On the conjugation length for oligo(ethynylnaphthalene)-based molecular rods

Article abstract of DOI:10.1002/chem.200701235

The synthesis is described for a small series of oligomers built from (2, 3, 4 or 6) ethynyl-naphthalene repeat units and end-capped with solubilising 1,2,3-tris-dodecyloxy-benzene groups. These compounds absorb in the near-UV region and exhibit strong fl

Full text of DOI:10.1002/chem.200701235

DOI: 10.1002/chem.200701235
On the Conjugation Length for Oligo(ethynylnaphthalene)-Based
Molecular Rods
Andrew C. Benniston,* Anthony Harriman,* Dorota B. Rewinska, Songjie Yang, and
Yong-Gang Zhi[a]
Abstract: The synthesis is described for
a small series of oligomers built from
ing increase in the Huang–Rhys factor
for the radiative process. The non-radi-
ative rate constants also depend on
molecular length and are well ex-
plained in terms of the energy-gap law.
In contrast, very weak phosphores-
cence is observed at 77 K for which the
peak maximum and lifetime remain in-
sensitive to the number of naphthalene
units. The triplet lifetimes recorded at
ambient temperature are also inde-
pendent of the molecular length but
the triplet–triplet absorption spectra
change throughout the series. These re-
sults are discussed in terms of the
degree of electronic coupling between
adjacent repeat units for each of the
relevant excited states. During these
studies it was noted that the rate of in-
tersystem crossing to the triplet mani-
fold is but weakly affected by heavy-
(2, 3,
4 or 6) ethynyl-naphthalene
repeat units and end-capped with solu-
bilising 1,2,3-tris-dodecyloxy-benzene
groups. These compounds absorb in the
near-UV region and exhibit strong
fluorescence in both fluid solution and
a glassy matrix at 77 K. The spectral
profiles are fully consistent with
a
atom perturbers. A non-fluorescent
structurally heterogeneous ground
state becoming more planar upon exci-
tation and with the low-temperature
glass further stabilising the co-planar
orientation. The absorption and fluo-
rescence maxima move towards lower
energy with increasing number of
repeat units and there is a correspond-
complex is formed between iodoethane
and the molecular rod but the corre-
sponding bimolecular process occurs at
well below the diffusion-controlled
limit. This behaviour is considered in
terms of spin-orbit coupling between
the excited states and takes account of
the differing conjugation lengths.
Keywords: conducting polymer
·
energy gap law · fluorescence ·
naphthalene
triplet state
·
photophysics
·
Introduction
meric structures capable of supporting charge transport
along the molecular axis.[7–9] Current interest in such highly
conjugated, one-dimensional organic wires[10] stems from
their potential applications in many diverse areas of materi-
als science, including light-emitting diodes,[11] single-mole-
cule conductors,[12] flash memory devices,[13] chemical sen-
sors,[14] light-harvesting arrays[15] and molecular rectifiers.[16]
Closely related research has been directed towards the con-
struction of molecular photonic devices based on highly con-
jugated organic substrates bridging photoactive termi-
nals.[17,18] The most popular organic modules for building
into conductive wires are simple aromatic units such a thio-
phene,[19] phenylene,[20] biphenylene,[21] fluorene[22] and an-
thracene.[23] In many cases, the aryl groups are connected
through vinyl or acetylene spacer groups that themselves
help to modify and control the electronic properties. Al-
though the field has grown rapidly over recent years, only
sparse attention has been given to naphthalene-based wires,
despite the favourable photophysical properties of such de-
rivatives relative to their smaller counterparts.[24] We report
Acquiring a deeper understanding of how electronic charge,
in the form of either electrons or positive holes, migrates
along conjugated organic molecules is of fundamental im-
portance, and indeed such charge-transfer processes have
been subjected to innumerable experimental and theoretical
studies over the past few decades.[1–6] The level of underlying
electron-transfer theory has now advanced to the point at
which most aspects of long-range charge migration can be
explained. More recently, attention has focussed on the
study of medium-length, wire-like supermolecules and oligo-
[a]Dr. A. C. Benniston, Prof. A. Harriman, D. B. Rewinska, Dr. S. Yang,
Dr. Y.-G. Zhi
Molecular Photonics Laboratory
School of Natural Sciences, Newcastle University
Newcastle upon Tyne, NE1 7RU (UK)
Fax : (+44)191-222-6929
E-mail: a.c.benniston@ncl.ac.uk
anthony.harriman@ncl.ac.uk
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FULL PAPER
here on the synthesis and photophysical examination of a
small series of ethynylnaphthalene-based molecular-scale
wires, with particular focus on their putative role in photonic
devices.
Apart from establishing the triplet energies of these mate-
rials it is also necessary to enquire into the mechanism and
efficacy of triplet formation. An important feature of many
conducting polymers is that the triplet state seems to be
highly localised on a small number of repeat units whereas
the corresponding singlet excited state is more extensively
delocalised.[29] This behaviour is evident from the disparate
delocalisation lengths extracted from spectral curve fitting
routines. In turn, this realisation raises concerns about the
exact nature of the intersystem-crossing process and also
about the delocalisation length of higher-lying triplet states.
