Nonlinear Optical Properties of Organic and Polymeric Materials

9 Nonlinear Optical Properties of Polydiacetylenes M. L. SHAND and R. R. CHANCE

Downloaded by NORTH CAROLINA STATE UNIV on May 9, 2015 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0233.ch009

Allied Corporation, Corporate Research and Technology, Morristown,NJ07960

The nonlinear optical properties of novel, soluble polydiacetylenes are reviewed and discussed. The nonlinear optical properties are determined using resonance Raman scattering, coherent anti-stokes Raman scattering and coherent stokes Raman scattering. The two-photon p o l a r i z a b i l i t y is found to be very large in these materials. General aspects of the third-order susceptibility of these materials are also discussed. The linear and nonlinear optical properties of polydiacetylenes have received considerable a t t e n t i o n recently (1-3). This i n t e r e s t i s a t t r i b u t a b l e to the fully-conjugated backbone s t r u c t u r e

which leads to e s s e n t i a l l y one dimensional e l e c t r o n i c p r o p e r t i e s . The acetylene mesomer (A) i s the lowest-energy conformation for the polymer, and numerous x-ray structures have y i e l d e d bonding sequences which are very close to A above (4_) · There i s some evidence for the higher-energy butatriene mesomer (5, 6) in systems, such as the ones we are considering here, where backbone s t r a i n i s introduced due to intramolecular i n t e r a c t i o n s of the substituent groups ( R ) .

0097-6156/83/0233-0187$07.25/0 © 1983 American Chemical Society

In Nonlinear Optical Properties of Organic and Polymeric Materials; Williams, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

188

NONLINEAR OPTICAL PROPERTIES

Extensive e l e c t r o n d e r e a l i z a t i o n along the chain d i r e c t i o n leads to an e l e c t r o n i c t r a n s i t i o n energy for one-photon absorption (E ) of t y p i c a l l y 15 000-16 000 cm" for an unstrained backbone (7) . This value i s very close to that of polyacetylene (or polyene) (HC=CH) which has E ~14 000 cm" ( 8 ) . For both polymers, E v a r i e s with chain length (or conjugation length) i n much the same manner ( 9 ) . This analogy to the polyene system leads to the expectation that the polydiacetylenes might have unusually large third-order susceptibilities ( χ ) as had been demonstrated for polyenes of varying l e n g t h . This expectation was borne out by the work of Sauteret et a l . (1_) who found χ values for p o l y d i a c e t y l e n e c r y s t a l s comparable to those of inorganic semiconductors. T h e i r r e s u l t s suggested a t t r a c t i v e p o s s i b i l i t i e s f o r these one-dimensional materials i n nonlinear o p t i c a l d e v i c e s . A p p l i c a t i o n s i n parametric o s c i l l a t o r s which u t i l i z e three-wave mixing were p a r t i c u l a r l y a t t r a c t i v e . However, three-wave-mixing experiments were unsuccessful due to phase-matching problems (10), as w e l l as n o n l i n e a r absorption problems which Lequime and Hermann (11) a t t r i b u t e d to absorption by photogenerated d e f e c t s . R e c e n t l y , polydiacetylenes have been discovered which are s o l u b l e i n common organic solvents (12). These polymers have urethane s u b s t i t u e n t groups of the form 1

0

1

n

0


Q

0 II

-(CH ) 0C-NH-X 2

m

where m=3 or 4. In the work discussed here X i s -COOC4H9 ; these materials are referred to as mBCMU (BCMU butoxycarbonylmethylurethane). The most i n t e r e s t i n g feature of these polymers i s that the v i s i b l e absorption spectra can be v a r i e d i n a c o n t r o l l e d manner by solvent v a r i a t i o n (see F i g . 1). T h i s phenomenon i s due to the fact that the degree of conjugation (or planarity) of the chains in solution is c o n t r o l l e d by i n t r a m o l e c u l a r hydrogen bonding of the C=0 and N-H f u n c t i o n a l i t i e s on adjacent urethane substituent groups. In good s o l v e n t s , such as C H C I 3 , about one out of every four polymer repeat u n i t s ( r . u . ) i s not hydrogen bonded, which leads to a conjugation length of ~4 repeat u n i t s ( r . u . ) and a large blue s h i f t i n o p t i c a l absorption ( E ~ 21 000 cm"") (12). In poor s o l v e n t s , such as CHCl3-hexane m i x t u r e s , e s s e n t i a l l y every polymer u n i t i s hydrogen bonded, which leads to E ~ 16 000 cm" for 3BCMU and E ~ 19 000 cm" for 4BCMU. The blue s h i f t of the l a t t e r compared to the usual c r y s t a l values i s a t t r i b u t e d to s t r a i n (12, 13). T h i s paper w i l l review the l i n e a r and nonlinear o p t i c a l p r o p e r t i e s of p o l y d i a c e t y l e n e s with an emphasis on our work with the nBCMU polymers. The f o l l o w i n g s e c t i o n w i l l discuss m a t e r i a l 1

0

1

0

1

0


9.

Polydiacetylenes

SHAND AND CHANCE

189


WAVE LENGTH (nm) _

700

7h

650

600

550

500

3BCMU

4BCMU

CHCI /Hexone

CHCIj/Hexane

3

400

450

4BCMU CHCI,

15

16

17

18

19

20

21 3

WAVE NUMBER ( I 0 c m

22 _ l

23

24

25

)

Figure I. Linear absorption spectra for polydiacetylene solutions. The three solutions are referred to in the text as blue, red, and yellow (from left to right in the figure). (Reproduced with permission from Ref. 24. Copyright 1979, American Institute of Physics.)


190


p r o p e r t i e s i n c l u d i n g the conformational t r a n s i t i o n . The next s e c t i o n s include the l i n e a r o p t i c a l p r o p e r t i e s of polydiacetylenes and how they r e l a t e to the m a t e r i a l p r o p e r t i e s , the nonlinear o p t i c a l p r o p e r t i e s and, f i n a l l y , the present understanding of these p r o p e r t i e s .


