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Indian Journal of Chemistry

Vo1.39A, Jan-March 2000, pp. I ) 4- 1 19

A computational study of the non-linear optical properties in 1t-a-1t coupled donor-acceptor organic molecules#

J Laxmikanth Rao & K Bhanuprakash*

Inorganic Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 007, Andhra Pradesh, India . Email: bprakash@iict.ap.nic.in

Received 2 October 1999; accepted 15 December 1999

First hyperpolarizabilities (�) are calculated using semi-empirical molecular orbital methods for some organic molecules containing 7t-electron-donor (D) and 7t-electron acceptors (A), and separated by saturated a-bonds. Intramolecular charge transfer between D and A is observed and the molecules show large �. These molecules are predicted to have more transparency in the region of interest for frequency doubling applications. This study suggests that these saturated bridges in general may have potential applications in the development of non-linear optical materials. .

Introduction

Non-linear optics (NLO) deals with the interaction of applied electromagnetic fields in various materials to generate new electromagnetic fields altered in frequency, phase or other physical properties I . Jh� design of NLO materials has become the focus of current research in view of their potential applications in various photonic technologies. A variety of systems like inorganic mate­

rials, organometallic materials, organic molecules and polymers etc., have been studied for NLO activity. Ow­

ing to the combination of chemical flexibility and a va­

riety of synthetic strategy, organic molecules in particu­

lar have received much attention. In addition to the ad­

vantage in synthesis organic materials have ultra fast re­

sponse time, photo stability and large hyperpolarizability values. Nevertheless, practical utility of these has not yet been achieved 2. While optical transparency is one major problem, the retention of the large NLO activity when incorporated into devices is the other one. To J;e­

tain the hyperpolarizability in the bulk state they either have to crystallize into a non-centrosymmetric crystal or in a poled polymer; the non-centrosymmetric align­

ment should be retained on removal of the applied elec­

tric field3. Summing up the recent and important devel­

opments in the field of organic materials for second - order non-linear optics, Verbiest et al. 4 concluded that even though there are a lot of organic materials showing NLO activity a better combination of nonlinearity-sta­

bility-transparency is needed in order to really out per-

IICT Communication No:44) 9

form the inorganic materials; while for applications in frequency doubling, organic molecular materials are still a long way off. On the other hand search for NLO prop­

erties in completely new systems can increase our un­

derstanding of the mechanism responsible for NLO ef­

fects.

Our effort is to design at the molecular level NLO materials based on the concept of charge transfer (CT) between

0

and A separated by O'-spacers. The CT in these molecules arises owing to the interaction of 1t or­

bitals of the donor and acceptor moiety through the 0'­

bond (1t-0'-1t bond coupling). The molecules were first . reported by White5, who observed the CT from spectro­

scopic studies. Later Verhoeven et al.6, studied the CT in these molecules in great detail. These rod shaped bichromophoric molecules show CT sometimes even with 5 O'-bonds in the middle and it has been suggested that in all these cases through bond interaction (TBI) rather than the through space interaction (TSI) plays an important role for CT 6. In this study we report the opti­

mized structures and the hyperpolarizability of some such molecules computed using the semi empirical molecu­

lar orbital methods. To study the nature of transitions to the excited states in these molecules computations were carried out using semi-empirical CI methods. Our ear­

lier studies on 1t-0'-1t molecules have shown that large

� can be obtained with increased transparency 7-9.

Computational methods

The AMI hamiltonian 10 in the MOPAC package" was used to predict the bond lengths and bond angles of the

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Stru:\t.re -51

Stru:\tJ"e-S2

Stru:\tJ"e - S3

Fig. I -Molecular structures considered in the present computational study

molecular structures shown in Fig. I . The structure S 1 represents the set of molecules where the

0

and A are coupled by CH2-CH2 a-bond. Only the acceptors were changed while the N,N-dimethylamino group remained as the

0

in this set. All these molecules can adopt both gauche and anti conformations. We have studied both the conformations to estimate the role of TBI and TSI.

