Terpyridine-fused polyaromatic hydrocarbons generated via cyclodehydrogenation and used as ligands in Ru(ii) complexes

Article abstract of DOI:10.1039/c2dt30548h

A series of novel fused 4′-substituted 2,2′:6′, 2′′-terpyridine ligands and their ruthenium(ii) complexes were prepared. The unusual 4′-substituents comprised 2,3,4,5-pentaphenylbenzene and its tert-butyl derivative (1 and 2) and the products from oxidative cyclodehydrogenation, i.e. polyaromatic fragments consisting of ten or thirteen fused benzene rings (3 and 4). The syntheses of all the ligands are discussed in terms of the demands and limitations of the Scholl reaction. The optical properties of the ligands, along with the single-crystal X-ray structures of 1 and 2, are presented. The latter show that the pentaphenylbenzene and terpyridine appendages of 1 and 2 are perpendicular in the solid state. Despite the inclusion of the large organic chromophore the absorption and emission properties of the Ru(ii) bis-terpy complexes (of ligands 1, 2 and 3) were found to be comparable to those of [Ru(terpy)₂]²⁺. They are non-emissive at room temperature but emit at 77 K with excited state lifetimes of 11-12 μs.

Full text of DOI:10.1039/c2dt30548h

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PAPER
Terpyridine-fused polyaromatic hydrocarbons generated via
cyclodehydrogenation and used as ligands in Ru(II) complexes†
Angelika Graczyk,‡ Frances A. Murphy,‡ Deanne Nolan, Vanesa Fernández-Moreira, Natasha J. Lundin,
Christopher M. Fitchett and Sylvia M. Draper*
Received 7th March 2012, Accepted 1st May 2012
DOI: 10.1039/c2dt30548h
A series of novel fused 4′-substituted 2,2′ : 6′,2′′-terpyridine ligands and their ruthenium(II) complexes
were prepared. The unusual 4′-substituents comprised 2,3,4,5-pentaphenylbenzene and its tert-butyl
derivative (1 and 2) and the products from oxidative cyclodehydrogenation, i.e. polyaromatic fragments
consisting of ten or thirteen fused benzene rings (3 and 4). The syntheses of all the ligands are discussed
in terms of the demands and limitations of the Scholl reaction. The optical properties of the ligands,
along with the single-crystal X-ray structures of 1 and 2, are presented. The latter show that the
pentaphenylbenzene and terpyridine appendages of 1 and 2 are perpendicular in the solid state. Despite
the inclusion of the large organic chromophore the absorption and emission properties of the Ru(II)
bis-terpy complexes (of ligands 1, 2 and 3) were found to be comparable to those of [Ru(terpy)2]2+.
They are non-emissive at room temperature but emit at 77 K with excited state lifetimes of 11–12 μs.
The ability of complexes to absorb strongly in the visible and
near-IR regions is also key for light-harvesting in solar cell appli-
Introduction
The extensive exploration of Ru(II) complexes of terpyridine-
based (terpy) ligands has been driven by the desirable set of
photophysical properties they present and the fact that they offer
a non-chiral coordination environment about an octahedral metal
centre.1 The slight distortion of the octahedral Ru(II) environ-
ment induced by terpy coordination reduces the ligand field
compared to analogous bipyridyl ligands and renders rapid non-
radiative deactivation thermally accessible (via non-emissive
3MC (metal-centred) states). As a result, triplet excited state life-
times at ambient temperature are generally short, e.g. that of the
parent complex [Ru(terpy)2]2+ is just 0.25 ns at room tempera-
ture.2,3 Longer excited state lifetimes are desirable for most
optoelectronic applications and a number of different approaches
can be used to modify terpy ligands and to extend the lifetimes
of their Ru(II) complexes, e.g. substitution with an electron with-
drawing–donating group or the attachment of another organic
chromophore.1,4,5 In order to achieve an elongation of the
excited state lifetime, an appended chromophore needs to have a
non-emissive excited state that is similar in energy to the
3MLCT (metal-to-ligand charge transfer) state.1
cations. Inducing a bathochromic shift of MLCT absorption
bands and increasing molar extinction coefficients by extension
of π-conjugation in the ancillary ligand of Ru(II) polypyridyl
complexes are system optimisations which have shown promis-
ing photovoltaic results, e.g. excellent incident photon-to-current
conversion efficiencies (IPCE).6,7
All-benzenoid polyaromatic hydrocarbons (PAHs) are planar,
aromatic platforms consisting of a number of fused benzene
rings. They display a strong propensity towards π–π induced
aggregation and interesting electronic properties due to their
high degree of electron delocalization.8 Typically, PAHs are all-
carbon sheets, with limited possibilities for the coordination of
transition metal centres. As a consequence, our interest was
drawn to the easily derivatised hexa-peri-hexabenzocoronene
(HBC) subunit, containing thirteen fused aromatic rings.
Recently, organometallic complexes of HBC have been gener-
ated via π-coordination of a {Rh(PiBu3)2}+ fragment, bound η6
to an outer aromatic ring,9 and σ-coordination of a Pt(II) acety-
lide to two aromatic HBC platforms.10 The preparation of all-
carbon HBCs which bear substituent tridentate ethynyl-terpy
ligands has been achieved11 and incorporating bidentate nitro-
gen-based binding sites into PAHs is a recent, though syntheti-
cally challenging, advance in the generation of new luminescent
materials.12,13
School of Chemistry, Trinity College Dublin, Dublin 2, Ireland.
E-mail: smdraper@tcd.ie; Fax: +3531 671 2826; Tel: +353 1 896 2026
†Electronic supplementary information (ESI) available: The crystallo-
graphic data and refinement details for compounds 1 and 2. ORTEP dia-
In this new work, we hoped to combine the photophysical
properties of the [Ru(terpy)2]2+ fragment with that of a PAH.