M a t e r i a l P r o p e r t i e s and L i n e a r O p t i c a l P r o p e r t i e s The s o l i d - s t a t e polymerization of diacetylenes i s an example of a l a t t i c e - c o n t r o l l e d s o l i d - s t a t e r e a c t i o n . Polydiacetylenes are synthesized v i a a 1 , 4 - a d d i t i o n r e a c t i o n of monomer c r y s t a l s of the form R - C E C - C E C - R . The polymer backbone has a p l a n a r , f u l l y conjugated s t r u c t u r e . The e l e c t r o n i c s t r u c t u r e i s e s s e n t i a l l y one dimensional with a lowest-energy o p t i c a l t r a n s i t i o n of t y p i c a l l y 16 000 cm" . The polydiacetylenes are unique among organic polymers i n that they may be obtained as large-dimension s i n g l e crystals. S o l u t i o n s of CMU polymers show dramatic c o l o r changes when the solvent/nonsolvent ratio is varied. The color changes have t h e i r o r i g i n s i n a conformational t r a n s i t i o n which can be viewed as a one-dimensional c r y s t a l l i z a t i o n of a s i n g l e chain i n the polymer s o l u t i o n . 3BCMU i s quite soluble i n chloroform (>5% by weight or >0.1 r . u . m o l e / l i t e r ) . The 3BCMU polymer s o l u t i o n , at s u f f i c i e n t d i l u t i o n , i s yellow i n appearance (E ~21 000 c m " ) · On a d d i t i o n of hexane, a dramatic r e v e r s i b l e c o l o r t r a n s i t i o n to a dark blue s o l u t i o n occurs (E ~16 000 cm" ); subsequent a d d i t i o n of hexane r e s u l t s i n the p r e c i p i t a t i o n of the polymer as a blue solid. The blue s o l u t i o n i s a true s o l u t i o n ; i t i s quite s t a b l e and does not p r e c i p i t a t e on standing or during c e n t r i f u g a t i o n . Since the color transition is independent of polymer concentration, it is clearly a single-chain phenomenon. Completely analogous r e s u l t s are obtained for 4BCMU except the t r a n s i t i o n i s from yellow ( E = 21 000 cm" ) to red ( E = 19 000 cm" ) . Absorption spectra for 3BCMU are shown i n Figure 2. The broad s t r u c t u r e l e s s spectra of the yellow s o l u t i o n s ( X C H C 1 3 0 - 8 7 , 1.0) are not unexpected; i n f a c t , the behavior of the l o n g e r - c h a i n polyenes i s quite analogous (14). The o p t i c a l properties of a conjugated system are determined not by the chain length but by the conjugation length λ , i . e . , the length over which p l a n a r i t y of the conjugated u n i t s i s maintained without i n t e r r u p t i o n . Rotation about the s i n g l e bond i n the polydiacetylene backbone would i n t e r r u p t conjugation at a cost of - 1 kcal/mole i n resonance energy (11) - a cost e a s i l y s u p l i e d by the increased entropy of the chain and the bulky, f l e x i b l e side groups. Such r o t a t i o n s would be f a c i l i t a t e d by the i n t e r a c t i o n of the chloroform solvent with the ester and urethane units of the R group, an i n t e r a c t i o n 1

1

0

1

0

1

0

Q

1

=


SHAND AND CHANCE

Polydiacetylenes

WAVELENGTH(nm) 750 700 650 600 Downloaded by NORTH CAROLINA STATE UNIV on May 9, 2015 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0233.ch009

ρ

1

1

1

550

500

450

Τ

I

I

Poly-3BCMU

14

16

18

20

22

3

WAVENUMBER(l0 crrf') Figure 2. Visible absorption spectra of solutions of 3BCMUfor various mole fractions of chloroform in hexane. The spectra are progressively offset by multiples of 5000 L/mol-cm. The polymer concentration is 1.94 * 10~ mol/L. 5




192

which i s undoubtedly a major f a c t o r i n the high s o l u b i l i t y of the CMU polymers. Based on the analogous polyene behavior, we can estimate that the absorption peak corresponds to I « 7 r . u . (or 14 conjugated m u l t i p l e bonds) i n the yellow s o l u t i o n (9, 16). Thus, i n the yellow s o l u t i o n the backbone has acquired a b a s i c a l l y nonplanar conformation. As the nonsolvent hexane i s added the solvent-polymer i n t e r a c t i o n i s decreased - a process which would e v e n t u a l l y lead to precipitation. In this case, however, a conformational t r a n s i t i o n occurs from the nonplanar polymer s t r u c t u r e of the yellow s o l u t i o n to a planar s t r u c t u r e i n the blue s o l u t i o n , g i v i n g rise to the 16 000 cm"! optical transition. This optical t r a n s i t i o n energy i s e s s e n t i a l l y the i n f i n i t e conjugation length l i m i t , since i t i s very c l o s e to that t y p i c a l l y observed for p o l y d i a c e t y l e n e c r y s t a l s . ( 7 ) C l e a r l y , the conjugation lengths are q u i t e long; i i s estimated to be >30 r.u. f o r the blue s o l u t i o n , based on comparison to polyene r e s u l t s (16) · Peak absorption c o e f f i c i e n t s f o r the blue s o l u t i o n are i n good agreement with the polyene r e s u l t s (17) and with r e s u l t s f o r p o l y d i a c e t y l e n e c r y s t a l s (18, 19). It i s important to point out that i n Figure 2, though intermediate spectra (XCHC1Q 0·79, 0.71) are observed i n the transition, there i s no i n d i c a t i o n of intermediate e l e c t r o n i c species. The observed changes are discontinuous ( i n terms of s p e c t r a l s h i f t s ) so that the 16 000 cm" optical transition is already evident at the e a r l i e s t stages of the c o l o r change. This suggests some cooperative behavior as would be expected with nucleation and growth in the proposed one-dimensional c r y s t a l l i z a t i o n process. The stabilization of the planar conformation in these m a t e r i a l s i s due p r i m a r i l y to i n t r a m o l e c u l a r hydrogen bonding between the N-H and C=0 (urethane) groups on adjacent R groups. The resulting polymer conformation i s shown s c h e m a t i c a l l y i n Figure 3. This explanation of the c o l o r t r a n s i t i o n i s c l e a r l y evident from i n f r a r e d spectra (12), which w i l l not be discussed here. Completley analogous results have been obtained for poly-4BCMU, though changes i n the v i s i b l e and i n f r a r e d s p e c t r a are c o n s i d e r a b l y l e s s dramatic ( 5 ) . For example, the frequency s h i f t i n the v i s i b l e absorption spectrum i s -2400 cm" f o r poly-4BCMU compared to -4400 cm" f o r poly-3BCMu ( F i g . 2 ) . Molecular models sugest that the hydrogen bonds are more e a s i l y formed i n the 3BCMU case. =

1

1

1

Nonlinear O p t i c a l P r o p e r t i e s The aspects of nonlinear optics as applied to p o l y d i a c e t y l e n e s which w i l l be discussed i n t h i s s e c t i o n include



9.

