Ground- and excited-state infrared spectra of an azacrown-substituted [(bpy)Re(CO)3L]+ complex: Structure and bonding in ground and excited states and effects of Ba2+ binding

Article abstract of DOI:10.1021/jp711260d

Ground- and excited-state infrared spectra are reported for a [(bpy)Re ^I(CO)₃L]⁺ complex (bpy = 2,2′-bipyridine) in which L contains an azacrown ether that is linked to Re via an amidopyridyl group. Ground-state band assignments are made with the aid of spectra from model complexes in which a similar electron-donating dimethylamino group replaces the azacrown, in which an electron-donor group is absent, and from the L ligands, in conjunction with DFT calculations. Picosecond time-resolved IR (TRIR) spectra in the v(CO) region show bands characteristic of a metal-to-ligand charge-transfer (MLCT) excited state, [(bpy^?-)Re ^II(CO)₃L]⁺, from the complex in which an electron-donor group is absent, whereas those from the azacrown complex show bands of an MLCT state evolving into those characteristic of a ligand-to-ligand charge-transfer (LLCT) excited state, [(bpy^?-)Re ^I(CO)₃(L^?+)]⁺, formed upon intramolecular electron transfer. Picosecond TRIR spectra of the azacrown complex in the fingerprint region show strong L ligand bands that indicate that significant charge redistribution occurs within this ligand in the MLCT state and that decay as the LLCT state forms. Picosecond TRIR spectra obtained when Ba²⁺ was complexed to the azacrown show bands of only an MLCT state at all times up to 2 ns, consistent with the presence of Ba²⁺ inhibiting electron transfer from the azacrown N atom to form the LLCT state, and the positions of the bands in the fingerprint region provide direct evidence for the proposal that charge redistribution within the L ligand induces Ba ²⁺ release from the azacrown in the MLCT state.

Full text of DOI:10.1021/jp711260d

3852
J. Phys. Chem. A 2008, 112, 3852-3864
Ground- and Excited-State Infrared Spectra of an Azacrown-Substituted [(bpy)Re(CO)3L]+
Complex: Structure and Bonding in Ground and Excited States and Effects of
Ba2+ Binding
Jared D. Lewis,† Michael Towrie,‡ and John N. Moore*,†
Department of Chemistry, The UniVersity of York, Heslington, York, YO10 5DD, United Kingdom, and
Central Laser Facility, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire,
OX11 0QX, United Kingdom
ReceiVed: NoVember 28, 2007; In Final Form: January 30, 2008
Ground- and excited-state infrared spectra are reported for a [(bpy)ReI(CO)3L]+ complex (bpy ) 2,2′-bipyridine)
in which L contains an azacrown ether that is linked to Re via an amidopyridyl group. Ground-state band
assignments are made with the aid of spectra from model complexes in which a similar electron-donating
dimethylamino group replaces the azacrown, in which an electron-donor group is absent, and from the L
ligands, in conjunction with DFT calculations. Picosecond time-resolved IR (TRIR) spectra in the ν(CO)
region show bands characteristic of a metal-to-ligand charge-transfer (MLCT) excited state, [(bpy•-)ReII(CO)3L]+,
from the complex in which an electron-donor group is absent, whereas those from the azacrown complex
show bands of an MLCT state evolving into those characteristic of a ligand-to-ligand charge-transfer (LLCT)
excited state, [(bpy•-)ReI(CO)3(L•+)]+, formed upon intramolecular electron transfer. Picosecond TRIR spectra
of the azacrown complex in the fingerprint region show strong L ligand bands that indicate that significant
charge redistribution occurs within this ligand in the MLCT state and that decay as the LLCT state forms.
Picosecond TRIR spectra obtained when Ba2+ was complexed to the azacrown show bands of only an MLCT
state at all times up to 2 ns, consistent with the presence of Ba2+ inhibiting electron transfer from the azacrown
N atom to form the LLCT state, and the positions of the bands in the fingerprint region provide direct evidence
for the proposal that charge redistribution within the L ligand induces Ba2+ release from the azacrown in the
MLCT state.
Introduction
We have been studying [(bpy)Re(CO)3L]+ complexes in
which L contains a terminal azacrown ether linked to Re via
an amide, alkene, or alkyne group and that can act as metal
cation sensors and light-controlled ion switches.1-7 The amide
complex 1 was first reported in 1991 by MacQueen and
Schanze,8 along with model complex 3 in which L does not
have a terminal electron-donor group, and their emission studies
showed that the metal-to-ligand charge-transfer (MLCT) state
lifetime of 1 is <1 ns, whereas that of 3 is ca. 150 ns. They
proposed that the MLCT state of 1 is quenched rapidly by
intramolecular electron transfer from the azacrown nitrogen to
Re to generate a ligand-to-ligand charge-transfer (LLCT) state,
and we have reported pico- and nanosecond time-resolved UV-
visible absorption (TRVIS) studies that show that forward
electron transfer to form the LLCT state occurs in ca. 500 ps
(kFET-1) and that back electron transfer to return to the ground
state occurs in ca. 19 ns (kBET-1), as shown in Scheme 1.1
illustrated in Scheme 2 for 1-Ba2+. We also showed that
MacQueen and Schanze showed that binding a metal cation
subsequent cation rebinding to the ground state restores the
to the azacrown ether to form 1-Mn+ lengthens the MLCT-state
starting thermal equilibrium in ca. 0.1 µs to 1 ms, with the rate
lifetime, and they proposed that the bound cation inhibits the
constants and detailed mechanism depending on the identity of
electron-transfer process;8 the MLCT lifetime of 1-Mn+ was
the cation.
found to be shorter than that of 3 and to depend on the identity
TRVIS measurements provide good kinetic information, and
of the bound cation, leading to the proposal that a bound cation
they have enabled the excited states of these systems to be
is released in the MLCT excited state for some metals. We
identified and the photophysical and ion-switching mechanisms
recently investigated this mechanism using nanosecond TRVIS
to be established, but the broad TRVIS bands do not provide
spectroscopy,2 showing conclusively that release occurs in ca.
detailed information on the changes in structure, bonding, and
5-100 ns for Li+, Na+, Ca2+, and Ba2+, with the rate constant
charge distribution that drive metal cation release upon excita-
(k*off) decreasing in the order Na+ g Li+ > Ba2+ > Ca2+, as
tion. Time-resolved vibrational spectroscopy can generally
provide such information,9-11 and the excited states of several
(bpy)Re(CO)3L and related complexes have been studied
† The University of York.
‡ CCLRC Rutherford Appleton Laboratory.
10.1021/jp711260d CCC: $40.75 © 2008 American Chemical Society
Published on Web 04/05/2008

