Angewandte Communications Chemie How to cite: Angew. Chem. Int. Ed. 2021, 60, 17887 – 17892 Molecular Electronics International Edition: doi.org/10.1002/anie.202104294 German Edition: doi.org/10.1002/ange.202104294 A Trapezoidal Octacyanoquinoid Acceptor Forms Solution and Surface Products by Antiparallel Shape Fitting with Conformational Dipole Momentum Switch Samara Medina Rivero, Javier Urieta-Mora, Agustn Molina-Ontoria, Cristina Martn-Fuentes, Jos I. Urgel, Maria Zubiria-Ulacia, Vega Lloveras, David Casanova, Jos I. Martnez, Jaume Veciana,* David cija, Nazario Martn,* and Juan Casado* Abstract: A new compound (1) formed by two antiparallelly disposed tetracyano thienoquinoidal units has been synthe- sized and studied by electrochemistry, UV/Vis-NIR, IR, EPR, and transient spectroscopy. Self-assembly of 1 on a Au(111) surface has been investigated by scanning tunneling microsco- py. Experiments have been rationalized by quantum chemical calculations. 1 exhibits a unique charge distribution in its anionic form, with a gradient of charge yielding a neat molecular in-plane electric dipole momentum, which trans- forms out-of-plane after surface deposition due to twisted! folded conformational change and to partial charge transfer from Au(111). Intermolecular van der Waals interactions and Figure 1. Coupling modes in bichromophoric systems, A and B. Tetracyano substituted thienoquinoidal MQ and BQ compounds antiparallel trapezoidal shape fitting lead to the formation of represent the constitutional fragments of trapezoid-like compound an optimal dense on Au(111) two-dimensional assembly of 1. 1 together with the benchmark tetracyano quinodimethane, TCNQ. The realization of novel properties emerging upon electronic covalent coupling between chromophores (i.e., bichromo- storage[7] and in organic electronics.[8] In addition to this phoric systems) is a critical issue for the development of electronic provision, its embedment in different molecular photo- and electrically active systems.[1–3] In this regard, the forms is of relevance as these can define unique ways of shape relative topology and orientation of the p-subchromophores, fitting in supramolecular and surface assemblies. Joint such as in A + B type p-systems in Figure 1, with 1D linear electronic and molecular shape designs thus allow to build conjugation, 2D parallel conjugation,[4] 3D orthogonal spiro- molecular-based synthons in a tailored manner towards new conjugation[5] or 3D conjugation[6] are key factors. On the bulk and nano organized materials. Accordingly, herein, we other hand, studies of the distribution of the excess of charge present a trapezoidal-shape molecule having a double p- in p-conjugated moieties in post-electron transfer events are electron quinoidal array, 1 in Figure 1. central issues in photophysics and photochemistry, in energy [*] S. Medina Rivero, Prof. J. Casado Dr. V. Lloveras, Prof. J. Veciana Department of Physical Chemistry, University of Mlaga Department of Molecular Nanoscience and Organic Materials, Andalucia-Tech Campus de Teatinos s/n, 29071 Mlaga (Spain) Institut de Cincia de Materials de Barcelona (ICMAB-CSIC) and E-mail: casado@uma.es Networking Research Center on Bioengineering, Biomaterials and J. Urieta-Mora, Dr. A. Molina-Ontoria, C. Martn-Fuentes, Nanomedicine (CIBER-BBN) Dr. J. I. Urgel, Prof. D. cija, Prof. N. Martn Campus de la UAB, 08193 Bellaterra (Spain) IMDEA-Nanociencia, C/Faraday 9 E-mail: vecianaj@icmab.es Ciudad Universitaria de Cantoblanco Dr. J. I. Martnez 28049 Madrid (Spain) Department of Nanostructures and Low-dimensional Materials, E-mail: nazmar@ucm.es Institute of Materials Science of Madrid (ICMM-CSIC) J. Urieta-Mora, Prof. N. Martn Ciudad Universitaria de Cantoblanco Department of Organic Chemistry, Faculty of Chemistry C/Sor Juana Ins de la Cruz 3, 28049 Madrid (Spain) Complutense University of Madrid Supporting information and the ORCID identification number(s) for 28040 Madrid (Spain) the author(s) of this article can be found under: M. Zubiria-Ulacia, Dr. D. Casanova https://doi.org/10.1002/anie.202104294. Donostia, International Physics Center (DIPC) & IKERBASQUE—  2021 The Authors. Angewandte Chemie International Edition Basque Foundation for Science published by Wiley-VCH GmbH. This is an open access article under Paseo Manuel de Lardizabal, 4, 20018 Donostia-San Sebastin, the terms of the Creative Commons Attribution Non-Commercial Euskadi (Spain) License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Angew. Chem. Int. Ed. 2021, 60, 17887 –17892  2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 17887 Angewandte Communications Chemie Recently, the inspection of the organization of organic p- conjugated molecules on surfaces is a major topic of research given the emerging features of the adsorbates, their assem- blies and surface reactivity.