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Synthesis and third-order nonlinear optical properties of linear p-acenequinones
SVflTHETII[ [iflETRLS ELSEVIER
Synthetic Metals 92 (1998) 235-243
Synthesis and third-order nonlinear optical properties of linear p-acenequinones T.C. Wen *, C.J. Tiao, L.C. Hwang, C.Y. Tsai School of Chemistry, KaohsiungMedical College, PO Box 72-94, Kaohsiung 80708, Taiwan Received 26 August 1997; accepted 29 October 1997
Abstract
The synthesis of 1,4-tetracenequinone (Q [ 3,0] ) has been described, and the nonlinear optical properties of this quinone together with pbenzoquinone (Q[0,0]), 9,10-anthraquinone (Q[ 1,1 ] ) and 6,13-pentacenequinone (Q[2,2]) have been investigated with 532 nm and 8 ns pulses. For the quinones of Q [ 0,0], Q [ 1,1 ] and Q [ 2,2], we applied the nonlinear transmittance and the z-scan techniques to determine their two-photon absorption (TPA) coefficient/3 and the third-order nonlinear refractive index r~ff. The results show that their/3 values increased with increasing aromatic tings of quinone, and the magnitudes of n~ff and X~ff (3) are all approximately the order of 10-,o e.s.u, with negative sign. The feasible vertical transition states among their low-lying electronic energy levels have been predicted. For Q [ 3,0], its excited-state absorption cross section o-~Vxand the excited-state refractive-index cross section o'r have been measured with z-scan methods. The magnitudes of o'r and n~f~are comparable with those of well-known highly conjugated planar molecules of tetrabenzoporphyrin and metallophthalocyanines. © 1998 Elsevier Science S.A. Keywords: Linear acenequinones; Synthesis; Two-photon absorption; Nonlinear refraction; z-Scan
I. Introduction
9,10-Anthraquinone (Q[ 1,1] ) and its derivatives play an important role in the area of industrial Synthetic dyes and pigments. The newly synthesized [ 6 ] -1,4-cyclophaneanthraquinone and 1,2,3-tri-t-butyl-6,13-pentacenequinone have demonstrated photochromic behavior between their valence type isomers [ 1,2]. They have received considerable attention because of their potential for a wide range of molecular devices. The complexes of 1,5-diaminoanthraquinone with its oxidized form exhibit a semiconductive property as reported in Ref. [ 3 ]. There are only nine unsubstituted linear p-acenequinones which have been studied spectroscopically up to now [ 4]. In the present paper we have investigated four of them, as illustrated in Fig. 1. According to the symbol Q[m,n] introduced by Nepras and Titz [5], Q[m,n] with m = n stands for the D2h quinones, and with m v~n stands for the C2~, quinones. The electronic ground states are characterized by Ag and A~ symmetry, respectively, for the D2h and C2v quinones. For the D2h quinones, their ground state MOs of oxygen are n + (b2,) and n_ (b3g), and their -rr orbitals have big , b2g, b3u, or a. * Corresponding author. Tel.: +886 07 312 1101 2192; fax: +886 07 312 5339. 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved PIIS0379-6779(97)04094-0
symmetry. Hence, the excited ( n , v * ) states are BIg, B2g, B3u, or A,, and the excited (-rr,-rr*) states are B,,, B2,, B3g, or Ag. For the C2L quinones, their n+ and n MOs are of al and b2 symmetry, respectively, while the 7r orbitals are either a 2 or b.. Therefore, the excited ( n,~r* ) orbitals are either A2 or B ,, while the excited (~r,~r*) states are either A, or B2. The absorption spectra of four linear quinones have been obtained in CH2C12 as sketched in Fig. 1, and their correlated lowest lying electronic excited states are shown in Fig. 2. These excited-state energies have been calculated with semiempirical methods and have confirmed very well the experimental values [4]. For p-benzoquinone ( Q [ 0 , 0 ] ) , the absorption bands at about 240 and 300 nm are both due to transitions to the l(~r,,rr.) states. The former band has been assigned as due to the lAg-~ iB,, transition, while the latter band has been attributed to the JAg--~ JB3g transition which borrows the intensity through vibronic coupling with the former transition [4]. The very weak absorption band at about 500 nm corresponds to the excitation into the ~(n,~r*) state with the IBlg symmetry [4]. For 1,9-anthraquinone (Q [ 1,1 ] ), the bands starting from 260 nm and those starting from 320 nm are due to transitions to the '(-rr,Tr*) states, while the weak absorption band at about 420 nm corresponds to the transition to the lowest '(n,'tr*) state with the 1B,g
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72.C. Wen et al. /Synthetic Metals 92 (1998) 235-243
2.5 2.0
UJ tJ Z < 1.5 m n, O 1.0 (/)
2,
i~"
[~ o
Q[O,O]
CO
,<
1.s
O 1.0 ¢O "
i/x1 o ~, 1~.
0.5
x18
ILl 2.0 O Z
0.0 200 300 400 500 600 700 800
200 300 400 500 600 700 800
WAVELENGTH (rim)
WAVELENGTH (nm) 2.5
2.5 2.0
IB2u
LLI 0 Z 1.5 m n,, O or) 1.0 m ,< 0.5
LU O Z
O
xl
m
Q[1,1]
2.0
1.5
0.5
IB (~,~*)
1 2u
1A. I
Q[3,0]
o
1(~,~,)
O 1.0 U) m <
1Blu
X 10
/~
0.0
0.0 2O0 300 400 500 600 700 800
WAVELENGTH (nm)
200
300 400 500 600 700 800
WAVELENGTH (nm)
Fig. 1. Absorption spectra of quinones Q [ 0,0], Q [ t, 1], Q [ 2,2 ] and Q [ 3,0] in dichloromethane at room temperature.
1B1=
1B2,,
'T
/
~"~-~.
1~2 u
hv
~'~--~. l(tr, if*)
2
2-_~--
------JBlg
- hv
.~g_
-
Q(o,o)
I I T I I I I
hu
i
'A,I
I
Q(1,1)
r I
Q(2,2)
_
Q (3,o)
Fig. 2. The low-lying singlet excited-state energy levels of quinones. For Q[0,0], the quantum chemical calculations with the experimental values shown in parentheses are: ~B~g=2.46 (2.486); 'A.=2.60 (2.38); tB3g=4.13 (4.05); and LBI,=5.10 eV (5.0 eV) [4]. For Q[1,1]: 1]Big=2.768 (2.939); ]A,=2.909; lIB2u=3.808 (3.72); 1~B1,=4.482 (4.56); and 2]B2,=4.916 eV (4.92 eV) [41. The conversion factor of 1 eV = 8065 c m - ~is used here.
symmetry. The reported absorption spectral data for linearpacenequinones other than Q[0,0] and Q[1,1] are significantly limited [4]. However, as recorded in Fig. 1, their first singlet 7r ~ ~r* absorption bands were red shifted with maximum peaks from about 420 nm to about 500 nm for the quinones of Q[2,2] and Q[3,0], respectively. This absorption band can also be moved to the longer wavelength by the electron-withdrawing substituting groups such as - O H and -NH2, and the effect of the intramolecular hydrogen bond has attributed to this red shift [6]. Furthermore, it is also suggested that, when one of the phenyl rings of anthraquinone is bent in chair form, the J(rr,Tr*) state is very stabilized and comes below the ( n m * ) state as the bent angle becomes large [ 1 ]. In spite of a large number of papers concerning the spectroscopy and corresponding photophysical processes of linear aceneqinones in the literature, there has been almost no information with regard to their nonlinear optical properties to our knowledge. In this paper, we will report the synthesis of 1,4-tetracenquinone ( Q [ 3,0 ] ). The nonlinear optical responses of these four quinones are studied with a Q-switched 532 nm and near-Gaussian laser beams. The generated nonlinear processes such as the two-photon absorption (TPA), nonlinear refraction and excited-state absorptions (ESAs) are determined by measuring their transmittance as a function of inci-
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T.C. Wen et al. / Synthetic Metals 92 (1998) 235-243
CI CI~ , , , C I
c! Cl~Ci
C!~~'~ CI~ H CI ~ ~ 1
~
~CI
O
hemiacetal
l CH3 CH3 CH3 B12/CC14 I, c C l ! / ~ ~ Pyrolysis ~ B r Fe,12 CI~ . ~ C I ~" "Br Ib INBS el Ia |benzoyi peroxide
1 n-butyllithium~ / ~ CH2OH CaCO3 i D1VIF ~fl~-~Br diOxane/H20~ v OH Id form O
1Ltautomer
aldehyde
CH2Br v "Br Ic
O
O
form
If 1,4-TetracenequinoneQ[3,0]
Ie
Scheme1. dent optical intensity, or as a function of sample position along the light propagation direction, which is known as the z-scan method. The results are explained with their correlated electronic energy states.
-~ [ ~ C oven
The synthetic procedures of 1,4-naphthacenedione Q [ 3,0] appear in Scheme 1; the details follow. During the preparation of intermediates Ia-Ie, most methods are modifications from Ref. [ 7 ]. The yield of the intermediate lb was improved here with an efficient pyrolysis method (Scheme 2). The pbenzoquinone Q [ 0,0], 9,10-anthraquinone Q [ 1,1 ] and 6,13pentacenequinone Q [ 2,2] were purchased from Aldrich. The p-benzoquinone was purified by vacuum sublimation, and the other two quinones were recrystallized from chloroform before use.