The resultant bichromophoric ligand would electronically
connect the π-conjugated platform to the terpyridine ligand, pro-
viding a concomitant red-shift of MLCT absorption band and an
increased molar extinction coefficient. Such a system would
1
grams of 1 and 2. H NMR spectra for novel compounds 3, 4, 6 and 8.
CCDC reference numbers 843688 and 843689. For ESI and crys-
tallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30548h
‡These authors contributed equally to this work.
7746 | Dalton Trans., 2012, 41, 7746–7754
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of ligands 1–3 were key targets for optical investigation as parent
complexes, prior to any subsequent piperazinyl dithiocarbamate
functionalisation of the unsubstituted terpyridine ligand.
Results and discussion
Synthesis
The terpyridine motif was introduced into pentaphenylbenzenes
1 and 2 through the versatile intermediate 4′-[{(trifluoromethyl)-
sulfonyl}oxy]-2,2′ : 6′,2′′-terpyridine.21 This was reacted with
either phenylacetylene or 4-tert-butyl-phenylacetylene in
order to produce the appropriate acetylene precursor; 4′-(phenyl-
ethynyl)-2,2′ : 6′,2′′-terpyridine22 or 4′-(4-tert-butyl-phenylethy-
nyl)-2,2′ : 6′,2′′-terpyridine (5, Scheme 1), respectively. Novel
5 was characterized by NMR spectroscopy and ESI-MS.
4′-(phenylethynyl)-2,2′ : 6′,2′′-terpyridine was then reacted with
cyclopentadienone derivatives tetraphenylcyclopentadienone and
2,3,4,5-tetra-(4-tert-butylphenyl)cyclopentadien-1-one,12 respec-
tively, producing 1-(2,2′ : 6′,2′′-terpyrid-4′-yl)-2,3,4,5,6-penta-
phenylbenzene (1) and 1-(2,2′ : 6′,2′′-terpyrid-4′-yl)-2,3,4,5,6-
penta(4-tert-butylphenyl) benzene (2). Typical Diels–Alder
cycloaddition procedures were employed (heating the reagents at
280 °C in benzophenone under an argon atmosphere for 7.5 h).
Various reaction conditions and oxidants were used to attempt
the oxidative cyclodehydrogenation of 1 and 2 (AlCl3,23
Fig. 1 Terpyridine ligands 1–4.
24
extend current synthetic routes and avoid the use of the acetyle-
nic spacer unit, previously used to augment solar-to-electrical
energy conversion efficiency.6
MoCl5 and FeCl3 8). No products could be isolated from the
cyclisation of 1, which suggests the formation of multiple C–C
bonds and consequently highly insoluble products. With this in
mind, the reaction was repeated, modifying the ligand design, to
incorporate solubilising tert-butyl groups on the periphery.
Cyclodehydrogenation of 2 using FeCl3 yielded two products
after column chromatography, which proved to be partially
cyclised 3 as the major product (32%) and an intriguing nitro-
gen-substituted hexa-peri-hexabenzocoronene derivative 4 (5%).
3 and 4 were fully characterized by NMR spectroscopy and mass
spectrometry. Although unusual, in both 3 and 4 there remain
several incomplete C–C bond closures, between the generated
PAH and the terpyridine moiety. Two of these (to the terminal
pyridyl rings) are clearly sterically hindered but the fact that 3 is
the major product would suggest that the formation of the C–C
bonds to the central pyridine ring is also disfavoured. The latter
can be rationalized by giving some consideration to the mechan-
ism of the cyclodehydrogenation reaction.
In this way two essentially isoenergetic triplet excited states
3
would be provided: a MLCT ([Ru(terpy)2]2+, λ ∼ 629 nm)14,15
and
a λ ∼
HBC-localised 3ππ* excited state (HBC,
578 nm).10,16,17 Excitation into one of these might then give rise
to efficient energy equilibration between the two complementary
energy levels and substantially prolong the excited state life-
times. Terpy units have also been shown to be effective synthetic
platforms on which to introduce dithiocarbamates as highly
stable and robust anchoring units for attachment to Au sur-
faces.18 In our Ru(II) complexes, surface self-assembly would
be driven by π-stacking interactions between the HBC-fused
aromatic platforms, providing uniform surface coverage. The
inherent hydrophobicity of the PAH segment would also
decrease the probability of water-induced dye degradation in any
Ru(II)-dye thin film.19
The precursor ligand 1 was not new. A recent publication
demonstrated the vapochromic behaviour of a platinum(II)
complex containing 1 (Fig. 1) and its potential as a colorimetric
sensor.20 Using a similar procedure we synthesised 1, as well as
its tert-butyl analogue, 2 (Fig. 1). 2 was subjected to oxidative
cyclodehydrogenation to produce two unique ligands, 3 and 4
(Fig. 1), which contain planar aromatic platforms of ten and
thirteen fused benzene rings, respectively. C–C bond formation
at all the possible cyclisation sites in 2 would have produced a
system composed of 17 fused aromatic rings. Such an outcome
was sterically disfavoured and probably undesirable, given that
the resulting extensive increase in electron delocalization would
have lowered the energy of the lowest-lying triplet excited state
to an extent that the lifetime would be expected to shorten sig-
nificantly (energy gap law).1 In any event, the Ru(II) complexes
The most plausible mechanism for the Scholl reaction (the
condensation of aromatics under the influence of a Lewis acid) is
reported to involve the generation of radical cations which can
take part in a series of cascade reactions.25,26 Under highly
acidic conditions, the initial reaction relies on protonation of the
non-planar polyphenylene to give a radical cation that takes part
in electrophilic attack and undergoes deprotonation and sub-
sequent oxidation. In the known systems where nitrogen atoms
are present in the hexaphenylbenzene precursor, cyclodehydro-
genation is initiated at pyrimidyl rings.13,27 These basic nitrogen
atoms are protonated preferentially, giving an arenium cation
which is positively charged at positions ortho and para to the
site of protonation. For pyrimidyl-substituted hexaphenyl-
benzenes such as the N-HSB and N-1/2HSB precursor, (a) in
Scheme 2, this results in the formation of an ortho C–C bond on
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Scheme 1 The synthesis of ligands 2, 3 and 4 and terpyridine–acetylene precursor 5. (i) Pd(PPh3)2Cl2, CuI, triethylamine, toluene 70 °C, 4.5 h,
75%; (ii) benzophenone 280 °C, 7 h, 52%; (iii) CH2Cl2, FeCl3 (20 eq. in CH3NO2), 24 h, 3: 34%, 4: 5%.
