High two-photon cross-sections in bis(diarylaminostyryl) chromophores with electron-rich heterocycle and bis(heterocycle)vinylene bridges

Article abstract of DOI:10.1039/b618265h

Chromophores in which vinylene units and either pyrrole or dialkoxythiophene groups form the bridge between two diarylaminophenyl groups exhibit two-photon cross-sections of 1000-5000 GM at 600-650 nm. The Royal Society of Chemistry.

Full text of DOI:10.1039/b618265h

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High two-photon cross-sections in bis(diarylaminostyryl) chromophores
with electron-rich heterocycle and bis(heterocycle)vinylene bridges{
Shijun Zheng,a Luca Beverina,a Stephen Barlow,a Egbert Zojer,ab Jie Fu,c Lazaro A. Padilha,c
Christian Fink,a Ohyun Kwon,a Yuanping Yi,d Zhigang Shuai,d Eric W. Van Stryland,ce David J. Hagan,ce
Jean-Luc Bre´dasa and Seth R. Marder*a
Received (in Cambridge, UK) 14th December 2006, Accepted 8th January 2007
First published as an Advance Article on the web 26th January 2007
DOI: 10.1039/b618265h
Chromophores in which vinylene units and either pyrrole or
dialkoxythiophene groups form the bridge between two
diarylaminophenyl groups exhibit two-photon cross-sections
of 1000–5000 GM at 600–650 nm.
The applications of two-photon absorption (2PA), ranging from
3D fluorescence imaging, nonlinear optical transmission, and 3D
microfabrication, have motivated recent interest in the develop-
ment of new chromophores with large 2PA cross-sections, d.1 To
realize the full potential of these applications it is necessary to
develop more complete structure-property relationships for d.
Several classes of organic molecules have been investigated
including quadrupolar p-systems with donor end groups (D–p–
D);2 species such as D–p–D bis(styryl)benzenes exhibit peak d
values at least an order of magnitude larger than their
unsubstituted counterparts. d can be increased further and the
2PA band can be red-shifted when the p bridge is elongated or
acceptor-substituted (D–A–D). Here we report on the 2PA
properties of 2,5-bis[4-(diarylamino)styryl]thiophenes, a 2,5-bis[4-
(diarylamino)styryl]pyrrole (D–D9–D, 1, 3, 5, Fig. 1),3 and of
Fig. 1 Structures of 1–7 and model compounds 19–49, 79 and 30.
extended chromophores with additional heterocycle–vinylene units
previously reported,2a,c 7 also shows a well-defined moderate peak
in the bridge (D–D9–D9–D, 2, 4, 6) and compare them to those of
the previously investigated chromophore 7.2a,c
in d in this region. For compounds 2–6 a more intense
The 2PA spectra were determined by two-photon-induced
(.1000 GM) 2PA feature is observed at higher energy (state
fluorescence (for lower photon energies) and Z-scan (for higher
energy .3.5 eV; photon energy .1.75 eV; photon wavelength
photon energies) and are shown for some of the compounds in
Fig. 2. 1–6 all show moderate (,1000 GM; 1 GM = 1 6 10250 cm4
,700 nm). For 1 and 7 only the onset of a high-energy peak is seen
in the energy range studied. We note that, as with 1PA (see ESI{),
s photon21 molecule21) 2PA in the 3.0–3.5 eV state-energy range
the 2PA peaks of the pyrrole-bridged compounds are blue-shifted
(i.e., corresponding to absorption of photons of energy 1.5–1.75 eV,
equivalent to a wavelength of 700–830 nm), with distinct maxima
resolvable for most cases, but only a shoulder observed for 2. As
aCenter for Organic Photonics and Electronics and School of Chemistry
and Biochemistry, Georgia Institute of Technology, Atlanta, GA, 30332-
0400, USA. E-mail: seth.marder@chemistry.gatech.edu;
Fax: +1 404 894 5909; Tel: +1 404 385 6048
bInstitute of Solid State Physics, Graz University of Technology,
Petersgasse 16, A-8010, Graz, Austria
cCREOL, College of Optics and Photonics, University of Central
Florida, Orlando, FL, 32816-2700, USA
dCenter for Molecular Sciences, Institute of Chemistry, The Chinese
Academy of Sciences, 100080, Beijing, P. R. China
eDepartment of Physics, University of Central Florida, Orlando, FL,
32816-2700, USA
{ Electronic supplementary information (ESI) available: Experimental
details for synthesis, 2PA measurements and calculations; characterizing
data including electrochemical and spectroscopic data; 2PA spectra for 1–
7 with data points acquired by Z-scan and two-photon fluorescence
Fig. 2 Two-photon spectra of 1–4 and 7 in THF.
