Investigation of two-photon absorption behavior in symmetrical acceptor-π-acceptor derivatives with dimesitylboryl end-groups. Evidence of new engineering routes for TPA/transparency trade-off optimization

Article abstract of DOI:10.1039/b416605a

Investigations of the non-linear optical properties of a novel series of A-π-A quadrupoles, based on dimesitylboron end-groups, reveal the promising potential of elongated vinylboranes derivatives for combined enhanced two-photon absorption cross-section and improved transparency in the visible region. In addition, the excited state lifetime can be significantly enhanced in A-π-A′-π-A derivatives. This opens a new route towards optimized molecules for optical power limiting. The Owner Societies 2005.

Full text of DOI:10.1039/b416605a

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R E S E A R C H P A P E R
Investigation of two-photon absorption behavior in symmetrical
acceptor–p–acceptor derivatives with dimesitylboryl end-groups.
Evidence of new engineering routes for TPA/transparency trade-off
optimization
a
s,ab Christopher D. Entwistle, Andrew Beeby,
b
b
Marina Charlot, Laurent Porre
`
Todd B. Marder* and Mireille Blanchard-Desce*
b
a
a
Synthe`se et Electrosynthe`se Organiques (CNRS, UMR 6510), Institut de Chimie, Universite´
de Rennes 1, Campus Scientifique de Beaulieu, Baˆt. 10A, F-35042 Rennes, France.
E-mail: Mireille.Blanchard-Desce@univ-rennes1.fr; Fax: þ33 (0)2 2323 6277;
Tel: þ33 (0)2 2323 6277
b
Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE.
E-mail: Todd.Marder@durham.ac.uk; Fax: þ44 (0) 191 384 4737;
Tel: þ44 (0) 191 334 2037
Received 29th October 2004, Accepted 24th December 2004
First published as an Advance Article on the web 18th January 2005
Investigations of the non-linear optical properties of a novel series of A–p–A quadrupoles, based on
dimesitylboron end-groups, reveal the promising potential of elongated vinylboranes derivatives for combined
enhanced two-photon absorption cross-section and improved transparency in the visible region. In addition, the
0
excited state lifetime can be significantly enhanced in A–p–A –p–A derivatives. This opens a new route towards
optimized molecules for optical power limiting.
ingly, only a few very strong electron-releasing (e.g. diphenyl-
5
Introduction
,10–12,15,19
or dialkyl-amino
7
) and electron-withdrawing (e.g.
22
and triflyl ) substituents
,16,17,20,21
9,17,21–23
Molecular two-photon absorption (TPA) has attracted a lot of
interest over recent years due to the many applications it offers
both in biological imaging and in material science. These
include two-photon laser scanning microscopy imaging
(TPLSM), 3D optical data storage, localized photodynamic
,2
therapy, microfabrication and optical power limitation.
These applications have driven the design of compounds
displaying enhanced TPA cross-sections along with specific
features depending on the application. For TPLSM, high
fluorescence quantum yield (f), suitable photostability and
reduced phototoxicity are desirable in addition to TPA cross-
nitro,
sulfonyl,
have been used in the design of chromophores with enhanced
TPA. In addition, only very recently has the use of electron-
withdrawing moieties including boron atoms been considered
although such compounds are known to lead to interesting
1
24
25
photoluminescence and nonlinear optical properties.
Basically, two types of molecules have been examined for
TPA up to now (Fig. 1): D–p–A derivatives with trivalent
2
6,27
boron acceptor moieties
boron is part of an electron-withdrawing heterocycle.
or dioxaborines where tetravalent
2
8,29
In
addition, only a few examples of A–p–A derivatives bearing
dimesitylboryl acceptors have been reported quite recently
(compounds 11–13 in Fig. 1) and were shown to exhibit
sections (s
genous chromophores in the biological spectral window (700–
200 nm). On the other hand, for optical power limitation,
2
) that are orders of magnitude larger than endo-
3
0,31
1
moderate TPA properties in the 700–900 nm range.
In that
3
both high solubility, very good linear transparency and very
large multiphoton absorptivities (including excited-state ab-
context, our goal has been the design of A–p–A compounds
showing enhanced TPA in the visible-NIR region. Our strategy
is based on the symmetrical functionalization of a conjugated
core with dimesitylboryl acceptors. By modifying the nature
(and length) of the core (Scheme 1), we aimed at increasing the
TPA. We emphasize that except for compound 1, the dimesi-
tylboryl acceptor groups are connected to the core via vinylene
linkers. This is expected to allow better electronic interaction
between the core and the electron-withdrawing end-groups. In
particular, the steric hindrance between the close mesityl boron
substituents and peri hydrogens of the anthryl core is respon-
sible for a significant twist of the end-groups with respect to the
4
sorption) are needed.
