OPTICAL REVIEW Vol. 2, No. 2 (1995) 128-131

Linear and NOnlinear Optical PrOperties Of Racemic (1!~)2-(a-MethylbenZylaminO)-5-Nitropyridine Single CrystalS

Takashi KONDO,1 Fumiaki AKASE'2 Masashi KUMAGA12 and Ryochi ITO1

*Department of Applied Physics, Faculty of Engineering, The University of Tokyo, 7-8-1 Hongo, Bunkyo-ku, Tokyo, 113 Japan, 'Materials Development Research Laboratories, Japan Energy Co', Ltd., 3-17-35 Niizo-Minami, Toda, Saitama, 335 Japan

(Received December 16, 1994; Accepted January 27, 1995)

Linear and nonlinear optical properties of racemic (~)2-(c!-methylbenzylamino)-5-nitropyridine ((~)MBANP) single crystals have been comprehensively investigated and compared with those of the enantiomorph (-)2-(c!-methylbenzylamino)-5-nitropyridine ((-)MBANP) crystals. (~)MBANP crystal exhibits very high chemical and physical stability, but relatively small nonlinear optical coefiicients (d31 =6.8 pm/V, d32=4.7 pm/V,

d33=0.84 pm/V). A comparison between the nonlinear optical coefficients of (~)MBANP and (-)MBANP demonstrates the validity of the oriented-gas model in molecular crystals that neglects all the contributions from intermolecular interaction.

Key words : organic crystal, racemic L0rm, second-harmonic generation, refractive index, nonlinear optical coefflcient, crystal structure, oriented-gas model

1. Introduction

Since the discovery of extremely large quadratic optical nonlinearities in some organic molecular crystals, a large

number of organic crystals have been studied for the purpose of applications to efficient frequency conversion by second-harmonic generation (SHG).1) (_)2_(c!-Methyl-benzylamino)-5-nitropyridine ((-)MBANP) is one of ex-tensively investigated nonlinear optical organic crystals.2~4)

The very large quadratic optical nonlinearity found in this

crystal4) stems from its highly asymmetric structure5) that

is promoted by a chiral handle in the methylbenzylamino

group. The racemic form of MBANP, (~)MBANP, on the other hand, has attracted little attention although a

powder measurement by Cheng et al. 6) showed that (i)MBANP is SHG active.

In this paper we present the result of a comprehensive investigation of the linear and nonlinear optical properties

of (~)MBANP crystal. To the best of our knowledge, this is the first systematic study 0L nonlinear optical properties

of racemic crystal. Since (~)MBANP and (-)MBANP differ only in the alignment of chiral molecules, the com-parison of their nonlinear optical properties will be an ideal

test of the oriented-gas model, in which the nonlinear optical properties of a crystal are predicted solely based on

the molecular configuration and hyperpolarizability.4)

2. Crystal Growth and Crystal Structure

Single crystals of (~)MBANP have been grown from a dichloromethane solution by the solvent evaporation method. Slow evaporation at room temperature for about three weeks yielded good-optical-quality single crystals with typical dimensions of 40 X 30 X 10 mm3 . Their natural major faces and cleavage planes are parallel to the crystallo-

graphic (O I O) planes. The single crystals showed no degradation or decomposition at room temperature. The

melting point of (i)MBANP, 124'C, is much higher than that of (-)MBANP, 83'C, indicating higher stability of (~)MBANP crystals. Furthermore, (~)MBANP crystals are much harder than (-)MBANP crystals; compare the Vickers hardness of (~)MBANP, 27, with that of (-)MBANP, 9.

The crystal and molecular structures of (i)MBANP were determined by X-ray diffraction analysis on a sin-gle crystal with dimensions of 0.3 x 0.15 >< 0.Imm3. (~)MBANP belongs to the orthorhombic system with space group Aba2 (Z=8), point group mm2. The unit cell pararneters are a= 17.149(3) A, b= 18.665(3) A, c=7.936(2) A

and V=2540.2(8) A3. Table I summarizes the atomic co-ordinates determined with R=0.045 including isotropic hydrogen atoms for 1323 independent reflections.

