Bithiophenes in Acceptor

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Extending Hexaazatriphenylene with Mono/Bi-Thiophenes in Acceptor-Donor Diads and Acceptor-Donor Acceptor Triads María Moreno Oliva, Alberto Riaño, Iratxe Arrechea-Marcos, Maria Mar Ramos, Rafael Gomez, Manuel Algarra, Rocio Ponce Ortiz, Juan T. López Navarrete, José Luis Segura, and Juan Casado J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08123 • Publication Date (Web): 16 Sep 2016 Downloaded from http://pubs.acs.org on September 24, 2016

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The Journal of Physical Chemistry

Extending Hexaazatriphenylene with Mono/Bi-Thiophenes in Acceptor-Donor Diads and Acceptor-Donor Acceptor Triads María Moreno Oliva,1 Alberto Riaño,2 Iratxe Arrechea-Marcos,1 María M. Ramos, 2 Rafael Gómez, 2 Manuel Algarra, 1 Rocío Ponce Ortiz,1 Juan T. López Navarrete, 1 José L. Segura,2,* Juan Casado,1,* 1

Department of Physical Chemistry, University of Málaga, Campus de Teatinos s/n, Málaga 29071, Spain Department of Organic Chemistry, Complutense University of Madrid, Faculty of Chemistry, Madrid 28040, Spain

2

Corresponding Authors: *E-mail: [email protected], [email protected]

ABSTRACT Three new HAT (Hexaazatriphenylene)-based electron accepting molecules with octupolar disc-like symmetry that combine the HAT core with six branches of electron donor thiophenes in two modalities have been synthesized: i) with six donor thiophenes and bithiophenes delineating a six-donor-to-one-acceptor (6-1) profile, and ii) with six donor-acceptor branches configuring a 6-6-1 acceptor-donor-acceptor triad. The six-fold accumulation of donors and acceptors in the periphery of the HAT core is expected to tune the molecular electronic and optical properties. An exhaustive analysis of these properties as a function of the 6-1 and 6-61 stoichiometry of the molecules is described by combining a palette of experimental spectroscopic techniques such as electronic absorption (from the ground electronic and excited states), emission (fluorescence and phosphorescence), ultraviolet photoelectron spectroscopy, spectroelectrochemistry, and vibrational Raman have been implemented, all combined with electrochemistry and molecular theoretical modeling. A particular focus on the charged species and the charge distribution around the 6-1 and 6-6-1 patterns is conducted. Structureproperty relationships have been outlined. The complete understanding of all these properties

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might help to design improved chromophores based on the HAT structure and to anticipate new properties.

I. INTRODUCTION Azaheterocycles play an important part in the chemical household of all living creatures. Indeed our genetic material, the very blueprint of life, is encoded by azaheterocyclic pyrimidine and purine bases. The versatility of these structures allows to tune their chemical, physical and biological properties and has led to a wide variety of applications in medicine, as solvents, dyes, photographic sensitizers, antioxidants, etc.1,2 Especially the electron deficient nature of these aromatic systems, effected by the incorporation of the electronegative nitrogen heteroatoms, makes them attractive for many applications in supramolecular chemistry. While electron rich π-conjugated systems are generally good hole conductors and can be converted in p-type (donor) materials by oxidation, electron deficient aromatic systems are good electron conductors and can be reduced to n-type (acceptor) units. Both p- and n-type materials are necessary to arrive at true improved organic devices. HAT can be regarded as an extended π-system where three pyrazine rings share a central benzene core in an aromatic, disc-like platform. Since the first synthesis of HAT was reported in 1981,3 many of its derivatives have found extensive applications as metal ligands, and more recently this system has been the basis for the synthesis of novel materials with very interesting properties.4 On the other hand, C3 symmetric octupolar molecules have found application in nonlinear optics.5 In this regard, the HAT core with branched arms (usually six) plays an important role.6-10 Thiophenes and oligothiophenes are typical electron donors given the low aromatic character of the five member ring, further incremented by the sulphur atom. These compounds have been utilized as p-type semiconductors in many organic electronic devices and are recurrent building blocks when electron-donor and electron-acceptor synergistic interactions 2


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for enhanced opto-electronic properties are demanded. The two free terminal positions of the thiophene or oligothiophenes, are available for further functionalization giving rise to donoracceptor dyads or acceptor-donor-acceptors triads depending on the nature of the substituting groups. These donor/acceptor concepts have had a revival thanks to their implementation in donor-acceptor polymers in optolectronic applications, mainly in organic solar cell photovoltaic devices.

Figure 1. Chemical structures of the molecules investigated in this article.

