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
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1475