Efficient intramolecular charge transfer in oligoyne-linked donor-π-acceptor molecules

Article abstract of DOI:10.1002/chem.200902099

Studies are reported on a series of triphenylamine-(C≡C)_n-2,5-diphenyl-1,3,4-oxadiazole dyad molecules (n = 1-4, 1, 2, 3 and 4, respectively) and the related triphenylamineC₆H₄-(C≡C)₃-oxadiazole dyad 5. The oligoyne-linked D-π-A (D = electron donor, A = electron acceptor) dyad systems have been synthesised by palladium-catalysed cross-coupling of terminal alkynyl and butadiynyl synthons with the corresponding bromoalkynyl moieties. Cyclic voltammetric studies reveal a reduction in the HOMO-LUMO gap in the series of compounds 1-4 as the oligoyne chain length increases, which is consistent with extended conjugation through the elongated bridges. Photophysical studies provide new insights into conjugative effects in oligoyne molecular wires. In non-polar solvents the emission from these dyad systems has two different origins; a locally excited (LE) state, which is responsible for a π*-→π fluorescence, and an intramolecular charge transfer (ICT) state, which produces charge-transfer emission. In polar solvents the LE state emission vanishes and only ICT emission is observed. This emission displays strong solvatochromism and analysis according to the Lippert-Mataga-Oshika formalism shows significant ICT for all the luminescent compounds with high efficiency even for the longer more conjugated systems. The excited-state properties of the dyads in non-polar solvents vary with the extent of conjugation. For more conjugated systems a fast non-radiative route dominates the excitedstate decay and follows the Engelman-Jortner energy gap law. The data suggest that the non-radiative decay is driven by the weak coupling limit.

Full text of DOI:10.1002/chem.200902099

DOI: 10.1002/chem.200902099
Efficient Intramolecular Charge Transfer in Oligoyne-Linked
Donor–p–Acceptor Molecules
Lars-Olof Pꢀlsson,*[a] Changsheng Wang,[a] Andrei S. Batsanov,[a] Simon M. King,[b]
Andrew Beeby,[a] Andrew P. Monkman,[b] and Martin R. Bryce*[a]
Abstract: Studies are reported on a
tended conjugation through the elon-
gated bridges. Photophysical studies
provide new insights into conjugative
effects in oligoyne molecular wires. In
non-polar solvents the emission from
these dyad systems has two different
origins: a locally excited (LE) state,
which is responsible for a p*!p fluo-
rescence, and an intramolecular charge
transfer (ICT) state, which produces
charge-transfer emission. In polar sol-
vents the LE state emission vanishes
and only ICT emission is observed.
This emission displays strong solvato-
chromism and analysis according to the
ꢀ
series of triphenylamine–(C C)n–2,5-
diphenyl-1,3,4-oxadiazole dyad mole-
cules (n=1–4, 1, 2, 3 and 4, respective-
ly) and the related triphenylamine-
Lippert–Mataga–Oshika
formalism
shows significant ICT for all the lumi-
nescent compounds with high efficien-
cy even for the longer more conjugated
systems. The excited-state properties of
the dyads in non-polar solvents vary
with the extent of conjugation. For
more conjugated systems a fast non-ra-
diative route dominates the excited-
state decay and follows the Engelman–
Jortner energy gap law. The data sug-
gest that the non-radiative decay is
driven by the weak coupling limit.
ꢀ
C6H4–(C C)3–oxadiazole dyad 5. The
oligoyne-linked D–p–A (D=electron
donor, A=electron acceptor) dyad sys-
tems have been synthesised by palladi-
um-catalysed cross-coupling of termi-
nal alkynyl and butadiynyl synthons
with the corresponding bromoalkynyl
moieties. Cyclic voltammetric studies
reveal a reduction in the HOMO–
LUMO gap in the series of compounds
1–4 as the oligoyne chain length in-
creases, which is consistent with ex-
Keywords: charge transfer · fluo-
rescence · oligoynes · oxadiazole ·
photodynamics · triarylamine
Introduction
i) the development of new materials for the emerging tech-
nologies of molecular and nanoscale electronics,[2] and ii) un-
derstanding and mimicking key biological molecules that
mediate electron-transfer processes, for example, photosyn-
thetic reaction centres, redox proteins and nucleic acids.[3]
Many successful strategies have been developed to control
and tune the magnitude of charge transfer through conjugat-
ed bridges by end capping with various organic donor and
acceptor groups, that is, covalent D–p–A systems (D=elec-
tron donor, A=electron acceptor)[4] or organometallic moi-
eties.[5] In these molecules the systematic variation of param-
eters such as the redox potentials of the end groups, free
energy changes in charged states, intramolecular distances
(i.e., length of the bridge), topology and relative orienta-
tions of the component units has enabled charge separation
and storage characteristics to be probed in great detail by
electrochemical and spectroscopic techniques. On the basis
of this information, organic molecules—including oligomers
and polymers[6]—can now be tailored for specific electrical
or optoelectronic applications, such as wires,[7] switches,[8]
photovoltaic cells,[9] electroluminescent devices,[10] field-
Charge- and electron-transfer dynamics through organic mo-
lecular wires are of major interest in chemistry, physics and
biology.[1] Important incentives for studying this topic stem
from both fundamental and practical aspects, in particular:
[a] Dr. L.-O. Pꢀlsson, Dr. C. Wang, Dr. A. S. Batsanov, Dr. A. Beeby,
Prof. M. R. Bryce
Department of Chemistry, Durham University
Durham DH1 3LE (UK)
Fax : (+44) 191 384 4737
E-mail: lars-olof.palsson@durham.ac.uk
m.r.bryce@durham.ac.uk
[b] Dr. S. M. King, Prof. A. P. Monkman
Department of Physics, Durham University
Durham DH1 3LE (UK)
Supporting information for this article (General experimental meth-
ods, synthetic details and characterisation data for compounds 3–5, 7,
8, 10, 13 and 14; additional photophysical data; X-ray crystallographic
data for 3, 4, 5 and 8 in CIF format. This material is available on the
WWW under http://dx.doi.org/10.1002/chem.200902099.
