Ultrafast excited-state dynamics preceding a ligand trans-Cis isomerization of fac-[Re(Cl)(CO)3(t-4-styrylpyridine)2] and/fac-[Re(t-4-styrylpyridine)(CO)3(2,2′-bipyridine)] +

Article abstract of DOI:10.1021/jp044753+

UV-vis absorption and resonance Raman spectra of the complexes fac-[Re(Cl)(CO)₃(stpy)₂] and fac-[Re-(stpy)(CO) ₃(bpy)]⁺ (stpy = t-4-styrylpyridine, bpy = 2,2′-bipyridine) show that their lowest absorption bands are dominated by stpy-localized intraligand (IL) ππ* transitions. For the latter complex a Re → bpy transition contributes to the low-energy part of the absorption band. Optical population of the ¹IL excited state of fac[Re(Cl)(CO)₃(stpy)₂] is followed by an intersystem crossing (≤0.9 ps) to an ³IL state with the original planar trans geometry of the stpy ligand. This state undergoes a ～90° rotation around the stpy C=C bond with a 11 ps time constant. An electronically excited species with an approximately perpendicular orientation of the phenyl and pyridine rings of the stpy ligand is formed. Conversion to the ground state and isomerization occurs in the nanosecond range. Intraligand excited states of fac-[Re(stpy)(CO)₃(bpy)]⁺ show the same behavior. Moreover, it was found that the planar reactive ³IL excited state is rapidly and efficiently populated after optical excitation into the Re → bpy ¹MLCT excited state. A ¹MLCT → ³MLCT intersystem crossing takes place first with a time constant of 0.23 ps followed by an intramolecular energy transfer from the Re¹(CO) ₃-(bpy) chromophore to a stpy-localized ³IL state with a 3.5 ps time constant. The fast rate ensures complete conversion. Coordination of the stpy ligand to the Re¹ center thus switches the ligand trans-cis isomerization mechanism from singlet to triplet (intramolecular sensitization) and, in the case of fac-[Re(stpy)(CO)₃(bpy)] ⁺, opens an indirect pathway for population of the reactive ³IL excited state via MLCT states.

Full text of DOI:10.1021/jp044753+

3000
J. Phys. Chem. A 2005, 109, 3000-3008
ARTICLES
Ultrafast Excited-State Dynamics Preceding a Ligand Trans-Cis Isomerization of
fac-[Re(Cl)(CO)3(t-4-styrylpyridine)2] and fac-[Re(t-4-styrylpyridine)(CO)3(2,2′-bipyridine)]+
Michael Busby,† Pavel Matousek,‡ Michael Towrie,‡ and Anton´ın Vlcˇek, Jr.*,†
Department of Chemistry and Centre for Materials Research, Queen Mary, UniVersity of London, Mile End
Road, London E1 4NS, United Kingdom, and Central Laser Facility, CCLRC Rutherford Appleton Laboratory,
Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom
ReceiVed: NoVember 17, 2004; In Final Form: January 24, 2005
UV-vis absorption and resonance Raman spectra of the complexes fac-[Re(Cl)(CO)3(stpy)2] and fac-[Re-
(stpy)(CO)3(bpy)]+ (stpy ) t-4-styrylpyridine, bpy ) 2,2′-bipyridine) show that their lowest absorption bands
are dominated by stpy-localized intraligand (IL) ππ* transitions. For the latter complex a Re f bpy transition
1
contributes to the low-energy part of the absorption band. Optical population of the IL excited state of fac-
[Re(Cl)(CO)3(stpy)2] is followed by an intersystem crossing (e0.9 ps) to an 3IL state with the original planar
trans geometry of the stpy ligand. This state undergoes a ∼90° rotation around the stpy CdC bond with a 11
ps time constant. An electronically excited species with an approximately perpendicular orientation of the
phenyl and pyridine rings of the stpy ligand is formed. Conversion to the ground state and isomerization
occurs in the nanosecond range. Intraligand excited states of fac-[Re(stpy)(CO)3(bpy)]+ show the same behavior.
3
Moreover, it was found that the planar reactive IL excited state is rapidly and efficiently populated after
1
1
3
optical excitation into the Re f bpy MLCT excited state. A MLCT f MLCT intersystem crossing takes
place first with a time constant of 0.23 ps followed by an intramolecular energy transfer from the ReI(CO)3-
3
(bpy) chromophore to a stpy-localized IL state with a 3.5 ps time constant. The fast rate ensures complete
conversion. Coordination of the stpy ligand to the ReI center thus switches the ligand trans-cis isomerization
mechanism from singlet to triplet (intramolecular sensitization) and, in the case of fac-[Re(stpy)(CO)3(bpy)]+,
3
opens an indirect pathway for population of the reactive IL excited state via MLCT states.
Introduction
of the phenyl rings. Nonradiative transition to the maximum
on the ground-state potential-energy surface follows through a
Photochemical trans-cis isomerization of an olefinic CdC
bond is one of the fundamental photochemical processes. Its
investigations by ultrafast spectroscopic techniques1-5 and
quantum chemical calculations6 keep revealing important new
insights into the nature of the excited states involved, their
relaxation and dynamic couplings, dynamics of barrier crossing,
and the isomerization itself. Besides its fundamental interest,
photoisomerization is responsible for the capture of optical
information by retinal7-10 in the processes of vision and for
functioning of important biomolecules such as the photoactive
yellow protein11 or rhodopsin.12 It can be envisaged that
photoisomerization can perform similar functions in man-made
molecular devices, where photoisomerizable molecules could
serve as switches or memory units.12
conical intersection. The molecule then either rotates further
90° to the cis isomer or twists back to the trans configuration.
