Electrogenerated Chemiluminescence of Cationic Triangulene Dyes: Crucial Influence of the Core Heteroatoms

Reaction-based energy level modulation of a cyclometalated iridium complex for electrochemiluminescent detection of formaldehyde

Formaldehyde is a toxic compound present in both the environment and living systems, and its detection is important due to its association with various pathological process. In this study, we report a new electrochemiluminescence (ECL) probe based on a cyclometalated iridium complex (IrHAA) for the selective detection of formaldehyde. The homoallylamine moiety in IrHAA reacts with formaldehyde, undergoing a 2-aza-Cope-rearrangement reaction to form a formyl group. Significant changes in the electronic properties and molecular orbital energies of the iridium complex through the functional group transformation result in enhanced ECL and radiometric phosphorescence changes, enabling the quantitative and selective detection of formaldehyde. The energetic requirements for ECL sensing were investigated, highlighting the importance of the excited state energy for achieving efficient ECL. The sensing mechanism was elucidated using NMR spectroscopy and MALDI-TOF analysis.

Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes ‐ Access to Red Emitters and Planar Chiral Stereochemical Traits

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late‐stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron‐donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near‐infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG‐functionalized derivatives, C≡C− functionalized products remain broadly fluorescent, with red‐shifted absorptions (Δλ_abs up to 25 nm) and emissions (Δλ_em up to 73 nm, Φ_PL up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in ¹H NMR spectroscopy (218 K), N−CH₂ protons become diastereotopic with chemical shifts differences (Δδ) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of ∼12 kcal mol⁻¹. This phenomenon is due to planar chiral conformations (S_p and R_p configurations), induced by the geometry of the alkyl (n‐propyl) side‐chains next to the acetylenic substituents. Ion pairing studies with Δ‐TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.

Triangulenium dyes: the comprehensive photo-absorption and emission story of a versatile family of chromophores

The triangulenium dyes constitute a family of versatile chromophores whose impressive photo-absorption and emission properties are currently highlighted in numerous novel experimental applications. In this investigation, we provide a comprehensive TDDFT characterization of their spectroscopic properties elucidating the origin of their large and complex absorption and emission vibronic spectra spread over the (whole) visible region. More precisely, by benchmarking the performance of 10 commonly-used exchange-correlation density functionals belonging to different classes of approximation, we develop and validate a computational protocol allowing the accurate modeling of both the position and optical line-shape of their vibrationally-resolved absorption and emission band structures. We find that semilocal approximations provide the best estimate of the structure of the vibronic spectra, however they spuriously and strongly underestimate their position. We finally show that global-hybrid density functionals mixing between 20 and 30% of exact-like exchange are an excellent compromise to get a satisfactory estimate of both of these properties.

Synthesis, Resolution, Configurational Stability, and Properties of Cationic Functionalized [5]Helicenes

A straightforward approach to the synthesis of two different series of cationic [5]helicenes has been achieved including, in dioxa series, the possibility to introduce aromatic functional groups at the periphery of the helical structure. While photophysical study highlights that the introduction of aryl substituents at position 23 of the helical moieties has a negligible impact on the optical properties, styryl substituents allow a welcoming extension of the conjugation pathways. Finally, a red shift of the optical properties was evidenced upon introduction of nitrogen atoms in the helicene scaffold, leading to particularly good fluorescence efficiencies in the red domain for a helicenic dye. Detailed information on racemization kinetics was collected for the most stable species upon direct high-performance liquid chromatography (HPLC) resolution or, when configurational lability was too high, through VT-HPLC analysis on the chiral stationary phase (ΔG^‡ values ranging from 85.0 to 137.1 kJ·mol^-1 and above).

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.

Self-enhanced multicolor electrochemiluminescence by competitive electron-transfer processes

Controlling electrochemiluminescence (ECL) color(s) is crucial for many applications ranging from multiplexed bioassays to ECL microscopy. This can only be achieved through the fundamental understanding of high-energy electron-transfer processes in complex and competitive reaction schemes. Recently, this field has generated huge interest, but the effective implementation of multicolor ECL is constrained by the limited number of ECL-active organometallic dyes. Herein, the first self-enhanced organic ECL dye, a chiral red-emitting cationic diaza [4]helicene connected to a dimethylamino moiety by a short linker, is reported. This molecular system integrates bifunctional ECL features (i.e. luminophore and coreactant) and each function may be operated either separately or simultaneously. This unique level of control is enabled by integrating but decoupling both molecular functions in a single molecule. Through this dual molecular reactivity, concomitant multicolor ECL emission from red to blue with tunable intensity is readily obtained in aqueous media. This is done through competitive electron-transfer processes between the helicene and a ruthenium or iridium dye. The reported approach provides a general methodology to extend to other coreactant/luminophore systems, opening enticing perspectives for spectrally distinct detection of several analytes, and original analytical and imaging strategies.

