2,5-Bis(p-R-arylethynyl)rhodacyclopentadienes Show Intense Fluorescence: Denying the Presence of a Heavy Atom

Article abstract of DOI:10.1002/anie.200905697

Heavy-metal light show: Photophysical studies show unprecedented high fluorescence quantum yields (Φ_f up to 69%, τ_f' 1-3 ns) and unexpectedly slow intersystem crossing for a series of rhodium complexes. This new class of compounds challenges our understanding of the behavior of excited electronic states and the role of the heavy atom in intersystem crossing processes. Mes = mesityl, THF = tetrahydrofuran, Tol = toluene. (Figure Presented).

Full text of DOI:10.1002/anie.200905697

Angewandte
Chemie
DOI: 10.1002/anie.200905697
Photophysics
2,5-Bis(p-R-arylethynyl)rhodacyclopentadienes Show Intense
Fluorescence: Denying the Presence of a Heavy Atom**
Andreas Steffen, Meng Guan Tay, Andrei S. Batsanov, Judith A. K. Howard, Andrew Beeby,*
Khuong Q. Vuong, Xue-Zhong Sun, Michael W. George,* and Todd B. Marder*
The photophysics and photochemistry of transition-metal
compounds are of great interest, particularly since such
materials have been exploited for a wide range of applications
including photocatalysis, photosynthesis and photosynthetic
model compounds, artificial light-harvesting antenna systems
for solar energy conversion, sensing and imaging, supra-
molecular photochemically driven machines, multiphoton-
absorption materials, probes for monitoring biological pro-
cesses, and the fabrication of high-performance organic light-
emitting diodes (OLEDs).[1] A full understanding of the
excited-state behavior of organometallic compounds is crucial
for the design of new materials for all of these applications.
An attractive feature of this class of compounds is that subtle
changes in the ligand environment or metal can be used to
tune the properties, thereby allowing the control required for
a particular application. Diimine complexes of RuII, ReI, and
PtII have been extensively studied.[2] Recently there has also
been considerable interest in the photophysics of C^N
cyclometalated complexes, particularly IrIII,[1,3] and both the
diimine and C^N cyclometalated complexes can exhibit
highly emissive triplet excited states.
Mononuclear metal complexes usually show very rapid
conversion from singlet into triplet excited states, which is
attributed to the “heavy-atom effect”. The heavy-atom effect
is the promotion of intersystem crossing (ISC) processes by
the spin-orbit coupling (SOC) of the metal atom. These
effects can begin to be observed with elements as light as
sulphur (z = 16).[4] For example, the formation of the 3MLCT
(MLCT= metal-to-ligand charge transfer) excited state of
[Ru(bpy)3]2+ (bpy = 2,2’-bipyridine) occurs on a timescale of
less than 20 fs.[5] The precise factors governing the singlet-to-
triplet excited state interconversion have recently been
questioned by observations which have shown that formation
of the 3MLCT state of the first-row complex [Fe(bpy)3]2+
occurs in less than 20 fs, whereas the second-row complexes
[Re(X)(CO)3(bpy)]+ (X = Cl, Br, I) show a much slower
interconversion (ca. 100 fs).[6] Furthermore, the order was
found to be Cl (85 fs) < Br (128 fs) < I (152 fs), which is
contrary to that predicted by the simplistic consideration of
the effect of the heavy atom. Tetrahedral [Pt(binap)2]
(binap = 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl)
and
[Cu{bis(diimine)}]+ complexes have been shown to have
1
unusually long-lived MLCT states of t = 3 ps and t = 15 ps,
respectively, attributed to a distortion towards a square-
planar geometry which reduces the mixing of the 1,3MLCT
states.[7]
Our long-standing interest in rhodium–acetylide com-
pounds[8] and luminescent bis(arylethynyl)arenes[9] led us to
the development of a high-yielding, one-pot synthesis of a 2,5-
bis(phenylethynyl)rhodacyclopentadiene, which we reported
to be luminescent.[10] Our subsequent investigations, reported
herein, indicate that this new class of luminescent rhodium
complexes shows unprecedented excited-state behavior. Our
luminescence spectroscopic studies are supported by pico-
second time-resolved IR (TRIR) vibrational spectra of the
ground and excited states as a means by which to obtain
accurate kinetic data on the processes involved. Herein we
demonstrate that despite the presence of the second-row
transition metal the compounds show remarkable photo-
physical properties: specifically, long-lived, highly emissive
singlet excited states. This new class of material challenges
our understanding of the behavior of excited electronic states
and the role of the heavy atom in intersystem-crossing
processes.