2.1.1. 3-Bromo-2-methylnaphthalene-bis( hexachlorocyclopentadiene ) (Ia ) A mixture of 2-metbylnaphthalene-bis(hexachlorocyclopentadiene) ( 172 g, 0.25 mol), I2 (0.85 g), Fe powder (0.45 g) and dry CC14 (300 ml) was refluxed with stirring, and a solution of 40.5 g Br in 30 ml CC14 was added over 2 h. The mixture was refluxed 4 h around 77-80 °C, cooled, and the product dissolved with CH2C12, the precipitate filtered off and pumped to dry with rotavap. The solid product was recrystallized with CC14 and dried in air to give a white product (143 g, 75%): m.p. 212-213 °C. IR (KBr): 2937,
|,.
[ ~[.~l-N0)
Scheme2.
2. Experimental 2.1. Sample preparation
_
VaCHi.HD.
1616, 1487, 1257, 1083, 894, 769, 699, 584 cm i. ~H NMR (CDC13) 6:2.41 (s, 3H, CH3), 3.48 and 3.53 (dd, 2H, CH), 3.87 and 3.92 (dd, 2H, CH), 7.58 (s, 1H, ArH), 7.90 (s, 1H, ArH). 13C NMR (CDC13) 6: 22.61, 41.48, 41.53, 46.46, 46.71, 82.20, 84.59, 101.23, 124.40, 127.48, 127.60, 128.88, 128.93, 131.24, 132.79, 133.20, 133.29, 137.52.
2.1.2. 2-Bromo-3-methylnaphthalene (Ib ) This reversed Diels-Alder pyrolysis reaction was performed in a cylindrical flask under high vacuum as illustrated in Scheme 2. Ia (50 g) was heated first in an oven at 215 °C. After the sample was transferred completely from solid powder to liquid, the oven temperature was then raised to 280 °C. The pale yellow product was distilled into a flask which was trapped with liquid nitrogen. The solid product was chromatographed on silica gel (0.04-0.06 ram) and eluted with n-hexane. The final product was obtained by recrystallization from n-hexane (5.5 g, 38%); m.p. 132-136 °C. IR (KBr): 2987, 1601, 1511, 1387, 993, 884, 769, 495 cm-~. JH NMR (CDC13) 6:2.56 (s, 3H, CH3), 7.41-7.47 (m, 2H), 7.697.75 (m, 3H), 8.06 (s, 1H). 13C NMR (CDC13) 6: 23.25, 123.60, 125.94, 126.23, 126.57, 127.11, 128.70, 130.73,
133.01, 135.28.
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T.C. Wen et al. / Synthetic Metals 92 (1998)235-243
2.1.3. 2-Bromo-3-(bromomethyl)naphthalene (Ic ) Half of a mixture of NBS ( 16.2 g, 0.091 mol ) and benzoyl peroxide (0.4 g, 1.65 mmol) was first added into a refluxing solution of Ib (24 g, 0.109 tool) in CCI 4 (300 ml). After 2 h of refluxing at 76 °C, the remaining mixture solution was then added in portions over 30 rain, and the solution was refluxed for another 2 h. The product was cooled and washed with saturated NaHCO3 solution. The organic phase was dried with MgSO4, filtered, and then the solvent was removed with rotavap. The white solid product was chromatographed on silica gel (0.04-0.06 ram) and eluted with n-hexane. The solid product was recrystallized from hexane, and the white plate crystals were obtained ( 12.84 g, 39.43%); m.p. 113114 °C. IR (KBr): 2925, 2345, 1211, 1129, 988, 878, 752, 689, 668 cm i. tH NMR (CDC13) 8:4.78 (s, 2H, CH2), 7.9-7.56 (m, 2H), 7.73-7.83 (m, 2H), 7.95 (s, 1H), 8.11 (s, IH). ~C NMR (CDC13) 8:34.00 (CH2Br), 121.41, 126.71, 126.85, 127.56, 127.87, 130.50, 132.11, 132.28, 134.16, 134.21. 2.1.4. 3-Bromo-2-naphthalenemethanol (Id) A mixture of Ie (9.81 g, 32.7 mmol), CaCO3 (16.08 g, 0.161 tool) and dioxane/H20 ( 1:1,200 ml) was refluxed for 10 h. The product was cooled, mixed with methanol (20 ml), and extracted with warm CHCI3 ( 100 ml). The organic phase was dried over MgSO4. The solid product was recrystallized from toluene/hexane (1:1) to obtain a white needle solid powder (6.84 g, 88.2%); m.p. 113-115 °C. IR (KBr): 3231, 1589, 1491, 1201, 1126, 1057, 977, 876, 744, 471 cm ~. JH NMR (CDCI3) 8:2.11 (s, 1H, OH), 4.90 (s, 2H, CH20), 7.48-7.54 (m, 2H), 7.73-7.86 (m, 2H), 7.93 (s, 1H), 8.07 (s, 1H). 13C NMR (CDC13) 8: 65.19, 120.18, 126.54, 126.64, 126.77, 127.50, 127.87, 131.21, 132.27, 133.72, 136.83. 2.1.5. 3-( Hydro xvmethyl )- 2-naphthaldehyde (le ) A stirred mixture o f l d ( 3 g, 12.65 mmol) and dried diethyl ether ( 100 ml) was cooled to - 78 °C, and n-butyllithium in hexane ( 16.7 ml, 2.6N) was added by syringe. After 10 min, the solution was warmed to 0 °C for another 30 min, then dry DMF ( 1.5 ml) was injected and the mixture was stirred for 10 h. H20 ( 15 ml) was added to the product, and the precipitate was stirred in a mixture of NH4C1 and ether. The ether layer was separated and dried over MgSO4. The solid product was recrystallized from toluene/hexane ( 1:1 ) and a white solid product was obtained (2.18 g, 92%) ; m.p. 110-111 °C. IR (KBr): 3351, 3054, 2869, 1685, 1101, 1005, 885, 743, 482 cm ~. JH NMR (CDC13) 8: (hemi-acetal form) 3.04 (d, IH, OH), 5.19 and 5.42 (dd, 2H, CH2), 663 (d, 1H, CHOH), 7.49-8.02 (m, 6H, ArH); (aldehyde form) 3.81 (t, 1H, OH), 4.97 (d, 2H, CH2), 7.49-8.02 (s, 1H, ArH), 10.19 (s, 1H, CHO). 13C NMR (CDC13) 8: 64.42, 71.29, 101.11, 119.63, 122,18, 125.96, 126.62, 127.93, 128.00, 128.56, 129.13, 129.26, 130.02, 132.03, 132.98, 133.30, 134.18, 135.84, 137.05, 137.35, 140.30, 194.95. MS ( E l ) m / z = 185 (M-l).
2.1.6. 1,4-Tetracenequinone (If) Q[3,0] A mixture of Ie (0.125 g, 0.7 mmol), p-benzoquinone (0.09 g, 0.84 mmol) and glacial acetic acid (3 ml) was refluxed for 8 h. The solution was cooled and the red-brown precipitate was washed with ethanol. The crude product was recrystallized from toluene to give reddish-brown plate crystals (0.02 g, 12%); m.p. 262 °C dec. IR (KBr): 1664, 1606, 1418, 1307, 1244, 1136, 1054, 929, 872, 749 cm J. ~H NMR (CDC13) 6:7.11 (s, 2H, C=CH), 7.46-7.59 (q, 2H), 8.068.11 ( q, 2H), 8.67 ( s, 2H), 8.84 ( s, 2H). 13C NMR ( CDCI3 ) 6: 127.88, 128.69, 130.30, 130.70, 131.35, 133.71, 140.66, 184.44. 2.2. Spectra and nonlinear optical measurements The spectra of quinones were recorded on a UV-Vis Shimadzu UV-160A spectrophotometer, IR on a Perkin-Elmer FT-IR 2000 spectrophotometer and ~H NMR on a Varian Gemini-200 (200 MHz) VT proton/carbon FT-NMR spectrometer. The mass spectra were determined on a GC-mass HP-5989A system at 70 eV. Melting points determined on a capillary tube apparatus (Electrothermal IA9100) were uncorrected. The nonlinear transmittance was studied with 8 ns and 532 nm laser pulses from a frequency-doubled Qswitched Nd:YAG laser. The intensity varied without changing the pulse polarization by rotating the first of two Glan-laser polarizers, while the input pulses were Ibcused with a 20-cm focal-length lens. The incident and transmitted energies were detected simultaneously by two probes Rjp735 and -765, then averaged with the RJ-7620 energy meter individually. Each data point was an average of over 50 shots with about 1 Hz repetition rate. A 10-mm quartz cell was used for the nonlinear transmittance measurements with various input laser powers up to 2 GW cm-2. The laser beam passed through the sample with its focus spot near the middle of the sample cell, and was totally refocused onto the energy probe. A l-ram quartz cell was used during the z-scans. The energy of input laser pulses was kept around 12 txJ, and an energy probe was placed at a distance of 90 cm away from the focal point. The z-scan experimental set-up has been described elsewhere [ 8]. 2.3. Determination of TPA coefficient fl The TPA coefficient/3 has been determined in two ways. First, we measured the transmitted intensity/passing through a sample with thickness L by various input intensities Io. The value of/3 can be estimated with the equation [ 9,10]: /
loLfl) L/3
ln(1 +
(1)
In the second approach, we used the open-aperture z-scan technique to measure/3. The sample was scanned across the focal region of the lens along the z direction of the beam propagation, while the change of the normalized transmit-
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T.C. Wen et al. / Synthetic Metals" 92 (1998) 235-243
tance T(z) was recorded and analyzed with the equations [11]:
+
ln[l
qo ] 1 +x2J
T= =
v" _[ -_q
qo 1 +x 2
")1
_ 9 _ 2
T(z)
m.~_o(m + 1 )3/2 ,
q(z)
1 + (Z/zo) 2
(2)
where q( z,r,t ) = li,( z,r,t ) /3L~n.,LCff= ( 1 - exp( - oiL) )/ol, lo is the input intensity at the focus (z = 0), z is the distance of the sample from the focus, and zo = ('rr Wo2 ) /A. The numerical fittings are performed by choosing the most favorable parameter of q(z) first. Then the conversion from q(z) to /3 followed.