Molecular engineering of organic compounds with very large
5
TPA cross-section is still a recent field calling for extensive
experimental and theoretical studies. These include compre-
hensive investigations of structure–TPA relationships aiming
at better understanding of the factors that govern the TPA
magnitude as well as designing new molecules that display
giant TPA cross-sections (in the required spectral range) and
satisfy the criteria required for the applications sought after. A
6
–9
number of structures have been examined including dipolar,
2
,5,8–13
32
quadrupolar,
4
and multidimensional and branched struc-
plane of the core. In contrast, due to the presence of the
vinylene spacers, the steric hindrance is relieved in chromo-
1
tures. In the case of quadrupolar chromophores (i.e. D–p–D
5
,11,15
5,10,16,17
and D–p–AA–p–D,
molecules), it is known that the TPA magnitude is related to a
A–p–A and A–p–DD–p–A
phores 2–7 and crystal structures reveal relatively planar
3
conjugated systems.
3
quadrupolar type of charge transfer between the periphery and
5
In this paper we will briefly present the synthesis of the series
of chromophores then describe their photophysical and TPA
properties in detail and derive structure-property relationships.
Finally, we show that this experimental study allows us to
,15
the core of the molecules.
strength of the end-groups as well as the length of the con-
In addition, by modifying the
5
,11,12,15,18
jugated system it is possible to increase s
2
.
Surpris-
6
00
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 6 0 0 – 6 0 6
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5

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Scheme 1 Synthetic routes to boron based quadrupoles 1–7.
Fig. 1 Boron and TPA chromophores: some known compounds.
From ref. 27 (8), ref. 29 (9), ref. 28 (10), ref. 30 (11), ref. 31 (12, 13).
characterization, crystal and molecular structures of the vinyl-
boranes are beyond the scope of this paper and will be
described elsewhere.
identify appropriate structural combinations to optimize the
TPA/transparency and TPA/fluorescence trade-offs. It also
allows us to define a strategy for both enhancement of TPA and
fluorescence quantum yield that is based on elongated A–p–A
derivatives including phenylene-vinylene connectors. In addi-
tion, by combining electron-withdrawing core and end-groups,
derivatives showing both excellent transparency in the visible
region and longer excited-state lifetime have been obtained.
Such compounds are interesting new candidates for optical
limiting in the visible range in the nanosecond regime.
3
3
Optical experiments
UV/VIS spectra were recorded on a Jasco V-570 spectrophoto-
meter.
Fluorescence measurements were performed at room tem-
ꢀ6
perature in diluted CH
2
Cl
2
solutions (ca. 10 M) using an
Edinburgh Instruments (FLS 920) spectrometer in photon-
counting mode. Emission spectra were obtained, for each
compound, at lex ¼ lmax(abs) with Alex ¼ 0.1 to minimize
internal absorption. Fluorescence quantum yields were deter-
mined on degassed samples at room temperature, relative to
fluorescein in 0.1 M NaOH as a standard (lex ¼ 470 nm), and
Experimental
Chemicals
3
Compound 1 was prepared as previously described by lithia-
tion of 9,10-dibromoanthracene followed by reaction with
FBMes as shown in Scheme 1. Compounds 2–7 were synthe-
2
sized by hydroboration of the respective diethynyl compounds
2
3
4
refractive index correction was performed. Lifetimes were
recorded using the technique of time-correlated single photon
counting (TCSPC).
