Coronenes, Benzocoronenes and Beyond: Modern Aspects of Their Syntheses, Properties, and Applications

Synthesis and characterizations of highly luminescent 5-isopropoxybenzo[rst]pentaphene

A benzo[rst]pentaphene (BPP) substituted by an isopropoxy group (BPP-OiPr) was synthesized in a facile manner. Its photophysical properties were investigated by UV-vis absorption and fluorescence spectroscopy in compassion to pristine BPP and its oxidation product, benzo[rst]pentaphene-5,8-dione (BPP-dione). BPP-OiPr exhibited a significantly enhanced photoluminescence quantum yield (PLQY), reaching 73% in comparison to pristine BPP (13%). BPP-dione, when compared to the parent BPP, also displayed improved absorption and emission from the first excited singlet (S1) state with a PLQY of 62% and an intramolecular charge-transfer character. The rod-like nano- to microcrystals as well as longer wires of these BPPs were also revealed by scanning electron microscopy. The intriguing optical properties of BPP and its derivatives may lead to their application as fluorophores.

Heterocoronenes Containing Pyridine and 1,2-Azaborine Units

Several coronenes containing pyridine and azaborine units have been readily prepared and structurally confirmed by X-ray crystallographic analysis. The codoping results in interesting findings and properties such as the first observation of BN-H---NPy hydrogen bonds in crystals of BN-PAHs, short π-π stacking distances, lowered HOMO-LUMO levels, narrow band gap, and unique dual response to fluoride ion and proton in solution.

Substituent Effects in Scholl-Type Reactions of 1,2-Terphenyls to Triphenylenes

A series of 3,3''- and 4,4''-dimethoxy terphenyls with different second substituents on their ortho-positions have been synthesized and investigated upon the possibility to be oxidatively cyclodehydrogenated to the corresponding triphenylenes under Scholl-type conditions. The experimentally obtained selectivities were supported and explained by quantum chemical calculations and conclusions on the involved mechanisms (acid catalyzed arenium-ion mechanism (AIM) vs radical cation mechanism) were drawn.

Helix-to-Disc Conversion of Thia[6]helicenes into Coronenes Facilitated by Sulfur Oxidation and Fluorination

Helicenes, with their unique helical structures, have long captured the interest of synthetic chemists, not only as end products, but also as versatile platforms for further chemical transformations. However, transforming [6]helicene into planar coronene typically requires harsh conditions and poses significant challenges. Herein, we demonstrate that replacing the terminal benzene ring of [6]helicene with a thiophene ring enables its photochemical transformation into coronene. Sulfur oxidation of the thiophene ring enables the corresponding thermal transformation, and the terminal tetrafluorination of the opposite benzene ring further accelerates this process, yielding 1,2-difluorocoronene, as confirmed by X-ray crystallography. The transformation begins with an intramolecular Diels-Alder reaction, whose activation energy is significantly lowered by these structural changes. Our findings underscore the utility of strategic modifications such as sulfur oxidation and fluorination in promoting this "helix-to-disc" conversion and opening new avenues for synthesizing functional polycyclic aromatics.

Fluorescence Excitation and Dispersed Fluorescence Spectra of the First Electronic Excited (S1) State of peri-Hexabenzocoronene (C42H18) Isolated in Solid para-Hydrogen

