When copper dimethylphenanthroline absorbs a photon (bottom), it transitions from a ground state (S0) to an electronically excited (S1) state that causes the molecular structure to flatten. Ultrafast laser spectroscopy has revealed that the molecule coherently vibrates for a short time (upper center) before distorting. Reproduced, with permission, from Ref. 1, 2011 American Chemical Society |
Molecules
that suddenly transform into new structures when stimulated by photons
or electrons play key roles in many chemical and biological processes.
Recently, chemists have discovered that adding transition metals such as
copper to photo-responsive organic ligands produces materials with high
solar conversion efficiencies, owing to the metal’s ready supply of
light-activated electrons. But despite the interest in these substances
for opto-electronic devices, their inner workings remain mostly
inscrutable because the charge-transfer dynamics happen too quickly for
detection by typical instruments.
Tahei
Tahara and colleagues from the RIKEN Advanced Science Institute, Wako,
have spearheaded development of ultrafast laser spectroscopy that can
capture these high-speed reactions by taking ‘snapshots’ of
photochemical transformations with quadrillionths-of-a-second (10-15 s)
accuracy. Now, an unprecedented finding by the research team—a
picosecond (10-12 s) time delay during a theoretically instantaneous
distortion—is set to overturn current thinking about light-driven
rearrangements in transition metal complexes.
Copper
dimethylphenanthroline is a compound containing two propeller-shaped
wings, made out of thin aromatic sheets. Chemists regularly use it to
explore photo-induced structural changes. In its unexcited state, the
complex’s wings are oriented perpendicular to each other. But when
illuminated at a specific wavelength, the copper ion absorbs a photon
and transfers an electron to the sheets—an action that flattens the
structure by disrupting critical copper—phenanthroline bonds.
The
exact flattening mechanism, however, has been controversial because
copper electrons can be photo-excited in two different ways: through an
easily accessible high-energy state called S2, or a harder-to-spot,
low-energy transition called the S1 state. Tahara and colleagues tracked
the extremely fast relaxation process from both states and found that
S1 electrons provoked the flattening. This finding will allow
researchers to eventually squeeze as much efficiency as possible from
these devices.
When
the team examined how the molecule behaved in the S1 excited state,
they saw unexpected oscillations in the absorption signals during its
picosecond-long lifetime. According to Tahara, these signals are
unmistakable evidence that the excited complex vibrates coherently in
place and waits a short while before distorting.
Because
this result contradicts traditional understandings of transition metal
processes—atomic movements were theorized to immediately follow
excitation to S1-type electronic states—it may spark revolutionary
changes in how chemists conceive and control photo-initiated reactions.
“This is a fundamental and deep issue,” says Tahara.
By
expanding this technique to other poorly understood metal complexes,
the team hopes to produce ‘textbook-type’ results that can guide future
development of these remarkable materials.
The corresponding author for this highlight is based at the Molecular Spectroscopy Laboratory, RIKEN Advanced Science Institute.
