The photochromic reaction of two derivatives of bisthiazolylindenols was investigated in three solvents of various polarities (C6D12, THF-d8 and CD3CN) by NMR spectroscopy. Photoirradiation can generate two diastereomeric closed forms. High conversion ratio and large diastereomer excess were obtained and the reasons of the excellent properties were discussed in terms of intramolecular hydrogen bonds, steric bulkiness of substituents, and solvent polarities. Two sets of intramolecular nitrogen-hydrogen interactions fix the conformation of the two investigated bisthiazolylindenols in favor of cyclization, which also lead to high diastereoselectivity. The N-H interactions are partially disrupted when polarity of the solvent increases.