The nonlinear refractive index n 2 was measured with closed aperture z-scan. At this time, we should consider the radial phase shift A q~(r) of the transverse profile emerging from the exit face of the medium. Combining the equations dA ~ / dz = ('rrl/ A )n 2 and dl/ dz = - c d - /312, followed by integration, we can obtain the phase shift just after the sample plane as
(5)
where x=z/zo, v is the frequency of input laser pulses (5.64× l014 S I), and Fo=2E/Tro)o 2, where E is in joules for the input laser intensity, and c~= [In( 1/TI~,) ]/L. Thus, T can be evaluated with the value of o'¢x
qow~7
o 'xc , _- (6.24 X 10 -23 j c m ) E ( 1 - Tli,,) h
T=I
(6)
qo
( 9 + x 2 ) ( 1 + x 2) ×((3
+x2)
{
20-r]'/
(3)
where y is the nonlinear index, which is related through a conversion formula y ( m 2 W - i ) =n2 (e.s.u.) (40v/cno), where c is the speed of light. After some tedious algebraic processes, we derive the normalized transition equation as [121
(7)
where B = (dn/dt) ( 2khog/3 pCpO'Tx ), and dn/ dt is the change in index of the solution with temperature, which can be written as
dT
A @o(z,t) [ 4 x - 2 y x 2 - - 6y] T~- 1 q(X2~- 1) (X2 + 9 )
0-eTxOZFO( r = O)Len2hv
The equation used to determine the value of 0-r is [ 13 ]
2.4. Determination of nonlinear refractive index n f
-/k A~(z,r,t) =--~- ln[ 1 +q(z,r,t)]
qo =
~Op]\OT}
6np
(8)
where y is the expansivity.
3. Results and discussion
3.1. Two-photon absorption (4)
where y =/3/2ky. The numerical fittings are carried out first by choosing the most favorable value of y, while the values o f y and A qbo(Z,t) in Eq. (4) are converted by y = / 3 / 2 k y a n d Eq. (3), respectively. Then the value of n~ff is calculated with "}' ( m 2 W - 1) = n~ff ( e . s . u . ) (40"rr/cno).
2.5. Determination of the absorptive and refractive cross sections o'~, and 0-,. T The nonlinear excited-state absorptive cross section 0-cx and the excited-state refractive cross section 0-r were determined by the open-aperture and closed-aperture z-scan methods, respectively. The 'four-level' model (So--* S ~ T~--* T,,) is suggested here to correlate both the absorptive and refractive nonlinear optical processes. It is also suggested that there is no saturation effect during the pumping with nanosecond laser pulses. Thus, the theoretical equations used to fit the normalized transmittance from our open-aperture zscans are [ 13 ]
First of all, we have observed the nonlinear transmission with 8 ns and 532 nm laser pulses for all the quinones in CH2C12. The transmitted intensities were measured each time by varying the input laser energy. The results all fitted very well with the model Eq. (1), except for the 1,4-tetracenequinone ( Q [ 3 , 0 ] ) . Some of these results are shown in Fig. 3, where the continuous curves are given by Eq. ( 1 ) using the best-fitting parameter/3 = 0.24 and 0.73 cm G W - 1 for the pbenzoquinone (Q [ 0,0] ) and 9,10-anthraquinone (Q [ 1,1 ] ), respectively. In addition, the TPA cross section 0-2 can be determined by/3 = 0-2Nad × 10- 3, where 0-2 is in units of cm e GW ~, Na is the Avogadro constant, and d is the concentration in units of mol 1- t. The values of 13 and 0-2 are listed in Table 1. For the Q[3,0] which belongs to the Ce~ symmetry group, its lowest excited ~(~,-rr*) state is very close to our 532 nm input laser beam, so that the one-photon absorption process should be preferred during the beginning of excitation, and we have observed its excited-state absorption and refraction properties in Section 3.3. On the other hand, the TPAs attributed from those linear quinones have also been investigated by the z-scan method
240
T.C. Wen et a l . / Synthetic Metals 92 (1998) 235-243
~
mechanism causing the observed nonlinear absorption behavior, and that the/3 values increased with increasing aromatic subsystems.
(a) 5
3.2. Nonlinear refractions
~4
I-..
o
+
i
+
d
i
2
4
6
8
10
12
Input Intensity (GWIcm2) 3.0
(b) 2.5 ,,~ 2.0 g 1.5 "~ 1.0 ~'l 0.5 P
o.o
2
4
6
8
10
Input Intensity (GW/cm2) Fig. 3. Transmitted intensity of quinones Q[0,0] and Q[ 1,1] in CH2C12 vs. input laser beam intensity. The concentrations are 1.2 × 10- 3 mol for Q [ 0,01 and 1.69x 10 -3 mol for Q[ 1,11.
without aperture. The results for Q[0,0] and Q[ 1,1 ] have been plotted in Fig. 4(a) and (c), respectively. The continuous curves are the best numerical fittings with Eqs. (2). The correlating values of I m x ~3) are converted by Im X ~3) = (noc2/3) / 8172to, as listed in Table 1. Comparing the /3 values obtained from these two different methods, we found that the deviations between each of them are no more than _+30%, which is within the errors of our experiments. The results demonstrated that the TPA could be the dominant
The nonlinear refractive indices r~ff have been determined with closed-aperture z-scans for the above quinones exhibiting TPA properties. The measured normalized transmittances versus Z/Zo are fitted numerically with Eq. (4) as sketched in Fig. 4(b) and (d) for the quinones Q[0,0] and Q[ 1,1 ], respectively. The values of n~ff and Re A{eff • (3) are listed in Table 2, while the real part of the third-order nonlinear sus~/(3) -_ n~ff ( 2no~ 3~r ), ceptibility Re ,-~eff • (3> is converted from Re aeff where no is the linear refractive index of our sample solution. As listed in Table 2, all the values of n~ff a n d ,/~eff ~3) obtained here are negative in sign. Our results also indicated that the values of aeff"c3~for these linear quinones are all at least one order of magnitude bigger than the value of Im X ~3>. It is well known that the instantaneous nonlinear optical responses in conjugated organic molecules have significant contributions from the strong electron-electron interactions among the 'essential states', which consist of ground state and a few of the lowest lying excited states. The dipole moments connecting them to one another have contributed the sign and magnitudes of their third-order nonlinear responses. Recently, essential states models have been used successfully to explain the TPA, the X ~3~ and X ~5>dispersion data of a number of conjugated organic materials such as polyenes [ 14], polydiacetylene dyes [ 15] and squaraines [16]. For the polydiacetylenes, they have numerically confirmed that four states, 1Ag, 1Bu, mAg and nBu, are necessary to describe the dominant channels for third-order optical nonlinearity [ 15 ]. The relatively large dipole moment between the 'nBu' and 'mAg' states provides evidence that 'nBu' must be involved as an essential state in the THG and TPA processes [15]. For the bis(4-aminophenyl)squaraine (SQR) dye molecule, which involved a typical 'donor-acceptordonor' structure as shown in Fig. 5(a), the theoretical calculations show that three essential states, lAg, 1B3u and mAg, are necessary to describe the following two types of dominant channels for their third-order nonlinear optical processes [16]: ( i ) l l A g ---> llB3u ---~ l l A g -'+ llB3u -''> l l A g
(ii) llAg --->l I B 3 u ~ m i A g ~ llB3u --->liAg where channel (ii) consists of two virtual transitions with
Table 1 Two-photon absorption data of TPA coefficient/3, TPA cross section 0"2 and Im X 13~ for Q[0,0], Q[ 1,1 ] and Q[2,2] in CH2C12 at room temperature. The/3 values listed in parentheses are measured with open-aperture z-scans Compound
Measured conc. (10 3M)
Measured TPA coeff, fl ( c m G W i)
TPA cross section 0+2 (10-19cm4GW-i)
Im X ~3~ (10-L+e.s.u.)
Q[0,0] Q[ 1,1 ] Q[2,2]
1.20 1.69 1.32
0.24 (0.40) 0.73 (0.61) 2.71 (2.59)
2.88 7.22 34.2
3.74 11.33 42.10
241
T.C. Wen et al. / Synthetic Metals 92 (1998) 235-243 1.005
1.04 1.02
1.000.
oee
1.00
•
g0.9s
~ 0.995
0.96 c0.94
m
~. 0.990
0.92 .~ 0,90
=m 0 . 9 8 5
~
.E
o 0,980
0,88 o 0.86 0.84 0,82
(a)
0.975 -40 -30 -20 -10
0
10 20 30 40
(c) •.40 .30 -20 -10
x - - z/7~
10 20
0
30 40
x =7./~
1.4
1.15
1.3
._~1.10
1.2
._~
~
1.05
1.1
¢-
~ 1.00
1.0
.... .y ? . . : ,
0.9
~ 0.95
0.8
E ~0.90
0 ;Z
(b)
0.7
(d)
0.6
0.85
-40 - 3 0 - 2 0 - I 0
0
10 20
30 40
-40 -30 -20 -10
10 20 30 40
0
x=z/z o
X = 7../Ze
Fig. 4. (a) Q[0,0] and (c) Q[ 1,1 ] are the results of open-aperture z-scans fitted with Eqs. (2); (b) Q[0,0] and (d) Q[ 1,1 ] are the results of closed-aperture z-scans fitted with Eq. (4). All the samples are measured in solutions of dichloromethane with 1.0 mm thickness, and the linear transmissions T~, are around 0.95 _+0.05 at 532 nm. Table 2 The parameters of the third-order nonlinear index of refraction y, the effective nonlinear refractive index n~jf and the effective third-order nonlinear coefficient X~3: of quinones Q[0,0], Q[ 1,1] and Q[2,2] Compound
Q[0,0] Q[1,1I Q[2,2]
y
(10 12cm2W i)
n~'1 (10 llle.s.u.)
