Increasing Solvent Polarity Research Articles

Synthesis and spectroscopic characterization of 2-furancarbothioamide: N-(4-fluorophenyl)furan-2-carbothioamide

This investigation deals with an alternative thionation synthetic strategy for the preparation of N-(4-fluorophenyl)furan-2-carbothioamide using elemental sulphur and applying the Willgerodt-Kindler reaction. The carbothioamide was obtained with ca 30% yield assessing the purity (higher than 90%) with GC/MS spectrometry, and the compound was fully characterised by NMR (1H, 13C), FTIR and UV-visible spectroscopies. Indeed, 1H-NMR spectroscopic analysis demonstrated that a single conformer with anti-anti geometry was formed attributed to the observation of only one N-H signal (9.37 ppm) of the NH-C=S group and, FTIR spectroscopy (N-H stretching at 3372 cm−1) also reinforces this observation. UV-visible absorption and fluorescence emission spectra were recorded in different polar and nonpolar solvents (acetonitrile, ethanol and n-heptane) and the λ0,0 cross point displays a bathochromic shift of 11 nm with the increasing solvent polarity. The fluorescence quantum yield (φf) has been measured at room temperature concluding that this carbothioamide derivative is not a fluorescent compound showing a φf value of 0.0002. Finally, theoretical calculations have been carried out to estimate the NMR chemical shifts as well as vibrational frequencies values of the anti-anti isomer and these values were compared with the experimental data obtaining satisfactorily correlations.

TICT, and Deep-Blue Electroluminescence from Acceptor-Donor-Acceptor Molecules.

Donor-acceptor (D-A) materials based on butterfly-shaped molecules could inhibit exciton-migration-induced quenching due to molecular twist. To explore this attribute towards beneficial photophysical properties, three novel bipolar acceptor-donor-acceptor (A-D-A) molecules with triphenyl triazine end capping along with substitution ortho to the Tröger's base (TB) scaffold varying from H, Me, and F were explored. The installation of H/Me/F imparted an electron push-pull effect with concomitant maneuvering of photophysical properties. On increasing solvent polarity, a remarkable bathochromic shift with a significant decrease in emission efficiency was observed due to the twisted intramolecular charge transfer state (TICT). Emission enhancement in the ethylene glycol-water mixture and diminution in the THF-water mixture further confirmed the existence of TICT states in these TBs. The torsional dynamics in the excited state were also evidenced by the time-dependent density-functional theory (TD-DFT) calculations. Owing to the butterfly architecture of the TB that suppressed TICT, TB-Trzs exhibited a significant blue shift, accompanied by a favorable quantum yield in the solid state. Among the three compounds, Me-TB-Trz exhibited deep-blue photoluminescence and was explored as a dopant in organic light-emitting diodes (OLEDs) to obtain deep-blue electroluminescence of brightness 4128 cdm-2 and CIE coordinates of (0.16, 0.09).

Solvatochromism, preferential solvation and excited state dipole moments of flufenamic acid in different solvent polarities

The influence of pure solvent and binary solvent mixtures on the optical properties of non-steroidal anti-inflammatory drug (NSAID) molecule, specifically flufenamic acid (FLA), has been studied using absorption and fluorescence spectroscopic techniques. A bathochromic shift is observed in the emission wavelength of FLA with an increase in the solvent polarity of pure solvents, attributed to the highest stability of the excited state. The spectral properties are correlated with Lippert–Mataga, solvent polarity parameter, Kamlet, and Catalan solvent polarity parameters, suggesting that both general and specific solvent effects influence the spectral properties, with general solvent effects dominating over specific solvent effects. Preferential solvation studies have been carried out in tetrahydrofuran (THF) and water solvent mixture, revealing variations in the preference for solvation of FLA as a function of the mole fraction of water. Solvatochromic data are utilized to estimate the excited state dipole moment in pure solvents using different solvatochromic methods, revealing that the excited state dipole moment of FLA is higher than its ground state counterpart. The increase in dipole moment in the excited state compared to the ground state is explained based on intramolecular charge transfer (ICT), with elaboration and discussion on the possible resonance structure corresponding to ICT, inferring the more polar nature of the excited state compared to the ground state. The effect of solute polarizability on the excited state dipole moment and change in dipole moment is also discussed.

