Push-pull Molecules Research Articles

Benchmarking luminescent properties of the arylvinylpyrimidine scaffold.

The exploration of the photophysical properties of push-pull molecules incorporating pyrimidine rings as electron-attracting moieties in their structure continues to be a fascinating area of investigation. A thorough examination of these properties not only contributes to fundamental knowledge but also provides crucial insights for the rational design of emissive materials in prospective applications. In this context, this work conducts an in-depth analysis of four families of 4,6-bis(arylvinyl)pyrimidines, evaluating the influence of substituents on both the aryl groups and position 2 of the pyrimidine ring. While previous research has primarily focused on solution studies, this work emphasizes the importance of examining solid-state photophysics. Through a multidisciplinary approach encompassing optical techniques, x-ray diffraction, and quantum chemical calculations, a comprehensive understanding of the structure-property relationships is achieved. This study underscores the intricate interplay between molecular structure, aggregation, and fluorescence behavior in pyrimidines, offering valuable insights with broader implications beyond academic realms.

Tuning the Thermal Stability of Tetra-o-chloroazobenzene Derivatives by Transforming Push-Pull to Push-Push Systems.

Molecular photoswitches provide interesting tools to reversibly control various biological functions with light. Thanks to its small size and easy introduction into the biomolecules, azobenzene derivatives have been widely employed in the field of photopharmacology. All visible-light switchable azobenzenes with controllable thermostability are highly demanded. Based on the reported tetra-o-chloroazobenzenes, we synthesized push-pull systems, by introducing dialkyl amine and nitro groups as strong electron-donating and electron-withdrawing groups on the para-positions, and then transformed to push-push systems by a simple reduction step. The developed push-pull and push-push tetra-o-chloroazobenzene derivatives displayed excellent photoswitching properties, as previously reported. The half-life of the Z-isomers can be tuned from milliseconds for the push-pull system to several hours for the push-push system. The n-π* and π-π* transitions have better resolution in the push-push molecules, and excitation at different wavelengths can tune the E/Z ratio at the photostationary state. For one push-pull molecule, structure and absorption spectra obtained from theoretical calculations are compared with experimental data, along with data on the push-push counterpart.

Quantum rate electrodynamics and resonant junction electronics of heterocyclic molecules

The quantum rate theory provides a framework to understand electron-transfer reactions by correlating the electron-transfer rate constant (ν) with the quantum capacitance (Cq) and the molecular conductance (G). This theory, which is rooted in relativistic quantum electrodynamics, predicts a fundamental frequency ν=E/h for electron-transfer reactions, where E is the energy associated with the density of states Cq/e2. This work demonstrates the applicability of the quantum rate theory to the intermolecular charge transfer of push-pull heterocyclic compounds assembled over conducting electrodes. Remarkably, the observed differences between molecular junction electronics formed by push-pull molecules and the electrodynamics of electrochemical reactions on redox-active modified electrodes can be attributed solely to the adiabatic setting of the quantum conductance in push-pull molecular junctions. The electrolyte field-effect screening environment plays a crucial role in modulating the resonant quantum conductance dynamics of the molecule-bridge-electrode structure. In this context, the intermolecular electrodynamics within the frontier molecular orbital of push-pull heterocyclic molecules adhere to relativistic quantum mechanics, consistent with the predictions of the quantum rate theory.

Calculation of the geometry, absorption spectrum, and first hyperpolarizability of 4,5-dicyanoimidazole derivatives in solution. A multiscale ASEC-FEG study.

We investigate the effects of solvents on the geometry, absorption spectrum, and first hyperpolarizability of six push-pull molecules, each containing a 4,5-dicyanoimidazole group as an electron acceptor and a N,N-dimethylamino group as an electron donor, with systematically extended π-conjugated systems. Geometry optimizations in dichloromethane, methanol, water, and formamide under normal thermodynamic conditions were performed using the average solvent electrostatic configuration-free energy gradient method, which employs a discrete solvent model. The conformational structure of molecules is moderately affected by the environment, with the π-conjugated system becoming more planar in protic solvents. Solvent effects on the first hyperpolarizability result in marked increases that are in line with the red shifts of the absorption spectrum. The hyperpolarizability of smaller molecules within the set may be significantly influenced by the effects of geometry relaxation in highly polar protic solvents. The results illustrate the role of hydrogen bonds in the structure and electronic properties of push-pull molecules in protic environments. For smaller molecules, hydrogen bonds significantly contribute to enhancing the hyperpolarizability, but the effect of these specific interactions becomes less significant with the length of the π-conjugated system.