X~,~t' (10-1°e.s.u.)
-0.95 -0.40 -2.01
-3.24 -1.35 -6.84
-0.98 -0.41 -2.07
m = 3 and 4 for each. The large negative X ~3) for SQR is due to the fact that the transition dipole moment of I tB3, ~ 1lAg is much larger than the excited-state transitions such as 1 lAg ~ 11B3, a n d mlAg e-- 1JB3, from the above virtual channels (i) and (ii) [16]. For the quinones of Q [ 0,0], Q [ 1,1 ] and Q [ 2,2 ] which all belong to the D2h symmetry group, the axis parallel to their two C=O bonds is defined as z, and the y-axis is taken along their conjugated rr electron propagation direction as indicated in Fig. 5(b). Because their conjugated paT-orbitals should change sign under O-y=operation, their electronic states must be located among the four irreducible representatives such as Blg , Bzg , A. or B3.. The D2h character table also shows that the electronic transition dipole moment is represented by Bzu in the y direction. Therefore, the correlated one-photon and two-photon vertical transition channels might consist of the following essential states: lAg, JAu, IBlg, m l A g a n d n~B2,
(a)
G
H2N~
NH2
O®
Y
D2h x
(b)
o
D2h
°t
Ic I
C2v ~
o
I ° DZ
Fig. 5. Molecular structures of (a) bis(4-aminophenyl)sqnaraine (SQR), (b) 1,9-anthraquinone (Q[ 1,1 ] ), and (c) 1,4-tetracenequinone (Q[3,0]). The symmetry point groups and coordinate systems are also listed for each.
During the excitation with 532 nm laser pulses, the possible one-photon transitions w e r e JBig~---IAg a n d IAu~---]Ag, which could be induced mainly through the coupling of outof-plane vibrations with the ~(w,w*) states [4]. One of the two-photon vertical transitions could be m~Ag,--- lAg, where the energy level of m~Ag should be located somewhere between the 21B2, and 1~B~ustates of the Q[ 1,1 ]. Another feasible two-photon process could be nIBzu <-- lAg. This transition can be induced by the coupling between the vibrational
242
T.C. Wen et al. / Synthetic Metals 92 (1998) 235-243
modes and the B2u state, i.e., B2,®[b2,blu] = [ A i g A 2 g ] . Moreover, if the relative dipole moment between the 'nB,' and one in its neighborhood such as the 'm JAg ' state is very large, another transition such as nlB2,~-mtAg could be involved. Nevertheless, because the magnitudes and sign of n~ff and aeff" ~3~ of these quinones have been observed for the first time, we need to study further the nonlinear dispersion properties to confirm their essential states and the correlated vertical transition channels.
3.3. Excited-state absorption and refraction For 1,4-tetracenequinone (Q [ 3,0] ), the z-axis is the chain direction (Fig. 5) along which 7r electrons are delocalized. The C2~ character table indicates that its prr-orbitals could be represented either by A2 or B t, and the "rr electron transition 1.1 Z~
. 0 1.0 (n (n "~ 0.9
, /
¢) P" 0.8
I-- 0.7 "0 0.6
.__
i
t~ 0.5
E
(a),
0.4
O
0.3
i
-30
-20
i
-10
0
10
20
30
X = Z/Z 0 1.2
C 0 v) 1.0 u) o~
E U) r"
•
o~l, k
.,g-~r'/---.
dipole moment is represented by A~ in the z-direction. On the bases of the vibrational modes appearing in the phosphorescence spectrum of 1,4-naphthoquinone Q[ 1,0] [4], we tentatively assigned 3B I a s the Tl state for the Q [ 3,0]. Thus, the absorption of the 532 nm laser pulses promoted the Q[3,0] molecule from the So (1A~) to the excited singlet state S~ (~A~). For simplification, further excited-state absorption was concerned from Tj ( 3B 1) to higher T, states only. At this time, the excited-state cross section tTeTxof Q[3,0] has been determined with the open-aperture z-scan. The results are plotted in Fig. 6(a), where the curves are the numerical fittings by Eqs. (5). For example, put E = 3.0 txJ, linear transmittance T,. = 0.956, A = 532 × 10 - 7 cm, laser beam radius at the focus point OJo= l 3 × 10 - 4 cm, and the best fitting value ofqo = 0.445 into Eq. (6); this gives ~re~x= 1.84 × 10- J8 cm 2. This measurement has been repeated by raising the input energy. When E was increased from 13.2 to 67.8 txJ individually, it yielded o%~ T -_ 1.80× 10 -18 and 1.45 × 10 -~8 c m 2, respectively. The results indicated that there is no saturation of the excited-state population during our z-scan experiments. Finally, we performed closed-aperture z-scans on Q [ 3,0] to determine its excited-state refractive index cross section. For our CHeC12 solution sample, y = 1 . 2 4 × 1 0 -3 K -~, p = 1.326 g cm 3, n = 1.424, Cp= 1.191 J K - l g - i , so that it gives dn/dT= - 6.011 × 10 -4 K - ~ as evaluated with Eq. T = 1.84× 10-18 (8). With the values parameters and the O%x cm e, we obtained B = - 6 . 2 2 . The value of qo in Eq. (7) was estimated from the relations of qo=[E(close)/ E(open) ] qo(open), where qo(open) was obtained from the above open-aperture z-scans. Thus, our z-scan data were numerically fitted with Eq. (7), and o'~= 1.63 × 10- J7 c m 2. The values of n~ff were evaluated with n~ff(e.s.u.)= (cno/401r) y (m 2 W - l ) , where y = o'~N/kl. The results are listed in Table 3.
0.8
4. C o n c l u s i o n s
I'- 0.6
._.N 0.4 m
=0.2 o z
(b)
0.0 -40 -30 -20 -10
0
10
20
30
40
X = Z/Z 0
Fig. 6. (a) Open-aperture z-scan data for a 1.0 mm sample of Q[3,0] in CH2C12. The input intensities are 3.0 tzJ ( A ) , 13.2 ~J ( O ) and 67.8 gJ (C)), and the curves are fitted with Eqs. (5). T~,=0.956 at 532 nm. (b) Closed-aperture z-scan data for the same sample. The continuous curve is given by Eq. (7). T~. =0.82 at 532 nm.
In this work, the two-photon absorptive coefficient fl and the effective nonlinear refractive index n~ff of the quinones of Q[0,0], Q[ 1,1] and Q[2,2] have been determined. We found that their/3 values are increased from 0.24 for Q[0,0] to 0.73 for Q[ 1,1 ], and then to 2.71 cm G W - ~ for Q[2,2] as listed in Table 1. Also their magnitudes of n~ff and . (3) Re Xeff are all approximately the order of 10-lo e.s.u, with negative sign. The possible 'essential states' during the TPA and the third-order nonlinear processes have been assigned tentatively a s lAg, I A u, IBlg, mlAg and n l B 2 u according to their low-lying electronic energy levels. For Q [ 3,0 ], a simple
Table 3 The parameters of constant B, the excited-state absorption cross section tre~, T the excited-state refractive index cross section or, the effective nonlinear refractive index n~ff and the effective third-order nonlinear coefficient X~/3f~ of Q [ 3,0 ] B
O'e~T(10 18cm2)
O'r (10 17 cm2)
n~ef( e . s . 1u 0. )-2' '
X~3f~ ( 10 12 e.s.u.)
- 6.22
1.84
1.63
+ 1.0
+ 3.04
T.C. Wen et al. / Synthetic Metals 92 (19981 235-243
' f o u r - l e v e l ' m o d e l is u s e d to d e s c r i b e the e x c i t e d - s t a t e a b s o r p t i o n a n d r e f r a c t i o n p r o c e s s e s . T h e m a g n i t u d e s o f ~rr a n d rt~ff are + 1.63 × 1 0 - 1 7 c m 2 a n d + 1.0 × 10-11 e.s.u., r e s p e c t i v e l y , a n d t h e s e v a l u e s are c o m p a r a b l e w i t h t h o s e o f w e l l - k n o w n h i g h l y ~r-conjugated p l a n a r m o l e c u l e s o f tetrabenzoporphyrin and metallophthalocyanines.
Acknowledgements T h i s r e s e a r c h w a s c a r r i e d o u t w i t h the s u p p o r t o f the N a t i o n a l S c i e n c e C o u n c i l o f the R e p u b l i c o f C h i n a , G r a n t N S C 86-21 1 3 - M - 0 3 7 - 0 0 5 .
References [ l ] s. Miki, H. Shimizu and H. Nakatsuji, Tetrahedron Lett., 33 ( 19921 953.