Large polycyclic aromatic hydrocarbons (PAH) and their cationic, hydrogenated, and protonated derivatives have long been considered as promising candidates for the carriers of the diffuse interstellar bands. peri-Hexabenzocoronene (peri-HBC, C42H18) is a large, compact PAH, and, to the best of our knowledge, the largest centrosymmetric all-benzenoid PAH for which electronic spectroscopy data has been published. In this work, we present the dispersed fluorescence and fluorescence excitation spectra of the first electronic excited (S1) state of peri-HBC isolated in solid para-H2 and provide the first detailed vibronic analysis of observed features. The observed spectra agree with the emission and absorption spectra simulated according to optimized geometries and scaled harmonic vibrational frequencies calculated at the density functional theory (DFT) level using a Franck-Condon Herzberg-Teller approach; the spectral bands are associated solely with vibrational normal modes of approximate e2g symmetry and their combinations with vibrational modes of approximately a1g symmetry. We clearly observed the position of the S1-S0 electronic transition origin of peri-HBC at 22,088 cm-1 (452.7 nm), which was unreported previously. The matrix shift of ∼110 cm-1 to the red relative to the gas-phase value was estimated by comparison of two reported gas-phase bands with our work. Because of the significant deviation from the reported wavelengths of DIB, the weakness of the S1-S0 electronic transitions, and the lack of reported DIB at <400 nm where the intense S4 ← S0 band of peri-HBC is located, peri-HBC is unlikely to contribute to DIB.

Supramolecular Polymerization as a Tool to Reveal the Magnetic Transition Dipole Moment of Heptazines

Heptazine derivatives have attracted significant interest due to their small S1-T1 gap, which contributes to their unique electronic and optical properties. However, the nature of the lowest excited state remains ambiguous. In the present study, we characterize the lowest optical transition of heptazine by its magnetic transition dipole moment. To measure the magnetic transition dipole moment, the flat heptazine must be chiroptically active, which is difficult to achieve for single heptazine molecules. Therefore, we used supramolecular polymerization as an approach to make homochiral stacks of heptazine derivatives. Upon formation of the supramolecular polymers, the preferred helical stacking of heptazine introduces circular polarization of absorption and fluorescence. The magnetic transition dipole moments for the S1 ← S0 and S1 → S0 are determined to be 0.35 and 0.36 Bohr magneton, respectively. These high values of magnetic transition dipole moments support the intramolecular charge transfer nature of the lowest excited state from nitrogen to carbon in heptazine and further confirm the degeneracy of S1 and T1.

Coronenes with push-pull geometries from macrocycle-forming Perkin condensations

Although the Perkin reaction has been successful in producing ester-substituted conjugated macrocycles with four or six building blocks, macrocycles composed of only two elements remained elusive until now. Through the development of a building block derived from phenanthrene with two glyoxylic acid substituents in a pincer-like arrangement, formation of a two-block macrocycle was induced when paired with a complementary phenylenediacetic acid unit. The addition of ether functions to the phenanthrene building block not only improved the yields, but led to macrocycles with push-pull geometries. Photocyclisation of the resulting cyclophanes efficiently yield tetra- and hexasubstituted coronenes.

Influence of Steric Effects on the Emission Behavior of Pyrene-Based Blue Luminogens

Pyrene-based derivatives have been widely deployed in organic luminescent materials because of their bright fluorescence, high charge carrier mobility, and facile modification. Nevertheless, the fluorescence output of conventional pyrenes is prone to quenching upon aggregation due to extensive intermolecular π-π stacking interactions. To address this issue, a set of new Y-shaped pyrene-containing luminogens are synthesized from a new bromopyrene chemical precursor, 2-hydroxyl-7-tert-butyl-1,3-bromopyrene, where the bromo and hydroxyl groups at the pyrene core can be readily modified to obtain the target products and provide great flexibility in tuning the photophysical performances. When the hydroxy group at the 2-position of pyrene was replaced by a benzyl group, the steric hindrance of the benzyl group not only efficiently inhibits the detrimental intermolecular π-π stacking interactions but also rigidifies the molecular conformation, resulting in a narrow-band blue emission. Moreover, the TPE-containing compounds 2c and 3c possessed characteristic aggregation-induced emission (AIE) properties with fluorescence quantum yields of up to 66% and 38% in the solid state, respectively. Thus, this article has methodically investigated the factors influencing the optical behavior, such as intermolecular interactions, and the steric effects of the substituent group, thereby opening up the potential to develop narrow-band pyrene-based blue emitters for OLED device applications.