A perspective view of an S-isomer molecule is shown in Fig. 1(a). The MBANP molecule in (-)MBANP crystal5) is shown in Fig. 1(b) for comparison. The molecular structure of (~)MBANP is almost the same as that of (-)MBANP. The only apparent difference is the direction of the pyridine ring; the nitrogen atom (N2) in the pyridine ring in (~!~)MBANP is located opposite to the methyl carbon (C6) whereas that in (-)MBANP is on the same side as the C6 atom. In both crystals, the electron accepting nitro groups and the electron donating amino groups are on the plane of the pyridine rings. Therefore, the large optical nonlinearity of the MBANP molecule is maintained in (i)MBANP, just as in (-)MBANP, as a result of strong intrarnolecular charge transfer from the

amino group to the nitro group. Figure 2 shows the crystal structures of (~)MBANP

and (-)MBANP. Only four molecules of the (O, O, O)+ set are shown in Fig . 2(a) for simplicity. The other four molecules of the (O, 1/2, 1/2)+ set are located at the equiv-

alent positions of the displayed molecules translated by (b+c)/2.

128

OPTICAL REVIEW Vol. 2, No. 2 (1995)

Table I . Fractional atomic coordinates with estimated standard deviations in parentheses.

Atom x y z O1 02 N1 N2 N3 C1 C2 C3 C4 C5 C6 C7 C8 C9 CIO C11 C12 C13

0.2281(2)

0.1410(2)

0.2074(2)

0.3989(1)

0.4353(1)

0.2654(2)

0.3433(2)

0.2988(2)

0.2430(2)

0.3771(2)

0.4238(2)

0.4967(2)

0.4064(2)

0.3516(2)

0.3371(2)

0.3781(3)

0.4317(2)

0.4472(2)

0.1740(1)

0.1116(2)

0.1220(2)

0.0341(1)

-0.0763(1) 0.0685(2)

0.0807(2)

-0.0436(2) 0.0052(2)

-0.0289(1) -0.1478(2) -0.1928(2) -0.1464(2) -0.1927(2) -0.1957(2) -0.1512(2) -0.1047(2) -0.1023(2)

0.2353(6)

0.3582(8)

0.3154(6)

0.3645(4)

0.4625(4)

0.3609(6)

0.3281(5)

0.4778(5)

0.4387(5)

0.4364(4)

0.5336(4)

0.4977(5)

0.7197(4)

0.7866(5)

0.9573(5)

1.0633(5)

1.0035(5)

0.8299(5)

(a) 02

Ol

N1 Cl

C3 C2

C4 N2 C5

C9 N3 CIO C8 C6

(b)

C1 1 C13 C7 C12

Fig. 1. Perspective views of the MBANP molecules. (a) shows the molecular structure of the S isomer in (1;1)MBANP crystal with the

numbering scheme, and the molecule in (-)MBANP crystal is shown in (b) for comparison.

Molecular alignment in (~)MBANP is quite different frorn that in (-)MBANP. Molecules in (~)MBANP are arranged so that the intramolecular charge-transfer axis of each molecule makes a relatively large angle of 102.2' with

the polar c axis. The nearly antiparallel alignment of the charge-transfer axes leads to considerable cancellation of the molecular nonlinearities and, consequently, relatively small nonlinear optical coeflicients. The antiparallel align-

ment is probably a common tendency in racemic crystals. On the other hand, in (-)MBANP, the charge-transfer axes are strongly aligned along the polar b axis resulting in

the very large nonlinear optical coef~cient d22'4)

3. Linear Optical Properties

The optical transmission spectra have been measured using a 0.6-mm-thick b-plate (i)MBANP crystal with flat cleaved surfaces. Polarized transmission spectra in the visible region are shown in Fig. 3. The absorption cutoff in

the visible range is 0.44 fecm for the polarization along the

a axis and 0.43 Jccm for the polarization along the c axis.

T. KoNDO et al. 129

(a)

O

(b)

O

b

a

Fig. 2. Crystal structures of (i)MBANP (a) and (-)MBANP (b). For (~)MBANP, only four molecules are shown and the other four in the unit cell are omitted for simplicity.

1 oo

_ 80 ~~OO

~ (D o 60 c ce ~!

E 40 a) c: cr;

-F 20

o o

/

l

/

,

/

f

l

/

/

/ /

(=)MBANP E ll c

(~)MBANP E // a

---- (-)MBANP E // b

.4 0.5

Wavelength o .6

(um)

Fig. 3. Polarized transmission spectra of a b-plate of (~)MBANP crystal (0.6 mm thick). Optical transmission spectrum of an a-plate (-)MBANP crystal for light polarized along the b axis is shown by the dashed line.

These are blue-shifted by 30-40 nm compared with that of (-)MBANP; this arises from the difference in intermole-cular interactions.

