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Nonconjugatively linked donor—acceptor compounds: a new class of nonlinear optical materials
Volume 179, number I,2
12April 1991
CHEMICAL PHYSICS LETTERS
Nonconjugatively linked donor-acceptor compounds: a new class of nonlinear optical materials W. Schuddeboom, B. Krijnen, J.W. Verhoeven Laboratory oforganic Chemistry, UniversityofAmsterdam,
NieuweAchlergrachi
129. 1018 WS Amsterdam. The Netherlands
E.G.J. Staring,G.L.J.A. R&ken Philips Research Laboratories, PO Box 80000, 5600 JA Eindhoven. The Netherlands
and H. Oevering DSh4 Research, PO Box 18, 6061 MD Geleen, The Netherlands
Received I1 December 1990;in final form 25 January 1991
Whereas commonly studied organic inolecules for nonlinear optics contain an electron-donor group D and an electron-acceptor group A linked by a K-conjugated system, we now report the behaviour of some molecules in which a rigid, saturated hydrocarbon bridge separates D and A groups by at least three a-bonds. The electronic spectra of these systems and their hyperpolarizability values as determined by second-harmonic-generation (SHG) measurements reveal that the chromophores cannot be regarded as electronically isolated, but that a nonnegligible electronic interaction occurs, probably mediated via the interconnecting a-bonds. The relative weakness of this through-bond interaction results in a smaller value for the oscillator strength of the intramolecular charge-transfer transition in such nonconjugated D/A systems as compared to fully conjugated ones. The lowering effect of the smaller transition dipole moment on the magnitude of the molecular hyperpolarizability, 8, appears to be partly compensated, however, by the large difference between ground- and excited-state dipole moment achievable in these systems.
1. Introduction
In recent years, a plethora of new organic materials [ 11 has become available with interesting nonlinear optical properties. Compounds that show high second-order molecular polarizabilities are useful for applications such as second-harmonic generation and electro-optic phase modulation. Molecular engineering and determination of the molecular hyperpolarchizahility, /3, are essential in order to gain a better understanding of the chemical structure-hyperpolarizability relationship. So far, it has been well established that structures which incorporate an electron-rich donor group and an electron-deficient acceptor group, interconnected by a conjugated TCsystem, in general give rise to large molecular second-order effects (e.g., 4-nitroaniline has a proto-
type molecular structure [ 21). Furthermore, the hyperpolarizability has been related to the presence of a low-lying charge-transfer excited state resulting in a large change in dipole moment upon excitation to the first excited state as well as in a high oscillator strength for the corresponding electronic transition [ 3 1. Within a two-level model, eq. ( I ) can be shown [4] to describe the relation between the contribution &) of this charge-transfer state to the molecular hyperpolarizability, the transition dipole moment, ke, the difference between ground- and excitedstate dipole moments, A,E~~, the absorption angular velocity, ma, and the excitation angular velocity, w. In eq. ( 1), it has furthermore been assumed that kse and ApSchave the same direction (one-dimensional mqdel ) :
0009-2614/91/s 03.50 0 199I - Elsevier Science Publishers B.V. (North-Holland )
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pig. 1. Structure of the compounds studied.
(1) Many studies [ 5-81 have been directed to the influence of the length as well as the structure of the interconnecting n-system on the molecular hyperpolarizability p. Thus, Barzoukas et al. [ 71 demonstrated an almost quadratic dependence of p on the number of double bonds separating the donor and acceptor groups in their “push-pull” systems. Here, we report the results of a study on the nonlinear optical properties of bichromophoric molecules l-3 (see fig. 1). These molecules consist of a phenylsubstituted nitrogen eltctrori donor and an electronegatively substituted ethyl&e acceptor interconnected by a saturated hydrocarbon unit. Thus, the donor and acceptor chromophores are expected classically to be isolated from each other.
2. Experimental The syntheses of compounds l-3 have been described elsewhere [ 9, lo]. Electronic absorption spectra were measured on a Gary 17D spectrophotometer, while electronic emission spectra were determined using a Spex fluorolog2 spectrofluorimeter. The spectral distribution of the emission spectra for l-3 is known [ lo,11 ] to be independent of the excitation wavelength, in the present measurements, excitation was performed at 350 nm.~Emission.spectra were corrected for the spectral response of the detection system to represent relative fluorescence intensities in quanta per wavelength interval. All solvents used were of the highest grade‘of 74
., 12ADril 1991
purity commercially available and carefully deoxygenated by purging with argon before measurement of the emission spectra. Electric-field-induced second-harmonic generation on polymethyl methacrylate films, containing typically 1 wt$~of thecompounds under study, at a fundamental wavelength of 1064 nm was measured under constant corona-poling with an applied electric field of = 1.2 MV/cm at 95”C, just below the TB of the matrix. Molecular volumes of the systems studied were calculated with the CHEM-X molecular modeling package (Chemical Design Ltd., Oxford, UK) using the following van der Waals radii: C: 1.60, N: 1.50, 0: 1.40, H: I .20 A. Semi-empirical SCF MO calculations were performed with the AM1 Hamiltonian employing MOPAC 5.0 [ 121.
