Charge Transfer Character Research Articles

Density functionals with asymptotic-potential corrections are required for the simulation of spectroscopic properties of materials.

Five effects of correction of the asymptotic potential error in density functionals are identified that significantly improve calculated properties of molecular excited states involving charge-transfer character. Newly developed materials-science computational methods are used to demonstrate how these effects manifest in materials spectroscopy. Connection is made considering chlorophyll-a as a paradigm for molecular spectroscopy, 22 iconic materials as paradigms for 3D materials spectroscopy, and the VN− defect in hexagonal boron nitride as an example of the spectroscopy of defects in 2D materials pertaining to nanophotonics. Defects can equally be thought of as being “molecular” and “materials” in nature and hence bridge the relms of molecular and materials spectroscopies. It is concluded that the density functional HSE06, currently considered as the standard for accurate calculations of materials spectroscopy, should be replaced, in most instances, by the computationally similar but asymptotically corrected CAM-B3LYP functional, with some specific functionals for materials-use only providing further improvements.

Solar Energy Storage: Competition between Delocalized Charge Transfer and Localized Excited States in the Norbornadiene to Quadricyclane Photoisomerization.

We describe for the first time the full reaction coordinate regarding the photoisomerization of red-absorbing norbornadienes (NBDs) to quadricyclanes (QCs). Our studies go beyond steady-state investigations by using an arsenal of time-resolved techniques. Importantly, the red absorption of NBDs is made possible by a different charge-transfer character; adjusting its strength enables control over the photoreversibility of the rearrangement. In the case of strong charge-transfer character (a weakly electron-withdrawing ester and a strongly electron-donating dimethylaniline), photoirradiation with visible light into the delocalized charge-transfer absorption of NBD affords QC reversibly. In stark contrast, UV photoirradiation into the NBD localized excited state leads to a photoinduced degradation and cannot be back-isomerized to NBD under any circumstances. If the charge-transfer character is weak (a weakly electron-withdrawing ester and a weakly electron-donating phenyl), reversibility is seen independently of the photoirradiation light.

Finely tuning electronic and steric structures of spiro[fluorene-9,9′-xanthene] (SFX)-based emitters by isomerization strategy towards efficient electroluminescence

Considering the advantages of spiro[fluorene-9,9′-xanthene] (SFX) on facilitating radiative transition and suppressing concentration quenching, a pair of isomers, namely F-BT-F and X-BT-X, were designed and synthesized with benzo[c][1,2,5]thiadiazole (BT) as an acceptor core to bridge fluorene moieties (F-BT-F) or xanthene moieties (X-BT-X) of SFXs. The pair of isomers with D-A-D architecture presented almost the same energy gap in chloroform solution, providing a delicate model to probe the electronic and steric differences between binary structure of SFX. Single-crystal diffractions reveal that SFX-donor units are in antiparallel arrangement on the both sides of the BT core, and X-BT-X molecules have more discrete and ordered packing compared to F-BT-F in crystal phase. Photophysical, electrochemical measurements and DFT calculations manifest that the pair of emitters possess excellent photoluminescence quantum yields up to 95%, and the X-BT-X shows weaker charge-transfer character compared with the F-BT-F, attributed to the strong electron-donating nature and partly interrupted conjugation pathway of xanthene-end relative to fluorene-end. In doped organic light-emitting devices (OLEDs), the F-BT-F and X-BT-X displayed nearly uniform electroluminescence (EL) with the emission peaks at 515 and 518 nm, as well as the maximum external quantum efficiencies (EQEs) approaching 5.0%. In non-doped OLEDs, however, the X-BT-X had more remarkable EL performance with the emission peak at 525 nm, maximum current efficiencies of 14.02 cd A−1, maximum power efficiencies of 8.81 lm W−1 and maximum EQE of 4.25%, exceeding the efficiency values of 13.75 cd A−1, 7.71 lm W−1, and 4.08% of F-BT-F based device. By illuminating the electronic and steric features of xanthene moiety, this work opens a new door to construct high-efficiency and low-cost SFX-based fluorescence emitters.

