Intramolecular Steric Hindrance Research Articles

Intramolecular Through‐Space Charge‐Transfer Effect for Achieving Room‐Temperature Phosphorescence in Amorphous Film

AbstractOrganic emitters that exhibit room‐temperature phosphorescence (RTP) in neat films have application potential for optoelectronic devices, bio‐imaging, and sensing. Due to molecular vibrations or rotations, the majority of triplet excitons recombine rapidly via non‐radiative processes in purely organic emitters, making it challenging to observe RTP in amorphous films. Here, a chemical strategy to enhance RTP in amorphous neat films is reported, by utilizing through‐space charge‐transfer (TSCT) effect induced by intramolecular steric hindrance. The donor and acceptor groups interact via spatial orbital overlaps, while molecular motions are suppressed simultaneously. As a result, triplets generated under photo‐excitation are stabilized in amorphous films, contributing to phosphorescence even at room temperature. The solvatochromic effect on the steady‐state and transient photoluminescence reveals the charge‐transfer feature of involved excited states, while the TSCT effect is further experimentally resolved by femtosecond transient absorption spectroscopy. The designed luminescent materials with pronounced TSCT effect show RTP in amorphous films, with lifetimes up to ≈40 ms, comparable to that in a rigid polymer host. Photoluminescence afterglow longer than 3 s is observed in neat films at room temperature. Therefore, it is demonstrated that utilizing intramolecular steric hindrance to stabilize long‐lived triplets leads to phosphorescence in amorphous films at room temperature.

Geometric Signatures as Important Factors to Control the Photo-Stabilities of the Phosphorescent Pd(II)/Pt(II) Complexes: A Case Study.

Operation lifetime, as an important parameter, determines the performance of phosphorescent organic light-emitting diodes (OLEDs). Unveiling the intrinsic degradation mechanism of emission material is crucial for improving the operation's lifetime. In this article, the photo-stabilities of tetradentate transition metal complexes, the popular phosphorescent materials, are explored by means of density functional theory (DFT) and time-dependent (TD)-DFT, aiming to illustrate the geometric signatures as important factors to control the photo-stabilities. Results indicate that for the tetradentate Ni(II), Pd(II), and Pt(II) complexes, the coordinate bonds of the Pt(II) complex exhibit stronger strength. It seems that the strengths of coordinate bonds are closely related to the atomic number of the metal center in the same group, which could be attributed to the various electron configurations. The effect of intramolecular and intermolecular interactions on ligand dissociation is also explored here. The large intramolecular steric hindrance and strong π-π interaction between the Pd(II) complexes caused by aggregation could effectively raise the energy barriers of the dissociation reaction, leading to an unfeasible reaction pathway. Moreover, the aggregation of Pd(II) complex can change the photo-deactivation mechanism as compared to that of monomeric Pd(II) complex, which is favored for avoiding the TTA (triplet-triplet annihilation) process.

Adjusting the valence band center of Co-Ni-bimetallic sulfides through lattice expansion and stacking faults triggered by strain engineering to boost oxygen evolution reaction

Stress engineering can improve catalytic performance by straining the catalyst lattice. An electrocatalyst, Co3S4/Ni3S2-10%Mo@NC, was prepared with abundant lattice distortion to boost oxygen evolution reaction (OER). With the assistance of the intramolecular steric hindrance effect of metal–organic frameworks, slow dissolution by MoO42- of the Ni substrate and recrystallization of Ni2+ was observed in the process of Co(OH)F crystal growth with mild temperature and short time reaction. The lattice expansion and stacking faults created defects inside the Co3S4 crystal, improved the material conductivity, optimized the valence band electron distribution of the material, and promoted the rapid conversion of the reaction intermediates. The presence of reactive intermediates of the OER under catalytic conditions was investigated using operando Raman spectroscopy. The electrocatalysts exhibited super high performance, a current density of 10 mA cm−2 at an overpotential of 164 mV and 100 mA cm−2 at 223 mV, which were comparable to those of integrated RuO2. Our work for the first time demonstrates that the dissolution–recrystallization triggered by strain engineering is a good modulation approach to adjust the structure and surface activity of catalyst, suggesting promising industrial application.

Controlling the Triplet Potential Energy Surface of Bimetallic Platinum(II) Complex by Constructing Structure-Property Relationship: A Theoretical Exploration.

