An experimental and theoretical approach to the photophysical properties of some Rh and Ir complexes incorporating the dipyrromethene ligand

Article abstract of DOI:10.1002/ejic.201201427

Rh and Ir complexes that contain meso-(p-methoxyphenyl)dipyrromethene (dipy) and phenylpyridine (ppy) ligands {i.e., [Rh(dipy)₃], [Rh(ppy)₂(dipy)] and [Ir(ppy)₂(dipy)]} have been prepared and their electrochemical and luminescence behaviour are discussed and compared to results of theoretical calculations. The oxidation and reduction potentials, in agreement with the time-dependent (TD)-DFT calculations, indicate clearly that the HOMO-LUMO transitions are governed by the dipy ligand, which controls also the triplet excited-state emission. However, in luminescence, the two Rh complexes exhibit, in addition to a dipy-centred triplet emission, luminescence from the singlet excited state, which predominates at room temperature, due to a less important influence of the heavy-metal effect in Rh than in Ir complexes. Although the TD-DFT results are in very good agreement with the experimental emission spectra from the dipy-centred triplet excited state, this is not the case for the absorption spectra of the complexes and free dipy ligand. Copyright

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DOI:10.1002/ejic.201201427
An Experimental and Theoretical Approach to the
Photophysical Properties of Some Rh and Ir Complexes
Incorporating the Dipyrromethene Ligand
Diane Ramlot,[a] Mateusz Rebarz,[a] Leen Volker,[b]
Margriet Ovaere,[c] David Beljonne,[d] Wim Dehaen,[b]
Luc Van Meervelt,[c] Cécile Moucheron,[a] and
Andrée Kirsch-De Mesmaeker*[a]
Keywords: Rhodium / Iridium / N ligands / Luminescence / Photophysics / Density functional calculations
Rh and Ir complexes that contain meso-(p-methoxyphenyl)-
dipyrromethene (dipy) and phenylpyridine (ppy) ligands
{i.e., [Rh(dipy)3], [Rh(ppy)2(dipy)] and [Ir(ppy)2(dipy)]} have
been prepared and their electrochemical and luminescence
behaviour are discussed and compared to results of theoreti-
cal calculations. The oxidation and reduction potentials, in
agreement with the time-dependent (TD)-DFT calculations,
indicate clearly that the HOMO–LUMO transitions are gov-
erned by the dipy ligand, which controls also the triplet ex-
cited-state emission. However, in luminescence, the two Rh
complexes exhibit, in addition to a dipy-centred triplet emis-
sion, luminescence from the singlet excited state, which pre-
dominates at room temperature, due to a less important influ-
ence of the heavy-metal effect in Rh than in Ir complexes.
Although the TD-DFT results are in very good agreement
with the experimental emission spectra from the dipy-
centred triplet excited state, this is not the case for the ab-
sorption spectra of the complexes and free dipy ligand.
transition-metal complexes,[3] it was shown that different
types of excited states such as MLCT (metal-to-ligand
charge-transfer), LC (ligand-centred), or LLCT (ligand-to-
ligand charge-transfer) states can contribute to the emis-
sion, which sometimes consists of a mixture of different
types of luminescence because the corresponding excited
states are quite close in energy. Their relative energy de-
pends also on the solvent and temperature so that the origin
of the luminescence can change from one medium to the
other.[4] For these Ir or Rh compounds, the ligands were in
most of the cases 2,2Ј-bipyridine (bpy) or 2,2Ј:6Ј,2ЈЈ-terpyri-
dine (terpy) or other polyazaaromatic ligands (such as TAP
= 1,4,5,8-tetraazaphenanthrene) and quite often combined
with phenylpyridine for producing carbometalation of the
metal ion.[5]
Introduction
During the past decade, an increasing amount of studies
have been devoted to the syntheses and emission properties
of Ir complexes. The motivation and interest for these com-
pounds originate from their tuneable and intense lumines-
cence and electroluminescence properties that could lead to
potential applications in light-emitting devices.[1] Moreover,
Ir and Rh complexes have also been examined as attractive
luminescent probes or sensors for different biomolecules
such as DNA and peptides.[2] Although these complexes
have been the subject of abundant literature, the nature of
their emitting excited states has not always been quite clear
because the photophysics is generally much more compli-
cated than that of the RuII complexes. In the pioneering
works that explored the first emission properties of these
A quite different type of ligand has also been used, such
as dipyrrin compounds, also called pyrromethene or dipyr-
romethene (see, for example, Figure 1), which has some
similarities with the tetrapyrrole ring of porphyrins but is
constituted by half of a ring. Several derivatives of these
dipyrromethene compounds have been tested as ligands for
the complexation of different metal ions and more particu-
larly with transition-metal ions.[6] However, in those origi-
nal studies, no attempt was made to examine in detail the
emission properties of these complexes. The dipyrrome-
thene ligands have also been used for their complexation
with BF2, thus generating the well-known boron–dipyrro-
methene (BODIPY) derivatives that were studied, in con-
[a] Organic Chemistry and Photochemistry, Université Libre de
Bruxelles, CP 160/ 08,
50 Avenue F. D. Roosevelt, 1050 Brussels, Belgium
E-mail: akirsch@ulb.ac.be
Homepage: http://cop.ulb.ac.be
[b] Molecular Design & Synthesis, Department of Chemistry,
University of Leuven (KU Leuven),
Celestijnenlaan 200F, 3001 Heverlee, Belgium
[c] Molecular and Structural Biology, KU Leuven,
Celestijnenlaan 200F, 3001 Heverlee, Belgium
[d] Laboratory of Chemistry of Novel Materials, University of
Mons,
Place du Parc 20, 7000 Mons, Belgium
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201201427.
