Synthesis, characterisation and theoretical study of ruthenium 4,4′-bi-1,2,3-triazolyl complexes: Fundamental switching of the nature of S1 and T1 states from MLCT to MC

Article abstract of DOI:10.1039/c2dt30510k

The series of complexes [Ru(bpy)_3-n(btz)_n][PF ₆]₂ (bpy = 2,2′-bipyridyl, btz = 1,1′-dibenzyl-4,4′-bi-1,2,3-triazolyl, 2n = 1, 3n = 2, 4n = 3) have been prepared and characterised, and the photophysical and electronic effects imparted by the btz ligand were investigated. Complexes 2 and 3 exhibit MLCT absorption bands at 425 and 446 nm respectively showing a progressive blue-shift in the absorption on increasing the btz ligand content when compared to [Ru(bpy)₃][Cl]₂ (1). Complex 4 exhibits a heavily blue-shifted absorption spectrum with respect to those of 1-3, indicating that the LUMO of the latter are bpy-centred with little or no btz contribution whereas that of 4 is necessarily btz-centred. DFT calculations on analogous complexes 1′-4′ (in which the benzyl substituents are replaced by methyl) show that the HOMO-LUMO gap increases by 0.3 eV from 1′-3′ through destabilisation of the LUMO with respect to the HOMO. The HOMO-LUMO gap of 4′ increases by 0.98 eV compared to that of 3′ due to significant destabilisation of the LUMO. Examination of TDDFT data show that the S ₁ states of 1′-3′ are ¹MLCT in character whereas that of 4′ is ¹MC. The optimisation of the T ₁ state of 4′ leads to the elongation of two mutually trans Ru-N bonds to yield [Ru(κ²-btz)(κ¹-btz) ₂]²⁺, confirming the ³MC character. Thus, replacement of bpy by btz leads to a fundamental change in the ordering of excited states such that the nature of the lowest energy excited state changes from MLCT in nature to MC.

Full text of DOI:10.1039/c2dt30510k

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PAPER
Synthesis, characterisation and theoretical study of ruthenium 4,4′-bi-1,2,3-
triazolyl complexes: fundamental switching of the nature of S1 and T1 states
from MLCT to MC†
Christine E. Welby,a Stev Grkinic,a Adam Zahid,a Baljinder S. Uppal,a Elizabeth A. Gibson,b Craig R. Ricea
and Paul I. P. Elliott*a
Received 2nd March 2012, Accepted 27th April 2012
DOI: 10.1039/c2dt30510k
The series of complexes [Ru(bpy)3−n(btz)n][PF6]2 (bpy = 2,2′-bipyridyl, btz = 1,1′-dibenzyl-4,4′-bi-1,2,3-
triazolyl, 2 n = 1, 3 n = 2, 4 n = 3) have been prepared and characterised, and the photophysical and
electronic effects imparted by the btz ligand were investigated. Complexes 2 and 3 exhibit MLCT
absorption bands at 425 and 446 nm respectively showing a progressive blue-shift in the absorption on
increasing the btz ligand content when compared to [Ru(bpy)3][Cl]2 (1). Complex 4 exhibits a heavily
blue-shifted absorption spectrum with respect to those of 1–3, indicating that the LUMO of the latter are
bpy-centred with little or no btz contribution whereas that of 4 is necessarily btz-centred. DFT
calculations on analogous complexes 1′–4′ (in which the benzyl substituents are replaced by methyl)
show that the HOMO–LUMO gap increases by 0.3 eV from 1′–3′ through destabilisation of the LUMO
with respect to the HOMO. The HOMO–LUMO gap of 4′ increases by 0.98 eV compared to that of 3′
due to significant destabilisation of the LUMO. Examination of TDDFT data show that the S1 states of
1′–3′ are 1MLCT in character whereas that of 4′ is 1MC. The optimisation of the T1 state of 4′ leads to the
elongation of two mutually trans Ru–N bonds to yield [Ru(κ2-btz)(κ1-btz)2]2+, confirming the 3MC
character. Thus, replacement of bpy by btz leads to a fundamental change in the ordering of excited states
such that the nature of the lowest energy excited state changes from MLCT in nature to MC.
coordination chemistry due to the photophysical properties of
their resultant complexes, include 2,2-bipyridyl (bpy) and
Introduction
The Huisgen–Sharpless copper catalysed alkyne–azide 1,3-
dipolar cycloaddition (CuAAC) to form 1,4-disubstituted-1,2,3-
triazoles1 has in the past decade become an invaluable synthetic
tool in organic synthesis,2 materials and polymer science3–7 and
in the derivatisation of biological macromolecules.8,9 The reac-
tion benefits from high efficiency, selectivity and broad func-
tional group tolerance with a minimal work-up often required for
product isolation.
2,2′:6′,2′′-terpyridyl (tpy). Several CuAAC-derived analogues of
these ligands are now known. The complexes [Ru(tpy)(dtzpy)]2+
and [Ru(dtzpy)2]2+ (dtzpy = 2,6-di(1,2,3-triazol-4-yl)pyridine)32
have been prepared, for example. These show blue-shifting of
the 1MLCT bands on replacement of tpy by btzpy which is
expected as a result of the truncation of the ligand π-system,
resulting in the destabilization of the LUMO relative to the
HOMO in these complexes.
Recently the reaction has begun to attract significant attention
for its application in the design of new hybrid ligand systems for
transition metal complexes.10–39 Ligand systems, ubiquitous in
Complexes of the bidentate bpy analogue 4-(pyrid-2-yl)-
1,2,3-triazole (pytz) have been prepared with rhenium,40
iridium41–44 and ruthenium.45,46 Complexes of the form [Ru-
(dcb)(pytz)(NCS)2] (dcb = 2,2′-bipyridyl-4,4′-dicarboxylic acid)
have been shown to be promising candidates for dye-sensitized
solar cell applications.47 These complexes also display blue-
shifted absorption spectra relative to those of their bpy analogues
and also exhibit blue-shifted and often highly quenched emission
spectra. The complex [Re(pytz)(CO)3Cl] is however observed to
have an elongated luminescent lifetime and a greater emission
quantum yield compared to its bpy analogue.40 Cyclometallated
Ir(III) complexes with pytz ancillary ligands also show attractive
photophysical properties. Incorporation of a cyclodextrin into the
1-position of the pytz ligand has been shown to promote
aDepartment of Chemical & Biological Sciences, University of
Huddersfield, Queensgate, Huddersfield HD1 3DH, UK.