3
at room temperature. The reactions are
2
using (HBMes
2
)
2
TPA (two-photon absorption) spectra were obtained by
measuring the TPEF (two-photon-excited fluorescence) ac-
cording to the experimental protocol established by Xu and
very clean with only the single isomers being isolated, and the
products are stable to air for long periods. Terminal acetylenes
were prepared by palladium catalyzed cross-coupling reactions
of aryl halides with trimethylsilylacetylene followed by depro-
tection using sodium carbonate in methanol. All compounds
were purified by recrystallization and/or column chromatogra-
phy using silica gel and purity was assessed using GC-MS,
NMR and CHN analysis. Compounds 2 and 5 have been
3
5
Webb, by using a mode-locked Ti/sapphire laser that delivers
80 fs pulses at 80 MHz. The quadratic dependence of the
fluorescence intensity on the excitation intensity was verified
for every data point, indicating that the measurements were
carried out in intensity regimes in which saturation or photo-
degradation do not occur. TPEF measurements were cali-
brated relative to the absolute TPEF action cross-
section determined by Xu and Webb for fluorescein in water
3
2
reported previously along with their second molecular hy-
perpolarizabilities. Full details of the synthesis, spectroscopic
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 6 0 0 – 6 0 6
601

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Table 1 Photophysical properties of compounds 1–7 and 11–13
ꢀ1
ꢀ1
a
Stokes shift /cm
ꢀ1
b
max
TPA
c
c
/MW /GM
Compound
lmax/nm
e/cm
M
lem/nm
t /ns
F
l
/nm
s
2
/GM
s
2
d
1
455
373
360
361
370
386
399
371
372
389
14 500
45 100
38 200
48 000
52 000
57 400
70 100
—
497
433
505
454
419
430
447
384
407
431
1900
3700
8000
5700
3200
2700
2700
900
8.10
0.37
2.39
0.53
0.15
0.22
0.72
1.1
0.816
0.032
0.045
0.0068
0.084
0.043
0.548
0.52
705
715
710
25
24
28
22
34
86
0.037
0.038
0.040
0.035
0.049
0.120
0.517
0.011
0.061
0.096
ꢀ1
d
2
d
3
d
4
o735
710
710
705
730
720
720
d
5
d
6
d
7
377
8
30,e
31,e
31,e
1
1
1
a
1
2
3
39 900
48 000
b
2300
2500
c
0.9
1.2
0.83
0.78
41
75
max
ꢀ50
4
Stokes shift ¼ (1/labs ꢀ 1/lem). Experimental fluorescence lifetime. TPA cross-section at lTPA , 1 GM ¼ 10
cm s photon , TPEF
measurements were performed using a mode-locked Ti:sapphire laser delivering 80 fs pulses at 80 MHz, calibrating with fluorescein.
d
e
2 2
In CH Cl . In THF.
contrast, chromophores 2–6 where the dimesitylboryl end-
groups are attached to an aromatic core via vinylene linkers
show weak fluorescence efficiency. Only chromophore 7 (built
from an elongated stilbenyl core) recovers high photolumines-
cence efficiency.
Core effect in aromatic connectors. As observed from the
3
0
comparison with a fluorenyl derivative (Fang and colleagues,
1), we notice bathochromic shifts in the maxima of the anthryl
1
chromophore (our work, 1), and higher F (0.82 vs. 0.52). The
Stokes shift increases drastically as well, suggesting that im-
portant molecular reorganization takes place after excitation
prior to emission. This should be correlated with the larger
dihedral angle (531) between the aromatic core and the BC
planes due to the steric hindrance caused by anthracene
3
2,37
3
3
0
(
respectively, about 271 in the fluorene 11).
Fig. 2 Normalized absorption (left) and emission (right) of 1 in
CH Cl (the dotted trace is normalized on the 259 nm peak).
2
2
Core effect with vinylic spacers. On the other hand, the
insertion of a vinyl linkage between the boron atoms and the
conjugated core (2–7) reduces the steric hindrance of the boron
3
5,36
(
pH ¼ 11) in the 690–1000 nm range.
The experimental
uncertainty amounts to ꢁ10%.
3
atoms, allowing the twist angle (conjugated-core/BC ) to be
3
7
nearly zero, as derived from DFT calculations. These planar
structures allow an extended electronic delocalization along the
conjugated backbone and result in bathochromic shifts of the
absorption and the emission maxima (Figs. 3 and 4).
We note that the quantum yields of these chromophores are
low (between 3–9%) but still noticeable, except for the thio-
phene derivative (4) for which F is about 0.7%. However,
lower fluorescence efficiencies are generally expected with
thiophene-containing compounds, due to the heavy atom
effect.
Results and discussion
Linear spectroscopy investigations
The photophysical characteristics of chromophores 1–7 in
CH Cl are summarized in Table 1. All the chromophores
2
2
show an intense absorption band in the near UV–blue visible
region (Figs. 2, 3a, 4a). In contrast, the photoluminescence
efficiency is strongly dependent on the nature of the core.
Compound 1, built from the anthryl core, exhibits a large
fluorescence quantum yield (see also compound 11, Fig. 1). In
On going from phenylene (2) to perfluorophenylene (3), via
thienylene (4) core, we notice large increasing red shifts in the
2 2
Fig. 3 Normalized absorption (a) and emission (b) of quadrupoles 2–4 in CH Cl .
