Synthesis and electropolymerization of an ion sensing and fluorescent fluorene derivative bearing a quinoxaline moiety and its analogues with different donor units

A series of new fluorene derivatives bearing pendant quinoxaline moieties with different donor groups, namely 5′H-spiro[fluorene-9,4′-pyrrolo[1,2-a]quinoxaline] (FQ), 2,7-di(thiophen-2-yl)-5′H-spiro[fluorene-9,4′-pyrrolo[1,2-a]quinoxaline] (TQT), 2,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-5′H-spiro[fluorene-9,4′-pyrrolo[1,2-a]quinoxaline] (EQE) and 2,7-bis(3,3-dihexyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepin-6-yl)-5′H-spiro[fluorene-9,4′-pyrrolo[1,2-a]quinoxaline] (PQP) were synthesized. The monomers were electropolymerized via potentiodynamic methods in order to understand the effects of donor units on the electrochemical and optoelectronic properties of the resulting polymers. It was found that polymer films exhibit reversible redox behavior accompanied with a reversible electrochromic behavior (Epox = 0.95 V for poly(2,7-di(thiophen-2-yl)-5′H-spiro[fluorene-9,4'-pyrrolo[1,2-a]quinoxaline]) (PTQT), Epox = 0.90 V for poly(2,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-5′H-spiro[fluorene-9,4′-pyrrolo[1,2-a]quinoxaline]) (PEQE) and Epox = 0.67 V for poly(2,7-bis(3,3-dihexyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepin-6-yl)-5′H-spiro[fluorene-9,4′-pyrrolo[1,2 a]quinoxaline]) (PPQP)). The band gap values (Eg) of the polymers were found to be 2.47, 1.93 and 1.66 eV for PTQT, PEQE and PPQP, respectively. Furthermore, the ion sensitivity of PQP and its polymer PPQP was also investigated by monitoring the change in the fluorescence intensity. Among various common ions, both PQP and PPQP were found to be selective towards Fe2+ ions by quenching the fluorescence efficiency with a Stern-Volmer constant (Ksv) of (5.9 × 103 M−1) and (2.7 × 104 M−1) for monomer and polymer solutions, respectively.