In an attempt to explore new singular properties, we have prepared and report in this article three new molecules having a common central acceptor HAT group which is fully functionalized in its six peripheral positions with six hexyl monothiophene (HT) and six hexyl bithiophenes (HBT), both delineating a (6)-donor-(1)-acceptor pattern. In a further extension, the hexyl groups of HBT are replaced by dicyanovinylene groups (HBTV) which confer another electron accepting perimeter thus resulting in a (6)-acceptor-(6)-donor-(1)-acceptor disposition from the periphery to the core. We conduct a multidisciplinary physico-chemical spectroscopic study to systematically analyze the size/functionalization evolution of the optoelectronic

properties

on

HTHBT HBTV

by

simultaneously

combining

electrochemistry, electronic absorption (ground state and excited state absorptions), emission (fluorescence

and

phosphorescence),

ultraviolet

3


photoelectron

spectroscopy,


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spectroelectrochemistry and Raman spectroscopy together with theoretical modeling at the DFT level of theory.

II. EXPERIMENTAL AND THEORETICAL METHODOLOGIES II. 1. Synthesis. The structures and synthetic routes for the preparation of the three dyes are shown in Figure 1 and Scheme 1, respectively. All of the dyes have been synthesized according to several classical reactions, and detailed synthetic procedures and physical properties (Figure S1) are described in the Supporting Information. Among the three different methods reported for the building of the HAT skeleton,3,11,12 we have found that the condensation between hexaaminobenzene (HAB, Scheme 1) and different α-diketones is the most efficient route for the synthesis of our target compounds HT, HBT and HBTV (Figure 1).

Br Br S S N

NH2 H2N

+ H2N

NH2

AcOH

O

NH2 Br

Br

N

S S

Br

N

O

S

NH2 HAB

N

S

N N

S Br

Br

2

S Br

HT-6Br

Scheme 1. Synthesis of compounds HT and HT-6Br.

HAB can be obtained by reducing 1,3,5-triamino-2,4,6-trinitrobenzene using sodium in liquid ammonia following the procedure described by Rogers.13 Then, the target system HT bearing six hexyl-thiophene units attached to the peripheral positions of the HAT core, was 4


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obtained by reaction between HAB and the diketone 1,2-bis(5-hexylthiophen-2-yl)ethane-1,2dione (1).14 In spite of the large discotic π-conjugated system formed, the presence of six hexyl chains in the peripheral position of the molecule endows this material with good solubility and processability. By following an identical synthetic route, intermediate HAT derivative HT-6Br (Scheme 1) was obtained by reaction between the HAB and the diketone 1,2-bis(5-bromothiophen-2-yl)ethane-1,2-dione (2)15 in acetic acid in a good 80 % yield. The six-fold Stille cross-coupling reaction between HT-6Br and stannyl derivative (5hexylthiophen-2-yl)trimethylstannane (3)16 afforded the second target system HBT in a 70 % yield (Scheme 2).

Scheme 2. Synthesis of compounds HBT and HBTV.

Similarly, hexaaldehyde HBT-6CHO is obtained by a six-fold Stille cross-coupling reaction, between HT-6Br and stannyl derivative 3-hexylthiophene-2-carbaldehyde 417 in 53% yield. The key intermediate 4 was obtained as reported by Chen et al. through two steps: (1) the aldehyde function of 3-hexylthiophene-2-carbaldehyde was protected by a conversion into the glycol acetal, and (2) subsequent reaction with n-BuLi and tributyltin chloride. Finally, a six-fold Knoevenagel condensation reaction between malononitrile and HBT6CHO afforded the target HBTV as a deep red solid in 12% yield. Whereas HBT-6CHO is soluble in common organic solvents, HBTV exhibits only limited solubility. This is due to the 5



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additional planarization characteristic of the molecules containing the dicyanovinylene moiety which is produced through intermolecular interactions between the olefinic protons and the nitrile nitrogens18 thus enhancing the tendency toward π-π staking.

II. 2. Spectroscopic and Electrochemical Measurements. UV-Vis absorption spectra were recorded on an Agilent 8453 instrument equipped with a diode array detection system. Emission spectra were measured using a spectrofluorometer from Edinburgh Analytical Instrument (FLS920P) equipped with a pulsed xenon flash-lamp, at room temperature, and at 77 K by using a liquid nitrogen dewar. Fluorescence decays were measured by using a Single Photon Photomultiplier Detection System (S900) with Picosecond Pulsed Diode Laser (PDL 800-B), from Edinburgh Instruments. All solvents used were of spectroscopic grade from Aldrich. Fluorescence quantum yields, φF, were measured for all the solutions using 1 ⋅ 10-7 mol L-1 quinine sulphate in 0.1 mol L-1 H2SO4 as the standard (φF=0.546). No fluorescent contaminants were detected upon excitation in the wavelength region of experimental interest. Transient triplet-triplet absorption spectra were measured with a laser flash photolysis system (Luzchem LPF-111), with Xe ceramic lamps operating at 300 W (UV-Vis), a 125 mm monochromator, a Tektronix TDS 2001C oscilloscope with a bandwidth of 50 MHz, and a compact photomultiplier. Samples were excited by third-harmonic generation (355 nm) of an Nd:YAG laser (Lotis TII LS-2132 UTF) controlled by a Quantum Composers 9520 pulse generator with tunable frequencies. Ultraviolet Photoelectron Spectroscopy (UPS) measurements were carried out by a Thermo Scientific Multilab 2000 spectrometer using He(I) and He(II) UV source with a photon flux > 1.5 ⋅ 1012 s-1 and a 110 mm hemispherical sector analyzer fitted with a seven channeltron detector. Thin films of molecules HT, HBT and HBTV were deposited on silicon substrates. Binding energies in UPS data are measured with respect to the Fermi energy of the

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spectrometer system and can be easily calibrated by measuring the Fermi edge emission of an Au metallic simple. Electrochemical data were obtained with an Autolab PGSTAT 302. For working and counter electrodes Pt was used. Before each experiment the working electrode was submitted to the following cleaning procedure: Pt was polished with fine emery paper followed by polishing with an alumina suspension over cloth and then ultrasonically cleaned in distilled water. A reference electrode, Ag/Ag+ (0.01M AgNO3 in acetonitrile) for non-aqueous solvent, was used in the electrochemical cell. The solvent employed was anhydrous and desoxygenated dichloromethane. The supporting electrolyte was tetrabutylammonium hexafluorophosphate (TBAPF6, Aldrich, electrochemical grade). The voltammograms were obtained using a scan rate of 100 mV/s and all the values were referenced to the pair Fc/Fc+. Spectroelectrochemical data were obtained by using an optically transparent thin-layer electrochemical (OTTLE) cell positioned in the sample compartment of a Cary 5000 Spectrophotometer. The spectroelectrochemical cell consisted of a platinum wire as counter electrode, an Ag minigrid as the pseudo-reference electrode, and a Pt minigrid (32 wires per cm) as the working electrode. The potential of the three electrodes was controlled using a BAS 100B Electrochemical Analyzer referenced against the Fc/Fc+ couple. The supporting electrolyte consisted of tetrabutylammonium hexafluorophosphate (TBAPF6, Aldrich, electrochemical grade) 0.1M in dry desoxygenated dichloromethane. Chemical oxidation: oxidised species were generated in CH2Cl2 solutions by progressive addition of FeCl3. FT-Raman scattering spectra with excitation at 1064 nm were collected on a Bruker FRA106/S apparatus and a Nd:YAG laser source (λexc = 1064 nm), in a back-scattering configuration. The operating power for the exciting laser radiation was kept to 100 mW in all the experiments. Samples were analyzed as pure solids averaging 1000 scans with 4 cm-1 of spectral resolution. Resonant Raman spectra of the stable dicationic species of HT and HBT

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compounds were measured on a Bruker Senterra apparatus with excitation wavelengths at 532 nm and 785 nm.

II. 3. Theoretical Methods. The ground-state molecular geometries in their neutral and oxidized states, vibrational frequencies and Raman intensities were calculated using Density Functional Theory (DFT) by means of the Gaussian 09 programming package.19 The Becke’s three parameter (B3) gradient-corrected exchange functional combined with the correlation Lee-Yang-Parr (LYP) correlation functional was utilized20,21 the 6-31G** basis set was used.22,23 Theoretical Raman spectra were obtained for the resulting ground-state optimized geometries. Harmonic vibrational frequencies and Raman intensities were calculated analytically and numerically respectively. For the open-shell radical cations and trications, the unrestricted UB3LYP functional was used. The time-dependent DFT (TD-DFT) approach has been used for the evaluation of, at a minimum, the thirtieth lowest-energy vertical electronic excited states including both singlet and triplet states (last one with UB3LYP).24-26 In order to evaluate the optimized geometries of higher-lying excited states, we have relied on the restricted configuration interaction with singles approach (CIS) within the Hartree-Fock (HF) aproximation (RCIS/HF) in which the single determinant RHF wavefunction is used as the reference determinant in a CIS calculation of excited states.27 The ground-state molecular geometries of dimers were calculated using CAM-B3LYP/6-31G**.

III. RESULTS AND DISCUSSION III. 1. Electronic Spectra and Electronic Structure. Figure 2 shows the electronic absorption and emission spectra of all compounds. The UV-Vis absorption spectra are characterized by the presence of broad bands of medium-high intensity in the 300-550 nm range (Table 1 for molar absorptivities). TD-DFT excited-state calculations assuming a C3h 8


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geometry for the molecule helped us to assign these experimental features (Figure 2). In the case of the HT molecule, the lowest energy (430 nm) and strongest band (ε = 71497 M−1cm−1) corresponds to the sum of two double mono-electronic excitations, S0 → S5 [HOMO-1 → LUMO; HOMO → LUMO+1] and S0 → S6 [HOMO-1 → LUMO+1], predicted at 422 nm (oscillator strength 0.76). This band is followed by another band at 321 nm arising from the S0 → S13 [HOMO-3 → LUMO] transition calculated at 341 nm (oscillator strength 0.26) and S0 → S15 [HOMO-3 → LUMO+1], at 339 nm (oscillator strength 0.23). The S0 → S1 excitation is predicted with zero oscillator strength given the C3h geometry. These absorption bands show red-shift to longer wavelengths and a progressive broadening on HTHBT HBTV, the latter aspect likely due to an increasing number of C-C single bonds in the arms thus conferring increasing molecular flexibility.

Figure 2. UV-Vis spectra (solid line, black) of molecules HT, HBT and HBTV in CH2Cl2 together with their TD-DFT//B3LYP/6-31G** theoretical excitations (bars, wavelength versus oscillator strength). The fluorescence emission spectra (dotted line, red) in CH2Cl2 are also showed. Note that for the theoretical calculations, the hexyl groups have been replaced with methyl groups.

The orbital diagram with the absolute energies and topologies of the relevant frontier molecular orbitals are shown in Figure 3. As expected the HOMO/HOMO-1 and LUMO/LUMO+1 are degenerated for the C3h molecular symmetry. As a consequence of the degeneration, there are not single orbitals that might show full simultaneous wavefunction delocalization in all branches and core. For instance, the HOMO-2 and LUMO-2 are fully 9



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localized on the thiophene and HAT parts respectively, without intermixing, whereas the H-1, H, L and L+1, spread out the core and some of the arms.

Figure 3. (Left) DFT//B3LYP/6-31G** molecular orbital energies of molecules HT, HBT and HBTV. (Right) Topologies of molecule HT. Note that for the theoretical calculations, the hexyl groups have been replaced with methyl groups.

By increasing the size of the arms to a bithiophene unit, i. e. HTHBT, there is a slight ≈0.12-0.18 eV stabilization of the unoccupied molecular orbitals accompanied of a significant ≈0.4-0.6 eV destabilization of the occupied ones, resulting in a reduction of the HOMO-LUMO gap by 0.6 eV which correlates with an experimental optical gap reduction of 0.41 eV. The discrepancy between optical gap reduction and HOMO-LUMO reduction on HTHBT is due to the contribution by other orbital transitions to the low energy lying electronic excitation. On the other hand, after inclusion of the dicyanovinylene groups (HBTV) there is a considerable stabilization of the frontier molecular orbitals which is asymmetric since the unoccupied orbitals are stabilized by ≈1.3 eV whereas those occupied are less affected, by ≈1 eV, resulting in an overall decrease of the HOMO-LUMO theoretical gap (2.67 eV in HBT → 2.31 eV in HBTV). The change in the experimental optical gap is from 579 nm in HBT to 647 nm in HBTV (Figure 2) by 0.22 eV which contrasts with the 0.41 eV reduction in HTHBT revealing the different nature of the electronic effects

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(conjugative versus cross-conjugated donor-acceptor) in despite of the equal addition of 6 pzelectrons in both cases.

Figure 4. DFT//B3LYP/6-31G** molecular orbital diagram showing the coupling between the 2T+DCV and HAT fragments of HBTV. Note that for the theoretical calculations, the hexyl groups have been replaced with methyl groups.

Table 1. Summary of Photophysical and Electrochemical (in Volts) Data in CH2Cl2 at 25ºC, as well as experimental and theoretical ionization energies (in eV). λabs [nm]

-1

-1

ε [M cm ]

λF [nm]

ΦF

τF

Ered1a

Ered2a

Eox1a

Eox2a

IEexp

IEtheo

[ns]

[V]

[V]

[V]

[V]

[eV]

[eV]

+0.97

+1.16

7.95

6.06

7.86

5.60

7.23

6.62

HT

430

71497

480

0.05

0.6

-1.80

HBT

477

115879

567

0.06

0.7

-1.28

-1.67

HBTV

497

137434

583

0.21

0.5

-0.86

-1.14

+0.29

a

Conditions: electrochemical potentials (V) vs Fc/Fc+ of the indicated compounds in CH2Cl2

Figure 4 displays the MO diagram describing the interaction between 2T+DCV (bithiophene and dicyanovinylene groups) and the HAT core, coupled to form HBTV. The main feature is that the LUMO level is stabilized in HBTV in comparison with those of 2T+DCV and HAT while the HOMO is much less affected. It is clear that the LUMO of 2T+DCV interacts with the LUMO+1/LUMO+2 orbitals of HAT giving rise to a bonding

