Rotational Isomers Research Articles

Exploring Rotational Isomerism of Fluorescent Diarylethenes Comprising 2-Naphthyl and Regioisomeric Oxazoyl Groups. A Combined Experimental and Computational Study

In the paper a phenomenon of rotational (conformational) isomerism within 1,2-diarylethenic compounds (DAE) comprising heteroarene ring of oxazole is explored on the example of the fluorescent naphthyl-vinyl-oxazole framework. In this regard three regioisomers of a n-(2-(naphthalen-2-yl)vinyl)oxazole molecule (nNVOx), differing in the oxazole substitution pattern (n = 2, 4, 5), are synthesized and methodically investigated by means of electronic UV-Vis spectroscopy combined with chemometric methods for spectral data analysis. Design of the experiments and interpretation of their output is additionally supported by quantum-chemical simulations based on the (TD) DFT methodology. As the result, all the studied nNVOx compounds are unveiled to simultaneously occur in form of at least two non-equivalent rotational isomers, varying in the conformation taken by the arene moieties with respect to the central vinyl fragment. The individual rotamers are then demonstrated to exhibit non-identical spectroscopic properties expressed by differences in corresponding absorption and fluorescence profiles, thereby ultimately confirming the impact of rotamerism onto the investigated molecular systems.

Distinct High-Pressure Responses of Halide Perovskites from Bulk to Low Dimension

Hybrid organic-inorganic perovskites (HOIPs) are low-cost and highly efficient optoelectronic and photovoltaic materials for applications in solar cells, light emitting diodes (LEDs), and so on, which is correlated with their intrinsic structures. Now the hybrid perovskite families include three-dimensional (3D) (CH3NH3PbBr3), two-dimensional (2D) ((C4H9NH3)2(CH3NH3)n-1PbnI3n+1) and nanostructured ((CH3NH3PbBr3 nanoparticles) materials. Herein, well understanding the relationship between the cystal structures and the related functional properties of HOIP materials is essential to the application point of view. High pressure up to gigapascal, offers a comprehensive way to study the structure-property correlation of solid materials in the atomic level, where both crystal structures and electronic properties are changed dramatically.In this presentation, high pressure induced dramatically inorganic and organic part changes in 2D HOIPs are comprehensively studied[1,2], as shown in Figure 1. On the other hand, structural phase transition and morphology changes are also explored in 3D HOIP and their nanoparticles[3], as shown in Figure 2. All the high pressure-induced inorganic structural transition is resolved by in situ X-ray powder diffraction (XRD) measurements, morphology change is resolved by transmission electron microscopy (TEM), the correlated optical responses are investigated by absorption and photoluminescence spectroscopy. And the rotational isomerism is evidenced by the vibrational properties of the organic BA chain by Raman spectroscopy combined with the Ab initio calculations.Reference:[1] T Yin, B Liu, J Yan, Y Fang, M Chen, WK Chong, S Jiang, J-L Kuo, J Fang, P L, S-H W, K-P Loh, T-C Sum, T J. White, and Z Xiang, J. Am. Chem. Soc. 141, 1235 (2019).[2] Yin T, Yan H, Abdelwahab I, Lekina Y, Lü X, Yang W, Sun H, Leng K, Cai Y, Shen Z, Loh KP, Nat. Commun. 14, 411 (2023).[3] T Yin, Y Fang, WK Chong, KT Ming, S Jiang, X Li, J-L Kuo, J Fang, T-C Sum, T J. White, J Yan, and Z Xiang, Adv. Mater. 30, 1705017 (2018). Figure 1

Rotational isomerism of amide units in the ring of rotaxanes in response to base/acid stimuli

Abstract Rotational isomerism of 4 amide moieties of the threaded macrocyclic component of [2]rotaxanes featuring mono- and bis-ammonium moieties in dumbbell-shaped axle components was investigated in response to base, and the rotational isomerization of both rotaxanes could be controlled reversibly through the addition of appropriate amounts of base and acids.