These upper-lying triplets might become involved as reactive
intermediates in certain cases.[30] To explore the triplet mani-
fold we have used the external heavy atom effect[31] in order
to perturb the inherent intersystem crossing rates; this fea-
ture is also considered to be a viable model for the electron-
exchange reactions carried out with ruthenium(II) and os-
It was shown previously that ethynylnaphthalene residues
were especially effective bridges for electron exchange be-
tween terminal transition metal complexes.[25] A key feature
of such systems relates to the relative positioning of excited
states localized on the donor, bridge and acceptor units.[24]
According to super-exchange theory,[26] the triplet energy of
the bridge must be positioned well above those of both
donor and acceptor such that the bridge triplet is involved
only as a virtual intermediate. In contrast, a hopping mecha-
nism demands that the triplet energy of the bridge lies be-
tween those of donor and acceptor.[27] That the triplet
energy of a single ethynylnaphthalene unit lies close to that
of the donor, but well above that of the acceptor, has been
demonstrated[24] but it is quite unknown how this energy will
evolve with incremental numbers of added ethynylnaphtha-
lene units. This is critical information for the design of next-
generation molecular-scale bridges based on electron ex-
change and we have sought to explore the systematic evolu-
tion of the excited-state properties as a function of molecu-
lar length. For this purpose, we have synthesized a series of
linear molecular rods containing 2, 3, 4 or 6 ethynylnaphtha-
lene residues capped with solubilising terminal groups
(Scheme 1). The longest such system stretches some 50 Š
end-to-end and represents a useful model for the bridges en-
visioned in future photoactive devices.[28]
mium(II) poly(pyridine) complexes. It will be shown that
A
this effect is extremely inefficient for this series of com-
pounds, despite the fact that triplet formation takes place
under ambient conditions.
Results and Discussion
Synthetic procedure: Preparation of the molecular rods
(Scheme 1) followed standard Sonogashira cross-coupling
reactions and deprotection procedures.[32] The 1,2,3-trisdode-
cyloxybenzene moiety was chosen as the terminal unit in an
effort to help raise the solubility and to aid in purification of
Scheme 1. i) [Pd
N
N
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A. C. Benniston, A. Harriman et al.
both precursors and final products. Coupling two equiva-
lents of the readily prepared compound 1 with derivative 2
afforded, after work-up and purification, the dimer NAP2 in
a respectable 89% yield. Alternatively, coupling 1 with com-
pound 3 afforded derivative 4 which was readily deprotected
to give the valuable precursor 5 in 86% yield (two steps).
Oligomers NAP3 and NAP4 were obtained by coupling 5
with either 1,4-diiodonaphthalene or with precursor 2, re-
spectively. The longest oligomer, NAP6, had to be prepared
by first coupling 5 with 3 followed by deprotection to give 7,
which was subsequently coupled to derivative 2. All at-
tempts to produce higher-order oligomers failed because of
the poor solubility and extreme difficulty in purifying the
products by column chromatography.
It has to be emphasized that purification was a tedious
and time-consuming process, involving numerous chromato-
graphic separations. Even allowing for the low concentra-
tions needed for emission spectroscopy, it proved difficult to
identify suitable solvents for these compounds. Eventually, it
was found that methyltetrahydrofuran (MTHF) was a satis-
factory solvent for the entire series and this was used
throughout the investigation. Although attempts were made
to study the effects of solvent polarity on the photophysical
properties of these compounds, this line of enquiry was
abandoned because of limited solubility.
Photophysical properties: The lowest-energy absorption
transition observed for each of these compounds in MTHF
at room temperature appears as a relatively broad band
with hints of underlying vibrational fine structure. In each
case, there is a higher-lying transition with a maximum at
around 280 nm. The lowest-energy transition can be decon-
structed into a series of Gaussian-shaped bands of common
half-width (fwhm=1200 cmꢀ1).[33] The best fit requires two
sets of overlapping bands, each separated by a spacing of
about 1000 cmꢀ1. This spacing corresponds to a medium-fre-
quency skeletal vibronic mode coupled to the absorption
process. The broadness of the overall band arises from the
close positioning of the two transitions. An example of the
absorption profile is given in Figure 2 for NAP2. The maxi-
mum of the lowest-energy Gaussian (A00) component is as-
signed to the Franck-Condon absorption transition and is
clearly dependent on the number of naphthalene-based
repeat units (Table 1). Indeed, the energy of the derived A00
transition decreases smoothly with increasing molecular
length. The intensity and position of the lowest-energy ab-
sorption band are suggestive of the vertical transition being
Photophysical examination demands small quantities of
absolutely pure material. In order to remove all fluorescent
impurities, the molecular rods, NAP(n), were subjected to
multiple chromatographic separations on silica gel using pe-
troleum ether/CH2Cl2 mixtures. Basic compound identifica-
tion was carried out by mass spectrometry, elemental analy-
1
sis and H NMR spectroscopy. Depicted in Figure 1 is a typi-
1
cal H NMR spectrum recorded for NAP2, which shows the
associated signals for the alkyl chains and the aromatic pro-
tons, together with their assignments. With addition of each
ethynylnaphthalene unit, the regions corresponding to pro-
tons d,g, b,c and e,f become increasingly more complicated.
However, the clear singlet at about d = 6.8 ppm for protons
a remains diagnostic and its relative integral (1:1 NAP2; 2:3
NAP3; 1:2 NAP4; 1:3 NAP6) with respect to protons d,g
confirmed isolation of the required target compound. For
spectroscopic measurements, these oligomers were subjected
to repeated preparative TLC and were deemed of sufficient
purity when fluorescence lifetime measurements were strict-
ly mono-exponential.
!