Azacrown-Substituted [(bpy)Re(CO)3L]+ Complex
J. Phys. Chem. A, Vol. 112, No. 17, 2008 3853
SCHEME 1: Photochemical Mechanism for 1
and fingerprint regions can be used report on the charge
redistribution that occurs upon excitation and how it is modified
when a metal cation is bound to the azacrown.
Experimental Procedures
The ligands and complexes were synthesized according
to reported methods3,8,23 and characterized using 1H NMR
and electrospray ionization mass spectrometry, as reported
previously.1-3 Barium perchlorate (Aldrich) was dried overnight
under a vacuum at 230 °C, and a large excess was added to a
solution of 1 under nitrogen to prepare 1-Ba2+; UV-vis
absorption spectra were recorded to verify full complexation
to the azacrown.2 Steady-state and time-resolved IR spectra were
recorded using acetonitrile solvent (Aldrich) because it dissolves
both the complexes and the barium perchlorate used to generate
1-Ba2+; CH3CN was used for studies in the ν(CO) region (ca.
1800-2100 cm-1), whereas CD3CN was used for studies in
the fingerprint region (ca. 1000-1800 cm-1) because of its much
weaker bands. Steady-state IR spectra were also recorded using
dichloromethane solvent (Aldrich) because it dissolves all of
the ligands and complexes (including L1 and L2, which are
only sparingly soluble in acetonitrile), it gives good-quality
spectra upon addition of HCl (unlike acetonitrile), and it enables
both regions to be studied readily with one solvent because there
are only two strong solvent bands (at ca. 1260 and 1425 cm-1);
however, the metal perchlorates are not soluble in dichlo-
romethane, precluding studies of the 1-Mn+ complexes that are
the main target of our work. Where direct comparisons were
possible, the steady-state IR spectra from acetonitrile and
dichloromethane samples were found to be similar to each other.
Steady-state IR spectra were recorded using a Nicolet Impact
410 FTIR spectrometer, with samples of ligands and complexes
at ca. 10-2 to 10-3 mol dm-3 held in a 100 µm path length cell
with calcium fluoride windows. The spectra of samples contain-
ing barium perchlorate are presented after the scaled subtraction
of spectra from barium perchlorate solutions at comparable
concentrations, and the spectrum from a sample in acidified
dichloromethane is presented after the scaled subtraction of a
spectrum from the acidified solvent.
Picosecond TRIR spectra were recorded at the Rutherford
Appleton Laboratory, and the instrumentation has been described
in detail.16 Samples at ca. 0.4-1.0 × 10-3 mol dm-3 were
contained in a 100 µm path length cell mounted on an x-y
translation stage. They were pumped at 400 nm (ca. 1 µJ energy;
ca. 200 fs pulse width; 1 kHz repetition rate; and ca. 300 µm
beam diameter), and each experiment involved recording a series
of overlapping IR probe windows (ca. 150 cm-1 width), with
the pump and probe polarizations set at the magic angle (54.7°).
The TRIR spectra presented here were obtained by joining
adjacent windows, where possible at an appropriate wavenumber
where the transient absorption was near the baseline.
DFT calculations were performed with the Gaussian 98
package,24 using the B3LYP functional and the 6-31G(d) basis
set, and output files were parsed and analyzed using the
MOLEKEL25 and MOLDEN26 packages. The calculated vibra-
tional frequencies were scaled by the recommended factor of
0.9613,27 and a calculated spectrum was created by applying a
5 cm-1 (fwhm) Lorentzian function scaled to the calculated
intensity of each band.
SCHEME 2: Photochemical Mechanism for 1-Ba2+
successfully with time-resolved infrared (TRIR) spectroscopy
bymonitoringshiftsinthestrongν(CO)bandsuponexcitation.9-22
We have shown that ground-state IR and resonance Raman (RR)
spectra report effectively on changes within the L ligand on
metal cation binding to the azacrown of [(bpy)Re(CO)3L]+
complexes with alkene and alkyne linkers,7 suggesting that time-
resolved vibrational spectroscopy may be used to study the
changes that drive the ion-release mechanisms in the excited
states of such systems.
We recently reported a steady-state and nanosecond transient
resonance Raman study of the amide-linked system 1, its
azacrown-bound forms 1-Li+, -Na+, -Ba2+, and -Ca2+, and its
models.3 The ground-state RR spectra generally provided a rich
set of bands from both the (bpy)Re(CO)3 group and the L ligand,
whereas the excited-state RR spectra generally provided several
bpy bands and a single ν(CO) band from the (bpy)Re(CO)3
group, with few bands from the L ligand. The resonance effect
enabled spectra from the MLCT and LLCT states to be observed
selectively, and the data provided strong support for the
proposed ion-switching mechanism as well as information on
structure and bonding in these excited states.
Here, we report steady-state infrared spectra of 1, its
azacrown-bound form 1-Ba2+, the model complexes 2-4, and
the L ligands L1 and L2. We assign the ground-state IR bands
of 1 by comparison to those of the model complexes and ligands
and with the aid of DFT calculations on L2. We then report
picosecond TRIR spectra of 1, 3, and 1-Ba2+, which we find
generally provide a full set of two or three ν(CO) bands from
the (bpy)Re(CO)3 group and a rich set of fingerprint-region
bands from the L ligand that provide valuable information on
the changes that occur within this ligand upon excitation. This
TRIR work extends our time-resolved vibrational studies of this
system into the picosecond regime, where the MLCT f LLCT
conversion occurs (Scheme 1), and it complements our earlier
Raman work because it provides information on changes within
the L ligand, from bands that were not accessible in those
studies. More generally, this study provides a novel example
of the application of TRIR spectroscopy to a light-controlled
ion-release system, demonstrating how IR bands in both ν(CO)
Results and Discussion
Ground-State IR Spectra of 1-4. Steady-state IR spectra
in the fingerprint region are shown in Figure 1 for 1, 2, L1, and
L2, along with the DFT calculated IR spectrum of L2, and in

3854 J. Phys. Chem. A, Vol. 112, No. 17, 2008
Lewis et al.
intense IR bands at ca. 1472, 1447, and 1317 cm-1 from
complexes 1-4 but not from ligands L1 and L2 (Figures 1 and
2) occurs also for (bpy)Re(CO)3Cl and other [(bpy)Re(CO)3L]+
systems,7 and they can be assigned readily to modes of the bpy
ligand (Table 1).30,31
L Ligand Bands of 1 and 2. A comparison of the spectra of
1 and 2 with those of L1 and L2, respectively (Figure 1), shows
clearly that the remaining bands in the fingerprint region can
be assigned to modes of the respective L ligand; these bands
dominate the IR spectra in this region. The similarities between
these spectra indicate that the band assignments can be based
on the relatively simple L2 ligand, and so its IR spectrum was
calculated using DFT with the B3LYP functional and the 6-31G-
(d) basis set, consistent with our earlier calculation of its Raman
spectrum using this method.3 This approach has been shown to
provide good results for similar donor-acceptor organic com-
pounds;32 the use of a dimethylamino rather than an azacrown
substituent, and the absence of the (bpy)Re(CO)3 group,
simplifies the calculations.7 The calculated IR spectrum of L2
is shown in Figure 1, and calculated displacement vectors for
selected modes that give IR-active bands are shown in Figure
3. The calculations indicate that there is coupling between the
various groups within the L ligand because many modes involve
the motion of atoms throughout the molecule; some involve
vibrations within both phenyl and pyridyl rings, and many
involve vibrations of the amide group. However, the calculated
coupling between the two rings is generally weaker for this
amide ligand than that we reported for the alkene and alkyne
analogues,7 and its relative weakness can be attributed to a small
twist between the rings in the amide group, which is not fully
planar.3 Generally, the calculations on L2 indicate that the same
modes give strong bands in both IR and Raman3 spectra. Table
1 lists assignments that we discuss in detail next; they are based
on the calculated IR- and Raman-active modes of L2, on a
consistent approach to the IR assignments made here and the
Raman assignments we made earlier,3 and on the IR and Raman
assignments we have discussed in detail for the alkene and
alkyne analogues.7 Four strong IR bands of the amide ligands
and complexes that were not observed for the alkene and alkyne
analogues7 can be assigned to modes involving the amide group.
Figure 1. Fingerprint-region IR spectra of 1, 2, L1, and L2 in CH2Cl2
(regions of strong solvent absorption omitted, and spectra offset for
clarity), along with the calculated IR spectrum of L2.
The moderately strong and relatively broad IR band at ca.
1683 cm-1 from 1, 2, L1, and L2 is very strong in their RR
spectra,3 and it is assigned unambiguously to an amide I
mode,33-35 which involves amide CO stretching and NH bending
and which is calculated at 1695 cm-1 for L2 (Figure 3). Three
IR bands at ca. 1600 cm-1, from both ligands and complexes,
are readily assigned from the calculated IR spectrum of L2:
the strong band at ca. 1618 cm-1, observed distinctly for the
free ligands and as a shoulder for the complexes, is assigned to
a phenyl-centered mode involving Wilson 8a and 9a vibrations
Figure 2. Fingerprint-region IR spectra of 1, 1-H+, 3, and 4 in
CH2Cl2 (regions of strong solvent absorption omitted, and spectra offset
for clarity).
calculated at 1607 cm-1; the strong band at ca. 1604 cm-1
,
which becomes very strong on attachment to the Re group and
which was also observed to be very strong in the RR spectra,3
is assigned to a pyridyl-centered mode involving Wilson 8a and
9a vibrations calculated at 1588 cm-1; and the strong band at
ca. 1583 cm-1 is assigned to a mode involving NH bending
coupled to pyridyl ring Wilson 8b and 3 vibrations calculated
at 1576 cm-1 (Figure 3). A weak IR band at 1558 cm-1 and a
weak IR shoulder at ca. 1526 cm-1 were observed as moderately
strong bands in the RR spectrum of 1,3 and they are assigned
to phenyl modes involving coupled Wilson 8b and 3 and Wilson
Figure 2 for 1, 1-H+, 3, and 4. Ground-state IR band positions
are given in Table 1, including those from the ν(CO) region
(spectra of 1 and 3 in this region are included within Figures 6
and 7).
(bpy)Re(CO)3 Group Bands of 1-4. The ground-state IR
spectra of 1-4 give bands at ca. 2035 and 1930 cm-1 that are
assigned to ν(CO)Re modes (Table 1);28 the observation of two
rather than three bands is attributable to pseudo-C3V symmetry
at the Re center, arising from coordination of the L ligand via
a pyridyl group.7,17,29 The close similarity in the pair of ν(CO)Re
bands from 1-3 indicates that the variation between the
respective azacrown, -NMe2, and -H substituents at the terminus
of the L ligand has a negligible effect on the Re center in the
ground state. A set of three relatively narrow and moderately
18a and 19a vibrations calculated at 1542 and 1519 cm-1
respectively.
,
Several weak IR bands are calculated at ca. 1450-1500 cm-1
,
including alkyl vibrations, and several may contribute to the
broad feature observed at ca. 1480-1520 cm-1. An amide II
vibration, involving NH bending and CN stretching, typically

