A group of highly fluorescent 2,1,3-benzothiadiazole derivatives (BH0-BH3), including two fluorene derivatives (AB2 and C01) were synthesized and characterized. The electrochemical, spectroscopic, and electrogenerated chemiluminescence (ECL) properties of the compounds were determined. Benzothiadiazole derivatives BH1, BH2, and BH3 show reversible oxidation and reduction waves and produce strong green ECL in nonaqueous solutions. This ECL could be seen by the naked eye, even in a well lit room. The fluorene derivatives, C01 and AB2, also produce bright, easily observable ECL. Since the ECL spectra are at essentially the same wavelengths as the photoluminescence (PL) spectra, and the energies of the electron transfer reactions are greater than the singlet state energies, we propose direct formation of the excited singlet state during ion annihilation. BH0, which shows a quasi-reversible oxidation wave, only produced weak ECL via direct annihilation but gave strong ECL with benzoyl peroxide (BPO) as a coreactant. The ECL quantum efficiencies of the series, compared to that of 9,10-diphenylanthrace, was estimated to range from 0.05 to 7%. This series shows rare green photoluminescence (lambda(PL) = 490-556 nm) with a high PL quantum efficiency in solution (Phi = 5 to 90%).