[9–15] In spite of the relevance of on- surface discoveries, connections between solution properties of molecules and their behaviors on surfaces become mandatory to develop a rational and unified understanding of both worlds. We show here how the trapezoidal shape of 1 provides basic driving forces for the assembly mode on surface. The vanguard double antiparallel quinoid concept of 1 stresses on the efforts towards fine control of intramolecular properties, but also on the necessity of acting on new molecular forms pursuing intermolecular shape fitting which could bring on the table emerging ways to control assembly properties. For instance, our study of 1 discloses the ability to tailor the electrical dipole momentum of the species from solution to the surface driven by changes in the molecular Figure 2. a) UV/Vis absorption spectrum of 1 and TCNQ, and b) their conformation. cyclic voltamograms in CH2Cl2 0.1 M PF6NBu4. Spectroelectrochemical Solution studies on 1 have been carried out by stationary measurements of 1 and TCNQ in the cathodic wave carried out in infrared and time-resolved transient infrared (TRIR tech- CH2Cl2 0.1 M PF6NBu4. c) UV/Vis-NIR spectra and d) infrared spectra. nique)[16] spectroscopies. The presence of cyano groups in 1, through their CN stretching frequency [n(CN)], has provided the way for electron distribution studies.[17–22] On angle of about 208 (twisted conformer in Figure 3, and the other hand, on-surface studies of 1 have been carried out Figure S5) between the planes containing the two subchro- on a relatively weak interacting Au(111) surface by scanning mophores owing to the steric crowding between the dicyano tunneling microscopy (STM) under ultra-high vacuum groups of the two quinoidal moieties. The second local (UHV) conditions, which have led to the formation of an minimum exhibits a folded structure with CN groups of the optimal dense on-surface 2D antiparallel molecular assembly. two moieties (MQ and BQ) pointing towards the same side The new 2D p-conjugated 1 is formed by coupling of two regarding the main molecular plane, which is computed at thienoquinoidal cores in antiparallel fashion: a tetracyano DG8 = 1.4 kcal mol1 above the twisted conformer. The inter- quinoidal thiophene[23] (MQ) and a tetracyano quinoidal conversion between the two forms can be achieved by bithiophene[24] (BQ) in Figure 1, the three thiophenes sur- surmounting an energy barrier of DG° = 5.0 kcal mol1 with rounding a benzenoid ring (i.e., generating a trapezoidal-like respect to the twisted minimum. In general, computed form).[25, 26] Tetracyanoquinoidal or tetracyanoquinodime- electronic structure properties for the two conformers are thane (TCNQ)-based p-extended molecules are important rather similar. Comparison between the two forms can be electroactive units in the field of organic conductors[18–24] and found in the Supporting Information (Figures S6–S8, and organic electronics.[27] With 1 we wish to address the following Tables S2–S4). Another aspect of 1 is that its closed-shell issues: 1) the shape of the electron distribution in its anionic wavefunction is slightly unstable by 1.23 kcal mol1 with species by spectroelectrochemistry; and 2) the way in which respect to the open-shell broken symmetry solution at the the trapezoidal shape of 1 dictates its 2D organization on UB3LYP/6-31G** level of calculation disclosing a ground a relatively weak interacting surface; 3) the influence of state open-shell singlet with the lowest triplet at 5.33 kcal conformation in the direction of the electrical dipole momen- mol1 (4.50 kcal mol1 at UB3LYP/6–31 + G(d) level in tum upon adsorption. The aforementioned experimental vacuum and 6.52 kcal mol1 in solution). information will be underpinned by quantum chemical The cyclic voltammogram of 1 is in Figure 2 which, such as modeling. TCNQ, shows two consecutive reversible one-electron pro- The synthetic route to compound 1 starts with the cesses. Interestingly, TCNQ shows its first potential reduction tetrabrominated bbc-benzotrithiophene derivative which earlier than 1, while 1 displays its second reduction