Azacrown-Substituted [(bpy)Re(CO)3L]+ Complex
J. Phys. Chem. A, Vol. 112, No. 17, 2008 3855
TABLE 1: Observed IR Band Positions (cm-1) for Ground and Excited States of 1-4, L1, and L2;a,b Calculated Positions for
Ground-state L2; and Assignments
1
1-Ba2+
1-H+
2
3
4
L1
L2
assignment
L-centeredc
IR
TRIR TRIR
IR TRIR
IR
IR
IR
TRIR
IR
IR
IR
DFT
ground MLCT LLCT groundb MLCT ground ground ground MLCT ground ground ground ground
Re-centeredd
2035
1930
2069 2010 2035
2010 1901 1930
1973
2073 2035
2020 1932
1975
2035
1931
2035
1931
2073 2036
2014 1932
1973
ν(CO)Re
ν(CO)Re
ν(CO)Re
1683
1694
1685
1683 1696
1682
1696
1654
1676
1677
1695 ν(CO) + NH bend
(amide I)
1607 8a/9a (ph)
1618 sh 1616
1604
1583
1558
1616 sh
1605
1593
1617e 1617 sh 1615
1621
1609
1605 sh 1586
1555
1534
1605
1596
1606
1596
1586
1557
1533
1599
1579
1593 1604e 1605
1603
1586
1588 8a/9a (py)
1576 NH bend + 8b/3 (py)
1542 8b/3 (ph)
1560 1593
1583
1558
1534
1558
1529
1526 sh
1514 sh
1507
1521
1512
1519 18a/19a (ph) + Me +
ν(ph-N)
1512
1497
1461
1492 1514
1512
1505
1504
1504
1490 NH bend + ν(C-N)
(amide II) + 18a/19a (py)
1501 sh
1487
1505
1506
1502 sh 1502
1489
1492
1487
1500
1490
1490
1480 NH bend + ν(C-N)
(amide II) + 18a/19a (py)
1472
1447
1428b
1395
1473
1447
1430
1472
1447
1472
1447
1472
1447
1472
1447
ν27 (bpy)
ν18 (bpy)
1406
1399 18b/14 (py)
1351 18a/19a (ph) +
ν(ph-N) + Me
azacrown
azacrown
azacrown
1316 3 (py)
1383 1290 1299
1374
1369
1394
1372
1328
1363
1355
1347
1332
1317
1301b
1273b
1358
1353
1363
1355
1348
1328
1358
1338
1308
1335
1318
1299
1275
1342 1335
1303
1332
1317
1300
1333
1317
1298
1328
1318
1294
ν
29 (bpy)
1305 3 (ph)
1279 NH bend (amide III) +
18a (py) + 3 (ph)
1245b
1246
1247 18a (py) + 18a (ph) +
ν(py-am) + ν(ph-am)
1236 Me rock
1237b
1213
1198
1188 sh
1125
1090
1225 NH bend + 9a (py) + 9a (ph)
1204 9a (py)
1184 9a (ph)
1163 Me + 18a/1 (ph)
azacrown ν(C-O-C)
1088 18b (py) + NH bend + 18a (ph)
1213
1207
1213
1213
1197
1172
1214
1094
1205
1198
1173
1212
1197
1212
1196
1171
1129
1104
1125
1090
1089
1090
a Positions from ground states in dichloromethane, except where indicated, and from excited states in acetonitrile (CH3CN or CD3CN, as indicated
in the text). b Positions in acetonitrile. c Assigned motions given in order of decreasing contribution, generally based on calculations on L2, with ph
) phenyl, py ) pyridyl, and numbers indicating Wilson vibrations. d bpy ) Bipyridyl, with numbers indicating modes according to ref 30. e Small
uncertainty in positions and intensities due to a strong overlying solvent band for HCl-acidified dichloromethane.
gives a band at ca. 1510-1550 cm-1 that is strong in the IR
and weak or absent in the Raman spectrum.33-35 The calcula-
tions here indicate that the amide II vibration of L2 couples
strongly to the pyridyl 18a vibration to give two modes at 1490
and 1480 cm-1 arising from in-phase and out-of-phase combi-
nations (Figure 3), with the higher wavenumber band calculated
to be the stronger of the two (ca. 6:1) and the strongest band in
the IR spectrum (by ca. 2×). We assign two strong IR bands at
ca. 1512 and 1487 cm-1, which were not observed in the RR
spectra,3 to these modes; they are of comparable strength to
each other and to the other strong bands in the IR spectra.
A moderately strong IR band of 1 at 1428 cm-1 (obscured
by a solvent band in CH2Cl2 but seen in CD3CN; see Figure 4)
is assigned to a mode involving pyridyl 18b and 14 vibrations
calculated at 1399 cm-1. The assignment of the other IR bands
in this region, at ca. 1340-1400 cm-1, is not straightforward,
but it is aided not only by the calculation on L2 but by
similarities with the IR spectra and calculations for the alkene
and alkyne systems,7 as well as those from detailed studies
reported for para-dimethylaminobenzonitrile (DMABN) and its
isotopomers.32 A mode involving ph-N stretching is calculated
to be strongly mixed with phenyl 18a and 19a and alkyl
vibrations for the amide, alkene, and alkyne models, and it is
calculated to give a strong IR band at 1351 and 1353 cm-1 for
the phenyl-NMe2 systems L2 and DMABN,32 respectively.
Hence, we assign a moderately strong and broad IR band
observed at ca. 1374 cm-1 from the -NMe2 systems 2 and L2
to this mode, comparable to the assignment of a strong IR band
at 1372 cm-1 to this mode in DMABN.32 We assign the
equivalent mode from the azacrown systems 1 and L1 to a
moderately strong and broad IR band at ca. 1395 cm-1
comparable to the assignment of similar IR bands at ca. 1395
cm-1 from the alkene and alkyne azacrown analogues.7,36
,
A
band assigned to a mode involving ph-N stretching has been
observed at ca. 1350-1400 cm-1 in several related donor-
acceptor 1,4-disubstituted dialkylaminophenyl systems,32,37-40
and it has been proposed32 that its relatively high wavenumber
is indicative of ph-N double-bond character arising from
significant charge transfer in the ground state. The canonical
form of L1 and L2 labeled A in Scheme 3 illustrates this
proposal, and it may contribute significantly due to the presence
of both an azacrown or dialkylamino electron donor and an
amido-pyridyl electron acceptor; the calculated structure of L2
is close to planar at the dimethylamino N atom, consistent with
this effect.3,41
A moderately strong three-peaked IR band at ca. 1355 cm-1
is seen from the azacrown systems 1 and L1 but not from the
dimethylamino systems 2 and L2, and it is assigned to azacrown
modes because it was also observed from phenyl-azacrown
itself.7 A mode arising from a phenyl 3 vibration calculated at

