromatic sulfonohydrazides as corrosion inhibitors for XC38 carbon steel in acidic media

Two newly synthesized sulfonohydrazide derivatives, namely N '- {(E) - [4- (dimethylamino) phenyl] methylidene}benzenesulfonohydrazide (MBSH), and N '- {(E) - [4- (dimethylamino) phenyl] methylidene} -4- methylbenzenesulfonohydrazide (MpTSH) have been investigated as corrosion inhibitors for XC38 carbon steel in 1 M HCl using electrochemical measurements (potentiodynamic polarization, linear polarization resistance, electrochemical impedance spectroscopy) and theoretical calculations.Results obtained show that these derivatives are good corrosion inhibitors for carbon steel in molar HCl. Their inhibition efficiency increases with their concentration, but decreases with increasing temperature. Substituting a methyl group in MpTSH for a hydrogen atom in MBSH slightly improves its effectiveness. Both inhibitors display a mixed-type. Their adsorption on the steel surface proceeds essentially via physisorption mode and obeys the Langmuir adsorption isotherm. Good correlations were obtained between the anti-corrosion performance of these donor-acceptor compounds and their molecular properties.

Triarylborane-Based Helical Donor-Acceptor Compounds: Synthesis, Photophysical, and Electronic Properties.

The synthesis and characterization of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthren-4-amine (3-B(Mes)2 -[4]helix-9-N(p-Tol)2 1) and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthren-8-amine (3-B(Mes)2 -[5]helix-12-N(p-Tol)2 2) are reported herein. Their electrochemical and photophysical properties have been studied experimentally and theoretically. The donor and acceptor-substituted helicene derivatives exhibit moderate fluorescence quantum yields in THF (Φf =0.48 and 0.61 for 1 and 2, respectively), which are higher than unsubstituted ones (Φf =0.18 for [4]helicene; Φf <0.05 for [n]helicenes (n≥5)). In the solid state, the Φf values are higher (Φf =1.00 and 0.55 for 1 and 2, respectively) than those in solution, most likely due to the restrictions of molecular motions. The S1 ←S0 transitions of 1 and 2 are predominately HOMO→LUMO transitions. Upon excitation with UV light, the interplanar angle between the two terminal aryl rings of the [5]helix core of 2 decreases (S1 state compared with S0 state), which is similar to placing a spring under an external force.

Quantum Coherence Enhances Electron Transfer Rates to Two Equivalent Electron Acceptors.

When a molecular electron donor interacts with multiple electron acceptors, quantum coherence can enhance the electron transfer (ET) rate. Here we report photodriven ET rates in a pair of donor-acceptor (D-A) compounds that link one anthracene (An) donor to one or two equivalent 1,4-benzoquinone (BQ) acceptors. Subpicosecond ET from the lowest excited singlet state of An to two BQs is about 2.4 times faster than ET to one BQ at room temperature, but about 5 times faster at cryogenic temperatures. This factor of 2 increase results from a transition from ET to one of two acceptors at room temperature to ET to a superposition state of the two acceptors with correlated system-bath fluctuations at low temperature.

Bipolar thianthrene derivatives exhibiting room temperature phosphorescence for oxygen sensing

The series of donor-acceptor (D-A) compounds consisting of a thianthrene donor and either a benzophenone or a diphenylsulfone acceptor units were designed and synthesized via Suzuki cross-coupling. By varying the number of donor units, as well as by introducing heavy atom (Br) into the molecular structure the impact of these substituents on the thermal, electrochemical and emissive properties of the compounds was studied. The compound containing two thianthrene units appeared to have higher thermal stability and higher glass transition temperature than monosubstituted derivative. All thianthrenyl substituted benzophenone or diphenylsulfone compounds showed bipolar behavior. Strong room temperature phosphorescence and dual fluorescence-phosphorescence were observed at room temperature. The compounds containing stronger electron acceptor diphenylsulfone demonstrated a two-fold higher photoluminescence quantum yield values up to 19% in rigid Zeonex® films than that benzophenone derivatives. The crystals of some synthesized compounds demonstrated oxygen sensing ability.

Push-pull molecules bearing a hydrazonocyclopentadiene acceptor moiety: from the synthesis to organic photovoltaic applications

First representatives of a novel class of compounds containing an electron-accepting hydrazonocyclopentadiene moiety, π-conjugated phenylene spacer, and electron-donating triphenylamine moiety have been synthesized. Investigation of their optical, electrochemical and photovoltaic properties revealed a high potential of the hydrazonocyclopentadiene acceptor moiety in the design of donor-acceptor compounds for organic photovoltaics.

Synthesis of Pyridinothienogermoles as Unsymmetrically Condensed Germoles

Pyridinothienogermoles (PyTGs) were prepared by the reaction of dilithiopyridylthiophene with dichlorodi(n-octyl)germane and dichlorodiphenylgermane, and their electronic states were investigated by optical measurements and density functional theory (DFT) calculations. Bromination of trimethylsilyl-substituted PyTG with N-bromosuccinimide gave PyTG bromide, and palladium-catalyzed Stille coupling reactions of bromide with diphenyl[(trimethylstannyl)phenyl]amine and N-[(trimethylstannyl)phenyl]carbazole provided donor–acceptor compounds with pyridine as the acceptor and triphenylamine or phenylcarbazole as the donor. These compounds showed clear solvatochromic properties in the photoluminescence (PL) spectra. Bi(pyridinothienogermole) was prepared by the Stille coupling of DPyG bromide and trimethylstannyl-substituted DPyG. The Lewis basicity of the pyridine ring made it possible to use these DPyG derivatives as ligands for complex formation. Protonation and complex formation of the PyTG pyridine unit with...

