Fifteen heptamethine dyes in which a tricyanofuran acceptor is linked to donors of different strength were synthesized, and their absorption, thermal, electrochemical, and second-order nonlinear optical properties were investigated. While the variation of the peripheral bulky substituents allowed a decrease of the intermolecular interactions in the solid state, subtle engineering of the heterocyclic donor provided dyes with electronic structures that varied between dipolar and cyanine-like (i.e. with little bond length alternation and with ground and excited states having similar dipole moment) and remarkably high quadratic hyperpolarizabilities (µβ 1.9) of up to 115,000×10-48 esu.

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...