Three-Branched Dendritic Dipolar Nonlinear Optical Chromophores

Oct 30, 2008 - Department of Chemie, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Department of Chemistry - Physical Chemistry,...
0 downloads 0 Views 675KB Size
Download PDF
J. Phys. Chem. B 2008, 112, 14751–14761

14751

Three-Branched Dendritic Dipolar Nonlinear Optical Chromophores, More than Three Times a Single-Strand Chromophore? Jan Holtmann,† Edita Walczuk,† Markus Dede,† Christian Wittenburg,† Ju¨rgen Heck,*,† Graziano Archetti,‡ Ru¨diger Wortmann,⊥,‡ Hans-Georg Kuball,*,‡ Yan-Hua Wang,§ Kai Liu,§ and Yi Luo*,§ Department of Chemie, UniVersity of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Department of Chemistry - Physical Chemistry, Technische UniVersita¨t Kaiserslautern, Erwin-Schro¨dinger-Str., D-67663 Kaiserslautern, Germany, Theoretical Chemistry, Royal Institute of Technology, AlbaNoVa, S-106 91 Stockholm, Sweden ReceiVed: March 18, 2008; ReVised Manuscript ReceiVed: July 29, 2008

To elucidate the dependence of the nonlinear optical (NLO) response on the conformation of triply branched derivatives, a new series of D-π-A dendrimers has been synthesized. A combined approach of experiments (UV-vis and EOA measurements) and computational predictions (semiempirical and ab initio) was applied both on the dendrimers and on the corresponding single-strand chromophores. It has been shown that depending on the surrounding media the NLO activity of a flexible dendrimer can be very different. Two limiting cases are proposed: (i) the dendrimer resembles a solution of the corresponding single-strand chromophores with about 3-fold concentration, where the hyperpolarizability is the sum of the effect of three noninteracting single-strand subunits (“independent chromophores” limit); (ii) the dendrimers show nearly parallel or helical alignments of the single-strand subunits. Because of this change of conformation the NLO activity can be enhanced up to nine times the value of the “independent chromophores” limit and, thus, are more than a single strand chromophore. Conformers of dendrimers with interacting single-strand chromophores have been identified experimentally in nonpolar solutions by the EOA spectroscopy and possible structures have been revealed by numerical calculations, which could moreover show the tendency of the effects on the hyperpolarizability due to structural changes of the flexible dendritic architecture. Implications for future research developments are given to implement the “more than three times” concept. Introduction 3-Fold dendritic architectures of dipolar chromophores have been shown to be superior in NLO activities in polymers compared to the corresponding single-strand dipolar chromophores.1 For example, 3D-shaped dendritic material provided with dipolar nonlinear optical chromophores (NLOphores) embedded into a polymer demonstrates considerable enhancement of the electro-optic (EO) activity with respect to polymers containing the corresponding single-strand dipolar NLOphores in a guest-host relationship (r33 ) 60 pm/V vs 30 pm/V).1a The dendritic structure imposes a local order on the NLOphores, counteracting the antiferroelectric interaction between the dipolar NLOphores1a in particular with increasing concentration of the dipolar chromophores (Figure 1). In this publication we want to report under which conditions a three-branched dendritic design of dipolar D-π-A NLOphores (Figure 1) demonstrate intrinsic advantages in NLO activities, at the molecular scale, compared to the single-strand dipolar chromophores, which adds to the mentioned material-level benefits. To elucidate their NLO activity, single-strand dipolar NLOphores and dendrimers were subjected to electro-optical absorption (EOA) measurements in 1,4-dioxane and methylcyclohexane as well as to computations as alternative methods to determine the ground-state dipole and * To whom correspondence should be addressed. E-mail: juergen.heck@ chemie.uni-hamburg.de. † University of Hamburg. ‡ Technische Universita ¨ t Kaiserslautern. § Royal Institute of Technology. ⊥ Deceased on March 13, 2005.

Figure 1. Dendritic target structure containing dipolar D-π-A NLOphores: a donor (D) and an acceptor (A) are linked via a π-bridge, which warrants electronic communication between D and A.

the static first hyperpolarizability and to analyze the relation between the geometry of the dendritic compounds and their NLO response in the limiting case of nonpolar solvents, that is, the gas phase. Results and Discussion Synthesis. The initial step toward the three-branched dendritic structures was a hydrosilation reaction of the tris(allyl) silane derivative 1 with chlorodimethyl silane in the presence of H2PtCl6 as the catalyst. Compound 2 was obtained in good yield (Scheme 1).2 In a subsequent condensation reaction, the dipolar single-strand D-π-A compounds 3a-c (single-strand chromophores), which were synthesized with slight modifications

10.1021/jp802369c CCC: $40.75  2008 American Chemical Society Published on Web 10/31/2008

14752 J. Phys. Chem. B, Vol. 112, No. 47, 2008 SCHEME 1: Hydrosilation of Tris(allyl)phenylsilane

SCHEME 2: Condensation Reaction for Generating the Dendritic Structures 4a-c

according to the literature,3,4 are linked to the Si functions (Scheme 2). The new dendritic compounds 4a-c were characterized by means of NMR techniques, elemental analysis, and MALDI-TOF mass spectrometry. Spectroscopy. UV-Visible. The optical absorption spectra of the single-strand chromophores 3a-c and of the correspond-

Holtmann et al. ing dendritic compounds 4a-c possess a strong intramolecular charge transfer (CT) band in the long-wavelength region of the UV-vis spectrum (Figure 2, Figure 3, and Supporting Information). Both, the increase in conjugation length (3a,b) and the change of the acceptor group (3b,c) lead to bathochromic shift of the CT band. While solutions of the former in 1,4-dioxane lead also to hyperchromism, the latter result in a remarkable hypochromism (Table 1). The UV-vis spectra of the dendritic compounds 4a-c in 1,4-dioxane resemble in shape and position those of the single-strand chromophores 3a-c with exception to the size of the absorption coefficients (Table 1). The absorption bands and the integral absorptions of the dendritic chromophores are about three times larger than those of the related single-strand chromophores. Thus there is an additivity of the absorption spectra in going from 3a, 3b, and 3c to 4a, 4b, and 4c.5 No exciton splitting has been found (Figure 2 and Supporting Information) and up to a concentration of about 10-4 M in 1,4-dioxane no change in the structure of the absorption bands of the single-strand chromophores was observed, which suggests that in 1,4-dioxane no aggregation exists.6 Small changes in the UV-vis spectra in 1,4-dioxane appearing with concentrations up to 10-2 M may be originated through aggregation. Small differences between the absorption band of single-strand chromophores and dendrimers have been also observed in methylcyclohexane (MCH) (Figure 3 and Supporting Information) for which no quantitative data can be given, since the very low solubility did not allow for the investigation of the concentration dependence of the UV-vis spectra in MCH. NLO Properties. HRS Measurements. The single-strand dipolar chromophores 3a and 3c as well as the three-branched dendritic chromophores 4a and 4c were in first attempt subjected to hyper-Rayleigh scattering (HRS) studies.7-10 As the compounds absorb substantially in the area of 532 nm (18797 cm-1) [i.e., I(2ω) when the incident laser beam has the wavelength of 1064 nm (9398 cm-1)], the wavelength of the stimulating laser beam was shifted to 1500 nm (6667 cm-1). Thus, the superposition of the low energy absorption and the HRS signal will be minimized, making the calculated static hyperpolarizability11 β(0;0,0) ) β0 more reliable.12-16 Another important reason for using higher wavelengths for the stimulating beam is an attempt

Figure 2. UV-vis absorption spectra of single-strand chromophores 3a (---) 3b (s), and 3c ( · · · ) in 1,4-dioxane (T ) 298 K, ca. 10-6 M).