differentiated. See DOI: 10.1039/b618265h
1372 | Chem. Commun., 2007, 1372–1374
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relative to the corresponding dialkoxythiophene species, and those
of the n = 2 molecules are red-shifted relative to their n = 1
analogs. The R = Ph compounds, 5 and 6, show similar 1PA and
2PA spectra to the R = p-MeOC6H4 chromophores, 3 and 4,
respectively (see ESI{).
For (approximately) centrosymmetric systems, the cross-section
for 2PA from the ground state, g, into a state, e9, is often
dominated by three-state terms of the form
with the measured low-energy 2PA profiles; however, d for these
features is underestimated in the calculations. In part this can be
attributed to replacing the terminal aryl with methyl groups (39 vs.
30). We also cannot exclude additional vibrational channels
contributing to the overall magnitude of those maxima. Such
contributions have been discussed before5 and have also been
identified as being responsible for deviations between theory and
experiment for relatively weak 2PA features.6 The trends in the
energies of the 1PA- and 2PA-active states for the different
compounds are well reproduced by the calculations; typically for
the methodology used,6 the absolute values of the excited-state
energies are, however, somewhat overestimated. Calculations also
suggest that the onset we seen in the high-energy region for 1 and 7
are tails of strong 2PA peaks similar to those seen for 2–6.
Mg2eMe2e0
de ,3-state!Eg2e0
(1)
0
2
0
(Ege{(Ege =2))
where E and M are energies and transition dipole moments
respectively, and where the e subscript denotes an intermediate
1PA-allowed state.4 Quantum-chemical calculations on the model
compounds (Fig. 1) were used to obtain values of d (obtained
including the 50 lowest singlet excited states in the perturbative
description, see ESI{) and of the quantities entering eqn (1) (see
Table 1) for the lowest lying 2PA states. The calculated d values
are in qualitative agreement with experiment. In all cases, there is a
strong 2PA peak in the high-energy region; the magnitude of d is
also consistent, considering the significantly larger peak-widths in
the experimental data (ca. 0.6 eV compared to 0.2 eV for
calculations; see ESI{). The theoretical results show several weaker
2PA-active states in the 3.0–3.5 state-energy eV region, consistent
According to eqn (1), states with small detuning energies, Ege
2
(Ege9/2), should exhibit strong 2PA;7 this has been demonstrated
experimentally in several classes of compounds with sharp low-
energy cut-offs to 1PA and low detuning energies of 0.2–0.5 eV;8
however, often 2PA maxima at small detuning energy will overlap
with the tail of 1PA, reducing the usefulness of these systems for
practical applications. In the strong high-energy peaks of the
present compounds, however, the detuning energies are still
relatively large (see Table 1), but the large transition dipoles,
Mee9, of ca. 20 D obtained for the higher lying states are ca. 3 6
larger than those into the low-energy states. These enter the
expression for d as their square (see eqn (1)), amounting to
approximately an order-of-magnitude increase in d for the high- vs.
low-energy states. The rest of the difference results from the
smaller detuning and larger energy pre-factor that always apply to
high-energy peaks. In addition it is worth noting that the 1PA and
2PA absorptions of the chromophores referred to in ref. 8 are con-
siderably red-shifted relative to those of the present chromophores.
