Bis-1,4-(p-diarylaminostryl)-2,5-dicyanobenzene derivatives with large two-photon absorption cross-sections

Article abstract of DOI:10.1021/ol027343h

(Matrix presented) Synthesis and physical properties of novel multibranched two-photon materials are reported. The compound with three units of 4-(p-diphenylaminostyryl)-2,5-dicyanostyryl moieties attached to the central triphenylamine core exhibits a ver

Full text of DOI:10.1021/ol027343h

ORGANIC
LETTERS
2003
Vol. 5, No. 5
645-648
Bis-1,4-(p-diarylaminostryl)-2,5-
dicyanobenzene Derivatives with Large
Two-Photon Absorption Cross-Sections
Jun Yoo, Si Kyung Yang, Mi-Yun Jeong, Hyun Cheol Ahn, Seung-Joon Jeon, and
Bong Rae Cho*
Molecular Opto-Electronics Laboratory, Department of Chemistry and
Center for Electro- and Photo-ResponsiVe Molecules, Korea UniVersity,
1-Anamdong, Seoul 136-701, Korea
chobr@korea.ac.kr
Received November 23, 2002
ABSTRACT
Synthesis and physical properties of novel multibranched two-photon materials are reported. The compound with three units of
4-(p-diphenylaminostyryl)-2,5-dicyanostyryl moieties attached to the central triphenylamine core exhibits a very large two-photon absorption
cross-section.
Extensive research is being conducted to develop organic
materials with large two-photon absorption (TPA) cross-
sections because of their potential applications in optical
power limiting,1-3 two-photon upconversion lasing,4,5 two-
photon fluorescence excitation microscopy,6-10 three-dimen-
sional optical data storage,11-13 and photodynamic therapy.14
A variety of compounds including donor-bridge-acceptor
(D-π-A) dipoles,15 donor-bridge-donor (D-π-D) quadru-
poles,16-18 multibranched compounds,19 dendrimers,20 and
octupoles21 have been synthesized and their structure-
property relationships established.15-22 The results of these
studies reveal that that the TPA cross-section increases
monotonically with the extent of charge transfer. Also, a
significant increase in the TPA cross-section was noted in
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10.1021/ol027343h CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/07/2003

Figure 1. Structures of 1-3.
the multibranched structures indicating a cooperative en-
hancement.19 Therefore, a useful design strategy for the
synthesis of efficient TPA materials would be to incorporate
D-π-A- or D-π-D-types of TPA dye into the multi-
branched structure. We have now synthesized a series of bis-
1,4-(p-diphenylaminostryl)-2,5-dicyanobenzene derivatives
with unprecedentedly large two-photon crosssections by
using this design strategy (Figure 1).
All of the compounds were synthesized by the condensa-
tion between phosphonate ylids and benzaldehyde derivatives
as shown in Scheme 1. Compounds 1a-c were prepared by
Scheme 1a
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a Reagents and condition: (a) p-Ph2NC6H4CHO (1 equiv)/LDA/
THF, 0 °C-rt, 40-43%. (b) PhN(C6H4CHO)2 (0.5 equiv)/LDA/
THF, 0 °C-rt, 56 %. (c) N(Ph2NC6H4CHO)3 (0.3 equiv)/LDA/
THF, 0 °C-rt, 21-62 %.
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the condensation between A and the appropriate benzalde-
hyde as reported.17c,23 To prepare 2 and 3, intermediate B
was prepared by the same procedure as described for 1 except
that 1 equiv of benzaldehyde was used. Reactions of N,N-
bis(4-formylphenyl)aniline and N,N,N-tris(4-formylphenyl)-
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Y.; Norman, H.; Ågren, H. J. Chem. Phys. 2000, 113, 7055.
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Swiatkiewicz, J.; Prasad, P. N. Chem. Mater. 2000, 12, 2838.
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Jeon, S.-J.; Choi, J.-H.; Lee, H.; Cho, M. J. Am. Chem. Soc. 2001, 123,
10039. (b) Lee, W.-H.; Lee, H.; Kim, J.-A.; Choi, J.-H.; Cho, M.; Jeon,
S.-J.; Cho, B. R. J. Am. Chem. Soc. 2001, 123, 10658. (c) Cho, B. R.;
Piao, M. J.; Son, K. H.; Lee, S. H.; Yoon, S. J.; Jeon, S.-J.; Choi, J.-H.;
Lee, H.; Cho, M. Chem. Eur. J. 2002, 8, 3907.
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646
Org. Lett., Vol. 5, No. 5, 2003

