Triplet MLCT photosensitization of the ring-closing reaction of diarylethenes by design and synthesis of a photochromic rhenium(I) complex of a diarylethene-containing 1,10-phenanthroline ligand

Article abstract of DOI:10.1002/chem.200501325

Synthesis of the diaryle-thene-containing ligand LI based on Suzuki cross-coupling reaction between thienyl boronic acid and the dibromophenanthroline ligand is reported. On coordination to the rhenium(I) tricarbonyl complex system, the photochromism of L1 could be photosensitized and consequently extended from intraligand excitation at λ≤340nm in the free ligand to metal-to-ligand charge-transfer (MLCT) excitation atλ≤480nm in the complex. The photo-chromic reactions were studied by ¹HNMR, UV/Vis, and steady-state emission spectroscopy. Photosensitization was further probed by ultrafast transient absorption and time-resolved emission spectroscopy. The results provided direct evidence that the formation of the closed form by the MLCT-sensitized photochromic process was derived from the 3MLCT excited state. This supports the photosensitization mechanism, which involves an intramolecular energy-transfer process from the ³MLCT to the ³IL(L1) state that initiated the ring-closure reaction. The photophysical and electrochemical properties of the complex were also investigated.

Full text of DOI:10.1002/chem.200501325

DOI: 10.1002/chem.200501325
Triplet MLCT Photosensitization of the Ring-Closing Reaction of
Diarylethenes by Design and Synthesis of a Photochromic Rhenium(i)
Complex of a Diarylethene-Containing 1,10-Phenanthroline Ligand
Chi-Chiu Ko, Wai-Ming Kwok, Vivian Wing-Wah Yam,* and David Lee Phillips*[a]
Abstract: Synthesis of the diaryle-
thene-containing ligand L1 based on
Suzuki cross-coupling reaction between
thienyl boronic acid and the dibromo-
phenanthroline ligand is reported. On
coordination to the rhenium(i) tricar-
bonyl complex system, the photo-
chromism of L1 could be photosensi-
tized and consequently extended from
intraligand excitation at lꢀ340 nm in
the free ligand to metal-to-ligand
charge-transfer (MLCT) excitation at
lꢀ480 nm in the complex. The photo-
chromic reactions were studied by
1H NMR, UV/Vis, and steady-state
emission spectroscopy. Photosensitiza-
tion was further probed by ultrafast
transient absorption and time-resolved
emission spectroscopy. The results pro-
vided direct evidence that the forma-
tion of the closed form by the MLCT-
sensitized photochromic process was
3
derived from the MLCT excited state.
This supports the photosensitization
mechanism, which involves an intramo-
lecular energy-transfer process from
3
3
the MLCT to the IL(L1) state that in-
itiated the ring-closure reaction. The
photophysical and electrochemical
properties of the complex were also in-
vestigated.
Keywords:
ligands
luminescence
photochemistry
·
·
N
·
photochromism · rhenium
Introduction
ploitation of these diarylethenes as ligands to form metal
complexes are rare and less explored.[2] The combination of
diarylethene ligands and metal complex systems is envisaged
to give rise to novel properties, such as nondestructive data
processing,[2a] electron-transfer switching,[2b] and fluores-
cence modulation,[2c–f] and may extend the excitation wave-
length to lower energies, which are usually less destructi-
ve.[2g,h] Recently, we communicated the isolation of a versa-
tile diarylethene-containing 1,10-phenanthroline ligand and
its rhenium(i) complex, both of which are capable of photo-
chromic reactivity.[2h] Herein we report the detailed synthe-
ses, characterization, photophysics, and electrochemical
properties of this class of photochromic compounds. Insights
into the mechanism of their photochromic reactivity were
also provided by transient absorption and time-resolved
emission studies. Direct evidence for the photosensitization
of the cyclization reaction of the diarylethene unit by the
3MLCT excited state of the rhenium(i) diimine chromophore
are presented, and a mechanism for the cyclization process
that involves the 3IL state of the diarylethene-containing
ligand has been demonstrated for the first time.
Photochromic compounds are those that are capable of un-
dergoing light-induced reversible transformation between
two forms having distinguishable absorption characteristics.