Theoretical study on precise regulation of ESIPT reaction based on ICT effect of o-carborane derivatives

The unique stereocarborane fluorophore design has attracted widespread attention because of its ability to effectively enhance the performance of thin-film fluorescence sensors. This study provides a detailed theoretical investigation of the excited-state intramolecular proton transfer (ESIPT) reactions of three o-carborane derivatives, NaCBO, PaCBO and PyCBO, using density functional theory and time-dependent density functional theory. We found that as the number of benzene rings in the electron donor groups increases, the interaction between the first excited-state electron donor–acceptor fragments is enhanced, weakening the intramolecular hydrogen bonding strength, which in turn inhibits the ESIPT. Notice that the intramolecular charge transfer (ICT) is proportional to the interaction between electron donor and acceptor fragments with the analysis of electron structure. The higher the ICT level of the target system, the more unstable the keto form generated by ESIPT. Therefore, the ESIPT can be precisely manipulated by rationally adjusting the ICT effect. Furthermore, with increasing solvent polarity, the photophysical properties of the chromophores exhibit non-solvatochromism. This is attributed to the rigid o-carborane structure, which serves as a spatial scaffold to increase the adaptability of the molecule to the environment. This study is expected to provide theoretical guidance and new ideas for preparing high-performance thin-film fluorescence sensing.

High quantum yield blue fluorescent molecule of hydroxybenzo[c]chromen-6-ones: Synthesis and fluorescence characteristics

Hydroxybenzo[c]chromen-6-ones had been synthesized by the reaction of hydroxy‑substituted chalcone and ethyl acetoacetate in the presence of Cs2CO3 and their optical properties were then investigated. We found it particularly that dimethylamino-substituted hydroxybenzocoumarin 4e had extremely high quantum yield blue fluorescence (λem = 487 nm, ФF = 0.83) in dichloromethane and the fluorescence red shifted with the increase of solvent polarity, which had a good solvatochromic effect on the polarity of the solvent. Additionally, DFT and TD-DFT calculations were applied to explain and verify the good fluorescence properties of hydroxybenzo[c]chromen-6-one 4e.

Ab initio electronic absorption spectra of para-nitroaniline in different solvents: Intramolecular charge transfer effects.

Intramolecular charge transfer (ICT) effects of para-nitroaniline (pNA) in eight solvents (cyclohexane, toluene, acetic acid, dichloroethane, acetone, acetonitrile, dimethylsulfoxide, and water) are investigated extensively. The second-order algebraic diagrammatic construction, ADC(2), ab initio wave function is employed with the COSMO implicit and discrete multiscale solvation methods. We found a decreasing amine group torsion angle with increased solvent polarity and a linear correlation between the polarity and ADC(2) transition energies. The first absorption band involves π → π* transitions with ICT from the amine and the benzene ring to the nitro group, increased by 4%-11% for different solvation models of water compared to the vacuum. A second band of pNA is characterized for the first time. This band is primarily a local excitation on the nitro group, including some ICT from the amine group to the benzene ring that decreases with the solvent polarity. For cyclohexane, the COSMO implicit solvent model shows the best agreement with the experiment, while the explicit model has the best agreement for water.