Atropisomeric Properties of 5N‐Benzoyl‐1,5‐benzodiazepin‐2‐ones: Substituent Effects on Rotation around the Ar−N(CO) Axis

Abstract5N‐Benzoyl‐1,5‐benzodiazepin‐2‐ones with various substituents were synthesized and their atropisomeric properties were analyzed. For the rotational isomers (E/Z) based on the amide bond [N−C(=O)], the E‐isomers were preferentially formed regardless of the substituents. Atropisomers (aR/aS) due to the Ar−N(CO) (sp2‐sp2) axis were isolated using chiral HPLC, and the rotational barrier (ΔG≠) in thermal isomerization was measured. The rotation barrier about the Ar−N(CO) axis was affected by both the substituents at the C6‐position and the p‐substituents on the benzoyl group. The substituents at C6 (R1) caused both steric and electronic effects. On the other hand, the p‐substituents (R2) on the benzoyl group caused electronic effects: the electron‐donating group (EDG) lowered the ΔG≠ value, whereas the electron‐withdrawing group (EWG) raised it. The ΔG≠ values were linearly correlated with the magnitude of the electronic effect (Hammett σ constants) of functional groups (R2). The EDG causes an increase in the electron density on the carbonyl moiety in the amide, which may disturb the planarity of the amide bond, resulting in a decrease in the ΔG≠ value.

Direct and efficient catalytic conversion of biomass-derived 2-methyltetrahydrofuran to 1,3-pentadiene via acid-base coupling of lanthanum phosphate

It is critical to increase the reaction selectivity and atom economy, avoid side reactions during the catalytic process, and achieve the desired single-target product. Here, an efficient and novel method of producing 1,3-pentadiene via the acid-base coupling of rare-earth phosphate-catalyzed dehydra-decyclization of 2-methyltetrahydrofuran (2-MTHF) is reported, resulting in a previously unreported 1,3-pentadiene yield of 97.2% at 350 ℃. This method efficiently addresses two significant problems encountered in the dehydra-decyclization of 2-MTHF to generate 1,3-pentadiene research: the occurrence of retro-Prins condensation and the competitive reaction of 1,4-pentadiene. Density-Functional-Theory calculations demonstrated that 2-MTHF was adsorbed on the surface of rare-earth phosphate catalysts in a multi-site parallel mode, resulting in a lower apparent activation energy and ring-opening energy barrier. Space-time investigations and in situ diffuse reflectance infrared Fourier transform spectroscopy provided evidence for direct synthesis of pentadiene on rare-earth phosphate catalysts with Lewis acid-base sites. The acid-base coupling catalysis was demonstrated by poisoning experiments. In-situ self-regeneration of Brønsted acid sites was confirmed by infrared spectroscopy of adsorbed pyridine, facilitating the isomerization of 1,4-pentadiene to 1,3-pentadiene. Therefore, a unique reaction pathway for the dehydra-decyclization of 2-MTHF on the surface of rare-earth phosphates was proposed, and a new rotational isomer was discovered.

VT‐NMR Analysis of Rotation‐Inversion of N‐(4‐hydroxybutyl)‐N‐(2,2,2‐trifluoroethyl) tert‐butyl Carbamate: Utilizing the −CH2CF3 Appendage as a Reporter on E/Z‐Isomerization

AbstractThe rotational barrier about the CN carbamate bond of N‐(4‐hydroxybutyl)‐N‐(2,2,2‐trifluoroethyl) tert‐butyl carbamate 1 was determined by variable temperature (VT) 13C and 19F NMR spectroscopy. The −CH2CF3 appendage reports on rotational isomerism and allows for the observation of separate signals for the E‐ and Z‐ensembles at low temperature. The activation barrier for E/Z‐isomerization was quantified using Eyring‐Polanyi theory which requires the measurements of the maximum difference in Larmor frequency Δνmax and the convergence temperature Tc. Both Δνmax and Tc were interpolated by analyzing sigmoidal functions fitted to data describing signal separation and the quality of the superposition of the E‐ and Z‐signals, respectively. Methods for generating the quality‐of‐fit parameters for Lorentzian line shape analysis are discussed. Our best experimental value for the rotational barrier ΔGc≠(1)=15.65±0.13 kcal/mol is compared to results of a higher level ab initio study of the model N‐ethyl‐N‐(2,2,2‐trifluoroethyl) methyl carbamate.

