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Modulating Electronic Properties of Dinitrosoarene Polymers

We present a comprehensive analysis on how the electronic structure and the optical properties of an organic polymer can be modulated, based on the example of the dinitrosobenzene polymer (1). Using a combination of computational and experimental tools, we explore the effects of solid-state packing, backbone torsion, surface adsorption, the conjugation in the aromatic core, and substituents. The band gap (Eg) and optical spectrum of 1 are calculated using both GW-BSE with zero-gap renormalization (ZGR) and hybrid TD-DFT, with the former method predicting a value (2.41 eV) in excellent agreement with our diffuse reflectance spectroscopy measurements (2.39 eV). Using GW-BSE-ZGR, changes occurring upon solidstate packing are separated into a contribution arising from (i) the change in the torsional angle and (ii) the change in the screened Coulombic interaction, which strongly affects the exciton binding energies. Comprehensive hybrid TD-DFT calculations find that the effects of substituents on Eg and on transport properties can mostly be explained through changes in the torsional angle , and predict a linear dependence between  and Eg. Extending the conjugation in the aromatic core is found to enhance transport properties and narrow Eg, identifying future synthetic targets. Atomic force microscopy and spectroscopic ellipsometry are used to study 1 adsorbed to a (111) gold surface (1@Au), with the latter method showing a significant narrowing of the band gap to 0.68 eV, in good agreement with TD-DFT predictions.


INTRODUCTION
Due to their exceptional versatility, organic semiconductors (OSCs) are a very attractive alternative to traditional silicon-and GaAs-based semiconductors. OSC properties can be tuned to enable their use in field-effect transistors (FETs), 1-4 photovoltaics, 5-8 sensors, [9][10][11] and flexible electronics. [12][13][14] However, rational design of OSCs for a particular purpose is still very challenging, requiring a combination of synthesis, analytical techniques, and modelling. 15 On the modelling side, calculations based on density functional theory (DFT) with periodic boundary conditions are being increasingly used to investigate the optoelectronic properties of OSCs. Recently, hybrid DFT and its time-dependent formulation (TD-DFT) have been employed to study periodic porphyrin nanostructures, 16 polyacenes, [17][18][19] as well as polythiophenes and polypyrroles, 20,21 exploring the changes in the band gap (Eg) and electronic structure occurring upon structural modifications, and thus identifying interesting systems and potential synthetic targets.
More accurate methods for describing the band structures and optical properties of solids are the GW approximation and the Belthe-Salpeter equation (BSE), respectively. [22][23][24] GW-BSE introduces dynamic (frequency-dependent) screening of the Coulombic interaction, resulting in a nearly parameter-free method that goes beyond density functional theory. Due to its computational complexity (typically quartic scaling with system size), the use of GW-BSE has mostly been limited to inorganic systems with small unit cells. [25][26][27][28] In this paper, we present and explore a new class of OSCs based on the dinitroso aromatic moiety, which allows for the formation of 1-D polymers. The simplest such compound is 1,4dinitrosobenzene, which forms a remarkably stable polymer 1, whose crystal structure (shown in Figure 1; labelled C1 to distinguish it from single-strand 1) we recently reported. 29 Aromatic nitroso compounds are attractive for several reasons: (i) the azodioxide bond (which connects the monomers) is formed spontaneously, meaning that polymer formation is very straightforward; 30,31 (ii) in cryogenic conditions, UV irradiation can induce reversible cleavage of the azodioxide bond, providing a possibility for ON-OFF switching; [32][33][34] (iii) C1 has a herringbone-type packing, 20 which is well-known to promote intermolecular charge transfer; 35,36 (iv) it has been proposed 20,30 that the angle  between the azodioxide and the phenyl planes (see Figure 1) strongly influences the electronic properties of C1, by modulating the extent of conjugation along the polymer. Finally, in case of monomers with more than two nitroso groups, two-and three-dimensional polymer networks can be formed. 