To understand the origin of the large Mee9 values in 1–6, it is
necessary to look at the quantum-mechanical nature of the excited
states (i.e., their configuration interaction, CI, descriptions), which
are essentially equivalent for all the molecules investigated. As a
representative example, we consider 30: while the CI description
(given for spin-adapted determinants; see ESI{ for details) of the
1PA state, S1, is dominated by a single determinant (H A L), the
two low-energy 2PA states (S2 and S3) and the strong high-energy
2PA state (S7) are dominated by three main determinants:
Table 1 INDO/MRDCI-calculated state energies (eV) and transition
dipole moments (D) for the most strongly allowed low-lying 1PA
(italics, denoted e) and 2PA (denoted e9) states of 19–49, 30 and 79,
along with calculated detuning energies (eV) and cross-sections (GM)
for the 2PA states
a
c
State
Ege or Ege9
Ege 2 (Ege9/2)b Mge or Mee9
dd
19 S1 (e)
3.41
S2 (e9) 3.53
11.3
7.8
7.3
19.1
13.6
9.5
23.6
5.7
10.2
5.5
7.0
20.2
11.1
8.7
1.65
0.95
0.81
132
855
9471
S7 (e9) 4.92
S8 (e9) 5.20
29 S1 (e)
3.08
S2 (e9) 3.33
1.42
0.71
1.75
341
22453
50
S5 (e)
39 S1 (e9) 3.24
S2 (e) 3.37
4.73
S3 (e9) 4.19
S7 (e9) 4.79
S8 (e9) 4.94
1.27
0.98
0.90
179
543
6104
39 S1 (e)
3.21
S2 (e9) 3.36
S2 = 0.52(H,H A L,L) 2 0.38(H A L+1) 2 0.42(H21 A L)
S3 = 0.67(H21 A L) 2 0.58(H A L+1)
1.53
1.30
0.86
0.82
1.59
175
406
1643
6781
71
S3 (e9) 3.82
S6 (e9) 4.70
9.2
10.2
18.9
6.2
S7 (e9) 4.77
49 S1 (e9) 2.89
S7 = 0.49(H,H A L,L) + 0.42(H A L+1) + 0.33(H21 A L)
S2
3.04
12.1
4.0
22.8
11.4
12.0
3.7
Thus, the phases with which the doubly-excited and the two
singly-excited determinants mix is different for S2 and S7; this
mixing results in a correlation-induced redistribution of the
oscillator strength between those two states, i.e., the higher lying
state gains transition dipole moment (and consequently 2PA cross-
section) at the expense of the two lower 2PA states. A detailed
theoretical discussion of this effect can be found in refs. 6 and 9.
The behavior of the present D–D9–(D9)–D systems (which is
similar to that calculated for stilbene)9 is the exact opposite of what
happens in D–A–D type molecules, where the breaking of that
redistribution has been identified as the microscopic origin of the
strong 2PA states in the low-energy region.9 This is also evidenced
S3 (e9) 3.71
S5 (e9) 4.42
1.18
0.83
120
10031
79 S1 (e)
3.68
S2 (e9) 4.11
S3 (e9) 4.31
S8 (e9) 5.50
1.62
1.52
0.93
447
77
8867
19.9
a
b
Ege and Ege9 for 1PA and 2PA states, respectively. Detuning
energy between the virtual state and the lowest energy strongly 1PA-
allowed state. Mge and Mee9 for 1PA and 2PA states respectively, e
in each case denoting the lowest energy strongly 1PA-allowed state.
Calculated cross-sections obtained including the 50 lowest singlet
excited states in the perturbative description of d with the term given
in eqn (1) (with the lowest strongly allowed 1PA state as the
intermediate state, e) making the dominant contribution (see ESI).
c
d
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G. Subramanian, W. W. Webb, X.-L. Wu and C. Xu, Science, 1998,