Three-Branched Dendritic NLOphores

J. Phys. Chem. B, Vol. 112, No. 47, 2008 14753

Figure 3. Normalized UV-vis absorption spectra of 3b (s,---) and 4b ( · · · ,-••-) in methylcyclohexane (s, · · · ) and 1,4-dioxane (---,-••-) (T ) 298 K, ca. 10-6 M).

TABLE 1: CT Band Absorption Maxima λeg, Electronic Transition Dipole Moment µeg, Molar Decadic Absorption Coefficient ε, Ground-State Dipole Moment µg, Change of the Dipole Moment upon Electronic Excitation ∆µ, Static First Hyperpolarizability β0, and Figure of Merit µgβ0 of 3a–c, 4a–c in 1,4-Dioxane and Methylcyclohexane (MCH) (T ) 298 K)a 3a 4a 3b 4b 3c 4c

solvent

λeg

MCH 1,4-dioxane MCH 1,4-dioxane MCH 1,4-dioxane MCH 1,4-dioxane MCH 1,4-dioxane MCH 1,4-dioxane

472.6 481.3 474.0 479.0 477.6 488.0 481.8 486.2 491.4 507.4 493.8 508.1

28.6 45.9 31.3 32.8 56.4 26.0 47.0

ε

µgb

∆µ

c2

β0c,d

µgβ0

35333 92990 42313 44259 129649 25988 86578

26.3 ( 0.8 25.1 ( 0.5 27.4 ( 0.7 24.7 ( 0.4 23.8 ( 0.5 23.2 ( 0.4 40.9 ( 0.3 24.2 ( 0.5 16.6 ( 0.8 16.9 ( 0.4 29.0 ( 0.3 21.1 ( 0.3

76.8 ( 2.3 80.0 ( 1.9 72.8 ( 2.0 70.1 ( 1.5 105.4 ( 1.9 96.9 ( 1.4 77.1 ( 1.4 85.4 ( 1.8 106.2 ( 3.5 103.4 ( 1.9 83.7 ( 1.1 89.9 ( 1.2

0.093 ( 0.003 0.070 ( 0.002 0.086 ( 0.002 0.053 ( 0.002 -

212.9 229.8 524.3 515.8 358.1 376.2 865.2 974.9 263.3 273.4 686.7 780.3

5600 5760 14365 12760 8525 8715 35400 23620 4371 4632 19887 16422

λeg (/nm); µeg (/10 C m); ε (/M cm-1); µg (/10-30 C m); ∆µ (/10-30 C m); β0 (/10-50 C V-2 m3); µgβ0 (/10-80 C2 V-2 m4). b The dipole moments for the dendrimers determined by the EOA spectroscopy are given as partial dipole moments of the single-strand subunits of the dendrimers (see text and Supporting Information). The dipole moments of the dendrimers as given in Table 2 are dipole moments of the conformers as given in Figure 9. c Data have been calculated according to the Taylor convention (ref 18, 23); dipole moments and hyperpolarizabilities are solvent corrected by Onsager’s continuum theory. d Except for 3b, β0 and µgβ0 in methylcyclohexane (MCH) have been calculated using the µeg determined in 1,4-dioxane. a

-30

µeg

-1

to avoid a two-photon absorption-induced fluorescence (TPF). The HRS examinations of the 1500 nm incident beam were achieved by using a tunable optical parametric oscillator (OPO)based setup.13 The optical first hyperpolarizability β could only be determined for 3c in DMSO (β ) 1050 × 10-30 esu ) 390 × 10-50 C V-2 m3; Taylor convention), while the single-strand chromophore 3a and the dendritic chromophores 4a and 4c displayed strong intensities of the scattered light not only at 750 but also at 700 nm. The β of 3c is considerably smaller than the originally published value (β ) 21834 × 10-30 esu ) 8100 × 10-50 C V-2 m3),4 where the unusual large β is certainly due to an underestimation of resonance enhancement. In this context it is noteworthy to point out that the experimental β0 value of 3c from our experiment very well fits the theoretically calculated β0 value published by J. Hua et al.:4 β01500nm (3c) ) 155 × 10-50 C V-2 m3, β0calcd ) 122 × 10-50 C V-2 m3. Because of the difficulties of separating the HRS signal from

fluorescence, especially for the case of the dendrimers, we opted for a different spectroscopic method, namely the electro-optical absorption (EOA) spectroscopy, which can be applied also to very dilute solutions.17 EOA Measurements. Working Equations-Flexible Molecules. EOA spectroscopy is a suitable method for obtaining dipole moments of the ground (µg) and excited (µe) electronic states and allows for the evaluation of hyperpolarizabilities by measuring the absorption of an isolated band of a rigid compound in a dilute solution in an electric field F.17,18 Experimentally the very small absorption difference εF(φ,ν˜ ) - ε(ν˜ ) is measured with light linearly polarized parallel (φ ) 0°) and perpendicular (φ ) 90°) to F as a function of the wavenumber ν˜ (eq 1). εF(φ,ν˜ ) and ε(ν˜ ) are the molar decadic absorption coefficients of the solution in the presence and in the absence of the externally applied electric field, respectively (see also Supporting Information):

14754 J. Phys. Chem. B, Vol. 112, No. 47, 2008

L(φ, ˜ν ) )

˜) 1 εF(φ, ˜ν ) - ε(ν ε(ν ˜) F2

Holtmann et al.

(1)

The electrodichroism is positive (negative) when the absorption in the electric field increases (decreases) for parallel (φ ) 0°) and decreases (increases) for perpendicular (φ ) 90°) detection in comparison to the isotropic solution.17 For the dendrimers 4 equilibria of conformers with very different geometries may exist. The EOA is then given by the sum over all contributions of all conformers (j) in solution (εF(φ,ν˜ ) - ε(ν˜ ))j. Thus, within the limits of Lambert-Beer’s law, eq 1 has to be replaced by

L(φ, ˜ν )E(ν ˜ ) ) (EF(φ, ˜ν ) - E(ν ˜ ))

1 ) F2

∑ Lj(φ, ˜ν )εj(ν˜ )cjd j

(2) with L(φ,ν˜ )E(ν˜ ) being the experimental measured EOA spectrum as a mean over all contributions of all conformers (j). E(ν˜ ) (eq 3) is the extinction of the solution without the electric field; εj(ν˜ ) and cj are the molar decadic absorption coefficient and the concentration of the conformer j; and Lj(φ, ν˜ ) is the electrooptical dichroism (εF(φ,ν˜ ) - ε(ν˜ ))j of a conformer j in solution, respectively.