They are among the most promising materials for optical
data storage, optoelectronics, and molecular switching devi-
ces. Diarylethene derivatives, especially those with hetero-
cyclic aryl groups, have recently received much attention, as
they have outstanding fatigue resistance and thermally irre-
versible photochromic behavior.[1] While studies on diaryle-
thenes are mainly confined to the molecular design and syn-
thesis of the organic framework, investigations into the ex-
[a] Dr. C.-C. Ko, Dr. W.-M. Kwok, Prof. Dr. V. W.-W. Yam,
Prof. Dr. D. L. Phillips
Center for Carbon-Rich Molecular and Nano-Scale Metal-Based Ma-
terials Research
Department of Chemistry and HKU-CAS Joint Laboratory on New
Materials
The University of Hong Kong
Pokfulam Road, Hong Kong (P. R. China)
Fax : (+852)2857-1586
E-mail: wwyam@hku.hk
phillips@hku.hk
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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FULL PAPER
Results and Discussion
intraligand (IL) p!p* transition of the phenanthroline
moiety, probably mixed with the n!p* and p!p* transi-
tions of the thiophene rings. The electronic absorption spec-
trum of 1 showed, in addition to the intense IL (p!p*) ab-
sorption in the UV region, a moderately intense absorption
shoulder with molar extinction coefficients on the order of
103 dm3 molꢁ1 cmꢁ1 at about 400 nm, which is assigned as the
metal-to-ligand charge transfer (MLCT) transition
Synthesis and characterization: The synthetic route to
diarylethene-containing 1,10-phenanthroline ligand L1 is
summarized in Scheme 1. Suzuki cross-coupling of 2,5-dime-
A
[dp(Re)!p*
C
On excitation at l=300 nm, the open form of L1 in ben-
zene solution produced luminescence at about 385 nm (to <
0.1 ms). The close resemblance of the excitation spectrum to
the electronic absorption spectrum of L1, together with the
small Stokes shift and short excited-state lifetimes, suggests
assignment of the emission as fluorescence from states of IL
(p!p*) character, most probably derived from the phenan-
throline moiety. Attempts to record the weakphosphores-
cence of the open form of L1 were made by using the back-
scattering geometry at 77 K in MeCN. Under these condi-
tions, a vibronic-structured phosphorescence band at about
596 nm, with vibrational progressional spacings of about
1180 cmꢁ1, was observed (see Supporting Information). In
contrast, the open form of 1 in benzene solution produced
strong luminescence at 595 nm (to =0.26 ms) on excitation
into the MLCT absorption bands at about 400 nm. With ref-
erence to previous spectroscopic studies on related rhen-
ium(i) tricarbonyl diimine complexes[5] and the observed
low-temperature emission spectrum of L1 in MeCN, this
Scheme 1. Synthetic route to the diarylethene-containing 1,10-phenan-
throline ligand L1.
thylthien-3-ylboronic acid with 5,6-dibromo-1,10-phenan-
throline in THF gave L1 in moderate yield. Coupling of the
second equivalent of boronic acid was inefficient and re-
quired long reaction times, probably as a result of the steric
bulkof the system. Reaction of L1 with [ReCl(CO) ] in re-
5
fluxing anhydrous benzene afforded [ReCl(CO)3(L1)] (1) as
yellow crystals in high yield. The identities of the ligand and
the complex were confirmed by satisfactory elemental anal-
yses, 1H NMR and IR spectroscopy, and FAB mass spec-
trometry. The X-ray crystal structure of L1 has also been de-
termined.[2h] Both L1 and 1 showed two well-resolved sets of
1H NMR signals corresponding to the resonances of parallel
(photochromically active) and antiparallel (photochromi-
cally inactive) conformers. These two conformations are
commonly found in the open forms of diarylethene sys-
tems.[1,3] Unlike most other diarylethene systems, in which
rapid interconversion of the two forms results in only one
emission band is assigned as phosphorescence derived from
3
states of MLCT [dp(Re)!p*
A
of 3IL character. The close resemblance of the emission
3
energy to the MLCT emission maximum of about 600 nm
of the related complex [ReCl(CO)
3(phen)][5e] and the 3IL
phosphorescence of L1 is also supportive of the assignment.
The open form of 1 also displayed photoluminescence in the
solid state and in EtOH/MeOH (4/1) glass at 77 K. These
emissions are also assigned as derived from states of MLCT
3
[dp(Re)!p*
A
1
set of time-averaged H NMR signals,[1,3] the sterically de-
lar to that reported in the related complex [ReCl(CO)3-
manding 1,10-phenanthroline ring with protons in the 4- and
7-positions hinders rotation of the thiophene moieties, and
thus two sets of signals are observed. At room temperature,
interconversion between the two forms is very slow, as re-
flected by the ratio of the two sets of signals remaining un-
changed even on prolonged standing. When the sample was
heated to about 658C, the conformers started to intercon-
vert, and a ratio of about 1:1 was reached at thermal equili-
brium. Complex 1 showed three intense stretches in the car-
bonyl region (1890–2025 cmꢁ1), which confirm the fac ar-
rangement of the tricarbonyl unit in an octahedral environ-
ment.[4]
ACHTREUNG
(535 nm) relative to the solution state (595 nm) is attributed
to the increased rigidity in the glass, in which the solvent di-
poles could not reorient themselves around the complex to
stabilize the excited state and thus resulted in an energeti-
cally high lying excited state. Such luminescence rigido-
chromism has also been observed in other related systems.[6]
Photophysical data for L1 and 1 are summarized in Table 1.