bathochromic shift and hence a larger Stokes shift in
comparison to 1a,b, indicating that the donor stabilizes the
lowest excited state more than the ground state (Table 1).
On the other hand, the Stokes shift decreased from 1a to
2a, probably because two dicyanophenyl groups in 2a must
share the central nitrogen donor to diminish the excited-state
charge transfer. A further increase in the branching to 3a
had little influence on the Stokes shift, indicating that there
is little difference in the excited-state charge transfer between
2a and 3a.
The slight decrease in the fluorescence quantum yields
with a stronger donor could be due to the increased charge
transfer. This would facilitate the solute-solvent interaction,
by which the excited state could be rapidly quenched and
decrease the quantum yield.25 Note that compound 1c with
the largest Stokes shifts shows the smallest quantum yield.
Also, the quantum yields of 2a and 3a are nearly the same
probably because their Stokes shifts are very similar.
The two-photon cross-section δ was measured by using
the two-photon-induced fluorescence measurement technique
with the following equation,
Figure 2. Normalized one-photon absorption and emission spectra
of 1a-3a in toluene.
amine with B afforded 2 and 3, respectively (Scheme 1).
The experimental procedures for the synthesis of 2 and 3
are described in Supporting Information.
SsΦrφrc
One-photon absorption and fluorescence spectra of 1a-
3a in toluene solution are depicted in Figure 2. The
δ )
rδr
SrΦsφscs
corresponding spectra for 1b,c are shown in Figure S1 in
(1)
max
Supporting Information. The results indicate that λ
of
where the subscripts s and r stand for the sample and
reference molecules, respectively.17 The intensity of the
signal collected by a PMT detector was denoted as S. Φ is
the fluorescence quantum yield. φ is the overall fluorescence
collection efficiency of the experimental apparatus. The
number density of the molecules in solution was denoted as
c. δr is the TPA cross-section of the reference molecule (see
Supporting Information).
1a-c increases with a stronger donor (Table 1). It also
Table 1. One- and Two-Photon Properties of 1-3 in Toluene
(1)
max
{fl}
max
(2)
max
a
b
e
f,g
cmpd
λ
λ
∆νjc
Φd
λ
δmax
1a
1b
1c
2a
3a
473 (77.2)
485 (66.2)
488 (70.9)
492 (125)
495 (239)
527
543
560
534
536
2170
2200
2640
1600
1550
0.73
0.62
0.53
0.69
0.67
840
880
890
840
840
1370 (1.0)
1350
2250
3130 (1.4)
5030 (1.6)
a λmax of the one-photon absorption spectra in nm. The numbers in
b
parentheses are the molar extinction coefficients (103ꢀ). λmax of the one-
photon emission spectra in nm. c Stokes shift in cm-1
.
d Fluorescence
e
quantum yield. λmax of the two-photon absorption spectra in nm. f Peak
two-photon absorptivity in 10-50 cm4 s photon-1 (GM). g Numbers in the
parenthesis are relative δmax/MW.
increases as the number of branching increases from 1a to
2a, indicating that the central triphenylamine moiety par-
ticipates in the conjugation. However, a further increase in
the branching to 3a resulted in only a small increase in the
(1)
max
λ
probably because the cross-conjugation effect is rela-
Figure 3. Two-photon excitation spectra of 1a-3a in toluene.
tively unimportant.24 It is interesting to note that the ratio
between the molar absorptivity of 1a and 3a is close to 1:3,
as expected if the three branches in 3a are effectively
separated and noninteracting.
Figure 3 shows the two-photon-induced fluorescence
excitation spectra of 1a-3a. The corresponding spectra for
1b,c are shown in Figure S2 in Supporting Information. All
of the compounds show two peaks at 840-890 nm, indicat-
ing the existence of the common two-photon allowed states.
A similar result was observed in the fluorescence spectra
except that the substituent effect was larger. Compared with
{fl}
max
the absorption spectra, the λ
of 1c shows a greater
(24) Smith, M. B.; March, J. AdVanced Organic Chemistry; Wiley &
Sons: New York, 2001; pp 39-40.
(25) Ko, C.-W.; Tao, Y.-T.; Danel, A.; Krzeminska, L.; Tomasik, P.
Chem. Mater. 2001, 13, 2441.
Org. Lett., Vol. 5, No. 5, 2003
647