E(ν ˜) )

∑ εj(ν˜ )cjd

(3)

j

The electrostriction of the solution is neglected in eq 2. Taking coupled equilibria of conformers cj ) Kjrcr into account, then

L(φ, ˜ν )E(ν ˜) )

(∑ L (φ, ˜ν )ε (ν˜ )K )c d j

j

jr

r

(4)

j

The index r in eq (4) refers to the concentration of the chosen reference conformer r. For each conformer at least three similarly absorbing groups, the single-strand subunits, determine by overlapping bands εkj (ν˜ ) the absorption coefficient εj(ν˜ ) of a conformer j of dendrimers like 4:

εj(ν ˜) )

∑ εjk(ν˜ )

(5)

k

Then, the electro-optical dichroism Lj(φ, ν˜ ) is given by the sum over differences (εF(φ,ν˜ ) - ε(ν˜ ))kj of different transitions (see Supporting Information). For very flexible dendrimers as 4a-c, the application of eq 2-5 is restricted since it requires the knowledge of the equilibrium constants Kjr and the contribution of every single transition k to the electro-optic absorption. Therefore, molecular properties of flexible molecules are only accessible under suitable conditions (see Supporting Information). Depending on the mutual interactions between the single-strand subunits, two possibilities will be discussed for dendrimers with equivalent single-strand subunits such as 4: (i) The “independent chromophores” limit: the dendrimers are very flexible, and no interactions between the subunits exist. The absorption bands of the single-strand subunits of the dendrimers 4, εkj (ν˜ ) ) εj(ν˜ ) for k ) 1, 2, 3 of the single-strand chromophores 3a-c, are in 1,4-dioxane approximately equal in position and band shape. Therefore, the dendrimer’s absorption results approximately in ε(ν˜ ) ) 3εj(ν˜ ). Then, from eq 2 it follows that

L(φ, ˜ν )E(ν ˜) )

∑ [εj(˜ν)cjd(∑ Ljk(φ, ˜ν ))] j

(6)

k

The orientation of the single-strand subunits in the electric field is no longer correlated by the rest of the molecule and, thus, all

TABLE 2: Calculated Resultsa,b for Ground-State Dipole Moment µg, Dipole Change upon Electronic Excitation ∆µ, Projection of the Static First Hyperpolarizability on the Dipole Moment βµ, and Product µg · β0 of 3a-c and Selected Conformers of 4a and 4c 3a 4a_1c 4a_2c 4a_3c 4a_4c 3b 3c 4cc

µg

∆µ

βµb

µg · β0

µg · β0 ratio

energy

36.9 38.1 42.4 54.7 69.8 39.1 30.6 38.6

75.4 133.9 113.4 140.5 107.1 80.7 -

65.3 117.9 127.4 144.3 152.1 129.0 129.2 340.4

2406 4490 5405 7882 10623 5037 3950 13131

1 1.87 2.25 3.28 4.41 1 1 3.32

-1084.382 -4352.978 -4353.110 -4355.475 -4354.257 -1432.662 -1758.183 -6376.167

a µg (/10-30 C m); ∆µ (/10-30 C m); βµ (/10-50 C V-2 m3); µg · β0 (/10-80 C2 V-2 m4). b According to Taylor convention (ref 18, 23). c Ground state dipole moment of the conformer (see Supporting Information).

Lkj (φ,ν˜ ) are also approximately equal. The sum over j and k (eq 6) corresponds to the sum over all independent single-strand j p of L(0°,ν˜ ) and subunits. Then also a linear combination L L(90°,ν˜ )19 has to be frequency independent. The partial dipole moments of the ground state and the transition moments of the subunits are apparently parallel; the value of the partial dipole moments equals that of the single strand molecules 3. The EOA spectrum Lεν˜ -1 spectrum is approximately equal to the EOA spectrum of a solution with the 3-fold concentration of corresponding single-strand chromophores. (ii) The dendrimers are rigid and the orientation of every single-strand subunit is well defined with respect to the other by intramolecular interaction or chemical bonds. For dendrimers like 4 the spectral position and band shape of all transitions localized in the subunits are equal and, thus, the linear j p can be frequency dependent or independent. The combination L obtained mean of the ground-state dipole moment lies between zero and three times the one of the single-strand subunits when, for example, all three subunits are parallelly oriented. In solution, the situation of very rigid conformers is almost improbable for dendrimers such as 4. Neighboring chromophores in a fixed orientation with respect to each other give often rise to an exciton coupling, as proven for chiral compounds with the help of circular dichroism (CD) spectroscopy20 and, as recently discussed, for nonlinear optical effects.21 By the exciton coupling differently polarized transitions with different positions in spectral regions of the original j p results CT band come into play and a frequency dependent L if the coupling is not negligibly small (exciton splitting; see Supporting Information). EOA Results-Coupled and Independent Subunits. Experimentally for 3a-c and 4a-c a positive electrochromism has been found in 1,4-dioxane. The drastic enhancement of the L(φ,ν˜ )εν˜ -1 spectrum of the dendrimers in comparison to that of the single-strand chromophores is mainly determined by the increase of ε(ν˜ ) (Figure 4 and Supporting Information). No significant band shift between the single-strand chromophores and the dendrimer CT bands can be observed (Table 1, Figures 3, 5, and Supporting Information). Thus, if exciton coupling exists than it is negligibly small. Furthermore, the analysis of j p of the dendrimers 4a-c in 1,4-dioxane leads to the conclusion L that the long-wavelength CT absorption band is apparently uniformly polarized (Figure 6 and Supporting Information).19 The experimental findings point out that the single-strand subunits in 1,4-dioxane act as “independent chromophores”. In MCH more complex EOA spectra for 4b and 4c are obtained (Figure 7 and Supporting Information). In contrast to

Three-Branched Dendritic NLOphores

J. Phys. Chem. B, Vol. 112, No. 47, 2008 14755

Figure 4. Electro-optical absorption spectra for φ ) 0° (O,3,0) and φ ) 90° (b,1,9) and multilinear regression curves (s, · · · ) for Lεν˜ -1 of 3b (3,1, · · · ) and 4b (O,b,s), and (Lεν˜ -1)3b(εD/εM) of 3b (0,9, · · · ) in 1,4-dioxane (T ) 298 K, E ≈ 4 × 106 V m-1, ca. 10-6 M). εM and εD are the molar decadic absorption coefficients in the maximum of the CT band of the single-strand chromophore 3b and the dendrimer 4b, respectively.

Figure 5. Comparison of the L(φ ) 0°) values of 3b (b,1) and 4b (O,3) in MCH (1,3) and 1,4-dioxane (b,O).

the results in 1,4-dioxane, a clear change of the shape of the L(φ,ν˜ )εν˜ -1 spectra are detected in going from the single-strand chromophores to the dendrimers. Furthermore, in MCH solution j p’s of the dendrimers 4b and 4c are frequency dependent the L and thus the CT band is no longer uniformly polarized (Figure 8 and Supporting Information). A shift of the CT bands of the dendrimers can be observed, and the EOA band changes in its shape. Since the calculated dipole moment (see next section) differs significantly from that of the corresponding single-strand chromophores, in MCH conformers must exist in which differently polarized transitions determine the long-wavelength absorption band of the dendrimers. In MCH the limit of “independent chromophores” is no longer valid. This different experimental finding in MCH must be addressed to the role of the solvent molecules, which take care that in 1,4-dioxane the subunits are far from each other, isolated by solvation. Thus, in MCH the interaction of the subunits give rise to conformers with new spectroscopic features.