SHAND AND CHANCE

Polydiacetylenes

193

N - H - - - 0 =C

/

N - H - - - 0 =C N - H-

-c = c

- —0 = C

(CH ) 2

0

(CH )

1

2

(CH ) 2

(CH ) 2

(CH > 2

n

;

2

n

c-c=c-c

c-c=c-c

(CHp) 2 n

(CH )

2

n

n

n

C - C E C - C

I 0

1 (CH )

1

0

I

0 0

\

Ν- H —

n

C=0

n

I 0 Η—Ν

C = 0- - - Η - Ν •Η - Ν

-Η- Ν

Figure 3. CMU polymer in planar (hydrogen bonded) configuration. The structure shown is that suggested by molecular models of3BCMU where η = 3 and Z= -C H», i.e. the wavy line in the figure at the end of the substituent group is -CH COOC H . (Reproduced with permission from Ref. 12. Copyright 1979, American Institute of Physics.) 4

2


4

ç

N O N L I N E A R O P T I C A L PROPERTIES


194

Raman s c a t t e r i n g (RS) and resonance Raman s c a t t e r i n g (RRS), which show c o n c l u s i v e l y that the broad absorption l i n e shape i s due to a d i s t r i b u t i o n of chains of varying conjugation lengths rather than a homgeneously broadened l i n e width, and three wave mixing (TWM) which shows that the large χ ( 3 ) i n polydiacetylenes i s dominated by a s t r o n g l y allowed two-photon t r a n s i t i o n . We begin with a b r i e f mention of the experiments of Sauteret et al. (_1) who measured third harmonic generation in p o l y d i a c e t y l e n e c r y s t a l s and found χ ( ^ ) values comparable to those of inorganic semiconductors. They observed a 1000 fold enhancement of χ ( ^ ) i n the polymer compared to the monomer and an a n i s o t r o p y of at l e a s t 100, with χ ( ^ ) l a r g e s t i n the polymer chain direction. X^^ was measured for two polymers and at two i n c i d e n t frequencies, 3820 cm"" and 5290 c m " . The l a r g e s t value of χ ( 3 ) (~ i C f i O esu) was obtained for the p o l y d i a c e t y l e n e PTS [ R i s -CH2SO3C6H4CH3] at 5290 cm" . The h y p e r p o l a r i z a b i l i t y ( γ = χ / Ν i g n o r i n g l o c a l f i e l d e f f e c t s ) i s then about 5 χ 10~^ esu i n the polymer chain d i r e c t i o n . Note that 3ω (15 870 cm" ) i s quite near the band edge of PTS (16 200 cm" ) and resonance e f f e c t s are undoubtedly important. In f a c t , with 3820 cm" excitation, χ(3) i found to be about f i v e times l e s s . S i m i l a r l y i n another polymer with a higher band gap, X < values were s i g n i f i c a n t l y lower. This work opened up a t t r a c t i v e possibilities for p o l y d i a c e t y l e n e as nonlinear o p t i c a l materials ( 1 , 20). 1

1

1

2

1

1

1

s

3 )

Raman S c a t t e r i n g . The RS and RRS (21, 22) measurements a c t u a l l y were done a f t e r the TWM (23-25) , but w i l l be described f i r s t because the TWM a n a l y s i s used important assumptions later confirmed by the RRS work. The RRS process enhances the ordinary RS process when the i n c i d e n t l i g h t energy (ω^) or the scattered l i g h t energy ( ω ) i s near resonance with e l e c t r o n i c states i n the s c a t t e r i n g medium. In a sample with s e v e r a l components, RRS i s s e l e c t i v e , enhancing only those chromophores which have e l e c t r o n i c s t a t e s near or ω . In t h i s study i s s h i f t e d throughout the absorption band of polydiacetylene solutions to test the hypothesis that these s o l u t i o n s are composed of a number of chromophoric components each of which has d i f f e r e n t electronic s t a t e s c o n t r i b u t i n g to d i f f e r e n t regions of the absorption band. As 0)£ i s s h i f t e d the RS from d i f f e r e n t components w i l l be enhanced and the observed v i b r a t i o n a l frequencies w i l l be those of the photoselected component. The e l e c t r o n i c p r o p e r t i e s and thus the absorption bands of a polymer depend on the conjugation length A . The absorption energy decreases as I increases reaching a l i m i t i n g value of - 16 000 cm" as I approaches i n f i n i t y . If the polymer s o l u t i o n c o n s i s t s of a d i s t r i b u t i o n of conjugation lengths (i.e. chromophores), photoselection of the different chromophores within the 8

8

1


9.

SHAND AND CHANCE

195

Polydiacetylenes

inhomogeneously broadened absorption band should be p o s s i b l e with RRS. The Raman frequencies for backbone carbons are expected to vary with I for b a s i c a l l y the same reason that o p t i c a l absorption v a r i e s with £ , i . e . , e l e c t r o n d e r e a l i z a t i o n . In p a r t i c u l a r , as e l e c t r o n d e r e a l i z a t i o n increases with i n c r e a s i n g i, the C=C bonds and the C^C bonds become weaker and the Raman a c t i v e v i b r a t i o n a l frequencies associated p r i m a r i l y with these bonds, v C C and vC=C, will decrease. As u s u a l , i n the following, the 3BCMU i n CHCl3-hexane i s r e f e r r e d to as the "blue s o l u t i o n " and 3BCMU i n CHCL3 i s r e f e r r e d to as the "yellow s o l u t i o n " . An example of a RRS spectrum from the yellow s o l u t i o n with λ^=488 nm (a)j=20 492 c m ) i s shown i n F i g . 4. A l s o shown are s p e c t r a i n the v C C r e g i o n s . The Raman modes of i n t e r e s t are the 667 c m CHCI3 peak which i s used for polymer mode n o r m a l i z a t i o n , vC C at 1520 cm" and v = at 2120 cm" . A n a l y s i s of the peaks i n the yellow s o l u t i o n i s s t r a i g h t f o r w a r d because the Raman bands do not o v e r l a p . Raman spectra of v^ ^ i n the blue s o l u t i o n are complicated because of hexane l i n e s at 1450 cm"" and by modes which do not s h i f t with ω-£. Peak i n t e n s i t i e s can be i n f e r r e d by comparison to the nonresonant hexane s i g n a l near 1450 cm"" which shows only the u s u a l nonresonant ω - ^ v a r i a t i o n . A l i n e at 1460 c m i s present which does not s h i f t with i n c i d e n t frequency. v^=C f 3BCMU polymer s i n g l e c r y s t a l i s 1460 c m " . Thus, for greater than approximately 540 nm the RS i n d i c a t e s an absorption spectrum with chains having a s i n g l e v ^ Raman frequency - the frequency value i n d i c a t i n g an e f f e c t i v e l y i n f i n i t e conjugation l e n g t h , as does the E v a l u e . The v a r i a t i o n of v^-C i s shown i n F i g . 5 for both the yellow and blue s o l u t i o n s . For comparison the absorption spectrum of each s o l u t i o n i s a l s o shown i n F i g . 5. In the yellow s o l u t i o n at small ω-£ a l l chromophores are i n preresonant conditions and a l l c o n t r i b u t e to the Raman l i n e , which i s broadened (width l a r g e r than 30 cm"" ) and peaks at an average value of ^1520 cm" . At lowest energy and f a r t h e s t from resonance the Raman l i n e is asymmetrically broadened as expected ( F i g 4 ) . As i s increased, approaching the absorption band, chromophores with l a r g e s t I reach resonant conditions, their contribution to the Raman line dominates, and the peak frequency decreases to the value c h a r a c t e r i s t i c of these c h a i n s . As ω-£ i s increased f u r t h e r through the absorption band, chrompohores of s u c c e s s i v e l y smaller average i values are i n resonance (photoselected) and dominate the spectrum. Thus, the peak frequency increases with ω-£. In a l l RRS s p e c t r a the l i n e w i d t h i s nearly constant at 26 cm" , decreasing only s l i g h t l y with i n c r e a s i n g ω^. The blue s o l u t i o n r e s u l t s are s i m i l a r i n the high energy region though the short chain component to these s o l u t i o n s i s c l e a r l y l e s s than i n the yellow s o l u t i o n .