6
02
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 6 0 0 – 6 0 6
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5

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2 2
Fig. 4 Normalized absorption (a) and emission (b) of quadrupoles 2 and 5–7 in CH Cl .
fluorescence emission bands (Fig. 3b), while it is the opposite in
the absorption maxima (Fig. 3a). As a result, a marked increase
in the Stokes shift values is observed, which reaches 8000 cm
for compound 3 and reveals a large nuclear reorganization in
Stokes shift and red shifts both in absorption and emission. It
would be particularly interesting to extend this study to tris-
anthryl boron derivatives that have been reported to present
ꢀ1
3
9
attractive stability and electron-accepting properties.
the relaxed S state following electronic redistribution.
1
2
BMes grafted via vinylic bonds. The TPA cross-section is
Size effect. Increasing the size of the central connector
always results in higher molar absorption coefficient, usually
together with red shifts of the absorption and emission bands
enhanced when the dimesitylboryl end-groups are grafted via a
vinylene linkage, as seen from the comparison of compounds 6
and 11 (Table 1). The insertion of these double bonds (increas-
ing the weight by only 8%) seems to have a profound effect on
the TPA cross-section, most probably due to the planarized
structure allowed by the relief of steric hindrance.
Indeed, if the planarity is prevented by removing the methy-
lene bridge (biphenyl compound 5), the TPA is kept down
Fig. 5b), again following the bathochromic shifts of the linear
(
Table 1).
Lengthening the phenyl p-connector (compound 2) to a
stilbenyl unit (compound 7) results in bathochromic shifts of
both absorption and emission (Fig. 4). This red shift is less
pronounced in emission as seen from the decreasing values of
the Stokes shift, suggesting a more pronounced nuclear reor-
ganization in the short compounds in the excited state.
In addition, it must be stressed that the stilbenyl p-connector
restores high fluorescence quantum yield (F ¼ 55%), while
having only an additional vinyl bond as compared to the
biphenyl compound 5 (F ¼ 8.4%).
(
optical transitions. Hence, a planar structure allowing efficient
electronic delocalization along the conjugated backbone is
crucial to obtain a large TPA cross-section.
We have also investigated the effect on the TPA properties of
three other aromatic rings with compounds 2–4: phenylene,
perfluorophenylene, thienylene. The values are in the order
phenylene o perfluorophenylene and phenylene o thienylene
Lengthening the p-connector does not always induce a
bathochromic shift, as best seen from the biphenyl derivative
(
5) which is blue shifted with respect to the parent phenyl
(
Fig. 5c), and follow both the increasing electron-withdrawing
compound (2). In contrast, the position of absorption and
emission maxima strongly depends on the planarity of the p-
connector: the bridged biphenyl compound (fluorene 6) indeed
is red shifted compared to both 2 and 5 (Fig. 4).
and electron-releasing strengths of the aromatic core. Whereas
the effect of the thienylene unit as compared to the phenylene
unit is expected for A–p–A compounds, the positive (and again
1
8
expected based on a simple model ) effect of the replacement
of a phenylene core by a perfluorophenylene core has only been
It is known from X-ray structures and ab initio calculations
0
that the twist angle usually reaches 401 in 4,4 -disubstituted
biphenyls, whereas fluorene has an almost planar structure.
9
3
8
observed before in D–p–D compounds. Indeed, the similar
trend observed here for A–p–A derivatives has never been
established before, to the best of our knowledge, in A–p–A
chromophores. The TPA cross-section of compound 3 is about
The better p orbital overlap allowed from planar structures
explains the bathochromic shifts in the fluorene chromophore.
Moreover, in 6, the rigid methylene bridge prevents a large
2
0% larger than that of compound 2. Here, the source of the
0 1
reorganization between the S and S states, resulting in a
large TPA cross-section of 3 is more intricate than a simple
quadrupolar electronic redistribution from the core to the
periphery. It should be stressed that perfluoroaryl rings have
rarely been used in the design of TPA chromophores. Yet the
present study converges with the research of Strehmel et al.
emphasizing the promises of the perfluorobenzene core in a
smaller Stokes shift (Table 1) than the biphenyl derivative,
while having the same p system. We also observe a hyperchro-
mic shift with the fluorene chromophore.