Photochromic behavior: On UV excitation into the IL ab-
sorption band at lꢀ340 nm, an intense absorption band and
two moderately intense absorption bands at 366, 510, and
540 nm for L1 and 386, 546, and 580 nm for 1 emerged
(Figure 1). These new absorption bands were ascribed to ab-
sorptions of the closed form, generated by photocyclization
of the open form (Scheme 2). The large bathochromic shift
of the absorption bands of the closed form with respect to
Electronic absorption and emission properties: The elec-
tronic absorption spectrum of L1 in benzene showed an in-
tense band at about 300 nm, with a molar extinction coeffi-
cient on the order of 104 dm3 molꢁ1 cmꢁ1. This is assigned as
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5841

V. W.-W. Yam, D. L. Phillips et al.
Table 1. Photophysical data for the open and closed forms of L1 and 1.
result of the photochromic
backreaction. For the complex,
in addition to excitation into
the IL absorption band, excita-
tion into the MLCT absorption
band at lꢀ480 nm also gave
rise to these new absorptions,
indicative of generation of the
closed form with MLCT excita-
tion. This is suggestive of sensi-
tization of the photochromic ac-
tivity of the diarylethene-con-
taining phenanthroline with the
MLCT excited state, which ex-
tends the excitation wavelength
from the UV region in the free
ligand to the visible region in
Compound
Absorption
Medium (T [K])
Emission
lem [nm] (to [ms])
[a]
labs [nm] (e [dm3 molꢁ1 cmꢁ1])
L1 (open form)
300 (8670)
benzene (298)
solid (298)
385 (<0.1)
[c]
–
–
–
[c]
solid (77)
glass[b] (77)
MeCN (77)[d]
benzene (298)
glass[b] (77)
benzene (298)
solid (298)
[c]
596
L1 (closed form)
366 (31730), 510 (3950), 540 (3730)
338 (4930), 400 (4690)
644 (<0.1)
577 (5.2)
595 (0.26)
558 (<0.1)
562 (1.9)
535 (7.2)
626 (<0.1)
1 (open form)
solid (77)
glass[b] (77)
benzene (298)
1 (closed form)
386 (36670), 546 (5390), 580 (5050)
[a] In benzene at 298 K. [b] EtOH/MeOH (4/1). [c] Emission not detected. [d] Recorded in backscattering ge-
ometry.
the complex. The photosensitization reaction probably in-
volves intramolecular energy transfer from the 3MLCT to
3
the IL state of L1, similar to that reported in our previous
studies on spirooxazine-, pyridyl–azo-, and pyridyl–stilbene-
containing rhenium(i) complexes,[5i,7] and in a dithienylper-
fluorocyclopentene-containing bipyridyl ruthenium(ii) com-
plex.[2g] Further insights into the photosensitization mecha-
nism were provided by transient absorption spectroscopy
and time-resolved emission studies (vide infra). Photochro-
mic interconversion between the open and closed forms was
1
further supported by H NMR studies. On excitation at l=
313 nm, the signals due to the open form of the antiparallel
configuration decreased due to conversion to the closed
form. The concomitant decrease in the signal intensities of
the open form of the antiparallel configuration and the in-
crease in the signal intensity of the closed form (Figure 2) is
Figure 1. UV/Vis absorption spectral changes of a) L1 (5.52”10ꢁ5 m) and
b) 1 (7.16”10ꢁ5 m) in benzene solution on excitation at 313 and 440 nm,
respectively.
Scheme 2. Photochromic reactions of L1 and 1.
the open form has been attributed to an increase in the ex-
tended p delocalization across the condensed thiophene
rings (8a,8b-dimethyl-1,8-dithia-as-indacene) leading to a re-
duced HOMO–LUMO energy gap for the p!p* and n!p*
transitions. On excitation into the absorption bands of the
closed form, absorptions due to the closed form decreased
in intensity, indicative of regeneration of the open form as a
Figure 2. 1H NMR (300 MHz) spectral changes of 1 in C6D6 solution at
298 K on irradiation, showing the time course of closed-form formation
&
(from bottom to top). The signals corresponding to the antiparallel ( )
^
*
)
and parallel ( ) conformations of the open form and the closed form (
are labeled. (Ia, Ip, and Ic are the integrals of the methyl protons corre-
sponding to signals of antiparallel and parallel conformations of the open
and closed forms, respectively).
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Photochromic Reactions of Rhenium(i) Complexes
FULL PAPER
indicative of a photochromic reaction originating solely
from the antiparallel conformation. This observation is also
in agreement with the prediction of the Woodward–Hoff-
man rule that the photocyclization reaction is active in the
conrotatory mode.[8] The photochemical quantum yields of
the photochromic reactions in benzene solution, corrected
for the active conformer, were determined with excitation at
l=313 nm for L1 and 313 and 440 nm for 1 for the forward
reaction, and 510 nm for both L1 and 1 for the backward re-
action. The quantum yields for the photocyclization of L1
(f313 =0.33) and 1 (f313 =0.55, f440 =0.65) were much higher
than those for photocycloreversion (L1, f510 =0.012; 1,
f510 =0.009; Table 2). Similar findings have also been report-
ed for related diarylperfluorocyclopentene systems.[1,2]
Table 2. Photochemical quantum yields of L1 and 1 in benzene solution
at 298 K.
Compound
Photochemical quantum yield/f
photocyclization[a]
photocycloreversion
Figure 3. Cyclic voltammograms of a) oxidation of the open form of 1
and b) repetitive reductive scans during the course of the photochromic
conversion of 1 from the open form (c) to the closed form in MeCN
f313
f440
0
f510
(0.1m nBu4NPF6). Scan rate: 100 mVsꢁ1
.