For phosphorescent materials, managing the triplet potential energy surface stands for controlling the phosphorescence quantum yield. However, due to the complexity and variability, the triplet potential energy surface can be managed with difficulty. In this work, a series of bimetallic Pt(II) complexes, namely Pt-1, Pt-1-1, Pt-1-2, Pt-2, Pt-3-5, and Pt-6-7, are employed as models to construct a relationship between the structures and triplet potential energy surfaces, aiming to achieve meaningful information to manage the triplet potential energy surface. On the basis of the results, it is observed that the triplet potential energy surface has an intimate connection with the structures of bimetallic Pt(II) complexes. In the case of the primordial Pt(II) complex, the triplet potential energy surface consists of two minimal points, illustrating various properties, which can largely affect the phosphorescence quantum yield. Once the intramolecular steric hindrance, restriction effect, and metallophilic interaction (Pt-Pd/Pd-Pd) are employed by tailoring the structures of primordial Pt(II) complexes, the triplet potential energy surface can be reconstructed via one minimal point-charactered short metal-metal distance, resulting in different photophysical properties. The relationship between the triplet potential energy surface and structure is essentially unveiled from the structural and electronic viewpoints. The conclusions originated from the structural and electronic investigations can be regarded as indicators to accurately and expediently predict the triplet potential energy surfaces of bimetallic Pt(II) complexes. The results presented here are helpful in addressing the designed strategies as they show that the triplet potential energy surfaces of bimetallic Pt(II) complexes can be properly tuned.

Improving the Performance of Photocatalytic Hydrogen Production Through Adjusting the Size of Defective Quantum Dots Co‐Catalyst Affected by Intramolecular Steric Hindrance on Thermal Stability of Functional Groups

Improving the Performance of Photocatalytic Hydrogen Production Through Adjusting the Size of Defective Quantum Dots Co‐Catalyst Affected by Intramolecular Steric Hindrance on Thermal Stability of Functional Groups

Improving the Performance of Photocatalytic Hydrogen Production through Adjusting the Size of Defective Quantum Dots Co‐Catalyst Affected by Intramolecular Steric Hindrance on Thermal Stability of Functional Groups

Quantum dots (QDs) co‐catalysts have been extensively studied in the field of photocatalytic hydrogen production (PHP) due to their limited domain effect. However, how to adjust the size of QDs co‐catalysts remains a big challenge. Herein, Ni2P QDs co‐catalysts are synthesized via phosphating metal–organic frameworks (MOFs). Due to the intramolecular steric hindrance on the thermal stability of functional groups: electron‐giving functional groups ‐NH2, ‐OH, and electron‐absorbing functional group ‐COOH, the electronic symmetry of organic ligands and the coordination with metal ions is affected, resulting in three sizes of Ni2P QDs co‐catalysts in the phosphating process, and the smallest one has the highest interfacial charge injection efficiency and transfer rate. Meanwhile, the carbon defects of the carbon skeleton produced in the phosphating process, in which the band position is lower than the conduction band of the Zn0.5Cd0.5S (ZCS) catalyst, can effectively collect electrons and inhibit the recombination of photocarriers to a greater extent, and Ni defects adjust the internal electronic structure of Ni2P QDs. The results of experiments and density functional theory calculations demonstrate that Ni2P@ZCS has excellent electronic structure and electron transfer efficiency. Thus, the best PHP performance of Schottky junctions Ni2P@ZCS is achieved with the combined efforts of QDs co‐catalysts and defects.

Interplay of Steric Effects and Aromaticity Reversals to Expand the Structural/Electronic Responses of Dihydrophenazines.