Symmetry Breaking Charge Separation in Far-Red Absorbing Bisstyryl-Bodipy Dimers

Symmetry breaking charge transfer is one of the important photo-events occurring in photosynthetic reaction centers that is responsible for initiating electron transfer leading to a long-lived charge-separated state and has been successfully employed in light-to-electricity converting optoelectronic devices. Here, we report the synthesis of far-red absorbing and emitting BODIPY-dimer to undergo symmetry-breaking charge transfer leading to charge-separated states of appreciable lifetimes in polar solvents. Compared to its monomer analog, both steady-state and time-resolved fluorescence originating from the S1 state of the dimer revealed quenching which increased with an increase in solvent polarity. The electrostatic potential map from DFT and the time-dependent DFT calculations suggested the existence of a quadrupolar type charge transfer state in polar solvents, and the singlet excited state to be involved in the charge separation process. The electrochemically determined redox gap to be smaller than the energy of the S1 state supporting thermodynamic feasibility of the envisioned symmetry-breaking charge transfer and separation. The spectrum of the charge-separated state arrived from spectroelectrochemical studies, revealing diagnostic peaks helpful for transient spectral interpretation. Finally, ultrafast transient pump-probe spectroscopy provided conclusive evidence of diabatic charge separation in polar solvents by far-red pulsed laser light irradiation. The measured lifetime of the final charge-separated states was found to be 165 ps in dichlorobenzene, 140 ps in benzonitrile, and 43 ps in dimethyl sulfoxide, revealing their significance in light energy harvesting, especially from the less-explored far-red region.

A Fluorescent Organic Probe for Naked-Eye Detection of Chloromethanes.

An organic fluorescent probe (OFP-TAR) with a propeller-like structure was designed and synthesized. The photoluminescence of OFP-TAR in solution exhibited a significant red shift with the increase of solvent polarity, enabling a transition of fluorescence emission from blue (445 nm) to yellow (540 nm). The organic thin-film materials based on OFP-TAR/PMMA exhibit significant color changes upon exposure to CH2Cl2, CHCl3, and CCl4, with their maximum fluorescence wavelengths measured at 445, 471, and 494 nm respectively. The device facilitates the visual detection of chloromethanes and is capable of enduring more than 7 cycles of testing. These materials can also be prepared as binary-coded microarray data storage devices or applied in the field of anti-counterfeiting. The quantum yields of guest-loaded crystals CH2Cl2@OFP-TAR, CHCl3@OFP-TAR and CCl4@OFP-TAR are observed as 19.13 %, 8.79 %, and 0.83 % respectively, which are consistent with the tendency of OFP-TAR in CH2Cl2 (47.30 %), CHCl3 (34.27 %) and CCl4 (3.10 %). The fluorescence properties of OFP-TAR, OFP-TAR/PMMA, guest-loaded and guest-free crystals provided insights into the special response mechanism of OFP-TAR towards different chloromethanes.

Unraveling the effect of molecular medium on the fluorosolvatochromism of the biphenyl- salicylate derivative drug Diflunisal: Experimental and computational study

Understanding the photochemical behavior of a photosensitive drug through its molecular properties is crucial for assessing potential phototoxicity. In this study, we examine how the molecular microenvironment influences the physicochemical and photochemical properties of Diflunisal (DIF), an organofluorine salicylate derivative drug. To elucidate the photophysical properties of DIF, we employed absorption and fluorescence spectroscopy techniques combined with various DFT-based computational methods. The solvatochromic and fluorescence behaviors of DIF were investigated employing various neat solvents of various polarity and hydrogen bonding (HB) capabilities. The molecular behavior of DIF exhibited mixed fluorosolvatochromism, where a negative fluorosolvatochromism is noted as solvent polarity increases, where DIF exhibited positive fluorosolvatochromism within the examined set of protic polar solvents. These fluorosolvatochromic behaviors suggest that a combination of factors, including solvent polarity, hydrogen bonding capacity, and specific solvation effects, influences the fluorosolvatochromic properties of DIF. To elucidate the impact of specific and non-specific interactions between DIF and solvent on the fluorosolvatochromism of DIF, we utilized Catalán’s four empirical scales model. The results revealed that the solvent’s acidity, basicity, and polarizability had approximately equivalent and notable impacts on the fluorosolvatochromic behavior of DIF. For a molecular-level explanation of the experimental spectral properties, we employed DFT and TD-DFT/B3LYP/6–31 + G(d) computational methods, incorporating the IEFPCM implicit solvation approach. It is also revealed that the negative fluorosolvatochromism of DIF is attributed to the solvent’s impact on the main electronic transition, namely HOMO → LUMO. It is revealed as well that not only the salicylate moiety is affected by the solvent, but also the biphenyl moiety can exhibit a major contribution to the observed behavior. The results presented here offer insights into the photochemical behavior of DIF within various microenvironments at the molecular level. This understanding could inform future endeavors aimed at mitigating induced toxicity and side effects of DIF.