31,37  To better understand the factors influencing the band gap of C1 and related compounds, we recall that band gap engineering typically assumes that the band gap (Eg) depends on five parameters, which can be written as follows: 38,39 Eg = E 3D + E  + E ASE + E SUB + E BLA (1) In equation (1), (i) E 3D is the solid-state packing of the material; (ii) E  is torsion, or the backbone deviation from planarity; (iii) E ASE is the aromatic stabilization energy; (iv) E SUB is the presence of substituents; and (v) E BLA is the bond length alternation. In this paper, we employ a combination of computational and experimental techniques to systematically explore the influence of parameters (i)-(iv) on the band gap of dinitrosoarene polymers (Chart 1). Experimentally, we determine the Eg of C1 using diffuse reflectance spectroscopy and compare the IR spectra of 1 and C1. Using atomic force microscopy and spectroscopic ellipsometry, we also determine the surface morphology and optical response of thin films of 1 adsorbed to a 111-gold surface (1@Au), respectively. Computationally, we use hybrid TD-DFT based on the range-separated HSE06 functional 40 (TD-HSE) and GW-BSE to evaluate the properties of 1 and C1. The former method is also used to study the effects of torsion, adsorption on a gold surface (1@Au monolayer) and predict the properties of derivatives of 1 (Chart 1) differing in their substituents (S2-S9) or aromatic core (A10-A18). These derivatives were chosen as they are amenable to synthesis (e.g., the preparation of S8 41 and A17 42 has been reported). Finally, we evaluate the effect of phonons on Eg by calculating the zero-gap renormalization (ZGR) using an approach recently developed by Kresse et al., 43 which has, to our knowledge, not yet been applied to organic semiconductors, although the ZGR effects in inorganic systems were found to be significant. 34,44 Chart 1. Structures of investigated compounds.
The aim of this study is to (i) explore the extent to which the individual parameters influencing Eg can be separated and estimate their individual contributions, (ii) determine the possible extent of modulation of electronic, transport, and optical properties of 1 and identify future synthetic targets, and (iii) compare the predictions made by a rigorous (GW-BSE-ZGR) approach with those obtained by the much more affordable hybrid TD-DFT.

EXPERIMENTAL RESULTS
(a) Diffuse reflectance spectroscopy. The band gap of C1 was measured using UV-vis diffuse reflectance spectroscopy. A Tauc plot was used: where F(Rinf) is the Kubelka-Munk reemision function at energy E (details in experimental), and Eg is the band gap. The value of Eg obtained using equation (2) was 2.39 ± 0.02 eV ( Figure  2), confirming C1 is a wide-gap semiconductor. (b) Infrared spectroscopy. Infrared spectra of 1 and C1 are nearly identical in the 400-4000 cm -1 range. They are well reproduced by the PBE-revD3BJ 45 functional (details in experimental), allowing for the complete assignment of IR-active modes ( Figure 3 and Table  1). The most prominent signal, corresponding to the asymmetric E-ON=NO azodioxide stretching, occurs in the 1200-1300 cm -1 range. Other notable signals are the C-H stretches at 3050-3150 cm -1 , in-plane aromatic ring breathing at ~1100 cm -1 , and out-of-plane C-H bending at 770-860 cm -1 . The modelled spectra overestimate the C-H stretching frequencies by about 70-80 cm -1 , but otherwise produce an excellent agreement with a mean unsigned error of 11 cm -1 relative to experiment, without any scaling. Signal corresponding to the free nitroso group (around 1520 cm -1 ) 46,47 is completely absent, suggesting that the length of polymer strands is very large.    (d) Ellipsometry. The thickness of a thin film of 1 produced by drop-casting on a (111) gold surface (1@Au) is 178 ± 4 nm, as determined by ellipsometry. The imaginary part of the dielectric function displays a peak at 1.03 eV in the IR region, and a broad signal with a maximum around 2.1 eV in the visible part of the spectrum ( Figure 5). The band gap, as determined from the Tauc-Lorentz oscillator model, is 0.68 ± 0.01 eV. The index of refraction of 1@Au is ~1.5 in the NIR and visible range, and increases in the UV, which is typical for organic polymers. However, the most notable feature of 1@Au is that it exhibits considerable attenuation in the visible and the NIR parts of the spectrum ( Figure 5).