243
[2] S. Miki, J. Ito, R. Noda, N. Nishijima and K. Fukunishi, Tetrahedron, 52 ( 19961 4269. [3] K. Alagesan and A.G. Samuelson, Synth. Met., 87 ( 19971 37. [4] T. Itoh, Chem. Rev., 95 (1995) 2351 and Refs. therein. [5] M. Nepras and M. Titz, J. Quantum Chem., 16 ( 19791 543. [6] S.R. Flora and P.F. Barbara, J. Phys. Chem., 89 ( 19851 4489. [ 7] J.G. Smith, P.W. Dibble and R.E. Sandbom, J. Org. Chem., 51 ( 19861 3762. [8] T.C. Wen and I.D. Lian, Synth. Met., 83 ( 19961 I 11. [9] T.F. Boggess, K.M. Bohnert, K. Mansour, S.C. Moss, I.W. Boyd and A.L. Smirl, IEEE J. Quantum Electron., 22 (1986) 360. [ 10] L.W. Tutt and T.F. Boggess, Prog. Quantum Electron., 17 ( 1993 ) 299. [11] M. Sheik-Bahae, A.A. Said, T.H. Wei, D.J. Hagan and E.W. Van Stryland, IEEE J. Quantum Electron., 26 (1990) 760. [ 12] I.D. Lian and T.C. Wen, Chemistry (Chin. Chem. Soc., Taipei), 54 ( 19961 83. [ 13] G.L. Wood, M.J. Miller and A.G. Mott, Opt. Lett., 20 ( 19951 973. [ 14] Z. Shuai, D. Beljonne and J.L. Br6das, J. Chem. Phys., 97 ( 19921
1132. [15] D. Guo, S. Mazumdar, S.N. Dixit and F. Kajzar, Phys. Rev. B. 48 ( 19931 1433. [ 16] Q.L. Zhou, R.F. Shi, O. Zamani-Khamari and A.F. Garito, Nonlinear Opt., 6 ( 19931 145.
Synthetic Metals 92 (1998) 235-243
Synthesis and third-order nonlinear optical properties of linear p-acenequinones T.C. Wen *, C.J. Tiao, L.C. Hwang, C.Y. Tsai School of Chemistry, KaohsiungMedical College, PO Box 72-94, Kaohsiung 80708, Taiwan Received 26 August 1997; accepted 29 October 1997
Abstract
The synthesis of 1,4-tetracenequinone (Q [ 3,0] ) has been described, and the nonlinear optical properties of this quinone together with pbenzoquinone (Q[0,0]), 9,10-anthraquinone (Q[ 1,1 ] ) and 6,13-pentacenequinone (Q[2,2]) have been investigated with 532 nm and 8 ns pulses. For the quinones of Q [ 0,0], Q [ 1,1 ] and Q [ 2,2], we applied the nonlinear transmittance and the z-scan techniques to determine their two-photon absorption (TPA) coefficient/3 and the third-order nonlinear refractive index r~ff. The results show that their/3 values increased with increasing aromatic tings of quinone, and the magnitudes of n~ff and X~ff (3) are all approximately the order of 10-,o e.s.u, with negative sign. The feasible vertical transition states among their low-lying electronic energy levels have been predicted. For Q [ 3,0], its excited-state absorption cross section o-~Vxand the excited-state refractive-index cross section o'r have been measured with z-scan methods. The magnitudes of o'r and n~f~are comparable with those of well-known highly conjugated planar molecules of tetrabenzoporphyrin and metallophthalocyanines. © 1998 Elsevier Science S.A. Keywords: Linear acenequinones; Synthesis; Two-photon absorption; Nonlinear refraction; z-Scan
I. Introduction
9,10-Anthraquinone (Q[ 1,1] ) and its derivatives play an important role in the area of industrial Synthetic dyes and pigments. The newly synthesized [ 6 ] -1,4-cyclophaneanthraquinone and 1,2,3-tri-t-butyl-6,13-pentacenequinone have demonstrated photochromic behavior between their valence type isomers [ 1,2]. They have received considerable attention because of their potential for a wide range of molecular devices. The complexes of 1,5-diaminoanthraquinone with its oxidized form exhibit a semiconductive property as reported in Ref. [ 3 ]. There are only nine unsubstituted linear p-acenequinones which have been studied spectroscopically up to now [ 4]. In the present paper we have investigated four of them, as illustrated in Fig. 1. According to the symbol Q[m,n] introduced by Nepras and Titz [5], Q[m,n] with m = n stands for the D2h quinones, and with m v~n stands for the C2~, quinones. The electronic ground states are characterized by Ag and A~ symmetry, respectively, for the D2h and C2v quinones. For the D2h quinones, their ground state MOs of oxygen are n + (b2,) and n_ (b3g), and their -rr orbitals have big , b2g, b3u, or a. * Corresponding author. Tel.: +886 07 312 1101 2192; fax: +886 07 312 5339. 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved PIIS0379-6779(97)04094-0
symmetry. Hence, the excited ( n , v * ) states are BIg, B2g, B3u, or A,, and the excited (-rr,-rr*) states are B,,, B2,, B3g, or Ag. For the C2L quinones, their n+ and n MOs are of al and b2 symmetry, respectively, while the 7r orbitals are either a 2 or b.. Therefore, the excited ( n,~r* ) orbitals are either A2 or B ,, while the excited (~r,~r*) states are either A, or B2. The absorption spectra of four linear quinones have been obtained in CH2C12 as sketched in Fig. 1, and their correlated lowest lying electronic excited states are shown in Fig. 2. These excited-state energies have been calculated with semiempirical methods and have confirmed very well the experimental values [4]. For p-benzoquinone ( Q [ 0 , 0 ] ) , the absorption bands at about 240 and 300 nm are both due to transitions to the l(~r,,rr.) states. The former band has been assigned as due to the lAg-~ iB,, transition, while the latter band has been attributed to the JAg--~ JB3g transition which borrows the intensity through vibronic coupling with the former transition [4]. The very weak absorption band at about 500 nm corresponds to the excitation into the ~(n,~r*) state with the IBlg symmetry [4]. For 1,9-anthraquinone (Q [ 1,1 ] ), the bands starting from 260 nm and those starting from 320 nm are due to transitions to the '(-rr,Tr*) states, while the weak absorption band at about 420 nm corresponds to the transition to the lowest '(n,'tr*) state with the 1B,g
236
72.C. Wen et al. /Synthetic Metals 92 (1998) 235-243
2.5 2.0
UJ tJ Z < 1.5 m n, O 1.0 (/)
2,
i~"
[~ o
Q[O,O]
CO
,<
1.s
O 1.0 ¢O "
i/x1 o ~, 1~.
0.5
x18
ILl 2.0 O Z
0.0 200 300 400 500 600 700 800
200 300 400 500 600 700 800
WAVELENGTH (rim)
WAVELENGTH (nm) 2.5
2.5 2.0
IB2u
LLI 0 Z 1.5 m n,, O or) 1.0 m ,< 0.5
LU O Z
O
xl
m
Q[1,1]
2.0
1.5
0.5
IB (~,~*)
1 2u
1A. I
Q[3,0]
o
1(~,~,)
O 1.0 U) m <
1Blu
X 10
/~
0.0
0.0 2O0 300 400 500 600 700 800
WAVELENGTH (nm)
200
300 400 500 600 700 800
WAVELENGTH (nm)
Fig. 1. Absorption spectra of quinones Q [ 0,0], Q [ t, 1], Q [ 2,2 ] and Q [ 3,0] in dichloromethane at room temperature.
1B1=
1B2,,
'T
/
~"~-~.
1~2 u
hv
~'~--~. l(tr, if*)
2
2-_~--
------JBlg
- hv
.~g_
-
Q(o,o)
I I T I I I I
hu
i
'A,I
I
Q(1,1)
r I
Q(2,2)
_
Q (3,o)
Fig. 2. The low-lying singlet excited-state energy levels of quinones. For Q[0,0], the quantum chemical calculations with the experimental values shown in parentheses are: ~B~g=2.46 (2.486); 'A.=2.60 (2.38); tB3g=4.13 (4.05); and LBI,=5.10 eV (5.0 eV) [4]. For Q[1,1]: 1]Big=2.768 (2.939); ]A,=2.909; lIB2u=3.808 (3.72); 1~B1,=4.482 (4.56); and 2]B2,=4.916 eV (4.92 eV) [41. The conversion factor of 1 eV = 8065 c m - ~is used here.
symmetry. The reported absorption spectral data for linearpacenequinones other than Q[0,0] and Q[1,1] are significantly limited [4]. However, as recorded in Fig. 1, their first singlet 7r ~ ~r* absorption bands were red shifted with maximum peaks from about 420 nm to about 500 nm for the quinones of Q[2,2] and Q[3,0], respectively. This absorption band can also be moved to the longer wavelength by the electron-withdrawing substituting groups such as - O H and -NH2, and the effect of the intramolecular hydrogen bond has attributed to this red shift [6]. Furthermore, it is also suggested that, when one of the phenyl rings of anthraquinone is bent in chair form, the J(rr,Tr*) state is very stabilized and comes below the ( n m * ) state as the bent angle becomes large [ 1 ]. In spite of a large number of papers concerning the spectroscopy and corresponding photophysical processes of linear aceneqinones in the literature, there has been almost no information with regard to their nonlinear optical properties to our knowledge. In this paper, we will report the synthesis of 1,4-tetracenquinone ( Q [ 3,0 ] ). The nonlinear optical responses of these four quinones are studied with a Q-switched 532 nm and near-Gaussian laser beams. The generated nonlinear processes such as the two-photon absorption (TPA), nonlinear refraction and excited-state absorptions (ESAs) are determined by measuring their transmittance as a function of inci-
237
T.C. Wen et al. / Synthetic Metals 92 (1998) 235-243
CI CI~ , , , C I
c! Cl~Ci
C!~~'~ CI~ H CI ~ ~ 1
~
~CI
O
hemiacetal
l CH3 CH3 CH3 B12/CC14 I, c C l ! / ~ ~ Pyrolysis ~ B r Fe,12 CI~ . ~ C I ~" "Br Ib INBS el Ia |benzoyi peroxide
1 n-butyllithium~ / ~ CH2OH CaCO3 i D1VIF ~fl~-~Br diOxane/H20~ v OH Id form O
1Ltautomer
aldehyde
CH2Br v "Br Ic
O
O
form
If 1,4-TetracenequinoneQ[3,0]
Ie
Scheme1. dent optical intensity, or as a function of sample position along the light propagation direction, which is known as the z-scan method. The results are explained with their correlated electronic energy states.