Synthesis, optical properties, DNA, β-cyclodextrin interactions, and antioxidant evaluation of novel isoxazolidine derivative (ISoXD2): A multispectral and computational analysis

ISoXD2 are well explored among versatile and outstanding class of pharmacophores for the preparation and discovery of drugs. Herein, the electronic absorption and emission spectra of ISoXD2 were investigated in three different solvents. The observed transition was attributed to π-π* with charge transfer character. Changes in the excited state and shift of the absorption and emission peaks to longer wavelengths are observed as a result of increasing solvent polarity, due to the interactions between the ISoXD2 molecule and the solvent molecules surrounding it. Changing the solvent confirms its solvatochromic effect. UV–vis and fluorescence analysis revealed that ISoXD2 binds to deoxyribonucleic acid (DNA) by intercalation mode, with a stoichiometric ratio of 1:1.5. Moreover, the fluorescence intensity of DNA bound to ethidium bromide (EB) in the presence of ISoXD2 was investigated. From this analysis, the Stern-Volmer quenching constant (Ksv), quenching rate constant (kq), binding constant (Kb) and binding sites number (n) were found to be 5.654 × 103 M−1, 2.827 × 1011 M−1 s−1, 3.81 × 104 M−1 and 1.225, respectively. The interaction between ISoXD2 and β-CD was investigated through absorption spectra analysis. Kb for this interaction was determined to be 4.9 × 104 M−1. The free radical-scavenging ability of the prepared ISoXD2, examined by DPPH and ABTS assays have shown a good antioxidant activity. Furthermore, modeling study was conducted to explore their plausible binding mechanism with ISoXD2 and to support the experimental results.

Radical scavenging activity of bromophenol analogs: analysis of kinetics and mechanisms.

This theoretical study explores the antioxidant activity of five bromophenol analogs, with a particular focus on their interaction with different solvent environments of varying polarities. Key findings include the correlation between increased solvent polarity and enhanced antioxidant activity of these analogs, comparable in some instances to ascorbic acid. Notably, compound 5, developed by our research team, demonstrates superior antioxidant activity in both lipid and aqueous solutions, surpassing that of ascorbic acid and other tested analogs. This research contributes to the understanding of bromophenol analogs, presenting the first known kinetic and chemical stability data such as rate constants, pKa values, and branching ratios for reactions with the methylperoxyl radical (CH3OO•). The computational analyses were conducted using the Gaussian 09 software suite at the M05-2X/6-31 + G(d) computational level. These analyses employed conventional transition state theory to account for various potential mechanisms and effects of solvent polarity on the antioxidant activities of bromophenol analogs. The study meticulously calculated enthalpy under standard conditions (298.15K and 1atm) with necessary thermodynamic corrections. Additionally, the Quantum Mechanics-based Test for Overall Radical Scavenging Activity (QMORSA) protocol guided the evaluation of radical scavenging activity, ensuring a comprehensive assessment of the antioxidant potential of the compounds.

Excited Charge Transfer Promoted Electron Transfer in all Perylenediimide Derived, Wide-Band Capturing Conjugates: A Mimicry of the Early Events of Natural Photosynthesis.

Fundamental discoveries in electron transfer advance scientific and technological advancements. It is suggested that in plant and bacterial photosynthesis, the primary donor, a chlorophyll or bacteriochlorophyll dimer, forms an initial excited symmetry-breaking charge transfer state (1CT*) upon photoexcitation that subsequently promotes sequential electron transfer (ET) events. This is unlike monomeric photosensitizer-bearing donor-acceptor dyads where ET occurs from the excited donor or acceptor (1D* or 1A*). In the present study, we successfully demonstrated the former photochemical event using an excited charge transfer molecule as a donor. Electron-deficient perylenediimide (PDI) is functionalized with three electron-rich piperidine entities at the bay positions, resulting in a far-red emitting CT molecule (DCT). Further, this molecule is covalently linked to another PDI (APDI) carrying no substituents at the bay positions, resulting in wide-band capturing DCT-APDI conjugates. Selective excitation of the CT band of DCT in these conjugates leads to an initial 1DCT* that undergoes subsequent ET involving APDI, resulting in DCT +-APDI - charge separation product (kCS~109 s-1). Conversely, when APDI was directly excited, ultrafast energy transfer (ENT) from 1APDI* to DCT (kENT~1011 s-1) followed by ET from 1DCT* to PDI is witnessed. While increasing solvent polarity improved kCS rates, for a given solvent, the magnitude of the kCS values was almost the same, irrespective of the excitation wavelengths. The present findings demonstrate ET from an initial CT state to an acceptor is key to understanding the intricate ET events in complex natural and bacterial photosynthetic systems possessing multiple redox- and photoactive entities.

