Reversible trans ? cis photoisomerizations of [Re(CO)3(ph2phen)(stpyCN)]+ towards molecular machines

Article abstract of DOI:10.1039/c7dt01648d

In our search for light powered molecular devices, a novel fac-[Re(CO)₃(ph₂phen)(trans-stpyCN)]⁺ complex was synthesized to show switchable trans-cis configurations of the coordinated stpyCN ligand through efficient and re

Full text of DOI:10.1039/c7dt01648d

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

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REVERSIBLE TRANS ⇌ CIS PHOTOISOMERIZATIONS OF
[Re(CO)3(ph2phen)(stpyCN)]+ TOWARDS MOLECULAR MACHINES
Kassio P.S. Zanoni and Neyde Y. Murakami Iha*
Laboratory of Photochemistry and Energy Conversion, Departamento de Química Fundamental,
Instituto de Química, Universidade de São Paulo – USP, Av. Prof. Lineu Prestes, 748, 05508-900,
São Paulo, SP, Brazil; E-mail: neydeiha@iq.usp.br
ABSTRACT
In our search for light powered molecular devices, the novel fac-[Re(CO)3(ph2phen)(trans-
stpyCN)]+ complex was synthesized to show switchable trans-cis configurations of the
coordinated stpyCN ligand through efficient and reversible photoassisted isomerizations.
Controlled photolyses of acetonitrile solutions led to spectral changes ascribed to reversible trans
⇌ cis photoisomerization processes. A remarkable quantum yield for the back cis-to-trans
isomerization was obtained, with the same magnitude of the trans-to-cis photoprocess, and,
curiously, both trans- and cis-isomers are emissive at room temperature. Photochemical and
photophysical characterizations for fac-[Re(CO)3(ph2phen)(stpyCN)]+ provided insights on the
isomerization and emissive light-driven pathways that are resulted from the interaction between
close-lying intraligand and charge transfer states. The reversible cis-trans photoisomerization have
potential to be exploited as light-powered molecular motors and geometry regulators in molecular
machines.
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TABLE OF CONTENTS ENTRY
TABLE OF CONTENTS SYNOPSIS
The fac-[Re(CO)3(ph2phen)(stpyCN)]+ complex is capable of switching through extended
trans- and contracted cis-configurations upon irradiation under proper energies.
INTRODUCTION
The understanding of molecular-level devices can lead to potential applications in light-
controlled devices, such as optical data storage systems, molecular photoswitches, ion transport
controllers in membranes, as well as geometry regulators for organic materials, biomolecules and
supramolecular complexes.1–11 Thus, the search for photoresponsive materials capable of changing
their properties under light absorption – in special, switching through extended trans and
contracted cis configurations – has attracted considerable efforts, with numerous reports on the
photobehavior of azobenzene, stilbene and their derivatives.1–8,12–21
The complexation of these organic substrates to rhenium(I) tricarbonyl polypyridyl (NN)
compounds ([Re(CO)3(NN)(trans-L)]+, trans-L = trans-stilbene-like ligand) triggers new metal-
to-ligand charge transfer (1MLCT) pathways to the trans-to-cis photoisomerization in the
visible.1,4,14 In the cis-complex photoproducts, a low-lying 3MLCT state usually leads to a radiative
2

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deactivation and quenches the reverse cis-to-trans photoreaction.4,7,8,21,22 Therefore, these Re-L
systems are usually attractive candidates for luminescent sensors, although not properly useful as
on-off (cis-trans) switchable luminescent devices or light-driven geometry regulator machines.2,4
In our previous work,4 we demonstrated the trans ⇌ cis photoreversibility of the
coordinated 4-(4-cyano)styrylpyridine ligand (stpyCN) in the fac-[Re(CO)3(phen)(stpyCN)]+
complex; however, such a complex is non-emissive for both trans- and cis-isomers. Here, we
proceeded our investigations within the stpyCN-complex series by means of synthesis and
characterization of the fac-[Re(CO)3(ph2phen)(trans-stpyCN)]+ complex (ph2phen = 4,7-diphenyl-
1,10-phenanthroline). The photochemical and photophysical behaviors were investigated, and an
unusual and remarkable quantum yield for the cis-to-trans isomerization has been observed, of the
same magnitude to the trans-to-cis photoprocess.