A lifetime of 90 ps has been determined16 for the 1ππ* state in
n-hexane at room temperature. A similar mechanism operates
on the triplet potential-energy surface. However, intersystem
crossing from 1ππ* is not competitive with isomerization, and
3
the reactive ππ* state can be populated only by using triplet
sensitizers.17-19 The efficiency of intersystem crossing to the
3ππ* state and the relative contribution of the triplet mechanism
to the trans-cis isomerization can also be enhanced by a heavy-
atom effect of bromo or chloro substituents in 4-halogenated
stilbenes.20 The perpendicular triplet state of stilbene is nearly
isoenergetic with the perpendicular form of the ground state.6,15
Nevertheless, because of its spin-forbidden character, the
intersystem crossing to the ground state is rather slow, ca. 60
ns.17,21
Over many years stilbene has been the model photoisomer-
izable molecule.13 The generally accepted mechanistic picture6
originates in the work by Saltiel14 and Orlandi.15 The reaction
occurs on the potential-energy surface of the optically populated
1ππ* state, overcoming a small energy barrier to produce an
intermediate singlet excited state with a perpendicular geometry
Introduction of a nitrogen atom into one or two of the stilbene
aromatic rings yields styrylpyridine or 1,2-bis-pyridylethylene,
respectively. These compounds undergo the same trans-cis
photoisomerization as stilbene, again following the singlet and
triplet mechanisms.22,23 Moreover, they can also act as ligands,
binding to transition-metal atoms via their pyridyl rings.
Photoisomerization reactivity of stilbene-like molecules is
* To whom correspondence should be addressed.
† University of London.
‡ CCLRC Rutherford Appleton Laboratory.
10.1021/jp044753+ CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/11/2005

Ultrafast Excited-State Dynamics
J. Phys. Chem. A, Vol. 109, No. 13, 2005 3001
Experimental Section
Materials. All starting chemicals were obtained from Aldrich
and used as received. Solvents of spectroscopic quality were
used. The stpy ligand and the complexes were prepared
according to modified literature procedures.
trans-4-Styrylpyridine (stpy).33 An equimolar mixture of
benzaldehyde (10.61 g, 0.1 mol) and 4-picoline (9.31 g, 0.1
mol) was refluxed in 200 mL of acetic anhydride for 18 h. The
mixture was allowed to cool, quenched with 60 mL of water,
and made pH 8-9 with 30% NaOH. The brown solid was
separated via vacuum filtration, washed with water, and dried
over silica under reduced pressure for 24 h. The crude compound
was twice sublimed at 135 °C, 0.1 mmHg, and then recrystal-
lized from methanol, producing white needles: 61% yield.
1H NMR (CD3CN, 270 MHz): δ 7.17 (1H, d, J ) 16.2,
ethylenic CH), 7.38 (4H, m), 7.47 (2H, d, J ) 8.1, aromatic
CH), 7.51 (2H, d, J ) 8.1, aromatic CH), 8.54 (2H, d, J ) 5.4,
aromatic CH).
Figure 1. Schematic structures of fac-[Re(Cl)(CO)3(stpy)2] (left) and
fac-[Re(stpy)(CO)3(bpy)]+ (right).
generally retained upon coordination to transition-metal com-
plexes. A relatively slow (∼106 s-1) trans-cis isomerization
of the stilbene unit in [Re(stilbene-amido-pyridine)(CO)3(bpy)]+
(bpy ) 2,2′-bipyridine) follows optical excitation of the Re f
bpy metal to ligand charge transfer (MLCT) transition.24
Efficient (φ ) 0.54 at 366 nm) isomerization was found19 in
fac-[Re(Cl)(CO)3(stpy)2], where the 4-styrylpyridine (stpy)
ligand is directly coordinated to the Re atom. Less efficient
isomerization was reported25 for fac-[Re(stpy)(CO)3(NN)]+ (NN
) bpy or phen ) 1,10-phenanthroline), while quantum yields
≈ 0.2 were found26,27 in complexes with 1,2-bis(4-pyridyl)-
ethylene (bpe), fac-[Re(bpe)(CO)3(NN)]+. Bridging pyridyl-
ethylene-type ligands can also link two Re(CO)3(NN) units,
leading to photoactive polynuclear28-30 complexes. Trans-cis
isomerization of complexes containing pyridyl-ethylene ligands
coordinated to a Re(CO)3(NN) unit is accompanied by emer-
gence of a strong emission, a function which could possibly be
utilized in molecular devices.31,32
Coordination of stilbene-like isomerizable molecules to
transition metals presents new opportunities and brings up
further mechanistic questions: (i) Is the singlet isomerization
mechanism still operational or does metal coordination steer
the reaction to the triplet potential-energy surface? (ii) What is
the nature of the photoreactive state(s) and intermediates
involved? (iii) Can MLCT excitation to a stilbene-type ligand
induce its isomerization? (iv) Is it possible to trigger isomer-
ization of a stilbene-type ligand by exciting a MLCT transition
directed to a polypyridine ligand simultaneously present in the
coordination sphere? The latter question is especially important.
Population of reactive intraligand (IL) ππ* excited states upon
Re f NN MLCT excitation of fac-[Re(L)(CO)3(NN)]+ com-
plexes (L ) photoisomerizing stilbene-type ligand) was often
assumed25,28,31 to occur with modest rates but never proven
directly. Such an intramolecular energy transfer, if sufficiently
fast, would provide an alternative way to trigger trans-cis
isomerization using relatively low-energy light, with possible
device applications.
fac-[Re(Cl)(stpy)2(CO)3].19
[Re(Cl)(CO)5] (0.52 g, 1.44 mmol) and stpy (0.58 g, 3.2
mmol) were refluxed in dry toluene for 12 h. The solution was
cooled, and the volume was reduced to a minimum. The
precipitate was filtered under suction and then washed with pet
spirits (60-80 °C) to remove any excess free ligand: 92% yield.
FT IR (CH2Cl2): νCO ) 2024(s), 1920(m), 1886(m) cm-1
.
1H NMR (CDCl3, 270 MHz): δ 6.98 (2H, d, J ) 16.3,
ethylenic CH), 7.39 (12H, m, CH), 7.52 (4H, d, J ) 8.1,
aromatic CH), 8.67 (4H, d, J ) 6.7, aromatic CH).
fac-[Re(stpy)(CO)3(bpy)]PF6.25
fac-[Re(OTf)(CO)3(bpy)]34 (1.18 g, 2.06 mmol) and stpy (0.41
g, 2.26 mmol) were refluxed in 30 mL of dry THF for 12 h.