before was synthesized according to previously reported protocols TCNQ. Regarding the first reduction, the aromaticity recov- (see ESI for details). With this central scaffold in hand, 1 was readily prepared by reaction with tetracyanoethylene oxide and copper in refluxing 1,2-dibromoethane in 18 % yield (characterization in the Supporting Information, Figures S1– S3). The electronic absorption spectrum of 1 is shown in Figure 2. It displays two main absorptions at 425 and 538 nm, in fairly good agreement with the absorption profiles simulated at different computational levels (Table S1 and Figure 3. Relative Gibbs free energies of the twisted and folded con- Figure S4). 1 presents two structural conformers. The most formers, and the transition state (TS) for their interconversion for the stable form has an optimized geometry featuring a dihedral ground state of the neutral species of 1. 17888 www.angewandte.org  2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021, 60, 17887 –17892 Angewandte Communications Chemie ery of benzene in TCNQ might be greater than the sum of the tion (Tables S10 and S11 for the bond length and NICS aromaticity recoveries in the BQ/MQ moieties due to the changes on reduction). The final situation is that either in 1C competition effect. The situation is reversed in the second or 12 there is a larger amount of charge on the BQ moiety reduction owing to the larger mitigation of electrostatic than in MQ or negative charge gradient (see Figure 4) that repulsion in the dianion of 1 than in that of TCNQ. Further will produce an in-plane electric dipole moment in the reduction of 12 (Figure S9) is observed at more cathodic charged species. potentials. The reduction process was followed by electron para- UV/Vis-NIR absorption spectroelectrochemistry of 1 in magnetic resonance (EPR) spectroscopy (Figure 5). The Figure 2 (spectroelectrochemical UV/Vis-NIR details in Fig- neutral species of 1 was EPR silent in solution at room ure S12 and theoretical bands assignment in Tables S6 and S7) temperature in accordance with its singlet ground state. shows that initial reduction produces the disappearance of the However, upon first reduction, the generated species pos- neutral bands and the development of a spectrum charac- sesses a neat EPR spectrum at g = 2.0020 composed of 9 main terized by three new ones at 479, 644, and 750 nm. Further charging gives way to the growth of only one new band at 514 nm. Infrared spectroelectrochemistry of 1 in the region of the n(CN) band is also shown in Figure 2. The IR band of neutral 1 at 2223 cm1 gives rise to a very strong and broad band at 2183 cm1 in 1C , a spectrum which is similar (i.e., 2180 cm1) to that of the radical anion of TCNQ[28] also displayed in Figure 2 (spectroelectrochemical IR reductions of 1 in Figure S13 with theoretical IR calculations in Figures S14 and S15, and spectroelectrochemical UV/Vis- NIR and IR for TCNQ in Figures S16 and S17) confirming Figure 5. Left: EPR experimental (blue) spectrum of the first reduced that most of the anionic charge in 1C is distributed between species of 1 and simulated (gray) spectrum of the radical anion of 1. Right: enlarged experimental and simulated spectra where satellite the dicyanomethylene groups of the BQ moiety (with lines, due to the coupling with the 33S nuclei, are shown. a smaller amount on MQ). Interestingly, this charge distribu- tion generates a permanent electric dipole moment in 1C such as revealed in Figure S11 from calculations. Posterior electro- lines due to the coupling of the unpaired electron with N and chemical reduction of 1C produces a spectrum with two well H nuclei of the BQ moiety. This species also shows weak defined IR absorptions at 2160 and 2106 cm1 which compare emerging satellite lines as a result of the coupling of the with the 2152/2105 cm1 pair in the dianion of TCNQ unpaired electron with the 33S nuclei (isotope abundance confirming that a larger portion of the negative charge in 0.75 %) of BQ and MQ moieties (Table S12). These results 12 is on the cyano groups of the BQ moiety (see Table S5 and evidence that the charge is mostly delocalized on the BQ Figure S10 for charge distribution analysis). To further moiety with a residual amount on MQ, such as discussed rationalize this, we carried out quantum chemical calculations above. The dianion does not give EPR signal due to the on the structures of Figure 4. D1, D1 a and D1 b. DFT singlet ground state. calculations predict