Two-photon absorption properties of alkynyl-conjugated pyrene derivatives

Full text of DOI:10.1021/jo800363v

studies reveal that the TPA cross sections (δ) increase with the
donor-acceptor strength, conjugation length, and planarity of
the π-center.
Two-Photon Absorption Properties of
Alkynyl-Conjugated Pyrene Derivatives
Hwan Myung Kim,† Yeon Ok Lee,† Chang Su Lim,
Jong Seung Kim,* and Bong Rae Cho*
The most attractive π-center for two-photon materials is the
pyrene moiety because it not only is planar but also has been
extensively studied in various areas of chemical biology as a
fluorophore by introducing appropriate structural modification.
Due to its durable electronic properties, its derivatives have also
been used in many applications including microenvironment
sensors,9 liquid crystals,10 organic light-emitting diodes,11 as
components of various types of fluorescent polymers and
dendrimers,12 photoactive polypeptides,13 and genetic probes.14
Moreover, π-extended pyrene derivatives emitted significantly
red-shifted fluorescence with enhanced quantum yield compared
to the parent pyrene.15–20 However, except for one example
employing pyrene as the edge substituent,21 there has been no
report on the two-photon materials utilizing pyrene as the
π-center.
Department of Chemistry, Korea UniVersity,
Seoul 136-701, Korea
jongskim@korea.ac.kr; chobr@korea.ac.kr
ReceiVed February 14, 2008
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A series of pyrene derivatives having 4-(N,N-dimethylami-
no)phenylethynyl groups as the substituent (1-5) have been
synthesized and their two-photon absorption properties were
investigated. Comparison of two-photon cross section (δmax
)
with related compounds reveals that pyrene is as efficient a
π-center as anthracene in two-photon materials. Moreover,
the two-photon cross section (δmax) increased with the
number of substituents reaching at the maximum value of
1150 GM for the tetra-substituted derivative (5). Furthermore,
the two-photon action cross section (Φδmax) of 5 is compa-
rable to that the most efficient two-photon materials. This
result provides a useful guideline to the design of efficient
two-photon materials bearing pyrene as a π-center.
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Much effort has been devoted to develop organic materials
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microscopy.8 The results of structure-property relationship
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* Corresponding authors.
† These authors contributed equally to this work.
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10.1021/jo800363v CCC: $40.75
Published on Web 06/12/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 5127–5130 5127

SCHEME 1. Synthesis of 1-5a
TABLE 1. Photophysical Data for 1-5
b
e
f
g
compd λmax (10-4 ε)a λfl
∆ν˜c
Φd
λ
δmax Φδmax
(2)
max
1
2
3
4
5
396 (1.59)
431 (6.69)
430 (4.06)
478 (6.06)
508 (6.94)
474 4155 0.90
481 2412 1.00
483 2552 1.00
506 1158 1.00
830
760
760
780
820
55
490
230
530
1150
50
490
230
530
1150
534
958 1.00
a λmax of the one-photon absorption spectra in nm. The numbers in
b
parentheses are molar extinction coefficients. λmax of the one-photon
fluorescence spectra in nm. c Stokes shift in cm-1
.
d Fluorescence
e
quantum yield, (10%. λmax of the two-photon absorption spectra in
f
nm. Two-photon absorptivity in 10-50 cm4 s per photon (GM), (15%.
g
(2)
Two-photon action cross section in GM at λ
.
max
a Reagents and conditions: (a) A (1-4 equiv), PdCl2(PPh3)2, CuI, PPh3,
Et3N, THF, 70 ° C, 12 h.
In this work, we have synthesized a series of pyrene
derivatives having 4-(N,N-dimethylamino)phenylethynyl groups
(1-5) as the substituent. The usefulness of pyrene as a π-center
and the effects of increasing the number of substituents and
molecular symmetry on the one- and two-photon spectroscopic
properties have been studied.
Pyrene derivatives (1-5) have been synthesized by the
Sonogashira coupling reaction22 between bromopyrenes (6-10)
and 4-ethynyl-N,N-dimethylaniline (A) as shown in Scheme 1.
1-Bromo- (6), a mixture of 1,6- and 1,8-dibromo- (7 and 8),
1,3,6-tribromo- (9), and 1,3,6,8-tetrabromopyrenes (10) were
prepared by the bromination23 of pyrene with 1 to 4 equiv of
bromine. Mono-, tris-, and tetrakis[4-(N,N-dimethylamino)phe-
nylethynyl]pyrene (1, 4, and 5) have been synthesized by the
ethynylation reaction. To obtain 2 and 3, a mixture of 7 and 8
was reacted with A and the product mixture was separated by
crystallization followed by column chromatography.
