Donor–Acceptor Dihydropyrenes Switchable with Near-Infrared Light

The
use of low-intensity NIR light to operate molecular switches
offers several potential advantages including enhanced penetration
into bulk materials, in particular biological tissues, and reduced
radiation damage due to the limited photon energies. The latter, however,
pose a challenge for designing reasonably bistable systems. We have
developed a general design strategy for direct one-photon NIR photoswitches
based on negative photochromic dihydropyrenes carrying opposing strong
donor–acceptor substituents either along the long axis of the
molecule or across it. Thus, two series of 2,7- and 4,9-disubstituted
dihydropyrenes were synthesized, and their photothermal properties
investigated as a function of the type, strength, and position of
the attached donor and acceptor substituents as well as the polarity
of the environment. By shifting the excitation wavelength deep into
the NIR, both NIR one-photon absorption cross-section and photoisomerization
efficiency could be maximized while retaining a reasonable thermal
stability of the metastable cyclophanediene isomer. Thus, the lowest
optical transition was shifted beyond 900 nm, the NIR cross-section
was enhanced by two orders of magnitude, and the thermal half-lives
vary between milliseconds and hours. These unique features open up
ample opportunities for noninvasive, optically addressable materials
and material systems.