To understand the effect of replacement with powerful acceptors, the acceptor part was substitu ted by pyridinium ion. This set of molecules is represented by structure S2. Like in the earlier set, here too we have computed the � values for both the gauche and the anti forms. Structure S3 represents the molecules where in the spacer one CH2 is replaced py NH or O. The "PRE­

CISE" option in the MOPAC program was used for all geometry optimizations. For the set S2 the keyword

"CHARGE = + 1 " was used. The dipole moments of this set were calculated by Stewart's dipole definition for an ion incorporated in MOPACI I.

The geometry optimized molecules were then used to compute the static � values using the time-dependent Hartree-Fock (TOHF) methodl2 incorporated in the MOPAC program. �vc}he vector component along the dipole moment is reported here, which is defined as

where

�·Il = �i'Il;+ �j'llj + �k·llk

�; = <Piii + Pijj + Pikk)

Using the ZINOO programl3 for the AMI optimized structures of the molecules, we calculated the change in the dipole moment �Il, wavelength of the transition, 1."

and the oscillator strength,!, for transition from ground to excited states. Singly excited configuration interac­

tion studies using ZINOO have proven to be reliable in predicting the spectroscopic constants for similar kind of molecules 14.

Results and Discussion

c -C (J -bonded donor-acceptor molecules

The AMI optimized bond lengths of the central bond and the side bonds do not change with variation of the acceptor. The central bond length remains at 1 .521

A

and the side bonds at 1 .488

A.

This is the case in both anti and the gauche conformers. A predicted feature in molecules having TBI is the decrease in bond order of the central bond (increase in bond length) and increase in the bond order (decrease in bond length) of the side bonds in the ground statelS• Recent work, however, us­

ing high level ab initio calculations has shown that these bond lengths do not have a correlation with the TBI and any deviation is basically due to the steric effectsl6. The dihedral angle between the

0

and A in the case of the anti conformer remains around 1 80" for all the substitu­

ents while that in the gauche conformer is around 80".

The heats of formation along with the dipole moments and hyperpolarizabilities for all the molecules studied here are shown in Tables I , 2 and 3. Comparing the heats of formation in Table 1 , we see that in general the anti conformer is more stable than the gauche conformation by - 1 kcaVmol. The dipole moments of the anti con­

former due to greater charge separation, are larger than those of the corresponding gauche conformers and the largest value is for molecule 1 which has a powerful ac­

ceptor, N02. The dipole moment of molecule 1 is of the same magnitude as that of p-nitroaniline (PNA) and N,N dimethyl-4-nitroaniline (OM A) while other molecules have smaller values. The lower dipole moments should help the molecule to attain non-centrosymmetry in a crys­

ta1 17• At the orbital level HOMO-LUMO energy differ­

ence remains the same in the gauche conformation as in anti. Molecule 6 and 1 2 with COCF3 as the acceptor has the smallest HOMO-LUMO gap followed by molecule

1 and 7 with NO] as the acceptor.

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1 1 6 INDIAN J CHEM, SEC. A, JAN - MARCH 2000

Table 1 - Calculated heats of fonnation, Hr (kcaIlmol), dipole The hyperpolarizability values of the anti confonn­

ers are quite large and in some cases even larger than that of DMA. The molecule 6 has the largest value fol­

lowed by 1 . The gauche conformers in general have smaller � value. While the gauche conformers can have TSI between D and A, the anti conformers, because of the spatially far apart D and A, can only have TBL To understand the nature of CT transitions we carried out the semi-empirical CI studies on both conformers. The results are listed in Table 4 (anti) and Table 5 (gauche).

In Table 4 it is seen that the oscillator strength of the first transition is zero and the next excited state corre-

moments, !1 (Debye) and hyperpolarizabilities,

(x 1 0'lO esu) values obtained using MOPAC for structure S I Molecule

no. A Hr !1

N02 46.3 7.3 8.3

2 CN 74.6 5 . 1 7.4

3 COMe 6.2 4.5 8.2

4 CHO 1 2.3 4.2 8.3

5 COCl 5.3 5.3 9.7

6 COCF) - 1 38.6 6.2 1 1 .2

7 N02 47. 1 5.4 5.3

8 CN 75.6 3.5 5.0

9 COMe 7.2 3.9 5.3

1 0 CHO 1 3.2 4.3 4.9

1 1 COCl 6.2 5.0 5.7

1 2 COCF) - 1 37.7 4.3 7. 1

PNA 2 1 .4 7.3 5.5

DMA 3 1 .7 7.8 8.9

Molecules 1 -6 correspond to the anti confonner and molecules 7-

\ sponds to the wavelength of 295 nm. But this has a very small oscillator strength of 0.046. The dipole moment difference between the ground and the excited state is also very small. To analyze this at the orbital level, the molecular orbital picture obtained from ZINDO program was used. The molecular orbital HOMO is basically lo­

calized on the donor ring while the LUMO is localized on the acceptor ring. The spacer atoms C-C show a small

1 2 correspond to the gauche confonner.

Table 2 - Calculated heats of formation, Hr (kcal/mol), dipole moments, !1 (Debye) and hyperpolarizabilities, (x 1 Q-JO esu) values obtained using MOPAC for structure S2 Molecule

No. D R HI !1

1 3 NMe2 CN 249.9 7.0 30.9

1 4 NMe2 COOMe 1 42.7 2.3 28.7

1 5 NMe2 H 209.7 1 2.8 23.0

1 6 OMe CN 205.4 7.5 1 3.0

1 7 OMe COOMe 98. 1 3.2 1 2. 1

1 8 OMe H 1 65.0 1 3.3 1 1 .0

1 9 NMe2 CN 248.0 6.8 1 5.3

20 NMe2 COOMe 1 40.7 6.0 1 4.2

2 1 NMe2 H 208.0 9.2 1 2.0

22 OMe CN 204.0 7.4 6.7

23 OMe COOMe 97.0 6.7 6.2

24 OMe H 1 64.0 9.9 5.8

Molecules 1 3- 1 8 correspond to the anti confonner and molecules 1 9-24 correspond to the gauche confonner.

Molecule no.

25 26 27 28 29 30 3 1 32

Table 3 - Calculated heats of formation, Hr (kcaIlmol), dipole moments, !1 �Debye) and hyperpo1arizabilities,

(x 1 0.30 esu) values obtained using MOPAC for structure S3

D X A Hr !1

NMe2 NH N02 59.8 1 0.0 20.9

NMe2 NH CN 89.3 7 . 1 1 7. 1

OMe NH N02 1 3.4 8.5 1 5.2

OMe NH CN 42.8 5.7 1 2.2

NMe2 0 N02 23.7 7.9 1 4.2

NMe2 0 CN 52.5 5.5 1 2.4

OMe 0 N02 -22.7 7.0 9.2

OMe 0 CN 6. 1 4.8 9.7

*

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Table 4 - Excited states contributions to in molecule 6 (anti) calculated using ZINDO

Transi- �� A f Major C I

tion (Debye) (nm) transitions

12 3.04 399 0.000 HOMO - 5 LUMO (0.88)

HOMO - 5 LUMO+4 (0.41 )

13 - 1 .57 295 0.046 HOMO - I LUMO+3 (0.44)

HOMO - I LUMO +2 (0.85)

14 -2.07 283 0. 1 49 HOMO - 3 LUMO (0.76)

HOMO - 2 LUMO+ 1 (0.54)

15 - 1 2.37 268 0.795 HOMO - 2 LUMO (0.76)

HOMO LUMO (0.47)

1 � 6 -29.28 252 0.047 HOMO - 2 LUMO (0.46)

HOMO LUMO (0.81 )

17 -6.37 248 0.299 HOMO LUMO +3 (0.87)

1 � 8 -9.84 2 1 6 0. 1 99 HOMO - 3 LUMO (0. 6 1 )

HOMO - 3 LUMO+4 (0.35)

HOMO - 2 LUMO+ I (0.55)

HOMO LUMO+ I (0.35)

19 -0.32 2 1 4 0.000 HOMO LUMO+8 (0.84)