The mixture was filtered hot, and the solvent was then removed
in vacuo. The solid was dissolved in 30 mL of methanol, and
a saturated solution of methanol containing 1 g of NH4PF6 was
added to form a precipitate, which was then filtered and
repeatedly recrystallized from methanol: 63% yield.
FT-IR (CH2Cl2): νCO ) 2035(s), 1934 (br) cm-1
.
1H NMR (CD3CN, 270 MHz): δ 7.05 (1H, d, J ) 16.2,
ethylenic CH), 7.39 (6H, m, CH), 7.56 (2H, d, J ) 7.9, aromatic
CH), 7.79 (2H, dd, J ) 7.9, aromatic CH), 8.13 (2H, d, J )
6.7, aromatic CH), 8.26 (2H, dd, J ) 7.9, aromatic CH), 8.36
(2H, d, J ) 8.2, aromatic CH), 9.23 (2H, d, J ) 5.4, aromatic
CH).
13C NMR (CD3CN, 67.9 MHz): δ 124.23, 124.87, 125.79,
128.55, 129.85, 130.22, 130.67, 136.69, 137.98, 142.19, 149.43,
152.93, 154.90, 156.83, 196.89 (CO).
Spectroscopic Techniques. Ground-state resonance Raman
spectra were obtained using a Coherent Innova 90-5 UV argon-
ion laser with a power output of ca. 50 mW at 457.9 nm and
30 mW at 351.1 nm. The Raman spectra were recorded using
2 mm quartz flow tubes, and the scattered light was collected
at 90° from the excitation beam passed through a Spex
Triplemate 187 Series monochromator and dispersed across a
Princeton Instruments LN/CCD-1024 CCD camera. Spectra
were calibrated and solvent bands subtracted using CSMA CCD
Spectrometric Multichannel Analysis Software, version 2.4a,
Princeton Instruments Inc.
Time-resolved UV-vis absorption spectra, TA, were mea-
sured using the experimental setup at the van’t Hoff Institute
for Molecular Sciences, University of Amsterdam.35 A ∼130
fs, 390 nm pump pulse was generated by frequency doubling
of the Ti:Sapphire laser output. White-light continuum probe
pulses were generated by focusing the 780 nm fundamental in
a sapphire plate. Some TA spectra were obtained using the
Herein, we studied complexes fac-[Re(Cl)(CO)3(stpy)2] and
fac-[Re(stpy)(CO)3(bpy)]+ (Figure 1), which are further ab-
breviated as Re(stpy)2 and Re(stpy)(bpy), respectively. A
combination of stationary and ultrafast time-resolved spectro-
scopic techniques was employed with the aim of understanding
the nature and dynamics of the first steps leading to isomer-
ization of metal-bound stilbene-type ligands. It is shown that
stpy-localized 3IL excited states are populated very rapidly and
1
efficiently, following optical excitation to either IL or Re f
bpy 1MLCT states. Intramolecular energy transfer from an
3MLCT to 3IL state is directly observed for the first time.
Moreover, a picosecond structural reorganization of the stpy
3IL state was clearly demonstrated and attributed to a trans f
perpendicular twist around the CdC bond.

3002 J. Phys. Chem. A, Vol. 109, No. 13, 2005
Busby et al.
Figure 2. UV-vis absorption spectra of stpy in acetonitrile (‚‚‚‚), [Re-
(Cl)(CO)3(stpy)2] in CH2Cl2 (- - -), and [Re(stpy)(CO)3(bpy)]+ in
acetonitrile (s).
Figure 3. Resonance Raman spectra of (a) stpy in CH2Cl2, excited at
351.1 nm, (b) [Re(Cl)(CO)3(stpy)2] in CH2Cl2, excited at 457.9 nm,
and (c) [Re(stpy)(CO)3(bpy)]+ in acetonitrile, excited at 457.9 nm.
Solvent bands have been subtracted.
equipment at the Rutherford Appleton Laboratory,36 using ∼200
fs, 400 nm pulses and a white-light continuum generated with
800 nm fundamental in H2O flowing through a quartz cell.
Time-resolved IR (TRIR) and Kerr-gate resonance Raman
(TR3) experiments used the equipment and procedures described
in detail previously.36-42 In short, the sample solution was
excited (pumped) at 400 nm using frequency-doubled pulses
from a Ti:sapphire laser of ∼150 fs duration (fwhm) for TRIR,
while pulses of 1-2 ps duration were used for Raman studies.
TRIR spectra were probed with IR (∼150 fs) pulses obtained
by difference-frequency generation. The IR probe pulses cover
a spectral range ca. 200 cm-1 wide and are tunable from 1000
to 5000 cm-1 (i.e., 2-10 µm). Kerr-gate TR3 spectra were
probed at 475 nm using an OPA. Kerr gate was employed to
remove all long-lived emission from the Raman signal. TR3
spectra were corrected for the Raman signal due to the solvent
and the solute ground state by subtracting the spectra obtained
at negative time delays (-50, -20 ps) and subtracting any weak
residual fluorescence emission that passed through the Kerr gate.
The sample solutions for picosecond TR3 and TRIR experiments
were flowed as a 0.5 mm open jet and through a 0.5 mm CaF2
raster-scanned cell, respectively.
These spectral assignments are confirmed by (pre)resonance
Raman spectra shown in Figure 3. The spectrum of free stpy,
excited at 351.1 nm, shows strongly enhanced bands at 1593
(s) and 1635(vs) cm-1 and weaker ones at 1311, 1191, and 992
cm-1. The similarity with the spectrum of stilbene allows us to
assign the 1635 cm-1 band to stretching vibration of the
ethylenic CdC bond, coupled to the ethylenic C-H symmetric
in-plane bend, and the 1593 cm-1 band to the ring CC stretches,
with minor contributions from the ethylenic CdC and C-Ph/
py stretches.45 The preresonance Raman spectrum of Re(stpy)2
is similar to that of the free ligand. Three bands at 1600(s),
1612(vs), and 1632(m) cm-1 are seen. On the basis of their
similarity to the stpy 1593-1635 cm-1 doublet, they are thought
to originate in vibrations with a significant ν(CdC) character.