1
of La
Ag character.[34]
The various molecular rods
display intense fluorescence in
deoxygenated MTHF and, as
shown in Figure 2 for NAP2,
the emission band is relatively
narrow and displays vibrational
fine structure. There is no obvi-
ous indication for mirror sym-
metry. In each case, the spectral
profile remains independent of
illumination wavelength and
the excitation spectrum shows a
good match to the correspond-
ing absorption spectrum over
the entire region. The fluores-
cence spectra could be readily
deconstructed into
a
single
series of Gaussian bands, typi-
cally four or five such bands of
common half-width (fwhm=
1000 cmꢀ1) were needed to fully
reconstruct the entire spectrum,
Figure 1. 1H NMR (300 MHz, 258C, CDCl3) spectrum for the molecular rod NAP2 and the aromatic proton
assignments. The signal marked X is a residual solvent signal and the water signal is seen at ca. 1.56 ppm. each band being separated by
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FULL PAPER
a low-frequency vibrational mode of about 300 cmꢀ1 is cou-
pled to decay of the excited state, this being in addition to
the medium-frequency mode of about 800 cmꢀ1. The low-
frequency vibration is most probably connected with a sol-
vent libration.[37]
sꢀSSn
n!
ð1Þ
I0,n
¼
Although there are marked similarities between the emis-
sion spectra recorded for the various members of this series,
it is clear that there are subtle differences in addition to the
red shift. In particular, the ratio of the intensities of the first
two vibronic bands depends on the number of repeat units
and this is indicative of a change in the Huang–Rhys factor
(S).[38] Crude estimates for S were first determined from
Equation (1) where I0,n refers to the integrated fluorescence
area for the transition from the zero vibrational level in S1
to the nth vibronic level in S0.[39] These S values were then
used in conjunction with the other parameters derived from
emission spectral curve fitting to reconstitute the entire fluo-
rescence according to Equation (2).[30]
Figure 2. Absorption and fluorescence spectra recorded for NAP2 in de-
oxygentated MTHF at 258C.
Table 1. Photophysical properties recorded for the singlet-excited states
of the various molecular rods in deoxygenated MTHF at room tempera-
ture.
Property
NAP2
22498
21459
0.558
1.0
4.7
5.3
1180
1090
0.45
NAP3
22129
21008
0.526
0.85
NAP4
21859
21859
0.437
0.70
NAP6
A00 [cmꢀ1]23367
E00 [cmꢀ1]22222
fF
0.318
ꢀꢁ
ꢂ
ꢃ
ꢁ
ꢂ ꢄꢅ
7
2
E00ꢀnhw 3 Sn
nꢀE00 þ nhw
Dn1=2
X
tS [ns]1.1
IðnÞ ¼
exp ꢀ4ln2
kNR [108 sꢀ1]4.1
kRAD [108 sꢀ1]5.1
6.6
5.2
1280
9.7
4.5
1120
E00
n!
n¼0
hwM [cmꢀ1]1095
ð2Þ
SS [cmꢀ1
S[b]
]
1060
0.53
1125
0.59
1255
0.63
[a]
Here, I(n) is the normalised fluorescence intensity at wave-
number n and hwM, E00 and S are as defined above. The
term Dn1/2 refers to the inhomogeneously broadened band
half width and the index n indicates the vibrational quantum
number. The maximum value of n was restricted to 7; higher
values had no effect on the quality of the fit. This spectral
analysis was used to refine the value of S (Table 1). It is
seen that S increases systematically with increasing number
of repeat units, changing from 0.45 for NAP2 to 0.62 for
NAP6.
The fluorescence quantum yields (FF) measured in deoxy-
genated MTHF at room temperature are relatively high but
decrease notably with increasing number of repeat units
(Table 1). The corresponding excited-singlet state lifetimes
(tS), obtained from the single exponential decay profiles, are
short but also decrease progressively as the molecular
length increases (Table 1). The radiative rate constants
(kRAD = FF/tS) vary only slightly across the series, in line
with the modest changes in the mean emission energy. As
such, the transition dipole must remain independent of the
number of repeat units. In contrast, the non-radiative rate
constants (kNR = (1/tS)ꢀkRAD) increase significantly with in-
creasing number of naphthalene units (Table 1).
[a]Stokes ’ shift. [b]Huang-Rhys factor.
an average spacing of 800 cmꢀ1. This spacing corresponds to
a relatively low-energy skeletal bending mode (hwM) and is
clearly much smaller than that extracted from fitting the ab-
sorption spectra. The highest-energy Gaussian band (E00),
attributed to the 0–0 electronic transition, decreases smooth-
ly with increasing number of repeat units but is always sig-
nificantly lower than the energy of the corresponding ab-
sorption transition (Table 1). The Stokesꢁ shifts (SS), calcu-
lated as the minimum energy gap between the deconstructed
absorption and emission bands, are quite large and increase
progressively with increasing number of repeat units in the
molecular backbone (Table 1).
These large Stokesꢁ shifts, taken together with the nature
of the absorption/fluorescence profiles, indicate a reasonably
large geometry change upon excitation. The structured emis-
sion and featureless absorption profiles are fully consistent
with a non-planar ground state evolving into a more planar
excited singlet state.[35] This behaviour is somewhat analo-
gous to that found for quaterphenyl[36] where the central di-
hedral angle is reduced upon promotion to the first-excited
singlet state. On cooling rapidly to 77 K, the fluorescence
spectra undergo a red shift of about 800 cmꢀ1, while the
half-widths are decreased by about 20%. This situation can
be explained in terms of the frozen glass favouring forma-
tion of the fully planar geometry for the excited-singlet
state. For the glassy matrix, spectral curve fitting shows that
No phosphorescence could be detected for these com-
pounds in glassy MTHF at 77 K, even after addition of 10%
(v/v) iodoethane. Using an optical chopper to remove resid-
ual fluorescence, it was possible to observe extremely weak
phosphorescence from a snow formed from iodoethane
(80%) and MTHF (20%) at 77 K. It was necessary to
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A. C. Benniston, A. Harriman et al.
employ signal averaging techniques in order to properly re-
solve this triplet emission (Figure 3). Nonetheless, the phos-
phorescence signal was found to decay via first-order
Figure 4. Transient differential absorption spectrum recorded for NAP3
in deoxygenated MTHF following excitation at 355 nm. The insert shows
a decay trace recorded at 480 nm.