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FULL PAPER
effect transistors[11] and single-
molecule electrode junctions.[12]
To enhance electronic com-
munication over nanometer dis-
tances,
conjugated
D–p–A
dyads that possess delocalised p
systems and tunable HOMO–
LUMO gaps are attractive tar-
ꢀ
gets. With sufficient overlap be-
the related triphenylamine-C6H4–(C C)3–diphenyloxadia-
tween the wave functions, electron transfer along the wire
can be facilitated. In this context, a range of conjugated p-
electron bridges have been studied. Specific examples in-
clude: oligo(para-phenylene)s,[13] oligo(para-phenyleneviny-
zole 5 with the aim of probing their optoelectronic proper-
ties as a function of the D–A distance. This work provides
new insights into conjugative effects of oligoyne bridges
and, in particular, establishes that non-radiative decay in oli-
goynes follows the Engelman–Jortner energy gap law.[24]
lene)s
(OPVs),[14]
oligo(para-phenyleneethynylene)s
(OPEs),[15] oligofluorenes,[16] oligothiophenes,[17] oligoenes
and carotenoids.[18]
ꢀ ꢁ
Oligoynes [(C C )n] comprise an array of sp-hybridised
Results and Discussion
carbon atoms with approximately cylindrical electron deloc-
alisation along a one-dimensional, rigid-rod backbone[19] and
they have only rarely been studied as bridges in organic D–
p–A systems. This is largely due to the significant synthetic
challenges posed by unsymmetrical derivatives with n>
2.[19d,e] To our knowledge, the only previous systematic study
Synthesis: The general protocol for synthesising the mole-
cules 1–5 involved palladium-catalysed cross-coupling reac-
tions of terminal alkynes with terminal alkynyl bromides.
Compound 6 reacted with 1-bromo-2-trimethylsilylacetylene
(TMSA bromide) to give compound 7, which was desilylat-
ed by using potassium carbonate in MeOH/CH2Cl2 at room
temperature to give the terminal butadiyne reagent 8, which
was isolated as a crystalline solid (Scheme 1, 35% overall
yield for the two steps).
ꢀ
concerns a series of Zn-porphyrin–(C C)n–C60 dyads (n=1–
4) in which photoinduced charge separation and charge re-
combination were studied as a function of D–A distance.[20]
The observed very efficient electron transfer (small attenua-
tion factor (b) of 0.06ꢂ0.005 ꢂꢁ1) was rationalised in terms
of full p conjugation between the phenyl ring of the porphy-
rin donor, the oligoyne linker and the C60 acceptor. Oli-
goynes are linear structures through which electron trans-
port is independent of rotation around the single bonds—a
feature that makes them clearly distinct from OPVs and
OPEs, where the barrier to rotation around single bonds is
low and conjugation is interrupted when the phenyl rings
are rotated with respect to each other.[21] Previous experi-
mental studies of the optical properties of symmetrical silyl
end-capped oligoynes (n=2–10) established that they pos-
sess extensive conjugation, which was supported by theoreti-
cal models of the third-order non-linear optical response as
a function of the number of alkyne units.[22]
Scheme 1. Synthesis of terminal butadiyne derivative 8 (TMS=trimethyl-
silyl).