The weakness of the ν(CO) A′(1) band at 2026 cm-1 effectively
rules out any significant contribution of a Re f stpy MLCT
transition to the resonant enhancement. (Preresonance with
MLCT transitions is known46,47 to strongly enhance Raman
bands due to the in-phase symmetric CO vibrations, as seen,
for example, in the rR spectrum44 of [Re(Cl)(CO)3(4-benzoylpy-
ridine)2].) It can be concluded that the predominant resonant
electronic transition, which is also responsible for the 320 nm
absorption band, has a ππ* IL character, very similar to that of
free stpy.
The spectral bands observed in TRIR spectra were fitted by
Lorentzian functions to determine accurately their center posi-
tions, widths, and areas. All spectral fitting procedures were
performed using Microcal Origin 7 software.
The rR spectrum of Re(stpy)(bpy) is very similar to that of
Re(stpy)2. The three vibrations with ν(CdC) character are again
seen at 1601(sh), 1615(vs), and 1635(m) cm-1 together with a
band at 1226(w) cm-1. The A′(1) ν(CO) metal-carbonyl mode
is observed at 2036 cm-1, though its resonant enhancement is
minute compared to other bands, indicating that the Raman
intensity enhancement originates predominantly in the stpy-
localized IL ππ* transition. However, the presence of a weak
band at 1494 cm-1, which corresponds to a bpy-localized
vibration,48,49 indicates a small contribution from a Re f bpy
MLCT transition.
Ultrafast Excited-State Dynamics. Transient Visible Ab-
sorption Spectroscopy (TA). The TA spectra of free stpy show
a broad intense band at ca. 515 nm, Figure 4a. Its decay is
accompanied by a small shift of the band maximum to shorter
wavelengths caused by vibrational cooling50 and appearance of
a shoulder at ca. 450 nm. The decay kinetics are biexponential
with time constants of 1.7 ( 0.1 (84%) and 8.6 ( 0.4 ps (16%)
at 510 nm (Figure 5a) and 3.1 ( 0.5 (46%) and 14.4 ( 10.0 ps
Results
UV-Vis Absorption and Resonance Raman Spectroscopy.
The UV-vis spectrum of the free stpy ligand (Figure 2) shows
a strong absorption at 295 nm that was assigned43 to almost
1
1
degenerate ππ* and nπ* transitions. For Re(stpy)2 there is a
strong absorption band at 320 nm (ꢀ ) 53 100 M-1 cm-1) which
resembles that of protonated stpyH+.19 This band has been
assigned, through low-temperature studies, to an intraligand (IL)
ππ* transition.19 Re(stpy)(bpy) exhibits a similar absorption
spectrum with a broad band centered at ∼332 nm. Its molar
absorptivity (∼31 650 M-1 cm-1) is by ∼5100 M-1 cm-1 higher
than one-half of that of the IL band of Re(stpy)2. This indicates
that the band arises from a stpy-localized IL transition with
contributions from MLCT and bpy-localized IL transitions, the
latter being manifested by the spike at 320 nm. A weak tail
extending to about 420 nm is attributed to a Re f bpy MLCT
transition, based on comparison with spectra of [Re(L)(CO)3-
(bpy)]+ complexes, where L is a pyridine-type ligand.34,44

Ultrafast Excited-State Dynamics
J. Phys. Chem. A, Vol. 109, No. 13, 2005 3003
The TA spectrum of Re(stpy)2 (Figure 4b) is distinctly
different from that of free stpy. An intense, broad transient band
appears within the instrument time resolution (<500 fs) at
wavelengths shorter than ca. 550 nm. The maximum at ca.470
nm is only apparent, resulting from overlapping negative
absorption due to a bleached ground state at shorter wavelengths.
This band rapidly decays in intensity, narrows, and blue shifts
during the first few picoseconds. Further decay occurs between
2 and 40 ps, leaving a broad, weak featureless signal that extends
from 430 to 700 nm and does not change during the 1 ns time
interval investigated. The decay kinetics determined at 470 nm
is biexponential: 0.9 ( 0.1 (64%) and 11.4 ( 1.3 ps (27%)
with a time-independent term (9%) that reflects the long-lived
(.1 ns) signal, Figure 5b. Similar biexponential decay kinetics
are seen in the region of a weak absorbance between 620 and
700 nm. The large relative weight of the 0.9 ps kinetic
component indicates that it corresponds to excited-state decay
1
of the optically prepared ILt excited state, as opposed to its
vibrational cooling. (The subscript “t” denotes the original trans
geometry of the stpy CdC bond.) The resulting spectrum is
3
assigned to a triplet ILt state of a ππ* origin because of its
similarity with the excited-state spectra of t-stilbene,17,18 whose
3ππ* state absorbs only in the near UV region, while the
corresponding singlet state shows a strong band in the visible
for both stilbene17,18 and free stpy (Figure 4a). The 0.9 ps step
is thus assigned as a 1ILt f 3ILt intersystem crossing. The 3ILt
state then undergoes a further 11.4 ps process which yields a
long-lived, weakly absorbing species. TA spectra provide no
information on its character. Finally, it should be noted that
the TA spectra show no evidence of a Re f stpy MLCT state,
which would be expected to show a sharp strong absorbance at
ca. 490 nm and a weaker absorbance at ca. 690 nm, based on
the UV-vis spectrum of the stilbene radical anion.51
Figure 4. Difference transient absorption spectra of (a) stpy in
acetonitrile, measured after 276 nm excitation (spectra shown at 2
(black), 3 (red), 4 (green), 6 (dark blue), 10 (light blue), and 50 (pink)
ps), (b) [Re(Cl)(CO)3(stpy)2] in CH2Cl2, measured after 400 nm
excitation (spectra shown at 0 (black), 1 (red), 2 (green), 4 (dark blue),
6 (light blue), 12 (red), 20 (yellow), and 500 (brown) ps), and (c) [Re-
(stpy)(CO)3(bpy)]+ in acetonitrile, measured after 400 nm excitation
(spectra shown at 2 (black), 3 (red), 6 (green), 16 (dark blue), 20 (light
blue), 30 (pink), and 500 (yellow) ps). Chirp distorts early spectral
timing between 0 and 2 ps.