Figure 3. Phosphorescence spectrum recorded for NAP2 in a deoxygenat-
ed 1:4 mixture of MTHF and iodoethane at 77 K.
the compound and moves steadily towards lower energy
with increasing number of repeat units (Table 2). This latter
band decays with the same lifetime as found for the 470 nm
band and both spectral features are formed within the laser
pulse.
kinetics with an averaged lifetime (tP) of 6Æ2 ms under
these conditions. This lifetime was insensitive to the number
of repeat units (Table 2). Likewise, the phosphorescence
spectra remained very similar across the series and, in par-
ticular, the 0,0 band (lPHOS) was fairly constant at 670 nm
(Table 2). This would suggest that the triplet energy (T00) re-
mains fixed at around 1.85Æ0.05 eV throughout this series
of compounds.
Effects of increasing molecular length: The most notable
spectroscopic changes that accompany an increase in the
number of repeat units relate to the absorption and fluores-
cence spectra (Table 1). There is a progressive decrease in
the Franck-Condon absorption maximum (A00) for the first
allowed transition and for the corresponding fluorescence
process (E00). There is a concomitant increase in the Huang-
Rhys factor (S), although other parameters, such as the vi-
brational spacings and spectral profiles, remain unchanged
throughout the series. These effects can be explained within
the confines of Figure 5. Here, increasing the molecular
Table 2. Photophysical properties recorded for the triplet-excited states
of the various molecular rods in deoxygenated MTHF.
Property
NAP2
670
1.88
5.1
NAP3
665
1.85
7.2
NAP4
670
1.86
5.3
NAP6
lPHOS [nm]660
ET [eV][a]
tP [ms][b]
1.85
6.5
tT [ms]1.7
1.3
470
660
1.3
470
710
1.6
460
720
lTT1 [nm][c]
lTT2 [nm][d]
490
760
[a]Triplet energy determined from low temperature phosphorescence in
4:1 iodoethane/MTHF. [b]Phosphorescence lifetime recorded at 77 K.
[c]High-energy peak in the transient absorption spectrum observed by
laser flash photolysis. [d]Low energy peak seen in the transient absorp-
tion spectrum.
Further information about the triplet-excited states was
sought by nanosecond laser flash photolysis in deoxygenated
MTHF and a typical transient differential absorption spec-
trum is shown in Figure 4. These spectra were weak and it
proved difficult to remove all contamination from residual
fluorescence. The transient absorption band possessing a
maximum at about 470 nm (lTT1) is common to all four com-
pounds and decays cleanly to the pre-pulse baseline with ki-
netics that are exclusively first order. The triplet lifetimes
(tT) measured in deoxygenated solution lie within the range
of 1–2 ms (Table 2), and are significantly shortened in the
presence of air. At longer wavelength, a much weaker ab-
sorption band can be resolved from the baseline but, in this
case, the peak maximum (lTT2) is sensitive to the nature of
Figure 5. Potential energy diagram showing the main excited states in-
volved in the photophysics of these molecular rods. The processes in-
volved include Franck-Condon excitation (FC), fluorescence (FL), phos-
phorescence (PH) and triplet–triplet absorption (TT). The Figure implies
a large structural change on formation of S1 and Tn but negligible geome-
try change between S0 and T1.
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Molecular Rods
FULL PAPER
length leads to a general decrease in the HOMO–LUMO
energy gap and, as a consequence, a decrease in A00. This
situation demands that the conjugation length increases
smoothly with incremental number of naphthalene units, de-
spite the realisation that the ground-state geometry is un-
likely to be planar along the molecular axis. Even so, there
must be sufficient electron delocalisation for the conjugation
length to increase with each added repeat unit. The fluores-
cence spectral profile is consistent with the excited-singlet
state attaining a more planar geometry,[35] such that E00 is
considerably smaller than A00. Since the Stokesꢁ shift in-
creases steadily with increasing molecular length, it follows
that the size of the geometry change accompanying relaxa-
tion of the initially formed Franck-Condon state must also
increase with each additional naphthalene unit. This geome-
try change is reflected by S, which increases throughout the
series (Table 1).
Figure 6. Correlation between the energies of particular transitions and
the parameter describing the molecular length, according to Eqn 4. The
*
transitions are: Franck-Condon absorption (&), fluorescence ( ), phos-
~
*
phorescence ( ), triplet–triplet absorption from T1 to Tn
( ),and triplet–
&
triplet absorption from T1 to Tnꢀ1 ( ).
Indeed, the Huang-Rhys factor can be related to the
change in the equilibrium geometries between the ground
state and the relaxed singlet-excited state (DQ), by way of
Equation (3).[40]
Table 3. Summary of the parameters derived from the length dependence
for the optical properties of the various excited states.
State
FC[a]
EM [cmꢀ1
]
jbj [cmꢀ1
1900
2000
2400
450
]
25240
24240
23300
15615
17510
22465
[b]
S1
ꢁ
ꢂ
[c]
S77K
T1
1
2
MwM
ꢀh
DQ2
ð3Þ
[d]
S ¼
[e]
T(nꢀ1)
T(n)
2360
1040
[f]
Here, M is the reduced mass of the vibrator and wM is the
frequency of the coupled vibrational mode. The latter pa-
rameter, as extracted from the emission spectral curve fit-
ting routine,[30] remains independent of molecular length so
that changes in S can be attributed to variations in DQ.