Scheme 2 Radical cation mechanism for the intermolecular Scholl reaction in formation of N-HSB derivatives (top) and the carbocation intermediate
formed by protonation of 2 (bottom), where C–C bond formation at sites adjacent to the pyridyl ring is disfavoured.
an adjacent ring and C–C bond formation at the pyrimidyl end
of the molecule.
between the five posterior tert-butylphenyl rings, leaving the
terpy unit less connected to the platform and 3 as the major
product.
In the case of terpyridine 2, preferential protonation of the
central pyridyl ring generates a cationic charge at the ortho and
para positions, but C–C bond formation requires activation at
the carbon in the meta position (Scheme 2). The same is also
true of protonation at the peripheral pyridyl rings. Cyclodehydro-
genation in this case proceeds in a more efficient manner
Crystal structures
Crystals suitable for single-crystal X-ray diffraction were
obtained for 1 and 2 by slow evaporation of CHCl3 and CH2Cl2
7748 | Dalton Trans., 2012, 41, 7746–7754
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Fig. 2 Perspective view of crystal packing in 1 (solvent molecules
omitted for clarity).
solutions, respectively.§ In the structure of 1 the asymmetric unit
contains one molecule of the ligand and three molecules of
CHCl3. The terpy end of ligand 1 is bent by 56.8° with respect
to the central benzene ring and the tilt angles for the external
phenyl rings vary from 65° to 70°. Fig. 2 shows the organisation
between molecules of 1, which are not involved in any form of
π-stacking. The perspective view shows that the molecules form
head-to-tail columns.
Fig. 3 (a) The π–π stacking interaction between two molecules of 2.
(b) The packing arrangement between molecules of 2 as viewed along
the y axis (solvent molecules omitted for clarity).
In the crystal structure of 2, the asymmetric unit contains one
molecule of 2 and two molecules of CH2Cl2. The terpyridine
unit is almost perpendicular to the central benzene ring (86.9°),
while the five tert-butylphenyl rings are typically propeller-like
and twisted between 64° and 82° with respect to the central
benzene ring. Ligand 2 shows intermolecular π-stacking at the
terpy end of the molecule with a distance of 3.74 Å between the
molecules (Fig. 3a). Fig. 3b shows the extended packing
arrangement between molecules of 2 along the y axis. The mole-
cules are shown in yellow and grey layers which associate
through this dimeric π–π interaction.
§Crystallographic data. A suitable crystal of each compound 1 and 2
was selected and mounted using inert oil on a 0.30 mm quartz fibre tip
and immediately placed on the goniometer head in a 123 K N2 gas
stream. The data was collected on a Bruker Smart Apex CCD Diffracto-
meter using graphite-monochromated MoKα (λ = 0.71073 Å) and
Bruker Smart Version 5.625 software run in multirun mode. 2400 image
frames of 0.3° per frame were recorded. Data integration and reduction
were carried out using Bruker Saint+ Version 6.45 software31 and cor-
rected for absorption and polarization effects using SADABS Version
2.10 software.32 Space group determination, structure solution and
refinement were obtained using Shelxtl ver. 6.14 software.33 The struc-
tures both converged to a reasonable R factor. All the non-hydrogen
atoms were refined anisotropically, and the hydrogen atoms were fixed
from the residual electron density using appropriate HFIX commands.
Intermolecular interactions were computed using the PLATON program.
Further details and ORTEP diagrams are provided in the Supporting
Information.† CCDC-843688 (1) and CCDC-843689 (2) contain the
supplementary crystallographic data for this paper.†
Ru(II) complexes
Ru(II) complexes of the terpyridine ligands were synthesised;
homoleptic complex [Ru(1)2](PF6)2 (6) was produced by react-
ing ligand 1 directly with RuCl3 under reducing conditions
while the heteroleptic complexes [Ru(terpy)(2)](PF6)2 (7) and
[Ru(terpy)(3)](PF6)2 (8) were prepared by reacting the respective
ligands with the precursor [Ru(terpy)Cl3].28 Unfortunately, it
was not possible to produce ligand 4 in sufficient amounts
to facilitate synthesis of its corresponding Ru(II) complex,
[Ru(terpy)(4)](PF6)2. Heteroleptic complexes 7 and 8 were pre-
pared so that the influence of the large aromatic ligands on the
excited state of the [Ru(terpy)2]2+ could be determined. All the
compounds were characterized by NMR spectroscopy and gave
satisfactory ESI-MS results.
Crystal data for 1.3CHCl3. C54H38Cl9N3, Mr = 1047.92, monoclinic,
space group = P2(1)/n, a = 11.5279(5) Å, b = 17.3151(8) Å, c =
25.3288(12) Å, β = 95.0820°, U = 5035.9(4) Å3, T = 153(2) K, Z = 4,
44 768 reflections measured, 10 440 unique (Rint = 0.0285) which were
used in all calculations. The final R1 (all data) was 0.0595 and wR2
(all data) was 0.1707.
