Lone Pair Delocalization Effect within Electron Donor Molecules: The

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C: Energy Conversion and Storage; Energy and Charge Transport

Lone Pair Delocalization Effect within Electron Donor Molecules: The Case of Triphenylamine and Its Thiophene-Analog Teng Zhang, Iulia Emilia Brumboiu, Cesare Grazioli, Ambra Guarnaccio, Marcello Coreno, Monica de Simone, Antonio Santagata, Håkan Rensmo, Barbara Brena, Valeria Lanzilotto, and Carla Puglia J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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The Journal of Physical Chemistry

Lone Pair Delocalization Effect within Electron Donor Molecules: The Case of Triphenylamine and Its Thiophene-Analog 1

T. Zhang,* I. E. Brumboiu,

2, 3

4

4

4

5

C. Grazioli, A. Guarnaccio, M. Coreno, M. de Simone, A.

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1

1

1

Santagata, H. Rensmo, B. Brena, V. Lanzilotto and C. Puglia

*1

1. Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden. 2. KTH Royal Institute of Technology, Department of Theoretical Chemistry and Biology, Stockholm 10691, Sweden 3. Korea Advanced Institute of Science and Technology (KAIST), Department of Chemistry, Daejeon 34141, Republic of Korea 4. ISM-CNR, Tito Scalo (Pz) and Trieste LD2 Unit, Italy 5. IOM-CNR, Laboratorio TASC, Sincrotrone Trieste, Basovizza, Trieste, Italy

*

Corresponding authors: [email protected], [email protected]

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(Abstract)

Triphenylamine (TPA) and its thiophene-analog, N,N-Diphenyl-2-thiophenamine (DPTA), are both well known as electron-donating molecules implemented in optoelectronic devices such as organic solar cells and LEDs. Comprehensive valence and core level PhotoElectron Spectroscopy (PES), as well as Near Edge X-ray Absorption Spectroscopy (NEXAFS) measurements have been performed on gas-phase TPA and DPTA. The experimental results have been compared to Density Functional Theory (DFT) calculations, providing a detailed description of the molecular electronic structure. Specifically, the C 1s photoelectron lines of both TPA and DPTA were resolved in the different C atom contributions and their binding energies explained as the result of two counter-acting effects: (1) the electronegativity of the nitrogen atom (and sulphur atom in DPTA) and (2) the delocalization of the N (and S in DPTA) lone pair electrons. In addition, the C K-edge NEXAFS spectrum of DPTA reveals that the Lowest Unoccupied Molecular Orbital (LUMO) energy position is affected differently if the core-hole site is on the phenyl compared to the thiophene ring. The electron-donating properties of these two molecules are largely explained by the significant contribution of the N lone pair electrons (pz) to the Highest Occupied Molecular Orbital (HOMO). The contribution to the LUMO and to the empty density of states of the sulphur of the thiophene ring in DPTA explains the better performance of Donor-π-Acceptor molecules containing this moiety and implemented in photo-energy conversion devices.


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Introduction First synthesized by Merz and Weith in 18731, triphenylamine (TPA, (C6H5)3N) has been intensively studied due to its versatile and tuneable properties, like, for example the adsorption edge (related to the HOMO - LUMO gap) via functionalization or via modification of the molecular structure in which TPA represents the donor moiety.2 For these reasons it has become an attractive component in technological devices.2–5 The molecular structure of TPA is shown in Figure 1 (a). It is an aromatic amine and can be considered a derivative of aniline. The central N binds three phenyl rings forming three co-planar N-C bonds via sp2-hybridization, which leaves a valence lone pair of electrons on the N 2pz orbital. Like other aromatic amines, the N lone pair has low propensity for protonation being delocalized throughout the phenyls rings (see Figure 2a). Due to steric hindrance, the phenyl groups are not on the same plane defined by the three CN bonds, but are twisted, giving the molecule a “propeller-like shape”.4 This non-planar molecular structure prevents the intermolecular aggregation and favours instead the formation of isotropic amorphous films of stable and homogeneous morphology.2 This property and the low ionization potential make TPA suitable for hole injection and as hole transport material (HTM), required for improving the performance and stability of optoelectronic devices5 such as OLEDs, dye-sensitized solar cells (DSSCs and, more recently, perovskite solar cells (PSCs) )2–4,6–8. In 2004, Yanagida and co-workers first introduced the TPA as electron donor in organic dye molecules.9 During the last 10 years, TPA and its derivatives, among which DPTA (N,NDiphenyl-2-thiophenamine, C16H13NS), have become recurring moieties implemented into Donor-π-Acceptor (D-π-A) molecules.9-11 In such a system, the Highest Occupied Molecular


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Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) are localized on two different portions of the molecule (the donor and the acceptor moieties, respectively) usually connected through a π-conjugated bridge. The D-π-A design provides not only the advantage to generate long-living charge separated states but also the possibility to enhance solar light absorption by varying the amount of π−conjugation. TPA-based dyes are considered very promising and implemented in highly efficient DSSCs,7 with a validated efficiency over 10% for metal-free organic dyes.10

Figure 1: Relaxed molecular structure of (a) TPA and (b) DPTA. For TPA, following the nomenclature of benzene-derived compounds, the carbon atoms directly bound to the N atom are denoted as Cipso. The carbon atoms binding to the Cipso are denoted Cortho followed further away from the central N atom by Cmeta and, outermost, by Cpara. For DPTA, the carbon atoms within the phenyl rings are named accordingly, while the four carbon atoms in the thiophene ring are named C1, C2, C3 and C4.


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Understanding the fundamental electronic structure of TPA and its derivative will help in recognizing their function and in the design of new compounds. In this work, we focus on the characterization of the electronic structure of the isolated TPA (Figure 1a) and its thiopheneanalog, DPTA (Figure 1b), i.e. with a phenyl replaced by a thiophene ring. DPTA shows almost equivalent electron donating properties with respect to TPA but at the same time is bearing an important functionality in the thiophene ring. Indeed, the use of thiophene-based donors facilitates the π-electron delocalization and the formation of mesomeric structures keeping the πconjugation over the whole molecular backbone. For this reason thiophene moieties are usually used as π-conjugated linkers in D-π-A molecules, and specifically DPTA is found in a very promising donor-acceptor-acceptor molecular complex/system called DTDCTB (2-{[7-(5-N,NDitolylaminothiophen-2-yl)-2,1,3-benzothiadiazol-4-yl]methylene}malononitrile). In DTDCTB, DPTA acts as an electron donor that, through the thiophene ring, binds directly a benzothiadiazol unit which, in turn, is bonded to a dicyanovinylene electron-acceptor group.11–13


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Figure 2: (a) The resonant electronic structures for the N lone pair electrons of TPA: the lone pair electron can delocalize to ortho and para positions of the phenyl ring. The delocalization shown in (a) can happen equally to all 3 phenyl rings of TPA. (a) is also valid for the two phenyl rings of DPTA, whereas DPTA has two additional resonances, (b) - (c), associated to the thiophene ring. (b) Resonant electronic structures of S pz lone pair electrons of DPTA: the S lone pair electron can delocalize to all positions in the thiophene ring of DPTA. (c) The resonant electronic structures for the N lone pair of DPTA to the thiophene ring. According to (c), the C3carbon cannot accept the N lone pair electron.

