Excited-state relaxation dynamics of Re(I) tricarbonyl complexes with macrocyclic phenanthroline ligands studied by time-resolved IR spectroscopy

Article abstract of DOI:10.1039/b820748h

Two sets of supramolecular rhenium carbonyl-phenanthroline complexes were prepared: fac-[Re(Cl)(CO)₃(N,N)] and fac-[Re(Etpy)(CO) ₃(N,N)]⁺, where N,N is 2,9-di-anisyl-1,10-phenanthroline (dap) or two related macrocyclic lig

Full text of DOI:10.1039/b820748h

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Excited-state relaxation dynamics of Re(I) tricarbonyl complexes with
macrocyclic phenanthroline ligands studied by time-resolved IR spectroscopy†
Ana Mar´ıa Blanco-Rodr´ıguez,a Michael Towrie,b Jean-Paul Collin,c Stanislav Za´lisˇd and Anton´ın Vlcˇek Jr.*a,d
Received 20th November 2008, Accepted 4th February 2009
First published as an Advance Article on the web 10th March 2009
DOI: 10.1039/b820748h
Two sets of supramolecular rhenium carbonyl-phenanthroline complexes were prepared:
fac-[Re(Cl)(CO)3(N,N)] and fac-[Re(Etpy)(CO)3(N,N)]+, where N,N is 2,9-di-anisyl-1,
10-phenanthroline (dap) or two related macrocyclic ligands, where the two anisyl groups are connected
by a polyether chain of different length and rigidity (m27, m30), which wraps around and above the
equatorial Re(CO)2 group. The excited-state character and relaxation dynamics of these complexes were
∫
investigated by picosecond time-resolved IR spectroscopy in the spectral region of C O stretching
vibrations, n(CO). The results were interpreted with the aid of DFT and TD-DFT calculations of the
molecular structures and electron-density redistribution upon excitation. [Re(Cl)(CO)3(phen-
macrocycle)] in CH2Cl2 have the same type of lowest excited state as analogous acyclic phen or bpy
complexes, that is a mixed Re(CO)3→phen and Cl→phen MLCT/XLCT, together with some
pp*(phen) IL character. Its relaxation dynamics are qualitatively similar to those of phen or bpy
complexes. However, relaxation of [Re(Cl)(CO)3(m30)] shows a slow kinetics component (~22 ps) which
arises from confined local solvent molecules and/or from conformational movements of the flexible
m30 polyether ring. In contrast, attaching anisyl groups at the 2 and 9 phen positions in
[Re(Etpy)(CO)3(phen-macrocycle)]+ effectively “freezes” excited-state relaxation in MeCN, regardless
of the presence of the macrocyclic ring. The lowest excited triplet state has a mixed MLCT/IL
character. Restricting the solvent access to the excited chromophore clearly affects both the character
and dynamics of the lowest excited state.
Recently, it was found that excited-state relaxation of Re(I)
Introduction
carbonyl-polypyridyl complexes fac-[Re(L)(CO)3(N,N)]n+ (L =
halide, n = 0; L = 4-Et-pyridine (Etpy), imidazole, n = 1;
N,N = 2,2¢-bipyridine (bpy) or 1,10-phenanthroline (phen))
depends sensitively on the nature of the surrounding medium
and its dynamical properties.6–11 While investigating relaxation
dynamics of Re(CO)3(phen) chromophores attached to proteins,
we have realized that Re(I) carbonyl-diimines can be employed
as probes reporting on solvation, flexibility and dynamics of
binding sites of biomolecules and other complex molecular
systems.10 However, the nature of the medium motions involved
and the mechanism whereby they are coupled with the relaxation
dynamics of the excited state of the molecular probe are far from
understood.
