Achieving yellow emission by varying the donor/acceptor units in rod-shaped fluorenyl-alkynyl based π-conjugated oligomers and their binuclear gold(i) alkynyl complexes

Abstract

Fluorenyl-alkynyl based π-conjugated rod-shaped oligomers bearing different central aromatic moieties and functionalizable di-alkynyl termini, such as H––Fl––Fl––Fl––H (OH1), H––Fl––Btz––Fl––H (OH2) and H––Fl––Btd––Fl––H (OH3) where Fl = 9,9-dioctylfluorene, Btz = N-hexylbenzotriazole, and Btd = benzothiadiazole, were successfully synthesized by a Pd(0) catalyzed Stille coupling protocol. Electron withdrawing benzothiadiazole and benzotriazole as strong to moderate acceptors and fluorene as the donor have been incorporated to adjust the Donor–Acceptor (D–A) strength for fine-tuning the bandgap (E_g) as well as the emission wavelength. The corresponding digold(I) σ-complexes (PPh₃)Au––Fl––Fl––Fl––Au(PPh₃) (OM1), (PPh₃)Au––Fl––Btz––Fl––Au(PPh₃) (OM2) and (PPh₃)Au––Fl––Btd––Fl––Au(PPh₃) (OM3) have also been prepared by a reaction of Au(PPh₃)Cl and methanolic NaOMe in DCM with the corresponding alkynyl functionalized oligomers to take advantage of the heavy-atom effect on their emissive properties. The synthesized rod-shaped π-conjugated fluorene based oligomers and their binuclear Au(I) σ-complexes have been unambiguously characterized by various spectroscopic tools such as FTIR and multinuclear NMR as well as MALDI-TOF and CHN analyses. The absorption and emission spectral studies exhibited a progressive red shift with increasing the electron withdrawing character of the central aromatic unit. The rod-like oligomers having alkynyl termini and the corresponding digold(I) complexes are found to be blue, cyan and yellow emissive, demonstrating the fine-tuning of the emission wavelength. Most importantly, the fluorene based π-conjugated yellow light emitters OH3 and OM3 are successfully achieved by varying the donor/acceptor moiety to the fluorenyl-alkynyl backbone. The digold(I) diacetylide organometallic wires exhibit phosphorescence at 77 K in degassed CH₂Cl₂ due to the efficient intersystem crossing from the S₁ to the T₁ excited state as induced by heavy atoms.