Electronic structure and magnetic properties of the triangular nanographenes with radical substituents: a DFT study

DFT modeling of triangular polycyclic hydrocarbons bearing radicals provided insights into dependence of electronic ground states on their structural peculiarities

Extended Triangulenium Ions: Syntheses and Characterization of Benzo-Bridged Dioxa- and Diazatriangulenium Dyes

The very limited class of fluorophores, with a long fluorescence lifetime (>10 ns) and fluorescence beyond 550 nm, has been expanded with two benzo-fused triangulenium derivatives and two cationic [5]-helicene salts. The syntheses of the benzo-bridged dioxa- and diazatriangulenium derivatives (BDOTA⁺ and BDATA⁺, respectively) required two different synthetic approaches, which reflect the structural and physiochemical impact on the reactivity of [5]-helicenium precursors. Spectroscopic investigations show that the introduction of the benzo bridge into the triangulenium chromophore significantly redshifts the absorption and emission while maintaining fluorescence lifetimes above 10 ns. The combination of a high quantum yield, long fluorescence lifetime, and emission above 600 nm is possible only if the structural aspects of the triangulenium framework are perfectly harmonized to secure a low rate of nonradiative deactivation. The new benzo bridge may be a general motif to obtain red-shifted derivatives of other dye classes.

Specific labeling of mitochondria of Chlamydomonas with cationic helicene fluorophores

Twelve cationic helicenes and one triangulene were tested for the specific labeling of mitochondria from algal cells. Octyl ester derivative 5 readily penetrates algal cells and gives rise to clear fluorescence patterns when it is used at concentrations in the μM range. Under these conditions, cell structures are well preserved and cell survival is not compromised. Cationic helicene compounds such as 5 provide new useful tools for examining the mitochondrial network and its dynamics including fission and fusion events.

Electrogenerated chemiluminescence of black phosphorus nanosheets and its application in the detection of H2O2

Black phosphorus nanosheets (BPNS) were synthesized from BP crystals through liquid exfoliation coupled with ultrasonic methods under aqueous conditions.

C-Functionalized Cationic Diazaoxatriangulenes: Late-Stage Synthesis and Tuning of Physicochemical Properties

A series of nine C-functionalized cationic diazaoxatriangulene (DAOTA) dyes have been successfully synthesized and fully characterized, including X-ray structural analysis of four derivatives. The introduction of electron-withdrawing or -donating functions enables the tuning of both electro- and photochemical properties with, for instance, two consecutive (reversible) reductions or oxidations observed for nitro or amino derivatives, respectively. The substituents also impacted on the optical properties, with absorption maxima varying from λ=528 to 640 nm and fluorescence being shifted from the yellow to the red range, up to λ=656 nm.

Influence of electronic and molecular structure on the fragmentation dynamic of even-electron carbocationic triangulenes and helicenes in the gas phase

Stable, long-lived organic cations are directly transferred by electrospray ionization (ESI) from solution into the gas phase where their collision-induced dissociations (CID) are studied by tandem mass spectrometry. Three related types of triphenyl carbenium ions are investigated, in which the meta positions are either substituted by methoxy groups or tertiary nitrogen bridges, including tetramethoxyphenylacridinium (TMPA⁺ ), dimethoxyquinacridinium (DMQA⁺ ), and triazatriangulenium (TATA⁺ ) cations. These ions are triangular in shape with increasing degrees of planarity. Fragmentation occurs at the periphery of the triangular molecule, involving the methoxy groups and the substituent of the nitrogen bridge. Each initial precursor cation is an even electron (EE) system and shows competing dissociations into both even (EE) and odd electron (OE) fragment ions. The latter reaction is a breach of the classic 'even-electron rule' in mass spectrometry. While the EE fragment dissociates similar to the precursor, the OE fragment ion shows a rich radical-induced fragmentation pattern. Two driving forces direct the fragmentation of the EE precursor ion toward OE fragment ions, including the release of stabilized radicals and the extension of the π-system by increasing planarization of the triangulene core. Copyright © 2017 John Wiley & Sons, Ltd.

