含Benzo[a]azulene 單元的鋸齒狀梯形共軛聚合物的表面在位合成

摘要：梯形共軛聚合物（CLPs）因其獨特的光電性質而受到廣泛關注。絕大多數CLPs是通過溶液方法合成的，但近年來，在超高真空環(huán)境中進行的表面原位合成策略逐漸嶄露頭角，成為CPL合成的新方法。表面原位合成方法能夠克服傳統(tǒng)溶液合成的限制，如隨著聚合度增加而受限的溶解度和結構穩(wěn)定性，從而實現復雜共軛結構的精確合成。Azulene衍生物是在表面合成非苯型CLPs的有吸引力的前體。與傳統(tǒng)的只含六元環(huán)的CLPs相比，使用烷基取代的azulene作為前體分子，有望獲得具有復雜骨架結構的CLPs，從而調控其電子性質，但目前很少有人探索這種策略。本文報道了3，3'-二溴-2，2’-二甲基-1，1’-聯薁（DBMA）在Au（111）表面上的熱化學反應。在室溫的Au（111）襯底上，我們發(fā)現沉積的分子在重構表面的fcc （面心立方堆積）區(qū)域形成無定型的聚集體，并在100 °C以下保持形貌不變。當退火溫度高于150 °C后，DBMA發(fā)生脫溴反應并與金原子絡合形成具有復雜空間立體結構的2，2’-二甲基-1，1’-聯薁有機金屬聚合物，并展現出迥異的圖像特征。隨后在更高溫度下退火，有機金屬聚合物脫去金屬原子并經歷碳碳偶聯反應。該過程伴隨著甲基與相鄰薁單元之間的分子內環(huán)化反應，形成了含有benzo[a]azulene單元的梯形共軛聚合物。有趣的是，我們發(fā)現當一側甲基參與反應并在聚合物中形成六元環(huán)時，會顯著地彎折聚合物鏈，使得另一側甲基與薁單元之間的距離增加，并抑制預期的環(huán)化過程。我們通過鍵分辨掃描探針顯微鏡對反應過程中的相關結構進行了研究，發(fā)現反應結果與反應中間結構的應力關聯緊密。我們的結果表明，烷基取代的azulene前體可應用于非苯型碳納米結構的表面合成，并有望實現擴展的非苯型二維碳納米結構。

關鍵詞：表面在位合成；薁；非苯型碳納米結構；梯形共軛聚合物；掃描探針顯微鏡；鍵分辨原子力顯微鏡

中圖分類號：O649

Abstract： Conjugated ladder polymers （CLPs） haveattracted broad interest due to their intriguing opticaland electronic properties. While many of these CLPshave been synthesized using solution-basedreactions， on-surface synthesis under high vacuumconditions has gradually gained prominence in recentdecades. This new approach holds promise forovercoming some of the limitations of conventionalsolution-based methods， such as the low solubility and stability of newly formed large π-conjugated systems. Azulenederivatives are attractive precursors for on-surface synthesis of CLPs that incorporate non-benzenoid moieties. The useof alkyl-substituted azulene precursors shows potential in providing CLPs with more complex backbone structures andmodulated electronic properties compared to traditional CLPs that containing solely of six-membered rings. However， thisstrategy has been scarcely explored to date. In this study， we report on the thermal reactions of 3，3’-dibromo-2，2’-dimethyl-1，1’-biazulenyl （DBMA） on Au（111） surfaces. At room temperature， we observed that the deposited molecules formedamorphous aggregates in the fcc （face center cubic） regions of the reconstructed Au（111） surface， remaining unchangedbelow 100 °C. Debromination of DBMA was induced above 150 °C， leading to the formation of 1，1’-biazulenyl-2，2’-dimethyl-3，3’-diyls-Au organometallic polymers. These polymers exhibited complex stereostructures and distinct imaging features.At higher temperatures， the organometallic polymer underwent C―C coupling， followed by dehydrocyclization betweenthe methyl groups and the adjacent azulene units， resulting in the ladder polymer containing benzo[a]azulene units.Interestingly， we observed that the formation of hexagonal rings between the methyl groups and the adjacent azulene unitscaused the polymeric chain to bend， increasing the distance between the corresponding reaction sites （methyl group andazulene） on the other side of the polymer chain. Due to the ring strain， the second ring closure did not occur within theazulene dimer as expected. Instead， this methyl group cyclized toward the other azulene unit， resulting in CLPs with achevron shape and the absence of long-range periodicity. The evolution of related chemical species and the structures ofCLPs were analyzed using scanning tunneling microscopy （STM） and bond-resolved atomic force microscopy （BR-AFM），and the reaction mechanism was discussed. This study thus demonstrates the feasibility of utilizing alkyl-substitutedazulenic precursors in the synthesis of non-benzenoid carbon nanostructures on surfaces and suggests the possibility ofdeveloping two-dimensional nanostructures containing non-benzenoid units through on-surface azulene chemistry.