EXPERIMENTAL PROCEDURES
All chemicals and solvents for syntheses were purchased from Sigma-Aldrich or Synth,
and used as supplied. HPLC grade solvents were employed for the photochemical and
photophysical measurements.
Precursor compounds fac-[Re(CO)3(ph2phen)(tfms)] and fac-[Re(CO)3(ph2phen)Cl] were
synthesized as previously described.4,23
Syntheses
The syntheses were based on a procedure previously described,4 with slight modifications.
trans-stpyCN. 4-cyanobenzaldehyde (7.0 g, 48 mmol) and 4-picoline (5.0 mL, 50 mmol) were
refluxed for 8 hours in acetic anhydride (5.0 mL, 50 mmol). The solution was distilled and the
residual oil was dropped in water to yield a light-yellow precipitate. The crude product was
recrystallized from ethanol by slow addition of water to obtain 5.2 g of pure product (24 mmol, 50
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1
% yield). The H NMR spectrum was recorded in a 500 MHz Bruker AIII, using deuterated
acetonitrile as solvent, with the residual solvent as an internal standard; /ppm: 8.57 (dd, 2H); 7.75
(m, 4H); 7.49 (dd, 2H); 7.45, (d, 1H); 7.31 (d, 1H). Elemental analysis data were obtained on a
Perkin-Elmer CHN 2400; calculated for C14H10N2.1/2H2O: C, 78.11%; N, 13.01%; H, 4.65%;
found: C, 78.41%; N, 12.56%; H, 5.06%.
trans-HstpyCN+. The protonation of the isomerizable ligand is an approximation of the
coordination effects and was obtained by addition of HCl to a trans-stpyCN saturated solution to
yield a pure yellow precipitate. CHN analyses calculated for C14H11N2Cl.1/5H2O: C, 54.17%; H,
3.65%; N, 9.03%; found: C, 54.17%; H, 3.12%; N, 9.07%.
fac-[Re(CO)3(ph2phen)(trans-stpyCN)][PF6]. The precursor fac-[Re(CO)3(ph2phen)(tfms)]
complex (0.80 g, 1.1 mmol) and the trans-stpyCN ligand (0.60 g, 3.3 mmol) were dissolved in 60
mL of methanol (Synth) and heated to reflux for 9 hours. After cooling to room temperature, solid
NH4PF6 was added in excess to the solution to precipitate a yellow complex. The solid was
collected by filtration and washed with water then with isopropanol and, finally, with diethyl ether.
The product was dried under vacuum (10 mmHg) to obtain 0.38 mg of pure product (0.40 mmol,
38% yield). CHN analyses calculated for ReC41H26N4O3F6P: C, 51.63%; H, 2.75%; N, 5.87%;
found: C, 51.80%; H, 2.86%; N, 5.87%. 
Photolyses
Photolyses at 255, 313, 334, 365 and 404 nm (corresponding to spectral lines of the Hg(Xe)
lamp) were carried out using an Oriel 500 W Hg(Xe) arc lamp powered by an Oriel universal
power supply model 68700. Light intensities were obtained by potassium tris(oxalate)ferrate(III)
actinometry before and after each photolysis experiment. Irradiations were performed in a special
arrangement consisted of a 1.000 cm optical length quartz cuvette for light irradiation connected
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to another 0.100 cm optical length quartz cuvette for probing absorption changes (Figure 1).
Solutions under photolyses were adjusted to absorb more than 99.99% (i.e. A(irr) > 4 OD in the
1.000 cm irradiation cuvette, therefore A(irr) > 0.4 OD in the 0.100 cm probing cuvette) of the
incident light and were kept under stirring while irradiated. Concentrations of trans- and cis-
1
isomers after photolyses were obtained by H NMR or by changes in absorption spectra, as
previously described.17 Photoisomerization quantum yields were obtained based on the average of
at least three independent experiments.
Figure 1. Picture of the special arrangement consisted of a 1.000 cm optical length quartz
cuvette for light irradiation connected to another 0.100 cm optical length quartz cuvette for
probing absorption changes.
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Photophysical measurements
UV-Vis absorption spectra were recorded in 0.15 s on an Agilent 8453 diode array
spectrophotometer using the same solvent as blank.