The TA spectrum of Re(stpy)(bpy) is shown in Figure 4c. A
weak transient with an apparent maximum at 460-470 nm
grows with biexponential (0.23 ( 0.02 and 3.5 ( 0.3 ps) kinetics
(Figure 5c), reaching its maximum intensity by ∼6 ps. Its shape
and intensity closely resemble that of Re(stpy)2 at time delays
3
g2 ps and is thus attributed to the same ILt state. The band
subsequently decays with a 11.7 ( 0.8 ps lifetime into a weak
broad signal that covers most of the visible spectral region and
shows no further temporal evolution between 100 and 1000 ps.
Time-Resolved Resonance Raman Spectroscopy, TR3. TR3
spectra of free stpy (Figure 6a) were obtained after 300 nm
pumping using a 475 nm probe that is directed into the blue
side of the excited-state absorption band (Figure 4a). Lorentzian
fitting of the weak signal obtained at a 5 ps delay has identified
TR3 peaks at 1576 ( 2 (s), 1280 ( 4 (m), 1164 ( 2 (s), 986 (
5 (w), and 846 ( 4 (w) cm-1. On the basis of the comparison
with TR3 spectra of the 1ππ* and 3ππ* states of t-stilbene,18,52
these bands are tentatively assigned to ν(CC), in-plane δ(CH),
ring δ(CC) + in-plane δ(CringH), in-plane δ(CringH), and
ν(CC)+δ(CCC) modes, respectively. The Raman bands decay
with a lifetime (ca. 4 ps for the 1576 cm-1 band) that
approximately corresponds to that of the TA band. Concomi-
tantly, they undergo a small dynamic shift to higher wavenum-
bers, caused by vibrational cooling.52,53
Figure 5. Kinetic absorbance profiles and fits of transient absorption
of (a) stpy at 510 nm in acetonitrile, excited at 276 nm, (b) [Re(Cl)-
(CO)3(stpy)2] at 470 nm in CH2Cl2, excited at 400 nm, and (c) [Re-
(stpy)(CO)3(bpy)]+ at 470 nm in acetonitrile, excited at 400 nm.
Absorbance values were measured as a 10 pixel average around the
probe wavelength. The corresponding TA spectra are shown in Figure
4.
For Re(stpy)2 TR3 spectra (Figure 6b) were measured in CH2-
Cl2 at 400 nm with the probe beam directed into the maximum
of the TA band at 475 nm. Very weak broad bands were
acquired that have been fitted to Lorentzians at 1521 ( 1.1 (s),
1251 ( 4.0 (m), and 1022 ( 5.2 (w) cm-1, measured in the 20
ps spectrum. As discussed for the TA spectra, the resonant
(53%) at 462 nm. The band at 515 nm resembles the TA band
1
seen for t-stilbene at 585 nm16,50 and is attributed to a ππ*
1
state. The 450 nm shoulder is tentatively assigned to a nπ*
state. The short decay component is similar to the fluorescence
lifetime of 2.6 ps measured in n-hexane.43
transition in the TR3 spectra originates in an ILt state. This is
3

3004 J. Phys. Chem. A, Vol. 109, No. 13, 2005
Busby et al.
Figure 7. Difference time-resolved infrared spectra of (a) [Re(Cl)-
(CO)3(stpy)2] in CH2Cl2 (spectra shown at 2 (black), 6 (red), 10 (green),
20 (dark blue), 50 (light blue), 100 (pink), and 1000 (yellow) ps) and
(b) [Re(stpy)(CO)3(bpy)]+ in CH2Cl2 (spectra shown at 1 (black), 1.5
(red), 2 (green), 3 (dark blue), 6 (light blue), 10 (pink), 20 (yellow),
30 (brown), and 1000 (gray) ps). Both complexes are measured after
400 nm excitation. Experimental points are separated by 4-5 cm-1
.
Figure 6. Kerr-gate time-resolved resonance Raman spectra of (a) stpy
in CH2Cl2, probed with 475 nm laser pulses at 1, 3, 5, 20, and 500 ps
after a 300 nm excitation, (b) [Re(Cl)(CO)3(stpy)2] in CH2Cl2, probed
with 475 nm laser pulses, at 0, 2, 6, 10, and 50 ps after 400 nm
excitation, and (c) [Re(stpy)(CO)3(bpy)]+ in acetonitrile, probed with
475 nm laser pulses at 0, 2, 5, 8, 30, and 100 ps after 400 nm excitation.
Spectra evolve from bottom to top.
now confirmed by the presence of Raman bands that resemble
those of the ππ* state of free stpy. Fitting the time evolution
of the area of the 1521 cm-1 band from 2 ps onward reproduces
the ∼11 ps decay measured in the TA.
Figure 8. Difference time-resolved infrared spectra of [Re(Cl)(CO)3-
(stpy)2] in CH2Cl2, measured after 400 nm excitation (spectra shown
at 3 (black), 4 (red), 6 (green), 10 (dark blue), 20 (light blue), 30 (pink),
50 (yellow), 100 (brown), and 1000 (gray) ps). Experimental points
Re(stpy)(bpy) was excited at 400 nm and its TR3 spectra
(Figure 6c) probed at 475 nm. As in the TA spectra, the TR3
peaks exhibit a fast rise with a time constant of 3.2 ( 0.9 ps.