Key Words： On-surface synthesis; Azulene; Non-benzenoid carbon nanostructure; Conjugated ladder polymer;Scanning probe microscopy; Bond-resolved atomic force microscopy

1 Introduction

Conjugated ladder polymers （CLPs） have attracted broadinterest due to their intriguing optical and electronic properties，including a low bandgap， intense light absorption， and highcharge-carrier mobility. These properties arise from their fullyconjugated， rigid backbone and unique electronic bandstructures 1–4. Potential applications range from field-effecttransistors and organic light-emitting diodes to single molecularelectronic devices 5. While many CLPs have been synthesizedusing solution reactions 6， on-surface synthesis under highvacuum conditions has only gradually gained prominence in therecent decade. This novel approach holds promise forovercoming some of the limitations associated with conventionalsolution-based methods， such as the low solubility and stabilityof newly formed large π-conjugated systems and limited reactiontypes for ring formation through direct C（sp3）―C（sp2） coupling 7–13.Despite successful engineering of on-surface synthesizedcovalent ribbons， the conversion of these 1D chains into 2Dnanoarchitectures remains a rarely reported challenge 14.

CLPs with various widths and edge geometries have beensynthesized on metal surfaces using rationally designedmolecular precursors 15–19. While most previously reported CLPsare composed of benzene rings， CLPs containing non-benzenoid aromatic moieties have been theoretically investigated for theirsuperior optoelectrical response， charge-carrier transport， andtunable magnetic properties 20–28. Nevertheless， the controlledincorporation of non-benzenoid moieties into the CLP backboneremains a big challenge.

Azulene is a nonalternant aromatic hydrocarbon composed offused pentagonal and heptagonal carbon rings. Azulenederivatives have attracted considerable attention as precursorsfor on-surface synthesis of non-benzenoid carbonnanostructures 29–32. Fan et al. reported the first on-surfacesynthesis of 2，6-polyazulene， achieved through Ullmanncoupling of 2，6-dibromoazulene as a key step. These azulenepolymers undergo lateral fusion at elevated temperatures，resulting in non-benzenoid carbon nanostructures containingperiodic six-membered rings or four- and seven-memberedrings 30 （Scheme 1a）. Subsequently， Hou et al. reported the onsurfacereactions of 3，3’-dibromo-1，1’-biazulenyl （DBDA） onAu（111）， yielding curved non-benzenoid carbon nanostructuresdue to the rotational freedom of azulene moieties along thebridging C―C bonds， as well as the rearrangement of azuleneunits at elevated temperatures 31 （Scheme 1b）. The precursormolecules in these cases are all halogenated azulenes， and thecarbon nanostructure’s skeleton could only originate from cyclodehydrogenation of the azulene moieties. We anticipatethat by introducing alkyl substituents at appropriate positions ofazulene precursors， cyclization involving new C（sp3）―C（sp2）bonds could yield more complex non-benzenoid CLPs withtunable electronic properties 30.

In this study， we report the synthesis of CLPs containing nonbenzenoidbenzo[a]azulene moieties on an Au（111） surfaceunder ultrahigh vacuum conditions. To achieve this， we firstsynthesized the monomer 3，3’-dibromo-2，2’-dimethyl-1，1’-biazulenyl （DBMA）， featuring two methyl groups on the fivememberedrings， in a solution. After sublimation onto Au（111）surfaces， DBMA underwent debromination at 150 °C， withthe diradical intermediate stabilized by Au atoms， resultingin the formation of 1，1’-biazulenyl-2，2’-dimethyl-3，3’-diyls-Auorganometallic polymers. At higher temperatures， theorganometallic polymer underwent C―C coupling， followed bydehydrocyclization between the methyl groups and the adjacentazulene， leading to the ladder polymer containingbenzo[a]azulene units. Notably， the formation of hexagonalrings between methyl groups and the adjacent azulene bent thepolymeric chain， increasing the distance between thecorresponding reaction sites （methyl group and azulene） on theother side of the polymer chain. Due to the resulting ring strain，the second ring-closure did not occur within the azulene dimeras expected. Instead， this methyl group cyclized toward the otherazulene unit， resulting in CLPs with a chevron shape. Theevolution of related chemical species and the structures of CLPswere analyzed using scanning tunneling microscopy （STM） andbond-resolved atomic force microscopy （BR-AFM） to elucidatethe reaction pathway.