HOMO LUMO+ I O (0.33)

II O -29. 1 5 208 0.01 4 HOMO LUMO+ I (0.9 1 )

Table 5 - Excited states contributions to � i n molecule 1 2 (gauche) calculated using ZINDO

Transi- �� A f Major C I

tion (Debye) (nm) transitions

12 3.40 400 0.000 HOMO - 5 LUMO (0.88)

HOMO - 5 � LUMO+4 (0.43)

13 -0.57 295 0.042 HOMO - I LUMO+3 (0.42)

HOMO - I LUMO +2 (0.85)

1� 4 -2.67 283 0.008 HOMO - 3 LUMO (0.75)

HOMO - 2 LUMO+ I (0.48)

15 -24.96 27 1 0. 1 05 HOMO LUMO (0.89)

1� 6 -9.29 263 0.560 HOMO - 2 LUMO (0.72)

HOMO - I LUMO (0.49)

HOMO LUMO (0.33)

1� 7 -2.54 249 0.368 HOMO LUMO +3 (0.91 )

18 -23.66 22 1 0.0 1 5 HOMO LUMO+1 (0.96)

19 -7.66 2 1 5 0.2 1 1 HOMO - 3 LUMO (0.61 )

HOMO - 3 LUMO+4 (0.40)

HOMO - 2 LUMO+ I (0.52)

contribution in both these orbitals. While in the HOMO the spacer atoms have a larger coefficient the LUMO has slightly smaller one. This functions as a pathway

tor have wavelength of 268/252 nm with a fairly strong oscillator strength for the first and a weaker one for the next. But the change in dipole moment in the two transi­

tions is very large, being 1 2.4/29.3 Debye. This corre­

sponds to a mixture of transitions from the D aromatic ring to the A ring and from the donor group itself to the acceptor. This mixing increases the intensity of the tran­

sition. While the next transitions also involve charge movement from the donor ring to the acceptor, the in- . for the CT between the donor ring and the acceptor. At

the orbital level the transition (295 nm) is assigned to a local transition on the D ring. The next transition corre­

sponds to a local transition on the acceptor ring. This has a stronger oscillator strength. The transitions which correspond to the CT from the donor ring to the accep-

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1 1 8 INDIAN J CHEM, SEC. A, JAN - MARCH 2000

Table 6 - Excited states contributions to P for molecule 25 calculated using ZINDO

Transi- �� A- I Major C I

tion (Debye) (nm) transitions

1----7 2 2. 1 5 450 0.000 HOMO - 6 ----7 LUMO (0.94)

HOMO - 6 ----7 LUMO+2 (0.33)

1----7 3 1.71 383 0.000 HOMO - 7 ----7 LUMO (0.94)

HOMO - 7 ----7 LUMO +2 (0.33) 1----7 4 - 1 1 .72 327 0.603 HOMO - 1 ----7 LUMO+2 (0.96)

1----7 5 -2.05 294 0.043 HOMO - 2 ----7 LUMO+4 (0.48)

HOMO ----7 LUMO+3 (0.84)

1----7 6 -2.34 293 0.004 HOMO -3 ----7 LUMO (0.60)

HOMO - 1 ----7 LUMO+ I (0.7 1 )

1----7 7 -35 . 1 7 253 0.081 HOMO ----7 LUMO (0.88)

HOMO ----7 LUMO+4 (0.34)

1----7 8 - 1 0.69 25 1 0.396 HOMO ----7 LUMO (0.38)

HOMO ----7 LUMO+4 (0.85)

1----7 9 - 1 0.21 . 241 0. 1 00 HOMO - 3 ----7 LUMO (0.78)

HOMO -1----7 LUMO+ 1 (0.55)

1----7 1 0 -5.4 1 234 0.058 HOMO -I ----7 LUMO+2 (0.90)

tensity is weaker. In the gauche conformer (Table 5), the first transition corresponds to wavelength 295 nm which is again a local transition on the D ring. Here again the D to A transitions are mixed and these transitions occur at nearly the same energy as in Table 4 but with weaker oscillator strengths. The �fl between the ground state and excited states is not as large as that observed in anti conformation. Thus the hyperpolarizability of the gauche conformer is smaller. The absorption wavelength of the two conformers is still less than that of PNA which has a calculated A value of 320 nm. The condition for max increasing the intensity of the intramolecular CT in the anti conformer would be to substitute groups which in­

crease the contribution of the spacer carbons in both HOMO and the LUMO. With larger overlap the transi­

tion dipole moment would increase. This would then en­

hance the hyperpolarizability.