Compound 10 was obtained in 90% yield by bromination
of 9 as shown in Scheme 2. Reactions of 10 with 6 and 8, re-
spectively, catalysed by [PdCl2ACHTNUGTRNEUNG(PPh3)2]/CuI, under basic con-
ditions gave the corresponding hexatriyne and octatetrayne
We recently described compounds 1 and 2 as part of a
study on D–p–A dyads with different electron-donor moiet-
ies (D=tetrathiafulvalene (TTF), bithiophene, 9-(4,5-di-
methyl-1,3-dithiol-2-ylidene)fluorene and triphenylamine)
connected to electron-accepting 2,5-diphenyl-1,3,4-oxadia-
zole (OXD) units. The latter was chosen because of its ex-
cellent chemical and thermal stabilities and high photolumi-
nescence quantum yield (PLQY). We established that both
1 and 2 are strongly luminescent although the PLQY was
lower for 2 and it was concluded that intramolecular energy
transfer was less efficient through the butadiynylene bridge
(2) than the ethynylene bridge (1).[23] Based on these initial
results, we have now focused our attention on a more exten-
products 3 and 4 in 56% and 39% yields. Scheme 3 shows
ꢀ
sive series of triphenylamine–(C C)n–2,5-diphenyl-1,3,4-oxa-
diazole molecules including novel hexatriyne and octate-
trayne analogues (1, 2, 3 and 4, n=1–4, respectively) and
Scheme 2. Synthesis of compounds 3 and 4 (NBS=N-bromosuccinimide).
Chem. Eur. J. 2010, 16, 1470 – 1479
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1471

L.-O. Pꢀlsson, M. R. Bryce et al.
the synthesis of compound 5. Compound 12[25] was convert-
ed into 13 for which the polar 2-hydroxy-2-propyl protecting
group facilitated purification. Standard deprotection of 13
under basic conditions[26] gave the target alkyne reagent 14
(70% yield from the 2 steps). By analogy with the synthesis
of 3, the cross-coupling reaction of 14 with 10 gave 5 in
42% yield.
Figure 1. Molecular structures of 3, 4, 5·1= CH2Cl2 and 8, showing 50%
2
thermal ellipsoids. Minor positions of the disordered CH2Cl2 molecule
and tBu group in 5 are omitted.
tween the amino and butadiyne substituents, in spite of the
Scheme 3. Synthesis of compound 5.
ꢁ
22.58 twist around the N C(phenylene) bond.
Solution electrochemistry: The solution redox properties of
1,[23] 2,[23] 3, 4 and 5 have been studied by cyclic voltammetry
(CV) (Figure 2). As expected, the CVs of these compounds
are characterised by oxidation and reduction waves due to
the triarylamine and oxadiazole moieties, respectively.
These are irreversible waves except for the reduction of 1
that is quasi-reversible. In addition, for 2–5 irreversible re-
duction waves associated with the oligoyne bridges are ob-
served at more negative potentials. The difference between
the first oxidation and the first reduction potentials,
X-ray crystallography: Single-crystal X-ray structures were
determined for 3, 4, 5 (as a CH2Cl2 hemi-solvate) and 8
(Figure 1). In each case, the N3 atom has sp2 geometry, with
the three adjacent benzene rings inclined to its plane in a
propeller-like fashion. The triyne “rod” in 3 is bent in the
“symmetrical bow” fashion[19] with an additional spiral twist.
ꢁ
The angle between the terminal C C bonds, q, of 27.68 and
the contraction of the overall chain length, d, (compared to
the sum of the bond lengths) of 1.4% are rather high: of the
46 previously reported triynes[27] only 11 have q>148 and
only one[28] (with q=28.98, d=1.8%) is bent more strongly
than 3. The tetrayne rod in 4 adopts an “asymmetric bow”
bend of the magnitude (q=13.98, d=0.5%) not uncommon
for tetraynes with bulky, especially organometallic, end
groups.[19a,b] It can be argued that the bending is caused by
incompatible packing demands of the rods and the end
groups, whereas in 5 the bending is slight (q=3.88, d=
0.2%) as the higher packing density is achieved by solva-
tion.
Bonds in these derivatives, as in other oligoynes,[19] alter-
nate sharply between triple and single, and show no tenden-
cy to equalisation. The butadiyne geometry in 8 is similar to
those in recently reported (surprisingly stable) terminal bu-
tadiynes.[29] The phenylene ring shows a quinoidal distortion
(DCꢁC =0.023(2) ꢂ) similar to that in the planar 4-ethynyla-
niline molecule[30] (D=0.021(2) ꢂ), but only half of that in
p-nitroaniline.[31] This may indicate some conjugation be-
Figure 2. Cyclic voltammograms of compounds 1–5 in MeCN. All solu-
tions contained tetrabutylammonium hexafluorophosphate (TBAPF6,
0.1m) as the supporting electrolyte, scan rate 100 mVsꢁ1, using Pt disk
(F=1.8 mm) as the working electrode, Pt wire as the counter electrode
and Ag/AgNO3/DMF as the reference electrode.