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trast to the previous case, for their unique and important geometry for Rh due to the dipyrromethene ligand. As ex-
quantum yields of emission[7] and their phosphorescence pected, an N-trans orientation of the two phenylpyridine
detected in multicomponent species.[7a] Therefore the moieties is observed around the metal core [N2–Rh1–N2i
BODIPY compounds have been extensively developed as 173.11(12)°, symmetry code (i): 1 – x, y, ½ – z]. The bond
photosensors of different media.[8] More recently, dipyrro- length between the metal centre and Ndipy is the longest
methene ligands have been complexed to IrIII ions[9] to form [2.134(4) Å] (Table 1), whereas the shortest is observed be-
not only mononuclear complexes but also polymetallic spe- tween the metal and the carbon of the phenylpyridine li-
cies[9a] with the aim of evaluating and understanding their gand [2.001(4) Å] (Table 1). The dihedral angle between the
resulting emission properties.
best planes through both six-membered rings in phenylpyr-
idine is 6.2(2)° and between the two pyrrole rings is 7.8(2)°.
The two phenylpyridine rings make an angle of 82.4(2)°.
Figure 1. The nonprotonated dipyrromethene (dipyH) of this work,
the meso-(p-methoxyphenyl)dipyrromethene [or 5-(p-meth-
oxyphenyl)-4,5-dipyrrin] with its two tautomers.
We are interested in the construction of polynuclear or
polymeric materials with dipyrromethene derivatives as li-
gands and with RuII, RhIII and IrIII as metal ions. The
interest in this type of ligand lies in their possible symmetri-
cal derivatization in the meso position (instead of a p-meth-
oxyphenyl group), which allows the design of symmetrical
bridging ligands. Along our way, we focused on the emis-
sion properties of the novel mononuclear dipyrromethene
components and more particularly we studied their spectro-
scopic behaviour as a function of the nature of the transi-
tion-metal ion with the same dipyrromethene derivative. In
the meantime, interesting studies on the luminescence prop-
erties of other Ir complexes have been published by De Cola
et al.[9a] and Thompson et al.[9b] In the present study, we
report on a joint experimental–theoretical investigation of
a comparison between the redox, absorption and emission
properties of the complexes that contain RhIII and IrIII as
metal ions and either one or three meso-(p-methoxyphenyl)-
dipyrromethene (dipy) ligands (Figure 1).
Figure 2. View of the crystal structure of [Rh(ppy)2(dipy)] (disorder
of p-methoxyphenyl substituent omitted for clarity).
Table 1. Bond lengths and angles for [Rh(ppy)2(dipy)] from X-ray
diffraction data and DFT calculations (see text).
Theoretical
Experimental
Bond length [Å]
Rh–N (dipy)
Rh–N (ppy)
Rh–C (ppy)
2.182
2.075
2.032
2.134(4)
2.043(4)
2.001(4)
Bond angles [°]
N–Rh–N (dipy)
N–Rh–C (ppy)
85.86
80.47
87.28(16)
81.04(16)
Results and Discussion
Before studying the electrochemical and photophysical
properties of the complexes, we first checked their stability
in different solvents. Therefore their absorption and emis-
Synthesis and Characterization
The [Rh(dipy)3] homoleptic complex was synthesized by sion spectra were measured as a function of time after dis-
mixing dipyrromethene ligand (3 equiv) with RhCl3·3H2O solution in CHCl3 (Figures S1–S6 in the Supporting Infor-
in the presence of NEt3, thus leading to a yield of 18%. mation). It is clear that a partial or even a total dechelation
The two heteroleptic complexes [Rh(ppy)2(dipy)] (ppy = of a dipyrromethene ligand occurs in this solvent; such de-
phenylpyridine) and [Ir(ppy)2(dipy)] were prepared from a gradation does not take place in MeCN or EtOH/MeOH
dichloro-bridged precursor, [Rh(ppy)2]2Cl2 and [{Ir- (Figures S7–S18 in the Supporting Information). The reac-
(ppy)2}2Cl2], respectively, which were mixed with dipyrro- tion that takes place in CHCl3 could be due to traces of
methene ligand (2 equiv.) in the presence of a base. acid, which protonates the negatively charged dipy– in the
[Rh(ppy)2(dipy)] and [Ir(ppy)2(dipy)] were obtained with complex at the level of the nitrogen atom, followed by a
yields of 68 and 61%, respectively. The three complexes complete loss of ligand. Therefore, to avoid destruction of
1
were characterized by H NMR spectroscopy and electro- the complex in chlorinated solvent, drops of triethylamine
spray mass spectrometry. X-ray diffraction of [Rh(ppy)2- 10–2 molL–1 in CHCl3 were added to the solution of each
(dipy)] (Figure 2, Table 7) shows a distorted-octahedral complex in chloroform.
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Electrochemistry
Rh and Ir complexes with 2,2Ј-bipyridine (bpy) or 1,4,5,8-
tetraazaphenanthrene (TAP) ligands (Table 2), the oxi-
dation of which is metal-centred. These considerations led
to the conclusion that the oxidations of the dipy complexes
are ligand-centred. In reduction, the waves for the free li-
gand and the three complexes are also irreversible (Fig-
ures S23–S26 in the Supporting Information) and do not
appear at a sufficiently negative potential to be attributed
to a ppy ligand {see, for example, [Ir(ppy)2(bpy)]+ at
–2.06 V at –54 °C}. Therefore, the reduction waves for the
two Rh and the Ir complexes should correspond to those
of the dipy ligand.
