Pentagon-Containing π-Expanded Systems: Synthesis and Photophysical Properties

Straightforward Synthesis of Pentagon-Embedded Expanded [11]Helicenes for Radiative Cooling Property

Two new pentagon-embedded carbo[11]helicenes have been designed and synthesized in a three-step process, which are the first example of carbo[11]helicenes through the post-functionalization of twistacene. TD-DFT analyses indicate that both of them possess high enantiomerization barriers of 42.29 kcal/mol and 40.76 kcal/mol, respectively. They emit strong red fluorescence and can be chemically oxidized into stable cationic radicals upon addition of AgSbF6 evidenced by the bathochromic-shifted absorption spectra and the appearance of electronic paramagnetic resonance (EPR) signals. In addition, such helical derivatives can be chosen as radiative cooling materials in a glass model house, and the maxima of 5.4 °C for the former and 6.5 °C for the latter are found in the comparative tests, which might be caused by the NIR reflective response.

A Negatively Curved π-Expanded Pyracylene Comprising a Tropylium Cation

We disclose π-expanded pyracylenes and their cationic species comprising 7-membered rings. The compounds were synthesized by stepwise oxidative cyclodehydrogenation to monitor the effect of successive cyclization on the structural and optoelectronic properties. As shown by X-ray crystallography, the complete cyclization leads to a boat-shaped scaffold featuring negative curvature provided by the 7-membered ring. The embedded tropone unit enabled the convenient generation of a stabilized tropylium cation, showing bathochromically shifted absorption bands reaching into the near-infrared region beyond 1000 nm. The altered structural features, supported by theoretical calculations, point towards the positively charged 7-membered ring having aromatic character.

Heptagon-Embedded Helicene Derivatives: Synthesis, Crystal Structural Analyses, and Circularly Polarized Luminescence

Two pairs of isomers of heptagon-embedded helical arenes (3/6 and 10/13) have been strategically prepared, where the molecular structures of 3 and 13 have been identified through single crystal X-ray diffraction analysis. The effect of the heptagon unit on the physical properties of 3, 6, 10, and 13 is investigated in a comparative manner, and the results indicate that the optical enantiomers of 13 obtained from HPLC exhibit promising chiroptical properties.

Fluorescent boron difluoride curcuminoides as perspective materials for bio-visualization

Curcuminoids of boron difluoride, 1-aryl(hetaryl)-5-phenylpenta-2,4-dien-1-onates of boron difluoride, have been synthesized. A comparative study of the electronic structure, luminescent properties and their potential for applications in bio-imaging has been carried out. The influence of the electronic structure of α-substituents on the luminescence of compounds was studied by the methods of stationary and time-resolved luminescence spectroscopy and DFT modeling. The introduction of π-donor substituents leads to a noticeable bathochromic shift and an increase in the Stokes shift in the luminescence spectra. On going from σ-donor substituents in the phenyl ring to π-donor substituents, the luminescence quantum yield increases from 0.03 to 0.22. The maximum Stokes shift and high quantum yield of luminescence is exhibited by the complex with a stilbene substituent, which has the longest π-system and the maximum efficiency of charge transfer. Dyes are able to penetrate into the cells of the model cell line and accumulate, moreover, accumulation occurs mainly in the cytoplasm of cells. The compounds penetrate into the cells by 12 h of incubation without damaging it's structure and without causing rapid cell death. The submicromolar range of non-toxic concentrations during long-term incubation for a model cell line was determined, which is a characteristic of fluorescent imaging. Due to uniform distribution in the cytoplasm of cells dye with naphtyl substituent is promising for visualization of the cell cytoplasm. This leader compound has the lowest cytotoxicity for cells from the synthesized series of dyes, which makes it promising for further studies as a fluorescent imaging agent. The leader compound has the lowest cytotoxicity for cells from the synthesized series of dyes, which makes it promising for further studies as a fluorescent imaging agent.

Enhanced Ultrafast Broadband Reverse Saturable Absorption in Twistacenes with Enlarged π-Conjugated Central Bridge

Optical nonlinearities of two all-carbon twistacenes, DPyA and DPyN, with the different π-conjugated central bridges were investigated. The nonlinear absorption properties of these compounds were measured using the femtosecond Z-scan with wavelengths between 650 and 900 nm. It has been found that the nonlinear absorption originated from two-photon absorption (TPA) and TPA-induced excited state absorption (ESA), wherein DPyA demonstrates higher performance than DPyN. The TPA cross section of DPyA (4300 GM) is nearly 4.3 times larger than that of DPyN at 650 nm. Moreover, the different central structures modulate the intensity of ESA at 532 nm, and DPyA exhibits an excellent ESA at 532 nm with multi-pulse excitation. Meanwhile, the result of data fitting and quantum chemistry calculation shows that the enhancement of nonlinear absorption in DPyA is due to the extended π- conjugated bridge and improved delocalization of π-electrons. These all-carbon twistacenes could yield potential applications in optical power limiting (OPL) technology.

Inducing Curvature to Pyracylene upon π‐Expansion

We disclose a successive π‐expansion of pyracylene towards boat‐shaped polycyclic scaffolds. The unique structural features of the resulting compounds were revealed by X‐ray crystallographic analysis. Depending on the extent of π‐expansion the compounds display intense bathochromically shifted absorption bands in their UV/Vis spectra and are prone to several redox events as documented by cyclic voltammetry. The experimental observations are in line with the computational studies based on density functional theory, suggesting progressive narrowing of the HOMO–LUMO gap and distinct evolution of the electronic structure and aromaticity.

Double π-Extended Helicene Derivatives Containing Pentagonal Rings: Synthesis, Crystal Analyses, and Photophysics

Two novel double π-extended [n]helicene (n = 5, 6) derivatives (7b and 9) including pentagonal rings have been synthesized and characterized. Both of them have three isomers containing two enantiomers (P₆), (M₆), and a diastereoisomer in a meso form (P,M). X-ray single crystal analyses suggest that molecules 7b and 9 exhibit offset packing models of (P₆,P₅)- and (M₆,M₅)-isomers. Optical resolution of the resultant compound 7b was finished, and their chiroptical properties, as well as the DFT calculations, were also examined.