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Intramolecular Charge Transfer
FULL PAPER
Eox1ꢁEred1, is a measure of the
HOMO–LUMO gap. A notable
trend in Figure 2 is a reduction
in the HOMO–LUMO gap in
the series of compounds 1–4 as
the oligoyne chain length in-
creases, which is consistent with
extended conjugation through
the elongated bridges. A similar
trend was noted in the CV data
ꢀ
of two Zn-porphyrin–(C C)n–
C60 dyads (n=1, 2) studied pre-
viously.[20] Interestingly, an in-
crement of +30 to +50 mV in
the value of Eox was observed
with each additional triple bond
for compounds 1–4. As noted
previously for 1 and 2,[23] and
for some oligoyne-bridged binu-
clear
organometallic
com-
pounds,[32,33] this can be as-
cribed to the electron-with-
drawing nature of the extra
triple bond. In the present D–
Figure 3. Absorption (dashed line) and luminescence (solid line) spectra for 1, 2, 3 and 5 in methylcyclohexane
(blue) and toluene (red): excitation at 390 nm (25641 cmꢁ1).
ꢀ
(C C)n–A systems we cannot
exclude the possibility that
charge delocalisation across the bridge might play a role in
causing these anodic shifts. Compound 5 has the lowest oxi-
dation potential and this may be due to the biphenyl system
interrupting conjugation with the electron-withdrawing oli-
goyne bridge. However, the irreversibility of the waves for
1–5 means that only tentative conclusions should be drawn
from the CV data. It can be noted that there is a sequential
decrease in the potential of the irreversible oxidation waves
of the triarylamine building blocks 6, 8 and 14 (see Fig-
ure S1 in the Supporting Information). This is not a case of
electron transfer along the lines of classical Marcus theory,
rather it is intramolecular charge transfer (CT), so it is not
appropriate to analyse the data by using the Weller equa-
tion.
Table 1. Data on the optical transitions of compounds 1–5.
n˜abs n˜em
Dn˜[b] mg [D] mg [D] Dm [D][d] me [D][e] me [D][d
[cmꢁ1 [cmꢁ1 [cmꢁ1 AM1[c] PM3[c]
AM1 PM3
G
]
E
]
E
]
1
2
3
4
5
25575[a] 24570[a] 1005[a] 4.91
24631[a] 23866[a] 764[a] 5.26
23474[a] 22936[a] 538[a] 4.15
4.68
5.15
4.15
4.64
4.79
3.2ꢂ0.1 8.1ꢂ0.1 7.9ꢂ0.1
3.9ꢂ0.2 9.2ꢂ0.2 9.1ꢂ0.2
4.3ꢂ0.3 8.5ꢂ0.3 8.5ꢂ0.3
21740[a]
–
–
4.27
–
–
–
24450[a] 22830 1620[a] 4.08
4.9ꢂ0.3 9.0ꢂ0.2 9.7ꢂ0.2
solvent (methylcyclohexane).
[a] Recorded
in non-polar
a
~
~
~
[b] Dn ¼ nabs ꢁ nem . [c] AM1 and PM3 are the two semiempirical calcula-
tion methods used. [d] Dm according to Equation (2). The luminescence
of 3 was recorded with relatively large slit widths (ꢃ10 nm) of the spec-
trometer. Accordingly, the centre of weight of the luminescence is poorly
defined, which prevents an accurate calculation of the Stokes shift and
thus Dm. [e] Excited-state dipole moment me calculated from the ground-
state dipole moment and the difference dipole moment: me =mg+Dm.
Photophysical studies: Table 1 summarises the absorption
and emission data (when observed) and the Stokes shift
(when observed) of 1–5 in methylcyclohexane (MCH) as ob-
tained from the data shown for 1, 2, 3 and 5 shown in
Figure 3. The data are extracted from the lowest energy ab-
sorption band (or shoulder) and the sharp peak in the emis-
sion spectra. The obtained Stokes shifts show a clear de-
crease as the conjugation is extended along the series from
one to three alkyne units, that is, compounds 1, 2 and 3, re-
spectively. We note that this is likely to be an underestimate
as different rotational conformations also contribute. The
data does, however, imply that as the electronic system be-
comes more conjugated there is less displacement of the ex-
cited-state potential energy curve relative to the ground
state.