Herein, we have set out to investigate the dynamics of a
system where solvation is restricted by a bulky ring-shaped
ligand (Fig. 1) which partly shields the Re(CO)3 group. To this
effect, we prepared a series of complexes [Re(Cl)(CO)3(N,N)]
and [Re(Etpy)(CO)3(N,N)]+ with N,N = 2,9-macrocycle-1,10-
phenanthroline ligands (phen-macrocycle), and investigated their
excited-state relaxation dynamics employing picosecond time
Macrocyclic polypyridine ligands form interesting metal com-
plexes in which a polyether ring wraps around a metal center,
affecting its electrochemical and photochemical behavior.1–5 Lig-
ands of this type were used to construct various metal catenanes
and rotaxanes. Some of them act as “molecular motors” in
which mechanical movement is accomplished electrochemically
or photochemically by changing the oxidation or electronic state
of the metal center, respectively.1–3 Wrapping the polyether ring of
a macrocyclic polypyridine ligand ring around Ru(II)-containing
chromophores was found to change their photoreactivity. Gener-
ally, the metal center is stabilized against photosubstitution by sol-
vent. Instead, irradiation leads to intriguing photoisomerizations
and rearrangements.4 In mixed-polypyridine Ru(II) complexes,
the sterically demanding macrocyclic ligand is least amenable to
photodissociation.
aSchool of Biological and Chemical Sciences, Queen Mary, University of
London, Mile End Road, London, United Kingdom E1 4NS
bCentral Laser Facility, STFC Rutherford Appleton Laboratory, Chilton,
Didcot, Oxfordshire, United Kingdom OX11 0QX
∫
resolved IR spectroscopy (TRIR) in the spectral region of C O
cLaboratoire de Chimie Organo-Mine´rale, UMR 7177 du CNRS, Institut
Le Bel, Universite´ Louis Pasteur, 4 rue Blaise Pascal, 67000, Strasbourg,
France
stretching vibrations, n(CO). If the macrocycle surrounding the
Re(CO)3 moiety induces constraints on the solvation mechanism,
or its own motions couple with the relaxation of the 3MLCT lowest
excited state of the Re(CO)3(phen) moiety, it should be possible to
follow these effects by means of time-dependent shifts of excited-
state n(CO) bands in TRIR spectra.8–10
dJ. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of
the Czech Republic, Dolejsˇkova 3, CZ-182 23, Prague, Czech Republic.
E-mail: a.vlcek@qmul.ac.uk
† Electronic supplementary information (ESI) available: Additional exper-
imental details and Fig. S1–S4. See DOI: 10.1039/b820748h
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3941–3949 | 3941
©

Fig. 1 Schematic structures of the 2,9-macrocycle-1,10-phenanthroline ligands (phen-macrocycle). The ligands m27 and m30 differ in the ring size and
flexibility, while dap was chosen as an open-ring reference.
NH4PF6 in methanol (~3 mL) was added. Water was added to form
a precipitate, which was then filtered and repeatedly recrystallized
from methanol. The products were characterized as single-species
tricarbonyls by FTIR, 1H-NMR and MS, Table 1 and electronic
supplementary information, ESI.†
Experimental
Materials
MeCN and CH2Cl2 solvents (spectroscopic quality) and
Re(CO)5Cl were used as obtained from Aldrich. The ligands
dap, m27 and m30 (Fig. 1) were synthesized and characterized
as described previously.12,13
The [Re(Cl)(CO)3(phen-macrocycle)] and [Re(Etpy)(CO)3-
(phen-macrocycle)]+ complexes were synthesized by a well-
established route, known9,14–18 to selectively produce the de-
sired tricarbonyl-polypyridine complexes. [Re(Cl)(CO)3(phen-
macrocycle)] was obtained by refluxing the phen-macrocycle
Spectroscopic measurements. Time resolved IR (TRIR) spec-
tra were measured using the equipment and procedures described
in detail previously.17,19–24 In short, the sample solution was
excited (pumped) at 400 nm, using frequency-doubled pulses
from a titanium sapphire laser of ~150 fs duration (FWHM).
TRIR spectra were probed with IR pulses (~150 fs) obtained
by difference-frequency generation. The IR probe pulses cover
a spectral range ca. 200 cm-1 wide, tunable from 1000 to 5000 cm-1
(i.e., 2–10 mm). The cell was rapidly oscillated in the plane
perpendicular to the direction of the laser beams in order to
minimise the potential build-up of decomposition products on
the windows. The sample integrity was checked by FTIR spectra
measured before and after each experiment. All spectral fitting
procedures were performed using Microcal Origin 7 software.
