Azacalixquinarenes: From Canonical to (Poly-)Zwitterionic Macrocycles

Fluorescence emission mechanism for the π-conjugated zwitterion 2,4-Bisimidazolylphenol base on ESIPT: A TDDFT theoretical reconsideration

Molecules with zwitterionic characteristics exhibit significant potential for utilization in nonlinear optics, optoelectronics, and organic lasers owing to their large dipole moments. Recently, the synthesized compound 2,4-bis (4,5-diphenyl-1H-imidazol-2-yl) phenol (2,4-bImP) by Sakai et al. has been noticed for its unique photochromic properties in solvents [J. Phys. Chem. A, 125 (2021), 4784-4792]. The observed fluorescence in chloroform was attributed to the keto tautomer. Based on the excited state intramolecular proton transfer, the photochromism of 2,4-bImP in chloroform was interpreted as zwitterion production. However, the zwitterion with a specific electronic structure can be in resonance with the conventional neutral structure. The impact of the resonance contribution from the zwitterion and the conventional neutral structure on fluorescence attribution was not taken into account in the previous studies. In this investigation, the ESIPT mechanism of the 2,4-bImP in chloroform has been explored using both the density functional theory and the time-dependent density functional theory. The optimized geometric configuration parameters illustrate the molecular resonant properties. The calculated fluorescence spectra on the basis of the optimization results further corroborate that the fluorescence peaks after proton transfer originates from the resonance of the zwitterionic and the neutral configuration. The zwitterionic nature of the molecule was demonstrated by electrostatic potential and atomic dipole modified Hesfeld atomic charge (ADCH) analysis. Furthermore, the characterization of potential energy curves and IR spectrum further verified the resonance of both the zwitterionic and neutral structures. The results reveal that the 2,4-bImP molecule generates the neutral o-quinoid structure and the zwitterionic structure resonance phenomenon following ESIPT. The aforementioned resonance structure offers novel insights into the ascription of fluorescence. These discoveries establish the theoretical foundation for the exploration and development of zwitterions.

Insights into extended coupled polymethines through the investigation of dual UV-to-NIR acidochromic switches based on heptamethine-oxonol dyes

A series of heptamethine-oxonol dyes featuring different heterocyclic end groups were designed with the aim to explore structure-property relationships in π-extended coupled polymethines. These dyes can be stabilised under three different protonation states, affording dicationic derivatives with an aromatic core, cationic heptamethines, and zwitterionic bis-cyanine forms. The variation of the end groups directly impacts the absorption and emission properties and mostly controls reaching either a colourless neutral dispirocyclic species or near-infrared zwitterions. The acidochromic switching between the three states involves profound electronic rearrangements leading to notable shifts of their optical properties that were investigated using a parallel experiment-theory approach, providing a comprehensive description of these unique systems.

Breaking Azacalix[4]arenes into Induline Derivatives

Tetraamino-tetranitro-azacalixarene 5 is at the crossroad of two different families of compounds depending on the conditions and the agent used to reduce the NO2 groups: (1) azacalixphyrin 7 in neutral medium, or (2) phenazinium of type 8 in acidic medium. The key role of the N-substituted amino functions at the periphery is highlighted by investigating octaaminoazacalixarene as a model compound, and by using the corresponding tetrahydroxy-tetranitro-azacalixarene 15 as a precursor, which behaves differently.

Molecular rotor based on an oxidized resorcinarene

Rate of rotation of substituents in a molecular single stator-double rotor based on an oxidized resorcinarene with unsaturated hemiquinonoid groups at its meso positions (i.e., a fuchsonarene) has been controlled according to solvent polarity and acidity.

Modified Indulines: From Dyestuffs to In Vivo Theranostic Agents

The efficient, versatile, and straightforward synthesis of the first N-alkyl analogues of induline 3B (8a and 8b) is reported. Thanks to the introduction of lipophilic substituents and their attractive photophysical properties (far-red emission and production of singlet oxygen), phenazinium 8b can be used as a theranostic agent and shows, at very low concentrations (100 nM), a remarkable ability to (i) image cells and zebrafish embryos with high quality under both mono- (514 nm) and biphotonic (790 and 810 nm) excitations, (ii) efficiently and quickly penetrate cancer cells rather than healthy fibroblasts, and (iii) induce a total or almost total cancer cell death in vitro and in vivo after illumination (λ_exc = 540-560 nm). The molecular structure of 8b is based on a triamino-phenazinium core only, with no need for additional components, highlighting the emergence of a minimalistic and versatile class of fluorescent probes for targeted photodynamic cancer therapy.

Stabilization of a 12-π electrons diamino-benzoquinonediimine tautomer

We explore tautomerism and its consequences in the first example of diamino-benzoquinonediimine bearing both electron-donating and electron-withdrawing groups on the same 6-π electron subunit.

Small Panchromatic and NIR Absorbers from Quinoid Zwitterions

The transamination of oxoaminobenzoquinonemonoimine (BQI derivatives), an unconventional zwitterionic quinone, allows isolation of a series of compounds featuring electron-donating aryl auxochromes. The substitution has a very strong impact on the electrochemical and optical features, which is rationalized by theoretical calculations. Protonation and alkylation of the BQIs toward the corresponding cations lead to surprising red-shifts of the absorption, especially in the instance of the most electron-rich dyes that exhibit panchromatic absorption spanning up to the near-infrared (NIR) region, a remarkable achievement for such small molecules.

MixedN-aryl/alkyl substitution favours an unusual tautomer of near-infrared absorbing azacalixphyrins

This work depicts the synthesis of a new azacalixphyrin presenting both aryl and alkyl substituents. The joint experimental and theoretical study supports that the substitution pattern favour an unusual 5–7 tautomer.

One-step synthesis of cross-linked and hollow microporous organic-inorganic hybrid nanoreactors for selective redox reactions

Hollow microporous nanostructures (HMNs) are powerful platforms for multiple promising applications, including energy storage, drug/gene delivery, nanoreactors/catalysis, adsorption, and separation. Herein, we report a facile one-step method to synthesize highly cross-linked organic-inorganic hybrid poly(cyclotriphosphazene-co-4,4'-sulfonyldiphenol) (PZS) HMNs via a salt-induced liquid-liquid separation process. The size of inner cavities can be properly tuned by modulating the concentration of the NaOH solution. The regulation mechanism of the PZS HMNs was further confirmed by encapsulating water-dispersed Pt nanoclusters into the cavities. Interestingly, the resulting yolk-shell Pt@PZS serves as nanoreactors and exhibits excellent substrate selectivity and recyclability for the catalytic oxidation of 1,3,5-trimethylbenzene.

First principles investigation of the spectral properties of neutral, zwitterionic, and bis-cationic azaacenes

An in-depth investigation of the optical properties of recently-synthesized linear azaacene derivatives of various electronic nature (neutral, dicationic, and zwitterionic) is presented.