The efficiency of intramolecular charge transfer (ICT) in
D–A dyads can be examined through optical absorption and
emission experiments in media with different solvent polari-
ty. In a low polarity solvent with a solvent density parameter
of Df=0, where Df is defined by Equation (1):
ꢀ
ꢁ
er ꢁ 1
n2D ꢁ 1
ð1Þ
Df ¼
ꢁ
2n2D þ 1
2er þ 1
where er is the dielectric constant and nD is the refractive
index, the luminescence spectra of 1, 2, 3 and 5 in MCH
(Figure 3) consist of two distinct features—a sharp and
narrow band followed by a considerably broader and gener-
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L.-O. Pꢀlsson, M. R. Bryce et al.
ally structureless component at lower energy. Interestingly,
for all other solvents (dielectric medium) where Df> 0, the
narrow high energy component is no longer observed and
the luminescence gives only a broader band devoid of any
fine structure. This is demonstrated in Figure 3 that shows
the photoluminescence (PL) in two different environments,
MCH and toluene. With respect to Equation (1), the solvent
density parameter is Df=0 for MCH, whereas Df=0.013
for toluene. This observation indicates how sensitive the ex-
cited state is to the environment and how a very small
change to more polar media will stabilise the CT state. It is,
therefore, clear that for low polarity media with Df=0 the
luminescence is due to two different contributions, namely a
higher energy narrow band caused by singlet fluorescence
(p*!p) and a broader lower energy ICT band. The exis-
tence of these two states was suggested by Khundkar
et al.[34a,b] in studies of asymmetrically end-capped dipheny-
lethyne and diphenylbutadiyne molecules—the singlet excit-
ed state has also been referred to as a locally excited (LE)
state—although the LE state eluded detection in that study.
The hypothesis that the lower energy band is due to ICT
is strengthened by absorption and emission experiments on
the solvatochromism of 1, 2, 3 and 5, by using a number of
solvents with varying polarity, as defined by the solvent den-
sity parameter [Eq. (1)]. The data give an apparent Stokes
on the centre of weight of the absorption while the lumines-
cence centre of weight is substantially more affected by the
polarity of the medium. This in turn suggests that the
ground-state dipole moment is largely unaffected, in con-
trast to the excited-state dipole moment that is considerably
more sensitive to the medium. Semiempirical calculations
(by using two different methods, see Table 1) show that the
ground-state dipole moment is approximately 4–5 D and in-
dependent of the extent of conjugation, that is, the number
of alkyne units in the bridge. This, in turn, implies that when
the D–A separation increases there is a slightly less charge
localisation in the ground state for the longer, more conju-
gated compounds. The luminescence is, however, strongly
dependent upon the properties of the dielectric medium and
considerable bathochromic shifts of the PL centre of weight
with a more polar solvent are observed. The experimental
observations were analysed by using the Lippert–Mataga–
Oshika formalism [Eq. (2)],[35]
À
Á
ꢀ
ꢁ
2
2 me ꢁ mg
er ꢁ 1
n2D ꢁ 1
abs
em
~
~
~
Dv ¼ vmax ꢁ vmax
¼
ꢁ
þ constant
ð2Þ
hca3
2er þ 1
2n2D þ 1
which relates the optical properties in various media to the
difference dipole moment Dm=meꢁmg and the radius of the
Onsager cavity, a. As shown in Figure 4, plots of the Stokes
abs
em
~
~
~
shift Dn ¼ nmax ꢁ nmax, where the Stokes shift is now calcu-
lated from the lowest energy band in the absorption and the
centre of weight of the CT emission. Figure 4 shows the
Stokes shift for 1, 2, 3 and 5 in various dielectric media.
These experiments reveal that there is a moderate impact
~
shift Dn against the solvent density parameter Df for the
compounds 1, 2, 3 and 5 results in a linear dependence as
would be expected from the Lippert–Mataga–Oshika for-
malism. From the slope of the
linear fits the difference dipole
moment Dm=meꢁmg can be ob-
tained from an estimate of the
Onsager radius a. In estimating
this radius we used bond
lengths and dihedral angles ob-
tained from the X-ray crystal
structures. We acknowledge
that assuming
a
spherical
volume for these molecules is
an approximation; however, we
are not aware of any other way
forward in this context.
The excited-state dipole mo-
ments calculated here are simi-
lar to those reported by Stieg-
man et al. for this type of D–A
system.[36] However, we find
that the difference dipole
moment is larger for the more
conjugated molecules 3 and 5.
This observation is in contrast
to Stiegman et al., who ob-
served a reduction in the differ-
ence dipole moment for diphe-
nylbutadiynyl D–p–A dyads
Figure 4. Stokes shifts for 1, 2, 3 and 5 as a function of solvent density parameter as defined in Equation (1).
The line represents a fit of the data according to the Lippert–Mataga–Oshika relationship as defined in Equa-
tion (2) by using a linear regression fit. The quality of the fit is judged by the corresponding R values, which
are as follows: 1: R=0.95, 2: R=0.96, 3: R=0.95, 5: R=0.94.