ligand with [Re(Cl)(CO)5] for
4 h in toluene, followed
by filtration and repeated washing with hexane. To make
the [Re(Etpy)(CO)3(N,N-macrocycle)]PF6 complexes, the corre-
sponding [Re(Cl)(CO)3(phen-macrocycle)] was first converted to
[Re(OTf)(CO)3(phen-macrocycle)] by stirring 1.88 mmol with
Ag(CF3SO3) (1.3 m. eq.) in dry THF (~20 mL) for 4–5 h. The
mixture was filtered through Celite, and its volume doubled with
petroleum spirits (60–80 ◦C). The solvent was removed to a
minimum volume and the precipitate formed was filtered and
dried in vacuo. The product was immediately used for the next
synthetic step, whereby it was reacted with 4-ethyl-pyridine (Etpy)
(1.1 m. eqv.) in THF for 12 h. The mixture was filtered hot and
the solvent was removed in vacuo. The solid was dissolved in
the minimum amount of methanol and a saturated solution of
DFT and TDDFT calculations. The electronic structures were
calculated by density functional theory (DFT) methods using
the Gaussian 0325 program package. For H, C, N, O and Cl
atoms 6-31G* polarized valence double - z basis sets26 were used,
together with quasirelativistic effective core pseudopotentials
and corresponding optimized set of basis functions for Re.27
Table 1 Ground- and excited-state n(CO) energies of [Re(Cl)(CO)3(phen-macrocycle)] and [Re(Etpy)(CO)3(phen-macrocyle)]+ measured in CH2Cl2 and
MeCN, respectively. Excited-state A¢¢ and A¢(2) energies are estimated from spectra measured at 1 ns. A¢(1) energies are extrapolated to an infinite time
delay (n(•)). Band assignment is based on ref. 34–36
Excited state (extrapolated
to t = •)
Ground state
A¢(2)
A¢¢
A¢(1)
A¢¢
A¢(2)
A¢(1)
2050
2043
2049
Dn A¢¢
Dn A¢(2)
Dn A¢(1)
[Re(Cl)(CO)3(dap)], exp.
[Re(Cl)(CO)3(dap)], calc.b
[Re(Cl)(CO)3(m27)]
1884
1888
1884
1885
1923
1924
1921
1924
1920
1927
1924
1923
1924
1921
2022
2022
2023
2022
2029
2029
2028
~1960 sh
1951
1953
~1972
1978
1973
~36
31
26
—
38
44
37
~88
90
89
28
21
26
[Re(Cl)(CO)3(m30)]
1950–1980a
2049
—
27
[Re(Etpy)(CO)3(dap)]+
[Re(Etpy)(CO)3(m27)]+
[Re(Etpy)(CO)3(m30)]+
1961
1968
1958
~2010
~2010
~2010
<2041
<2042
<2045
~87
~86
~89
<12
<13
<17
a Broad unresolved band. b UKS (B3LYP, CPCM for CH2Cl2) Calculated energies scaled by 0.973.
3942 | Dalton Trans., 2009, 3941–3949
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The Royal Society of Chemistry 2009
©

Calculations employed the hybrid functional B3LYP.28 The solvent
was described by the polarizable conductor calculation model
(CPCM).29 Geometry optimizations were performed without any
symmetry constraints. The unrestricted Kohn–Sham approach
(UKS) was used to optimize the lowest triplet states. Calculations
of the vibrational frequencies for ground and lowest triplet states
[Re(Cl)(CO)3(dap)] and [Re(py)(CO)3(dap)]+ were performed at
the corresponding optimized geometries. Low-lying singlet and
triplet excited states were calculated by time-dependent DFT (TD-
DFT) at the ground-state geometry. The difference density plots
were drawn using the GaussView software.
calculated by DFT. It can be seen that the two anisyl groups at
the 2 and 9 positions of the phen ligand lie nearly perpendicular
to the equatorial Re(CO)2 plane, twisted toward the axial ligand,
i.e. Cl or Etpy. The phen ligand is tilted below the equatorial
Re(CO)2 plane, unlike in other Re tricarbonyl-diimine complexes,
where the (N,N)Re(CO)2 moiety is planar.17,18,30–32 The bulky
anisyl groups in [Re(py)(CO)3(dap)]+ force the pyridine ligand
to adopt a highly unusual configuration in the symmetry plane
of the molecule, i.e. bisecting the plane of the phen ligand
(Fig. 3), in contrast with analogous Re complexes17,18,30,31 where the
pyridine plane always lies perpendicularly to the symmetry plane,
bisecting the equatorial OC–Re–N(diimine) angles. Macrocyclic
rings in m27 and m30 are partly wrapped around the equatorial
Re(CO)2 group. They are folded above the equatorial plane, in
the direction of the axial ligand. DFT calculations have found
at least three stable conformations of [Re(Cl)(CO)3(m30)], which
differ only in the polyether chain folding, to occur within a 0.1 eV
energy range. This indicates a considerable flexibility of the m30
Results
Ground-state structures and spectra
Ground-state structures of [Re(Cl)(CO)3(phen-macrocycle)]
(Fig. 2) and [Re(Etpy)(CO)3(dap)]+ (Fig. 3) in vacuum were
Fig. 2 Two different views of optimized structures of [Re(Cl)(CO)3(m27)] (top) and [Re(Cl)(CO)3(m30)] (bottom) in vacuum. Left: the Cl ligand points
towards the viewer. Right: the Cl ligand points up and phen points toward the viewer. Calculated by DFT (G03/B3LYP).