Steady-state emission spectra were recorded on a PC1 photon-counting spectrofluorometer
(ISS) with a photomultiplier based, photon-counting detector with a detector sensitivity correction.
Emission decays were recorded on a ChronosBH time-resolved fluorometer (ISS) using a diode
laser (λex = 378 nm, frequency = 20 kHz, ISS) as an excitation light source. The absorbances at the
excitation wavelength of samples were set between 0.1 and 0.2 in four-polished-face quartz
cuvettes with a 1.000 cm optical path length. Solutions were deoxygenated with argon for, at least,
10 minutes prior to measurements. Emission quantum yields were obtained as described previously
24,25 using the precursor fac-[Re(CO)3(ph2phen)Cl] complex (em = 0.031, exc = 365 nm) 26 in the
same solvent as an emission standard. For experiments at 77 K, samples were prepared in a mixture
of ethyl ether, isopentane and ethanol (5/5/2, v/v) (EPA) in quartz tubes and inserted into a quartz
Dewar flask containing liquid N2.
Transient absorption (TA) and time-resolved emission measurements were performed by
using 355 nm nanosecond laser pulses (1 Hz; 11 mJ/pulse) produced by a Spectra-Physics Quanta-
Ray Lab-170 Nd:YAG laser integrated into an Edinburgh LP920 laser flash photolysis
spectrometer system. The probe pulse was generated by a pulsed 450 W Xe lamp and detected by
a photomultiplier tube (Hamamatsu R928). Appropriate band-pass and/or long-pass filters were
placed before the detector to reject unwanted scattered laser light. Single wavelength kinetic data
were averaged over 15 to 30 laser shots.
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Emission and TA experiments of pure trans-solutions were conducted using a quartz flow
cuvette (0.5 mL min-1) to ensure that the observed emission was not from residual cis-
photoproducts formed during measurements.
Data treatment
Apparent photoisomerization quantum yields (Φ푡푎→푝푝푐 ) were obtained by directly using
absorption changes after photolyses, Equation 1; they are apparent because both trans- and cis-
isomers absorb in the same region.17
(
)
퐴푡푟푎푛푠 − 퐴푖푟푟 . 푁퐴 . 푉
푖푟푟
1
푎푝푝
푡→푐
( )
1 ,
Φ
=
×
퐼0 . 푡푖푟푟
휀푡푟푎푛푠 . 푏
where Atrans is the absorbance of the trans-solution before irradiation, Airr is the absorbance of the
solution after irradiation, tirr is the irradiation time (s), I0 is the light intensity (quanta s-1), b is the
optical path length of the irradiated cuvete (cm), Virr is the volume of the irradiated solution, NA is
the Avogadro number and trans() is the molar absorptivity of the trans-isomer (L mol-1 cm-1). For
푎푝푝
푡→푐
the calculation of Φ
, absorption changes during photolyses (less than 10% apparent
conversion) were monitored at 320, 330 and 340 nm.
On the other hand, distinct chemical shifts and coupling constants are observed in the 1H
NMR spectra of the two isomers, thus signal integrals can be used to calculate their true fractions
after photolyses. Based on these concentration ratios, the molar absorptivities of the fac-
7,17
[Re(CO)3(ph2phen)(cis-stpyCN)]+ complex was calculated by use of Equation 2
even not
having synthesized nor isolated the cis-complex.
NMR
(
( )
( )
)
퐴푖푟푟 휆 − 퐴푡푟푎푛푠 휆 . %푡푟푎푛푠
( )
( )
( )
2 ,
휀푐푖푠 휆 = 휀푡푟푎푛푠 휆 ×
NMR
( )
퐴푡푟푎푛푠 휆 . %푐푖푠
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and %NMR are percentages of isomers in the irradiated solution obtained by the
푐푖푠
NMR
푡푟푎푛푠
where %
distinct integrals in the 1H NMR spectrum.
Therefore, true quantum yields (Φ푡푡→푟푢푐푒) can be obtained directly by using the integrals of
the distinct 1H NMR signals, Equation 3a, or indirectly by just correcting Φ푡푎→푝푝푐 to the cis influence,
Equation 3b, with excellent agreement between both methods.17
1
퐴푡푟푎푛푠. 푁퐴 . 푉
푖푟푟
NMR
푐푖푠
푡푟푢푒
( )
3푎 ,
Φ
=
=
×
×
×%
푡→푐
퐼0 . 푡푖푟푟
휀푡푟푎푛푠 . 푏
(
)
퐴푖푟푟 − 퐴푡푟푎푛푠 . 푁퐴 . 푉
푖푟푟
1
푡푟푢푒
푡→푐
(
)
3푏 .