The electrochemical properties of the nonprotonated di-
pyH ligand, the protonated dipyH2 species and the three
+
complexes were examined in dry deoxygenated benzonitrile
by cyclic voltammetry. The potential values are collected in
Table 2 with those of some reference compounds for com-
parison. In oxidation, the free ligand dipyH and the three
complexes show irreversible waves (Figures S19–S22 in the
Supporting Information). The potential values of the first
oxidation peaks are not very different and are similar to
that of an Ir complex studied in the literature[9b] {[Ir(ppy)2-
(dipyЈЈ)] in which dipyЈЈ
= 5-mesityldipyrromethene;
Table 2}. These behaviours stand in contrast to those of the
Absorption
+
Table 2. Redox potentials for dipyH, dipyH2 and the three com-
The absorption spectra of the free nonprotonated dipyH
ligand and the protonated form dipyH2 were recorded in
+
plexes in benzonitrile, measured against SCE with ferrocene as in-
ternal reference and a sweep rate of 100 mVs–1.[a]
chloroform in the presence of NEt3 and TFA, respectively
(Figure 3, a and b). The spectrum of the nonprotonated
dipyH has a maximum of absorption at 438 nm with a mo-
lar absorption coefficient of approximately 28000 m–1 cm–1.
E versus SCE [V]
Ox1
Ox2
Red1
Red2
dipyH
0.91 (ir)
1.94 (ir)
1.15 (ir) (L)
0.94 (ir) (L)
0.94 (ir) (L)
1.21 (ir) –1.40 (ir)
+
(dipyH)H+
[Rh(dipy)3]
[Rh(ppy)2(dipy)]
[Ir(ppy)2(dipy)]
The absorption band of the protonated dipyH2 is charac-
1.30 (ir) –1.35 (ir)
–1.60 (ir)
–1.60 (ir)
terized by a maximum at 468 nm with a higher molar ex-
tinction coefficient (ε ≈ 47000 m–1 cm–1). Such a difference
of absorption in basic and acid solution likely reflects the
existence of a tautomeric equilibrium for the nonprotonated
ligand (Figure 1), which is absent for the protonated
form[6a] (see below). Moreover, it is generally accepted that
a protonated ligand prefigures the corresponding ligand
complexed to a metal, at least to some extent. A batho-
–1.55 (ir)
–1.49 (r)
–0.95 (r)
[Rh(ppy)2(bpy)]+[5a] 1.42 (ir) (M)
[Rh(ppy)2(TAP)]+[5a] 1.65 (ir) (M)
[Ir(ppy)2(bpy)]+[5a]
1.22[b] (r) (M)
0.96[d] (ir) (L)
–1.41[c] (r) –2.06[c] (r)
–1.46[d] (r)
[Ir(ppy)2(dipyЈЈ)][9b]
[a] The redox potentials of the reference complexes were measured
with Pt electrode in CH3CN that contained 0.1 m [But4N]+[PF6]–.
L = ligand-centred process, M = metal-centred process; (r) = re-
versible, (ir) = irreversible. [b] In CH3CN at –40 °C, 1 Vs–1. [c] In
DMF at –54 °C, 1 Vs–1. [d] In DMF, measured against Fc+/Fc (for
oxidation, +0.51 V versus Fc+/Fc, thus +0.96 versus SCE, and for
reduction, –1.91 V versus Fc+/Fc, thus –1.46 versus SCE).
+
chromic shift as observed from dipyH to dipyH2 indeed
appears from the free to the complexed ligand (Tables 3 and
5). Interestingly, the splitting of the visible absorption band
into two maxima is clearly observed in the case of
Figure 3. Comparison of the absorption spectra with the calculated transition energies. (a) For the free dipyH ligand, experimentally in
CHCl3, λmax = 438 nm, ε = 28100 m–1 cm–1, and by calculation when one tautomer is considered (TD-DFT) and when the hydrogen atom
is localized between the two pyrrole rings (TD-DFTЈ); (b) for dipyH2+, experimentally in CHCl3, λmax = 468 nm, ε = 46700 m–1 cm–1;
and (c) for [Rh(ppy)2(dipy)] (see also Table 2).
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Table 3. Absorption and emission data at 298 and 77 K for [Rh(dipy)3], [Rh(ppy)2(dipy)] and [Ir(ppy)2(dipy)], with the frequencies
corresponding to the vibrational structure of the emission spectra at 77 K and vibrational energy differences (Δυ).
Absorption
CHCl3 + NEt3
Emission at 298 K
CHCl3+NEt3 CH3CN
Emission at 77 K
EtOH/MeOH (4:1)
CH3CN
λ [nm]
ε [m–1 cm–1]
λ [nm]
λ [nm]
λ [nm]
λ [nm]
υ [cm–1]
Δυ [cm–1]
[Rh(dipy)3]
464
499
30700
22000
459
499
532
683
744
528
682
738
530
678
740
817
515
668
728
802
–
–
–
14750
13513
12240
–
14970
13736
12469
–
1237
1273
[Rh(ppy)2(dipy)]
[Ir(ppy)2(dipy)]
488
482
49000
35300
484
478
514
670
730
515
669
730
–
1234
1267
–
–
–
680
738
680
740
674
738
812
14837
13587
12315
1250
1272
[Rh(dipy)3] (Table 3 and Figures S7 and S13 in the Support- triplet emission, as also observed by Thompson and co-
ing Information), which would be in agreement with the workers for [Ir(ppy)2(dipyЈЈ)].[9b]
predictions from the theory of exciton coupling as proposed
Similar luminescence spectra have been recently reported
by McLean et al. for coordination chemistry with dipy de- for some PtII, PdII and ReI dipyrrinato complexes.[11] More-
rivatives.[10]