E-mail: p.i.elliott@hud.ac.uk; Tel: +44 (0)1484 472320
bSchool of Chemistry, University of Nottingham, University Park,
Nottingham NG7 2RD, UK
†Electronic supplementary information (ESI) available: NMR spectra of
complexes, graphical plots of molecular orbitals for complexes from
DFT calculations, x,y,z coordinate files for optimised geometries of
singlet ground and triplet excited states for complexes and crystallo-
graphic information for complex 2. CCDC reference number 866315.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2dt30510k
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Chart 1 Complexes [Ru(bpy)3−n(btz)n]2+ (n = 0–3).
elevated emission quantum yields, presumably through the hin-
dering of solvent and oxygen interactions that deactivate emis-
sive excited states.41
Quenched emission in these systems may in part derive from
the thermal population of metal centred 3MC states from 3MLCT
states.48–50 This would be expected due to the destabilisation of
the LUMO on replacement of pyridyl by triazolyl moieties,
which in turn would destabilise 3MLCT states relative to the
ground and 3MC states.
The symmetrical bpy analogue 4,4′-bi-1,2,3-triazolyl (btz,
Chart 1) has received much less attention. Complexes of the type
[Re(btz)(CO)3Cl] and [Ru(btz)3]2+ are known and do not show
any luminescent emission at room temperature.51,52 Mattiuzzi
et al. have recently conducted investigations of the spectroscopic
and electrochemical properties of complexes of the form [Ru-
(tap)2(btz)]2+ and [Ru(tap)2(pytz)]2+ (tap = 1,4,5,8-tetraazaphe-
nanthrene) and have shown that these complexes should behave
as highly photo-oxidising agents under illumination.53
In order to gain a deeper understanding of the photophysical
and electronic properties imparted by this ligand, we report here
results from experimental and theoretical investigations of the
series of complexes [Ru(bpy)3−n(btz)n]2+ (n = 0–3, Chart 1).
Scheme 1 Synthesis of the btz ligand and complexes 1 to 4.
MeCN of the product is in agreement with that previously
reported52 and exhibits a singlet for the triazole ring protons at δ
8.17, a singlet for the methylene protons at δ 5.62 and a multi-
plet for the phenyl ring protons at δ 7.36–7.41.
[Ru(bpy)2Cl2] was combined with an equivalent of the btz
ligand in ethanol and heated to reflux. The product [Ru-
(bpy)2(btz)][PF6]2 (2) was then isolated on addition of
ammonium hexafluorophosphate as an orange powder. Upon
coordination to the metal, the resonance for the triazole protons
in the 1H NMR spectrum shifts to a slightly lower field and
appears at δ 8.37. The methylene protons result in a pair of
geminal doublets at δ 5.50 and 5.55, which show an AB pattern
with significant roofing (2JHH = 14.9 Hz). Seven signals are
observed for the bpy ligands with the resonances of the bpy H3
and H3′ protons being coincident. This reflects the asymmetry
Results and discussion
The benzyl substituted btz ligand 1,1′-dibenzyl-4,4′-bi-1,2,3-tria-
zole was prepared in good yield in a one-pot procedure from 1,4-
bis(trimethylsilyl)buta-1,3-diyne and benzyl azide in the pres-
ence of tetrabutyl ammonium fluoride, copper sulfate and
sodium ascorbate (Scheme 1). The 1H NMR spectrum in d3-
7638 | Dalton Trans., 2012, 41, 7637–7646
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Table 1 Selected bond lengths (Å) and angles (°) for [Ru(bpy)2(btz)]-
[PF6]2·2MeCN
Ru1–N1
Ru1–N3
Ru1–N4
N4–Ru1–N4
2.0630(19)
2.0529(19)
2.0593(18)
77.68(10)
N1–Ru1–N3
N1–Ru1–N4
N4–Ru1–N3
N1–Ru1–N1
78.88(8)
93.58(7)
170.01(7)
174.26(10)
cooling, 3 precipitated as an orange–yellow microcrystalline
powder which was purified by recrystallization from dichloro-
1
methane–diethyl ether. The H NMR spectrum of 3 shows two
resonances for the btz ligand triazole ring protons at δ 8.30 and
8.31 consistent with the asymmetric coordination environment
where one triazole ring of each btz ligand is trans to bpy whilst
the others are mutually trans to each other. The methylene
protons of the btz ligand similarly give rise to two signals at δ
5.49 and 5.53. The first of these signals initially appears to be a
broad singlet. On closer examination this shows evidence of an
AB pattern, as for the methylene signal for 2, as a pair of doub-
lets (JHH = 15 Hz). The inner lines are almost coincident and the
outer lines of each doublet are very small.
The homoleptic complex [Ru(btz)3][PF6]2 (4) was prepared by
an analogous route to that of 3 using the precursor complex [Ru-
(p-cymene)(btz)Cl]PF6. Firstly, [Ru(p-cymene)Cl2]2 was stirred
in methanol with an equivalent of btz for 2 hours after which
time excess NH4PF6 was added and the volume reduced to pre-
Fig. 1 ORTEP plot of the [Ru(bpy)2(btz)]2+ cation (the counter ions,
hydrogen atoms and solvent are removed for clarity).
within the bpy ligands where one pyridyl ring of each bpy is
trans to btz whilst the other rings of the bpy ligands are mutually
trans to each other.
