Technologies which utilize near-infrared (700 – 1000 nm) and short-wave infrared (1000
– 2000 nm) electromagnetic radiation have applications in deep-tissue imaging, telecommunications
and satellite telemetry due to low scattering and decreased background signal in
this spectral region. However, there are few molecular species, which absorb
efficiently beyond 1000 nm. Transition dipole moment coupling (e.g.
J-aggregation) allows for redshifted excitonic states and provides a pathway to
highly absorptive electronic states in the infrared. We present aggregates of two cyanine dyes whose
absorption peaks redshift dramatically upon aggregation in water from ~ 800 nm to 1000 nm and 1050
nm with sheet-like morphologies and high molar absorptivities (e ~ 10<sup>5 </sup>M<sup>-1</sup>cm<sup>-1</sup>).
To describe this phenomenology, we extend Kasha’s model for J- and H-aggregation to describe the excitonic states of <i> 2-dimensional aggregates</i> whose slip is controlled
by steric hindrance in the assembled structure. A consequence of the increased
dimensionality is the phenomenon of an <i>intermediate
</i>“I-aggregate”, one which redshifts yet displays spectral signatures of
band-edge dark states akin to an H-aggregate. We distinguish between H-, I- and
J-aggregates by showing the relative position of the bright (absorptive) state
within the density of states using temperature dependent spectroscopy. Our
results can be used to better design chromophores with predictable and tunable aggregation
with new photophysical properties.
Probing intramolecular vibronic coupling through vibronic-state imaging