-~ [ ~ C oven
The synthetic procedures of 1,4-naphthacenedione Q [ 3,0] appear in Scheme 1; the details follow. During the preparation of intermediates Ia-Ie, most methods are modifications from Ref. [ 7 ]. The yield of the intermediate lb was improved here with an efficient pyrolysis method (Scheme 2). The pbenzoquinone Q [ 0,0], 9,10-anthraquinone Q [ 1,1 ] and 6,13pentacenequinone Q [ 2,2] were purchased from Aldrich. The p-benzoquinone was purified by vacuum sublimation, and the other two quinones were recrystallized from chloroform before use.
2.1.1. 3-Bromo-2-methylnaphthalene-bis( hexachlorocyclopentadiene ) (Ia ) A mixture of 2-metbylnaphthalene-bis(hexachlorocyclopentadiene) ( 172 g, 0.25 mol), I2 (0.85 g), Fe powder (0.45 g) and dry CC14 (300 ml) was refluxed with stirring, and a solution of 40.5 g Br in 30 ml CC14 was added over 2 h. The mixture was refluxed 4 h around 77-80 °C, cooled, and the product dissolved with CH2C12, the precipitate filtered off and pumped to dry with rotavap. The solid product was recrystallized with CC14 and dried in air to give a white product (143 g, 75%): m.p. 212-213 °C. IR (KBr): 2937,
|,.
[ ~[.~l-N0)
Scheme2.
2. Experimental 2.1. Sample preparation
_
VaCHi.HD.
1616, 1487, 1257, 1083, 894, 769, 699, 584 cm i. ~H NMR (CDC13) 6:2.41 (s, 3H, CH3), 3.48 and 3.53 (dd, 2H, CH), 3.87 and 3.92 (dd, 2H, CH), 7.58 (s, 1H, ArH), 7.90 (s, 1H, ArH). 13C NMR (CDC13) 6: 22.61, 41.48, 41.53, 46.46, 46.71, 82.20, 84.59, 101.23, 124.40, 127.48, 127.60, 128.88, 128.93, 131.24, 132.79, 133.20, 133.29, 137.52.
2.1.2. 2-Bromo-3-methylnaphthalene (Ib ) This reversed Diels-Alder pyrolysis reaction was performed in a cylindrical flask under high vacuum as illustrated in Scheme 2. Ia (50 g) was heated first in an oven at 215 °C. After the sample was transferred completely from solid powder to liquid, the oven temperature was then raised to 280 °C. The pale yellow product was distilled into a flask which was trapped with liquid nitrogen. The solid product was chromatographed on silica gel (0.04-0.06 ram) and eluted with n-hexane. The final product was obtained by recrystallization from n-hexane (5.5 g, 38%); m.p. 132-136 °C. IR (KBr): 2987, 1601, 1511, 1387, 993, 884, 769, 495 cm-~. JH NMR (CDC13) 6:2.56 (s, 3H, CH3), 7.41-7.47 (m, 2H), 7.697.75 (m, 3H), 8.06 (s, 1H). 13C NMR (CDC13) 6: 23.25, 123.60, 125.94, 126.23, 126.57, 127.11, 128.70, 130.73,
133.01, 135.28.
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T.C. Wen et al. / Synthetic Metals 92 (1998)235-243
2.1.3. 2-Bromo-3-(bromomethyl)naphthalene (Ic ) Half of a mixture of NBS ( 16.2 g, 0.091 mol ) and benzoyl peroxide (0.4 g, 1.65 mmol) was first added into a refluxing solution of Ib (24 g, 0.109 tool) in CCI 4 (300 ml). After 2 h of refluxing at 76 °C, the remaining mixture solution was then added in portions over 30 rain, and the solution was refluxed for another 2 h. The product was cooled and washed with saturated NaHCO3 solution. The organic phase was dried with MgSO4, filtered, and then the solvent was removed with rotavap. The white solid product was chromatographed on silica gel (0.04-0.06 ram) and eluted with n-hexane. The solid product was recrystallized from hexane, and the white plate crystals were obtained ( 12.84 g, 39.43%); m.p. 113114 °C. IR (KBr): 2925, 2345, 1211, 1129, 988, 878, 752, 689, 668 cm i. tH NMR (CDC13) 8:4.78 (s, 2H, CH2), 7.9-7.56 (m, 2H), 7.73-7.83 (m, 2H), 7.95 (s, 1H), 8.11 (s, IH). ~C NMR (CDC13) 8:34.00 (CH2Br), 121.41, 126.71, 126.85, 127.56, 127.87, 130.50, 132.11, 132.28, 134.16, 134.21. 2.1.4. 3-Bromo-2-naphthalenemethanol (Id) A mixture of Ie (9.81 g, 32.7 mmol), CaCO3 (16.08 g, 0.161 tool) and dioxane/H20 ( 1:1,200 ml) was refluxed for 10 h. The product was cooled, mixed with methanol (20 ml), and extracted with warm CHCI3 ( 100 ml). The organic phase was dried over MgSO4. The solid product was recrystallized from toluene/hexane (1:1) to obtain a white needle solid powder (6.84 g, 88.2%); m.p. 113-115 °C. IR (KBr): 3231, 1589, 1491, 1201, 1126, 1057, 977, 876, 744, 471 cm ~. JH NMR (CDCI3) 8:2.11 (s, 1H, OH), 4.90 (s, 2H, CH20), 7.48-7.54 (m, 2H), 7.73-7.86 (m, 2H), 7.93 (s, 1H), 8.07 (s, 1H). 13C NMR (CDC13) 8: 65.19, 120.18, 126.54, 126.64, 126.77, 127.50, 127.87, 131.21, 132.27, 133.72, 136.83. 2.1.5. 3-( Hydro xvmethyl )- 2-naphthaldehyde (le ) A stirred mixture o f l d ( 3 g, 12.65 mmol) and dried diethyl ether ( 100 ml) was cooled to - 78 °C, and n-butyllithium in hexane ( 16.7 ml, 2.6N) was added by syringe. After 10 min, the solution was warmed to 0 °C for another 30 min, then dry DMF ( 1.5 ml) was injected and the mixture was stirred for 10 h. H20 ( 15 ml) was added to the product, and the precipitate was stirred in a mixture of NH4C1 and ether. The ether layer was separated and dried over MgSO4. The solid product was recrystallized from toluene/hexane ( 1:1 ) and a white solid product was obtained (2.18 g, 92%) ; m.p. 110-111 °C. IR (KBr): 3351, 3054, 2869, 1685, 1101, 1005, 885, 743, 482 cm ~. JH NMR (CDC13) 8: (hemi-acetal form) 3.04 (d, IH, OH), 5.19 and 5.42 (dd, 2H, CH2), 663 (d, 1H, CHOH), 7.49-8.02 (m, 6H, ArH); (aldehyde form) 3.81 (t, 1H, OH), 4.97 (d, 2H, CH2), 7.49-8.02 (s, 1H, ArH), 10.19 (s, 1H, CHO). 13C NMR (CDC13) 8: 64.42, 71.29, 101.11, 119.63, 122,18, 125.96, 126.62, 127.93, 128.00, 128.56, 129.13, 129.26, 130.02, 132.03, 132.98, 133.30, 134.18, 135.84, 137.05, 137.35, 140.30, 194.95. MS ( E l ) m / z = 185 (M-l).
2.1.6. 1,4-Tetracenequinone (If) Q[3,0] A mixture of Ie (0.125 g, 0.7 mmol), p-benzoquinone (0.09 g, 0.84 mmol) and glacial acetic acid (3 ml) was refluxed for 8 h. The solution was cooled and the red-brown precipitate was washed with ethanol. The crude product was recrystallized from toluene to give reddish-brown plate crystals (0.02 g, 12%); m.p. 262 °C dec. IR (KBr): 1664, 1606, 1418, 1307, 1244, 1136, 1054, 929, 872, 749 cm J. ~H NMR (CDC13) 6:7.11 (s, 2H, C=CH), 7.46-7.59 (q, 2H), 8.068.11 ( q, 2H), 8.67 ( s, 2H), 8.84 ( s, 2H). 13C NMR ( CDCI3 ) 6: 127.88, 128.69, 130.30, 130.70, 131.35, 133.71, 140.66, 184.44. 2.2. Spectra and nonlinear optical measurements The spectra of quinones were recorded on a UV-Vis Shimadzu UV-160A spectrophotometer, IR on a Perkin-Elmer FT-IR 2000 spectrophotometer and ~H NMR on a Varian Gemini-200 (200 MHz) VT proton/carbon FT-NMR spectrometer. The mass spectra were determined on a GC-mass HP-5989A system at 70 eV. Melting points determined on a capillary tube apparatus (Electrothermal IA9100) were uncorrected. The nonlinear transmittance was studied with 8 ns and 532 nm laser pulses from a frequency-doubled Qswitched Nd:YAG laser. The intensity varied without changing the pulse polarization by rotating the first of two Glan-laser polarizers, while the input pulses were Ibcused with a 20-cm focal-length lens. The incident and transmitted energies were detected simultaneously by two probes Rjp735 and -765, then averaged with the RJ-7620 energy meter individually. Each data point was an average of over 50 shots with about 1 Hz repetition rate. A 10-mm quartz cell was used for the nonlinear transmittance measurements with various input laser powers up to 2 GW cm-2. The laser beam passed through the sample with its focus spot near the middle of the sample cell, and was totally refocused onto the energy probe. A l-ram quartz cell was used during the z-scans. The energy of input laser pulses was kept around 12 txJ, and an energy probe was placed at a distance of 90 cm away from the focal point. The z-scan experimental set-up has been described elsewhere [ 8]. 2.3. Determination of TPA coefficient fl The TPA coefficient/3 has been determined in two ways. First, we measured the transmitted intensity/passing through a sample with thickness L by various input intensities Io. The value of/3 can be estimated with the equation [ 9,10]: /
loLfl) L/3
ln(1 +
(1)
In the second approach, we used the open-aperture z-scan technique to measure/3. The sample was scanned across the focal region of the lens along the z direction of the beam propagation, while the change of the normalized transmit-
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T.C. Wen et al. / Synthetic Metals" 92 (1998) 235-243
tance T(z) was recorded and analyzed with the equations [11]:
+
ln[l
qo ] 1 +x2J
T= =
v" _[ -_q
qo 1 +x 2
")1
_ 9 _ 2
T(z)
m.~_o(m + 1 )3/2 ,
q(z)
1 + (Z/zo) 2
(2)
where q( z,r,t ) = li,( z,r,t ) /3L~n.,LCff= ( 1 - exp( - oiL) )/ol, lo is the input intensity at the focus (z = 0), z is the distance of the sample from the focus, and zo = ('rr Wo2 ) /A. The numerical fittings are performed by choosing the most favorable parameter of q(z) first. Then the conversion from q(z) to /3 followed.