over, in line with a ligand-centred structured emission for
the Ir and Rh complexes, no important shift in maximum
wavelength is observed by changing the polarity of the sol-
vent or by going from fluid solutions to a rigid matrix at
77 K.
Emission
The emission spectra are shown in Figure 4 at 298 and
The emission lifetimes (Table 4) have also been deter-
77 K for the three complexes, and the maxima values are mined at 298 and 77 K for the Rh and Ir complexes. For
gathered in Table 3. Although the emissions of the three the Rh complexes at their maximum of luminescence at ap-
complexes are quite similar at 77 K (Figure 4, b, d and f), proximately 520 nm at 298 K, these lifetimes correspond to
the luminescence spectrum of the two Rh complexes at a few nanoseconds, which is in agreement with a singlet
298 K (Figure 4, a and c) exhibits an additional hypsoch- excited-state emitter. In contrast, at longer wavelengths
romic band centred around 520 nm, which is much more from 660 nm, the excited-state lifetimes for both Rh com-
intense than the weak emission at wavelengths Ͼ650 nm. In plexes are much longer (2 ms at 77 K), which is in agree-
contrast, the emission at wavelengths Ͼ650 nm is the only ment with a triplet excited-state assignment.
luminescence that appears for the Ir complex (Figure 4, e).
As mentioned above, the luminescence of [Ir(ppy)2(dipy)]
The supplementary Rh emission around 520 nm is not at- exhibits only the structured triplet emission centred on the
tributable to some remaining free dipyH ligand. Indeed this dipy at 680 nm, at room temperature and at 77 K, with life-
latter emits at 490–500 nm at 298 and 77 K, thus not at the times that are shorter than those for the two Rh complexes
same λmax of emission as that of the complexes that origi- at both temperatures. This could be attributed to the fact
nate from pure crystals analyzed by X-ray diffraction and that the Ir is a heavier atom than the Rh, and so the rate
dissolved in conditions under which there is no dechelation. of decay from the triplet excited state to the ground singlet
This Rh complex emission around 520 nm could in contrast state is faster for the Ir complex than for the Rh complex.
correspond to Rh fluorescence, which would be in agree- The same heavy-atom effect should be observed on the rate
ment with the absorption maxima around 500 and 488 nm of intersystem crossing from the singlet excited state to the
for [Rh(dipy)3] and [Rh(ppy)2(dipy)], respectively (Table 3). triplet excited state. This is indeed observed and the spin–
At 77 K this fluorescence can no longer be detected. This orbit coupling constants have been reported as ξIr
=
is probably due to the fact that the emission around 670 nm 3909 cm–1 and ξRh = 1259 cm–1.[12] For Rh, less than 100%
is much more likely at lower temperatures, so that the of the excited states cross to the triplet state since some
520 nm fluorescence is no longer observable at the lower fluorescence is detected. In contrast, intersystem crossing
detection sensitivity that had to be chosen for this increased from the singlet to the triplet manifold is quantitative in the
670 nm emission. For the Rh complexes, the results thus Ir complex. With regard to the triplet emission, it is also
indicate that two excited states contribute to light emission, interesting to note that for the two Rh and the Ir complexes,
one from the singlet state and one from the triplet state, each compound presents the same vibrational energy pro-
with the latter gaining in intensity from room temperature gression of approximately 1250 cm–1 (measured between the
to 77 K. The triplet emission is vibrationally structured and three maxima of the phosphorescence band; Table 3), which
features a pattern similar to the emission signal from the Ir is typical of the dipy ligand and is also similar to the vi-
complex (Table 3). This emission is typical of dipy-centred brational energy measured for some metalloporphyrins.[13]
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Figure 4. Emission spectra of [Rh(dipy)3], [Rh(ppy)2(dipy)] and [Ir(ppy)2(dipy)] at 298 K (a, c, e) in chloroform in the presence of NEt3
and (b, d, f) 77 K in EtOH/MeOH (4:1).
Table 4. Emission wavelengths and luminescence lifetimes at 298
and 77 K.
TD-DFT Calculations
298 K
CH3CN (Ar)
77 K
EtOH/MeOH (4:1)
Free Dipyrromethene Ligand
Since the experimental results suggest that the optical
properties of the Rh and Ir complexes are entirely governed
by the dipyrromethene ligand, we decided to start the theo-
retical study by DFT geometry optimization and TD-DFT
excited-state calculations for the free dipyH ligand and its
protonated form dipyH2+. For the free neutral ligand, in
the form of one tautomer (thus with one hydrogen atom
localized on one of the pyrrole rings), the optimized geome-
try exhibits a completely planar dipyrrin moiety with a
twisted meso-substituted phenyl moiety (Figure S27 in the
Supporting Information). For this geometry the calculated
HOMO–LUMO energy gap is equal to 3.23 eV (Table 5),
which is significantly higher than the energy value that cor-
responds to the experimental maximum of the dipyH ab-
λ [nm]
τ [ns][a]
λ [nm]
τ [μs][b]
[Rh(dipy)3]
528
570
680
515
560
665
6
6
490
5
5
530
700
0.007
2000
[c]
[Rh(ppy)2(dipy)]
515
1100
660
722
2000
2000
11
[Ir(ppy)2(dipy)]
680
740
135[b]
139[b]
665
727
11
12.9[d]
[Ir(ppy)2(dipyЈЈ)]
677[d]
6300[d]
665[d]
[a] Conducted with single-photon counting equipment, excitation
by laser diode. [b] Excitation by a pulsed laser and emission re-
corded with an oscilloscope. [c] Emission too weak for the determi-
nation of the excited-state lifetime. [d] In 2-MeTHF under N2 at-
mosphere.[9b]
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sorption (2.83 eV). Consequently, the simulated absorption the free ligand dipyH is removed by protonation into
spectrum is strongly blueshifted relative to the experimental dipyH2+. The protonated dipyrrin moiety is no longer
one (Figure 3, a and Table 5), even if the polarizable contin- planar (Figure S28 in the Supporting Information) due to
uum model (PCM) is applied to take into account the sol- some steric hindrance brought by the proton. In that case
vent. This discrepancy between the experimental and theo- too, the energies of the calculated singlet excited states are
retical data could be due to the fact that the dipyH is pres- overestimated by TD-DFT calculations relative to the ex-
ent in the form of two tautomers in equilibrium (Figure 1), perimental data (Figure 3, b and Table 5). The comparison
+
which is not taken into account in the calculations in which of the calculated energies for the protonated ligand dipyH2
a hydrogen atom is localized on one of the two pyrrole and for one tautomer of the nonprotonated ligand dipyH
rings. We therefore performed a calculation on the free di- (compare Figure 3, a and b) indicates a bathochromic shift
pyH ligand in which we elongated the N–H bond so that due to protonation, which is in agreement with a redshift
the hydrogen atom is fixed at equal distance from both N observed experimentally. However, the calculated spectrum
atoms of the two pyrrole rings, in an attempt to reflect more for dipyH2+ is also at too high energy relative to the experi-
closely the proton exchange between the two N atoms. The mental one.