=

-1

=

- 1

=

1

c

c

1

=

1

1

- 1

o r

1

= c

Q

1

1

1


196



τ

Γ

—X~=458nm 1

1400

I

ι

I

ce

1500 1600

ι

•

I

2000 2100

R a m a n Shift

.

I

ι

2200

(cm)

Figure 4. Raman spectra of the yellow solution. Key: top, entire spectrum with λ, = 488 nm; bottom, spectra of the v ~ and v regions with λ, = 458 nm (-)and λ, - 556 nm (—) and v ~ regions with λ, = 752 nm (—). (Reproduced with permission from Ref 21. Copyright 1982, American Institute of Physics.) c

c

c

c=c

c


9.

SHAND AND CHANCE

Polydiacetylenes


CONJUGATION

14

16

18

LENGTH

20

ω ( 1 0

3

197

22

α η

1

24

26

)

Figure 5. Absorption spectra and Raman data for 3BCMU yellow and blue solutions. The data points indicate the ω, dependence of v . The conjugation length scale is an estimate ofthe length ofa chromophore with absorptionfrequency ω. (Reproduced with permission from Ref 21. Copyright 1982, American Institute of Physics.) c=c




198

A model has been given (25) which attempts to quantify the q u a l i t a t i v e i n t e r p r e t a t i o n given above. As p r e v i u o s l y discussed the results are described in terms of photoselection of chromophores ( i . e . , conjugated units of length I r . u . ) . The f i r s t step i n t h i s a n a l y s i s i s to model the d i s t r i b u t i o n of chain lengths using the absorption spectrum, which is shown i n F i g . 6 ( s o l i d curve) for the yellow s o l u t i o n . The absorption peak for t h i s s o l u t i o n (21 000 c n f l ) corresponds to £ - 6 - 7 u n i t s . However, i n f r a r e d studies of side group i n t e r a c t i o n s c l e a r l y i n d i c a t e an average I of 3-4 u n i t s . We a t t r i b u t e t h i s d i f f e r e n c e to an i n c r e a s e i n the e l e c t r o n i c linewidth with decreasing I. This increase can be expected due to, for example, l i f e t i m e shortening because of i n t r a c h a i n , nonradiative energy t r a n s f e r from short to long chain segments. A model based on these ideas and a Gaussian d i s t r i b u t i o n of chain lengths gives the r e s u l t s shown i n F i g . 6. There are two free parameters i n t h i s f i t . The Raman modes v ^ C and ν ^ = £ depend on chain length i n a manner which p a r a l l e l s the absorption energy dependence on chain length (25) . The c o n t r i b u t i o n of each chromophore to the observed Raman peak i s determined by the RS i n t e n s i t y for i n c i d e n t l i g h t at energy ω^. The RS peak p o s i t i o n i s then a weighted average of a l l the Raman modes of a l l chromophores. The f i t to the data with the model (same as i n F i g . 6) i s shown i n F i g . 7 for v and for C=C i the yellow s o l u t i o n . In both cases the experimental data are quite w e l l represented considering that there i s only one parameter i n the f i t . The model also adequately describes the Raman e x c i t a t i o n p r o f i l e . The good agreement between the model and the experimental r e s u l t s confirms the basic premise of the model that the i n d i v i d u a l polymer chains i n s o l u t i o n contain a d i s t r i b u t i o n of chain lengths which determine the absorption c h a r a c t e r i s t i c s of the s o l u t i o n s . We should note i n passing that the above work on Raman photoselection i s d i r e c t l y relevant to the current controversy regarding similar solid-state experiments on polyacetylene, -(HC=CH) -. The l i n e a r o p t i c a l p r o p e r t i e s of polyacetylene and p o l y d i a c e t y l e n e are very s i m i l a r and the close analogy between the two systems has been emphasized i n previous work ( 9 ) · Kuzmany et a l . (26) observe e f f e c t s i n Raman s p e c t r a of polyacetylene which are quite s i m i l a r to those seen i n our blue s o l u t i o n s and apply basically the same i n t e r p r e t a t i o n , i.e., conjugation length dispersion. Mele (27) and Lauchlan et a l . (28) argue against t h i s i n t e r p r e t a t i o n and suggest a model based on hot luminescence. The l a t t e r model i s not c o n s i s t e n t with our yellow s o l u t i o n d a t a . T h e r e f o r e , the i n t e r n a l consistency between our yellow and blue solution data and the similarity of the latter to the polyacetylene case argues heavily i n favor of the conjugationl e n g t h d i s p e r s i o n model for p o l y a c e t y l e n e . =

c = c

V

n

n



SHAND AND

Polydiacetylenes

CHANCE

ω (ΙΟ

3

cm')

Figure 6. Comparison of experimental (—) and theoretical (—) absorption spectrafor the yellow solution. Peaks in the theoretical curve are due to contributionsfrom chains of length 1 and are labeled with the appropriate 1. (Reproduced with permission from Ref. 21. Copyright 1982, American institute of Physics.)

^ 1

1

2l25h

2120

V

• Yellow Solution

2115

Έ 1525 1520 1515 1510 1505 12

ι

ι

ι

I

14

16

18

20

ω (ΙΟ

3

22

cm"') c

c

Figure 7. Experiment and theoryfor the excitation energy dependence of ν ^ and ν (Reproduced with permission from Ref 21. Copyright 1982, American institute of Physics.)


c = c

.