The reaction of [Rh(CꢀCSiMe3)(PMe3)3] (1) with the
bis(diyne)s 2a–d leads to the formation of the metallacyclic
complexes 3a–d (Scheme 1, top), which have been unambig-
uously characterized by 1H and 31P NMR and IR spectrosco-
py, mass spectrometry, elemental analysis, and by single-
crystal X-ray diffraction studies on 3a–c (Figure 1). In situ
NMR spectroscopic studies show that the reactions occur
quantitatively, and the products have been isolated in 23–
82% yield after several recrystallizations, to ensure high
purity for photophysical studies. The photophysical data are
summarized in Table 1 and Table 2.
[*] Dr. A. Steffen, M. G. Tay, Dr. A. S. Batsanov, Prof. Dr. J. A. K. Howard,
Dr. A. Beeby, Prof. Dr. T. B. Marder
Department of Chemistry, Durham University
South Road, Durham, DH1 3LE (UK)
Fax: (+44)191-384-4737
E-mail: andrew.beeby@durham.ac.uk
todd.marder@durham.ac.uk
Homepage: http://www.dur.ac.uk/chemistry/staff/profile/?id=194
Dr. K. Q. Vuong, Dr. X.-Z. Sun, Prof. Dr. M. W. George
School of Chemistry, University of Nottingham
University Park, Nottingham, NG7 2RD (UK)
Fax: (+44)115-951-3555
E-mail: mike.george@nottingham.ac.uk
[**] A.S. thanks the DAAD and the EU (Marie-Curie) for postdoctoral
fellowships. M.W.G. gratefully acknowledges receipt of a Wolfson
Merit Award. M.G.T. thanks the Universiti Malaysia Sarawak for a
Ph.D. scholarship.
Compounds 3a–d absorb light with extinction coefficients
of 15000–44000 Lmolꢁ1 cmꢁ1 and emit in the visible region
(Figure 2). A vibrational progression typical of aromatic
stretching modes (ca. 1360 cmꢁ1) is observable in the absorp-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905697.
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Communications
donors and acceptors can be rationalized
by the fact that acceptors have a greater
influence on the lowest unoccupied molec-
ular orbital (LUMO) than on the highest
unoccupied molecular orbital (HOMO),
and donors show an opposite effect, there-
fore both decrease the HOMO–LUMO
gap as previously reported for related 2,5-
bis(arylethynyl)thiophenes.[11] A solvato-
chromic effect has been found for 3c and
3d. Comparison of the emission data from
n-hexane to MeCN for 3d shows an 85 nm
shift of the emission maximum and a
1600 cmꢁ1 increase in the Stokes shift
(Table 1), indicating charge-transfer char-
acter in the emissive excited state.
The results from luminescence meas-
urements suggest that the emission occurs
from the excited singlet state S1, since small
Scheme 1. Synthesis of 2,5-bis(arylethynyl)rhodacyclopentadienes.
Table 1: Emission and absorption properties of 3a–d and 5a–c.
Solvent
lmax (abs)
[nm]
e
lmax (em) Stokes shift
[nm]
[molꢁ1 cmꢁ1 dm3]
[cmꢁ1
]
3a toluene
3b toluene
3c n-hexane
3c toluene
3c THF
3c MeCN
3d n-hexane
3d toluene
3d THF
3d MeCN
5a toluene
5b toluene
5c n-hexane
5c toluene
5c THF
456
467
487
497
497
497
518
529
534
535
476
487
508
518
519
518
22000
41000
501
518
538
560
580
588
581
606
631
666
526
541
566
586
599
611
2000
2100
1900
2300
2900
3100
2100
2400
2900
3700
2000
2000
2000
2200
2600
2900
44000
39000
43000
24000
21000
Figure 1. Molecular structure of 3a. Hydrogen atoms omitted for
15000
clarity, thermal ellipsoids drawn at 50% probability.[17]
5c MeCN
Stokes shifts (ca. 2000 cmꢁ1, Figure 3) are observed, which is
unexpected for coordination and organometallic compounds.