Theoretical investigation of the conformational preference and spectroscopic analysis of cinnamoyl chloride and cinnamide

The rotational isomers of cinnamoyl chloride and cinnamide have been computed using the B3LYP method and the 6-311++G (d, p) basis set to calculate their molecular structure, conformational stability, and spectroscopic properties. Computations revealed that the s-cis rotamers of both compounds are the most stable rotamers in both the gas phase and in the solution. The s-trans rotational barrier is 8.76 kcal/mol and 5.63 kcal/mol for cinnamoyl chloride and cinnamide, respectively. Replacing the Cl group of the cinnamoyl chloride with the NH2 group in the cinnamide has reduced the highest occupied molecular orbital – lowest unoccupied molecular orbital (HOMO-LUMO) gap. The solvation effects on the conformational stability of rotamers have been studied in nine solvents (heptane, chloroform, tetrahydrofuran, dichloroethane, acetone, ethanol, methanol, dimethyl sulfoxide, and water) using the integral equation formalism of the polarizable continuum model. The chemical shifts of 13C and 1H NMR spectra have been simulated in the gas phase, dimethyl sulfoxide (DMSO), and chloroform using the gauge-independent atomic orbital (GIAO) method. The UV absorption spectral analysis has been computed in different solvents (chloroform, methanol, and water), and atomic charges calculated. The examination of vibrational wave numbers and their corresponding assignments revealed excellent agreement between simulated and experimental infrared spectra for the studied molecules.

Evaluating the impact of structural and rotational isomerization, as well as halogenation, on Y18 and Y18-ID small molecule acceptors: A DFT and TD-DFT study

In the present work, using a systematic approach with DFT and TD-DFT techniques, we investigate the isomerization impact on Y18 and Y18-ID, two electron acceptors belonging to the Y family. Furthermore, the effect of the halogen atom was assessed by changing the original fluorine atoms with chlorine, bromine, and iodine. Two structural isomers were proposed for each acceptor, and a configurational (rotamer) for each structural isomer. A benchmarking was performed for relative energies, molecular planarity, light-harvesting features, frontier molecular orbitals alignment, electron-accepting indexes, reorganization energies, and average electrostatic potential. Mainly, structural isomerization yields the most significant modification to the studied properties and, to a lower extent, rotamerization. Nonetheless, the impact of rotamers might not be ignored, mainly in electron-accepting character and intermolecular electric field. Indeed, the modification that rotamers induce in some properties is comparable to that induced by the modification of halogen atoms.

A computational study of a light-driven artificial device: a third generation rotational photo-molecular motor in dilute solutions.

A third-generation artificial photo-molecular motor, featuring two photo-switchable rotating moieties in connection with a pseudoasymmetric molecular centre, is investigated by combining quantum-mechanics (QM) algorithms with classical molecular dynamics (MD) propagators. In particular, in the present contribution we have addressed such a molecular motor in different rotational isomers following the experimental observations arising from the application of multiple spectroscopic techniques in dilute solutions. At first, we focused our attention on the reproduction of the UV/Vis absorption spectrum in two solvents (acetonitrile and cyclohexane) with different gradient-corrected density functional theory (B3LYP, Cam-B3LYP, PBE, PBE0) functionals in conjunction with the conductor-like and polarizable continuum model (C-PCM). Furthermore, we refined the absorption signals by combining a classical MD sampling at room-temperature with DFT-based electronic degrees of freedom to compute perturbed excitation wavelengths driven by thermal fluctuation and solvation effects. In this respect, we have modelled the investigated artificial motor within solution nanodroplets with solvent molecules treated contextually at atomistic level and via a dielectric and polarizable continuum model.

