Novel linear and V-shaped D-π-A + -π-D chromophores by Sonogashira reaction

Dipolar V-shaped chromophores derived from the pyridinium cation as the acceptor have been synthesized by Sonogashira reaction and their linear and nonlinear optical properties have been studied and compared to those of analogous one-dimensional derivatives


Introduction
The growing interest in organic materials for electrooptic device applications has led to the development of NLO chromophores that have large molecular second order nonlinearity (β). 1 Typical second-order NLO chromophores are one-dimensional (1D) π-conjugated systems represented as D-π-A, with donor (D) and acceptor (A) moieties for which hyperpolarizabilities β can be tailored by optimizing the D/A strengths and/or extending the conjugation path. 2 Chromophores of this type usually exhibit a single, intense low-lying longitudinal chargetransfer (CT) excitation and possess optical nonlinearities that are essentially dominated by a single β tensor component.Most attempts to design molecules with large β values have relied on endcapping an optimal π-conjugated bridge with different donors and acceptors.
However, a major problem associated with these 1D dipolar chromophores is the trade-off between optimizing the nonlinearity and transparency, such that the increase in the second-order hyperpolarizability β is usually accompanied by a bathochromic shift of the electronic transition.Thus, in recent years, a new synthetic strategy based on introducing multiple D and/or A substituents has emerged that gives rise to multi-dimensional (MD) organic NLO-chromophores.These systems offer potential advantages over classical 1D dipolar chromophores due to the contribution of the large, off-diagonal component, 3 which provides increased β responses, and the possibility of overcoming the nonlinearity-transparency trade-off. 4To this end, several kinds of dual (multiple) donor-acceptor substitution chromophores have been developed to circumvent these drawbacks by extending the charge transfer from one to higher dimensions (2D, 3D) ranging from octupolar, 5 star-shaped, 6 Λ-shaped (also called V-shaped), 7 X-shaped, 8 Y-shaped, 9 U-shaped, 10 etc.
On the other hand, the use of charged moieties as acceptor units is restricted to diazonium salts 11 and some heteroaromatic cations such as benzothiazolium 12 and pyridinium salts, 13 which have been studied as cationic and dicationic acceptor 1D chromophores.However, very few charged 2D chromophores have been reported to date 13b,14 (Figure 1).In this communication, we report our initial results on the synthesis and properties of a number of D-π-A + -π-D noncentrosymmetric molecules based on a charged heterocycle, represented by the pyridinium cation (A + ), as potential acceptor units in a variety of NLOcatiophores.The main objective was to relate the nonlinear optical responses to the electronic structures and molecular geometry in order to progress our understanding of these fields.The synthesis of charged chromophores D-π-A + -π-D was achieved by Sonogashira 15 cross-coupling reactions as a basic strategy and this gave rise to 1D and 2D chromophores.The linear and nonlinear optical properties of 2D systems were studied and compared with those of doubly substituted 1D chromophores.The β responses of D-π-A + -π-D systems were determined by Hyper-Rayleigh scattering (HRS) 16 and significant responses were only observed in twodimensional Λ-shaped molecules in which two charge transfer (CT) axes form an angle 17 between the two donors (Figure 2).

Results and Discussion
As part of a project focused on the development of the chemistry and applications of heteroaromatic cations, 18 we considered the use of charged moieties as acceptor units in the design of NLO-active cations and the feasibility of using the Sonogashira reaction as a method to synthesize a number of D-π-A + -π-D 1D and 2D charged chromophores.
The synthesis of the chromophores, which have the general structure represented in Figure 2 and are denoted as 1D (D-π-A + -π-D), was carried out by reacting 2,5-dibromo-1methylpyridinium iodide 1a as the dihalogenated starting material with different alkynes under Sonogashira conditions (Scheme 1).The salt 1a was initially used to determine the best conditions for the coupling of D-π-A + .Thus, the reaction was carried out with Pd2Cl2(PPh3)2 (5 mol%), CuI (10 mol%) and Et3N in DMF at room temperature with the only change being the number of equivalents of alkyne (2.4 equiv) and base (3 equiv).Only monocoupling products or Scheme 1 low yields of the double coupling products were obtained.However, it was found that yields could be improved by prolonging the reaction times and/or carrying out the reaction in highly dilute solution.For instance, a change in the dilution conditions from 0.05 to 0.002 M led to an improvement in the yields of all compounds shown in Table 1, and the yields of cations 2b and 2f increased from 51% to 66% and from 53% to 75%, respectively, when the reaction was carried out for 10 h.However, heating the reaction at 60 ºC gave only low yields in the case of 2d and decomposition products for 2a-c.Some of these results can be understood by considering the difficulty of the double Sonogashira reaction with some acetylenes, as observed experimentally, because transformation of monocoupled product into the dicoupled compound was only achieved on increasing the reaction time.The differences in yield are probably due to the difficulty of purification process rather than to electronic effects associated with the coupling partners.a Yields refer to isolated yields.
The synthesis of the V-shaped chromophores denoted as 2D (D-π-A + -π-D) was carried out from 3,5-dibromopyridinium derivative 1b (Scheme 2).Attempts to overcome problems encountered in the work-up, purification, and reaction time, as well as to improve the yields, led us to use the 3,5-dibromo-1-butylpyridinium material as the PF6 salt 19 ; this salt is soluble in THF and avoids the use of DMF, thus facilitating the work-up.Compound 1b was reacted with different acetylenes to afford the coupling products 3 as hexafluorophosphates in excellent yields (Table 2).
Compounds 3 were also prepared under similar Sonogashira conditions using analogous catalytic systems, the same number of equivalents of the corresponding acetylene derivative and base in THF at room temperature for 1 h.The main differences observed in the reactivity of 1b were that monocoupling products were not detected -likely due to the more similar reactivity on C3/C5 positions when compared with reactivity on C2/C5 in 1a-and its reaction with compounds bearing electron-withdrawing substituents, which gave products in good 3d (86%) or moderate 3e (41%) yields.
A comparative study of the linear and nonlinear optical properties of the 1 D and 2D (D-π-A + -π-D) chromophores was carried out on systems bearing electron-donating substituents, although chromophores bearing electron-withdrawing substituents were also considered for comparative purposes.

Linear optical properties
The absorption spectra of the compounds depicted in Tables 1 and 2 were recorded in the 220-800 nm range.In general, the absorption spectra shown in Figure 3, for chromophores 2 and 3, display a characteristic pattern of three absorption bands.The relatively intense peak located at the highest wavelength is ascribed to the -* charge transfer absorption band, which is typical for highly conjugated compounds.The maximum of this band is usually quite sensitive to the degree of conjugation and the nature of the electron-donating/withdrawing groups.Most of the D-π-A + -π-D chromophores (a, b, c and f) show a sharp cut off for the low energy -* band and they do not exhibit any absorption above this band that could be attributed to a low-lying n-* transition due to the presence of heteroatoms.However, this sharper cut-off was not observed for 3d, 3e or even 2d, the substituents of which have a certain withdrawing character and whose spectra display -* bands that are slightly shifted to the blue.In fact, a distinguishable band was observed at around 400-425 nm for 3e and a very weak one for 3d.Nevertheless, these n-* transitions are characterized by molar absorptivities that are usually at least a factor of 100 lower than those for -* transitions.
The 1D chromophores denoted as D-π-A + -π-D with two substituents located at the C2 and C5 positions in the pyridinium acceptor moiety (Table 1) contribute significantly to the extension of the π conjugation that stabilizes the system.As a consequence, the -* band is significantly shifted to the red as compared to the corresponding band for the D-π-A + type compounds studied previously, 19 where a single donor group is attached at the C3-position of the pyridinium system.This effect is less marked for the series of compounds 3 (Table 2), whose -* absorption bands except for 3cappear at wavelengths close to those of the D-π-A + type compounds.The extension of conjugation is more effective in the linear 1D substituted compounds ) than in the V-shaped 2D ones 3. It is noteworthy that the molar absorptivity values for the maxima of the -* absorption band are much larger for V-shaped compounds 3 than for compounds 2.
As shown in Table 3, a monotonic change towards longer wavelengths (bathochromic effect) is also exerted on the -* band due to additional stabilization by donor-to-acceptor intramolecular charge transfer (ICT).This stabilization is greater on increasing the electrondonating character of the substituent.A polarized excited state by ICT is usually more stabilized than the ground state, thus narrowing the transition gap and consequently leading to a bathochromic shift in the absorption band.This phenomenon is especially prevalent in polar solvents. 20Thus, the maximum of the π-π* absorption band for compound 2b, 3b, which contains MeOC6H4-groups, appears almost 40 nm (46 nm) above the band for 2d, 3d, which contains a deactivating CF3C6H4-moiety.Nevertheless, the ICT stabilization effect for the same substituents seems to be quite similar for compounds of both series 3 and 2. It should be pointed out that there is significant ICT stabilization due the strong activating Me2NC6H4-group in 3c as compared to those containing a strongly withdrawing F3CC6H4-substituent 3d.The location of the maxima for the π-π* absorption bands for the two compounds differs by more than 100 nm.
The fluorescence emission obtained upon excitation at the wavelength (exc) of the π-π* band collected in the 350-800 nm rangewith the exceptions of 3c and 3e, which did not show emissionexhibits features similar to those of the absorption spectra.Thus, monotonic displacements of the emission to longer wavelengths were observed upon increasing the electron-donating strength (CF3 < Br < Me < MeO).The fluorescence quantum yields (Φf) and lifetime averages obtained from the intensity decay profiles, which are superimposed in Figure 3 and included in Table 3, were also evaluated for all of the systems under investigation.In general, with the exceptions of 2a, 2c and 3f, most of the samples exhibit null or very weak fluorescence, with Φf values below 0.1.As stated previously, the presence of donor and/or acceptor groups usually leads to efficient charge transfer-like transitions and the fluorescence is quenched in relatively polar solvents. 21Fluorescence intensity profiles for all samples were fitted to bi-exponential decays.This indicates that additional processes other than the simple radiative emission from the singlet state were involved and ICT is among them.It is quite difficult to find any correlation between the quantitative values for f and <> and the electron-donating strength or location of the substituents on C2,5 (compared with C3,5) for these systems where complex deactivation excited state processes are involved.Table 3. Wavelength for the maximum of the -* charge transfer absorption band (λmax) and emission band (λem) upon excitation of exc, molar absorptivity (), fluorescence quantum yields f, and lifetime averages <> for selected compounds in methanol at 25 ºC.

