Photochemistry of Metal Complexes Studied by Time

1A9
Photochemistry of Metal Complexes Studied by Time-resolved Infrared
Spectroscopy
(1PRESTO-JST, 2Tokyo Tech.) Ken Onda1,2, Sei'ichi Tanaka2, Tatsuhiko Mukuta2,
Masataka Funaki2, Kyohei Tannno2
I. Introduction
Transition metal complexes are widely used in various photo-functional materials, for example, a
photosensitizer for organic solar cells and artificial photosynthetic systems, a photoredox reagent for
organic synthesis, and biological applications such as a fluorescence probe of protein and a
photo-anticancer drug. In spite of this wide application, the fundamental photoexcited processes in
metal complexes have not been well understood yet because of their complicated structures and
excited states. Time-resolved infrared vibrational spectroscopy is one of the best tools for studying
such excited state dynamics because vibrational peaks are sensitive to local charge and structure even
in complex molecular systems. However, the application of TR-IR to metal complexes was limited to
those including a CO group, which has strong infrared absorption. Using this method, we previously
studied various organic crystals consisting of hetero-cyclic conjugated molecules and showed that
stretching vibrations of double bonds in these molecules are a good probe for ultrafast variations of
charge and structure in the crystals [1-4]. Since similar molecules are often used in metal complexes as
a ligand, we have applied the same way to studies on photoexcited dynamics in metal complexes by
developing a suitable time-resolved infrared spectroscopy system.
II. Experimental
Time-resolved infrared (TR-IR) vibrational spectra were acquired using the pump-probe method
using a femtosecond Ti:sapphire chirped pulse amplifier (CPA; 800 nm, 120 fs, 1 kHz). A tunable
mid-infrared pulse (1000 - 4000 cm-1) was generated by optical parametric amplification (OPA) and
difference frequency generation (DFG) from the output of the CPA. The bandwidth of the IR pulse
was 150 cm−1. A tunable pump pulse (266, 400, 480 - 800 nm) was obtained by nonlinear optical
processes from the outputs of CPA and/or OPA. To acquire the transient IR absorption spectrum, the
probe pulse was dispersed by a 19 cm-1 polychromator and data were acquired using a 64-channel
MCT (mercury cadmium telluride) infrared
detector array.
III. Results and discussion
We first studied a prototypical ruthenium
complexes, [Ru(bpy)3]2+ and [Ru(bpy)2(bpm)]2+
(bpy = 2,2'-bipyridine, bpm = 2,2'-bipyrimidine)
[5]. Figure 1 shows the vibrational spectra of
these complexes at 500 ps after photoexcitation
with a 400 nm pulse. By comparing these
Fig. 1. TR-IR spectra of [Ru(bpy)2(bpm)]2+ (top)
and [Ru(bpy)3]2+ (bottom). Colored circles
represent assignments of the peaks.
complexes and isotope substituted complexes and by using
(a) non-reactive condition
quantum chemical calculations, we assigned all the observed peaks
at 1000 - 1700 cm-1 in the triplet metal to ligand charge transfer
state (3MLCT). In addition, from a careful analysis of temporal
3MC
3MLCT
variations of all these peaks, we found that only a peak at 1600
cm-1 shows a blue shift over 15 ps as shown in Fig. 2 (magnified
the oval area in Fig. 1). We revealed that this peak is assigned to
(b) reactive condition
the metal centered state (3MC) from dependence of the temporal
behavior of the peak on various parameters such as solvent, ligand,
and excitation wavelength. Figure 2 compares the temporal
3MLCT
3MC
variations of the peak under (a) non-reactive and (b) reactive
conditions. Because it is considered that the photoreaction occurs
via 3MC, disappearance of the peak at 1600 cm-1 strongly indicates
that this peak is assigned to the 3MC state.
Fig. 2. TR-IR spectra at around
1600 cm-1 under (a) non-reactive
and (b) non-reactive conditions
We learned from the result that temporal variations of vibrational peaks of ligands are a good probe
for photochemical process in metal complexes, so that we applied the same way to more practical
metal complexes. One example is the study on internal rotation of a phenyl group via - interaction
in [Re(dmb)(CO)2{P(p-FPh)3}2]+, which is one of CO2 reduction photocatalysts. Figure 3 shows the
temporal variations of this complex. Because
ph
only the peak assigned to phenyl group
(indicated by the red arrow) gradually increases
dmb
over 20 ps, we concluded that it takes 20 ps for
the phenyl groups to rotate via a - interaction
ph
after charge transfer to the dmb ligand. We are
now studying photoexcited dynamics in more
practical
metal
complexes,
for
example,
Fig. 3. Temporal variations of TR-IR spectra of
[Re(dmb)(CO)2{P(p-FPh)3}2]+.
characterization of excited states in water oxidation complexes, [Ru(tpy)(pynp)]2+, in aqueous solution
[6] and even multi-reagent and multi-step chemical reactions in supramolecular systems.
References
[1] Y. Matsubara, et al. "Photoinduced Ionic to Neutral Phase Transition in TTF-CA Studied by Time-resolved
Infrared Vibrational Spectroscopy", J. Phys. Soc. Jpn, 80, 124711 (2011).
[2] N. Fukazawa, et al. "Charge and structural dynamics in photoinduced phase transition of (EDO-TTF)2PF6
examined by picosecond time-resolved vibrational spectroscopy", J. Phys. Chem. C, 116, 5892 (2012).
[3] N. Fukazawa, et al. "Time-Resolved Infrared Vibrational Spectroscopy of the Photoinduced Phase Transition
of Pd(dmit)2 Salts Having Different Orders of Phase Transition", J. Phys. Chem. C, 117, 13187 (2013).
[4] K. Onda, et al. "Diverse Photoinduced Dynamics in an Organic Charge-Transfer Complex Having Strong
Electron–Phonon Interactions", Acc. Chem. Res. 47, 3494 (2014).
[5] T. Mukuta, et al. "Infrared Vibrational Spectroscopy of [Ru(bpy)2(bpm)]2+ and [Ru(bpy)3]2+ in the Excited
Triplet State" Inorg. Chem. 53, 2481 (2014).
[6] S. Tanaka, et al. "Characterization of the Excited States of Distal- and Proximal-[Ru(tpy)(pynp)OH2]2+ in
Aqueous Solution Using Time-Resolved Infrared Spectroscopy" submitted.