Predicting the UV−Vis Spectra of Tetraarylcyclopentadienones: Using DFT Molecular Orbital Energies to Model Electronic Transitions of Organic Materials

21-Carba-23-oxaporphyrinoids and 21-oxo-21-carba-23-oxaporphyrinoids: macrocyclic π-conjugation involving the carbonyl moiety

Aromaticity of 21-oxo-21-carba-23-oxachlorin resulted from a predominant dipolar contributor. Nonaromaticity of 21-oxo-21-carba-23-oxaporphyrin reflects a participation of canonical aromatic and antiaromatic forms engaged in a peculiar “tug of war”.

Synthesis of [2.2]Paracyclophane/9-Alkylfluorene Hybrids and the Discovery of a Solvent-assisted Rearrangement

Work on the synthesis of [2.2]paracyclophane/9-alkylfluorene hybrids led to the discovery of the rearrangement of cyclopentadienone 7 to cyclophane 6. A DFT computational study revealed that this formal 1,3-alkyl shift occurs in two steps, but requires the participation of a solvent molecule (ethanol). The rearrangement could be avoided by generating 7 under mild conditions and using benzynes as dienophiles to afford the targeted cyclophanes 14 and 16, the latter of which exhibits dual fluorescence emission.

Investigating the change in the photophysical properties of a trio of tetraphenylcyclopentadienone derivatives with varied groups on the aromatic rings in the 3- and 4-positions

Organic compounds with electronic properties, such as a small band gap, are useful in areas ranging from organic field effect transistors to solar cells. Such organic compounds can possess conjugation and/or aromatic systems, with one example being tetraphenylcyclopentadienone and its derivatives. A trio of dramatically coloured tetraphenylcyclopentadienone derivatives with varied substituents on the aromatic rings in the 3- and 4-positions were prepared. Their identities were confirmed using the usual methods, for example ¹ H nuclear magnetic resonance (NMR) spectroscopy, and their purity quantified using elemental analysis. The X-ray crystal structure of compound 2 was determined. Its notable structural features involved the cyclopentadienone core with its distinct C-C and C=C bond lengths and its overall nonplanarity, both of which served to mitigate its antiaromatic nature. Chloroform solutions of compounds 2-4 exhibited absorption spectra with three absorption bands at approximately 250, 350, and 500 nm that were assigned to (π)→(π*) transitions. Computational chemistry methods assisted in assigning the observed transitions to a specific molecular orbital combination in the structures of 2-4. Emission in the red end of the visible spectrum (550-625 nm) was observed from chloroform solutions of all three of the prepared compounds.

Regio- and Stereoselectivity in the 1,3-Dipolar Cycloaddition Reactions of Isoquinolinium Ylides with Cyclopenta[a]acenaphthylen-8-ones

A convenient regio- and diastereoselective synthesis of functionalized 5a,5b-dihydro-5H,13H-naphtho[1′′,8′′:4′,5′,6′]pentaleno[1′:3,4]pyrrolo[2,1-a]isoquinolin-5-ones via 1,3-dipolar cycloaddition reaction of 8H-cyclopenta[a]acenaphthylen-8-ones with carbonyl-stabilized isoquinolinium N-ylides, is described. Based on DFT calculations at b3lyp/6-311+g(d,p) level of theory, a nonconcerted mechanism is proposed to explain the regioselectivity of this reaction. The structure of a typical product was confirmed by X-ray crystallographic analysis.

Tuning of the Electro-Optical Properties of Tetraphenylcyclopentadienone via Substitution of Oxygen with Sterically-Hindered Electron Withdrawing Groups

In this report, the substitution of the oxygen group (=O) of Tetraphenylcyclopentadienone with =CR2 group (R = methyl ester or nitrile) was found to have tuned the electro-optical properties of the molecule. Although both groups are electrons withdrawing in nature, their absorption from UV-vis spectra analysis was observed to have been blue-shifted by methyl ester substitution and red-shifted by nitrile substitution. Interestingly, these substitutions helped to enhance the overall intensity of emission, especially in the context of methyl ester substitution whereby the emission was significantly boosted at higher concentrations due to hypothesized restrictions of intramolecular motions. These observations were explained through detailed descriptions of the electron withdrawing capability and steric properties of the substituents on the basis of density functional theory calculations.