3. Results and discussion 3.1. Second-order nonlinear susceptibility Comparison of the second-harmonic intensities measured with a known sample (quartz) allowed calculation of the inner product of the ground-state dipole moment and the molecular hyperpolarizability (Q) as compiled in table 1. 3.2. Ground- and excited-state dipole moments For a discussion of the nonlinear optical properties and extraction of the molecular parameters from those determined via measurements on electric-fieldokiented samples, knowledge of the dipole moment bf the molecules in their electronic ground state (&) and in their electronically excited state (&) is required. Table I Electric-field-induced second-harinonic generation data for 1-3 estimated in a PMMA matrix at 95’C and at a fundamental wavelength of 1064 nm Compound
@(lO-“esuD)
1 2 3
26f 10 802 13 90? 15
Volume 179, number 1,2
12April 1991
CHEMICAL PHYSICS LETTERS
Tt u
pg= 1.74 D
the local dipole moment of the acceptor group which determines the overall magnitude and direction of K. For 2, the value of pg depends rather strongly on the orientation of the carboxymethyl substituent. From X-ray diffraction data [ lo], this substituent is known to occupy, an s-cis orientation (&z= 1.74 D) with respect to the exocyclic double bond in the solid state. From AMI calculations, this orientation is found to be x 1.5 kcal/mol more stable than the alternative s-trans orientation (~~~3.94 D), implying that under the conditions of the SHG measurements (95”C), both orientations may be populated, the scis, however, being present predominantly. It is well documented [ 13,141 that the solvent dependence (solvatochromism) of electronic absorption and emission spectra pravi&d a useful experimental tool to estimate the change in dipole moment between ground and excited state. As published earlier [ 10,151, efficient through-bond interaction between donor and acceptor via the intervening saturated hydrocarbon skeleton in systems like l-3 gives rise to a discrete intramolecular charge-transfer (CT) absorption in the 350 nm region not overlapped by the absorptions of the skparate donor and acceptor chromophores, that occur at significantly shorter wavelength ( ~300 nm). Furthermore, these molecules fluoresce [ 111 from the intramolecular chargetransfer state populated either by direct excitation in their CT absorption or via internal conversion from higher excited states. Table 2 compiles absorption and emission data for 1-3 in a series of solvents, while fig. 3 displays the typical spectra of 3 in two such solvents_ As evident from table 2 and from fig. 3, the eompounds display a huge solvatochromism of their fluorescence and a minor solvatochromism of the ab-
1 2
J 3
Fig. 2. Orientation and magnitude ofthe ground-state dipole moment (tip) in l-3 derived from AM1 calculations.‘For 2, the results for both the s-cis and the s-trans rotational minima of the carboxymethyl group are shotin.
Ground-state dipole moments of the molecules l3 were calculated employing the SCF MO semi-empirical AM1 .method. Results are compiled in fig. 2, which also shows the orientation of the calculated dipole moments with respect to the molecular framework. For 1 and .3, rather similar values (4.09 and 3.37 D, respectively) were found although the orientation of the donor phenyl group was taken to be equatorial in 1 and axial in 3 #I. Evidently, it is mainly ” X-ray analysis [ 9,IO] has shown that for all compounds studied the phenyl group adopts an axial orientation in the solid state, In solution, 2 and 3 retain the axial orientation while both axial and equatorial conformations are populated by 1.
Table 2 Absorption and fluorescence maxima, u, (ur) (in 10m3cm-’ ), of l-3 in various solvents (see text for definition ofthe solvent parameters’
Sandf’ ) Solvent
f
f’
1
2
3
n-hexane cyclohexane di-n-butyl ether diisopropyl ether diethyl ether ethylacetate
0.1850 0.2020 0.2920 0.3290 0.3400 0.3850
0.1864 0.2040 0.1952 0.1840 0. Ii75 0.1854
29.50 (22.30) 29.24 (22.20) 29.07 (19.50) 29.07 (19.00) 29.07 (18.50) 28.99 (16.90)
29.41 (21.60) 29.24 (21.40) 29.24 (19.00) 29.24 (18.80). 29.24 (18.10) 29.15 {16.80)
28.17 (21.00) 28.01 (20.80) 27.78 (19.00) 27.78 (18.30) 27.78 (17.90) 27.78 (16.40)
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Volume 179, number 1,2
CHEMICAL PHYSICS LETTERS
12April 1991
sorption. This is typical for systems with a small dipole moment in the ground state (H) and a large dipole moment in the emissive excited state &). It has been shown [ 13,141 that treating the molecular dipole as a point dipole and the solvent as a continuous dielectric, the change in dipole moment between ground and excited state (A,u~~=p,-- & in D) can be derived #2from the solvatochromic shifts of the absorption and emission frequencies (v,, respectively, vf, in cm-‘) via 250
350
550
450
650
Wavelength
=const.t 10070Cf-f’)(A&)2/p3.
Fig. 3. Electronic absorption and emission spectra of 3 in cyclohexane () and in ethylacetate (---).
f-f 0-r -0.10
’
1
0.00
.
1
0.10
r
’
0.20
Fig. 4. Solvatochromic shifts (v,,- or) of l-3 as a function of the solvent polarity If-f ’ ). For clarity, the data points for 2 and 3 have been vertically offset by IO000and 20000cm-‘, respectively.