L1
1
0.329
0.552
0.012
0.009
0.648
[a] Corrected to the ratio of the photochromically active conformation,
that is, with respect to the antiparallel configuration
reduction. The irreversible reduction wave at ꢁ1.79 V,
which is absent in [ReCl(CO)3
(phen)],[5e,11a] was assigned to
AHCTREUNG
thiophene-based reduction. The electrochemical changes on
conversion of the open form to the closed form were also
obtained from the cyclic voltammograms of a mixture of the
open and closed forms in their photostationary state. The
lackof observable changes in the irreversible oxidation
wave at +1.40 V and the quasireversible reduction couple at
ꢁ1.31 V is suggestive of insignificant perturbation of the p-
accepting ability of the phenanthroline moiety on condensa-
tion of the thiophene units. A drop in the electrochemical
signal of the reduction wave at ꢁ1.79 V with concomitant
formation of a new reduction wave at less cathodic potential
(ꢁ1.61 V) was observed on conversion of the open form to
the closed form on light excitation, and this confirms the thi-
ophene-based nature of the reduction, as conversion of the
thiophene rings in the open form to the condensed thio-
phene units in 8a,8b-dimethyl-1,8-dithia-as-indacene, which
has better p conjugation and hence a better p-accepting
ability, would cause a shift of the reduction wave to less neg-
ative potential. The much smaller perturbation of the phe-
nanthroline-localized reduction than that of the thiophene-
based reduction on photochromic ring closure is understand-
able in view of the indirect involvement and minimal dis-
turbance of the aromaticity of the phenanthroline moiety in
the ring-closure process. Similarly, the open form of L1 dis-
played two irreversible reduction waves at ꢁ1.89 and
ꢁ2.13 V versus SCE in acetonitrile containing 0.1 moldmꢁ3
nBu4NPF6 (Supporting Information). They are assigned to
phenanthroline- and thiophene-based reduction, respective-
ly. On conversion to the closed form, a new reduction wave
at less cathodic potential (ꢁ1.67 V versus SCE), which was
assigned to reduction of the condensed thiophene units, was
also observed.
Apart from the UV/Vis absorption changes on conversion
to the closed form, the emission maxima of L1 and 1 were
also found to shift to the red, from 385 (to <0.1 ms) to
644 nm (to <0.1 ms) and 595 (to =0.26 ms) to 626 nm (to <
0.1 ms), respectively. This red shift is in line with an increase
in the extent of p conjugation on photocyclization. Similar
emission energy was reported for the closed form of a relat-
ed diarylethene.[9] The closed forms of L1 and 1 in EtOH/
MeOH (4/1) glass at 77 K also showed vibronic-structured
emission peaking at 577 nm (to =5.2 ms) and 620 nm (to =
6.4 ms), respectively, with fairly well-resolved vibronic struc-
tures that show vibrational progressional spacings of about
1250 cmꢁ1, typical of n(CPS) stretching modes.[10] The rela-
A
tively long emission lifetimes in the microsecond range sug-
gest assignment to a phosphorescence origin. Thus, the emis-
sion of L1 in its closed form is assigned as being derived
from states of IL
close resemblance of the emission energy and vibronic struc-
A
tures of 1 to those of L1 in their closed forms, an emission
origin of metal-perturbed IL
A
Electrochemical studies: The open form of 1 displayed an ir-
reversible oxidation wave at +1.40 V, a quasireversible re-
duction couple at ꢁ1.32 V, and an irreversible reduction
wave at ꢁ1.79 V versus SCE in acetonitrile containing
0.1 moldmꢁ3 nBu4NPF6 (Figure 3). With reference to previ-
ous electrochemical studies on other related rhenium(i) tri-
carbonyl diimine systems,[5e,11] the first irreversible oxidation
was tentatively assigned as the metal-centered oxidation of
ReI to ReII, while the first quasireversible reduction couple
at ꢁ1.33 V was assigned as phenanthroline ligand-centered
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V. W.-W. Yam, D. L. Phillips et al.
Transient absorption and time-resolved emission studies: Ul-
trafast spectroscopy has been commonly employed in the
study of photosensitization reactions, including those of re-
lated rhenium(i) polypyridine systems.[12] Thus, to provide
further insights into the photophysical and photochromic be-
havior of 1 in acetonitrile, transient absorption spectroscopy
on the pico- to nanosecond timescale after femtosecond
laser excitation at 400 nm was carried out. Transient absorp-
tion spectra of 1 in acetonitrile in the wavelength range of
325 to 670 nm recorded at various delay times after photo-
excitation are displayed in Figure 4, and the absorption time
profiles at selected wavelengths in the insets of Figure 4. Im-
close agreement of these absorption decays at the early
times with the decay of the 3MLCT state in the time-re-
solved emission measurements under the same conditions
(vide infra). The fast decay observed in the early stages was
3
attributed to cooling of the hot MLCT excited state to its
relaxed state. The evolution of this fast decay was monitored
at 460 nm, and a time constant of about 9 ps (see Figure 4a)
3
was determined for cooling of the MLCT state. Since the
3MLCT state underwent noticeable vibrational cooling, this
1
3
implied that the ISC from MLCT to MLCT occurred very
3
fast and reasonably far away from the equilibrium MLCT
geometry. Between 50 ps and 6 ns, an absorption feature at
around 420 nm increased initially and then decreased in in-
tensity (see Figure 4b and c). The time dependence of this
feature was monitored at 420 nm (inset of Figure 4b). A
best fit to the time dependence data of Figure 4b with single
exponential growth and decay functions gave time constants
of 1.8 and 7 ns, respectively. We note that there appeared to
be an isosbestic point at 336 nm between 50 ps and 1.5 ns.