Two-photon properties (TPA)
9
modular approach towards enhanced TPA
We aim here at scrutinizing the structure–TPA property rela-
tionships. The TPA spectra have been determined using the
two-photon excited fluorescence (TPEF, see experimental
section) technique and are illustrated in Fig. 5a–d.
Moreover, the perfluorophenylene derivative 3 has potential
for optical limiting applications, being both more TPA-active
than its phenylene analogue 2 and showing the best transpar-
4
0
ency in the visible region (cut-off wavelength ¼ 419 nm in
CH Cl ). In this respect, it is important to note that chromo-
2
2
BMes
derivative (1), compared to its fluorenyl analogue (11), leads
to TPA cross-section (s ) more than three times larger resulting
in a comparatively large TPA cross-section for a compound of
similar size. This should be related to the more pronounced
intramolecular charge transfer in 1, as denoted by the larger
2
grafted directly onto the aromatic rings. The anthryl
phore 3 shows a nearly one order of magnitude longer excited-
state lifetime than its phenylene analogue 2 (Table 1 and Fig.
6). This effect can be correlated with the significant red shift of
the emission of chromophore 3 (by 72 nm) as compared to
chromophore 2. This behavior is most probably related to a
very large reorganization energy and might be an indication
3
0
2
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 6 0 0 – 6 0 6
603

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Fig. 5 (a) Two-photon absorption spectra of 1 in CH Cl
2
2
. (b) Two-photon absorption spectra of 5–6 in CH
Cl
2
Cl
2
. (c) Two-photon absorption
spectra of 2–4 in CH Cl and (d) Two-photon absorption spectra of 2 and 7 in CH
2
2
2
2
.
that a significant photo-induced change of geometry (such as
4
such a phenomenon could be involved. In such a viscous
ꢀ1
1
TICT ) is taking place after excitation prior to emission.
Preliminary experiments performed in squalane indicate that
solvent, the Stokes shift is smaller (6000 cm ) and the emis-
sion spectrum is significantly blue-shifted as compared to the
emission spectrum in a medium polarity but low-viscosity
2 2
solvent (such as CH Cl ) and is a mirror image of the absorp-
tion spectrum. This indicates that the higher viscosity of
squalane hampers strong geometrical changes that lead to the
longer-lifetime red-shifted emissive state. Indeed, such a pheno-
menon has been reported in D–p–A–p–D systems where a
4
2
twisted intramolecular charge transfer was observed. We
report here that a similar phenomenon may occur even with a
0
neither dipolar nor quadrupolar A–p–A –p–A compound, un-
derlining the particular interest of the perfluorophenylene moi-
4
2
ety firstly underlined by Strehmel et al.
The longer lifetime obtained for molecule 3, in addition to its
excellent transparency in the visible region, is particularly
interesting for optical limiting applications in the nanosecond
regime where excited-state absorption (ESA) can play a sig-
4
3
nificant role in enhancing the nonlinear absorption behavior.
0
Hence, A–p–A –p–A compounds represent a promising novel
route for optical power limiting in the visible region calling for
nonlinear transmission experiments in the ns regime and
excited-state absorption experiments. This new route could
prove to be quite advantageous for further engineering of such
chromophores by replacing for instance the central phenyl ring
with phenyl moieties bearing much stronger electron-with-
drawing substituents such as triflyl moieties. Indeed, such a
strategy has been demonstrated recently to be quite efficient in
enhancing the TPA properties in a different type of deriva-
2
2
tives. Interestingly, in those octupolar derivatives, the ex-
cited-state lifetime has also been found to increase with
2
2
increasing acceptor strength.
2 2
Fig. 6 Fluorescence decays (dots) of 1–3 in CH Cl , using the TCSPC
Effect of the connector size. Most interestingly, the stilbenyl
compound 7 reveals the highest TPA cross-section of the series,
having a TPA cross-section at 740 nm more than 16 times as
method. The very short dotted profile is the pulse shape of the
excitation laser (lex ¼ 371 nm). Lower traces are the residues of the
fitting curves (lines).