1
cipitate [Ru(p-cymene)(btz)Cl]PF6. The H NMR spectrum of
the complex in d3-MeCN exhibits the expected pair of doublet
resonances at δ 5.64 and 5.85 characteristic of the aryl ring
protons of the cymene ligand. These straddle an AB pattern of
doublets (δ 5.70 and 5.76, 2JHH = 14.9 Hz) that show significant
roofing and correspond to the endo and exo methylene protons
of the btz benzyl substituents. The resonance for the triazole ring
protons appears as a singlet at δ 8.27 and is marginally shifted to
a lower field by 0.10 ppm relative to that of the free ligand in the
same solvent.
Crystals of X-ray diffraction quality were grown from an
acetonitrile solution with slow vapour diffusion of diethyl ether.
The complex crystallizes as the solvate [Ru(bpy)2(btz)]-
[PF6]2·2MeCN in the Pbcn space group. An ORTEP plot of the
structure of the cation is shown in Fig. 1, whilst Table 1 contains
selected bond lengths and angles. The cation that exhibits a dis-
torted octahedral geometry lies on a C2 axis such that only half
of it (one bpy ligand and half of the btz ligand) is crystallogra-
phically unique. The Ru–N(btz) bond lengths are comparable at
2.0593(18) Å to the Ru–N(bpy) ligands (2.0529(19) Å trans to
btz and 2.0630(19) Å cis to btz). These bond distances are in the
range quoted for the Ru–N bond lengths of the structure of [Ru-
(btz)3]Cl2 reported by Monkowius et al. (2.05(3) Å, although the
authors noted the low quality of their data in their report of this
structure) and are similar to those reported for [Ru(bpy)3]2+
(2.056(2) Å).52 The btz bite angle of 77.68(10)° is also similar
to those observed for the homoleptic complex [Ru(btz)3]Cl2
(77.1(12), 77.9(11) and 74.9(12)°). The most notable feature in
the structure is a significant twist in between the planes of the
triazole rings characterised by an N–C–C–N torsion angle about
the inter-ring C–C bond of 11.91°. However, the DFT optimised
geometry of the cation (vide infra) with a methyl analogue of the
btz ligand does not show a comparable twist (only 2.3°), nor
does the reported structure of [Ru(btz)3]Cl2.52 This twisting may
therefore be due to the crystal packing effects necessary to
accommodate the benzyl substituents or as the result of intermo-
lecular π–π stacking type interactions in the solid state. Indeed,
the btz phenyl rings lie only ∼3.4–3.5 Å away from a bpy ligand
of a neighbouring cation.
The [Ru(p-cymene)(btz)Cl]PF6 complex was then heated to
reflux in 3 : 1 ethanol–water with two further equivalents of btz
1
and NaPF6 to yield 4. The complex exhibits a very simple H
NMR spectrum with a singlet resonance for all six triazole ring
protons at δ 8.34 and a multiplet for the phenyl groups over the
range δ 7.11 to 7.13. The resonance for the methylene protons
appears as a singlet at δ 5.52 rather than the geminal pattern
observed for 2 and 3.
UV-visible absorption spectra were recorded in acetonitrile for
all the complexes and are shown in Fig. 2. Spectroscopic data
for all the complexes are presented in Table 2. The spectra for
complexes 1–3 all show an intense absorption band at approxi-
mately 290 nm, assigned to bpy-centred π → π* excitation. For
complexes 2 and 3 this band exhibits a shoulder at longer wave-
lengths (vide infra).
Each of these complexes also exhibits a broad band between
1
400–500 nm, assigned to MLCT-based transitions. On replace-
ment of bpy by btz these MLCT bands are observed to a blue-
shift appearing at 446 nm for 2 and 425 nm for 3, compared to
455 nm for 1. This is consistent with the expectation that repla-
cement of bpy with btz will destabilise the LUMO relative to the
HOMO, resulting in a larger HOMO–LUMO gap.
The complex [Ru(bpy)(btz)2][PF6]2 (3) was prepared from
[Ru(p-cymene)(bpy)Cl]PF6 by reaction with two equivalents of
the btz ligand and NaPF6 in 3 : 1 ethanol–water under reflux. On
On replacement of the final bpy ligand to give 4, the absorp-
tion profile is observed to be dramatically blue-shifted relative to
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Fig. 2 UV-vis absorption spectra for complexes 1–4 in acetonitrile.
Table 2 UV-vis absorption and luminescence data for complexes 1–4
in acetonitrile
Complex
λ
abs/nm
RT λmaxem/nm 77 K λem/nm
[Ru(bpy)3]Cl2 (1)
288, 455 610a
[Ru(bpy)2(btz)][PF6]2 (2) 287, 446 ∼590a,b
[Ru(bpy)(btz)2][PF6]2 (3) 286, 425
544, 585c
565, 610c
[Ru(btz)3][PF6]2 (4)
303
a Not corrected for detector response. b CH2Cl2. c 4 : 1 EtOH–MeOH
glass.
Fig. 3 (a) Emission spectra for complexes 1 (in MeCN) and 2 (in
CH2Cl2, vertical scale magnified ×200 with respect to that of 1), (b) nor-
malised emission spectra for complexes 2 and 3 in 4 : 1 EtOH–MeOH
glass at 77 K.
those of 1–3 such that the complex has very little absorption at
wavelengths longer than 370 nm. Indeed, compared to the red,
orange and yellow solutions of 1, 2 and 3 respectively, the sol-
utions of 4 are almost colourless to the eye. The spectrum con-
tains a broad band centred at about 300 nm which we assign to
1MLCT excitations.52 We therefore also tentatively assign the
shoulders at ∼300 nm in the spectra of 2 and 3 as arising from
btz-centred 1MLCT transitions.