200

In the next s e c t i o n , we w i l l discuss three wave mixing experiments (two-photon absorption) i n these s o l u t i o n s . The Raman r e s u l t s provide important background information f o r two reasons: 1) the conjugation-length d i s p e r s i o n model i s confirmed by the Raman data and suggests a s i m i l a r d i s p e r s i o n i n the energies of two-photon a c c e s s i b l e states and 2) the Raman r e s u l t s demonstarte t h a t , over the pre-resonance frequency range of our three wave mixing experiments, the l a s e r "sees" the ensemble average of the conjugation length d i s t r i b u t i o n . The l a t t e r i s r e f l e c t e d i n the average value of ν ^ = 0 d C=C observed up to - 17 500 cm""l i n the yellow s o l u t i o n . Downloaded by NORTH CAROLINA STATE UNIV on May 9, 2015 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0233.ch009

a n

V

Three Wave M i x i n g . A number of resonances are p o s s i b l e i n the third-order susceptibility, χ ( 3 ). These resonances may be due to phonons or vibronic excitation such as in CARS (Coherent A n t i - S t o k e s Raman S c a t t e r i n g ) , e l e c t r o n i c states a c c e s s i b l e to one or two photons ( t y p i c a l l y i n t h i r d harmonic g e n e r a t i o n ) , or any other e x c i t a t i o n i n the nonlinear m a t e r i a l . In g e n e r a l , χ (3) represents "four-photon" or "four-wave" processes which have been c a l l e d four-wave mixing (FWM). The process described here i n which only two input beams are present (one beam i s u t i l i z e d twice i n each process) has been c a l l e d three-wave mixing (TWM) (29) as w e l l as FWM. The t r a d i t i o n a l TWM w i l l be used i n t h i s paper. TWM i s accomplished by focusing two l a s e r beams with frequencies and u)2 i n the polymer s o l u t i o n . Incident l a s e r beams are generated by two dye l a s e r s pumped by a s i n g l e N2 l a s e r . Wavelengths for the i n c i d e n t beams ranged from 760 nm to 590 nm (or to the region of large a b s o r p t i o n ) . The beams are focused i n t o the sample with a 15-cm f o c a l length l e n s . The c r o s s i n g angle between the two beams i s adjusted f o r phase matching (approximately 4° outside the sample c e l l ) . The sample s o l u t i o n s are held i n a 1-cm c u v e t t e . The beam generated at 0)3 = 2ω^ -0)2 i s f i l t e r e d with a double monochromator and i t s i n t e n s i t y measured with a p h o t o m u l t i p l i e r and boxcar e l e c t r o n i c s . Our a n a l y s i s u t i l i z e s the measurement of the 0)3 i n t e n s i t y as a f u n c t i o n of polymer concentration to determine the r e a l and imaginary parts of the two-photon h y p e r p o l a r i z a b i l i t y (24, 30). The 0)3 i n t e n s i t y of the solvent serves as an i n t e r n a l i n t e n s i t y r e f e r e n c e . The f i r s t discussion of TWM experiments will be of the two photon absorptions. Finally, the Raman resonances i n χ ( ^ ) w i l l be discussed. In order to extend the range of 2ω^ p o s s i b l e with a l i m i t e d range of l a s e r e x c i t a t i o n , both CARS (Coherent Anti-Stokes Raman S c a t t e r i n g ) and CSRS (Coherent Stokes Raman S c a t t e r i n g ) are used. In both cases 0)3 = 2u)j - ω 2 · In the CARS mode 01)3 > > ω 2 ; i n the CSRS mode 0)2 > > 003. One-photon resonance e f f e c t s are the same i n both cases as described l a t e r . Phase matching i s also the same i n both cases with £ 3 = 2Έ\ -^2 ·



9.

SHAND AND CHANCE

Polydiacetylenes

201

L i n e a r absorption spectra for the three s o l u t i o n s considered h e r e i n are given i n F i g . 1. The concentration range a v a i l a b l e i n the yellow solution is 0-120 χ 1 0 ^ r . u . / c m ^ . For the other s o l u t i o n s an upper l i m i t i s imposed on the concentration range due to polymer p r e c i p i t a t i o n : ~4 χ 1 0 ^ r . u . / c m ^ for the red soution and ~9 χ 10*' r . u . / c m ^ for the blue s o l u t i o n . The two-photon h y p e r p o l a r i z a b i l i t y i s determined from the v a r i a t i o n of the 0)3 i n t e n s i t y with polymer c o n c e n t r a t i o n . This c o n c e n t r a t i o n dependence i s determined most a c c u r a t e l y for the yellow s o l u t i o n , since the concentration range i s l a r g e s t i n that case. In a d d i t i o n , the low-energy t a i l of the l i n e a r absorption (which corresponds to the high-frequency cut off of our experimental range) i s at higher energies i n the yellow s o l u t i o n than i n the blue or red s o l u t i o n s . Therefore, the yellow s o l u t i o n s y i e l d the more extensive data sets and these r e s u l t s are presented and discussed i n d e t a i l here. Six samples of yellow s o l u t i o n were prepared with various c o n c e n t r a t i o n up to 1.2 χ 1 0 ^ r . u . / c m ^ . At higher concentrations and i n regions of l i n e a r absorption the phase-matching c o n d i t i o n s depend on c o n c e n t r a t i o n . These higher concentrations are not used in order to avoid changing the phase-matching angle during a s e r i e s of measurements. The t h i r d - o r d e r nonlinear mixing i s determined i n both the CARS and CSRS geometries, ωι - u)2 i s tuned away from any Raman resonances at about 1300 c m * . The output i n t e n s i t y i s measured for each sample and for the solvent without polymer (pure chloroform for the yellow s o l u t i o n ) . The t o t a l susceptibility is -

Xtot - Xs + Xp + XT = Xs + Ν γ + « Y V ^ ' T )

Χ

( )

ρ

where χ i s the solvent s u s c e p t i b i l i t y (assumed to be p o s i t i v e and r e a l ) , χρ i s the polymer nonresonant s u s c e p t i b i l i t y , χ τ i s the two-photon s u s c e p t i b i l i t y , Ν i s the polymer c o n c e n t r a t i o n , γ ρ i s the polymer nonresonant h y p e r p o l a r i z a b i l i t y , and γ-p = y'lj+ly''^ is the two-photon h y p e r p o l a r i z a b i l i t y . (Note that we take γ = χ / Ν , ignoring l o c a l - f i e l d effects (24)). The (1)3 i n t e n s i t y I is p r o p o r t i o n a l to |χ|2 so that 8

I

S +

P _

2

\xmr\

1

,

2NR ,

N2( 2 + R

Ύ Τ

s +

"2)

(

s

2

)

where I P i s the (1)3 i n t e n s i t y for the polymer s o l u t i o n , I i s the 0)3 i n t e n s i t y for the pure s o l v e n t , and R^Yp+Y'f. I t has been shown p r e v i o u s l y that the c o n t r i b u t i o n of γ to R i s very small over our frequency range (23-25). T h e r e f o r e , R can be i d e n t i f i e d ρ


202


as the r e a l part of the two-photon h y p e r p o l a r i z a b i l i t y γ ' τ .