The eXact integral simplified time-dependent density functional theory (XsTD-DFT).

In the framework of simplified quantum chemistry methods, we introduce the eXact integral simplified time-dependent density functional theory (XsTD-DFT). This method is based on the simplified time-dependent density functional theory (sTD-DFT), where all semi-empirical two-electron integrals are replaced by exact one- and two-center two-electron integrals, while other approximations from sTD-DFT are kept. The performance of this new parameter-free XsTD-DFT method was benchmarked on excited state and (non)linear response properties, including ultra-violet/visible absorption, first hyperpolarizability, and two-photon absorption (2PA). For a set of 77 molecules, the results from the XsTDA approach were compared to the TDA data. XsTDA/B3LYP excitation energies only deviate on average by 0.14eV from TDA while drastically cutting computational costs by a factor of 20 or more depending on the energy threshold chosen. The absolute deviations of excitation energies with respect to the full scheme are decreasing with increasing system size, showing the suitability of XsTDA/XsTD-DFT to treat large systems. Comparing XsTDA and its predecessor sTDA, the new scheme generally improves excitation energies and oscillator strengths, in particular, for charge transfer states. TD-DFT first hyperpolarizability frequency dispersions for a set of push-pull π-conjugated molecules are faithfully reproduced by XsTD-DFT, while the previous sTD-DFT method provides redshifted resonance energy positions. Excellent performance with respect to the experiment is observed for the 2PA spectrum of the enhanced green fluorescent protein. The obtained robust accuracy similar to TD-DFT at a fraction of the computational cost opens the way for a plethora of applications for large systems and in high throughput screening studies.

Resolution of 9,10-Diketo[7]helicene and Its Use in One-Step Preparation of Helicene-Based D-A-D Push-Pull Systems.

Racemic 9,10-diketo[7]helicene was successfully separated into enantiomers using a reversible and stereoselective reaction with 2,2'-diamino-1,1'-binaphthalene with moderate yields but with remarkable purity (>99% de). The enantiomerically pure diketone was used as a convenient starting material for the preparation of helicene-based push-pull molecules, which incorporated aza-aryl acceptors and diarylaminophenylene donor groups in a single step. A series of six push-pull systems, along with three reference molecules without donors, were prepared and studied using UV/vis and fluorescence measurements, circular dichroism, and DFT calculations.

Strong Bases and beyond: The Prominent Contribution of Neutral Push-Pull Organic Molecules towards Superbases in the Gas Phase.

In this review, the principles of gas-phase proton basicity measurements and theoretical calculations are recalled as a reminder of how the basicity PA/GB scale, based on Brønsted-Lowry theory, was constructed in the gas-phase (PA-proton affinity and/or GB-gas-phase basicity in the enthalpy and Gibbs energy scale, respectively). The origins of exceptionally strong gas-phase basicity of some organic nitrogen bases containing N-sp3 (amines), N-sp2 (imines, amidines, guanidines, polyguanides, phosphazenes), and N-sp (nitriles) are rationalized. In particular, the role of push-pull nitrogen bases in the development of the gas-phase basicity in the superbasicity region is emphasized. Some reasons for the difficulties in measurements for poly-functional nitrogen bases are highlighted. Various structural phenomena being in relation with gas-phase acid-base equilibria that should be considered in quantum-chemical calculations of PA/GB parameters are discussed. The preparation methods for strong organic push-pull bases containing a N-sp2 site of protonation are briefly reviewed. Finally, recent trends in research on neutral organic superbases, leaning toward catalytic and other remarkable applications, are underlined.

