Synthesis and photochemistry of two quinoline analogs of the perimidinespirohexadienone family of photochromes

We report the detailed synthesis and photochemistry of two analogs (specifically 3,5-di-tert-butyl-7′-methyl- and 3,5-di-tert-butyl-7′,9′-dimethyl-1′,3′-dihydrospirocyclohexa[2,5]diene-1,2′-pyrido[4,3,2-de]quinazolin-4-one) of the perimidinespirohexadienone (3,5-di-tert-butyl-1′,3′-dihydrospirocyclohexa[2,5]diene-1,2′-perimidin-4-one) family of photochromes in which the naphthalene moiety of the parent is replaced by a quinoline, and compare them to the parent compound. Molar absorptivities of both the short wavelength spirocyclic isomer (SW) and long wavelength quinonimine isomer (LW) of each were determined by a combination of proton NMR and UV–vis spectroscopy in solvents of varying polarity. Quantum yield measurements for photoisomerization of SW to LW are reported in those same solvents, with qualitative extrapolation to additional solvents. The position and rate of the thermal equilibrium reverting LW to SW is estimated for these compounds. The 9′-methyl in SW (6-methyl in LW) is found to be essential for complete reversion of LW to SW in the dark. Finally one-dimensional NOE NMR spectroscopy was used to conclusively determine the structure of LW for the quinoline analogs as the 4-(5-aminoquinolin-4-ylimino)-2,6-di-tert-butylcyclohexa-2,5-dienone resulting from opening toward the quinoline nitrogen, rather than the 4-(4-aminoquinolin-5-ylimino) structure that would result from spirocyclic ring opening away from the quinoline nitrogen which had been initially proposed by Minkin et al. for very similar compounds [V.I. Minkin, V.N. Komissarov, V.A. Kharlanov, Perimidinespirocyclohexadienones, in: J.C. Crano, R.J. Guglielmetti (Eds.), Organic Photochromic and Thermochromic Compounds, vol. 1, Plenum Press, New York, 1999, pp. 315–340, and references therein].