Electrogenerated chemiluminescence (ECL) is a light emission
phenomenon
caused by electrochemically generated radical anions (R•–) and cations (R•+), in which the ion annihilation
results in the formation of a pair of excited (R*) and ground state
(R) of a luminescent molecule. Here, the ECL properties of pyrene
(Py) and 2,7-di-tert-butylpyrene (di-t-BuPy) are reported. It was found that at a commonly employed concentration
(1 mM), the ECL spectra were time-dependent because of increasing
the oligomer emission and increasing the concentration of R near R*,
leading to an enhancement of the excimer emission. At a low concentration
range (20–30 μM), the shape of the ECL spectra containing
the monomer and excimer emission was determined by isolated pairs
of R* and R, which were generated through ion annihilation of R•– and R•+. It was found that
in the ECL of Py and di-t-BuPy originated from the
isolated pairs of R•– and R•+, 58 and 48% of the excited states were the excimer states, respectively.
Diffusion equation analysis indicates that the lower excimer formation
in the case of di-t-BuPy is because of a farther
initial separation distance between R* and R, i.e., a longer electron transfer distance between the radical ions. The
Marcus model for the electron transfer kinetics suggests that the
farther electron transfer distance is mainly caused by the larger
molecular size, which resulted in a smaller reorganization energy
of the solvent acetonitrile molecule. Taking advantage of the photophysical
and electrochemical properties of Py and di-t-Bu
Py, the monomer and excimer emission in ECL is discussed.