The electrogenerated chemiluminescence of novel organic donor-acceptor emitters as well as study heterogeneous electron transfer kinetics using scanning electrochemical microscopy

Abstract

New modalities and novel emitters were investigated for the production of electrogenerated chemiluminescence (ECL). In annihilation ECL, a light-emitting excited state is formed upon reaction of two electrochemically generated species, typically a radical anion and a radical cation. Donor-acceptor (DA) molecules provide a means of generating these two reactive species within the same molecule but where the oxidized and reduced centers are separated; furthermore, they allow one to explore the ECL properties of multiply charged radical ions. Three new efficient ECL-emitting donor-acceptor molecules were investigated. The effects of conjugation in the electrochemistry of diphenylaminospirobifluorenylfumaronitrile (FPhSPFN), which has the structure of D-X-A-A-X-D, where X is a linker, as well as the effects of the stability of its (multiply charged) radical ions on its red ECL emission (λmax= 708 nm) were studied; the molecule shows solvatochromism and different emission yields on both photoluminescence and ECL in benzene:acetonitrle mixtures. The possibility of generating ECL through multiply charged radical ions was further tested with the very efficient 1b emitter (4,7-bis(4-(4-sec-butoxyphenyl)-5-(3,5-di(1-naphthyl)phenyl)thiophen-2-yl)-2,1,3-benzothiadiazole). Two reversible oxidations and one reduction were observed. The more sluggish reduction is proposed to be a consequence of a long distance electron transfer to the buried acceptor center; further confirmation of this effect was pursued by application of the scanning electrochemical microscope (SECM) to model systems. 1b emits intense ECL with λmax= 635 nm and with an efficiency 330% of the ECL standard 9, 10-diphenylanthracene and similar intensity to the red emitting standard tris(2,2′-bipyridine)ruthenium(II) perchlorate (Rubpy). The generation of asymmetric chronoamperometric ECL pulses upon generation of radical anion-radical dication annihilation events was explained by the use of digital simulation, and proven to be a consequence of asymmetry in the amount of generated charges rather than instability of the electrogenerated species. ECL was also produced from a film of a red fluorophore 1a (4,7-bis(4-(n-hexyl)-5-(3,5-di(1-naphthyl)phenyl)thiophen-2-yl)-2,1,3-benzothiadiazole) with a coreactant in PBS buffer solution. The electrochemical synthesis of carbon quantum Dots (C QDs) in inert atmosphere was explored using highly oriented pyrolytic graphite as the starting material, for its later use in the production of ECL in the radical annihilation mode. FT-IR (ATR), mass spectrometry (desorption chemical ionization), Raman and TEM analysis were used to characterize the C QDs.