Combined Spectroelectrochemical and Theoretical Study of Electron

Article pubs.acs.org/JPCA

Combined Spectroelectrochemical and Theoretical Study of Electron-Rich Dendritic 2,5-Diaminothiophene Derivatives: N,N,N′,N′‑Tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine Vladimír Lukeš,*,†,‡ Peter Rapta,†,‡ Kinga Haubner,† Marco Rosenkranz,† Horst Hartmann,§ and Lothar Dunsch*,† †

Center of Spectroelectrochemistry, Department of Electrochemistry and Conducting Polymers, Leibniz Institute for Solid State and Materials Research, D-01069 Dresden, Germany ‡ Institute of Physical Chemistry and Chemical Physics, Slovak University of Technology Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovak Republic § Institute of Applied Photophysics, University of Technology Dresden, D-01069 Dresden, Germany S Supporting Information *

ABSTRACT: The in situ spectroelectrochemical and electron spin resonance (ESR) behavior of the recently prepared N,N,N′,N′-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine 11 is presented. The results are compared to the ones of the parent 2,5-bis-diphenylamino-thiophene 41 as well as to the corresponding high-molar third dendrimer generation 8 containing the same thiophene-2,5-diamine core. The dendritic compound 11 can be reversibly oxidized in three separated steps to yield the corresponding stable monocation 11•+, dication 112+, and tetracation 114+. A well resolved ESR spectrum of the corresponding cation radical 11•+ with dominating splittings from two nitrogen atoms and two hydrogen atoms was observed at the first oxidation peak similar to 41•+. The shape of the SOMOs orbitals very well correlates with the proposed distribution of the unpaired electron mainly on the thiophene center and neighboring nitrogen atoms. The spin delocalization on the central thiophene moiety in the monocations for all three model compounds 41•+, 11•+, and 8•+ was confirmed. The computed single occupied molecular orbital (SOMO) for trication 11•3+ is completely different compared to the SOMO of the corresponding monocation 11•+, and it confirms a largely delocalized unpaired spin density. Dominating diamagnetic product was determined at the third oxidation peak, confirming the formation of a tetracation by a two electron oxidation of ESR silent dication. The positive charge is fully delocalized over the lateral parts of the molecule leading to the high stability of tetracation 114+. The estimated theoretical limit energy of the lowest optical transition S0 → S1 is 2.90 eV, and it can be achieved for the 3D dendrimer generation.

■

INTRODUCTION In the past, N-perarylated di-, tri-, and tetra-amines, such as the compounds 1−3 (Scheme 1), received a lot of interest as they are able to form after cooling their melts amorphous solids in which the positively charged species obtained by oxidation exhibit a high stability and mobility.1−6 Therefore, these compounds have been used as the hole transport materials for manufacturing electro-optical devices.7−12 The same is valid for the N,N,N′,N′tetraaryl-substituted 2,5-diaminothiophenes 41 and their thienyl homologues 4N (N > 1) as well as for the starburst compounds 5N and 6N (Scheme 2), which have been synthesized and studied recently by us and some other authors.13−19 By using in situ spectroelectrochemistry including a combination of in situ electron spin resonance (ESR) and UV−vis−near infrared (NIR) with nuclear magnetic resonance (NMR) spectroelectrochemistry, the detailed study of electron transfer mechanisms and structures of intermediates and products for different substituted thiophenes was demonstrated.13,17,18 Similar to their carbocyclic analogues 1−3, the thiophene analogues 4N−6N can © 2013 American Chemical Society

be oxidatively transformed into rather stabilized radical cations 4N•+−6N•+ and dications 4N2+−6N2+. In the series 4N, the stability of the charge states increases with increasing number N of thiophene rings. The same trend was observed for the starburst compounds 5N and 6N.20,21 Another approach to increase stability of the charge states and to tune the redox behavior of electron-rich compounds was found by dendritic molecules.22−24 For instance, the third generation (3G) dendrimer 7 with 10 amino groups and Ar = p-methoxyphenyl (Scheme 3) exhibits in cyclic voltammetric measurements six reversible oxidation steps indicating the stability of highly charged species in dendritic molecules.25,26 From this result and some other ones found in the series of carbazolyl-substituted dendrimers,27−32 it seems of interest to study the redox properties and stability of charged species in Received: May 27, 2013 Revised: July 1, 2013 Published: July 2, 2013 6702

dx.doi.org/10.1021/jp405212e | J. Phys. Chem. A 2013, 117, 6702−6711

The Journal of Physical Chemistry A

Article

Scheme 1. N-Perarylated Di-, Tri-, and Tetra-Amines

Scheme 2. N,N,N′,N′-Tetraphenyl-Substituted α,ω-Diamino-oligothiophenes and 2-(N,N-Diarylamino)thienyl-Substituted Starburst Compounds

Scheme 3. Diarylamino-Substituted Dendrimers

characterized at yet. This finding contrasts to the behavior of the unsubstituted 2,5-bis-diphenylamino-thiophene parent compound 41 and its tetra-bromophenyl derivative 10, which can be oxidized only in two reversible steps. To obtain more detailed information on the properties of cation radical, dication, and higher oxidized species generated by the electrochemical oxidation of compound 11, cyclic voltammetry and in situ ESR/UV−vis−NIR spectroelectrochemistry were used. These combined techniques give detailed structural

dendritic compounds containing N,N-diaryl-substituted 2-aminothiophene building blocks. The first prototype 11 of such compounds has been described recently.33 The dendritic compound 11, containing 2,5-diaminothiophene core connected with thiophenes as building blocks, was prepared by a palladium-catalyzed coupling reaction of diphenylamine 9 with tetra-2,3,4,5-(4-bromophenyl)thiophene 10 (Scheme 4). This compound 11 could be reversibly oxidized in three separated steps to yield species that have not been further 6703