The Dipole Moments of the Ground and Excited State. According to the results evaluated by eq 1 the transition dipole moments µeg and the dipole moments µg for 3a-c are oriented approximately parallel (Table 1). Because of the high polarity of compounds 3 the evaluation of µg and of the dipole change upon electronic excitation ∆µ ) µe - µg from the EOA spectra (Figure 4, Figure 7) have been performed by neglecting effects depending on the polarizabilities of the ground and excited state (Supporting Information).22 L(0°,ν˜ ) is in 1,4-dioxane for 4a-c, like for 3a,b, a linear function of L(90°,ν˜ ) with approximately equal slopes of about 3, which is a further hint that the transition moments of the CT bands are polarized approximately parallel to the dipole moment of the ground-state of the subunits. From this follows unequivocally that the independent chromophore model can be applied for the 1,4-dioxane solution of 4. Then, with the assumption that Lkj (φ,ν˜ ) is equal for all subunits, the dipole moments of the single-strand subunits can be evaluated by eq 6. As expected these values for the dendrimers are equal

14756 J. Phys. Chem. B, Vol. 112, No. 47, 2008

Holtmann et al.

j p(1) and long-wavelength CT absorption band of 4b (s) in 1,4-dioxane. L j p ) 6[L(0°, ν˜ ) - 3L(90°, ν˜ )] - 6[G - F] t(ν˜ ) - 6[I - H] u(ν˜ ) Figure 6. L is frequency independent for uniformly polarized absorption bands. G, F, I, and H are multilinear fitting parameters, and t(ν˜ ), u(ν˜ ) are functions characterizing the band shape of the absorption band (see Supporting Information).

Figure 7. Normalized Lεν˜ -1 spectra for φ ) 0° (O,3) and φ ) 90° (b,1) and normalized multilinear regression curves (s, · · · ) of 3b (3,1, · · · ) and 4b (O,b,s) in methylcyclohexane (T ) 298 K, E ≈ 4 × 106 V m-1, ca. 10-6 M). Since the absolute value of Lεν˜ -1 for MCH cannot be calculated because of the uncertainty of the absorption band ε(ν˜ ), the Lεν˜ -1 curves of the single-strand chromophore and dendrimer have been normalized for 3b and 4b.

to µg of the single-strand chromophores 3 (Table 1 and Supporting Information). The increase in conjugation length of the single-strand chromophore (3a,b) leads to a slight decrease in µg, while the change of the acceptor group (3b,c) demonstrates a more dramatic decrease. The dipole moments of 4a and 3a in both 1,4-dioxane and MCH, and of 4b, 3b only in 1,4-dioxane are approximately the same. On the contrary, in MCH solution an increase of µg of about 70% has been detected for 4b and 4c in comparison to their single-strand chromophores (Table 1). In jp addition to this enhancement the frequency dependence of L is a further proof that in MCH solution the single-strand subunits of 4b and 4c do not behave as “independent chromophores”, that is, a stronger interaction leads to a correlated orientation of the subunits. Since the size of µg of the dendrimers with

interacting subunits is expected to be larger than the one found for 4b and 4c in 1,4-dioxane, the detected L(φ,ν˜ ) in MCH is a mean of the effect of different conformers. One should notice also that the µg of 4c and 3c differ somewhat beyond the experimental error in both solvents (Table 1), thus also in polar solvents there are hints for interacting subunits and thus for different conformers. Both the dendrimers and the single-strand chromophores show a distinct dipole change upon electronic excitation (Table 1), which confirms that the long-wavelength absorption bands possess an intramolecular CT character. Whereas µg is approximately constant or increases in 1,4-dioxane by going from the single-strand chromophore to the dendritic structure, the difference of the dipole moments ∆µ decreases by about 13%. For 3b, 4b and 3c, 4c in MCH a detailed analysis is prevented

Three-Branched Dendritic NLOphores

J. Phys. Chem. B, Vol. 112, No. 47, 2008 14757

j p (1) and long-wavelength CT absorption band of 4b (s) in methylcyclohexane (see also caption of Figure 6 and Supporting Information). Figure 8. L

by the unknown equilibrium between the conformers with interacting and independent subunits. Thus, in this case the numbers in Table 1 represent mean values. Still a significant difference is observed for 4b and 4c where the enhancement of µg in MCH is concomitant of a larger decrease in ∆µ. Electronic Character of the Ground State. The ground-state character of a push-pull chromophore can be characterized according to the c2 parameter (eq 7),23 where 0 and 1 represent the neutral (NE) and the zwitterionic structure (ZW), respectively, and 0.5 defines the so-called cyanine limit. Within the scope of the two center model the c2 parameter is for a onedimensional chromophore12 a function of ∆µ and µeg:18b

c2 )

1 [1 - ∆µ(4µ2eg + ∆µ2)-1 ⁄ 2] 2

(7)

According to eq. 7 all single-strand chromophores 3a–c in 1,4-dioxane possess a predominant NE character for the ground state (Table 1). For compounds with independent chromophores like 4a and 4b in 1,4-dioxane, three independent resonance forms contribute to the strengthening of the zwitterionic character. But despite the existence of independent or approximately independent subunits, the c2 values are not very useful for the characterization of dendrimers. NLO Analysis. For 3a-c, within the assumption of onedimensional chromophores, the static hyperpolarizability tensor coordinate along the direction of µg can be calculated according to the two-level-model (TLM) by the use of the Taylor convention:18,23

β(0;0,0) ) β0 )

6(µegλeg)2∆µ (hc)2

(8)

Because of the similar polarity (ET30,24 εr) of MCH and 1,4dioxane and, thus, the small solvent dependence of the integral absorption of the NLOphores, µeg in 1,4-dioxane can be used in good approximation also for MCH. This allows for calculating the β0 values of 3a-c in MCH according to eq 8 (Table 1). The same assumptions as for 3a-c hold also for the independent subunits of 4a-c in the limiting case of the independent chromophore model, for which eq 8 can be applied. Here the subunits are expected to give additive contribution to β0 of the