Table 3 Molecular volume ( V), and effective radius (p) of the molecules l-3asdetermined from molecular modeling, the slope of the correlation (see fig. 4) between the difference of their absorption and emission maxima as a function off-f ’ Bnd the change in dipole moment between ground and first excited state (Ape,) calculated therefrom via eq. (2) Compound
Y (AS)
p (A)
~(&Z/~P’ (cm-‘)
A/+ (D)
1
478 592 577
4.85 5.21 5.16
22958 20968 18652
16.1 17.1 16.0
2 3
76
(2)
In eq. (2),fandS ’ are parameters related to the solvent static dielectric constant (E) and refractive index (n) as f=(t-1)/(2Etl) and f=(n’-l)/ ( 2n2 t I), while p (in A) represents the effective radius of the solvent cavity occupied by the molecule. Thus, by plotting the difference of the fluorescence and absorption frequencies versus the solvent parameters (j-f ’ ) , a linear correlation should be found with a slope proportional to (A@‘, under the assumption that ,uL,and pg have (nearly) the same direction. In view of the charge-transfer nature of the excited state, pe must evidently be directed from D to A along the long axis of the molecules. The direction of ps, as estimated from AM1 calculations (see fig. 2), indeed closely coincides with this axis. As demonstrated in fig. 4, the absorption and emission data of l-3 nicely obey the correlation predicted by eq. (2). From the slopes of these correlations (see table 3), A,u~~can then be calculated if p is known. For nonspherical molecules, an effective pvalue is obtained by taking the radius of a sphere with a volume equal to the molecular volume (V) according top = ( 3 V/4x) ‘I3 . Molecular volumes were determined from molecular-modeling calculations. The results compiled in table 3 show that upon excitation the dipole moment for the present systems w2In principle, iftheir relative orientation is known, both & and a can be extracted from the combined solvatochromism of absorption and fluorescence. Under the present conditions (k z+ H), however, this procedure leads to a large degree of uncertainty ink. Furthermore, the AM 1 calculations showed the charge distribution in the ground state to be multipolar in nature, making the dipolar model underlying eq. (2) less ap plicable for the ground-state solvation.
Volume I 79, numberI,2
CHEMICAL PHYSICS LETTERS
increases by 16.5 + 0.5 D units. This corresponds to full charge separation over 3.44 + 0.1 A, a distance compatible with that between the donor nitrogen and the center of the acceptor. 3.3. Molecular hyperpolarizabilities Employing the ground-state dipole moment values estimated above, the molecular hyperpolarizability, p, was calculated from the macroscopic hyperpolarizabilities observed (see table 1). The results (&) are given in the last column of table 4. Although the macroscopic hyperpolarizabilities of these materials do not seem of direct interest for nonlinear optical application, the molecular hyperpolarizabilities are surprisingly high if one regards the chromophores as electronically#isolated. In fact, compounds 2 and 3 display Pvalues comparable to that of a strongly coupled D-A system like 4nitroaniline for which /?( 1064 nm ) = 34.5 x 10B3’ esu has been reported
12 April 1991
bond interaction is optimized for an axial orientation of the phenyl group in systems like 1-3. Evidently such an axial orientation is more highly populated in the tropane derivatives 2 and 3, where the additional bridging ethylidene unit sterically destabilizes the equatorial orientation of the phenyl group, than in 1. This difference in conformation has been demonstrated [ 9,101 to result in stronger chargetransfer absorption of the tropane derivatives and now also may be inferred to be related to their higher P-values (see table 4). It is, therefore, of interest to investigate the applicability of the two-level model underlying eq. ( 1) for estimating the CT contribution to the molecular hyperpolarizability of 1-3. The oscillator strength (F) of the charge-transfer transition (see table 4) was calculated from the absorption spectra via eq. (3 ), F=4.3x 1O-9 E,,,Av,,~,
(3)
where emaxis the maximum molar absorption coefficient and Av,,~ is the full band width at half height (in cm-‘), which then can be related to the transition dipole moment via eq. (4),
PI. From absorption and X-ray diffraction measurements, Krijnen et al. [ 9,10,15] stipulated that for molecules like l-3 significant electronic interaction between the chromophores occurs via the intervening o-bonds. As discussed above, this interaction is manifest from the long-wavelength absorption and the corresponding emission of these compounds (see fig. 3), which demonstrate that in spite of the lack of direct x-conjugative interaction between the donor and acceptor sites, a low-lying intramolecular charge-transfer state is available, a feature generally considered to be essential for second-order nonlinear optical properties. It was shown by Krijnen et al. [ 9, lo], that through-
Fc4.7~ 1O29v,,, 1~~ I*,
(4)
where v,, is the position of the maximum of the charge-transfer absorption band (in cm-l ) and &e is the transition dipole moment. Comparison of the calculated &--values with the experimental molecular hyperpolarizabilities (see table 4) shows that indeed PCTcan account for a major component of /?, thus substantiating the hypothesis that the hyperpolarizability of the present system is mainly governed by the presence of the through-bond-mediated charge-transfer interaction, We have shown before
Table 4
Molarexctinction (c), halfwidth (Av, ,a) and oscillator strength (F) aftbe intramolecular charge-transfer absorption of l-3 (in a-hexane at 20°C) as well as the CT contribution to the molecular hyperpolarizability &) calculated from these data via eq. (1) and the experimental hyperpolarizability (&_~) determined from the electric-field-induced second-harmonic-generation efficiencies (table I ) employing AM1 ground-state dipole moments e (!?mol-’ cm-‘)
k/l (cm-‘)
F
lb ( lO-‘Oesu)
B-4 ( 10e30esu)
2
2970 5800
4500 4700
0.057 0.117
3.7 8.1
6.4k2.5 46 f7.5 *’ 20.3k3.3 b,
3
6390
4200
0.115
9.1
26.7k4.5
1
a) EmployingH value calculated for the s-cis conformation.
b, Employing& value calculated for the s-trans conformation.