Orange-red emissive Cu(I) complexes bearing Schiff base ligands: Synthesis, structures, and photophysical properties

Herein, the three novel copper (I) complexes bearing Schiff base ligands of the types [Cu(Phen-PyMethylene-Adam-Amine)(POP)]PF6 (P1), [Cu(Methoxyl-Phen-PyMethylene-Adam-Amine)(POP)]PF6 (P2), and [Cu(TriflMethyl-Phen-PyMethylene-Adam-Amine)(POP)]PF6 (P3) (Phen-PyMethylene-Adam-Amine = ((5-phenylpyridin-2-yl)methylene)adamantan-1-amie, Methoxyl-Phen-PyMethylene-Adam-Amine = ((5-(4-methoxyphenyl)pyridin-2-yl)methylene)adamantan-1-amine, TriflMethyl-Phen-PyMethylene-Adam-Amine = ((5-(4-(trifluoromethyl)phenyl)pyridin-2-yl)methylene)adamantan-1-amine, POP=bis[2-diphenylphosphino]-phenyl)ether) have been prepared and investigated. Incorporation of the bulky adamantyl group into the complexes can enhance the stability of the complexes. The different electron-donating/withdrawing groups such as –H, –OCH3, and –CF3 substituents were functionalized onto the pyridine ring part of Schiff base ligands to tune the emission behavior of the resulting Schiff base Cu(I) complexes. All complexes exhibit the lowest energy metal-to-ligand charge transfer (MLCT) characters in 400–450 nm region showing a slight blue-shift of the absorption wavelengths in the order P3 > P1 > P2. Organe-red emissions in the region of 611-619 nm with photoluminescence quantum yields (PLQY) of 1.0–1.3% can be observed in the solid state. Additionally, the photophysical properties are also investigated theoretically using density functional theory (DFT) and time dependent DFT.

Tuning the Excited State of Tetradentate Pd(II) and Pt(II) Complexes through Benzannulated N‐Heteroaromatic Ring and Central Metal

Comprehensive SummaryA series of tetradentate Pd(II) and Pt(II) complexes containing fused 5/6/6 metallocycles with phenyl N‐heteroaromatic benzo[d]imidazole (pbiz), benzo[d]oxazole (pboz) or benzo[d]thiazole (pbthz)‐containing ligands was developed. Systematic studies by experiments and theoretical calculations reveal that both the central metal and the benzannulated N‐heteroaromatic ring have significant influence on the electrochemical, photophysical and excited‐state properties of the Pd(II) and Pt(II) complexes. In identical condition, compared to pbiz‐based Pd(II) and Pt(II) complexes, the corresponding metal complexes with pboz and pbthz‐containing ligand show significant red‐shift emission spectra. Pd(II) complexes exhibit blue‐shift emission spectra in comparison with the corresponding Pt(II) complexes. The Pt(II) complexes possess more metal‐to‐ligand charge transfer (3MLCT) character in their T1 states, which enables the Pt(II) complexes to have much higher quantum efficiencies (ΦPL) and shorter excited‐state lifetimes (τ), resulting in large radiative decay rates (kr). Especially, Pt(pbiz) has a ΦPL of 94% with a τ of 4.0 μs in 5 wt% doped PMMA film at room temperature. Pt(pbiz)‐doped green organic light‐emitting diode (OLED) using 26mCPy as host demonstrated a peak external quantum efficiency (EQE) of 21.6% and a maximum brightness (Lmax) of 55481 cd/m2, which still maintained an EQE of 16.5% and 10.8% at 1000 and 10000 cd/m2, respectively.

Amino-Functionalized Cage-Opened C60 Derivatives.

The amino-functionalized cage-opened [60]fullerene derivatives were synthesized by reactions with phenylenediamine. In this reaction, the diamine undergoes direct addition to the α,β-unsaturated carbonyl moiety. Further C-C bond scission is promoted by the intramolecular SN2 reaction. These amino-functionalized derivatives possess high-lying highest occupied molecular orbital levels as suggested by electrochemical analyses. These compounds showed intense near-infrared absorption bands that tail to 900 nm, reflecting the optical transition with π-π* and charge transfer character.

