Guest Molecular Assembly Strategy in Covalent Organic Frameworks for Electrochemiluminescence Sensing of Uranyl

The application of covalent organic frameworks (COFs) in electrochemiluminescence (ECL) is promising in environmental monitoring. Developing an emerging design strategy to expand the class of COF-based ECL luminophores is highly desirable. Here, a COF-based host-guest system was constructed through guest molecular assembly to deal with nuclear contamination analysis. The efficient charge transport network was formed by inserting an electron-withdrawing guest tetracyanoquinodimethane (TCNQ) into the open space of the COF host (TP-TBDA; TP = 2,4,6-trihydroxy-1,3,5-benzenetricarbaldehyde and TBDA = 2,5-di(thiophen-2-yl)benzene-1,4-diamine) with an electron-donating property; the construction of the COF-based host-guest system (TP-TBDA@TCNQ) triggered the ECL emission of non-emitting TP-TBDA. Furthermore, the dense active sites in TP-TBDA were utilized to capture the target substance UO₂²⁺. The presence of UO₂²⁺ broke the charge-transfer effect in TP-TBDA@TCNQ, resulting in the weakening of the ECL signal, thus the established ECL system integrating the low detection limit with high selectivity monitors UO₂²⁺. This COF-based host-guest system provides a novel material platform for constructing late-model ECL luminophores and creates an opportunity for the vigorous ECL technology.

Internal conversion rate constant calculations considering Duschinsky, anharmonic and Herzberg-Teller effects

A novel method for calculating rate constants for internal conversion (k_IC) that simultaneously accounts for Duschinsky, anharmonic and Herzberg-Teller effects has been developed and implemented. This method has been applied to robust planar molecules like tetraoxa[8]circulene (4B), free-base porphyrin (H2P) and pyrometene (PM567) with small Duschinsky rotation (i.e. with almost identical normal coordinates in the ground and excited states) and to poly[n]fluorenes (P[n]F) (n = 2-14) with a substantial Duschinsky rotation. The obtained results show that the Duschinsky effect is large in the harmonic approximation, whereas it is in general much smaller in the anharmonic approximation. The Duschinsky effect is found to be large for high frequency vibrational modes with energies of ∼3300 cm^-1 such as the X-H (X = C, N and O) stretching modes that mix in the S₁ → S₀ electronic transition. However, even in this case, the increase in k_IC due to the Duschinsky effect does not exceed one order of magnitude. The calculations show that anharmonic contributions to k_IC are larger than Herzberg-Teller contributions which in turn are larger than contributions from the Duschinsky effect ANH > HT > Du. We also show that an approximation, where only X-H bonds are considered in the k_IC calculation, is accurate even for P[n]F (n = 2-14).

Photophysics, Electrochemistry and Efficient Electrochemiluminescence of Trigonal Truxene-Core Dyes

We synthesized and characterized a series of dyes built from a spirofluorene or truxene core. The quadrupolar spirofluorene system is the initial building unit for the design and preparation of more complex star-shaped dyes consisting of a truxene core bearing three di- or triphenylamine moieties with or without a thiophene connector. Their photophysical, electrochemical, and electrochemiluminescence (ECL) properties were first investigated in solution. Structure/activity relationships were derived and rationalized by comparing the quadrupolar system and trigonal truxene-core derivatives using computational studies. The photophysical and redox characteristics are drastically tuned by the introduction of a thiophene bridge and electron-donor substituents at their terminal branches. These comparative studies show the essential role of the stability of both radical cations and anions to obtain efficient ECL dyes. The stabilization of the radicals is directly related to the charge delocalization due to the π-conjugation by the thiophene bridge. The brightest ECL is achieved by annihilation and coreactant (benzoyl peroxide) pathways with the blue-emitting truxene dye, which is 2- and 4.5-times greater than that of the quadrupolar compound and reference [Ru(bpy)₃ ]²⁺ emitter, respectively. Such an extensive study on these extended π-conjugated molecules presenting different core structures may guide the design and synthesis of new ECL dyes with a strong efficiency.