Φ
(
)
휀푐푖푠 − 휀푡푟푎푛푠 . 푏
퐼0 . 푡푖푟푟
RESULTS AND DISCUSSION
Absorption
The electronic absorption spectrum from 200 to 500 nm of fac-[Re(CO)3(ph2phen)(trans-
stpyCN)]+ in acetonitrile at 298 K, Figure 2A, shows a major band around 325 nm that extends the
absorption up to 460 nm ascribed – by comparison to precursors’ spectra, Figure 2B – to an overlap
between trans-stpyCN intraligand (1ILtrans-stpyCN) and Re to ph2phen charge transfer
1
(1MLCTRe→ph2phen) transitions, yet the MLCTRe→ph2phen state is more-likely the lowest-lying
singlet extending the absorption to the visible. The intense absorption around 295 nm is assigned
to a 1ILph2phen excited state related to a →* transition in the ph2phen ligand.
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Figure 2. Absorption spectra of fac-[Re(CO)3(ph2phen)(trans-stpyCN)]+ (blue ——) and fac-
[Re(CO)3(ph2phen)(cis-stpyCN)]+ (green ─••─••, see data treatment in the experimental section)
(A), as well as of fac-[Re(CO)3(ph2phen)Cl] (black ─ ─) and trans-HstpyCN+ precursors
(red ••••) (B) in acetonitrile.
The spectrum is very similar to the one reported4 for [Re(CO)3(phen)(trans-stpyCN)]+
(Figure S1 in Supporting Information). A stabilization of 1ILNN (red shift from ~275 to ~295 nm)
1
and MLCTRe→NN (red shift from ~375 to ~400 nm) transitions is observed for the ph2phen
complex arising from the electron-withdrawing phenyl substituents. The oscillator strengths of
these ph2phen-related transitions are stronger than for phen transitions, leading to an overall
increase of molar absorptivities (Figure S1 in Supporting Information), as similarly observed for
other series of compounds, e.g. Ir(III) complexes.25 The [Re(CO)3(ph2phen)(trans-stpy)]+ complex
(without the cyanide group) also have an analogous absorption spectrum14 (Figure S1 in
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1
Supporting Information), yet the CN-substituted complex exhibits a blue-shifted ILtrans-L band
(from ~325 to ~232 nm).
trans-to-cis photoisomerization
Irradiation of [Re(CO)3(ph2phen)(trans-stpyCN)]+ upon 313, 334, 365 and 404 nm results
in the trans-to-cis isomerization of the trans-stpyCN ligand, leading to spectral changes, Figure
3A. The photoisomerization pathways are exclusively through deactivations of 1/3ILtrans-stpyCN
excited states located at the trans-stpyCN ligand, in accordance to those reported in the literature.14
Figure 3. Absorption (A) and 1H NMR (500 MHz) (B) spectral changes of the fac-
[Re(CO)3(ph2phen)(trans-stpyCN)]+ complex in acetonitrile as a function of photolysis time (exc
= 365 nm, t = 20 s) towards a photostationary state (PS).
Refer to Table 1 for 1H NMR signal attribution.
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The clear and well-defined isosbestic point at 280 nm indicates no competitive
photodegradations within the time range of the experiment. The lower absorptivity of the cis-
photoproduct in the 280-470 nm region, Figure 2A and 3A, is a consequence of a decreased 1ILcis-
oscillator strength that is weakened by a distortion around the C=C double bond with the
stpyCN
CN-ring away from the pyridyl ring.4 For example, the dihedral angle for the similar fac-
[Re(CO)3(phen)(bpe)]+ complex (bpe = 1,2-bis(4-pyridyl)ethylene) is reported to be 51.0º in the
cis-complex.27 This distorted configuration also leads to a slightly destabilization of ILcis-stpyCN
states in comparison to ILtrans-stpyCN states, as ascertained by a slight blue shift of  = 4 nm.