Controlling excited-state dynamics via protonation of naphthalene-based azo dyes.

Azo dyes are a class of photoactive dyes that constitute a major focus of chemical research due to their applications in numerous industrial functions. This work explores the impact of protonation on the photophysics of four naphthalene-based azo dyes. The pKa value of the dyes increases proportionally with decreasing Hammett parameter of p-phenyl substituents from 8.1 (R = -H, σ = 0) to 10.6 (R = -NMe2, σ = -0.83) in acetonitrile. Protonation of the dyes shuts down the steady-state photoisomerization observed in the unprotonated moieties. Fluorescence measurements reveal a lower quantum yield with more electron-donating p-phenyl substituents, with overall lower fluorescence quantum yields than the unprotonated dyes. Transient absorption spectroscopy reveals four excited-state lifetimes (<1 ps, ∼3 ps, ∼13 ps, and ∼200 ps) exhibiting faster excited-state dynamics than observed in the unprotonated forms (for 1-3: 0.7-1.5 ps, ∼3-4 ps, 20-40 ps, 20-300 min; for 4: 0.7 ps, 4.8 ps, 17.8 ps, 40 ps, 8 min). Time-dependent density functional theory (TDDFT) elucidates the reason for the loss of isomerization in the protonated dyes, revealing a significant change in the lowest excited state potential energy nature and landscape upon protonation. Protonation impedes relaxation along the typical rotational and inversion isomerization axes, locking the dyes into a trans-configuration that rapidly decays back to the ground state.

Rotational conformers and nuclear spin isomers of carbonyl diisothiocyanate.

Nuclear spin isomers of molecules play a pivotal role in our understanding of quantum mechanics and can have significant implications for various fields. In this work, we report the isolation and characterization of stable nuclear spin isomers as well as conformational isomers of a reactive compound, namely carbonyl diisothiocyanate. It can exist as three rotational conformers, two of which, the syn-syn and syn-anti, were observed in a pulsed supersonic jet by chirped pulse Fourier transform microwave spectroscopy in the 2-12 GHz frequency region. The rotational spectra of two distinct nuclear spin isomers of syn-syn-carbonyl diisothiocyanate, ortho and para, were recorded and analyzed. Experimental molecular rotational parameters for the identified rotational and nuclear spin isomers were determined, including rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants. The two nuclear spin isomers are distinguished by unique hyperfine splitting signatures in their rotational spectra as an outcome of their different nuclear spin states. The relative abundances of the two observed conformers in the gas phase were estimated from the intensity of their rotational transitions. Following detection of singly substituted rare isotopologues of the syn-syn conformer, a partial substitution (rs) structure was determined.

Improved Synthesis of Chiral 1,4,7‐Triazacyclononane Derivatives and Their Application in Ni‐Catalyzed Csp3−Csp3 Kumada Cross‐Coupling

AbstractHerein, we report four new chiral 1,4,7‐triazacyclononane (TACN) derivatives and their corresponding nickel(II) chloride complexes. All TACN ligands are bearing one chiral N‐substituent and two alkyl (methyl or tert‐butyl) N‐substituents, and we have developed a new synthetic method for the dimethyl‐substituted TACN derivative, in order to prevent the rotational isomers that hinder the cyclization reaction. The nickel complexes change their coordination geometry significantly depending on the steric bulk of the N‐alkyl substituents, from a dinuclear tris(μ‐chloro)dinickel complex to mononuclear Ni‐dichloride and Ni‐chloride complexes. These complexes were then employed in the alkyl‐alkyl Kumada cross‐coupling reaction and revealed that the more sterically hindered ligands produced more homocoupled product rather than the cross‐coupled product, while the mononuclear Ni‐dichloride complex exhibited significantly lower catalytic activity. These chiral complexes were also employed in enantioconvergent cross‐coupling reactions as well, to afford significant enantioenrichment. Overall, the least sterically hindered Ni complex yields the best yields in the alkyl‐alkyl Kumada cross‐coupling reaction among the four complexes investigated, as well as the highest enantioselectivity.