The observations of the changes in the absorption spectra
across the series lead us to conclude that the contribution from
the btz ligands to the LUMO in 2 and 3 is very small and that
these orbitals are therefore delocalised, primarily over the
remaining bpy ligands. The large blue-shift in the absorption
observed for 4 is therefore the result of the mandated change in
localisation of the LUMO from a bpy ligand to the btz ligands.
We surmise that the LUMO of the isolated btz is significantly
destabilised with respect to that of bpy.
Of the btz-containing complexes only 2 shows observable,
albeit very weak, luminescent emission at room temperature in
an aerated solution. The emission spectrum of 2 is shown in
Fig. 3a along with the trace for 1 for comparison (the spectra
were not corrected for detector response). The emission profile is
blue-shifted relative to that of 1 (610 nm), as expected from the
blue-shift in the MLCT absorption band, with λmax appearing at
approximately 590 nm. Whilst it is too weak to obtain reliable
lifetime measurements, the emission band is assigned as arising
from a 3MLCT state.
spectra of complexes 2–4 were recorded at 77 K (Fig. 3b) and
structured emission bands are observed for 2 (544 & 585 nm)
and 3 (565 & 610 nm), consistent with this proposal. No emis-
sion is observed for 4 however. The 77 K emission spectrum for
2 undergoes the expected rigidochromic blue-shift relative to that
recorded at room temperature.
All the complexes were analysed by cyclic and pulsed voltam-
metry in order to obtain the first oxidation and reduction poten-
tials to probe the effect of the btz ligand on the energies of the
HOMO and LUMO for complexes 2–4. Complexes 2–4 show
reversible Ru(II)–Ru(III) redox couples at 0.95, 0.98 and 1.01 V
(vs. Fc–Fc+), respectively. Complex 2 exhibits a reversible
reduction centred at −1.22 V assigned to a bpy-centred
reduction. Reductions also seem to occur for 3 and 4, but the
samples appear to undergo some degradation at these negative
potentials. Due to this degradation, our electrochemical measure-
ments are not particularly informative with regard to the frontier
orbital make up of these complexes and are not discussed
further.
The complexes described here were therefore additionally
studied by density functional theory (DFT) methods to further
evaluate the electronic and spectroscopic effects of increasing
the btz ligand content. The singlet ground state geometries of all
four complexes [Ru(bpy)3−n(btz)n]2+ (1′–4′) were optimized in
the gas phase at the B3LYP level of theory using the Stuttgart–
Dresden small core potential for ruthenium and 6-311G* basis
sets for all other atoms. To limit the computational expense, the
The largely quenched room temperature emission observed for
2 and the lack of emission for 3 and 4 are proposed to be at least
in part due to the thermal population of non-emissive 3MC states
3
from elevated MLCT states, as mentioned above. The emission
7640 | Dalton Trans., 2012, 41, 7637–7646
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Fig. 4 Energy level diagram for the molecular orbitals for complexes [Ru(bpy)3−n(btz)n]2+ with representative graphical plots of HOMO, LUMO
and dσ* orbitals shown (principle d-orbital interactions and ligand centred LUMOs are in black, intervening ligand orbitals are in grey).
benzyl side chains of the btz ligands were simplified to methyl.
The optimized geometries of complexes 1′–4′ are depicted in the
ESI.† All the complexes adopt distorted octahedral geometries.
In the heteroleptic complexes 2′ and 3′, the average Ru–N(btz)
bond lengths of 2.127 and 2.137 Å, respectively, are elongated
relative to the corresponding Ru–N(bpy) bond lengths (2.104
and 2.100 Å, respectively). These calculated bond lengths for 2′
are slightly elongated relative to the crystallographic data for 2
by ca. 0.07 Å for btz and ca. 0.08 Å for bpy. The average Ru–N
bond length for the homoleptic btz complex, 4′, is shorter than
those for 2′ and 3′ at 2.100 Å, but again elongated relative to the
reported crystallographic structure for [Ru(btz)3]Cl2 by approxi-
mately 0.05 Å.52
The energies and plots of the principle molecular orbitals of
interest in complexes 1′–4′ were calculated for the optimized
ground state geometries. Fig. 4 shows a simplified molecular
orbital energy level diagram for the series of complexes. The
HOMO in all cases is composed primarily of the metal 4dz2
orbital, whereas the LUMOs are composed primarily of ligand-
centred π* orbitals. The HOMO − 1 and HOMO − 2 orbitals for
all the complexes are metal d-orbitals. In the case of complexes
1′ and 2′ the LUMO is spread equally over the bpy ligands, and
in complex 3′ resides on the sole bpy ligand. In complex 4′ after
the replacement of the third bpy ligand, the LUMO is spread
equally across the three btz ligands.
showed a dependence on the nature of the btz ligand substitu-
ents.51 The addition of the first and second btz ligands in com-
plexes 2′ and 3′ leads to greater destabilization of the LUMO
relative to that of the HOMO such that the HOMO–LUMO gap
increases by 0.3 eV on going from 1′ to 3′ from 3.43 to 3.73 eV.
The replacement of the third bpy ligand by btz in complex 4′
results in a dramatic destabilization of the LUMO compared to
that of 3′ and an increase in the magnitude of the HOMO–
LUMO gap by 0.98 eV to 4.71 eV. Consistent with this, the
LUMO for the free btz ligand fragment (derived by excision and
optimisation of a btz ligand from the geometry of 4′) lies
1.02 eV higher in energy than the LUMO of free bpy (similarly
derived from the geometry of 1′). From our calculations, the
degenerate LUMO + 1 and LUMO + 2 orbitals for 1′ are dπLπ*
in character, as expected, and lie 0.1 eV above the LUMO. For
2′, the comparable orbitals are LUMO + 1 and LUMO + 4 and
are separated by 0.95 eV, straddling two intervening largely bpy
π*-based orbitals. Here, LUMO + 1 has a largely bpy character
whereas LUMO + 4 has a btz character. For 3′, the dπLπ* orbi-
tals are both significantly elevated with respect to the LUMO
(0.96 eV higher in energy) and appear as LUMO + 2 and
LUMO + 3, exhibiting significant btz character. For 4′, the ana-
logous orbitals are again LUMO + 1 and LUMO + 2, and now
lie only 0.16 eV above the LUMO due to the dramatic destabili-
sation of the latter on replacement of the final bpy ligand by btz.