Fig. 3 Two different views of the optimized structure of [Re(py)(CO)3(dap)]+ in vacuum. Left, right: phen and equatorial CO ligands point toward the
viewer, respectively. Calculated by DFT (G03/B3LYP, CPCM).
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3941–3949 | 3943
©

polyether ring. On the other hand, the m27 ring appears rather
rigid.
a single band in the case of the Etpy complexes, indicating that the
local symmetry of the Re(CO)3 unit approaches C3v. The spectra of
the dap and cyclic complexes being essentially identical, it follows
that wrapping by the macrocyclic ligand does not significantly
affect the Re(CO)3 geometry. The shape of the A¢(1) band is
predominantly Lorentzian with a smaller Gaussian contribution.
UV-vis spectra of [Re(Cl)(CO)3(phen-macrocyle)] are typical37,38
of ReI tricarbonyl-phen complexes. The lowest absorption band of
the chloro complexes occurs as a shoulder at about 400 nm while
the cationic Etpy complexes show a lowest-lying band (shoulder)
at about 330 nm, tailing to ~440 nm.
∫
FTIR spectra measured in the region of C O stretching
vibrations, n(CO), are shown in Fig. 4 and the IR band energies
are summarized in Table 1. The chloro complexes show the
characteristic splitting of the low-energy A¢¢ and A¢(2) bands,
caused33 by different values of stretching force constants of the
axial and equatorial CO ligands. These two vibrations merge into
Picosecond Time-Resolved IR spectroscopy, TRIR. TRIR
spectra were obtained at selected time delays after excitation with
400 nm, ~150 fs laser pulses. They are presented as differences
of the spectra obtained after and before excitation. Positive and
negative features in the spectra thus correspond to photoproduced
transients and depleted ground-state population, respectively.
Chromophore reorientation effects on the spectra were excluded
by setting the polarization directions of the optical pump and IR
probe beams at the magic angle. Excitation was directed into the
onset of the metal to ligand charge transfer (MLCT) absorption
band.
Excited-state IR bands of [Re(Cl)(CO)3(phen-macrocycle)]
(Fig. 5) occur at higher energies than their ground-state coun-
3
terparts, as is typical8,19,35,36,39–41 for a Re→polypyridine MLCT
excited state. The constant signal intensity over the 500 ps
time range is in accord with the expected37,42 long excited-state
lifetime. A large dynamic upward shift of the A¢(1) excited-state
band accompanied by a much smaller A¢(2) shift is the only
time-evolution seen in the spectra. To quantify this effect, the
Fig. 4 Ground-state FTIR spectra of [Re(Cl)(CO)3(phen-macrocyle)]
(top) and [Re(Etpy)(CO)3(phen-macrocyle)]+ (bottom) measured in
CH2Cl2 and MeCN, respectively.
Fig. 5 Difference TRIR spectra of [Re(Cl)(CO)3(phen-macrocycle)] in CH2Cl2, excited at 400 nm, ~150 fs FWHM. Left: full spectra measured over
a 1–500 ps time range. Experimental points are separated by 4–5 cm-1. Right: high-resolution spectra of the A¢(1) band measured over a 1–30 ps time
range. Experimental points are separated by ~2 cm-1. IR band assignment34 is shown in the top left spectrum in blue and red for the electronic ground
and excited state, respectively.