The nonlinear refractive index n 2 was measured with closed aperture z-scan. At this time, we should consider the radial phase shift A q~(r) of the transverse profile emerging from the exit face of the medium. Combining the equations dA ~ / dz = ('rrl/ A )n 2 and dl/ dz = - c d - /312, followed by integration, we can obtain the phase shift just after the sample plane as
(5)
where x=z/zo, v is the frequency of input laser pulses (5.64× l014 S I), and Fo=2E/Tro)o 2, where E is in joules for the input laser intensity, and c~= [In( 1/TI~,) ]/L. Thus, T can be evaluated with the value of o'¢x
qow~7
o 'xc , _- (6.24 X 10 -23 j c m ) E ( 1 - Tli,,) h
T=I
(6)
qo
( 9 + x 2 ) ( 1 + x 2) ×((3
+x2)
{
20-r]'/
(3)
where y is the nonlinear index, which is related through a conversion formula y ( m 2 W - i ) =n2 (e.s.u.) (40v/cno), where c is the speed of light. After some tedious algebraic processes, we derive the normalized transition equation as [121
(7)
where B = (dn/dt) ( 2khog/3 pCpO'Tx ), and dn/ dt is the change in index of the solution with temperature, which can be written as
dT
A @o(z,t) [ 4 x - 2 y x 2 - - 6y] T~- 1 q(X2~- 1) (X2 + 9 )
0-eTxOZFO( r = O)Len2hv
The equation used to determine the value of 0-r is [ 13 ]
2.4. Determination of nonlinear refractive index n f
-/k A~(z,r,t) =--~- ln[ 1 +q(z,r,t)]
qo =
~Op]\OT}
6np
(8)
where y is the expansivity.
3. Results and discussion
3.1. Two-photon absorption (4)
where y =/3/2ky. The numerical fittings are carried out first by choosing the most favorable value of y, while the values o f y and A qbo(Z,t) in Eq. (4) are converted by y = / 3 / 2 k y a n d Eq. (3), respectively. Then the value of n~ff is calculated with "}' ( m 2 W - 1) = n~ff ( e . s . u . ) (40"rr/cno).
2.5. Determination of the absorptive and refractive cross sections o'~, and 0-,. T The nonlinear excited-state absorptive cross section 0-cx and the excited-state refractive cross section 0-r were determined by the open-aperture and closed-aperture z-scan methods, respectively. The 'four-level' model (So--* S ~ T~--* T,,) is suggested here to correlate both the absorptive and refractive nonlinear optical processes. It is also suggested that there is no saturation effect during the pumping with nanosecond laser pulses. Thus, the theoretical equations used to fit the normalized transmittance from our open-aperture zscans are [ 13 ]
First of all, we have observed the nonlinear transmission with 8 ns and 532 nm laser pulses for all the quinones in CH2C12. The transmitted intensities were measured each time by varying the input laser energy. The results all fitted very well with the model Eq. (1), except for the 1,4-tetracenequinone ( Q [ 3 , 0 ] ) . Some of these results are shown in Fig. 3, where the continuous curves are given by Eq. ( 1 ) using the best-fitting parameter/3 = 0.24 and 0.73 cm G W - 1 for the pbenzoquinone (Q [ 0,0] ) and 9,10-anthraquinone (Q [ 1,1 ] ), respectively. In addition, the TPA cross section 0-2 can be determined by/3 = 0-2Nad × 10- 3, where 0-2 is in units of cm e GW ~, Na is the Avogadro constant, and d is the concentration in units of mol 1- t. The values of 13 and 0-2 are listed in Table 1. For the Q[3,0] which belongs to the Ce~ symmetry group, its lowest excited ~(~,-rr*) state is very close to our 532 nm input laser beam, so that the one-photon absorption process should be preferred during the beginning of excitation, and we have observed its excited-state absorption and refraction properties in Section 3.3. On the other hand, the TPAs attributed from those linear quinones have also been investigated by the z-scan method
240
T.C. Wen et a l . / Synthetic Metals 92 (1998) 235-243
~
mechanism causing the observed nonlinear absorption behavior, and that the/3 values increased with increasing aromatic subsystems.
(a) 5
3.2. Nonlinear refractions
~4
I-..
o
+
i
+
d
i
2
4
6
8
10
12
Input Intensity (GWIcm2) 3.0
(b) 2.5 ,,~ 2.0 g 1.5 "~ 1.0 ~'l 0.5 P
o.o
2
4
6
8
10
Input Intensity (GW/cm2) Fig. 3. Transmitted intensity of quinones Q[0,0] and Q[ 1,1] in CH2C12 vs. input laser beam intensity. The concentrations are 1.2 × 10- 3 mol for Q [ 0,01 and 1.69x 10 -3 mol for Q[ 1,11.
without aperture. The results for Q[0,0] and Q[ 1,1 ] have been plotted in Fig. 4(a) and (c), respectively. The continuous curves are the best numerical fittings with Eqs. (2). The correlating values of I m x ~3) are converted by Im X ~3) = (noc2/3) / 8172to, as listed in Table 1. Comparing the /3 values obtained from these two different methods, we found that the deviations between each of them are no more than _+30%, which is within the errors of our experiments. The results demonstrated that the TPA could be the dominant
The nonlinear refractive indices r~ff have been determined with closed-aperture z-scans for the above quinones exhibiting TPA properties. The measured normalized transmittances versus Z/Zo are fitted numerically with Eq. (4) as sketched in Fig. 4(b) and (d) for the quinones Q[0,0] and Q[ 1,1 ], respectively. The values of n~ff and Re A{eff • (3) are listed in Table 2, while the real part of the third-order nonlinear sus~/(3) -_ n~ff ( 2no~ 3~r ), ceptibility Re ,-~eff • (3> is converted from Re aeff where no is the linear refractive index of our sample solution. As listed in Table 2, all the values of n~ff a n d ,/~eff ~3) obtained here are negative in sign. Our results also indicated that the values of aeff"c3~for these linear quinones are all at least one order of magnitude bigger than the value of Im X ~3>. It is well known that the instantaneous nonlinear optical responses in conjugated organic molecules have significant contributions from the strong electron-electron interactions among the 'essential states', which consist of ground state and a few of the lowest lying excited states. The dipole moments connecting them to one another have contributed the sign and magnitudes of their third-order nonlinear responses. Recently, essential states models have been used successfully to explain the TPA, the X ~3~ and X ~5>dispersion data of a number of conjugated organic materials such as polyenes [ 14], polydiacetylene dyes [ 15] and squaraines [16]. For the polydiacetylenes, they have numerically confirmed that four states, 1Ag, 1Bu, mAg and nBu, are necessary to describe the dominant channels for third-order optical nonlinearity [ 15 ]. The relatively large dipole moment between the 'nBu' and 'mAg' states provides evidence that 'nBu' must be involved as an essential state in the THG and TPA processes [15]. For the bis(4-aminophenyl)squaraine (SQR) dye molecule, which involved a typical 'donor-acceptordonor' structure as shown in Fig. 5(a), the theoretical calculations show that three essential states, lAg, 1B3u and mAg, are necessary to describe the following two types of dominant channels for their third-order nonlinear optical processes [16]: ( i ) l l A g ---> llB3u ---~ l l A g -'+ llB3u -''> l l A g
(ii) llAg --->l I B 3 u ~ m i A g ~ llB3u --->liAg where channel (ii) consists of two virtual transitions with
Table 1 Two-photon absorption data of TPA coefficient/3, TPA cross section 0"2 and Im X 13~ for Q[0,0], Q[ 1,1 ] and Q[2,2] in CH2C12 at room temperature. The/3 values listed in parentheses are measured with open-aperture z-scans Compound
Measured conc. (10 3M)
Measured TPA coeff, fl ( c m G W i)
TPA cross section 0+2 (10-19cm4GW-i)
Im X ~3~ (10-L+e.s.u.)
