The protonation behavior of bicyclo[2.2.2]octadiene (BCOD)-fused thiaporphyrin and thiabenzoporphyrin was examined by titration with trifluoroacetic acid. Neutral, monorotonated, and diprotonated species of thiaporphyrins were easily detected by UV-vis spectral measurement. The first (K-1) and second (K-2) equilibrium constants were found to be K-1 = 520 and K-2 approximate to 7.1 for BCOD-fused thiaporphyrin and K-1 = 1.4 x 10(4) and K-2 approximate to 1.4 for thiabenzoporphyrin. The difference between K-1 and K-2 values of thiaporphyrins is larger than that observed for porphyrins (N-4-porphyrins). The main contributing conjugation pathways for thiaporphyrins in both protonated species were investigated by NICS and AICD quantum calculations using their atomic geometries obtained by X-ray structural analysis. The 18 pi-conjugation pathways of the neutral, monoprotonated, and diprotonated species thiaporphyrins were found to contain 18, 19, and 20 atoms, respectively. Results of H-1 NMR studies also support the major contributing pathways. (C) 2016 Elsevier Ltd. All rights reserved.