To gain insights into the coupling of conformational and electronic variables, we exploited steric hindrance to modulate a polycyclic skeleton with a bent conformation in the S0 state and a twisted conformation in the S1 state under the guidance of photoexcited aromaticity reversals. Polycyclic 5,10-dihydrophenazine (DHP) adopted a bent structure in S0 but involved a bent-to-planar transformation in S1 due to the excited-state aromaticity of the 8π-electron central ring. The N,N'-locations and 1,4,6,9-sites of the DHP skeleton provided a versatile chemical handle for fine-tuning intramolecular steric hindrance. Specifically, N,N'-diphenyl-5,10-dihydrophenazine (DPP-00) and its derivatives DPP-10-DPP-22 were synthesized with different numbers of methyl groups on the 1,4,6,9-sites. X-ray crystal analyses suggested that the DHP skeletons of DPP-00-DPP-22 had more bending configurations along the N···N axis with an increase in the number of methyl groups. Following the bending-promoted interruption of π-conjugation, the absorption spectra of DPP-00-DPP-22 significantly blue-shifted from 416 to 324 nm. By contrast, the emission bands exhibited a reverse shift to longer wavelengths from 459 to 584 nm as the number of methyl substituents increased. Theoretical calculations revealed that introducing methyl groups caused the planar DHP skeleton in S1 to further twist along the N···N axis, resulting in a twisted high-strain conformation. The greater Stokes shift of the more steric-hindered structure can be attributed to the release of larger strain and aromatic stabilization energy. This research highlighted the potential promise associated with the interplay of steric effects and aromaticity reversals in a single fluorophore.

Efficient circularly polarized thermally activated delayed fluorescence hetero-[4]helicene with carbonyl-/sulfone-bridged triarylamine structures

This work presents the first circularly polarized thermally activated delayed fluorescence hetero-[4]helicene with a carbonyl-/sulfone-bridged triarylamine structure (QPO-PhCz), demonstrating circularly polarized light emission through effective intramolecular steric hindrance and intramolecular space charge transfer.

Side chain engineering of semiconducting polymers for improved NIR-II fluorescence imaging and photothermal therapy

Development of efficient agents for NIR-II fluorescence imaging (NIR-II FI) offers opportunities to facilitate NIR-II FI in biomedicine and life science. Although semiconducting polymers (SPs) are proved to be excellent candidates for NIR-II FI, their fluorescence quantum yields are generally low mainly because of the accelerated nonradiative decay by the vibrational overlap between the ground state and excited state due to the small energy gap of NIR-II SPs. We herein propose a side chain engineering of SPs for enhanced NIR-II FI and imaging-guided photothermal therapy (PTT). The fluorescence optimization is realized by tuning the conformation and bulk of side chains of SPs, which demonstrates that the bulky branched side chains remarkably contribute to the fluorescence enhancement relative to linear or short branched side chains. Density functional theory (DFT) calculation indicates that bulky branched side chain-attached SP (P3) has the largest dihedral angle between electron donor and acceptor units, which suggests the largest intramolecular steric hindrance that restricts the free rotation in the molecule, thus boosting the radiative decay to generate fluorescence. Water-dispersible nanoparticles with high tumor specificity fabricated from the optimal P3 are employed for in vivo applications, which show excellent NIR-II FI performance and PTT efficacy toward tumor. Our study therefore proposes a feasible molecular guideline to amplify the NIR-II brightness of SPs for improved phototheranostics.

Azobispyrazole Family as Photoswitches Combining (Near-) Quantitative Bidirectional Isomerization and Widely Tunable Thermal Half-Lives from Hours to Years*.

Azobenzenes are classical molecular photoswitches that have been widely used. In recent endeavors of molecular design, replacing one or both phenyl rings with heteroaromatic rings has emerged as a strategy to expand molecular diversity and access improved photoswitching properties. Many mono-heteroaryl azo molecules with unique structures and/or properties have been developed, but the potential of bis-heteroaryl architectures is far from fully exploited. We report a family of azobispyrazoles, which combine (near-)quantitative bidirectional photoconversion and widely tunable Z-isomer thermal half-lives from hours to years. The two five-membered rings remarkably weaken the intramolecular steric hindrance, providing new possibilities for engineering the geometric and electronic structure of azo photoswitches. Azobispyrazoles generally exhibit twisted Z-isomers that facilitate complete Z→E photoisomerization, and their thermal stability can be broadly adjusted regardless of the twisted shape, overcoming the conflict between photoconversion (favored by the twisted shape) and Z-isomer stability (favored by the orthogonal shape) encountered by mono-heteroaryl azo switches.