The TR3 bands are broad with Lorentzian-fitted maxima at 1535
( 1.3 (s), 1343 ( 13.4 (m), 1244 ( 3.5 (m), and 1015 ( 3.6
(m) cm-1, measured at 30 ps. The bands are at comparable
positions to those seen for Re(stpy)2 and are hence assigned to
a stpy-localized 3ILt state. It is obvious that this state is formed
from another precursor state that has neither strong absorption
in the visible region nor strongly resonance-enhanced Raman
bands. A Re f bpy MLCT is a likely candidate for the precursor
are separated by ca. 2 cm-1
.
band at 2012 cm-1 and a weak, broad feature at ∼1864 cm-1
,
see Figures 7a and 8a. This IR spectral pattern is characteristic54-57
of a ππ* IL state, as seen, for example, for [Re(4-Etpy)(CO)3-
(dppz)]+ and similar complexes. TRIR spectra thus confirm the
3
stpy-localized ILt character of the initially populated excited
state of Re(stpy)2, in full accordance with the TA and TR3
evidence discussed above. The 2012 cm-1 band narrows and
decreases in intensity between 2 and 40 ps, while another sharp
band grows in at 2019 cm-1. Concomitantly, the 1864 cm-1
feature shifts to higher wavenumbers, into the region of bleached
ground-state absorption, giving rise to two apparent maxima at
3
state. The IL TR3 bands decay completely with a lifetime
estimated as 20 ( 9 ps.
Time-Resolved Infrared Spectroscopy (TRIR). TRIR spec-
tra were investigated in the spectral regions of ν(CO) and stpy-
localized vibrations following excitation at 400 nm. Spectra
measured at the earliest time delays are complicated by
coherence effects which usually subside at 1-3 ps. The TRIR
spectra in the ν(CO) region of both complexes are shown in
Figure 7, while Figures 8 and 9 provide detailed views of the
high-frequency region.
1926 and 1894 cm-1. An isosbestic point is seen at ∼2017 cm-1
,
indicating that the spectral change is caused by a clean
conversion between two states. The new spectral pattern is again
3
characteristic of an IL state. As will be discussed below, we
propose this state to have perpendicular orientation of the
3
The spectrum of Re(stpy)2 measured 2 ps after 400 nm laser-
pulse excitation shows bleached ground-state ν(CO) bands at
2024, 1917, and 1884 cm-1 and a transient consisting of a sharp
pyridine and phenyl rings. Hereafter it will be known as ILp.
Once formed the 2019 cm-1 band stays constant from ca. 70
ps until the end of the time interval investigated, i.e., 1000 ps.

Ultrafast Excited-State Dynamics
J. Phys. Chem. A, Vol. 109, No. 13, 2005 3005
slight narrowing and upward shift to 2074 cm-1 with a time
constant estimated as 4 ps. Such a behavior is typical for
vibrational cooling.61,62 In acetonitrile the A′(1) bleach and
excited-state bands are seen at 2034 and 2076 cm-1, respec-
tively. Vibrational cooling shifts the latter band to 2085 cm-1
with a 3-4 ps time constant. The larger overall shift of the
A′(1) band upon excitation in acetonitrile (+51 cm-1) than in
CH2Cl2 (+40 cm-1) is caused by the higher polarity of the
former solvent, which favors a larger shift of electron density
from Re(CO)3 to bpy. This solvent effect further supports the
assignment of the early TRIR spectral pattern seen for Re(bpy)-
(stpy) to a 3MLCT(bpy) state. Vibrational cooling severely
affects the decay dynamics of the A′(1) MLCT band. This is
manifested by the increase of the decay constant from ca. 2.4
to 8.9 ps with increasing the probe wavenumber across the A′(1)
MLCT band. The band area, determined by Lorentzian fitting,
decays with a time constant of 6.6 ( 1.7 ps in CH2Cl2. Similar
decay dynamics were estimated in the region of the 1963 cm-1
band. Considering interfering effects of vibrational cooling, the
Figure 9. Difference time-resolved infrared spectra of [Re(stpy)(CO)3-
(bpy)]+ in acetonitrile, measured after excitation at 400 nm (spectra
shown at 3 (black), 6 (red), 8 (green), 10 (dark blue), 15 (light blue),
20 (pink), 30 (yellow), 50 (brown), and 1000 (gray) ps). (Inset) MLCT-
(bpy) decay at 3 (black), 4 (red), 6 (green), 8 (dark blue), 10 (light
blue), 15 (pink), 20 (yellow), 30 (brown), and 50 (gray) ps. Experi-
3
decay of the MLCT TRIR pattern can be identified with the
3.5 ps rise kinetics of the TA and TR3 signals.
3
Decay of the MLCT bands is accompanied by growth of a
sharp transient band at 2027 cm-1 and a broad absorption
between 1910 and 1960 cm-1. The latter strongly overlaps with
the A′(2) + A′′ negative bleach bands, nearly canceling
absorption in this spectral region. This secondary transient IR
spectrum is characteristic of an IL excited state. It closely
resembles the initial IR spectral pattern seen for Re(stpy)2.
mental points are separated by ca. 2 cm-1
.
Therefore, it is identified with the long-lived, weakly absorbing
transient observed in TA. Quantitative analysis of the evolution
of bleached ground-state bands is precluded by a strong overlap
3
with the transient features. This overlap is higher for the ILp,
3
Hence, it is attributed to the same stpy-localized ILt excited
as manifested by a decrease of the bleach during conversion.
The area of the 2012 cm-1 band, obtained by Lorentzian
fitting, decays with a time constant of 11.8 ( 0.4 ps. The
absorbance at 2019 cm-1 grows with a comparable rise time,
12 ( 3 ps. The decay constant of the 2012 cm-1 band is
identical, within the instrumental precision, to the 11.4 ps
component of the TA decay.
The assignment of the primary and secondary IR transients
as 3ILt and 3ILp states with different orientations of the pyridine
and phenyl rings of the stpy ligand implies that their ∼12 ps
conversion occurs as a ∼90° rotation around the CdC bond.
To confirm this assumption we studied this step in a PMMA
film, where the rotation should be hindered by the medium
rigidity. Indeed, the TRIR spectra of Re(stpy)2 in PMMA show
only the bleach at 2023 cm-1 and a broad 3Ilt band at ca. 2012
cm-1, which narrows between 1 and 40 ps due to cooling. It
neither decays in intensity nor converts into any other band
during the 1-2000 ps interval investigated.
state. Rise times of 4.5 ( 0.7 ps in CH2Cl2 and 4.1 ( 0.2 ps in
acetonitrile were estimated for the 2027 cm-1 band. This kinetics
can be identified with the 3.5 ps rise of the TA and TR3 signals.
