Triphenylamine-modified ruthenium(II) terpyridine complexes: Enhancement of light absorption by conjugated bridging motifs

Article abstract of DOI:10.1021/ic9025427

The photophysical properties of a family of heteroleptic [Ru(tpy) ₂]²⁺ (tpy = 2,2′:6′,2″-terpyridine) complexes modified with triphenylamine donor units with different bridging units are reported.

Full text of DOI:10.1021/ic9025427

Inorg. Chem. 2010, 49, 5335–5337 5335
DOI: 10.1021/ic9025427
Triphenylamine-Modified Ruthenium(II) Terpyridine Complexes: Enhancement
of Light Absorption by Conjugated Bridging Motifs
Kiyoshi C. D. Robson, Bryan D. Koivisto, Terry J. Gordon, Thomas Baumgartner, and Curtis P. Berlinguette*
Department of Chemistry and Institute for Sustainable Energy, Environment & Economy, University of Calgary,
2500 University Drive NW, Calgary, Alberta, Canada T2N-1N4
Received December 20, 2009
The photophysical properties of a family of heteroleptic [Ru(tpy)2]2þ
(tpy = 2,20:60,200-terpyridine) complexes modified with triphenyl-
amine donor units with different bridging units are reported.
The design of molecules for light-harvesting applications
requires structural elements that balance a myriad of electro-
nic properties.1 Compounds with high molar extinction
coefficients (ε) often feature a highest occupied molecular
orbital (HOMO) distributed over a significant portion of the
molecule and a spatially separated lowest unoccupied mole-
cular orbital (LUMO). Organic chromophores designed for
dye-sensitized solar cells,2 for example, often contain a conju-
gated linker between the donor (D) and acceptor (A) groups,
an arrangement that facilitates a light-driven π-π* tran-
sition.3,4 Ruthenium(II) polypyridyl complexes, on the other
hand, typically make use of a metal-to-ligand charge-transfer
(MLCT) process for use in light-harvesting applications.5
While ε values are usually lower for ruthenium-based dyes
relative to their organic counterparts, the absorption profiles
of the metal complexes are generally broader and are ex-
tended to longer wavelengths, thereby offering greater utility
in solar energy conversion schemes.6
Taking these collective observations into account, we set
out to combine the favorable attributes of organic and
inorganic chromophores in pursuit of molecules with intense,
low-energy electronic transitions. To realize these hybridized
systems, we installed the triphenylamine (TPA) donor motif
common to organic dyes3,7,8 onto a ligand platform capable
Figure 1. Structural representations of ligands L1-L3 and complexes
1-4 (counterion = NO3-) investigated in this study.
of binding to a metal [i.e., 2,20:60,200-terpyridine (tpy)]. There
is limited precedent for polypyridyl ruthenium complexes
containing terminal TPA substituents,9-11 and even fewer
examples exist where conjugated linkers are installed between
the coordinating ligand fragment and the TPA group.12,13
In this work, we incorporate acetylene and 2,5-thiophene linkers,
which serve to circumvent steric repulsion between adjacent aro-
matic six-membered rings, between the {Ru(tpy)} and TPA units
of 1 (Figure 1). An evaluation of the photophysical properties
*To whom correspondence should be addressed. E-mail: cberling@
ucalgary.ca.
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r
2010 American Chemical Society
Published on Web 05/18/2010
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5336 Inorganic Chemistry, Vol. 49, No. 12, 2010
Robson et al.
Table 1. Electronic Absorption and Emission Data for L1-L3 and 1-5
UV-vis dataa
emission datac
b
λmax
(nm)
λem (nm)
ε (M-1 cm-1
)
(λex (nm)) τ (ns) (χ2)
Φd
L1 361
L2 376
L3 395
29 300
27 200
25 100
29 300
12 300
34 900
17 900
42 000
19 400
44 200
20 200
15 500
498 (400) 6.6 (1.05)
530 (400) 4.9 (0.83)
542 (400) 3.7 (0.81)
0.15
0.23
0.29
1
2
3
4
5
493
414
498
430 (sh)
505
440 (sh)
508
445 (sh)
475
660 (493) 1.1 (0.86)
1.8 ꢀ 10-4 f
9.6 ꢀ 10-4 f
e
689 (509) 18 (1.11)
538 (425) 4.6 (1.10)
720 (510) 410 (0.98) <0.01
549 (425) 3.4 (1.18)
723 (510) 580 (0.97) <0.03
548 (425) 3.5 (1.16)
619 (449)
Figure 2. (a) Absorption profiles for 1-5 in CH3OH (counterion =
NO3- for 1-4; PF6- for 5). (b) Three-dimensional emission profiles for 4