Crystal data for 2.2CH2Cl2. C73H79Cl14N3, Mr = 1140.2, triclinic,
Absorption spectroscopy
ˉ
space group = P1, a = 12.2579(7) Å, b = 16.5216(9) Å, c = 18.2312(10)
Å, α = 66.3110°, β = 70.3760°, γ = 76.7660°, U = 3165.7(3) Å3, T =
The UV-vis absorption spectra for ligands 1–4 in acetonitrile are
presented in Fig. 4. 1 and 2 show very similar absorption
profiles, arising from π–π* and n–π* transitions, with absorption
153(2) K, Z = 2, 31 713 reflections measured, 12 437 unique (Rint
=
0.0297) which were used in all calculations. The final R1 (all data) was
0.0801 and wR2 (all data) was 0.2070.
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Fig. 5 Normalized absorption data of Ru(II) complexes 6, 7 and 8 in
CH3CN at room temperature (∼10−5 M).
Fig. 4 Normalized absorption spectra for ligands 1–4, in CH3CN at
room temperature (∼10−5 M).
Table 1 Absorption maxima for ligands 1–4 and Ru(II) complexes 6–8
in CH3CN at room temperature (∼10−5 M)
abs
Compound
λ
(nm)
max
1
2
3
4
221, 254, 279
220, 248, 279
224, 244, 280sh, 323, 343, 372, 439
231, 335sh, 350sh, 371, 405sh
6
7
8
278, 310
271, 308
271, 309, 338
270, 307
486
484
488
475
[Ru(terpy)2]2+ 29
maxima around λ 250 and 279 nm. In the spectrum of 3, a
number of additional lower energy transitions involving the aro-
matic platform appear (Table 1). These are in the same region as
the π–π* transitions observed in HBC derivatives, although the
shape of the absorption profile differs due to the smaller aromatic
platform in this case. For 4, absorptions related to the π–π* tran-
sitions of the planar aromatic platform are shifted to longer
wavelengths (around λ 371 nm), as is typical for HBC-type com-
pounds and are broadened and less structured due to enhanced
aggregation effects.10,11
Ru(II) complexes 6, 7 and 8 show a number of ligand-centred
π–π* bands at higher energy wavelengths and a lower energy
MLCT (d–π*) transition at around λ 485 nm (Fig. 5).30 The
ligand-centred (LC) bands show a red shift of 25–30 nm on
coordination; for example the band observed at λ 280 nm in the
free ligands appears at approximately λ 310 nm in all the com-
plexes. The spectrum of 8 has a larger number of LC bands com-
pared with the other complexes (Fig. 5) due to the extended π
conjugation of its heteroleptic ligand. The relative energies of
the MLCT transitions are similarly affected by the ligands and
are lowest for 8 (λ 488 nm) and highest for 7 (λ 484 nm), where
the electron donating effect of the five tert-butyl substituents
exerts an influence. On comparison with [Ru(terpy)2]2+, all three
complexes show red-shifted MLCT-based absorptions, however
the effect of the differing 4′-substituents is surprisingly small.
(Unsubstituted [Ru(terpy)2]2+ has an MLCT transition at λ
475 nm29 whereas [Ru(phterpy)2]2+ complex absorbs at λ
487 nm5).
Fig. 6 Normalised emission of Ru(II) complexes 6, 7 and 8 at 77 K in
4 : 1 ethanol–methanol (∼10−5 M).
Emission spectroscopy
All three complexes are non-emissive at room temperature but
luminesce at 77 K (Fig. 6). Such behaviour is similar to that of
the parent complex [Ru(terpy)2]2+ and several other Ru(II)
complexes containing terpyridine-based ligands.30 The effect is
3
due to the coupling of the emitting MLCT state with a non-
emissive metal-centred triplet state (3MC), which provides an
efficient non-radiative deactivation pathway at ambient tempera-
ture. At lower temperature the 3MC is no longer thermally acces-
sible and 3MLCT-based emission is observed. The emission
profiles of 6–8 exhibit significant vibrational fine structure and
3
are typical of MLCT emission at low temperature. 8 emits at a
maximum of λ 620 nm at 77 K, representing a red shift of 12 nm
compared with 7 (λ 607 nm), as a consequence of its increased
electron delocalization and the lowering of the LUMO. Extended
aromatic frameworks are known to lower the energy of the
LUMO, resulting in emission at longer wavelengths, e.g. [Ru-
(bpy)2(NHSB)]2+ and [Ru(bpy)2(1/2-NHSB)]2+ emit in the near-
IR region (λ 880 nm12 and λ 868 nm13) whereas emission from
[Ru(bpy)3]2+ occurs at λ 626 nm. The relatively small stabili-
zation of the emission energy of 8 vs. 7 suggests that communi-
cation between the terpyridine and polyaromatic regions of the
ligand is limited, in agreement with the UV-Vis data.
7750 | Dalton Trans., 2012, 41, 7746–7754
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Table 2 Emission data for Ru(II) complexes 6, 7 and 8 (77 K, 4 : 1
ethanol–methanol) (∼10−5 M)
complexes which were found to have absorption and emission
characteristics similar to those of the parent complex [Ru-
(terpy)2](PF6)2. This implies minimal interaction between the
[Ru(terpy)2]2+ unit and the polylphenyl/PAH appendages in
these complexes, and is thought to be a consequence of the non-
planar arrangement of the substituent and terpy end groups. In
order to optimise electronic communication, the work points to
the need to generate chromophores that adopt a co-planar
arrangement and gives further impetus to the formation of 4 and
derivatives like it.
Complex
Emission (λmax/nm)
τ (μs)
6
7
8
606, 660
607, 660
620, 671
11 ( 0.02)
11 ( 0.02)
12 ( 0.01)
For the tris-bpy type [Ru(bpy)2(NHSB)]2+ complexes, the
fused NHSB ligand is directly attached to the metal centre whilst
in 8, the chelating terpy unit does not form part of the fused aro-
matic structure. In fact, given the crystal structure of 2 and the
additional unfavourable interactions between the hydrogen atoms
ortho to the inter-annular bond and tert-butyl substituent groups
in 3, it is highly likely that the [Ru(terpy)2]2+ chromophore and
the fused aromatic sheet have limited interaction both in the
ground state and excited state due to a forced perpendicular
arrangement.