The 2027 cm-1 band is most prominent at 10-15 ps. Then it
decays, being replaced by another band at ca.2031 cm-1 (Figures
7b and 9). This conversion is fully completed between 50 and
100 ps. The 2031 cm-1 band then remains constant beyond 1
ns. Because of a close similarity with the Re(stpy)2 system
3
discussed above, it is attributed to a stpy-localized ILp state
with a perpendicular geometry.
TRIR spectra in the region of stpy ν(CdC) vibrations exhibit
a bleach at 1616 cm-1 which becomes fully developed at about
3
15-20 ps, supporting the delayed formation of an IL state.
No well-developed transient IR feature was observed. Weakness
of the signal and overlapping broad background absorption
prevented any quantitative analysis.
Discussion
TRIR spectra in the region of stpy ν(CdC) vibrations, 1580-
1640 cm-1, show a bleached ground-state band at 1612 cm-1
which becomes fully developed at 4-6 ps when the initial fast
relaxation is completed. Overlapping broad background absorp-
tion and overall signal weakness prevents further analysis. No
distinct transient IR feature was observed. Persistent bleaching
of an IR band corresponding to a stpy-localized vibration
supports the assignment of both transient features seen in the
ν(CO) region to IL states.
The ν(CO) TRIR spectrum of Re(stpy)(bpy) (Figures 7b and
9) measured at 1 ps in CH2Cl2 shows the ground-state bleach
bands at 1932 (A′(2) + A′′) and 2034 cm-1 (A′(1)) and three
transient bands at higher wavenumbers: 1963, 1995, and 2061
cm-1. Such a spectral pattern is characteristic of a Re f bpy
3MLCT excited state, as observed for [Re(4-Et-pyridine)(CO)3-
(bpy)]+ and other complexes.44,54,55,57-61 These spectral features
essentially disappear within the first 6-10 ps. Concurrent with
this fast decay the MLCT A′(1) 2061 cm-1 band undergoes a
1
UV excitation of the free stpy ligand populates a ππ* and,
probably, also 1nπ* excited states43 which decay with lifetimes
of ca. 3 and 10 ps. Photoisomerization to the cis isomer occurs
with a quantum yield of 0.37,22 slightly smaller than that of
t-stilbene (0.48).63 The ππ* lifetime of stpy is much shorter
1
than that of t-stilbene (90 ( 5 ps) due to mixing between the
1ππ* and 1nπ* characters.16 With respect to the following
discussion, it is important to note that the trans-cis isomeriza-
tion under direct irradiation of both stilbene and stpy occurs on
singlet potential-energy surfaces. The triplet mechanism is
possible if triplet sensitizers, such as benzophenone, are used.
Quantum yields of 0.50 and 0.40 were reported for sensitized
isomerization of t-stilbene and t-stpy, respectively.22,63
Information on the nature of the lowest allowed electronic
transitions and, hence, the optically populated Franck-Condon
states of Re(stpy)2 and Re(stpy)(bpy) is provided by UV-vis
absorption and resonance Raman spectra. The structured low-

3006 J. Phys. Chem. A, Vol. 109, No. 13, 2005
Busby et al.
3
3
SCHEME 1: Proposed Excited-State Behavior of
[Re(Cl)(CO)3(stpy)2]a
state ILt with the same trans geometry. The ILt excited state
of Re(stpy)2 is characterized by near-UV absorption below ∼500
nm, resonantly enhanced Raman bands due to stpy-localized
vibrations, and the characteristic54-57 ν(CO) TRIR pattern. The
excited-state ν(CO) IR bands are shifted to lower wavenumbers
relative to the ground state (ca. -12 cm-1 for the highest A′(1)
band) due to donation of electron density from the π* stpy
orbital, occupied in the ILt state, to dπ Re orbitals.54
3
The 3ILt state undergoes in a fluid solution an 11 ps
conversion to another long-lived (.1 ns) excited state, which
is characterized by a much weaker absorption in the visible
region, lack of resonantly enhanced Raman signal, and ν(CO)
3
pattern that again points to its IL character. The downshift of
the A′(1) band from the ground-state position is relatively small
(ca. -7 cm-1), and the A′(2) and A′′ bands are even shifted
slightly upward, by e+8 cm-1. We assign this species as an
3ILp excited state of the stpy ligand in which the pyridine and
phenyl rings are oriented approximately perpendicularly to each
other. The loss of π-delocalization in the perpendicular con-
figuration accounts for the observed near-disappearance of
absorption in the visible region and for the ν(CO) wavenumbers
a Excited-state localization is shown in brackets
SCHEME 2: Proposed Excited-State Behavior of
[Re(stpy)(CO)3(bpy)]+ a
3
being slightly higher than those for the ILt state. The latter
effect is caused by weakening of the π donation to the Re-
3
(CO)3 unit. The ILp excited state is the immediate precursor
of the trans-cis isomerization, which occurs for Re(stpy)2 with
3
a quantum yield of 0.49.19 It is assumed that ILp undergoes a
nanosecond ISC to the ground state in a perpendicular geometry,
from which either the trans starting configuration is reformed
or isomerization to the cis isomer is completed by further ∼90°
rotation.
Compared to free t-stilbene and stpy, it follows that coordina-
tion to a Re atom in Re(stpy)2 switches the isomerization
mechanism from singlet to triplet ππ* potential-energy surfaces.