Effect of the introduction of ethylenedioxythiophene and styrilnitroisooxazol groups on the properties and photovoltaic performances of tryphenylamine based [formula omitted] −conjugated push-pull molecules

Properties of π-conjugated push-pull molecules based on triphenylamine have been modified through the introduction of ethylenedioxythiophene and a styrilnitroisooxazol group. It is shown that the obtained new molecule called GC8 exhibits light absorption in the visible, between 400 nm and 700 nm and a high hole mobility of 210−5 cm2V− 1s− 1, which places this organic donor among those with good mobility in this family of compounds. It behaves as electron donor in organic photovoltaic cells, with a HOMO of 5.35 eV, a LUMO of 2.94 eV which gives a band gap of 2.41 eV, these values being obtained from theoretical estimation using TD-DFT theory. The measured experimental optical band gap is 1.9 eV, this difference in value between theory and experiment is reasonable for small molecules. GC8 has been introduced as electron donor in planar heterojunctions organic photovoltaic cells (PHJ-OPVs), fullerenes (C60 or C70) being used as electron acceptor. It is shown that the optimum GC8 layer thickness is 20 nm, while a 5 min annealing at 100 °C improves the power conversion efficiency (PCE) by 10%. The high light absorption of C70 allows improving cells performances compared to that using C60, the champion cell having a 6.15% efficiency. The PCE improvement after the annealing is due to an increase of the hole mobility and optical absorption.

Toward Accurate Two-Photon Absorption Spectrum Simulations: Exploring the Landscape beyond the Generalized Gradient Approximation.

In this Letter, we present a pioneering analysis of the density functional approximations (DFAs) beyond the generalized gradient approximation (GGA) for predicting two-photon absorption (2PA) strengths of a set of push-pull π-conjugated molecules. In more detail, we have employed a variety of meta-generalized gradient approximation (meta-GGA) functionals, including SCAN, MN15, and M06-2X, to assess their accuracy in describing the 2PA properties of a chosen set of 48 organic molecules. Analytic quadratic response theory is employed for these functionals, and their performance is compared against the previously studied DFAs and reference data obtained at the coupled-cluster CC2 level combined with the resolution-of-identity approximation (RI-CC2). A detailed analysis of the meta-GGA functional performance is provided, demonstrating that they improve upon their predecessors in capturing the key electronic features of the π-conjugated two-photon absorbers. In particular, the Minnesota functional MN15 shows very promising results as it delivers pleasingly accurate chemical rankings for two-photon transition strengths and excited-state dipole moments.

Nanoparticles of Push-Pull Triphenylamine-Based Molecules for Light-Controlled Stimulation of Neuronal Activity.

Organic semiconductor materials with a unique set of properties are very attractive for interfacing biological objects and can be used for noninvasive therapy or detection of biological signals. Here, we describe the synthesis and investigation of a novel series of organic push-pull conjugated molecules with the star-shaped architecture, consisting of triphenylamine as a branching electron donor core linked through the thiophene π-spacer to electron-withdrawing alkyl-dicyanovinyl groups. The molecules could form stable aqueous dispersions of nanoparticles (NPs) without the addition of any surfactants or amphiphilic polymer matrixes with the average size distribution varying from 40 to 120 nm and absorption spectra very similar to those of human eye retina pigments such as rods and green cones. Variation of the terminal alkyl chain length of the molecules forming NPs from 1 to 12 carbon atoms was found to be an efficient tool to modulate their lipophilic and biological properties. Possibilities of using the NPs as light nanoactuators in biological systems or as artificial pigments for therapy of degenerative retinal diseases were studied both on the model planar bilayer lipid membranes and on the rat cortical neurons. In the planar bilayer system, the photodynamic activity of these NPs led to photoinactivation of ion channels formed by pentadecapeptide gramicidin A. Treatment of rat cortical neurons with the NPs caused depolarization of cell membranes upon light irradiation, which could also be due to the photodynamic activity of the NPs. The results of the work gave more insight into the mechanisms of light-controlled stimulation of neuronal activity and for the first time showed that fine-tuning of the lipophilic affinity of NPs based on organic conjugated molecules is of high importance for creating a bioelectronic interface for biomedical applications.