Experimental
General methods
Tetraphenylcyclopentadienone (Aldrich) and RuCl3 (Johnson
Matthey) were used as received. 4′-[{(trifluoromethyl)sulfonyl}-
oxy]-2,2′ : 6′,2′′-terpyridine,21 4′-(phenylethynyl)-2,2′ : 6′,2′′-ter-
pyridine,34 2,3,4,5-tetra-(4-tert-butylphenyl)cyclopentadien-1-
one12 and [Ru(terpy)Cl3]28 were prepared according to literature
procedures. 1 was prepared by a different method to that pre-
viously published20 thus the procedure is presented below. All
reactions were carried out under inert conditions. Nuclear mag-
netic resonance spectra were recorded in deuterated acetonitrile
or chloroform with a Bruker Avance DPX-400 MHz spectro-
meter at the following frequencies: 400.13 MHz for 1H and
The excited-state lifetimes of complexes 6, 7 and 8 were
measured at low temperature in 4 : 1 ethanol–methanol solutions
using a single photon photomultiplier detection system. The
excitation source used was a picosecond NanoLED laser fixed at
em
max
λ 460 nm; emission was detected at λ
= 610 nm for 6 and 7
and at λ 620 nm for 8. Complex 8 has a slightly longer-lived
excited state lifetime than the other two complexes, at 12 μs
(Table 2).
1
100.6 MHz for 13C. The signals for H and 13C spectra were
The excited state lifetimes of 6, 7 and 8 are very similar to
those of [Ru(terpy)2]2+ (τ = 10.6 μs) and [Ru(phterpy)2]2+ (τ =
11.9 μs).5 This is indicative of the relatively limited influence
exerted by the polyphenyl and PAH groups in complexes 6–8 on
the [Ru(terpy)2]2+ excited state. Work is currently underway
toward modifying the synthesis of 4 and the preparation of
[Ru(terpy)(4)](PF6)2. This ligand would not be expected to
demonstrate the same steric constraints as a portion of the
terpyridyl fragment is directly incorporated into the PAH back-
referenced to TMS at δ 0.0 ppm, coupling constants
were recorded in hertz (Hz) to two decimal places. 13C signals
were assigned with the aid of DEPT 145 and DEPT 90 experi-
ments. 2-D correlation spectra were employed to assign the
1H and 13C peaks. Homonuclear correlation spectroscopy was
1
performed using TOCSY or H–1H COSY experiments, hetero-
nuclear correlation spectroscopy was performed using HSQC,
HMQC or HMBC (long-range) experiments. Electrospray mass
spectra were recorded on a Micromass-LCT spectrometer and
accurate mass spectra were referenced against leucine enkephalin
([M + H]+ m/z 556.2771 g mol−1) and reported to within 5 ppm.
The MALDI-TOF mass spectrum of 4 was recorded on a Waters
MALDI-QTOF premier spectrometer using a DCTB matrix
(trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malo-
nonitrile) and accurate mass spectra were referenced against
[Glu1]-Fibrinopeptide B ([M + H]+ m/z 1570.6768 g mol−1) and
reported to within 5 ppm. Elemental analyses were performed in
the Microanalytical Laboratory, University College Dublin.
Compounds were thoroughly dried in vacuo. However, despite
multiple attempts, it was not possible to obtain consistent, accu-
rate elemental analysis results for 4. This finding is not uncom-
mon where large thermally robust polyaromatic compounds are
involved and can be attributed to incomplete combustion of the
3
bone. Therefore, a more ready lowering of the MLCT excited
state would be anticipated in addition to more efficient equili-
bration between the 3MLCT and 3ππ* energy levels. These
results emphasize the electronic and steric factors that need to be
taken into consideration in designing ligands that successfully
tune the excited state of [Ru(terpy)2]2+ complexes. Optimisation
of the synthesis of 4 and its complex, in combination with
further modification to attach a piperazinyl dithiocarbamate
moiety to the unsubstituted terpyridine ligand, should produce
systems with MLCT absorption bands sufficiently red-shifted to
optimise absorption of sunlight photons throughout the visible
light spectrum.
Conclusion
1
carbon framework. In the alternative the H NMR spectrum and
the experimental (+)-MALDI-TOF spectrum and (M)+ simulated
isotopic distribution pattern are provided for compound 4 in the
ESI (Fig. S3 and S4†).
Four 4′-substituted terpyridine ligands have been prepared, two
of which contain large polyaromatic hydrocarbon fragments.
Of these, 3 and 4, support fused systems comprising ten and thir-
teen rings and are the newest members of an exclusive family of
ligand-functionalised PAHs. The dominance of 3 as the major
product on the oxidative cyclodehydrogenation of polyphenylene
2 led us to rationalize the effect of pyridyl rings on the Scholl
reaction. Ligands 1–3 were used to prepare a series of Ru(II)
Photophysical measurements
All photophysical studies were carried out with solutions con-
tained within 1 × 1 cm2 quartz cells in HPLC grade solvents and
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1-(2,2′ : 6′,2′′-Terpyrid-4′-yl)-2,3,4,5,6-penta-(4-tert-butylphenyl)-
benzene (2). 5 (0.127 g, 0.33 mmol), 2,3,4,5-tetra-(4-tert-butyl-
phenyl)cyclopentadienone (0.2 g, 0.33 mmol) and benzophe-
none (1.5 g) were heated at 280 °C for 7.5 h under argon.