The photoisomerization process can be better monitored by 1H NMR spectroscopy, Figure
3B, following chemical shifts and coupling constants of the protons,14,17 Table 1, since they are
distinct for the two isomers. The proton signals were assigned by analyses as well as comparisons
with the 1H NMR spectra of fac-[Re(CO)3(ph2phen)Cl] and of the uncoordinated stpyCN ligand.
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Table 1. 1H NMR spectral data for fac-[Re(CO)3(ph2phen)(stpyCN)]+ complexes.
Complex
Proton
J / Hz
/ppm
fac-[Re(CO)3(ph2phen)(trans-stpyCN)]+
9,65 (d, 2H)
8,07 (d, 2H)
8,09 (s, 2H)
7,66 (m, 14H)
8,31 (dd, 2H)
7,36 (dd, 2H)
7,15 (d, 1H)
7,43 (d, 1H)
5,5
5,3
H
H
H
H, He, Hf
Ha
5,3; 1,3
5,5; 1,3
16
Hb
Hc
Hd
16
fac-[Re(CO)3(ph2phen)(cis-stpyCN)]+
9.59 (d, 2H)
8.04 (d, 2H)
8.10 (s, 2H)
7.66 (m, 10H)
8.14 (dd, 2H)
6.95 (dd, 2H)
6.54 (d, 1H)
6.90 (d, 1H)
7.51 (d, 2H)
7.19 (d, 2H)
5.5
5.5
H
'
H
'
H
'
H
'
Ha'
Hb'
Hc'
Hd'
He'
Hf '
6.0; 1.3
5.9; 1.3
12
12
7.5
7.5
The Hc and Hd olefin proton signals at 7.15 and 7.43 ppm for the trans-isomer (J = 16 Hz)
decrease gradually upon irradiation while new upfield-shifted signals, Hc’ and Hd’, build up in
intensity at 6.54 and 6.90 ppm, ascribed to the cis-configuration (J = 12 Hz). The continuous
photolysis of the trans-isomer results in a photostationary (PS) state with ~ 55% cis- and ~ 45%
trans-isomer, as obtained by 1H NMR.
푎푝푝
푡→푐
푡푟푢푒
푡→푐
Table 2 summarizes Φ
and Φ
obtained for the four different irradiation
wavelengths. For [Re(CO)3(ph2phen)(trans-stpyCN)]+, true quantum yields are constant within the
experimental error at all irradiation wavelengths and twice higher than apparent ones, as similarly
observed for most fac-[Re(CO)3(NN)(trans-L)]+ complexes,4,7,15,17 showing the importance of
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correcting the apparent values by the cis-contribution in the absorption. Worth noticing, the
occurrence of the photoisomerization process at 404 nm irradiation – where neither the free nor
the protonated trans-stpyCN ligand absorb – is ascertained by an efficient sensitization of the
3
stilbene-localized ILtrans-stpyCN state by the rhenium polypyridyl portion through 1/3MLCT
intramolecular energy transfers.1,7,14,18,28
Table 2. Apparent and true quantum yields for the trans-to-cis photoisomerization of fac-
[Re(CO)3(ph2phen)(trans-stpyCN)]+, trans-stpyCN and trans-HstpyCN+ in acetonitrile at
different irradiation wavelengths (see data treatment in the experimental section).
Compound
irr
313 nm
334 nm
365 nm
404 nm
푡푟푢푒
푡→푐
0.49  0.04 4 0.43  0.04 4
trans-stpyCN
No absorption No absorption
Φ
Φ
Φ
trans-HstpyCN+
No absorption
푡푟푢푒
푡→푐
0.47  0.05 4 0.49  0.05 4
0.52  0.05 4
0.17 ± 0.02
0.38 ± 0.03
푎푝푝
푡→푐
fac-[Re(CO)3(Ph2phen)
(trans-stpyCN)]+
0.17 ± 0.04
0.38 ± 0.08
0.17 ± 0.04
0.38 ± 0.08
0.20 ± 0.03
0.37 ± 0.02
푡푟푢푒
푡→푐
Φ
Reverse cis-to-trans photoisomerization
Irradiation at 255 nm of the PS solution also leads to changes in the absorption spectrum
(Figure S2 in Supporting Information) ascribed to the reverse cis-to-trans photoisomerization4
with a clear isosbestic point at 280 nm identical to the trans-to-cis photoprocess, thus with no signs
of decomposition during the experiment. True quantum yields for the cis-to-trans photoreaction
(Φ푐푡푟→푢푡푒 ) were calculated by Equation 4 using these spectral changes, where APS is the initial
absorption of the photostationary solution. The quantum yield obtained by Equation 4 is in
excellent agreement to the one probed by 1H NMR.