Azobispyrazole Family as Photoswitches Combining (Near‐) Quantitative Bidirectional Isomerization and Widely Tunable Thermal Half‐Lives from Hours to Years**

AbstractAzobenzenes are classical molecular photoswitches that have been widely used. In recent endeavors of molecular design, replacing one or both phenyl rings with heteroaromatic rings has emerged as a strategy to expand molecular diversity and access improved photoswitching properties. Many mono‐heteroaryl azo molecules with unique structures and/or properties have been developed, but the potential of bis‐heteroaryl architectures is far from fully exploited. We report a family of azobispyrazoles, which combine (near‐)quantitative bidirectional photoconversion and widely tunable Z‐isomer thermal half‐lives from hours to years. The two five‐membered rings remarkably weaken the intramolecular steric hindrance, providing new possibilities for engineering the geometric and electronic structure of azo photoswitches. Azobispyrazoles generally exhibit twisted Z‐isomers that facilitate complete Z→E photoisomerization, and their thermal stability can be broadly adjusted regardless of the twisted shape, overcoming the conflict between photoconversion (favored by the twisted shape) and Z‐isomer stability (favored by the orthogonal shape) encountered by mono‐heteroaryl azo switches.

Computational Investigation on ESIPT-driven Luminescence of Imidazo[1,2-a]pyridine Derivatives Regulated by Inter/Intramolecular Hydrogen bonding

The three newly synthesized imidazopyridine derivatives bearing inter/intramolecular hydrogen bond were computationally investigated. The quasi stable seven-membered ring systems of the compounds assisted by the intramolecular hydrogen bond exhibited distinct luminescence depending on the surrounding media; in solution, in frozen solution and in solid state. The interesting luminescent properties were studied by means of a series of quantum chemical calculations; i.e., DFT, TDDFT, CASSCF/CASPT2, ADC(2), CC2 and CCSD(T). The stability of the quasi π-conjugated rings was found to be regulated by the delicate balance between intramolecular steric hindrance and intra/intermolecular hydrogen bond strength. The excited state dynamics was explored by the surface hopping trajectory calculations which reproduced the ESIPT process in the first excited singlet state.

Strategic molecular design of closo-ortho-carboranyl luminophores to manifest thermally activated delayed fluorescence.

In this paper, we propose a strategic molecular design of closo-o-carborane-based donor–acceptor dyad system that exhibits thermally activated delayed fluorescence (TADF) in the solution state at ambient temperature. Planar 9,9-dimethyl-9H-fluorene-based compounds with closo- and nido-o-carborane cages appended at the C2-, C3-, and C4-positions of each fluorene moiety (closo-type: 2FC, 3FC, 4FC, and 4FCH, and nido-type: nido-4FC = [nido-form of 4FC]·[NBu4]) were prepared and characterized. The solid-state molecular structure of 4FC exhibited a significantly distorted fluorene plane, which suggests the existence of severe intramolecular steric hindrance. In photoluminescence measurements, 4FC exhibits a noticeable intramolecular charge transition (ICT)-based emission in all states (solution at 298 K and 77 K, and solid states); however, emissions by other closo-compounds were observed in only the rigid state (solution at 77 K and film). Furthermore, nido-4FC did not exhibit emissive traces in any state. These observations verify that all radiative decay processes correspond to ICT transitions triggered by closo-o-carborane, which acts as an electron acceptor. Relative energy barriers calculated by TD-DFT as dihedral angles around o-carborane cages change in closo-compounds, which indicates that the structural formation of 4FC is nearly fixed around its S0-optimized structure. This differs from that for other closo-compounds, wherein the free rotation of their o-carborane cages occurs easily at ambient temperature. Such rigidity in the structural geometry of 4FC results in ICT-based emission in solution at 298 K and enhancement of quantum efficiency and radiative decay constants compared to those for other closo-compounds. Furthermore, 4FC displays short-lived (∼0.5 ns) and long-lived (∼30 ns) PL decay components in solution at 298 K and in the film state, respectively, which can be attributed to prompt fluorescence and TADF, respectively. The calculated energy difference (ΔEST) between the first excited singlet and triplet states of the closo-compounds demonstrate that the TADF characteristic of 4FC originates from a significantly small ΔEST maintained by the rigid structural fixation around its S0-optimized structure. Furthermore, the strategic molecular design of the o-carborane-appended π-conjugated (D–A) system, which forms a rigid geometry due to severe intramolecular steric hindrance, can enhance the radiative efficiency for ICT-based emission and trigger the TADF nature.