molecular orbitals and their energies for the modified opti-
mized geometry of the dipyH ligand are presented in Fig-
Rh and Ir Complexes
ure 5 and Table 5. In this modified structure, the LUMO
The issues encountered to quantitatively account for the
orbital is lower in energy and has an enhanced degree of experimental absorption data of the free dipyrromethene li-
delocalization across the pyrrole rings (Figure 5). Therefore gand at the TD-DFT level have also been faced for the two
the calculated vertical transition energies to the lowest sing- Rh and the Ir complexes. The calculated singlet excited
let excited states and thus the simulated absorption bands states and simulated absorption spectra are again strongly
correspond better to the experimental results (Figure 3, a), blueshifted relative to the experimental absorption bands
although the relative intensities of the two main bands are {Figure 3, c for [Rh(ppy)2(dipy)], for example, and for all
poorly reproduced by the calculations. The asymmetry of the TD-DFT data, Tables S1 and S2 in the Supporting
Information}. Despite this discrepancy in transition ener-
gies, the calculated data clearly show that the lowest elec-
Table 5. TD-DFT data for the free ligand: dipyH, for one of the
tronic excitations with the highest oscillator strengths are
dipy-ligand-centred transitions (Table S1), which is in line
with the conclusions drawn from the experimental data.
Moreover, by comparing the DFT-optimized geometry of
[Rh(ppy)2(dipy)] with the XR data obtained for this com-
plex, we found a very good agreement (Table 1). The main
geometrical parameters, Rh–N and Rh–C bond lengths and
the N–Rh–C and N–Rh–N angles are similar within 0.05 Å
and 1.5°, respectively. In conclusion, the fact that all the
calculated data for the electronic absorption are not in full
agreement with all the experimental data for the absorption
non protonated tautomer; dipyHЈ, when the H atom is localized
between the two pyrrole rings; dipyH2+, for the protonated free
ligand.
State
E
[eV]
λ
[nm]
Osc.
Dominant
strength transition[a]
dipyH
dipyHЈ
dipyH2
S1
S2
S3
S1
S2
S6
S1
S2
S3
3.23
3.43
3.52
2.80
2.83
3.49
2.99
3.31
3.42
383.3
361.3
352.7
443.3
438.2
355.3
414.1
374.3
362.8
0.23
0.18
0.27
0.03
0.05
0.19
0.43
0.42
0.02
HǞL
H–1ǞL
H–2ǞL
H–2ǞL
HǞL+1
H–1ǞL+1
HǞL
+
+
H–1ǞL
H–2ǞL
spectraϪand this holds for both the free dipyH, dipyH2
and the corresponding complexesϪindicates that the
B3LYP TD-DFT calculations do not apply well to this type
of compound. We stress that the poor quantitative agree-
ment between the computed and measured excitation ener-
gies likely arises from a combination of multiple effects, in-
cluding the limited size of the basis set, the neglect of spin–
orbit couplings and the choice of the DFT functional. Yet,
preliminary calculations performed by using different hy-
brid functionals (namely, BYLYP and PBE0)[14] did not
yield substantial differences. More work is definitively
needed to assess the origin of the discrepancy between
theory and experiment.
[a] H: HOMO, L: LUMO.
Next, we compare the relative energies of the HOMO
and LUMO levels of the three complexes as evaluated from
the oxidation and reduction potentials, respectively (thus in
E versus SCE), with the relative energy levels from the DFT
calculations (Figure 6). Inspection of these two sets of data
shows that from one complex to the other, the variation of
the HOMO and LUMO levels is similar for the experimen-
tal and theoretical data. Indeed the HOMO and LUMO
Figure 5. HOMO and LUMO orbitals for the free dipyH ligand:
on the left, calculated for one tautomer; on the right, calculated
when the hydrogen atom is localized between the two pyrrole rings
(TD-DFTЈ).
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Figure 6. Energy levels of the HOMO and LUMO orbitals for the three complexes. Left: as determined from electrochemistry. Right: as
determined from theoretical calculations.
Table 6. TD-DFT data for the triplet excited states of the three complexes.
Triplet states
T1
Energies of the corresponding triplets [eV]
1.840
λ [nm] Dominant transitions
[Rh(dipy)3]
673.7
671.9
669.8
666.3
479.9
H–1ǞL+1
LC (dipy)
HǞL, H–2ǞL+2
LC (dipy)
HǞL+2, H–2ǞL
LC (dipy)
HǞL
T2
T3
T1
T2
1.845
1.851
1.861
2.583
[Rh(ppy)2(dipy)]
LC (dipy)
H–3ǞL
LC (dipy)
MLCT (Rh/dipy)
H–2ǞL
T3
2.600
476.8
LC (dipy)
MLCT (Rh/dipy)
H–1ǞL
LC (dipy)
HǞL, H–3ǞL
MLCT (Ir/dipy)
LLCT (ppy/dipy)
LC (dipy)
[Ir(ppy)2(dipy)]
T1
T2
1.864
2.302
665.2
538.6
T3
2.360
525.3
H–2ǞL
MLCT (Ir/dipy)
LC (dipy)
levels are found to increase in energy when going from calized on the dipy ligand (Table 6), in agreement with the
[Rh(dipy)3] to [Rh(ppy)2(dipy)] at both the electrochemical experimental results.