Article

Scheme 4. Synthetic Route to N,N,N′,N′-Tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine 11

Figure 1. (a) Schematic structure of studied molecules (41, 11, and 8) and the numbering of selected bonds and nitrogen atoms. (b) The side view on the studied molecules with the fixed Cs symmetry. The mirror plane is perpendicular to the thiopene ring, and it is oriented through the sulfur atom and the center of bond No. 3.

information on both diamagnetic and paramagnetic species formed in complex redox reactions. Next, the density functional theory (DFT) calculations of three dendritic 2,5-diaminothiophene model structures, namely, compounds 41, 8, and 11, at the TDB3LYP level in the neutral and charged states allow us to interpret the experimental results obtained. Additionally, the charge and spin distribution as well as optical properties for the

high-molar dendrimer generation containing thiophene-2,5-diamine core will be discussed.

■

EXPERIMENTAL SECTION Commercially available dichloromethane (CH2Cl2, Aldrich) and ferrocene (Fc) (p.a., ≥98.0%) purchased from Merck were used without further purification. Tetrabutylammonium hexafluorophosphate 6704



Article

Table 1. The selected bond lengths (in Angstrom) and dihedral angles (in degrees) for the neutral and charged states of molecule 11. The bond and atom numbering see in Figure 1 Distances 1/1′ 2/2′ 3 4/4′ 5/5′ 6/6′ 9/9′ 10/10′ 11/11′ 14/14′ 15/15′ 16/16′ N(1)−thiophene−N(1′) N(1)−phenyl−N(2) N(1)−phenyl−N(3) Dihedral Angles 4−5−8/4′−5′−8′ 4−6−7/4′−6′−7′ 9−11−12/9−11−12′ 9′−10′−13′/9′−10′−13′ 14−16−17/14−15−18 14′−16′−17′/14′−15′−18′

11

11•+

112+

11•3+

1.770 1.372 1.424 1.402 1.426 1.422 1.423 1.421 1.421 1.422 1.421 1.421 5.244 5.674 5.678

1.772 1.394 1.400 1.372 1.429 1.430 1.398 1.433 1.430 1.398 1.432 1.432 5.179 5.654 5.659

1.769 1.404 1.389 1.367 1.421 1.423 1.383 1.435 1.435 1.382 1.437 1.437 5.173 5.633 5.638

1.765 1.389 1.405 1.390 1.416 1.417 1.382 1.435 1.433 1.389 1.429 1.429 5.209 5.634 5.627

1.763 1.381 1.413 1.406 1.407 1.412 1.379 1.429 1.429 1.382 1.427 1.427 5.237 5.613 5.626

36/−36 41/−41 41/−41 41/−41 41/−41 41/−41

47/−47 49/−49 52/−52 52/−52 52/−52 52/−52

45/−45 45/−45 54/−54 54/−54 54/−54 54/−54

42/−42 43/−43 51/−51 51/−51 51/−51 51/−51

38/−38 41/−41 46/−46 46/−46 46/−46 46/−46

114+

pseudoreference electrode were used. To reach the thin layer conditions, the electrolyte volume was reduced by inert foil sheets inserted into the flat cell.35 1 H NMR spectra were recorded with a Bruker Avance II 500 MHz instrument equipped with an autotunable BBO probe and controlled with the Bruker Topspin 2.1 software and BSMS keyboard. Calculation Details. The optimal geometries of studied molecules were calculated using the quantum chemical density functional theory (DFT).36 The Becke’s three parameter exchange functional (B3) with Lee−Yang−Parr correlation functional (LYP) was used. The unrestricted version of DFT (UB3LYP functional) was used for the electron open shell species. The optimized neutral and charged structures 41, 8, and 11 were confirmed to be real minima by frequency analysis (no imaginary frequency). On the basis of optimized geometries, the vertical transition energies and oscillator strengths between the initial and final states were computed by the time-dependent (TD)-DFT method. All theoretical calculations were performed using the Gaussian03 program package (energy cutoff of 10−5 kJ mol−1, final RMS energy gradient under 0.01 kJ·mol−1·Å−1).37 The standard 6-31G(d,p) basis set was used for C, H atoms and the N, S atoms were described using the 6-31+G(d,p) basis set. The Mulliken population analysis was performed using the basis set without polarization and diffusion functions (6-31G). The vertical ionization potential (VIP) was calculated as the (U)B3LYP energy difference for two corresponding electric states, which have fixed geometry. However, the geometry relaxation was accounted for each electric state in the evaluation of the adiabatic ionization potential (AIP).