Thus, all spectroscopic evidence points to a more substantial
change in molecular geometry on illumination as the molec-
ular length increases. Given that the excited-singlet state ge-
ometry tends towards planarity, the ground state structure
must become more disordered as the molecular length in-
creases. It should be emphasized that the excited-singlet
state becomes more planar in a glassy matrix at 77 K. This
means that the geometries found under ambient conditions
are subject to conformational defects that tend to be elimi-
nated at lower temperature.
[a]Franck-Condon state measured by optical absorption. [b]Relaxed sin-
glet excited state measured by fluorescence. [c]Fully relaxed singlet-ex-
cited state measured by fluorescence in a glassy matrix at 77 K. [d]First-
excited triplet state measured by low temperature phosphorescence.
[e]An upper-lying triplet state measured from the low energy peak seen
in the transient absorption spectrum. [f]The highest-energy triplet state
measured from the high-energy peak in the transient absorption spec-
trum.
rather heterogeneous structure with respect to the mutual
alignment of the repeat units, a non-linear, least-squares fit
affords values for EM of 25240 cmꢀ1 (396 nm) and jbj of
1900 cmꢀ1 (0.23 eV). The latter value indicates pronounced
electronic coupling between adjacent ethynylnaphthalenes
but this is very much less than that observed for poly(p-
phenylene vinylene) where jbj is 8800 cmꢀ1 (1.1 eV).[43]
A
We can consider that, for each excited state, the exciton is
confined to a characteristic conjugation length set by confor-
mational restrictions and/or electronic factors.[41] Within this
model, the energy associated with any particular transition
(EN) is related to the number of repeat units (N) in accord-
ance with Equation (4).[42]
somewhat higher jbj (=2000 cmꢀ1) value is found for the
relaxed excited-singlet state, where the geometry is more
planar, and there is a further increase to jbj=2400 cmꢀ1 in
the glassy matrix. This latter environment stabilises the co-
planar excited-singlet state. Thus, jbj seems to provide a
crude estimate of the degree of planarity along the molecu-
lar backbone.
ꢁ
ꢂ
A similar jbj (=2360 cmꢀ1) is found for the upper-lying
triplet state Tnꢀ1 but a very low jbj of about 450 cmꢀ1 is de-
rived for the first triplet state T1 and an intermediate value
for the other triplet state Tn (Table 3). As such, we can con-
clude that adjacent repeat units are but weakly coupled at
the T1 level such that the exciton is essentially confined to a
single naphthalene residue. An obvious inference is that the
T1 state possesses a perpendicular geometry that minimizes
electronic coupling along the backbone, even at 77 K, but
p
EN ¼ EM þ 2bcos
ð4Þ
N þ 1
Here, EM is the monomer transition energy and b is the exci-
tation exchange interaction between neighbouring naphtha-
lene units. Given the limited number of compounds, linear
plots to Equation (4) were obtained for each of the excited
states (Figure 6) and the derived parameters are collected in
Table 3. For the Franck-Condon state, believed to possess a
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A. C. Benniston, A. Harriman et al.
that Tnꢀ1 adopts a co-planar structure and Tn takes up a
structure between these two extremes. This need not be the
case, however, since the coupling parameter depends on fac-
tors other than the mutual geometry.[44] More reasonably,
the atomic orbital coefficients for the carbon atoms connect-
ing naphthalene and ethyne groups are severely reduced for
T1 relative to the other states because of a change in elec-
tron distribution or symmetry of the excited state. It should
be noted that similar strong confinement of the T1 triplet ex-
citon has been observed for platinum polyyne derivatives.[45]
This finding and ours are somewhat different to those re-
ported for ethylenedioxythiophene oligomers[46] and
oligo(9,9-dihexyloligofluorene-2,7-diyl)s,[47] where the triplet
energies gradually decrease with an increase in monomer
units. We speculate of the possibility that triple bonds help
decouple organic units at the triplet level, but further studies
are clearly required to see if this is a general phenomenon.
The radiative rate constant (kRAD) remains almost unaf-
fected by changes in molecular length (Table 1) and it is
clear that the various excited-singlet states share a common
parentage and transition dipole moment. In contrast, the
rates of non-radiative decay (kNR) increases with increasing
length of the molecular rod (Table 1). This latter effect is
easily explained in terms of the Englman–Jortner energy
gap law,[48] where kNR is predicted to increase exponentially
with increasing S and decreasing E00. The observed rate con-
stants are well described in terms of this model, taking due
account of changes in S and E00, but since it is not possible
to discriminate between non-radiative decay to form the
ground or triplet states, a quantitative fit cannot be attempt-
ed. The much smaller energy gap between S1 and T1, com-
pared to that involving S0, could be used to argue in favour
of intersystem crossing to the triplet manifold.
found in the far-red region. Here, the absorption maximum
evolves from 660 nm for NAP2 to 760 nm for NAP6
(Table 2). This latter absorption band corresponds to the
T1ꢀT(nꢀ1) transition and, as such, the energy of the T(nꢀ1)
state must decrease with increasing number of repeat units.
The spectral evolution is comparable to that found for the
S1 state, thereby raising the possibility that these two states
share a common conjugation length that is longer than those
of either T1 or Tn.
Intersystem crossing in the molecular rods: Transient ab-
sorption spectroscopy shows that the triplet-excited state is
populated in fluid solution at room temperature but that, in
each case, the signal is weak. Likewise, phosphorescence
was extremely difficult to detect in a glassy matrix at 77 K
even in the presence of 10% iodoethane. These findings are
consistent with inefficient intersystem crossing to the triplet
manifold. Systematic addition of iodoethane to MTHF solu-
tions of each fluorophore shows that extremely high concen-
trations are needed to effect reasonable levels of fluores-
cence quenching. Furthermore, in each case, quenching of
the steady-state emission exhibits positive deviation from
Stern–Volmer kinetics but the singlet-excited state lifetime
follows simple Stern–Volmer kinetics (Figure 7), albeit with
Quite remarkably, most of the triplet state properties
remain essentially independent of the number of added
repeat units in the molecular backbone (Table 2). The triplet
energies (ETT), derived from the low temperature phosphor-
escence spectra remain fairly constant at about 1.86 eV.