(Figure 6, left) and DFT (Figure 6, right) levels; switching
from [Rh(ppy)2(dipy)] to [Ir(ppy)2(dipy)] does not affect the
energetic position of the HOMO level, whereas the LUMO
level goes down. With regard to the triplet energy levels, the
Conclusion
calculated energy for the three complexes corresponds very
As detailed in the Introduction, the main goal of this
well to the maximum of phosphorescence at 77 K (Tables 6 study was to compare the photophysical properties of dipy-
and 4). Moreover, for the three complexes, the lowest triplet containing transition-metal complexes of Rh and Ir. It is
states correspond to transitions between orbitals entirely lo- clear that, when studying their photophysics, any source of
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dation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), as
described in the literature.[15]
proton traces has to be excluded from the solution, other-
wise the proton attacks the nitrogen–metal coordination
bond, a process that is followed by degradation of the com-
plex.
Dichloro-bridged
Precursors:
Dichloro-bridged
precursors
[{Rh(ppy)2Cl}2] and [{Ir(ppy)2Cl}2] were synthesized according to
literature procedures[16] by reaction of RhCl3·3H2O (1 equiv.;
46.77 mg, 178 μmol) or IrCl3·3H2O (1 equiv.; 49.72 mg, 140 μmol)
with 2-phenylpyridine (2 equiv.; 50 and 40 μL) in a mixture of me-
thoxyethanol/water (5:1 v/v). After evaporation of the solvent, the
final solid was washed with water, then diethyl ether and dried. The
obtained yields are 58% (45.3 mg) for the Rh compound and 79%
(23.9 mg) for the Ir compound.
The spectroscopic data for absorption and emission show
that the lowest excited states are π–π* states centred on the
dipy ligand, as shown by the same series of vibronic transi-
tions observed for each complex. In electrochemistry, the
oxidation and reduction waves originate from the HOMO
and LUMO levels also centred on the dipy ligand, respec-
tively. Interestingly, however, is that a difference is observed
between the Rh and Ir complexes. The Rh compounds emit
also from the singlet excited state, thus not exclusively from
the triplet state as is characteristic of the Ir complex. This
can be attributed to the heavy-atom effect of the metal cen-
tre, which is more pronounced on the dipy-centred excited
state with the Ir than with the Rh. This difference of heavy-
metal effect is also marked in the rate of deactivation of the
dipy-centred excited triplet state, which is indeed higher
with Ir than with Rh. The experimental data are partly sup-
ported by the results of the TD-DFT calculations. Indeed
the emission spectra at 77 K are in excellent agreement with
the calculated values for the electronic transitions from the
excited triplet state centred on the chelated dipy. Moreover,
the relative evolution of the calculated HOMO and LUMO
energy levels of the two Rh compounds and the Ir complex
is quite comparable to that determined from the oxidation
and reduction potentials, respectively, for the three com-
plexes.
Intriguing problems have, however, been found in trying
to reproduce the absorption spectra from TD-DFT calcula-
tions. First, with the free ligand, probably because of the
equilibrium of the tautomers, the calculated transition ener-
gies are too high relative to the experimental values. This
can be partly corrected by enforcing the hydrogen atom to
be delocalized between the two N atoms of the dipyH. In
addition, the predicted singlet excitation energies for the
complexes are significantly blueshifted relative to the exper-
imental absorption data. This is surprising as TD-DFT cal-
culations performed at the same level provide a more quan-
titative agreement with experimental results in other Ir
complexes that do not contain dipyrromethene ligands.
[Rh(dipy)3]:
Dipyrromethene
(24.9 mg,
99.5 μmol)
and
RhCl3·3H2O (8.1 mg, 30.8 μmol) were dissolved in EtOH/H2O (1:1,
20 mL) and heated under reflux. After the addition of Et3N
(13.9 μL, 99.9 μmol), a precipitate occurred instantaneously in
solution. After 1 h of heating, the reaction mixture was cooled,
then filtered and the orange solid was collected. The purification
was carried out by preparative TLC on silica with C6H12/AcOEt
(7:3) as eluent, yield 4.7 mg (18%). 1H NMR (300 MHz, CD3CN):
δ = 7.38 (d, J = 8.7 Hz, 6 H), 7.02 (d, J = 8.7 Hz, 6 H), 6.66 (dd,
J = 4.2 and 1.2 Hz, 6 H), 6.61 (br. s, 6 H), 6.36 (dd, J = 4.2 and
1.2 Hz, 6 H), 3.87 (s, 9 H) ppm. ESI-MS: m/z = 889.2 [M + K]+,
873.2 [M + Na]+, 851.3 [M + H]+, 619.2 [Rh(dipy)2(H2O)]+, 601.2
[Rh(dipy)2]+, 251.1 [(dipyH)H+].