Carbon dioxide was evolved and colour changed from purple to
brown. After cooling the product was purified by column chrom-
atography (SiO2, CHCl3–MeOH, 30 : 1) to give 2 as a white
powder (0.17 g, 56%). Anal. Calcd for C71H75N3: C, 87.88;
H, 7.79; N, 4.33. Found: C, 87.68; H, 7.71; N, 4.24. ESI-MS
(CH3OH) m/z = 970.6018 [M + H]+ (calcd for C71H76N3:
970.6039). δH (400 MHz; CDCl3; Me4Si) 8.60 (d, 2H, J =
4 Hz), 8.11 (d, 2H, J = 8 Hz), 7.83 (s, 2H), 7.67 (ddd, 2H, J = 8,
1.5 Hz), 7.20 (ddd, 2H, J = 4.5 Hz), 6.84–6.72 (m, 20H), 1.13
(s, 9H), 1.12 (s, 18H), 0.95 (s, 18H) ppm. δC (101 MHz; CDCl3;
Me4Si) 155.1 (2C), 153.5 (2C), 150.8, 149.0 (2C), 147.4, 147.0,
140.9, 140.2, 139.3, 136.7 (2C), 135.7, 131.1, 131.0, 124.3
(2C), 123.0, 122.6, 122.4 (2C), 121.0 (2C), 33.5 (5C), 30.7
(9C), 30.4 (6C) ppm.
were degassed using argon bubbling. UV-visible absorption
spectra were recorded on a Shimadzu UV-2450 spectrophoto-
meter and the data was analysed using UVProbe software.
Fluorescence spectra were recorded on a Varian Cary Eclipse
luminescence spectrometer. The excited-state lifetimes of com-
plexes 6, 7 and 8 were measured at low temperature in 4 : 1
ethanol–methanol solutions using a single photon photomulti-
plier detection system. The excitation source used was a pico-
second NanoLED laser fixed at 460 nm; emission was detected
em
max
at λ
= 610 nm for complexes 6 and 7 and at 620 nm for
complex 8. The resulting data were collected and lifetimes were
determined from the observed decays with the F900 software
(Edinburgh Instruments). The decay was fitted using a first order
exponential decay model (Fit = A + Be(−τ/τ ) where A = back-
1
ground constant, B = the pre-exponential factor and τ1 is the life-
time of the excited state) and the goodness of fit was evaluated
for each result by considering the χ2 values and inspecting the
residual plots.
1-(2,2′ : 6′,2′′-Terpyrid-4′-yl)-2,3,4,5,6-pentaphenylbenzene (1).
4′-(phenylethynyl)-2,2′ : 6′,2′′-terpyridine (0.3 g, 0.9 mmol),
tetraphenylcyclopentadienone (0.33 g, 0.9 mmol) and benzo-
phenone (3.5 g) were heated at 280 °C for 7.5 h under argon.
Carbon dioxide was evolved and the colour changed from purple
to brown. After cooling the product was purified by column
chromatography (SiO2, CHCl3–MeOH, 30 : 1 then the polarity
was increased by elution with CHCl3–MeOH–NH3aq, 10 : 1 : 1)
giving 1 as a white powder (0.3 g, 48%). Anal. Calcd for
C51H35N3·1/2(CH3OH): C, 87.63; H, 5.28; N, 5.95. Found:
C, 87.78; H, 5.30; N, 5.86. ESI-MS (MeOH) m/z 690.2918 [M +
H]+ (calcd for C51H36N3: 690.2909). δH (400 MHz; CDCl3;
Me4Si) 8.61 (d, 2H, J = 4 Hz), 8.36 (d, 2H, J = 8 Hz), 8.05 (s,
2H), 7.35 (ddd, 2H, J = 7.6 and 1.5 Hz), 7.24 (ddd, 2H, J = 4.5
and 1 Hz), 7.02 (d, 4H, J = 7 Hz), 6.86 (m, 20H), 6.79 (t, 2H,
J = 7.5 Hz) ppm. δC (101 MHz; CDCl3; Me4Si) 155.6 (2C),
153.2 (2C), 150.6 (2C), 148.5 (2C), 140.5, 140.0 (2C), 139.9,
139.8 (2C), 139.4 (2C), 139.3 (2C), 137.7, 136.0 (2C), 130.8,
130.7 (4C), 126.3, 126.1, 125.1 (2C), 124.8, 123.4 (2C), 122.8
(2C), 120.4 (2C) ppm.
4′-3,6,9,12,15-Penta-tert-butyldibenzo[fg,ij]phenanthro[9,10,1,
2,3-pqrst]pentaphen-18-yl)-2,2′ : 6′,2′′-terpyridine (3) and (4). A
solution of iron(III) chloride (0.5 g, 3 mmol) in nitromethane
(4.5 mL) was added drop-wise to a stirring solution of 2 (0.15 g,
0.154 mmol) in dichloromethane (35 mL). Argon was bubbled
though the mixture for 2 h, then the mixture was stirred under an
argon atmosphere for a further 48 h. The reaction was quenched
with methanol (50 mL), washed with water and extracted into
dichloromethane, dried MgSO4 and the solvent removed under
reduced pressure. The crude product was purified by column
chromatography (SiO2). Terpyridine 4 was eluted first (8 mg,
5%), using a solvent mixture of CHCl3–MeOH (30 : 1, and
increasing the polarity to 4 : 1). Partially fused 3 was eluted by
increasing the solvent polarity to CHCl3–MeOH (1 : 1) and
finally to CHCl3–MeOH–NH3(aq) (1 : 1 : 0.5) giving 3 (50 mg,
32%).