COMPUTATIONAL RESULTS
(e) Geometry and phonons. Geometry optimization using the PBE-revD3BJ method yields a geometry and unit cell parameters for C1 in good agreement with its XRD-determined 13,17 structure, with a mean unsigned error of 2.0% (details in Figure S1 in SI). The same method gives IR spectra in good accord with experiment (cf. section (b)). Vibrational structures of C1 and 1 are very similar, with small differences (<10%) only at energies below 400 cm -1 , as illustrated by a comparison of their phonon band structures ( Figure 6; full phonon band structure of C1 can be found in Figure S5). The ZGR value at 0 K was determined to be -0.27 eV for C1 and -0.28 eV for 1. (f) Electronic structure and optical properties. To describe 1 and C1, two approaches were employed. In the first, the electronic structure and transport properties were calculated at the hybrid DFT level using the HSE06 functional, 32 while its time-dependent variant (TD-HSE) was used for calculations of optical spectra. The second approach (GW-BSE-ZGR) involved a partially self-consistent eigenvalue GW0 calculation 48 on orbitals obtained using PBE. This was followed by the use of the Bethe-Salpeter equation (BSE) to account for electron-hole interactions and the addition of ZGR corrections to include electron-phonon coupling. These calculations were done on optimized geometries of 1 and C1.
Results are summarized in Table 2  The band structure of C1 reveals a direct gap at A and nearly degenerate pairs of conduction and valence bands close to the Fermi level, with flat regions associated with the variation of ky (G-Y; A-E; C-Y). Relative to HSE, GW opens the gap by ~1.0 eV, and otherwise differs from it only along the A-E path, where it produces two distinctly separate bands, while HSE predicts them to be separated in A (0,) and degenerate in E (,). The first bright transition, is strongly excitonic in nature, and corresponds to the transfer of electron density from the N-N bond and oxygen to the C-N bond (Figure 7e), which may be associated with the cleavage of the azodioxy bond.       Figure S7).
The  value and correlation coefficient for dimers (0.22 ± 0.05 eV; R 2 = 0.72) show a significant correlation between  and Eg, which is completely absent in polymers S2-S9 ( = -0.3 ± 0.2 eV; R 2 = 0.003). Instead, introducing a substituent invariably widens the band gap and decreases dispersion, resulting in larger hole and electron effective masses ( Figure 10 and Table 4). Introducing substituents also affects the torsional angle , which is shown in Figure   8.  (j) Influence of the aromatic core. Substituting the phenyl group for a nitrogen-containing heterocycle (A10-A12) widens Eg and enhances the transport properties ( Figure 11 and Table  5). The change is relatively small (Eg ≈ 0.1 eV) for pyridine (A10) and pyrazine (A12), but much larger for pyridazine (A11; Eg ≈ 0.7 eV), which also displays a roughly five-fold decrease in the intensity associated with the first absorption maximum relative to 1. Increasing the aromatic stabilization energy by replacing benzene with naphthalene (A13-A15) results in compounds with properties which are highly dependent on the positions of the two nitroso groups. In A13, Eg does not significantly change relative to 1, but effective masses are decreased by 30-40%, and the position of the fundamental gap changes from X to . On the other hand, in A14 and A15 the conduction and valence bands become strongly localized, which is accompanied by gap widening. Introducing anthracene (A16) narrows Eg to 1.90 eV and strongly decreases the electron (~50%) and hole (~30%) effective mass. Finally, exchanging phenyl for biphenyl (A17) narrows the gap by about 0.2 eV, but increases the absorption intensity as well as the electron and hole mobilities by 20-30%.

TD-DFT vs GW-BSE.
