Benzo[d]thiazol-2-ylmethanol undergoes progressive oligomerization when treated under solvothermal condition with FeCl 3·6H 2O to give heterocycle aggregate (1,2,3-tris(benzo-[ d]thiazol-2-yl)-2,9-dihydrobenzo-[ b]cyclopenta-[ e][1,4]-thiazine. Four single-crystal structures analyses have been performed on compounds isolated during the reactions and 15 consecutive steps have been deciphered from ESI-MS of both solid products and the intermediate reaction solutions. These progressive steps involve intermolecular C-C coupling (9 steps) and intramolecular ring expansion (6 steps). Each of the non-carbon atoms (N, O, and S) has its particular function due to their position on the heterocycle: (i) The N and O coordinated with Fe(III) to activate the reaction site, ( ii) C-O homolytic cleavage promoted C-C coupling reactions, and ( iii) C-S migration caused intramolecular ring expansion. Importantly, through theoretical calculations, decrease in Gibbs free energy of intramolecular reaction pathways support this mechanism and activation mode, which indicated that the Fe(III) is necessary for the reaction to progress. The investigation of photophysical properties revealed that the heterocyclic aggregates exhibited good luminescent behavior in the wavelength range of 535–610 nm, approaching the near infrared region. This finding underscores the significance of this reaction pathway and the identification of its mechanistic steps in facilitating the synthesis of functional oligomers and polymers from monomers, particularly through the catalytic promotion by inexpensive metal ions.