824
Chemistry Letters 2001
Synthesis and Characterization of a Novel Charge Transfer Compound
with Large Three-Photon Absorption Cross Section
Junxiang Zhang,†,†† Yiping Cui,*† Mingliang Wang,†† Chunxiang Xu,† Yuan Zhong,† and Juzheng Liu††
†Department of Electronic Engineering, Southeast University, Nanjing 210096, P. R. China
††Department of Chemistry, Southeast University, Nanjing 210096, P. R. China
(Received May 14, 2001; CL-010444)
A new intramolecular charge transfer compound containing
diethylamino group as electronic donor and oxadiazole group as
electronic acceptor has been synthesized using Wittig–Horner
reaction. This long conjugated molecule has large three-photon
absorption cross section excited at 1.06 µm lasing.
There has recently been a considerable amount of interest in
frequency up-conversion process because of their potential
applications to frequency-upconverted lasing,1 optical power
limiting,2 frequency-upconverted fluorescence microscopy,3
photorefractive effect in organic materials,4 three dimensional
optical data storage,5 and two-photon photodynamic therapy.6 A
major impetus to this application has been provided by reports
of molecules with large nonlinear absorption cross sections.7
The delocalization of π-electrons occurring in the conjugat-
ed molecules can lead to significant enhancement of the high-
order optical nonlinearity. Recently theoretical investigations8
reveal that the major contribution to the static value of the third-
order polarizabilities originates from the transition dipole term
that involves the difference between the dipole moments in the
ground state and in the lowest charge transfer excited state.
The effective charge transfer can increase the transition dipole
term. Those reported molecules, which are specially designed
for multi-photon process, could be mainly divided into two
types. One is the symmetric types9 whose charges transfer
from the end to the middle in the molecule chains or versa; the
other is push–pull asymmetric types10 whose charges transfer
from one end to another. In our present work, we designed a
donor–acceptor–donor system, in which charge transfer may be
more effective from the end to the middle than that of popular
donor–conjugated bridge–donor system.
In this letter we report the synthetic route of that new
intramolecular charge transfer compound named 2,5-bis[4-(4-
N,N-diethylaminostyryl)phenyl]oxadiazole (BASDO). We also
use intensity dependent transmission measurement to obtain the
three-photon absorption cross section of BASDO at 1.06 µm
pumped with nanosecond laser.
The molecular structure of BASDO is presented in Figure 1.
BASDO was synthesized by Wittig–Horner reaction (Figure 1).
The detailed synthetic steps are depicted as follows:
Polyphospharic acid (50 mL) was added to 50% hydrazine
hydrate (5 g, 0.05 mol) at 50 °C under N2. The mixture was heat-
ed to 80 °C. p-Toluic acid (13.6 g, 0.1 mol) was added. The
resulting mixture was heated to 130 °C and stirred for 10 h. After
cooling, the mixture was poured into water. The solid was fil-
tered and washed with 5% Na2CO3 aqueous solution (2 × 200
mL) and water (2 × 200 mL) to give 12.6 g of product 1 (mp
243–245 °C). 1H NMR (DMSO-d6, ppm) δ 2.35 (s, 6H, –CH3),
7.32 (d, 4H, ArH), 7.81 (d, 4H, ArH), 10.35 (s, 2H, –NH).
A mixture of compound 1 (12.5 g, 46.64 mmol) and POCl3
(250 mL) was refluxed under N2 for 7 h. The excess POCl3 was
distilled out and the residue was poured into water. The filtered
solid was purified by recrystallization from CHCl3/methanol to
give compound 2 (mp 172–173 °C). 1H NMR (CDCl3, ppm) δ
2.43 (s, 6H, –CH3), 7.32 (d, 4H, ArH), 8.05 (d, 4H, ArH).
A mixture of compound 2 (3 g, 12 mmol), N-bromosuccin-
imide (NBS) (4.7 g, 26 mmol), benzoxy peroxide (0.05 g), and
CCl4 (60 mL) was refluxed for 6 h. The mixture was filtered
while it was still hot. The solid was washed with hot chloro-
form. The residual solid was recrystallized from THF/methanol
1
to give product 3 (mp 226–227 °C). H NMR (DMSO-d6, ppm)
δ 4.80 (s, 4H, –CH2Br), 7.72 (d, 4H,ArH), 8.15 (d, 4H, ArH).
A mixture of compound 3 (2.0 g, 4.90 mmol) and P(OEt)3
(5 mL) was stirred at 150 °C for 5 h. The excess P(OEt)3 was
distilled out. The residual solid was recrystallized from
1
THF/hexane to give product 4 (mp 113–115 °C). H NMR
(DMSO-d6, ppm) δ 1.18 (t, 12H, –CH3), 3.45 (d, 4H, –CH2P),
3.95 (m, 8H, –OCH2–), 7.50 (d, 4H, ArH), 8.10 (d, 4H, ArH).
NaH (0.136 g, 5.65 mmol) was added into a solution of 4-
diethylaminobenzaldehyde (0.678 g, 3.83 mmol) in 10 mL of
diethene glycol dimethyl ether (DME) under N2. The resulting
suspension was stirred for 5 min and was then added dropwise
to a solution of compound 4 (1.000 g, 1.91 mmol) in DME at
room temperature. The resulting solution was stirred at 80 °C
overnight and then poured into methanol. The filtered solid
was purified by silica gel column to give BASDO (mp 156–159
°C) 1H NMR (CDCl3, ppm) δ 1.23 (t, 12H, –CH3), 3.44 (d,
8H,–NCH2–), 6.90 (m, 4H, –CH=CH–), 7.49 (d, 4H, ArH), 7.72
(d, 4H, ArH), 8.07–8.10 (d, 8H, ArH).
The linear absorption single peak of BASDO in CHCl3 at
1.0 × 10–5 M/L was located around 347 nm (Figure 2), where is
the three-photon energy region of the 1.06 µm incident lasing.
So the three-photon absorption in this long conjugated com-
Copyright © 2001 The Chemical Society of Japan