4: MALDI-TOF MS (CH3CN) m/z 957.5010 [M]+ (calcd for
C71H63N3: 957.5022). δH (101 MHz; CDCl3; Me4Si) 9.33
4
4
(s, 2H), 9.32 (d, 2H, J = 1.5 Hz), 9.26 (d, 2H, J = 1.5 Hz),
9.15 (d, 2H, 4J = 1.5 Hz), 8.78 (d, 2H, 3J = 4.3 Hz), 8.45 (d, 2H,
3
4′-(4-tert-Butyl-phenylethynyl)-2,2′ : 6′,2′′-terpyridine (5). Copper
iodide (25 mg, 0.125 mmol) was added to a degassed solution of
4-tert-butyl-phenylacetylene (0.389 g, 2.46 mmol), 4′-[{(trifluoro-
methyl)sulfonyl}oxy]-2,2′ : 6′,2′′-terpyridine (0.62 g, 1.63 mmol)
and [Pd(PPh3)2Cl2] (89 mg, 0.127 mmol) in toluene–triethyl-
amine (1 : 1, 25 ml). The reaction mixture was stirred at 70 °C for
3 h. Solvents were removed under reduced pressure and then the
product was purified by column chromatography (SiO2, DCM–
acetone 49 : 1) giving 5 as a white powder (0.49 g, 77%). Anal.
Calcd for C27H23N3·1/3H2O: C, 82.00; H, 6.03; N, 10.62. Found:
C, 81.82; H, 5.94; N, 10.54. ESI-MS (CH3OH) m/z = 390.1976
[M + H]+, (calcd for C27H24N3: 390.1970). δH (400 MHz; CDCl3;
Me4Si) 8.75 (d, 2H, J = 5.52 Hz), 8.65 (d, 2H, J = 8 Hz), 8.59 (s,
2H), 7.89 (ddd, 2H, J = 8 Hz, 2 Hz), 7.55 (m, 2H, J = 8.5 Hz),
7.44 (m, 2H, J = 8.5 Hz), 7.37 (ddd, 2H, J = 4.5 Hz, 1 Hz), 1.36
(s, 9H) ppm. δC (101 MHz; CDCl3; Me4Si) 155.3 (2C), 155.0
(2C), 154.7, 151.9 (2C), 148.7, 136.5 (2C), 133.3 (2C), 131.3,
125.1 (4C), 123.6 (2C), 122.4 (2C), 120.8 (2C), 119.0, 116.2
(4C), 34.5, 30.1 (3C) ppm.
4J = 1.5 Hz), 8.22 (d, 2H, 4J = 7.8 Hz), 7.98 (dd, 2H, J =
4
3
4
7.7 Hz, J = 1.8 Hz), 7.44 (ddd, 2H, J = 7.7, 5.0 Hz, J =
1.1 Hz), 1.83 (s, 9H), 1.81 (s, 18H), 1.35 (s, 18H) ppm. δC
(101 MHz; CDCl3; Me4Si) 150.0 (2C), 149.7 (4C), 149.2 (4C),
134.1 (2C), 131.0 (4C), 130.5 (4C), 130.4 (4C), 130.3 (2C),
126.7 (6C), 125.2 (2C), 124.2 (2C), 123.6 (2C), 121.6 (2C),
120.7 (2C), 119.3 (5C), 119.2 (4C), 35.9 (2C), 35.8 (4C), 35.2
(4C), 32.0 (9C), 31.5 (6C) ppm.
3: Anal. Calcd for C71H67N3·CH3OH: C, 87.32; H, 6.82; N,
4.24. Found: C, 87.03; H, 6.61; N, 4.37. ESI-MS (MeOH) m/z
962.5399 [M + H]+ (calcd for C71H68N3: 962.5413). δH
(400 MHz; CDCl3; Me4Si) 9.16 (s, 2H), 9.11 (s, 2H), 8.98
(s, 2H), 8.69 (s, 4H), 8.67 (d, 2H, J = 7.5 Hz), 8.62 (d, 2H, J =
4 Hz), 7.91 (ddd, 2H, J = 7.5, 1.5 Hz), 7.51 (d, 2H, J = 9 Hz),
7.33 (ddd, 2H, J = 6, 1 Hz), 6.98 (d, 2H, J = 8.5 Hz), 1.82 (s,
9H), 1.78 (s, 18H), 1.38 (s, 18H) ppm. δC (101 MHz; CDCl3;
Me4Si) 156.2 (2C), 155.8 (2C), 155.2 (2C), 148.9 (2C), 148.7,
136.2 (2C), 132.0, 131.5 (2C), 130.9, 129.8, 129.3, 127.3 (2C),
125.5, 124.5, 123.1 (2C), 122.7, 122.5, 122.3, 121.7 (2C), 120.8
7752 | Dalton Trans., 2012, 41, 7746–7754
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(2C), 118.8 (2C), 118.5 (2C), 118.4 (2C), 117.5 (2C), 35.3, 35.2
(2C), 34.3 (2C), 31.6 (3C), 31.5 (6C), 30.7 (6C) ppm.
N, 5.24. Found: C, 64.21; H, 5.31; N, 5.15. ESI-MS (CH3CN)
m/z 648.775 [M − 2PF6]2+ (calcd 648.550). δH (400 MHz;
CD3CN; Me4Si) 9.36 (s, 2H), 9.32 (s, 2H), 9.21 (s, 2H), 9.09
(s, 2H), 8.93 (s, 2H), 8.88 (d, 2H, J = 8 Hz), 8.65 (d, 2H, J = 8
Hz), 8.52 (t, 1H, J = 8 Hz), 8.19 (d, 2H, J = 8 Hz), 8.11 (dd, 2H,
J = 7.5 Hz), 7.85 (m, 4H), 7.78 (d, 2H, J = 5.5 Hz), 7.51 (dd,
2H, J = 6 Hz), 7.47 (d, 2H, J = 5 Hz), 7.27 (d, 2H, J = 8.5 Hz),
7.21 (dd, 2H, J = 6 Hz), 1.85 (s, 9H), 1.83 (s, 18H), 1.42 (s,
18H) ppm. δC (101 MHz; CD3CN; Me4Si) 158.3, 158.1, 156.0,
155.2, 152.8, 152.5 (2C), 151.6 (2C), 150.8, 150.6, 150.4, 138.3
(2C), 137.9 (2C), 135.9, 132.7, 132.0 (2C), 130.4, 130.2, 130.0,
129.7, 127.9 (2C), 127.4 (2C), 127.3 (2C), 127.1 (2C), 125.1,
124.6 (2C), 124.1 (2C), 123.9 (2C), 122.9, 122.6, 122.0, 121.8,
121.0, 120.2 (2C), 119.8 (2C), 119.7 (2C), 118.7 (2C), 35.6,
35.5 (2C), 34.5 (2C), 30.98 (6C), 30.92 (9C) ppm.