This suggested that the growth of the 420 nm feature is asso-
3
ciated with a process in which the MLCT evolves cleanly
into another species with a stronger absorption at around
420 nm and a weaker absorption at around 540 nm. We ten-
3
tatively assigned this new species to the IL excited state,
and the time constant of 1.8 ns appeared to be associated
with internal conversion of the 3MLCT state to the 3IL excit-
ed state, or alternatively, it could be visualized as intramo-
lecular energy transfer from the 3MLCT state to the 3IL
state. Such internal conversion or intramolecular energy-
transfer processes could be substantiated by energetics con-
3
sideration, since the MLCT state of 1 (lem =595 nm in ben-
zene) should be of sufficient energy to transfer its energy to
the 3IL state of the open form of L1 (lem =596 nm in
MeCN; Table 1). The slow decay of the 420 nm absorption
was found to parallel the growth of the absorption bands at
about 386, 540, and 580 nm, which were ascribed to the
ring-closing process, since the steady-state absorption of the
closed form of 1 also occurred in the same region. This indi-
Figure 4. Transient absorption spectra of
1 in MeCN obtained with
400 nm excitation at a series of pump–probe delays: a) 1, 2, 3, 4, 5, 7, 8.5,
14, 30, and 50 ps; b) 50, 100, 250, 400, 600, 800, 1000, 1250, and 1500 ps;
c) 1.5, 1.75, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, and 6 ns. The insets show absorp-
tion–time profiles at a) 460 nm, b) 420 nm, and c) 386 nm. The lines are
best-fit exponential curves to the data. The sharp features around 400 nm
are due to artifacts from the pump pulses.
3
cated that the IL excited state evolved into the characteris-
tic absorption bands associated with formation of the closed
form of 1. We monitored the growth of the strong absorp-
tion feature at 386 nm to follow the formation of the closed
form of 1, and a best fit of a single exponential growth func-
tion gave a time constant of 7 ns (see Figure 4c and inset).
mediately after laser flash excitation, an intense transient
absorption feature extending beyond 330 nm evolved, which
then underwent fast decay, followed by slower decay to gen-
erate a new band at about 386 nm and growth of intensity at
about 540 and 580 nm. With reference to previous excited-
3
We note that the time constant for decay of the IL excited
state (t=7 ns) matches well with the time constant for the
growth of the closed form of 1 (t=7 ns). This, as well as the
observation of apparent isosbestic points at about 345 nm
and 612 nm, indicates clean interconversion of these two dis-
tinct species. This provides direct evidence that the forma-
tion of the closed form of 1 by an MLCT-sensitized photo-
state absorption studies on [ReCl(CO)
3(R-phen)],[5e] which
A
3
showed that the MLCT excited state exhibited two absorp-
tion bands peaking at about 300 and 480 nm, the absorption
feature extending beyond 330 nm was ascribed to the ab-
3
3
sorption of the MLCT excited state of 1. Similarly, the ini-
chromic process was derived from the MLCT excited state
tial formation of the absorption features at about 460 nm
immediately after laser flash and the subsequent fast decay
prior to the growth in intensity on a longer timescale was at-
tributed to the fact that the MLCT of 1 also absorbs in this
region. Such assignments were further supported by the
via its internal conversion or intramolecular energy transfer
to the IL state that then underwent ring closure to produce
the closed form of 1. In contrast to photocyclization process-
es on the picosecond timescale observed in other dithienyl-
perfluorocyclopentenes,[13] in which the reactions originated
3
3
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Photochromic Reactions of Rhenium(i) Complexes
FULL PAPER
1
3
from the singlet excited states, the relatively slower rate of
formation of the closed form on the nanosecond timescale is
supportive of the involvement of a triplet excited state. Re-
action on a similar timescale has also been reported for a
triplet-state photosensitized dithienylperfluorocyclopente-
ne.[2g] Similarly, in contrast to the singlet-state trans–cis pho-
toisomerizations, which occur on the picosecond time-
scale,[14] slower reactions on the nanosecond timescale were
also commonly found in triplet-state sensitized photoisome-
rization processes in azo- and stilbene-containing rhenium(i)
complexes.[12]
ISC from MLCT to MLCT. Since the time resolution of
our Kerr gated emission experiment is about the same as
the 0.8 ps time constant, this value should be treated as an
1
3
upper limit for ISC of MLCT to MLCT. This timescale is
reasonable since ISC of a related [Ru
AHCTREUNG
system has been shown to occur on a similar timescale.[15]
The slower decay of the signal monitored at 590 nm with a
time constant of about 8 ps (see Figure 5, inset) correlated
with the decay of the signal at 460 nm in the transient ab-
sorption spectra that has a time constant of about 9 ps (see
Figure 4a). This is consistent with these changes in the time-
resolved emission and absorption spectra being due to relax-
ation of a highly excited 3MLCT state to an equilibrated
3MLCT state.