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Intramolecular Charge Transfer
FULL PAPER
ꢀ
(D-C6H4–(C C)n–C6H4-A, n=2) compared to diphenyle-
smaller relative increase in changing from a non-polar to a
polar dielectric medium, compared to 1 and 3. The polar
solvents will stabilise the structure of the excited state in
favour of a CT state over an LE state. This, in turn, has sym-
metry implications with possible conformation alterations
that explain the enhanced TPA cross sections in polar
media. In this context, it is necessary to make additional
comments about the PLQY of these compounds. The data
show that the PLQY is not only affected by conjugation (as
previously discussed) but also by the properties of the di-
electric medium. For 1 and 2, there is a clear decrease of the
PLQY in more polar media, as compared to MCH
(Table 2). We observed this trend for a number of other
more polar solvents. However, there was no systematic var-
iation of PLQY versus solvent density parameter (data not
shown). The impact of the medium on the PLQY of 3 is
more difficult to assess as the PLQY value is low. In parallel
to the effect of the medium on the TPA cross sections, there
is also an effect due to increased conjugation. Compound 3,
with the more extended conjugation, also shows the largest
TPA cross sections, as expected.[37] (Note that although s2
for 3 is high, there is also a large associated error due to an
uncertainty in the PLQY value, as previously discussed).
The luminescence decay of the excited state was also af-
fected by the extent of conjugation through oligoyne bridg-
ing (Figure 5). For 1 in a non-polar medium the decay was
dominated by a 1.1 ns component, while data for 2 was do-
minated by an approximate 500 ps component. For the more
conjugated compounds 3 and 5 the luminescence decay was
very fast and most likely not fully resolved due to insuffi-
cient time resolution of the detection system. Accordingly,
the time constants for 3 and 5 (Table 3) are somewhat un-
certain. In a polar environment a longer decay phase was
observed for all four compounds. This supports the sugges-
tion that the luminescence in polar media originates from an
ICT state for which longer decay times are expected.[38] We
are confident that there is no degradation of the compounds
in chloroform as the PL spectra of 1 and 2 are similar to
those in other polar environments.[23] We note that the lumi-
nescence decays appear to be independent of the excitation
wavelength. Essentially similar kinetics were obtained for
excitation at 300 nm and 400 nm. There were only moderate
variations of the PL dynamics with different detection wave-
length (in the PL spectra), within the time resolution of our
time-correlated single photon counting (TCSPC) apparatus.
However, we cannot rule the presence of PL decay phases
in the 10ꢁ12–10ꢁ13 s range that would escape detection in our
TCSPC system. In combination, the PLQY data and the lu-
minescence decay data reveal that for the more conjugated
systems 3 and 5 the excited-state decay is dominated by
non-radiative processes. Probing the excited-state decay
with transient absorption spectroscopy supports this conclu-
sion. Figure 6 shows pump-probe absorption data for 1, 2, 3
and 4 in MCH at 390 nm after excitation with optical pulses
with a temporal width of 200 fs.
thynyl analogues (n=1). This is significant as a relatively
smaller excited dipole moment is indicative of less extensive
ICT. The data obtained in the present study do, however,
show that for these compounds ICT is equally efficient for
the more conjugated longer molecules.
Khundkar et al. observed that when the ethynylene bridge
was replaced by a butadiynylene bridge in compounds with
the same donor and acceptor moieties (i.e., MeS-C6H4–(C
C)n–C6H4-CN, n=1, 2) there was a substantial reduction in
the PLQY.[34b] The present series of compounds shows simi-
lar behaviour with a dramatic drop in PLQY from 93% for
1 to 1% for 3 (Table 2). Insertion of a phenyl ring in 5 re-
sults in a slight increase in PLQY although the value re-
mains very low. We note that 4 appears to be luminescent,
although the PLQY is extremely low, about 10ꢁ3.
ꢀ
Table 2. PLQY values and s2 data for 1–5.
PLQY[a]
s2 [GM]
MCH
acetone
MCH
Acetone
1
2
3
4
5
0.93
0.36
0.56
0.28
0.01
–
50ꢂ10
140ꢂ10
190ꢂ50
–
150ꢂ10
200ꢂ10
0.01
590ꢂ100
ꢃ10ꢁ3
0.04
–
–
–
–
[a] PLQY values were measured in two different solvents based upon the
absolute method by using an integrating sphere (MCH=methylcyclohex-
ane). Errors ꢂ5%. The two photon absorption (TPA) cross-sections (s2)
were measured by using fluorescein/NaOH as a standard (PLQY=0.9
and s2 =36 GM at lexc =800 nm). The concentrations of the standard (flu-
orescein) was, cR =5ꢃ10ꢁ6 m and the concentration of the samples typical-
ly cS ꢃ10ꢁ6 m. See the Supporting Information for more information.
