ARTICLE pubs.acs.org/joc
Donor-Substituted Diphenylacetylene Derivatives Act as Electron Donors and Acceptors Christine Onitsch,† Arnulf Rosspeintner,† Gonzalo Angulo,‡ Markus Griesser,† Milan Kivala,§ Brian Frank,§ Franc-ois Diederich,§ and Georg Gescheidt*,† †
Institute of Physical und Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland § Laboratory of Organic Chemisty, ETH Z€urich, H€onggerberg, HCI, 8093 Z€urich, Switzerland ‡
bS Supporting Information ABSTRACT: Despite the predominant electron donor character of p-phenylenediamine, our studies on extended pphenylenediamine derivatives show that they can not only be chemically oxidized, giving well-known Wurster-type radical cations, but also be chemically reduced, giving radical anions. Making use of EPR/ENDOR spectroscopy and supported by DFT calculations, we were able to reveal the extent of πelectron delocalization in the paramagnetic species and to shed light onto the geometry and bond lengths. While for the radical anions spin was found to be mostly delocalized into the π-system, the radical cations can be described as essentially N-centered. Furthermore, we performed electrochemical characterizations using cyclic voltammetry to gain insight into the thermodynamics of the redox processes. The photophysical properties of the parent extended p-phenylenediamine were investigated by absorption, emission, and excitation spectroscopy. The fluorescence quantum yield and the excited-state lifetime of the neutral precursors in hexane and acetonitrile were determined to establish elementary differences originating from solvent effects.
1. INTRODUCTION p-Phenylenediamines constitute building blocks of conjugated organic polymers and are thus of interest for their application as optoelectronic devices, e.g., organic light-emitting diodes,1 or as molecular wires in chemosensors.2 This is particularly due to their persistent oxidized stages. The one-electron oxidation products of p-phenylenediamines and their derivatives have a long history in chemistry since they were first described by Wurster in 1879.3 Being one of the most stable organic radicals, Wurster’s radical cations were even among the first to be investigated by EPR spectroscopy.4 Since then, numerous papers have been published dealing with the radical cations of various p-phenylenediamines.58 The persistence of the radicals of the type of Wurster’s salts is known to be associated with the choice of solvent and pH value of aqueous solutions.9 The magnetic properties depend on size and type of the counterions.10,11 Particularly, the one-electron oxidation product of N,N,N0 ,N0 -tetramethyl-p-phenylenediamine, also called Wurster’s Blue radical cation, owes its stability to the π-electron delocalization in its semiquinoid structure (Scheme 1).12,13 Accordingly, the persistence of Wurster’s salts is improved in derivatives where the amino nitrogen and the benzene ring are substituted with alkyl groups, as long as a coplanar arrangement between the alkyl groups on the amino nitrogen and on the benzene ring at the stage of the radical cation is not disabled by steric effects.9,14 The electron distribution in r 2011 American Chemical Society
the radical cations is well reflected by EPR spectroscopic parameters, i.e., the isotropic hyperfine coupling constants (hfcs). Thus, different alkyl substitution on the amino N atoms which influences the spin distribution in the radical cations leads to well distinguishable differences in the EPR spectra.5,7 In contrast to the radical cations, only a few examples of radical anions of substituted p-phenylenediamines are present in the literature. In 1969, a one-electron reduction of an N-trimethylsilyl-substituted p-phenylenediamine to a radical anion was established by means of EPR,15,16 where the success in reducing an electron-rich molecule was explained in terms of a distortion of the nN/π-delocalization due to a twisting of the bulky (Me3Si)2N groups around the bond between the N and the ring C atom.17 Further examples are the formation of radical anions of substituted o-phenylenediamine upon treatment with diphenylthallium hydroxide8 or reaction with alkali-metal mirrors combined with irradiation.18 Angulo et al. published their work on a spectroscopic characterization of tetracyano-p-phenylenediamine, which, although containing the p-phenylenediamine moiety, due to the substitution with four cyano groups becomes a strong oxidant and can be regarded as a tetracyanobenzene rather than as a p-phenylenediamine derivative.19 Received: March 19, 2011 Published: May 23, 2011 5628
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Table 1. Oxidation Peak Potentials (vs Fc/Fcþ) of 1, 2, and TMPPD in Acetonitrile (þ 0.1 M Bu4NPF6) vs Fc/Fcþ, Recorded with a Scan Rate of 100 mV/s at Room Temperature Ep(1)
Ep(2)
Ep(3)
1
0.235
0.330
0.534
2
0.165
0.355
0.450
0.280a
0.295a
TMPPD a
Scheme 2. Molecular Structures
