Efficient and Stable Blue Electrogenerated Chemiluminescence of Fluorene‐Substituted Aromatic Hydrocarbons

Abstract

Tailored molecular structure is the key to tune the electronic and spectroscopic properties of a molecule. This is always a goal in electrogenerated chemiluminescence (ECL), that is, to have a material with high photoluminescence (PL) quantum yield and stable, long-lived radical ions in solution upon oxidation and reduction. Among various ECL active materials, polyaromatic hydrocarbons (PAHs) were the earliest subjects of ECL studies, [1] and substituted PAHs have been widely investigated.[2] Indeed, the PAHs 9,10-diphenylanthracene (DPA) and rubrene (R) have usually been used as ECL standards.[1a] These molecules are intense ECL emitters because of their high fluorescence quantum yields and stable radical cations and anions in aprotic media. However, the ECL emission of DPA and R eventually decays. This might be attributed to the irreversibility of the second oxidation because of the instability of the dication, for example, DPA 2+. Although for generation of ECL spectra, the radical ions are typically obtained by pulsing the electrode potentials, slightly beyond ( % 100 mV) the first reduction and oxidation peak potentials, there is always some disproportionation of the radical cation leading to dication production. Thus, instability of doubly charged ions leads to slow decomposition. If the second oxidation wave is made more electrochemically reversible, the stability of ECL should be significantly improved. A similar argument has been made in considering the stability of electrochromic devices involving the generation of radical ions. [3] In aprotic media, ECL involves light emission due to the electron transfer process in the vicinity of the electrode with the intensity largely governed by the stability of the electrogenerated radical ions over the time scale of the potential sweeps (or steps) and the quantum efficiency of the generated excited state.[1a] The ECL spectrum of a compound is usually the same as the fluorescence spectrum because both methods produce the same excited state. However, the ECL spectra of some PAHs, like pyrene, show another extra broad and structureless peak at longer wavelengths, which is attributed to the formation of excimer.[4] Long wavelength emission can also be attributed to the formation of a byproduct during the electrolysis, for example, by the decomposition of the radical cation, as seen in ECL of anthracene.[1e]Fluorene-based molecules, like oligofluorenes [2a] and terfluorenes [5] are of interest in ECL and organic light emitting devices (OLED) because of their good electrochemical and thermal stabilities and high quantum yields. The present work reports that capping a DPA derivative, pyrene and anthracene, with two fluorene derivatives produces new aromatic hydrocarbons (FDF, FPF, FAF) (Figure 1) with very interesting ECL behavior, with enhanced ECL efficiency and stability as compared to their parent PAHs.Figure 2 depicts a comparison of the cyclic voltammograms (CV) of FAF, FDF, and FPF and their parent counterparts. The electrochemical data are summarized in Tabl...

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Twisted configuration pyrene derivative: exhibiting pure blue monomer photoluminescence and electrogenerated chemiluminescence emissions in non-aqueous media

“…Moreover, broad tailing above 430 nm can be observed, which is ascribed to the excimer of pyrene formed in MeCN : Bz (v/v ¼ 1 : 1). 12 It is clear that the maximum emission wavelength of Bz-Py is red-shied (by 33 nm) from Py, and is characterized by three dened vibrational structures around 407, 430, and 457 nm, which are similar to the vibrational structures observed in the PL spectrum of pyrene. The bump in the spectral line of Bz-Py around 480 nm is attributed to the Bz-Py excimer emission, which was conrmed in a previous report.…”

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