QM/MM Studies on Thermally Activated Delayed Fluorescence of a Dicopper Complex in the Solid State

Dicopper complexes with thermally activated delayed fluorescence (TADF) phenomena are important in enriching the arsenal of organic light-emitting diodes materials. However, the TADF mechanism is still elusive, especially in the solid state. Herein, we chose a TADF dicopper complex and investigated its geometric and electronic structures and absorption and emission spectra using DFT, TD-DFT, and QM/MM methods. On the basis of these results, we further estimate the fluorescence emission rate from the S1 state, phosphorescence emission rate from the T1 state, and forward and reverse intersystem crossing (ISC and rISC) rates between S1 and T1. The present work shows that both the S1 and the T1 states have mixed metal-to-ligand and interligand charge-transfer character. Good spatial separation between the HOMO and the LUMO makes the S1–T1 energy gap small, ca. 2.8 kcal/mol. This small energy gap leads to efficient ISC and rISC processes, whose rates are much larger than the fluorescence and phosphorescence emission rates at 300 K, therefore enabling TADF. In contrast, at 77 K, the rISC process is blocked because its rate is much smaller than the phosphorescence emission. Thus, TADF disappears at 77 K. Further analysis shows that high-frequency deformation and low-frequency torsional vibrational modes make a large contribution to the Huang–Rhys factors, but Duschinsky rotation effects are essentially negligible for both the ISC and the rISC processes.

Solvent Effects on Ultrafast Photochemical Pathways.

Photochemical reactions are increasingly being used for chemical and materials synthesis, for example, in photoredox catalysis, and generally involve photoexcitation of molecular chromophores dissolved in a liquid solvent. The choice of solvent influences the outcomes of the photochemistry because solute-solvent interactions modify the energies of and crossings between electronic states of the chromophores, and they affect the evolving structures of the photoexcited molecules. Ultrafast laser spectroscopy methods with femtosecond to picosecond time resolution can resolve the dynamics of these photoexcited molecules as they undergo structural and electronic changes, relax back to the ground state, dissipate their excess internal energy to the surrounding solvent, or undergo photochemical reactions. In this Account, we illustrate how experimental studies using ultrafast lasers can reveal the influences that different solvents or cosolutes exert on the photoinduced nonadiabatic dynamics of internal conversion and intersystem crossing in nonradiative relaxation pathways. Although the environment surrounding a solute molecule is rapidly changing, with fluctuations in the coordination to neighboring solvent molecules occurring on femtosecond or picosecond time scales, we show that it is possible to photoexcite selectively only those molecular chromophores transiently experiencing specific solute-solvent interactions such as intermolecular hydrogen bonding.The effects of different solvation environments on the photodynamics are illustrated using four selected examples of photochemical processes in which the solvent has a marked effect on the outcomes. We first consider two aromatic carbonyl compounds, benzophenone and acetophenone, which are known to undergo fast intersystem crossing to populate the first excited triplet state on time scales of a few picoseconds. We show that the nonadiabatic excited-state dynamics are modified by transient hydrogen bonding of the carbonyl group to a protic solvent or by coordination to a metal cation cosolute. We then examine how different solvents modify the competition between two alternative relaxation pathways in a photoexcited UVA-sunscreen molecule, diethylamino hydroxybenzoyl hexyl benzoate (DHHB). This relaxation back to the ground electronic state is an essential part of the effective operation of the sunscreen compound, but the dynamics are sensitive to the surrounding environment. Finally, we consider how solvents of different polarity affect the energies and lifetimes of excited states with locally excited or charge-transfer character in heterocyclic organic compounds used as excited-state electron donors for photoredox catalysis. With these and other examples, we seek to develop a molecular level understanding of how the choice of solution environment might be used to control the outcomes of photochemical reactions.

Photophysical properties of push-π-pull type chromophores of aryl-substituted bipyridines in different solvents

Photophysical studies of 4′-methyl-2,2′-bipyridine derivatized with the polycyclic aromatic hydrocarbons (PAHs) anthracene (mbpy-anth), naphthalene (mbpy-naph), and pyrene (mbpy-pyr) were acquired in different solvents. While the absorption spectra were not affected by the solvent nature, the emission ones showed red shifts with increase polarity consistent with an intramolecular charge-transfer (ICT) character. Theoretical calculations found a dominant “Hole” → “Particle” contribution for the first five transitions involving local excited-state (LE) and ICT transitions, with the latter being dominant at higher wavelengths (S0 → 1CT). The fluorescence solvatochromic data suggested a hybridized local and charge transfer (HLCT) state for the mbpy-R compounds with the CT contribution prevailing in high polar solvents; a result that was corroborated by the monoexponential decay lifetimes. The Stokes shifts difference between heptane and dimethylsulphoxide, the most and least polar solvents used in this work, was calculated as 1315, 2969, and 808 cm−1 for mbpy-naph, mbpy-anth, and mbpy-pyr, respectively. The increase of the solvent polarity results in a red shift and a broadening of the emission and excitation bands for mbpy-anth and mbpy-naph with the spectra of the latter being the most affected. For mbpy-pyr, on the contrary, the emission spectra were narrower than the excitation ones indicating the La transitions lie at lower energy than the Lb. For this species, the results indicated an effective mixing of LE and CT states in the moderately polar solvents being in accordance with the observed single exponential decay which is likely related to one hybridized state.