3944 | Dalton Trans., 2009, 3941–3949
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The Royal Society of Chemistry 2009
©

Table 2 IR spectral shift parametersa of the A¢(1) peak energy of
[Re(Cl)(CO)3(phen-macrocycle)] and [Re(Cl)(CO)3(phen)] in CH2Cl2 at
21 ◦C. Fitted from 2 ps
region of the A¢(1) band was investigated with a higher spectral
resolution, Fig. 5-right. The energy of the A¢(1) band maximum as
a function of time was determined by Gaussian fitting, as shown
in Fig. 6. Very good Gaussian fits were obtained despite the small
asymmetry of the A¢(1) band. To exclude any drifts in the fitted
values, exactly the same fitting procedure was applied to the spectra
obtained at each time-delay and for each sample investigated.
phen
dap
m27
m30
n
n
GS, cm-1
(•), cm-1
2023.7
2054.1
30.4
21.2
9.2
2.4 0.04
6.8 0.1
11.2 0.2
1.4 0.03
2022.1
2050.4
28.3
10.9
17.4
5.1 0.3
12.3 1.6
10.0 0.6
1.4 0.2
2023.1
2049.1
26.0
12.0
14.0
2.7 0.4
11.3 0.7
13.4 2.2
2.1 0.2
2021.6
2049.4
27.8
12.6
15.2
3.5 0.3
11.7 0.7
22.0 2.9
2.5 0.2
Total shift, cm-1
Inst. shift, cm-1
Dyn. shift, cm-1
As, cm-1
Af, cm-1
ts, ps
tf, ps
a Total shift = Inst. shift + Dyn. shift. As + Af = Dyn. shift.
Fig. 7 Time dependences of the peak energy of the excited-state A¢(1)
n(CO) band of [Re(Cl)(CO)3(phen-macrocycle)] in CH2Cl2. The curves
show the bi-exponential fits. The horizontal lines at a zero time delay
correspond to the ground-state positions of the A¢(1) band. Experimental
data are shown from t = 1 ps, biexponential fitting starts at 2 ps because
of poorly defined band shapes at earlier time delays.
Fig.
6
Typical fits of the high resolution TRIR spectra of
dynamic band shift is accompanied by narrowing. Widths (fwhm)
of the excited-state A¢(1) band of the three [Re(Cl)(CO)3(phen-
macrocycle)] complexes increase sharply between 1 and 2 ps and
then decay single-exponentially with a lifetime of ca. 10 ps, see
Fig. S1–S3 in the ESI.† The A¢(1) width of [Re(Cl)(CO)3(phen)]
only decays monotonically with lifetimes of 2.0 0.2 and 10.6
1.6 ps, without showing any rise, Fig. S4.†
[Re(Cl)(CO)3(phen-macrocycle)] in CH2Cl2, measured at 5 ps (left) and
at 300 ps (right). The macrocyclic ligand is specified in the legend.
Black: experimental points. Green: individual Gaussian bands. Red: final
simulated spectrum that in most cases overlaps with the green curve. Top
to bottom: dap, m27, m30.
The time-dependences of the A¢(1) peak energy for individual
complexes are shown in Fig. 7. Experimental data can be fitted to
biexponential kinetics, eqn (1):
TRIR spectra of the complexes [Re(Etpy)(CO)3(phen-
macrocycle)]+ (Fig. 8) point to a very different excited-state
behavior. While the overall excited-state IR pattern is still typical
˜
˜
n(t) = n(•) - Af exp(-t/tf) - As exp(-t/ts)
˜
(1)
3
of a MLCT excited state, the highest A¢(1) band is upshifted
-1
Herein, n(t) is the time-dependent A¢(1) peak energy (in cm ),
upon excitation by an unusually small amount, ca. 10 cm-1, partly
overlapping with the bleach. The A¢(1) band is rather broad and
neither narrowing nor a dynamic shift occur over the time range
investigated. Overall, the spectral shape observed is unprecedented
for [Re(L)(CO)3(N,N)]n complexes in solution, as can be seen from
the comparison with the spectrum of [Re(Etpy)(CO)3(phen)]+ in
MeCN, shown in Fig. 9-left. The unusual excited-state band shape,
small shift and lack of dynamic evolution are reminiscent of the
behavior of cationic [Re(Etpy)(CO)3(N,N)]+ complexes in PMMA
polymer matrices, see Fig. 9-right.