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coupling between the two units along the connecting C-C bonds (see L, L+1, and L+2). This results in the stabilization of these two LUMO+1 and LUMO+2 orbitals in HAT which becomes the LUMO and LUMO+1 whereas the LUMO of HAT converts in the LUMO+2 of the HBTV. The situation is rather different in the occupied orbitals since the HOMO, HOMO1 and HOMO-2 are essentially due to the 2T+DCV units (i.e., -6.10 eV of energy for the HOMO of 2T+DCV and -6.16 eV for the HOMO/HOMO-1 of HBTV). This description highlights the fact that the occupied-to-unoccupied one-electron transitions have an intrinsic charge transfer character from the periphery to the core. Figure 2 also displays the emission spectra of a dilute solution of molecules HT, HBT and HBTV in dichloromethane, whereas Table 1 summarizes the relevant spectral and photophysical data. The fluorescence emissions present the same pattern as the absorption spectra, that is, a bathochromic shift on HTHBT HBTV. The fluorescence quantum yield (Φ F) is very similar in HT and HBT, but molecule HBTV presents the highest value. The fluorescence lifetimes are very similar, with bi-exponential decays, on the order of 0.6 ns as the main component. Molecule HBTV shows the lowest τF according with the largest Φ F. This is interesting given that the (6)-acceptor-(6)-donor-(1)-acceptor coupling blocks the nonradiative deactivation channels, more intensively operating in the (6)-donor-(1)-acceptor case. A tentative explanation of this could be on the basis of the well-known decrease of the fluorescence associated with optical bands with A-D charge transfer character as in HT and HBT, while this quenching route is partially cancelled in the A-D-A case of HBTV (less electron-withdrawing effect in D). To complement the study of fluorescence emission, we have measured in the case of HT the fluorescence spectra at different concentrations and temperatures (Figure S2). Upon increasing the concentration, the absorption spectra gradually become more intense, whereas the fluorescence spectral intensity decreases after reaching an emission maximum. This indicates the formation of some intermediate aggregates which

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quench the effective emission. The formation of aggregates in these disc-like molecules is a well-known fact for HAT derivatives and will be of central relevance to interpret the UPS data bellow, which need explanation by invoking aggregation in the solid state.

III. 2. Triplet-Triplet Absorption and Triplet Emission. It is now pertinent to explore the triplet excited state properties of the new compounds in correlation with the already discussed singlet ground and excited state properties. We thus analyze the triplet excited state manifold by measuring the phosphorescence spectra (at 77 K) and by performing microsecond time resolved flash photolysis or excited state absorption measurements. On this series of molecules, the phosphorescence spectrum28 is only obtained in the case of HT (Figure 5), which is in agreement with its lower ΦF value compared to HBT and HBTV.

Figure 5. (Left) UV-Vis spectra (solid line, black) in CH2Cl2, fluorescence emission spectra (dotted line, red) in CH2Cl2, phosphorescence emission spectra (dotted line, green) in methylcyclohexane at 77 K, and transient triplet-triplet spectra (dashed line, blue) in CH2Cl2 of molecule HT. (Right) Energy diagram for the electronic singlet and triplet states of HT comparing experimental (bold italics) and DFT//B3LYP/6-31G** theoretical data.

The phosphorescence spectrum of HT has a band maximum at 620 nm (2.00 eV) with a similar vibronic structure as the fluorescence emission recorded at room temperature. This wavelength correlates very well with that predicted by TD-DFT/B3LYP/6-31G** calculations in Figure 5 which estimates a value of 634 nm (1.97 eV) for the vertical T1→ S0 transition. The phosphorescence lifetime (τphos) has a mono-exponential decay with a lifetime of 8.9 ms.

13



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Figure 6. (Top) Transient triplet-triplet spectra of molecules HT (left), HBT (middle) and HBTV (right) in CH2Cl2 at room temperature (293 K). (Bottom) Kinetic curves after the laser-pulse pump at 355 nm and triplet lifetimes.

Triplet-triplet excited state absorptions have been also characterized in combination with DFT calculations in Figure 6. The excited state spectrum of HT presents a broad and unstructured band around 675 nm, whereas HBT and HBTV show more intense and wellstructured bands with absorption maxima at 790 and 795 nm, respectively. All of them present a first-order kinetics associated with the depletion of the ground electronic singlet state (photo-bleaching) and the formation of a sole transient species. The conversion processes show well defined isosbestic points. The excited state lifetimes vary from 22.63 µs in HT to 36.69 µs in HBT and to 24.97 µs in HBTV giving the first indication that we are dealing with long lived triplet species. The shorter conjugation in HT and the blockage of charge transfer character (such as in the fluorescence ΦF) in the acceptor-donor-acceptor extended triad might justify their shorter lifetimes compared to HBT. T1→Tn excited state transitions have been calculated by means of TD-DFT//(U)B3LYP/6-31G** (Figure 6), showing a suitable correlation between the experimental and theoretical data. The red-shift from HT to HBT and HBTV can be attributed to the larger triplet delocalization in the more π-extended thiophene systems for the first triplet excited state T1 in consonance with the behavior of the fluorescence spectra and of their analogous first singlet excited state S1. To summarize the

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photophysical data, Figure 5 displays an energy diagram with absorption (S0 → S5/S6), fluorescence emission (S1 → S0), phosphorescence emission (T1 → S0) and excited state triplet-triplet absorption (T1 → T15) in the case of molecule HT from TD-DFT//(U)B3LYP/631G** calculations. III.

3.

Electrochemical,

Solid

State

UPS

Properties

and

UV-Vis-NIR

Spectroelectrochemistry. HAT derivatives29 exhibit three characteristic quasi-reversible waves in the cathodic regime, corresponding to the three consecutive reduction steps of the three electron-deficient pyrazine moieties. However, for the HT, HBT and HBTV derivatives only one or two reduction waves are observed (Figure S9 and Table 1). In any case, the first electron reduction displays a shift to more positive potentials in qualitative agreement with the decrease in absolute energy of the unoccupied molecular orbitals (Figure 3) although with different nature since on HTHBT the decrease is a π−conjugation effect while on HBTHBTV the orbital stabilization is due to the electron-withdrawing effect of the dicyano groups. The same explanation is valid for the trend in the second reduction potentials. This experimental (reduction potential) /theoretical (absolute energy values of the lowest unoccupied molecular orbitals) correlation is not quantitative because of the raw use of the Koopman´s approach when degenerate orbitals are involved. In the anodic potentials regime, only in molecule HT the oxidation potentials corresponding to the formation of the radical cation (+0.97 V) and dication (+1.16 V) could be observed. For HBT the oxidation potential is at +0.29 V and no further oxidations are observed. In HBTV oxidations are precluded by the electron-withdrawing effect of the acceptors. Aimed to explore the ionization properties of these thienyl HAT derivatives in solid state, we have measured their ultraviolet photoelectron spectra (i.e., UPS) as solid thin films (Figure 7) from which we obtained the ionization energies (i.e., IE) in Table 1.29 Furthermore,

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IE has been theoretically estimated by calculating the difference between the absolute formation energies of the neutral and radical cation species considered as individual entities in the vacuum without any effect of the environment (neither solvent or solid state). From HT to HBT the IE decreases by 0.09 eV, qualitatively in agreement with the theoretical decrease of 0.46 eV. However, quantitatively, the discrepancy between theory and experiment reveals the significance of solid state interactions. This is not unexpected since it is well known that HAT dyes easily aggregate by π−π stacking,4 an effect likely facilitated here by the presence of intermolecular donor-acceptor coupling (see Scheme 3) which likely confront the HAT acceptor part with the thiophene donor part of the vicinal molecule in order to couple the respective local dipolar momenta originated by the BTHAT charge polarization effect in each arm (the molecules as a whole would display nearly zero dipolar momentum by symmetry reasons). This situation is likely reversed in HBTV given the acceptor-donoracceptor pattern which minimizes the intrinsic local dipoles and allows a more columnar stacking by π−π interactions facing donor groups of neighbor molecules, on one hand, and acceptors on the other (see Scheme 3). In this solid state organization, once the electron is ejected from the solid surface (within the few nanometers layer), the positive charge can be delocalized among the donor sites justifying the smaller IE measured in HBTV regarding HT and HBT, and also accounting for the deviation of the experimental IEs regarding the values predicted by calculations as isolated entities. In this regard, theoretical calculations in dimeric stacked structures corroborate the interpretation of the IE data (see Figure S11). Thus, in the case of HT and HBT, these dimers show branch-to-core stacking orientation whereas HBTV dimers prefer to stack in a core-to-core disposition. Aimed to explore the behavior of these materials as semiconductors in organic field effect transistors, substrates of the three samples have been prepared either by sublimation of the organic semiconductors or as a spin-coated thin films. Thermal annealing of the substrates was also carried out at room temperature (25

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ºC), 80 ºC and 150 ºC in the SiO2 surfaces previously treated by HDMS. It turns out that for HT and HBTV (the smallest and largest) no field effects were obtained. The medium size compound HBT was electrically active and the field effect mobility detected was of p-type (no n-type) that amounts to 3.9 10−7 cm2•V–1s–1 at room temperature while at 150 ºC, the mobility was 7.3 10−7 cm2•V–1s–1 (see Supporting Information). These are certainly very poor data which can be the result of the electronic intramolecular polarization due to the acceptor and donor which somehow interferes intramolecular transport. In fact, the p-type behavior detected is for that molecule with greater contain of thiophenes electron donors. Hence for HT and HBTV that the acceptor contains are larger, no mobilities are detected. Nonetheless, these very small mobilities are not surprising in the context of other HAT derivatives where the largest mobility obtained was around 10−4 cm2•V–1s–1.30

Figure 7. UPS He I spectra of molecules HT (black), HBT (red) and HBTV (blue), and Au (pink) as reference in solid state.

H-T

hν

H-T

H-BT

hν

H-BT

H-BT-V

hν

H-BT-V

H-T

hν

H-T + e

H-BT

hν

H-BT + e

H-BT-V

hν

H-BT-V + e

δ+

δ+

H-T

H-T

δ+

hν

δ+

H-BT δ+

H-T

H-T

δ+

δ+

H-T

H-T

+ e

δ+

H-BT

hν

δ+

δ+

H-BT-V

H-BT-V

δ+

H-BT

H-BT

δ+

+ e

δ+

H-BT

H-BT

H-BT-V

hν

H-BT-V

δ+

δ+

H-BT-V

H-BT-V

+ e

Scheme 3. Photoionization processes: Top, according to calculations, as isolated entities in the vacuum. Middle, in gas phase (not measured here). Bottom, in solid state. HT, HBT and HBTV from the left to the right.