Tuning the Quantum Chemical Properties of Flavins via Modification at C8.

Flavins are central to countless enzymes but display different reactivities depending on their environments. This is understood to reflect modulation of the flavin electronic structure. To understand changes in orbital natures, energies, and correlation over the ring system, we begin by comparing seven flavin variants differing at C8, exploiting their different electronic spectra to validate quantum chemical calculations. Ground state calculations replicate a Hammett trend and reveal the significance of the flavin π-system. Comparison of higher-level theories establishes CC2 and ACD(2) as methods of choice for characterization of electronic transitions. Charge transfer character and electron correlation prove responsive to the identity of the substituent at C8. Indeed, bond length alternation analysis demonstrates extensive conjugation and delocalization from the C8 position throughout the ring system. Moreover, we succeed in replicating a particularly challenging UV/Vis spectrum by implementing hybrid QM/MM in explicit solvents. Our calculations reveal that the presence of nonbonding lone pairs correlates with the change in the UV/Vis spectrum observed when the 8-methyl is replaced by NH2, OH, or SH. Thus, our computations offer routes to understanding the spectra of flavins with different modifications. This is a first step toward understanding how the same is accomplished by different binding environments.

Tuning the Quantum Chemical Properties of Flavins via Modification at C8.

Flavins are central to countless enzymes but display different reactivities depending on their environments. This is understood to reflect modulation of the flavin electronic structure. To understand changes in orbital natures, energies, and correlation over the ring system, we begin by comparing seven flavin variants differing at C8, exploiting their different electronic spectra to validate quantum chemical calculations. Ground state calculations replicate a Hammett trend and reveal the significance of the flavin π-system. Comparison of higher-level theories establishes CC2 and ACD(2) as methods of choice for characterization of electronic transitions. Charge transfer character and electron correlation prove responsive to the identity of the substituent at C8. Indeed, bond length alternation analysis demonstrates extensive conjugation and delocalization from the C8 position throughout the ring system. Moreover, we succeed in replicating a particularly challenging UV/Vis spectrum by implementing hybrid QM/MM in explicit solvents. Our calculations reveal that the presence of nonbonding lone pairs correlates with the change in the UV/Vis spectrum observed when the 8-methyl is replaced by NH2, OH, or SH. Thus, our computations offer routes to understanding the spectra of flavins with different modifications. This is a first step toward understanding how the same is accomplished by different binding environments.

Structural, electronic and magnetic properties of weakly correlated metal Sr2CrTiO6: a first principles study

Structural, electronic and magnetic behaviour of a less-explored material Sr2CrTiO6 has been investigated using ab initio calculations with generalized gradient approximation (GGA) and GGA + U methods, where U is the Hubbard parameter. For each of the three possible Cr/Ti cationic arrangements in the unit cell, considered in this work, the non-magnetic band structure shows distinct traits with significant flat-band regions leading to large t 2g density of states around the Fermi energy. The Cr4+ ion in Sr2CrTiO6, which is a d 2 system, shows a reverse splitting of the t 2g orbitals. The calculated hopping matrix contains non-zero off-diagonal elements between the d xz and d yz orbitals, while the d xy orbitals remain largely unmixed. A minimal tight binding model successfully reproduces the six t 2g bands around the Fermi energy. The Fermi surface shows a two-dimensional nesting feature for the layered arrangement of Cr and Ti atoms. Fixed spin moment studies suggest that the magnetism in this compound cannot be explained by Stoner’s criterion of an itinerant band ferromagnet. In the absence of Hubbard U term, the ground state is a half-metallic ferromagnet. Calculations for the anti-ferromagnetic spin arrangement show re-arrangement of orbital character resulting in (a) narrow d xz /d yz bands and sharp peaks in the density of states at the Fermi energy and (b) highly dispersive d xy bands with a broader density of states around the Fermi energy. The metallicity persists even with increasing U for both the spin arrangements, thus suggesting that Sr2CrTiO6 belongs to the class of weakly correlated metals, while the shifting of O 2p states towards the Fermi energy with U indicates a negative charge-transfer character in Sr2CrTiO6.