Consistent with this finding, the triplet decay rates are in-
sensitive to the molecular length, as might be expected on
the basis of the Englman–Jortner energy-gap law,[48] at both
77 and 293 K. It follows that the triplet states must share
closely comparable S values. Furthermore, the transient dif-
ferential absorption spectra exhibit a common absorption
peak at around 470 nm. This suggests that the energy of the
relevant transition, T1ꢀTn, is also independent of molecular
length. The obvious inference is that, for these two triplet
states, the conjugation length is the same throughout the
series. Since phosphorescence is measured in a solid matrix
at 77 K and triplet–triplet absorption refers to fluid solution
at room temperature, it is difficult to attribute this fixed
conjugation length to structural distortions. Indeed, for the
first-excited singlet state the planar geometry is found at
77 K but a more heterogeneous distribution of geometries
abounds at room temperature.
Figure 7. Stern-Volmer plots for the effect of iodoethane on the fluores-
*
*
cence intensity ( ) and lifetime ( ) of NAP3 in MTHF at room tempera-
ture.
an unexpectedly low bimolecular quenching rate constant
(kQ ꢁ2Æ1”107 mꢀ1 sꢀ1). This behaviour indicates that the
heavy-atom quencher forms a weak complex with the mo-
lecular-scale rod;[49] the stability constants for formation of
the corresponding 1:1 complexes are of the order of 1mꢀ1
and show no obvious correlation with the molecular length.
These complexes do not fluoresce with any appreciable effi-
ciency but show enhanced triplet formation. It is also nota-
ble that the triplet lifetime is not much shortened by the
heavy-atom effect but it is surprisingly short in all cases.
The presence of excess iodoethane has little effect on the
absorption spectrum and, in agreement with the limited
fluorescence quenching, does not provide significant en-
hancement of the S0–T1 absorption transition. It is generally
considered that the external heavy-atom effect promotes
spin orbit coupling that is already present in the molecule,
The one exception to this generic triplet-state behaviour
concerns the weak transient differential absorption band
10200
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Chem. Eur. J. 2007, 13, 10194 – 10203

Molecular Rods
FULL PAPER
rather than introduces a new process, and that the effect is
most notable when the inherent levels of spin orbit coupling
are high.[31] That intersystem crossing seems to be ineffective
in these molecular-scale rods provides at least partial ration-
alization for the inability of the heavy-atom effect to pro-
mote triplet formation. The nature of the 1:1 complex
formed between the organic moiety and iodoethane is
mostly likely of charge-transfer origin, although exchange
interactions cannot be ruled out, whereby the complex
steals intensity from the corresponding singlet-state charge-
transfer complex. Spin-orbit coupling of this type is efficient
only for states of the same total symmetry and this is clearly
not the case for the molecular orbitals involved here. Elec-
tron delocalization is greatly reduced for T1 relative to S1
and is more in keeping with the ground state. As such, inter-
system crossing from T1 to S0 is effective, hence the relative-
ly short lifetime for the lowest-energy triplet state, but inter-
conversion from S1 to T1 is symmetry forbidden.
Experimental Section
Solvents were dried by standard literature methods before being distilled
and stored under nitrogen over 4 Š molecular sieves. 1H and 13C NMR
spectra were recorded with a Bruker AVANCE 300 MHz spectrometer.
Routine mass spectra and elemental analyses were obtained using in-
house facilities. Compounds 1,[50] 2[25] and 3[25] were prepared by literature
methods. Even though the final compounds were vacuum dried for sever-
al hours, it was not always possible to remove all traces of residual water;
this could be seen clearly in the 1H NMR spectra (e.g., Figure 1) at ca.
1.56 ppm. Elemental analyses were also consistent with trace amounts of
water impurity. Samples for spectroscopic measurements were subjected
to preparative TLC immediately before making the measurement.
Preparation of 4: To a 100 mL two-necked flask were added 1 (2.0 g,
3.1 mmol), 3 (1.09 g, 3.10 mmol), [Pd(PPh3)2Cl2](150 mg, 0.210 mmol),
N
copper(I) iodide (90 mg, 0.47 mmol) and dry THF (50 mL). The mixture
was purged thoroughly with N2 for one hour before addition of diisopro-
pylamine (15 mL). The solution was refluxed overnight, cooled and the
solvent removed under reduced pressure. The crude material was purified
by column chromatography on silica gel using petroleum ether/CH2Cl2
2:1. Yield 2.37 g, 2.70 mmol, 89%. 1H NMR (300 MHz, 258C, CDCl3):
d=0.31 (s, 9H), 0.85 (t, 9H, J=6 Hz), 1.23 (m, 48H), 1.46 (m, 6H), 1.73
(m, 6H), 3.98 (m, 6H), 6.80 (s, 2H), 7.60 (dd (overlapping peaks), 2H,
J=6, J’=6 Hz), 7.64 (s, 2H), 8.33 (dd, 1H, J=6, J’=6 Hz), 8.40 ppm (dd,
1H, J=6, J’=6 Hz).
Conclusion
Preparation of 5: To a solution of compound 4 (1.0 g, 1.1 mmol) in dry
THF (20 mL) was added TBAF (2 mL, 1m). The mixture was stirred
overnight at room temperature before removing the solvent under re-
duced pressure. The residue was purified by column chromatography on
silica gel using petroleum ether/CH2Cl2 2:1. Yield 0.86 g, 1.1 mmol, 97%.