in MeCN as a function of the excitation wavelength (λex).
a Collected in CH3OH. b Values corresponding to lowest-energy
maxima, and shoulders (sh) are reported. c Collected in CH3CN.
d Absolute values; measured with an integrating sphere unless noted
otherwise. e Not observed. f Relative to [Ru(bpy)3]2þ in MeCN: 0.059.14
that arise from the varying degrees of electronic coupling
between these neighboring chromophore units is detailed
herein.
The syntheses of TPA-substituted tpy ligands L1-L3 were
all achieved in reasonably high yields on a relatively large
scale (Scheme S1 in the Supporting Information). Coordina-
tion of L1-L3 to the synthon [Ru(tpy-R2)Cl3] (R2 = -H,
-COOH) yields hybridized organic/inorganic chromo-
phores 1-4. The structural identities of all ligands and
complexes were confirmed by a combination of NMR
spectroscopy, mass spectrometry, and/or elemental analysis
(see the Supporting Information).
The cyclic voltammograms for L1-L3 and 1-4 all reveal a
reversible oxidation wave (E1/2ox) ascribed to oxidation of the
TPA unit (i.e., TPA/TPA•þ). The proximity of the electron-
withdrawing pyridine and alkyne groups of L1 and L2
increases the oxidation potential by 150 and 160 mV, respec-
tively, relative to the thiophene spacer of L3 (E1/2ox = 1.38,
1.39, and 1.23 V vs NHE for L1-L3, respectively). In the
absence of a conjugated linker, the electron-withdrawing
character of the tpy fragment is significantly diminished
upon ligation to the metal, thereby rendering the TPA unit
more susceptible to oxidation (E1/2ox for 1 is observed at 1.26
V vs NHE). Conversely, the inclusion of either a thiophene or
alkyne spacer results in a TPA oxidation potential that is
Figure 3. Principle electronic transitions for 3 predicted by TD-DFT.
The ILCT (red) and MLCT (blue) transitions are offset for clarity (Ru =
magenta; N = blue; S = gold).
actions between protons of adjacent six-membered rings,
insertion of the acetylene and thiophene spacers circum-
vents these interactions, resulting in a bathochromic shift
of the absorption band maxima (λmax) for L2 and L3. The
longer λmax value for L2 relative to L1 is ascribed to the
electron-withdrawing character of the alkyne linker, lower-
ing the LUMO. Similarly, the low-resonance energy of the
thiophene unit stabilizes the LUMO.
The absorption profiles for 1-4 over the 375-600 nm
range each contain a broad, intense band centered at ca.
500 nm with a higher-energy shoulder (or peak) between
400 and 450 nm (Figure 2a). Both features display a sensi-
tivity to the identity of the spacer in terms of the intensity and
position of the absorbance bands. The maxima of the lowest-
energy absorption bands are bathochromically shifted by
132, 122, and 110 nm and progressively more intense for 1-3,
respectively, relative to the free ligand (Table 1). The ε value
for 1 is 2-fold greater than that of 5, which highlights the
positive effect that the D-A arrangement of the TPA-tpy
motif has on the absorption profiles of ruthenium(II) co-
ordination complexes of this type. The higher ε values (and
longer λmax values) for 2-4 relative to 1 are manifest in the
enhanced conjugation between the D and A units.
ox
effectively unaltered upon ligation; i.e., the E1/2 values of
1.38, 1.24, and 1.24 V vs NHE for 2-4, respectively, are
congruent with the formal oxidation potentials of the corre-
sponding free ligands. Scanning to higher potentials reveals
an irreversible signal that is assigned to the oxidation of the
TPA•þ unit; a reversible RuII/RuIII redox couple could not be
delineated. The ligands are not reduced within the solvent
window, but the corresponding metal complexes show two
pseudoreversible reduction waves at ca. -0.8 and -1.2 V vs
NHE (Table S1 in the Supporting Information), which are
assigned to successive reductions of the tpy ligand and the
opposing TPA-tpy fragment.
Electronic absorption and emission data for L1-L3, 1-4,
and benchmark complex [Ru(tpy)2](PF6)2 (5) in MeOH are
provided in Table 1. The absorption bands centered in the
350-450 nm range for L1-L3 are ascribed to intraligand
charge-transfer (ILCT) transitions (Figure S1 in the Support-
ing Information). While ligand L1 is subject to steric inter-