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CHEMICAL PHYSICS LETTERS
[ 10,161, that the CT transition of systems like those presented here derives part of its intensity from coupling to higher (locally) excited states, implying that a simple two-level model cannot be expected to describe quantitatively the hyperpolarizability connected to this transition.
l2April 1991
Acknowledgement We express our sincere gratitude to Dr. A.M. Brouwer performance and interpretation of the AM1 calculations; furthermore, use of the services and facilities for molecular modeling of the Dutch National NWO/SURF Expertise Center CAOSjCAMM is acknowledged with special thanks to Dr. J.H. Borkent for assistance.
4. Co~eludingremarks In this Letter, we have demonstrated that molecules in which donor and acceptor are connected by a Saturated o-bond system (D-o-A) .can exhibit efficient s&ond-harmonic generation, probably as a result of through-bond interaction. It should be noted that a similar mechanism has been .proposed earlier to explain SHG by the mono N-oxide of diazabicyclooctane [ 171, for which, however, no molecular hyperpolarizability value appears to have been determined. The Bvalues now estimated for 2 and,3 are comparable to such commonly studied D-n-A molecules as p-nitroaniline. From the analysis given above in the context of a two-level model, it is evident that the present D-o-A systems derive their remarkably large molecular hyperpolarizability mainly from a high value of Afl=, corresponding to complete charge separation in the excited state as opposed to virtually none in the ground state. For fully conjugated systems of comparable size, ApScis expected to be less, due to significant mixing of the charge-transfer state into the ground state. On the other hand, however, the transition dipole moment, ,u~~for a nconjugated system will in general be larger, implying a trade-off between ApgBe and bgeas a function of the electronic interaction between donor and acceptor sites. At the moment, it is hard to say what degree of electronic interaction will establish qptimum conditions for achieving high molecular hyperpolarizability and it is, therefore, of considerable interest to investigate systems in which the interaction is systematically varied between the extremes presented by pure x-conjugation and mere through-a-bond interaction.
78
References [,I 1D-J. Williams, ACS Symp. Ser. No. 233 (Am. Chem. Sot., Washington, 1983); D.S. Chemla and I. Zyss, eds., Nonlinear optical properties of organic molecules and crystals, Vols. 1, 2 (Academic Press, New York, 1987). [2] J.F. Nicoud and R.J. Twieg, in: Nonlinear optical properties oforganic molecules and crystals, Vol. 1, eds. D.S. Chemla and J. Zyss (Academic Press, New York, 1987) p, 227. [3] D.J. Williams, Angew. Chem. 96 (1984) 637. [4] B.F. Levine, Chem. Phys. Letters 37 (1976) 516; J.L. Oudar and D.S. Chemla, J. Chem. Phys. 66 (1977) 2664; J.L. Oudar, J. Chem. Phys. 67 (1977) 446; A. Dulvicand C. Dauteret, I. Chem. Phys. 69 (197%) 3453. [5] J.O. Morley, V.Y. Docherty and D. Pugh, J. Chem. Sot. Perkin. Tram II (1987) 1351. [6] T.M. Leslie, R.N. Martino, E.W. Choe, G. Khanarian, D. Haas, G. Nelson, J.B. Stamatoff, D. Stuetz, CC. Teng and Y.N. Yoon, Mol. Cryst. Liq. Cryst. ,I53 (1987) 451. [ 71 M. Banoukas, M. Blanchard-Desce, D. Josse, J.-M. Lehn and J. Zyss, Chem. Phys. 133 ( 1989) 323. [ 81L.T. Cheng, W. Tam, G.R. Meredith, G.L.J.A. Rikken and E.W. Meijer, in: Proc. SPIE, Vol. 1147. Nonlinear optical properties of organic materials, ed. G. Kbanarian ( 1990) p. 61. [ 91 B. Krijnen, H.B. Beverloo, J.W. Verhoeven, C.A. Reiss, K. Goubitz and D. Heijdenrijk, J. Am. Chem. Sot. 111 ( 1989) 4433. [ lo] B. Krijnen, Thesis, University of Amsterdam, Amsterdam (1990). [ ll] R.M. Hermani, N.A.C. Bakker, B. Krijnen and J.W. Verhoeven, J. Am. Chem. Sot. 112 (1990) 1214. [ 121J.J.P. Stewart, QCPE program 455 (1989). [ 13] E. Lippert, Z. Elektrochem. 61 ( 1957) 962. [ 14 ] P. Suppan, J. Photochem. Photobiol. SO( 1990) 293. [ 15] B. Krijnen, H.B. Beverloo and J.W. Verhoeven, Rec. Trav. Chim. 106 (1987) 135. [ 161 P. Pasman, F. Rob and J.W. Verhoeven, J. Am. Chem. Sot. 104 (1982) 5127. [ 17] P. Mihailovic, P. Bassoul and J. Simon, Chem. Phys. Letters 141 (1987) 462; J. Simon, P. Bassouland S. Norvez, New J. Chem. 13 ( 1989) 13.
