A. Belén Marcoa,
Paula Mayorga Burrezob,
Laura Mosteoa,
Santiago Francoa,
Javier Garína,
Jesús Ordunaa,
Beatriz E. Diosdadoc,
Belén Villacampad,
Juan T. López Navarreteb,
Juan Casado*b and
Raquel Andreu*a
aDepartamento de Química Orgánica, ICMA, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain. E-mail: randreu@unizar.es
bDepartamento de Química Física, Universidad de Málaga, 29071 Málaga, Spain
cServicio de Difracción de Rayos X y Análisis por Fluorescencia, ICMA, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
dDepartamento de Física de la Materia Condensada, ICMA, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
First published on 21st November 2014
Push-pull systems, in which the proaromatic 4H-pyranylidene electron donor is conjugated with a dicyanomethylene acceptor through a quinoid thiophene as (part of) the electron relay, have been prepared, and their properties have been compared to those of a parent compound featuring an aromatic thiophene moiety. Different experimental techniques (X-ray diffraction, 1H NMR, IR, Raman, UV-vis, cyclic voltammetry, spectroelectrochemistry, and NLO measurements) combined with theoretical calculations have been used for the study of the chromophores. Quinoidal derivatives, although neutral, show strongly polarized structures, with positive μβ values and a progressive increase of the intramolecular charge transfer (ICT) on lengthening the π-spacer. Comparison between compounds that only differ in the character (quinoid or aromatic) of the thiophene unit shows a more efficient ICT for the quinoid thiophene-containing chromophore, which influences the second-order NLO response. Furthermore, the thienyl ring has been also found to play a significant role in the ICT process for the analogous aromatic derivative.
In the field of second-order NLO, polyenic or aromatic bridges have been extensively used in spite of having serious drawbacks, as the former imparts poor thermal and chemical stabilities, and the latter leads to ineffective charge polarization. In this context, the use of moderately aromatic structures (with resonance energies below that of benzene), such as thiophene,5 and the introduction of pro-aromatic6 bridges7,8 are two widely-used strategies for tuning the molecular polarization and, in consequence, the second-order nonlinearities. Although several chromophores with a thiophene ring as a part of the π-relay5 have been studied, the use of the thienoquinoid moiety as spacer7d,8,9 in merocyanines for second harmonic generation remains much less explored, being their NLO properties very influenced by the solvent polarity.10 Further, as far as we know, there is only a pair of related chromophores differing only in the quinoid/aromatic character of the thiophene ring for which their polarization and photorefractive figures of merit have been reported.11 Comparison of this couple shows a more polarized structure for the quinoidal system, displaying a bathochromic shift of the absorption band in the UV-vis and a higher photorefractive response. Also in the field of organic dyes for DSSC, a D–A structure connected through a quinoidal thiophene as the π-conjugated relay has been recently reported12 pursuing to effectively extend the π-conjugation, resulting in a further bathochromic shift of the absorption maximum compared to related compounds with similar π-conjugation length.13
Taking all these data into account, in this paper we describe the synthesis, characterization, study of the ground-state polarization and the NLO response of a series of D–π–A compounds with dicyanomethylene as the acceptor group, the pro-aromatic ring 4H-pyranylidene ring as donor14 and a quinoid thiophene as (part of) the electron relay (1a–c), together with a comparable pair of quinoid/aromatic thiophene derivatives (1b/2) having the methine bridge in different positions (Scheme 1) Given the strong dependence of the absorption properties of these compounds with the degree of electron charge transfer, we also conducted a study of their electro-chromic properties by spectroelectrochemical experiments.
Compound 2 (with an aromatic thiophene moiety) was obtained from the aldehyde 617 by reaction with malononitrile in absolute ethanol (Scheme 3).
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Fig. 1 Molecular structure of compound 1c. Selected bond lengths (Å): C15–C16 1.369(4), C16–C17 1.412(4), C17–C18 1.360(4), C18–C19 1.417(4), C19–C20 1.390(4). |
A detailed comparison between the bond lengths of the quinoid thiophene moiety and C19–C20 bond (see values in caption of Fig. 1) and the corresponding bond distances in compounds 718 and 8
9a (Fig. 2) taken as references, indicates that: (i) the C19–C20 distance in 1c is considerably longer than the analogous bond in compound 7, thus confirming a certain zwitterionic (dicyanomethanide) contribution to the ground state of 1c. The same conclusion can be obtained if CC distances b and c in 7 are compared with C18–C19 and C17–C18 respectively in 1c, pointing to a partial aromatic character for the thiophene moiety; (ii) the aforementioned bond lengths C19–C20, C18–C19 and C17–C18 in compound 1c are similar to the corresponding bonds a, b, c in the 1,3-dithiole derivative 8, for which a certain zwitterionic character of the ground state was established.7d,9a Moreover, the fact that the C15–C16 bond in 1c is shorter than the corresponding e in 8 is consistent with the decrease of polarization in the former.