It is well documented that hybrid DFT gives good predictions of Eg for both molecular 50,53-57 and polymer 55,58 OSCs, as well as MOFs, 59-62 which is partially due to the possibility of using different functionals with varying levels of Hartree-Fock exchange. Our HSE and TD-HSE calculations show that admixing 25% HF exchange produces effective masses that are similar to GW and results in a reasonable agreement of predicted optical spectra with experiment. This strongly suggests that results obtained on related systems in sections (h), (i), and (j) are useful, although some caveats apply. First, correcting the DFT-obtained Eg for ZGR ( Figure 12) results in a much worse agreement with experiment. This is partially a consequence of the fact that hybrid TD-DFT results are most frequently directly compared to experimental Eg values, leading to an increase in popularity of functionals which produce good results "out of the box", such as B3LYP or HSE. 53,56,57 Ignoring the effect of electron-phonon coupling on the band gap is certainly valid for inorganics with heavy atoms, but both our results and previous experimental and computational work find values around 0.2-0.3 eV for carbonbased compounds, 63 highlighting the importance of including zero-gap renormalization in OSCs. This may be especially important in cases where a precise value of Eg is needed, such as in sensors or light-gathering devices.
The second issue with TD-HSE is its approximate treatment of charge screening. This results in a severe underestimation of Exb compared to GW-BSE, which is known to show very good agreement with experiments. 64 These results suggest that TD-HSE likely profits from fortuitous error cancellation (i.e., underestimation of both the electronic band gap and Exb), as well as from the fact that ZGR has not been extensively studied in OSCs. Therefore, the usefulness of TD-HSE for the estimation of precise values of Eg and Exb, which is important for determining applicability in devices such as LEDs (high Exb) or solar cells (low Exb) may be limited, and the use of GW-BSE(-ZGR) may be more appropriate.
On the other hand, while the result obtained by GW-BSE-ZGR is in excellent agreement with experiment, it also partly a product of good fortune, as the GW calculation is only converged to about 0.05 eV (see Figure S6), the vertex corrections are ignored, and there is a slight discrepancy between the experimental and optimized geometries. A more serious issue with GW calculations is their very poor scaling with regard to system size (although low-scaling approaches [65][66][67][68] are emerging), which is a serious limitation when considering OSCs; however, our results suggest that even an approximate calculation might provide important insights over hybrid DFT. The effect of solid-state packing can be estimated through its influence on the following: (i) the vibrational structure, (ii) the geometry, and (iii) the screening of Coulombic interaction. The effect of (i) is very minor (Figure 3), which is also illustrated by virtually identical ZGR values for 1 (-0.28 eV) and C1 (-0.27 eV), and similar phonon band structures ( Figure 6). The influence of packing on the polymer geometry (ii) can mostly be described through changes in  (° in 1 vs 53.8° in C1) which result in an Eg increase of 0.67 eV at the GW-BSE level.
The electron-hole interactions (iii) are screened much more strongly in C1, which significantly reduces Exb (0.55 eV for C1 vs 1.47 eV for 1) of the first optical transition. Therefore, (i) is unimportant, while the effects of (ii) and (iii) are significant and opposite to each other ( Figure  13). The geometry-independent effect of solid state packing on Eg, denoted as E 3D , can be estimated by comparing the Eg values of C1 and a 1 with the geometry (i.e. ) of C1, which produces a value of 0.65 eV (Figure 13) at the GW-BSE-ZGR level.
A qualitatively similar result is obtained by DFT on the geometries modulated along normal modes which correspond to a rotation about  (Figure 8). In that case, E 3D can be estimated as 0.24 eV from linear regression for 1.