Kerr gated time-resolved emission measurements on the
open form of the complex in acetonitrile solution were also
made. Figure 5 shows the time-resolved emission spectra of
Our current studies revealed that the ring-closing process
of 1 with MLCT excitation proceeds initially with formation
of the 1MLCT excited state, which undergoes intersystem
3
3
crossing to the MLCT excited state (tꢀ0.8 ps). The MLCT
excited state then undergoes internal conversion or intramo-
lecular energy transfer to produce the IL excited state (t=
3
1.8 ns), which subsequently formed the closed form of 1 (t=
7 ns). A mechanism for the photochromic reactivity of 1
with MLCT excitation has thus been proposed (Figure 6).
Figure 5. Kerr gated time-resolved emission spectra of 1 in MeCN ob-
tained by 300 nm excitation. The time delays from top to bottom are 0.2,
0.5, 1, 2, 5, 10, 30, and 1000 ps. The inset shows the emission time profile
at 590 nm. The line represents the best fit of a two-exponential decay
curve to the data.
1 in acetonitrile recorded at various delay times after excita-
tion at 300 nm. An intense emission with maximum at about
500 nm predominated at very early times and disappeared in
less than 2 ps after excitation. This emission band is assigned
Figure 6. Proposed qualitative energetic scheme for photosensitized pho-
tochromism of 1 by MLCT excitation.
1
as the fluorescence derived from the MLCT excited state.
At a later time (>2 ps), an emission band at about 550 nm
was observed, which showed a gradual wavelength shift to
the red, eventually reaching a wavelength maximum at
about 590 nm after 40 ps. This emission maximum at 590 nm
was similar to that of the 3MLCT emission in the steady-
state emission spectrum. The emission at about 550 nm was
ascribed to the phosphorescence derived from the vibration-
ally excited 3MLCT state, which gradually relaxes to the
ground 3MLCT excited state, which subsequently emits at
about 590 nm. The time-dependence profile of the vibration-
Conclusion
The synthesis of diarylethene-containing ligand L1 by
Suzuki cross-coupling reaction between thienylboronic acid
and the dibromophenanthroline ligand has been reported.
On coordination to the chlororhenium(i) tricarbonyl com-
plex system, the photochromism of L1 could be photosensi-
tized and consequently extended from IL excitation at lꢀ
340 nm in the free ligand to MLCT excitation at lꢀ480 nm
in the complex. The photosensitization was studied by ultra-
fast transient absorption and time-resolved emission spec-
3
ally excited MLCT emission was monitored at 590 nm and
1
is shown in the inset of Figure 5 as open circles. A best fit of
these data with a two-exponential decay function gave time
constants of 0.8 and 8 ps. A wavelength of 590 nm was used
to fit the late weakemission from the 3MLCT, since it was
less influenced by the early intense emission from the
1MLCT state and we found this wavelength more conven-
troscopy. Emission from the MLCT state was also observed
for the first time in the chlororhenium(i) tricarbonyl phe-
nanthroline system. The results provide direct evidence that
formation of the closed form by the MLCT sensitized pho-
3
tochromic process is derived from the MLCT excited state.
This supports the photosensitization mechanism which in-
3
ient to probe the cooling of the MLCT state. The very fast
volves intramolecular energy transfer from the 3MLCT to
3
decay of the emission monitored at 590 nm corresponded to
the IL(L1) state that initiates the ring-closure reaction. The
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V. W.-W. Yam, D. L. Phillips et al.
formation), 2.15 (s, 3H; 2-CH3 of parallel conformation), 6.28 (s, 1H;
thienyl 4-H of parallel conformation), 6.29 (s, 1H; thienyl 4-H of antipar-
allel conformation), 6.98 (m, 2H; phenanthrolyl 3,8-H), 7.89 (m, 2H;
phenanthrolyl 4,7-H), 9.11 ppm (m, 2H; phenanthrolyl 2,9-H); positive-
ion EI-MS: m/z (%): 400 (100) [M+], 385 (12) [M+ꢁCH3], 370 (6) [M+
ꢁ2CH3]; elemental analysis calcd (%) for C24H20N2S2·0.5H2O (409.6): C
70.38, H 5.17, N 6.84; found: C 70.57, H 4.96, N 6.71.
photophysical properties of the complex were also found to
change in the photochromic reactions. With our current syn-
thetic strategy, different diarylethene-containing photochro-
mic ligands could be designed, synthesized, and incorporat-
ed into various metal complex systems. It is envisaged that,
through judicious design of the photochromic ligands, the
photophysical properties of the complexes could be readily
tuned, the photochromic behavior perturbed, and the source
of excitation extended towards the use of lower energy
light.
[ReCl(CO)3(L1)] (1): The complex was prepared by modification of a lit-
erature method for related Re(i) diimine complexes.[19] [ReCl(CO)5]
(100 mg, 0.27 mmol) and L1 (113 mg, 0.28 mmol) were suspended in ben-
zene (25 mL). The suspension was heated to reflux for 10 h under nitro-
gen, during which the starting materials dissolved gradually to give a
yellow solution. After removing the solvent under reduced pressure, the
residue was purified by column chromatography with chloroform as
eluent. Yellow crystals of 1 were obtained by slow diffusion of n-hexane
into a dichloromethane solution of 1. Yield: 155 mg, 0.22 mmol, 82%.