In the above-mentioned examples the electron-accepting character of substituted p-phenylenediamines is caused by substantially electron-withdrawing substituents at the phenyl ring. Here, we report on the electronic properties of 1, 1,2-bis[(p-N,N-dimethylamino)phenyl]ethyne, and 2, 1,4-bis[p-(N,N-dihexylamino)phenylethynyl]benzene (Scheme 2).20,21 Their electronic properties are investigated by optical spectroscopy, and the one-electron reduced and oxidized stages are characterized by EPR/ENDOR and DFT calculations. As reference compounds, we chose N,N,N0 ,N0 -tetramethyl-pphenylenediamine (TMPPD) and the unsubstituted “cores”, diphenylacetylene (tolane, DPA) and 1,4-bis(phenylethynyl)benzene (BPB), to distinguish the interplay of the electrondonating properties of the dialkylamino substituents and of the central aromatic moiety in 1 and 2. Both BPB and DPA already served as reference compounds for our solvatochromic study, which we performed recently on 1 and 2 in 25 solvents. There, we found solute polarizability to be responsible for the bathochromic shifts in the optical spectra upon changing the solvent properties.22
2. RESULTS AND DISCUSSION 2.1. Redox Behavior. For both molecules, 1 and 2, only oxidation peaks could be detected with our experimental setup presenting an electrochemical window between 2.5 and þ2.5 V vs the ferrocene couple, Fc/Fcþ (in acetonitrile), and partly in THF (3.2 V vs Fc/Fcþ) . The cyclic voltammogram indicates oxidation peaks at 0.235, 0.330, and 0.534 V vs Fc/Fcþ, for 1, whereas the corresponding values for 2 are 0.165, 0.355, and 0.450 V (Table 1). A somewhat higher reversibility than in the case of 2 is observed for 1, but still the peaks have to be considered irreversible. Compared with the
Reversible potentials, E1/2.23
stable and persistent radical cation of TMPPD, the observed irreversibility for 1 and 2 is rather unexpected. This is even more the case since the calculated HOMO energies of 1 and 2 do not deviate substantially (4.414 and 4.467 eV, respectively, B3LYP/6-31G(d)) and are not much different from the corresponding value of TMPPD (4.273 eV, it has to be borne in mind, however, that the calculated values are only rough estimates in view of the substantial energy shifts caused by solvation). The shape of the observed cyclic voltammograms points to an ECE (electrochemicalchemicalelectrochemical) process. This observation of a nonreversible electron transfer of Wurster’s salt derivatives is rather unusual in view of the substantial persistence of these radical cations: For TMPPD, two separate reversible oxidation steps are reported at clearly lower potentials (0.280 and þ0.295 V vs Fc/Fcþ, Table 1),23 whereas the oxidation potential is decreasing when going to longer alkyl substituents.5 Accordingly, the irreversible character of the oxidation of 1 and 2 is very likely based on the central phenylacetylene moietie(s). It has been reported that arylacetylenes, when oxidized in rigorously dried solvents and under inert conditions, react with even minute traces of moisture and undergo rapid addition of OH to one of the acetylenic carbon atoms. The subsequently formed neutral radical then adds to solvent molecules or parent phenylacetylenes finally leading to stilbene derivatives.24 Such reactivity patterns are very likely the reason for the unexpectedly irreversible shape of the cyclovoltammograms of 1 and 2 where rearrangements have already taken place before the expected Wurster-type radical cation can be formed at amounts detectable for cyclovoltammetry. Within the experimental electrochemical window (from þ2.5 to 2.5 V vs Fc/Fcþ) no reduction was observed for 1 and 2, and even the use of THF as the solvent offering the possibility of observing reduction potentials down to 3.2 V vs Fc/Fcþ did not lead to the detection of reduction processes. This is very likely due to the rather strongly negative potentials required for reducing these two molecules. For the parent hydrocarbon DPA, a reduction potential of 2.49 V vs Fc/Fcþ was reported25 (the original value was determined vs a saturated calomel electrode, SCE, and recalculated to the reference Fc/Fcþ based on the data given in refs 26 and 27). Already the value of DPA is at the border of detectability for our system and, owing to the substitution with the electron-donating dialkylamino groups, is certainly shifted to even more negative values in 1 and 2 thus impairing the determination of the corresponding potentials. 2.2. Radical Ions of 1 and 2 Characterized by EPR/ENDOR Spectroscopy. The experimental conditions and sensitivity of EPR spectroscopy differ from those in electrochemical experiments. The samples for EPR are prepared under high-vacuum conditions using rigorously dried solvents and reactants. Accordingly, together 5629
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Table 2. Hyperfine Coupling Constants in mT (the Numbers in Parentheses Describe the Number of Equivalent Nuclei) for the Radical Cation and Anion of 1 and References DPA•þ and DPA•a m-H (4)
N (2)
N-CH3 (12)
(exp)
0.349
0.257
0.066
0.382
0.377
0.125
0.068
0.427
DPA•þ (exp)30
0.075
0.222
(exp)
0.048
0.071
0.280
0.035
1• (calc)
0.078
0.087
0.318
0.027
0.059
0.271
1
•
DPA• (exp)30,31 a
o-H (4)
1•þ (calc)
1
Figure 1. Experimental (upper) and simulated (lower) EPR spectra attributed to 1•þ.
•þ
0.314b
0.485b
b
For labeling see Scheme 2. 2 p-H.
Table 3. Hyperfine Coupling Constants in mT (the Numbers in Parentheses Describe the Number of Equivalent Nuclei) for the Radical Cation and Anion of 2a N (2)
Figure 2. Experimental (upper) and simulated (lower) EPR spectra attributed to 2•þ.
with the substantial sensitivity of EPR techniques, detection of the oxidized and reduced forms of 1 and 2 would not be unexpected. This is consistent with results reported recently,28 where small p-phenylene-based oligomers terminated with two triphenylamines were shown to be irreducible electrochemically as well. Remarkably, the radical cations and, in particular, radical anions of 1 and 2 are established to be astonishingly persistent, despite the nonreversible oxidation peaks and their strongly negative reduction potentials. 2.2.1. Radical Cations. Contrary to the generation of radical anions, where the standard procedure is reduction of the species on a K-metal mirror, there is no such standard procedure for the generation of radical cations. The choice of which oxidizing agent to use can be facilitated taking ionization energies or electrode potentials into account, but in most cases, it is the experience with structurally related substances which provides the fastest way to success.29 Oxidations were attempted with AgClO4 in HFP (1,1,1,3,3,3-hexafluoropropan-2-ol), AlCl3 in CH2Cl2, and PIFA (phenyliodine(III) bistrifluoroacetate) in acetonitrile, respectively, but without success. The use of PIFA in CH2Cl2 and thallic trifluoroacetate in CH2Cl2, respectively, did not give EPR spectra with a lifetime sufficient for precise analysis. For the oxidation of the parent molecules 1 and 2, the combination of PIFA and TFA serving as oxidants in HFP under high vacuum was most successful, yielding spectra of an intensity which allowed straightforward analysis (see Figure 1 for the EPR spectrum of 1•þ and Figure 2 for that of 2•þ). This very selective response toward the whole array of oxidants shown above underpins that oxidation of 1 and 2 requires rather subtle interactions between the oxidizing agent and the substrates (in terms of the redox potential and, very likely, counterions) to obtain persistent radical cations, in line with the irreversible shape of the cyclovoltammograms. The EPR spectrum of 1•þ is fairly resolved and allows the determination of the dominating hfcs, which are assigned to the
c-H (4)
m-H (4)
o-H (4)
N-CH2 (8)
2•þ (exp)
0.224
0.066
0.091
0.041
0.481
2•þ (calc)b
0.306
0.052
0.110
0.027
0.179
2• (exp)
0.093
0.045
0.112
0.136
0.023
2• (calc)b
0.030
0.101
0.090
0.224
0.034
a
For labeling, see Scheme 2. b The calculations were performed using propyl instead of hexyl groups on both nitrogens. The differences between the calculated and experimental hfcs are partly caused by the flexible N-alkyl substituents. Their conformational flexibility is not mirrored in the single-point DFT calculations.
protons of the methyl groups (0.382 mT, 12 H) and to the nitrogen atoms (0.349 mT, 2 N). The smaller hfcs of 0.257 mT (4 H) and 0.066 mT (4 H) are attributed to the meta- and orthoprotons, respectively (Table 2). The EPR spectrum obtained upon oxidation of 2 shows a structured, but not fully resolved, pattern. In analogy with 1•þ, the hfcs of 0.481 mT (8 H) and 0.224 mT (2 N) can be assigned to the dialkylamino groups (methylene protons and nitrogen atoms). The three smaller hfcs of 0.091 mT (4 H), 0.066 mT (4 H), and 0.041 mT (4 H), not resolved in the EPR spectrum but used for the simulation, describe the spin density at the position of the four meta-protons, the protons on the central ring, and the ortho-protons, respectively (Table 3), and were included in the simulation based on the calculated data. At the stage of the radical cations, a predominate portion of the spin resides at the nitrogen atoms thus shifting the hybridization at the nitrogen atoms toward sp2 (cf. Scheme 1). A shorter CN bond length as well as double-bond character can be postulated for the radical cations, as revealed by the quantum-mechanical calculations: Upon going from 1 to 1•þ, the bond length between the C of the benzene ring and N decreases by ca. 3 pm (139.0 to 135.8 pm, respectively) leading to a quinoidal structure additionally causing an essentially planar geometry around the N atoms in 1•þ. The high spin population at the nitrogen and the adjacent methyl and methylene groups, respectively, of 1•þ and 2•þ is related to those of Wurster’s Blue-type radical cations like TMPPD•þ (14N hfc of 0.705 mT and 1H hfc of the CH3 group of 0.677 mT in absolute methanol at 293 K).6,5 The somewhat lower 14N hfc in 1•þ and 2•þ reflects the delocalization of the spin into the acetylenic π systems. The smaller conjugated 5630
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Figure 3. Singly occupied orbitals of 1 and 2 representing the spin populations of 1•þ and 2•þ, respectively.
Figure 5. Top: Experimental (upper) and simulated (lower) EPR spectra of 2•. Bottom: ENDOR spectrum of 2•.
Figure 4. Top: Experimental (upper) and simulated (lower) EPR spectra of 1•. Bottom: ENDOR spectrum of 1•.
π-system of 1 leaves a higher portion of the spins at the N atoms than in 2 (cf. Figure 3 and Tables 2 and 3). 2.2.2. Radical Anions. Prior to contact of the solution with the K-metal mirror, no EPR signal was observed. After reduction of the parent molecules 1 and 2 with the K-metal mirror under high vacuum, the solution turned from colorless to purple (see absorption spectrum of the radical anion in the Supporting Information) and yielded EPR spectra which could be very well analyzed using hyperfine coupling constants derived from ENDOR spectroscopy (Figures 4 and 5 for 1• and 2•, respectively). For the radical anion of 1•, the EPR spectrum is made up by four different hyperfine coupling constants. The largest hfc of 0.280 mT (4 H) belongs to the four equivalent ortho-protons, followed by the four equivalent meta-protons with a hfc of 0.071 mT (4 H). The hfc of 0.035 mT (12 H) can be attributed to the 12 protons of the methyl groups. The hfc of 0.048 mT (2 N) of the two nitrogen atoms is well in line with the quantummechanical calculations (see Table 2, Figure 6). The observed spectral pattern of 2• is produced by five different hfcs. The two largest hfcs of 0.136 mT (4 H) and 0.112 mT (4 H) can be assigned to the four equivalent
Figure 6. Singly occupied orbitals of 1 (top) and 2 (bottom) mirroring the spin distribution in 1• and 2•.