FIGURE 1. Normalized one-photon absorption (9), emission (O,
excitation at λmax), and two-photon excitation spectra (2) for 1 in
toluene. The two-photon spectrum is plotted against λ/2 (twice the
photon energy).
One-photon absorption spectra of 1-5 in toluene are broad
and complicated due to the complexity of the molecular
structure. However, a gradual bathochromic shift in the λmax is
clearly observed in the order 1 < 2 ≈ 3 < 4 < 5 (Figure S1
(Supporting Information) and Table 1), indicating that the extent
of intramolecular charge transfer (ICT) increases with the
conjugation length. The molar extinction coefficient (ε) increases
sharply from 1 to 2, and remains nearly the same after further
increase in the molecular size (Table 1).
The fluorescence spectra of 1-5 exhibit a single peak (Figure
S1, Supporting Informaton), which indicates that the emission
occurs from the lowest excited state with the largest oscillator
strength. The emission spectra show systematic bathochromic
shifts with the conjugation length increase, that is, 1 < 2 ≈ 3
< 4 < 5, implying that the energy gap between ground and
excited states decreases in this order (Table 1). The fluorescence
Stokes shift decreases in the order 1 > 2 ≈ 3 > 4 > 5 (Table
1); the energy gap between the Franck-Condon and emitting
states decreases in the same order. This is presumably because
the ICT is more similar between the ground and excited states
of the extended molecules. A similar result was reported for
9,10-bis(arylethynyl)anthracene derivatives.24 All compounds
are strongly fluorescent with high fluorescence quantum yields.
The two-photon cross section was measured by the two-
photon-induced fluorescence measurement technique, using the
femtosecond laser pulses as described.4e In all cases, the output
intensity of two-photon excited fluorescence (TPEF) was linearly
dependent on the square of the input laser intensity, thereby
confirming the nonlinear absorption (Figure S3, Supporting
Information).
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For compounds 1, 3, and 4, one- and two-photon spectra
overlap reasonably well in terms of the total absorption energy
(Figures 1 and S4 (Supporting Information)). This indicates that
the two-photon allowed states of the dipoles are close to the
one-photon allowed states, as predicted by symmetry. On the
other hand, the λ(m2a)x/2 of 2 is located at a shorter wavelength
than λmax. This is again consistent with the prediction that the
two-photon allowed states for the quadrupoles should be located
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5128 J. Org. Chem. Vol. 73, No. 13, 2008

Experimental Section
General Procedure for Alkyne Coupling. Bromopyrenes were
synthesized by bromination of pyrene.23 The bromopyrenes [1- (6),
1,6-di- (7), 1,8-di- (8), 1,3,6-tri- (9), and 1,3,6,9-tetra- (10)] were
coupled with 4-ethynyl-N,N-dimethylaniline under Sonogashira
conditions to give compounds 1-5.
1-[4-(N,N-Dimethylamino)phenylethynyl]pyrene (1). 1-Bro-
mopyrene (100 mg, 0.36 mmol), [PdCl2(PPh3)2] (25 mg, 0.030
mmol), CuI (7.0 mg, 0.030 mmol), PPh3 (9.0 mg, 0.030 mmol),
and 4-ethynyl-N,N-dimethylaniline (72 mg, 0.50 mmol) were added
to a degassed solution of Et3N (20 mL) and THF (20 mL) under
N2. After the mixture was stirred at 70 °C for 12 h, the product
was poured into CH2Cl2 (200 mL) and water (200 mL). The organic
layer was separated and dried over anhydrous MgSO4, then the
solvent was removed in vacuo. Column chromatography using silica
gel with hexane:ethyl acetate (3:1) gave 78 mg (63%) of yellow
solid. Mp 169 °C. IR (KBr pellet, cm-1) 2915, 2845, 2195 (CtC),
1592, 1520, 1345 (CsN). 1H NMR (300 MHz, CDCl3) δ 8.69 (d,
J ) 9.1 Hz, 2 H), 8.22-7.99 (m, 8 H), 7.62 (d, J ) 9.1 Hz, 2 H),
6.74 (d, J ) 9.1 Hz, 2 H), 3.03 (s, 6 H). 13C NMR (100 MHz,
CDCl3) δ 150.4, 133.1, 131.7, 131.5, 131.4, 130.8, 129.5, 128.2,
127.9, 127.6, 126.3, 126.1, 125.6, 125.5, 124.8, 124.6, 119.2, 112.2,
96.9, 86.9, 40.5. FAB MS m/z (M+) calcd 345.42, found 345.23.