281, 1653; (b) L. Ventelon, M. Blanchard-Desce, L. Moreaux and
J. Mertz, Chem. Commun., 1999, 2055; (c) M. Rumi, J. E. Ehrlich,
A. A. Heikal, J. W. Perry, S. Barlow, Z. Hu, D. McCord-Maughon,
T. C. Parker, H. Ro¨ckel, S. Thayumanavan, S. R. Marder, D. Beljonne
and J.-L. Bre´das, J. Am. Chem. Soc., 2000, 122, 9500; (d) M. Drobizhev,
A. Karotki, A. Rebane and C. W. Spangler, Opt. Lett., 2001, 26, 1081;
(e) A. Abbotto, L. Beverina, R. Bozio, A. Facchetti, C. Ferrante,
G. A. Pagani, D. Pedron and R. Signorini, Org. Lett., 2002, 4, 1495; (f)
S. J. K. Pond, M. Rumi, M. D. Levin, T. C. Parker, D. Beljonne,
M. W. Day, J. L. Bre´das, S. R. Marder and J. W. Perry, J. Phys. Chem.
A., 2002, 106, 11470; (g) S. K. Lee, W. J. Yang, J. J. Choi, C. H. Kim,
S.-J. Jeon and B. R. Cho, Org. Lett., 2005, 7, 323; (h) S.-J. Chung,
M. Rumi, V. Alain, S. Barlow, J. W. Perry and S. R. Marder, J. Am.
Chem. Soc., 2005, 127, 10844; (i) B. Strehmel, S. Amthor, J. Schelter and
C. Lambert, ChemPhysChem, 2005, 6, 893.
3 A 2PA study of two related bis(aminostyryl)thiophenes has previously
been reported (W. Wang, G. M. Xia, G. B. Xu, Z. Q. Liu and Q. Fang,
Chin. Chem. Lett., 2005, 16, 85); however, this study was limited to a
single wavelength of 800 nm. Similar chromophores with dithienothio-
phene bridges have also been studied experimentally and computation-
ally (ref. 2b;O.-K. Kim, K.-S. Lee, H. Y. Woo, K.-S. Kim, G. S. He,
J. Swiatkiewicz and P. N. Prasad, Chem. Mater., 2000, 12, 284;
by the experimental finding that here very strongly 2PA active
states are found only at higher energies, while the primary
advantage of the more ‘‘conventional’’ D–A–D systems is that
there the lowest 2PA states are already characterized by high cross-
sections.
It is worth noting that the heterocycles in the bridges lead to 1–6
being rather electron rich; 1 and 2 are oxidized at 20.19 and
20.37 V, respectively, vs. FeCp2+/0 and 3–6 all oxidize in the range
+0.05–0.15 V (see ESI{). Excited-state potentials, E1/2+/*, can be
estimated from E1/2+/0 2 E0,0 where E0,0 is the energy of the lowest
singlet excited state (estimated from the onset of 1PA), and
indicate that 1 (22.71 V) and 2 (22.69 V) are superior excited-
state reducing agents to 7 (22.58 V), whereas 3–6 (22.15 to
22.35 V) are somewhat poorer than 7. Since 7 has been shown to
be a sufficiently powerful excited-state reductant to initiate the
radical polymerization of acrylates, it has found application as an
initiator for 2PA microfabrication.10 The similarity of the excited-
state redox potentials to that of 7, along with the enhanced d of the
present compounds at higher energy, suggest the present
compounds, particularly the more electron-rich pyrrole examples,
might be useful 2PA photoinitiators.
˚
C.-K. Wang, P. Macak, Y. Luo and H. Agren, J. Chem. Phys., 2001,
114, 9813).
4 (a) C. W. Dirk, L. T. Cheng and M. G. Kuzyk, Int. J. Quantum Chem.,
1992, 43, 27; (b) R. R. Birge and B. M. Pierce, J. Chem. Phys., 1979, 70,
165.
˚
5 (a) P. Macak, Y. Luo, P. Norman and H. Agren, J. Chem. Phys., 2000,
˚
113, 7055; (b) P. Macak, Y. Luo and H. Agren, Chem. Phys. Lett., 2000,
Finally, we have performed preliminary pump–probe experi-
ments on compound 4; these indicate excited-state absorption at
ca. 670 nm, a wavelength for which there is strong 2PA from the
ground state (corresponding to a Ege9 state energy of 3.7 eV),
suggesting the possibility of optical pulse suppression11 via a 2PA-
induced excited-state absorption mechanism.12
346, 447; (c) A. Painelli, L. Del Freo and F. Terenziani, Chem. Phys.
Lett., 2001, 346, 470.
6 E. Zojer, W. Wenseleers, M. Halik, C. Grasso, S. Barlow, J. W. Perry,
S. R. Marder and J. L. Bre´das, ChemPhysChem, 2004, 5, 982.
7 (a) S. K. Pati, T. J. Marks and M. A. Ratner, J. Am. Chem. Soc., 2001,
123, 7287; (b) E. Zojer, D. Beljonne, P. Pacher and J. L. Bre´das, Chem.