Synthesis of indazole-benzothiadiazole push-pull molecules as solid-state fluorescent compounds

Synthesis of indazole-benzothiadiazole push-pull molecules as solid-state fluorescent compounds

A quantum chemical investigation of the second hyperpolarizability of p-nitroaniline.

Recent measurements of the third harmonic scattering responses of molecules have given a new impetus for computing molecular second hyperpolarizabilities (γ) and for deducing structure-property relationships. This paper has employed a variety of wavefunction and density functional theory methods to evaluate the second hyperpolarizability of the p-nitroaniline prototypical push-pull π-conjugated molecule, addressing also numerical aspects, such as the selection of an integration grid and the impact of the order of differentiation vs the achievable accuracy by using the Romberg quadrature. The reliability of the different methods has been assessed by comparison to reference Coupled-Cluster Singles and Doubles with perturbative treatment of the Triples results. On the one hand, among wavefunction methods, the MP2 scheme offers the best accuracy/cost ratio for computing the static γ. On the other hand, using density functional theory, γ remains a challenging property to compute because all conventional, global hybrid or range-separated hybrid, exchange-correlation functionals underestimate static γ values by at least 15%. Even tuning the range-separating parameter to minimize the delocalization errors does not enable to improve the γ values. Nevertheless, the original double-hybrid B2-PLYP functional, which benefits from 27% of PT2 correlation and 53% Hartree-Fock exchange, provides accurate estimates of static γ values. Unfortunately, the best performing exchange-correlation functionals for γ are not necessarily reliable for the first hyperpolarizability, β, and vice versa. In fact, the β of p-nitroaniline (pNA) could be predicted, with a good accuracy, with several hybrid exchange-correlation functionals (including by tuning the range-separating parameter), but these systematically underestimate γ. As for γ, the MP2 wavefunction method remains the best compromise to evaluate the first hyperpolarizability of pNA at low computational cost.

The design of push-pull substituted coronene molecules for optoelectronic applications

The optoelectronic and electronic properties of coronene ([6]circulene) and push-pull coronene molecules were predicted by using different computational chemistry programs. For this aim, the electron/hole reorganization energies, the adiabatic/vertical ionization potentials, the adiabatic/vertical electron affinities, the chemical hardness values, the frontier orbital shapes and energy levels of the coronene and its derivatives were obtained at B3LYP/6-31G(d) level. Then the energies of the studied molecules in S0, S1, and T1 states were calculated at PBE0/TZP level. From the obtained results, the emission values and TADF parameters of the investigated compounds were determined. Based on the theoretically obtained results, it was found that 6ethynyl-coronene and 12ethynyl-coronene molecules can be used as electron transfer materials and 6cyano-coronene and 12cyano-coronene compounds can be utilized as hole transfer materials. Additionally, it was noted that 6NO2-coronene and 12NH2-coronene derivatives can be good candidates for electron blocking materials, while 6NH2-coronene and 12NH2-coronene molecules can be utilized as hole blocking materials. Furthermore, it was emphasized that both 6NH2-coronene and 12NH2-coronene molecules can be good candidates as both electron injection and hole injection materials. Lastly, it was reported that 6ethynyl-coronene and 12cyano-coronene structures can be considered the most suitable candidates for near infrared organic emitting diodes.

Novel Y-shaped multi-stimuli responsive pyrene substituted quinoxaline and pyridopyrazine-based push-pull molecules showing solvatochromism, aggregation induced excimer emission, acidofluorochromism and moisture detection