For all four complexes in the series, the dσ* anti-bonding
orbitals are LUMO + 9 and LUMO + 10. For complexes 1′–3′
these orbitals lie between 2.32 to 1.98 eV above the LUMO, but
for 4′ this separation is reduced to only 1.19 eV. This could
therefore allow for dramatically destabilised MLCT states in 4
that would be in close proximity to metal-centred states and thus
provide thermally accessible routes to non-radiative deactivation.
Sequential replacement of bpy by btz leads to an overall de-
stabilization of the orbital energies across the series with the
HOMO increasing in energy by 0.48 eV. This is the opposite
trend to that suggested by CV data, but it must be noted that
these calculations are carried out in the gas phase and therefore
in the absence of solvent interactions. Fletcher et al. also noted
that the oxidation potential of [Ru(btz)3]2+-type complexes
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Table 3 TDDFT calculated energies, wavelengths and principle compositions of selected excitation transitions for complexes 1′–4′
Energy/eV
Complex
Excited state (oscillator strength) Wavelength/nm Composition
[Ru(bpy)3]2+ (1′)
S1
2.61 (0.00013)
476
HOMO → LUMO, HOMO → LUMO + 1,
HOMO → LUMO + 2
HOMO − 1 → LUMO, HOMO − 2 → LUMO
HOMO → LUMO + 7
MLCT
S7
S19
S24
2.98 (0.102)
3.88 (0.0259)
4.00 (0.0551)
416
319
310
MLCT
MLCT
HOMO − 1 → LUMO + 6, HOMO − 1 → LUMO + 4 MLCT
[Ru(bpy)2(btz)]2+ (2′) S1
2.72 (0.00099)
3.05 (0.114)
3.18 (0.0406)
4.00 (0.0682)
4.13 (0.0293)
4.13 (0.0348)
456
406
389
310
300
300
HOMO → LUMO
MLCT(bpy)
MLCT(bpy)
MLCT(bpy)
MLCT(bpy)
MLCT(bpy&btz)
MLCT(bpy&btz)
S5
S6
S19
S24
S25
HOMO − 2 → LUMO, HOMO − 1 → LUMO + 1
HOMO − 2 → LUMO + 1
HOMO − 1 → LUMO + 3, HOMO → LUMO + 5
HOMO − 1 → LUMO + 5
HOMO → LUMO + 5, HOMO → LUMO + 7
[Ru(bpy)(btz)2]2+ (3′) S1
2.85 (0.00062)
3.22 (0.0847)
3.95 (0.0504)
3.70 (0.0400)
4.16 (0.0819)
434
386
314
312
298
HOMO → LUMO
MLCT(bpy)
MLCT(bpy)
S3
HOMO − 2 → LUMO
S12
S13
S18
HOMO − 1 → LUMO + 1, HOMO − 2 → LUMO + 2 MLCT(bpy&btz)
HOMO − 1 → LUMO + 2, HOMO → LUMO + 7
HOMO → LUMO + 7, HOMO − 2 → LUMO + 1,
HOMO − 1 → LUMO + 2
MLCT(btz)
MLCT(btz)
[Ru(btz)3]2+ (4′)
S1
3.65 (0.00006)
4.08 (0.0697)
4.09 (0.0761)
4.38 (0.123)
340
304
303
283
HOMO → LUMO + 9
HOMO − 2 → LUMO
HOMO − 2 → LUMO + 1, HOMO − 1 → LUMO
HOMO − 2 → LUMO + 6, HOMO → LUMO + 9
MC
S10
S11
S21
MLCT
MLCT
MLCT/MC
Time-dependent DFT (TDDFT) calculations were performed
on the optimized geometries of the singlet ground states for each
complex to determine the vertical excitation energies and their
associated oscillator strengths (selected major transitions are
listed in Table 3). The resultant calculated absorption spectra for
1′–4′ are presented in Fig. 5 with the experimentally observed
spectra for 1–4 overlaid. Consistent with the trend of the increas-
ing HOMO–LUMO gap and the spectroscopic data for com-
plexes 1–4, a visual analysis of the positions of the major
vertical excitations show that the absorptions shift towards
shorter wavelengths on replacement of bpy with btz when going
from complex 1′ to 3′. On addition of the third btz ligand in
complex 4′, the excitation energies are observed to undergo a
large blue-shift in comparison to the trend from 1′ to 3′. This is
in line with the observed large blue-shift in the absorption
profile of 4 in the experimental spectra and the observation that
the LUMO of 4′ is dramatically destabilized relative to those of
1′–3′. For complexes 2′ and 3′, the lowest energy singlet exci-
tations, S1, appearing at 456 and 434 nm, respectively, corre-
spond to HOMO → LUMO transitions and are therefore 1MLCT
in character. As with 1′, these transitions contribute little to the
absorption spectra due to having very small oscillator strengths.
For 2′, the first transitions of significance correspond to exci-
tation to the S5 and S6 states at 406 and 389 nm, respectively,
and are largely composed of transitions from HOMO − 1 and
HOMO − 2 to the LUMO and LUMO + 1 orbitals. This
confirms the assignment of the lower energy absorption bands in
our assignment of the shoulders observed for 2 and 3 that appear
in this region of their absorption spectra to MLCT transitions
with charge transfer to btz. These absorptions are also coincident
with the lowest energy major band in the spectrum for 4, and so
would be consistent with this assignment.