X-ray based spectroscopies such as PhotoElectron Spectroscopy (PES) and Near Edge X-ray Absorption Spectroscopy (NEXAFS) are experimental tools to obtain atomic level understanding of electronic structures. In this paper, we show comprehensive valence and core level PES, as well as NEXAFS results of gas-phase TPA and DPTA. This work is a new study among the very


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few spectroscopic investigations on these molecules in gas phase,14–16 aiming to provide a detailed and fundamental understanding of their own electronic structure. Density functional theory (DFT) was used to understand the experimental findings and specially to shed light on the effect of the lone-pair electrons of the nitrogen and sulphur atoms. For both TPA and DPTA, the C 1s PE spectra were resolved into the different C atom contributions. Their binding energies (BEs) are determined by two counter-acting effects, namely (1) the electron-withdrawing ability of the N and S atoms, and (2) the electron-donating properties of their lone pairs. Thanks to DFT simulations, we also clarified the contribution to the highest occupied molecular orbital (HOMO) coming from N/S lone pair electrons, which are responsible for the promising electron-donating properties of both molecules. Surprisingly the thiophene substitution does not affect the HOMO, but contributes significantly to the unoccupied density of states introducing a slightly acceptor character which is enhanced in D-π-A systems.

Experimental details Gas phase measurements were performed at the GasPhase beamline of Elettra Synchrotron in Trieste, Italy.17 The chamber is equipped with an in-house built Knudsen effusion cell for the vaporisation of solid samples in-situ.18 During the TPA measurements, a few mg of TPA (powder, purity 98%, Sigma Aldrich) were put into the effusion cell and further purified in-situ by slowly increasing the temperature (about 10 °C/hour) and keeping it at about 70 °C for more than 6 hours before starting the measurements. By these means, contaminants within the TPA sample, notably H2O, were removed. TPA was then sublimated at about 75°C during the PES and NEXAFS measurements, resulting in a pressure in the low 10−6 mbar range.


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Similar evaporation procedures were used for the DPTA molecule (Georganics Ltd., 97+%) with a purging temperature of about 50 °C and an evaporation temperature of 61 °C resulting in a pressure in the low high 10−7 mbar range. For the calibration of binding energy of the PE spectra and photon energy of the NEXAFS spectra, a proper reference gas was introduced into the analysis chamber simultaneously with the sample, resulting in a pressure of about 10−5 mbar. Photoelectron Spectroscopy (PES) measurements The PES measurements were recorded by a Scienta SES20019 spectrometer mounted at the magic angle (54.7°) with respect to the beam of the linearly polarized incident light. For TPA, the C 1s spectra were taken with a photon energy of 382 eV and an overall resolution of about 80 meV for the main lines (TPA in Figure 3) and 380 meV for the shake-up spectrum (TPA in Figure 4). The N 1s main line was measured with a photon energy of 495 eV and a resolution of about 200 meV (inset in Figure 5). The TPA N 1s shake-up region was recorded with a photon energy of 455 eV and a resolution of 360 meV (Figure 5, main graph). For DPTA, photon energies of 382 eV, 495 eV and 260 eV and overall resolutions of 210 meV, 270 meV and 94 meV, were used for C 1s, N 1s and S 2p spectra, respectively. The experimental valence photoelectron spectra of TPA and DPTA, shown in Figure 9, were taken with 100 eV photon energy and a resolution of 70 meV. CO2 (BE of 297.7 eV20), N2 (BE of 409.9 eV21) and SF6 (180.21 eV for 2p3/222) reference gases were used to calibrate the C 1s, N 1s and S 2p PE spectra binding energy scales, respectively. The Ar 3p3/2 line, set to its nominal binding energy of 15.76 eV23, was used to calibrate the binding energy scale of valence spectra. NEXAFS measurements


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The NEXAFS results at the C K-edge of TPA and DPTA, as well as S L2,3-edge of DPTA were obtained by recording the partial electron yield (PEY) signal by means of a channeltron electron multiplier, while the N K-edge of TPA and DPTA were measured by Auger yield using the SES200 electron analyser. The NEXAFS spectra were normalized by the transmitted photon flux measured from a calibrated Si photodiode. CO2 and N2 absorption peaks at 290.77 eV (C 1s → π*)24 and 401.10 eV25 (N 1s → π∗, ν’= 1) were used to calibrate photon energy scales of C and N K-edge spectra, respectively. The SF6 absorption peaks at 172.5 and 173.6 eV (T1u (a1g)-3/2 T1u (a1g)-1/2)22 were used to calibrate DPTA S L2,3-edge NEXAFS. The overall resolutions of the long-range C K-edge absorption spectrum of TPA shown in Figure 7 (a) was about 470 meV while the short-range C K-edge NEXAFS spectrum shown in Figure 7 (b) was approximately 70 meV. For the N K edge absorption spectrum, the resolution was about 75 meV. The resolutions of the NEXAFS spectra of DPTA were 70 meV, 120 meV and 50 meV at the C K-edge, N K-edge and S L-edge, respectively.

Computational details The molecular structures as well as the ground state valence electronic structure of TPA, DPTA and thiophene were relaxed using the B3LYP functional26 in combination with the 631g(d,p) basis set27 as implemented in the Gaussian 1628 quantum chemistry software. The calculated valence level spectra were broadened using Gaussian functions of 0.3 eV FWHM and shifted +2.36 eV and +2.18 eV towards higher binding energy for TPA and DPTA, respectively to align with the experimental valence spectra (Figure 9). The atomic orbital (AO) contributions to valence levels (i.e. partial density of states) were determined using the c2 method,29 as


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performed in previous studies30,31. For Tables 3 and 4, the c2 contribution of each AO was converted to a percentage, where the sum of all c2 contributions to a particular MO was taken to represent 100%. The optimized geometry of each molecule was then used to compute the C 1s and N 1s PES and NEXAFS spectra. The NEXAFS and PES calculations were performed in the StoBe 201432 program package using the exchange-correlation functional by Perdew, Burke and Ernzernhof (PBE)33. The total NEXAFS spectrum was calculated as the sum of individual atomic contributions. An individual spectrum was computed for each atom in the molecule by first relaxing the electronic structure in the presence of a half core hole (HCH) localized on that particular atom and then calculating the dipole transition matrix elements between the 1s orbital and the relaxed unoccupied molecular orbitals. The core-excited atom was described using the IGLO-III triple ζ basis set,34 while the other atoms of the same species were described using a 4 electrons effective core potential provided by the StoBe package. The remaining atoms were described using the ccpVTZ basis set.35 Each individual NEXAFS spectrum was shifted such that the calculated eigenvalue of the 1s level should match the Kohn-Sham calculated ionization energy for the same level. The ionization energy was calculated as the energy difference between the electronic ground state and the ionized state, described using a full core hole on the core-excited atom. A Gaussian broadening of 0.4 eV full width at half maximum (FWHM) was added to the bar graphs in order to facilitate the comparison to experiment. An additional shift of +1.6 eV (N 1s NEXAFS) and +0.4 eV (C 1s NEXAFS), respectively, towards higher photon energies, was performed to align the calculated π* peak to the experimental one. This shift is required because of relativistic effects not included in the calculation36. For the N 1s NEXAFS spectrum, the out-