To gain further insights into the optoelectronic properties
we also performed two photon absorption (TPA) measure-
ments on 1, 2 and 3. Seminal work by Albota et al. clearly
suggest that introduction of charge-transfer moieties and ex-
tended conjugation of the p-electron system are prerequi-
sites for high TPA cross sections.[37] It was, therefore, most
relevant to examine 1, 2 and 3 in different dielectric media
to assess the impact of conjugation within the homologous
series. All three compounds exhibited clear and well-defined
TPA PL for excitation at 790 nm, similar in shape to the cor-
responding linear one photon excited PL spectra with re-
spect to centre of weight of bands (see Figure S3 in the Sup-
porting Information). One difference that could be observed
was that the relative intensity of the LE state emission (in
methylcyclohexane) is slightly lower relative to the lower
energy CT bands. This may be a reflection of different sym-
metry implications of the TPA process. To examine the
effect of changing the dielectric medium, the data for 1, 2
and 3 were obtained in a non-polar solvent MCH (Df=0)
and acetone (Df=0.355), which was the most polar medium
in which the compounds were soluble. The data shown in
Table 2 clearly show that the TPA cross sections increase
significantly for both 1 and 3 with increased polarity of the
solvent. At present it is not clear why compound 2 shows a
All the systems studied have an initial fast decay to a
longer-lived state, which is estimated to have a lifetime of
Chem. Eur. J. 2010, 16, 1470 – 1479
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www.chemeurj.org
1475

L.-O. Pꢀlsson, M. R. Bryce et al.
ent absorption measurements
(Figure 6 and Table 3), addi-
tional experiments where per-
formed on 3 and 4, where this
phase is more prominent. Excit-
ed-state absorption experiments
revealed a strong absorption at
approximately 470 nm for both
3 and 4 (see the Supporting In-
formation for details). Probing
the decay dynamics of this band
at 470 nm did not result in any
detected transient species. The
reason for this is the time reso-
lution of our experimental ap-
paratus that is in the order of
ꢃ70 ns. Hence, the recovery in
this band must in the ꢃ10 ns
range. Interestingly, when the
probe wavelength was 639 nm
recovery to the ground state
was sufficiently slow to be
monitored. Furthermore, the
decay dynamics of this species
showed a marked sensitivity to
the presence of oxygen. Hence,
we conclude that this absorp-
tion is due to triplet–triplet ab-
sorption. It then remains to
assign the strong band around
470 nm to excited-state absorp-
tion of a dark charge-transfer
state with a lifetime in the ns
regime as no corresponding
decay phase was observed in
the time-resolved PL experi-
ments as previously men-
tioned.[39] This conclusion corre-
lates well with the observation
of fast luminescence dynamics
observed for 3 (see Figure 5).
The initial faster decay shows
Figure 5. Luminescence decays recorded in the emission maxima of compounds 1, 2, 3 and 5. Data recorded in
polar solvent (rigorously purified chloroform: data points a) and a non-polar solvent (methylcyclohexane: data
points b). Excitation wavelength lex =295 nm. See text and the Supporting Information for details of the data
analysis.
Table 3. Data obtained in time domain experiments. Luminescence decay data were obtained in TCSPC ex-
periments.
Lum. decay (MCH)[a]
Lum. decay (CHCl3)[b]
Pump-probe abs. (MCH)[a]
DE
t1 [ns]
t2 [ns]
t1 [ns]
t2 [ns]
t1 [ns]
t2 [ns]
[eV]
1
2
3
4
5
1.0
0.5
–
–
–
–
–
0.2 (10%)
0.1 (5%)
0.1 (5%)
–
2.0 (90%)
1.0 (95%)
1.0 (95%)
–
0.78
–
3.17
3.05
2.91
2.71
–
0.35 (50%)
0.30 (50%)
0.03 (50%)
–
ꢃ15 (50%)
ꢃ10 (50%)
ꢃ10 (50%)
–
ꢃ0.06
–
ꢃ0.06
0.15 (5%)
1.0 (95%)
[a] MCH=methylcyclohexane. [b] Percentage number in brackets is the relative contribution of the particular
decay phase. The energy gap is taken as the lowest energy absorbing species in the absorption spectrum. See
text for details.
tens of ns. We note that there was no corresponding decay
time found in the luminescence experiments. However, the
same trend as in the time-resolved PL experiments was ob-
served in the ultrafast transient absorption. This trend shows
that the absorption recovery time of the first component be-
comes faster with increasing length of the oligoyne bridge,
that is, increasing conjugation (see Table 3). The PL dynam-
ics are, therefore, limited by the excited-state lifetime. The
excited-state decay, as monitored in the transient absorption
measurements in a non-polar medium is, therefore, due to
two phases: a fast direct relaxation from the excited state to
the ground state, or via a charge separated state that will re-
combine non-radiatively on relatively slower timescales
compared to the PL dynamics. In order to learn more about
the nature of the slow ns component observed in the transi-
interesting behaviour depending on the length of the oli-
goyne bridge. This aspect will now be discussed in more
detail. Compound 1, with the shortest bridge, exhibits a slow
decay of the luminescence in conjunction with a slow return
to the ground state in the transient absorption measure-
ments. For the more extended analogues 2, 3 and 4 the ki-
netics become faster in both luminescence and transient ab-
sorption (Table 3). In conjunction with the PLQY data, the
scenario is, therefore, that strong non-radiative pathways
dominate the excited-state decay for more conjugated sys-
tems. The same phenomena have been observed experimen-
tally for a number of related systems.[40] The mechanism
behind the non-radiative decay has not yet been clarified
but the possibility that the non-radiative relaxation is con-
nected to the ground- and excited-state energy gap has been
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Chem. Eur. J. 2010, 16, 1470 – 1479

Intramolecular Charge Transfer
FULL PAPER
The reorganisation energy
l
must be equal to, or less than,
the average vibrational energy
ꢀhwave. For the systems studied
here prominent modes are the
C=C stretch vibration at n˜
ꢁ1
ꢀ
ꢃ1600 cm , the C C stretch
vibration at n˜ ꢃ2200 cmꢁ1 and
ꢁ
the C H stretch vibration at n˜
ꢃ3000 cmꢁ1. We remark that
the energy in all these vibra-
tions exceeds the reorganisation
energy as estimated from the
Stokes shift obtained from the
lowest energy band in the ab-
sorption spectra and the sharp
LE emission (see Figure 1 and
Table 1).