12April 1991
CHEMICAL PHYSICS LETTERS
Nonconjugatively linked donor-acceptor compounds: a new class of nonlinear optical materials W. Schuddeboom, B. Krijnen, J.W. Verhoeven Laboratory oforganic Chemistry, UniversityofAmsterdam,
NieuweAchlergrachi
129. 1018 WS Amsterdam. The Netherlands
E.G.J. Staring,G.L.J.A. R&ken Philips Research Laboratories, PO Box 80000, 5600 JA Eindhoven. The Netherlands
and H. Oevering DSh4 Research, PO Box 18, 6061 MD Geleen, The Netherlands
Received I1 December 1990;in final form 25 January 1991
Whereas commonly studied organic inolecules for nonlinear optics contain an electron-donor group D and an electron-acceptor group A linked by a K-conjugated system, we now report the behaviour of some molecules in which a rigid, saturated hydrocarbon bridge separates D and A groups by at least three a-bonds. The electronic spectra of these systems and their hyperpolarizability values as determined by second-harmonic-generation (SHG) measurements reveal that the chromophores cannot be regarded as electronically isolated, but that a nonnegligible electronic interaction occurs, probably mediated via the interconnecting a-bonds. The relative weakness of this through-bond interaction results in a smaller value for the oscillator strength of the intramolecular charge-transfer transition in such nonconjugated D/A systems as compared to fully conjugated ones. The lowering effect of the smaller transition dipole moment on the magnitude of the molecular hyperpolarizability, 8, appears to be partly compensated, however, by the large difference between ground- and excited-state dipole moment achievable in these systems.
1. Introduction
In recent years, a plethora of new organic materials [ 11 has become available with interesting nonlinear optical properties. Compounds that show high second-order molecular polarizabilities are useful for applications such as second-harmonic generation and electro-optic phase modulation. Molecular engineering and determination of the molecular hyperpolarchizahility, /3, are essential in order to gain a better understanding of the chemical structure-hyperpolarizability relationship. So far, it has been well established that structures which incorporate an electron-rich donor group and an electron-deficient acceptor group, interconnected by a conjugated TCsystem, in general give rise to large molecular second-order effects (e.g., 4-nitroaniline has a proto-
type molecular structure [ 21). Furthermore, the hyperpolarizability has been related to the presence of a low-lying charge-transfer excited state resulting in a large change in dipole moment upon excitation to the first excited state as well as in a high oscillator strength for the corresponding electronic transition [ 3 1. Within a two-level model, eq. ( I ) can be shown [4] to describe the relation between the contribution &) of this charge-transfer state to the molecular hyperpolarizability, the transition dipole moment, ke, the difference between ground- and excitedstate dipole moments, A,E~~, the absorption angular velocity, ma, and the excitation angular velocity, w. In eq. ( 1), it has furthermore been assumed that kse and ApSchave the same direction (one-dimensional mqdel ) :
0009-2614/91/s 03.50 0 199I - Elsevier Science Publishers B.V. (North-Holland )
73
Volume 179, number 1,2
CHEMICAL PHYSICS LETTERS
pig. 1. Structure of the compounds studied.
(1) Many studies [ 5-81 have been directed to the influence of the length as well as the structure of the interconnecting n-system on the molecular hyperpolarizability p. Thus, Barzoukas et al. [ 71 demonstrated an almost quadratic dependence of p on the number of double bonds separating the donor and acceptor groups in their “push-pull” systems. Here, we report the results of a study on the nonlinear optical properties of bichromophoric molecules l-3 (see fig. 1). These molecules consist of a phenylsubstituted nitrogen eltctrori donor and an electronegatively substituted ethyl&e acceptor interconnected by a saturated hydrocarbon unit. Thus, the donor and acceptor chromophores are expected classically to be isolated from each other.
2. Experimental The syntheses of compounds l-3 have been described elsewhere [ 9, lo]. Electronic absorption spectra were measured on a Gary 17D spectrophotometer, while electronic emission spectra were determined using a Spex fluorolog2 spectrofluorimeter. The spectral distribution of the emission spectra for l-3 is known [ lo,11 ] to be independent of the excitation wavelength, in the present measurements, excitation was performed at 350 nm.~Emission.spectra were corrected for the spectral response of the detection system to represent relative fluorescence intensities in quanta per wavelength interval. All solvents used were of the highest grade‘of 74
., 12ADril 1991
purity commercially available and carefully deoxygenated by purging with argon before measurement of the emission spectra. Electric-field-induced second-harmonic generation on polymethyl methacrylate films, containing typically 1 wt$~of thecompounds under study, at a fundamental wavelength of 1064 nm was measured under constant corona-poling with an applied electric field of = 1.2 MV/cm at 95”C, just below the TB of the matrix. Molecular volumes of the systems studied were calculated with the CHEM-X molecular modeling package (Chemical Design Ltd., Oxford, UK) using the following van der Waals radii: C: 1.60, N: 1.50, 0: 1.40, H: I .20 A. Semi-empirical SCF MO calculations were performed with the AM1 Hamiltonian employing MOPAC 5.0 [ 121.