Since the crystal class is orthorhombic, (~)MBANP is optically biaxial and three principal dielectric axes are parallel to the crystallographic axes. The principal re-fractive indices were measured in the following way. First,

the refractive indices n* and n. were determined over the

wavelength range of 0.47-0.70 pm by the immersion method using thin b-plate crystals. Then, n~ and n. at 1 064 ,clm and nb at 1.064 pm and 0.532 Jclm were deter-mined from the periods of the Maker-fringe patterns. Figure 4 shows the dispersion of the refractive indices thus

130 OPTICAL REVIEW Vol. 2, No. 2 (1995)

1 ,9

>< 1,8 o ID c 0>_ 1.7

~= o cu L H-o O: 1.6

o

na

o nb

nc

1 .5

0.4 o 5 o 6 o 7 o 8 o 9 1 .o 1 Wavelength (um)

.1

Fig. 4. Dispersion of the refractive indices, na (open squares), nb (closed circles), and n. (open circles), of (~)MBANP crystal. The solid lines represent the least-squares fit to Sellmeir formula.

determined. The measured dispersion of the refractive indices is fitted with the Sellmeir formulas:

0.550 n~2=2.260+ 1-(0.339/~)2 ' (1)

O . 488 n.2=1.998 + 1-(0.265/~)2 ' (2)

where ~ is the wavelength in microns.

4. Nonlinear Optical Properties

Nonlinear optical coefficients of (dl)MBANP have been determined by the Maker-fringe method using a Q-switch-ed Nd:YAG Iaser (~=1.064 pm). Quartz (dll) was used as a standard for the relative SHG measurements. Since (i)MBANP belongs to mm2 point group, the second-harmonic polarization is given by

E*2

P~1 O O O O dl50 Pyl~:e O O O d240 O

JJJ Pz/ a31 u32 ce33 O O O

Ey2 Ezz

2EyEz 2EZEX 2E*E y

, (3)

where dij's are the nonlinear optical coeflicients and E*, Ey, and E are the electnc field components of the funda-mental wave along the crystallographic a, b, and c axis, respectively. From the Maker-fringe patterns shown in Fig. 5, the relative values of the d31' d32' and d33 compo-

nents have been determined:

d31((~)MBANP)=+22.8 x dll I (quartz) , d32((~)MBANP)=+15.8 x dll I (quartz) , (4) d33((i)MBANP)=+2.8 x dll I (quartz) .

Using the standard value, dll(quartz)=0.30 pm/V,7) the following results have been obtained:

d31((~!~)MBANP)=+6.8i0.7 pm/V , d32((~)MBANP)=+4.7i0.5 pm/V , (5) d33((i)MBANP)=+0.84l~0.08 pm/V .

As expected from its crystal structure, the d values of (i)MBANP are not so large as those of other highly

T. KONDO et al.

1

~ ~ 0.5 ccf

= (T

o ~ o O ~ 30 cls

o * ~ ~ 20 co c:

o - 10 c:

o Z: o o E * 15 c~S

I ~ c ~O o o o (/) 5

( a) ~l

~~!

e

~!D D~!~ D

~1 Q:]

(b)

(c)

O 20 -60 40 External Angle (degree)

Fig. 5. Maker fringes for a 0.6-mm-thick (~)MBANP b-plate crys-tal. Second-harmonic power is relative to that of dl I of quartz. Open

squares are experimental data, and the solid lines represent theoreti-cal fitting. Experimental configurations are: (a) Rotation axis parallel

to c with s-polarized fundamental and second-harmonic light; (b) Rotation axis parallel to a with s-polarized fundamental and p-polarized second-harmonic light; (c) Rotation axis parallel to c with

p-polarized fundamental and p-polarized second-harmonic light.

nonlinear organic crystals including (-)MBANP. And, moreover, the dispersion and birefringence do not allow one to achieve phase matching for any nonzero compo-nent of the nonlinear optical coefficient. These properties

make (i)MBANP crystals of limited use in practical frequency conversion devices.

On the other hand, it is very interesting to compare the obtained d values with those of the enantiomorph (-)MBANP. Since the molecular structures are almost identical in (~)MBANP and (-)MBANP crystals, a com-parison between the d tensor cornponents of these two crystals should give important information on the influ-ence of intermolecular interaction on the nonlinear opti-

cal properties. For (-)MBANP,4) we have performed a theoretical calculation of the nonlinear optical coefflcients

based on the oriented-gas model8) combined with the molecular hyperpolarizability calculated by the semiernpi-rical method. The predicted nonlinear optical coefflcients

closely agreed with the experimental result, indicating the validity of the oriented-gas model that neglects all the intermolecular interaction contribution to the non-linearity.