the most stable form to be D1 followed The transient electronic absorption spectra (or excited by D1 a, and D1 b (Table S8). In addition, such calculations state absorptions, ESA bands) of 1 in the pico-second time show that the ground state of 12 is a closed-shell singlet with regime are shown in Figure S18 (Table S13 for excited state a rather large energy gap to the triplet (Table S9). The calculations). The observed rise and decay times correspond reduction of the electronic repulsion of the negative charges to ultrafast excited state processes justifying the absence of by spreading them around the whole structure of 1 together fluorescence of 1. Interestingly, the time resolved spectra with the recovery of the aromaticity in the central benzene show the simultaneous rise of two ESA bands at 520 nm and and in two thiophenes in D1 might justify its larger stabiliza- at 660 nm. The femtosecond excited state absorption spec- trum of TCNQ[29] in acetonitrile features a main absorption at 460 nm with a major decay time of hundreds of femtoseconds followed by a secondary long component of 24 ps. The TRIR spectra of 1 in the interval of tens of picoseconds show a strong infrared absorption that initially displays its max- imum at  2170 cm1 and that progresses with time with a displacement up to 2181 cm1. The appearance of the infrared band at values close to that of the radical anion could be ascribed to a symmetry breaking effect in the excited state Figure 4. Left: Relative energies of the dianion species of 1 showing due to equalization of the CC bonds in the conjugated path the qualitative charge distribution on its optimized geometry, D1, and followed by pseudo Jahn Teller distortion along a suitable D1 a and D1 b, which are the forms with the charge constrained in the backbone CC stretchimg vibration. The distortion of such BQ and MQ units, respectively. Right: drawn of the negative charge gradient (qualitatively marked by the blue-yellow colors change) in the fused rigid core is feasible by out-of-plane distortions of the dianion of 1 according to calculations in Table S5 and Figure S10. The outermost CN groups that localize the excitation distortion arrow marks the electric dipole momentum generated (Figure S11). (producing large changes of the n(CN) wavenumbers) and Angew. Chem. Int. Ed. 2021, 60, 17887 –17892  2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org 17889 Angewandte Communications Chemie minimizes steric crowding. This structure might resemble that Figure 6 b, revealing a T trapezoidal molecular shape together of the folded structure of 1 in Figure 4 revealing a fluxional with two bright peripheral lobes with a maximum apparent twisted!folded conversion upon photexcitation. height of 2.2  (measured at a sample bias of 0.5 V). The We have further characterized 1 upon adsorption on experimental features, observed in Figure 6 b, are well a relatively weak interacting surface such as Au(111). For this reproduced by the DFT optimized geometry and the corre- purpose, a submonolayer coverage of 1 is deposited onto an sponding Keldish–Green STM simulation of 1 on the Au(111) atomically clean Au(111) surface held at room temperature surface (Figures 6 c,d). According to optimized model in under ultra high vacuum conditions. The STM image dis- Figure 6 d, a single molecule of 1 adopts on-surface a highly played in Figure 6 a shows molecules self-assembled into nonplanar configuration corresponding to the metastable close-packed islands. This antiparallel top&down organiza- folded conformer obtained in vacuum (Figure 3). In the BQ tion is primarily promoted by the trapezoidal shape of 1 and it moiety, one CN of each dicyano group is bent towards the is intermolecularly fastened by van der Waals interactions surface, establishing CN···Au interactions, while the other between adjacent species. Such self-assembly is in agreement vicinal CN is displaced upright. In the MQ moiety the CN with DFT simulations of a supramolecular domain (see the groups are both bent pointing to the same side. Furthermore, Supporting Information). the S atom of the MQ unit is closer to the surface, likely by To study an isolated molecule, we have performed STM S···Au interaction. Altogether, the molecules adopt a non- manipulation experiments, by laterally displacing the STM tip planar folded conformation which provokes the appearance at close tip-sample distances across the edge of a molecular of a waving-like tiled roof aspect for