The design, synthesis, and characterization of a series of novel Y-shaped push-pull compounds based on the quinoxaline and pyrido[2,3-b]pyrazine moieties as electron acceptors, 1,4-phenylene rings as spacers, and pyrene units as electron donors was carried out. Their structural, photophysical, solvatochromic, aggregation, acidofluorochromic, moisture sensing and thermal characteristics were investigated. It was observed that the substitution of an electron withdrawing group (EWG) on the quinoxaline moiety led to red shifts in the absorption and emission spectra due to enhanced intramolecular charge transfer (ICT). Crystal structure of 3a showed twisted structure with herringbone packing arrangement and intermolecular π-π stacking of pyrene units. Compounds 3a-c displayed aggregation induced excimer emission in aggregated states in THF-water mixtures due to intermolecular π-π interactions among pyrene units. The concentration dependent studies of 3a-d showed monomer/excimer conversion at higher concentrations in THF solvent. The excited lifetime analysis revealed a biexponential decay at lower concentrations with a major contribution from monomeric species while at higher concentrations, a triexponential decay was observed with a major contribution from the excimeric species. The lifetimes for the monomeric species (2.9–3.3 ns) were found to be higher than the excimeric species (1.3–2.6 ns). These compounds displayed reversible acidofluorochromism, in solution and in solid state, with TFA followed by subsequent treatment with TEA. Compounds were also able to detect up to 0.005 v/v % moisture in THF solvent through TICT mechanism. Filter paper strips coated with these compounds were able to detect TFA vapors and moisture, which shows the practical application of these compounds.

Phenalenone derivatives: The voyage from photosensitizers to push-pull fluorescent molecules

Phenalenone and several phenalenone derivatives have the remarkable capacity to produce singlet oxygen (1O2), via photosensitization, with singlet oxygen quantum yield values close to unity in most solvents. Considerable efforts have been made to obtain phenalenones that red-shifted their absorption but maintaining the high capacity of generation of singlet oxygen.The presence of electron donors on specific positions of the aromatic ring promotes important changes. Herein, with the presence of a dimethylamino substituent in position 3 or 6, we were able to modulate singlet oxygen generation capacity and fluorescence. Both, 3DMAPNF and 6DMAPNF show red-shifted absorption (compared with phenalenone) associated with ππ* transitions. Solvent effect in absorption and emission properties were observed for 6DMAPNF, while that for 3DMAPNF no fluorescence response was observed. Both, show a decrease in singlet oxygen quantum yield with polarity of solvent, always with 3DMAPNF singlet oxygen quantum yield values greater than for 6DMAPN. No singlet oxygen generation was observed for both compounds in H2O.

The search for a maximum of the D-π-A paradigm for second order nonlinear optical molecular materials.

Push-pull π-conjugated molecules are one of the paradigms of second order nonlinear optical (NLO) materials and have been extensively explored. However, high-performance second order NLO materials with an optimum electron donor (D), π-bridge (π) and acceptor (A) under this paradigm are still the most sought-after. In the present work, D-π-A molecules with optimal D, π and A combination for strong second order NLO properties are proposed based on molecular orbital theories. The optimal D-π-A push-pull molecule achieves an unprecedentedly strong NLO response under the D-π-A paradigm, i.e., the static first hyperpolarizability reaches -453.92 × 10-30 esu per heavy atom using azulene as part of the π-bridge and acceptor to synergistically reinforce the strength of the acceptor. The protocols of D-π-A NLO molecule design through frontier molecular orbital matching of D, π and A with optimal combination of electron donating and accepting strengths shed light on future molecular NLO materials exploration. The simulated two-dimensional second order spectra provide useful information (e.g., sum frequency generation) on the applications of those D-π-A push-pull molecules in nonlinear optics.

Theoretical Study on Non-Linear Optics Properties of Polycyclic Aromatic Hydrocarbons and the Effect of Their Intercalation with Carbon Nanotubes.

Results of a theoretical study devoted to comparing NLO (non-linear optics) responses of derivatives of tetracene, isochrysene, and pyrene are reported. The static hyperpolarizability β, the dipole moment μ, the HOMO and LUMO orbitals, and their energy gap were calculated using the CAM-B3LYP density functional combined with the cc-pVDZ basis set. The para-disubstituted NO2-tetracene-N(CH3)2 has the highest NLO response, which is related to a large intramolecular charge transfer. Adding vinyl groups to the para-disubstituted NO2-tetracene-N(CH3)2 results in an increase in the NLO responses. We further investigated the effect of the intercalation of various push-pull molecules inside an armchair single-walled carbon nanotube. The intercalation leads to increased NLO responses, something that depends critically on the position of the guest molecule and/or on functionalization of the nanotube by donor and attractor groups.

The Impact of Backbone Fluorination and Side-Chain Position in Thiophene-Benzothiadiazole-Based Hole-Transport Materials on the Performance and Stability of Perovskite Solar Cells.

