Tuning Azoheteroarene Photoswitch Performance through Heteroaryl Design

Photoswitchable
compounds, which can be reversibly switched between
two isomers by light, continue to attract significant attention for
a wide array of applications. Azoheteroarenes represent a relatively
new but understudied type of photoswitch, where one of the aryl rings
from the conventional azobenzene class has been replaced with a five-membered
heteroaromatic ring. Initial studies have suggested the azoheteroarenesthe
arylazopyrazoles in particularto have excellent photoswitching
properties (quantitative switching and long Z isomer
half-life). Here we present a systematic computational and experimental
study to elucidate the origin of the long thermal half-lives and excellent
addressability of the arylazopyrazoles, and apply this understanding
to determine important structure–property relationships for
a wide array of comparable azoheteroaryl photoswitches. We identify
compounds with Z isomer half-lives ranging from seconds
to hours, to days and to years, and variable absorption characteristics,
all through tuning of the heteraromatic ring. Conformation perhaps
plays the largest role in determining such properties: the compounds
with the longest isomerization half-lives adopt a T-shaped ground
state Z isomer conformation and proceed through a
T-shaped isomerization pathway, whereas the most complete photoswitching
is achieved for compounds that have a twisted (rather than T-shaped) Z isomer conformation. By balancing these factors, we report
a new azopyrazole 3pzH, which can be quantitatively switched
to its Z isomer (>98%) with 355 nm irradiation,
near-quantitatively
(97%) switched back to the E isomer with 532 nm irradiation,
and has a very long half-life for thermal isomerization (t1/2 = 74 d at 25 °C). Given the large tunability
of their properties, the predictive nature of their performance, and
the other functional opportunities afforded by usage of a heteroaromatic
system, we believe the azoheteroaryl photoswitches to have huge potential
in a wide range of optically addressable applications.