The Effect of Bulky Substituents on Two π-Conjugated Mesogenic Fluorophores. Their Organic Polymers and Zinc-Bridged Luminescent Networks

Abstract

From a dicyano-phenylenevinylene (PV) and an azobenzene (AB) skeleton, two new symmetrical salen dyes were obtained. Terminal bulky substituents able to reduce intermolecular interactions and flexible tails to guarantee solubility were added to the fluorogenic cores. Photochemical performances were investigated on the small molecules in solution, as neat crystals and as dopants in polymeric matrixes. High fluorescence quantum yield in the orange-red region was observed for the brightest emissive films (88% yield). The spectra of absorption and fluorescence were predicted by Density Functional Theory (DFT) calculations. The predicted energy levels of the frontier orbitals are in good agreement with voltammetry and molecular spectroscopy measures. Employing the two dyes as dopants of a nematic polymer led to remarkable orange or yellow luminescence, which dramatically decreases in on-off switch mode after liquid crystal (LC) order was lost. The fluorogenic cores were also embedded in organic polymers and self-assembly zinc coordination networks to transfer the emission properties to a macro-system. The final polymers emit from red to yellow both in solution and in the solid state and their photoluminescence (PL) performance are, in some cases, enhanced when compared to the fluorogenic cores.

A Novel DR/NIR T-Shaped AIEgen: Synthesis and X-Ray Crystal Structure Study

“…The intramolecular hydrogen-bond interactions in the half-salen moieties cause the excited-state intramolecular proton-transfer (ESIPT) [60,61] known to lead to emission in solution [46,59,[61][62][63][64][65][66]. The same effect is known to occur in the solid state [12,67] if the intramolecular H-bond produce hindrance to the torsion of sterically encumbered parts of the molecule [57,68,69]. In our case, the acetyl substituents on the furan rings and the bulky naphthol moieties make NBDF a RIR probe.…”

“…Compound NBDF is the condensation product between a diamine and a salicylic aldehyde in 1:2 ratio. The probe contains two half-salen [12,[57][58][59] Schiff base sites working as electron-donor arms and an electron-acceptor core. As shown in Scheme 1, the (4-nitrophenyl) benzodifuran unit represents the acceptor moiety due to the strong electron-withdrawing properties of the nitro substituent.…”

“…The fundamental issue is designing a conjugate system depressing the non-radiative deactivation processes through restriction of intramolecular rotation (RIR), restriction of intramolecular vibrations (RIV), and restriction of intramolecular motions (RIM) [10,11]. Therefore, the introduction of sterically encumbered substituents or bulky groups [9,12,13] and the construction of highly twisted skeleton [14][15][16][17] are the possible solutions. Unlike the common π-conjugated fluorogens, this kind of molecules undergo aggregation-induced enhanced emission (AIE) effect providing photoluminescence (PL) in the solid state.…”

Spectroscopic Behaviour of Two Novel Azobenzene Fluorescent Dyes and Their Polymeric Blends

“…As summarized in Scheme 1, the dyes A1 and A2 were obtained by condensation of the diamino derivative AB-NH 2 [39] with 4-(diethylamino)-2-hydroxybenzaldehyde and 2-hydroxy-4-nitrobenzaldehyde, respectively. Though structurally similar, a different conjugation pattern is recognizable, D-π-D-π-D respectively for A1 and A-π-D-π-A for A2 (where D = electron donor moiety, A = electron acceptor moiety, and π = conjugated system).…”

“…Two ESIPT undergoing sites are generated by the condensation of the amino-terminal groups of the precursor AB-NH 2 with the salicylic aldehydes, guaranteeing emission in solution. According to a recent approach [26,39,40], it was found that the fast proton transfer in ESIPT sterically encumbered probes are impeded due to restriction of intramolecular rotation (RIR effect), also providing solid-state emission. The central methoxy groups cause steric hindrance on the conjugated skeleton without too much lowering solubility.…”