A Metal-Free Triazacoronene-Based Bimodal VOC Sensor

Developing suitable sensors for selective and sensitive detection of volatile organic compounds (VOCs) is crucial for monitoring indoor and outdoor air quality. VOCs are very harmful to our health upon inhalation or contact. Bimodal sensor materials with more than one transduction capability (optical and electrical) offer the ability to extract complementary information from the individual analyte, thus improving detection accuracy and performance. The privilege of manipulating the optoelectronic properties of the polycyclic aromatic hydrocarbon-based semiconducting materials offers rapid signal transduction in multimodal sensing applications. A thiophene-functionalized triazacoronene (TTAC) donor-acceptor-donor (D-A-D) type sensor is reported here for VOC sensing. The single-crystal X-ray structure analysis of the TTAC revealed that a distinctive supramolecular polymer architecture was formed because of cooperative π-π and intermolecular D-A interactions and exhibited rapid signal transduction upon exposure to specific VOCs. The TTAC-embedded green luminescent paper-based test strip exhibited an on-off fluorescence response upon nitrobenzene vapor exposure for 120 s. The selective and rapid response is due to the fast photoinduced electron transfer, as is evident from the time-resolved excited-state dynamics and density functional theory studies. The thick-film-based prototype chemiresistive sensor detects harmful VOCs in a custom-made gas sensing system including benzene, toluene, and nitrobenzene. The TTAC sensor rapidly responds (200 s) at relatively low temperatures (180 οC) compared to other reported metal-oxide-based sensors.

Mesomorphism Modulation of Perfluorinated Janus Triphenylenes by Inhomogeneous Chain Substitution Patterns

Two isomeric series of compounds with "inverted" chains' substitution patterns, 7,10-dialkoxy-1,2,3,4-tetrafluoro-6,11-dimethoxytriphenylene and 6,11-dialkoxy-1,2,3,4-tetrafluoro-7,10-dimethoxytriphenylene, labelled respectively p-TPFn and m-TPFn, and two non-fluorinated homologous isomers, 3,6-dibutoxy-2,7-dimethoxytriphenylene and 2,7-dibutoxy-3,6-dimethoxytriphenylene, p-TP4 and m-TP4, respectively, were synthesized in three steps and obtained in good yields by the efficient transition-metal-free, fluoroarene nucleophilic substitution via the reaction of appropriate 2,2'-dilithium biphenylenes with either perfluorobenzene, C6 F6 , to yield p-TPFn and m-TPFn, or o-difluorobenzene, C6 H4 F2 , for p-TP4 and m-TP4, respectively. The single-crystal structures of p-TPF4, m-TPF4 and p-TP4, unequivocally confirmed that the cyclization reactions occurred at the expected positions, and that the fluorinated molecules stack up into columns with short separation, a propitious situation for the emergence of columnar mesophases. The mesomorphous properties were found to be greatly affected by both chains' length and positional isomerism: a Colhex phase is found for p-TPF4 and m-TPF4, but mesomorphism vanishes in p-TPF6, and changes for the isomeric homologs m-TPFn, with the induction for n≥6 of a lamello-columnar phase, LamColrec . As expected, both non-fluorinated compounds are deprived of mesomorphism. These compounds emit blue-violet colour in solution, independently of the chains' substitution pattern, and the absolute fluorescence quantum yields can reach up to 46 %. In thin films, fluorescence is slightly redshifted.

Tetraazacoronenes and Their Dimers, Trimers and Tetramers

Tetraazacoronenes were synthesized from bay-functionalized tetraazaperylenes by Zr-mediated cyclization and four-fold Suzuki-Miyaura cross coupling. In the Zr-mediated approach, an η4 -cyclobutadiene-zirconium(IV) complex was isolated as an intermediate to cyclobutene-annulated derivatives. Using bis(pinacolatoboryl)vinyltrimethylsilane as a C2 building block gave the tetraazacoronene target compound along with the condensed azacoronene dimer as well as higher oligomers. The series of extended azacoronenes show highly resolved UV/Vis absorption bands with increased extinction coefficients for the extended aromatic cores and fluorescence quantum yields of up to 80 % at 659 nm.