1
We had reasoned that the observed quenching of the emission
from the complexes on inclusion of the btz ligand may be in part
due to the rise in energy of the 3MLCT state. The resultant proxi-
3
mity to the lowest MC state could then allow the thermal popu-
lation of the latter, leading to non-radiative deactivation to the
ground state. We would of course expect this to be most pro-
nounced for 4, based on spectroscopic data and the calculated
orbital energies. The lowest energy singlet excited states that
have significant 1MC contributions are S11 (343 nm, 1.01 eV
above S1) for 1′, S9 (344 nm, 0.89 eV above S1) for 2′ and S4
(360 nm, 0.59 eV above S1) for 3′. These transitions are all of
low oscillator strength (>0.004) and therefore contribute little to
the absorption spectrum. Nevertheless, replacement of bpy by
btz and the consequent raising of the LUMO with respect to the
dσ* orbitals does indeed result in a narrowing of the gap
between the 1MLCT and 1MC states. For complex 4′ on the
other hand, the lowest energy singlet excited state with an exci-
1
tation at 340 nm is not of btz-centred MLCT character but, to
1
our surprise, is instead a MC state corresponding to the exci-
tation of an electron from the Ru 4dz2 orbital to the LUMO + 9
1
dσ* orbital. MLCT transitions are instead observed to appear at
1
303–304 nm (S10 and S11) in the same region as the MLCT
1
this region as arising from MLCT with charge transfer to the
transitions with significant btz character observed for 2′ and 3′.
Thus, replacement of the final bpy ligand results in a switching
bpy ligands.
1
1
For 3′, the first major excitation, S3, is similarly MLCT in
in the order of the lowest energy excited states from MLCT in
character (HOMO − 2 → LUMO) and again involves charge
transfer to bpy rather than btz. Both 2′ and 3′ show 1MLCT tran-
sitions in the region around 300–314 nm that involve excitation
to orbitals with significant btz character. This therefore confirms
nature to 1MC.
To further understand the photophysical properties of these
complexes we proceeded to calculate the optimised geometries
of the lowest lying triplet excited states using the constraint of a
7642 | Dalton Trans., 2012, 41, 7637–7646
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Fig. 6 Optimised geometry of the 3MC T1 excited state of 4′.
complexes, but through the use of appropriate Hartree–Fock
initial guess vectors.54,55 Less pronounced elongation of the Ru–
N bonds to 2.46 and 2.45 Å were observed for the 3MC states of
1′ and [Ru(bpz)3]2+ respectively but the corresponding Ru–N
distances for [Ru(tap)3]2+ (2.52 Å) are comparable to those
found for 4′. As a consequence of the structural deformation
occurring during the optimisation of the T1 state starting from
the S0 geometry of 4′, the dσ* LUMO + 9 orbital for the ground
state geometry undergoes a dramatic stabilisation and becomes
the HOMO. Optimising the geometry of the S0 state starting
from this four-coordinate T1 state results in the re-coordination
of the two κ1-btz ligands and the restoration of the ground state
geometry.
Whilst the DFT calculations presented here have been carried
out in the gas phase and the calculated MLCT states will there-
Fig. 5 TDDFT calculated absorption spectra for complexes
[Ru(bpy)3n(btz)n]2+
3
.
fore be stabilised through solvation, the photoexcitation of 4 may
well proceed exclusively through this non-radiative photoreactive
3MC state. This will therefore have an impact on the design of
photoactive complexes and molecular devices that contain the
btz ligand framework. Indeed, we have noticed that NMR
samples of 3 in d3-MeCN that have been left to stand in daylight
over two weeks appear to undergo conversion to at least one new
complex along with the formation of a small amount of free
ligand. At a much slower rate, 2 also converts into at least one
new metal complex, again with a small amount of free btz for-
mation. No discernable differences are observed in the spectra of
4 however. We are currently working to identify these new pro-
ducts and are probing the mechanism of formation to determine
whether this is a thermal and/or photochemical process. Results
from these further studies will be published in due course.
spin multiplicity of 3. The optimisation of the T1 state of 1′
results in the slight shortening of the Ru–N bonds lengths to two
of the bpy ligands from an average of 2.114 to 2.095 Å, consist-
ent with the convergence toward C2 symmetry observed in
similar calculations by Alary and co-workers.54 For the hetero-
leptic complex 2′, the Ru–N bond lengths for the bpy ligands
trans to btz are observed to shorten by 0.053 Å. Here, the Ru–N
bonds to btz elongate from 2.127 to 2.153 Å. For 3′, the Ru–N
bonds to bpy again shorten upon optimisation of the T1 state
from 2.100 for S0 to 2.035 Å.
For complexes 1′–3′, the SOMOs are very similar in appear-
ance to the HOMO and LUMO orbitals of the singlet ground
state. This then confirms these triplet excited states as being
3MLCT in character.
We reasoned that since the lowest lying singlet excited state
1
for 4′ has MC character, the lowest lying triplet excited state
would also have MC character. Indeed, when the geometry of
Conclusions
3
the lowest energy triplet excited state of 4′ is allowed to optimise
starting from the geometry of the singlet ground state, two
mutually trans Ru–N bonds are observed to elongate to 2.53 Å.
Thus, the formation of the lowest energy triplet excited state of
4′ involves dechelation of two ligands to yield [Ru(κ2-btz)(κ1-
btz)2]2+ (Fig. 6). Alary et al. were able to arrive at similar geo-
In conclusion, we have prepared and characterized complexes in
the series [Ru(bpy)3−n(btz)n]2+ (n = 1 to 3) and investigated the
resultant photophysical effects of the btz ligand. The experimen-
tal data reveal a blue-shifting in the absorption bands consistent
with the destabilization of the LUMO relative to the HOMO,
with a larger blue-shift being observed on replacing the final bpy
ligand by btz. The LUMO in all the bpy-containing complexes
is bpy-centred, whereas that for 4 is btz-centred. The inclusion
3
metries for the MC states of 1′ and the analogous homoleptic
2,2′-bipyazyl (bpz) and 1,4,5,8-tetraazaphenanthrene (tap)
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separated and dried over MgSO4, filtered, and the solvent
removed under reduced pressure to yield the product as a white
solid. The characterisation data match those previously reported.