[Rh(ppy)2(dipy)]:
A
solution of [{Rh(ppy)2Cl}2] (24.8 mg,
27.77 μmol), dipyrromethene (13.8 mg, 55.2 μmol) and AcONa
(4.6 mg, 56.31 μmol) in CH3CN (10 mL) was stirred at room tem-
perature.[17] The reaction was followed by TLC and stopped after
3 h and 15 min. The solvent was removed by evaporation under
vacuum and the crude yellow product was purified by column
chromatography (silica, cyclohexane) to yield 24.9 mg (68%) of
[Rh(ppy)2(dipy)]. 1H NMR (300 MHz, CDCl3): δ = 7.82 (d, J =
8.1 Hz, 2 H), 7.77 (d, J = 5.7 Hz, 2 H), 7.69 (td, J = 7.5 and 1.2 Hz,
2 H), 7.64 (dd, J = 7.5 and 0.9 Hz, 2 H), 7.38 (d, J = 8.4 Hz, 2 H),
6.89 (m, 10 H), 6.59 (dd, J = 4.2 and 0.9 Hz, 2 H), 6.38 (d, J =
7.5 Hz, 2 H), 6.20 (dd, J = 4.2 and 0.9 Hz, 2 H), 3.88 (s, 3 H) ppm.
ESI-MS: m/z = 661.2 [M + H]+, 452.1 [Rh(ppy)2(CH3CN)]+, 411.0
[Rh(ppy)2]+, 251.1 [(dipyH)H+].
[Ir(ppy)2(dipy)]: The dichloro-bridged precursor [{Ir(ppy)2Cl}2]
(21 mg, 19.5 μmol) and dipyrromethene (9.9 mg, 39.8 μmol) were
dissolved in CH3CN. AcONa (3.3 mg, 56.9 μmol) was added and
the mixture was heated under reflux.[3c] The reaction mixture was
followed by TLC on silica and after 2 h and 30 min, was allowed
to cool and the solvent evaporated under vacuum. The complex
was purified by column chromatography (silica gel C6H12) and ob-
tained as a red powder (17.8 mg, 61%). 1H NMR (300 MHz,
CDCl3): δ = 7.82 (m, 4 H), 7.60 (m, 4 H), 7.37 (d, J = 8.7 Hz, 2
H), 6.90 (m, 6 H), 6.81 and 6.79 (td and br. s, J = 7.5 and 1.2 Hz,
4 H), 6.55 (dd, J = 4.2 and 1.2 Hz, 2 H), 6.39 (dd, J = 7.5 and
0.9 Hz, 2 H), 6.22 (dd, 4.J = 2 and 1.5 Hz, 2 H), 6.22 (dd, J = 4.2
and 1.5 Hz, 2 H), 3.88 (s, 3 H) ppm. ESI-MS: m/z = 751.2 [M +
H+], 542.2 [Ir(ppy)2(CH3CN)]+, 501.1 [Ir(ppy)2]+, 251.1 [(dipyH)-
H]+.
Experimental Section
Chemicals: 2-Phenylpyridine, iridium chloride, triethylamine, aceto-
nitrile, ethyl acetoacetate, cyclohexane (Acros), rhodium chloride
(Johnson, Mattey and Co), sodium acetate (Aldrich), ethanol (Rie-
del-de Haën) and 2-methoxyethanol (Alfa Aesar) were used as
such. Silica for column and thin-layer chromatography were pur-
chased from Merck and Analtech, respectively. The solvents for the
photophysical studies were spectroscopic-grade acetonitrile
(Acros), methanol (Acros), ethanol (Riedel de Haen) and chloro-
form (HPLC grade).
Electrochemistry: The redox potentials were determined by cyclic
voltammetry with an Autolab PGSTAT100 potentiostat in a one-
compartment cell. A platinum disk (2 mm in diameter, Princeton
Applied Reasearch) was used as working electrode and a large
platinum foil as counter-electrode with a saturated calomel elec-
trode (SCE) as reference. If not otherwise stated, the cyclic voltam-
mograms were recorded after 30 min deoxygenation by bubbling
nitrogen, using a sweep rate of 100 mVs–1, in anhydrous benzo-
nitrile with tetrabutylammonium perchlorate (0.1 molL–1) as sup-
Dipyrromethene: The dipyrromethene derivative used in this study
corresponds to the meso-p-methoxyphenyldipyrromethene (Fig-
ure 1), which is prepared through the condensation of anisaldehyde
with pyrrole to the corresponding dipyrromethane, followed by oxi-
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porting electrolyte. The concentration of the complexes was 3–
7 mmolL–1. Ferricenium/ferrocene (Fc+/Fc) was added as an in-
ternal reference.
Table 7. X-ray crystal structure data.
Complex
[Rh(ppy)2(dipy)]
Formula
C38H29N4ORh•2CHCl3
Instrumentation: The 1H NMR spectra were recorded with a Bruker
Spectrospin 300 MHz at 25 °C. The chemical shifts are indicated
in ppm, with the reference being residual protonated solvent. The
mass spectra were recorded with a Waters QTOF2 for ESI mass
analyses in positive mode. All the complexes were dissolved in ace-
tonitrile before the measurements.
Mr [gmol–1]
899.30
Crystal dimensions [mm3]
0.33ϫ0.28ϫ0.20 (orange-brown
block)
monoclinic
C2/c
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
12.2345(5)
15.0631(7)
20.2724(8)
90.00
Photophysical Characterization: The absorption spectra were re-
corded with a Perkin–Elmer Lambda UV/Vis spectrophotometer
and the emission spectra with a Shimadzu RF-5001 PC spectroflu-
orimeter with a Xe lamp (150 W) as excitation source and a Hama-
matsu R928 PM for the detection.