1H NMR (400 MHz, C6D6, 258C, TMS): d=1.59 (s, 6H; 5-CH3 of parallel
conformation), 1.66 (s, 3H; 5-CH3 of antiparallel conformation), 1.74 (s,
3H; 5-CH3 of antiparallel conformation), 2.12 (s, 3H; 2-CH3 of antiparal-
lel conformation), 2.14 (s, 3H; 2-CH3 of antiparallel conformation), 2.17
(s, 6H; 2-CH3 of parallel conformation), 5.89 (s, 1H; thienyl 4-H of anti-
parallel conformation), 6.17 (s, 1H; thienyl 4-H of antiparallel conforma-
tion), 6.23 (s, 2H; thienyl 4-H of parallel conformation), 6.47 (m, 4H;
phenanthrolyl 3,8-H), 7.60 (m, 4H, phenanthrolyl 4,7-H), 8.79 ppm (m,
4H; phenanthrolyl 2,9-H); positive-ion FAB-MS: m/z (%): 706 (60) [M+
], 678 (16) [M+ꢁCO], 671 (100) [M+ꢁCl], 650 (14) [M+ꢁ2CO], 622
Experimental Section
Materials: [ReCl(CO)5] was obtained from Strem Chemicals, Inc. 1,10-
Phenanthroline, 20% fuming sulfuric acid, bromine, 2,5-dimethylthio-
phene, tri-n-butyl borate, n-butyllithium (1.6m in hexane) and hydrazine
hydrate were obtained from Aldrich Chemical Company and were used
as received. 3-Bromo-2,5-dimethylthiophene was prepared according to a
reported procedure for 3-bromo-4-deuterio-2,5-dimethylthiophene[16]
except water was used in place of deuterium oxide. 5,6-Dibromo-1,10-
phenanthroline was synthesized by bromination of 1,10-phenanthroline
with bromine in 20% fuming sulfuric acid according to the modification
of a procedure reported by Mlochowski et al.[17] Tetrakis(triphenylphos-
phine)palladium(0) as catalyst for Suzuki cross-coupling was synthesized
according to a literature procedure.[18] THF (Lab Scan, AR) was distilled
over sodium benzophenone ketyl before use. All other reagents were of
analytical grade and were used as received.
(14) [M+ꢁ3CO]; IR (KBr disk): n˜ =1890, 1912, 2025 cmꢁ1 (C O); ele-
ꢂ
mental analyses calcd (%) for C27H20ClN2O3ReS2 (706.2): C 45.91, H
2.83, N 3.96; found: C 45.86, H 2.41, N 3.94.
Physical measurements and instrumentation: Electronic absorption spec-
tra were recorded on a Hewlett-Packard 8452 A diode-array spectropho-
tometer. Steady-state emission and excitation spectra at room tempera-
ture and 77 K were recorded on a Spex Fluorolog-2 Model F 111 fluores-
cence spectrophotometer equipped with a Hamamatsu R928 photomulti-
plier tube detector. The 77 K low-temperature phosphorescence spectrum
of the open form of L1 was obtained with 309 nm excitation by a single-
shot 5-ns laser pulse. By employing a single laser pulse to obtain the
spectrum, one can avoid emission from any closed form that may be pro-
duced from any previous laser pulse. The emission was collected in back-
scattering geometry to increase the collection efficiency. The emission
was detected by a liquid-nitrogen-cooled CCD spectrometer. Excited-
state lifetimes of solution samples were measured by using a convention-
al laser system. The excitation source was the 355 nm output (third har-
Syntheses: All reactions were performed under strictly anaerobic and an-
hydrous conditions in an inert atmosphere of nitrogen by standard
Schlenktechniques.
2,5-Dimethylthien-3-ylboronic acid: n-Butyllithium (1.6m in hexane,
9.7 mL, 15.6 mmol) was slowly added to a stirred solution of anhydrous
3-bromo-2,5-dimethylthiophene (2.7 g, 14.1 mmol) in anhydrous THF
(40 mL) at ꢁ788C, and the reaction mixture was then stirred at this tem-
perature for 90 min.
A solution of tri-n-butyl borate (3.85 mL,
14.2 mmol) in THF (10 mL) was then added over 15 min. After stirring
for 5 h, aqueous HCl (2m) solution was added and the mixture was stir-
red at room temperature for 10 h. This was followed by extraction with
diethyl ether and the combined extracts were then washed with copious
water. The product was then obtained by extracting the ethereal layer
with aqueous sodium hydroxide solution (2m, 20 mL), followed by acidi-
fication with HCl (12m) to commence the precipitation of 2,5-dimethyl-
monic, 8 ns) of
a Spectra-Physics Quanta-Ray Q-switched GCR-150
pulsed Nd:YAG laser (10 Hz). Luminescence decay traces were recorded
on a Tektronix Model TDS 620A digital oscilloscope and the lifetime t
was determined by single exponential fitting of the luminescence decay
A
7.8 mmol, 87%. 1H NMR (300 MHz, [D6]DMSO, 258C, TMS): d=2.33
(s, 3H; 5-CH3), 2.50 (s, 3H; 2-CH3), 6.85 (s, 1H; thienyl 4-H), 7.72 ppm
(brs, 2H; B(OH)2).