Geometric and Hydrophilic Effects of Oxirane Compounds with a Four-Carbon Backbone on Clathrate Hydrate Formation.

The physicochemical properties of clathrate hydrates are influenced by the chemical nature and three-dimensional (3D) geometry of the added molecules. This study investigates the effects of five oxirane compounds: cis-2,3-epoxybutane (c23EB), trans-2,3-epoxybutane (t23EB), 1,2-epoxybutane (12EB), 1,2,3,4-diepoxybutane (DEB), and 3,3-dimethylepoxybutane (33DMEB) on CH4 hydrate formation. Despite having a four-carbon backbone, these compounds differ in their 3D geometries. The structures and stabilities of CH4 hydrates containing each compound were analyzed using high-resolution powder diffraction, solid-state 13C NMR, and phase equilibrium measurements. The experimental results revealed that c23EB, 12EB, and 33DMEB act as sII/sH hydrate formers and thermodynamic promoters, whereas t23EB and DEB have opposite roles. These results were analyzed in relation to the 3D geometries and relative stabilities of various rotational isomers using DFT calculations. Hydrate structure was influenced by both the length and thickness of the added compounds. Moreover, an appropriate level of (not excessive) hydrophilicity induced by an oxirane group appeared to enhance the thermodynamic stability of the hydrates. This study provides insights into how the chemical nature of additives influences the structure and stability of clathrate hydrates.

Rotational Isomerism of the Side Chains of Hydroxypropyl Cellulose in Aqueous Solution Observed Using Attenuated Total Reflectance Infrared Spectroscopy

Etherified cellulose derivatives, in contrast to cellulose, are soluble in water at room temperature and have a wide variety of applications. One of their most important characteristics is their decrease in solubility with temperature. The objective of this work was to study the rotational isomerism of the side chains of hydroxypropyl cellulose (HPC) in aqueous solution as sole solute and in the presence of chloride, sulfate, and barium ions as a function of temperature. Infrared Attenuated Total Reflectance spectroscopy was used to measure changes in the side-chain rotational isomerism using the structurally sensitive methylene wagging region as the probe. Decreases in end-gauche and kink conformers were observed. Principal component analysis revealed the presence of multiple forms of HPC at higher molecular weight. The precipitation of HPC as the temperature was increased was accompanied by a reduction in the numbers of end-gauche and kink conformers.

A rotational isomerization of diiron complexes of ferrole type involved in the catalytic hydroboration of aldehydes under mild conditions

In this work, two new diiron(I) complexes of ferrole or ferracyclopentadiene type, [Fe2(CO)6{RC2(CH2)4C2R}] (R = H, 1; CH2CH3, 2) were synthesized by reacting the [Fe2(CO)9] with diynes (1,7-octadiyne and 3,9-dodecanediyne). The ferrole-type complexes were fully characterized, wherein the absolute structure of 2 was determined by single-crystal X-ray diffraction analysis. Interestingly, a rotational isomerization of the diiron(I) complexes (1 and 2) between the sawhorse conformer and the non-sawhorse conformer is verified, originated from the switch of CO coordination. Additionally, the sawhorse conformer with the terminal CO is dominant in solution due to the tricarbonyl iron units undergo a drastic geometrical turnstile rotation, whereas the non-sawhorse conformer with a semi-bridging CO is rigid in a solid state. Particularly, the isomerism of the diiron(I) complexes of ferrole type leads to a unique catalytic activity for hydroboration of aldehydes with pinacolborane (HBpin). Notably, twelve aldehydes (3a-3l) were exampled to be converted quantitatively into the corresponding organoboronates (4a-4l) mediated by complex 2 under mild conditions. Mechanistically, the 1H NMR and FTIR spectroscopic results proved that the ferrole-type complex 2 bound with the aldehyde to activate the carbonyl of the substrate rather than to cleave the H-B bond of pinacolborane. Collectively, a plausible mechanism for the catalytic hydroboration is proposed, with the rotational isomerization of the diiron(I) complexes involved in.