Nonlinear optical properties. Hyper-Rayleigh scattering (HRS) studies
Femtosecond hyper-Rayleigh scattering measurements performed at 800 nm on chromophores 3 confirm the potential of these small ionic compounds, which are denoted as linear and V-shaped chromophores (D-π-A + -π-D), for second-order nonlinear optics.In good agreement with the findings from the linear optical (absorption) experiments, the first hyperpolarizability value for the compounds with a pyridinium acceptor is larger when the N,N-dimethylamino or methoxyphenyl group is present as the aryl donor, i.e. 3c and 3b, respectively.The double Sonogashira coupling adds a conjugated carbon-carbon triple bonded bridge to the total conjugated system, thereby lowering the transition energy and enhancing the value for the first hyperpolarizability with a significant increase in the nonlinear response.However, in compounds (D-π-A + -π-D) a different type of behaviour is observed.The first hyperpolarizability of linear 1D chromophores (D-π-A + -π-D) with two substituents located at positions C2 and C5 in the pyridinium unit show βHRS values ranging from 3 to 10.10 -30 esu as these compounds have two contributions that could be cancelled out between the two D-π-A + units.The results for V-shaped chromophores (D-π-A + -π-D) give high first hyperpolarizability values (Table 3).The replacement of substituents with donors that have a higher electron density contributes to a large molecular optical nonlinearity and chromophore 3c exhibits an exceptionally high  value and chromophore 3b gives a value that is nearly double that of the monosubstituted system. 19The dominant contribution to β in 3c is expected to arise from the off diagonal βzyy tensor component.Calculations are in progress to determine the relative magnitudes between the diagonal and the off-diagonal β tensor components.

Conclusions
In addition to D-π-A + chromophores, in this study we synthesized linear and V-shaped (D-π-A +π-D) catiophores by Sonogashira cross-coupling reactions on dihalogenated pyridinium salts.This procedure afforded good yields of 2D catiophores in 1 hour after a simple work-up procedure.The first hyperpolarizabilities of the series (D-π-A + -π-D) based on heteroaromatic cations as acceptor units were determined by HRS.The results indicate that a large first hyperpolarizability was obtained in compòunds 3b and 3c.The low transition energy and high level of CT are the decisive factors that give a large first hyperpolarizability in 3c.Theoretical TD-DFT calculations at the B3LYP/6-31G level are in progress in order to gain a better understanding of the NLO properties.The results will be reported in due course to probe the key factors that determine β values in 2D structures.

Experimental Section
General.Thin layer chromatography (t.l.c.) was carried out using Merck TLC aluminium sheets (silica gel 60 F254).Flash column chromatography was carried out on Merck silica gel (230-400 mesh).Melting points are uncorrected.All new compounds were fully characterized by standard spectroscopic techniques. 1 H NMR and 13 C NMR spectra were recorded on Varian UNITY-300, Varian-Mercury-VX-300 (300 MHz for 1 H and 75 MHz for 13 C) or Varian XL-200 NMR spectrometers.Infrared spectra were recorded on a Perkin-Elmer FTIR 1725X or a Perkin-Elmer system 1760 FTIR spectrophotometer as KBr or NaCl pellets and spectral bands are reported in cm -1 .Microanalyses were performed on a Heraeus CHN Rapid instrument.The mass spectra (MS) obtained by CI were recorded on a Hewlett-Packard 5988A (70 eV) and by ESI + on an HP 1100MSD (LCQ deca XP Thermo) spectrometer.Dry solvents were obtained from a MBRAUN SPS-800 automatic purification system.

General procedure for the synthesis of D-π-A +π-D pyridinium salts
A flame-dried flask was charged under argon with 1 equiv. of the appropriate pyridinium salt 1 (0.6834 mmol), 10 mol% CuI (0.0683 mmol, 0.013 g), 5 mol% PdCl(PPh3)2 (0.0341 mmol, 0.0239 g) in dry THF or DMF (10 mL).The appropriate acetylene (2.4 equiv., 1.6401 mmol) and Et3N (3 equiv., 2.0502 mmol, 0.2815 mL) were added.The mixture was stirred at room temperature for the time indicated in each case.The resulting solution was filtered through a small pad of Celite and washed with methanol.The solution was concentrated, the solvent was evaporated under reduced pressure, and the solid was purified by column chromatography on silica gel using CH2Cl2/MeOH (9.5:0.5) as eluent.

Figure 3 .
Figure 3. Absorption and fluorescence emission spectra for compounds 2 and 3 in methanol at 25 ºC.Some examples did not exhibit fluorescence.Superimposed are fluorescence quantum yields (f) and lifetime averages <>.