6
04
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 6 0 0 – 6 0 6
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5

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ment of the TPA efficiency. This led to an elongated derivative
(
(
7: l B 2.7 nm) showing the largest normalized TPA efficiency
/MW) reported up to now for symmetrical transparent
s
2
derivatives with dimesitylboryl end-groups. The advantage of
such derivatives with respect to improvement of the TPA/
transparency trade-off is clearly evidenced in Fig. 7, compound
7
being the more TPA active in the visible region.
ii) Another important observation is that increasing the
(
electron-poor nature of the aromatic core results in both a
slight increase of the TPA efficiency, a definite improvement of
the linear transparency, and in major increase of the excited-
state lifetime. In combination with the above observation, this
finding suggests a new molecular engineering route towards
optimization of the TPA/transparency trade-off based on
0
elongated A–p–A –p–A systems. Indeed examination of mole-
cules 2 and 3 (same normalized TPA efficiency but much
improved transparency for 3) as well as molecules 2 and 7
much higher normalized TPA efficiency for 7 but lower
(
transparency) from Fig. 7 suggests that the analogous deriva-
tives of 7 where the two phenylene units in the stilbenyl core
would be replaced by perfluorophenylene units should display
superior TPA/transparency trade-off. Such a route holds pro-
mise for optical power limiting applications in the visible
region.
Fig. 7 Weight normalized TPA values of known boron TPA chro-
max
mophores, plotted versus labs
(restricted to transparent compounds:
o 400 nm ). Compounds 1–7 are from this work. IV and VIII
are dipoles 4 and 8 in ref. 27, respectively. 11, g, and d are compounds
, 3 and 4 in ref. 30, respectively. 12 and 13 are quadrupoles 1 and 3 in
max
l
abs
2
ref. 31, respectively. c, f, h and i are compounds 3, 6, 8 and 9 in ref. 29,
respectively. BP is quadrupole 1 in ref. 28.
(
iii) Lastly, we have shown that minor modifications of the
conjugated backbone can significantly influence the fluores-
cence quantum yield, lengthening inducing either a decrease or
an increase of the fluorescence quantum yield.
large as that of its shorter analogue with the phenylene p-
connector (2), though its molecular weight is increased by only
1
6% (Fig. 5d). In addition, this chromophore is highly fluor-
escent, and could therefore be used in TPA-induced photo-
luminescence applications. In that respect, another main
advantage is the long excited-state lifetime.
Acknowledgements
We are very grateful to Prof. J. Mertz (ESPCI, now at Boston
University) for access to his TPEF facilities and to T. Pons for
help in recording the TPA spectra. We acknowledge partial
financial support from CNRS and MENRT. M.C. and C.D.E.
thank the DGA and the EPSRC, respectively, for studentships.
T.B.M. and A.B. thank One NorthEast for financial support of
this work, through the Nanotechnology UIC programme.
M.B.D. and T.B.M. thank the Royal Society (UK) and
CNRS (France) for financial support for their collaboration
through the International Joint Project – Europe (EPES)
Program.
This study thus provides evidence that it is possible to
enhance significantly the TPA characteristics in A–p–A deri-
vatives bearing dimesitylboryl acceptors while maintaining
excellent photoluminescence properties via suitable molecular
engineering of the p–connector. Indeed, chromophore 7 shows
a TPA cross-section (Table 1) that is 5–9 times larger than
others A–p–A analogues (13 and 12, respectively) reported
3
1
very recently (Fig. 1). In addition, based on the experimental
observations, extending the p-connector from a phenyl to a
stilbenyl unit raises the lowest energy two-photon allowed (but
one-photon forbidden) state above the first one-photon al-
4
4
lowed excited state, thus restoring fluorescence. Such an
interpretation is indeed supported by the observations that
molecule 7 shows a low-energy TPA maximum which is blue–
shifted with respect to twice the maximum one-photon absorp-
tion (OPA) wavelength, whereas the TPA spectra of molecules
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2
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2
and 3 have low-energy weak TPA bands (shoulders), which
are red-shifted with respect to twice the maximum OPA
wavelength.
1
106.
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3
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Conclusion
We have scrutinized the two-photon absorption as well as the
linear photophysical properties of a series of dimesitylboron-
substituted chromophores. The structure-property relation-
ships arising from our measurements reveal several points that
could guide further molecular TPA-chromophore design:
4
5
(
i) First of all, higher TPA cross-sections have been achieved
1
653.
by conferring a planar ground-state structure. So far, vinyl-
boranes have not been considered for TPA, and they show here
very promising properties compared with the traditional aryl-
borane. Although the fluorescence efficiency decreases with the
insertion of a vinyl unit between the aryl–boron bond for the
shortest compounds, such an effect can be circumvented by
using an appropriate elongated conjugated core (i.e. stilbenyl
instead of fluorenyl or biphenyl) allowing both for restoration
of a significant fluorescence quantum yield and major enhance-
6
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T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
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