Novel Symmetrical Benzazolyl Derivatives Endowed with Potent Anti-Heparanase Activity

Heparanase is the only mammalian endo-β-d-glucuronidase involved in a variety of major diseases. The up-regulation of heparanase expression increases tumor size, angiogenesis, and metastasis, representing a validated target in the anti-cancer field. To date, only a few small-molecule inhibitors have been described, but none have gotten through pre-clinical development. Previously, we explored 2-(4-(4-(bromo-methoxybenzamido)benzylamino)phenyl) benzazole derivatives as anti-heparanase agents, proposing this scaffold for development of broadly effective heparanase inhibitors. Herein, we report an extended investigation of new symmetrical 2-aminophenyl-benzazolyl-5-acetate derivatives, proving that symmetrical compounds are more effective than asymmetrical analogues, with the most-potent compound, 7g, being active at nanomolar concentration against heparanase. Molecular docking studies were performed on the best-acting compounds 5c and 7g to rationalize their interaction with the enzyme. Moreover, invasion assay confirmed the anti-metastatic potential of compounds 5c, 7a, and 7g, proving the inhibition of the expression of proangiogenic factors in tumor cells.

Rational Design of Push-Pull Fluorene Dyes: Synthesis and Structure-Photophysics Relationship

Our work surveyed experimental and theoretical investigations to construct highly emissive D-π-A (D=donor, A=acceptor) fluorenes. The synthetic routes were optimised to be concise and gram-scalable. The molecular design was first rationalised by varying the electron-withdrawing group from an aldehyde, ketotriazole or succinyl to methylenemalonitrile or benzothiadiazole. The electron-donating group was next varied from aliphatic or aromatic amines to saturated cyclic amines ranging from aziridine to azepane. Spectroscopic studies correlated with TD-DFT calculations provided the optimised structures. The selected push-pull dyes exhibited visible absorptions, significant brightness, important solvatofluorochromism, mega-Stokes shifts (>250 nm) and dramatic shifts in emission to the near-infrared. The current library includes the comprehensive characterization of 16 prospective dyes for fluorescence applications. Among them, several fluorene derivatives bearing different conjugation anchors were tested for coupling and demonstrated to preserve the photophysical responses once further bound.

Tuning the intramolecular charge transfer of alkynylpyrenes: effect on photophysical properties and its application in design of OFF-ON fluorescent thiol probes

Green and yellow-emitting 1,6- and 1,8-bis(phenylethynyl) pyrenes (dyes 7, 8, 9, and 10) with different intramolecular charge transfer (ICT) feature were synthesized and the effect of ICT on the photophysical properties of these derivatives were studied by UV-vis absorption spectra, fluorescence emission spectra, and DFT/TDDFT calculations. For the dyes with electron-pushing group (e.g., -dimethylamino, dye 8 and dye 10), structureless and solvent polarity-sensitive fluorescence emission spectra were observed. Conversely, dye with electron-withdrawing group (e.g., -CN, dye 7) shows structured and solvent polarity-independent emission spectra. OFF-ON fluorescent thiol probes 11 and 12 with 2,4-dinitrobenzenesulfonyl protected ethynylpyrene fluorophore were designed based on DFT/TDDFT calculations, which predicts dark state (S(1)) for these thiol probes (e.g., oscillator strength f = 0.0086 for S(1)<--S(0) transition of the probe 11). This dark state is induced by the ICT effect with ethynylated pyrene fluorophore as electron donor and 2,4-dinitrobenzenesulfonyl unit as electron acceptor. Cleavage of the 2,4-dinitrobenzenesulfonyl unit by thiol releases the free fluorophore, for which the lowest-lying excited state S(1) is no longer a dark state, but an emissive state (f = 0.9776 for S(1)<--S(0) transition). These theoretical predictions on the photophysical properties of the molecular probes were fully proved by experimental results. Our results demonstrated that the fluorescence OFF-ON switching of this kind of thiol probe is due to the termination of the ICT effect (which quenches the emission, by a dark S(1) state) by cleavage of the 2,4-dinitrobenzenesulfonyl unit (as acceptor of ICT effect) with thiols, not the re-establishment of the D-pi-A feature of the fluorophore. These investigation on the pyrene derived green-emitting fluorophores and the DFT/TDDFT calculation aided probe design suggest that future application of these results may prove useful toward the rational design of fluorophores or fluorescent probes with predetermined photophysical properties.