Note

t h a t , because of the i n t e r f e r e n c e of terms i n |xtot I > both the magnitude and sign of γ ' χ i s determined. Furthermore, we emphasize again that the solvent acts as an i n t e r n a l reference so that the r e s u l t s do not depend on measurement of absolute l i g h t intensities. I P/I as a f u n c t i o n of Ν i s shown i n F i g . 8 for two values of ω ι . The closed and open c i r c l e s are the data with 2u>i = 32 362 cnfl and = 26 554 cm""", r e s p e c t i v e l y . For the former, the i n t e n s i t y decreases with i n c r e a s i n g Ν for small N, which i n d i c a t e s γ < 0 for 2d)! = 32 362 cm" ; for the l a t t e r , the i n t e n s i t y increases with i n c r e a s i n g Ν i n d i c a t i n g γ χ > 0 for 2ω^ = 26 554 cm' . Thus the energy of the two-photon s t a t e , determined by γ ' χ = 0 , i s between these two values of 2u>\. The s o l i d l i n e s i n F i g . 8 are obtained from Eq.(2) with Y'T/XS and γ''τ/Xs determined from a l i n e a r regression a n a l y s i s . The χ values are taken from Ref. 31: χ = 0 . 8 7 x 1 0 ~ esu for CHCI3 and χ = 1 . 0 6 x 1 0 " esu for hexane. There are no two-photon resonances i n the solvents i n the frequency range of i n t e r e s t here (31). The r e s u l t i n g values of γ'τ and γ"τ> with 90% confidence l i m i t s expressed as e r r o r b a r s , are shown i n F i g . 9. Each data p a i r i s determined from the concentration dependence of I S + P / J S value of in either a CARS or a CSRS c o n f i g u r a t i o n . The s o l i d l i n e s i n F i g . 9 are a least-squares f i t of the r e a l and imaginary parts to the t h e o r e t i c a l form of γ χ (21): s +

s

1

!

1


τ

!

1

8

14

14

8

8

a

γ

Χ

t

o

n

e

Ξ Α/(-2ω -ίΓ £) β

1

(3)

β

where A -

3

f »

f

g

i

f

"

16hm'Ka)gi>

1

f

\ I

1

1 +

\ < ω ι > - ω ^ \gi>-u>2 β

J

~^3

J

where u) f and T f are the energy and width of the s t a t e a c c e s s i b l e to two-photon e x c i t a t i o n (with only one such state assumed for an i n d i v i d u a l chromophore). The ground s t a t e i s represented by g and the f i n a l (two-photon) state by f. The bracketed frequencies i n Eqs. 3 and 4 represent averages over the ensemble of chromophores i n t e r a c t i n g with the photon f i e l d s . The terms i n parentheses i n E q . 4 represent the one-photon resonance enhancement to the twophoton absorption term i n χ-ρ. I t i s assumed that one state ( l a b e l e d i ) with energy Mg± and width Tg± i s responsible for these resonances. In general a summation over a l l e l e c t r o n i c states and a l l conjugation lengths would be r e q u i r e d . However, our l a s e r s do not come i n resonance with any of the a v a i l a b l e one photon states and we have demonstrated the appropriateness of t h i s averaging g

g



9.

SHAND AND CHANCE

203

Polydiacetylenes

25

50 N(IO r.u./cm ) ,7

3

Figure 8. Variation in the normalized ω output intensity with polymer concentration for the yellow solution at two incident ω values. The solid lines represent theoreticalfits to the data according to Equation 2. (Reproduced with permission from Ref. 25. Copyright 1980, American institute of Physics.) 3

ι



204


~i

3h

1

1

1

1

1

1

I inear abs. peak · 2 1 3 0 0 c m "

1. ι ι ι

Imaginary Part

2h

Φ

ro to

Η

1

1-

Real Part Theory

—

= 30500 cm' = 4 6 0 0 cm"

1

1

-3

26

27

28

29

30

32

33

2ω,(l0 cm~ ) , ^

,

Figure 9. Variation of the real (m) and imaginary (o)parts of the two photon hyperpolar izability (y ) with two-photon energy (2ω^ for the yellow solution. The error bars represent 90% confidence limits. All data are taken in a 1111 geometry (all beams polarized \\). The solid lines are theoretical fits to the data according to Equation 3. (Reproduced with permission from Ref. 24. Copyright 1979, American Institute of Physics.) T



9.

205

Polydiacetylenes

SHAND AND CHANCE

procedure, as already mentioned, with our Raman r e s u l t s . (The width i s ignored, since u>gi - ω » ?g±*) The f a c t o r Φ accounts for the fact that the TWM s i g n a l i s an average over a l l molecular orientations in solution; Φ varies from 1 for an i s o t r o p i c chromophore to 1/5 for a one-dimensional chromophore (our c a s e ) , fjfc i s the o s c i l l a t o r strength for the t r a n s i t i o n from j to k. The average over the ensemble of chromophores (i.e., chain lengths) has been approximated i n E q . (3) by a L o r e n t z i a n with a width Tgf which represents the inhomogeneous width of the twophoton resonance due to the d i s t r i b u t i o n of chain lengths plus any v i b r a t i o n a l progressions. The frequency and o s c i l l a t o r s t r e n g t h fg^ for the one-photon g+i t r a n s i t i o n are obtained from l i n e a r absorption data such as those i n F i g . 1. Therefore, the only parameters v a r i e d i n the fitting procedure are , r^f (the average frequency and inhomogeneous width of the d i s t r i b u t i o n of states a c c e s s i b l e by two-photon a b s o r p t i o n ) , and fif the oscillator strength for the transition from the intermediate s t a t e to the two-photon f i n a l s t a t e . Polarization measurements and t h e o r e t i c a l c a l c u l a t i o n s show that the s t a t e observed i n l i n e a r absorption measurements i s , as expected, the intermediate s t a t e i n the two-photon resonance. The two-photon state as determined by the f i t to the yellows o l u t i o n data has =30 500 cm" . The e f f e c t of the one-photon resonance enhancement on γ-j can be seen i f we consider the form of the s o l i d curves i n F i g . 9 under conditions that no one-photon resonance i s present. In t h i s case, γ ' χ would be a d i s p e r s i v e type curve with i n v e r s i o n symmetry around the γ ' χ = 0 point at 30 500 cm" and γ " would be an absorptive l i k e curve centered at 30 500 cm" . The large increase i n the magnitudes of γ χ and γ"τ on the high-energy side of the spectrum i s therefore a t t r i b u t e d to one-photon resonance. The peak expected for at 30 500 cm" i s barely d i s c e r n i b l e as a broad shoulder, since i t i s almost completely obscured by the one-photon resonance. The strong one-photon absorption i s observed for p o l a r i z a i o n p a r a l l e l to the chain axis ( 7 ) . Our p o l a r i z a t i o n studies of the three-wave mixing demonstrate that the two-photon absorption observed i n these experiments i s also p o l a r i z e d along the chain axis. S i m i l a r measurements and a n a l y s i s have been c a r r i e d out for the red s o l u t i o n . In t h i s case the concentration of polymer i n the chloroform-hexane solvent i s l i m i t e d to l e s s than 4 χ 1 0 ^ r.u./cm^. Therefore, the e r r o r s for γ ' χ and γ "χ are l a r g e r than i n the case of the yellow s o l u t i o n . In a d d i t i o n , the absorption band of the red s o l u t i o n i s s h i f t e d to the red with respect to that of the yellow s o l u t i o n . Therefore, 2ω^ can only be extended up to 32 000 cm" . The blue s o l u t i o n , 3BCMU i n chloroform and hexane, has 1

g

1

τ

1

?

1

1


206


s i m i l a r l i n e a r o p t i c a l p r o p e r t i e s to the 4BCMU red s o l u t i o n but w i t h a considerable red s h i f t of the l i n e a r absorption spectrum (Fig. 1). T h e r e f o r e , the a c c e s s i b l e range of 2ω\ i s effectively l i m i t e d to a s i n g l e frequency (26 554 cm" ), and we can only make estimates of and T g f . A comparison of the concentration dependence for the yellow s o l u t i o n and the blue s o l u t i o n i n F i g . 10 at the same such that (blue) < 2ω^ < < U ) f > ( y e l l o w ) c l e a r l y shows the e f f e c t of the change of sign of γ ' χ . The values and Tgf determined from the f i t t i n g to γ χ ( 2 ω ι ) from the red and yellow s o l u t i o n s along with estimates f o r the blue s o l u t i o n are shown i n Table I . The r e s u l t s for fg-^ determined from l i n e a r absorption data and f^f determined from the γ-ρ data for a l l three s o l u t i o n s are also included i n Table I . Note that these o s c i l l a t o r strengths correspond to the values a p p r o p r i a t e for p o l a r i z a t i o n p a r a l l e l to the chain d i r e c t i o n . Though the restricted data for the blue solution yields s i g n i f i c a n t u n c e r t a i n t i e s i n the and Tgf v a l u e s , f-^f for the blue s o l u t i o n i s reasonably w e l l determined n e v e r t h e l e s s . 1


g

TABLE I . E n e r g i e s , widths, and o s c i l l a t o r strengths for one- and two-photon absorption i n p o l y d i a c e t y l e n e s o l u t i o n s (energies and widths i n c m ) . - 1

Solution

Yellow Red Blue

21 300 18 900 15 900

a

30 500 28 100 -22 000

B o t h fgj_ and f^f represent o s c i l l a t o r p a r a l l e l to the chain d i r e c t i o n .

r

gf

fif

4600 4000 -3000

strengths

a

2.7 2.5 2.5

for p o l a r i z a t i o n

The consistency of the o s c i l l a t o r strength for a l l three m a t e r i a l s i n d i c a t e s that the only d i f f e r e n c e i n the experimental r e s u l t s for each case are the resonance terms due to changes i n o)gi and and Tgf values correspond to a L o r e n t z i a n d e s c r i p t i o n of the energy d i s t r i b u t i o n



9.

SHAND AND CHANCE

207

Polydiacetylenes

Figure 10. Variation of the normalized ω output intensity (CSRS) vs. polymer concen trationfor the blue solution (φ) and the yellow solution (o)at 2ω - 26,554 cm' . The solid lines are theoreticalfits to the data: y - (0.15 + Ί0.30) 10~ esu for the yellow solution and y - (-5.36 + Ί3.52) * 10~ esu for the blue solution. (Reproduced with permission from Ref 25. Copyright 1982, American institute of Physics.) χ

1

ι

x

33

T

33

T


208


of two-photon a c c e s s i b l e s t a t e s . The inhomogeneous width Tgf i s attributed to a distribution of conjunction lengths plus v i b r a t i o n a l sidebands. It i s reasonable to expect t h i s width to be s i m i l a r to or somewhat greater than the inhomogenous width of the one-photon t r a n s i t i o n (~ 2500 c m ; see F i g . 1). As a consequence of t h i s f i t t i n g procedure, represents an average energy for the two-photon s t a t e and the e l e c t r o n i c o r i g i n (0-0 two-photon t r a n s i t i o n energy) would probably be l e s s than by at most a few thousand wave numbers, i . e . , ~Tgf. This e f f e c t would be most important for the red and blue s o l u t i o n s where a f a i r l y w e l l defined 0-0 t r a n s i t i o n i s evident i n the s o l u t i o n s p e c t r a of F i g . 1. Thus, for the blue s o l u t i o n a near coincidence of u)gi and u)gf i s p o s s i b l e . Based on these r e s u l t s , and on t h e o r e t i c a l work which i s i n r e l a t i v e l y good agreement with experiment (25), we suggest that the large two-photon h y p e r p o l a r i z a b i l i t y i s a general property of polydiacetylenes. The o r i g i n of the large γ-ρ values is a combination of large o s c i l l a t o r strengths and large one-photon resonance e f f e c t s . P o l y d i a c e t y l e n e c r y s t a l s , which have Mg± = 15 000-19 000 cm , should also display large two-photon a b s o r p t i o n TPA. This i s an important point i f these m a t e r i a l s are to be considered f o r use i n nonlinear o p t i c a l devices based on the l a r g e s u s c e p t i b i l i t i e s measured with i n f r a r e d l a s e r s ( 1 ) . There have been two previous measurements of TPA i n p o l y d i a c e t y l e n e crystals. Reimer and B a s s l e r (33) found a two-photon absorption coefficient, 3, of - 5 χ 1 0 ~ c m ^ - s e c / p h o t o n - r . u . at 2ω=18 880 cm" for PTS (R i n p o l y d i a c e t y l e n e s t r u c t u r e i s -CH2SO3C6H4CH3; =16 200 c m " ) . This value i s l a r g e , though not remarkably so and is consistent with our values for γ"·ρ which i s d i r e c t l y p r o p o r t i o n a l to 3· Lequime and Hermann (11) measured TPA over an extended range in two polymers: PTS "arid TCDU [R i s -(CH )4 OCONHC5H5; =18 500 cm""] . They u t i l i z e d a 9400-cm" e x c i t a t i o n beam and a l o w - i n t e n s i t y probe beam. T h e i r r e s u l t s are i n good agreement with the single-frequency result of Ref. 30 and indicate remarkably large 3 v a l u e s , e s p e c i a l l y for PTS. They a t t r i b u t e d t h i s absorption to a two-step process i n v o l v i n g defect formation and defect a b s o r p t i o n . However, we are able to describe t h e i r data quite a c c u r a t e l y based on TPA s o l u t i o n r e s u l t s (25) · We conclude that the data of Lequime of Hermann, i n c l u d i n g the large difference they observed for two c r y s t a l s with d i f f e r e n t u)g£ v a l u e s , are e a s i l y explained by TPA without invoking d e f e c t s . We f u r t h e r conclude that strong TPA i s a fundamental property of the p o l y d i a c e t y l e n e backbone. The large values of the h y p e r p o l a r i z a b i l i t y i n these mole c u l e s imply l a r g e s u s c e p t i b i l i t i e s for more concentrated s o l u t i o n s or s o l i d - s t a t e f i l m s . This large t h i r d - o r d e r s u s c e p t i b i l i t y can


- 1

- 1

48

1

1

2

1

1

g



9.

