Rémi Métivier

ortho‐, meta‐, and para‐Dihydroindenofluorene Derivatives as Host Materials for Phosphorescent OLEDs

This work reports the first structure-properties relationship study of ortho [2,1-c]-, meta [1,2-a]-, and para [1,2-b]dihydroindenofluorenes, highlighting the influence of bridge rigidification on the electronic properties. This study has made it possible to devise an extended π-conjugated molecule with both a high triplet state energy level and excellent thermal and morphological stability. As a proof of concept, dihydroindenofluorenes were used as the host in sky-blue phosphorescent organic light-emitting diodes (PhOLEDs) with high performance.

Dependence of the Properties of Dihydroindenofluorene Derivatives on Positional Isomerism: Influence of the Ring Bridging

The future of organic electronics is driven by the synthesis and the study of novel molecular fragments for the construction of highly efficient polymers or oligomers. In this context, polyand oligophenylene derivatives constitute an important class of highly promising molecules, which have been widely studied for the last two decades. Of particular interest in the chemistry and physics of oligophenylenes is the bridged-para-terphenyl unit, namely, 6,12dihydroindeno[1,2-b]fluorene (Scheme 1). Although it has been known since the 1950s, investigations of the dihydroindeno[1,2-b]fluorenyl core only started a decade ago thanks to the pioneering work of M llen, which made this molecule a key building block for electronics. There are nowadays numerous examples of efficient dihydroindeno[1,2-b]fluorenylbased semiconductors that have found application in various fields, such as fluorescent 5–8] and phosphorescent organic light-emitting diodes (OLEDs), organic field-effect transistors, and organic solar cells. This wide range of applications clearly shows the high potential of this building block, but also its versatility. However, the dihydroindeno[1,2-b]fluorene is not the only member of the bridged-terphenyl family, since it possesses four other positional isomers with different phenyl linkages (para/meta/ortho) and different ring-bridging positions (anti vs. syn ; Scheme 1). There are hence five dihydroindenofluorene positional isomers, each possessing its own ring topology, which in turn has structural and electronic consequences. However, in contrast to the dihydroindeno[1,2-b]fluorene, other positional isomers remain very scarce in the literature owing to synthetic difficulties. For example, the dihydroindeno[2,1-a]fluorenyl (syn para-terphenyl) unit (Scheme 1) has only been investigated for organic electronics very recently, and thanks its particular syn geometry has emerged as a promising scaffold for a new generation of excimer-based OLEDs. Similarly, antiaromatic fully conjugated indenofluorene derivatives have recently attracted particular attention; Haley and co-workers have for example reported a new class of (2,1-c)indenofluorenes with high electron affinities. However, the anti and syn meta-terphenyl isomers, that is, dihydroindeno[1,2-a]fluorene and dihydroindeno[2,1-b]fluorene, although known for 60 years, are almost absent from the literature, and their intrinsic properties have never been studied. As the design of novel molecular fragments is of key importance for the future of organic electronics, we report herein the first examples of the use of dihydroindeno[1,2b]fluorene (1) and dihydroindeno[2,1-a]fluorene (2 ; Scheme 1. The five positional dihydroindenofluorene isomers.

Unprecedented Stability of a Photochromic Bisthienylethene Based on Benzobisthiadiazole as an Ethene Bridge