1H NMR (300 MHz, 258C, CDCl3): d=0.81 (t, 9H, J=6 Hz), 1.20 (m,
48H), 1.42 (m, 6H), 1.74 (m, 6H), 3.50 (s, 1H), 3.95 (m, 6H), 6.72 (s,
2H), 7.47 (m, 2H), 7.55 (s, 2H), 8.25 (m, 1H), 8.35 ppm (m, 1H).
In probing the properties of the triplet manifold in these pu-
tative molecular-scale rods, two significant factors have
emerged. Firstly, the magnitude of electronic coupling be-
tween adjacent repeat units at the triplet level depends on
the state of excitation. Some triplets, including T1, are local-
ised onto a small number of repeat units but more extended
conjugation is apparent for certain higher-lying triplet states
and for the first-excited singlet state. Similar behaviour has
been noted previously for other conjugated oligomers but
the origin of this property is unclear. Secondly, the com-
pounds are surprisingly resistant to the external heavy-atom
effect despite the realisation that intersystem crossing
occurs from S1 to the triplet manifold in the absence of spin
orbit perturbation and that the lifetime of the T1 state is rel-
atively short. It seems likely that these two effects are relat-
ed and arise as a consequence of changes in the nature of
the relevant molecular orbitals. The restricted conjugation
length observed for the first triplet state is not due to con-
formational effects but to poor electronic communication
between adjacent naphthalene-based units as imposed by
symmetry restrictions. Interestingly, spin orbit coupling is
promoted by the internal heavy-atom effect using terminal
transition metal complexes where charge-transfer effects are
significant. Thus, fluorescence is absent from the closely-re-
lated binuclear ruthenium(II) bis(2,2’:6’,2’’-terpyridine) com-
plexes and is replaced by fast intersystem crossing to the
triplet manifold.[25] The key feature here seems to be the
ability to form an intramolecular charge-transfer complex
with the perturbing species. Complexation of this type might
align the molecular rod in such a way that communication
between individual naphthalene residues increases and this
might help to circumvent the problems of triplet localiza-
tion.
Preparation of 6: The procedure described for the preparation of NAP2
was used: 5 (740 mg, 0.920 mmol), 3 (330 mg, 0.940 mmol), [Pd(PPh3)4]
C
(32 mg, 0.027 mmol), diisopropylamine (6 mL), THF (60 mL). Yield
830 mg, 0.81 mmol, 88%. 1H NMR (300 MHz, 258C, CDCl3): d=0.33 (s,
9H), 0.80 (t, 9H, J=6 Hz), 1.23 (m, 48H), 1.43 (m, 6H), 1.73 (m, 6H),
3.95 (m, 6H), 6.79 (s, 2H), 7.70 (m, 8H), 8.33 (m, 1H), 8.41 (m, 1H),
8.50 ppm (m, 2H).
Preparation of 7: The procedure described above for the preparation of 5
was used: 6 (830 mg, 0.810 mmol), THF (20 mL), TBAF (3 mL, 1m).
Yield 450 mg, 0.470 mmol, 58%. 1H NMR (300 MHz, 258C, CDCl3): d=
0.80 (t, 9H, J=6 Hz), 1.20 (m, 48H), 1.43 (m, 6H), 1.70 (m, 6H), 3.50 (s,
1H), 3.95 (m, 6H), 6.79 (s, 2H), 7.60 (m, 4H), 7.74 (m, 4H), 8.36 (m,
1H), 8.42 (m, 1H), 8.52 ppm (m, 2H).
General procedure for preparation of final compounds (NAP2): To a
100 mL two-necked flask was added 1 (200 mg, 0.310 mmol), 2 (60 mg,
0.11 mmol), [Pd(PPh3)4](16 mg, 0.014 mmol), copper(I) iodide (6 mg,
N
0.031 mmol) and dry THF (40 mL). The mixture was purged thoroughly
with N2 for one hour and diisopropylamine (5 mL) added. The solution
was refluxed overnight, cooled and the solvents removed under reduced
pressure to afford a residue that was purified by column chromatography
on silica get using petroleum ether/CH2Cl2 2:1. Yield 160 mg,
0.100 mmol, 89%. 1H NMR (300 MHz, 258C, CDCl3): d=0.82 (t, 18H,
J=6 Hz), 1.20 (m, 96H), 1.43 (m, 12H), 1.73 (m, 12H), 3.95 (m, 12H),
6.79 (s, 4H), 7.63 (m, 4H), 7.70 (d, 2H, J=8 Hz), 7.80 (d, 2H, J=8 Hz),
8.42 (m, 2H), 8.53 ppm (m, 2H); 13C NMR (75 MHz, 258C, CDCl3): d=
153.36, 139.37, 132.37, 132.28, 129.04, 128.65, 126.40, 126.25, 125.91,
125.74, 121.39, 120.43, 116.67, 110.18, 95.90, 93.24, 85.40, 72.69, 68.69,
30.97, 29.48, 28.77, 28.73, 28.68, 28.47, 28.37, 25.20, 21.68, 13.00 ppm; ES-
MS: m/z: calcd for C110H166O6: 1584.49; found: 1584.20 [M +]; elemental
analysis calcd (%) for C110H166O6: C 84.38, H 10.56; found: C 84.15, H
9.87.
