Synthesis, photophysics and photochemistry of substituted 2,7-Di-(t-butyl)-trans-10b,10c-dimethyl-10b,10c-dihydropyrenes




Sheepwash, Molina Audrey Lorraine

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The photochromism of several simple substituted and [e]-annelated 10b, 10c-dimethyl-10b, 10c-dihydropyrene derivatives was studied using steady state and time resolved fluorescence techniques as well as chemical actinometry and laser flash photolysis (LFP). The purpose of this study was to determine the mechanism of the photoisomerization between the closed, coloured dimethyldihydropyrene isomer and the open, colourless metacyclophanediene isomer. A detailed understanding of the switching mechanism of such compounds will allow for the rational design of multichromophoric switches in the future. Fluorescence from the dimethyldihydropyrene isomers was weak (ɸ < 0.03). The simple substituted systems exhibited sharp emissions comprised of a single transition while the [e]-annelated derivatives emission were broader and resolved into two bands at low temperature (77 K). The fluorescence lifetimes for the dimethyldihydropyrene isomers were between 2.4 and 5.6 ns. The emission for the metacyclophanediene isomers was found to be very structured with lifetimes between 12 and 17 ns for most derivatives. The photoisomerization was found to proceed through the singlet excited state and bond breakage / formation occurred on the nanosecond timescale as determined by LFP. The triplet excited state, although formed, was not involved in the switching mechanism. The ring opening isomerization quantum yields were found to be low for the simple substituted systems (≤ 0.012) but were improved upon [e]-annelation (0.042 - 0.095). The ring closing isomerization quantum yields for the annelated systems were found to be much higher than the ring opening efficiencies (0.28 — 0.42) and were insensitive to substitution or the nature of the fused arene moiety. Future synthesis and studies should be based on the [e]-annelated architectures.



Photochemistry, Photophysics, Metacyclophanediene, Dimethyldihydropyrenes