dendrimer. To avoid an overestimation of β0 within the approximation of eq 8, an adequate value for µ2eg has to be taken into account. Adequate values are those between 2 (subunit) < µ2 < µ2 (dendrimer) because other tensor µeg eg eg coordinates of the dendrimer βijk * βzzz gain intensity on the expense of β0 ) βzzz. In agreement with the observation of Liao et al.,25 the contribution per single-strand subunit within the dendritic structure to β0 is smaller than β0 of the single-strand chromophores (Table 1). Since in 1,4-dioxane the µg does not significantly change in going to the dendritic architecture, the molecular nonlinear optical efficiency, the product µgβ0,17b,18 shows the same trend as for β0. According to the figure of merit (Table 1) in 1,4-dioxane the most efficient single-strand chromophore is 3b (µgβ0 ) 8715 × 10-80 C2 V-2 m4) and the most efficient dendrimer is 4b (up to µgβ0 ) 23620 × 10-80 C2 V-2 m4). Remarkably, this value increases even more for MCH where the interaction between the subunits, respectively, and the induced structural change give their contribution to the nonlinearity of the dendrimer (µgβ0 (4b) ) 35400 × 10-80 C2 V-2 m4). Within the scope of the EOA results a main part of this enhancement is due to the increase of the ground-state dipole moment. Only the quantum chemical calculation can (next section) evaluate the contributions due to the variation of the transition dipole moments by the intermolecular interaction. Exciton coupling should in principle lead to a decrease of the tensor coordinate parallel to the ground-state dipole moment because transition moments change their direction by this type of coupling (see Supporting Information). Computations. General Remarks. The exciton model and the model of independent chromophores can be discussed to interpret the experimentally obtained molecular properties from the electro-optical measurements going from a more polar solvent 1,4-dioxane to a nonpolar solvent MCH. Theoretical calculations of the dipole moments and hyperpolarizabilities have been additionally performed in order to check whether the observed trend by going from higher to lower polarity of the surrounding medium can be extrapolated further into the gaseous phase, leading to an even more enhanced nonlinear optical effect.

14758 J. Phys. Chem. B, Vol. 112, No. 47, 2008

Holtmann et al.

Ground State Geometries and First Hyperpolarizabilities. In this study, we focused on the D-π-A conjugated molecule 3a and its corresponding dendritic structures. The optical properties of the other two single-strand chromophores, 3b,c, have also been calculated for comparison. In Table 2 we have listed the calculated results for the ground-state dipole moment µg, the projection of static first hyperpolarizability on the dipole moment βµ, the product µg · β0, and the µg · β0 ratios for 3a-c, 4a of different conformations and for 4c of one particular conformation of low energy, respectively. The projection βµ is defined according to eq 9

βµ )

µg · β0 |µg|

(9)

Here we use compound 4a and 4c as examples for studying the relationship between the dendritic structures and their first hyperpolarizabilities. We have found several local minima for the dendritic compounds. Comparing the four conformers of dendrimer 4a (4a_1, 4a_2, 4a_3, and 4a_4) as given in Figure 9, one can find that the three branches in structure 4a_4 are more stretched than those in other structures. The dipole moments of the conformers 4a_1 and 4a_2 increase a little compared to that of 3a. However, the dipole moments of the conformers 4a_3 and 4a_4 are almost 1.5 and 1.9 times as large as that of 3a. One conformer of 4c, shown in Supporting Information, revealed the largest βµ value compared to 3c. The calculated ratio of µg · β0 compared to 3c is 3.32 (Table 2). It is worth mentioning that the more the dendritic structure is stretched, the more the dipole moment increases thereby intensifying µg · β0 ratio. The calculated µg · β0 ratios for dendritic structures 4a_1, 4a_2, 4a_3, and 4a_4 are 1.87, 2.25, 3.28, and 4.41, respectively. These results indicate the possibility for obtaining larger µg · β0 ratios experimentally if compounds with suitable geometries can be synthesized. Comparison of Experimental and Theoretical Results. The tendency of the variation of µg and β0 of the single-strand chromophores, due to the structural changes shows an acceptable agreement between the theoretical and experimental results (1,4dioxane) as can be demonstrated by the ratios of dipole moments and hyperpolarizabilities for the single-strand chromophores (Table 3). For 3a and 3b as well as 3c the theoretically predicted absolute value of the ground-state dipole moments for the gas phase are larger than the solvent-effect corrected moments measured in 1,4-dioxane by a factor of 1.5, 1.7, and 1.8, respectively (Tables 1 and 2). Thus, theory overestimates the dipole moments of the ground-state in contrast to the result obtained for the dipole moments of the excited state (Tables 1 and 2). The ratios of the theoretically and the experimentally obtained µg · β0 for 3a-c yield an acceptable agreement (Tables 1 and 2) because the overestimation of the dipole moments is compensated by underestimated β0 values. The structures of the dendrimers are sensitive to solvents as shown experimentally by the different findings for 1,4-dioxane and MCH. The theoretical calculations for the gaseous state do not allow a prediction of the conformations of the dendrimers in different solvents. However, the various structures in the gas phase (Figure 9 and Supporting Information) give a hint that relevant structures with independent chromophores and interacting chromophores (single strand subunits) are possible. The calculated CT state of 3a is located at 489.1 nm with oscillator strength of 0.66. The formation of conformer 4a_3 results in three close-lying CT states, at 489.6, 489.2, and 488.4 nm, with oscillator strengths of 0.65, 0.69, and 0.78, respectively, shows

Figure 9. Some optimized molecular structures of dendrimers 4a with AM1 method.

no observable band shift with respect to the single-strand chromophore. Furthermore, the direction of the calculated transition dipole moments (see Supporting Information) resembles the orientation in space of the single-strand subunits.

Three-Branched Dendritic NLOphores

J. Phys. Chem. B, Vol. 112, No. 47, 2008 14759

TABLE 3: Comparison of the Experimental and Theoretical Ratios of Dielectric Properties of the Single-Strand Chromophores {X(3b)/X(3a)}

{X(c)/X(3a)}

X

theor

exptl

theor

exptl

µg β0 µg · β0

1.1 2.0 2.1

0.9 1.6 1.5

0.8 2.0 2.1

0.7 1.2 1.5

This is in agreement with the presupposition for the independent chromophore model in which the measured L(φ,ν˜ ) is the sum over the Lkj (φ,ν˜ ) values of all individual single strand subunits possessing the same wavenumber dependence. Also the results are consistent with those of the exciton model. Several nonhelical and helical-like elongated conformers of 4a and 4c (Figure 9 and Supporting Information) possess differently polarized transitions in the UV-vis spectral region. These results resemble the findings in the unpolar solvent MCH in which the interaction of the single-strand subunits in the dendrimer lead to a set of conformers where at least one of these conformers possesses differently polarized transitions in the long-wavelength region of the UV-vis absorption spectrum as proven by EOA measurements. Conclusion The comparison of the experimental and theoretical results has shown that the applied theoretical method can be a tool for predicting the response of the single-strand chromophores as well as the variation of µg · β0 as a function of the geometry of the dendrimers. The analysis of the electro-optical and UV-vis absorption measurements of the dendritic structures has led to the conclusion that in 1,4-dioxane a strong interaction of the three identical single-strand subunits is prohibited probably due to the solvation of the subunits. As expected, additional EOA measurements in a lower polar environment (methylcyclohexane) hint at the appearance of conformers where a stronger intramolecular interaction between the single-strand subunits leads to an enhancement of both the dipole moment of the ground-state and the first hyperpolarizability by probably an alignment of the subunits as shown by the numerical calculations. The dendrimers in MCH are not yet in the limit of fully aligned interacting subunits, which means that an even less polar solvent is needed for allowing the dendrimers to align (Supporting Information Figure 4c) or to develop a predominant exciton coupling. A less polar environment is out of the reach of the available EOA setup. The goal of investigating the influence due to conformational changes and/or exciton coupling could be reached by way of computational calculations. Several structures for the dendrimers could be found, where no partially and strongly coupled subunits exist. For structures with strong interaction an unprecedent increase for β0 and µg · β0 up to five times the performances of the single-strand chromophores have been predicted. This increase is governed by two mechanisms: (i) a structural change of the conformers by which the partial dipole moments and the transition moments of the longwavelength transitions of the single strand subunits will be oriented in a suitable parallel or helical arrangement and (ii) an interaction between the subunits which changes their electronic structure. Because the UV-vis spectroscopy of the dendrimers do not show essential deviations from that of the single strand chromophores the structural change must be the predominant effect. Furthermore, exciton coupling itself might be contraproductive because one of the possible exciton transitions will

