D149, a metal-free indoline dye, is one of the most promising
sensitizers
for dye-sensitized solar cells (DSSCs) and has shown very high solar
energy conversion efficiencies of 9%. Effective electron injection
from the excited state is a prerequisite for high efficiencies and
is lowered by competing deactivation pathways. Previous investigations
have shown surprisingly short-lived excited states for this dye, with
maximum lifetime components of 100–720 ps in different solvents
and less than 120 ps for surface-adsorbed D149. Using steady-state
and time-resolved fluorescence, we have investigated the photochemical
properties of D149 in nonpolar and polar solvents, polymer matrices,
and adsorbed on ZrO2, partially including a coadsorbent.
In solution, excitation to the S2 state yields a product
that is identified as a photoisomer. The reaction is reversible, and
the involved double-bond is identified by NMR spectroscopy. Our results
further show that lifetimes of 100–330 ps in the solvents used
are increased to more than 2 ns for D149 in polymer matrices and on
ZrO2. This is in part attributed to blocked internal motion
due to steric constraint. Conversely, concentration-dependent aggregation
leads to a dramatic reduction in lifetimes that can affect solar cell
performance. Our results explain the unexpectedly short lifetimes
observed previously. We also show that photochemical properties such
as lifetimes determined in solution are different from the ones determined
on semiconductor surfaces used in solar cells. The obtained mechanistic
understanding should help develop design strategies for further improvement
of solar cell dyes.