PBPK Modeling of PAXLOVIDTM: Incorporating Rotamer Conversion Kinetics to Advanced Dissolution and Absorption Model

PAXLOVID™ is a combination medicine of nirmatrelvir tablets co-packaged with ritonavir tablets. Nirmatrelvir is a peptidomimetic inhibitor of SARS-CoV2 main protease (Mpro), developed for the treatment of COVID-19. Ritonavir is co-administered as a pharmacokinetics (PK) enhancer to inhibit CYP3A mediated metabolism increasing exposures of nirmatrelvir. In the solid form, nirmatrelvir exists in a stable single conformational state (ANTI form). However, nirmatrelvir exhibits atropisomerism in solution whereby upon dissolution the ANTI rotational isomer reversibly converts to another conformation state (SYN form). Nirmatrelvir rotamer conversion follows pseudo first order kinetics with a conversion half-life of approximately 15 min in aqueous solutions, which is on a similar time scale of diffusion mediated dissolution from the solid form. In vitro dissolution studies further indicated that rotamer conversion is one of the processes controlling nirmatrelvir dissolution. It was hypothesized that rotamer conversion kinetics would affect oral absorption of nirmatrelvir in vivo. Consequently, a physiologically based pharmacokinetic (PBPK) model for Paxlovid was developed in Simcyp™ using the advanced dissolution, absorption, and metabolism model (ADAM) by incorporating rotamer conversion kinetics to achieve a more mechanistic description of nirmatrelvir oral absorption. The results demonstrate that the established absorption model with rotamer kinetics adequately described observed clinical data from various nirmatrelvir doses, dosage forms, and dosing regimens. The predicted vs. observed AUCinf and Cmax ratios were within 2-fold. The model has been internally used to inform clinical studies and dose recommendations for pediatrics.

Organoruthenium Complexes Containing Phosphinodicarboxamide Ligands

Ruthenium complexes of phosphinocarboxamide ligands, and their use to form metallacycles using halide abstraction/deprotonation reactions are reported. Thus, [Ru(p-cym){PPh2C(=O)NHR}Cl2; R = iPr (1), Ph (2), p-tol (3)] and [Ru(p-cym){PPh2C(=O)N(R)C(=O)N(H)R}Cl2; R = Ph (4), p-tol (5)] were synthesized from [(p-cym)RuCl2]2 (p-cym = para-cymene) and phosphinocarboxamides or phosphinodicarboxamides, respectively. Single-crystal X-ray diffraction measurements on 1–5 reveal coordination to ruthenium through the phosphorus donor, with an intramolecular hydrogen bond between the amine-bound proton and a metal-bound chloride. Six-membered metallacycles formed by halide abstraction/deprotonation of complexes 4 and 5 afforded [Ru(p-cym){κ2-P,N-PPh2C(=O)N(R)C(=O)NR}Cl] [R = Ph (6), p-tol (7)]. These species exist as a mixture of two rotational isomers in solution, as demonstrated by NMR spectroscopy.

Molecular networking-guided isolation strategy of a new C-glycosyl flavone rotamer from Stellaria alsine and evaluation of anti-inflammatory and antioxidant activities.

