Tuning the solid-state emission of small push-pull dipolar dyes to the far-red through variation of the electron-acceptor group

Abstract

Series of solid-state emitters based on the D-π-A dipolar structure and featuring various electron-donor and electron-acceptor groups were designed, and their spectroscopic properties studied. From weak emission in dilute solutions, intense emissions in aggregated state (AIE) and in the crystalline state were obtained. Analysis in light of crystal structures obtained by X-ray diffraction revealed specific crystal packing and presence of long chain of emitting aggregates. This simple molecular engineering around the D-π-A dipolar structure provides easy access to a wide range of effective solid-state emitters allowing modulation of emission wavelengths up to the near infrared (λem reaching 735 and 768 nm for compound 2f and 3f bearing the strongest electron-withdrawing group). Highlights • Library of small push-pull dipolar solid-state emitters has been obtained featuring three different electron-donor groups and various electro-acceptor groups. • New electron-acceptor groups based on substituted 2(5H)-furanone rings are presented. • Fluorescence properties in the aggregated state and in the solid-state are described, and correlated to the presence of specific aggregates in the crystal structure. • The best dyes displayed near-infrared emission in the solid-state with emission quantum yield of 11% at λem=735 nm and 4% at λem=768 nm. • Keywords Solid-state fluorescence; Push-pull dyes; Organic dyes; Near-infrared; Aggregation-induced emission; 2(5H)-furanone rings library of push-pull fluorophores featuring 4-(N,N-diphenylamino)phenyl-(Series 1, Chart 1), 9-ethyl-9H-carbazolyl-(Series 2) and 4-(9H-carbazol-9-yl)-(Series 3) as electron-donor groups and various electron-acceptor groups (A-J, Scheme 1), in particular 2(5H)-furanone rings with a weak electronwithdrawing group at the C2 position (G-J). If some of these acceptor groups such as indanedione (A), dicyanoisophorone (E) or TCF (F) are quite commonly used in several high efficiency solid-state emitters, [32, 38-41, 44-50] other 2(5H)-furanones have rarely been utilized as acceptors entities in dye design and only within the frame of non-linear optical chromophores design (acceptor group G).[51, 52] Chart 1. Structures of fluorophores 1a-1i, 2a-2j and 3b-3i. 2. Experimental 2.1 General Information Commercially available materials and reagent grade solvents were used as received. Microwave syntheses were conducted in 20 mL sealed tube on a Biotage Initiator 2.5 single-mode reactor using external IR temperature control. The reaction monitoring was performed by analytical thin-layer chromatography (TLC) on Merck 60 F254 precoated silica gel plate (0.2 mm thickness) with visualization using a UV lamp. Purification by column chromatography was carried out using 35-70 m silica gel. 1 H and 13 C NMR spectra were recorded at ambient temperature on a Bruker Advance 300, 400 or 500 operating at 300.1, 400.0 or 500.0 MHz for 1 H and 75.0, 101.00 or 125.0 MHz for 13 C, respectively. Chemical shifts are reported as δ values (ppm) with reference to the residual solvent peaks...

Push-Pull Chromophores Based on the Naphthalene Scaffold: Potential Candidates for Optoelectronic Applications

“…The fluorescent properties of all compounds were investigated in dichloromethane and toluene, in diluted solutions and the results are summarized in the Figure 10 and the Table 5. Interestingly, most of the chromophores were not emissive whatever the solvent is and this behavior is consistent with results classically reported in the literature for indane-1,3-dione derivatives [57,58,59,60,61]. However, it has to be noticed that a series of indane-1,3-dione derivatives was reported as being highly emissive, in a specific context, by use of oligo(phenylene)vinylene as electron donors [62].…”

Push–Pull Dyes for Yellow to NIR Emitting Electrochemical Cells

“…Dyes with a large angle sum (A πD plus A πA ) form distorted structures projected from the central π-bridge. These distorted structures were confirmed by the single crystal X-ray diffraction (XRD) data [31,33,37] of dyes I-III and V, which show larger A πD than A πA that matched well to the calculated results (Table S3, Supporting Information). The interchromophore dye-dye stacking distance increased in the order I < II < V << III ( Figure S7, Supporting Information).…”

“…The synthesis of the other dyes, also by Knoevenagel condensations, was previously described. [30,31,33] The thermal properties of the dyes showed high melting points (>225 °C) as well as excellent thermal stability (>290 °C), as determined by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) ( Figure S2 and Table S1, Supporting Information). In particular, dyes I and II showed the best thermal stability and the highest thermal degradation temperature for 5% weight loss (T 95% , ≈340 °C).…”

“…[34,35] The Onsager radii were estimated from Suppan's equation [36] (see the Supporting Information) using the density obtained from X-ray diffraction on single crystals. [31,33,37] From those Onsager radii (Table S2, Supporting Information), dipole moment variations from 16-19.1 (I-IV) to 9.6 D (V) were determined. Thus, the dipole moment increase upon excitation of dyes I-IV is approximately twice as strong as that of dye V. Finally, it is interesting to note that all dyes present large Stokes shifts (5000-7500 cm −1 ), also characteristic of their strong intramolecular CT, so low autoabsorption can be expected in the films.…”

Unprecedented Nucleophilic Attack of Piperidine on the Electron Acceptor during the Synthesis of Push‐Pull Dyes by a Knoevenagel Reaction

“…During the past decades, push‐pull dyes have been extensively studied with regards to their numerous applications ranging from sensors, energy conversion, field effects transistors, organic light‐emitting diodes, nonlinear optics or photoinitiators of polymerization . To access to these structures consisting in an electron donor connected to an electron acceptor by means of a conjugated or none‐conjugated spacer, the Knoevenagel reaction is undoubtedly the most popular reaction, opposing an aldehyde with an electron acceptor bearing an activated methylene group, in the presence of a catalytic amount of a base.…”