3. Results and discussion 3.1. Second-order nonlinear susceptibility Comparison of the second-harmonic intensities measured with a known sample (quartz) allowed calculation of the inner product of the ground-state dipole moment and the molecular hyperpolarizability (Q) as compiled in table 1. 3.2. Ground- and excited-state dipole moments For a discussion of the nonlinear optical properties and extraction of the molecular parameters from those determined via measurements on electric-fieldokiented samples, knowledge of the dipole moment bf the molecules in their electronic ground state (&) and in their electronically excited state (&) is required. Table I Electric-field-induced second-harinonic generation data for 1-3 estimated in a PMMA matrix at 95’C and at a fundamental wavelength of 1064 nm Compound
@(lO-“esuD)
1 2 3
26f 10 802 13 90? 15
Volume 179, number 1,2
12April 1991
CHEMICAL PHYSICS LETTERS
Tt u
pg= 1.74 D
the local dipole moment of the acceptor group which determines the overall magnitude and direction of K. For 2, the value of pg depends rather strongly on the orientation of the carboxymethyl substituent. From X-ray diffraction data [ lo], this substituent is known to occupy, an s-cis orientation (&z= 1.74 D) with respect to the exocyclic double bond in the solid state. From AMI calculations, this orientation is found to be x 1.5 kcal/mol more stable than the alternative s-trans orientation (~~~3.94 D), implying that under the conditions of the SHG measurements (95”C), both orientations may be populated, the scis, however, being present predominantly. It is well documented [ 13,141 that the solvent dependence (solvatochromism) of electronic absorption and emission spectra pravi&d a useful experimental tool to estimate the change in dipole moment between ground and excited state. As published earlier [ 10,151, efficient through-bond interaction between donor and acceptor via the intervening saturated hydrocarbon skeleton in systems like l-3 gives rise to a discrete intramolecular charge-transfer (CT) absorption in the 350 nm region not overlapped by the absorptions of the skparate donor and acceptor chromophores, that occur at significantly shorter wavelength ( ~300 nm). Furthermore, these molecules fluoresce [ 111 from the intramolecular chargetransfer state populated either by direct excitation in their CT absorption or via internal conversion from higher excited states. Table 2 compiles absorption and emission data for 1-3 in a series of solvents, while fig. 3 displays the typical spectra of 3 in two such solvents_ As evident from table 2 and from fig. 3, the eompounds display a huge solvatochromism of their fluorescence and a minor solvatochromism of the ab-
1 2
J 3
Fig. 2. Orientation and magnitude ofthe ground-state dipole moment (tip) in l-3 derived from AM1 calculations.‘For 2, the results for both the s-cis and the s-trans rotational minima of the carboxymethyl group are shotin.
Ground-state dipole moments of the molecules l3 were calculated employing the SCF MO semi-empirical AM1 .method. Results are compiled in fig. 2, which also shows the orientation of the calculated dipole moments with respect to the molecular framework. For 1 and .3, rather similar values (4.09 and 3.37 D, respectively) were found although the orientation of the donor phenyl group was taken to be equatorial in 1 and axial in 3 #I. Evidently, it is mainly ” X-ray analysis [ 9,IO] has shown that for all compounds studied the phenyl group adopts an axial orientation in the solid state, In solution, 2 and 3 retain the axial orientation while both axial and equatorial conformations are populated by 1.
Table 2 Absorption and fluorescence maxima, u, (ur) (in 10m3cm-’ ), of l-3 in various solvents (see text for definition ofthe solvent parameters’
Sandf’ ) Solvent
f
f’
1
2
3
n-hexane cyclohexane di-n-butyl ether diisopropyl ether diethyl ether ethylacetate
0.1850 0.2020 0.2920 0.3290 0.3400 0.3850
0.1864 0.2040 0.1952 0.1840 0. Ii75 0.1854
29.50 (22.30) 29.24 (22.20) 29.07 (19.50) 29.07 (19.00) 29.07 (18.50) 28.99 (16.90)
29.41 (21.60) 29.24 (21.40) 29.24 (19.00) 29.24 (18.80). 29.24 (18.10) 29.15 {16.80)
28.17 (21.00) 28.01 (20.80) 27.78 (19.00) 27.78 (18.30) 27.78 (17.90) 27.78 (16.40)
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Volume 179, number 1,2
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sorption. This is typical for systems with a small dipole moment in the ground state (H) and a large dipole moment in the emissive excited state &). It has been shown [ 13,141 that treating the molecular dipole as a point dipole and the solvent as a continuous dielectric, the change in dipole moment between ground and excited state (A,u~~=p,-- & in D) can be derived #2from the solvatochromic shifts of the absorption and emission frequencies (v,, respectively, vf, in cm-‘) via 250
350
550
450
650
Wavelength
=const.t 10070Cf-f’)(A&)2/p3.
Fig. 3. Electronic absorption and emission spectra of 3 in cyclohexane () and in ethylacetate (---).
f-f 0-r -0.10
’
1
0.00
.
1
0.10
r
’
0.20
Fig. 4. Solvatochromic shifts (v,,- or) of l-3 as a function of the solvent polarity If-f ’ ). For clarity, the data points for 2 and 3 have been vertically offset by IO000and 20000cm-‘, respectively.
