127.1, 126.4, 114.2, 63.5, 14.73. MS (MALDI-TOF): m/z 420.2
(M+). Anal. calc. for C22H22N2O2: C, 79.98; H, 5.75; N, 6.66.
Found: C, 79.62; H, 5.69; N, 6.91%.
Two-photon induced polymerization
Two-photon induced polymerization (TPP) experiments were
performed with the irradiation of 80 fs pulses from a mode-
locked Ti-sapphire femtosecond laser (Tsunami, Spectra-
Physics) with a repetition rate of 80 MHz at 800 nm. The
photoresist resin was prepared by mixing methyl acrylic acid
(MAA) as monomer, DEP-6A as cross-linker, and a dye as
photoinitiator. In the control experiment, commercially
available benzil was used as initiator. The lasing source was
tightly focused by a 100 ꢁ 100 oil-immersion objective lens
with a high numerical aperture (N.A. = 1.40, Olympus). The
focal point was focused on the liquid photopolymerisable resin
which was placed on a cover glass above the xyz-step motor-
ized stage controlled by a computer. After laser fabrication,
the unpolymerized resin was washed out with ethanol. The
images of fabricated lines were observed using a field-emission
scanning electron microscope (FE-SEM, JOEL, JSM-6330F).
2,3-Bis[4-(di-p-tolylamino)phenyl]quinoxaline (TPAQ). This
compound was prepared from 2b and o-phenylenediamine
according to the same procedure as that for EOQ to give
the product as a yellow solid in 91% yield; mp 239–241 1C. 1H
NMR (CDCl3, 400 MHz) d 8.20–8.10 (m, overlapped, 2H),
7.72 (dd, 2H, J1 = 6.42 Hz, J2 = 3.36 Hz), 7.34 (d, 4H, J =
8.56 Hz), 7.03 (d, 8H, J = 8.28 Hz), 6.97 (d, 8H, J = 8.28 Hz),
6.90 (d, 4H, J = 9.04 Hz), 2.33 (s, 12H). 13C NMR (CDCl3,
100 MHz) d 153.2, 149.0, 144.8, 140.7, 133.2, 131.2, 130.7,
130.0, 129.6, 128.8, 125.3, 120.8, 20.9. MS (MALDI-TOF):
m/z 672.3 (M+). Anal. calc. for C48H40N4: C, 85.68; H, 5.99;
N, 8.33. Found: C, 85.70; H, 6.09; N, 8.31%.
2,3-Bis[4-(di-p-tolylamino)phenyl]benzoquinoxaline (TPABQ).
This compound was prepared from 2b and 2,3-naphthalene-
diamine according to the same procedure as that for EOQ to
give the product as a yellow solid in 87% yield; mp 273–275 1C.
1H NMR (CDCl3, 400 MHz) d 8.66 (s, 2H), 8.08 (dd, 2H,
J1 = 6.45 Hz, J2 = 3.23 Hz), 7.53 (dd, 2H, J1 = 6.45 Hz, J2 =
3.23 Hz), 7.46 (d, 4H, J = 8.72 Hz), 7.11 (d, 8H, J = 8.39 Hz),
7.06 (d, 8H, J = 8.39 Hz), 6.99 (d, 4H, J = 8.72 Hz), 2.34
(s, 12H). 13C NMR (CDCl3, 100 MHz) d 154.1, 149.0, 144.7,
138.0, 133.8, 131.6, 130.7, 129.9, 128.4, 127.0, 126.3, 125.3,
120.7, 20.8. MS (MALDI-TOF): m/z 722.5 (M+). Anal. calc.
for C22H22N2O2: C, 86.39; H, 5.86; N, 7.75. Found: C, 85.99;
H, 5.75; N, 7.60%.
Results and discussion
Photophysical properties
Photophysical properties of the target compounds
(EOQ, EOBQ, TPAQ and TPABQ) were studied in CHCl3
solution and the data are summarized in Table 1.
Linear spectroscopy
From Table 1 and Fig. 2, we can see that all the target
compounds show an intense absorption band with absorption
maxima being located in the range of 350–470 nm. As
expected, a red shift in the absorption maximum occurred
from EOQ to EOBQ or from TPAQ to TPABQ due to the
conjugation expansion in the benzoquinoxaline system as
compared to the quinoxaline one. On the other hand, the
red-shift from EOQ to TPAQ or from EOBQ to TPABQ
indicated that triphenylamine moiety provides the molecule
with a larger degree of conjugation than does the alkoxyaryl
group. Thus, the maximum emission wavelength in CHCl3
from EOQ to TPABQ could be red-shifted by 134 nm. The
absorption maxima of EOBQ and TPAQ appeared at 405 and
419 nm, respectively, indicating that the half of wavelength of
their two-photon absorption maxima is around 400 nm. In
contrast, the absorption maximum of benzil is located at
290 nm. Therefore, the absorption maximum of benzil can
be adjusted to about 400 nm by modification to enlarge its
conjugated system.
Measurements of fluorescence quantum yields
The quantum yields of the materials were measured by using
Coumarin 307 as the standard according to a known procedure.23
Absorbance was limited to equal or less than 0.1. The
quantum yield was calculated by the following equation:
ISARn2S
IRASn2R
QYS ¼ QYR
where QYS is the quantum yield, AS the absorbency and IS
fluorescence area of the tested compound, respectively; QYR
(0.58) is the quantum yield of Coumarin 307 in acetonitrile
solution,24 AR the absorbency and IR fluorescence area of
coumarin 307, respectively; and nS and nR are the refractive
index of toluene and acetonitrile, respectively.
Measurements of two-photon absorption cross sections
The fluorescence quantum yields of the target compounds
were determined (Table 1). The quinoxaline-type compounds
exhibited significantly higher fluorescence quantum yields than
the benzoquinoxaline-type ones, indicating that the benzo-
quinoxaline is a relatively weak p-acceptor. Moreover, the
triphenylamine-modified species showed higher fluorescence
quantum yields than the corresponding ethoxyphenyl-
modified ones. This may be attributed to the stronger electron-
donating ability of the triphenylamine moiety than that of the
alkoxyaryl group. It is noteworthy that the conjugation length
of the molecules does not produce a significant effect on the
radiative lifetimes of these chromophores for the one-photon
excited fluorescence lifetime of the excited state.
2PA spectra were obtained by the two-photon-induced excited
fluorescence (TPEF) method reported by Xu et al.25 TPEF
spectra were recorded on SD2000 spectrometer (Ocean Optical)
with the excitation by a mode-locked Ti-sapphire femtosecond
laser (Tsunami, Spectra-Physics), where oscillating wave-
length, pulse width and repetition rate were 780 nm, 80 fs
and 82 MHz, respectively. It is assumed that fluorescence
quantum yields after two-photon excitation are the same as
those after one-photon excitation. 2PA cross sections
are obtained by calibration against fluorescein in aqueous
0.1 M NaOH solution (pH = 11) for the femtosecond
measurements.
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
1580 | New J. Chem., 2009, 33, 1578–1582