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(
)
퐴푃푆 − 퐴푖푟푟 . 푁퐴 . 푉
푖푟푟
1
푡푟푢푒
푐→푡
Φ
=
×
(4)
(
)
휀푐푖푠 − 휀푡푟푎푛푠 . 푏
퐼0 . 푡푖푟푟
The efficient cis-to-trans photoisomerization shows a notable quantum yield of 0.16  0.01.
It’s worth reinforcing that this value is related to the PS solution, in which trans- and cis-isomers
푡푟푢푒
푐→푡
are competing for the light absorption, hence the calculated Φ
value might be even higher for
pure (100%) cis-solutions; an estimative by linear extrapolation using data from Table S5 in the
Supporting Information indicates a real value within the 0.20 – 0.26 range (Figure S3 in the
Supporting Information). In other words, the ratio between cis-to-trans and trans-to-cis
푡푟푢푒
푐→푡
photoisomerization quantum yields (Φ
/Φ푡푡→푟푢푐푒) lies between 0.53 and 0.68, showing that both
processes have fairly the same order of magnitude.
The same order of magnitude between the trans ⇌ cis quantum yields is remarkable yet
unusual for most fac-[Re(CO)3(NN)(cis-L)]+ complexes.14 Only two other Re(I) systems were
reported to show similar cis-to-trans photobehaviors: fac-[Re(CO)3(phen)(cis-stpyCN)]+ (Φ푐푡푟→푢푡푒
/
4
푡푟푢푒
푡→푐
Φ푡푡푟→푢푐푒 ≈ 0.44) and fac-[Re(CO)3(Me4phen)(cis-L)]+ (Φ푐푡푟→푢푡푒/Φ
≈ 0.27 for L = stpy and
푡푟푢푒
푐→푡
푡푟푢푒
푡→푐
Φ
/Φ
≈ 0.46 for L = bpe)29.
Similar isomerization quantum yields for the forward and reverse reactions is a desired
feature for molecular motors and geometry regulators in molecular machines based on interactions
with light.
Unusual Photophysical Behavior
Excitations of both trans- and PS acetonitrile solutions also leads to greenish emissions
with broad spectra, Figure 4, which are suppressed in the presence of O2. Their emission lifetimes
are in the microsecond range (em = 1.2 ± 0.2 for trans; em = 1.8 ± 0.4 s for PS), obtained from
their time-resolved emission decays (Figure S4 in Supporting Information). Both isomers exhibit
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analogous emission energies (with max = 540 nm) and intensities, with similar emission quantum
yields between the trans- (em = 0.021 ± 0.002) and the PS (em = 0.023 ± 0.002) degassed
solutions.
These
analogous
photophysical
characteristics
between
both
fac-
[Re(CO)3(ph2phen)(stpyCN)]+ isomers at 298 K are unusual; for most fac-[Re(CO)3(NN)(L)]+
complexes, cis-isomers usually show a far more intense emission than trans-isomers (frequently
non-emissive).13,14
Figure 4. Phosphorescence spectra of trans- (blue ——) and PS (green ─••─••) solutions of fac-
[Re(CO)3(ph2phen)(trans-stpyCN)]+ in acetonitrile at 298 K (A)
or EPA at 77 K (B) (exc = 300 nm).
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Their microsecond-lived emission and broad spectra are typical of a phosphorescence from
a charge transfer (3CT) excited state. By similarities to other analogous Re complexes,14 this CT
3
1
state is probably ascribed to MLCTRe→ph2phen, the triplet correspondent of the MLCTRe→ph2phen
observed in the visible portion of the absorption spectrum. Still, the 3MLCTRe→ph2phen state could
be expected to lie at a lower energy, similarly to the one observed for fac-[Re(CO)3(ph2phen)(Cl)],
for example, that shows a broad emission around 610 nm (Figure S5 in Supporting Information).
