Fluorescence imaging has become an indispensable tool for the rapid, simple and minimally invasive visualization of the cellular structures and processes. Among the diverse families of emissive probes, triamino-phenazinium structures, part of the broader azine dyes family, have recently emerged as promising candidates thanks to their significant theranostic potential. In this study, we report an attractive synthetic route that enables access to diamino-phenazinium dyes directly from their triamino-phenazinium precursors. The resulting dyes display bathochromically shifted optical bands with absorption of red light and emission bands spanning up to the near-infrared region. Theoretical calculations reveal that this spectral shift originates from an enhanced intramolecular charge transfer character in the diamino derivative. Quantitative colocalization analysis further demonstrates that the diamino-substituted dye exhibits improved mitochondrial targeting efficiency, achieving a Pearson's correlation coefficient of 0.8. These findings highlight the potential of rationally modified diamino-phenazinium fluorophores as modern tools for NIR-light enabled mitochondrial biolabeling.

We present the synthesis along with structural and photophysical properties of a series of 2‐(2′‐hydroxyphenyl)benzoxazole (HBO) and dimethylindole (HDMI) derivatives, in an effort to modulate their fluorescent properties by changing either electronic substitution or introducing planarization via ethynyl bridges. HBO and HDMI dyes are reported to undergo excited‐state intramolecular proton transfer, leading to red‐shifted emission with typically low quantum yield in solution, owing to pronounced non‐radiative deactivation channels. Introducing electron acceptors, such as benzonitrile, emerges as an effective strategy to boost the radiative transitions in these systems. In this context, this study aims at building structure–property relationships by a stepwise investigation of the influence of several parameters, that is, the nature of proton acceptor heterocycle (HBO vs. HDMI), the presence of an ethynyl moiety, and the addition of substituents (CN vs. Me). In each case, the nature of the transition(s) involved in the optical profile was also investigated by theoretical calculations.

A trigonal prismatic coordination cage endowed with a large cavity is built by self-association of a bis(rhodium) complex and a planar triazatruxene-based ligand. The remarkable versatility of this molecular building in promoting reversible structural transformations is demonstrated through manipulating three orthogonal external stimuli, i.e., cage concentration, guest identity, and pH. Four distinct stable discrete structures can be formed on-demand: a monomeric cage, two distinct host-guest complexes, and a mechanically interlocked cage dimer, and all these structures are fully and readily interconvertible. The cage uniquely couples (i) selective double guest encapsulation, (ii) stimulus-controlled reversible catenation, and (iii) pH-triggered pairwise guest release-uptake. The resulting structures were comprehensively characterized using 1D and 2D NMR spectroscopy, high-resolution mass spectrometry, theoretical calculations, and, in most cases, single-crystal X-ray diffraction.

[This corrects the article DOI: 10.1039/D4QM00632A.].

This Viewpoint aims at providing a concise overview of the photophysical properties of recent examples of 2-iminophenol ligands, sometimes referred to as anils and their corresponding boron complexes, boranils, formed upon boron coordination in the N'̂'O chelating site. Due to the presence of a strong internal hydrogen bond, anil derivatives are prone to display excited-state intramolecular proton transfer (ESIPT) processes, which can induce thermochromic and photochromic properties, due to the presence of an isomerizable imine moiety. While these motions of the molecular scaffold of anils are inevitably detrimental for solution-state fluorescence, aggregation-induced emission behavior has been evidenced in confined media, leading to fluorescent nanoaggregates. Coordination to trivalent boron ion drastically modifies the optical properties, by rigidifying the molecular backbones, enabling environment-sensitive intense fluorescence in both solution and solid states.

ABSTRACT Fluorescence‐based bioimaging enables noninvasive visualization of molecular and cellular processes with high sensitivity and without ionizing radiation. However, conventional fluorophores emitting in the visible or near‐red infrared I (NIR‐I) (650–800 nm) regions suffer from limited tissue penetration and scattering. Extending fluorescence emission into the deeper NIR region represents a promising strategy to overcome these drawbacks, yet achieving high brightness and stability in organic dyes remains a major challenge. We report an original family of hetero‐substituted‐fused boron‐dipyrromethene (BODIPY) dyes bearing carbazole and thienyl donors that exhibit record brightness and emission maxima up to 852 nm in toluene. The synthetic route combines successive Stille couplings from a 2,6‐dibromo‐3,5‐diiodo‐BODIPY precursor and an unprecedented silver(I)‐mediated oxidative cyclization, affording high yields and suppressing undesired chlorination. The resulting dyes display intense absorption ( ε = 1.8–2.5 × 10 5 M − 1 cm − 1 ) and exceptional fluorescence quantum yields ( Φ up to 0.73). Encapsulation in silica nanoparticles (NPs) preserves their photophysical properties and enables efficient NIR‐II in vivo imaging in mice, allowing tumor detection at doses as low as 0.2 nmol with tumor‐to‐muscle ratios > 4. These fused BODIPY derivatives rank among the brightest NIR fluorophores reported to date and open new avenues for high‐contrast deep‐tissue imaging and image‐guided surgery.