Yield 111 mg, 70%.
1H NMR (500 MHz) CD3CN δH 5.62 (s, 4H, CH2), 7.36–7.41
(m, 10H, Ph), 8.17 (s, 2H, CHN3).
Synthesis
of
[Ru(bpy)2(btz)][PF6]2
(2). [Ru(bpy)2Cl2]
(100 mg, 0.21 mmol) and btz (68 mg, 0.23 mmol) were sus-
pended in ethanol (25 cm3) and refluxed under nitrogen for
4 hours. The solution was allowed to cool to room temperature,
concentrated and passed through a silica column using MeCN/
H2O/sat with KNO3(aq) as the eluent. The orange band for the
product was collected and reduced to dryness. The residues were
dissolved in ethanol (10 cm3) and the product collected as an
orange precipitate by addition of excess NH4PF6. Yield 139 mg,
65%.
Fig. 7 Qualitative energy level diagram showing the change in the
nature of the lowest energy excited states for [Ru(bpy)3]2+ and [Ru-
(btz)3]2+ from MLCT to MC, respectively.
of btz also results in blue-shifted luminescent emission for [Ru-
(bpy)2(btz)]2+, which is highly quenched compared to that of
[Ru(bpy)3]2+. Theoretical studies show that the lowest lying
singlet and triplet excited states for all the bpy-containing com-
plexes are MLCT in character, with the excited electron residing
on the bpy ligands. The replacement of the final bpy ligand by
btz in [Ru(btz)3]2+ results in a change in the nature of the lowest
1H NMR (500 MHz) CD3CN δH 5.50 (d, 2JHH = 14.9 Hz, 2H,
2
CHH of Bz), 5.55 (d, JHH = 14.9 Hz, 2H, CHH of Bz),
7.11–7.12 (m, 4H, ortho-Ph), 7.34–7.40 (m, 8H, meta- & para-
1
1
Ph & H5′-bpy), 7.45 (ddd, 4JHH = 1.2 Hz, 3JHH = 5.6 Hz, 3JHH
=
lying excited state from MLCT to MC (Fig. 7). The lowest
3
7.6 Hz, 2H, H5-bpy), 7.90 (at, 3JHH = 6.0 Hz, 4H, H6-bpy), 8.05
lying triplet excited state is similarly MC in nature and results
in the dechelation of two btz ligands to yield [Ru(κ2-btz)-
(κ1-btz)2]2+. The excitation of this complex will therefore result
in exclusive non-radiative deactivation to the ground state.
4
3
3
(td, JHH = 1.6 Hz, JHH = JHH = 8.1 Hz, 2H, H4′), 8.10 (td,
4JHH = 1.5 Hz, JHH = JHH = 7.9 Hz, 2H, H4), 8.37 (s, 2H,
3
3
3
CHN3), 8.48 (at, JHH = 8.7 Hz, 4H, H3-bpy); 13C NMR
(125.8 MHz) CD3CN δC 55.3 (CH2), 123.4, 123.5, 123.8, 126.7,
127.4, 128.0, 128.9, 129.1 (all CH), 134.0 (C), 137.6 (CH),
140.4 (C), 137.7, 152.0, 152.4 (all CH), 157.3, 157.8 (all C).
HRMS (ESI) calcd for C38H32N10Ru 365.092197 (M2+),
found 365.092938.
Experimental
General methods
[Ru(bpy)3]Cl2, 2,2′-bipyridyl and 1,4-bis(trimethylsilyl)-1,3-
butiadiyne were purchased from Aldrich and used as supplied.
[Ru(p-cymene)Cl2]2 and [Ru(p-cymene)(bpy)Cl]PF6 were pre-
pared following the procedures in the literature.56 NMR spectra
were recorded on Bruker 500 Avance and 400 AMX spec-
trometers, UV-visible absorption spectra on a Varian Cary 300
spectrophotometer and luminescence spectra on a Hitachi
F-4500 spectrophotometer. The 77 K emission spectra were
recorded on a Jobin Yvon Fluoromax spectrometer. The samples
were dissolved in acetonitrile and the spectra were recorded
immediately. Mass spectra were collected on a Bruker Micro-
Q-TOF instrument. Electrochemical measurements on the com-
plexes were carried out at concentrations of 1 mM in 100 mM
solutions of N(Bu)4PF6 in acetonitrile. The data were referenced
against Ag–AgCl (3 M KCl(aq)) and all samples are quoted rela-
tive to ferrocene–ferrocenium. A glassy carbon electrode was
used for the working electrode and a platinum counter electrode
was used.
Synthesis of [Ru(bpy)(btz)2][PF6]2 (3). [Ru(p-cymene)(bpy)-
Cl]PF6 (50 mg, 0.082 mmol), btz (52 mg, 0.16 mmol) and
NaPF6 (60 mg, 0.18 mmol) were suspended in a 3 : 1 EtOH–
H2O mixture (8 cm3) under nitrogen and heated to 90 °C over-
night. On cooling, a bright yellow–orange precipitate formed,
which was isolated by filtration and washed with ether. The
product was then recrystallized from dichloromethane–ether.
Yield 69 mg, 71%.
1H NMR (500 MHz) CD3CN δH 5.49 (br s, 4H, CH2 of Bz),
5.53 (s, 4H, CH2 of Bz), 7.09–7.12 (m, 4H, Ph), 7.13–7.14 (m,
4H, Ph), 7.30–7.33 (m, 10H, Ph), 7.34–7.39 (m, 4H, Ph & H5-
5
4
3
bpy), 7.98 (ddd, JHH = 0.6 Hz, JHH = 1.4 Hz, JHH = 5.7 Hz,
4
3
3
2H, H6-bpy), 8.03 (td, JHH = 1.5 Hz, JHH = JHH = 7.9 Hz,
2H, H4-bpy), 8.30 (s, 2H, CHN3), 8.31 (s, 2H, CHN3), 8.40
(dt, 5JHH & 4JHH = 1.0 Hz, 3JHH = 8.7 Hz, 2H, H3-bpy).