3856 J. Phys. Chem. A, Vol. 112, No. 17, 2008
Lewis et al.
Figure 3. Displacement vectors for selected calculated vibrational modes of L2.
SCHEME 3: Canonical Forms of Ligands L1 and L2
a pyridyl 3 vibration, and a weaker band from 1 at ca. 1301
cm-1 (obscured by a solvent band in CH2Cl2 but seen in
CD3CN; see Figure 4) is assigned to a calculated mode at 1305
cm-1 involving an equivalent phenyl 3 vibration.
An amide III vibration, involving NH bending and CN
stretching, typically gives a band at ca. 1250-1300 cm-1 in
secondary amides that is moderately strong in the IR and strong
in the Raman spectrum.33-35 Our calculations indicate that the
amide III vibration of L2 couples to pyridyl 18a and phenyl 3
vibrations to give a mode at 1279 cm-1, and we assign a strong
IR band from 1 at ca. 1273 cm-1 (obscured by a solvent band
in CH2Cl2 but seen in CD3CN; see Figure 4) that is also strong
in the RR spectrum3 to this mode. Two other weak IR bands
seen from 1 in CD3CN at 1245 and 1237 cm-1 are assigned to
delocalized modes calculated at 1247 and 1225 cm-1 for L2,
the latter being another mode in this region that involves an
NH bending vibration.
Two IR bands at ca. 1213 and 1198 cm-1 from 1, 2, L1, and
L2 are assigned to modes involving 9a vibrations centered on
the pyridyl and phenyl rings, respectively, calculated at 1204
and 1184 cm-1 for L2, and for which matching bands were
observed in the RR spectra; the phenyl 9a mode typically gives
anintensebandat1100-1200cm-1 insimilarcompounds.32,37-40,42
An IR band observed as a shoulder at ca. 1188 cm-1 for the
1328 cm-1 for L2 also may contribute to this feature, but no
such bands were observed from the dimethylamino models
studied here, in contrast to the IR bands at ca. 1350 cm-1
observed from the alkene and alkyne diethylamino models that
resulted in some bands in this region being assigned to L ligand
bands for the respective azacrown systems.7 A strong IR band
at ca. 1332 cm-1 from 1 and 2, and at ca. 1328 cm-1 from L1
and L2, is assigned to a calculated mode at 1316 cm-1 involving

Azacrown-Substituted [(bpy)Re(CO)3L]+ Complex
J. Phys. Chem. A, Vol. 112, No. 17, 2008 3857
attributed to the presence of additional bands arising directly
from the CH2 group, to weaker bands arising from smaller dipole
changes during vibrations along the long axis of the L ligand
because of weaker interactions between the phenyl and pyridyl
rings in 4,44 or to band shifts arising from changes in bonding
or the forms of the modes. The most notable change is that the
amide I band of 2 shifts down by 28 cm-1 to occur at 1654
cm-1 for 4, indicating a decrease in CO bond strength that can
be rationalized by charge transfer from the NMe2 group being
more localized on the amide group because it is transmitted
less effectively through to the pyridyl ring (Scheme 3; form B
contributes less). The relatively large downshift of this band
on introducing an insulator CH2 group indicates that the
electronic communication between the phenyl and the pyridyl
rings of the L ligand is quite strong when it is absent, despite
the small twist at the amide link, which is consistent with the
other evidence from electronic and vibrational spectroscopy and
also from the excited-state electron-transfer kinetics of 1 and
4.1 There are relatively small shifts in many of the other bands
of the L ligand, as may be expected, and several are slightly
weaker for 4 than for 2, particularly those involving motion
along the donor-acceptor axes of the phenyl and pyridyl groups.
It is also notable that the band at 1374 cm-1 from 2, assigned
to a mode involving a strong contribution from ph-N stretching,
is present as a moderately strong band at 1369 cm-1 in 4,
consistent with this assignment and with weaker charge transfer
from the NMe2 group due to weaker electron demand from the
insulated Re-pyridyl group.
Ground-State IR Spectrum of 1-H+. The addition of HCl
to 1 in dichloromethane protonates the azacrown nitrogen atom
to generate 1-H+, as determined by a strong blue-shift of the
intraligand charge-transfer (ILCT) UV-vis absorption band;1,2,8
the electron-donor character of the azacrown is removed as the
nitrogen atom protonates to become fully pyramidal and sp3
hybridized. The changes in the IR fingerprint region on
protonation of 1 to give 1-H+ are significant, as shown in Figure
2, and they are similar in nature but generally larger in
magnitude than those observed between the spectrum of 1 and
the spectrum of 3; they may be attributed, similarly, to weaker
bands arising from smaller dipole changes44 or to band shifts
arising from changes in structure and bonding.
The 1395 cm-1 IR band of 1, assigned to a mode involving
ph-N stretching, is lost upon protonation, but a new band can-
not be seen clearly, possibly being obscured by solvent or other
sample bands. This band is well-established to be diagnostic of
electronic structure in dialkylamino systems;32,37,39,40,45-48
for example, a band of dimethylaniline at 1348 cm-1 is
reported to shift down to 1245 cm-1 upon protonation,37
consistent with the change to a ph-N single bond and an
sp3-hybridized nitrogen atom. The observation here is con-
sistent with the changes we have reported for the alkene and
alkyne analogues, where a band at ca. 1395 cm-1, assigned to
this mode, was strongly downshifted upon cation binding to
the azacrown.7
The bands of 1 at 1618, 1604, 1512, 1487, and 1198 cm-1
assigned to modes involving 8a, 9a, 18a, and 19a vibrations
lose intensity upon protonation, consistent with the loss of
charge-transfer character. The amide I band of 1 shifts up by
13 cm-1 to 1696 cm-1 for 1-H+, as also observed on going
from 1 to 3, and this can be rationalized similarly by an increase
in the amide CO bond strength due to a decreased contribution
from the charge-transfer canonical form A in Scheme 3. The
coupled amide II and pyridyl 18a/19a band of 1 shifts up slightly
by 2 cm-1 to 1514 cm-1 for 1-H+, consistent with its position
Figure 4. Fingerprint-region IR spectra of 1 and 1-Ba2+ in CD3CN
(regions of strong perchlorate absorption omitted, and spectra offset
for clarity).54
azacrown systems and as a distinct band at ca. 1172 cm-1 for
the dimethylamino systems is assigned to a mode involving an
alkyl vibration coupled to phenyl 18a and 1 vibrations calculated
at 1163 cm-1 for L2; its position is similar to that reported for
other dimethylaminophenyl compounds.32,37-40,42 The broad
band at ca. 1125 cm-1 that occurs for the azacrown compounds
1 and L1 is assigned to ν(C-O-C) modes of the azacrown
ring.43 A moderately strong IR band at ca. 1090 cm-1 from 1,
2, L1, and L2 is assigned to a calculated mode at 1088 cm-1
,
in which the pyridyl 18b vibration is coupled to NH bending
and phenyl 18a vibrations; a Raman band at 1060 cm-1 was
assigned to a calculated mode at 1075 cm-1 in which essentially
the same coupled vibrations occur but with the pyridyl 18b
displacements having the opposite phase.
L Ligand Bands of 3. There are significant differences
between the IR spectra of the electron-donor systems 1 and 2
and that of 3, in which a terminal electron-donor substituent is
absent, as shown by the spectra of 1 and 3 given together in
Figure 2. The differences may be attributed to the absence of
bands arising directly from the absent substituents, to weaker
bands arising from smaller dipole changes during vibrations
along the long axis of the L ligand of 3,44 or to band shifts
arising from changes in bonding or the forms of mixed modes.
The most notable absence in the spectrum of 3 is the band at
1395 cm-1 from 1 (and at 1374 cm-1 from 2), consistent with
its assignment to a mode involving a strong contribution from
a ph-N stretching vibration that cannot occur in 3. The bands
at ca. 1618, 1604, 1512, 1487, and 1198 cm-1 are much weaker
from 3 than from 1 and 2, consistent with their assignment to
modes involving 8a, 9a, 18a, and 19a vibrations that occur along
the donor-acceptor axes of the phenyl and pyridyl groups and
that may be expected to give smaller dipole changes due to the
absence of the phenyl electron-donor substituent in 3. The amide
I band of 1 and 2 shifts up by ca. 13 cm-1 to occur at 1696
cm-1 for 3, and this shift can be rationalized by an increase in
CO bond strength due to a decreased contribution from the
charge-transfer canonical form A in Scheme 3. There are also
small upshifts of 3 and 1 cm-1 in bands assigned to modes
involving pyridyl 8b/3 and 3 vibrations that occur at 1586 and
1333 cm-1, respectively, in 3.
L Ligand Bands of 4. There are significant differences
between the IR spectra of 1 and 2 and that of 4, in which a
phenyl electron-donor substituent is present but in which the
CH2 spacer may be expected to increase the nonplanarity at
the amide link and thereby decrease the electronic and vibra-
tional coupling between the pyridyl ring and the other groups
within the L ligand. The differences in the spectra may be