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of-plane (Iz) and in-plane (Ixy) contributions, where the xy plane is defined by the three N-C bonds, were also determined32: Iz=2/3·E·||2 Ix=2/3·E·||2 Iy=2/3·E·||2, and Ixy=Ix+Iy. Here, E represents the excitation energy, LUMO+n is the final state (n is a nonnegative integer) and the factor 2/3 results from averaging over all light polarization directions.32 The N 1s and C 1s PES spectra of TPA and DPTA were constructed using the Kohn-Sham ionization energies. The N 1s (C 1s) bar graphs were broadened using Gaussian functions of 0.4 eV FWHM and shifted by +1.02 (+1.09) eV towards higher binding energy to match the corresponding experimental peaks.

Results and discussion Core levels and shake-up satellites

Figure 3 (a) shows the experimental C 1s PE spectra of gas phase TPA (top panel) and DPTA (bottom panel), along with the corresponding results from DFT calculations. The fitting results of the experimental C 1s spectra are shown in Figure 3 (b), considering a linear background and using a pseudo-Voigt profile with a Lorentzian fraction m = 0.3,37 and the details are reported in Table 1. The fitting curves of TPA has applied an additional asymmetry coefficient of 0.8 to take into account the line profile of benzene-like carbons.38 For both molecules, the spectrum consists of two main peaks. The intensity ratio between peak 2 and peak 1 in the spectrum of TPA and between 1’ and 2’ for DPTA are found to be 1 : 4.92 (TPA) and 1 : 4.32 (DPTA) respectively. These values are very close to the expected intensity ratios between the C atoms bound to the N and the remaining ones, i.e. 1 : 5 (TPA) and 1 : 4.3


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(DPTA). Therefore, peak 2 and 2’ can be associated with the C atoms bound to the central N atom, while all the remaining carbon atoms are contributing to peak 1 and peak 1’. Such assignment is further confirmed by the higher BEs of peak 2 (+ 1.09 eV for TPA) and peak 2’ (+1.16 eV for DPTA), consistent with carbons linked to the more electronegative N atom. As it will be discussed later, the broad, low-intensity features, marked with S, at 2.45 eV (TPA) and with S’ at 2.55 (DPTA) are ascribed to shake-up transitions.

(a)

(b) 1

TPA Exp. TPA Cal.

1

2

Cortho

2

Cpara

S 1'

1'

C4

Cmeta

2'

C2

C1

Cortho

tis y .In a (u )e

C3

Cipso

2'

Cpara

S' 293

TPA

S

DPTA Exp. DPTA Cal.

294

C 1s

Exp. Fit

Cmeta

Cipso

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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292

291

290

289

S'

288

294

293

Binding Energy (eV)

DPTA

292

291

290

289

288

287

Binding Energy (eV)

Figure 3: (a) Experimental C 1s PE spectra of gas phase TPA and DPTA measured with 382 eV photon energy, compared with the corresponding DFT-calculated spectra. The bars mark the calculated ionization energies of individual carbons labeled according to Figure 1. The binding energy scale of the calculated results is shifted +1.09 eV to align to the experimental first peak. The C1s eigenvalue bar graphs were broadened using Gaussian functions of 0.4 eV FWHM. (b) Experimental C 1s PE spectra (grey markers) of TPA and DPTA with their corresponding fit, where the total fit (black line) and individual fit curves (red line) are also shown.


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The C 1s photoelectron line shapes of TPA and DPTA are quite similar except a little shift to higher binding energy and the slight broadening of the DPTA lines. Despite the slight different resolution of these two spectra, the broadening of the C 1s line of DPTA is most likely related to the extra components contributing to the C 1s spectrum with respect to TPA. DFT calculations were performed for a more detailed analysis of the C 1s spectra. The binding energies calculated for the chemically non-equivalent carbon atoms are shown as bars in both Figure 3 (a), and also reported in Table 1. In the case of TPA, the theoretical BEs underneath peak 1 are grouped around three main values: 289.71, 289.80, and 290.07 eV. The C 1s lines with lower BEs correspond to the C atoms in the para position of the benzene rings. In comparison to the other carbon atoms, those in para position are more distant from the central N atom and therefore less affected by its electronegativity. Moreover, they experience the electron-donating effect of the amino lone pair which is delocalized on both para and ortho positions.39 The carbon atoms in the ortho position are at slightly higher BE because of their vicinity to the electronegative N atom. The C 1s contributions at further higher BE derive from the carbon atoms in meta positions that do not experience the amino-lone pair electron donation but only the nitrogen electronegativity. Finally, the theoretical BEs underneath peak 2 are due to the carbon atoms directly bound to the central N atom, C-ipso, confirming the conclusions drawn by the experimental fit.


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C 1s / TPA Experimental Peak

Theoretical

BE (eV)

FWHM (eV)

Peak

BE (eV)

289.64

0.58

C-para

289.71

289.76

0.53

C-ortho

289.80

290.01

0.50

C-meta

290.07

2

290.94

0.38

C-ipso

291.06

S

292.32

0.80

1

C 1s / DPTA Experimental Peak

1’

2’

BE (eV)

290

291.16

Theoretical FWHM (eV)

0.72

0.47

Peak

BE (eV)

C-para

289.73

C-ortho

289.83

C2*2

289.92

C-meta

290.10

C3

*2

290.20

C4

*1

290.26

C-ipso C1

S’

292.55

*1

291.12 291.30

1.06

Table 1: Experimental (left column) and theoretical (right column) binding energies (BEs) of peaks 1, 2 and S for TPA and peaks 1’, 2’ and S’ for DPTA obtained by a fit of the C 1s photoelectron spectra. The fittings are presented in Figure 3 (b). The right column contains the calculated C 1s BEs of the non-equivalent C atoms of TPA and DPTA, shown as bars in Figure 3 (a). The calculated values have been shifted by +1.09 eV to align the main peaks of the experimental and theoretical spectra. The superscript label *1 (*2) denotes the carbon atom of the thiophene unit in DPTA, placed at the same position as the C1 (C2) atoms in the isolated thiophene, according to the labelled used in Ref. [40].


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The analysis of the C 1s photoelectron spectrum of DPTA is slightly more complex due the substitution of one phenyl with a thiophene ring. The BEs of the C atoms within the phenyl rings do not change much with respect to the TPA results. The calculation shows a shift of about 0.06 eV to higher BE for C atoms bound to the N (C-ipso) and an average shift of about +0.03 eV for the other C atoms of the phenyl rings. Since the BE of the C atoms within the phenyl rings follows the same trend already observed for the TPA, we will mostly discuss the BEs of the C atoms of the thiophene moiety. In this case, the lone pair of the sulphur atom is delocalized over the thiophene ring (Figure 2b). Focusing on the C atoms not directly bound to the heteroatoms (i.e. N, S), we found that C2 has lower BE than C3, although it is closer to the N. This is due to the involvement of C2 in the delocalization of the N lone pair, that also explains the lower BE of the C4 atom compared to the corresponding C atom in the free thiophene molecule (C1*1 and C1 (C-C-S), shown in Table S1). Regarding instead C4 and C1, the latter shows a spectral line at higher BE as it binds both the N and S atoms. For the same reason, the C1 BE is also higher than those of the C-ipso atoms.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C1s TPA DPTA

8.4 7

10.9

5.8 4.8

S/S'

X4

15

10

5

0

Relative Binding Energy (eV)

Figure 4: Comparison between C 1s shake-ups of gas phase TPA and DPTA measured with 382 eV photon energy on a relative binding energy scale where the main lines are aligned. The black arrows point the shake-ups features.


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A comparison between the C 1s BEs of the DPTA thiophene moiety and free thiophene is reported in Table S1. Figure 4 shows the C 1s shake-up spectra of TPA and DPTA displayed on a relative BE scale. The black arrows indicate the shake-up features of TPA and DPTA. The shake-up peaks between 4.8 and about 11 eV correspond to shake-up transitions of the phenyl carbons.41 In previous studies of biphenyl and p-terphenyl, the shake-up features at about 10, 7, 6 and 4.8 eV were ascribed to intra-ring benzene like transitions.42 Considering that DPTA has 1 phenyl ring less than TPA, it is expected that the satellites at 7 eV and 4.8 eV, (due to the benzene π – π* intraring transitions42) become less intense in the DTPA spectrum. The new shake-ups features, appearing at 2.45 eV for TPA and 2.55 eV for DPTA (indicated by S and S’ in Figure 3 and Figure 4) are representative of the HOMO-LUMO transitions of each molecule, as also supported by the valence–NEXAFS energy level alignment that we show later. The N 1s PE spectra for gas phase TPA and DPTA are compared in the inset of Figure 5 with the corresponding DFT calculations. Both spectra feature an asymmetric peak found at 404.96 eV (TPA) and 405.19 eV (DPTA) with FWHM 0.56 eV and 0.62 eV, respectively. The bars indicate the theoretical ionization energies, shifted of +1.02 eV to match the experimental peaks. The experimental BE difference between N 1s of TPA and DPTA (0.23 eV) is well reproduced by the theoretical values (0.27 eV). The comparison of the shake-up region of the N 1s spectra, aligned on a relative BE scale, is shown in Figure 5. It is seen that the molecules have similar shake-ups at around 7 eV, whereas DPTA has an extra shake-up transition at 4.7 eV (indicated in the figure by the arrow) and attributed to the thiophene-substitution of DPTA, which opens new transition channels.


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Figure 5: Comparison of N1s of TPA (black) and DPTA (red) PE spectra on a relative BE scale. The black arrow shows an additional shake-up in the spectrum of DPTA located at 4.7 eV from the main line. Inset: the experimental N1s PE spectra of gas phase TPA (top) and DPTA (bottom) compared with the corresponding DFT-calculated spectra (black line). The bars indicate the calculated ionization energies of the N1s state. The N1s bar graphs were broadened using Gaussian functions of 0.4 eV FWHM and shifted by +1.02 eV to match the experimental peaks.


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Peak

BE (eV)

FWHM (eV)

S2p3/2

169.43

0.53

S2p1/2

170.62

0.53

S1

172.66

0.80

S2

173.87

0.59

S3

176.43

2.00

Table 2: Fitting parameters used for the S 2p PE spectrum of DPTA in Figure 6 (a) where the red and black lines refer to individual peaks and the total fitting results, respectively.

Figure 6: (a) Comparison of S 2p PE spectra of gas phase DPTA and thiophene (1T, taken from Ref. [22]) measured with 260 eV photon energy. The 5-peak fit applied to the S 2p PE spectrum of DPTA with a linear background gives the total result (black line), considering two main feature contributions from S 2p1/2 at 170.62 eV and S 2p3/2 at 169.43 eV together with three


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shake-up features, S1, S2 and S3. (b) S L-edge X-ray absorption spectrum of DPTA compared to free thiophene.22

The S 2p spectrum of DPTA is shown in Figure 6 (a) together with previous results of free thiophene (1T).22 The fitting parameters of DPTA S 2p are reported in Table 2. The intensity ratio of S 2p3/2 : S 2p1/2 is found to be 1.98, corresponding well to the expected ratio for the p spin-orbit splitting. The chemical shift between the S 2p3/2 and S 2p1/2 is 1.2 eV and the closest shake-ups are located at 3.23 eV from the main line, i.e. S1 at 172.66 eV and S2 at 173.87 eV. These values are quite close to an electron transition from the HOMO to the S-derived unoccupied states as shown in Figure 10 (b) (i.e. 3.38 eV from the energy level alignment). Noting that the energy shift between the two shake-up features (S1 and S2) is similar to the spinorbit splitting of the S 2p1/2 and S 2p3/2 components, S2 is likely due to the same kind of transition as S1. Since these two shake-up satellites, (S1 and S2) are only observed in the spectrum of DPTA and not in the spectrum of thiophene (Figure 6a), they are ascribed to transitions within the DPTA valence electronic structure, i.e. resulting from the variation of the electronic structure of the thiophene ring when it is part of the DPTA molecule. The comparison between the S 2p PE spectra of DPTA and free thiophene (Figure 6a) shows that the S 2p line is broader for DPTA (FWHM of 0.53 eV vs. 0.4 eV of thiophene). As presented in the Ref. [22], the line broadening can be explained by the molecular field splitting, which would have a stronger effect in the DPTA case than for thiophene. The bonding to the molecule through the central N, would affect the electronic static field of the thiophene ring in DPTA, resulting in a different molecular field splitting than the free thiophene. The comparison shows a shift (of about 0.52 eV) to lower BE of the spectrum of DPTA. This can be attributed to


19


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the more effective intra-molecular screening due to the increasing molecular size of DTPA, a similar effect experienced by the C2, C3, C4 carbons, as it has been already discussed and observed for other molecules like linear alkanes and C60.43,44 However, the shift can also be related to the effect of the N lone pair delocalization in DPTA.