Perovskite solar cells (PSCs) currently reach high efficiencies, while their insufficient stability remains an obstacle to their technological commercialization. The introduction of hole-transport materials (HTMs) into the device structure is a key approach for enhancing the efficiency and stability of devices. However, currently, the influence of the HTM structure or properties on the characteristics and operational stability of PSCs remains insufficiently studied. Herein, we present four novel push-pull small molecules, H1-4, with alternating thiophene and benzothiadiazole or fluorine-loaded benzothiadiazole units, which contain branched and linear alkyl chains in the different positions of terminal thiophenes to evaluate the impact of HTM structure on PSC performance. It is demonstrated that minor changes in the structure of HTMs significantly influence their behavior in thin films. In particular, H3 organizes into highly ordered lamellar structures in thin films, which proves to be crucial in boosting the efficiency and stability of PSCs. The presented results shed light on the crucial role of the HTM structure and the morphology of films in the performance of PSCs.

Luminescent push-pull triphenylamine-based molecules end-capped with various electron-withdrawing groups: Synthesis and properties

Organic luminescent materials are widely used in various electronic and optoelectronic devices upon growing demands of science and technology. Enhancement of spectral-luminescence characteristics for such materials and in depth understanding of “structure-property” relationships remain challenging tasks. Herein, we report on synthesis and comprehensive investigation of the series of novel luminescent push-pull molecules with triphenylamine unit as an electron donor block and thiophene as a π-spacer, which end-capped with various types of electron-withdrawing groups (EWGs), which are commonly used for the molecular design of various functional materials in organic electronics. The results allowed us to evaluate the impact of EWG type used on the target materials characteristics. Phenyl-substituted EWGs were found to be more suitable for the design of highly thermally and electrochemically stable materials with relatively high melting temperatures and melting enthalpies. Depending on the EWG nature luminescence maxima of the luminophores demonstrated significant variability, e.g. from 509 nm to 750 nm, while the photoluminescence quantum yield (PLQY) values laid in the range of 1–89%. All luminophores showed good compatibility with a polystyrene (PS) matrix, in which PLQYs were generally higher (up to 25-fold enhancement) compared to the corresponding solutions or polycrystalline films. The changes of spectral characteristics observed for these luminophores were well described using basic relations of the semi-empirical theory of solvatochromism. Based on lifetime of excited states measurements, it was shown that the excited state non-radiative deactivation constants values the major contributors to PLQY values in THF solutions, while increase of the PLQY values in PS films can be associated with decrease of the probability of non-radiative deactivation of the excited states.

Connecting two intrinsic electron donor and acceptor moieties through distinctive π-bridging units to explore their optical and nonlinear optical properties

Boraphenalene and pyrido(2,1,6-de)quinolizine moieties are intrinsically electron acceptor and donor groups, owing to their electron deficient and electron rich characteristics, respectively. In the current study, quantum computations are executed for investigating the charge transfer character and nonlinear optical (NLO) response for various combinations of boraphenalene and pyrido(2,1,6-de)quinolizine derivatives. The effect of various π-conjugated connectors i.e. direct ring attachment (1AB), ethylene (2AB), acetylene (3AB) and phenylene (4AB) units on optical and NLO properties were studied. Density functional theory (DFT) techniques are employed for computing the responses properties for designed compounds (A, B, 1AB, 2AB, 3AB and 4AB). From the studied compounds, 4AB exhibits the leading average second-order NLO polarizability of 954.8 × 10-30 esu, which was about 148 times greater than para-Nitroaniline (p-NA) molecule (a typical push-pull NLO molecule). The source of greater values of hyperpolarizability is tracked through TD-DFT computations where electronic transitions having better oscillator strengths along with lower transition energies are seen more contributing in hyperpolarizability. Additionally, the assessment of electrostatic potentials (ESPs), density of states (DOS), frontier molecular orbitals (FMOs) and transition density matrix (TDM) diagrams provide a molecular-level insight into the structures and their relationship to NLO properties of designed derivatives. Hence, the current investigation would arouse the curiosity of other scientists in the field for such different and novel strategy for designing NLO materials.