Stability of Odd- Versus Even-Electron Gas-Phase (Quasi)Molecular Ions Derived from Pyridine-Substituted N-Heterotriangulenes

Electrospray ionisation of N-heterotriangulenes (i.e., dimethylmethylene-bridged triphenylamines) with up to three pyridyl groups at their periphery, produces the true radical cation ([M]^{+ .} ) and the protonated molecule ([M+H]⁺ ) simultaneously. These ions are studied as model systems to illustrate the stability alternation of odd- versus even-electron ions in energy-dependent collision-induced dissociation (CID) experiments. All ions show the same fragmentation pattern, the consecutive loss of three methyl radicals (^. CH₃ ) from the dimethylmethylene bridges of the central triangulene core. [M]^{+ .} ions dissociate at much lower collision energies than their [M+H]⁺ counterparts. The radical cation forms a singlet fragment with an extended aromatic system that is energetically favoured. Ab initio and density functional theory calculations support this interpretation and allow the assignment of the electronic structure of the fragment ions. Consecutive collision-induced dissociations provide a better match with theory when studied with an ion trap, rather than a linear quadrupole. This is attributed to the resonant nature of the excitation of intermediate ions.

Azadioxatriangulenium and Diazaoxatriangulenium: Quantum Yields and Fundamental Photophysical Properties

Over the last decade, we have investigated and exploited the photophysical properties of triangulenium dyes. Azadioxatriangulenium (ADOTA) and diazaoxatriangulenium (DAOTA), in particular, have features that make them useful in various fluorescence-based technologies (e.g., bioimaging). Through our work with ADOTA and DAOTA, we became aware that the reported fluorescence quantum yields (ϕ_fl) for these dyes are lower than their actual values. We thus set out to further investigate the fundamental structure-property relationships in these unique conjugated cationic systems. The nonradiative processes in the systems were explored using transient absorption spectroscopy and time-resolved emission spectroscopy in combination with computational chemistry. The influence of molecular oxygen on the fluorescence properties was explored, and the singlet oxygen sensitization efficiencies of ADOTA and DAOTA were determined. We conclude that, for these dyes, the amount of nonradiative deactivation of the first excited singlet state (S₁) of the azaoxa-triangulenium fluorophores is low, that the rate of such deactivation is slower than what is observed in common cationic dyes, that there are no observable radiative transitions occurring from the first excited triplet state (T₁) of these dyes, and that the efficiency of sensitized singlet oxygen production is low (ϕ_Δ ≤ 10%). These photophysical results provide a solid base upon which technological applications of these fluorescent dyes can be built.

Physicochemical and Electronic Properties of Cationic [6]Helicenes: from Chemical and Electrochemical Stabilities to Far-Red (Polarized) Luminescence

The physicochemical properties of cationic dioxa (1), azaoxa (2), and diaza (3) [6]helicenes demonstrate a much higher chemical stability of the diaza adduct 3 (pKR+ =20.4, Ered1/2 =-0.72 V) compared to its azaoxa 2 (pKR+ =15.2, Ered1/2 =-0.45 V) and dioxa 1 (pKR+ =8.8, Ered1/2 =-0.12 V) analogues. The fluorescence of these cationic chromophores is established, and ranges from the orange to the far-red regions. From 1 to 3, a bathochromic shift of the lowest energy transitions (up to 614 nm in acetonitrile) and an enhancement of the fluorescence quantum yields and lifetimes (up to 31 % and 9.8 ns, respectively, at 658 nm) are observed. The triplet quantum yields and circularly polarized luminescence are also reported. Finally, fine tuning of the optical properties of the diaza [6]helicene core is achieved through selective and orthogonal post-functionalization reactions (12 examples, compounds 4-15). The electronic absorption is modulated from the orange to the far-red spectral range (560-731 nm), and fluorescence is observed from 591 to 755 nm with enhanced quantum efficiency up to 70 % (619 nm). The influence of the peripheral auxochrome substituents is rationalized by first-principles calculations.