(TBAPF6) of puriss. quality (Fluka) was dried under reduced pressure at 70 °C for 24 h and stored in a glovebox. Cyclic voltammograms (CV) were carried out in dichloromethane with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte using a one-compartment electrochemical cell with a platinum wire as working and counter electrodes and a Ag/AgCl wire as a pseudoreference electrode (prepared by partial oxidation of silver wire in 0.1 M HCl at current density of 10 mA/cm2 for about 1 min). Compound 11 was dissolved at a concentration of 0.2 mM in dichloromethane. All electrochemical measurements were performed under inert nitrogen atmosphere. Data were recorded on an Autolab electrochemical analyzer equipped with a PGSTAT 100 potentiostat. All potentials were measured against the oxidation potential of ferrocene in the same medium. In situ ESR/UV−vis−NIR spectroelectrochemical experiments were performed in the optical ESR cavity (ER 4104OR, Bruker Germany). ESR spectra were recorded by the EMX X-band CW spectrometer (Bruker, Germany). UV−vis−NIR spectra were measured by the Avantes spectrometer AvaSpec2048 × 14-USB2 with the CCD detector and AvaSpec-NIR2562.2 with the InGaAs detector applying the AvaSoft 7.5 software. Both the ESR spectrometer and the UV−vis−NIR spectrometer are linked to a HEKA potentiostat PG 390, which triggers both spectrometers. Triggering is performed by the software package PotMaster v2 × 40 (HEKA Electronik, Germany). For standard in situ ESR/vis-NIR spectroelectrochemical experiments, an ESR flat cell was used.34 A laminated platinum mesh as the working electrode, a silver wire as the quasi-reference electrode, and a platinum wire as the counter electrode were used in all spectroelectrochemical experiments in dichloromethane (CH2Cl2) with 0.2 mol dm−3 TBAPF6 as supporting electrolyte. For thin layer spectroelectrochemistry, a three-electrode arrangement with a laminated working electrode with a gold-μmesh, a platinum wire as a counter electrode, and a silver wire as a

■

RESULTS AND DISCUSSION Electronic Properties of the Neutral States. Although the molecular structures of the studied compounds 41, 8, and 11 are very simple (Figure 1a), the large number of possible conformations 6705



Article

can occurr for the spatial orientations of diphenylamine moieties. With respect to this fact, we have accounted the full point group Cs symmetry for the neutral states. As it is illustrated in Figure 1b, the mirror plane is perpendicular to the thiophene ring, and it is oriented through the sulfur atom and the center of bond No. 3. The comparison of the optimal geometries for the neutral species can be focused on the mutual arrangement of the aromatic planes. In the case of the smallest molecule 41 considered, the dihedral angle between the nitrogen atom and neighboring carbon atoms is 175° (see bonds 4−5−6 or 4′−5′−6′ in Figure 1a). The next branching leads to the planarization of these atoms in the vicinity of the thiophene as well as triphenyl-amino fragments and dihedral angles mentioned above were changed to approximately 180° for 11 and 8. The absolute mutual orientation of the thiophene and phenyl planes is similar for the discussed molecules, and the dihedral angles between the bonds 4−5−6/4−7−8 are 40 and 35 degrees. The data collected in Table 1 show that the mutual orientations of the phenyl planes in the triphenyl-amino fragments of 11 is ±41 degrees (see last two rows). The equilibrium distances between the nitrogen atoms separated by phenylene ring is practically constant (5.66 Å for 41 to 5.68 Å for 8), and it is independent of the molecular branching. Similarly, the constant value of 5.24 Å for all three structures 41, 8, and 11 in the neutral state was obtained for the distance between the inner nitrogen atoms separated by thiophene ring. Next the electrical charge distribution is considered. It is connected with the partial charges of the heteroatoms, which can be evaluated as the multiple of elementary charge. For the smallest compound 41, the partial electric charge on the sulfur atom is positive 0.48e, and the nitrogen atoms have the negative atomic charges −0.79e. The molecular branching has the negligible influence on these electric quantities. The partial charges of the largest molecule 8 are 0.47e (for S), −0.79e (for the inner N), and −0.83e (for the rest N). The molecular architecture has direct influence on the electronic excited states. The lowest TD-B3LYP vertical optical transitions S0 → S1 for all three molecules have A″ symmetry. The corresponding excitation energy for the smallest compound 41 is 3.40 eV (365 nm), and the oscillator strength is 0.3256. The addition of the next four lateral diphenylamino-fragments leads in the compound 11 to the red-shifted 11A″ optical transition with the energy of 3.00 eV (414 nm), and the oscillator strength grows up (0.4427). The theoretical excitation energy found for 11 agree well with the corresponding experimental value (3.01 eV) estimated as the onset of the lowest energy absorption band (Figure 2a). The largest dentritic molecule 8 exhibits the lowest excitation energy at 2.91 eV (426 nm), and the oscillator strength is 0.5133. If the dependence of the evaluated energies on the number of nitrogen atoms is assumed exponential (see Figure 1S, Supporting Information), the estimated limit for higher-branched species is 2.90 eV. This result indicates that the lowest energy saturation is achieved for the largest studied model compound 8. The results presented in Figures 2 and 3 show that the energy difference between the lowest 11A″ and next calculated higher optical transitions decreases with the molecular size, i.e., 0.6 eV for 41, 0.4 eV for 11, and 0.30 for 8. In order to understand how the molecular geometry of investigated molecules modifies the electronic structure, the relevant occupied and unoccupied molecular orbitals playing a dominant role in the electronic transitions are examined. In this context, the shape and symmetry of B3LYP molecular orbitals