The effect of solid-state packing on the transport properties is also significant. Although C1 has significantly larger effective masses than 1, its band curvatures in the kx and kz directions are similar. This translates to similar mobility along the polymer chain (x) and the direction of π-π stacking (z), suggesting that even polycrystalline films of C1 may show electrical conductivity with doping, which we will attempt to measure by four-point and contactless 44,48 measurements in a forthcoming study. The effect of torsion presents itself as a linear dependence of Eg on , as illustrated by DFT results in Figure 8. In the case of 1, GW-BSE results predict a larger slope (0.041 eV/°) than DFT (0.024 eV/°), which may be due to a better description of exciton (de)localization upon changes in . DFT predicts a stronger Eg- dependence (slope of 0.034 eV/°) for C1 than for 1 (0.024 eV/°), which can be attributed to the effect through-space π-π interactions between the polymer strands. These interactions also cause a relatively large (16.3°) change in  of C1 relative to 1, showing that packing effects and torsion are coupled, but may be separated ( Figure   13). Therefore, E  may be understood as a parameter causing a linear change of Eg with , the magnitude of which can be determined computationally.
Changes in Eg occurring upon substitution (Figure 8) can mostly be attributed to a change in .
Also, estimating Eg of the 1@Au monolayer by assuming that the changes occur only due to torsion (Table 3, method B) gives a very good estimate of the value determined by spectroscopic ellipsometry. Therefore, many changes occurring upon modification of 1 can be understood purely through their effects on , illustrating that the control of torsion is highly important for tuning the properties of conjugated polymers such as 1.
Effect of chemical modification. In all cases, the addition of substituents to the phenyl ring (S2-S9) or introducing nitrogen in the aromatic core (A10-A12) results in band localization, which is reflected in a significant increase in effective masses (Table 4) and Eg ( Figure 8). As mentioned in the previous section, this can mostly be explained by an increase in  caused by steric hinderance between the substituent and the azodioxide group. The nitro group (S6) is an exception, being the only one below the trendline in Figure 8, which is likely due to a very strong electron-withdrawing resonance effect ( = 0.78) stabilizing the excited state by moving the transition density from the C-N bond to the nitro group (cf. Figure 7e with Figure 14).

Figure 14.
Transition density for the first optical transition in S6.
Replacing benzene with naphthalene (A13-A15) has more dramatic effects, which are highly dependent on the substituent position. The 2,6-substituted derivatives A13 and A16 show an increase in band curvature, while an almost complete flattening of the conducting and valence bands occurs in 2,7-substituted A14 and 1,5-substitued A15. The differences between these compounds can be rationalized by visualizing their orbitals or looking at the aromatic delocalization pathways (Figure 15), which suggest delocalization across multiple unit cells is present in A13, but absent in A14. Comparing 1 (benzene) with A13 (naphthalene), A16, (anthracene) and A18 (tetracene), we note that both Eg and effective masses decrease continuously with increasing conjugation. However, the differences between effective masses of A16 and A18 are minimal, suggesting that transport becomes limited by the azodioxide group. On the other hand, Eg continues to decrease significantly (1.84 eV in A16 vs 1.49 eV in A18), presenting an interesting opportunity for band gap engineering by combining an appropriate (Eg-decreasing) aromatic core with an (Eg-increasing) substituent. This may be especially attractive for A18, as tetracene-based compounds are attractive candidates for singlet fission. 70 Finally, the biphenyl-based A17 (which has been synthesized) 42 shows a significant (30%) enhancement of transport properties relative to 1. Adsorption on gold. AFM reveals that the drop-casting method can be used to adsorb 1 on a (111) gold surface, producing thin films of 1@Au. Their morphology is characterized by islands with heights reaching 15 to 30 nm, which is consistent with multilayer formation. Beneath the on-top islands, additional multilayers are likely to be present. This is further corroborated by ellipsometry, which estimates the average film thickness of 1@Au at around 180 nm. The presence of the IR peak (~1.0 eV) in the imaginary dielectric function of 1@Au obtained by spectroscopic ellipsometry may be accounted for by calculations of the 1@Au monolayer, which suggest that the Eg narrowing is caused by polymer planarization. In case of nitroso dimers, such planarization was already observed. 30 Due to its interesting electronic properties and high stability, 1 could be possibly used as an organic field effect transistor (OFET). However, for application in such devices, highly ordered OSC films are needed. 71 To achieve this goal, a comprehensive investigation of the influence of experimental parameters (e.g. solution concentration, solvent, deposition method, substrate) on the surface morphology of thin films of 1, and its derivatives such as A16, A17, and A18, is planned.