be polarized more or less perpendicular to the axis of the dipole moment of the dendrimer and, thus, enhance other tensor coordinates of βijk * βzzz to the debit of the tensor coordinate βzzz parallel to the orienting dipole moment. The analysis given in this paper shows the importance of the influence of the solvent,26 or more general of the matrix, on the NLO properties of the analyzed three-branched architecture beyond effects which can be described by the Onsager’s continuums theory. To avoid this strong solvent effect, which may turn to be detrimental, and to maximize the effect due to the dipole moment, the idea of synthesizing tethered dendrimers is more than intriguing. For such structures, as shown by quantum mechanical calculation (to be published), an even stronger increase of µg · β0 is expected in comparison to the corresponding flexible architecture. Furthermore, a tethered structure will lead to an increase in dipole moment independently on environmental effects by at least a factor between 3 and 9. Experimental Section General. The reactions were carried out under a nitrogen atmosphere using Schlenk technique. Tetrahydrofuran and triethylamine were distilled over drying agents under nitrogen before use. Column chromatography was carried out on silica gel (70-230 mesh) and aluminum oxide (70-230 mesh). Mass spectroscopy (MS) measurements were carried out using the matrix-assisted laser desorption ionization time-of-flight (MALDITOF) technique. All samples for MALDI-TOF MS were prepared in a dithranol matrix in tetrahydrofuran. Compounds 1, 2, and 3a-c were prepared according to literature procedures.2-4 (E)-Tris[3-({2-[N-(4-{2-[4-(2,2-dicyanovinyl)styryl]vinyl}phenyl)N-ethylamino]ethoxy}dimethylsilanyl)propyl]silane (4a). Compound 2 (873 mg, 1.70 mmol) was added dropwise to a solution of chromophore 3a (1.96 g, 5.71 mmol) in tetrahydrofuran (200 mL) and triethylamine (1.5 mL). After the addition, the solution was stirred for 19 h. The solvent was removed in vacuo, and the pure product was isolated by column chromatography (Al2O3/10% H2O; dichloromethane and petroleum ether (4:1)). The pure product was obtained as a dark red solid. Yield: 1.53 g (63%). MS (MALDI-TOF): m/z (%) ) 1431.6 (67) [M]+, 1470.6 (40) [M + K]+; IR (KBr pellet): ν˜ ) 3025, 2954, 2912, 2872, 2222, 1693, 1609, 1592, 1569, 1520, 1420, 1400, 1355, 1321, 1273, 1248, 1177, 1143, 1095, 915, 904, 822, 785, 700, 610, 539 cm-1. 1H NMR (400 MHz, CD2Cl2, rel. TMS, 293 K): δ ) 7.85 (d, 3J ) 8.5 Hz, 6H), 7.68 (s, 3H), 7.55 (d, 3J ) 8.5 Hz, 6H), 7.50-7.45 (m, 2H), 7.38 (d, 3J ) 8.9 Hz, 6H), 7.34-7.28 (m, 3H), 7.23 (d, 3J ) 16.2 Hz, 3H), 6.88 (d, 3J ) 16.2 Hz, 3H), 6.64 (d, 3J ) 8.9 Hz, 6H), 3.68 (t, 3J ) 6.2 Hz, 6H), 3.48-3.30 (m, 12H), 1.47-1.30 (m, 6H), 1.13 (t, 3J ) 7.0 Hz, 9H), 0.94-0.80 (m, 6H), 0.72-0.58 (m, 6H), 0.03 (s, 18H) ppm; 13C NMR (100 MHz, CD Cl , rel. TMS, 293 K): δ )-2.0, 12.4, 2 2 17.3, 18.2, 21.3, 45.9, 52.7, 60.2, 79.5, 111.9, 113.8, 114.9, 121.7, 123.9, 125.6, 126.7, 128.0, 129.1, 129.2, 131.8, 134.0, 134.4, 138.2, 145.4, 149.0, 159.0 ppm. Anal. Calcd (%) for C87H101N9O3Si4: C, 72.91; H, 7.10; N, 8.80. Found: C, 72.57; H, 7.29; N, 8.31. (E,E)-Tris{3-[(2-{N-[4-(2-{4-[2-(3-dicyanomethylene-5,5-dimethylcyclohex-1-enyl)vinyl]styryl)phenyl]-N-ethylamino}ethoxy)dimethylsilanyl]propyl}phenylsilane (4b). Compound 2 (778 mg, 1.52 mmol) was added dropwise added to a solution of chromophore 3b (2.30 g, 4.96 mmol) in tetrahydrofuran (200 mL) and triethylamine (1.3 mL). After the addition, the solution was stirred for 19 h. The solvent was removed in vacuo, and the pure product was isolated by column chromatography (Al2O3/10% H2O; dichloromethane and ethyl acetate (9:1)). The

14760 J. Phys. Chem. B, Vol. 112, No. 47, 2008 pure product was obtained as a dark red solid. Yield: 2.07 g (77%). MS (MALDI-TOF): m/z (%) ) 1792.8 (67) [M + H]+, 1831.8 (27) [M + K + H]+; IR (KBr pellet) ν˜ ) 3022, 2954, 2915, 2868, 2217, 1607, 1587, 1558, 1519, 1396, 1326, 1248, 1174, 1149, 1098, 957, 842, 816, 785, 701, 553 cm-1; 1H NMR (400 MHz, CD2Cl2, rel. TMS, 293 K) δ ) 7.49-7.46 (m, 14H), 7.36 (d, 3J ) 8.9 Hz, 6H), 7.34-7.31 (m, 3H), 7.10 (d, 3J ) 16.3 Hz, 3H), 7.07-7.00 (m, 6H), 6.87 (d, 3J ) 16.3 Hz, 3H), 6.82 (s, 3H), 6.65 (d, 3J ) 8.9 Hz, 6H), 3.69 (t, 3J ) 6.4 Hz, 6H), 3.43-3.37 (m, 12H), 2.59 (s, 6H), 2.48 (s, 6H), 1.42 (m, 6H), 1.14 (t, 3J ) 7.0 Hz, 9H), 1.07 (s, 18H), 0.89 (m, 6H), 0.68 (m, 6H), 0.05 (s, 18H) ppm; 13C NMR (100 MHz, CD2Cl2, rel. TMS, 293 K) δ ) -2.0, 12.4, 17.3, 18.2, 21.4, 28.1, 32.2, 39.4, 43.3, 45.9, 52.8, 60.2, 78.3, 112.0, 113.4, 114.1, 122.9, 123.6, 124.7, 126.7, 128.0, 128.3, 128.4, 128.6, 129.0, 130.6, 134.4, 134.5, 134.8, 137.1, 138.2, 140.3, 148.3, 154.6, 169.6 ppm. Anal. Calcd (%) for C114H137N9O3Si4: C, 76.33; H, 7.70; N, 7.03. Found: C, 75.86; H, 7.77; N, 6.52. (E,E)-Tris(3-{[2-(N-{4-[2-(4-{2-[5,5-dimethyl-3-(5-oxo-3phenylisoxazol-4-ylidene)cyclohex-1-enyl]vinyl}styryl]phenyl}N-ethylamino)ethoxy]dimethylsilanyl}propyl)phenylsilane (4c). Compound 2 (185 mg; 0.361 mmol) was added dropwise added to a solution of chromophore 3c (796 mg, 1.42 mmol) in tetrahydrofuran (50 mL) and triethylamine (1.0 mL). After the addition, the solution was stirred for 19 h. The solvent was removed in vacuo, and the pure product was isolated by column chromatography (SiO2/ethyl acetate). The pure product was obtained as a dark red-violet solid. Yield: 539 mg (72%). MS (MALDI-TOF): m/z (%) ) 2116.0 (56) [M + K]+, 2078.1 (8) [M + H]+; IR (KBr pellet) ν˜ ) 3023, 2952, 2914, 2867, 1728, 1605, 1587, 1541, 1519, 1374, 1272, 1249, 1174, 1161, 1101, 1074, 956, 906, 865, 840, 817, 786, 770, 699, 656, 551 cm-1; 1H NMR (400 MHz, CD Cl , rel. TMS, 293 K) δ ) 8.22 (s, 2 2 3H), 7.45-7.22 (m, 35H), 7.21-7.00 (m, 9H), 6.94-6.82 (m, 3H), 6.79-6.68 (m, 3H), 6.63 (d, 3J ) 8.8 Hz, 6H), 3.75-3.61 (m, 6H), 3.45-3.29 (m, 12H), 2.41 (s, 6H), 2.07 (s, 6H), 1.56-1.30 (m, 6H), 1.24-1.10 (m, 9H), 1.09 (s, 9H), 0.93-0.84 (m, 6H), 0.82 (s, 9H), 0.72-0.61 (m, 6H), 0.05 (s, 18H) ppm; 13C NMR (100 MHz, CD Cl , rel. TMS, 293 K) δ ) -2.0, 2 2 12.4, 17.3, 18.2, 21.4, 28.0, 32.3, 39.4, 42.9, 45.9, 52.8, 60.2, 112.0, 124.8, 125.2, 126.6, 126.7, 128.0, 128.2, 128.4, 129.0, 129.1, 129,2, 129.3, 129.8, 130.3, 130.6, 131.6, 131.7, 134.5, 136.6, 140.2, 148.3, 154.8, 156.4, 162.6, 163.4, 163.6, 170.3 ppm. Anal. Calcd (%) for C132H152N6O9Si4: C, 76.26; H, 7.37; N, 4.04. Found: C, 75.15; H, 7.46; N, 3.58. UV-Visible Spectroscopy. All UV-vis spectra were recorded in anhydrous 1,4-dioxane and methylcyclohexane, prepared by distillation from Na under Argon prior to use. All of the spectra required for the evaluation of the integral absorption, band maxima, and the first and second derivatives of the optical absorption spectrum were recorded with a Perkin-Elmer Lambda 900 spectrophotometer at 298 K (scan speed of 50 nm min-1; bandwidth 1 nm; 3 cm quartz glass cuvette). Because of the problem of solubility, the molar decadic absorption coefficient (ε) of 3a, 3c, 4a, 4b, and 4c could not be determined in methylcyclohexane. Hyper-Rayleigh Scattering. The hyper-Rayleigh scattering measurements were performed for liquid solution samples with a pulsed Nd:YAG laser. Since the single-strand chromophores 3a-c and the dendrimers 4a-c absorb substantially in the area of 532 nm [i.e., I(2ω) when the incident light has a wavelength of I(ω) ) 1064 nm], the stimulating laser light was shifted to 1500 nm by an optical parametric oscillator (OPO). The experiments were carried out in CH2Cl2 as described earlier.9

Holtmann et al. As a reference Disperse Red 1 (DR1) (β(CH2Cl2) ) 420 × 10-30 esu ) 156 × 10-50 C V-2 m3; Taylor convention)14 was used. Electro-Optical Absorption Measurements (EOA). All EOA spectra were recorded in anhydrous 1,4-dioxane and methylcyclohexane, prepared by distillation from Na under Argon prior to use. The EOA signal has been recorded with an integration time of 0.5-1 min (depending on the quality of the signal) and p-amino-p′-nitrobiphenyl was used as a calibrating reference. Because of problems of solubility the molar decadic absorption coefficient of 3 and 4 could not be determined in methylcyclohexane and thus Lεν˜ -1 for MCH has been normalized. From a multilinear fitting of Lεν˜ -1 as a function of the wavelength dependence of the absorption bands [t({ν˜ }), u({ν˜ })] five parameters E, D, F, G, H, and I can be obtained which are the basis for the evaluations of the dipole moments µg and ∆µ ) µg - µa and by this µa along the transition dipole moment direction. It should be mentioned that the relations between the molecular parameter and the fitting parameter, given by Liptay et al.,17a have to be modified with respect to the effect of flexible molecules (eq 2-4). For uniformly polarized absorption bands jp with dipole moment parallel to the transition dipole moment L ) Lp({ν˜ }) - 6[G - F] t(ν˜ ) - 6[I - H] u(ν˜ ) has to be independent of the wavelength. Lp({ν˜ }) ) 6[L(0°, ν˜ ) - 3L(90°, ν˜ )] (see Supporting Information). Computational Details. Geometry optimizations were performed with the Austin Model 1 (AM1) 27,28 method. Many local minima have been found for the dendritic structures, among them four for 4a, named as 4a_1, 4a_2, 4a_3, and 4a_4, and one for 4c, labeled as 4c, were selected to demonstrate the structure dependence of the first hyperpolarizability. Calculations for the first hyperpolarizability were performed on the optimized geometries with the hybrid density functional B3LYP.29,30 For the Si atom we have chosen basis set LanL2DZ 31-34 and for all the other atoms basis set 3-21G 35,36 was used. All quantum chemical calculations were carried out using the Gaussian 0337 program package. Acknowledgment. This work is part of, and is supported by, the EU-STREP project ODEON: Design and fabrication of optoelectronic devices based on innovative second-order nonlinear organic nanomaterials. ODEON receives research funding from the European Community’s Sixth Framework Programme. The contents of this article reflect only the authors’ views and the European Community is not liable for any use that may be made of the information contained herein. Supporting Information Available: Additional spectra and calculations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Pereverzev, Y. V.; Prezhdo, O. V.; Dalton, L. R. Chem. Phys. Lett. 2003, 373, 207–212. (b) Ma, H.; Chem, B.; Sassa, T.; Dalton, L. R.; Jen, A. K.-Y. J. Am. Chem. Soc. 2001, 123, 986–987. (c) Ma, H.; Jen, A. K.-Y. AdV. Mater. 2001, 13, 1201–1205. (d) Li, Z.; Qin, A.; Lam, J. W. Y.; Dong, Y.; Dong, Y.; Ye, C.; Williams, I. D.; Tang, B. Z. Macromolecules 2006, 39, 1436–1442. (e) Okuno, Y.; Yokohama, S.; Mashiko, S. J. Phys. Chem. B 2001, 105, 2136–2169. (f) Yamaguchi, Y.; Yokomichi, Y.; Yokohama, S.; Mashiko, S. J. Mol. Struct. 2001, 545, 187– 196. (2) (a) Deng, X.; Mayeux, A.; Cai, C. J. Org. Chem. 2002, 67, 5279– 5283. (b) Speier, J. L.; Webster, J. A.; Barnes, G. H. J. Am. Chem. Soc. 1956, 79, 974–979. (3) (a) Lemke, R. Synthesis 1974, 359–361. (b) Zhang, C.; Ren, A. S.; Wang, F.; Zhu, J.; Dalton, L. R. Chem. Mater. 1999, 11, 1966–1968. (4) Hua, J.; Luo, J.; Qin, J.; Shen, Y.; Zhang, Y.; Lu, Z. J. Mater. Chem. 2002, 12, 863–867.