Table 3 Molecular volume ( V), and effective radius (p) of the molecules l-3asdetermined from molecular modeling, the slope of the correlation (see fig. 4) between the difference of their absorption and emission maxima as a function off-f ’ Bnd the change in dipole moment between ground and first excited state (Ape,) calculated therefrom via eq. (2) Compound
Y (AS)
p (A)
~(&Z/~P’ (cm-‘)
A/+ (D)
1
478 592 577
4.85 5.21 5.16
22958 20968 18652
16.1 17.1 16.0
2 3
76
(2)
In eq. (2),fandS ’ are parameters related to the solvent static dielectric constant (E) and refractive index (n) as f=(t-1)/(2Etl) and f=(n’-l)/ ( 2n2 t I), while p (in A) represents the effective radius of the solvent cavity occupied by the molecule. Thus, by plotting the difference of the fluorescence and absorption frequencies versus the solvent parameters (j-f ’ ) , a linear correlation should be found with a slope proportional to (A@‘, under the assumption that ,uL,and pg have (nearly) the same direction. In view of the charge-transfer nature of the excited state, pe must evidently be directed from D to A along the long axis of the molecules. The direction of ps, as estimated from AM1 calculations (see fig. 2), indeed closely coincides with this axis. As demonstrated in fig. 4, the absorption and emission data of l-3 nicely obey the correlation predicted by eq. (2). From the slopes of these correlations (see table 3), A,u~~can then be calculated if p is known. For nonspherical molecules, an effective pvalue is obtained by taking the radius of a sphere with a volume equal to the molecular volume (V) according top = ( 3 V/4x) ‘I3 . Molecular volumes were determined from molecular-modeling calculations. The results compiled in table 3 show that upon excitation the dipole moment for the present systems w2In principle, iftheir relative orientation is known, both & and a can be extracted from the combined solvatochromism of absorption and fluorescence. Under the present conditions (k z+ H), however, this procedure leads to a large degree of uncertainty ink. Furthermore, the AM 1 calculations showed the charge distribution in the ground state to be multipolar in nature, making the dipolar model underlying eq. (2) less ap plicable for the ground-state solvation.
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increases by 16.5 + 0.5 D units. This corresponds to full charge separation over 3.44 + 0.1 A, a distance compatible with that between the donor nitrogen and the center of the acceptor. 3.3. Molecular hyperpolarizabilities Employing the ground-state dipole moment values estimated above, the molecular hyperpolarizability, p, was calculated from the macroscopic hyperpolarizabilities observed (see table 1). The results (&) are given in the last column of table 4. Although the macroscopic hyperpolarizabilities of these materials do not seem of direct interest for nonlinear optical application, the molecular hyperpolarizabilities are surprisingly high if one regards the chromophores as electronically#isolated. In fact, compounds 2 and 3 display Pvalues comparable to that of a strongly coupled D-A system like 4nitroaniline for which /?( 1064 nm ) = 34.5 x 10B3’ esu has been reported
12 April 1991
bond interaction is optimized for an axial orientation of the phenyl group in systems like 1-3. Evidently such an axial orientation is more highly populated in the tropane derivatives 2 and 3, where the additional bridging ethylidene unit sterically destabilizes the equatorial orientation of the phenyl group, than in 1. This difference in conformation has been demonstrated [ 9,101 to result in stronger chargetransfer absorption of the tropane derivatives and now also may be inferred to be related to their higher P-values (see table 4). It is, therefore, of interest to investigate the applicability of the two-level model underlying eq. ( 1) for estimating the CT contribution to the molecular hyperpolarizability of 1-3. The oscillator strength (F) of the charge-transfer transition (see table 4) was calculated from the absorption spectra via eq. (3 ), F=4.3x 1O-9 E,,,Av,,~,
(3)
where emaxis the maximum molar absorption coefficient and Av,,~ is the full band width at half height (in cm-‘), which then can be related to the transition dipole moment via eq. (4),
PI. From absorption and X-ray diffraction measurements, Krijnen et al. [ 9,10,15] stipulated that for molecules like l-3 significant electronic interaction between the chromophores occurs via the intervening o-bonds. As discussed above, this interaction is manifest from the long-wavelength absorption and the corresponding emission of these compounds (see fig. 3), which demonstrate that in spite of the lack of direct x-conjugative interaction between the donor and acceptor sites, a low-lying intramolecular charge-transfer state is available, a feature generally considered to be essential for second-order nonlinear optical properties. It was shown by Krijnen et al. [ 9, lo], that through-
Fc4.7~ 1O29v,,, 1~~ I*,
(4)
where v,, is the position of the maximum of the charge-transfer absorption band (in cm-l ) and &e is the transition dipole moment. Comparison of the calculated &--values with the experimental molecular hyperpolarizabilities (see table 4) shows that indeed PCTcan account for a major component of /?, thus substantiating the hypothesis that the hyperpolarizability of the present system is mainly governed by the presence of the through-bond-mediated charge-transfer interaction, We have shown before
Table 4
Molarexctinction (c), halfwidth (Av, ,a) and oscillator strength (F) aftbe intramolecular charge-transfer absorption of l-3 (in a-hexane at 20°C) as well as the CT contribution to the molecular hyperpolarizability &) calculated from these data via eq. (1) and the experimental hyperpolarizability (&_~) determined from the electric-field-induced second-harmonic-generation efficiencies (table I ) employing AM1 ground-state dipole moments e (!?mol-’ cm-‘)
k/l (cm-‘)
F
lb ( lO-‘Oesu)
B-4 ( 10e30esu)
2
2970 5800
4500 4700
0.057 0.117
3.7 8.1
6.4k2.5 46 f7.5 *’ 20.3k3.3 b,
3
6390
4200
0.115
9.1
26.7k4.5
1
a) EmployingH value calculated for the s-cis conformation.
b, Employing& value calculated for the s-trans conformation.
