Study on the Conformation of an Isopropyl-Substituted Furylfulgide. Photochromic Coloring Reaction and Thermal Racemization

The conformations of the E-forms of photochromic furylfulgides with a methyl substituent (1E) and an isopropyl substituent (2E) were investigated by X-ray crystallographic analyses, NMR measurements, and molecular orbital (MO) calculations in relation to the large quantum yield of the coloring reaction of 2E upon the irradiation of UV light. The conformations of 1E and 2E obtained by X-ray crystallographic analyses are the photocyclization-possible ones (Eα), in which the distances of the bond-forming carbon atoms of 1E and 2E are essentially the same. NOESY of 1H NMR measurements of 1E and 2E showed that they take both the Eα and cyclization-impossible Eβ conformations in solution. Although MO calculations of 2E proved that the population of 2Eα is dominant when the entropy contributions of the rotation of substituents are considered, the large difference in the quantum yields was not fully elucidated. The difference may have come from a difference in the nature of the excited states of 1E and 2E, which is evidenced by the large difference in the molar absorption coefficient (1E: 6700, 2E: 4250 mol−1 dm3 cm−1 in toluene). The thermodynamic parameters of the enantiotopomerization process of 2E were determined by variable-temperature 1H NMR measurements. The mechanism of enantiotopomerization was specified by MO calculations.