Electrochemistry and electrogenerated chemiluminescence of three phenanthrene derivatives, enhancement of radical stability, and electrogenerated chemiluminescence efficiency by substituent groups.

The electrochemistry and electrogenerated chemiluminescence (ECL) of three phenanthrene derivatives, 3,6-diphenyl-9,10-bis-(4-tert-butylphenyl)phenanthrene (TphP, T1), 3,6-di(naphthalen-2-yl)-9,10-bis(4-tert-butylphenyl)phenanthrene (TnaP, T2), and 3,6-di(pyrene-1-yl)-9,10-bis(4-tert-butylphenyl)phenanthrene (TpyP, T3), are investigated in an acetonitrile:benzene (v:v = 1:1) solvent. Cyclic voltammetry (CV) of the three derivatives shows reversible reduction waves and less chemically reversible oxidation waves at low scan rates. The CV character becomes more reversible, and the stability of the radical cations increases as the conjugation of the substituent groups appended to the phenanthrene increases. This finding indicates that the radical ion stabilities in phenanthrene derivatives are drastically improved by increasing the conjugation of the substituent groups; thus, electrochemically stable radical ions can be obtained by introducing more conjugated groups to the phenanthrene center. Additionally, ECL is generated for all compounds by radical ion annihilation, and the ECL spectrum shows good agreement with the fluorescence emission, assigned as emission by the S-route. ECL efficiencies for radical ion annihilation are 0.004 for TphP, 0.16 for TnaP, and 0.25 for TpyP, respectively, and the ECL efficiency increases with the conjugation of the substituent groups appended to the phenanthrene increases. Radical ion annihilation produced by potential steps exhibits asymmetric ECL transients in which the cathodic ECL pulse is smaller than the anodic pulse due to the instability of the radical cation. These molecules can produce a stronger ECL, which can be observed by the naked eye in a lighted room, on reduction in the presence of a coreactant (benzoyl peroxide).