Detection mechanism and solvent effects of the 3‐hydroxy‐2‐(4‐(pyrrolidin‐1‐yl)phenyl)benzo[g]quinolin‐4(1H)‐one (PBQ) probe

AbstractHydroxy‐2‐(4‐(pyrrolidin‐1‐yl)phenyl)benzo[g]quinolin‐4(1H)‐one (PBQ) is a ratiometric fluorescent probe based on excited‐state intramolecular proton transfer (ESIPT). PBQ‐1 is the reaction product following its exposure to phosgene. Density functional theory (DFT) and time dependent density functional theory (DFT) have been used to study the excited state dynamics of PBQ and PBQ‐1 in different solvents. The results show that the reaction of PBQ with a transition from charge‐transfer excitation to local excitation before and after the reaction. It becomes more difficult for PBQ in the excited state to transfer proton with increasing solvent polarity. The product PBQ‐1 undergoes a molecular structure twist, and the angle of twisting decreases with increasing solvent polarity, resulting in a lower degree of rotational freedom of the hydroxyl group (5‐OH) at the 5th carbon position, which makes it more susceptible to ESIPT reactions. Therefore, PBQ‐1 is more susceptible to ESIPT as solvent polarity increases. Our theoretical calculations also elucidate the cause of the blue shift of PBQ fluorescence and the impact of the twisting intramolecular charge transfer phenomenon on the solvent effect. Furthermore, our study provides the theoretical guidance for the designing probe based on excited state intramolecular proton transfer.

Multicomponent synthesis and photophysical properties of meso-thienyl BODIPY-pyrrolo[3,4-b]pyridin-5-ones. An experimental and theoretical study

Seven new pyrrolo[3,4-b]pyridin-5-ones linked to a meso-thienyl BODIPY moiety by an ethynylbenzene π-conjugated system were synthesized in one-pot manner via an Ugi-Zhu reaction coupled to a cascade process [aza Diels-Alder cycloaddition/N-acylation/decarboxylation/dehydration]. The photophysical properties of the polyheterocyclic compounds were evaluated by UV–Vis and fluorescence spectroscopy in five solvents of different polarities. All synthesized products showed two distinct absorption/emission bands, corresponding to both fluorophores separately. The fluorescence intensity of the pyrrolo[3,4-b]pyridin-5-one was the highest in toluene, and it decreased with increasing solvent polarity. On the other hand, the fluorescence intensity of the meso-thienyl BODIPY fragment was insensitive to solvent polarity. The effect of different substituents bonded to the pyrrole ring was evaluated, finding that strong electron-donating groups influence the fluorescence intensity and decay rate of the compounds, even with the presence of a methylene spacer. Moreover, the electronic structure was assessed by DFT and TD-DFT calculations, which reproduced the experimental results reasonably well, showing that the HOMO is located on the pyrrolo[3,4-b]pyridin-5-one core, while the LUMO is on the BODIPY fragment.