We report the synthesis and full characterization of a family of four phospha-squaraine dyes, obtained in a single step from squaric acid derivatives. Their structure-property relationships are investigated through a joint experimental/theoretical approach. All of these exotic ylides display strong absorbtion and emission in the red region of the spectrum, a rare feature for phosphorus ylides. Furthermore, they exhibit enhanced photostability compared to a standard squaraine reference.

The straightforward synthesis of the first difluoro dipyridomethene boron complexes as heavy-atom-free photosensitizers is described. These molecules show good singlet oxygen generation (ϕΔ = 10 to 47%) and significant nonplanar arrangement, inducing axial and boron-centered chirality.

Fluorescence imaging has become an indispensable tool for the rapid, simple and minimally invasive visualization of the cellular structures and processes. Among the diverse families of emissive probes, triamino-phenazinium dyes, closely related to rhodamines, have recently emerged as promising candidates thanks to their significant theranostic potential. In this study, we report a facile synthetic route that enables direct access to diamino-phenazinium dyes directly from their triamino-phenazinium precursors. The resulting dyes display bathochromically shifted optical bands with absorption of red light and emission bands spanning up to the near-infrared region. Theoretical calculations reveal that this spectral shift originates from an enhanced intramolecular charge transfer character in the diamino derivative. Quantitative colocalization analysis further demonstrates that the diamino-substituted dye exhibit improved mitochondrial targeting efficiency, achieving a Pearson’s correlation coefficient of 0.8. These findings highlight the potential of rationally modified diamino-phenazinium fluorophores as next-generation tools for high-performance mitochondrial biolabeling.

Switching the handedness of circularly polarized luminescence (CPL) at the molecular level remains a challenge in the development of responsive chiral materials. We report a light-driven molecular motor covalently linked to two perylenediimide (PDI) chromophores, enabling reversible and directional modulation of CPL. The different motor states, which are accessible in a unidirectional fashion via light-irradiation and thermal helix inversion steps, respectively, display significantly distinct chiroptical properties. In addition, our system allows switching of the chiral induction process and hence, the observation of a CPL signal. The present work discloses the first example of reversible CPL sign inversion triggered by light irradiation at the single molecular level, offering a new starting point for designing emitters with light-responsive chirality modulation.

We report a systematic computational investigation of the (chiro)ptical properties of a series of intrinsically chiral organic ketones, considering the prediction of one-photon absorption, electronic circular dichroism, fluorescence, and circularly polarized luminescence spectra, together with the absorption and luminescence dissymmetry factors, gabs and glum, typically used to quantify chiral responses. A set of TD-DFT functionals, namely B3LYP, MN15, M06-2X, CAM-B3LYP, and ωB97X-D, is first assessed within the vertical approximation. Such an approach qualitatively reproduces the main trends in dipole and rotatory strengths, but provides rather unreliable gabs and glum factors. For the same set of functionals, vibrationally resolved spectra were then simulated using a panel of vibronic models. For absorption and electronic circular dichroism spectra, vertical hessian and vertical gradient vibronic models reproduce the experimental vibronic structure more accurately than the adiabatic hessian model, which often yields overly broad and blueshifted bands. The inclusion of Herzberg-Teller effects significantly improves the prediction of gabs, mainly by increasing the computed absorption dipole strength associated with the weak n → π* transition, with the range separated functionals, ωB97X-D and CAM-B3LYP, providing the best overall results. In contrast, the fluorescence and circularly polarized luminescence spectra are less sensitive to the vibronic model, as they are dominated by a single broad emission band. Nevertheless, Herzberg-Teller effects allow improving glum values for all the tested functionals. Overall, this work shows that reliable predictions of dissymmetry factors in chiral ketones require a balanced treatment of electronic-structure and vibronic effects. While non-Condon contributions are essential for accurately describing gabs, the calculation of glum is comparatively more dependent on the electronic-structure method for the investigated set of compounds.