Eur. J., 2004, 10, 2668.
8 (a) K. Ogawa, T. Zhang, K. Yoshihara and Y. Kobuke, J. Am. Chem.
Soc., 2002, 124, 22; (b) M. Drobizhev, Y. Stepanenko, Y. Dzenis,
A. Karotki, A. Rebane, P. N. Taylor and H. L. Anderson, J. Am.
Chem. Soc., 2004, 126, 15352; (c) D. S. Correˆa, S. L. Oliveira,
L. Misoguti, S. C. Zilio, R. F. Aroca, C. J. L. Constantino and
C. R. Mendonc¸a, J. Phys. Chem. A, 2006, 110, 6433; (d) S.-J. Chung,
S. Zheng, T. Odani, L. Beverina, J. Fu, L. A. Padilha, A. Biesso,
J. M. Hales, X. Zhan, K. Schmidt, A. Ye, E. Zojer, S. Barlow,
D. J. Hagan, E. W. Van Stryland, Y. Yi, Z. Shuai, G. A. Pagani,
J. L. Bre´das, J. W. Perry and S. R. Marder, J. Am. Chem. Soc., 2006,
128, 14444.
In summary, we have shown that D–D9–(D9)–D chromophores
can show high d into a state with energy in the range ca. 3.8–4.1 eV
(corresponding to photon energies of 600–650 nm), which in some
cases is detuned sufficiently from double resonance that these
states can be easily accessed experimentally without interference
from 1PA. The high cross-sections associated with these peaks can
be understood in terms of a correlation-induced redistribution of
oscillator strength. The 2PA spectral features, along with the
excited-state redox potentials for the present molecules, suggest
potential use in 2PA microfabrication.
This material is based upon work supported in part by the STC
Program of the NSF under Agreement No. DMR-0120967. We
also thank the National Science Foundation (CRIF 04-43564), the
AFOSR (FA95500410200), the DARPA (MORPH program,
ONR N00014-04-0095), and NSF-China (Nos. 20420150034,
20433070, 10425420). We thank Greg Walker for supplying a
sample of 7.
9 E. Zojer, D. Beljonne, T. Kogej, S. R. Marder, J. W. Perry and
J. L. Bre´das, J. Chem. Phys., 2002, 116, 3646.
10 (a) B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer,
J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I.-Y. S. Lee,
D. McCord-Maughon, J. Qin, H. Ro¨ckel, M. Rumi, X.-L. Wu,
S. R. Marder and J. W. Perry, Nature, 1999, 398, 51; (b) S. H. Park,
T. W. Lim, D.-Y. Yang, N. C. Cho and K.-S. Lee, Appl. Phys. Lett.,
2006, 89, 173133.
11 C. W. Spangler, J. Mater. Chem., 1999, 9, 2013.
12 The strong near-IR absorptions expected for the radical cations and
dications derived from these species also suggest the possibility of optical
pulse suppression via 2PA-induced charge separation (we have
previously reported that [3]2+ absorbs at 1072 nm: S. Zheng,
S. Barlow, C. Risko, T. L. Kinnibrugh, V. N. Khrustalev, S. C. Jones,
M. Yu. Antipin, N. M. Tucker, T. V. Timofeeva, V. Coropceanu,
J. L. Bre´das and S. R. Marder, J. Am. Chem. Soc., 2006, 128,
1812).
Notes and references
1 T.-Z. Lin, S.-J. Chung, K.-S. Kim, X. Wang, G. S. He, J. Swiatkiewicz,
H. E. Pudavar and P. N. Prasad, Adv. Polym. Sci., 2003, 161, 157 and
references therein.
2 (a) M. Albota, D. Beljonne, J.-L. Bre´das, J. E. Ehrlich, J.-Y. Fu,
A. A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder,
D. McCord-Maughon, J. W. Perry, H. Ro¨ckel, M. Rumi,
1374 | Chem. Commun., 2007, 1372–1374
This journal is ß The Royal Society of Chemistry 2007