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The electronic absorption spectra obtained for the oxidized species of HT and HBT in CH2Cl2 are shown in Figure 8, together with that of neutral HBTV. For HT and HBT stepwise addition of the oxidant induces the disappearance of the absorption bands of the neutral species and the emergence of new electronic absorption bands in the visible region for the oxidized species, an inter-conversion that progresses through well-defined isosbestic points.

Figure 8. (Top) Experimental absorption spectra of molecules HT (left), and HBT (middle) as neutral (red), radical cation (blue), dication (green), and trication (pink) species together with TD-DFT//B3LYP/6-31G** theoretical excitations (bars, wavelength versus oscillator strength). In the case of HBTV (right), the neutral (red) is compared with HBT dication (green). (Bottom) Raman and theoretical spectra of molecules HT (left), and HBT (middle): a) Neutral theoretical; b) Neutral experimental; c) Dication theoretical; d) Dication experimental. Note that for the theoretical calculations, the hexyl groups have been replaced with methyl groups.

The absorption spectra of the oxidized species of HT have two bands at 460 nm and 580 nm while the corresponding species in HBT shows these at 550 nm and 700 nm. Therefore, by increasing the arm size on HT → HBT, the absorption bands are red-shifted due to extended conjugation and delocalization of the charges in molecules with more π−size. TD-DFT/ B3LYP/6-31G** calculations have been carried for a series of oxidized species

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consisting in the following species, radical cation, dication and radical trication of HT and HBT. By comparing the experimental and theorerical spectra of the oxidized species, we see that for the divalent species the correlation and fit is the best either in HT or in HBT. From the theoretical charge distribution, we observe that the dications allow a more symmetric distribution of the charge on the thiophene moieties what would justify its stabilization in preference with the lower and higher oxidized species. In the spectroelectrochemical experiment for HT, an intermediate species between the neutral and the dication appears and quickly vanishes that could be associated with the unstable radical cation. The direct oxidation to the dications and the apparent non-persistency of the radical cations of both compounds could explain the complex anodic behavior in the cyclic voltammetry experiments.

III. 4. Raman Spectroscopy. The experimental Raman spectra of neutral HT, HBT and HBTV in the solid state are shown in Figure 8 together with those of the oxidized species. Their theoretical Raman spectra of the neutral and oxidixed species of HT and HBT have been also obtained and presented in Figure 8. The relevant normal modes associated with the main Raman features described are displayed in the Supporting Information file. These spectra are dominated in all cases by the strong bands in the interval 1400-1480 cm−1 due to the C=C/C-C stretching vibrational modes, ν(C=C), of the thiophene parts. The frequencies of these bands are typically associated with the thiophene conjugational properties in the ground electronic state and, in this case, the changes of these π−properties with the evolution of the geometrical parameters from molecule to molecule. The main band in HT is at 1468 cm−1 while for HBT this upshifts to 1473 cm−1 indicating a reinforcement of the C=C bonds of the thiophene rings which takes place on those more separated from the HAT core. The analogous ν(C=C) Raman bands of the innermost thiophene rings however appear at 1445/1418 cm−1 which are at lower frequencies due to the quinoidization effect (the C=C are

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weakened while the C-C are reinforced) on these thiophene as a result of the electronwithdrawing effect by the central HAT core. In HBTV these quinoidal bands at 1447/1426 cm−1 dominate the spectrum revealing that the two thiophenes progressively converts to quinoidal by the simultaneous action of the electron-withdrawing effect of the HAT core in the innermost one and the dicyano-vinylene groups31,32 on the external one. For the stable dication species of HT and HBT, the Raman spectra were recorded for and shown in Figure 8 as well. The Raman spectrum of HT+2 displays full bleaching of the Raman bands due to the neutral molecule followed by the appearance of a spectral profile typical of oxidized cations. This spectrum has two main bands at 1436 and 1416 cm-1 in HT+2 due to the ν(C=C) thiophene modes (see deconvolution in Figure 8) and representing a clear frequency downshift regarding the neutral band of HT recorded at 1468 cm-1. The frequency downshift on HT HT+2 gives account of the quinoidization on the peripheral thiophenes with oxidation. On HBT HBT+2 the main ν(C=C) thiophene Raman band downshifts from 1473 cm-1 in neutral HBT to 1419 cm-1 in HBT+2 owing again to the quinoidization of the bithiophene rings. To close this discussion, we observe that the main Raman bands at 1447/1426 cm-1 of HBTV compare with the main bands of oxidized (quinoidized) HBT+2 at 1419 cm-1 corroborating that substitution with electron-withdrawing dicyanovinylene groups imparts in the ground electronic state of this acceptor-donor-acceptor molecule a quinoidization effect on the bithiophenes with the only difference that in HBT+2 quinoidization is induce by oxidation selectively in the bithiophene moieties, while in HBTV this is attained by electron-withdrawing of the vicinal HAT/DCV acceptors.