β [°]
96.036(2)
90.00
3715.3(3)
4
1.608
71.52
γ [°]
V [Å3]
Z
Dcalcd. [gcm3]
θmax [°]
The luminescence lifetimes at room temperature were determined
with time-correlated single-photon counting (TC-SPC) equipment
Edinburgh Instruments FL-900CDT. The samples were excited at
445 nm by a diode laser EPL-445 (Picosecond Pulsed Laser, Edin-
burgh Instruments). The detection was performed with a Peltier-
cooled Hamamatsu R955s photomultiplier tube. The luminescence
decays were analyzed with the Edinburgh Instruments software
(version 3.0) on the basis of a nonlinear least-squares regression
using Marquardt algorithms. The luminescence lifetimes at low
temperature were determined with a Nd:YAG Q-switch laser (Con-
tinuum Inc.) coupled to an Optical Parametric Oscillator (Contin-
uum Inc.) with a wavelength range from 410 to 2300 nm. The emis-
sion decays were recorded with a R-928 Hamamatsu photomulti-
plier tube, the output of which was applied to a digital oscilloscope
(Hewlett–Packard HP 54200A) interfaced with a Dell Dimension
DE051 computer. The signals were averaged over 16 shots and cor-
rected for the baseline. Kaleidagraph software was used for the de-
cay analyses. The measurements at low temperature were per-
formed with an Oxford Instruments DN 1704 nitrogen cryostat
controlled by an Oxford Intelligent Temperature Controller (ITC
4) instrument.
λ [Å]
F(000)
1.54178 (Cu-Kα)
1816
T [K]
100(2)
17990
3558
3279
Measured reflections
Unique reflections
Observed reflections
[Io Ͼ2σ(Io)]
Parameters refined
R1
273
0.0557
0.1369[a]
0.0599
0.1401
1.044
[a]
ωR2
R1 (all data)
ωR2 (all data)
GoF
μ [mm–1]
8.005
[a] Weighting scheme ω = 1/[σ2(F2o) + (0.0504P)2 + 44.6350P], P =
(F2o + 2F2c)/3.
Computational Methods: The ground-state geometric and electronic
structures of the complexes and the free dipy ligand were computed
using Becke’s three-parameter density functional in combination
with the nonlocal correlation functional of Lee, Yang, and Parr
(B3LYP).[21] A 6-31G(d) basis set was employed for the ligand and
the relativistic effective core potential of the Los Alamos/double-ξ
basis set (LANL2DZ) was used for the central metal.[22] The geo-
metries were fully optimized without symmetry constraints. At the
respective ground-state geometries, TD-DFT calculations, utilizing
the B3LYP functional with the 6-31G(d) basis, were performed to
obtain the vertical singlet and triplet excitation energies. The polar-
izable continuum model (PCM) was applied to take into account
the solvent (acetonitrile) environment.[23] Calculation of the lowest
100 singlet–singlet excitations at the ground-state-optimized geo-
metries allowed us to simulate a large portion of the absorption
spectrum. All the simulations were performed using the GaussSum
2.0 software.[24] In addition, the lowest ten vertical triplet excitation
energies were obtained from TD-DFT calculations and the fully
relaxed geometry of the lowest triplet excited state calculated at
the spin-unrestricted UB3LYP level. All the theoretical calculations
were carried out with the Gaussian 03 program package.[25]
X-ray Crystallographic Analysis of [Rh(ppy)2(dipy)]: Crystals of
[Rh(ppy)2(dipy)] obtained by slow evaporation of a solution in
chloroform gave rise to orange-brown crystals. X-ray diffraction
data were collected at a temperature of 100 K using Cu-Kα radia-
tion and φ and ω scans on a SMART 6000 diffractometer equipped
with a CCD detector and crossed Göbel mirrors. Cell refinement
and data reduction were carried out by the program SAINT.[18] For
the absorption correction the program SADABS was used.[19] The
structure was solved by direct methods and refined by full-matrix
least-squares on |F2| using the SHELXTL program package.[20]
Non-hydrogen atoms were anisotropically refined and hydrogen
atoms were placed in calculated positions with temperature factors
fixed at 1.2Ueq of the parent atoms. Hydrogen atoms for the methyl
group were placed on the basis of the difference density around the
methyl group and refined by using a riding model and temperature
factors fixed at 1.5Ueq of the parent atom. The asymmetric unit
consists of a half molecule with atoms Rh1 and C19 on a special
position. The second half is generated by a twofold axis (symmetry
operation 1 – x, y, ½ – z). Due to the internal symmetry of the
molecule, the p-methoxyphenyl substituent is disordered over two
positions with population parameter 0.50. The crystallographic
data are given in Table 7.
Supporting Information (see footnote on the first page of this arti-
cle): Evolution of absorption and emission spectra over time for the
complexes dissolved in CHCl3, CH3CN and EtOH/MeOH (4:1);
oxidation and reduction voltammograms of the complexes; calcu-
lated optimal geometries of nonprotonated and protonated ligand;
CCDC-909363 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ calculated molecular orbitals and the lowest excited singlet states
data_request/cif.
of the complexes.
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Received: November 26, 2012
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I21427
/KAP1
Date: 12-02-13 17:56:52
Pages: 11
www.eurjic.org
FULL PAPER
Dipyrromethene Rh/Ir Complexes
The spectroscopic properties of Rh and Ir
complexes that contain meso-p-methoxy-
phenyldipyrromethene (dipyH) and phen-
ylpyridine (ppy) ligands, [Rh(dipy)3],
[Rh(ppy)2(dipy)] and [Ir(ppy)2(dipy)], are
described and discussed from experimental
and theoretical approaches.
D. Ramlot, M. Rebarz, L. Volker,
M. Ovaere, D. Beljonne, W. Dehaen,
L. Van Meervelt, C. Moucheron,
A. Kirsch-De Mesmaeker* .............. 1–11
An Experimental and Theoretical Ap-
proach to the Photophysical Properties of
Some Rh and Ir Complexes Incorporating
the Dipyrromethene Ligand
Keywords: Rhodium / Iridium / N ligands /
Luminescence / Photophysics / Density
functional calculations
Eur. J. Inorg. Chem. 0000, 0–0
11
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim