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Novel nlo-phores with proaromatic donor and acceptor groups

R. Andreu, J. Garín, J. Orduna, R. Alcalá, B. Villacampa

Donor (D)–acceptor (A) substituted organic molecules with large second-order nonlinear optical (NLO) properties constitute a topic of burgeoning interest due to their possible applications in areas such as frequency doubling and photorefractive polymers, among others. These molecules can show large first molecular hyperpolarizabilities ( b ), related to an electronic intramolecular charge transfer excitation between the ground and excited states of the molecule, and it has long been recognized that D- p -A NLO-phores with dominantly aromatic ground state do not favor electronic polarization in an applied field, as a result of the decreased resonance energy upon charge transfer. This led Marder et al . to design D-p -A systems in which the loss of aromaticity in one end (D) on charge separation was offset by a gain in aromaticity in the opposite end (A) [1]. Given the success of this approach to obtain NLO-phores with high b values, a great deal of effort has been devoted to the study of derivatives bearing proaromatic acceptors.

In this work we report the first systematic experimental and theoretical study of the second-order NLO properties of merocyanines incorporating both a proaromatic donor and a proaromatic acceptor.

The target merocyanines incorporating an acyclic spacer ( 3 , 5a and 5b ) were prepared by reaction of 1 with dithiolium salt 2 or aldehydes 4a and 4b , respectively (Scheme 1).

Since the instability of some push-pull polyenes is a well-known issue, we reasoned that incorporation of a partially rigidified spacer could increase the stability of the desired NLO-phores. In fact, reaction of compound 6 with 4a and 4c afforded 9a and 9c , with four and six double bonds in the spacer, respectively. Moreover, compound 8 , directly analogous to 5b , was prepared (by reaction of 6 with salt 7 ) with a view to study the effect of the introduction of the cyclohexene moiety on the linear and nonlinear optical properties of these NLO-phores.



Scheme 1. Synthetic approach

Table 1. Redox Potentials (V), Absorption Spectra (nm) and mb (10–48 esu) for CH2Cl2 Solutions

compound

E oxa

Ereda

lmax

mbb

mb0c

3

1.55

–1.48

403

–12

–9

5a

1.16

–1.00

558

100

60

5b

0.87

–0.85

642

1100

525

8

0.81

–0.96

630

440

221

9a

0.65

–0.81

638

2315

1135

9c

0.48

–0.77

638

2746

1350

a vs Ag/AgCl, glassy carbon working electrode, 0.1 M TBAPF6 , 0.1 V s -1 . b Measured at 1907 nm. c Calculated using the two-level model.

Relevant electrochemical and optical characterization data are gathered in Table 1. Cyclovoltammetric data indicate that oxidation of the dithiafulvene moiety and reduction of the thiobarbituric unit (both chemically irreversible processes) become increasingly easier on lengthening the p -spacer, indicating a smaller contribution of the charge-separated form to the ground state of the longer derivatives.

The second-order NLO properties of the NLO-phores were determined by EFISH. The measured mb0 values, determined using the two-level model, range from –9´10-48 esu for 3 to 1350´10-48 esu for 9c .


Inspection of Table 1 reveals that, for the same type of spacer, mb0 values increase with the conjugation path, and that introduction of the isophorone-derived bridge gives rise to a decrease of mb0 when compared to an acyclic spacer of the same conjugation length ( 5b and 8 ). It should be noted that mb0 for 9c is about four times larger than for DANS (4,-dimethylamino-4'-nitrostilbene), a common benchmark for second-order NLO-phores. Moreover, compound 3 is the only one to show a small but negative value.

Ab initio and DFT calculations have been carried out on model compounds 10 (Figure 1) in order to disclose the role of the proaromaticity of the end groups on the properties of these NLO-phores.

Figure 1. Theoretically studied compounds

It can be seen that lengthening the p -spacer results in an increase of | BLA | , together with a decrease of the charge on the dithiole ring. This implies that the gain in aromaticity at the end groups and the contribution of the zwitterionic limiting form to the ground state of these molecules are more important for the shorter derivatives. This is confirmed by the small negative mb0 value displayed by compound 3 , implying that mg> me (where g and e refer to the ground and excited states, respectively). Its very weak solvatochromism also suggests that this molecule is close to the cyanine limit and that, therefore, lies in region C, according to Marder's classification.

Although CPHF calculations correctly reproduce the observed trend in the mb0 values of these NLO-phores, a more intuitive picture of their NLO properties comes from the two-level model. In this simplified treatment b µ  Dmxfge /E3 , where Dm = memg , and fge and E are the oscillator strength and the energy of the intramolecular charge transfer transition, respectively. On passing from 10a to 10d , TD-DFT calculations show an increase in f ge and a decrease in E but, more important, predict a smooth increase in mg and a steeper increase in me. Therefore, Dm rapidly increases along this series and, being negative for 10a , an inversion in the sign of Dm is calculated to occur near 10b (Figure 2).

This explains the low mb values calculated for some dithiole- p -acceptor NLO-phores incorporating two C=C bonds (n = 1) in the spacer, a fact that cannot be attributed to a poor electron-donating ability of the 1,3-dithiole unit. A comparison of the optical properties of the 1,3-dithiole NLO-phores herein reported with those of the TTF-thiobarbituric series reveals that: a) lengthening the p spacer in the former series results in a bathochromic shift, in sharp contrast to the unusual hypsochromic shift observed with the TTF derivatives [2]; and b) the 4,5-dimethyl-1,3-dithiole unit is a more efficient donor than the TTF moiety for a given number of conjugated atoms.

To sum up, the proaromatic character of the 1,3-dithiole group gives rise to high mb values, provided long spacers are used; lengthening the polyenic chain results in large photoinduced changes in the dipole moment and enhanced hyperpolarizabilities. The negative mb value for the shortest derivative demonstrates the importance of the zwitterionic form in the description of its ground state. Experimental and theoretical results demonstrate that the 1,3-dithiole moiety is a more efficient p electron-donor than TTF in dipolar D– p –A NLO-phores.


Figure 2. Calculated mg and me values for 10

The most relevant results of the calculations are gathered in Table 2.
Table 2.
Calculated BLA (Å), Charge on the Dithiole Ring (e) and mb0 (10 –48 esu) for Model Compounds 10 .

compound

BLA a

chargeb

mb0 (calc)b

10a

––

+0.344

–62

10b

–0.029

+0.207

2

10c

–0.040

+0.129

437

10d

–0.043

+0.086

1363

a B3P86/6-31G*. b HF/6-31G*//B3P86/6-31G*.

Principal publication

R. Andreu, J. Garín, J. Orduna, R. Alcalá, B. Villacampa, Org. Lett. 2003, 5 , 3143.

Acknowledgements

MCyT-FEDER (BQU2002-00219) and Gobierno de Aragón-Fondo Social Europeo (P009-2001 and E39).

References

  1. S.R. Marder, B. Kippelen, A.K.-Y. Jen, N. Peyghambarian, Nature 1997, 388 , 845.
  2. M. González, J.L. Segura, C. Seoane, N. Martín, J. Garín, J. Orduna, R. Alcalá, B. Villacampa, V. Hernández, J.T. López Navarrete, J. Org. Chem. 2001, 66 , 8872.
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