traces with the model I(t)=Io exp
A
nescence intensity at time=t and time=0, respectively. Solution samples
for measurements of luminescence lifetime were degassed with at least
four freeze–pump–thaw cycles. 1H NMR spectra were recorded on a
Bruker DPX-400 (400 MHz) FT NMR spectrometer. Chemical shifts (d,
ppm) were reported relative to tetramethylsilane. All positive-ion FAB
and EI mass spectra were recorded on a Finnigan MAT95 mass spec-
trometer. Elemental analyses of the newly synthesized compounds were
performed on a Carlo Erba 1106 elemental analyzer at the Institute of
Chemistry in the Chinese Academy of Sciences in Beijing.
5,6-Bis(2,5-dimethylthien-3-yl)-1,10-phenanthroline (L1): The ligand was
synthesized by the bis-coupling reaction of 2,5-dimethylthien-3-yl boronic
acid with 5,6-dibromo-1,10-phenanthroline according to standard Suzuki
coupling procedure in a heterogeneous mixture of water and THF. Aque-
ous sodium carbonate solution (2m, 40 mL, 80 mmol) was added to a sol-
ution of 5,6-dibromo-1,10-phenanthroline (600 mg, 1.78 mmol), 2,5-dime-
thylthien-3-yl boronic acid (692 mg, 4.44 mmol), and tetrakis(triphenyl-
phosphine)palladium(0) (200 mg, 0.17 mmol) in THF (50 mL). The re-
sulting heterogeneous mixture was vigorously stirred and heated under
reflux, and the course of reaction monitored by TLC. It was then extract-
ed with diethyl ether (3”50 mL), and the combined extracts were dried
over anhydrous magnesium sulfate. After filtration and removal of the
solvent under reduced pressure, the residue was purified by column chro-
matography on silica gel (70–230 mesh) with chloroform as eluent. Fur-
ther purification was achieved by recrystallization from chloroform/etha-
nol. Yield: 312 mg, 0.78 mmol, 44%. 1H NMR (400 MHz, C6D6, 258C,
TMS): d=1.79 (s, 3H; 5-CH3 of antiparallel conformation), 1.95 (s, 3H;
5-CH3 of parallel conformation), 2.12 (s, 3H; 2-CH3 of antiparallel con-
The transient absorption measurements were performed with a Ti:Sap-
phire regenerative amplified source operated at 1 mJ/pulse, 150 fs, 1 kHz,
and 800 nm that has been described in detail elsewhere.[20] The pump
pulses were obtained at 400 nm by frequency doubling of the regenera-
tive amplifier fundamental, providing 2 mJ after attenuation. A white-
light continuum, used for the probe, was generated by focusing 1–2 mJ of
the 800 nm light onto a rotating CaF2 crystal plate. The white light con-
tinuum was separated into two beams by a fused-silica Al beam splitter.
One beam, used as the probe, crossed the pump in the sample (1 mm
thickness), and the other, which passed through an unpumped spot in the
sample, was used as reference to monitor the intensity and spectral char-
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Photochromic Reactions of Rhenium(i) Complexes
FULL PAPER
acteristics of the white light. The probe and reference beams were colli-
mated and then focused into the sample. The beams were then focused
into a monochromator and detected separately in different stripes by a
CCD. The signals from the CCD were downloaded to a PC and analyzed
by comparing the spectrum with the pump beam blocked and without
the pump beam being blocked to obtain the absorbance difference spec-
trum. The reference spectrum permits correction for variation over time
of the white-light fluctuations. The pump and probe spot sizes at the
sample were about 200 and 100 mm, respectively. The instrument re-
sponse function was about 200 fs.
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For the Kerr gated time-resolved emission measurements, the 300 nm ex-
citation pulses were generated by mixing of the 800 nm pulses and the
480 nm output from a home-built OPA system pumped by the 400 nm
laser pulses.[20] The excitation pulses (about 1 mJ) were focused (about
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of about 1.5 mm was circulated for the measurements. To keep the closed
form of 1 at a very low concentration in the overall sample solution, an
unfocused 527 nm laser beam constantly irradiated the glass sample res-
ervoir to convert the closed form of 1 backto the open form of 1.
Acknowledgements
We acknowledge support from the University Development Fund and
the Faculty Development Fund of The University of Hong Kong, the
URC Seed Funding for Strategic Research Theme on Organic Optoelec-
tronics, the HKU Foundation for Educational Development and Re-
search Limited, and the RGC Central Allocation (HKU 1/01C), and
C.-C.K. acknowledges the receipt of a University Postdoctoral Fellowship
and W.-M.K. the receipt of a Research Assistant Professorship. The work
has been supported by a grant from the Research Grants Council of the
Hong Kong SAR, China (Project No. HKU 7050/04P).
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Received: October 24, 2005
Revised: April 4, 2006
Published online: May 24, 2006
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