ortho-protons and the four equivalent meta-protons, respectively, of the outermost phenyl groups. The smaller hfc of 0.045 mT (4 H) is attributed to the four equivalent protons on the central benzene ring. The smallest hfc of 0.023 mT (8 H) describes the spin density on the eight equivalent protons of the four R-methylene groups attached to the amino nitrogen atoms. Furthermore, EPR simulation revealed a hfc of 0.093 mT (2 N) for the amino nitrogen atoms (see Table 3) as for 1•, and this spin distribution is well reproduced by the corresponding singly occupied molecular orbital (Figure 6). As has been shown in a previous work,32 the 14N hfc can depend —among other factors—on the hybridization or, so to speak, the extent of pyramidalization on the N-center. Contrary to the radical cations, a pyramidal geometry around the nitrogen atoms exists in the radical anions leading to a substantially smaller spin population. Accordingly, only a very small portion of spin is delocalized at the nitrogen centers. Moreover, the very small orbital coefficient at the adjacent aromatic carbon amplifies this effect. The hyperfine data obtained for the DPA-derived molecule 1 are well in line with those of DPA• itself (Table 2). For both 5631
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molecules, the spin population is higher for the ring protons closer to the center than for those pointing toward the dialkylamino groups. This also illustrates the substantially different character of the radical anions and the radical cations as illustrated in Figures 7 and 8. In the former, the charge and the spin are confined to the hydrocarbon moiety with a significant contribution of the acetylenic units. On the other hand, the properties of the radical cations are mainly determined by the amino donor moieties. This is mirrored by the shapes of the singly occupied orbitals displayed in Figures 3 and 6, respectively. 2.3. Photophysics. Absorption/Extinction Coefficients. The absorption maximum of 1 in hexane lies at 322 nm, whereas the absorption is again red-shifted to 329 nm in acetonitrile (Figure 9). The maximum extinction coefficients in both hexane and acetonitrile are high: 48 300 and 47 600 M1 cm1 as expected based on the less extended π system of 1 compared to 2 (Table 4). In acetonitrile, unexpectedly, the respective absorption bands are broadened for both molecules. The absorption of 2 is broad and unstructured in hexane with a maximum at 365 nm. In more polar acetonitrile, the absorption band is broadened and red-shifted to 380 nm (Figure 10). The maximum extinction coefficients in both hexane and acetonitrile are extraordinarily high: 75 600 and 74 400 M 1 cm1 ,
Figure 7. Spin distribution of 1•þ (black) and 1• (gray), mirrored by the corresponding hfcs. For labeling, see Scheme 2.
respectively (Table 4) also accounting for their relatively short fluorescence life times and large quantum yields. Emission. In hexane, the emission spectrum of 1 shows peaks at 349, 362, and 376 nm (Figure 11). However, the fluorescence emission of 1 in acetonitrile is too weak to supply conclusive information; i.e., the lifetime was too short to be determined with our setup. The emission of 2 in hexane is structured and peaks at 393, 412, and 429 nm, whereas in acetonitrile, the emission is broad and unstructured with a maximum at 466 nm being red-shifted compared to 1 (Figure 12). A striking resemblance is noticed comparing the spectrum of 2 to that of BPB. The emission of BPB in chloroform (εS = 4.81 at 293 K, η = 1.44 cP at 298 K)33 is of analogous shape like the one presented here in hexane, but blue-shifted.34 Table 4 summarizes the main photophysical properties, namely, the maximum molar decadic extinction coefficient, wavelength of maximum emission, fluorescence quantum yield, excited-state lifetime, and 00 transition energy, of 2 and 1 in hexane and acetonitrile, respectively. The 00 transition energy is given as the average of the absorption and first emission peak maxima.35 The shortening of the conjugated system when going from the three-unit scaffold in 2 to the two-unit core in 1 leads to the observed blue-shift in the fluorescence maximum. This trend is well-known for para-connected phenylacetylenic compounds, such as the reference compounds BPB and DPA.34,36 Furthermore, whereas BPB is known to be strongly fluorescent (j = 0.58 in cyclohexane,36 j = 0.79 in toluene, and j = 0.90 in dioxane37), DPA has only a very small fluorescence yield (j = 0.006 in cyclohexane)36 at 25 °C. This goes hand in hand with what we observed: the molecule with the more extended π-conjugation, 2, shows a quantum yield of 0.70 in hexane and 0.69 in acetonitrile, whereas for the smaller molecule, 1, the quantum yield was determined to be 0.041 in hexane and 0.003 in acetonitrile. As far as the maximum extinction coefficients of 2 are concerned, it can be summarized that they are about one-fifth higher than those known for BPB (ε = 62 000 M1 cm1 in dioxane)37 but much higher compared to TMPPD with a
Figure 8. Spin distribution of 2•þ (black) and 2• (gray), mirrored by the corresponding hfcs. For labeling see Scheme 2.
Figure 9. Normalized absorption spectra of 1 in hexane (solid line) and acetonitrile (dashed line), respectively.
Table 4. Photophysical Properties of 1 and 2 in Hexane and Acetonitrile, Respectivelya 1
εmax/M
cm
1
1 in hexane
1 in MeCN
2 in hexane
2 in MeCN
48 300 at 322 nm
47 600 at 329 nm
75 600 at 365 nm
74 400 at 380 nm
λem,max/nm
349
368
393
466
j
0.041
0.003
0.70
0.69
τ/ns