Anal. Calcd for C21H21N3O2: C, 90.40; H, 5.54; N, 4.05. Found:
C, 90.58; H, 5.21; N, 4.19.
FIGURE 2. Two-photon excitation spectra of 1-5 in toluene. The
values are an average of three measurements.
at a higher energy than that of the one-photon allowed states.25
An exception to this trend is the reasonable overlap between
the one- and two-photon spectra of 5. Although the origin of
this dichotomy is not clear, perhaps this might be the reason
for the larger δmax value of 5 than others (vide infra).
All compounds show modest to large two-photon cross
sections ranging from 55 to 1150 GM at 780-830 nm (Figure
2). The δmax value increased by approximately 9-fold as the
conjugation length was increased from 1 to 2. The value of δmax
) 490 GM for 2 is similar to δmax ) 540 GM reported for
9,10-bis[4-(N,N-didecylamino)phenylethynyl]anthracene.24 More-
over, the two-photon action cross section of 2 (Φδmax ) 490
GM) is significantly larger than Φδmax ) 290 GM for the latter
due to the larger fluorescence quantum yield, which is of
particular advantage in applications that use TPEF. This result
implies that pyrene is as efficient a π-center as anthracene in
two-photon materials. Interestingly, the δmax value of 3 is nearly
half that of 2. A similar result was reported for 2,6-bis(p-
dihexylaminostyryl)anthracene (δ ) 1100 GM) whose δ value
decreased significantly in the 2,7-isomer (δmax ) 720 GM).4f
Hence, the linear molecules seem to have a certain advantage
in enhancing the two-photon cross section. When the molecular
size was increased to 4, the δ value increased slightly. However,
a further increase in the molecular size from 4 to 5 enhanced
the δmax value by more than 2-fold despite a modest increase
in MW. Moreover, the Φδmax value of 5 is comparable to that
of the most efficient two-photon materials4f due to the significant
two-photon cross section and large fluorescence quantum yield.
Furthermore, the δmax values of quadrupolar molecules (2, 5)
are always larger than those of dipolar molecules (1, 3, 4) having
comparable MW. This result underlines the importance of
molecular symmetry consideration in the design of efficient two-
photon materials.
1,6-Bis[4-(N,N-dimethylamino)phenylethynyl]pyrene (2) and
1,8-Bis[4-(N,N-dimethylamino)phenylethynyl]pyrene (3). A mixture
of 7 and 8 (200 mg, 0.55 mmol), [PdCl2(PPh3)2] (19 mg, 0.028
mmol), CuI (10 mg, 0.050 mmol), PPh3 (14 mg, 0.050 mmol), and
4-ethynyl-N,N-dimethylaniline (190 mg, 1.4 mmol) was added to
a degassed solution of Et3N (20 mL) and THF (20 mL) under N2.
After stirring at 70 °C for 12 h, the reaction mixture was poured
into CH2Cl2 (200 mL) and water (200 mL). The organic layer was
separated and dried over anhydrous MgSO4 and the solvent was
removed in vacuo. A 50 mL solution of hexane:ethyl acetate (4:1)
was added and yellow solid was precipitated from the solution.
Filtration of the solid gave 53 mg (19%) of 2 in a yellow solid.
The filtrate was evaporated in vacuo and the crude product was
then subjected to column chromatography (silica gel) with hexane:
ethyl acetate (3:7) to give 40 mg (15%) of 3 in a yellow solid.
Compound 2: Mp >300 °C dec. IR (KBr pellet, cm-1) 2917,
2850, 2194 (CtC), 1596, 1523, 1350 (Cs-N) 1H NMR (300 MHz,
CDCl3) δ 8.66 (d, J ) 8.81 Hz, 2 H), 8.10-8.18 (m, 6 H), 7.58 (d,
J ) 9.05 Hz, 4 H), 6.73 (d, J ) 9.05 Hz, 4 H), 3.30 (s, 12 H). 13
C
NMR could not be determined due to its low solubility. FAB MS
m/z (M+) calcd 488.62, found 488.27. Anal. Calcd for C21H21N3O2:
C, 88.49; H, 5.78; N, 5.73. Found: C, 88.28; H, 5.71; N, 5.59.