2 Experimental methods

2.1 Synthesis of DBMA

2-Methylazulene （1） was initially synthesized following theliterature procedure， employing readily available startingmaterials 33，34. Subsequently， selective bromination at the 1，3-position was carried out using N-bromosuccinimide， yielding1，3-dibromo-2-methylazulene （2） as a dark blue solid in asatisfactory yield. To synthesize DBMA， we employed aYamamoto coupling reaction with a sub-stoichiometric amountof bis（1，5-cyclooctadiene）nickel （Ni（cod）2）. This approach waschosen to suppress the formation of higher oligomers and tomaintain the integrity of the bromo groups for the on-surfacereaction. The utilization of 0.6 equivalents of Ni（cod）2 facilitatedthe formation of DBMA within 1.5 h， yielding a mixturecomprising the target product， unreacted starting material 2， anda small portion of higher oligomers. DBMA was subsequentlyeasily purified through silica gel chromatography. Furtherpurification was achieved by subjecting the compound torecrystallization five times， resulting in a highly pure precursorsuitable for on-surface studies. （Additional details regarding thesynthesis are provided in Scheme 2 and the supplementarymaterials）.

2.2 STM/AFM characterizations

In this experiment， a low-temperature scanning tunneling microscope （LT-STM） from Scienta Omicron， Germany，operating at a temperature of 4.6 K， was utilized. Theexperimental setup was housed within an ultra-high vacuumchamber with a base pressure of 5 × 10?8 Pa. To prepare thesingle crystal Au（111） substrate， it underwent a cleaning processinvolving argon ion sputtering and subsequent thermal annealingto achieve a flat surface. A K-cell organic molecular beamepitaxy device （OMBE） from Kentax GmbH was employed todeposit DBMA onto the Au（111） surface at room temperature.The sample was then subjected to gradual annealing， includingsteps at 150， 180， 210， 250， and 300 °C， with each temperaturebeing maintained for a duration of 30 min. For bond-resolvedcharacterization， tungsten tips were modified with CO moleculesusing the ramping method as described by Bartel et al. 35.

3 Results and discussion

To investigate the on-surface reactions， we initially sublimedthe precursor DBMA onto an Au（111） surface held at roomtemperature. This resulted in the deposition of moleculesforming amorphous aggregations aligned along thereconstruction line on the fcc region （Fig. 1a）. An individualDBMA molecule displayed a stereoscopic adsorptionconfiguration on the surface （Fig. 1b，c）. Although we couldn’tunequivocally identify the azulene moiety， the observedmolecular size （1.3 ± 0.1 nm in length and 0.9 ± 0.1 nm in width）closely matched the azulene dimer in the reference 31.Furthermore， we verified the presence of unobstructed precursormolecules by detaching a couple of DBMA molecules， as shownin Fig. 1d，e. This observation suggested that the debrominationreaction likely did not occur during the deposition. However， toconfirm this conclusion， further spectroscopic measurements，such as X-ray photoelectron spectroscopy， would be necessary.

Subsequently， we subjected the sample to stepwise annealingto trace the on-surface reactions of DBMA. At annealingtemperatures below 100 °C， no significant changes wereobserved in the STM images. When the substrate was annealedto 150 °C， we began to observe some straight chains withperiodic protrusions （Fig. 2a）. The average distance betweenthese protrusions was measured to be 1.8 ± 0.1 nm （Fig. 2b，c），which aligned well with the estimated size of an Au-mediatedstructure containing a periodicity of 2 nm. Additionally， someshort fragments dispersed on the surface appeared darker thanthe protrusions （Fig. 2d，e）. The periodicity of such structureswas determined to be 2.4 ± 0.1 nm in length and 1.2 ± 0.1 nm inwidth， and they were identified as Au-mediated dimers with aflat configuration. As the temperature increased， the Aumediatedpolymeric chains became distorted， and the brightprotrusions in STM images gradually disappeared， as illustratedby the statistics in Fig. 2f. This indicated the planarization ofinitially non-planar structures during annealing.