This can be viewed as intramolecular triplet sensitization. The
a Excited-state localization is shown in brackets
temperature UV absorption,19 enhancement of Raman bands due
to stpy vibrations, and absence of the intensity enhancement of
the in-phase ν(CO) A′(1) Raman band show that the near-UV
absorption spectrum of Re(stpy)2 is dominated by stpy-localized
ππ* IL transitions. The Re f stpy MLCT transitions, if present,
lie at higher energies. The situation is very similar for Re(bpy)-
(stpy), whose near-UV spectrum is also dominated by a stpy-
localized ππ* IL transition. However, observation of a weakly
enhanced Raman band due to a bpy-localized vibration at 1494
cm-1 indicates the presence of a Re f bpy transition, which
gives rise to an absorption tail between 390 and 420 nm. It
follows that the 400 nm laser-pulse irradiation, used for the
3
1ILt f ILt ISC is a subpicosecond process, facilitated by the
heavy-atom effect of the rhenium atom. The following ∼90°
3
rotation around the stpy CdC bond in the ILt state, to form
the perpendicular intermediate 3ILp, is herein observed directly
for the first time, and its time constant is determined as 11 ps
in CH2Cl2. It does not occur in a rigid PMMA film. The lifetime
3
of the perpendicular ILp excited state is in the nanosecond
3
range, similar to the ππ* state of t-stilbene.17,21 Apparently
3
the Re heavy-atom effect does not affect the ILp f GS ISC
much, whose rate might be limited by geometrical constraints
and shapes of the potential-energy surfaces.
1
ultrafast studies, initially populates the ILt ππ* state of Re-
Photochemistry of Re(stpy)(bpy) starts with population of a
1
3
(stpy)2 and the MLCT(bpy) state of Re(stpy)(bpy).
Re f bpy MLCT excited state by ISC from the optically
The proposed mechanisms of the excited-state behavior of
the complexes Re(stpy)2 and Re(stpy)(bpy) are shown in
Schemes 1 and 2, respectively.
In the case of Re(stpy)2 the optically prepared ILt state
undergoes an ultrafast (e 0.9 ps) ISC to the corresponding triplet
excited 1MLCT state, which occurs with a time constant of ca.
3
0.2 ps. The MLCT state is characterized by a typical TRIR
spectral pattern with upshifted ν(CO) bands.44,54,55,57-61 The
3MLCT state is formed vibrationally hot, as documented by the
broadness of the ν(CO) IR bands and their small dynamic shift
1
3
3
SCHEME 3: Schematic Orbital View of the MLCT f ILt Conversion in [Re(stpy)(CO)3(bpy)]+

Ultrafast Excited-State Dynamics
J. Phys. Chem. A, Vol. 109, No. 13, 2005 3007
to higher wavenumbers.61,62 The 3MLCT state decays into a 3ILt
(stpy)2] and fac-[Re(stpy)(CO)3(bpy)]+ switches the trans-cis
isomerization mechanism from a spin singlet to a triplet ππ*
excited-state potential-energy surface. The ReI moiety acts as
an intramolecular triplet sensitizer, facilitating the 1ππ* f 3ππ*
intersystem crossing to the femtosecond range.
state with a time constant of ∼3.5 ps. UV-vis absorption,
3
resonance Raman, and IR spectra of the ILt excited state of
Re(stpy)(bpy) are very similar to those discussed above for Re-
3
(stpy)2. All the ILt spectral features show rise times com-
3
An indirect population of the reactive intraligand 3ππ* excited
state by a picosecond internal conversion from a Re f bpy
3MLCT excited state occurs in the case of fac-[Re(stpy)(CO)3-
(bpy)]+. This process amounts to an intramolecular energy
transfer from a ReI(CO)3(bpy) chromophore to a stpy ligand.
The trans-cis isomerization of the stpy ligand is preceded
by a ∼90° rotation around the CdC bond, which occurs in an
mensurate with the decay time of the MLCT state. It should
be noted that the 3MLCT f 3ILt conversion involves a
3
vibrationally hot MLCT state since rates of conversion and
vibrational cooling are comparable. The following 3ILt f 3ILp
∼90° rotation around the stpy CdC bond occurs with a time
constant of 12 ps. The spectroscopic characteristics of the 3ILp
state are very similar to those of Re(stpy)2. The 3ILp lifetime is
in the nanosecond region, beyond the range of our experiment.
This can be compared to the 28 ns decay that was determined
for a 3IL state of a similar complex [Re(bpe)(CO)3(phen)]+, for
which a perpendicular bpe geometry was also postulated.64 The
3ILp state is assumed to decay to the ground state while
preserving the perpendicular geometry. Further backward or
forward 90° rotation will form the trans or cis isomer,
respectively. The isomerization quantum yield of Re(stpy)(bpy)
was reported to be very low.25 (A value of 0.21 was determined26
for [Re(bpe)(CO)3(bpy)]+.) It can be limited either by inefficient
3ILt population from the 3MLCT state or by asymmetric
evolution on the ground-state surface. On the basis of compari-
3
intraligand ππ excited state with a 11-12 ps time constant.
An intraligand 3ππ excited state with a perpendicular orientation
of the pyridine and phenyl rings is formed. The following
intersystem crossing to the ground state, which leads to
isomerization, is much slower, taking place in the nanosecond
range. Intraligand rotation and, hence, isomerization is hindered
by a rigid medium, as observed in a PMMA film.
Re f stpy MLCT excited states are not involved in the
isomerization mechanism.
Acknowledgment. This research was funded by EPSRC,
CCLRC, and COST Action D14. Dr. Ian P. Clark (Nanosecond
Laboratory, CCLRC Rutherford Appleton Laboratory) and Mr.
Michiel Groeneveld (Institute of Molecular Chemistry, Univer-
sity of Amsterdam) are thanked for their help in measuring
stationary resonance Raman and time-resolved UV-vis spectra,
respectively. Ms. Ana Maria Blanco Rodriguez is thanked for
making and measuring the PMMA films. Fruitful and stimulat-
ing discussions with Dr. A. W. Parker (CCLRC Rutherford
Appleton Laboratory) are gratefully appreciated.
3
son with analogous complexes, the MLCT state of Re(stpy)-
(bpy) would be expected34 to have a ca. 230 ns lifetime were
3
3
the conversion to the ILt state absent. The 3.5 ps MLCT
lifetime found for Re(stpy)(bpy) essentially implies that the
3MLCT f 3ILt conversion is complete.65 Therefore, the isomer-
ization quantum yield cannot be limited by a competitive
3
relaxation of the MLCT state to the ground state. Instead, it
appears to be diminished by steric constraints in the low-
symmetry Re(stpy)(bpy) molecule, which could make the
ground-state potential-energy surface asymmetric with respect
to the forward and backward rotation around the stpy CdC
bond.
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