An oxacalix[4]arene derived dual sensing fluorescent probe for the detection of As(v) and Cr(vi) oxyanions in aqueous media

An oxacalix[4]arene-Ce(iii) complex viz. L-Ce(III) has been introduced for the selective detection of As(v) and Cr(vi) oxyanions in aqueous medium. The binding mode of L-Ce(III) + AsO₄^3-/CrO₄^2- was completely investigated with fluorometric titration, time resolve fluorescent decay and FTIR analyses. Photoinduced electron transfer (PET) and chelation-enhanced fluorescence (CHEF) play an important role in the sensing of these oxyanions. The characteristic fluorescence of the L-Ce(III) complex has been quenched by AsO₄^3- and CrO₄^2- through cascading the ligating sites. Cyclic voltammetry (CV) experiments with various scan rates suggest that the electrochemical processes on the electrodes were controlled by diffusion. Both the analytes exhibit a lower limit of detection (LOD) below their standard EPA permissible limits. Moreover, the probe successfully detects the oxyanions in environmental real samples with excellent recovery ranging from 97 to 101%.

Heterocycle Effects on the Liquid Crystallinity of Terthiophene Analogues

Liquid crystalline self-assembly offers the potential to create highly ordered, uniformly aligned, and defect-free thin-film organic semiconductors. Analogues of one of the more promising classes of liquid crystal semiconductors, 5,5"-dialkyl-α-terthiophenes, were prepared in order to investigate the effects of replacing the central thiophene with either an oxadiazole or a thiadiazole ring. The phase behaviour was examined by differential scanning calorimetry, polarized optical microscopy, and variable temperature x-ray diffraction. While the oxadiazole derivative was not liquid crystalline, thiadiazole derivatives formed smectic C and soft crystal lamellar phases, and maintained lamellar order down to room temperature. Variation of the terminal alkyl chains also influenced the observed phase sequence. Single crystal structures revealed the face-to-face orientation of molecules within the layers in the solid-state, a packing motif that is rationalized based on the shape and dipole of the thiadiazole ring, as corroborated by density functional theory (DFT) calculations. The solution opto-electronic properties of the systems were characterized by absorption and emission spectroscopy, cyclic voltammetry, and time-dependent density functional theory (TD-DFT).

CdSe (quantum dots)–graphene oxide system for thiophene polymerization: a new strategy, a new material

This study presents a novel method for the oxidative polymerization of thiophene (T) by employing cadmium selenide (CdSe) quantum dots and CdSe–graphene oxide (GO) as activators of the polymerization system.

Contrasting electrogenerated chemiluminescence for a dissolved and surface-attached carbazole thiophene cyanoacrylate dye

The electrogenerated chemiluminescence (ECL) of a carbazole thiophene cyanoacrylate dye ((2-cyano-3-[5"'-(9-ethyl-9H-carbazol-3-yl)-3',3",3"',4-tetra-n-hexyl-[2,2',5',2",5",2"']-quarter-thiophenyl-5yl]acrylate) = MK-2) has been investigated in solution, where the maximum ECL wavelength occurs at 640 nm, and in a thin film on an ITO surface, where the ECL is substantially red-shifted to 730 nm. The ECL intensity for the solution annihilation reaction is relatively weak, whereas a much higher ECL intensity is measured with oxalate as a co-reactant. This result is attributed to the two Nernstian reversible oxidation waves of the thiophene moiety of MK-2, whereas the reduction is stabilized by the unblocked carbazole and cyanoacrylate groups.

NIR electrochemiluminescence from Au25− nanoclusters facilitated by highly oxidizing and reducing co-reactant radicals

By means of spooling spectroscopy, we demonstrate our discovery on near-infrared electrochemiluminescence of Au₂₅⁻ nanoclusters at 950 and 900 nm, which can be attributed to the Au₂₅⁻*, Au₂₅⁰* and Au₂₅⁺* excited species being controllable chemically and electrochemically.