[Ru(1)2](PF6)2 (6). 1 (50 mg, 0.73 mmol) was dissolved in a
1 : 1 mixture of ethanol–chloroform (5 mL). The solution was
degassed by bubbling a stream of argon through the flask. After
30 min RuCl3 (7.5 mg, 0.36 mmol) was added and then the reac-
tion mixture was flushed with argon for 20 min. 4 drops of
N-ethylmorpholine were added and then the reaction mixture
was refluxed for 6 h. After cooling, the mixture was filtered and
the chloroform was evaporated under reduced pressure. To the
ethanol solution a saturated solution of KPF6 was added and a
deep orange precipitate was formed. The crude product was
purified by column chromatography (Al2O3, CH3CN–sat. aq.
KNO3–H2O, 10 : 0.5 : 1.5) giving 6 (19 mg, 30%.). Anal. Calcd
for C102H70N6P2F12Ru: C, 69.19; H, 3.98; N, 4.75. Found:
C, 68.98; H, 4.04; N, 4.86. ESI-MS (CH3CN) m/z 740.2365
[M − 2PF6]2+ (calcd. 740.2352). δH (400 MHz; CD3CN; Me4Si)
8.14 (s, 4H), 8.06 (d, 4H, J = 8 Hz), 7.84 (dd, 4H, J = 7.5 Hz),
7.25 (d, 8H, J = 7.5 Hz), 7.18 (dd, 4H, J = 6.5 Hz), 7.09–6.96
(m, 38H), 6.90 (t, 4H, J = 7 Hz), 6.60 (d, 4H, J = 5.5 Hz) ppm.
δC (101 MHz; CD3CN; Me4Si) 158.1, 157.1, 156.2, 152.6,
150.5 (2C), 149.2, 141.9, 140.3, 139.6, 139.5, 139.3, 139.2,
137.6 (2C), 136.7, 131.3 (4C), 130.8 (4C), 130.7 (2C), 127.0
(2C), 126.7 (4C), 126.5 (4C), 126.4 (2C), 125.9 (2C), 125.5
(2C), 125.4 (1C), 125.3 (2C), 123.5 (2C) ppm.
Acknowledgements
The authors thank Dr John O’Brien, Dr Manuel Reuther and
Dr Martin Feeney for technical assistance. The work presented
was supported financially by Science Foundation Ireland
[SFIPICA-05] (AG, FM, CF), [SFIRFP-09-MTR2366] (DN), a
European Union FP6 Marie Curie award [EU-FP6-014472]
(VFM) and an Irish Research Council postdoctoral award
[IRCSET08] (NL).
[Ru(terpy)(2)](PF6)2 (7). 2 (20 mg, 0.021 mmol) was dis-
solved in ethylene glycol (4 mL) and the mixture was degassed
by bubbling a stream of argon through the flask for 30 min. Ru-
(terpy)Cl3 (9.1 mg, 0.021 mmol) was added and the reaction
mixture was heated at 90 °C for 6 h. After cooling, a saturated
solution of KPF6 was added to obtain a deep orange precipitate.
The crude product was purified by preparative TLC (SiO2,
CH3CN–KNO3–H2O, 10 : 0.5 : 1.5) giving 7 (9.7 mg, 30%).
Anal. Calcd for C86H86N6P2F12Ru: C, 64.77; H, 5.44; N, 5.27.
Found: C, 64.49; H, 5.28; N, 5.49. ESI-MS (CH3CN) m/z
652.2985 [M − 2PF6]2+ (calcd 652.2979). δH (400 MHz;
CD3CN; Me4Si) 8.74 (d, 2H, J = 8 Hz), 8.50 (d, 2H, J = 8 Hz),
8.39 (t, 1H, J = 8 Hz), 7.96 (dd, 2H, J = 8 Hz), 7.92 (s, 2H),
7.84 (d, 2H, J = 7.5 Hz), 7.77 (dd, 2H, J = 7.5 Hz), 7.34 (dd,
2H, J = 6.5 Hz), 7.19 (dd, 2H), 7.16–7.08 (m, 8H), 7.06 (d, 2H),
7.04 (d, 2H), 7.02 (d, 2H, J = 8 Hz), 6.96 (m, 6H, J = 8 Hz),
6.84 (d, 4H, J = 8 Hz), 1.16 (s, 27 H), 0.81 (s, 18 H) ppm. δC
(101 MHz; CD3CN; Me4Si) 158.1, 157.3, 155.0, 152.7, 152.3
(2C), 150.9 (2C), 149.3, 149.0, 148.6, 148.3, 143.2, 141.1,
140.0, 138.0 (2C), 137.7 (2C), 136.9, 136.5, 135.6, 135.5 (2C),
131.9, 130.8, 130.6, 129.6, 127.5 (2C), 127.0 (2C), 128.2,
126.2, 124.4 (2C), 123.9 (2C), 123.7 (2C), 123.5 (2C), 123.3,
33.7 (5C), 30.32 (3C), 30.30 (6C), 30.1 (6C) ppm.
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[Ru(terpy)(3)](PF6)2 (8). 3 (20 mg, 0.021 mmol) was dis-
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by bubbling a stream of argon through the flask for 30 min.
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CH3CN–KNO3–H2O, 10 : 0.5 : 1.5) yielding 8 (9.9 mg, 30%).
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7754 | Dalton Trans., 2012, 41, 7746–7754
This journal is © The Royal Society of Chemistry 2012