Pyridinium ion as the acceptor

Marder et al. 18, have demonstrated that organic salts where the cation has been designed to have a large hyperpolarizability can be used to obtain materials with large X. Duan et al. 19 have calculated and measured for some stilbazolium salts large � value and have shown that the pyridinium ion can be a good acceptor. Earlier, Gee et al.20 observed CT absorption in a series of mol­

ecules containing p-methoxyphenyl group as an electron donor and 4-methoxycarbonylpyridinium group as an

electron acceptor separated by saturated C - C bond.

This was also observed in rigid systems where the D and A are spatially far apart and this they concluded to be due to through boncl ':T interactions. We have stud­

ied this system which is shown in the Fig. 1 as structure S2. Again, both anti and the gauche forms were consid­

ered. The results are shown in Table 2. The central bond has a length of 1 .534

A

in the anti form and it reduces by only 0.004

A

in the gauche form. The sub­

stitution of various donors and acceptors does not make much difference in the bond lengths. The side bond�

essentially remain of the same length in both the confor­

mations. The angle between D and A in gauche form is around 63n for the various donors and acceptors and this compares well with the X-ray determined value21 of 68·'.

This angle is reduced when compared to structure S I , which is due to the larger electrostatic attraction here.

Unlike in the earlier set, here the gauche conformer is more stable than the anti isomer by - 1 - 2 kcaVmol. The

� values are quite large and in general the anti conformer has larger values. With NMe2 as donor, the � value is 30.0 which is around 6 times larger than that for PNA.

While it is seen from the heats of formation that the gauche conformer is more stable, the way to increase the stability of anti form would be to substitute bulky groups on the spacers.

Charge transfer over a heteroatom

The set S3 represents those molecules where one CH2 in the spacer is replaced by a heteroatom. In our studies

(6)

here we have replaced the CH2 by both NH and O. Charge transfer in such molecules has been observed in 1 968 by Oki and Mutai22 who reported intramolecular CT phe­

nomena in the compounds P-N02-CoH4-CH2-NH-CoHs' The computational results are given in Table 3. Only the results of the CH2 replacement in the spacer attached to the acceptor ring are shown in the table. The heteroatom attached to the donor ring does not show very high � value, hence, it is not discussed here. The AM

1

mini­

mized geometry shows that the two phenyl rings are nearly perpendicular to each other. The central bond (C­

N) has a bond length of

1 .444A

while the length of the C-O bond is

1 .440A.

These do not change much with change in D and A. At the molecular orbital level the effect of replacing the CH2 by heteroatom is that the HOMO of the molecule is lowered by 0.2 e V to 0.8 e V (for various substitutions) while the LUMO is raised by 0.4 e V when compared to set S I . Thus, there is A larger HOMO-LUMO gap in these molecules. The � values are much larger in this type of molecules. Comparison of molecule 25 with PNA or DMA shows that the � value is larger by 2-3 times. While the inductive effect plays a role in the enhancement of the hyperpolarizability, the CT does take place from D to A part of the molecule thus adding to the increased NLO activity of this mQl­

ecule. The largest � value obtained is for NMe2 as the D and N02 as the A. Replacing the heteroatom by O-atom reduces both dipole and the hyperpolarizability of the molecules. To understand CT in this type of molecules, the transitions obtained from the CI studies are shown in Table 6. The transition corresponding to the CT from the N-atom of the spacer to the N02 group has an oscil­

lator strength of 0.6. The spacer -N- due to the induc­

tive effect has larger charge (-0.3 I e) when compared to the charge on the N atom (-0.28e) in DMA. The CT corresponding to the movement of charge from D to A mixes with the local transitions of the donor ring (7 and 8). This increases the intensity of the transitions. The change of dipole is large which is seen particularly in transition 7. The absorption, A, in these molecules is less than 330 nm, as in PNA, but the � values are much larger. The decreased � in the 0- substituted molecules is due to the lower electron density on the 0-atom23.