Position isomers of Ru(II) polypyridine complexes with tunable photophysical properties, aggregation-induced phosphorescence enhancement and application in triplet-triplet annihilated upconversion

Three Ru(II) polypyridine complex isomers (Ru-2-DP, Ru-3-DP and Ru-5-DP) have been synthesized using phenanthroline-based isomers containing 4-(2,2-diphenyl-vinyl)-phenyl unit. Their photophysical properties are systematically investigated. Notably, three coordination isomers exhibit much longer triplet lifetimes compared to the model complex [Ru(bpy)2(Phen)]2+. The density function theory calculations reveal that the extension of triplet lifetimes is attributed to the establishment of an excited-state equilibrium between the intraligand (3IL) state and the metal-to-ligand charge-transfer (3MLCT) state. In aggregates, Ru-2-DP shows an interesting aggregation-induced phosphorescence enhancement (AIPE) property. The phosphorescence intensity of Ru-2-DP increases by 16.2-fold from acetonitrile solution to aggregated state. However, Ru-3-DP and Ru-5-DP are AIPE-inactive. Single-crystal analyses show substituent position has a dramatic effect on intramolecular steric hindrance, leading to different molecular conformation and packing pattern. Using three coordination isomers as triplet sensitizers for triplet-triplet annihilated (TTA) upconversion, Ru-3-DP and Ru-5-DP display obvious upconversion properties, but it's quite the opposite for Ru-2-DP. Experimental data demonstrate 3- and 5-positions of phenanthroline, especially 3-position, are beneficial to enhance intersystem crossing and triplet-triplet energy transfer and for the resulting upconverted efficiency enhancement. This work definitely suggests that minor structural change may have major effects upon the solid-state spectroscopic properties and TTA upconversion performances, which provides a rational basis for designing excellent solid phosphorescent materials and triplet sensitizers.

Effect of side chain on conformational properties of comb-like polycarboxylate in salt solutions

To investigate the effect of side-chain structure of comb-like polycarboxylates (PCE) on conformational properties in salt solutions, PCE with polyethyleneoxide (PEO) of different lengths were synthesized using methallyl polyethylene glycol (MPEG, Mw = 1200, 2400, 4000). Adding counter-ions (i.e., Na+, Ca2+) to dilute PCE solutions was found to induce a more complicated conformational changes, since the screening of the electrostatic intramolecular repulsion and the different complexation behaviors of Ca2+ with carboxylic groups. Laser light scattering (LLS) and conductivity measurements were used to investigate the conformations of PCE at various pH values. PCE of a long side chain polymer and the sparse grafting studied herein possesses a more coiled polymer backbone due to the intramolecular steric hindrance, which resulted in a more exposed extent of carboxylic groups on the backbone at the same pH values. Obviously, the solution conformation of PCE strongly impacts the accessible carboxylic groups contribution to complexation of the carboxylic oxygen atoms of PCE with Ca2+. In this way it may provide a new insight to design the polymer dispersants.

S1/S0 Potential Energy Surfaces Experience Different Types of Restricted Rotation: Restricted Z/ E Photoisomerization and E/ Z Thermoisomerization by an Out-of-Plane Benzyl Group or In-Plane m-Pyridinium Group?

Any method that can enhance the fluorescence of fluorophores is highly desirable. Fluorescence enhancement accomplished by restricted Z/ E photoisomerization through intramolecular steric hindrance or relatively high bond order of a C═C double bond in a S1 excited state has rarely been studied. In this article, we used green fluorescent protein (GFP) chromophore analogues as a model to get new physical insights into the restricted Z/ E photoisomerization and E/ Z thermoisomerization phenomena. We found that the S1 and S0 potential energy surfaces (PESs) of the GFP chromophore analogues experience two dramatically different types of restricted rotation, and 2b can be a representative example. In its S1 PES, it is not the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group but the relatively high bond order of the I-bond in the S1 excited state of 2b that makes it have a higher barrier for the Z/ E photoisomerization, a smaller Z/ E photoisomerization quantum yield, and a higher fluorescence quantum yield. In its S0 PES, it is not the reduced bond order of the I-bond in the S0 ground state of 2b but the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group that makes it have an extra higher barrier for E/ Z thermoisomerization and a much smaller E/ Z thermoisomerization rate constant.

