Intramolecular charge delocalization and nonlinear optical properties from vibrational spectra

Synthetic

Metals

102 (1999)

1582-1583

Intramolecular Charge Delocalization and Nonlinear Optical Properties from Vibrational Spectra M. Del Zoppo, C. Castiglioni, M. Tommasini, P. Mondini, C. Magnoni and G. Zerbi Dept. of Industrial Chemistry, Politecnico di Milano, Milan, Italy Abstract The vibrational method for the evaluation of molecular hyperpolarizabilities of polyconjugated the modulation of second order hyperpokuizability (v) induced by the effect of charge delocalization. Keywords: lnfrared and Raman Polyacetylene and derivatives.

Spectroscopy;

Non-linear

optical

The development of design criteria for new molecules and polymers for nonlinear optical (n.1.o.) applications has taught that it is important to modulate the extent of intramolecular charge delocalization in order to optimize physical and optical properties of conjugated molecules. In particular n.l.0. coefficients are very sensitive to x electron delocalization. The concomitant development of detailed structure/property relationships has made it possible to relate intramolecular charge delocalization to the degree of bond length alternation (BLA). Thus molecular n.1.o. coefficients are modulated by BLA. Along this line of thought, one possible way to enhance molecular nonlinearities is the incorporation within the molecule of charge defects which strongly affect BLA (e.g. doping) [l] or photoexcitation [2]). In this work we focus on second order hyperpolarizabilities evaluated with the vibrational method. [3]. According to this method the vibrational contribution to y is given by:

I (1)

The quantities $+/dQr, &ijl@,, &ijk/aQ, where i, j, k indicate the Cartesian components, can be obtained from tiared intensities, Raman and Hyper Raman cross sections, respectively. is the vibrational

Ab initio

quantum

chemical

methods and calculations;

1 this is possible only if the [p/3] terms are dominant contribution is alw-ays positive).

1. Introduction

jr

methods;

molecules is used in order to account for

frequency of the r-th normal mode Q,. It has

2. Results and discussion One possible way to enhance the third order optical response of polyene molecules is the introduction of a charge defect. A simple model of doped polyene chain which can be studied with ab initio calculations, is C~~HU’. The charge defect affects the extent of intramolecular charge delocalization and modifies the chain structure making the central part of the molecule (where the defect is localized) more equalized. The ab initio calculated spectra give an extremely strong and selective infrared spectrum contrary to what is found in the case of an undoped polyene of similar length (e.g. CzzH2,9. This means that to get a reliable estimate of y with the vibrational method, the [$I terms must be included in eq. 1. Since the available quantum chemical programs do not calculate analytical derivatives of p with respect to normal modes we use the finite difference method. In principle one should compute the quantities AP/AQ for each normal mode appearing in the tiared spectrum. Actually the calculations show that the [pp] contribution coming from only the strongest IR normal mode is already larger than the sum of all [a*] terms (
been shown [4,5] that the values thus obtained correlate very favorably with those obtained with the traditional techniques (e.g. THG). Up to now only systems for which [a*] is relevant have been considered because of the difficulty in obtaining experimentally Hyper Raman absolute cross sections which appear in the [pp] term, However this contribution becomes more and more important as the degree of bond length alternation decreases. For a cyanine-like structure a large negative y value is expected [6] and according to eq. 0379-6779/99/$ - see front matter PII: SO379-6779(98)00973-4

0 1999

Elsevier

Science

S.A.

All rights

(the [a*]

= [a*],9+LOI, reserved.

M. Del Zoppo

et al. I Synthetic

Table 1: Contributions to y” from Raman [a*] and Hyper-Raman [up] ab initio RHF/3-21G calculated cross-sections.The y axis is oriented along the polyenic chain.