77
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CHEMICAL PHYSICS LETTERS
[ 10,161, that the CT transition of systems like those presented here derives part of its intensity from coupling to higher (locally) excited states, implying that a simple two-level model cannot be expected to describe quantitatively the hyperpolarizability connected to this transition.
l2April 1991
Acknowledgement We express our sincere gratitude to Dr. A.M. Brouwer performance and interpretation of the AM1 calculations; furthermore, use of the services and facilities for molecular modeling of the Dutch National NWO/SURF Expertise Center CAOSjCAMM is acknowledged with special thanks to Dr. J.H. Borkent for assistance.
4. Co~eludingremarks In this Letter, we have demonstrated that molecules in which donor and acceptor are connected by a Saturated o-bond system (D-o-A) .can exhibit efficient s&ond-harmonic generation, probably as a result of through-bond interaction. It should be noted that a similar mechanism has been .proposed earlier to explain SHG by the mono N-oxide of diazabicyclooctane [ 171, for which, however, no molecular hyperpolarizability value appears to have been determined. The Bvalues now estimated for 2 and,3 are comparable to such commonly studied D-n-A molecules as p-nitroaniline. From the analysis given above in the context of a two-level model, it is evident that the present D-o-A systems derive their remarkably large molecular hyperpolarizability mainly from a high value of Afl=, corresponding to complete charge separation in the excited state as opposed to virtually none in the ground state. For fully conjugated systems of comparable size, ApScis expected to be less, due to significant mixing of the charge-transfer state into the ground state. On the other hand, however, the transition dipole moment, ,u~~for a nconjugated system will in general be larger, implying a trade-off between ApgBe and bgeas a function of the electronic interaction between donor and acceptor sites. At the moment, it is hard to say what degree of electronic interaction will establish qptimum conditions for achieving high molecular hyperpolarizability and it is, therefore, of considerable interest to investigate systems in which the interaction is systematically varied between the extremes presented by pure x-conjugation and mere through-a-bond interaction.
78
References [,I 1D-J. Williams, ACS Symp. Ser. No. 233 (Am. Chem. Sot., Washington, 1983); D.S. Chemla and I. Zyss, eds., Nonlinear optical properties of organic molecules and crystals, Vols. 1, 2 (Academic Press, New York, 1987). [2] J.F. Nicoud and R.J. Twieg, in: Nonlinear optical properties oforganic molecules and crystals, Vol. 1, eds. D.S. Chemla and J. Zyss (Academic Press, New York, 1987) p, 227. [3] D.J. Williams, Angew. Chem. 96 (1984) 637. [4] B.F. Levine, Chem. Phys. Letters 37 (1976) 516; J.L. Oudar and D.S. Chemla, J. Chem. Phys. 66 (1977) 2664; J.L. Oudar, J. Chem. Phys. 67 (1977) 446; A. Dulvicand C. Dauteret, I. Chem. Phys. 69 (197%) 3453. [5] J.O. Morley, V.Y. Docherty and D. Pugh, J. Chem. Sot. Perkin. Tram II (1987) 1351. [6] T.M. Leslie, R.N. Martino, E.W. Choe, G. Khanarian, D. Haas, G. Nelson, J.B. Stamatoff, D. Stuetz, CC. Teng and Y.N. Yoon, Mol. Cryst. Liq. Cryst. ,I53 (1987) 451. [ 71 M. Banoukas, M. Blanchard-Desce, D. Josse, J.-M. Lehn and J. Zyss, Chem. Phys. 133 ( 1989) 323. [ 81L.T. Cheng, W. Tam, G.R. Meredith, G.L.J.A. Rikken and E.W. Meijer, in: Proc. SPIE, Vol. 1147. Nonlinear optical properties of organic materials, ed. G. Kbanarian ( 1990) p. 61. [ 91 B. Krijnen, H.B. Beverloo, J.W. Verhoeven, C.A. Reiss, K. Goubitz and D. Heijdenrijk, J. Am. Chem. Sot. 111 ( 1989) 4433. [ lo] B. Krijnen, Thesis, University of Amsterdam, Amsterdam (1990). [ ll] R.M. Hermani, N.A.C. Bakker, B. Krijnen and J.W. Verhoeven, J. Am. Chem. Sot. 112 (1990) 1214. [ 121J.J.P. Stewart, QCPE program 455 (1989). [ 13] E. Lippert, Z. Elektrochem. 61 ( 1957) 962. [ 14 ] P. Suppan, J. Photochem. Photobiol. SO( 1990) 293. [ 15] B. Krijnen, H.B. Beverloo and J.W. Verhoeven, Rec. Trav. Chim. 106 (1987) 135. [ 161 P. Pasman, F. Rob and J.W. Verhoeven, J. Am. Chem. Sot. 104 (1982) 5127. [ 17] P. Mihailovic, P. Bassoul and J. Simon, Chem. Phys. Letters 141 (1987) 462; J. Simon, P. Bassouland S. Norvez, New J. Chem. 13 ( 1989) 13.