Simply planarizing nonfused perylene diimide based acceptors toward promising non-fullerene solar cells

The pyrene core and significantly reduced intramolecular steric hindrance endow a nonfused PDI-tetramer with moderate planarity and good self-assembly properties.

Designing better diffracting crystals of biotin carboxyl carrier protein from Pyrococcus horikoshii by a mutation based on the crystal-packing propensity of amino acids.

An alternative rational approach to improve protein crystals by using single-site mutation of surface residues is proposed based on the results of a statistical analysis using a compiled data set of 918 independent crystal structures, thereby reflecting not only the entropic effect but also other effects upon protein crystallization. This analysis reveals a clear difference in the crystal-packing propensity of amino acids depending on the secondary-structural class. To verify this result, a systematic crystallization experiment was performed with the biotin carboxyl carrier protein from Pyrococcus horikoshii OT3 (PhBCCP). Six single-site mutations were examined: Ala138 on the surface of a β-sheet was mutated to Ile, Tyr, Arg, Gln, Val and Lys. In agreement with prediction, it was observed that the two mutants (A138I and A138Y) harbouring the residues with the highest crystal-packing propensities for β-sheet at position 138 provided better crystallization scores relative to those of other constructs, including the wild type, and that the crystal-packing propensity for β-sheet provided the best correlation with the ratio of obtaining crystals. Two new crystal forms of these mutants were obtained that diffracted to high resolution, generating novel packing interfaces with the mutated residues (Ile/Tyr). The mutations introduced did not affect the overall structures, indicating that a β-sheet can accommodate a successful mutation if it is carefully selected so as to avoid intramolecular steric hindrance. A significant negative correlation between the ratio of obtaining amorphous precipitate and the crystal-packing propensity was also found.

Crystal structure and tautomerism of Pigment Yellow 138 determined by X-ray powder diffraction and solid-state NMR

The crystal structure of C.I. Pigment Yellow 138 was determined from X-ray powder diffraction data using real-space methods with subsequent Rietveld refinements. The tautomeric state was investigated by solid-state 1D and 2D multinuclear NMR experiments.In the crystals, the compound exhibits the NH-tautomer with a hydrogen atom situated at the nitrogen of the quinoline moiety. Direct evidence of the presence of the NH-tautomer is provided by 1H–14N HMQC solid-state NMR at very fast MAS. Solid-state dispersion-corrected density functional theory calculations with BLYP-D3 confirm the correctness of the crystal structure and support the NH tautomer. The NH hydrogen atom forms an intramolecular resonance-assisted NH⋯O hydrogen bond to the neighbouring indandione moiety. The indandione moiety is almost coplanar with the quinoline fragment, whereas the phthalimide group forms an angle of 57° with it. Quantum-mechanical calculations show that for an isolated molecule this tautomeric form is by approximately 60 kJ/mol more stable than the OH-form with a hydrogen atom placed at an oxygen atom of the indandione moiety. The out-of-plane rotation of the phthalimide group is not a packing effect, but caused by intramolecular steric hindrance.

Theoretical insight into the enhanced hindrance, thermal stability and optical properties of diarylethene with a benzobis(thiadiazole) bridge and benzothiophene rings

To rationalize the effects of intramolecular steric hindrance on the performance of diarylethene photoswitches, we here present a density functional theory study on the thermal bistability and optical properties of a photochromic diarylethene BBTE consisting of a benzobis(thiadiazole) bridge and benzothiophene rings, in which prominent steric hindrance exists owing to the extended structures of π-conjugated groups. Our calculations not only provide rational explanations for the isolation of enantiopure anti-parallel conformers of the open-ring isomer, but also elucidate the detailed pathway of a two-step ground-state ring-opening process, where the thermal stability of the closed-ring isomer is guaranteed by an overall free energy barrier of around 113 kJ mol−1 (27 kcal mol−1). In addition, the tunable intramolecular charge transfer of the donor–acceptor structure formed by the benzobis(thiadiazole) bridge and the bulky benzothiophene rings is also addressed within time-dependent density functional theory. Charge transfer excitations in the open- and the closed-ring isomers are characterized as long-range and medium-range, respectively. We show that the diarylethene derivative under investigation can serve as a promising platform for future development of optoelectronic materials.