HRMS (ESI) calcd for C46H40N14Ru (M2+) 445.129645,
found 445.131811.
Synthesis of [Ru(p-cymene)(btz)Cl]PF6. [Ru(p-cymene)Cl2]2
(100 mg, 0.16 mmol) and btz (209 mg, 0.66 mmol) were sus-
pended in methanol (10 cm3) and stirred for 3 hours. To the
resultant yellow solution was added NH4PF6 (108 mg, excess)
dissolved in methanol (1 cm3) and the volume of solution was
reduced until the product precipitated as a bright yellow powder.
The product was collected by filtration, washed with ether and
air dried. Yield 205 mg, 88%.
Synthesis of ligand and complexes
Synthesis of 1,1′-dibenzyl-4,4′-bi-1,2,3-triazolyl (btz). 1,4-bis-
(trimethylsilyl)-1,3-butadiyne (97 mg, 0.5 mmol), benzyl azide
(133 mg, 1 mmol), CuSO4·5H2O (32 mg, 0.13 mmol), sodium
ascorbate (79 mg, 0.4 mmol), K2CO3 (136 mg, 1 mmol) and
pyridine (0.4 cm3, 5 mmol) were added to a flask containing
1 : 1 tert-butanol–H2O (10 cm3). The mixture was stirred vigor-
ously for 24 hours and then partitioned between dichloro-
methane and 5% aqueous ammonia. The organic layer was
1H NMR (500 MHz) CD3CN δH 1.07 (s, 3H, CH(CH3)), 1.09
3
(s, 3H, CH(CH3)), 2.15 (s, 3H, CH3), 2.73 (sp, JHH = 6.9 Hz,
1H, CH(CH3)2), 5.64 (d, 3JHH = 6.3 Hz, 2H, p-cymene), 5.70 (d,
7644 | Dalton Trans., 2012, 41, 7637–7646
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2
2JHH = 14.9 Hz, 2H, CHH of Bz), 5.76 (d, JHH = 14.9 Hz, 2H,
Table 4 X-Ray crystallographic data for [Ru(bpy)2(btz)][PF6]2·2
MeCN
3
CHH of Bz), 5.85 (d, JHH = 6.3 Hz, 2H, p-cymene), 7.38–7.45
(m, 10H, Ph), 8.27 (s, 2H, CHN3); 13C NMR (125.8 MHz)
CD3CN δC 17.8 (CH3, p-cymene), 21.3 (CH3, CH(CH3)2), 30.7
(CH, CH(CH3)2), 55.8 (CH2), 83.0 (CH, p-cymene), 85.2 (CH,
p-cymene), 101.6 (C, p-cymene), 104.6 (C, p-cymene), 122.8,
128.5, 129.1, 129.2 (all CH, btz), 134.0, 138.5 (C, btz).
HRMS (ESI) calcd for C28H30NClRu (M+) 587.12549, found
587.126586.
Formula
C42H38F12N12P2Ru
1101.85
150
Mr/g mol−1
Temperature/K
Space group
a/Å
b/Å
c/Å
α/°
β/°
Pbcn
17.4573(7)
15.6748(6)
16.6134(6)
90
90
γ/°
90
4546.1(3)
1.610
Synthesis of [Ru(btz)3][PF6]2 (4). [Ru(p-cymene)(btz)Cl]PF6
(50 mg, 0.068 mmol), btz (44 mg, 0.14 mmol) and NaPF6
(66 mg, 0.39 mmol) were suspended in a 3 : 1 EtOH–H2O
mixture (8 cm3) under nitrogen and heated to 90 °C overnight.
On cooling, a pale yellow precipitate formed, which was isolated
by filtration and washed with ether. The product was then recrys-
tallized from dichloromethane–ether as a pale tan solid. Yield
71 mg, 78%.
V/Å3
Dc/g cm−3
Z
4
μMo/mm−1
‘T’min, max
2θmax
Nref
R1
wR2
0.512
0.790, 0.880
64.06
7884
0.0452
0.1445
0.977
S
1H NMR (500 MHz) CD3CN δH 5.52 (s, 12H, CH2),
7.11–7.13 (m, 12H, Ph), 7.28–7.35 (m, 18H, Ph), 8.34 (s, 6H,
CHN3); 13C NMR (125.8 MHz) CD3CN δC 55.2 (CH2), 122.7,
127.8, 128.9, 129.1 (all CH), 134.3, 141.0 (all C).
SADABS.65 All structures were refined until convergence (max
shift/esd < 0.01) and in each case, the final Fourier difference
map showed no chemically sensible features.
HRMS (ESI) calcd for C54H48N18Ru (M2+) 525.167093,
found 525.169291.
Computational details
Acknowledgements
DFT calculations were carried out using the GAMESS-UK57 and
NWChem58 software packages. Calculations were carried out
using the B3LYP hybrid functional (20% Hartree–Fock),59 Stutt-
gart relativistic small core ECP for ruthenium60 and 6-311G*
basis sets for all other atoms.61 Molecular structures and molecu-
lar orbitals were visualized using the ccp1 graphical user inter-
face. The ground state geometries of all complexes were first
optimized and molecular orbital energies determined. TDDFT
calculations were then used at the ground state geometries to
derive the vertical excitation energies and hence the simulated
absorption spectra. The geometries of the lowest lying triplet
states were optimised starting from the geometries of the ground
states using the constraint of a spin multiplicity of 3.
The authors thank the University of Huddersfield and the Lever-
hulme Trust for funding this research. We also thank the Hudder-
sfield Centre for High Performance Computing, the National
Grid Service and Science and Technology Facilities Council
(Daresbury Laboratories) for computational resources and
support. We also thank Dr Daniel Sykes (University of Sheffield)
for technical assistance.
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