Photochromic systems are of increasing interest as a result of their applications in ophthalmic lenses and as smart molecular materials that act as photoresponsive self-assembling systems, molecular switches, logic gates, and information storage media. In particular, bisthienylethenes (BTEs) have become one of the most promising families because of their thermal irreversibility and outstanding fatigue resistance since the 1,3,5-hexatriene section can adopt an appropriate conformation to undergo a conrotatory 6p-electron photocyclization. To date, the rational design of symmetric or asymmetric BTEs has mainly been carried out by modifying the thiophene rings. In contrast, the central ethene bridges necessary for the versatility of the BTE architectures reported so far have been mostly limited to a cyclopentene unit or its strong electron-withdrawing analogues, such as perfluorocyclopentene, maleic anhydride, or maleic imide. As alternatives to this, several research groups have investigated the use of six-membered rings containing a C=C bond, such as 1,10-phenanthroline and cyclohexene obtained by a [4+2] cycloaddition of butadiene with a dienophile, as the central ethene bridge of BTEs. Recently, we incorporated highly polar and electron-withdrawing chromophores of 2,1,3benzothiadiazole and naphthalimide units as a novel family of six-membered rings that act as ethene bridges and result in good photochromic performance with moderate fatigue resistance in solution or in organogel systems. Moreover, their fluorescence can be modulated by solvatoand photochromism, eventually leading to a combined NOR and INHIBIT logic operation system. However, the two building blocks have a degree of aromaticity, which can facilitate the undesirable thermal back reaction in the dark as a result of the large loss of aromatic stabilization energy upon photocyclization from the open to the closed form. Accordingly, a lower aromaticity of the central ethene moiety in BTEs is expected to lead to a higher thermal stability of the closed form. Since the ethene bridge within a six-membered ring has its own advantages, such as a higher quantum yield in its closed-ring form and longer absorption wavelength, we envisioned that the use of a non-aromatic sixmembered ring with a C=C bond as the central ethene bridge might widely extend the diversity of the thermally irreversible photochromic systems. With this in mind, we report herein an original BTE photochromic system (BTTE, Figure 1) based on the benzo[1,2-c :3,4-c’]bis[1,2,5]thiadiazole (abbreviated as benzobisthiadiazole) chromophore as a novel six-membered ring containing a central bridging ethene unit, which is expected to have a low aromaticity, and yield a thermally stable closed form (c-BTTE). As depicted in Figure 1A, BTTE was easily prepared by conventional Suzuki cross-coupling with the corresponding boronic acid. The key intermediate of 4,5-dibromobenzo[1,2c :3,4-c’]bis[1,2,5]thiadiazole (BBT) was conveniently synthesized from benzothiadiazole according to the reported procedure. The Suzuki coupling of BBT and 2,5-dimethylthien-3-yl boronic acid in the presence of a [Pd(PPh3)4] catalyst in a mixture of aqueous Na2CO3 (2m) and 1,4-dioxane under reflux gave the target molecule BTTE, which was fully characterized by H NMR and C NMR spectroscopy as well as HRMS (see the Supporting Information). Interestingly, the H NMR spectrum of BTTE shows two well-resolved sets of signals for the methyl protons which correspond to the parallel (d = 2.09 and 2.42 ppm) and antiparallel (d = 1.96 and 2.46 ppm) conformations commonly found in BTE systems (Figure 1D). In general, the two conformations, parallel (photochromic inactive) and antiparallel (photochromic active) conformations, undergo very fast single-bond rotation in most BTEs, thereby resulting in only one set of time-averaged signals in the H NMR spectrum. Only in the cases of diarylethenes bearing substituents can the rotation of the aryl groups be slowed down sufficiently to give two sets of signals. As found by integration of the H NMR spectrum, the ratio between the parallel and antiparallel conformations of BTTE in CDCl3 is 55:45. A solution of BTTE in THF is colorless, with an intense absorption band at 288 nm and a moderate one at 361 nm (Table 1). As expected, irradiation of the solution of BTTE with light at a wavelength of 365 nm results in the solution [*] Prof. Dr. W. Zhu, Y. Yang, Dr. Q. Zhang, Prof. Dr. Y. Xie, Prof. Dr. H. Tian Shanghai Key Laboratory of Functional Materials Chemistry Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology Shanghai 200237 (China) E-mail: whzhu@ecust.edu.cn

Photophysics of calixarenes bearing two or four dansyl fluorophores: charge, proton and energy transfers

The photophysical properties of calixarenes bearing two and four dansyl derivatives (Calix-DANS2 and Calix-DANS4) were investigated in a CH(3)CN/H(2)O mixture (60 : 40 v/v). The carboxydansyl (DANS1) was used as a reference compound. The pH titration of DANS1 permits determination of two pK(A) values (2.4 and 6.3). The charge-transfer character of this type of fluorophore is discussed. The protonated dansyl group LH(2)(+) undergoes photoinduced proton transfer. The photophysical characterization (absorption spectra, emission spectra, quantum yield and fluorescence decays) of Calix-DANS2 and Calix-DANS4 showed that nonradiative energy transfer occurs from a dansyl fluorophore in the basic form to another one in the neutral form. The transfer efficiencies are 0.1 and 0.5 for Calix-DANS2 and Calix-DANS4, respectively. Fluorescence data analysis allowed us to determine the fractions of dansyl fluorophores in the basic and neutral forms.

Tuning the optical properties of aryl-substituted dispirofluorene-indenofluorene isomers through intramolecular excimer formation.

Two families of positional isomers of dispirofluorene-indenofluorene substituted by phenyl groups at the 2,7-positions of the fluorene moieties present drastically different optical properties. The emission wavelengths may be gradually and conveniently modulated for one of the two isomers by the phenyl groups substituent whose bulkiness controls the extent of the excimeric interaction evidenced in this paper.

9,9′-Spirobifluorene and 4-phenyl-9,9′-spirobifluorene: pure hydrocarbon small molecules as hosts for efficient green and blue PhOLEDs

We report herein a new pure hydrocarbon material, 4-phenyl-9,9′-spirobifluorene (4-Ph-SBF), with a high triplet energy level (ET: 2.77 eV) as a host for blue phosphorescent organic light-emitting diodes (PhOLEDs). Structural, thermal, electrochemical and photophysical properties have been investigated in detail and compared to its constituting building block 9,9′-spirobifluorene (SBF) in order to precisely study the influence of the incorporation of a phenyl unit in C4. A surprising out of plane deformation of the fluorene ring and a highly twisted structure have been notably found for 4-Ph-SBF due to the substitution in C4. As both 4-Ph-SBF and SBF possess a high triplet energy level (2.77/2.87 eV resp.), they have been successfully used as host materials for green and blue PhOLEDs. The performance of blue PhOLEDs, ca. 20 cd A−1, appears to be among the highest reported for pure hydrocarbon derivatives.

A highly sensitive and selective fluorescent molecular sensor for Pb(II) based on a calix[4]arene bearing four dansyl groups

A new fluorescent molecular sensor based on a calix[4]arene bearing four dansyl groups exhibits very efficient binding in acetonitrile-water for lead and the changes of emission properties allows a detection limit of 4 microg L(-1).

Photoswitchable interactions between photochromic organic diarylethene and surface plasmon resonance of gold nanoparticles in hybrid thin films

Hybrid materials combining gold nanoparticles (GNP) of variable diameter and an organic thin layer of photochromic diarylethenes were achieved. Solid-state photoswitching based on ring-closure/ring-opening reaction was carried out under alternate UV and visible irradiations. In addition to the spectral changes due to the photochromism itself, the surface plasmon resonance related to the GNP is significantly modified, influenced by a photoinduced change in the refractive index of its environment. These two contributions were sorted out, showing the possibility of probing a photochromic switch by following the plasmon band. The shape change of the plasmon band was consistently compared to calculations based on the Mie theory. Additionally, with one given diarylethene compound, both UV-visible spectroscopy and surface enhanced Raman scattering (SERS) spectroscopy showed an acceleration of the ring-opening photochromic reaction in the presence of GNP.

Photoswitching in diarylethene nanoparticles, a trade-off between bulk solid and solution: towards balanced photochromic and fluorescent properties

Nanoparticles of 1,2-bis[4′-methyl-2′-(2″-pyridyl)thiazolyl]perfluorocyclopentene (1) were prepared by laser ablation (355 nm) of microcrystalline powder suspensions in a water–sodium dodecylsulfate medium. AFM studies coupled with microfiltrations reveal that the resulting suspension is mostly made of nanoparticles with 25 ± 10 nm diameters. The resulting yellow transparent colloidal aqueous suspension is both photochromic (ΦOF→CF = 0.20 and ΦCF→OF = 0.96) and fluorescent (ΦF = 0.017). 1 is not photochromic in the bulk solid, whereas it is only weakly fluorescent in CH3CN (ΦF = 0.005); thus, the nanoparticles present a trade-off between these two states where the two properties show reasonable efficiencies. Upon UV irradiation of the opened form of 1, the fluorescence of the colloidal suspension decreases and shows a bathochromic shift. The emission is recovered upon visible irradiation. Such a behaviour enables us to consider nanoparticles of 1 as a ratiometric fluorescent photoswitch.

Characterization of alumina surfaces by fluorescence spectroscopy

In order to characterize and compare the spatial distribution of hydroxyl groups on γ- and δ-alumina surfaces, a pyrene derivative was covalently grafted on these groups and studied by steady-state and time-resolved fluorescence spectroscopy. When the pyrene probes are close enough, they can form excimers with a characteristic emission band. The relative intensities of the monomer and excimer bands provide a global information on the proximity of pyrene probes, whereas time-resolved fluorescence experiments offer a more detailed characterization of the surface in terms of heterogeneity. The fluorescence decay curves were satisfactorily analyzed in terms of stretched exponentials that account for the distribution of decay times resulting from the distribution of distances between the pyrene probes that can form excimer, and the distribution of the energies of interaction between the alumina surface and the pyrene monomers or excimers. Comparison of samples having the same surface coverage of pyrene probes allowed us to conclude that there are indeed significant differences in the characteristics of γ- and δ-alumina surfaces. In particular, the reactive hydroxyl groups are not homogeneously distributed on alumina surfaces but they are rather clustered into regions of high density. The fraction of these reactive groups belonging to the clustered regions is more than four times larger for δ-alumina than for for γ-alumina.