Some structural parameters of the pyranylidene ring (Fig. 3) also reveal the ICT from the terminal donor of the molecule, showing intermediate values between the totally quinoid and the aromatic forms. The following features are highlighted: (i) shortening of the C–O bonds; (ii) lengthening of the pyran exocyclic bond; and (iii) a decreased degree of C–C bond length alternation, which can be evaluated through the parameter δr19 (Fig. 3) (δr = (a − b + c − d)/2, with δr = 0 for benzene and δr ≈0.10 for fully quinoid rings), is found for our compounds, in agreement with those of other 4H-pyranylidene–π–A systems previously described.8,14a
The Bird index (I6)20 of the donor, (Fig. 3) used as an estimation of the aromaticity of the ring, (I6 = 25.4 for fully quinoid pyrans; I6 = 50 for pyrylium cations14a) also indicates the partial contribution of the zwitterionic forms to the ground state of the chromophores. Nevertheless, this contribution is smaller than those found for other 4H-pyranylidene-containing merocyanines.8,14a,21
As an additional confirmation of the ICT, the bond length alternation (BLA) value along the spacer (defined as the difference between the average carbon–carbon single and double bond lengths22) is 0.029 Å. Unlike some other analogues with different acceptor moieties, (in particular 1,1,3-tricyano-2-phenylpropene14a and 2-dicyanomethylenethiazole14b) which show BLA values close to zero (from the X-ray structures), 1c has been found to possess a moderately-polarized structure. This fact may have a positive effect on the NLO response (see NLO properties) as places the chromophore, at least in the solid state, far from the cyanine limit.
Compound 2 (Fig. 4) also shows a completely planar structure. The thiophene exocyclic bonds (C1–C9 and C4–C5) have an s-trans geometry, as found in a related chromophore with a 1,3-dithiole unit as donor.23
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Fig. 4 Molecular structure of compound 2. Selected bond lengths (Å): C1–C2 1.389(10), C2–C3 1.388(10), C3–C4 1.404(11). |
Comparison of the standard CC and C–C bonds of 1.362 and 1.424 Å in unsubstituted aromatic thiophenes24 with C1–C2 (C3–C4) and C2–C3 respectively reveals the π-electron delocalization along the thiophene ring in 2. These distances are similar to those found for the thiophene unit in the related chromophore with a 1,3-dithiole unit as donor23 above mentioned. The structural parameters for 2 (Fig. 3) indicate a limited polarization of the donor fragment given that δr and Bird index parameters together with C4–Cexo bond lengths (C9
C10 bond in derivative 2) exhibit values corresponding to a fully quinoid 4H-pyranylidene ring. In fact, the C9
C10 bond is even shorter than the C4–Cexo one in 2,2′,6,6′-tetraphenylbipyranylidene (1.385 Å) taken as reference for a quinoidal derivative.25 Thus, surprisingly, in the solid state, the thiophene moiety seems to be the main donor involved in the ICT process towards the acceptor unit.
The chemical shifts of the thiophenic protons can also be useful to study the polarization of compounds 1a–c. Comparison of the NMR spectra of the quinoidal chromophores with that of the tautomer of 2-dicyanomethylthiophene 3, named as 3′15 (Fig. 5), shows that Ha (bonded to a carbon with partially positive electron density, more distant from the acceptor) is deshielded in the series 1a > 1b > 1c with respect to 3′, whereas Hb (bonded to a carbon with partially negative electron density, proximal to the acceptor) is shielded (1a < 1b < 1c). These data also support the important charge transfer from the donor to the acceptor, with a higher contribution of the zwitterionic form for the shortest merocyanine 1a.