Q[0,0] Q[ 1,1 ] Q[2,2]
1.20 1.69 1.32
0.24 (0.40) 0.73 (0.61) 2.71 (2.59)
2.88 7.22 34.2
3.74 11.33 42.10
241
T.C. Wen et al. / Synthetic Metals 92 (1998) 235-243 1.005
1.04 1.02
1.000.
oee
1.00
•
g0.9s
~ 0.995
0.96 c0.94
m
~. 0.990
0.92 .~ 0,90
=m 0 . 9 8 5
~
.E
o 0,980
0,88 o 0.86 0.84 0,82
(a)
0.975 -40 -30 -20 -10
0
10 20 30 40
(c) •.40 .30 -20 -10
x - - z/7~
10 20
0
30 40
x =7./~
1.4
1.15
1.3
._~1.10
1.2
._~
~
1.05
1.1
¢-
~ 1.00
1.0
.... .y ? . . : ,
0.9
~ 0.95
0.8
E ~0.90
0 ;Z
(b)
0.7
(d)
0.6
0.85
-40 - 3 0 - 2 0 - I 0
0
10 20
30 40
-40 -30 -20 -10
10 20 30 40
0
x=z/z o
X = 7../Ze
Fig. 4. (a) Q[0,0] and (c) Q[ 1,1 ] are the results of open-aperture z-scans fitted with Eqs. (2); (b) Q[0,0] and (d) Q[ 1,1 ] are the results of closed-aperture z-scans fitted with Eq. (4). All the samples are measured in solutions of dichloromethane with 1.0 mm thickness, and the linear transmissions T~, are around 0.95 _+0.05 at 532 nm. Table 2 The parameters of the third-order nonlinear index of refraction y, the effective nonlinear refractive index n~jf and the effective third-order nonlinear coefficient X~3: of quinones Q[0,0], Q[ 1,1] and Q[2,2] Compound
Q[0,0] Q[1,1I Q[2,2]
y
(10 12cm2W i)
n~'1 (10 llle.s.u.)
X~,~t' (10-1°e.s.u.)
-0.95 -0.40 -2.01
-3.24 -1.35 -6.84
-0.98 -0.41 -2.07
m = 3 and 4 for each. The large negative X ~3) for SQR is due to the fact that the transition dipole moment of I tB3, ~ 1lAg is much larger than the excited-state transitions such as 1 lAg ~ 11B3, a n d mlAg e-- 1JB3, from the above virtual channels (i) and (ii) [16]. For the quinones of Q [ 0,0], Q [ 1,1 ] and Q [ 2,2 ] which all belong to the D2h symmetry group, the axis parallel to their two C=O bonds is defined as z, and the y-axis is taken along their conjugated rr electron propagation direction as indicated in Fig. 5(b). Because their conjugated paT-orbitals should change sign under O-y=operation, their electronic states must be located among the four irreducible representatives such as Blg , Bzg , A. or B3.. The D2h character table also shows that the electronic transition dipole moment is represented by Bzu in the y direction. Therefore, the correlated one-photon and two-photon vertical transition channels might consist of the following essential states: lAg, JAu, IBlg, m l A g a n d n~B2,
(a)
G
H2N~
NH2
O®
Y
D2h x
(b)
o
D2h
°t
Ic I
C2v ~
o
I ° DZ
Fig. 5. Molecular structures of (a) bis(4-aminophenyl)sqnaraine (SQR), (b) 1,9-anthraquinone (Q[ 1,1 ] ), and (c) 1,4-tetracenequinone (Q[3,0]). The symmetry point groups and coordinate systems are also listed for each.
During the excitation with 532 nm laser pulses, the possible one-photon transitions w e r e JBig~---IAg a n d IAu~---]Ag, which could be induced mainly through the coupling of outof-plane vibrations with the ~(w,w*) states [4]. One of the two-photon vertical transitions could be m~Ag,--- lAg, where the energy level of m~Ag should be located somewhere between the 21B2, and 1~B~ustates of the Q[ 1,1 ]. Another feasible two-photon process could be nIBzu <-- lAg. This transition can be induced by the coupling between the vibrational
242
T.C. Wen et al. / Synthetic Metals 92 (1998) 235-243
modes and the B2u state, i.e., B2,®[b2,blu] = [ A i g A 2 g ] . Moreover, if the relative dipole moment between the 'nB,' and one in its neighborhood such as the 'm JAg ' state is very large, another transition such as nlB2,~-mtAg could be involved. Nevertheless, because the magnitudes and sign of n~ff and aeff" ~3~ of these quinones have been observed for the first time, we need to study further the nonlinear dispersion properties to confirm their essential states and the correlated vertical transition channels.
3.3. Excited-state absorption and refraction For 1,4-tetracenequinone (Q [ 3,0] ), the z-axis is the chain direction (Fig. 5) along which 7r electrons are delocalized. The C2~ character table indicates that its prr-orbitals could be represented either by A2 or B t, and the "rr electron transition 1.1 Z~
. 0 1.0 (n (n "~ 0.9
, /
¢) P" 0.8
I-- 0.7 "0 0.6
.__
i
t~ 0.5
E
(a),
0.4
O
0.3
i
-30
-20
i
-10
0
10
20
30
X = Z/Z 0 1.2
C 0 v) 1.0 u) o~
E U) r"
•
o~l, k
.,g-~r'/---.
dipole moment is represented by A~ in the z-direction. On the bases of the vibrational modes appearing in the phosphorescence spectrum of 1,4-naphthoquinone Q[ 1,0] [4], we tentatively assigned 3B I a s the Tl state for the Q [ 3,0]. Thus, the absorption of the 532 nm laser pulses promoted the Q[3,0] molecule from the So (1A~) to the excited singlet state S~ (~A~). For simplification, further excited-state absorption was concerned from Tj ( 3B 1) to higher T, states only. At this time, the excited-state cross section tTeTxof Q[3,0] has been determined with the open-aperture z-scan. The results are plotted in Fig. 6(a), where the curves are the numerical fittings by Eqs. (5). For example, put E = 3.0 txJ, linear transmittance T,. = 0.956, A = 532 × 10 - 7 cm, laser beam radius at the focus point OJo= l 3 × 10 - 4 cm, and the best fitting value ofqo = 0.445 into Eq. (6); this gives ~re~x= 1.84 × 10- J8 cm 2. This measurement has been repeated by raising the input energy. When E was increased from 13.2 to 67.8 txJ individually, it yielded o%~ T -_ 1.80× 10 -18 and 1.45 × 10 -~8 c m 2, respectively. The results indicated that there is no saturation of the excited-state population during our z-scan experiments. Finally, we performed closed-aperture z-scans on Q [ 3,0] to determine its excited-state refractive index cross section. For our CHeC12 solution sample, y = 1 . 2 4 × 1 0 -3 K -~, p = 1.326 g cm 3, n = 1.424, Cp= 1.191 J K - l g - i , so that it gives dn/dT= - 6.011 × 10 -4 K - ~ as evaluated with Eq. T = 1.84× 10-18 (8). With the values parameters and the O%x cm e, we obtained B = - 6 . 2 2 . The value of qo in Eq. (7) was estimated from the relations of qo=[E(close)/ E(open) ] qo(open), where qo(open) was obtained from the above open-aperture z-scans. Thus, our z-scan data were numerically fitted with Eq. (7), and o'~= 1.63 × 10- J7 c m 2. The values of n~ff were evaluated with n~ff(e.s.u.)= (cno/401r) y (m 2 W - l ) , where y = o'~N/kl. The results are listed in Table 3.
0.8
4. C o n c l u s i o n s
I'- 0.6
._.N 0.4 m
=0.2 o z
(b)
0.0 -40 -30 -20 -10
0
10
20
30
40
X = Z/Z 0
Fig. 6. (a) Open-aperture z-scan data for a 1.0 mm sample of Q[3,0] in CH2C12. The input intensities are 3.0 tzJ ( A ) , 13.2 ~J ( O ) and 67.8 gJ (C)), and the curves are fitted with Eqs. (5). T~,=0.956 at 532 nm. (b) Closed-aperture z-scan data for the same sample. The continuous curve is given by Eq. (7). T~. =0.82 at 532 nm.
In this work, the two-photon absorptive coefficient fl and the effective nonlinear refractive index n~ff of the quinones of Q[0,0], Q[ 1,1] and Q[2,2] have been determined. We found that their/3 values are increased from 0.24 for Q[0,0] to 0.73 for Q[ 1,1 ], and then to 2.71 cm G W - ~ for Q[2,2] as listed in Table 1. Also their magnitudes of n~ff and . (3) Re Xeff are all approximately the order of 10-lo e.s.u, with negative sign. The possible 'essential states' during the TPA and the third-order nonlinear processes have been assigned tentatively a s lAg, I A u, IBlg, mlAg and n l B 2 u according to their low-lying electronic energy levels. For Q [ 3,0 ], a simple
Table 3 The parameters of constant B, the excited-state absorption cross section tre~, T the excited-state refractive index cross section or, the effective nonlinear refractive index n~ff and the effective third-order nonlinear coefficient X~/3f~ of Q [ 3,0 ] B
O'e~T(10 18cm2)
O'r (10 17 cm2)
n~ef( e . s . 1u 0. )-2' '
X~3f~ ( 10 12 e.s.u.)
- 6.22
1.84
1.63
+ 1.0
+ 3.04
T.C. Wen et al. / Synthetic Metals 92 (19981 235-243
' f o u r - l e v e l ' m o d e l is u s e d to d e s c r i b e the e x c i t e d - s t a t e a b s o r p t i o n a n d r e f r a c t i o n p r o c e s s e s . T h e m a g n i t u d e s o f ~rr a n d rt~ff are + 1.63 × 1 0 - 1 7 c m 2 a n d + 1.0 × 10-11 e.s.u., r e s p e c t i v e l y , a n d t h e s e v a l u e s are c o m p a r a b l e w i t h t h o s e o f w e l l - k n o w n h i g h l y ~r-conjugated p l a n a r m o l e c u l e s o f tetrabenzoporphyrin and metallophthalocyanines.
Acknowledgements T h i s r e s e a r c h w a s c a r r i e d o u t w i t h the s u p p o r t o f the N a t i o n a l S c i e n c e C o u n c i l o f the R e p u b l i c o f C h i n a , G r a n t N S C 86-21 1 3 - M - 0 3 7 - 0 0 5 .
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