Stellaria alsine has traditionally been used as both a famine relief food and an alternative medicine in East Asia. Modern pharmacological studies have revealed that S. alsine has various biological effects such as anticancer, anti-hepatoma, anti-inflammatory, and antioxidative effects. However, the anti-inflammatory properties of chemical constituents derived from this plant have not been studied well. To identify potential therapeutic candidate for treating inflammatory diseases such as inflammatory bowel disease (IBD). The distribution of chemical compounds was investigated by Global Natural Product Social (GNPS)-based molecular networking (MN) analysis using UPLC-Orbitrap tandem mass spectrometry. The anti-inflammatory and antioxidative effects of S. alsine extracts and fractions were evaluated by measuring interleukin (IL)-8 and reactive oxygen species (ROS) productions. The active EA layer of S. alsine showed the highest percentage of major compounds by feature-based molecular networking. The top candidate structures of EA fraction were rapidly annotated as flavone C- or O-glycosides via an advanced analysis tool, Network Annotation Propagation (NAP). With the GNPS molecular networking-guided isolation strategy, a new C-glycosyl flavone rotamer (1) was isolated. The structures of the major (1a) and minor (1b) rotational isomers were determined by extensive NMR analysis and MS/MS fragmentation. Finally, the anti-inflammatory activity of 1 was predicted by molecular docking simulations with IL-8 protein. These results suggested that the compound 1 is a potential therapeutic candidate for inflammatory bowel disease (IBD).

Unexplored Isomerization Pathways of Azobis(benzo-15-crown-5): Computational Studies on a Butterfly Crown Ether.

Computational studies on trans → cis and cis → trans isomerizations of photoresponsive azobis(benzo-15-crown-5) have been reported in this work. The photoexcited ππ* state (S2) of the trans isomer relaxes through the planar S2 minimum and the planar S2/S1 conical intersection (both situated around 9 kcal/mol below the vertically excited S2 state) arising along the N═N stretching coordinate. The nπ* state (S1) of this isomer has both planar and rotated (clockwise and anticlockwise) minima, which may lead to a torsional conical intersection (S0/S1) geometry having a <CNNC dihedral angle value close to 90°. This rotational isomerization path is found to have an energy barrier. It has been noticed that an N-N-out-of-plane motion coupled with the torsion can bring down the barrier and may facilitate the isomerization process. On the other hand, the vertically excited S1 state of the cis-isomer undergoes a barrierless path to reach a torsional conical intersection (S0/S1) geometry (<CNNC = 88°), responsible for the trans-isomer formation. The cis-S2/S1 CI (conical intersection) torsional geometry (<CNNC = 45°) is situated around 16 kcal/mol above the vertically excited ππ* state of the cis-isomer and not reachable on S2 photoexcitation. The two optimized torsional S2 minima of this system are close to the S1 surface and seem to be involved in the S2-S1 internal conversion. A less efficient concerted inversion photoisomerization path through a linear geometry has also been identified. The thermal cis → trans isomerization has been found to undergo an inversion motion, which forms a less stable trans isomer. Possible differences between the isomerization channels of this azobis(benzo-15-crown-5) and the unsubstituted azobenzene system have also been highlighted in the work.

Synthesis of 3‐arylated chlorophyll‐a derivatives via Diels–Alder reaction and their atropisomerism

AbstractMethyl pyropheophorbides‐a possessing an o/m‐(methoxycarbonyl)phenyl group at the 3‐position were prepared by Diels–Alder reaction of 3‐(trans‐1,3‐butadienyl)chlorin with methyl propiolate and successive didehydrogenation of the resulting 1,4‐cyclohexadienes. The 3‐arylated chlorin bearing the sterically demanding o‐COOMe group as the major product was a 1:1 mixture of high‐performance liquid chromatography‐separable rotational isomers around the C3–C31 bond, while the minor product with the m‐COOMe was less sterically hindered to exhibit rapid atropisomerization at room temperature. The 3‐aryl group of the major product was nearly perpendicular to the chlorin π‐system and less conjugated with the chlorin moiety than that of the minor product wherein more π‐conjugation occurred to give slightly red‐shifted Qy bands. Although both the atropisomers of the o‐substitute in dichloromethane showed similar visible absorption bands and fluorescence emission data, their circular dichroism bands in the ultraviolet‐C light region were dependent on the stereochemistry.