In Equation (3) the parame-
ter g depends only weakly on
the energy gap DE and the ana-
lytical expression for g is given
by Equation (4):[36]
Figure 6. Ambient temperature transient absorption data for 1, 2, 3 and 4 for excitation and probing at 390 nm
in MCH. The squares represent the measured data and the line is the fit of the data. See text for details.
considered.[36] This dependency is commonly known as the
ꢀ
ꢁ
energy-gap law and the framework was outlined by Engel-
man and Jortner in the 1970s.[24] This law defines two limit-
ing cases, referred to as the strong and weak coupling limits,
which in turn relate to the extent of displacement of the
LUMO relative to the HOMO potential energy surface. In
the case of the weak coupling limit, the reorganisation
energy (l) is considered to be small and it also assumes a
moderate temperature-dependence of the PLQY, which was
observed for the present series of compounds (see the Sup-
porting Information for details). Prominent factors in the
weak coupling limit are high frequency vibrational modes,
which are common for organic-based materials. For the
strong coupling limit on the other hand, the potential surfa-
ces of the ground and excited state are identical. The strong
coupling case implies that the Stokes shift considerably ex-
ceeds the mean vibrational frequency.[24] From the recorded
Stokes shifts (Table 1) we conclude that those are likely to
be significantly less than the energies of prominent vibra-
tional modes of the systems studied here.
2DE
g ¼ ln
ꢁ 1
ð4Þ
nꢀhwMDQ2
where DQ is the average displacement between the ground-
and excited-state potential energy surfaces. Taking the natu-
ral logarithm of Equation (3) gives Equation (5).
1
2
DE
ꢀhwM
ð5Þ
ln knr / ꢁ ln DE ꢁ g
This is more useful as there is a linear relationship be-
tween lnknr and DE. Furthermore, the logarithmic term in
Equation (5) contributes only marginally to the dependence
of DE on lnknr. Consequently, the pre-exponential factor in
Equation (3) can be regarded as a constant and with these
approximations the relationship shown in Equation (6) is
obtained where C is the constant term:
DE
ꢀhwM
ð6Þ
ln knr ꢃ Cg
Assuming that the weak coupling limit is valid in this
case, the non-radiative decay rate knr is then given by Equa-
tion (3):[36,40]
Figure 7 displays the results obtained for compounds 1, 2,
3 and 4 where the length of the oligoyne bridge is systemati-
cally extended. The data fit reasonably well to a linear de-
pendence, as would be expected from Equation (6) and thus,
confirm a weak coupling behaviour as outlined previously.
A significant difference between D–p–A systems with oli-
goene bridges[18d] and the oligoynes in the present study is
that the oligoynes display the energy gap law dependence
while the oligoenes do not. This could be due to prevention
of isomerisation processes after excitation of the oligoynes
1
2
ꢂ
ꢃ
ꢂ
ꢃ
2
V ð2pÞ
l
gDE
ꢀhwM
ð3Þ
knr
¼
ꢄ exp ꢁ
ꢄ exp ꢁ
1
2
ꢀhwM
ꢀhðꢀhwMDEÞ
In this formalism V is the electronic coupling between the
ground and excited state, DE represents the energy gap
(LUMOꢁHOMO), wM is the vibrational frequency and g is
an analytical function which contains structural information.
Chem. Eur. J. 2010, 16, 1470 – 1479
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1477

L.-O. Pꢀlsson, M. R. Bryce et al.
X-ray facilities and EPSRC for funding the improvement to the X-ray in-
strumentation.
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Chem. Eur. J. 2010, 16, 1470 – 1479

Intramolecular Charge Transfer
FULL PAPER
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Received: July 28, 2009
Published online: December 18, 2009
Chem. Eur. J. 2010, 16, 1470 – 1479
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