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209

Polydiacetylenes

be u t i l i z e d as a frequency s h i f t e r , even though χ has i t s o r i g i n i n two-photon absorption ( 3 4 ) . L i n e a r absorption l i m i t s these devices to energies below 600 nm i n 4BCMU, f o r example. This energy range i s appropriate f o r extending the output of the a l e x a n d r i t e l a s e r which produces l i n e emission at 680 nm and tunable emission from 700 to 820 nm ( 3 5 ) . TWM leads to generation of l i g h t with wavelengths as long as 1.1 ym. The e f f i c i e n c y of c o n v e r t i n g 680 nm r a d i a t i o n to the i n f r a r e d f o r an oriented f i l m of 4BCMU i s calculated to be 1-10% depending on i n c i d e n t intensities. TPA l i m i t s the i n t e n s i t y of the i n c i d e n t beams which may be used. Two experiments were attempted with higher density polymer materials. CSRS was observed i n 4BCMU s i n g l e c r y s t a l , - 69 μπι t h i c k , with = 730 nm. Very l i t t l e energy (-1.8 y J) focused on the sample caused visible change; the sample turned r e d , presumably due to melting and conjugation length s h o r t e n i n g . In a second experiment CARS was observed i n a 4BCMU g e l (36) with a d e n s i t y of -10*8 r . u . / c m ^ . The i n c i d e n t beam at = 650 nm was not absorbed and no damage occurred. However, the CARS s i g n a l was s t r o n g l y dependent on p o s i t i o n of focusing i n the sample due to inhomogeneities i n the g e l and was not as large as p r e d i c t e d from s o l u t i o n measurements. T h e r e f o r e , these polymers are not at present p r a c t i c a l parametric o s c i l l a t o r s because of l i m i t a t i o n s of sample q u a l i t y and, p o s s i b l y , of TPA. Resonances i n v ( 3 ) a l s o occur f o r Raman modes. Enhancement of the 0)3 output beam occurs when Δω=ω^-ω2 i s near Raman a c t i v e molecular v i b r a t i o n s as w e l l as when 2ω^ i s near an e l e c t r o n i c excitation. In t h i s case there i s an a d d i t i o n a l term to be added to ( 3 ) , q . ( 1 ) , (23, 37): X

E

— ω -Δω-1Γ ρ

(7) K

U

ρ

where Ν i s the number density of polymer repeat u n i t s i n r . u . / c m ^ , u)p and Γρ are the Raman mode frequency and width i n the polymer. As Δω i s scanned over the polymer v i b r a t i o n at 1518 cm"" , which i s the C=C s t r e t c h mode, the t h i r d - o r d e r mixing s i g n a l v a r i e s as shown i n F i g . 11. The s o l i d curves are the experimental r e s u l t s . The dashed curves are l e a s t square f i t s of the d a t a . The f i t s to the data use the r e s u l t s of the two-photon susceptibilities determined p r e v i o u s l y . The f i t determines ω , Γ and A . S i m i l a r r e s u l t s are a l s o obtained f o r Raman modes i n the chloroform. The r e s u l t s are consistent with frequencies and widths determined by other means, and with the general model we have a p p l i e d to these solutions· 1

ρ

ρ



210


l7

3

l7

3

N = 4.0x10 r.u./cm — "

Experiment Theory

\ \\ \Λ\ \\ \\

* // ,

N = 2.0x10 r.u./cm

1450

1500

. _ Δ ω (cm )

A

nl

1550

1600

Figure 11. Third-order mixing (CA RS) in a solution of 4 BCMUin 2/3 hexane and 1 /3 chloroform for the frequency region of the polymer vibration. Key: —, experimental data; and —, theoretical fit. (Reproduced with permission from Ref. 23. Copyright 1978, American institute of Physics.)


9.

SHAND AND CHANCE

Polydiacetylenes

211

Acknowledgments


Many people have contributed i n various ways to t h i s work. In p a r t i c u l a r we would l i k e to thank M.A. G i l l e o for many h e l p f u l d i s c u s s i o n s and R . H . Baughman, R. S i l b e y , C . Hogg, M. Schott, and M. L e P o s t e l l e c , for t h e i r c o l l a b o r a t i v e input on v a r i o u s aspects of t h i s work. We also acknowledge G . N . P a t e l for supplying m a t e r i a l s and J . M . Sowa and M . C . Baker for v a l u a b l e technical asistance.

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20. Moran, M . J . ; She, C . Y . ; Carman, R . L . IEEE J. Quantum Electron. 1975, 11, 259. 21. Shand, M . L . ; Chance, R.R.; LePostollec, M.; Schott, M. Phys. Rev. Β 1982, 25, 4431. 22. Chance, R.R.; Shand, M . L . ; LePostollec, M.; Schott, M. J. Polymer S c i . , Polymer Lett. Ed. 1981, 19, 529. 23. Shand, M . L . ; Chance, R.R. J. Chem. Phys. 1978, 69, 4482. 24. Shand, M . L . ; Chance, R.R.; Silbey, R. Chem. Phys. Lett. 1979, 64, 448. 25. Chance, R.R.; Shand, M . L . ; Hogg, C.; Silbey, R. Phys. Rev. Β 1980, 22, 3540. 26. Kuzmany, H . ; Imhoff, E . A . ; Fitchen, D.B.; and Sarhangi, Α . , Phys. Rev. Β 1982, 26, 7109. 27. Mele, E . ; in Proceedings of the International Conference on Conducting Polymers,Les Arcs, 1982, J. de Physique (in press). 28. Lauchlan, L . ; Chen, S.P.; Etemad, E . ; Kletter, M.; Heeger, A.J.; MacDiarmid, A.G. preprint. 29. Levenson, M.D.; Bloembergen N. Phys. Rev. 1974, 10, 4447; Maker, P.D.; Terhune, R.W. Phys. Rev. 1965, 137, A801. 30. Frey, R.; Pradere, F. Opt. Commun. 1974, 12, 98; see also Infrared Phys. 1975, 15, No. 4. 31. Lotem, H . ; Lynch, R . T . ; Bloembergen, N. Phys. Rev. A 1976, 14, 1748. 32. Sondheimer, F.; Ben-Ephriam, D.A.; Wolorsky, R. J. Amer. Chem. Soc. 1961, 83, 1675. 33. Reimer, B.; B ä s s l e r , H. Chem. Phys. Lett. 1978, 55, 315. 34. Meisner, L . B . ; Khadzhiiski, N.G. Kvant. Elektron.(Moscow) 1979,6,345.[Sov. J. Quantum Electron. 1979, 9, 199.] 35. Walling, J . C . ; Peterson, O.G.; Morris, R.C. IEEE J . Quantum Electron. 1980, 16, 120. 36. Patel, G.M.; Khanna, Y.P. J. Polym. S c i . , Polym. Phys. Ed. (in press). 37. Lynch, R . T . , J r . ; Kramer, S.D.; Lotem, H . ; Bloembergen, N. Opt. Commun. 1976, 16, 372. RECEIVED June 6, 1983


Nonlinear Optical Properties of Organic and Polymeric Materials

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