Figure 2. (a) Experimental normalized UV−vis−NIR spectra of neutral (green line) and charged cationic states (monocation, black line; dication, red line; simultaneously formed trication and tetracation, blue line) of 11 (dashed arrow, estimation of the onset of the lowest energy absorption band; * artifact from equipment) with (b) the calculated TD(U)B3LYP vertical electronic transitions based on the optimal (U)B3LYP geometries for neutral 11 (green solid bar lines), monocharged 11•+ (black solid bar lines), bicharged 112+ (red solid bar lines), tricharged 11•3+ (violet dashed bar lines), and tetracharged 114+ (blue dashed bar lines). Note: H = HOMO and L = LUMO.

contributing to the lowest S0 → S1 optical transition are compared. For all studied molecules, the excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied orbital (LUMO) dominates, and the percentage contribution is ca. 90%. As it can be seen in Figure 4, all HOMOs are uniformly delocalized over the central thiophene moiety (see carbon−carbon bonds 2−3 and 2′−3′) as well as over the inner nitrogen atoms and neighboring phenyl moieties. The symmetry of all these orbitals is A″. However, the LUMOs possess A′ symmetry and are delocalized only over the heterocyclic moiety and neighboring atoms. The lobes are oriented along the central carbon−carbon bond 3−3′ and sulfur and neighboring carbon atoms. The presence of the next lateral biphenylamino moieties has the tendency to delocalize unoccupied orbitals in the central part. Electronic Properties of the Charged States. The shapes of depicted HOMOs indicate where the geometrical changes in the molecule can occur after the electron loss. The data collected 6706



Article

charged 11•3+ states where the values increase by about 11 to 13 degrees. Cyclic voltammogram of N,N,N′,N′-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine 11 in dichloromethane exhibits three reversible oxidation processes in a wide range of scan rates (from 3 to 500 mV s−1) at half-wave potentials E1/2 = −0.22, +0.03, and +0.65 V vs Fc+/Fc, respectively (Figure 5a). From these results follow that in the first two reversible processes the oxidation of the starting compound 11 to a radical cation 11•+ followed by the formation of a dication 112+ occurs. The third reversible oxidation peak at +0.65 V, which is almost twice as large as for the first two oxidation processes, indicates a nearly two-electron transfer process. ESR/UV−vis−NIR spectroelectrochemistry of the compound 11 shows spectroelectrochemical behavior quite similar to that for the first two electron transfers as observed for structure 41.13 For radical cation 41•+, intense absorption maxima at about 470 nm (2.64 eV) and 640 nm (1.94 eV) in the visible range and an ESR spectrum split into 7 lines was recorded (simulation parameters aN = 4.6 G and aH = 4.0 G).13 The calculated gas-phase isotropic Fermi coupling spin constant values are aN = 5.0 G and aH = 1.3 G for radical cation 41•+. The inclusion of the solvent effect within the continuum model (IEFPCM = CH2Cl2) gives much better agreement with the experimental observation aN = 4.8 G and aH = 3.5 G. It seems that the interaction with the solvent molecules affects mainly the magnetic properties of thiophene hydrogen atoms. Thin layer in situ ESR/UV−vis−NIR spectroelectrochemistry applying laminated gold micromesh electrode was used to monitor all redox states appearing by oxidation of compound 11. A well-defined UV−vis−NIR spectrum of cation radical 11•+ with dominating band maxima at 504 nm (2.46 eV) and 1135 nm (1.09 eV) was observed. It dominates the optical spectra in the potential range at first voltammetric peak (Figures 2a and 5b, black lines). Hyperfine structured stable ESR signal with 7 lines, centered at g = 2.0025 and with a line width of about 2 G by 0.5 G modulation, was detected, whose intensity increases at the first oxidation peak (Figure 6). The ESR spectrum of the cation radical 11•+ can be simulated with the following ESR parameters: aN = 4.6 G, aH = 3.2 G, ΔHpp = 2.7 G, and L/G 0.5. The corresponding calculated isotropic Fermi hyperfine coupling constants (hfcc) obtained for the gas-phase are aN = 4.4 G and aH = 2.3 G, and for the IEFPCM = CH2Cl2, aN = 4.6 G and aH = 2.3 G. The values of hfcc for the lateral nitrogen atoms in 11•+ are much smaller, 0.8 and 1.0 G (for the gas-phase) and 0.6 and 0.7 G, for the IEFPCM = CH2Cl2 and therefore are hidden in the line broadened experimental spectrum (see inset in Figure 6, black line). From the qualitative point of view, the distribution of the unpaired electron in 41•+, 11•+, and 8•+ also supports the interpretation of ESR measurements. Besides, the corresponding spin density distribution obtained as a difference between the alpha and beta electron densities is uniformly spread over the central part of the molecule and inner nitrogen atoms (Figure 7). It should be noted that in contrast to 41•+ no strong influence of the solvent on the hfcc of the thiophene hydrogen atoms is found. This might be connected with the location of the solvent cavity in the calculation model, which is out of the central part of the molecule. At the second oxidation peak of 11, the absorption at 504 nm (2.46 eV) decreases and a new optical band at 445 nm (2.79 eV) arises (Figures 2a and 5b, red lines). Additionally, a new intense band at 1073 nm (1.16 eV) dominates the spectrum of this species with the shoulder at 930 nm (1.33 eV). Low intense new

Figure 3. TD-B3LYP optical transitions for the neutral and charged states of studied molecules: 41 (a) and 8 (b). Note: H = HOMO and L = LUMO.

in Table 1 show that the formation of the charged states leads to consecutive structural changes. The equilibrium distance between the nitrogen atoms attached at the thiophene ring reaches the minimal value of 5.17 for bication 112+ where the most central bonds (1/1′ to 6/6′) are shortened. However, the averaged distance between the outer nitrogen atoms (N−phenyl−N) separated by phenyl moiety is minimal for the tetracation 114+. The next influence of the electron abstraction is connected with the mutual orientation of aromatic rings. As it can be seen in Table 1, the maximal dihedral angles for the central as well as lateral parts are indicated for the second 112+ and third 6707



Article

Figure 4. B3LYP molecular orbitals contributing to the lowest energy transition (S0 → S1) of studied molecules in the neutral state: 41 (a), 11 (b), and 8 (c). The depicted isosurface is 0.03 atomic unit. The values written in italic are oscillator strengths. The values in parentheses are the percentage of contribution.

We propose that instability of 11 on light is caused by photochemically induced formation of the corresponding radical cation, as already confirmed for other π-conjugated oligomers,38 and its follow up reactions in solution. Interestingly, compound 11 can be further oxidized at even higher oxidation potential at about 1.1 V vs Fc+/Fc probably up to hexacation 116+ because this step exhibits the same peak height as the third one. However, this step is already irreversible, and we observed consecutive products in the back voltammetric scan indicating a high reactivity of the hexacation 116+ as expected for such high charged states (Figure 3S, Supporting Information). The calculated energies of the lowest TD-UB3LYP transition of radical cations are 1.86 eV (for 41•+), 0.89 eV (for 11•+), and 0.59 eV (for 8•+), and the relevant oscillator strengths range from 0.3 to 0.5. As depicted in Figure 4S, Supporting Information, these transitions originate mainly to the excitation from HOMOβ to LUMOβ. The next computed dominant transitions with higher energy are at 2.59 eV (for 41•+), 1.19 eV (for 11•+), and 0.62 eV (for 8•+), and they corresponds mainly to the excitation from HOMOβ−1 to LUMOβ. Plots of the B3LYP molecular orbitals significantly contributing to the TD-B3LYP lowest energy transitions of compound 11 in higher charged states are shown in Figure 5S for dication and in Figure 6S for tetracation (Supporting Information). It seems that the excitation energy transfer has the tendency to propagate from the lateral atoms of the side groups to the central part.

band also arises at 1660 nm (0.75 eV). The observed bands can be attributed to the ESR silent bication 112+, as a decrease of ESR intensity was monitored in this potential region (Figure 6, full red circles). The third oxidation process is accompanied with a twoelectron transfer step leading to the dominating tetracation 114+ with band maxima at 480 nm (2.58 eV) and 955 nm (1.30 eV). However, despite the formation of diamagnetic species at the third redox peak, the low intense single line narrow ESR signal, where intensity slightly increases in this potential region, indicates the presence of a small amount of radical trication 11•3+ (Figure 6, full blue triangles). The shape of ESR line is characterized for delocalized spin (see inset in Figure 6, blue line) as will be discussed in more detail bellow using restricted openshell Hartree−Fock (ROHF) approach. At very small ESR intensity at the third oxidation peak, a new absorption band at 955 nm (1.30 eV) dominates the spectrum, and therefore, the product of the oxidation of compound 11 at the third oxidation peak is proved to be dominantly diamagnetic and corresponds to the tetracation 114+. In contrast to the starting compound 41, the 2,5-diaminothiophene derivative 11 is more light- and air-sensitive. By standing at daylight in bulk as well as in solutions, it changes its color from slight yellow to pink and then to dark red. This color change parallels with a significant broadening of the proton signals in the 1H NMR spectrum (Figure 2S, Supporting Information) and indicates the formation of radical species. 6708



Article

Figure 6. Potential dependence of double integral ESR intensity observed upon oxidation of 11 in 0.2 M TBAPF6 in CH2Cl2 (forward scan, scan rate 3 mV s−1) in the region of the first (black solid squares), second (red solid circles), and third (blue solid triangels) voltammetric peak. Inset: Representative ESR spectra of paramagnetic species observed simultaneously upon oxidation at the first (black line) and the third (blue line) voltammetric peak (the ESR spectrum of 11•+ monocation remaining in the solution in the region of the 3rd voltammetric peak was subtracted from the spectrum of the 11•3+ trication for clarity).

Figure 5. In situ ESR/UV−vis−NIR spectroelectrochemistry for the sample 11 in 0.2 M TBAPF6 in CH2Cl2 (scan rate 3 mV s−1). (a) In situ cyclic voltammogram with color highlighted regions where the corresponding optical spectra were taken and (b) corresponding UV− vis−NIR spectra detected simultaneously during the in situ voltammetric scan.

Empirical design rules in organic electronics are often based on the concept of engineering the energy levels of the frontier molecular orbitals.39 The molecular orbitals discussed above were obtained using the approach originating from the principles of unrestricted Hartree−Fock (UHF) method. For the radical systems, the theoretical results obtained from UHF or UB3LYP suffer from the spin contamination error, which can distort the electron density distribution. However, the visualization of the restricted open-shell Hartree−Fock (ROHF) orbitals can be more relevant with respect to the spin contamination and specification of the single occupied molecular orbital (SOMO). Plots of the ROHF molecular orbitals and their orbital energies obtained for the optimal B3LYP geometry of the neutral and charged states are summarized in Figure 7S, Supporting Information. The shape of the SOMOs very well correlate with the proposed distribution of the unpaired electron mainly on the thiophene center and neighboring nitrogen atoms for 41•+ (Figure 7Sa, Supporting Information) and 11•+ as already discussed in EPR experiments above. The computed SOMO for 11.3+ is completely different compared to the SOMO of the corresponding monocation 11•+ (Figure 7Sb, Supporting Information) and indicates delocalized spin over the large part of the molecule. However, the concentration of trication 11•3+ is very low as a dominating ESR silent product was determined at

Figure 7. B3LYP atomic spin densities for studied cation radicals: 41•+ (a), 11•+ (b), and 8•+ (c). The depicted isosurface is 0.005 atomic unit.

the third oxidation peak, confirming the formation of a tetracation 114+ by a nearly two electron transfer. Figure 7Sb, Supporting Information, also shows HOMO for optimized geometry of tetracation 114+. The positive charge is fully delocalized over the lateral parts of the molecule. This is obviously the reason of the high stability of tetracation 114+. It seems that that the increase of the molecular size open new oxidation centers in the region of the lateral arms for higher charged states. As illustrated for HOMO of tetracation 84+, the charge is mainly distributed over two side groups (Figure 7Sc, Supporting Information), and consequently, the other side groups could have the capacity for further oxidation. Finally, it is important to emphasize that one cannot measure molecular orbital energies, even in principle for the open shell 6709



Article

SOMO of the corresponding monocation 11•+ and confirms a largely delocalized unpaired spin density as was confirmed by ESR spectroscopy. The single narrow ESR line characteristic of delocalized spin was observed for the trication 11•3+. The concentration of this trication is, however, very low as a dominating ESR silent product was determined at the third oxidation peak, confirming the formation of a tetracation 114+ by a nearly two electron transfer. The positive charge is fully delocalized over the lateral parts of the molecule in this tetracation, which is the reason for its high stability. The good agreement between theoretical electronic transitions and experimental values seems to indicate that a rational design of tunable molecular layers in organic devices based on these dendrimers is possible. Furthermore, using the theoretical methodologies applied here, we showed that it is possible to predict and interpret the electronic properties of these systems. Therefore, these theoretical studies can contribute to synthetic efforts and may help in the understanding of the structure− properties relationships of materials with three-dimensional π-conjugation. If we suppose that the dependence of the evaluated quantities (optical transitions, ionization potentials, and spin distribution) tends to the limit with respect to the number of nitrogen atoms, our results predict that the saturation is achieved already for the studied third generation dendritic model compound 8.

systems. However, the electrochemical properties of organic molecules upon the oxidation are directly connected with the ionization energy, which can be theoretically evaluated as the vertical (VIP) or adiabatic (AIP) ionization potentials. The calculated gas-phase (U)B3LYP ionization potentials of the compounds studied are presented in Table 2. The results Table 2. (U)B3LYP Vertical (VIP) and Adiabatic (AIP) Ionization Potentials (in eV) Calculated for the (U)B3LYP Optimal Geometries in the Neutral and Charged States of 41, 11, and 8 Molecules molecule 41 11 8

VIP AIP VIP AIP VIP AIP

neutral

monocation

bication

trication

tetracation

5.90 5.51 5.21 4.90 4.82 4.62

9.25 8.16 7.24 7.17 6.30 6.22

14.25 14.05 9.42 9.35 7.56 7.52

11.41 11.35 8.98 8.94

13.32 13.26 9.96 9.95

obtained clearly indicate that both these quantities increase with the number of positive charges but decrease with the molecular size. Next, the geometry relaxation upon the electron abstraction is significant for the smallest molecule where the difference between VIP and AIP is maximal (0.61 eV for neutral state and 1.09 for cation). If we suppose that the dependence of the evaluated energies on the number of nitrogen atoms is exponential, the estimated gas-phase AIP limits for the infinity dendrimer generation here studied are 4.59 eV (for neutral state), 5.69 eV (for monocation), and 7.39 eV (for bication).

■

ASSOCIATED CONTENT

* Supporting Information S

Lowest vertical excitation energies, 1H NMR spectra, cyclic voltammograms, UB3LYP and B3LYP orbitals, ROHF HOMO/ SOMO, and corresponding orbital energies. This material is available free of charge via the Internet at http://pubs.acs.org.

■

CONCLUSIONS The electric neutral and positive charged states of the three model dendrimers 41, 11, and 8 containing the thiophene-2,5diamine moiety were investigated both from theoretical and spectroelectrochemical points of view. The quantum chemical calculations offered a deeper insight into the relations between the structure, optical, and magnetic as well as electrochemical properties of the investigated compounds. In this context, the in situ spectroelectrochemical and ESR studies of the dendritic N,N,N′,N′-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5diamine 11 were presented. These results were compared to the ones of the parent compound 41 previously investigated as well as to the corresponding high-molar dendrimer generation 8 containing the same thiophene-2,5-diamine core. The dendritic compound 11 can be reversibly oxidized in three separated steps to yield the corresponding stable monocation 11•+, dication 112+, and even stable tetracation 114+. This finding contrasts to the behavior of the unsubstituted 2,5-bisdiphenylamino-thiophene parent compound 41 and its tetrabromophenyl derivative 10, which can be oxidized only up to the stable dications 412+ and 102+, respectively. A well resolved ESR spectrum of the corresponding cation radical 11•+ with dominating splittings from two nitrogen atoms and two hydrogen atoms was observed at the first oxidation peak similar to 41•+. The shape of the SOMOs very well correlate with the proposed distribution of the unpaired electron mainly on the thiophene center and neighboring nitrogen atoms both for 41•+ and 11•+ as confirmed by ESR spectroelectrochemical experiments. The spin delocalization on the central thiophene moiety in the monocations for all three model compounds 41•+, 11•+, and 8•+ was confirmed also theoretically. The computed SOMO for trication 11•3+ is completely different compared to the

■

AUTHOR INFORMATION

Corresponding Author

*(V.L.) E-mail: [email protected]. Tel: ++421 2 59325 741. Fax: ++ 421 2 59325 751. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Technical support by F. Ziegs (IFW Dresden) is gratefully acknowledged. P.R. and V.L. are thankful for financial support from the Scientific Grant Agency of the Slovak Republic (Projects No. VEGA 1/1072/11, 1/0289/12) and IFW Dresden.

■

REFERENCES

(1) Shirota, Y. Organic Materials for Electronic and Optoelectronic Devices. J. Mater. Chem. 2000, 10, 1−25. (2) Shirota, Y. In Organic Light Emitting Devices, Müllen, K., Scherf, U., Eds.; Wiley-VCH: Weinheim, Germany, 2006; p 245−263. (3) Shirota, Y.; Kageyama, H. Charge Carrier Transporting Molecular Materials and Their Application in Devices. Chem. Rev. 2007, 107, 953− 1010. (4) Naito, K.; Miura, A. Molecular Design for Nonpolymeric Organic Dye Glasses with Thermal Stability: Relations between Thermodynamic Parameters and Amorphous Properties. J. Phys. Chem. 1993, 97, 6240− 6248. (5) Koene, B. E.; Loy, D. E.; Thompson, M. E. Asymmetric Triaryldiamines as Thermally Stable Hole Transporting Layers for Organic Light-Emitting Devices. Chem. Mater. 1998, 10, 2235−2250. (6) Jang, S.-H.; Jen, A. K.-Y. Electro-Optic (E-O) Molecular Glasses. Chem.Asian J. 2009, 4, 20−31. 6710



Article

(7) Strohriegl, P.; Grazulevicius, J. V. Charge-Transporting Molecular Glasses. Adv. Mater. 2002, 14, 1439−1456. (8) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Brèdas, J.-L. Charge Transport in Organic Semiconductors. Chem. Rev. 2007, 107, 926−952. (9) Fáber, R.; Mielke, F.; Rapta, P.; Staško, A.; Nuyken, O. Anodic Oxidation of Novel Hole-Transporting Materials from Tetraarylbenzidines. Electrochemical and Spectroscopic Characterization. Collect. Czech. Chem. Commun. 2000, 65, 1403−1418. (10) Hammerich, O.; Parker, V. D. Kinetics and Mechanisms of Reactions of Organic Cation Radicals in Solution. Adv. Phys. Org. Chem. 1984, 20, 55−189. (11) Baumgarten, M.; Müllen, K. Radical Ions: Where Organic Chemistry Meets Materials Science. Top. Curr. Chem. 1994, 169, 1−103. (12) Inada, H.; Shirota, Y. 1,3,5-Tris[4-(diphenylamino)phenyl]benzene and Its Methyl-Substituted Derivatives as a Novel Class of Amorphous Molecular Materials. J. Mater. Chem. 1993, 3, 319−320. (13) Rohde, D.; Dunsch, L.; Tabet, A.; Hartmann, H.; Fabian, J. Radical Ions of α,α‘-Bis(diphenylamino)-Capped Oligothiophenes: A Combined Spectro-Electrochemical and Theoretical Study. J. Phys. Chem. B 2006, 110, 8223−8231. (14) Tabet, A.; Schröder, A.; Hartmann, H.; Rohde, D.; Dunsch, L. Preparation and Characterization of α,α′-Bisdiarylamino-Capped Oligothiophenes. Org. Lett. 2003, 5, 1817−1820. (15) Schumann, J.; Kanitz, A.; Hartmann, H. Synthesis and Characterization of Some Heterocyclic Analogues of N,N′-Perarylated Phenylene-1,4-diamines and Benzidines as a New Class of Hole Transport Materials. Synthesis 2002, 1268−1276. (16) Kanitz, A.; Schumann, J.; Scheffel, M.; Rajoelson, S.; Rogler, W.; Hartmann, H.; Rohde, D. Arylated Mono- and Bifunctional 2Aminothiophenes and 2-Aminothiazoles as a New Class of Hole Transport Materials. Chem. Lett. 2002, 896−897. (17) Rapta, P.; Rohde, D.; Hartmann, H.; Dunsch, L. The Variety of Reactions of Radical Cations Derived from 2-Diphenylaminothiophene Oligomers. Tetrahedron Lett. 2006, 47, 7587−7590. (18) Rapta, P.; Zeika, O.; Rohde, D.; Hartmann, H.; Dunsch, L. Thiophene−Thiophene versus Phenyl−Phenyl Coupling in 2-(Diphenylamino)-Thiophenes: An ESR-UV/Vis/NIR Spectroelectrochemical Study. Chem. Phys. Chem. 2006, 7, 863−870. (19) Wu, I. Y.; Lin, J. T.; Tao, Y. T.; Balasubramaniam, E. Starburst Molecules Based on Hexathienylbenzene Units: Potential HoleTransport Materials. Adv. Mater. 2000, 12, 668−669. (20) Rapta, P.; Tabet, A.; Hartmann, H.; Dunsch, L. New Molecular Materials for Hole Injection: the Synthesis and in Situ ESR-UV/Vis/ NIR Spectroelectrochemistry of 2-Diarylaminothiophene-Based Starburst Compounds. J. Mater. Chem. 2007, 17, 4998−5007. (21) Šolc, R.; Lukeš, V.; Rapta, P.; Hartmann, H.; Dunsch, L. Structural Changes in 2-Diarylthiophene-Substituted Starburst Compounds upon Charging: A Theoretical and Spectroelectrochemical Study. ChemPhysChem 2008, 9, 2501−2509. (22) Frechet, J.; Tomalia, D. Dendrimers and Other Dendritic Polymers; Wiley: New York, 2001. (23) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. About Dendrimers: Structure, Physical Properties, and Applications. Chem. Rev. 1999, 99, 1665−1688. (24) Lo, S.-C.; Burn, P. L. Development of Dendrimers: Macromolecules for Use in Organic Light-Emitting Diodes and Solar Cells. Chem. Rev. 2007, 107, 1097−1116. (25) Louie, J.; Hartwig, J. F.; Fry, A. J. Discrete High Molecular Weight Triarylamine Dendrimers Prepared by Palladium-Catalyzed Amination. J. Am. Chem. Soc. 1997, 119, 11695−11696. (26) Hagedorn, K. V.; Varnavski, O.; Hartwig, J.; Goodson, T., II.I Exciton Trapping in an Organic Dendrimer Possessing No Energy Gradient. J. Phys. Chem. C 2008, 112, 2235−2238. (27) Kundu, P.; Justin Thomas, K. R.; Lin, J. T.; Tao, Y.-T; Chuen, C.H. High-Tg Carbazole Derivatives as Blue-Emitting Hole-Transporting Materials for Electroluminescent Devices. Adv. Funct. Mater. 2003, 13, 445−452.