Decoding the commutable first hyperpolarisability of keto–enol tautomer using DFT and AI-based soft computing method

ABSTRACT Non-linear optical (NLO) parameter of a series of donor–acceptor substituted N-salicylidene-4-benzenesulfonylaniline (SBSA) derivatives with a contrast of first hyperpolarisability has been investigated using density functional-based theory. The derivative of SBSA with both the donor and acceptor substitution (NMe2-SBSA-NO2) exhibits the highest first hyperpolarisability ( ∼ 3 × 103 a.u in gas phase) value. Frequency and solvent polarity responsive behaviours of first hyperpolarisability are utilised to demonstrate the variation of NLO as switchable electrical parameter. Each of the studied compounds displays very high NLO response at frequency corresponding to the absorption maxima of their electronic state. Likewise, the increase of solvent polarity induces a substantial increase of first hyperpolarisability. The computation of NLO responsive first hyperpolarisability by varying the frequency and solvent dielectric constant within a wide domain is very tedious and time-consuming. To overcome the lacuna, we implemented machine learning tools such as artificial neural networks (ANNs), fuzzy logic (FL) and adaptive neuro-fuzzy inference system (ANFIS) to predict the computation data of the NMe2-SBSA-NO2 and moderately good results were obtained using ANFIS as soft computing tool.

The Influence of the Alkylamino Group on the Solvatochromic Behavior of 5-(4-substituted-arylidene)-1,3-dimethylpyrimidine-2,4,6-triones: Synthesis, Spectroscopic and Computational Studies.

Advances in electronics and medical diagnostics have made organic dyes extremely popular as key functional materials. From a practical viewpoint, it is necessary to assess the spectroscopic and physicochemical properties of newly designed dyes. In this context, the condensation of 1,3-dimethylbarbituric acid with electron-rich alkylaminobenzaldehyde derivatives has been described, resulting in a series of merocyanine-type dyes. These dyes exhibit intense blue-light absorption but weak fluorescence. An electron-donating alkylamino group at position C4 is responsible for the solvatochromic behavior of the dyes since the lone electron pair of the nitrogen atom is variably delocalized toward the barbituric ring, which exhibits electron-withdrawing properties. This was elucidated, taking into account the different geometry of the amino group. The intramolecular charge transfer in the molecules is responsible for the relatively high redshift in absorption and fluorescence spectra. Additionally, an increase in solvent polarity moves the absorption and fluorescence to lower energy regions. The observed solvatochromism is discussed in terms of the four-parameter Catalán solvent polarity scale. The differences in the behavior of the dyes were quantified with the aid of time-dependent density functional theory calculations. The obtained results made it possible to find regularities linking the basic spectroscopic properties of the compounds with their chemical structure. This is important in the targeted search for new, practically important dyes.

Rational design and investigation of nonlinear optical response properties of pyrrolopyrrole aza-BODIPY-based novel push-pull chromophores.

Intramolecular charge transfer (ICT)-based chromophores are highly sought after for designing near-infrared (NIR) absorbing and emitting dyes as well as for designing materials for nonlinear optical (NLO) applications. The properties of these 'push-pull' molecules can easily be modified by varying the electronic donor (D) and acceptor (A) groups as well as the π-conjugation linker. This study presents a methodical approach and employs quantum chemical analysis to explore the relationship between the structural features, electro-optical properties, and the NLO characteristics of molecules with D-π-A framework. The one- and two-photon absorption (2PA), linear polarizability (α), and first hyperpolarizability (β) of some novel chromophores, consisting of a dimeric aza-Boron Dipyrromethene (aza-BODIPY) analogue, called, pyrrolopyrrole aza-BODIPY (PPAB), serving as the acceptor, have been investigated. The electronic donors used in this study are triphenylamine (TPA) and diphenylamine (DPA), and they are conjugated to the acceptor via thienyl or phenylene π-linkers. Additionally, the Hyper-Rayleigh Scattering (βHRS), which enables direct estimation of the second-order NLO properties, is calculated for the studied chromophores with 1064 nm excitation in acetonitrile. The β value shows a significant increase with increasing solvent polarity, indicating that the ICT plays a crucial role in shaping the NLO response of the studied molecules. The enhancement of the 2PA cross-section of the investigated molecules can also be achieved by modulating the combinations of donors and linkers. The results of our study indicate that the novel D-π-A molecules designed in this work demonstrate considerably higher hyperpolarizability values than the standard p-nitroaniline, making them promising candidates for future NLO applications.