IV. CONCLUSIONS We have synthesized three new molecules comprising a central HAT core substituted by six monothiophene or bithiophene moieties arms, as well as with bithiophene20


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dicyanovinylene arms, involving different electron acceptor-donor and acceptor-donoracceptor patterns. Their electronic and molecular structures are fully characterized by optical spectroscopies, UPS, electrochemistry, spectroelectrochemistry, Raman and theoretical calculations. The extension on the number of thiophene rings in the arms and also inclusion of the bithiophene-dicyanovinylene groups produces a bathochromic shift in the absorption spectra, emission and excited-state absoprtions, consistent with the reduction in orbital gaps. The variations on the fluorescence quantum yields are due to the modification of the acceptordonor and acceptor-donor-acceptor character immersed in the main optical excitations. We have also addressed the properties of the triplet excited state manifold, the T1 excited state by its emission in the phosphorescence or by means of their absorptions. The electrochemical properties have been also studied by cyclic voltammetry and UV-Vis and Raman spectrochemistry, as well as UPS. IE is an important parameter for consideration of these organic molecules for charge injection/transport applications. The particular 6-1 substitution in the donor-acceptor cases allows the preferential stabilization of dications, which is in contrast with the case of linearly conjugated oligothiophenes where oxidation goes progressively step by step from cation to dication. The conversion from linear conjugation to 2D extended conjugation produces this interesting effect. We address by Raman spectroscopy the structures of the oxidized species and conclude on the quinoidization of the thiophene moieties which is the same structural motif occurring in the acceptor-donor-acceptor triad as a result of the donor-acceptor coupling.

Supporting Information Detailed methods of materials and instruments, synthesis of compounds, spectroscopic measurements, and device integration.

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ACKNOWLEDGEMENTS We thank the MINECO of Spain (MAT2014-52305-P) and the UCM-BSCH joint project (GR3/14-910759), for financial support at Complutense University of Madrid. Financial support from MINECO (Project reference CTQ2012-33733) is gratefully acknowledged by University of Málaga. M.M.O. thanks the MINECO for a “Juan de la Cierva-Incorporación” research contract. A. R. acknowledges the MINECO of Spain for a predoctoral fellowship.

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(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision C.01, Gaussian, Inc.: Wallingford, CT, 2010. (20) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785-789. (21) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648-5652. (22) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 1972, 56, 2257-2261. (23) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. SelfConsistent Molecular Orbital Methods. XXIII. A Polarization-Type Basis Set for Second-Row Elements. J. Chem. Phys. 1982, 77, 3654-3665. (24) Runge, E.; Gross, E. K. U. Density-Functional Theory for Time-Dependent Systems. Phys. Rev. Lett. 1984, 52, 997-1000. (25) Gross, E. K. U.; Kohn, W. Time-Dependent Density-Functional Theory. Adv. Quantum Chem.; ed L. PerOlov, Academic Press, 1990. (26) Heinze, H. H.; Görling, A.; Rösch, N. An Efficient Method for Calculating Molecular Excitation Energies by Time-Dependent Density-Functional Theory. J. Chem. Phys. 2000, 113, 2088-2099. (27) Foreman, J. B.; Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Toward a Systematic Molecular Orbital Theory for Excited States. J. Phys. Chem. 1992, 96, 135-149. (28) Moreno Oliva, M.; Casado, J.; López Navarrete, J. T.; Hennrich, G.; van Cleuvenbergen, S.; Asselberghs, I.; Clays, K.; Ruiz Delgado, M. C.; Brédas, J.-L.; Seixas de Melo, J. S.; et al. Synthesis, Spectroscopy, Nonlinear Optics, and Theoretical Investigations of Thienylethynyl Octopoles with a Tunable Core. Chem.- Eur. J, 2009, 15, 8223-8234. (29) Juárez, R.; Moreno Oliva, M.; Ramos, M.; Segura, J. L.; Alemán, C.; Rodríguez-Ropero, F.; Curcó, D.; Montilla, F.; Coropceanu, V.; Brédas, J. L.; et al. Hexaazatriphenylene (HAT) versus tri-HAT: The Bigger the Better? Chem.- Eur. J. 2011, 17, 10312-10322. (30) Saragi, T. P. I.; Reichert, T.; Scheffler, A.; Kussler, M.; Salbeck, J. Electron Mobility in Hexaazatriphenylene Hexacarbonitrile Field-Effect Transistors. Synthetic Metals 2012, 162, 1572-1576. (31) Moreno Oliva, M.; Casado, J.; Raposo, M. M. M.; Fonseca, A. M. C.; Hartmann, H.; Hernández, V.; López Navarrete, J. T. Structure-Property Relationships in Push-Pull Amino/Cyanovinyl End-Capped Oligothiophenes: Quantum Chemical and Experimental Studies. J. Org. Chem. 2006, 71, 7509-7520. (32) Ruiz Delgado, M. C.; Hernández, V.; Casado, J.; López Navarrete, J. T.; Raimundo, J.-M.; Blanchard, P.; Roncali, J. Vibrational and Quantum-Chemical Study of Push-Pull Chromophores for Second-Order Nonlinear Optics from Rigidified Thiophene-Based π-Conjugating Spacers. Chem. Eur. J. 2003, 9, 3670-3682.

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Bithiophenes in Acceptor

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