3858 J. Phys. Chem. A, Vol. 112, No. 17, 2008
Lewis et al.
at higher wavenumbers for amides that are not attached to an
electron donor;49,50 this mode has substantial ν(C-N) character,
and its shift may be rationalized by an increase in the amide
C-N bond strength due to an increased contribution from
canonical form C in Scheme 3. There are also upshifts of 10
and 3 cm-1 in the bands assigned to modes involving pyridyl
8b/3 vibrations, which gave smaller shifts on going from 1 to
3 and which occur at 1593 and 1335 cm-1, respectively, in 1-H+.
Ground-State IR Spectrum of 1-Ba2+. The addition of
barium perchlorate to 1 in acetonitrile results in metal cation
complexation to the azacrown to generate 1-Ba2+, as determined
by a moderately strong blue-shift of the ILCT UV-vis
absorption band that is approximately half that observed upon
protonation;1,2,8 the electron-donor character of the azacrown
is lowered by Ba2+ complexation. The IR spectrum of 1-Ba2+
in acetonitrile shows ν(CO)Re and bpy bands that are essentially
identical to those of 1 (Figure 4 and Table 1),51 indicating that
Ba2+ complexation does not affect the Re center significantly
in the ground state, whereas the L ligand bands of 1 show
significant changes upon Ba2+ complexation (Figure 4) that are
similar to those observed upon protonation (Figure 2) but smaller
in magnitude.
Figure 5. Fingerprint-region IR spectra of 2 in CD3CN, in the absence
and presence of barium perchlorate at 0.19, 0.46, and 0.73 mol dm-3
(region of strong perchlorate absorption omitted, and spectra offset for
clarity).
amide I band is consistent with the absence of an azacrown
within 2, and the growth of a downshifted amide I band at 1657
cm-1 may be attributed to Ba2+ binding to the amide carbonyl
group at [Ba2+] g 0.4 mol dm-3, with an estimated very low
binding constant of ca. 2 dm3 mol-1. The only other significant
change in the IR spectrum of 2 at high Ba2+ concentrations is
a ca. 7 cm-1 upshift of the band at 1276 to 1283 cm-1, which
is consistent with its assignment to an amide III mode involving
CN stretching (Figure 5 and Table 1) and to an increased
contribution from canonical form C (Scheme 3) giving a stronger
C-N bond on Ba2+ binding to the amide carbonyl. Both the
positions and the intensities of the other IR bands are remarkably
unaffected, with the only other changes being a slight broadening
of the ca. 1605 and 1332 cm-1 bands to lower and higher
wavenumbers, respectively. The absence of other changes
indicates that the effect of Ba2+-carbonyl binding is essentially
localized at the amide group, and thus, it appears to be similar
to a carbonyl solvation effect. This limited effect of Ba2+ on
the IR bands of 2 contrasts its much more substantial effect on
the IR band positions and intensities of 1, for which the changes
are therefore attributable predominantly to Ba2+ binding to the
azacrown inducing changes throughout the L ligand, as dis-
cussed previously.
The 1397 cm-1 band of 1 in CD3CN, assigned to a mode
involving ph-N stretching, loses all intensity, and the equivalent
mode in 1-Ba2+ is tentatively assigned to be a contributor to
the broad feature that includes peaks at 1299 and 1275 cm-1
,
consistent with our assignment of IR bands at 1250 cm-1 to
this mode for the Ba2+-complexed alkene and alkyne analogues.7
The bands of 1 in CD3CN at 1618, 1604, 1493, 1273, and 1200
cm-1 assigned to modes involving 8a, 9a, 18a, 19a, and ν(ph-
N) vibrations become weaker, the amide I band shows a
moderate upshift of 4 cm-1 to 1685 cm-1 in 1-Ba2+, and the
bands assigned to modes involving pyridyl 8b/3 and 3 vibrations
show small upshifts of 5 and 3 cm-1 to occur at 1593 and 1335
cm-1, respectively, in 1-Ba2+ (all in relation to their positions
for 1 in CD3CN). All of these effects are rationalized in the
same way as those observed upon protonation, and the generally
smaller magnitudes of the changes are attributed to an interaction
with the azacrown N atom that is weaker for Ba2+ than for H+.
Ba2+ Binding to the Amide Carbonyl Group. An additional
IR band at 1662 cm-1 was observed from 1-Ba2+ at the high
barium perchlorate concentration required to ensure that the
proportion of 1 without Ba2+ bound to the azacrown was
minimal. This additional band can be attributed to a downshifted
amide I mode arising from Ba2+ binding not only to the
azacrown but also to the amide carbonyl group of 1, as we
discuss in detail next.
The IR and TRIR studies of 1-Ba2+ we report here were
carried out on samples in which the presence of 1 without Ba2+
bound to the azacrown was minimized to e2% to ensure that
the TRIR bands could be attributed unambiguously to excitation
of 1-Ba2+, with Ba2+ bound to the azacrown, and not to the
excitation of 1. This strategy dictated the use of high ionic
strengths and very high Ba2+ concentrations (g0.5 mol dm-3),
which were typically at g50-fold in excess of the already high
concentrations of 1 (e10-2 mol dm-3) that were needed due to
a combination of low sample IR absorption coefficients and a
short path length (100 µm) required because of strong solvent
absorption. From the binding constants of ca. 126 and 2 dm3
mol-1 for Ba2+ attaching to the azacrown2 and amide carbonyl
groups, respectively, these conditions gave samples comprising
almost entirely 1-Ba2+, with Ba2+ bound to the azacrown, but
with the azacrown-and-amide-bound form also present (typically
ca. 35:65).54 Earlier TRVIS2 and emission8 studies that estab-
lished the general photochemical mechanisms of 1-Mn+ were
carried out on samples with 1 at much lower concentrations of
ca. 10-4 to 10-5 mol dm-3, due to the higher sensitivity of these
techniques, and generally at lower ionic strengths and Mn+
concentrations at which binding to the amide carbonyl would
generally not be significant. Our photochemical studies under
conditions where Mn+ binding to the amide carbonyl of 1 occurs
In general, an amide I band shifts significantly with solvent
or other intermolecular interactions at the carbonyl group, and
a downshift is indicative of a weaker amide CdO bond.
Chalcone-substituted ferrocenes in acetonitrile have been shown
to give a downshifted ν(CO) IR band due to Ca2+ interacting
with a carbonyl group at very high Ca2+ concentration (ca. 0.1
mol dm-3); and, in the case of a chalcone substituent with both
azacrown and carbonyl groups, the addition of Ca2+ was
reported first to give an upshifted band due to binding to the
azacrown and then to give a downshifted band due to binding
also to the carbonyl group at very high Ca2+ concentrations.52
Our observations indicate that a comparable situation occurs
for 1, for which the 1685 cm-1 band is attributable to an
upshifted amide I mode due to Ba2+ binding only to the
azacrown, as discussed for protonation at the azacrown, and
the 1662 cm-1 band is attributable to a downshifted amide I
mode attributable to an additional amide CdO‚‚‚Ba2+ interaction
at very high Ba2+ concentrations.53
This effect was confirmed by the addition of Ba2+ to 2 in
CD3CN, as shown in Figure 5. The absence of an upshifted