Table 1 summarizes the λmax of one-photon absorption and
fluorescence spectra, wavelengths of the two-photon absorp-
tion maxima (λ(m2a)x), and the maximum values of the two-
ment is mainly caused by the vibronic coupling.19b The
electronic coupling is weak probably because the central
amino group is used as the connecting unit, which breaks
the conjugation of the whole network. On the other hand,
increasing the size of the system will increase the density of
state, providing more effective coupling channels, which
would in turn increase the TPA cross-section.
A similar explanation could be applied to the present result.
As stated above, δmax/MW increases with the applied
branching despite the smaller Stokes shift (Table 1). This is
in contrast to the parallel increase of δmax and Stokes shift
for 1a-c, where the electronic interactions are important
(vide supra). Hence, the possibility of the electronic coupling
being the cause of such an enhancement could be ruled out,
and the vibronic coupling appears to be the favored alterna-
tive.
In conclusion, we have synthesized a series of efficient
TPA molecules. The wavelength of the maximum TPA cross-
section could be tuned by using a different substituent. The
TPA cross-section increased with a stronger donor and with
the number of branching to reach the maximum value of
5030 GM for 3a. Noteworthy is the significant enhancement
of the TPA cross-section in the multibranched structure. This
result underlines the importance of incorporating the TPA
chromophores in the multibranched structure to obtain a very
large TPA cross-section.
photon absorption cross-section (δmax). The value of δmax
)
1370 GM for 1a is somewhat smaller than the reported value
(2)
max
by Marder’s group.17 The λ
values of 1a-c increase as
the electron-donating ability of the aryl substituent increases.
This result indicates the interesting possibility that the
wavelength of the maximum two-photon cross-section could
be tuned by using an appropriate substituent. It is also shown
that the δmax values of 1a,b are similar, but that of 1c is
{fl}
max
much larger. Note that 1c has a much larger λ than 1a,b
and, hence, a larger Stokes shift (Table 1). This could be
explained if the two-photon allowed states are close to the
lowest excited states. As discussed above, the donor stabilizes
the latter more than the ground state to diminish the energy
gap between the ground- and two-photon allowed states. This
would predict a larger two-photon cross-section, because the
smaller the energy gap is, the higher the probability of the
excitation will be.
The most interesting result of the present study is that the
value of δmax/MW for 1a-3a increases from 1.0 to 1.4 to
1.6 with increased branching, indicating significant enhance-
ment in the multibranched structure. To our knowledge, δmax
) 5030 GM determined for 3a is the largest value reported
in the literature.26
Chung et al. reported similar enhancement in the triph-
enylamine-based multibranched structure and attributed their
result partly to the electronic coupling between the branches.19a
However, a theoretical study revealed that such an enhance-
Acknowledgment. This work is supported by NRL-
MOST and CRM-KOSEF. S. K. Yang and H. C. Ahn were
supported by a BK21 scholarship.
Supporting Information Available: Experimental pro-
cedures for the synthesis of compounds 2 and 3 and one-
photon absorption and fluorescence, as well as two-photon
excitation spectra of compounds 1b,c. This material is
available free of charge via the Internet at http://pubs.acs.org.
(26) The most efficient two-photon chromophores and their two-photon
cross-sections measured by the two-photon-induced fluorescence using the
nanosecond pulses are 2,7-bis{[7-(p-dioctylaminostryl)-9,10-dihydrophen-
athren-2-yl]vinyl}-9,10-dihydrophenathrene (λmax ) 422 nm, Φ ) 0.66,
δmax ) 3760 GM),18a 4,4′-bis[4-(p-dihexylaminophenylethynyl)styryl]-
biphenyl (λmax ) 400 nm, Φ ) 0.80, δmax) 1050 GM), and 1,3,5-tricyano-
2,4,6-tris(p-diphenylaminostyryl)benzene (λmax ) 488 nm, Φ ) 0.25, δmax
)
2480 GM).21a
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Org. Lett., Vol. 5, No. 5, 2003