Communication
Inorganic Chemistry, Vol. 49, No. 12, 2010 5337
Time-dependent density functional theory (TD-DFT) in-
dicates that the ILCT transitions for L1-L3 are maintained
when ligated to the metal center in 1-3, respectively. The
principle electronic transitions predicted for 3 in solution
(MeCN) are presented in Figure 3. These results indicate that
an ILCT process originating from the TPA unit (HOMO f
LUMOþ4) makes a significant contribution to the lower-
energy absorbance band, while the shoulder centered at ca.
440 nm consists of transitions that are predominantly MLCT
in character (e.g., HOMO-2 f LUMOþ2 and HOMO-3 f
LUMO). Similar observations were made for 1, 2, and 4,
but symmetry considerations predict fewer transitions for 1
and 2 (Figure S2 in the Supporting Information). The sig-
nificant intensity of the absorbance band for 1-3 is attrib-
uted to the confluence of these ILCT and MLCT transitions.
Spectroelectrochemical experiments confirm that a combina-
tion of ILCT and MLCT transitions comprises both bands,
with the lower-energy band being primarily ILCT in char-
acter (Figure S3 in the Supporting Information).
Emission measurements reveal similar Stokes shifts (ca.
150 nm), excited-state lifetimes (3-7 ns), and quantum yields
(0.15-0.29) for L1-L3 (Table 1). Tracking the wavelength-
dependent emission behavior of the metal complexes reveals
dual-emission behavior for 2-4 (Figure 2b), an uncommon
feature among ruthenium(II) complexes.15-17 The dominant
ILCT character within the low-energy absorption bands for
1-4 is corroborated by monitoring the emission at variable
excitation wavelengths (Figures 2b and S4 in the Supporting
Information). Excitation of 3 (or 4) at 425 nm generates two
emission bands at ca. 550 and 725 nm, while irradiation at
longer wavelengths (e.g., 510 nm) only generates the lower-
energy emission band. Complex 1 does not exhibit dual-
emission behavior, which is in accordance with related
compounds that lack spacers between the donor substituent
and the tpy ligand.10 Note that 1 is also the only complex in
this study that exhibits electronic communication between
the TPA and Ru center in the ground state on the basis of
electrochemistry experiments. Dual emission is observed for 2,
but the excited-state lifetime of the lower-energy emission band
is significantly lower than that of 3 and 4. We contend that this
discrepancy is due to the lower rotational barrier of the alkyne
spacer, providing access to nonradiative decay pathways.
The dual-emission behavior is assigned to decay from a single
excited state to the spatially separated TPA and Ru chromo-
phores (Scheme 1). Transient absorption spectroscopy studies
are underway to unambiguously elucidate these processes.
Scheme 1. Qualitative Description of Key Radiative and Nonradia-
tive Processes for 2-4a
a Radiative and nonradiative decay processes indicated by solid and
dashed arrows, respectively; GS = ground state; ES = emissive state.
This work demonstrates the profound effect that the TPA
group, with a judicious choice of the conjugated spacer, has
on the photophysical properties of [Ru(tpy)2]2þ complexes.
Of the complexes used in this investigation, the influence of
the D-A arrangement on the optical properties is most
pronounced in the case where the thiophene is used as the
bridging unit (i.e., 3 and 4). This observation is ascribedto the
polarizability and low resonance energy of the thiophene
spacer, promoting a planar arrangement between the TPA
and tpy units. This notion is validated by TD-DFT calcula-
tions and by the relative excited-state lifetimes for the metal
complexes. The observed lifetimes can also be rationalized by
the spatial proximity of the excited-state electron density to
the TPA unit. This trend is also consistent with enhanced
intersystem crossing mediated by the RuII center.14
Ligand systems with the TPA moiety attached to poly-
pyridyl ligands are not without precedent but have not been
exhaustively developed to date.9,11-13 This examination
serves to elaborate on these studies by evincing the role that
the spacer unit has on the photophysical properties of these
types of complexes. The principle objective of this study was
realized in establishing that large increases in the ε values
(and excited-state lifetimes) are observed when appropriate
conjugated spacer units are positioned between the TPA
donor group and the {Ru(tpy)} acceptor fragment. Studies
are currently underway to explore the utility of these com-
plexes in artificial photosynthetic frameworks.
Acknowledgment. This work was financially supported
by the Canadian Natural Science and Engineering
Research Council, Canada Research Chairs, Canadian
Foundation for Innovation, Alberta Innovates, and the
Canada School of Energy and Environment. K.C.D.R. is
grateful for an NSERC graduate scholarship.
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Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85–277.
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2001, 40, 4063–4071.
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1986, 108, 7843–7844.
(17) Wang, X.-Y.; Del Guerzo, A.; Tunuguntla, H.; Schmehl, R. H. Res.
Chem. Intermed. 2007, 33, 63–77.
Supporting Information Available: Synthetic details and
TD-DFT, (spectro)electrochemical, and optical data. This material
is available free of charge via the Internet at http://pubs.acs.org.