NEXAFS NEXAFS measurements were performed for probing the empty density of states of the studied molecules to recognize the modification induced by a substitution, i.e. the thiophene ring in DPTA. We start by analyzing the S L2,3-edge NEXAFS of DPTA and then continue by comparing the C K-edge and N K-edge NEXAFS results of TPA and DPTA. In Figure 6 (b) the S L2,3-edge NEXAFS results of DPTA and isolated thiophene (1T) are shown. The resonance peaks in the NEXAFS spectrum of DPTA are found at 165.7 eV, 167.1 eV and 168.5 eV. The only appreciable differences between DPTA and thiophene are the features in the range of energy from 168 to 171 eV (labelled C and D in the figure), attributed for thiophene to transitions into mixed valence-Rydberg states. The C feature has been already shown to be very sensitive to the increase in the molecular complexity in a study dedicated to a series of oligothiophenes.22 The modification of such a band would be due to the increased number of possible transitions to final diffuse molecular states, causing an intensity distribution that modifies the resonance from a double peak to a less structured feature.


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Figure 7: (a) Experimental C K-edge NEXAFS of TPA and DPTA in gas phase. Experimental C K-edge NEXAFS results of TPA and DPTA with the corresponding calculated spectra are shown in (b) and (c), respectively. The contributions from the different carbon atoms, with labels indicating different final states, are also shown.

Figure 7 (a) compares the C K-edge NEXAFS spectra of TPA and DPTA and reports a detailed analysis of the spectra by theoretical calculations in (b) and (c), respectively. For TPA, the first two resonances, 285.07 eV (A) and 286.27 eV (B) (see Figure 7b), represent transitions to the lowest unoccupied molecular orbital (LUMO). In the case of resonance B, the transition


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involves the 1s electrons of the ipso carbon (C-N), while resonance A is due to the remaining carbon atoms. The energy difference between A and B (1.20 eV) roughly reflects the chemical shift between the core levels of these carbon atoms (1.09 eV). Equivalently, the shoulder at the higher photon energy side of resonance A (A’) is due to the carbon atoms in meta positions, which have slightly higher BE relative to the carbons in ortho and para positions. Resonance B is mainly due to the transition of C 1s (ipso) LUMO with a small contribution from the C 1s (ortho) LUMO+3. All different kinds of carbon atoms contribute to resonance C. A more detailed assignments of the C 1s NEXAFS peaks can be found in Figure 7 (b). Similar to TPA, the transitions of the phenyl C 1s to LUMO in DPTA split according to the corresponding core level BE shifts. In addition, the contributions due to the 1s LUMO of C atoms from the thiophene ring follow the same trend, hence C2, C3, C4 contribute to resonance A (285.15 eV), while C1 affects resonance B (286.35 eV). In comparison with TPA, resonance B of DPTA is broader (see feature B') as a consequence of new additional transitions which involve mainly the C atoms of the thiophene ring (transitions from C4, C3, C2). Finally, resonance C, at about 287.2 eV, mainly involves transitions of C 1s (C4) LUMO+5, C 1s (meta) LUMO+4, LUMO+5, C 1s (ipso) LUMO+2 and a further important contribution from C1 to LUMO+1. The detailed assignments of the C 1s transitions, which contribute to the NEXAFS peaks can be found in Figure 7 (c). However, by comparing the transitions from the carbon atoms of the phenyl and thiophene ring, we have found that their transition energies (Tr) do not exactly follow the core level BEs shifts. This is particularly evident for C1 (S-C-N) and C (ipso) (C-N). Although the C 1s BE of C1 is higher than the C 1s BE of C (ipso), the C 1s (C1) LUMO transition occurs at a photon energy lower than that required for the C 1s (ipso) LUMO transition, that is:


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Tr (C1) < Tr (ipso). Moreover, considering the BE sequence C2 > C-ortho > C-para, we should expect transitions to the LUMO level as follow: Tr(C-para) < Tr(C-ortho) < Tr(C2). Instead we find Tr(C2)< Tr(C-ortho)/Tr(C-para). The same relationships, even if less evident, can also be found for C3 and C-meta. According to the theoretical calculations, these findings can be explained by the fact that the ground state electronic structure is differently perturbed depending on whether the core-hole is located on the phenyl or thiophene rings (Figure S1). The LUMO in the ground state is mostly localized over the thiophene moiety, with only small contributions from the phenyl rings. In the presence of a (half)-core-hole (HCH) on the thiophene C atoms, the contribution from the phenyl rings further decreases and causes a shift of the LUMO energy (Figure S1). The situation drastically changes when the HCH is localized on the phenyl C atoms resulting in a LUMO mostly localized on the phenyl ring with the HCH, and only small contribution from the thiophene and the other phenyl ring. In this case, the energy shift of the LUMO is smaller. Since the HCH on the thiophene ring shifts the LUMO energy more than the HCH on the phenyl, the energy shift of the corresponding transitions is reversed relative to the BE shift. Figure 8 shows the N K-edge NEXAFS of TPA (top) and DPTA (bottom) in comparison with DFT-calculated spectra, discerning the contributions from the Iz (out-of-plane orbital, N 2pz) and Ixy (in-plane, N 2px, 2py orbitals), considering the plane defined by the C-N bonds. The N K-edge XA spectra of TPA and DPTA show resonance peaks located at quite similar photon energies (a first π* resonance at 403 eV and an increase of intensity from about 404.3 eV due to σ* resonances). The theoretical simulations reveal that the 1st resonance, for both TPA and DPTA,


23


can be ascribed to transitions to out-of-plane orbitals, LUMO+5 for TPA and LUMO+4 for DPTA.

N K-edge XAS TPA

Exp. Cal.

P

DPTA 4 0 1 .5

Int ensit y ( a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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TPA

P' 4 0 2 .0

4 0 2 .5

4 0 3 .0

P

Iz N1 s

LUMO+5

Ixy

P: N1 s --> LUMO+1, LUMO+2 , LUMO+3 , LUMO+4

DPTA

P'

Iz N1 s

LUMO+4

Ixy

P' : N1 s --> LUMO, LUMO+1 , LUMO+2 , LUMO+3

400

402

404

406

408

410

412

Phot on Energy ( eV)

Figure 8: Experimental N K-edge NEXAFS of TPA (up) and DPTA (bottom) shown together with the calculated contributions of Iz (out-of-plane orbitals, blue line) and Ixy (in-plane orbitals, green pattern). Inset: enlargement of the simulated pre-edge energy range of the N1s absorption spectra of TPA (P) and DPTA (P'). The DFT calculations indicate the low intensity P feature as due to the transition from N1s to LUMO+1, LUMO+2, LUMO+3, LUMO+4 of TPA and P' to the transition from N1s to LUMO, LUMO+1, LUMO+2, LUMO+3 of DPTA. The 1st resonance peak of significant intensity of TPA is due to the transition from N1s to LUMO+5, while for DPTA from N1s to LUMO+4.


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By a careful analysis of the theoretical results (inset of Figure 8), we find a small pre-edge feature in the N K-edge absorption spectra of both TPA and DPTA (feature P and P’) at about 402.2 eV, attributed to transitions from N 1s to LUMO+1, …, LUMO+4 for TPA and N 1s to LUMO, LUMO+1, …, LUMO+3 for DPTA, respectively. These unoccupied orbitals contain no contribution from the N 2pz atomic orbital and the intensity of the pre-edge peak is related only to the in-plane N 2pxy components. In fact, in the case of TPA, the first intensive resonance would correspond to the transition to LUMO+5, which is the first unoccupied state with significant contribution from nitrogen as shown in Table 3 and in Figure S3. Moreover, the calculations show that LUMO+4 is the first unoccupied state of DPTA with significant contributions from the N atom (Table 4 and Figure S5). Valence electronic structure The valence PE spectra of gas phase TPA and DPTA measured with a photon energy of 100 eV are shown in Figure 9 (a). The TPA and DPTA spectra resemble each other, especially in the outermost valence levels. The HOMO of TPA is found as an asymmetric peak at 7.17 eV with a FWHM of about 0.3 eV. The HOMO of DPTA is found as a more symmetric peak at 7.13 eV with slightly broader FWHM (0.45 eV).


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Figure 9: Valence PE spectra of TPA and DPTA measured with 100 eV photon energy (a), in comparison with the corresponding DFT-calculated spectra, respectively ((b) and (c)). In (b) and (c), the partial density of states (PDOS) of individual C (C-ipso, C-ortho, C-meta, C-para for TPA; C-ipso, C-ortho, C-meta, C-para, C1, C2, C3, C4 for DPTA), N (blue lines for N PDOS and light blue filled for N pz) and S (yellow line for S PDOS and yellow filled for S pz for DPTA) are also presented. The theoretical C atom spectra are normalized to the numbers of atoms present in the molecule.


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TPA

N (%)

Molecular Orbitals

Calculated. BE (eV)

s

px,y

pz

LUMO+5 LUMO+3, LUMO+4 LUMO+1, LUMO+2 LUMO HOMO HOMO-1, HOMO-2 HOMO-3 HOMO-4 HOMO-6

1.40 2.13 2.61 2.68 7.32 9.00 9.29 9.33 10.59

0.00 0.00 0.00 0.79 0.00 0.00 0.10 0.00 0.00

0.00 0.07 0.24 0.00 0.00 0.10 0.00 0.34 0.00

9.78 0.00 0.00 0.00 33.34 0.00 0.00 0.00 26.40

C-ipso (%) s p 0.00 0.06 6.06 2.26 0.00 0.41 0.20 2.54 0.00

29.55 0.79 31.25 0.14 7.68 6.41 0.02 26.62 13.54

C-ortho (%) s p 0.79 0.29 0.31 0.05 3.71 0.04 0.03 0.09 5.71

5.57 50.14 19.88 44.04 31.61 42.19 54.03 19.58 8.94

C-meta (%) s p 0.01 0.09 0.46 0.40 0.61 0.00 0.01 0.11 1.89

23.18 47.73 11.90 50.08 2.14 45.51 45.08 22.97 22.25

C-para (%) s p 0.00 0.01 0.38 0.37 0.00 0.04 0.03 0.04 0.00

30.45 0.12 28.47 0.07 20.08 4.83 0.01 27.10 16.96

Table 3: DFT calculated BEs of the indicated outermost molecular occupied and unoccupied valence levels of TPA. The calculations resolved the Atomic Orbital (AO) contribution (%) to each molecular orbital as describe in the Computational Details section. The calculated BEs have been shifted +2.36 eV to align them with the experimental valence spectrum.

The comparison of the experimental spectra with the theoretical total and partial density of states (DOS and PDOS, respectively) of TPA are shown in Figure 9(b) and DPTA in Figure 9(c). The contribution of each atomic orbital, namely s and p orbitals (pz & pxy only for N and S), to the molecular levels are reported in Table 3 for TPA and Table 4 for DPTA. The HOMO of TPA has a significant contribution from the pz orbital of the N atom (33.34%, Table 3), i.e. the lone pair electrons of N participate to the HOMO orbital (pale-blue curve in Figure 9b). Other important contributions are from the C 2p orbitals of the ortho (31.61%) and para (20.08%) atoms. An almost negligible contribution comes from the meta carbons (s + p total 2.75%). All these results well reflect the N lone pair delocalization, which we previously simply derived on the basis of the resonance structures of Figure 2 for the C 1s analysis. The calculation also shows (Table 3) a small contribution of the C-ipso atoms (7.68%) to the HOMO due to the π bonding with ortho carbons as shown in Figure S5. The calculations indicate that the HOMO-1/HOMO-2 (degenerate), HOMO-3 and HOMO-4 (8.7 to 10 eV) of TPA have C


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character only, while the inner valence states have both C and N character, with the N lone pair still greatly contributing to the HOMO-6. Similarly, also for DPTA (Figure 9c), the HOMO is mainly derived from the N pz orbital (29.54%) and the aforementioned C p orbitals of the phenyl rings (ortho and para C atoms as shown in Table 4). Moreover, the HOMO receives contributions from the thiophene ring, in particular from the pz orbitals of C2 and C4, as expected by considering the resonance structure in Figure 2 (c). The HOMO is also contributed by the px,y orbitals of the S atom (2.84%), but only 1.83% from the S 3pz orbital (lone pair). The latter mostly contributes to HOMO-2 and inner states as HOMO-4 and HOMO-5. On the basis of these results, it is clear that the electron donating properties of both the considered molecules are due to the N lone pair electrons which strongly contribute to the HOMO level. In DPTA, the S lone pair does not have particular influence for enhancing the electron-donating properties of the molecule, since it contributes mostly to inner occupied molecular orbitals. Nonetheless, the thiophene moiety slightly decreases the HOMO ionization potential, from 7.32 to 7.22 eV (theoretical values in Table 3 and Table 4). Note that the calculated spectra have been shifted to align to the experimental first peak and, therefore, the calculated HOMO BE coincides with the experimental one for both TPA and DPTA. Moreover, although the energy position of LUMO is not much affected (2.68 eV) by the thiophene insertion, the orbital character shows to be significantly contributed by the S 3pz orbital indicating that LUMO is delocalized on the thiopene ring (see Table 4 and Figure S4), with the phenyl mostly contributing to the LUMO+1, at slightly higher energy position.


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Molecular Orbital LUMO+4 LUMO+3 LUMO+2 LUMO+1 LUMO HOMO HOMO-1 HOMO-2 HOMO-3 HOMO-4 HOMO-5 HOMO-6

Molecular Orbital LUMO+4 LUMO+3 LUMO+2 LUMO+1 LUMO HOMO HOMO-1 HOMO-2 HOMO-3 HOMO-4 HOMO-5 HOMO-6

DPTA Calculated BE (eV) 1.56 1.97 2.35 2.46 2.68 7.22 8.73 8.83 8.98 9.11 9.19 10.38

DPTA Calculated BE (eV) 1.56 1.97 2.35 2.46 2.68 7.22 8.73 8.83 8.98 9.11 9.19 10.38

N (%)

C-ipso (%) s p

C-ortho (%) s p

s

pz

pxy

0.01 0.02 1.11 0.11 0.00 0.04 0.01 0.00 0.01 0.13 0.16 0.00

2.69 0.06 0.01 0.00 0.03 29.54 1.30 0.01 0.72 0.00 0.00 26.63

0.06 0.03 0.29 0.15 0.15 0.09 0.53 0.13 0.27 0.04 0.21 0.23

0.10 0.58 0.21 2.15 5.41 0.02 1.68 0.23 0.76 0.09 1.23 0.09

pxy

s

p

s

23.52 0.08 0.60 2.24 0.08 2.84 0.14 0.15 0.09 0.09 0.13 8.09

10.38 0.04 1.63 4.95 0.02 0.04 0.03 0.18 0.00 0.35 0.67 0.15

8.28 0.20 0.77 0.50 22.41 4.91 17.68 1.12 9.16 0.43 0.99 3.59

0.09 0.03 0.06 0.03 0.04 1.76 0.01 0.00 0.02 0.00 0.01 1.37

S (%) pz

s 0.49 0.34 0.28 0.04 0.03 0.32 0.02 0.04 0.06 0.02 0.02 0.16

1.32 0.03 0.19 0.04 16.67 1.83 2.14 37.93 0.11 10.69 22.85 0.89

10.88 0.86 0.83 28.86 5.51 5.35 5.61 6.61 2.84 4.32 19.17 9.91

0.21 0.16 0.21 0.14 0.09 1.35 0.31 0.06 0.32 0.06 0.04 3.13

C1 (%)

3.28 49.33 42.81 19.70 4.37 20.38 18.05 13.69 33.80 38.97 8.20 4.79

C2 (%) p 0.39 0.07 0.46 0.54 9.41 6.16 1.51 12.01 1.03 3.72 8.62 2.45

C-meta (%) s p 0.14 0.06 0.47 0.21 0.24 0.19 0.08 0.01 0.10 0.05 0.07 1.09

s

7.75 47.01 46.58 12.24 1.91 1.45 19.17 14.19 32.25 33.73 12.83 16.55

C3 (%) p

0.03 0.00 0.00 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.00 0.00

2.75 0.01 0.44 0.13 5.41 1.81 10.43 7.55 5.32 2.04 4.05 1.88

C-para (%) s p 0.00 0.01 0.20 0.05 0.33 0.01 0.08 0.01 0.02 0.02 0.01 0.00

11.47 0.07 1.42 25.79 4.66 13.47 4.53 5.09 2.67 4.68 20.00 13.35

C4(%) s

p

9.41 0.02 0.35 0.90 0.01 0.01 0.01 0.00 0.00 0.02 0.00 0.01

4.47 0.04 0.20 0.33 22.14 7.56 15.78 0.57 9.88 0.07 0.20 2.25

Table 4: DFT-calculated BEs and AO contributions (%) to the indicated outermost molecular occupied and unoccupied valence levels of DPTA obtained by the same method used in Table 3. The calculated Bes have been shifted +2.18 eV to align with the experimental valence spectrum.

Figure 10 shows the energy level alignment of the occupied and unoccupied valence states of TPA, DPTA and thiophene, using the valence photoemission and the absorption results, by a method introduced by Schnadt et al.45 In this way we get an image of the occupied and unoccupied valence states, considering however the effect of the core hole of the final state, i.e. NEXAFS does not probe the ground state DOS but the local core-hole perturbed density of states. In Figure 10 we took advantage that our core and valence level photoemission binding energy scales have been calibrated with respect to the vacuum level providing then the ionization potentials of the core and valence electrons. The NEXAFS spectrum can be aligned on the same energy scale as the valence PE spectrum, with the ionization potential as the zero of such a scale. In fact we can find the ionization energy in the NEXAFS spectrum considering that it is the excitation (photon) energy needed to excite the 1s core electron to the vacuum level. Then,


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subtracting this IP from the photon energy scale, we can get the NEXAFS and the valence PE results aligned on the same energy scale. The detailed energy alignment procedure is explained in the Supporting Information (SI).

Figure 10: Energy level alignment of occupied (Valence PES) and unoccupied states (NEXAFS) for TPA, DPTA and thiophene (1T) according to the method introduced by Schnadt et al. [45].


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The results give an estimation of the HOMO-LUMO gaps of 2.36 eV, 2.25 eV and 3.56 eV for TPA, DPTA and thiophene, respectively. These values are in quite good agreement with the HOMO-LUMO gap estimation obtained by the energy positions (2.45 eV for TPA and 2.55 for DPTA) of the shake-up features in the C 1s photoemission spectra (Figure 3), considering that the presence of the core hole perturbs the density of states as shown in Figure S1 and S2. The ground state DFT calculations give HOMO-LUMO gaps of 4.65 eV, 4.53 eV and 6.17 eV for TPA, DPTA and thiophene respectively, which are quite different from our estimations. However, it has to be observed that the HOMO-LUMO gap is generally overestimated by the B3LYP/DFT calculations, as discussed in details in Ref. [46]. In addition, the HOMO-LUMO gap is calculated in the ground state and does not, therefore, consider the effects of the core and valence holes which affect the measurements (e.g. by shifting the electronic levels towards higher binding energies). According to the calculations, the smaller energy gap of DPTA has to be mainly ascribed to a slightly decrease of the ionization potential (HOMO BE), while the LUMO level position seems almost unperturbed as reported in the Tables 3 and 4. The overall density of the empty states obtained by our valence calculations is in good agreement with the results of the level alignment (Figure 9 and Figure 10). Considering the ground state valence calculation for TPA (Figure 9), the LUMO is dominated by the C atoms of the phenyl rings with negligible contribution from the N (0.79%, Table 3), the latter contributes significantly to higher energies (9.78%, LUMO+5). For DPTA, the LUMO (Figure S4) in the ground state is mainly localized over the thiophene moiety, with major contributions from the C atoms but also with significant contribution from the S atom (s + p totally 16.78%, Table 4). The energy level alignment (Figure 10) well confirms the theoretical description of the NEXAFS spectra with exception of the S contribution to the LUMO of DPTA, not observed in


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the NEXAFS being a transition which is forbidden by the dipole selection rules that governs the absorption process.22