At a substrate temperature of 300 °C， the contrast in STMimages became uniform， suggesting complete planarization.Interestingly， within those long but twisted polymeric chains，some periodic structures （as labeled in white circles in Fig. 3a）were observed. The periodicity of these chain structures wasmeasured to be 0.74 ± 0.05 nm in Fig. 3b， corresponding to theperiodicity of the demetallized polymer structure （approximately0.78 nm）. Bond-resolved AFM imaging indicated that themethyl group reacted with a neighboring seven-membered ring，forming only one six-membered ring as presented in Fig. 3d（also known as poly-benzo[a]azulene in Scheme 1c）.Furthermore， we noticed some bright contrasts near the sevenmemberedring of azulene units （blue circles in Fig. 3c）. Giventhat Br atoms are likely to desorb from the surface at thistemperature， the much higher signal intensity might be attributedto the attachment of an individual Au adatom 35. Our AFMsimulation results， as shown in Fig. S1 （Supporting Information），unequivocally demonstrate the image features when an Auadatom is considered around the structure. This bondingpreference indicated the possibility of an unpaired electron atthis specific position when the methyl group participated in the cyclization.

In addition to the previously mentioned structure， we alsoobserved some defective chain structures （white circles inFig. 3a） that differed from the short-ordered polybenzo[a]azulene. By resolving the atomic arrangement of thesedefects （Fig. 3e and 3f）， we could gain further insights into thereaction behavior of DBMA on Au（111）. One typical defectresulted from the enantiotropy of DBMA precursors， as shownin Fig. S2. The presence of cis-isomers led to noticeable chaindistortion. Moreover， bond-resolved AFM imaging indicatedthat some defective structures exhibited key features of polynaphtha[ab]azulene， where the methyl group formed two sixmemberedrings， each adjacent to a seven-membered ring （redcircle in Fig. 3g）. However， there was apparent chain distortiondue to the built-up strain in this structure. An additional fivememberedring formed because the two adjacent sevenmemberedrings came too close to each other. Conversely， thedistance between two adjacent seven-membered rings expandedtoo much for C―C coupling. We concluded that theunsuccessful evolution of poly-naphtha[ab]azulene from polybenzo[a]azulene was due to such a strain effect （A schematicdescription of the possible process is shown in Scheme 1c）.Therefore， we propose that a larger substituent than a methylgroup， such as ethyl or iso-propyl， would be advantageous inrelieving the strain and facilitating ribbon formation usingazulene derivatives.

4 Conclusions

An azulene derivative， DBMA， featuring two pre-installedmethyl groups， was synthesized in solution and thoroughlyexamined on Au（111） surface using bond-resolved scanningprobe microscopy. This investigation aimed to explore the onsurfacesynthesis of non-benzenoid CLPs. Short-range orderedpoly-benzo[a]azulene structures were successfully obtained.However， the strategy of methyl substitution did not proveeffective in the transformation from poly-benzo[a]azulene topoly-benzo[ab]azulene. The introduction of additional methylgroups had the advantage of promoting the linear alignment ofthe precursors due to steric hindrance. However， this approachresulted in relatively weak molecule-substrate interactions， as evidenced by the non-planar adsorption of azulene units and theformation of defective arrangements in the resulting products.Furthermore， through the analysis of the defective ribbonstructures， we concluded that the introduction of excessivestrain， which occurs when two fused six-membered rings areformed during the hypothetical twofold dehydrogenation of amethyl group， had an adverse effect. This strain not onlydisrupted the long-range ordering of poly-benzo[a]azulene butalso encouraged unwanted dehydrogenative coupling on oneside while hindering the creation of desired polynaphtha[ab]azulene and the long-range ordering of polybenzo[a]azulene. Moreover， we believe that the organizedazulenic chain polymers may pave the way for the developmentof two-dimensional nanoarchitectures characterized by nonbenzenoidrings.

Author Contributions： Conceptualization， L.C. and K.M.;Methodology， M.W.， M.M. and X.L.; Validation， H.Z.; DataCuration， M.W.， M.M.， X.L.; Writing – Original DraftPreparation， M.W.; Writing – Review amp; Editing， X.L.， C.X.，Q.C. and H.Z.; Visualization， M.W.; Supervision， K.M. andL.C.; Project Administration， L.C.; Funding Acquisition， L.C.

Supporting Information： available free of charge via the internet at http：//www.whxb.pku.edu.cn.

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國家自然科學基金（51821002， 22072103， 22161132026）， 蘇州納米科技協(xié)同創(chuàng)新中心， 江蘇省高等學校重點學科建設項目（PAPD）， 高等學校學科創(chuàng)新引智計劃（“111”計劃）以及江蘇省卓越博士后計劃資助