Electronic and Photophysical Properties of 9,10‐Anthrylene‐Bridged B‐π‐N Donor‐Acceptor Molecules with Solid‐State Emission in the Yellow to Red Region

9,10-Anthrylene-bridged triarylborane-triarylamine donor-acceptor compounds were prepared to examine the influence of the bulky π-bridge on the electronic and photophysical properties of the compounds, with the aim of realizing their solid-state emission. The intramolecular charge-transfer (ICT) absorption and emission between the vacant p orbital on the boron center, p(B), and occupied p orbital on the nitrogen center, p(N), through the π orbital of the anthrylene, π(anthrylene), were observed, and it was demonstrated that the HOMO-LUMO gap decreased with increasing number of introduced anthrylene units because of the effective lowering of LUMO originating from the p(B)-π(anthrylene) orbital interaction. The compounds exhibited solid-state emission with emission maxima at 560 nm and 643 nm, respectively, in the yellow to red region, with the corresponding absolute solid-state quantum yield of 18 % and 31 %, as a result of the combination of the highly congested structure originating from the anthrylene π-bridge and the introduction of bulky tert-butyl protecting groups.

Preparation and Characterization of a π-Conjugated Donor-Acceptor System Containing the Strongly Electron-Accepting Tetraphenylborolyl Unit.

A novel thiophene-bridged donor-acceptor system was synthesized with a carbazole as donor and a borole as acceptor unit. The borole group was successfully installed via the tin-boron exchange reaction of 1,1-dimethyl-2,3,4,5-tetraphenylstannole with 9-(5-(dibromoboryl)thiophen-2-yl)carbazole. The effect of the borole on the optoelectronic properties of the donor-acceptor system was explored by spectroscopic (UV/Vis and fluorescence spectroscopy), electrochemical (cyclic voltammetry) and theoretical (TD-DFT) methods as well as by modifying its structure. The corresponding donor-acceptor compound bearing the widely employed dimesitylboryl acceptor group was also synthesized for comparison.

C−F Bond Activation by a Saturated N‐Heterocyclic Carbene: Mesoionic Compound Formation and Adduct Formation with B(C6F5)3

AbstractThe reaction of SIPr, [1,3‐bis(2,6‐diisopropylphenyl)‐imidazolin‐2‐ylidene] (1), with C6F6 led to the formation of an unprecedented mesoionic compound (2). The formation of 2 is made accessible by deprotonation of the SIPr backbone with simultaneous elimination of HF. The C−F bond para to the imidazolium ring in 2 is only of 1.258(4) Å, which is the one of the shortest structurally authenticated C−F bonds known to date. The liberation of HF during the reaction is unequivocally proved by the addition of one more equivalent of SIPr, which leads to the imidazolium salt with the HF2− anion. To functionalize 2, the latter reacted with B(C6F5)3 to give an unusual donor–acceptor compound, where the fluoride atom from the C6F5 moiety coordinates to B(C6F5)3 and the carbanion moiety remains unaffected. Such coordination susceptibility of the fluoride atom of a nonmetallic system to a main‐group Lewis acid (Fnon‐metal→BR3) is quite unprecedented.

C-F Bond Activation by a Saturated N-Heterocyclic Carbene: Mesoionic Compound Formation and Adduct Formation with B(C6 F5 )3.

The reaction of SIPr, [1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] (1), with C6 F6 led to the formation of an unprecedented mesoionic compound (2). The formation of 2 is made accessible by deprotonation of the SIPr backbone with simultaneous elimination of HF. The C-F bond para to the imidazolium ring in 2 is only of 1.258(4) Å, which is the one of the shortest structurally authenticated C-F bonds known to date. The liberation of HF during the reaction is unequivocally proved by the addition of one more equivalent of SIPr, which leads to the imidazolium salt with the HF2 - anion. To functionalize 2, the latter reacted with B(C6 F5 )3 to give an unusual donor-acceptor compound, where the fluoride atom from the C6 F5 moiety coordinates to B(C6 F5 )3 and the carbanion moiety remains unaffected. Such coordination susceptibility of the fluoride atom of a nonmetallic system to a main-group Lewis acid (Fnon-metal →BR3 ) is quite unprecedented.

Red-shifted delayed fluorescence at the expense of photoluminescence quantum efficiency - an intramolecular charge-transfer molecule based on a benzodithiophene-4,8-dione acceptor.

Employing the thiophene based quinone, benzo[1,2-b:4,5-b']dithiophene-4,8-dione, as the electron-accepting moiety alongside N-phenylcarbazole donors to produce a donor-π-acceptor-π-donor (D-π-A-π-D) molecule has yielded a new red emitter displaying delayed fluorescence. This new molecule shows strongly (over 100 nm) red-shifted emission when compared to an anthraquinone based analogue. Cyclic voltammetry complemented by computational insights prove that this red-shift is due to the significantly stronger electron-accepting ability of the thiophene quinone compared to anthraquinone. Photophysical and computational studies of this molecule have revealed that while the presence of the thiophene containing acceptor facilitates rapid intersystem crossing which is comparable to anthraquinone analogues, the reverse intersystem crossing rate is slow and non-radiative decay is rapid which we can attribute to low-lying locally excited states. This limits the total photoluminescence quantum efficiency to less than 10% in both solution and the solid state. These results provide a useful example of how very minor structural variations can have a defining impact on the photophysical properties of new molecular materials.