Push-pull 1,4-dithiafulvenes: a combined experimental and theoretical study

Novel push-pull 6,6-disubstituted 1,4-dithiafulvenes have been synthesized and their electrochemical and spectral properties were recorded. The proaromaticity of the donor end of these alkenes lies at the origin of their strong dipolar character, confirmed by DFT calculations, and their second-order NLO properties.


Introduction
Push-pull alkenes are substituted ethylenes containing electron-donor groups (D) at one end and electron-acceptor groups (A) at the other.These compounds have been widely studied on account of their low rotational barrier around the carbon-carbon double bond, 1 which is due to their high degree of polarization or, in valence-bond language, to the importance of zwitterionic limiting forms to the description of their ground states.Moreover, there is a renewed interest in this kind of compounds, since their high dipole moments and strong intramolecular chargetransfer (ICT) bands are at the origin of their first molecular hyperpolarizabilities (β) and, therefore, of their second-order nonlinear optical (NLO) responses. 2,3n this context, push-pull 1,4-dithiafulvenes of general formula 1 are expected to exhibit an increased dipolar character on account of the gain in aromaticity of the donor end on chargetransfer (Scheme 1).

Electrochemistry and optical spectroscopy
The electrochemical and optical properties of the synthesized compounds were studied by cyclic voltammetry and UV-vis spectroscopy, respectively, the most relevant results being collected in Table 1.The higher anodic reduction potentials of the polycyano groups (present in 8 and 10) indicate that they are stronger electron-withdrawing groups than the thiobarbituric and isoxazolone moieties.The higher oxidation potentials of the dithiafulvene moieties in 8 and 10, when compared to those of 4 and 6, confirm this fact.It is pertinent to note that the acceptor group in compound 10 is the only one to show two distinct reduction potentials.
The UV-vis spectra show that 10 has the longest wavelength ICT band among the newly prepared dithiafulvenes and that all, except 8, show a weak positive solvatochromism.

Theoretical calculations: electronic properties
In order to get a deeper understanding of the structure and properties of these push-pull dithiafulvenes, theoretical calculations on model compound 4a and compounds 6, 8, and 10 have been carried out at the B3P86/6-31G*// B3P86/6-31G* level as it has previously been demonstrated that geometries calculated at this level are in very good agreement with crystal structures for 1,3-dithiole derivatives.The most relevant data for the target compounds are gathered in Table 2.It can be seen that there is a good qualitative correlation between the calculated energies of the LUMOs and the experimentally determined reduction potentials.Both sets of values point to the fact that the isoxazolone group is the least electron-withdrawing fragment among those studied in this work.This is also confirmed by the smaller positive charge on the dithiole ring of 6 and by the higher energy of its HOMO.
The relatively high positive charge supported by the dithiole ring of all these dervatives suggests an important contribution of the charge-separated form (1a) to the ground state of these molecules, as confirmed by the high values of their ground state dipole moments.
Table 2 also shows that the HOMO-LUMO gap for compound 10 is smaller than for the other derivatives, in agreement with its experimentally determined λ max value (Table 1).This fact lends support to the ICT character of this band.

The proaromaticity of the acceptor
In compounds 4 and 6 the exocyclic C=C bond links two proaromatic moieties, as revealed by the following canonical forms (Scheme 3): This feature results in an important contribution of the zwitterionic limiting forms to the description of the ground state of these molecules, as confirmed by DFT calculations.Thus, selected bond lengths of compounds 4a and 6 are shown in Figure 1.The short C-S bond (1.737 Å) of 4a is nearly identical to the analogous one calculated for the cation-radical of tetrathiafulvalene (TTF), 7 which indicates that the dithiole ring supports a high degree of positive charge.Moreover, the C=O bond length of the thiobarbituric moiety is similar to those experimentally determined for 1,3-diethyl-5-(4-dimethylaminobenzylidene)thiobarbituric acid, a compound with a highly polarized structure. 8imilar features can be found in the optimized structure of 6, although the C-S bonds are longer and the exocyclic C=C bond and the C=O bond are shorter than the corresponding ones in compound 4a.This points to a weaker electron-withdrawing ability of the isoxazolone group, when compared to the thiobarbituric group, in agreement with cyclovoltammetric data.A comparison of the relevant bond lengths of 6 with those determined for a substituted 3-phenyl-4pyridylidenisoxazol-5-one 9 seems to indicate that the 4-pyridylidene group is a stronger electron donor than the 1,3-dithiol-2-ylidene moiety.

1,5 S ….. O interactions
The optimized geometry of compound 4a reveals the presence of two 1,5-interactions between the dithiole sulfur atoms and the oxygen atoms, with S--O distances of 2.660 Å, which are substantially less than the sum of van der Waals radii (3.32 Å) of both atoms, and are indicative of weak interactions 10,11 with a covalency ratio of χ = 0.420.These attractive interactions result from the opposite charges of +0.44 e on the sulfur atoms and -0.52 e on the oxygen atoms (Figure 2).
A similar feature can be found in the structure of 6, although the S--O distance of 2.747 Å (resulting in a smaller covalency ratio, χ = 0.365) is longer than those calculated for 4a.The different geometries of the six-and five-membered acceptor rings account for the weaker interaction found for the latter.For the sake of comparison, the experimentally determined (Xray diffraction) intramolecular S--O distance in compound 11 is 2.944 Å (Figure 2).