CONCLUSION
Experimental characterization of C1 demonstrates that it is a purely polymeric compound with an Eg of 2.39 eV. This value is very well reproduced by a GW-BSE calculation with ZGR corrections (2.41 eV), demonstrating a nearly parameter-free procedure for calculating OSC properties. These calculations also predict that C1 has a relatively large exciton binding energy, which is strongly influenced by the environment. Crystal packing causes a significant increase of the torsional angle  relative to a single strand of 1; however, this is nearly completely offset by the effect of 3D interactions. Significant band curvature along the polymer chain and - stacking directions is found, suggesting 2-D conductivity may be present upon excitation or doping.
Hybrid TD-DFT predicts a linear dependence of Eg on torsion and suggests that substituent effects can mostly be understood through their effect on . In contrast, extending the conjugation present in the aromatic core is predicted to enhance the transport properties and narrow Eg, which may, in combination with adding substituents, allow for precise manipulation of the properties of derivatives of C1.
Finally, drop-casting is demonstrated as a viable method for producing thin films of 1 on a (111) gold surface, opening a possibility for preparing OFETs. Atomic force microscopy reveals these films display island-like morphology, while spectroscopic ellipsometry determines their band gap to be 0.68 eV. According to hybrid TD-DFT calculations, this large perturbation can mostly be attributed to surface-induced polymer planarization.

EXPERIMENTAL PART
Calculations. Functional choice. To identify a functional which produces a geometry in accordance with the experimentally determined structure of C1, we optimized its geometry using periodic boundary condition DFT calculations with several popular functionals in combination with different schemes to account for van der Waals interactions ( Figure S1).
Smallest RMSD values and lowest deviations in  were obtained using the PBE-revD3BJ method, which combines the PBE 72 functional with the corrected D3BJ correction scheme. 45,73 This method was used in optimizations of C1 and all 1-D systems. All calculations except for the ZGR used a primitive unit cell (two dinitroso units for C1 and one unit for 1-D systems); unconstrained optimizations of supercells produced no significant desymmetrization. For modelling gold surface adsorption, the vdW-DF2 scheme was used, as it was shown to describe organic-metal interfaces well. [74][75][76][77] Optimization. Geometries were optimized with the above described PBE-revD3BJ method. A 700 eV kinetic energy cutoff for the planewave basis was used, and a 0.15 eV Gaussian smearing was employed. Calculations on 1-D systems utilized a Γ-centered reciprocal grid with 16 k-points along the Γ-X path. The structure of C1 was optimized using a 8x4x12 reciprocal grid. Geometry optimizations were carried out until the sum of Helmann-Feynman forces was less than 0.01 eV Å -1 , while the wavefunction energy was converged to 10 -6 eV. 78 HSE and TD-HSE calculations. The HSE wavefunction was converged to 10 -6 eV. Out of 324 bands (25 and 50 occupied for 1-D and 3D systems, respectively) that were used to construct the frequency dependent dielectric matrix, 12 occupied and 36 virtual bands were included in the Casida equation. Band structures interpolation and calculation of transport properties were done with the BoltZtraP2 code. 79 Band paths were drawn according to Hinuma et al. 80 GW calculations were done on a PBE wavefunction with 502 bands, a Gaussian smearing of 0.175 eV, and a 120 eV response function cutoff. For C1, an 8×1×6 Gamma-centered k-grid was used, while calculations on 1 used an equivalent 8×1×1 grid. Four iterations of were performed, which resulted in convergence within <25 meV in quasiparticle gaps. The Tamm-Dancoff approximation was used. Convergence tests can be found in Figure S6 in SI.