Recent Advances in Applications of Fluorescent Perylenediimide and Perylenemonoimide Dyes in Bioimaging, Photothermal and Photodynamic Therapy

The emblematic perylenediimide (PDI) motif which was initially used as a simple dye has undergone incredible development in recent decades. The increasing power of synthetic organic chemistry has allowed it to decorate PDIs to achieve highly functional dyes. As these PDI derivatives combine thermal, chemical and photostability, with an additional high absorption coefficient and near-unity fluorescence quantum yield, they have been widely studied for applications in materials science, particularly in photovoltaics. Although PDIs have always been in the spotlight, their asymmetric counterparts, perylenemonoimide (PMI) analogues, are now experiencing a resurgence of interest with new efforts to create architectures with equally exciting properties. Namely, their exceptional fluorescence properties have recently been used to develop novel systems for applications in bioimaging, biosensing and photodynamic therapy. This review covers the state of the art in the synthesis, photophysical characterizations and recently reported applications demonstrating the versatility of these two sister PDI and PMI compounds. The objective is to show that after well-known applications in materials science, the emerging trends in the use of PDI- and PMI-based derivatives concern very specific biomedicinal applications including drug delivery, diagnostics and theranostics.

Edge-Decorated Polycyclic Aromatic Hydrocarbons by an Oxidative Coupling Approach

Oxidative phenol coupling reduces reliance on halo/metalated substrates used in conventional redox neutral couplings. A new strategy for constructing polycyclic aromatic hydrocarbons (PAHs) that incorporates oxidative phenol coupling is outlined in a three-stage approach: oxidative fragment coupling, linking of the two resultant units, and oxidative cyclization. The protocol allows rapid assembly of both planar and helical systems with a high degree of edge functionalization. The incorporation of 12 alkoxy groups on systems with 12 rings gave rise to lower optical gaps compared to systems with a lesser degree of edge functionalization.

Solvation of Large Polycyclic Aromatic Hydrocarbons in Helium: Cationic and Anionic Hexabenzocoronene

The adsorption of helium on charged hexabenzocoronene (Hbc, C42H18), a planar polycyclic aromatic hydrocarbon (PAH) molecule of D6h symmetry, was investigated by a combination of high-resolution mass spectrometry and classical and quantum computational methods. The ion abundance of HenHbc+ complexes versus size n features prominent local anomalies at n = 14, 38, 68, 82, and a weak one at 26, indicating that for these "magic" sizes, the helium evaporation energies are relatively large. Surprisingly, the mass spectra of anionic HenHbc- complexes feature a different set of anomalies, namely at n = 14, 26, 60, and 62, suggesting that the preferred arrangement of the adsorbate atoms depends on the charge of the substrate. The results of our quantum calculations show that the adsorbate layer grows by successive filling of concentric rings that surround the central benzene ring, which is occupied by one helium atom each on either side of the substrate. The helium atoms are fairly localized in filled rings and they approximately preserve the D6h symmetry of the substrate, but helium atoms in partially filled rings are rather delocalized. The first three rings contain six atoms each; they account for magic numbers at n = 14, 26, and 38. The size of the first ring shrinks as atoms are filled into the second ring, and the position of atoms in the second ring changes from hollow sites to bridge sites as atoms are filled into the third ring. Beyond n = 38, however, the arrangement of helium atoms in the first three rings remains essentially frozen. Presumably, another ring is filled at n = 68 for cations and n = 62 for anions. The calculated structures and energies do not account for the difference between charge states, although they agree with the measurements for the cations and show that the first solvation shell of Hbc± is complete at n = 68. Beyond that size, the adsorbate layer becomes three-dimensional, and the circular arrangement of helium changes to hexagonal.