˜
n(•) is the A¢(1) energy extrapolated to t = • that corresponds
to a fully relaxed excited state, tf and ts are the “fast” and
“slow” relaxation times, while Af and As are the corresponding
amplitudes. In addition, we can define the total spectral shift
˜
˜
˜
˜
on excitation n(•)–n(GS), the “instantaneous” shift n(0)–n(GS),
which occurs within the experimental time resolution, and the
dynamic shift Af + As. n(0) is the A¢(1) energy extrapolated to t
˜
˜
˜
= 0, i.e. n(0) = n(•) - (Af + As). The fitted curves are shown in
Fig. 7 and all the shift parameters are summarized in Table 2. The
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3941–3949 | 3945
©

Fig. 10 Electron density difference between the lowest triplet state
and the ground state calculated by TD-DFT (B3LYP/CPCM) for
[Re(Cl)(CO)3(dap)] in CH2Cl2 (left) and [Re(py)(CO)3(dap)]+ in MeCN
(right). The axial ligands Cl and py point up.
experimental excited-state IR spectra, Table 1. The lowest triplet
state of [Re(py)(CO)3(dap)]+ (Fig. 10-right) is predominantly pp*
IL localized on phen, with a minor Re(CO)3→phen MLCT
contribution. UKS optimization of the lowest triplet state failed
because of a close energetic proximity of another triplet of A¢¢
symmetry. Excited states of both complexes show a minor charge
transfer from the anysil groups to phen, which is more extensive
for [Re(py)(CO)3(dap)]+.
Fig. 8 Difference TRIR spectra of the [Re(Etpy)(CO)3(macrocycle)]+
complexes in MeCN, excited at 400 nm, ~150 fs FWHM. Left: full spectra,
experimental points separated by 4–5 cm-1. Right: high-resolution spectra
of the A¢(2) and A¢(1) bands, measured over a 1.5–2000 ps time range.
Experimental points are separated by ~2 cm-1. IR band assignment34 is
shown in the top left spectrum in blue and red for the electronic ground
and excited state, respectively. The small intensity drop at longer time
delays is an instrumental artifact caused by decreasing overlap between
pump and probe pulses.
Discussion
Excited-state IR spectra presented above provide information
on the excited-state character, i.e. the extent of charge transfer
upon excitation. Their dynamic evolution reports on the inner-
and outer-sphere relaxation dynamics, involving structural re-
organization of the excited chromophore and its environment,
respectively.8–10
Concerning the excited-state relaxation dynamics, two experi-
mental observations deserve further discussion:
– [Re(Cl)(CO)3(phen-macrocycle)] in CH2Cl2 show qualitatively
the same type of relaxation that was observed for analogous phen
or bpy complexes, however, with important macrocycle-dependent
quantitative differences.
DFT characterization of the lowest 3MLCT state
The lowest triplet excited states of [Re(Cl)(CO)3(dap)] and
[Re(py)(CO)3(dap)]+ at the ground-state geometry were iden-
tified by TD-DFT calculations as a3A¢¢ and characterized by
electron-density difference plots, Fig. 10. The lowest triplet
state of [Re(Cl)(CO)3(dap)] involves a charge transfer from the
Re(Cl)(CO)3 unit to phen combined with a minor electron density
reorganization on phen. This is a predominantly MLLCT41 (or
MLCT/XLCT in another terminology;43 X = Cl) state, with
an admixture of a pp* intraligand (IL) character. The same
excited-state description is obtained by an UKS (B3LYP, CPCM
for CH2Cl2) structural optimization of the lowest triplet, which
was validated by an excellent match between the calculated and
– Attaching anisyl groups at the 2 and 9 phen positions in
[Re(Etpy)(CO)3(phen-macrocycle)]+ effectively “freezes” excited-
state relaxation.
Fig. 9 Left: Comparison of TRIR spectra of [Re(Etpy)(CO)3(dap)]+ (red) and [Re(Etpy)(CO)3(phen)]+ (blue) in MeCN, shown at 500 and 1000 ps
respectively. Right: Comparison of TRIR spectra of [Re(Etpy)(CO)3(dap)]+ in MeCN (red) and [Re(Etpy)(CO)3(bpy)]+ in a PMMA film (green), shown
at 500 ps.