Zero-gap renormalization. The phonon-induced band-gap renormalization was estimated using the one shot ZG configuration method. 43,81 A 4x2x3 supercell was used, with benchmarking details in Table S1 in SI.
All calculations were performed using the Vienna Ab initio Simulations Package (VASP). [82][83][84][85] Synthesis. Compound 1 was prepared by the method described recently. 29 The product was purified by column chromatography (silica gel, chloroform:acetone = 4:1) and the green solution of monomer of 1 was collected. Evaporation of the solvent yielded a yellow powder of E-azodioxy polymer 1 which was confirmed by IR spectroscopy.
Diffuse Reflectance Spectroscopy. The UV/Vis diffuse reflectance spectrum of a powder sample of C1 was obtained using the Cary 5000 (Agilent Technologies) integrating sphere spectrophotometer. Diffuse reflectance was modelled according to the Kubelka-Munk model, 86 described as: where K and S are phenomenological parameters and ( inf ) is the Kubelka-Munk reemission function. The absolute reflectance can be written as: where standard = 1. For perfect diffuse scattering K is equal to 2α, where α is the absorption coefficient, with the energy dependence: where C1 is a proportionality constant, and n is 0.5 for direct band gaps. As S is a constant too, in the case of a direct band gap we can write: ( ( inf ) ) 2 = ( 2 ) 2 = ( 2 1 ( − g ) 0,5 ) 2 = 2 ( − g ) which is equation (1) from which the Eg value of C1 was determined.
Infrared spectroscopy. The infrared spectrum of C1 was measured on a polycrystalline sample, while a spectrum of 1 was obtained by preparing a well-mixed 1:100 KBr matrix of C1. Spectra were recorded using a PerkinElmer SpectrumTwo spectrometer at a resolution of 4 cm -1 , averaging 10 scans per spectrum.
Preparation of 1@Au. All glassware was cleaned using a piranha solution (3:1 mixture of sulfuric acid and 30 % hydrogen peroxide heated to 90 °C). Prior to deposition, commercially available Au(111)/mica substrates (Phasis) were flame annealing followed by cooling in the stream of argon. Thin films of 1 (1@Au) were prepared by drop-casting method as follows: two droplets of 0.02 M solution of compound 1 in a 4:1 mixture of chloroform and acetone were placed on an Au(111) surface. After the solvent evaporated, the adlayers were characterized by atomic force microscopy (AFM) and ellipsometry.
Atomic force microscopy (AFM). AFM measurements were performed on a MultiMode 8 (Bruker) in soft tapping mode with silicon probes (Bruker, NCHV-A, nom. spring constant 40 N/m, nom. freq. 320 kHz) under ambient conditions and humidity of 40-50 %. AFM images were processed and analyzed by using the NanoScope Analysis 2.0 (Bruker) software.

Ellipsometry.
Optical characteristics of 1 deposited on a (111) gold surface were investigated by spectroscopic ellipsometry using a J. A. Woollam V-VASE ellipsometer in the spectral range from 0.57 to 4.73 eV. Measurements were carried out at three angles of incidence, 55°, 65° and 75°. Optical properties and layer thickness were both obtained by simultaneous fitting. Depolarization (due to thickness non-uniformity) was included into the model in order to ensure a satisfactory agreement between the experimental results and the data generated by the model. Due to complexity of the surface layer, the flexible multiple oscillator model was used.
Due to the importance of estimating the band gap, Tauc-Lorentz oscillators were used in the general oscillator model because they include the band gap energy (Eg) as one of the defining parameters. 87

ASSOCIATED CONTENT
Supporting Information. The Supporting Information is available free of charge on the ACS publication website.

Optimized geometries of all structures (POSCAR)
Benchmarking results for geometry optimization, GW calculations, and ZGR calculations.
Calculated conductivity of C1, full phonon band structure of C1, TD-HSE predicted optical spectra, Hammett graph for dinitroso dimers and polymers.

Notes
The authors declare no competing financial interest.