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Fig. 5 Chemical shifts (ppm, in CDCl3) of the thiophene ring protons for compounds 1a–c, together with those of 3′ (tautomer of 3). Data for 3′ were taken from ref. 15. D = donor unit. |
On the other hand, given that the hydrogen atoms at the 3- and 5-positions of a pyranylidene unit undergo downfield shifts with the increased aromatization of the ring,14a,28 the decrease in chemical shifts (in ppm) for such protons in CD2Cl2 on passing from 1a (6.57, 6.17) to 1b (6.46, 6.14) and 1c (6.36, 5.97) reveals a lower contribution of the zwitterionic form to the ground electronic state as the polyenic π-spacer lengthens, in agreement with other 4H-pyranylidene–π–A derivatives.5d,14a,29
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Fig. 6 Mulliken atomic charges on various molecular domains for compounds 1a–c, 2 at the PCM-M06-2X/6-311+G(2d,p)/PCM-M06-2X/6-31G* level (D = donor unit). |
For the series 1a–c, a decrease of the positive charge on the donor on lengthening the spacer was observed. This finding is in agreement with a lower contribution of the zwitterionic form to the ground state as the length of the π-spacer increases, as disclosed by the 1H NMR data. Compound 1a is the most polarized chromophore within this series.
Comparison of systems 1b and 2 reveals the strong difference about the charge on the thiophene ring: it is negligible for compound 2, whereas it becomes positive and even higher than the charge supported by the pyranylidene donor unit in 1b. Theoretical description of the geometry obtained for system 2 differs from its experimental X-ray structure, since in the solid state the charge transfer seems to take place only from the thiophene moiety to the acceptor unit likely due to the counter-polarizing effect of the surrounding molecules in the crystal. Concerning the electron-withdrawing group, the higher dicyanomethanide character found for system 1b regarding its aromatic isomer 2, supports the more efficient charge transference towards the acceptor provided by the quinoidal thiophene ring. This conclusion is in agreement with the previously reported quinoid/aromatic thiophene-containing pair of chromophores,11 as evaluated through the resonance parameter c2.11
Compd | ν(C![]() |
---|---|
a All ν data are in cm−1. Measured on KBr pellets. | |
1a | 2162, 2189 |
1b | 2197 |
1c | 2197 |
2 | 2210 |
Comparison of system 1b with its aromatic analogue 2 shows a lower value of the ν(CN) frequency for the quinoidal derivative, pointing to a higher degree of charge transfer (as disclosed by Mulliken charges analysis).
The parallel variation of the frequencies corresponding to the main groups (cyano, thiophene, and 4H-pyranylidene) in the Raman spectra for all the systems is a further indication of the extension of charge polarization through the whole conjugated pathway from the donor to the acceptor. In general, experimental (crystal structures, 1H NMR data, IR and Raman spectroscopies) and theoretical data all show a higher polarization, and hence a higher degree of charge transfer, for the quinoidal chromophores compared to the aromatic system 2.
Compd | Eoxa (V) | Ereda (V) | EHOMOb (eV) | ELUMOb (eV) | EgEChemb (eV) | EHOMOc (eV) | ELUMOc (eV) | EgTheorc (eV) |
---|---|---|---|---|---|---|---|---|
a 10−3 M in CH2Cl2 versus Ag/AgCl (3 M KCl), glassy carbon working electrode, Pt counter electrode, 20 °C, 0.1 M NBu4PF6 100 mV s−1 scan rate. Ferrocene internal reference E1/2 = +0.43 V.b HOMO and LUMO energies and electrochemical HOMO–LUMO gaps determined from the onset of the oxidation and the reduction waves in cyclic voltammograms.c Calculated at the PCM-M06-2X/6-311+G(2d,p) level in CH2Cl2.d Reversible oxidation wave (E1/2). | ||||||||
1a | 0.87d | −1.23 | −5.16 | −3.48 | 1.68 | −6.60 | −2.24 | 4.36 |
1b | 0.75 | −0.96 | −4.95 | −3.60 | 1.35 | −6.35 | −2.41 | 3.94 |
1c | 0.64 | −0.93 | −4.80 | −3.71 | 1.09 | −6.16 | −2.55 | 3.61 |
2 | 0.82 | −1.23 | −5.04 | −3.60 | 1.44 | −6.42 | −2.25 | 4.17 |
The electrochemical band gaps and HOMO/LUMO energy levels were estimated from the electrochemical data. Thus, electrochemical band gaps were calculated from onset potentials of the anodic and cathodic waves, by the empirical relationship: HOMO = −(Eox,onset + 4.4) eV; LUMO = −(Ered,onset + 4.4) eV.34
Both oxidation and reduction processes become easier on chain lengthening (1a → 1b → 1c). The observed trends are also confirmed by computational calculations (Table 2) which show that the EHOMO(ELUMO) values increase (decrease) with the length of the spacer. Thus, a larger conjugation length gives rise to a decrease of the electrochemical band gap. The calculated HOMO–LUMO gaps are overestimated with respect to the electrochemical ones (more than 2.0 eV), but similar trends using any of these two methods were encountered. It is worth mentioning that direct comparison of electrochemical and calculated band gaps assumes a “frozen orbital approximation” and must be done with caution.35
Comparison of compound 1b with its aromatic analogue 2 shows lower oxidation/reduction potentials (absolute values) for the former, due, presumably, to the stabilizing effect imparted on the electrochemically generated radical ions by the gain of aromaticity from the quinoid thiophene ring. Besides, 1b show lower electrochemical (and calculated) band gap than 2. The same behavior has been described in related systems with a thienothiophene moiety in the π-spacer.8
Compd | λmax (log![]() |
λmax (log![]() |
λmax (log![]() |
λmaxb | fb |
---|---|---|---|---|---|
a All λmax data are in nm.b PCM-TD-M06-2X/6-311+G(2d,p) calculations in CH2Cl2. | |||||
1a | 504 (4.53) | 507 (4.42) | 506 (4.29) | 507 | 1.37 |
539 (4.65) | 542 (4.65) | 540 (4.60) | |||
581 (4.48) | 585 (4.60) | 582 (4.68) | |||
1b | 573 (4.73) | 587 (4.67) | 584 (4.62) | 588 | 1.85 |
607 (4.74) | 627 (4.82) | 627 (4.88) | |||
662 (sh) | 684 (4.68) | 685 (4.91) | |||
1c | 619 (4.82) | 658 (4.79) | 656 (4.73) | 664 | 2.32 |
709 (4.81) | 715 (4.85) | ||||
788 (4.55) | 790 (4.79) | ||||
2 | 536 (4.60) | 568 (4.64) | 557 (4.61) | 542 | 1.22 |
The four molecules show very strong and broad electronic absorption bands in the visible region. For 1b–c the two bands on the high-frequency side are associated and the second one can be attributed to vibronic transitions. This assignment is based on: (i) the energy spacing between these sub-bands (1100 ± 100 cm−1; e.g. for 1c, 1093 cm−1 in CH2Cl2 and 1258 cm−1 in DMF) which correspond to totally symmetric CC/C–C vibrational modes coupling the ground electronic state to the pertinent excited state (S1) through the ICT vibrational coordinate along the π-spacer;38 and (ii) the variation of the spectral shape with solvent polarity, i.e., an increase in the lower energy band at the expense of the higher energy one on increasing the solvent polarity.39
For the series 1a–c, the λmax values increase on lengthening the spacer, showing an accentuated bathochromic shift with solvent polarity, and reaching a shift of ∼100 nm in CH2Cl2 or even greater in DMF. On the energy scale, these shifts decrease on increasing conjugation length, being 0.31 and 0.24 eV in CH2Cl2 and 0.32 and 0.24 eV in DMF. This behavior points to weakly alternated structures, suggesting an important zwitterionic character for these systems, in agreement with calculated Mulliken charges and IR/Raman spectroscopies.
The character of the thiophene (quinoid or aromatic) moiety has a significant effect on the electronic absorption properties of the chromophores 1b and 2. Thus, the presence of the quinoid ring causes a significant red shift of the maximum absorption wavelength in 1b when compared to 2. This shift is greater than 110 nm (0.37 eV in CH2Cl2 and 0.42 eV in DMF), surpassing that encountered when a thienothiophene unit (aromatic or quinoid) is evaluated in related compounds.8 In general, larger molar extinction coefficients ε in all the solvents studied are found for compound 1b. This trend parallels that of the calculated oscillator strengths f in CH2Cl2. These results, obtained from the comparison of D–π–A systems which only differ in the quinoid/aromatic character of the thiophene unit, are in agreement with the two examples reported in the literature.11,12
Concerning the dependence of the band position on solvent polarity, compound 1a presents an almost negligible solvatochromism. This point can be explained33 regarding the HOMO and the LUMO of 1a, (Fig. 8) whose topologies are not dominated by the polar end groups, being spread over the whole conjugated backbone.
For systems 1b–c, 2 positive solvatochromism for low polarity solvents (cf. dioxane and CH2Cl2) that becomes negligible/slightly negative for 1b/2 respectively when increasing polarity (cf. CH2Cl2 and DMF) was observed. This variety of behavior has already been reported for other D–π–A systems,40 including some 4H-pyranylidene derivatives.8,29
The red-shift of the bands of the radical anions from 350–400 nm in 1a to 434 nm in 1b and 340/402/491 nm in 1c is an indication of the increased conjugation of the injected charge along the whole path from the acceptor to the donor, promoted by the precursor quinoidal sequences in the neutral compounds. The comparison of the UV-vis absorption data of 1b and 1c with those of the aromatic compound 2, reveals the similitude of the radical anions of 2 and 1b, both with absorption maxima at 435 nm and 434 nm respectively. This fact points to the complete delocalization of the negative charge in the two isomers since both have the same number of π electrons (12 e− without considering the common CN groups) placed in different positions. The situation changes completely in the neutral forms, where the nature and placement of the active π electrons defines the position of the lowest energy absorption band and therefore the relevant optical properties.
A similar behavior is found in the oxidative processes compared to reductions. The electron extraction leads to stable radical cations for 1b and 1c with clear isosbestic points. In 1c the radical cation is characterized by a new broad band at 448 nm, whose wavelength is comparable to those of the bands of the radical anion (340/402/491 nm) revealing that although the extraction of the electron takes place on the donor part of the molecule, it has an effect in the optical properties similar to the inclusion of one electron in the acceptor. Such a similar response points towards a scenario promoted by the coalescence of the donor-acceptor interaction in the neutral compound either with oxidation or reduction. The same pattern than for 1c is found for the oxidation to the radical cation in 1b. However, the case of the oxidation of 1a is particular: the decrease of the neutral absorption band gives rise to a well-defined band with maxima at 554 and 593 nm, together with another one at 370 nm. These bands progressively evolve with the increment of the potential into one single feature at higher energies, with its maximum absorption value at 365 nm. The species with the highest potential is assigned to a radical cation, given its similitude with those of 1b–c. The spectra that appear between the neutral form and the cation might arise from a transitory meta-stable species (self-assembled dimers are possible) generated in the vicinity of the electrode which is reversibly transformed into the stable cation.
In compound 2, oxidation gives rise to a broad band with maximum at 356 nm, at rather different position than in 1b besides their similar π electron nature (number of π electrons), likely indicating that the electron extraction takes place mostly in the aromatic thiophene for 2, while in the 1 series oxidation comes from the thiophene–pyrane unit. In conclusion, both oxidation and reduction produce the disappearance of the strong visible absorptions of the neutral compounds and the formation of rather transparent solutions, highlighting a significant reversible electrochromic effect.
Fig. 10 displays the infrared spectra of 1b and 2 obtained during the electrochemical oxidation and reduction processes in CH2Cl2. In the significant region of the ν(CN) absorption bands, reduction of 1b gives rise to the disappearance of the neutral absorption associated to the ν(CN) vibration at 2202 cm−1, and to the growth of two well resolved peaks at 2163 cm−1 and 2108 cm−1.33 These two peaks display the characteristic two pattern bands of the tetracyano anionic species, such as those of TCNQ and TCNE derivatives.41 The emergence of the same two absorptions in a dicyano-substituted push-pull compound might be justified by a strong vibrational coupling of the two CN stretches in the anionic species, which is translated into two infrared bands. The 2202 cm−1 → 2163 cm−1/2108 cm−1 downshift is a result of the antibonding character of the LUMO wavefunction between the C and N atoms in the cyano groups (an increase of the charge on the LUMO upon reduction produces an enlargement of the CN bond and a frequency downshift). A similar two-band profile is observed for the aromatic compound 2 after reduction and generation of the radical anion. Noticeably, the frequency change is greater in 2 (68 cm−1; 2216 cm−1 → 2148 cm−1) than in 1b (39 cm−1; 2202 cm−1 → 2163 cm−1), which indicates a higher ability to accept additional electrons in 1b than in 2 (smaller frequency changes indicate less structural reorganization and energy requirements to accommodate electrons). In both cases, however, the whole charge in the anion is placed in the dicyano group, leading to the complete disappearance of the characteristic charge-transfer band of the neutral molecules, in agreement with the electronic absorption UV-vis data.
During oxidation, the neutral ν(CN) vibration at 2202 cm−1 in 1b gives rise to a single band at higher frequency, 2224 cm−1, which is in line with the extraction taking place in the donor group. The CN groups are then affected by the decreasing of the ground electronic state polarization, as the initial donor loses its electron releasing character upon the formation of a cation on it. The 2202 cm−1 → 2224 cm−1 frequency upshift shows the removal of charge from the antibonding state of the CN group upon oxidation, causing the strengthening of the bond and an increased vibrational frequency. A similar situation is found during oxidation in compound 2, with a 2216 cm−1 → 2229 cm−1 frequency upshift in accordance with the generation of a radical cation within the thiophene donor group. The frequency upshift with oxidation and the frequency downshift upon reduction follow a different pattern in the two almost π-isoelectronic compounds: now the upshift is smaller in 2 (13 cm−1; 2216 cm−1 → 2229 cm−1) than in 1b (22 cm−1; 2202 cm−1 → 2224 cm−1) in agreement with the greater ability to release electrons (less frequency changes indicate less structural reorganization and energy requirements to expulse electrons).
Inspection of Table 4 reveals that μβ values in the series 1a–c increase with the conjugation length. The low response for 1a together with its solvatochromism, 1H NMR data, IR frequencies and calculated data clearly supports the important polarization of the ground electronic state of this compound, which is near the cyanine limit. Lengthening the spacer gives rise to more alternated structures with higher second-order nonlinearities.
The influence of the character of the thiophene moiety (aromatic or quinoid) can be inferred from the analysis of μβ values of compounds 1b and 2. Thus, 1b presents a higher response than 2, in agreement with the more efficient ICT upon the quinoid ring introduction.
Finally, it could be also pertinent to compare the NLO properties of the herein reported compounds to those of related derivatives. Thus, comparison with the analogue of 1c bearing 1,1,3-tricyano-2-phenylpropene as acceptor14a (μβ = 1550 × 10−48 esu measured in the same conditions) shows higher response for compound 1c in consonance with their less polarized structure (as it has been seen in X-ray section) and closer, although surpassed, to the maximum of the Marder's plot.22b
On the other hand, while compounds 1a–c show positive μβ values in agreement with a predominantly quinoid form for their ground electronic state, their analogues in which the thiophene ring has been substituted by a thiazole14b show negative figures of merit, due to their higher polarization, being essentially zwitterionic molecules. Nevertheless, absolute values are of the same order, or even slightly higher for the series herein studied (e.g. analogue of 1c measured in the same conditions: μβ = −3900 × 10−48 esu).
Derivative 1b, with a quinoid thiophene ring in the π-spacer shows a more efficient ICT than its aromatic analogue 2, as shown by X-ray studies, calculated Mulliken charges, absorption frequencies of the CN groups in IR and Raman spectroscopy. As a consequence, a higher NLO response has been found for 1b when compared to 2. Moreover, CV and UV-vis spectra showed favored redox processes and bathochromically shifted λmax values respectively for the quinoid system 1b.
Concerning chromophore 2, the UV-vis and IR spectroelectrochemistry studies, along with the crystalline structure, point to the important role of the thiophene ring in the ICT process towards the dicyanomethylene moiety, acting as the main electron-donating fragment.
Finally, a reversible electrochromic effect has been observed from the spectroelectrochemical study associated with the interconversion of neutral/anion/cationic species derived from systems 1a–c, 2. The changes in the optical properties upon redox processes are consistent with the coalescence of the ICT feature in the neutral species giving rise to colorless solutions which describes a potential use of these compounds as electrochromic materials combined with the parallel changes (switch on → off) of the NLO property associated with any of the redox process.
Footnote |
† Electronic supplementary information (ESI) available: General experimental methods, NMR and UV-vis spectra, voltammograms and HRMS spectra of new compounds, X-ray crystallographic data and diagrams of the crystal structures of 1c and 2, NLO measurements, computed energies and Cartesian coordinates of optimized geometries. CCDC 1007965 and 1007966. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra12791a |
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