3946 | Dalton Trans., 2009, 3941–3949
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The Royal Society of Chemistry 2009
©

TRIR spectra as well as DFT and TD-DFT calculations
of [Re(Cl)(CO)3(phen-macrocycle)] clearly show that the lowest
triplet excited state has a MLCT/XLCT/IL character consisting
of a predominant Re(Cl)(CO)3→phen and minor pp* IL(phen)
contributions. This is typical32,41,43–45 for [Re(Cl)(CO)3(N,N)] com-
plexes. Total A¢(1) spectral shifts observed for [Re(Cl)(CO)3(phen-
macrocycle)] upon excitation (Tabs. 1, 2) are smaller than those
observed for analogous species such as [Re(Cl)(CO)3(4,4¢-bpy)],46
[Re(Cl)(CO)3(bpy)],39 or [Re(Cl)(CO)3(phen)] (Table 2) in CH2Cl2.
This is due to a slightly larger admixture of an IL character and/or
restricted solvent reorganization by the anisyl groups and the
macrocycle.
Electronic and structural changes observed upon excita-
tion of [Re(Cl)(CO)3(phen-macrocycle)] occur on three different
timescales, manifested by the bi-exponential character of the IR
shift dynamics and the rather large values of the instantaneous
shift, Fig. 7 and Table 2. These relaxation dynamics can be
explained by considering a “supermolecule” or a solute-solvent
complex {[Re(Cl)(CO)3(phen-macrocycle)](local solvent)}, which
includes ordered solvent molecules that intercalate into the
chromophore structure and interact specifically with individual
ligands. This “local solvent” is assumed to have very different
dynamics from the usual47 dielectric relaxation. The supermolecule
is solvated by bulk CH2Cl2 in the usual way and feels its
electrostatic field, so called reaction field.47
The instantaneous shift, which occurs within the experimental
time resolution, accounts for about 40–45% of the total shift. It
encompasses several processes such as electron-density redistribu-
tion upon Franck–Condon excitation and intersystem crossing,45
intramolecular vibrational redistribution (IVR), as well as the
inertial and part of the collective reorientational relaxation of
bulk CH2Cl2, which are known47 to occur with lifetimes of 0.14 ps
(52%) and 1.0 ps (48%), respectively.
The “fast” relaxation component tf accounts for most of the
dynamic shift. It involves the later stages of IVR and of the
collective reorientational relaxation of CH2Cl2. Within the present
model, IVR includes redistribution of vibrational energy between
the modes of the chromophore and the local solvent molecules.
Interestingly, tf slightly increases in the order dap < m27 < m30.
This order could reflect the decreasing access of the local as well as
bulk solvent to the chromophore, see Fig. 2. By and large, however,
the observed tf values are similar to those seen for analogous
“small” Re complexes in other dipolar solvents such as MeCN,8,9,48
D2O,10 or MeOH,6,7,48 indicating that they are mostly due to the
chromophore itself. The occurrence of IVR on this timescale is
further supported by the observation of a major 1.6 ps shift of
IR bands of vibrationally excited electronic ground state of p-
nitroaniline in MeOH, which is also accompanied by initial IR
band broadening.49
well as in UV-vis spectra of several excited transition metal
complexes. The observation8 that the excited-state UV band at
~380 nm and excited-state Raman bands due to bpy∑- vibrations
of [Re(Etpy)(CO)3(4,4¢-Me2-bpy)]+ and [Re(Cl)(CO)3(bpy)] grow
in intensity within the first 20 ps after excitation shows that
the underlying processes affect electronic structure of the excited
chromophore. Therefore, we can hardly attribute this process
only to a simple transfer of vibrational energy from excited
Re(Cl)(CO)3(phen) moiety to collective vibrations of the bulk
solvent. Instead, we propose vibrational relaxation of the whole
supermolecule {[Re(Cl)(CO)3(phen-macrocycle)](local solvent)},
which involves restructuring of the “intercalated” local solvent,
whose molecules adopt new orientations and enter new interac-
tions with the ligands, befit the excited-state structure and electron
density distribution. The “slow” relaxation lifetime ts increases
in the order dap < m27 << m30. While the values of 10 and
13 ps measured for dap and m27, respectively, are in the range
expected for Re carbonyl-polypyridines in solution, the 22 ps
relaxation time of m30 is distinctively slower. The increase of ts
on going to the m30 complex is not paralleled by an increase in
the band-narrowing time, which stays at ca. 10 ps for all three
complexes. We propose that the relatively slow relaxation rate of
[Re(Cl)(CO)3(m30)] is caused either by slow reorganization of a
few local CH2Cl2 molecules whose movement is hampered by their
confinement inside the folded polyether ring, or by conformational
motions of the polyether ring itself, which create a time-dependent
field acting on the equatorial CO groups. In reality, the slow
solvent- and conformational motions can be coupled, as has
been proposed for protein relaxations.55,56 The proposed active
role of m30 polyether ring motions in the excited-state dynamics
is supported by the DFT-calculated conformational flexibility.
