Synthesis, photophysics and photochemistry of substituted 2,7-Di-(t-butyl)-trans-10b,10c-dimethyl-10b,10c-dihydropyrenes
Date
2018-11-09
Authors
Sheepwash, Molina Audrey Lorraine
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Abstract
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.
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Keywords
Photochemistry, Photophysics, Metacyclophanediene, Dimethyldihydropyrenes