The rhodacyclopentadienes 3a–d exhibit anomalously long
fluorescence lifetimes and unprecedented high fluorescence
quantum yields (tf = 1.2–3.0 ns, Ff = 0.33–0.69), leading us to
conclude that the rates of radiative decay and ISC are on the
order of 108 sꢁ1. Neither the photoluminescence quantum
yield nor the emission lifetimes are influenced by the
presence of molecular oxygen.
The compounds act as potent sensitizers of singlet oxygen
(Table 2), and the 1O2 phosphorescence observed at l =
1270 nm showed a decay time characteristic of the solvent
in which the measurement was conducted. Partial degassing
of the solution led to a reduction in signal intensity, but the
temporal profile remained unchanged. Resaturation with O2
regenerated the original signal decay profile and intensity.
These observations imply that the lifetime of the sensitizing
state, which we assign as the T1 state, is on the order of the
risetime of the detection system, that is, t ꢂ 400 ns. However,
Figure 2. Photoluminescence spectra of 2,5-bis(p-R-arylethynyl)rhoda-
cyclopentadienes [R=H (3a), SMe (3b), CO2Me (3c), BMes2 (3d)]
recorded in degassed toluene at 298 K upon excitation at the respec-
tive lmax values.
tion and emission spectra. Both donor and acceptor substitu-
ents at the para position of the phenyl rings of the ligand lead
to a bathochromic shift, the influence of the acceptor
substituents exceeding that of the donors. The effect of
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than sulphur (380 cmꢁ1), the rhodacycles exhibit higher
fluorescence quantum yields and longer fluorescence life-
times than the thiophenes. The emission solvatochromism for
3c, d and 5c (Table 1) indicates significant charge transfer in
1
the excited state, which we propose is a mixed ML/ILCT
state (ILCT= intraligand charge transfer).
To probe the kinetics of the photophysical processes in
our compounds additionally, we measured time-resolved
infrared (TRIR) vibrational absorption spectra of 3a in
CH2Cl2 (Figure 4). The TRIR spectrum, obtained 11 ps after
excitation, clearly shows that the parent band (nCꢀ
=
C
2128 cmꢁ1) is bleached and a new absorption is produced at
2008 cmꢁ1.
Figure 3. Absorption and photoluminescence spectra of 3a recorded in
degassed toluene at 298 K.
Table 2: Photophysical data for 3a–d and 5a–c in degassed toluene at
298 K.
kf [108 sꢁ1
]
kD [108 sꢁ1
]
[a]
FPL
tf [ns]
t0 [ns]
FD
3a
3b
3c
3d
5a
0.33
0.34
0.69
0.69
0.07
1.2 (1.9[b])
1.8
3.6
5.3
4.3
3.8
0.65
0.40
0.26
2.75
1.89
2.30
5.42
2.22
0.87
3.0
2.6
1.0 (13%)
0.4 (87%)
1.1 (72%)
0.7 (28%)
2.5
0.32
0.19
0.20
0.09
1.52
1.05
0.64
1.84
5b
0.16
5c
0.46
5.4
0.80
[a] Quantum yield for O2 (1Dg) formation in O2 saturated solutions.
[b] Measured in degassed dichloromethane.
Figure 4. Selected picosecond TRIR spectra of 3a in CH2Cl2 under
argon at various time intervals after laser excitation at 400 nm.
no phosphorescence (between 400–1000 nm) could be
observed at room temperature or in an iso-pentane/Et2O/
EtOH glass at 77 K.
To examine the effect of modulating the metal participa-
This band is assigned to the S1 excited state which decays
at the same rate [t = 1.6 ꢃ 0.6 ns] as a new species is formed
with an IR band at 1941 cmꢁ1. The parent is also partially
reformed (ca. 35%) at a similar rate as the singlet band
decays. The partial recovery from the TRIR kinetics is
ꢁ ꢀ
tion in the frontier orbitals, we changed the C CSiMe3
ligand to N,N-diethyldithiocarbamate, as a strong s and
p donor should destabilize the filled d orbitals of Rh. The
quantitative reactions occurred as shown in Scheme 1
(bottom), representing an organometallic analogue of the
atom-economical click chemistry, which is well-established in
the organic synthesis of heterocyclic rings.[12] The dithiocar-
bamate rhodacyclopentadienes have been fully characterized
and a single crystal X-ray diffraction study was carried out for
5a. In general, compounds 5a–c show a similar behavior to
those of 3a–d upon optical excitation (Table 2).