OPTICAL REVIEW Vol. 2, No. 2 (1995)

Here, we reexamine the validity of the oriented-gas model L0r (i)MBANP and (-)MBANP on the basis of a simple structural consideration assuming the one-dimen-sional character 0L the molecular hyperpolarizability, i.e.,

the hyperpolarizability tensor be dominated by only one component fi along the axis of charge transfer from the nitrogen of the amino group (N3) to the nitrogen of the nitro group (Nl). Then, the nonlinear optical coeflicients

can be expressed by

d.. =Nf.2~fj"f,"fi cose cose cosek , (6) ,, J* ,

where N is the number of molecules per unit volume, f is the Lorentz local field factor for crystal axis l, and 6t is

the angle between the crystallographic I axis and the charge-transfer axis. For (~)MBANP, the angles are:

e*((~!~)MBANP)=135.2' ,

6y((i)MBANP)=47.8' , (7) 6.((~)MBANP)=102.2' .

The angle in (-)MBANP is

ey((-)MBANP)=33.2' . (8) This simple model calculation yields

d31((~)MBANP)/d22((-)MBANP)=-0.15 , d32((~)MBANP)/d22((-)MBANP)=-0.12 , (9) d33((~)MBANP)/d22((-)MBANP)= -0.012 .

These results are in good agreement with the experimental ratios obtained using d22((-)MBANP)=-36 pm/V,4)

d31((~)MBANP)/d22((-)MBANP)=-0.19 , d32((I~)MBANP)/d22((-)MBANP)= -0.13 , (10) d33((~!~)MBANP)/d22((-)MBANP)=-0.023 .

Considerable discrepancy for d33((~)MBANP) is probably due to contributions from off-diagonal components of the molecular hyperpolarizability tensor which will be non-negligible for such a small cornponent of the nonlinear optical coefiicients. The quite good agreement between the calculated and measured ratios indicates the validity of the

oriented-gas model that neglects the intermolecular inter-action contribution to the quadratic nonlinearities in both

two crystals composed of MBANP molecules. This will probably be true in many other molecular crystals as well.

T. KONDO et al. 131

5. Conclusions

Linear and nonlinear optical properties 0L racemic (~)MBANP single crystals have been comprehensively investigated and compared with those 0L the enantio-morph (-)MBANP crystals. Large single crystals with good optical quality and high chemical and physical stabil-

ity can be easily grown by the slow evaporation method. The eight MBANP molecules in a unit cell are arranged with their intramolecular charge transfer nearly antipar-allel with one another resulting in quite small nonlinear optical coefficients compared with those of the enantio-morph (-)MBANP with strong alignment 0L the charge-transfer axes along the polar axis. A comparison between the nonlinear optical coefEicients of (~)MBANP and (=)MBANP has demonstrated that the oriented-gas model, which neglects all the contributions from inter-molecular interaction, is very useful in predicting the nonlinear optical property of a molecular crystal.

Acknowledgments

This work was partially supported by the Grant-in-Aid for Scientific

Research, #4555012, Lrom the Ministry of Education, Science and Culture.

Ref erences

1) See, for example, D.S. Chemla and J. Zyss eds.: Nonlinear Q~)tical Properties of Organic Molecules and Crystals (Aca-demic Press, Orlando, 1987) Vol. 1.

2) R. Twieg, A. Azema, K. Jain and Y.Y. Cheng: Chem. Phys. Lett. 92 (1982) 208.

3) R.T. Bailey, F.R. Cruickshank, S.M.G. Guthrie, B.J. McArdle, H. Morison, D. Pugh, E.A. Shepherd, J.N. Sherwood, C.S. Yoon, R. Kashyap, B.K. Nayar and K.1. White: Opt. Commun. 65 (1988) 229.

4) T. Kondo, R. Morita, N. Ogasawara, S. Umegaki and R. Ito: Jpn. J. Appl. Phys. 28 (1989) 1622. (Molecular hyperpolar-izabilities and nonlinear optical coefiicients reported in this paper

have been derived assurning dll(quartz)=0.5 pm/V. These val-ues should be rescaled using dll(quartz)=0.3 pm/V.)

5) T. Kondo, N. Ogasawara, R. Ito, K. Ishida, T. Tanase, T. Murata and M. Hidai: Acta Cryst. C44 (1988) 102.

6) Y.Y. Cheng, J.1. Crowley, K. Jain and R.J. Twieg: IBM Tech. Disclosure Bull. 25 (1982) 1625.

7) D.A. Roberts: IEEE J. Quantum Electron. 28 (1992) 2057. 8) J. Zyss and J.L. Oudar: Phys. Rev. A26 (1982) 2028.