Conclusion

In this work we have carried out computational stud­

ies on organic molecules where the 1t-electron donor and the 1t-electron acceptor are separated by a-bonds. The 1t orbitals of the aromatic groups mix with the a-frame-

work and through bond interaction is observed. The in­

tramolecular CT in these type of molecules proceed with a large dipole moment difference between the ground and the excited states but the oscillator strengths are weaker. In general the hyperpolarizability of the mol­

ecule is enhanced suggesting that these NLO phores could play a role in the development of organic non- . linear optical materials. Further work including synthe­

sis of these materials is being carried out by our group .. Acknowledgement

We thank the Department of Science and Technolog

y

(DST), New Delhi, for the funding and Dr. B . M.

Choudary, Head, Inorganic Chemistry Division for his constant encouragement during this work

References

I Nonlinear opticaL properties of organic moLecuLes and crystaLs, edited by D S Chemla & J Zyss, (Academic Press, Orlando) 1 987.

2 Marder S R , Sohn J E & Stucky G D, Materials for nonlinear optics (ACS Symposium Series 455, American Chemical Soci­

ety, Washington, DC), 1 99 1 .

3 Williams D J, Angew Chern Inti Ed Eng, 23 ( 1 984) 690.

4 Verbiest T, Houbrechts S, Kauranen M, Clays K & Persoons A, J mater Chern., 7( 1 997)21 75.

5 White W N, J Am chern Soc, 8 1 ( 1 959)29 1 2.

6 Mes G F, de Jong B , van Ramesdonk H J, Verhoeven W J, Warman J W de Haas M P & Horsman-van den Dool L E W, J Am chern Soc, 1 06 ( 1 984) 6524 and references therein.

7 Laxmikanth Rao J & Bhanuprakash K, J moL Struct (Theochem),

458 ( 1 999) 269.

8 Bhanuprakash K & Laxmikanth Rao J, Chern Phys Lett,

3 1 4( 1 999)280.

9 Bhanuprakash K, Laxmikanth Rao J, Bandyopadhyay A K &

Likhar P R, (communicated).

1 0 Dewar M J S , Zoebisch E G, Healy E & Stewart J J P, J Am chern Soc, 1 07( 1 985)3902.

I I MOPA C 93, (Fujitsu Inc)

1 2 Dupis M & Kama S, J comput Chern, 1 2( 1 99 1 )487.

1 3 ZINDO version 960, 1 996, Biosym Technologies, Inc., ( Mo­

lecular Simulations, Inc).

1 4 Kanis D R , Ratner M A , & Marks T J, Chern Rev, 94 ( 1 994) 1 95.

1 5 Gleiter R , Angew Chern lnt Ed Engl, 1 3( 1 974)696.

1 6 Gleiter R, J Am chern Soc, 1 1 9 ( 1 997) 7048.

1 7 Morley J 0, J moL Struct (Theochem), 365( 1 996) I .

1 8 Marder S R, Perry J W & Schaefer W P, Science 248 ( 1 989) 626.

1 9 Duan X M, Konami H , Okada S , Oikawa H , Matsuda H &

Nakanishi H, J phys Chern, 1 00( 1 996) 1 7780.

20 de Gee A J, Verhoeven J W, Sep W J & de Boer T J, J chem Soc, Perkin II ( 1 975) 579.

2 1 Edixhoven L D & Starn C H, Rec Trav Chim, 8 8 ( 1 969) 577.

22 Oki M & Mutai K. Te/rahedron Lett, ( 1 968)201 9.

23 Whitaker C M , Patterson E V, Kott K L & McMahon R, J Am chem Soc, 1 1 8 ( 1 996) 9966.

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