Solvatochromic fluorescent ethynyl naphthalimide derivatives for detection of water in organic solvents

Three new derivatives of (4-methoxyphenyl) ethynyl-naphthalimide were synthesized by Sonogashira coupling reaction. These compounds showed robust and strong fluorescent signals, which shift from blue to orange as the solvent polarity increases. Computational calculations revealed that in low-polarity solvents, the molecules preferred a 90° rotation between the naphthalimide and ethynyl benzene fragments. In high-polarity solvents, where the energy barrier for rotation of the alkyne bond was higher, the molecules favored the 0° conformer, leading to longer emission wavelengths. The compound with a 2-methoxyethyl group tethered to the naphthalimide nitrogen demonstrated sensitivity to water presence in the ranges of 0–30 % (v/v) in THF, 0–15 % (v/v) in DMSO, 0–30 % (v/v) in MeCN, 0–15 % (v/v) in acetone, and 0–40 % (v/v) in EtOH. The detection limits for water were determined to be 0.0523 %, 0.1365 %, 0.0337 %, 0.0570 %, and 0.1170 % (v/v) in THF, DMSO, MeCN, acetone, and EtOH, respectively. The effectiveness of this compound in quantifying ethanol content in spirit samples was successfully demonstrated.

From Criegee to Breslow: How π-Donors Steer the Route of Olefin Ozonolysis.

While π-bonds typically undergo cycloaddition with ozone, resulting in the release of much-noticed carbonyl O-oxide Criegee intermediates, lone-pairs of electrons tend to selectively accept a single oxygen atom from O3, producing singlet dioxygen. We questioned whether the introduction of potent electron-donating groups, akin to N-heterocyclic olefins, could influence the reactivity of double bonds - shifting from cycloaddition to oxygen atom transfer or generating lesser-known, yet stabilized, donor-substituted Criegee intermediates. Consequently, we conducted a comparative computational study using density functional theory on a series of model olefins with increasing polarity due to (asymmetric) π-donor substitution. Reaction path computations indicate that highly polarized double bonds, instead of forming primary ozonides in their reaction with O3, exhibit a preference for accepting a single oxygen atom, resulting in a zwitterionic species formally identified as a carbene-carbonyl adduct. This previously unexplored reactivity potentially introduces aldehyde umpolung chemistry (Breslow intermediate) through olefin ozonolysis. Considering solvent effects implicitly reveals that increased solvent polarity further directs the trajectories toward a single oxygen atom transfer reactivity by stabilizing the zwitterionic character of the transition state. The competing modes of chemical reactivity can be explained by a bifurcation of the reaction valley in the post-transition state region.

Symmetry-breaking charge separation in a null-excitonic 3-dimensional rigid nonconjugated trimer.

Photoinduced symmetry-breaking charge separation (SB-CS) has been extensively observed in various oligomers and aggregates, which holds great potential for robust artificial solar energy conversion systems. It attaches great importance to the precise manipulation of interchromophore electronic coupling in realizing efficient SB-CS. The emerging studies on SB-CS suggested that it could be realized in null-excitonic aggregates, and a long-lived SB-CS state was observed, which offers an advanced platform and has gathered immense attention in the SB-CS field. Here, we unveiled the null-exciton coupling induced ultrafast SB-CS in a rigid polycyclic aromatic hydrocarbon framework, triperyleno[3,3,3]propellane triimides (TPPTI), in which three chromophores were attached through a nonconjugated bridge. Through a combination of theoretical calculations and steady-state absorption results, we demonstrated that this nonconjugated TPPTI possesses negligible exciton coupling. Increased solvent polarity was found to significantly enhance state mixing between local excited and charge transfer states. Using transient absorption spectroscopy, ultrafast SB-CS was observed in highly polar dimethylformamide, facilitated by a selective hole-transfer coupling and a favorable charge separation free energy (ΔGCS). Additionally, the rate ratio between SB-CS and charge recombination was at least high to 1800 in dimethylformamide. This investigation provides profound insights into the role of null-exciton coupling in dominating ultrafast SB-CS in multichromophoric systems.