Three-Branched Dendritic NLOphores (5) Perkampus, H.-H. Encyclopedia of Spectroscopy; VCH: Weinheim, Germany 1995 and references cited therein. (6) This observation is validated for 3a by the results from: Jen, A. K.Y. J. Chem. Soc., Chem. Commun. 1993, 90–92, where no deviation from linearity is reported for the EFISHG analysis in 1,4-dioxane. (7) (a) Terhune, R. W.; Maker, P. D.; Savage, C. M. Phys. ReV. Lett. 1962, 8, 404–406. (b) Clays, K.; Persoons, A. ReV. Sci. Instrum. 1992, 63, 3285–3289. (8) Meyer-Friedrichsen, T.; Wong, H.; Prosenc, M. H.; Heck, J. Eur. J. Inorg. Chem. 2003, 936–946. (9) Farrell, T.; Manning, A. R.; Mitchell, G.; Heck, J.; MeyerFriedrichsen, T.; Malessa, M.; Wittenburg, C.; Prosenc, M. H.; Cunningham, D.; McArdle, P. Eur. J. Inorg. Chem. 2002, 1677–1686. (10) Farrell, T.; Meyer-Friedrichsen, T.; Malessa, M.; Wittenburg, C.; Heck, J.; Manning, A. R. J. Organomet. Chem. 2001, 625, 32–39. (11) (a) Oudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66, 2664– 2668. (b) Hendricksx, E.; Clays, K.; Dehu, C.; Bre´das, J. L. J. Am. Chem. Soc. 1995, 117, 3547–3555. (12) Wolff, J. J.; Wortmann, R. AdV. Phys. Org. Chem. 1999, 32, 121– 138. (13) Stadler, S.; Dietrich, R.; Bourhill, G.; Bra¨uchle, C.; Pawlik, A.; Grahn, W. Chem. Phys. Lett. 1995, 247, 271–276. (14) Lambert, C.; No¨ll, G.; Schma¨lzlin, E.; Meerholz, K.; Bra¨uchle, C. Chem. Eur. J. 1998, 4, 2129–2135. (15) Clays, K.; Hendricks, E.; Verbiest, T.; Persoons, A. AdV. Mater. 1998, 10, 643–655. (16) (a) Wolff, J. J.; Wortmann, R. J. Prakt. Chem. 1998, 340, 99–111. (b) Stadler, S.; Diedrich, R.; Bourhill, G.; Bra¨uchle, C. J. Phys. Chem. 1996, 100, 6927–6934. (17) (a) Liptay, W. In Excited States, Vol. 1, Dipole Moments and Polarizabilities of Molecules in Excited Electronic States; LimE. C., Ed.; Academic Press: New York, 1974; pp 129-229. (b) Wortmann, R.; Elich, K.; Lebus, S.; Liptay, W.; Borowicz, P.; Grabowska, A. J. Phys. Chem. 1992, 96, 9724–9730. (18) (a) Beckmann, S.; Etzbach, K.-H.; Kra¨mer, P.; Lukaszuk, K.; Matschiner, R.; Schmidt, A. J.; Schumacher, P.; Sens, R.; Seibold, G.; Wortmann, R.; Wu¨rthner, F. AdV. Mater. 1999, 11, 536–541. (b) Wu¨rthner, F.; Thalacker, C.; Matschiner, R.; Lukaszuk, K.; Wortmann, R. Chem. Commun. 1998, 16, 1739–1740. (19) Wortmann, R. Habilitationsschrift, Mainz, 1993. (20) (a) Kasha, M. Radiat. Res. 1963, 20, 55–71. (b) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy-Exciton Coupling in Organic Stereochemisty; University Science Books: Mill Valley, CA, 1983. (21) Datta, A.; Pati, S. K. Chem.sEur. J. 2005, 11, 4961–4969.

J. Phys. Chem. B, Vol. 112, No. 47, 2008 14761 (22) Steybe, F.; Effenberger, F.; Kra¨mer, P.; Glania, C.; Wortmann, R. Chem. Phys. 1997, 219, 317–331. (23) Wortmann, R.; Poga, C.; Twieg, R. J.; Geletneky, C.; Moylan, C. R.; Lundquist, P. M.; DeVoe, R. G.; Cotts, P. M.; Horn, H.; Rice, J. E.; Burland, D. M. J. Chem. Phys. 1996, 105, 10637–10647. (24) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry; VCH: Weinheim, Germany, 1990. (25) Liao, Y.; Firestone, K. A.; Bhattacharjee, S.; Luo, J.; Haller, M.; Hau, S.; Anderson, C. A.; Lao, D.; Eichinger, B. E.; Robinson, B. H.; Reid, P. J.; Jen, A. K.-Y.; Dalton, L. R. J. Phys. Chem. B 2006, 110, 5434–5438. (26) Wu¨rthner, F.; Archetti, G.; Schmidt, R.; Kuball, H.-G. Angew. Chem. 2008, 120, 4605–4608. (27) Dewar, M.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 2338–2339. (28) Davis, L. P.; Guidry, R. M.; Williams, J. R.; Dewar, M. J. S.; Rzepa, H. S. J. Comput. Chem. 1981, 2, 433–445. (29) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (30) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789. (31) Dunning, T. H., Jr.; Hay, P. J. in Modern Theoretical Chemistry, Vol. 3, Methods of Electronic Structure Theory; Schaefer, H. F. III, Ed.; Plenum: New York, 1976; pp 1-28. (32) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (33) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298. (34) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (35) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102, 939–947. (36) Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Pietro, W. J.; Hehre, W. J. J. Am. Chem. Soc. 1982, 104, 2797–2803. (37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.; Gaussian 03, revision B.03; Gaussian, Inc.: Wallingford, CT, 2004.

JP802369C