Intramolecular charge transfer-induced solvatochromism and large Stokes shifts of furocoumarins

A series of furocoumarins (4a-h) was synthesized via a multi-component reaction and characterized by FTIR, 1H NMR, 13C NMR and ESI-MS. Compound 4g was further characterized with single-crystal X-ray diffraction for the molecular packing and interaction analysis. The photophysical and photochemical properties of the series have been experimentally and theoretically investigated, focusing on the absorption and fluorescence properties in solvents of varying polarity. Geometry optimization, HOMO-LUMO energy gap, NBO analyses and potential nonlinear optical (NLO) responses for all furocoumarins (4a-h) have been computed using DFT/TD-DFT method. The experimental results reveal that absorption and fluorescence of furocoumarins are highly sensitive to the solvent polarity and inter or intramolecular interactions. Furocoumarin exhibits strong solvatochromism through the intramolecular charge transfer (ICT) process, large Stokes shift up to 142 nm in polar solvents, long emission wavelength up to 520 nm in polar protic solvent through large polarization and relative high quantum efficiencies up to 94% in DMSO. Compound 4h exhibits Stokes shift value of 197 nm in toluene. The charge transfer character was correlated with the Lippert-Mataga and McRae plots while the effect of substituents on the fluorescence quantum yield was described by the Hammett plot. Based on the theoretical computations, furocoumarins with strong electron donating substituents (4b and 4d) possess high frequency-dependent first order hyperpolarizabilities, β in the range of 21.32–27.34 × 10−30 esu, suggesting their possible use in the NLO application. Fluorescence quenching in 4c, 4g and 4h were caused by site-specific hydrogen bonding strengthening and molecular packing that led to the formation of excimer species. This work will be valuable for the exploitation of ICT and solvatochromic effects and future molecular engineering of organic materials for practical utilities endowing low cost, facile and greener approach.

Rational Management of Charge Transfer Character of Benzothienopyrimidine‐Based N‐Type Host for Blue Phosphorescent Organic Light‐Emitting Diodes

AbstractTwo materials, 3‐(4‐(9H‐carbazol‐9‐yl)benzo[4,5]thieno[3,2‐d]pyrimidin‐2‐yl)‐5‐(triphenylsilyl)benzonitrile (CzBTPCNmSi) and 9‐(2‐(3‐(triphenylsilyl)phenyl)benzo[4,5]thieno[3,2‐d]pyrimidin‐4‐yl)‐9H‐carbazole‐3‐carbonitrile (CNCzBTPmSi), are synthesized as the N‐type host (electron‐transport type host) of a mixed host, harvesting triplet excitons of the blue phosphor by engineering the charge transfer character using a CN unit. The two host materials are based on benzothienopyrmidine modified using CN, tetraphenylsilyl, and carbazole groups to control the lowest unoccupied molecular orbital level, manage their intermolecular packing, and stabilize the molecule against holes, respectively. The two hosts are mixed with a P‐type 3,3‐di(9H‐carbazol‐9‐yl)biphenyl (mCBP) to offer a mixed host for sky and deep blue‐emitting phosphors. In the device applications, CzBTPCNmSi and CNCzBTPmSi show high external quantum efficiency (EQE) (around 24% in the sky‐blue devices), whereas only CNCzBTPmSi exhibited high EQE (above 20% in the deep blue devices) due to high triplet energy. Moreover, the CNCzBTPmSi demonstrates a longer device lifetime than the control host without the CN unit. Thus, CN engineering can manage the triplet energy, EQE, and device lifetime of blue phosphorescent devices by controlling the charge transfer character.

Polycyclic aromatic hydrocarbon-substituted push-pull chromophores: an investigation of optoelectronic and nonlinear optical properties using experimental and theoretical approaches.