[esu] 5.48 1o-34 4.28 1O-34 1.63 lo-”

I II Cd-h+

bul

[es4

102 (1999)

1582-1583

1583

Finally it must be noted that in the caseof C~IHU+we obtain a large, negative = -1.66x lo”* esu (if compared with that of undoped Cz2H24, = 1.86x 1o”3 esu) thus confiig the idea that the incorporation of charge states enhances the nonlinear optical response[ 11.

Gl

1.91 1o-34 7.39 1O-34 .127 -0.39 1o-34 3.89 1o-34 ,085 3. Conclusions -5.89 1O-32 -5.87 lo”* ,012’“’/ .094@:

(a) Average over four central CC bonds at the site of the defect (b) Average over the other CC bonds.

H3C

Metals

We have shown that a large intramolecular charge delocahzation such as that introduced by doping yields large negative y values. The vibrational method accounts for this behavior thanks to the [up] terms which become dominant because of the extremely strong IRAV (IR activatedvibrations) modes.

‘N AH3

H3Y--YCN CH3

II

CN

The resultsof such calculationsare reported in Table 1. Here we can observe that as the BLA decreasesthe [pp] contribution to y gains in relative importance. While in the caseof polyene molecules [S] the Raman contribution from the fl mode accountsfor the whole y in push-pull polyene systems the situation is intermediate. For weak donor and acceptor groups (molecule I) the situation is similar to what is observedfor polyenes (small, positive [up] contribution). As the strength of the push-pull system increases(molecule II) the [up] contribution changes sign even if its absolute value is rather small. Finally for CZIHB+ a large, negative [up] contribution is obtained. In this last case we report two values for the BLA parameter becausethe charge defect, localized in the central part of the molecule, inducesa pronounced equalization only over the four central bonds. Thus a mean BLA value over the whole chain can be misleading. This observation justifies also the fact that, as a consequenceof the alternation of large portions of the molecule (lateral wings), the calculated Raman spectrum still shows large intensities (for normal modes other than a collective, in phase II mode) and hence a non negligible [a*] contribution. We expect that considering even more equalized molecules (e.g. cyanine molecules) the situation becomesmore extreme,the [up] term being at least one order of magnitude larger than the [a*] contribution. Preliminary calculations on NHz(CH)3NH2 (vanishing BLA) indeed give: <(yldp (over all Raman active normal modes) = 4.46x 10”’ esu and
References

Ul

C.W. Spangler in T. Skotheim (ed.), Handbook of Conducting Polymers.II Edition, Dekker, New York, 1998, p. 743. PI D.C. Rodenberger, J.R Heflin, A.F. Garito, Nature 359 (1992) 309; ibidem Phys.Rev. A 51 (1995) 3234. r31 C. Castiglioni, M. Gussoni, M. Del Zoppo, G. Zerbi, Solid State Comm. 82 (1992) 13 I41 M.C. Rumi, G. Zerbi, K. Mullen, M. Rehahn, J. Chem. Phys. 106 (1997) 24. PI M. Del Zoppo, C. Castiglioni, P. Zuliani, G. Zerbi in Handbook of Conducting Polymers.II Edition, Dekker, New York, 1998, p. 765. if51 a. SR Marder, D. N. Beratan and L. -T. Cheng, Science, 252 (1991) 103; b. S.R. Marder, C.B. Gorman, F. Meyers, J.W. Perry, G. Bourhih, J.-L. Bredas and B.M. Pierce, Science, 265 (1994) 632 [71 a.M. Gussoni, C. Castiglioni and G. Zerbi in R.J.H. Clark and R.E. Hester teds.), Spectroscopy of Advanced Materials, Wiley, New York, 1991, p. 251; b. C. Castiglioni, M. Del Zoppo and G. Zerbi, J.Raman Spectr.24 (1993) 485. M a. M. Del Zoppo, C. Castiglioni, M. Veronelli, G. Zerbi, Synth. Met. 55 (1993) 3919; b. M. Tommasini, C. Castigliom G. Zerbi, submitted. [91 C. Magnoni, Thesis of the Advanced School on Polymer Science,Polite&co di Milano, 1998