The influence of the solvent
The geometry of very polar compounds, such as those reported in this work, is expected to be quite sensitive to a change in the polarity of the surrounding medium.In order to check this possibility, the geometries of compounds 4a and 6 in DMSO were optimized, using the PCM-B3P86/6-31G* level (Figure 3).A comparison of the bond lengths in Figure 3 with those in Figure 1 reveals that, in a more polar environment, the contribution of the generalized zwitterionic limiting form 1a to the description of the ground state of these molecules increases.This is also confirmed by the higher positive charge supported by the dithiole ring in DMSO than in vacuum (+0.307 e for 4a and +0.274 e for 6; cf.Table 2) and by the 1 H-NMR chemical shifts of the dithiole ring hydrogen atoms, which are progressively deshielded in solvents of increasing polarity: 7.60 (CDCl 3 ), 8.04 (acetone-d 6 ), and 8.11 (DMSO-d 6 ) for 4; 7.31 and 7.05 (CDCl 3 ), 7.80 and 7.62 (acetone-d 6 ), and 7.85 and 7.65 (DMSO-d 6 ) for 6.The chemical shift of the dithiole ring protons points to the proaromaticity of the ring.

Tricyano-containing dithiafulvenes
Compounds 8 and 10 can exist in two conformations, s-trans (a) or s-cis (b), around the C2-C3 bond (Figure 4).For compound 8 DFT calculations show that conformer b is 2.48 kcal/mol more stable than conformer a, and that the dihedral angle τ (C1-C2-C3-C4) is 43.0°.These structural features closely agree with the results of ab initio calculations on similar compounds bearing substituted benzylidene groups instead of the 1,3-dithiol-2-ylidene moiety. 13In a similar vein, compound 10 also adopts conformation b (0.96 kcal/mol more stable than a), with a dihedral angle τ (C1-C2-C3-C4) = 43.2°.Nevertheless, the difference between the energy of the two conformers is small and the more polar conformer a can be the favored one in a polar environment, such as that found in the solid state.

NLO properties
Given their dipolar character, the dithiafulvenes herein reported are expected to show moderate second-order NLO properties.In fact, for compound 4 µβ = -12 × 10 -48 esu (measured at 1907 nm in CH 2 Cl 2 using the EFISH technique).On the other hand, preliminary measurements indicate that compounds 6, 8, and 10 display low but positive µβ values.Taking into account that increasing the length of the conjugated spacer linking the two rings in 4 leads to a dramatic increase of the corresponding second-order NLO responses, 14 the synthesis and study of higher vinylogues of compounds 6, 8, and 10 are being actively pursued in our laboratory.

Conclusions
The dithiafulvenes herein reported are strongly polarized as a result of both the proaromatic character of the dithiolylidene moiety and the powerful electron-withdrawing ability of the acceptor groups present at C6. Cyclovoltammetric measurements and theoretical calculations indicate that the isoxazolone moiety is the weakest acceptor among those studied in this work.The title compounds show moderate second-order NLO properties and constitute the simplest derivatives of the corresponding dithiafulvene-based merocyanines, which are excellent candidates in the search for new NLO-phores.

Experimental Section
General Procedures.Melting points were obtained on an Olympus BH-Z polarizing microscope equipped with a hot stage or in a Gallenkamp apparatus and are uncorrected.Infrared measurements were carried out in nujol mulls using a Perkin-Elmer FTIR 1600 spectrometer. 1Hand 13 C-NMR spectra were recorded on a Bruker ARX300 or a Varian Unity-300 spectrometers operating at 300 MHz and 75 MHz respectively; δ values are given in ppm (relative to TMS) and J values in Hz.EI Mass spectra were recorded with a VG Autospec at 70 eV.Microanalyses were performed with a Perkin-Elmer 2400 microanalyzer.Electronic spectra were recorded with a UV-Vis-NIR Cary 500 Scan spectrophotometer.Cyclic voltammetry measurements were performed with a µ-Autolab ECO-Chemie potentiostat, using a glassy carbon working electrode, Pt counter electrode, and Ag/AgCl reference electrode.The experiments were carried out under argon, in CH 2 Cl 2 , with Bu 4 NPF 6 as supporting electrolyte (0.1 mol L -1 ).Scan rate was 100 mV s -1 .Molecular orbital calculations were performed on Intel Pentium III and Pentium IV based computers running under Windows XP.DFT calculations were performed with the Gaussian 98w program. 15Processing of the results of molecular orbital calculations was achieved with the MOLEKEL-4.3program. 163-Diethyl-2-thiobarbituric acid (3), 3-phenyl-5-isoxazolone (5), and 2-amino-1-propene-1,1,3-tricarbonitrile ( 7) are commercially available.Compounds 2 4 and 9 5 were prepared as previously described.
a cIn DMSO.

Table 2 .
Energies of the frontier orbitals (eV), Mulliken charges on the dithiole fragment (e), and dipole moments (D)