NAP3:
0.079 mmol), [Pd
0.03 mmol), diisopropylamine (6 mL), THF (40 mL). Yield 121 mg,
5
(165 mg, 0.200 mmol), 1,4-diidonaphthalene (30 mg,
AHCTREUNG
Chem. Eur. J. 2007, 13, 10194 – 10203
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10201

A. C. Benniston, A. Harriman et al.
0.0690 mmol, 88%. 1H NMR (300 MHz, 258C, CDCl3): d=0.79 (t, 18H,
J=6 Hz), 1.17 (m, 96H), 1.40 (m, 12H), 1.71 (m, 12H), 3.92 (m, 12H),
6.77 (s, 4H), 7.63 (m, 8H), 7.77 (m, 4H), 8.41 (m, 2H), 8.51 ppm (m,
4H); 13C NMR (75 MHz, 258C, CDCl3): d=152.37, 139.38, 132.34,
132.28, 129.11, 129.06, 128.66, 126.52, 126.44, 125.93, 125.87, 125.74,
121.38, 120.99, 120.36, 116.66, 110.19, 95.95, 93.57, 93.20, 85.41, 72.70,
68.71, 30.96,29.48, 28.77, 28.72, 28.68, 28.59, 28.47, 28.37, 25.20, 21.68,
13.00 ppm; ES-MS: m/z: calcd for C122H172O6: 1734.3; found: 1734.3
[M+]; elemental analysis calcd (%) for C122H172O6·H2O: C 83.60, H 10.01;
found C 83.89, H 10.08.
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J. Phys. Chem. A 2003, 107, 3970–3980.
NAP4:
5
(120 mg, 0.150 mmol),
2
(30 mg, 0.057 mmol), [Pd(PPh3)4]
A
(8 mg, 0.0069 mmol), copper(I) iodide (4 mg, 0.02 mmol), diisopropyla-
mine (5 mL), THF (40 mL). Yield 88 mg, 0.047 mmol, 82%. 1H NMR
(300 MHz, 258C, CDCl3): d=0.81 (t, 18H, J=6 Hz), 1.20 (m, 96H), 1.43
(m, 12H), 1.75 (m, 12H), 3.96 (m, 12H), 6.80 (s, 4H), 7.66 (m,10H), 7.84
(m, 6H), 8.44 (m, 2H), 8.57 ppm (m, 6H); 13C NMR (75 MHz, 258C,
CDCl3): d=152.19, 139.22, 132.16, 132.10, 128.93, 128.87, 128.47, 126.26,
126.09, 125.68, 125.55, 121.20, 120.89, 120.73, 120.16, 116.46, 110.03, 95.76,
93.43, 93.32, 92.99, 85.21, 72.53, 68.54, 30.77, 29.29, 28.57, 28.53, 28.49,
28.40, 28.28, 28.17, 25.01, 21.48, 18.57, 12.80 ppm; ES-MS: m/z: calcd for
C134H178O6: 1888.84; found: 1884.3 [M+]; elemental analysis calcd (%)
for C134H178O6·H2O: C 84.58, H 9.53; found: C 84.23, H 9.01.
NAP6:
7
(280 mg, 0.290 mmol),
2
(60 mg, 0.11 mmol), [Pd(PPh3)4]
A
[4]a) A. M. Funston, E. E. Silverman, J. R. Miller, K. S. Schanze, J.
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(20 mg, 0.017 mmol), diisopropylamine (10 mL), THF (50 mL). Yield
144 mg, 0.0660 mmol, 58%. 1H NMR (300 MHz, 258C, CDCl3): d=0.81
(t, 18H, J=6 Hz), 1.20 (m, 96H), 1.43 (m, 12H), 1.78 (m, 12H), 3.98 (m,
12H), 6.80 (s, 4H), 7.71 (m,16H), 7.88 (m, 8H), 8.43 (m, 2H), 8.58 ppm
(m, 10H); ES-MS: m/z: calcd for C158H190O6: 2185.2; found: 2185.4
A
8.84; found: C 84.12, H 9.06.
Methods: Absorption spectra were recorded with a Hitachi U3310 spec-
trophotometer while all fluorescence studies were made with an Yvon-
Jobin fluorolog tau-3 spectrometer. Fluorescence spectra were corrected
for spectral imperfections using a standard lamp. Measurements were
made using optically dilute solutions after deoxygenation by purging with
dried N2. Fluorescence quantum yields were measured with respect to
quinine sulphate (FF =0.58) in 0.1m H2SO4. Corrected excitation spectra
were also recorded under optically dilute conditions. Fluorescence life-
times were measured by time-correlated, single-photon counting condi-
tions following excitation with a laser diode at 370 nm. After deconvolu-
tion of the instrument response function, the temporal resolution of this
set-up was ca. 100 ps. Low-temperature studies were carried out with an
immersion well Dewar filled with liquid N2.
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11207.
Laser flash photolysis studies were made with an Applied Photophysics
Ltd LKS.60 instrument. Excitation was made at 355 nm using a fre-
A
[7]E. A. Weiss, M. R. Wasielewski, M. A. Ratner, Top. Curr. Chem.
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through a high-radiance monochromator and detected with a fast re-
sponse PMT. Transient differential absorption spectra were recorded
point-by-point with five separate records being averaged at each wave-
length. Kinetic data were obtained by averaging about 100 individual re-
cords collected at a particular wavelength. Solutions for flash photolysis
were prepared to possess an absorbance of ca. 0.20 at 355 nm. All solu-
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This work was supported by EPSRC (GR/S00088/01) and by Newcastle
University. The EPSRC-sponsored Mass Spectrometry Service at Swan-
sea is gratefully acknowledged for recording the electro-spray mass spec-
tra.
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Molecular Rods
FULL PAPER
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Received: August 8, 2007
Published online: October 23, 2007
Chem. Eur. J. 2007, 13, 10194 – 10203
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10203