the surface assembly. island (Figure S19). A high resolution image is shown in Furthermore, our DFT simulations of an individual molecule and supramolecular assembly of 1 on Au(111) reveal the following conclusions: 1) adsorption energies per molecule of 1.62 eV in the simulated layer and of 1.9 eV in the single molecule; 2) an intermolecular interaction per mole- cule of 0.32 eV stabilizing the waving-like self-assembled arrangement (Figure S20); 3) 1 as a single molecule is predicted with a folded structure with an adsorption height of 3.0  largely stabilized with respect to the twist form due to the steric hindrance between the two hydrogens and the two CN groups. In addition, we performed a Baders charge analysis[30] based on the DFT charge density distribution, obtaining a charge transfer of 0.40 e from the metal surface to a single molecule of 1 which is slightly reduced to 0.37 e in the self- assembled layer both confirming the acceptor character of 1 even on the metal surface. Interestingly, it is worth remarking that the electronic molecular in-plane dipole momentum of the planar conformer of 1 in solution dramat- ically changes upon adsorption on Au(111), adopting an out- of-plane disposition pointing perpendicularly down from the molecule towards the surface with a net computed value of 0.45 D for the single molecule and of 0.53 D in the simulated ad-layer. This in!out switch of the dipolar momentum has its origin in the confluence of two major factors: 1) the on- surface induced molecular distortion that produces a charge redistribution within the molecule unbalanced between both molecular basal planes, and 2) the charge transferred from the substrate, computed in 0.40 and 0.37 e for individual and self- assembled species, respectively. Part of this transferred charge redistributes within the molecule and another part accumu- lates on the linking molecule-substrate domains. In an intuitive scenario, one could visualize the gas phase twisted molecules approaching to the substrate during deposition and Figure 6. Structural characterization of 1 on Au(111). a) Self-assem- adopting the structure of the folded conformer upon adsorp- bled island of 1 after sublimation on the gold surface held at room tion, switching the electric dipole momentum orientation in temperature. Vb = 0.5 V, It = 80 pA, scale bar = 3 nm. b) Magnified the process. This phenomenon opens the door to additional view of an isolated molecular species evidencing its T-shape appear- tunning capabilities in designing molecular assemblies by ance with two bright peripheral lobes. Vb = 0.5 V, It = 100 pA, scale bar = 0.5 nm. c) Simulated Keldish-Green STM image of the molecule/ switching the dipole momentum orientation upon surface Au(111) interface. d) Top (upper panel) and side (lower panel) views adsorption with the subsequent modulation/impact in the of the DFT-optimized geometry of 1 on Au(111). electric, optical and chemical properties. 17890 www.angewandte.org  2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021, 60, 17887 –17892 Angewandte Communications Chemie In summary, we have prepared a new acceptor molecule Coto, E. T. Chernick, R. Casillas, B. S. Basel, M. Thoss, R. R. formed by two antiparallelly embedded tetracyano thieno- Tykwinski, D. M. Guldi, Proc. Natl. Acad. Sci. 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Chem. 2011, 3, 197 – chemical properties that guide the molecular behavior in 204. [8] a) A. Molina-Ontoria, M. Wielopolski, J. Gebhardt, A. Goulou- solution and in surfaces thus contributing to jointly rationalize mis, T. Clark, D. M. Guldi, N. Martn, J. Am. Chem. Soc. 2011, both worlds, while opening new avenues for tuning properties 133, 2370 – 2373; b) M. Wielopolski, A. Molina-Ontoria, C. such as the electrical dipole momenta. Schubert, J. T. Margraf, E. Krokos, J. Kirschner, A. Gouloumis, T. Clark, D. M. Guldi, N. Martn, J. Am. Chem. Soc. 2013, 135, 10372 – 10381; c) A. Ferrer-Ruiz, T. Scharl, P. Haines, L. Acknowledgements Rodrguez-Prez, A. Cadranel, M. . Herranz, D. M. Guldi, N. Martn, Angew. Chem. Int. Ed. 2018, 57, 1001 – 1005; Angew. Chem. 2018, 130, 1013 – 1017; d) M. Izquierdo, B. Platzer, A. J. The authors thank the Spanish Ministry of Science, Innova- Stasyuk, O. A. Stasyuk, A. A. Voityuk, S. Cuesta, M. Sol , D. 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Version of record online: July 7, 2021 17892 www.angewandte.org  2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021, 60, 17887 –17892