Azacrown-Substituted [(bpy)Re(CO)3L]+ Complex
J. Phys. Chem. A, Vol. 112, No. 17, 2008 3859
Figure 6. ν(CO)-region IR spectrum of 3 along with TRIR spectra
recorded upon 400 nm excitation, all in CH3CN.
Figure 7. ν(CO)-region IR spectrum of 1 along with TRIR spectra
recorded upon 400 nm excitation, all in CH3CN.
to some extent, both here and in our earlier Raman work,3 have
not shown any effects that we can attribute to it having a strong
influence on the overall photochemistry, with the only clear
effect that we have observed being on the excited-state amide
I IR band position. Hence, our observations suggest that an
amide-bound metal cation may act principally as a spectator in
the photochemistry of 1-Mn+; however, they indicate that the
effect should be considered carefully in designing and studying
metal cation sensors and switches containing both azacrown and
carbonyl groups, as noted also in a recent report on azacrown-
substituted chromenes.55
Excited-State TRIR Spectra of 1 and 3. The Re f bpy
MLCT transition in these [(bpy)Re(CO)3L]+ complexes gives
a broad UV-vis absorption band peaking at ca. 350 nm that is
distinct for 3 and that is responsible for the absorption tail at
>390 nm, which emerges from under the strong ILCT band of
1 that dominates at shorter wavelengths.1,2,8 The TRIR spectra
reported here were recorded upon pumping at 400 nm, which
populates the Re f bpy MLCT excited states of 1 and 3.1 Table
1 gives band positions and possible assignments of the excited-
state bands that are discussed in detail next.
ν(CO) Region. The TRIR spectrum of 3 in the ν(CO) region
(Figure 6) consists of bleaches of the ground-state bands at 2035
and 1931 cm-1 and three transient bands at higher wavenumbers
that persist for g2 ns, after showing narrowing and a small
upshift in wavenumber over the first ca. 15 ps that are consistent
with vibrational relaxation in the excited state.15-19,22,56 These
transient bands at 2073, 2014, and 1973 cm-1 (Table 1) are
upshifted from the ground-state positions by ca. 38, 83, and 42
cm-1, respectively, and they are characteristic of the MLCT
excited state of a (bpy)Re(CO)3L complex in which oxidation
to ReII (Scheme 1) results in weaker backbonding to the CO
ligands.9-22 The observation of three ν(CO)Re bands from the
MLCT state is attributable to a lowering of the pseudo-C3V
symmetry that resulted in the observation of only two ν(CO)Re
bands in the ground state.
Figure 8. TRIR kinetic traces recorded upon 400 nm excitation of 1;
ν(CO) region in CH3CN, fingerprint region in CD3CN.
MLCT excited state of 1 (Table 1). In contrast to that of 3, the
TRIR spectrum of 1 evolves at later times, with the MLCT-
state bands decaying and new bands at lower wavenumbers than
the ground-state bands growing in to give peaks at 2010 and
1901 cm-1 at g1 ns. The TRIR kinetics essentially match those
that we observed by TRVIS,57 as illustrated in Figure 8 by the
concomitant decay and growth of the bands at 2069 and 1901
cm-1, respectively, and so the TRIR spectrum at late times (g1
ns) is assigned to the LLCT state of 1 (Scheme 1).1 The bands
at 2010 and 1901 cm-1 support this assignment because their
downshifts of 25 and 29 cm-1 from the respective ground-state
positions indicate that the CO bonds are weaker, consistent with
stronger backbonding from a ReI center that is attached to a
bpy•- ligand in the LLCT state; the return to a two-band pattern
indicates a return to pseudo-C3V symmetry in going from MLCT
to LLCT states. Similar ν(CO) band positions and downshifts
have been reported upon electrochemical reduction of
(bpy)Re(CO)3Cl to form [(bpy•-)Re(CO)3Cl]12 and upon pho-
tochemical formation of similar LLCT states of Re(CO)3
systems with quite different diimine and L ligands.19,21 The
The TRIR spectrum of 1 at early times of ca. <100 ps (Figure
7) is very similar to that of 3, comprising bands at 2069, 2010,
and 1973 cm-1 at 25 ps, and hence, it is readily assigned to the

3860 J. Phys. Chem. A, Vol. 112, No. 17, 2008
Lewis et al.
strongly mixed in the ground state, and their forms may change
significantly in the excited state; the ground-state spectrum gives
several bands in this region that are notably sensitive to solvent,
as can be seen by comparing Figures 1 and 4. The broad TRIR
feature with peaks at ca. 1406 and 1383 cm-1 includes a
minimum due to bleaching of the ground-state band at 1397
cm-1, which was assigned to a mode involving ph-N stretching.
The TRIR band at 1406 cm-1 is tentatively assigned to a 22
cm-1 downshift of the ground-state band assigned to a pyridyl
18b/14 vibration and that at 1383 cm-1 to a 14 cm-1 downshift
of the ground-state band assigned to the mode involving ν(ph-
N) and 18a/19a phenyl vibrations. The TRIR bands at 1338
and 1308 cm-1 are assigned to ca. 6 and 7 cm-1 upshifts of the
ground-state bands at 1332 and 1301 cm-1, which were assigned
to pyridyl and phenyl 3 modes, respectively. Our assignment
of all these fingerprint TRIR bands to L ligand rather than bpy
modes is consistent with the reported TRIR spectra of [Ru-
(bpy)3]2+ and a related [Re(bpy)(CO)3L]+ system,31 which show
that the MLCT-state bpy•- bands are much weaker than the
ground-state bpy bleaching bands, which are very weak in the
TRIR spectra observed here. By contrast, the transient resonance
Raman spectra that we have reported gave strong bpy•- bands
of 1 in the MLCT and LLCT states.3
Although MLCT excitation is localized at the (bpy)Re(CO)3
group, the TRIR bands in the fingerprint region indicate clearly
that it results also in significant changes within the L ligand.
The 13 cm-1 upshift of the amide I band indicates that the amide
CO bond becomes stronger in the MLCT state, and this is
attributable to increased electron demand from the ReII center
withdrawing electron density from the amide carbonyl group,
such that donation from the phenyl-azacrown is transmitted
more effectively through to the pyridyl group (Scheme 3; form
B contributes more and C less). The ca. 17 and 32 cm-1
downshifts of two bands assigned to amide II modes, which
involve amide C-N stretching, may be attributed to the same
effect, with the withdrawal of electron density weakening the
amide C-N bond and strengthening the py-N bond in the
MLCT state (Scheme 3; form B contributes more and C less);
these shifts may arise also, in part, from a change in the form
of the modes, and particularly those involving the amide II
vibration, which gives two bands from strongly mixed modes
in the ground state. The downshift of the phenyl and pyridyl
8a/9a modes may be attributed to the pyridyl and phenyl rings
adopting a more quinoidal structure38,39 as a result of increased
charge-transfer character due to the greater electron demand by
the ReII center in the MLCT state (Scheme 3; canonical forms
B and A, respectively, contribute more); the observation of
features at lower wavenumbers than the ground-state bands at
ca. 1600 cm-1, along with the absence of any at higher
wavenumbers, supports this interpretation regardless of the
precise excited-state band positions and mode assignments.
Figure 9. Fingerprint-region IR spectrum of 1 along with TRIR spectra
recorded upon 400 nm excitation, all in CD3CN.
TRIR spectrum of 1 in the LLCT state we report here is
consistent with our nanosecond transient resonance Raman
studies,3 where we observed one resonance-enhanced Raman
ν(CO)Re band at 2012 cm-1 that matches the high-wavenumber
TRIR band observed here.
Fingerprint Region. The TRIR spectra of 1 in the fingerprint
region (Figure 9) show bleaches of many of the ground-state
bands and several new excited-state bands. Following small
upshifts and narrowing at <25 ps attributable to vibrational
relaxation, the spectra at early times (e200 ps) show strong
and distinct excited-state peaks at ca. 1694, 1599, 1579, 1461,
1383, and 1338 cm-1 that can be assigned to the MLCT state.
These bands show decay kinetics on a time scale that matches
the TRIR decay kinetics of the ν(CO)Re bands of the MLCT
state (Figures 8 and 9), supporting this assignment. The spectra
at early times also show weaker or less distinct features at ca.
1616, 1558, 1544, 1497, 1406, and 1308 cm-1 that also may
arise from the MLCT state, although their decay kinetics are
less clear.
The TRIR band at 1694 cm-1 is assigned to the amide I mode
in the MLCT state (Table 1), upshifted by 13 cm-1 from its
ground-state position at 1681 cm-1 in CD3CN. The assignment
of the other MLCT-state bands is less straightforward, but some
tentative assignments can be proposed. The TRIR peaks at 1616
(weak), 1599, and 1579 cm-1 are tentatively assigned to ca. 2,
5, and 9 cm-1 downshifted counterparts of the three ground-
state bands at 1618, 1604, and 1588 cm-1, respectively (in CD3-
CN), which were assigned to modes involving phenyl 8a/9a,
pyridyl 8a/9a, and NH bend coupled to pyridyl 8b/3 vibrations
(Table 1).58 The sloping feature at slightly lower wavenumbers
may include peaks at ca. 1558 and 1544 cm-1 or may arise as
the shoulder of broad bands giving strong peaks at ca. 1599
and 1579 cm-1. The TRIR bands at 1497 and 1461 cm-1, which
occur at either side of a bleaching peak at ca. 1484 cm-1, are
tentatively assigned to ca. 17 and 32 cm-1 downshifts of the
ground-state bands at 1514 and 1493 cm-1, respectively (in CD3-
CN), that were assigned to modes involving coupled amide II
and pyridyl 18a vibrations. These modes were calculated to be
This interpretation is supported by the observation that some
of the band shifts on going from the ground state to the MLCT
state of 1 are in the opposite direction to those observed on
going from 1 to 4 in the ground state. The TRIR spectrum of 1
shows an upshift of the amide I band in going to the MLCT
state, attributed to excitation increasing the extent of electron
transfer to the Re center, whereas it shifts down upon going
from 1 to 4, attributed to the CH2 insulator decreasing the extent
of electron transfer to the Re center. The three bands at ca. 1600
cm-1 assigned to phenyl 8a/9a, pyridyl 8a/9a, and coupled NH
bending and 8b/3 pyridyl modes show downshifts in going to
the MLCT state of 1, attributed to increased quinoidal character