1
consistent with the 65% ISC S1!T1 of the O2 sensitization
experiment. The lifetime of the S1 state obtained from the
TRIR measurements (Figure 5) is consistent with the results
from the luminescence measurements of 3a (Table 2). The
state associated with the band at 1941 cmꢁ1 decays to the
ground state with t = 55 ns in degassed solution and we
attribute this to be the T1 state. Of particular interest is the
exceptionally slow ISC rate of approximately kISC = 5 ꢀ
108 sꢁ1.
However, the emission is significantly red-shifted, but a
decrease in the fluorescence and 1O2 formation quantum
yields is observed, indicating that changing the ligand sphere
alters the photophysical properties of the compounds, there-
fore the rhodium center must be involved in the transitions.
Despite the presence of the rhodium atom, no phosphor-
escence is observed for 5a–c, even at 77 K.
1
The unexpected long-lived ML/ILCT state, the absence
of phosphorescence, and the unusually slow ISC of our 2,5-
bis(arylethynyl)rhodacyclopentadienes 3a–d and 5a–c having
rate constants in the range of 107–108 sꢁ1 are difficult to
explain given the fact that they contain a second row
transition-metal atom covalently bound to the organic
chromophore. Most organometallic RhIII complexes exhibit
no fluorescence, but phosphoresce in a glass at 77 K from
predominantly ligand-centered 3p!p* states with only small
3MLCT contributions; the lifetimes are generally between t =
It is informative to compare the rhodacyclopentadienes
3a–d and 5a–c with their “thiacyclopentadiene” analogues,
that is, 2,5-bis(arylethynyl)-substituted thiophenes (Ff = 0.2–
0.3, tf = 0.2–0.3 ns).[12] Counterintuitively, despite the much
higher spin-orbit coupling constant of rhodium (1200 cmꢁ1
)
Angew. Chem. Int. Ed. 2010, 49, 2349 –2353
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compounds with quantum yields between 10ꢁ5-10ꢁ4,[3] as well
as from PtII/terpyridine complexes (t = 1 ns, Ff = 0.001).[7,16]
1
An ML/ILCT excited-state structural distortion such as
that found to result in weak SOC in the tetrahedral complexes
[Pt(binap)2] and [Cu(diimine)2]+,[7] vide supra, is unlikely to
be responsible for the observed properties of octahedral
complexes 3a–d and 5a–c. Also, the emission spectrum at
77 K and the TRIR data prove that no delayed fluorescence
occurs from our rhodacycles.
We have reported for the first time intense fluorescence
from an octahedral organometallic compound, originating
from a long-lived S1 state. Deactivation of this state by
intersystem crossing is relatively inefficient. The rhodacyclo-
pentadienes exhibit unexpected slow ISC rates on the nano-
7
8
ꢁ1
ꢁ
second timescale (kISC = 10 –10 s ) despite a M C bound
ligand and a SOC of Rh (1200 cmꢁ1) similar to that of Ru
(1259 cmꢁ1) in [Ru(bpy)3]2+, for which ultrafast ISC of less
than 20 fs has been measured. These results raise questions
and contradict the traditional picture of the “cascade”
behavior of optical excited states of organotransition-metal
compounds, decaying by steps well-separated in time and
energy, and according to which ISC should be orders of
magnitude faster than the radiative decay S1!S0.
Received: October 9, 2009
Revised: November 25, 2009
Published online: March 4, 2010
Keywords: intersystem crossing · luminescence · metallacycles ·
.
photophysics · rhodium
Figure 5. Kinetic traces showing: (a) the decay of the singlet excited
state at 2008 cmꢁ1 (&) and the growth of the triplet excited state at
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*
*
0.04–48 ms.[13] Photoluminescence with microseccond life-
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1
4d/5d transition-metal centers, as the MLCT states are too
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1
isocyanohexane) undergo excitation to a A2u (ds*–ps)
1,3
state.[15] The
A
2u
states are only weakly coupled and
emission with nanosecond lifetimes even at room temper-
ature is observed with Ff = 0.08, as well as the expected
phosphorescence (Fp = 0.3, FISC > 0.8). As mentioned above,
1
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for the prompt fluorescence observed in octahedral [M-
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I) complexes with extremely low quantum yields (< 10ꢁ6).[5,6]
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1MLCT states (several picoseconds) of tetrahedral Pt0 and CuI
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