Conclusions Gas phase TPA and its thiophene-analog DPTA were characterized by core and valence PES and NEXAFS. DFT calculations were applied for a detailed understanding of the molecular electronic structures. The effects of the delocalization of lone pair N electrons were observed in the C 1s and valence photoemission spectra for both molecules, whereas for DPTA, also the S lone pair electrons have to be taken into account. We have shown that the C K-edge NEXAFS of DPTA is affected by the site of core hole, i.e. different energy transitions are observed depending if the core-hole is located on analogous atoms of the phenyl or thiophene rings. The nature of the electron-donating character of TPA and DPTA is found largely explained by the N lone pair contribution to HOMO of both molecules. Moreover, both the experimental and theoretical results show that the thiophene insertion in the DPTA has a significant impact on the electronic structure. Indeed, the theoretical results have shown that the thiophene insertion in the DPTA has a significant impact on the LUMO level, which becomes contributed by the S 3pz orbital and largely localized on the thiophene ring. An effect of such localization has been found in the analysis of the C K-edge NEXAFS of DPTA, where, for a specific orbital, different energy transitions are observed depending if the core-hole is located on the phenyl or thiophene rings. In conclusion, it seems that the thiophene substitution does not affect the electron donating properties of DPTA compared to TPA but it introduces a slight acceptor character (see LUMO) that together with its π-conjugated properties makes DPTA the ideal candidate for building up more extended and efficient D-π-A dyes.


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Conflicts of interest There are no conflicts to declare.

AUTHOR INFORMATION Corresponding Authors [email protected] [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgements We thank the Carl Trygger Foundation for financial support and for making available the VGScienta SES-200 photoelectron analyser at the Gas Phase beamline, Elettra, Italy. B. Brena acknowledges the Swedish Research Council for research grant (VR 2014-3776). TZ thanks the Vice Chancellor of Uppsala University for financial support through the U4 collaboration. The authors acknowledge the EU CERIC-ERIC Consortium for the access to experimental facilities and financial support. The DFT calculations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at HPC2N. We thank Mr. G. Bortoletto and Mr. C. Pedersini of the User Support Lab at Elettra.


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ASSOCIATED CONTENT Supporting information Calculated core-hole dependent transition energy of DPTA. Calculated molecular orbital shape of HOMO and unoccupied states of TPA and DPTA. Comparison between the valence PE spectra of gas-phase DPTA and thiophene with DFT calculations. The detailed energy alignment procedure.

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Table of Contents Graphic


41

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14


Page 42 of 52

(a) Resonant structure for TPA (lone pair e- of N) Page 43 of 52 N

The Journal of Physical Chemistry N

+

-

N

+

N

+

1 2 3 4 (b) Resonant structure for DPTA (lone pair e- of S) 5 6 s+ s s+ - s+ s+ 7 N N N N N 8 9 10 11 (c) Resonant structure for DPTA (lone pair e- of N) 12 13 ACS Paragon-Pluss Environment s s 14 + + N N N 15

(a) 1

TPA Exp. TPA Cal.

Cortho Cpara

S 1'

C4 C3

Cmeta

Cipso 2'

C2

C1

Cortho

293

292

291

290

Binding Energy (eV)

ACS Paragon Plus Environment 294 289 288

TPA

S 1'

2'

Cpara

S' 294

2

Intensity (a.u.)

Cipso

C 1s

Exp. Fit

Cmeta

DPTA Exp. DPTA Cal.

Page 44 of 52

1

2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

(b)


S'

293

DPTA

292

291

290

289

Binding Energy (eV)

288

287

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Intensity (a.u.)

Page 45 of 52


C1s TPA DPTA 10.9

8.4 7

5.8 4.8

S/S'

X4

15

5 ACS10 Paragon Plus Environment

0



TPA N1s Exp. Cal.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Page 46 of 52

N 1s shake-ups TPA DTPA

DPTA N1s Exp. Cal.

408 407 406 405 404 403

Binding Energy (eV)

X4

15

ACS 10Paragon Plus Environment 5


0

Page 47 of 52

(a)

(b) B

S2p Experiment Fit X4

1T S3

S L2,3 edge C

S2

S1

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32


A

DPTA Thiophene

E

D

X4 DPTA

178

176

174

172

170

Binding Energy (eV)

ACS Paragon Plus Environment 164 168

168

172

176

Photon Energy (eV)

180


(a) C K-edge NEXAFS

Intensity (a.u.)

A

(c) DPTA C K-edge A

TPA DPTA

B B

C

D

Exp.

C B'

Calc. LUMO LUMO+1

290

295

300

Photon Energy (eV) (b) TPA C K-edge A A' B

LUMO+5

D

C

C4

Intensity (a.u.)

285

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

Page 48 of 52

C-para

LUMO+4, 5

LUMO

LUMO

LUMO+2

C3

C-meta

LUMO+4, 5

LUMO

Exp.

LUMO+4

C2

Calc. LUMO

C-para

LUMO+5

LUMO

LUMO

LUMO+6 LUMO+7

LUMO

LUMO+3

LUMO+5

C-ipso

282

284

286

288

Photon Energy (eV)

LUMO+7

C-ortho LUMO+1 LUMO

LUMO+3

C1

LUMO LUMO+2 LUMO+5

LUMO+5

LUMO

LUMO+6 LUMO+7 LUMO+5

C-meta C-ortho

LUMO+4

LUMO

C-ipso

290 ACS Paragon Plus Environment 282

284

286

LUMO+2

LUMO+4

288

Photon Energy (eV)

290

Page 49 of 52

N K-edge XAS TPA

401.5

TPA

Exp. Cal.

P

DPTA

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43


P' 402.0

402.5

403.0

P

Iz N1s

LUMO+5

Ixy

P: N1s --> LUMO+1, LUMO+2, LUMO+3, LUMO+4

DPTA

P' Iz N1s

LUMO+4

Ixy

P': N1s --> LUMO, LUMO+1, LUMO+2, LUMO+3

400

402

ACS Paragon Plus406 Environment 408 404

Photon Energy (eV)

410

412


(a)

(c)

Intensity (a.u.)

Valence

DPTA

TPA DPTA

Exp. Cal. 25

20

15

10

N/Npz

(b) TPA Exp. Cal.

Intensity (a.u.)

Binding Energy (eV)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

Page 50 of 52

C-ipso C-ortho C-meta C-para C1

N/Npz

C2 C-ipso

C3

C-ortho

C4

C-meta S/Spz

C-para 15

10

5

Binding Energy (eV)

0 Plus Environment ACS Paragon

15

10

5

Binding Energy (eV)

0

Page 51 of 52

Occupied states

(a)TPA TPA C (non-ipso)

N

C (ipso)

2.36 eV

(b) DPTA DPTA

Occupied states

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49


2.25 eV

S

C

N

3.38 eV

Occupied states

(c) 1T C1

3.56 eV

C2 S

10

8

4 ACS6Paragon Plus Environment

2

Binding Energy (eV)

0

The Journal of Physical Chemistry Page 52 of 52 1 2 3 4 5 6


Lone Pair Delocalization Effect within Electron Donor Molecules: The

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