Compound 3: Mp 198 °C. IR (KBr pellet, cm-1) 2915, 2852,
2192 (CtC), 1591, 1527, 1357 (CsN). 1H NMR (300 MHz,
CDCl3) δ 8.77 (s, 2 H), 8.18-8.08 (m, 4 H), 8.02 (s, 2 H), 7.06 (d,
J ) 9.02 Hz, 4 H), 6.74 (d, J ) 9.02 Hz, 4 H), 3.04 (s, 12 H). 13
C
NMR (100 MHz, CDCl3) δ 150.4, 133.1, 131.7, 131.0, 129.7, 127.7,
126.6, 125.1, 124.6, 119.5, 112.1, 110.4, 97.2, 87.0, 40.5. FAB
MS m/z (M+) calcd 488.62, found 488.30. Anal. Calcd for
C21H21N3O2: C, 88.49; H, 5.78; N, 5.73. Found: C, 88.61; H, 5.55;
N, 5.81.
In conclusion, we have synthesized a series of [4-(N,N-
dimethylamino)phenylethynyl]pyrene derivatives (1-5). Com-
parison of the δmax values with related compounds reveals that
pyrene is an efficient π-center similar to anthracene as two-
phone absorbable materials. The δmax value of compounds
increased with the number of substituents reaching the maximum
value of 1150 GM for 5. Moreover, the Φδmax value of 5 is
comparable to that of the most efficient two-photon materials.
To the best of our knowledge, this is the first example of two-
photon materials bearing pyrene as the π-center, and will provide
a useful guideline to the design of TPA molecules.
1,3,6-Tris[4-(N,N-dimethylamino)phenylethynyl]pyrene (4).
1,3,6-Tribromopyrene (100 mg, 0.23 mmol) and 4-ethynyl-N,N-
dimethylaniline (116 mg, 0.79 mmol) were used and the reaction
was carried out by the same method as that for 1. Column
chromatography using silica gel with hexane:ethyl acetate (1:1) gave
103 mg (71%) of orange-red solid. Mp 210 °C. IR (KBr pellet,
cm-1) 2185 (CtC), 1594, 1527, 1359 (CsN). 1H NMR (300 MHz,
CDCl3) δ 8.75 (s, 2 H), 8.64 (d, J ) 9.00 Hz, 1 H), 8.37 (s, 1 H),
8.18-8.08 (m, 3 H), 7.60 (d, J ) 9.00 Hz, 6 H), 6.74 (d, J ) 9.00
Hz, 6 H), 3.04 (s, 18 H). 13C NMR (75 MHz, CDCl3) δ 150.4,
135.1, 134.9, 133.1, 132.4, 132.3, 132.2, 131.3, 131.1, 128.8, 128.6,
124.7, 124.7, 124.5, 119.8, 119.3, 112.1, 110.4, 110.3, 97.51, 97.25,
(25) McClain, W. M. Acc. Chem. Res. 1974, 7, 129.
J. Org. Chem. Vol. 73, No. 13, 2008 5129

97.1, 87.1, 86.5, 86.48, 40.4. FAB MS m/z (M+) calcd 631.81, found
631.00. Anal. Calcd for C21H21N3O2: C, 87.45; H, 5.90; N, 6.65.
Found: C, 87.33; H, 5.98; N, 6.49.
Acknowledgment. This work was supported by the SRC
program (R11-2005-008-02001-0(2008)), Basic Science Re-
search of KOSEF (R01-2006-000-10001-0), and KOSEF
grant funded by MOST (R0A-2007-000-20027-0).
1,3,6,8-Tetrakis[4-(N,N-dimethylamino)phenylethynyl]pyrene (5).
1,3,6,9-Tetrabromopyrene (100 mg, 0.19 mmol) and 4-ethynyl-N,N-
dimethylaniline (126 mg, 0.85 mmol) were used. Column chro-
matography on silica gel with hexane:CH2Cl2 (1:4) provided 58
mg (40%) of 5 ina red solid. Mp <300 °C dec. IR (KBr pellet,
cm-1) 2190 (CtC), 1608, 1523, 1361. 1H NMR (300 MHz, CDCl3)
δ 8.72 (s, 4 H), 8.35 (s, 2 H), 7.60 (d, J ) 9.00 Hz, 8 H), 6.74 (d,
J ) 9.00 Hz, 8 H), 3.04 (s, 24 H). 13C NMR of this compound
cannot be determined due to its low solubility. FAB MS m/z (M+)
calcd 774.99, found 774.00. Anal. Calcd for C21H21N3O2: C, 86.79;
H, 5.98; N, 7.23. Found: C, 86.92; H, 6.01; N, 7.35.
Supporting Information Available: General methods,
solubility test, measurement of two-photon cross section, and
UV/vis, fluorescence, and NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.
org.
JO800363V
5130 J. Org. Chem. Vol. 73, No. 13, 2008