(28) Wong, K.-T.; Lin, Y.-H.; Wu, H.-H.; Fungo, F. Synthesis and Properties of Dumbbell-Shaped Dendrimers Containing 9-Phenylcarbazole Dendrons. Org. Lett. 2007, 9, 4531−4534. (29) Lin, Y.-H.; Wu, H.-H.; Wong, K.-T; Hsieh, C.-C.; Lin, Y.-C.; Chou, P.-T. Modulate Photoinduced Electron Transfer (PET) Efficiency of Bipolar Dendritic Systems. Org. Lett. 2008, 10, 3211−3214. (30) Promarak, V.; Ichikawa, M.; Sudyoadsuk, T.; Saebgsuwan, S.; Jungsuttiwong, S.; Keawin, T. Thermally and Electrochemically Stable Amorphous Hole-Transporting Materials Based on Carbazole Dendrimers for Electroluminescent Devices. Thin Solid Film 2008, 516, 2881−2888. (31) Moonsin, P.; Prachumrak, N.; Rattanawan, R.; Keawin, T.; Jungsuttiwong, S.; Sudyoadsuk, T.; Promarak, V. Carbazole Dendronised Triphenylamines as Solution Processed High Tg Amorphous HoleTransporting Materials for Organic Electroluminescent Devices. Chem. Commun. 2012, 48, 3382−3384. (32) Ito, A.; Sakamaki, D.; Ichikawa, Y.; Tanaka, K. Spin-Delocalization in Charged States of para-Phenylene-Linked Dendritic Oligoarylamines. Chem. Mater. 2011, 23, 841−850. (33) Heichert, C.; Wrackmeyer, M.; Hartmann, H. A Versatile, HeavyMetal Free Route to N,N‘-Tetraaryl-Substituted 2,5-Diaminothiophenes. Synthesis 2011, 3375−3378. (34) Petr, A.; Dunsch, L.; Neudeck, A. In situ UV−Vis ESR Spectroelectrochemistry. J. Electroanal. Chem. 1996, 412, 153−158. (35) Matis, M.; Rapta, P.; Lukeš, V.; Hartmann, H.; Dunsch, L. Highly Charged Cations from N,N,N′,N′-Tetrakis(4-aminophenyl)benzidine and Its N,N,N′,N′-Tetrakis(4-methoxyphenyl)-Substituted Homologue Studied by Thin-Layer in Situ Electron Spin Resonance/UV−Vis−NIR Spectroelectrochemistry. J. Phys. Chem. B 2010, 114, 4451−4460. (36) Barone, V. Recent Advances in Density Functional Methods Part I; World Scientific Publishing: Singapore,1996. (37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision A.1; Gaussian, Inc.: Wallingford, CT, 2003. (38) Rapta, P.; Fáber, R.; Dunsch, L.; Neudeck, A.; Nuyken, O. In Situ EPR and UV−Vis Spectroelectrochemistry of Hole-Transporting Organic Substrates. Spectrochim. Acta, Part A 2000, 56, 357−362. (39) Braun, S.; Salaneck, W. R.; Fahlman, M. Energy-Level Alignment at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 2009, 21, 1450−1472.

6711


Combined Spectroelectrochemical and Theoretical Study of Electron

Recommend Documents