Plasmonic Surface of Metallic Gold and Silver Nanoparticles Induced Fluorescence Quenching of Meso-Terakis (4-Sulfonatophenyl) Porphyrin (TPPS) and Theoretical-Experimental Comparable

Colloidal metallic nanoparticles have attracted a lot of interest in the last two decades owing to their simple synthesis and fascinating optical properties. In this manuscript, a study of the effect of both gold nanoparticles (Au NPs) and silver nanoparticles (Ag NPs) on the fluorescence emission (FE) of TPPS has been investigated utilizing steady-state fluorescence spectroscopy and UV–Vis spectrophotometry. From the observed electronic absorption spectra, there is no evidence of the ground state interaction between metallic Au NPs or Ag NPs with TPPS. On the other side, the FE spectra of TPPS have been quenched by both Ag and Au NPs. Via applying quenching calculations, Ag NPs showed only traditional static fluorescence quenching of TPPS with linear Stern–Volmer (SV) plots. On the contrary, quenching of TPPS emission by Au NPs shows composed models. One model is the sphere of action static quenching model that prevails at high quencher concentrations leading to non-linear SV plots with positive deviation. However, at low Au NPs concentrations, traditional dynamic quenching occurs with linear SV plots. The quantum calculations for TPPS structure have been obtained using Gaussian 09 software: in which the TPPS optimized molecular structure was achieved using DFT/B3LYP/6-311G (d) in a gaseous state. Also, the calculated electronic absorption spectra for the same molecule in water as a solvent are obtained using TD/M06/6-311G +  + (2d, 2p). Furthermore, the theoretical and experimental results comparable to UV–Vis spectra have been investigated.

Synthesis and Properties of BN-embedded N-Perylene

A B-N embedded nitrogen-annulated perylene has been successfully synthesized. The resultant molecule BN-NP is isoelectronic to coronene , but owns a five-membered pyrrole ring. Experiments and DFT calculations indicated that peripheral pyrrole and BN modifications endow BN-NP with various unique properties like bent structure, dual emission, efficient Lewis acidic response, peripheral aromaticity, narrowest energy band gap among all coronene isoelectronic structures and so on.

Synthesis, Structure, and Photophysical Properties of BN-Embedded Analogue of Coronene

Two BN-embedded benzo[ghi]perylene (Bzp) and coronene derivatives (BN-Bzp and BN-Cor) have been successfully synthesized from binaphthyl precursors by new efficient one-pot-multibond routes, and their single crystal structures were analyzed. Both experimental spectra and DFT theoretical calculations indicated that the absorption and emission of these BN-embedded polycyclic aromatic hydrocarbons are significantly enhanced comparing with those of their all carbon analogues. Especially, the fluorescence quantum yield of BN-Cor is nearly 20 times higher than that of ordinary coronene.

Coronene-based quantum dots for the delivery of the doxorubicin anticancer drug: a computational study

The coronene family could serve as a useful platform for the delivery of and tracking the release of the anticancer DOX drug.

Contorted Heteroannulated Tetraareno[a,d,j,m]coronenes

Fused polycyclic aromatic compounds are interesting materials for organic electronics applications. To fine-tune photophysical or electrochemical properties, either various substituents can be attached or heteroatoms (such as N or S) can be incorporated into the fused aromatic backbone. Coronenes and heterocoronenes are promising compounds in this respect. Up until now, the possibilities for varying the attached fused heteroaromatics at the coronene core were quite limited, and realizing both electron-withdrawing and -donating rings at the same time was very difficult. Here, a series of pyridine, anisole and thiophene annulated tetraareno[a,d,j,m]coronenes has been synthesized by a facile two-step route that is a combination of Suzuki-Miyaura cross-coupling and a following cyclization step, starting from three different diarenoperylene dibromides. The contorted molecular π-planes of the obtained cata-condensed tetraarenocoronenes were analyzed by single-crystal X-ray crystallography, and the photophysical and electrochemical properties were systematically investigated by UV/Vis spectroscopy and cyclovoltammetry.