We describe the synthesis, photophysical properties, and ab initio calculations on a series of 2-(2'-hydroxyphenyl)benzazoles (HBX), functionalized by a benzonitrile, prone to boost the fluorescent quantum yield in solution. HBX are well known to display the excited-state intramolecular proton transfer (ESIPT) process, but they typically lead to a quenched fluorescence in solution. The additional insertion of an ethynyl-triisopropylsilane substituent in the vicinity of the benzonitrile group acts as a cooperative moiety to obtain strong multiple-state emissive transitions with distinct emission wavelengths. Quantum yields of the ESIPT transition are up to 52% in solution, which are among the record values in the literature. The photophysical data can be further modulated by the nature of the heteroatom constitutive of HBX, paving the way for the development of ultrasensitive ratiometric probes.

We benchmark a representative set of exchange-correlation functionals for the 2PA strengths of centrosymmetric quadrupolar dyes using CC2 as reference. Owing to the Ci symmetry, the dominant 2PA channel of these compounds proceeds through a symmetry-allowed Ag → Au → Ag pathway, significantly differing from the process in dipolar dyes. We analyze the response using a three-state framework. Many functionals reproduce the qualitative CC2 structure-property trends, indicating that TD-DFT can capture the main chemical effects. However, substantial deviations remain in absolute magnitudes and in the ordering of closely related systems. These errors primarily originate from the transition-dipole-moment contribution, especially the upper S1 → S2 transition, while energetic detuning plays a secondary role. Low-exact-exchange global hybrids, such as B3LYP and PBE0, tend to overestimate the 2PA strengths, whereas range-separated hybrids typically underestimate them. A reasonably balanced performance is obtained with higher-exact-exchange hybrids, particularly MN15, BMK, M06-2X, and BH&HLYP. Overall, MN15 yields the lowest global error for this benchmark set, while CAM-B3LYP remains effective for preserving relative trends despite its systematic underestimation of absolute responses.

This article reports the synthesis and characterization of cross-conjugated donor-acceptor dyes based on a N,N,N',N'-tetraphenyl-1,4-phenylenediamine core, substituted at the 2,5-positions with various electron-withdrawing groups (carbonyl, imines, nitriles, or electron-poor heterocycles). Electrochemical studies reveal electronic communication between para-substituted donor groups and, in some cases, between acceptor groups. Photophysical studies show tunable absorption and emission properties based on the strength of the electron-withdrawing groups, a trend that is rationalized through computational analysis. Notably, some compounds exhibit long emission lifetimes in solution, whereas all dyes emit in the solid state, with emission extending into the near-infrared (up to > 1000 nm).

We present a comprehensive benchmark of excited-state polarizabilities for a representative set of more than 40 singlet states from 27 small organic molecules. Reference data were obtained using the high-level coupled-cluster CC3 model in combination with the aug-cc-pVTZ atomic basis set, providing the first systematic data set of excited-state polarizabilities at this level of theory. The studied set includes both valence and Rydberg states, the latter exhibiting significantly larger polarizabilities, reflecting their diffuse character and enhanced sensitivity to external electric fields. The benchmark analysis includes lower-level wave function-based methods, namely, CCSD and CC2, as well as Time-Dependent Density Functional Theory (TD-DFT) with several common density functional approximations (B3LYP, MN15, M06-2X, CAM-B3LYP, and LC-BLYP). The statistical analysis enables the evaluation of the impact of orbital relaxation and highlights method-dependent differences across the different kinds of excited states. The results indicate that CCSD, in both its relaxed and unrelaxed forms, provides the most accurate description of excited-state polarizabilities, closely followed by CC2, which can therefore be generally employed as a computationally efficient yet reliable alternative. Among the TD-DFT functionals, range-separated hybrids─particularly LC-BLYP─perform best, while larger errors are observed for the three evaluated global hybrids.

Heterocycle-containing luminescent probes exhibit intricate polarity responses, notably due to the presence of lone pair(s), which increases the complexity of the excited state and brings additional challenges for their rational design. In this work, we present three classes of fluorescent probes with "on-off", "insensitive", and "off-on" polarity responses by attaching different substituents to the same fluorophore core. Class I probes with nonaromatic six- and five-membered bulky heterocycle moieties exhibit an "on-off" response due to the transition from a local excited state to a twisted intramolecular charge transfer state with increasing polarity. Class II probes with more compact nonaromatic four- and three-membered heterocyclic moieties display an "insensitive" response originating from an excited-state electronic structure that is insensitive to environmental polarity. Class III probes with bulky aromatic six-membered moieties show an "off-on" response due to the transition from a dark (π, σ*)/(n, π*) state to a bright (π, π*) state as the polarity increases. This design strategy is shown to be applicable to a wide variety of electron-accepting fluorophores. Additionally, a simple metric, Δα, is proposed to predict polarity-response behaviors: Δα < 0 suggests Class I; Δα ≈ 0 indicates Class II; Δα > 0 hints at Class III. Our theoretical predictions are experimentally corroborated, and the wash-free lipid droplet imaging of one of the predicted dyes exhibits relatively high contrast. In a nutshell, this work provides theoretical guidance for designing polarity-responsive probes for both bioimaging and disease diagnosis.