A series of new push–pull chromophores were synthesized in moderate to very high yields (65%–97%) by treating TCNE and TCNQ with alkynes substituted by electron-rich diethylaniline and polycyclic aromatic hydrocarbons. Some of the chromophores exhibit strong intramolecular charge-transfer bands in the near-IR region with λmax values between 695 and 749 nm. With the help of experimental and theoretical analysis, it is concluded that the trend in λ max values is affected by PAH substituents sterically, not electronically. Steric constraints led to the increased dihedral angles, reducing conjugation efficiencies. The absorption properties of push-pull compounds have been investigated in solvents possessing different polarities. All chromophores exhibited positive solvatochromism. As an additional proof of efficient charge-transfer in push–pull chromophores, quinoid character (dr) values were predicted using calculated bond lengths. Remarkably, substantial dr values (0.045–0.049) were predicted for donor diethylaniline rings in all compounds. The effects of various polycyclic aromatic hydrocarbons on optical and nonlinear optical properties were also studied by computational methods. Several parameters, such as band gaps, Mulliken electronegativity, chemical hardness and softness, dipole moments, average polarizability, first hyperpolarizability, were predicted for chromophores at the B3LYP/6-31++G(d,p) level of theory. The predicted first hyperpolarizability β(tot) values vary between 198 to 538 × 10–30 esu for the reported push–pull chromophores in this study. The highest predicted β(tot) value in this study is 537.842 × 10–30 esu, 8150 times larger than the predicted β(tot) value of benchmark NLO material urea, suggests possible utilization of these chromophores in NLO devices. The charge-transfer character of the synthesized structures was further confirmed by HOMO-LUMO depictions and electrostatic potential maps.

Making Nitronaphthalene Fluoresce.

Nitroaromatic compounds are inherently nonfluorescent, and the subpicosecond lifetimes of the singlet excited states of many small nitrated polycyclic aromatic hydrocarbons, such as nitronaphthalenes, render them unfeasible for photosensitizers and photo-oxidants, despite their immensely beneficial reduction potentials. This article reports up to a 7000-fold increase in the singlet-excited-state lifetime of 1-nitronaphthalene upon attaching an amine or an N-amide to the ring lacking the nitro group. Varying the charge-transfer (CT) character of the excited states and the medium polarity balances the decay rates along the radiative and the two nonradiative pathways and can make these nitronaphthalene derivatives fluoresce. The strong electron-donating amine suppresses intersystem crossing (ISC) but accommodates CT pathways of nonradiate deactivation. Conversely, the N-amide does not induce a pronounced CT character but slows down ISC enough to achieve relatively long lifetimes of the singlet excited state. These paradigms are key for the pursuit of electron-deficient (n-type) organic conjugates with promising optical characteristics.

Spiro−type TADF emitters based on acridine donors and anthracenone acceptor

Two rigid spiro-type TADF emitters (CBZANQ, PXZANQ) were designed and synthesized via a two-step route. CBZANQ and PXZANQ both contain an anthracenone acceptor along with a carbazole‒acridine-fused donor and a phenoxazine‒acridine-fused donor, respectively, where each donor and acceptor are linked by a spiro bridge to form D-σ-A structures with an orthogonal arrangement. Both CBZANQ and PXZANQ have the difference between the singlet and triplet energy values (ΔEST) of 0.05 eV and 0.03 eV, respectively, which are sufficiently small for efficient triplet harvesting. CBZANQ with a weak electron donor (carbazole) exhibited a weak intramolecular charge transfer (ICT) character, resulting in a low photoluminescence quantum yield (PLQY) of 18%. On the other hand, when combined with a strong electron donor (phenoxazine), PXZANQ exhibited a strong ICT character, resulting in a high PLQY of 71%. Organic light-emitting diode (OLED) devices based on CBZANQ emitter and bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) host exhibited sky-blue electroluminescence (EL) at 492 nm with a low external quantum efficiency (EQE) of 6.7%. Conversely, the OLED devices fabricated using PXZANQ emitter and DPEPO host exhibited green emission at 528 nm, along with a high EQE of 22.1%. Additionally, the efficiency of PXZANQ-based devices remained high at high luminance, indicating relatively small efficiency roll-off (13.9% EQE under 500 cd/m2).

Understanding and Tailoring Excited State Properties in Solution-Processable Oligo(p-phenyleneethynylene)s: Highly Fluorescent Hybridized Local and Charge Transfer Character via Experiment and Theory.