Therefore, TD-DFT calculations is a key complementary technique towards a detailed ascribing
of the 3CT state, to be addressed in forthcoming works.
A characteristic feature of emissions with such a charge transfer nature is their sensitivity
to the medium rigidity. This so-called rigidochromic effect arises from the dipole reorientation of
the surrounding molecules in the medium as a response to the new configuration of the excited
state: the reorientation takes place promptly in fluid solutions, yet is restrained in more constricted
media, destabilizing the charge transfer state.30 For example, the rigidochromism for fac-
[Re(CO)3(ph2phen)(Cl)], a pure 3MLCT emitter, leads to a blue shift of  ~ 60 nm (Figure S5 in
Supporting Information). By contrast, the energy of ligand-centered ππ* excited states are
relatively unaffected.
In a rigid medium at 77 K, the emission energy of fac-[Re(CO)3(ph2phen)(stpyCN)]+ is
slightly blue-shifted ( = 5 nm for the trans-solution and  = 11 nm for the PS solution), Figure
4. Their emissions exhibit thin vibronic modes, typical of IL ππ* origins, indicating a major
3
contribution of the ILstpyCN state after destabilization of the CT state at the rigid medium. The
3
emission spectra in Figure 4 also demonstrate that the ILcis-stpyCN energy is slightly blue-shifted
compared to the 3ILtrans stpyCN
-
energy, with max = 6 nm as similarly observed in their absorption
spectra.
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Transient Absorption at 298 K
In degassed acetonitrile solutions, TA spectra in the nanosecond range are similar for both
configurations of the fac-[Re(CO)3(ph2phen)(stpyCN)]+ system, Figure 5, i.e. excitation at 355 nm
of both isomers leads to absorption bleachings in the UV region and increased absorptions in the
visible as compared to the ground state spectra.
Figure 5. TA spectra at t nanoseconds after the laser pulse of degassed acetonitrile solutions of
the fac-[Re(CO)3(ph2phen)(stpyCN)]+ system (exc = 355 nm; Nd:YAG 11 mJ).
As demonstrated in Figure 6, the nanosecond TA decay of trans- and PS solutions follow
a double exponential decay with contributions of a shorter lifetime () in the nanosecond range
and a longer lifetime (2) in the microsecond range – the second one better distinguished in the
3
visible region. The faster  lifetime is usual for IL excited-states, being ascribed to the ILstpyCN
state, while the slower  counterpart is the correspondent to the em lifetime obtained by emission
decays and ascribed to 3CT deactivations.
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Figure 6. Nanosecond TA decays of fac-[Re(CO)3(ph2phen)(trans-stpyCN)]+ (blue, left side)
and PS (green, right side) solutions in degassed acetonitrile monitored at 330 and 490 nm (exc
355 nm; Nd:YAG 11 mJ; room temeprature).
=
Worth noting, the magnitude of the faster  lifetime in the TA decay profile is substantially
lower than the  component such that a regular double exponential fitting procedure may not be
reliable. In fact, by fitting the TA decays without fixing any parameters in the exponential equation,
the lifetimes obtained are: = 35 ± 4 ns and  = 1.62 ± 0.06 s for the trans-isomer;  = 38 ± 7
ns and  = 1.78 ± 0.05 s for the PS solution. In this case, the  contribution for the trans-decay
is significantly different than em, leading to an inaccurate describing of the actual physical events
of the molecular system. Therefore, to avoid fitting artifacts, the fittings shown in Figure 6 were
obtained by fixing the slower  component as equal to the values found by emission decays ( =
em), i.e. 1.2 s for trans and 1.8 s for PS.
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Energy Diagram and Deactivation Pathways
Both photochemical and photophysical pathways are accessible in the fac-
[Re(CO)3(ph2phen)(stpyCN)]+ system at room temperature through deactivations from 1/3ILstpyCN
3
and CT states, respectively. In terms of energy, the small difference in the emission energy
3
3
between 298 K (from CT) and 77 K (from ILstpyCN) as well as the observation of two related
3
lifetimes are decisive evidences that the 3CT and ILstpyCN states lie very-close in energy at room
temperature, Figure 7.
Figure 7. Energy diagram for the lowest-lying triplet excited states of the fac-
[Re(CO)3(ph2phen)(stpyCN)]+ system at 298 K and 77 K.