Analogous structural fluctuations are not expected for the much
more rigid m27 ring. Notably, relatively slow (206 ps) solva-
tion/conformational dynamics has been reported57 for free 4,13-
diaza-18-crown-6, which accelerates to 22 ps upon complexation
with K+.
[Re(Etpy)(CO)3(phen-macrocycle)]+ shows completely different
TRIR spectra. The shifts and splitting of the two lower-lying A¢¢
and A¢(2) bands are typical8,9,35,36,40,48 of a 3MLCT state, while the
highest A¢(1) band is shifted unusually little and does not show
any dynamic evolution. In fact, the total A¢(1) shift is ca. 3-times
smaller than that of [Re(Etpy)(CO)3(phen)]+ in MeCN (Fig. 8),
which shows the usual relaxation dynamics (tf = 1.5 ps, Af =
12.6 cm-1, ts = 13 ps, As = 3.1 cm-1, total shift = 32 cm-1)
The unusual excited-state behavior of [Re(Etpy)(CO)3(phen-
macrocycle)]+ cannot be attributed to wrapping the chromophore
by the macrocyclic ring, since it is the same (Fig. 8) for the open-
ring [Re(Etpy)(CO)3(dap)]+ and its cyclic m27 and m30 congeners.
The nature of the excited state is affected by the presence of
the anisyl groups. Interestingly, fixing [Re(Etpy)(CO)3(bpy)]+ in
a PMMA film has a very similar effect on the TRIR spectrum,
Fig. 8. The DFT-optimized structure of [Re(py)(CO)3(dap)]+
shows (Fig. 3) that the pyridine ligand together with tilted anisyl
groups effectively block solvent access to the equatorial CO ligands
from the top and the sides. Any rotational movement or out-
of-plane vibrations of the py ligand are also hindered. Ensuing
steric restrictions on solvent and conformational dynamics would
prevent restructuring of the local solvent and fix the excited state
structure and charge distribution in a configuration similar to that
The “slow” relaxation component ts does not have any counter-
part in solvent dielectric relaxation dynamics measured by time-
dependent fluorescence Stokes shift.47 Even the fastest process
observed herein, 10 ps, is about 10-times slower than the slowest
kinetics component of CH2Cl2 dielectric relaxation.47 Time con-
stants of 10–15 ps are generally considered as typical of vibrational
energy relaxation in dipolar solvents. Similar times were observed
by TRIR or time-resolved resonance Raman for analogous6–10,48
acyclic Re complexes including [Re(Cl)(CO)3(phen)] (Table 2),
t-stilbenes50–53 aromatic cations,54 and hot p-nitroaniline,49 as
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Dalton Trans., 2009, 3941–3949 | 3947
©

present in freely relaxing media at very early time delays after
excitation, just after the “instantaneous” shift. Indeed, the A¢(1)
n(CO) band of most Re carbonyl-diimines at the onset of the
dynamic relaxation is shifted only by about 15–20 cm-1. In the
case of [Re(py)(CO)3(dap)]+, this shift is even smaller because of a
large IL admixture to the lowest triplet state, revealed by TD-DFT,
see Fig. 10-right.
In conclusion, it follows that shielding the CO ligands in
organometallic molecules with a supramolecular ligand affects
both the excited-state character and its relaxation processes, that
is the dynamic coupling with the solvent. It is proposed that a
small number of ordered solvent molecules, a “local solvent”,
plays an important role in excited-state relaxation of the solute.
In addition, evidence has been obtained indicating a dynamic
coupling between excited-state relaxation of the chromophore and
conformational movements of its supramolecular surroundings,
that is the polyether chain.
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Acknowledgements
Financial support from EPSRC, STFC (CMSD43), QMUL,
the European collaborative programme COST Action D35, the
Ministry of Education of the Czech Republic (1P05OC68) and the
Grant Agency of the Academy of Sciences of the Czech Republic
(KAN 100400702) is gratefully acknowledged. We thank Dr Jean
Weiss for a gift of a m27 sample.
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