Rod-shaped oligo(p-phenyleneethynylene) (OPE) offers an attractive π-framework for the development of solution-processable highly fluorescent molecules having tunable hybridized local and charge transfer (HLCT) excited states and (reverse) intersystem crossing ((R)ISC) channels. Herein, an HLCT oligo(p-phenyleneethynylene) library was studied for the first time in the literature in detail systematically via experiment and theory. The design, synthesis, and full characterization of a new highly fluorescent (ΦPL-solution ∼ 1) sky blue emissive 4',4‴-((2,5-bis((2-ethylhexyl)oxy)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(N,N-diphenyl-[1,1'-biphenyl]-4-amine) (2EHO-TPA-PE) was also reported. The new molecule consists of a D'-Ar-π-D-π-Ar-D' molecular architecture with an extended π-spacer and no acceptor unit, and detailed structural, physicochemical, single-crystal, and optoelectronic characterizations were performed. A high solid-state quantum efficiency (ΦPL-solidstate ∼ 0.8) was achieved as a result of suppressed exciton-phonon/vibronic couplings (no π-π interactions and multiple (14 per dimeric form) strong C-H···π interactions). Strong solution-phase/solid-state dipole-dependent tunable excited state behavior (local excited (LE) → HLCT → charge transfer (CT)) and decay dynamics covering a wide spectral region were demonstrated, and the CT state was observed to be highly fluorescent despite extremely large Stokes shift (∼130 nm)/fwhm (∼125 nm) and significant charge separation (0.75 charge·nm). Employing the Lippert-Mataga model, along with detailed photophysical studies and TDDFT calculations, key relationships between molecular design-electronic structure-exciton characteristics were elucidated with regards to HLCT and hot exciton channel formations. The interstate coupling between CT and LE states and the interplay of this coupling with respect to medium polarity were explored. A key relationship between excited-state symmetry breaking process and the formation of HLCT state was discussed for TPA-ended rod-shaped OPE π-systems. (R)ISC-related delayed fluorescence (τ ∼ 2-6 ns) processes were evident following the prompt decays (∼0.4-0.9 ns) both in the solution and in the solid-state. As a unique observation, the delayed fluorescence could be tuned and facilitated via small dielectric changes in the medium. Our results and the molecular engineering perspectives presented in this study may provide unique insights into the structural and electronic factors governing tunable excited state and hot-exciton channel formations in OPEs for (un)conventional solution-processed luminescence applications.

Tuning Excited-State Properties of [2.2]Paracyclophane-Based Antennas to Ensure Efficient Sensitization of Lanthanide Ions or Singlet Oxygen Generation

The multistep synthesis of original antennas incorporating substituted [2.2]paracyclophane (pCp) moieties in the π-conjugated skeleton is described. These antennas, functionalized with an electron donor alkoxy fragment (A1) or with a fused coumarin derivative (A2), are incorporated in a triazacyclonane macrocyclic ligand L1 or L2, respectively, for the design of Eu(III), Yb(III), and Gd(III) complexes. A combined photophysical/theoretical study reveals that A1 presents a charge transfer character via through-space paracyclophane conjugation, whereas A2 presents only local excited states centered on the coumarin-paracyclophane moiety, strongly favoring triplet state population via intersystem crossing. The resulting complexes EuL1 and YbL2 are fully emissive in red and near-infrared, respectively, whereas the GdL2 complex acts as a photosensitizer for the generation of singlet oxygen.

Benzothiazole‐based chemosensor: a quick dip into its anion sensing mechanism

AbstractDensity functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) methods have been applied to decipher the F− anion sensing mechanism of a fluorescent probe N‐(2‐(benzothiazole‐2‐yl)phenyl)‐4‐methoxybenzamide (4). The amidic proton (N–H) of the selected probe may exhibit an intramolecular hydrogen bond (IMHB) with the nitrogen atom of benzothiazole moiety. The weak IMHB of N24–H26‐‐‐N12 in the excited state favors reverse proton transfer process in 4. In the ground state of 4, the F− anion interacts via F−‐‐‐H–N bond, which immediately deprotonates the amidic proton, leading to the distortion of the geometry. Subsequent to deprotonation, the significant intramolecular charge transfer (ICT) character was observed in 4. On account of the significant ICT process, the redshift in absorption and emission spectra was observed. These outcomes were employed to delineate the sensing mechanism of molecule 4.