At 298 K, the close-lying 3CT – 3ILstpyCN energies lead to a crucial interplay between both
states (probably by repopulation) that plays a key role in the unusual photobehavior. For example,
a direct consequence is the observation of emission for the trans-isomer, which usually is non-
emissive.13,14 Also, the CT-IL energy difference in the trans-isomer is smaller than in the cis-
isomer hence the interplay can occur more easily in the first isomer; as a result, the trans-complex
exhibits a slightly shorter 2 (CT related) and a higher Φ푡푡→푟푢푐푒 (IL related) than the cis-complex.
Regarding the photochemical pathways, the results so far does not distinguish the individual
importance of 1ILstpyCN and 3ILstpyCN states to the photoisomerization process, being both feasible;
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still, the most contributive might be the triplet counterpart, as usually observed for coordination
compounds of heavy-metals with high spin-orbit constants.
3
At 77 K, the CT state is strongly destabilized from 298 K to 77 K by the rigidochromic
3
effect, resulting in an inversion in the excited state energy order, with CT > 3ILstpyCN, Figure 7.
3
As a result, the CT-IL interplay is extinguished, leading to a pure ILstpyCN emission (related
observations have been made for MLCT polypyridyl-based emitters25,29). Also, the
photoisomerization process is not observed at 77 K since vibrational processes are hindered in
frozen media.
An open question is the electronic/molecular mechanism that results in such an intriguing
triplet CT-IL interaction for which our ongoing efforts are on monitoring C≡O stretching signals in
time-resolved infrared spectra and elucidating the electronic structures of fac-
[Re(CO)3(NN)(stpyCN)]+ complexes by TD-DFT calculations and the application of the Golden
Rule.24,25,30
CONCLUSIONS
The photobehavior of the fac-[Re(CO)3(ph2phen)(stpyCN)]+ system is remarkable as well
as unusual and can be rationalized in terms of the interplay among close-lying excited states of
different natures. The photoisomerization of the coordinated ligand is observed in both ways,
trans-to-cis and cis-to-trans, of the same magnitude (Φ푡푡→푟푢푐푒 = 0.38, Φ푐푡푟→푢푡푒 ≈ 0.26), through the
3
deactivation of 1/3ILstpyCN excited states. A greenish emission is also observed for a CT
deactivation.
The efficient cis ⇌ trans reversibility is a great achievement and inspiration for the design
of molecular devices able of mimetizing some of the biological machines that nature requires to
sustain life. For example, they can be exploited as geometry regulators in molecular machines
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capable of achieving a mechanical work by optically lengthening and contracting the coordinated
ligand in its trans- and cis-configurations. Moreover, the free CN group at the end of the
coordinated stpy ligand allows its incorporation in larger structures, e.g. multinuclear complexes,
where it can act as molecular motors, wires and gears.
ASSOCIATED CONTENTS
Supporting Information. Picture of the homemade cuvette, absorption spectra of similar
complexes, spectral changes for the cis-to-trans photoisomerization, time-resolved emission
decays, emission spectra of the fac-[Re(CO)3(ph2phen)(Cl)] complex and complete triplicate data
for the quantum yield calculations. This material is available free of charge via the Internet.
AUTHOR INFORMATION
Corresponding Author
* E-mail: neydeiha@iq.usp.br
CLONFLICTS OF INTEREST
There are no conflicts of interest to declare.
ACKNOWLEDGEMENTS
This work is supported by Fundação de Amparo à Pesquisa do Estado de São Paulo
(FAPESP) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). This
research made use of the laser flash photolysis spectrometer system in the UNC EFRC
Instrumentation Facility established by the UNC EFRC: Center for Solar Fuels, an Energy Frontier
Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences under Award Number DE-SC0001011. We also thank Prof. Thomas J. Meyer for
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making this facility available, and Rodolfo L. Coppo, Ronaldo C. Amaral and Dr. M. Kyle
Brennaman for general assistances.
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TABLE OF CONTENTS ENTRY
TABLE OF CONTENTS SYNOPSIS
The fac-[Re(CO)3(ph2phen)(stpyCN)]+ complex is capable of switching through
extended trans- and contracted cis-configurations upon irradiation under proper
energies.