Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors.

Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 105 cm-1) and a relatively large ΔEST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔEST in organic DF emitters

Synthesis and spectroscopic properties of molecular assemblies based on boron curcuminoid chromophore

Le développement de colorants organiques contenant des groupes donneurs (D) et accepteurs (A) d’électrons connectés via un pont conjugué est un domaine très actif. En effet, les systèmes D-π-A-π-D de type quadripolaire présentent des structures électroniques caractérisées par un transfert de charge intermoléculaire à l’état fondamental et excité qui permet d'obtenir des propriétés d'absorption et d'émission de fluorescence remarquables.Depuis 2012, notre équipe développe la synthèse de chromophores D-π-A-π-D à partir du squelette curcuminoïde dans lequel A est un chélate de bore. Une grande série de cycles de donneurs aromatiques a été envisagée, permettant une modulation efficace des propriétés optiques et électroniques en solution. Ces molécules ont également été étudiées dans le solide et des valeurs élevées de rendements quantiques de fluorescence dans le proche infrarouge ont été obtenues. Tout d’abord, afin d'obtenir des informations précieuses sur les corrélations entre les propriétés optiques et les interactions interchromophoriques supramoléculaires, nous avons conçu des bichromophores covalents dans lesquels deux unités curcuminoïdes-BF2 sont reliées par une liaison flexible et rigide.Ensuite, nous avons décidé d’étudier la dynamique de spin des chromophores curcuminoïdes excités. Nous décrivons ici la synthèse de deux familles de dérivés curcuminoïdes substitués en position centrale par un radical persistant TEMPO ou 6-oxoverdazyle.The development of organic dyes containing electron donor (D) and acceptor (A) groups connected via a π-conjugated bridge is a very active domain. Indeed, quadrupole-like D-π-A-π-D systems show electronic structures characterized by an intermolecular charge transfer in the ground and excited states which makes possible to obtain remarkable absorption and fluorescence emission properties. Since 2012, our team is developing the synthesis of D-π-A-π-D chromophores based on the curcuminoid skeleton in which A is a boron chelate. A large series of aromatic donor cycles could be considered, allowing efficient modulation of optical and electronical properties in solution. These molecules have also been studied in the solid and high values of near-infrared fluorescence quantum yields were obtained. Firstly, in order to get valuable insights into the correlations between optical properties and supramolecular interchromophoric interactions, we are designing covalent bichromophores in which two curcuminoid-BF2 units are connected by a flexible and rigid link. Secondly, we decided to explore the perturbation of the excited curcuminoid chromophore by an unpaired electron spin. We describe the synthesis of two families of curcuminoid derivatives substituted in the central position by a persistent TEMPO or 6-oxoverdazyl radical

Optical properties of quadrupolar and bi-quadrupolar dyes: intra and inter chromophoric interactions

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Optoelectronic properties of a self-assembling rigidly-linked BF2-curcuminoid bichromophore

International audienceWe describe the synthesis and spectroscopic study of the bichromophoric molecule DA-meso consisting of two boron difluoride curcuminoid (BF 2-curcuminoid) units tethered by the rigid diacetylenic bridge. This structural feature imparts the quadrupolar-like DA-meso with a strong ability to self-assemble in solution, which induces a profound change in the optical properties. The UV-vis absorption spectrum of the aggregate is characterized by the appearance of a redshifted band at low energy and its fluorescence emission occurs in the near infrared ( max = 700 nm) with a low quantum yield (5 %). This is in contradiction with the strong emission expected for J-aggregates considering the Frenkel exciton model. From concentration-dependent UV-vis absorption spectroscopy we demonstrate the formation of a dimer composed of four BF 2-curcuminoid units. Single-crystal X-ray diffraction structure analysis and in-depth quantum chemical calculations based on density functional theory (DFT) and subsequent decomposition of the electronic excited states on a diabatic basis enable to discuss the geometry of the DA-meso dimer, and to establish the presence of excited states close in energy to that of the unperturbed monochromophore, which are not predicted by the classical Frenkel exciton model. Different dimer structures characterized by π-stacks of two and up to four chromophores are investigated, in which low-lying excited states wit

Exciton Interactions, Excimer Formation, and [2π+2π] Photodimerization in Nonconjugated Curcuminoid‐BF 2 Dimers

International audienceWe describe the synthesis of a series of covalently linked dimers of quadrupolar curcuminoid‐BF2 dyes and the detailed investigation of their solvent‐dependent spectroscopic and photophysical properties. In solvents of low polarity, intramolecular folding induces the formation of aggregated chromophores, the UV/Vis absorption spectra of which display the optical signature characteristic of weakly‐coupled H‐aggregates. The extent of folding and, in turn, of ground‐state aggregation is strongly dependent on the nature of the flexible linker. Steady‐state and time‐resolved fluorescence emission spectroscopies show that the Frenkel exciton relaxes into a fluorescent symmetrical excimer state with a long lifetime. Furthermore, our in‐depth studies show that a weakly emitting excimer lies on the pathway toward a photocyclomer. Two‐dimensional 1H NMR spectroscopy and density functional theory (DFT) allowed the structure of the photoproduct to be established. To our knowledge, this represents the first example of a [2π+2π] photodimerization of the curcuminoid chromophore