Identification of Nonradiative Decay Pathways in Cy3

Photoexcited
fluorescent markers are extensively used in spectroscopy,
imaging, and analysis of biological systems. The performance of fluorescent
markers depends on high levels of emission, which are limited by competing
nonradiative decay pathways. Small-molecule fluorescent dyes have
been increasingly used as markers due to their high and stable emission.
Despite their prevalence, the nonradiative decay pathways of these
dyes have not been determined. Here, we investigate these pathways
for a widely used indocarbocyanine dye, Cy3, using transient grating
spectroscopy. We identify a nonradiative decay pathway via a previously
unknown dark state formed within ∼1 ps of photoexcitation.
Our experiments, in combination with electronic structure calculations,
suggest that the generation of the dark state is mediated by picosecond
vibrational mode coupling, likely via a conical intersection. We further
identify the vibrational modes, and thus structural elements, responsible
for the formation and dynamics of the dark state, providing insight
into suppressing nonradiative decay pathways in fluorescent markers
such as Cy3.