Azacrown-Substituted [(bpy)Re(CO)3L]+ Complex
J. Phys. Chem. A, Vol. 112, No. 17, 2008 3861
due to increased charge-transfer character, whereas they shift
up upon going to 4, attributed to decreased charge-transfer
character.
Several TRIR bands in the fingerprint region show similar
decay kinetics to those recorded in the ν(CO) region (Figures
7-9), consistent with the decay of the MLCT state, and the
fingerprint-region spectrum of 1 at g1 ns (Figure 9) can be
assigned similarly to the LLCT state. This TRIR spectrum shows
the same bleaching of ground-state bands observed at earlier
times, but the fingerprint-region bands of the LLCT state are
notably much weaker than those of the MLCT state, contrasting
the similar strengths of the transient bands from these states in
the ν(CO) region (Figure 7). Only one new TRIR band, a weak
feature at 1290 cm-1 (Figure 9), is observed unambiguously
from the kinetics (Figure 8) to grow in on the same time scale
as the formation of the LLCT state. Some small band shifts are
observed on the same time scale (Figure 9): the amide I band
at 1694 cm-1, the 8a/9a phenyl band at 1596 cm-1, and the
NH bend band at 1575 cm-1 all appear to shift slightly to higher
wavenumbers as the strong, overlying MLCT excited-state bands
decay; however, these bands are weak at later times, they overlap
with strong ground-state bleaches that may distort their profiles,
and so these possible assignments are uncertain.
Figure 10. ν(CO)-region IR spectrum of 1-Ba2+ along with TRIR
spectra recorded upon 400 nm excitation, all in CH3CN.54 Dashed lines
indicate the maxima at 25 ps.
Although the TRIR spectrum of the LLCT state of 1 is sparse,
it provides some structural information. The one new band at
1290 cm-1 that can be attributed unambiguously to the LLCT
state may tentatively be assigned to a mode involving ph-N
stretching, downshifted by ca. 107 cm-1 from a band at 1397
cm-1 in the ground state and by ca. 93 cm-1 from a possible
equivalent band at 1383 cm-1 in the MLCT state. It is reasonable
to propose that the TRIR difference spectrum will show bands
of the phenyl group, where the changes in the L ligand are most
significant between ground and LLCT states, and our tentative
assignment is based on studies reported for DMABN and its
isotopomers,32,47,48 for which the strongest (although weak) band
at 1276 cm-1 in the TRIR spectrum of the CT state has been
assigned to a mode involving ph-N stretching, shifted down
by 96 cm-1 from a ground-state band at 1372 cm-1. We
observed only one strong LLCT-state band in the time-resolved
resonance Raman spectrum in this region,3 which was a
dominant feature at 1514 cm-1 that we assigned to a ca. 100
cm-1 downshift of the phenyl 8a/9a band from its ground-state
position; it is not observed in the TRIR spectrum here, consistent
with the equivalent ground-state IR band being relatively weak.
The observation that the IR bands from the LLCT state are
significantly weaker than the strong bands from the ground and
MLCT states is indicative of smaller dipole changes upon
vibration, which is consistent with the electron-donating proper-
ties of the azacrown nitrogen atom being lost due to its oxidation
in forming the LLCT state and with the electron-acceptor
properties of the rhenium center being lowered upon its
reduction from ReII back to ReI in going from the MLCT state
to the LLCT state (Scheme 1).
Figure 11. TRIR kinetic traces recorded upon 400 nm excitation of
1-Ba2+; ν(CO) region in CH3CN and fingerprint region in CD3CN.54,60
clearly does not show such shifts, indicating that the presence
of Ba2+ extends the MLCT-state lifetime of 1, consistent with
the proposal that electron transfer to form the LLCT state is
thermodynamically unfavorable when a metal cation is bound
to the azacrown.2,8 The persistence of the TRIR bands of the
MLCT state indicates that Ba2+ remains bound to the azacrown
for at least ca. 1 ns after excitation, consistent with a time scale
of ca. 40 ns for its release to bulk solution (koff-1; Scheme 2)
obtained by analyzing nanosecond TRVIS data2 and supported
by time-resolved resonance Raman data.3
Excited-State TRIR Spectra of 1-Ba2+. ν(CO) Region. The
TRIR spectrum observed in the ν(CO) region upon excitation
of 1-Ba2+ (Figure 10) has a similar profile to that from 1 at
early times (Figure 7) and that from 3 at all delay times (Figure
6), showing a set of three excited-state bands at higher
wavenumbers than the ground-state bands; it may be assigned
similarly to an MLCT state, with peaks at ca. 2069, 2011, and
1970 cm-1 observed at 25 ps after upshifts and narrowing at
early times. Whereas the TRIR spectrum from 1 shows large
band shifts to lower wavenumber over ca. 1 ns as the LLCT
state forms (Figure 7), the spectrum from 1-Ba2+ (Figure 10)
Although the profile of the TRIR spectrum from the MLCT
state of 1-Ba2+ persists, all three ν(CO)Re bands show ca. 5-10
cm-1 upshifts between ca. 25 ps and 1-2 ns (Figure 10) to
give peaks at 2073, 2020, and 1975 cm-1 at 1 ns, whereas the
ν(CO)Re bands of the MLCT state of 3 do not show such shifts
on this time scale (Figure 6). The TRIR kinetic traces in Figure
11 illustrate this effect further, showing minor growth and decay
in the signals at 2087 and 2069 cm-1, respectively, as the upshift
occurs. The possible origins of this effect are discussed next.