Excited-state absorption (ESA) corresponds to the transition between two electronic excited states and is a fundamental process for probing and understanding light-matter interactions. Accurate modeling of ESA is indeed often required to interpret time-resolved experiments. In this contribution, we present a dataset of 53 ESA oscillator strengths in three different gauges and the associated vertical transition energies between 71 excited states of 21 small- and medium-sized molecules from the QUEST database. In a few cases, we additionally investigated the effect of geometry relaxation on excited-state geometries. The reference values were obtained within the quadratic response (QR) CC3 formalism using eight different Dunning basis sets. We found that the d-aug-cc-pVTZ basis set is always adequate while its more compact double-ζ counterpart, d-aug-cc-pVDZ, performs well in most cases. These QR-CC3 data allow us to assess the performance of QR-TDDFT, with and without applying the Tamm-Dancoff approximation, using a panel of global and range-separated hybrids (B3LYP, BH&HLYP, CAM-B3LYP, LC-BLYP33, and LC-BLYP47), as well as several lower-order wave function methods, i.e., QR-CCSD, QR-CC2, EOM-CCSD, ISR-ADC(2), and ISR-ADC(3). We show that QR-TDDFT delivers acceptable errors for ESA oscillator strengths with CAM-B3LYP showing particular promise, especially for the largest molecules of our set, and in the Franck-Condon (FC) region. We also find that ISR-ADC(3) exhibits excellent performance in this region. When using excited-state optimal geometries, the relative performance of wave function-based approaches remains consistent with trends observed in the Franck-Condon region. However, for TD(A)-DFT, the accuracy varies more significantly, as the performance of different exchange-correlation functionals significantly depends on the chosen geometry.

A series of twenty-two acceptor-donor-acceptor chromophores, comprising both dicationic and neutral dyes, were synthesized and characterized. These compounds were designed with a wide range of electron-withdrawing end groups to explore the balance between acceptor-donor-acceptor aromatic structures and hypothetical nonaromatic coupled polymethine electronic structures within the central six-membered ring. Structural, electrochemical, photophysical, and computational investigations were conducted to gain insights into the structure-property relationships of these systems. The experimental data revealed that the redox and optical properties were most consistent with acceptor-donor-acceptor aromatic structures, significantly impacted by the nature of the end groups. Solvatochromic studies further indicated that the dyes were unlikely to lose their aromatic character in favor of a coupled polymethine structure. Theoretical calculations suggested that a few dicationic derivatives exhibited a slightly reduced aromatic character, hinting at minor coupled polymethine contributions. However, the majority of the chromophores retained a predominantly aromatic central ring.

We report the straightforward one-pot synthesis of 5 or 6-membered P-heterocycles featuring internal ylidic bond: P-containing acenaphthylenes and phenanthrenes. The stability of the compounds tolerates post-functionalization through direct arylation to introduce electron-rich/poor substituents and the synthetic strategy is also compatible with the preparation of more elaborated polyaromatic scaffolds such acenes and helicenes. Using a joint experimental and theoretical approach, we show that the molecular engineering of this platform allows not only tuning their absorption/emission on the entire visible range but also endowing them with chiroptical or non-linear optical properties, making them valuable dyes for a large panel of photonic or opto-electronic applications.

This article describes the synthesis, along with comprehensive photophysical and ab initio characterization, of a series of 2-(2 '-hydroxyphenyl)benzoxazole (HBO) fluorophores, a family of compounds prone to undergoing an excited-state intramolecular proton transfer (ESIPT) process, functionalized with different positional isomers of quinoline or isoquinoline. We notably show that the position of the nitrogen atom at the azaheterocycle site has a key influence on both the emission profile and the photoluminescence quantum yield in solution. We also demonstrate the proton-sensitive nature of these dyes in solution, where not only does protonation trigger fluorescence enhancement, but it also acts as a transition switch between two excited states, with different emission profiles. HBO-isoquinoline displays very intense fluorescence not only in neutral and protonated dichloromethane solutions in the green-yellow region, but also in the solid state. Moreover, this dye exhibited a record Stokes shift of 11 000 cm-1.