3862 J. Phys. Chem. A, Vol. 112, No. 17, 2008
Lewis et al.
assigned to ca. 7 and 4 cm-1 upshifts of the ground-state bands
at 1335 and 1299 cm-1, which were assigned to pyridyl and
phenyl 3 modes, respectively, and which give a similar pattern
to the slightly stronger TRIR bands observed in this region from
1. Several of the TRIR bands from the MLCT state of 1-Ba2+
(Figure 12) are weaker than the equivalent TRIR bands from
the MLCT state of 1 (Figure 9): most notably the bands at
1593 and 1599 cm-1, respectively, assigned to a pyridyl 8a/9a
vibration. This observation is consistent with the occurrence of
smaller dipole changes upon vibration due to weaker electron
donation when Ba2+ is complexed to the azacrown, as observed
and discussed for the equivalent ground-state spectra (Figure
4) and as indicated by several TRIR bleaching bands that are
weaker for 1-Ba2+ (Figure 12) than 1 (Figure 9).
Whereas the TRIR spectra from 1-Ba2+ at ca. 1200-1640
cm-1 are essentially unchanged between ca. 50 ps and 2 ns,
the TRIR bands assigned to amide I modes at ca. 1640-1750
cm-1 show significant changes over ca. 1-2 ns (Figure 12),
with a time dependence (Figure 11)60 that appears to match that
of the small upshifts in the ν(CO)Re TRIR bands discussed
previously (Figures 10 and 11). The TRIR spectrum at early
times shows a bleach of the ground-state amide I band at ca.
1662 cm-1, assigned above to 1 with Ba2+ bound to both the
azacrown and the amide carbonyl group, and an increase in
absorption at ca. 1685 cm-1, which corresponds to the ground-
state amide I band assigned to 1 with Ba2+ bound only to the
azacrown. These bleaching and new absorption bands decay to
give a TRIR spectrum at ca. 2 ns that is essentially featureless
in this region (apart from baseline drift), indicating that the
amide I band position in the MLCT state present at this time is
similar to that of the ground state. Hence, the TRIR data seem
to suggest that excitation to the MLCT state results in rapid
disruption of CdO‚‚‚Ba2+ interactions at the amide carbonyl
group, which then recover in ca. 1-2 ns, and that the amide I
band position in the MLCT state is little changed from that in
the ground state once this interaction is restored. As discussed,
the ν(CO)Re TRIR bands show a small upshift in ca. 1-2 ns
that indicates a decrease in electron density at the Re center,
which is consistent with a shift in electron density toward the
L ligand that might be caused by the restoration of a Ba2+
interaction with the amide carbonyl. The absence of large
changes in any of the other TRIR bands in ca. 1-2 ns also
may be consistent with this interpretation because the introduc-
tion of the CdO‚‚‚Ba2+ interaction results in relatively little
changes in the other bands in the ground-state IR spectrum of
2 (Figure 5). Another possibility is that the changes on this time
scale may arise also from changes in the nature of the Ba2+
interaction with the azacrown, as has been discussed in
interpreting picosecond TRVIS data observed upon excitation
of Mn+-bound organic-azacrown systems.61-65 However, the
Figure 12. Fingerprint-region IR spectrum of 1-Ba2+ along with TRIR
spectra recorded upon 400 nm excitation, all in CD3CN.54,59
Fingerprint Region. The fingerprint-region TRIR spectra
observed upon excitation of 1-Ba2+ (Figure 12)59 show bleaches
of ground-state bands and several new excited-state bands.
Following small upshifts and narrowing at <50 ps, the spectra
show strong and distinct excited-state peaks at ca. 1593, 1560,
1522, and 1492 cm-1 and weak features at ca. 1342 and 1303
cm-1, which undergo almost no change up to ca. 2 ns, as shown
also by the TRIR kinetics (Figure 11). These bands can be
assigned to the MLCT state of 1-Ba2+
.
The TRIR bands from 1-Ba2+ in the MLCT state at 1593
and 1560 cm-1 are tentatively assigned to the ca. 12 and 33
cm-1 downshifted counterparts of the strong ground-state bands
of 1-Ba2+ at 1605 and 1593 cm-1, respectively, which were
assigned to modes involving pyridyl 8a/9a vibrations and NH
bending coupled to pyridyl 8b/3 vibrations (Table 1). This
assignment indicates that these pyridyl modes are more strongly
downshifted in going from the ground to MLCT state of 1-Ba2+
than they are in going from the ground to MLCT state of 1,
where they were tentatively assigned to respective bands at 1599
and 1579 cm-1, downshifted by ca. 5 and 9 cm-1 from the
ground-state counterparts in 1, which overlapped strongly with
the bleaching bands in the TRIR spectra (Figure 9).58 The strong
TRIR feature from 1-Ba2+ at ca. 1480-1530 cm-1 comprises
a strong broad peak at 1492 cm-1, a bleaching peak at 1514
cm-1, and a feature peaking at 1522 cm-1 that may arise as a
shoulder of the strong band at lower wavenumber or as a distinct
band; the overall pattern in this region resembles that of the
strong TRIR feature from 1 at ca. 1440-1510 cm-1 (Figure 9),
although the feature is ca. 30 cm-1 higher for 1-Ba2+. The strong
TRIR band from the MLCT state of 1-Ba2+ at 1492 cm-1 is
tentatively assigned to a 15 cm-1 downshift of the ground-state
band at ca. 1507 cm-1 assigned to coupled amide II and pyridyl
18a vibrations; as discussed previously, the bands in this region
are sensitive to solvent for 1 (Figures 1 and 5), and they are
assigned to mixed modes (Table 1) that may be expected to
change significantly in the excited state. The very weak TRIR
bands at 1342 and 1303 cm-1 from 1-Ba2+ are tentatively
TRIR spectrum can be attributed to the MLCT state of 1-Ba2+
,
regardless of the precise origin of the relatively subtle changes
over ca. 1-2 ns, and it is clearly different from that of the
MLCT state of 1 when Ba2+ is not bound to the azacrown.
The similarities and differences between the fingerprint-region
TRIR spectra of the MLCT states of 1-Ba2+ and 1 are notable.
They show a similar set of excited-state bands and a net increase
in absorbance from the ground state that are both attributable
to a significant charge redistribution within the L ligand whether
or not Ba2+ is bound to the azacrown and despite the MLCT
excitation nominally being localized on the (bpy)Re(CO)3 group.
From the tentative assignments proposed here, it appears that
at least some of the pyridyl bands show shifts that are larger
upon MLCT excitation of 1-Ba2+ than 1, whereas at least some

Azacrown-Substituted [(bpy)Re(CO)3L]+ Complex
J. Phys. Chem. A, Vol. 112, No. 17, 2008 3863
of the amide and phenyl bands show smaller shifts. These
differences in the shift patterns suggest that the increased
electron demand of the ReII center in the MLCT state results in
charge redistribution that is relatively localized at the pyridyl
ring for 1-Ba2+ and more delocalized for 1, and this effect may
reasonably be attributed to the electron-donor properties of the
phenyl-azacrown group being lowered when Ba2+ is bound to
the azacrown.
The use of time-resolved infrared spectroscopy has been
shown to be particularly useful in this study because it has
provided vibrational information that was not available from
time-resolved UV-visible absorption or emission spectros-
copy,1,2 and L ligand bands that complement the (bpy)Re(CO)3
bands that were the principal features of the transient resonance
Raman spectra.3 Overall, though, it is the combination of the
results from all four of these time-resolved techniques that has
enabled the generic light-controlled ion-switching mechanism
to be elucidated.
Conclusion
This study also illustrates a general effect in transition-metal
photochemistry that can be deployed in designing supramo-
lecular switches: MLCT excitation that is nominally localized
on one ligand, which is reduced upon excitation, results in an
oxidized metal center whose increased electron demand induces
significant charge redistribution that is sufficient to induce a
reaction at a relatively remote site within another attached ligand.
In the example studied here, Re f bpy MLCT excitation induces
significant charge redistribution that results ultimately in ion
release from a pyridyl-amido-phenyl-azacrown ligand that
is not fully conjugated and in which the azacrown lies at
>10 Å from the Re center.66
We have reported ground- and excited-state IR spectra of 1,
its model complexes, and its protonated and Ba2+-complexed
forms. These IR spectra have been shown to provide not only
characteristically strong CO stretching bands from the
(bpy)Re(CO)3 group but also a rich set of bands in the
fingerprint region that provide useful information on structure
and bonding within the L ligand.
The ground-state IR spectrum of 1 is dominated in the
fingerprint region by bands arising principally from L ligand
pyridyl and amide vibrations, and their assignments are con-
sistent with our earlier RR assignments,3 for which they provide
further support. The TRIR spectra of 1 give ν(CO)Re marker
bands that characterize the MLCT and LLCT states conclusively,
with their positions supporting the assignments made earlier by
nanosecond transient Raman spectroscopy3 and their time
dependence supporting the time scale for intramolecular electron
transfer to form the LLCT state determined earlier by picosecond
time-resolved UV-vis spectroscopy.1 Importantly, the fingerprint-
region TRIR spectra of 1 also provide completely new informa-
tion. The MLCT state gives strong L ligand TRIR bands, with
most tentatively assigned to pyridyl and amide vibrations, which
indicate that the increased electron demand at the ReII center
results in significant charge redistribution within the L ligand.
By contrast, the LLCT state gives a sparse TRIR spectrum,
which indicates that the return to a ReI center largely restores
the charge distribution of the ground state at the pyridyl group,
with one weak L ligand band being assigned tentatively to a
mode of the oxidized phenyl-azacrown group.
Acknowledgment. We thank L. C. Abbott, P. Matousek,
and A. W. Parker for assistance and helpful discussions and
the EPSRC for financial support.
References and Notes
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2
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